Solar Energy, Wind Power, Intermittency, and Storage

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In ordinary conversations about renewable energy, the issue of energy storage is often overlooked. Renewable sources generate energy on their own schedules, not customers' schedules. The difference must be met either by backup energy supplies or by energy storage. This article describes some storage calculations in the absence of fossil-fired or nuclear sources. The calculations can be downloaded from here.

This is a plot of electricity generation for the US. This writer doesn't have data for any other countries and wouldn't presume to offer advice if he did.

[DOE]

For the rest of this analysis, the average generation for the years 2003-2007 will constitute the model year.

First, compare the demand curve with the availability of wind energy. Wind energy is approximately proportional to the cube of wind speed. Density is also a factor, and there is considerable mismatch at very high and very low wind speeds, but those differences won't change the conclusions. This analysis is based on wind-speed cubed.

The data show wind speeds for 265 cities. We have deleted cities with low winds or high differences between high-wind and low-wind months. We also have deleted Alaska cities, owing to their unique characteristics and their separation from the US power grid. 244 cities are left.

[NOAA]

Clearly, wind energy doesn't match electricity demand well. Next, compare electricity generation with solar potential. Cities with poor solar characteristics were deleted from the data, leaving 221 out of 238.

[NREL]

So we see that solar energy matches the electricity demand somewhat better. For our first cut we shall calculate the maximum amount of solar energy that can be generated and used within a month, and we find that 80.6% of the yearly demand can be met with solar energy on these terms. Now we can consider the remaining demand after all that solar energy is accounted for.

Now we can compare the remaining demand with available wind energy.

The calculations show that 200 billion KWH of storage is required.

We can do the same calculations for other shares of supply from solar energy, with the results shown here:

Our calculations show that the storage requirement ranges from 141 to 386 billion KWH.

There is no way to store that amount of energy. In fact, we'll have to devise a fictional example to illustrate the problem.

Imagine that a lake exists, named Upper Lake Fead, which is equal in size to Lake Mead. Lower Lake Fead is the same size and is located at the bottom of Foover Dam, which is identical to Hoover Dam. However, all the water in Upper Lake Fead can drain through the water turbines.

Lake Volume = 30,000,000 acre-feet

Average head at dam = 520 feet

If the efficiency were 100%, then

  Energy = volume x pressure = volume x head x weight-density
         = 30,000,000 acre-feet x 43560 sq-ft/acre x 520 feet x 62.4 lb/cu-ft
         = 4.24 x 10^16 ft-lb
         = 16 billion KWH

We'll set the turbine efficiency at 85% and account for pump inefficiency by upsizing where necessary. Thus, Upper Lake Fead is good for 13.6 billion KWH.

So we have calculated that the US would need between 10 and 28 Foover Dams, each with Upper and Lower Lake Feads, depending on how much electricity is generated with solar energy. There are, in fact, no Foover Dams and no locations for building any.

Nuclear Energy Costs

Every responsible study has shown that nuclear electricity is as cheap as any of the non-fossil alternatives and is competitive with fossil-fired electricity.

For example, the International Energy Agency and the Organisation for Economic Co-operation and Development's Nuclear Energy Agency determined the costs as follows:

Cost per MWH in US Dollars

Discount Rate	5%	10%
Coal	25-50	35-60
Nat Gas	37-60	40-63
Nuclear	21-31	30-50
Wind	35-95	45-140
Micro Hydro	40-80	65-100
Solar PV	~150	200+

The University of Chicago compared several detailed calculations with a range of discount rates and summarized the results thus:

Cost per MWH in US Dollars

Coal	37-49
Nat Gas	56-68
Nuclear (assuming old designs)	65-77
Nuclear (assuming new designs)	36-55
Nuclear (assuming advanced-fuel designs)	57-64
Wind	55-77
Solar PV	202-308
Solar Thermal	158-235

A question that immediately presents itself is, why do the two studies give different numbers? The answer is that every study depends on assumptions, such as interest rates and fuel costs. Both these factors, and other factors such as taxes, pollution controls, and equipment lifetimes vary in time and place. This introduces an opportunity to do mischief, since a motivated commentator can pick-and-choose results to bolster his intended conclusion. These numbers only have significance if they're calculated on equal terms and only if they're read relatively, not absolutely.

A common argument being made now is that nuclear construction costs have risen so fast they have rendered nuclear plants too expensive to build. This argument is anchored on a report about some calculations made by Cambridge Energy Research Associates (CERA) that allegedly show a cost increase of 185% between 2000 and 2007. Imagine, an almost tripling of costs in seven years! However, CERA doesn't publish the results in a public forum; nor does it show the calculations so they can be verified. Indeed, there's no way even to know what methods it used.

It is true, though, that costs have risen strongly since China and India began their notable advances in material progress. These cost rises apply to all kinds of construction and, in particular, apply to alternative energy sources.

Here is some information on the cost of windpower construction, which has doubled:


And some data (Oct. 28, 2008) on solar-electric construction. It has essentially held constant, but at US$4700 per KW rated power or over US$20,000 per average KW, it still is hopelessly expensive. What this shows is that the pressure on material prices has kept solar energy from getting cheaper.
Finally, here is some information from Power Engineering International on nuclear construction costs, which shows a cost increase of 125%, not much different from the increase for windpower.


What all these numbers show is what energy analysts have been telling us right along. Nuclear energy is as cost-effective as any non-fossil energy source, even ignoring the intermittency problem of part-time energy sources. But if intermittency is considered, then the comparison widens. There aren't any practical ways to overcome intermittency, as shown here. But if there were some way, the economic and environmental costs would drive the total cost out of sight.

As the world grapples with this issue, one other point has to be considered. A new generation of nuclear power plants is being born. These new plants use passive safety systems so the active systems can be simpler, thereby reducing costs. Furthermore, they operate at higher efficiencies, lowering fuel costs. As shown in the University of Chicago data, these improvements make nuclear energy cheaper than any alternative other than coal.

Construction Times for Nuclear Power Plants

I had thought we were pretty much done but the kids in the basement came up with another reason why we can't use nuclear energy. Nukes take too long to build! they whined, their lips quivering. It took ten years to build them before. Jim Hansen at NASA says we only have ten years to stop all the greenhouse gases or we're gonna die! We'll build windmills instead.

Now, how could anyone know all that? The estimable Dr. Hansen actually says

"The Energy Department says that we're going to continue to put more and more CO2 in the atmosphere each year--not just additional CO2 but more than we put in the year before. If we do follow that path, even for another ten years, it guarantees that we will have dramatic climate changes that produce what I would call a different planet--one without sea ice in the Arctic; with worldwide, repeated coastal tragedies associated with storms and a continuously rising sea level; and with regional disruptions due to freshwater shortages and shifting climatic zones."

Dr. Hansen goes on to make concrete suggestions, such as a cessation of building new fossil-fired power plants. He offers the possibility that in ten years it might be necessary to bulldoze all the existing ones.

Actually, a better plan would be to replace all the boilers with nuclear reactors. Most of the construction cost and effort is in the generating portion of the plant, anyway, so replacing the boilers will be much cheaper and faster than building new nuclear plants from scratch.

His other recommendations, such as financial incentives and improving the efficiency of buildings and vehicles, agree with the views of just about everyone who's looked at the issue.

But besides oversimplifying Dr. Hansen's remark, the kids are presuming to know more than they do.

We don't know how long it will take to build nuclear plants. In Japan they can build them in less than five years. If other countries got serious and cranked up their capacities for building them they could do it even faster.

The new designs are simpler than the earlier designs. The designers have incorporated features into them that make them inherently safer so that the risk of accident is lower even while the safety systems are less complex. Furthermore, manufacturing and construction technology has advanced in the last few decades. Just as office buildings can be put up faster and cheaper, so can power-plant structures. Computers and laser-guided machine tools have revolutionized the manufacture of heavy machinery. New testing techniques ensure quality control both cheaper and more thorough.

Besides, wind farms take more construction effort than nukes. Consider that a 1000 MW nuke will average over 850,000 KW. A very big (rotor-tip height ~ 450 feet!) wind turbine rated at 1.5 MW will average less than 500 KW. So 1 nuke equals more than 1700 big turbines.

As luck would have it, the British House of Lords studied the question and compiled some comparative data (and you wondered what the lords did!). A good measure of the construction effort is the energy inputs required for manufacturing and construction. What they show is that a 1000 MW nuke takes 6280 terrajoules per average GW, while a 25 MW wind farm takes 20,575, more than three times as much.

This is not an energy comparison, because that's much more complicated. We're only using these numbers to represent the manufacturing and construction effort.

What it shows is that the kids got it wrong. Again. Even if wind could provide full-time power, it still couldn't outpace nuclear in converting away from fossil fuels.

The Academic Approach to Anti-Nuclearism

For a long time there's been a belief among anti-nukes that you can prove anything if you write enough. You just have to beat science with statistical analysis and smother it with paper.

This came up again on another blog, which uses a lot of scientific language but is dedicated to the proposition that the laws of nature can be over-ridden if they're inconvenient.

In this case, the writer of the article is determined to show that part-time energy sources can provide full-time power, if you just do enough mathematical manipulations.

First he cites "Supplying Baseload Power and Reducing Transmission Requirements by Interconnecting Wind Farms" by Cristina L. Archer and Mark Z. Jacobson, which argues that if enough wind turbines are interconnected they can provide base-load power. According to the authors, the part of the average output that can be considered 87.5% reliable is between 33% and 47%, depending on how many wind turbines are interconnected. However, the area they studied, centered on the Texas and Oklahoma panhandles, has the most reliable winds in the US and their results don't translate to the country as a whole. Even so, they show that wind farms would have to be oversized by a factor of at least 2. They elect to call it base load, but that's not appropriate. It only can be base load if there is also some form of load-following power.

That's a problem. Without fossil fuel and nuclear energy, load following is limited to whatever hydro and pumped storage can be made available, and at most that can only be a few percent.

The article writer also cites "Improving the Technical, Environmental and Social Performance of Wind Energy Systems Using Biomass-Based Energy Storage" by Paul Denholm, which recognizes that problem and suggests using biofuels for backup. But there are a couple of problems here. One is that nowhere does he consider the fuel required to grow the biomass and convert it into biofuel. Currently, it takes up to a gallon of fuel to produce a gallon of fuel, and certainly a big part of a gallon. It seems unlikely that it will ever take no fuel to produce a gallon of fuel. In the absence of better information, his study has to be considered extremely optimistic.

His optimistic estimate is that it would take 6.9 hectares or .0266 sq mi to produce biofuels that would generate 1000 MWH per year. The US uses 4 billion MWH/year, so the area required would be 106,400 square miles, out of 650,000 square miles of arable land. Suppose wind energy allows us to reduce that in half, which would require a half-million 1.5 MW wind turbines (rotor height = 450 feet!); we still need 53,000 square miles. Since we're using almost all the arable land for food and fiber, it's not clear where the 53,000 square miles will come from. Also, to farm land of this magnitude means using less-productive land. He assumes 11.3 tonnes/hectare yields, which would require prime Iowa land, so the land areas would be much greater and very likely would require irrigation, for which water will not be available. That's enough trouble already, but consider that the need for motor fuels will vastly outweigh the need for bio-electricity, because there is another, better, way to generate electricity but no alternative way to produce non-fossil motor fuels.

So we're still where we've always been. Wind energy doesn't work without a backup, and biofuels won't provide the backup.

As we explained in an earlier article, nuclear energy allows solar and wind to play their maximum part in providing electricity. Further, it allows them to contribute efficiently to the production of hydrogen, by taking some load off the nuclear plants. This is the kind of solution that will minimize global warming. Trying to paper over the limitations of renewable sources with scientific-looking obfuscations, if it's successful, can only keep the world on its present reckless path to self-destruction.

But anti-nukes don't get this. They believe you can change reality by manipulating data. You want windmills to turn when there's no wind? No problem. Just crank out fifteen pages of equations, tables, diagrams, and charts and they'll turn themselves!

An Energy Plan

To start, we should look at some energy numbers. These apply to the US only. Here are the quantities of energy the US used in 2006, in quadrillion British thermal units, usually called quads:

Source	quads	%
Renewable	0.329091699	0.216726496
Hydro	0.987196598	0.650127793
Nuclear	2.686778447	1.769403728
Fossil-fired Elec	9.844436722	6.483148269
Other Fossil	137.9990426	90.88059371
TOTAL	151.8465461	100

Sources:
http://www.eia.doe.gov/cneaf/electricity/epa/epates.html
http://www.eia.doe.gov/iea/convheat.html
http://www.eia.doe.gov/emeu/aer/txt/ptb0103.html


Note, if you will, that fossil-fired electricity accounts for only 6.5% of the energy even though it accounts for 40% of the CO2 emissions.

This analysis comes in two parts. First we'll cover electricity. We know the rate of electricity generation will go up because a lot of the schemes for reducing greenhouse-gas emissions require shifting fossil-fuel applications to electricity: battery-powered cars, light-rail transit systems, replacing furnaces with heat pumps, etc.

Renewable energy sources such as wind and solar can't replace fossil fuels owing to their part-time natures. But they can greatly reduce the amount of fossil fuels being burned during the transition period while renewable and nuclear sources are being installed. So our plan includes both renewable and nuclear.



But wait, there's more! Electricity is a big part of the problem but not the only part. We also have to replace petroleum-based motor fuels. At this point, there are only two possibilities in view, besides electrified vehicles, bicycles, foot travel, horseback, rickshaw and some other specialized transportation modes. The two possibilities are hydrogen and hydrogen-enriched biofuels, as we discussed in the article, "The Dimensions of the Challenge." Our plan needs to include the capability of producing large amounts of hydrogen. This plan does that, because nuclear plants allow for thermochemical production of hydrogen, by far the most efficient technique available.

Once all the fossil-fired power plants are replaced, nuclear and renewables can complement each other. The nuclear plants can provide whatever electricity is needed during times of dim sunlight and low winds, or no sunlight and no wind. When the sun is shining and the wind is blowing, and when demand for electricity is low, nuclear plants can divert some of their capacity to generating hydrogen.

This plan allows solar and wind to play their maximum part in providing electricity. Further, it allows them to contribute efficiently to the production of hydrogen.

I don't want to claim that this is the only energy plan that could work. But it is the only plan I've seen that could work. If you know a better plan we'll do it your way instead. However, if your plan doesn't allow for providing electricity when the sun isn't shining and the wind isn't blowing then you don't have a plan.

Obstacles to Minimizing Global Warming

In earlier articles we discussed the technical challenges of preventing massive economic and environmental dislocations because of climate change. Actually, the world has the capability to meet those challenges if it has the will.

For example, the United States transformed itself from an agricultural nation in the depths of an economic depression into an industrial giant able to manufacture the hardware needed to defeat the Axis powers in five years even with millions of its able-bodied men and women in military service. Compared with that, converting energy away from fossil fuels is easy.

The problems arise from attitude.

First there is the problem of skepticism about global warming. The evidence isn't just strong, it's conclusive. Yet people have made up their minds not to accept it. You can find them on the web even if you don't want to. Any criticism of the Intergovernmental Panel on Climate Change, justified or not, from any person, qualified or not, is touted to be the final "debunking" (a term much in fashion) of climate-change science.

Overcoming this obstinacy should be the easiest part of the problem to solve, but we can't even accomplish this much.

Next is the perverse refusal of nuclear opponents, all of whom claim the mantle of Defenders of the Environment, to acknowledge the clear necessity of nuclear energy for minimizing climate change. The same Defenders of the Environment deny both the environmental benefits of nuclear energy and the limitations of part-time energy sources.

We see some erosion of this monolithic inertia among more thoughtful members of the public, but the executives at the major international anti-nuclear political organizations aren't budging.

Then, assuming these problems can be overcome (or possibly ignored), we face the difficulty of implementing solutions.

Perhaps there will be local opposition to construction of nuclear power plants. But wind farms already are seeing fierce opposition. And the fights over solar panels haven't begun. When utility ratepayers realize how much they're paying to subsidize their neighbors' rooftop panels they'll do one of two things. Some of them will decide to cash in on the program. But if too many people do that the program will collapse from the expense. So all the people left out will see their rates go up dramatically. Before it's over you will hear people say they wish they'd never heard of solar energy.

What's left is conservation. And conservation is always the preferred nostrum; whenever energy and global warming come up, conservation is always our best and brightest hope.

But conservation means more than putting in compact-fluorescent lightbulbs and recycling wine bottles. It even means more than junking our SUVs and buying hybrid cars. It means smaller houses and no vacation homes. It means giving up motorhomes and cabin cruisers and recreational flying. No more flying vacation trips.

Sounds discouraging, no? It is discouraging if stubborn, misinformed people are allowed to dictate the world's energy future. But there is a way to solve this, and that will be the next articles's subject.

The Dimensions of the Challenge

Most people don't understand the scale of the energy we use. This article will try to put it in perspective. The data will apply to the United States; nationals of other countries will have to interpret it for themselves. Generally speaking, though, nationals of other advanced countries will face challenges of the same scale or higher and those living in developing countries will increasingly find themselves in the same dilemma.

We will compare the different non-fossil energy sources that have been proposed with respect to their capabilities.  Where appropriate, we will compare the land areas required for each with the land area available.

Electricity

First, consider the amount of electricity the US uses, a total of just over 4 billion MWH/year.[source]

Wind

What really limits wind power is the small amount of storage available; hydroelectric dams can treat a small part of their capacity as short-term storage for wind power.  For the purpose of this calculation, we shall pretend that the limitation doesn't apply but we'll discuss storage later in this article.

Currently, typical wind-turbines on wind farms are sized at 1.5 MW, with a rotor-tip height of 450 feet and a rotor diameter of 231 feet.[source][source]. Allowing a generous load factor of 0.35 [source], each turbine yields 4602 MWH/year, so 869,000 turbines would be required. The minimum turbine spacing recommended is five times the rotor diameter [source], so each 1.5 MW turbine requires (5 X 231 ft)^2 = 1,334,025 feet, or 0.048 sq mile.

To provide all the electricity the US uses would require more than 41,720 square miles.  That would be a strip of land 40 miles wide running from the Montana/Canada border to the Arizona/Mexico border. To get good efficiency, a strip 60 miles wide would be needed.

Solar

Solar energy has the same storage limitations as wind power, but we still shall pretend that the limitation doesn't apply.

For the US, an average insolation would be around 5.5 KWH/m^2/day[source], or 2 MWH/m^2/year.  Allowing a generous 20% efficiency[source], the output would be 0.4 MWH/m^2/year.  To provide all the electricity the US uses would require 10 billion square meters or 3861 square miles of solar panels.  That would be a panel 1-1/2 miles wide running from San Diego to Boston.

Nuclear

Nuclear plants are operating at about 90% capacity factors.[source] 

However, new ones will run somewhat lower, so an average capacity factor of 80% will be assumed. 

For 1000 MW power plants, 571 would be required to provide all the electricity the US uses, compared with 104 that currently are in operation.

Up to now, this discussion has ignored the difference between peak power demands and gross power generation.  In the case of solar and wind power, it didn't matter because neither can reliably supply energy at any time, let alone meet peak demands.  Nuclear power is available at all times, though, so peak demand can be met.  The current US electric capacity is about 890,000 MW[source], so about a thousand 1000-MW power plants would be required, or a smaller number of larger plants.

Motor Fuels

The US uses about 140 billion gallons of gasoline per year.[source]  Since ethanol has only 70% of the energy content of gasoline[source], at 439 gallons per acre[source], the US would have to plant 456 million acres, or 713,000 square miles in corn to displace gasoline with ethanol.  That is about one-fourth of the area of the 48 contiguous US states.

The US consumes 63 billion gallons of diesel fuel per year.[source]  The land area required to grow enough soybeans to displace the petrodiesel with biodiesel, at 63 gallons per acre[source], would be one billion acres or 1,563,000 square miles, about half of the area of the 48 contiguous US states.

These calculated land areas seem too high to be correct, but they are in line with calculations done by others.  For example, this analysis finds that, if all vehicles were diesel-powered, the land area required would be 58% of the US including Alaska.  Another calculation shows that if all the corn and soybean crops in the US were converted into biofuels they would replace just 12 percent of the gasoline used and just 6 percent of the diesel fuel.

Of the land in all of the US, only 18% or 650,000 square miles is arable[source], and most all of that is being cultivated for food and fiber.  Also, the calculations shown above assume that all the land would have the same yields as the prime farmland currently under cultivation and that there would be sufficient water for irrigation.  Neither of those conditions is true, of course, so plainly there isn't enough land.

Clearly, biofuels won't provide much liquid fuel.  There is a possibility that these land requirements can be reduced by two thirds if hydrogen is injected into the biomass during processing.  For example, 0.77 gallons of biodiesel can be produced by adding 1 kg of hydrogen[source], which requires 39.3 KWH of energy to produce from water.  The biodiesel equivalent of US diesel consumption is 70 billion gallons per year; to produce enough hydrogen would require 2.75 trillion KWH per year.  The fact remains, though, that biofuels can only be part of the solution.

For a long time, fuel cells have been the holy grail in the quest to free the world from fossil-based motor fuels. The barrier seems to be the catalyst; platinum so far is the only material that works. Not only is it very expensive, several thousand dollars per vehicle [source] but if fuel cells drove up the demand then the cost would be even higher. There also is the unsolved problem of storing enough hydrogen on board for a reasonable driving range. But suppose these problems could be overcome.

As a rough estimate, let's say the amount of hydrogen needed would be the energy-equivalent of 100 billion gallons of diesel fuel per year, chosen mainly because it's a round number about half of the total of gasoline and diesel fuel: 50 billion gallons probably is too little and 150 billion gallons probably is more than necessary.  The heat value of diesel fuel is about 38 KWH/gallon[source], so our energy equivalent is 3.8 billion MWH/year.  For our rough purposes, this is the same as our current electrical usage. 

Unfortunately, the process for converting water to hydrogen at normal temperatures is less than 30% efficient. So, the electricity required would be more than three times our current electrical usage.  To generate that much electricity with solar panels would require a panel 5 miles wide running from San Diego to Boston.  To generate the electricity with wind turbines would require a strip of land 130 miles wide running from the northern Montana border to the southern Arizona border with 2,870,000 turbines, all rated at 1.5 MW.

It is possible to produce hydrogen efficiently in a thermochemical process, using nuclear-generated heat.  The nominal efficiency is over 45%.[source] But the heat left over from the conversion can be used to generate electricity, so the hydrogen production is nearly 100% efficient.  The nuclear plants can produce electricity and hydrogen at the same time.  More power plants aren't required because the additional heat will be available during off-peak hours.

Currently, hydrogen storage is the weak link.  It's practical only for local transportation, but intense research is underway.

Bulk Energy Storage

Pumped Energy Storage
The purpose of this admittedly rough calculation is to estimate the amount of pumped storage that would be required if wind power provided all the electricity the US uses.

First, consider the amount of electricity the US uses, a total of just over 4 billion MWH/year.[source] On an average day, not a high-demand day, that's 11,000,000 MWH/day.

We have to make up a fictitious example because there aren't any real examples. Suppose we use Lake Erie, Niagara Falls, and Lake Ontario as our pumped-storage, pretending that we could increase the capacity of the turbines enough.

We know that Niagara Falls' power capacity is 2400 MW, using 375,000 gallons/second of water.[source] We also know that the capacity of Lake Erie is 484 cubic kilometers of water, which is 128,000 billion gallons.[source] At present, the falls could produce 57,600 MWH/day, using 32.4 billion gallons/day. So, to serve all of the US for one day, the water required would be (11,000,000/57,600) X 32.4 billion gallons = 6,187 billion gallons, which is 4.8% of Lake Erie. That means that Lake Erie could provide storage for three weeks.

How much storage would be required? Looking at data for all of the US, we see that, with surprising consistency, low-wind months have average speeds about 70% of the average speeds for the high-wind months.[source] Based on the cubic relationship between wind speed and power, we would expect seasonal variations in energy generation of around 0.35 to 1, so the average would be something like the rated capacity times (1 + 0.35)/2 or 0.675. Conversely, for the generation to equal the load, the rated capacity of the wind farms would have to be the yearly average load divided by 0.675, or multiplied by 1.48. Since we can expect the generation in a slow-wind month to be 0.35 times the rated capacity, the storage capacity would have to be about 1 - 0.35 X 1.48 (= 0.48) times the yearly average load times the length of the low-wind period.

21 days' capacity would be good for 21/0.48 = 43 days of low winds. But low-wind seasons last longer than 100 days.

The power grid allows for some redistribution of power from areas experiencing high winds to areas with low winds. But the wind-variation patterns cover large regions so there are limits to what can be achieved. Allowing for redistribution still leaves a need for more than three-weeks' capacity.

To provide adequate pumped-storage capacity for wind power as the main electical-energy source for the US would require damming canyon streams to create twin lakes around the country equal in volume to something bigger than Lakes Erie and Ontario. Even if enough locations could be found, the projects would not be permitted because of the high ecological cost.

Compressed Air
Another scheme that sometimes is mentioned is storing compressed air in caves. There is a facility in Huntorf, Germany that we can use for an example.[source] It compresses air to 1000 pounds per square inch pressure.

The data show that it stores 3 x 290 = 870 MWH of energy and the cave volume is 310,000 cubic meters.

For one day of electricity storage for the US, the volume needed would be

      11,000,000/870 X 310,000 = 3.92 billion cubic meters = 138.4 billion cubic feet.

Suppose a cave had an average cross-section of 50 ft X 50 ft = 2500 sq ft.

For one day's electricity storage, the cave's length would have to be 138.4 billion / 2500 = 55.34 million feet = 10,490 miles. Granted that most big caves have never been surveyed, it's still safe to say that there aren't ten-thousand miles of caves in the US. So there is no possible way enough energy could be stored to see the country through 100 days of low winds.

Conclusion

Barring some startling new energy development, what all this shows is that solar panels and wind turbines won't provide major parts of the world's energy; biofuels can only be important if a large amount of hydrogen is available.  If global warming is to be avoided, the only two technologies that can provide sufficient energy are nuclear and hydrogen.

In the next article we'll look at the obstacles to solving this problem. In the article after that we'll see a scheme for reducing the consumption of fossil fuels.

Solutions to Global Warming: Part 2

3) Renewable Energy Sources

In this article we'll look at alternative energy sources and appraise their effectiveness in minimizing global warming. The numbers apply to the US; nationals of other countries will have to figure this out for themselves.

Residential Energy Sources

Some important savings can be made by making greater use of natural energy sources. Home heating and residential water heating could be switched almost entirely to solar and solar-heat-pump systems. Passive solar heating techniques can be built into homes. The remaining residential applications would mainly be cooking, which could almost entirely be converted to electricity. These changes would reduce CO2 emissions by 367 million metric tons, or 6.1% of the total.[source]

Wind Power

Wind power is already providing some electricity at a price which is only a little higher than electricity from fossil-fired power plants.[source] What limits wind power is the need for storage, since neither homes nor businesses can stop functioning when the wind power is unavailable. Currently, only one form of bulk storage is available for energy: existing hydroelectric dams, which account for 6.6% of total US electrical capacity.[source] There are limits to how much storage can be used, since dam operators have to maintain minimum water flows and also have commitments to irrigators, but it's conceivable that wind power could provide a few per cent of the country's energy.

If some sort of bulk energy storage could be developed, that could make wind energy practical. The storage method closest to practicality is pumped storage. In fact, there is a small amount of pumped storage in use now. A rough calculation shows, however, that there aren't enough places to install pumped storage for wind power to become the main electricity source.

A different strategy would be to have fossil-fired power plants standing by to back up wind power. Since wind turbines have a load factor of 25 to 35%, that would seriously hamper the effort to reduce greenhouse-gas emissions because the fossil-fired plants would have to operate a large portion of the time.[source] But the existing fossil-fired plants will be around for some decades while replacement capacity is built. In the meantime, they can be used as backup for wind turbines and other renewable sources. So, in the short term, wind power can be a major power source until replacement sources are constructed and the wind turbines wear out.

Solar Energy

Solar photovoltaic systems are presently too expensive to compete with other energy sources, but over the years can be expected to become cheaper.[source] They already are becoming popular in remote locations where connecting to the electrical grid is impractical. If costs continue to fall, solar energy can complement wind power but the unavailability of bulk storage will apply to solar energy as well.

Geothermal energy

Geothermal energy presently supplies 0.34% of the energy used in the US.[source] There are two types of geothermal energy: wet and dry. The wet type is being exploited about as much as it can be. There is a lot more available in dry form; unfortunately, there aren't any practical ways of extracting it.

Biofuels

Biofuels represent a possibility. To use them unblended as motor fuels would require new engine designs, but that will be unavoidable with any change from petroleum-based fuels. Currently, the best estimate is that it takes 0.75 gallons of fuel to produce the energy-equivalent of 1 gallon of conventional fuel. That's only true if credit is given for the value of the leftover material as animal feed; once the demand for animal feed is satisfied, the payoff ratio won't be as good. It is believed that the fuel input could be reduced to as little as 0.4 gallons with advanced technology that allows agricultural waste to be used as the raw material.[source] Agricultural waste is what gardeners call mulch; it hasn't been determined what would be the adverse consequences of diverting mulch away from fields. Research is being done on different biomass plants and chemical processes that could give better results.

The International Energy Agency estimates[source] fossil-fuel use at 388 exajoules per year worldwide, which may be expected to double or triple in this century. It also estimates that to supply 300 exajoules/year, a goal attainable with moderate effort, would require 7% of Earth's landmass, requiring forest clearing and insecticides and synthetic fertilizers. In comparison, 13.3% of the landmass is arable, including 4.7% already under cultivation.[source] The IEA concludes that biofuels can be an important means of reducing greenhouse-gas emissions on a global scale. For the US and Europe, though, given their limited free agricultural land and their high dependence on liquid fuels, the main effect would be switching from oil-rich suppliers to land-rich ones. Presumably, the benefit of reducing global warming would justify the higher cost.

Silver Bullets

Many other systems have been proposed: wave engines, tidal engines, and ocean-thermal-gradient engines, to mention only a few. People have suggested micro-hydroelectric power, installing small turbines on thousands of creeks and streams, but never have addressed the legal obstacles to extinguishing hundreds of species. Fusion research continues apace, but no projections are made regarding when it could become practical. Schemes have been suggested for energy storage, such as compressing air in caves, or building mammoth flywheels. All of these ideas are exactly where they were over thirty years ago: nowhere.

Hydrogen

But one idea has real potential: hydrogen. Presently, hydrogen use is hampered mainly by the low energy efficiency (around 30%) of converting water to hydrogen at ordinary temperatures.[source] There are more efficient processes, but they require high temperatures and are poorly suited to renewable energy sources. Alternatively, research is going on to improve the efficiency of photosynthetic production, currently around 2%.[source] If the efficiency could be improved, then there is a real future for hydrogen. Storage technology is ahead of production technology, and fuel cells are already proven. Hydrogen could be a fuel substitute and could well be the main element in future energy delivery.

However, there is a trap in something this attractive. For over thirty years, Americans have chosen to stay with fossil fuels based on the promise that something new and better was almost ready to displace the use of fossil fuels. The new and better something never materialized, with the result that fossil-fuel use now is threatening the planet's climate. Is it safe to continue this way, or should we look for other solutions that are available now?

Summary

There are two time-frames to consider. With present technology, renewable energy can displace a big part of fossil-fired electric power, but will lose that capability as the backup fossil-fired plants wear out. In the long run, the world must focus on non-fossil energy sources. Unless some form of bulk energy storage is invented, renewable sources will only be able to provide about ten per cent of the energy the US uses. Conservation, if pursued aggressively, could hold energy consumption to its current level.

The next few articles will take a look at nuclear energy.