Preface

There are not many energy sources that are truly renewable in the very long term--wind, water, geothermal, and solar. They will be available as long as the earth is habitable. Other sources that we know of last only a time. If all available heavy nuclei in the earth are fissioned, this would supply present and near future energy needs for about a few thousand years. Fusion of ocean deuterium, including generated artificial tritium, gives almost long term energy. Perhaps a few hundred thousand or a million years. On the time scale of species, let alone geology, this is short.

Wind and water energy are really single products--electricity. Geothermal-energy from the deep earth--provides direct heat and indirect electricity. Solar energy is exceptional, since it can be turned both directly and indirectly into electric power. It can also provide indirectly liquid fuels and directly heat for a future without much or any fossil fuel heat. The earth’s plant and animal life depend on it. If any renewable sources becomes universal and usefully varied, it is solar energy. Solar energy is going to be our main replacement  for the fossil carbon that will soon begin its permanent inescapable decline, dictated largely by geology. This decline will affect everything we do, transportation, industry, housing, agriculture, and beyond. Everything. If we deliberately speed the decline due to our concern with the worsening global climate, the effect will hit us soon.

On the surface of the earth, solar energy is erratic. It varies during the day, it is largely intermittent because of clouds. It varies by season. Seasonal variation is a very serious problem, as it is for wind energy. Escaping the earth to space and the potential of at least constant vast electric power is an engineering feat that is now beyond us.

My objective is to estimate for the U.S. how we might replace future missing fossil carbon energy at something near the present total “useful energy”. (This is end energy available, without the large heat losses from fossil carbon.) It is about a third of what is usually called “primary energy”, that mostly got by burning fossil carbon. I think that this is an essential first step, or tentative goal, in organizing our thinking about what I have called “replacement energy”. Actually reducing the total amount of useful energy the U.S. requires is a very complex task, and not just a technical one.  It might be good to keep in mind, from earlier books, that the U.S. has always been a high energy use nation on a per person basis--a third or half of our admittedly large present primary energy use per person.

It may appear otherwise, but the exploitation of solar energy is still near its beginning. We do not see clearly how the technology will develop. But it will change the future greatly. So this book is just a personal and temporary view of solar energy and the problems we face exploiting it. As in the other books in this series, my key notions are physics facts, logical analysis and rough numbers. Anybody, not just experts, can form opinions and questions based on public information, and I hope that you do so. These books are, I hope, a start.

Introduction

Our solar energy generation now does not amount to much. As distributed electric power, solar amounted to only about 7% of the total in 2018. Wind energy was almost 3 times greater. Small as this is now, as fossil carbon declines, we will need replacement power sources such as solar.

Looking at our present electric power use in the U.S., it is about 400 gigawatts on average, a number I use often. But the actual capacity to produce power is a bit greater than twice that, to account for variable demand. So it is not unreasonable to suppose that future solar electric power that supplies 400 gigawatts on average would need the ability to produce twice that when required. This corresponds to a capacity of about twice the average need.

As we replace fossil carbon energy with other energy, electric power demand will go up drastically, as I will come to later.[1] If our future average electric power demand later this century is even twice that 400 gigawatts now, solar energy is at present perhaps short by a factor ten or twenty. The factor would be even greater when including future direct solar heat energy need, a minor use now.

Overall, there is a big shortfall, looking out to 2100.

This little book is an attempt to collect some information I find useful on solar energy and project a bit into the future.

Photovoltaic (PV) solar panels are widespread, but they are now mostly used on a small scale. Panel groupings that produce a few watts or even a few kilowatts are very useful, but they are not a critical part of our energy future. Large installations do exist now, built over the last ten years. But were few and very costly at the time. [2] That is slowly changing and the investment costs of big installations is dropping. As of 2020, the market for large “utility scale” solar panels is quite erratic and it is unclear where it is headed.

Despite this small scale now, much of the hope for future large scale solar electric energy rests on photovoltaic panels. They are now cheap enough to become common.  With present technology efficiencies of mass produced panels are commonly about 15%; this is increasing only slowly.

What most present PV installations alone cannot do is store energy, which will create a significant cost once storage is included.[3] As an example involving storage, one solar farm might produce power, without storage, during the day and a second would store power for later use. We will see that storage longer than about 12 hours can become very expensive (at least at present), so this sort of daily storage is still possible, if costly. Storage for seasonal variation in solar illumination is quite a different matter, as will become clear later.

Large scale indirect solar heat--in light concentrating “power towers”, for example--can be stored at very high temperatures, several hundred degrees C, for later conversion to electric power. It is a good way to store energy for electricity for short times, normally less than a day. If heat losses can be kept low it is even possible to imagine large scale storage of energy as heat for longer times, perhaps a month or so.

By contrast, direct solar heat (for example to hot water or other storage media) is already efficient even in the northern plains [4]. Such small scale direct solar heat is often used at low temperatures, up to about 150o C with the heat used later, possibly days later.

Intermittent energy supply, due to fluctuations in solar illumination, creates a big problem, as it does for wind power. At best, fluctuations can only be predicted on average in the short term--essentially due to local weather. For the most part these fluctuations are random and even large. Such intermittency will create serious difficulties. For example, rapidly fluctuating DC power from solar PV farms has to be dispatched to AC grids whose power demand cannot change on the same time scale. Also, these grids need accurate power at a frequency of 60 Hz. Our present grids have evolved to deal with this need by exploiting the constancy of rotation of mechanical generators. Grids with mostly intermittent solar and wind energy inputs will have to use different methods to get proper synchronized power.

Solar illumination varies a lot around the U.S., but not by as much as you might guess--a factor 1.5 is common for sunbelt cities versus those in the northern plains. [5] If PV panels (and their grid interface) are cheap enough, it can pay to use them even in northern parts of the nation.  This has led to the idea that local grids can generate their own power from PV farms. But keep in mind that a city with a million households, plus industry, can easily have an average power demand now of 3 gigawatts. Plus, there is a need for energy storage. This is a large scale effort, as we will see later on. [6] And this is not the total energy use of such a city--it leaves out building heat and other inputs to industry. Detaching a moderate size city from a national or regional grid is not easy to imagine.

As our fossil carbon fades away, much of our future energy will come some form of solar illumination. This will likely be significantly more important than energy from wind. Whether or not solar energy will be sufficient is beyond the scope of this small book.

Chapter 1-- Resources and Preview

The solar energy resources of the U.S. are large. To orient ourselves, consider the land area that would be needed for photovoltaic panels that provide just an average of 800 gigawatts, possibly what we will need by 2100. From earlier books, a rough number for solar panels is that they deliver an average of about 30 watts per square meter of panel. A simple calculation then gives for the land area needed, even allowing for filling only half the area with panels, about 0.7% of the U.S. land area. By comparison, arable land suitable for crops is about 17% of the total. So there is lots of land. Obviously, the real resource question is the amount of specialized materials needed for the panels, not the available land. This chapter will only deal with the easier resource question.

The geographic illumination of the U.S. is well known. Solar resources become more complicated when we have to consider the character and quality of the solar illumination resource. Much of this information comes from NREL-the National Renewable Energy Laboratory.

Averages

Here is the average yearly illumination plot from NREL

(There are lots of other plots at the NREL site.)

Because this is an average, it does not indicate the seasonal effects. For example, in Phoenix the day length varies from 10 to 14 hours depending on the season. In Minneapolis, the day length varies from 8 to 15 hours. That, and the changing inclination of the sun with season produces the following horizontal illumination plot (for Arizona)

The average plots do not show how the illumination changes with time. This is mainly influenced by cloud cover--how much clouds and how cloudiness changes. The average cloudiness is a bit surprising. Here is the average cloudiness, from Brian Brettschneider. Even Arizona has 30-40% average cloudiness!

Variability

So how rapidly during a day does the illumination change? Clearly some days are perfectly clear the whole day, others not. There is no terribly good public information on this. Here is a plot from Jacksonville, Florida over a number of years (ignore the glitch),

Illumination is very erratic on this time scale, but what would be more interesting would be plots of hourly illumination over a single year. (The book on Wind Energy had hourly Texas grid data on wind.) I will come back to this, as it properly belongs to a discussion later in the book. However, here is the solar power produced (not illumination) over the whole European Union in 2014; the hour count starts in January:

It is both erratic and shows a large seasonal variability.

Uses

Solar energy is much more useful than wind energy, which is at present entirely a source of electricity.

Solar energy has grown our food since the time we were hunters and gatherers. Indirectly, it later provided mechanical work by animals fed by foraging. But we now think of solar energy mostly as direct and indirect heat and also as a source of electric power, often through solar heat.

Most of what I will have to say concerns solar power as electricity. Perhaps the most popular are photovoltaic panels, which produce power without heat (although there is some heat, due to the inefficiency of solar panels). But solar heat can also be used to produce electric power by heating some fluid that is then run thru turbines.[1]

But solar heat can be used directly to warm houses or to provide hot water, either for direct use or to indirectly heat houses. In the form of heat, it can even be stored for later use.

I will later on focus on electric power.

Chapter 2 -- Technology: Photovoltaic Panels and Batteries

There are vast applications of photovoltaic cells, from wristwatches to home electric power. [1] On a small scale, milliwatts to tens of kilowatts, these cells are everywhere. Some are even integrated into local electric power grids.

I will mainly discuss large installations of PV panels in this chapter--megawatts and up. Even now, many of these systems employ battery backup and this will become common in the future.[2] So I want to consider not just PV panels and systems, but also the technology (and cost) of batteries here.

A vital issue here is the gigantic scale of a national PV panel power system. In earlier books, I used an average delivered power per square meter of solar panel of 30 watts. [3] Our present total average power demand is about 400 gigawatts--a ballpark number that is easy to remember. This amounts to a yearly electric power delivery of 3500 terawatt hours (the actual number is a bit bigger, around 4000 terawatt hours, but let me use my ballpark number here). Let’s suppose that once industrial and all other end, or useful, energy use is twice the present value--that would be around 800 gigawatts average demand. If a national PV solar panel system were to deliver, say 300 gigawatts average of this demand, it would come to ten billion square meters of panels; if a panel were to be around one square meter, this would mean manufacturing ten billion panels. If this were to take 30 years, it would amount to making some three hundred million panels per year. Since the panels would at best last 30 years, this means that once the system is complete, construction of a new system would begin on the same scale as before; the expense would then also be continuous. [4] It also means that the expired panels--to the tune of some 300 million per year--would have to be recycled. This recycling would be unavoidable because, as we ill see, many of the panels would be made of rare elements whose names we are not now familiar with (cadmium, tellurium, indium…). Recycling would not just be a matter of avoiding pollution from panel residue and mining.

On to some physics and technology. Here it would be good to keep in mind that much what follows is technology that took 3-4 decades to develop. It will not change much in just a few years.

Silicon Solar Panels

Sunlight can cause some special materials to release electrons inside, as the light is absorbed and, under the right conditions, a voltage is produced. This is the photovoltaic effect. (There is a Wikipedia entry on it.) The effect was discovered in 1839 around the time the United States was developing its railroads at a time that electricity was a novelty. Around the end of that century, early solar cells appeared; unfortunately they were not useful because they could not turn much light into free electrons in their interior. It was not well understood how a voltage  was produced. Early cells had low efficiency--not much energy produced from the light energy. Developing photovoltaic cells to become useful was a hundred year physics task.

The nature of these cells depends on the properties of atoms inside a solid material; it is an application of solid state physics. [5]

We think of common materials as either being conductors, like metal, or insulators. An example of a good insulator is common silicon. But it is possible to change silicon by adding other atoms, beyond rare contaminants. [6] One way of doing this is to add atoms that have an almost loose electron, one that is easily removed to float around in the solid; phosphorous is an example. Another way is to do the opposite--add atoms that want an electron; boron is an example of this. Silicon “doped” with atoms that release electrons easily, leading to electron conduction, is “n-type”, meaning negative. The opposite, with atoms that want to pick up an electron is “p-type”, meaning positive. [7] A block of pure silicon thick enough to absorb most of the light on it will release a lot of electrons, and holes as well. But where do they go? If the back of the silicon has p-type atoms, it can pick up the electrons. If you run a wire from the front of the n-type silicon to the back, a current will flow. With a resistor in the path, there will be a voltage difference between the front and back, hence “photovoltaic”. Here is a picture, from the Handbook mentioned earlier; it is a schematic of a pure crystal silicon solar cell:

When an electron is knocked out of one of the p-type atoms, it leaves a positive charge. This positive charge (called a “hole”) can move from atom to atom--it is the moving h+ in the picture. The tricky part is the needed thickness of the regions with one atom or another--n-type or p-type. For silicon, the atoms needed for “n-type” are usually phosphorous and for “p-type”, boron. (See the endnote for more on this.) Because the light must be absorbed to be useful, there are complicated tradeoffs between the thickness of the silicon and the thickness of the layers with their extra atoms. For pure silicon, this has meant that the cell has to be about 0.2 millimeters thick to absorb sunlight (this can vary). Oddly, silicon is not considered a very good light absorber despite this rather small thickness.

The picture here is very simplified. Two questions might occur to you. One is “where does the voltage come from ?” and the other is why the electrons and holes flow as drawn.

The voltage in the device in the picture is actually built in. No sunlight needed, although without sunlight no current can flow. The two layers shown are not precise--there is an excess of positively charge phosphorus atoms just above the line dividing the layers and an excess of negative boron atoms below, due to the motion of the electrons and holes. This leads to an electric field that forces electrons up and holes down, at least near the interface of the layers. More important is the fact that the electrons and holes wander around and can be picked up by the metal at the top and the metal at the bottom. [8]

The great advances in solid state physics over the last century led to the realization that the basic idea can be applied in very many different ways--not just with this “doped” silicon. But today it is still true that most commercial solar cells are of pure silicon with carefully added other atoms to make a semiconducting wafer that can produce electricity from sunlight.

Individual industrial silicon cells might be 10-15 centimeters on a side and producing around 0.6 volts. This is why the panels you see need 20 or more individual cells to reach even 12 volts, familiar from auto batteries. The individual cells have to be wired in series to get to this 12 or more volts. Here is a picture of such how cells are arranged (from the Handbook):

and here is a familiar picture of a collection of individual panels (from Wikipedia; these appear to use polycrystalline cells, which are square):

Individual silicon cells are fragile; making such panels with these cells and in the millions--and to last outdoors for 30 years--is a challenging task. It is done, but not at all simply or cheaply (the panel cost is almost twice the cost of the cells in it). But the panel price has been on a long decline. In 2020 you can buy single panels for under $1 per peak watt; the price per average watt delivered is about 5 times this, or under $5 per average watt delivered. Depending on the scale, the panels themselves cost from a third (large scale) to half the system cost.

In 2015 the largest system built of crystalline silicon cells was “Solar Star I and II” in California at around 0.75 gigawatts peak power. The average power is about 1/4 the advertised peak power--favorable due to sun tracking on one axis. It was not possible for me to find good project cost data; I think that it was around $3 per peak watt or $12 per average watt delivered. [9] There are now larger systems, but information is scarce. Of course, solar module prices decrease. It is less clear what is happening to construction costs. The overall cost per average watt for large installations may not decline a lot.

It might be worth pointing out that if the cost of “Solar Star” was really $12 per average watt delivered, this amounts to $12,000 capital cost per average kilowatt delivered, or $12 billion for a gigawatt. This is not terribly cheap compared to, say, nuclear power, for which the average power delivered is about 90% of the maximum. Plus, solar farms really need battery backup, not included in this number. Hopefully this comparison will prove too pessimistic as PV power expands.

I will not go into more detail on  silicon cells except to note that a big advance was the development of cells that do not need pure single crystal silicon, so-called “polycrystalline” cells. They are cheaper, but less efficient. There are also amorphous silicon cells. There is a later short endnote on these.

A big disadvantage of both single crystal and polycrystalline cells is the amount of material needed for them. The 10x10 centimeter or 15x15 centimeter individual silicon cells are only about 0.2 mm thick, sawed out of several meter length pure silicon chunks or block polycrystalline silicon.  But consider what happens if you imagine making 10 billion square meters of such cells for the U.S. This is about 24 million tons of PV grade silicon. Using 1/30 of this per year would be 80 thousand tons per year, about four times the present world production, according to the Handbook. The environmental consequences of such large scale production are not clear to me. [10]

Despite the demand for a lot of single or poly crystal silicon, this remains how most solar panels are made now.

Possibly large future silicon demand is why there has been a shift to thin film solar panels--”thin” meaning a cell thickness of several micrometers, not hundreds of micrometers. Such thin cells are made by chemical processes, usually some kind of deposition method. It is important that even the thinnest layer be able to absorb all the incident light. This is unlike crystalline silicon, which needs to be thick to do so.

Thin Film Cadmium Tellurium Solar Panels

There are many different types of thin film solar cells made in laboratories. Alas, there remains a very big gap between what can be done in the lab with test solar cells and what can be manufactured on a large commercial scale--million of panels. Understandably, laboratory press releases emphasize future possibilities, not the past many decade long effort to commercialize the results of lab studies. It is good to keep in mind that near term very large scale deployment will probably use existing technology, not cells just now emerging from labs.

So, I do not want to describe the many present options because one method has made it to large scale commercial production--Cadmium Telluride thin film panels. If thin film panels do become important in large systems, this is a good example to think about in a bit of detail. The first thing to note is that, with the exception of low efficiency thin film silicon cells, these panels use exotic elements. (The same is the case for many thin film lab cells.) Cadmium is notoriously toxic and Tellurium is very rare. Despite this, thin film panel production works well commercially, and the CdTe compound is itself not toxic, and quite stable.

CdTe panels have been in use in the half gigawatt peak “Topaz” solar farm in California since 2014 that I mentioned earlier. (It has its own Wikipedia entry, which is where I got the image that follows.) The nine million 16% efficient solar panels from the First Solar company look like this from space:

(Keep in mind that the ballpark ten billion meter square panels I mentioned earlier would occupy a thousand times this land area, but widely distributed over the U.S.)

The technology behind these now commercial panels was quite revolutionary; the original cells date from 60 or so years ago and there has been intensive research for over 40 years. The most impressive thing about all this is just how complex CdTe cells of all the various types are and how many different approaches have been tried before something commercially workable emerged. A key feature that I cannot go into is just how thin such a cell can be and still absorb all the incident sunlight: a few micrometers, far less than silicon.

One striking thing to me is how pervasive the physics concept of electron conducting “n layers” and hole conducting “p layers” has been. CdTe cells have no phosphorous and boron doping such as those in silicon, but the physics is similarly understandable, if much more complex in detail.

There was no straight line development from physics origins to commercial product.

The result of this decades long process is capsuled a bit in what follows (for details, see the Handbook)

Thin films are layers deposited on some material strong enough to hold them and, often, transparent. Glass is popular. Here is a general sort of picture from the Handbook:

where the CdTe layer is “p-type” and the much thinner CdS (cadmium sulfide) layer is “n-type”--that is, the one with electron charge carriers. Notice something very important: the crystals of CdTe are small, of order the film thickness or less. This ability to exploit small crystals in films is now an industrial process for CdTe. However, it is worth keeping mind the decades long development that led to this picture, which schematically indicates how a modern cell is made.

Thin film cells offer opportunity to introduce clever ideas on how individual cells are connected into a module. It is even possible to eliminate the complex wiring of silicon cells by incorporating it into the film itself:

This is, of course, still necessary since individual cells only produce 0.5-0.6 volts.

The question remains--is it possible to make vast numbers of panels based on this technology, given that they use often rare and sometimes toxic elements?

There is much interesting data on CdTe solar cells in the Handbook. One fact is  this: per peak megawatt per year of cells (with a 2 micrometer absorber layer), manufacture now uses 0.4 tons of Cd (always metric), 0.6 tons of Te. Cadmium is extracted from Zinc, Copper and Led refining. Tellurium comes as a byproduct of electrolytic refining of Copper.

Adjusting this to my 300 average gigawatts of PV cells (as panels), rashly assumed to be all CdTe, a 30 year installation would need 10 gigawatts average per year or peak 50 gigawatts or 20,000 tons of Cd and 30,000 tons of Te. Yearly production of Cd is about 20,000 tons worldwide per year; for Tellurium the number is about 1600 tons per year, a factor 190 smaller than the projected demand. My estimate here is, of course, extreme, but it indicates the scale of the problem.

I do not want to go into details, but one other quite different thin film solar cell technology might require Indium. Yearly production of this is now about 900 tons. It is likely that most present thin film cell technologies will run into element production issues.

I mentioned that a big advance with thin films has been the use of films with small crystals, of a size roughly similar to film thickness--a micrometer or less. This is quite unlike the present prevailing panels using large and sometimes ultrapure silicon crystals. Naturally, there is work in laboratories with silicon. Silicon is so common that if it is possible, thin film silicon cells would face no resource problem at all. If and when this technology  will be commercialized in the scale we will need is uncertain. [11]

What is unhappily clear is that it will be very far from simple to make PV systems on the scale we will need, totaling a hundred square kilometers of panels or so. That we now get a few percent of our electric power this way tells us very little--almost nothing, in fact--about the future

Systems

Integrating PV panels into a large system, hundreds or thousands of megawatts peak power, is growing rapidly as of 2020. (The largest system now, in China, has 2.2 gigawatts peak power.) But there is an unresolved question: can the power grid absorb huge amounts of PV energy--on the scale of hundreds of gigawatts average power?

There exist some computer simulations of grid connected PV farms, but I think that this is inadequate. What is needed is studies of actual operating and grid connected farms. There is only one that I know of, conducted by NREL, published in 2016 (NREL/TP-5D00-65368)

Unfortunately, the plants are pretty small at around 20 megawatts peak power, with no batteries. But the study is still important. There is an image here, of a Texas (ERCOT) system. Our present two national grids have a number of problems. Often local system failures can lead to large scale disasters, as individual power lines or generators are switched off to protect them. Such failures propagate and create blackouts (some of this is described in an earlier book). Another problem is caused by the vast number of electrical motors on the grids. They absorb electric power as they operate. But they also feed power back into the grids--power that is not used but can be thought of as sloshing around. This is “reactive power”. This NREL study has several points of interest. First, it is possible for PV systems to react quickly to unusual or dangerous grid conditions, such as a wrong grid power frequency or low voltage. Second, these farms have some capability to “ride through” such bad conditions without having to be shut off. They also can be configured to deal with reactive power. The image here shows one way; there is a large capacitor bank on the PV system (near the top of the image).

It is essential that a PV system turn the DC power from the panels into AC power and control its level and the amount injected into the grid. That is shown on the image as many DC/AC “inverters” that make the power in most of the system AC at different voltages, managed by numerous transformers. The control system is not shown here. This part of the system is quite complicated.

This all sounds quite positive for large scale PV power on our grids, but it would be good to study really big operating plants, on the gigawatt scale. There is, I think, rather too much reliance on computer modelling.

I already commented briefly on the cost of large PV systems. The Solar Star I and II crystalline silicon plants with one axis solar tracking had a capital cost of $12/Wav . [12] Similarly, the Topaz CdTe solar cell plant (no solar tracking) has produced 146 megawatts average power at a capital cost of $16/Wav. [13] It appears that for most PV systems the solar panels cost about a third of the total capital cost. To repeat: none of these are, at present, very cheap.

Batteries

Batteries are not really very effective energy stores, despite their popularity. Remember that, per kilogram, batteries store just over a hundred times less energy than contained in the same mass of gasoline. A typical modern lithium ion battery installation at a solar or wind power plant or electrical switchyard might amount to a few tens of megawatts to a few hundred megawatts. Present storage times might amount to one to four hours--not really a long time. These are “utility scale” systems and are much more complex than the battery in an electric car. A complete free-standing system would involve many thousands of battery cells, power controls, elaborate cooling systems, and connection to the grid. The Energy Information Agency estimated an average cost per kilowatt-hour of $650 in 2018. The cost is dropping, but if cells get cheaper, the rest of the system likely will not. A future ballpark number of $300 per kilowatt-hour is not unreasonable.

At a cost of $300,000 per megawatt hour, a hundred megawatt hour battery installation might cost $30 million and have a mass of almost ten tons. This is perfectly reasonable for this sort of application; whether or not it is good for other applications is not immediately clear. Keep in mind that our scale should be likely measured in gigawatt-hours, not megawatt hours.

Let us investigate a battery storage option--imagine storing the energy produced by a modest PV farm that produces an average of 100 megawatts. Storing ten hours of produced energy (one gigawatt hour) would need a lithium ion battery that would cost around $300 million, even at an unrealistic 100% efficiency. Using my earlier $12 per average Watt for the PV farm, this would amount to a third of the farm capital cost. So this sort of energy storage is feasible, but not cheap. A real problem with erratic solar energy is that, historically hourly fluctuations are common solar energy for a longer time is not unknown. Ten hours storage will help, but it will not be decisive. And such storage for my ballpark national PV demand estimate of 300 gigawatts average would amount to probably tens of millions of tons of batteries and enough power for many millions of electric cars. [14]

I will go into the erratic nature of PV electric power later, but I should note here that the resulting fluctuating strain on batteries is much greater than that on electric car batteries. It will limit battery lifetime, so that the total battery capital cost over the lifetime of a solar PV farm would be greater than I estimated here.

Eventually, the supply of lithium metal will become a problem as well. [15]

Lithium ion batteries are a good example to use to judge all battery technologies; that is why I am going into a bit of detail here. Lithium ion batteries have a long history of decreasing prices (but not mass). Over the next decade or two their price, per kilowatt-hour, might drop somewhat per kilowatt hour. A cost of $150 per kilowatt hour is approximately the goal set by our Department of Energy for the price of energy storage for all battery types, ignoring any problems this scale of production might lead to. [16] In the future, we will likely be looking at many tens of millions of tons of batteries of all types.

I want to briefly comment on other sorts of batteries that have emerged in the last decade. Many of them are more promising than lithium ion batteries, but just how their cost, resource demand and environmental consequences will evolve is not known. The number of novel types of batteries is large. [17] I will mention only one type, to illustrate where developments are headed.

Flow Batteries as an Example

There are many types of flow batteries. The common feature is that there are two electrolytes that are separated liquids. They can be flowed together by pumps and the current that is produced is an electron flow through a membrane. A currently popular version is the Vanadium redox flow battery, Vanadium dissolved in Sulfuric and Hydrochloric acid. The Vanadium is in both electrolytes, but in a different chemical state in each. [18] This sort of battery can have a lifetime of decades and low maintenance costs. They are expensive and heavy, about 2000 tons per 100 megawatt hours capacity. These batteries do have a higher energy capacity per cubic meter, roughly 60 megajoules. By comparison, a pumped storage plant that raises water 100 meters would store only one megajoule per cubic meter. A very big battery installation with, say, one gigawatt hour capacity would weigh almost a hundred thousand tons, be staggeringly expensive and cover a lot of land. These batteries will probably be limited to around a hundred megawatt-hours--a good use would be in power plants and switchyards. They can be turned on in milliseconds, which is good for grid control and for emergency situations where power is lost for less than an hour or a few hours. [19]

I have chosen these two because you can buy them now. Other batteries, some based on cheap Zinc compounds, are not yet commercial. I do not want to discuss these here; nobody really knows if or when they will be available in industrial quantities.

Over the next few decades many newer types of battery will become commercial. Virtually all of them will be limited in some way, mostly to energy densities below about one megajoule per kilogram. The power delivery, in watts, will be a difficult problem for many battery types. [20]

It will take a long time for costs to drop below many hundreds to a thousand dollars per kilowatt hour. The significant price decline of lithium ion batteries has taken two decades.

Keep in mind that these estimates here are for batteries with short term energy storage, to buffer the hourly or daily fluctuations in power needs. Long term storage, such as seasonal, is a completely different matter. That need is so huge that no battery technology is relevant. I will come to it later.

Chapter 3--Technology: Solar Troughs, Towers, Storage

For millennia, energy from the sun has been either via plant photosynthesis or just as heat. Photosynthesis as a source of energy for us and the other animals is incredibly inefficient, probably less than 0.1% of incident sunlight gets turned into edible or burnable plant matter energy. Heat has long been just environmental warmth, welcome or not. Avoiding solar heat or its lack, for us, was a matter of shade or insulation. The photovoltaic effect changed the first, replacing photosynthesis with a new way to turn sunlight into a more useful form of energy, with hundreds of times the efficiency of photosynthesis. The second has also changed because of the invention of ways to concentrate sunlight heat--by mirrors and lenses. It is not unusual now to concentrate sunlight by factors of a hundred or more. The effects of this are profound.

Solar panels that are 15% efficiency actually loose 85% of the incident energy as heat. Concentrating sunlight by mirrors, by contrast, produces heat with only a small loss. Alas, converting this heat to electric power suffers from the heat loss of thermal engines, typically 70% or more loss, less in exceptional cases. This still leads to an overall efficiency of maybe 30%. The price paid is in the complexity and cost of the engines needed--boilers, turbines, generators. The result is that this concentrating solar energy is not cheap and can often cost more than solar panels that produce the same output energy. But there is an advantage. The technology of the engines needed is well understood. It does not require rare elements or complex and often toxic chemical processes. If the secret of photovoltaic cell manufacture were ever lost, these technologies, evolved from the nineteenth century, would still work.

But at present, almost all speculation about future solar electric power concerns PV panels deployed on the gigantic scale I have mentioned. So, I only want to discuss two of the ways solar heat can be converted to electric power: solar troughs and solar power towers. Such plants exist on a large enough scale to give us an idea how this might work on an even bigger scale in the future.

I want to focus on large scale electricity generating plants, of which there are not many in the U.S. at present

Solar Troughs

The idea of solar troughs is really simple. We are used to parabolic mirrors that focus parallel light to a point, or sunlight to a very small intense image of the sun itself. The same idea works, but a bit differently, if the mirror is like an opened can whose sides are parabolas. If the can is pointed at the sun, each little section of the can focusses sunlight to a small straight line image. It looks a bit like this (from Wikipedia):

A long trough that is pointed at the sun will produce an image that is a straight line along the trough at the focus. Of course, it is impossible to always point a trough directly at the sun without moving it bodily around, since the sun moves. That is where the following trick comes in. If the trough is pointed at the exact altitude of the sun, even if the sun is off to the side, there is still an image, only a piece of the image is missing at one end and runs off the other end of the trough. For a long trough, this matters very little. Now it is easy to just keep the trough pointed at the altitude of the sun even if the sun itself is off to one side or the other. As the sun rises and sinks in the sky, the trough only needs to move up or down in the one direction, pointing at the altitude of the sun alone. Here is the result (also from Wikipedia; the tanks in the back right may be molten salt heat storage):

The trough system itself consists of the parabolic reflectors, the tilt mechanism and the tube running down the focus of the reflectors. That tube is important and is usually an evacuated glass shell around a metal tube with fluid in it. (The glass shell with a vacuum is to keep convective heat loss from the metal tubes.) Because of the large size of the panel field you see, there are limits on the heat transfer fluid. It has to be very stable, even at high temperature, easy to pump through the system and not toxic. Since the field needs some thousands of tonnes, it cannot be too expensive. The fluid used now satisfies all these requirements. The solar heat increases the fluid temperature about 100^0 C to just under 400^0 C. It then runs thru a heat exchanger to produce steam for a turbine and generator. It can also be used to heat molten salts that can store energy for when the sun is down. It would be more efficient if the steam produced were at a conventional 550 C, which would probably require a heat transfer fluid stable to maybe 600^0 C. Alas, the usual molten salts that can do this would freeze at night. [1] A whole field looks like this (from NREL; it includes extra options, beyond molten salt heat storage, that I will not discuss):

An interesting feature of conventional solar trough plants is the large amount of stored heat in the transfer fluid (there is a lot of it-over a thousand tonnes--in a large field). With some clever engineering, this stored heat can be used to even out the otherwise serious effects of clouds. [2] (It is also possible to use backup heat from natural gas or, in the future, hydrogen).

One of the early trough plants was the Nevada Solar One in 2007 to the present. [3] It produces an average 14 megawatts and had a capital cost of about $18/Wav . It had no energy storage. It uses about 2% energy from natural gas as backup heat for the heat transfer fluid. As a general rule, trough plants use about 10% of their output energy for pumping and other plant needs. Some do not use natural gas to start up in the morning.

A more recent one is the Solana generating station in Arizona, which produced an average 89 megawatts and cost, by my reckoning, $23/Wav . It has 6 hours of molten salt storage. I generally do not quote the capacity, but in this case it is advertised as  280 megawatts, which would make it 32% efficient. There are others, that can be found in an NREL compilation “Concentrating Solar Power Projects” The capital costs for trough plants, including energy storage, seem to be in the range around $25/Wav .

None of these I found seem to be cheap. They are about twice the cost of present PV systems with the same average delivered power. But keep in mind that this technology has only been commercial for less than 15 years and there remains  scope for improvement. Final systems at the huge level we need are far off in time, and development. To my mind, there have been rather too many early complaints about this factor 2 cost issue compared to PV.

Solar Power Towers

A disadvantage of solar troughs is the small light concentrating area, literally a trough, spread out along a tube. Typical numbers for their ability to concentrate light are about 50 times the incident solar energy. The result is that the temperature of the heat transfer fluid in the trough central tube is not high, 400^0 C or thereabouts. This limits the temperature of the steam produced when the system heat is transferred to steam for a turbine. And so the efficiency is limited. They can store heat in the usual hot salts now often used for storage. This heat can then produce steam for a turbine and generator at night.

Power towers do not have this temperature limitation. They concentrate sunlight by a quite different method. You may have noticed that the sun, reflecting off of a wristwatch cover glass or off a cell phone cover glass,  produces a bright spot the size and shape of the flat reflector. (The spot does have a slightly fuzzy edge due to the angular size of the sun.) This is what power towers do, with flat reflectors each of which produces a spot about its own size and shape at the top of the tower. With thousands to tens of thousands of mirrors, this produces in total a single spot, on an absorber, with an intensity a thousand or so times the incident solar intensity. The temperature of this spot can reach nearly 800^0 C or even more. It then becomes possible to use interesting heat transfer fluids. One option is simply to use steam itself, sending it directly to a turbine. Another is to use molten salts, mostly salts of sodium and potassium, whose heat can be either stored or transferred to steam. These work up to 550^0 C, a useful temperature to transfer heat to steam. The only disadvantage is that most salts freeze to a solid below roughly 200^0 C, so they have to be kept hot. This involves some clever engineering (and possibly extra heat sources) to deal with nighttime. A novel possibility is even to use molten metals as heat transfer fluids; they also have to be kept liquid somehow.

A remarkable thing about all power towers is that the very many reflecting mirrors, of some tens of square meters each, have to be computer controlled to reflect the sun onto the tower as the sun moves in the sky.

Power towers are not new. The first one in the U.S. was Power Tower 1 (producing steam directly), and its evolved Power Tower 2 (with molten salt) in the 1970’s.

There are now only two large power tower plants in the U.S., one Ivanpah in California and the other, Crescent Dunes in Nevada. The latter is not now producing power, apparently due to design and construction errors that caused the project to go bankrupt. It was (and may be in future) the only U.S. power tower that uses and stores molten salt. [4]

The Ivanpah plant in California is operational and very large, with three power towers and their fields of reflecting mirrors. Here is a view from the DOE:

I have no schematic or good view of a tower itself, but here are some of the hundreds of thousands of heliostats (by Chuck Jones):

The design is simple enough. The flat mirror is rotated around a vertical post to the right azimuth and around a horizontal axis to then reflect the sunlight to the tower. The whole field is controlled by a computer; this technology is quite old and well understood. One key to cheaper power tower plants is reducing heliostat cost.

The Ivanpah plant heats steam directly for the turbines. The used steam is condensed by air cooling. The plant needs  quite a bit of natural gas to start up in the morning; future plants might use hydrogen. Average produced electric power is about 91 megatwatts. Compared to the maximum turbine power, the efficiency is about 23%.

These plants mainly depend on direct sunlight, so they are affected by even light clouds. However, the stored heat in the steam must be large, affording some offset of light loss.

The Ivanpah plant, all three towers and heliostat fields, cost about $2.2 billion or roughly a capital cost of $23 per average watt, $23/Wav. This makes it about twice the cost of a PV system at the time a few years ago when California built them, as I mentioned earlier.

An interesting idea is to make concentrating solar power plants much larger than immediately needed and use the size to store energy as heat for many days, or even a week, rather than hours. The heat can then be later turned into electric power as needed. (This sometimes goes by the name “dispatchable power”.)  Of course, this is not a solution to the seasonal output of solar power.

Alas, most distributed energy has environmental and land use problems (wind turbines, as one example). There have been environmental concerns with concentrating solar power. The Ivanpah plants had to be fenced in to keep out desert wildlife (and photographers, as you can see). They also incinerate a relatively small number of birds, compared to the billions per year that we kill by other methods--pesticides, tall buildings, and even domestic cats. Industrial society has many difficult to cure downsides.

Concentrating solar power has other uses than as a source of electric power, since the heat can in principle be stored for later use in homes, district heating of buildings and in industry. I will come to this next.

Chapter 4--Technology: The Various Uses of Solar Heat

Besides light itself, a direct solar product is simply heat generated when light is absorbed. We just saw that the key word in “concentrated solar power” or CSP is the concentration. It is now common to concentrate solar energy by factors of 50 to 5000 and use this to produce sometimes extreme heat to create electric power.

But heat can also be created or moved using electric power itself. So there are two areas where heat can be exploited: directly by sunlight or indirectly thru electric power, possibly from a PV farm.

An example might be useful here.

In many homes and industries, heat is produced simply by burning natural gas. If what we want is the heat, gas can be burned quite efficiently to generate it. It is common now to create direct heat by burning gas at an efficiency of around 90%. (The rest is heat in exhaust gases.) For homes, we want heat in winter at a comfortable temperature--air at under about 25^0-30^0 C or generally hot water around 60^0 C. In industry, the heat of natural gas is used up to its flame temperature of 2000^0 C.

There is another way to generate heat directly by electric power, resistance heat--in homes with resistance heaters and in industry by electric arcs used to produce aluminum metal and silicon metal, among many other uses.

If all we want is heat at low temperatures--in homes and businesses--there is yet another way. We can move heat either from inside to outside (an air conditioner) or from outside to inside. The amazing devices that do both things are called “heat pumps” and are becoming common. Right now, an affordable air source heat pump in a home can be used as an air conditioner in summer and a heater in winter. Some can even serve as a source of hot water in a home or commercial building; this use will become much more common in time. The northern tier U.S. states and Canada need much better ones than available now and I think that they will arrive, possibly using carbon dioxide as a working fluid. A well insulated ordinary house or commercial space could probably be heated in winter and cooled in summer with an inexpensive air source heat pump alone. [1] (So called “ground source” heat pumps are too complex and expensive for most of us, but possibly useful for large buildings.)

In this chapter, I want to discuss several ways of using sunlight to generate and use heat, rather than employing electric power to create or move heat. There are so many ways to do this that I will have to be selective and incomplete.

Heat at Low Temperature

It will be good to keep in mind here that much of the U.S. has a dramatic climate: hot in summer and cold in winter--even dangerously cold in winter. [2] The following map shows cold temperatures in January (from Brian Brettschneider at http://us-climate.blogspot.com):

by comparison, summer maximum temperatures are high but in most of the country 100^0 F or less (see Brettschneider’s site). Modest insulation, clever shading, and cheap air conditioning has already become a routine way of keeping indoor temperatures under 80^0 F in even the hottest areas.

For most of the U.S. extreme winter cold is the problem.

The first obvious solar use, and even common now, is to use sunlight to create low temperature heat--mostly for hot water, usually at under 60^0 C (or 140^0 F) or air heat at still lower temperature. Sunlight for low temperature heat has been employed for a very long time, but now devices have become quite efficient. I only want to describe a very well known panel, the evacuated tube solar heat panel, which works well even in very cold climates. [3] They are particularly ingenious and interesting. Here is a picture of a tube panel on a roof, from NREL [4]. I chose this particular picture for a reason. The tubes are connected to a manifold at the top (which is to collect the generated heat), but spaced apart and not connected together at the bottom. This shows features of the evacuated tube collector idea. For example, the tubes can collect heat from broad angles (which is why they can be spaced well apart) and on cloudy days as well as clear ones.

There are quite a few evacuated tube designs, but the one that interests me is the heat pipe type. Here is a somewhat schematic NREL image of the insides

The tube is evacuated (no air), to eliminate convection heat loss from it. It contains some kind of solar absorber (there is more than one way to do this) and a heat pipe tube that carries heat from the tube up to the manifold in the picture. [5] At present, the tubes are usually less than 5 cm in diamter.

These remarkable devices can generate temperatures of nearly 200^0 C (390^0 F) if needed, although most work cooler than that, certainly if only hot water is wanted. But the potential is there at higher temperature if the heat is to be stored inside a building for later use. To expand on an earlier end note, storing panel heat from summer to use as supplement in winter would require a large mass. For one kilowatt heat power over a third of a year, the storage demand would be for about 10 gigajoules. To store this amount by heating rock by 100^0 C would use about 500 tons of rock, a cube almost 6 meters on a side, assuming that it is insulated. Such volumes would probably make more sense for storing heat for multiple houses or in the ground, but then heat conduction losses may become important. In this case, large evacuated tube collectors might be better than the current small ones intended for homes and commercial buildings. I do not see any reason why very large systems with 10 or 20 cm diameter tubes would not be possible.

There are interesting other multiple applications for these evacuated tube heat collectors. For example, a small one might be used to heat the input air for a conventional heat pump in the winter. (When the heat pump is used for summer air conditioning, it would have to be disabled somehow.) You can find many entertaining discussions, and alternative designs, on the web.

Present commercial hot water evacuated tube collectors are tested and rated by an organization, “SPCC”. Sometimes the manufacturer even supplies the SPCC rating sheet [6]

If the aim is to produce a lot of heat at high temperature, evacuated tube collectors may not be too useful. The same parabolic troughs that produce electric power can be used to simply produce heat, without the turbines, at quite high efficiency and temperatures up to 400^0 C. Because troughs focus sunlight on a tube of heat transfer fluid, they need better clear skies than evacuated tube collectors do. But they would be good for heat for commercial use--mainly what is called “industrial process heat”--or for large scale heat storage for winter needs.

Industrial process heat is likely an important and varied future use of solar heat. Here is an NREL plot of the cumulative industrial heat use versus the temperature:

About 57% of the process heat for industry could be met by advanced evacuated tube collectors up to 200^0 C. An advantage of these collectors would be that they can be used even in cloudier areas. About 70% of the process heat could be met by trough collectors, but probably mostly in sunnier parts of the country. It is certainly possible to concentrate solar energy enough to reach the top level at 1600^0 C, but the installations would be pretty complicated and the achievable hot areas not large, maybe under 100 square meters. Maybe most of the heat produced above 400^0 C will come from either electric power or nuclear plants designed for industrial process heat.

Chapter 5 Energy Storage Revisited

Short term storage of electric power in batteries--twelve hours or less--does not seem to be a severe problem, possibly apart from the huge scale needed. This sort of storage might reduce the erratic nature of both wind and solar energy by enough to be useful. I already discussed this in Chapter 2.

Short term heat energy storage does not seem to be of much interest at present. And, apart from the brief mention in the previous chapter, I do not want to go into long term (seasonal) heat energy storage. [1]

Long term storage of electric power will become a grave issue and is the subject of this chapter. The problem was already apparent for wind power in the previous book. I did not discuss it there much because of a lack of information. There are now several studies for both wind and solar power on a national scale. The key question is what fraction of national demand can be satisfied by large scale deployment of wind and solar power. The answer from the studies that I want to describe here is in the range of 70%-90% depending on assumptions. Above 90% seems not to be possible, given present ideas.

What is not discussed at present is quite important. There are two serious issues. First, electrifying everything probably means doubling yearly average electric power production. (I have mentioned this a few times.) The studies I will mention only concern replacing present electric power. Second, studies do not deal much with large scale power distribution in the U.S. or the stability of an electric grid or grids based almost entirely on wind and solar power.

It is at least clear that there is a serious seasonal deficit in the production of renewable energy that is of the order of ten to twenty percent of present yearly demand--somewhere in the range above 500 terawatt hours. This estimate does depend on the scale of deployment of wind and solar power--a huge power oversupply plus curtailment (or unexplained alternative use) of power for three quarters of the year to meet the deficit in one quarter seems unrealistic. So the deficit is surely real.

So far as I can see, the present notion is that this deficit of ten percent or (likely) more will be met by “dispatchable” and only seasonally used natural gas plants. This is based on governmental (EIA) projections of natural gas availability out to beyond 2050. These projections are almost certainly wrong. Natural gas availability (and the official EIA study) has been analyzed in great detail by David Hughes, and this is a good place to quote from his “Shale Reality Check” (2019)[2] . Unfortunately most of the renewable energy studies here are constrained by their input assumptions and they need to be thought about and kept in mind when reading reports. Some of these assumptions are very hard for authors to work around, even if they are aware of them. A good example is EIA natural gas projections. Another is cost extrapolations. The projections may be good. They are not gospel.

On to the studies.

A very early investigation was by Short and Diakov. [3] They distinguish energy storage from dispatchable power--essentially coal and gas. Absent easily varied coal and gas production, what they refer to as dispatchable would become stored energy. Here is the key plot from their paper:

While there is a need for dispatchable (or large scale stored) energy throughout the year, it is concentrated around late summer. The amount of stored energy, in my view, is very large, well above 500 terawatt-hours. (See the note for a table)

Perhaps the most assumption free study (and still the best, in my opinion) was by Matthew R. Shaner and collaborators. [4] It needs to be read to get a clear idea of the nature of the problem. They do not present a plot like that of Short and Diakov, but do comment that if the deficit has to be met by large scale storage, it would amount to several weeks of U.S. power generation. Several weeks would correspond to about 10% or more of all yearly electrical power, also around 500 terawatt-hours (not a number in their paper).

The most explicit and recent study (2020) is by a Berkeley group at the Goldman School of Public Policy. [5] They propose an actual plan for renewable energy by 2035. Their plan presumes that only 10% of yearly demand can and will be met by dispatchable power generated from natural gas. At least one graph in their paper seems to assume that this plan will remain workable out to 2050. It is based on a number of assumptions, backed by other studies. First, that present nuclear power will remain available despite the aging of the U.S. fleet of reactors. (They are not explicit about this assumption.) Second, that natural gas will remain available and cheap out beyond 2035. Third, that PV farms, wind power generation, and short term battery storage, will get cheaper out to that date. A secondary assumption is that not too much curtailment of renewable energy will be needed. Here is their key graph:

The most important result is the claim that natural gas (no coal) will supply the missing 10% of yearly energy need. This is about 450 terawatt-hours, based on one of the graphs I do not show. This corresponds to a ballpark estimate of the deficit bump on the above graph for July-October.

With some luck it might be possible to fill the deficit with natural gas power out to some time beyond 2035, but almost certainly not beyond 2050 as the Berkley study seems to assume. As mentioned in the wind energy book, natural gas is only going to be an interim solution. As part of that, residential and commercial heating will need another source of energy than natural gas in a decade or two. [6]

From my point of view, that fossil carbon will decline drastically during this century, this is all quite alarming. There is no present prospect that a deficit of the size in the studies I have mentioned can be covered by energy storage on the needed scale. To give one example, pumped hydro energy storage is now the cheapest large scale option that works. Seasonal storage of, say, 400 terawatt-hours would require about twenty thousand 20 gigawatt-hour plants.

Once fossil carbon is rare two obvious replacement fuels are hydrogen and synthetic ammonia. Both of these need large scale hydrogen production, either from novel high temperature fission reactors or, more likely, electrolysis of water. The issue for energy storage is the scale needed and the storage technology.

Consider hydrogen for energy storage (hydrogen as a fuel is another use [7]) There is interesting background to the storage idea, since natural gas is already stored at high pressure--around 100 atmospheres--in the caverns leached out of salt domes, converting fresh water to removable brine and leaving a void. It is a simple calculation to learn that an energy content of one terawatt-hour of hydrogen at 100 atmospheres occupies 3.3 million cubic meters. It happens that a number of caverns exist with volumes in this range. There are numerous smaller ones as well. (Salt caverns are good cracks tend to seal up under pressure.) Whether there are enough caverns--and repurposable salt mines--is not clear to me, if the needed scale is really hundreds of terawatt-hours. [8]

I have not found any detailed examination of how many such caverns there are in the U.S. that are really practical for storage (there are some hundreds of caverns along the gulf coast) or how many new caverns could be leached out of existing salt deposits. There is real interest in this in Europe and it seems that possible sites are numerous. There is one good study for the U.K. with a “theoretical capacity” of several thousand terawatt-hours and a practical one of perhaps a few hundred terawatt-hours. [9]

The scale of any hydrogen storage proposal needs to be understood. The hydrogen gas needs to be produced from electricity by electrolyzers at an efficiency of 70% or so and, after storage, turned back into electric power by turbines or another method at perhaps 50% efficiency. It is unlikely that the end-to-end efficiency is over 30%-40% and one third efficiency seems to me a good ballpark number. So a useful energy demand of, say, 400 terawatt-hours, means an input electric power consumption of around three times this or 1200 terawatt-hours. This is, in turn, over a quarter of all yearly national electric power demand today. If I use the typical large electrolyzer size from a few year ago, this would take some tens to hundreds of thousands of electrolyzers. This is a very, very large scale.

Perhaps there is a better way to deal with seasonal demand.

It might be that the additional electric power need, beyond that used today, might help the seasonal problem somewhat. This was mentioned in the preceding note. If much of this extra demand is for heating residential and commercial buildings, the demand would be largely in December thru February. (There is already a small peak there in the plot above.) Depending on how the new electric power is deployed--for example in heat pumps--the extra demand might even approach the 10% value in the Berkley study. Then there would be two large peaks per year in the above plot, labelled Figure 8 in the study. There would still be dips, this time in spring and fall. The need for storage would not go away, but it might be lessened. (As might be expected, new electric demand for industry would not show the same seasonal variation as we see with heating and cooling of buildings.) Maybe the answer to the storage problem will be to vastly expand the solar and wind power generation and simply turn it down in spring and fall--or find another use for the power at those times of year. This seems to me to be a pretty inefficient method.

There have been suggestions that hydrogen production could be by using the heat of new high temperature reactors to drive chemical processes that can turn water into hydrogen and oxygen gas. I do not want to go into this; at the present time this is very conjectural.

Might it be possible to store heat directly to moderate the seasonal demand problem? It is reasonable to use solar power to produce heat rather than electric power, from the troughs described earlier for example. Nuclear power plants could be configured to do the same. When needed, the stored heat could be used to generate electricity. What needs to be solved is the time scale for heat storage, as well as the sheer scale of the problem. It hardly seems likely that the storage problem can be reduced below a few months and some hundreds of terawatt hours--even if the overall scale can be reduced below the 500 or so terawatt hours above.

The long term storage problem, weeks to months, will bedevil solar and wind power beyond about 2030.

Chapter 6 The Grid and National Energy

Our national energy after fossil carbon has two parts--one is simply the scale of energy we use that, for the most part, will have to be delivered as electrical power. The other is the unfortunate fact that most of this energy will be from intermittent wind and solar power. [1]

Some of these flows are easy to think about. Transportation energy use by fossil fuels is staggeringly inefficient--somewhere around 20% with 80% wasted as heat. It can be replaced for the most part by electric power. Electric public transportation--light rail, streetcars and busses, and even people and goods transport by heavy rail--are quite efficient, probably above 80% efficient at moving large numbers of people and goods. Individual electric automobiles are a problem. [2]

Heat for homes, apartments and businesses will be a problem but probably a manageable one. Affordable air source heat pumps, described elsewhere in these books, can in principle do this. They can also provide hot water for homes and apartments and businesses. If heat pumps can be made with a  “coefficient of performance” (essentially, heat moved divided by electric power used) of around 3 at low winter temperatures, the total grid drain might be under 100 gigawatts--less for properly insulated buildings. [3] Alas, the level of active research and development here is pitiful. Most commercial heat pumps simply do not work below about 0^0 to 10^0 F. So resistance heaters have to be used, with no efficiency gain.

I cannot comment on future industrial energy use without fossil carbon (or much less of it), this is just too complex a problem. Most of our industrial production will have to change in unpredictable ways.

Just how the two vast existing national grids will deal with intermittent solar and wind power is unclear. In my view, this is unlikely. At present, wind and solar power are a small part of our total electric power generation. With a few exceptions, most wind and solar additions are unregulated and the grid operators are left to cope with any irregularities this causes.  So far, they have succeeded. There seems to me to be a broad expectation that gradual adaptation to erratic power sources will be possible. There are two key studies by the National Academies (NAS) on this subject, and I will use and comment on them. [4] They are professionally done and very thorough, with many recommendations on the future grid and grid stability. They do, somewhat indirectly, point out that it is difficult to project the future of such a complex system in the face of a change from the present to a grid at least dominated by erratic power sources.

To start, here is their present map of North American transmission lines, roughly the US grid:

The actual grid is split up into pieces, the major ones being the Western Interconnect, the Eastern Interconnect and the Texas grid (the image is from NERC),

(There is a different grouping of grid management that I won’t be concerned with here.) The main point is that the three 48 state grids are only weakly connected to one another. It is simplest to think of three independent grids, rather than one, even though we speak of the ‘US grid’.

If I think of a future dominated by erratic energy sources, there is a serious conceptual problem behind the structure of the grid and its management. It is best brought out by the NAS in the following picture:

Why is this a conceptual problem? There are three layers here--an organizational layer, a technical layer, and a physical layer. The top two are very complex and developed historically from a physical layer that was based mainly on fossil carbon power sources, plus some nuclear and hydro power. Yet in the future the bottom layer, the physical one, will change totally over a very few decades. Even with some nuclear and hydro power, almost none of the present physical layer will survive.[5]  This is the core problem that the present grid can very likely not adapt to--those evolved top two layers.

The NAS studies are based on the notion that those top layers can adapt, but reading the reports provides little to no hope that they will do so. The studies are full of logical and well thought out proposals and suggestions, none of which appear to me to be enforceable or even likely. By the time the physical layer is largely built out in the present chaotic way, it will be too late. Effective management of the grid will become extremely difficult or impossible. Multiple system collapses may very well follow. Much present concern is about generation capacity, but the real problem lies in those two layers in this image. Our entire grid system is too rigid.

This present grid evolved from very many small grids combined into larger ones over time. By 1964 there were eight grids in the continental U.S.-- now down to the three I mentioned. The idea of combining grids was both to enable power flow over large distances and, supposedly, to increase grid stability.  Electric power flow is now indeed common, using high voltage lines, across both the Eastern Interconnect and the Western Interconnect (Texas has only weak connections to these two). The grid these make up is mostly stable, but there have been major breakdowns in 1965 and 2003, described in an earlier book.

The challenge of stability will become very severe in the near future. Smaller grids, such as that in Texas, are much easier to oversee and manage than a single big grid. As technology has advanced, it is now possible to transfer power from one grid to another with large AC to DC to AC conversion facilities on the boundary of two grids. The two AC grids connected this way do not need to be synchronized. There is no need to have a purely AC grid synchronized over very large distances. Is supplementary power needs to be transferred over large distances, modern DC power lines can do it. (The longest in the U.S. is a decades old line from Washington state to Los Angeles.)

Rather than one or two large synchronized grids, it is now possible to build a number of smaller ones--essentially a “Grid of Grids”. It seems likely that a solution for the future of erratic power sources could be ten or so grids, each serving about 30 million people and industry. Power flow between them could be via AC-DC-AC stations of some ten or so gigawatts on the grid boundaries, supplemented by long distance DC lines where needed. Texas is a model for this, and California could become another. Both now have significant erratic wind and solar power sources.

Such a “grid of grids” could be expensive and time consuming to create, by segmenting the existing interconnects, but the risks we face from erratic wind and solar power are very great. Some major adjustment in our grid structure will be needed--preferably before it is forced on us.

Introduction

Endnotes

1. To anticipate, consider the Lawrence Lab Sankey U.S. yearly energy plot I have used frequently. If all of what they refer to as “useful energy” or “energy services” (i.e. actual use after all losses) is electrical, end electric power demand would go up to about 2.5 times what it is now. This is surely an overestimate, but you get an idea from this. A need for twice the average present electric power demand is not unreasonable, as will be clear later.

2. See the first book in this series--the textbook. Early large installations mentioned there in Germany and Spain had costs of over $50 per average watt delivered power, a huge number even when compared to expensive nuclear power. Even the “Topaz” PV farm in California had a capital cost of $16 per average watt for 2015-2018--without energy storage (see Wikipedia for the numbers I used to calculate this).

3. Present Lithium Ion battery storage of energy is very expensive, around $500 per kilowatt-hour for a large installation. At such prices, 12 hours of storage for the “Topaz” PV farm would cost a large fraction of the original capital cost. I will return to this later; it is important to keep in mind that a large capacity installation is much more expensive than simply the cost of a lot of individual batteries.

4. Common numbers for evacuated tube panels for hot water are 50-70% efficiency. Using such panels for household hot water is already somewhat common. For whole house (or business) heating, the necessary panels can get big. See my discussion later on.

5. Here is an illumination chart from the textbook. The unit is watts per square meter for panels that are laid flat/at local latitude/vertical. The numbers vary less than you might think.

6. To get an idea of the scale involved, for a city in the northern plains, no energy storage at 15% efficiency a compact mass of solar panels (no spacing) at the local latitude would occupy 140 square kilometers. If the future electric energy need is twice that at present, to account for the additional electrical energy need that comes now from directly burning fossil carbon, the land needed would be twice this. Besides, installations usually have a rather low density of panels, so the land need would increase still more, possibly pushing a thousand square kilometers--just for a city. (This assumes all power is from PV panels.)

Chapter 1-- Resources and Preview

1. As a general guide to renewable energy, including solar, I have relied on “Principles of Sustainable Energy Systems”, second edition, by Frank Kreith, CRC press. (There are a number of contributors to this book.)

Chapter 2 -- Technology: Photovoltaic Panels and Batteries

1. To get an idea of applications (by peak power generated) and their growth to 2016, here is a plot from the Solar Energy Industries Association

2. Early large photovoltaic installations could get by, just feeding any available power into the grid, but the more common these become, the less easy it will be to feed directly into the grid without some ability to deal with sunlight fluctuations and the need to shift power with time. This will mean PV installations will have batteries as part of the system--whether these batteries will provide 1 hour, 4 hour or longer power is not clear now.

3. This 30 watts per square meter delivered electric power is not unreasonable. It corresponds to 15% efficient panels at local latitude tilt. One often reads of solar cells with laboratory efficiencies above 20%, but there has always been a huge gap between what can be achieved in a lab and what actual panels produced industrially in the millions can get. I will come back to this later.

4. Producing 300 million or more solar panels per year, essentially forever, (plus recycling cost) might amount to a lot of money, perhaps 50-100 billion dollars per year.

5. There are very many books on solar cells and installations. I have relied mainly here on the Handbook of Photovoltaic Science and Engineering, second edition, Wiley publishers, 2018. Most of what I will write about is in the chapters on particular cell types. I often refer to this reference in the text as the Handbook.

6. Common contaminants are oxygen and carbon. They do not easily give up or accept electrons, so they do not affect silicon too much.

7. We are familiar with conductors, such as copper; they have free electrons that can move through the material. Such conducting solids are a bit unusual in that the electrons can be thought of as “free” to move around, despite the presence of vast numbers of atoms. Silicon “doped” with phosphorous atoms is a bit like this. The phosphorous atoms inside the regular silicon “lattice” are positively charged, having lost that electron that can move as if free. What is an amazing consequence of quantum mechanics is what happens  if silicon or a few other materials are doped with atoms like boron. The boron atoms are negatively charged, having pulled an electron from elsewhere. So, there is a missing positive charge in the lattice. This positive charge, called a hole, can move through the lattice as if it too is a free particle. It is not a real positively charged particle--it cannot exist outside the solid. But inside the solid, it can move as if free, like the electron. So these materials are described in terms of free electrons and holes in their interior. This is surprising enough, but what I find even more remarkable is how general this phenomenon of such quasiparticles is in quantum mechanics. They appear in many different forms in different materials--not just as holes in the silicon that interests us here.

8. There are about as many conduction electrons as positive phosphorous ions in the doped n layer, which is usually pretty thin and has one phosphorous ion for some hundreds of silicon atoms. Similarly, there are about as many holes as negatively charged boron ions--about one boron atom for a few ten million silicon atoms. The number of holes in the doped p layer is much greater than the number of free electrons in the n layer because holes do not move as easily as electrons. Incident light produces both free electrons and holes in the thick p layer, but their number is small compared to the number of holes on average. Because of the temperature, there are some free electrons in the p layer, but many fewer than the electrons liberated by the incident sunlight.

9. This is based on a federal loan guarantee of $1.46 billion for 550 megawatts of the project. This must have reduced the unknown total cost, given the advantage of a guarantee. So I just used $1.5 billion for 500 megawatts (peak) as a ballpark number. In an earlier note I mentioned a cost of $16 per average watt delivered for the Topaz solar farm in California. (This used quite different thin film CdTe panels that I will discuss later.) See Wikipedia for background. I have found it increasingly difficult to find credible total capital investment costs for both large U.S. PV farms and large battery installations.

10. There is a good overview of silicon ingot production, with pictures, from Sandia National Laboratories, SAND2018-139OPE by Babu Chandamala. There is also an IEEE article from 2014, “Solar Energy isn’t Always as Green as You Think”, and other reports on the Web.

11. Thin film “amorphous” (not crystalline) silicon cells and panels have been made for a long time, and there is a commercial use for them. But there is a view that large scale utility systems need 15% or better efficiency and amorphous silicon panels are usually under 8% efficient. So I have not mentioned them until now. If their efficiency should reach 15% or better, it would be revolutionary because they are cheap and use nontoxic silicon. Crystalline silicon thin film solar cells exist in the laboratory, but when they might become commercial is unclear. See the Handbook for details. Other innovative laboratory cell research can be found thru the NREL website.

12. The EIA has monthly electricity production numbers for Solar Star II. Despite one axis tracking, there is a strong seasonal variation

13. And here are the Topaz plant monthly numbers--net production in megawatt hours. Note the strong seasonal variation again.

14.  This number leads to a staggering total of 3000 gigawatt-hours storage in batteries. At 75 kilowatt hours battery storage for one electric auto, this would correspond to 40 million autos. This has led to the frequent suggestion to use the batteries in electric cars as a grid storage, so long as the cars are not being driven. However, I was thinking of using the battery backup at each solar farm.

15.  Lithium for batteries will eventually be a serious problem. Lithium can be recovered by evaporating seawater (which is how some of the existing mined deposits were formed). But then the price will go up. I am not sure that existing battery price estimates properly include future resources. (See the next note.)

16.  Tens of millions of tons of batteries are going to create at least some resource and environmental problems, beyond price, that almost everyone ignores. I have seen estimates of 100-200 grams of lithium per kilowatt-hour battery (from an Argonne lab study). For ten million batteries that becomes well over a hundred thousand tons of lithium metal. From USGS data, this amounts to about a hundred times present U.S. production or ten times world production (these are “ballpark” numbers again) In a previous book I noted that Lithium can be got from seawater. Here is a cost estimate based on future seawater extraction (Peter Greim et al, Nature Communications, 2019; the yellow and green lines are two estimates.). There is a lot of Lithium; what the extraction involves I do not know.

17.  Rechargeable batteries can be based on solids or liquids; the number of types is very large. There are several good Wikipedia articles on them. There are very few battery types that store even one megajoule per kilogram of battery cell mass. When packaged, the mass goes up, sometimes quite a bit. This reduces the real, usable number of megajoules per kilogram packaged battery mass. You might wonder why batteries are such an inefficient store of energy compared, for example, to gasoline or methane gas. There is a factor of nearly one hundred “missing”; the problem with batteries is that they are essentially two dimensional: the voltage is across a membrane separating two materials and the electric current must flow through the membrane. Burning gasoline is three dimensional, and irreversable.

18.  A new design Vanadium redox battery using a mix of sulfuric and hydrochloric acid  is sold by UniEnergy (uetechnology.com). They are not particularly energy dense, at about 60 megajoules per cubic meter and an installed mass of about 2000 tons per megawatt hour. Prices are unclear to me but probably quite a bit more than $500 per kilowatt-hour. Many older versions of this battery exist, using sulfuric acid in the electrolyte.

19.  It is likely that, in time, large batteries will become common. An interesting question is: where in the grid do you put them? At the power source, such as a PV farm, is one possibility. As another example, a typical electrical substation might supply power to ten thousand homes using an average of a kilowatt each. This is ten megawatts, so a ten megawatt hour battery could supply emergency power for about an hour, if power delivery upstream of the substation is lost.

20.  To illustrate, a battery pack for an automobile that can deliver, say, 200 kilowatts at 80% efficiency will also generate 40 kilowatts of heat. If the packaging is a metal, the heat capacity of the package will ensure that it heats up fast, requiring effective cooling.

Chapter 3 Technology: Solar Troughs, Towers, Storage

1. There has been some effort to use steam directly as the heat transfer fluid, which could then run a really high temperature turbine.

2. I got this observation from a Chinese Academy of Sciences paper by L. Xu et al, 2014.

3. Here is the EIA chart for Nevada Solar One monthly electric power output

4. I will have nothing to say concerning the Crescent Dunes project, but here is the EIA record of monthly power production:

Chapter 4 Technology: The Various Uses of Heat

1. An example might be my own house in Minneapolis, which is a hundred years old and insulated fairly well in 1965. In January, it needs about 10 kilowatts of average heat power, available now only from natural gas. A new and better insulated house now might use less than a about 2 kilowatts of heat power. If an air source heat pump could attain a “coefficient of performance” in January of even 3 and in the northern plains, such a house would require perhaps a half kilowatt of electric power to operate its heat pumps. (See the first book in this series, the textbook, for details on the physics of heat pumps.) Heat pumps can deliver both internal air heat and heated water, a big advantage. A big disadvantage of them now is their negligible coefficient of performance under 5^0-10^0F, far below what I assumed above. I will have no more to say on low temperature home and commercial air heat and hot water from heat pumps, or mixtures of heat pumps, possibly PV or solar energy boosted, and other mixes of electric power and solar heat . This is interesting, but beyond my scope here.

2. Many quite good solar heat books deal with merely somewhat harsh climates; Europe in particular is a poor model for the problems that face solar heat energy use in the broad continental U.S. and Canada. It may be a good model for areas in the U.S. a some hundreds of kilometers from the coasts. See the figures at Brettschneider’s web site.

3. Probably more common today are so-called flat plate collectors; but I want to stay with a type that likely will in time become more prevalent, and can be used efficiently in even very cold parts of the U.S. This issue is generally discussed in the very useful book “Principles of Sustainable Energy Systems”, Frank Kreith, CRC Press, that was mentioned in an earlier endnote. (This book has little on evacuated tube collectors--there are other and more recent books on these.) I am also avoiding “solar houses” that are designed to be site specific to efficiently use considerable solar energy. They are complex in both design and construction, and tend to be expensive. A famous early example is the Frank Lloyd Wright “Solar Hemicycle” built on a Wisconsin hillside. As another example, in Minneapolis, there exist row houses built into a large artificial earth berm. Neither is generalizable. Newer houses will be needed on a large scale and will likely be somewhat standardized by building codes that emphasize insulation and more modest solar energy input than that in specialized solar houses or multifamily dwellings. (Commercial buildings may be an exception.) Sophisticated solar houses are a good idea if done right and if you can afford one.

4. There is a broad survey of water heat systems using different collectors in NREL/TP-5500-54793 by K. Hudon et al. Systems that heat water can also be used to provide general heat to a building. At present, most systems just provide building hot water, general heating is from some other source, often from fossil fuel. As fossil fuels decline, building heating will become more common. Unfortunately, a system that does not also cool a building indirectly is useless in summer unless the heat produced then can be stored over a season. This means a potentially expensive in-ground heat storage system.

5. The heat pipes used here are tubes that contain a fluid at the bottom that is evaporated by the absorbed solar heat and is condensed at the top, the fluid then running back to the bottom. The heat energy is transported due to the heat absorbed by vaporizing the fluid and then released by its condensing back to fluid at the top. (Alcohol can be used in the heat pipe). The hot vapor cools and condenses at the top by transferring heat to a heat transfer fluid in the manifold. This second fluid is what carries the heat energy into a building. Because it is outside, the heat transfer fluid cannot be allowed to freeze at night. It also cannot be allowed to boil inside the manifold in the daytime, which might damage the piping. One good choice is propylene glycol, which is liquid even at -40^0 C and boils around 200^0 C.

6. Here is a detail of one of the rating sheets for evacuated tube collectors (for Duda Solar)

Some comments on this table: The column labelled Ti-Ta refers to the transfer fluid temperature increase in the panel and rows D and E refer to the increases for normal cool climate hot water and commercial hot water; the panel heat output drops as Ti-Ta increases; the 7.9 kWhr/m^2day entry corresponds to an average output of 330 Watts heat power per square meter, perhaps a good ballpark number for average conditions. A house that needs 2-3 kilowatts of heat power for hot water and general heat might get by with a 10 square meter panel.

Chapter 5--Energy Storage Revisited

1. This is not to say that long term heat storage is unimportant. It could be that the seasonal heat energy storage is a problem on the scale of that discussed in this chapter.  Our summers are hot and  winters cold and the idea that we might transfer heat energy from one to the other is obvious. I just think that there are too many contingencies to deal with the issue here. The only claim I know of is by Jacobsen and his collaborators at Stanford; a number with details is hard to find. But it seems to require roughly 500 terawatt hours of heat energy shifted from summer to winter.

2. Here is the quote from Hughes:

“The fact that all U.S. tight oil resources would be consumed by 2050 in this best-case scenario, assuming that the EIA estimates of proven reserves plus unproved resources are correct, should be extremely troubling for long term energy security planning and policy development. And given the extremely optimistic nature of most of the EIA’s play-level forecasts, it is by no means assured that even this much oil and gas can be produced. Assuming that production will remain at high levels after 2050 is wishful thinking.”

I think that Hughes is correct and that sometime in the next decade U.S. shale gas will begin a decline that will drastically affect both gas use and price. Since other gas sources are already in decline, this is very serious.

3. The original detailed renewable energy study is by Walter Short and Victor Diakov, “Renewable Energy Load Matching for Continental U.S.”, ASME, November 2011. The summary is in a table

and the total amount needed is 685-780 terawatt-hours depending on assumptions.

4. The most extensive study, with important commentary, is by Matthew R. Staner and collaborators, “Geophysical Constraints on the Reliability of Solar and Wind Power in the United States”, in Energy and Environmental Science, 2018. Their most useful plot combines 36 years of data for the key information.

The deficit is for the period July-September.

5. The Berkley report is from June 2020 at the 2035report.com with supplementary material linked to there. I think that the title is “2035 The Report”.

6. The scale of replacement energy needed for future heating can be estimated from EIA data on present natural gas for residential heating. Here is a plot for 2020; the vertical scale is monthly average gigawatts of heat power and the horizontal scale is month of the year.

Commercial heating is somewhat more than half this. Meeting both would amount to a few hundred additional terawatt-hours (of heat energy or equivalently of electric resistance power), happily at a different time of year than the deficit discussed in the text. Of course, this is apart from any electric replacement energy for industrial natural gas use.

7. Hydrogen as a fuel is another use. Stored liquid or pressurized hydrogen has been used as an engine fuel. Liquid hydrogen has even been fed, as a gas, into almost conventional jet engines on aircraft.

8. There are a lot of studies of caverns for natural gas storage, back to the 1980’s, and the technical problems of hydrogen storage are related. Unfortunately, most present caverns are near the gulf coast, and large fields of the necessary salt are not common. Here is a figure from a 1982 study on caverns for compressed air energy sorage (Pacific Northwest Lab PNL 4242; this has useful information on the technology of leaching salt caverns in salt domes.)

Another Brookhaven National Lab report from 1979 (“Underground Hydrogen Storage Final Report” , Stephen Foh et al) has more relevant information. Still another is “Texas Salt Domes” by Steven Steni et al from the University of Texas. This points out that salt dome caverns do leak and sometimes even collapse or slowly close up. This reports estimates the life of a cavern at around 400 years. It is not clear to me how much improved more recent studies are; they are too many to quote here.

9. A study for the U.K. by members of the British Geological Society, John Williams et al, is in the form of a presentation under the title ACT ELEGANCY. It seems likely that their estimate that practical storage is a tenth or so of the possible would also apply to the U.S., but our area is so huge that storage of hundreds of terawatt-hours is probable.

Chapter 5--The Grid and National Energy

1. This is a good place to emphasize the facts concerning energy flows in our economy. This is the Lawrence Lab Sankey diagram I have repeatedly put in these books.

There are two main key features of this plot. First is the vast amount of fossil energy flowing to transportation--mainly automobiles and trucks. Second is the almost equal flows of oil and natural gas energy to industry--both as heat energy and as inputs to chemical processes. The direct fossil carbon flows to residential and commercial properties is much less, but hardly small. Unfortunately, most people who concern themselves with energy focus on automobiles, homes and businesses, not on industry. Alas, I have nothing useful to say about industry energy use, which may turn out to be the thorniest problem.

2. The problem of producing electric replacements for fossil fuel cars is usually thought of as a problem because of the demands on rare materials (at present, mostly lithium for batteries and rare earths for motors). Manufacturing and maintaining, say, 200 million replacement electric cars is a highly nontrivial problem. But it is worse, because of the drain on the net that recharging autos will lead to. If even half of these, or 100 million cars, recharge their batteries by half over an 8 hour night--say 20 kilowatt hours per car over 8 hours or 2.5 kW during this time--it adds up to an amazing load on the grid. The total amounts to 250 gigawatts drain on the grid while they are charging--and most of this demand will be in large cities. Recall the total present average national power use is 400 gigawatts. Seen as a total use, electric autos are a fabulously inefficient use of electric power in the face of all the other demands that can be expected. By themselves, this scale of electric autos could plausibly crash even an efficient power grid (to say nothing of the consequences for local electric substations). I do not think that we will ever see electric autos deployed and used on this scale, even if they could be built.

3. It does not seem to be properly understood that large areas of the country are undergoing a building boom of large cheaply and poorly insulated apartment blocks. Very many are in areas with very cold winters.  I have watched the construction of these blocks for years now. They will last a long time. I think that it is an illusion to think that they will be significantly more energy efficient than homes, once natural gas heat is gone or significantly reduced.

4. The National Academies studies are  “The Future of Electric Power in the United States” and “Enhancing the Resilience of the Nation’s Electricity System”. Free versions can be found on the Web.

5. An exception here lies in the still vast US coal resources. In a climate of political panic, the actual real climate impact may be forgotten.

Table of Contents

SolarEnergy

Endnotes