By Ed Caryl
Can we provide enough energy to power the world using alternative sources, such as wind, solar, or bio-fuels?
This is a question that has largely been ignored, in hopes that the answer will prove to be in the positive. This article is the first in a series, taking the question one technology at a time.
- Figure 1. Solucar PS10 solar concentrator power plant near Seville, Spain. Source: Wikipedia Commons
Four problems with wind and solar power are: the area needed for the plants, energy storage when the wind or the sun is not available, transmission costs, and energy losses in storage and transmission. For solar power in the U. S., the power is available in the southwest of the country, and the power is needed in the northeast. In Europe, sunshine is abundant in Spain, but not so much in Germany. In general, environmentalists oppose additional transmission lines nearly everywhere. But let us assume that the transmission problem is solved. Is there enough area to collect sunlight to support all the energy needs? Let us take the area where this author lives as an example.
New Mexico – area 314,460 km2 – insolation 7 kWh/m2/day
Not all of the area is usable. Mountains, forest, wilderness, National Monuments, farm and urban areas, military training areas, bombing and missile ranges, are off limits. Perhaps 10% of the land area might be available for solar if all the ranchers are bought off and the environmentalists are bound and gagged. That likelihood seems very remote, considering that every patch of sagebrush seems to harbor an endangered lizard. (In California the animal holding up solar plants is the Desert Tortoise).
The power available on 10% of the area of New Mexico is:
31,446,000,000 m2 X 7 kWh/m2 X 365 = 80,344,530,000,000 kWh or 80.345 PetaWatthours (PWh)
This sounds like plenty of energy. The U. S. currently uses about 30 PWh per year. But there are problems. This figure is for a two-axis solar concentrator if it runs at 100% efficiency. The real efficiency of a solar thermal plant is in the range of 8 to 15%, about the same as photovoltaic panels. The Sierra SunTower plant in Lancaster California generates 5 MW peak, with no thermal storage, occupies 20 acres of land, about 81,000 m2, or about 62 W/m2. It is in a zone with 7 to 8 kWh of solar radiation available. That is an efficiency of about 8%. The Andasol 1 and 2 plants in Spain claim 15% annual average efficiency with 8 hours of thermal storage.
So, using the above efficiency figures, 10% of the area of New Mexico would supply 6 to 12 PWh of electricity or 20 to 40% of the energy needed for the US. We would need to build the same number of plants in at two or three more states, California, Nevada, and Arizona would be candidates.
Storing the heat for use at night is the next biggest problem. With current technology the most efficient thermal storage (molten salt) stores heat for about 8 hours. The salt used is a mixture of sodium and potassium nitrate (fertilizer). It takes about 28,000 metric tonnes of salt to support the Andasol 1 plant for 8 hours of heat storage. Sodium and potassium nitrate are also known as saltpeter, the oxygen source in gunpowder. Now visualize a terrorist-driven airliner full of jet fuel (the carbon source) crashing into the storage tanks at one of these plants. It would be the equivalent of a small tactical nuclear bomb.
The total world production last year for nitrate fertilizers was 154 million tons, but most of that was ammonium nitrate. Chile, the chief supplier, only produced about a million tons of sodium and potassium nitrate. If all one million tons were used for energy storage, that would supply thirty-six 50 MW plants, or 0.018 PWh of electricity annually, 0.06% of that needed for the whole US, and less than 0.15% of that needed for all the above plants in New Mexico. One hundred years of Chilean production would build 6% of the needed solar plants for the U. S. alone.
There are other heat sinks that can be used, but they have problems of their own. The most efficient heat storage is the above described nitrate salts. The other proposals are more costly or have less volume specific heat capacity. One of the cheapest is concrete blocks with embedded pipes. But the structural stability of the pipes and concrete over many temperature cycles is a problem, and the mass needed is 250% of that needed using nitrate salt. For a plant the size of Andasol 1, 70,000 metric tonnes of concrete would be required. That volume takes up ground area, lowering the overall plant efficiency, and requiring more area for a given power output.
Let’s say we have beaten the above problems. We have paved 10% of four western US states with solar plants of 15% efficiency and are providing more than 30 PWh of electricity to the US, all the power we need. The problem then is that we are dumping nearly 200 PetaWh of heat into the environment, almost seven times what we are dumping now, and twice what the whole world is producing. What will happen to the temperature? The southwest US will be uninhabitable at something like 10 or 15°C hotter than at present. If the same thing is done in Europe, the rain in Spain will no longer fall anywhere. It will bring on the catastrophe that Hanson et al have been warning us will happen.
A solar power solution requires vast areas, with a high environmental cost that probably cannot be paid, and the waste heat load would make any projected CO2 warming look benign. Solar plants are not the long-term solution to our energy requirements.