Why is renewable energy ALWAYS expensive?

As a researcher in Photovoltaics(read solar cells), I think a lot about renewable energy. For the Rip Van Winkles of the world, we seem to be running out of oil. Also most scientists believe that all the fossil fuel burning is heating up the planet too much (Let me be precise here. We know that earth is heating up but we are not sure if it is caused by humans.). So we need alternative sources of energy that are not exhaustible and do not produce prodigious amounts of excess greenhouse gasses.

Enter renewable energy. By definition it refers to energy derived from sources that can regenerate ("renew") naturally. Biggest examples are wind, water, geothermal, sunlight, etc. Also included are solutions like bio-diesel, which do emit carbon dioxide but that carbon is subsequently recycled it back into fuel.

Now the big question. Compared to fossil fuels, why are all the renewable energy sources so expensive? No matter what technology you chose, solar-electric, solar-thermal, algae-diesel, wind, etc. compared to fossil fuels it is always more costly. It turns out that the problem is not availability but the low density of energy. Let me explain....

What is the ultimate source of all energy on the planet, other than nuclear power, it is the Sun. Everything from weather to food is ultimately being powered by our star. On an average day the Sun beats down around 174 pentawatts of power; that is more energy in one hour than what the total energy consumed by the world in 2002 [1]. Where does all this energy go? A substantial portion is radiated back to space. In fact it is the reduction in this reflected power, due to green house gasses, that is responsible for global warming. The rest of the energy gets dissipated as wind, stored as biomass, etc.

While this is a lot of energy, it is very dilute. The amount of energy falling on the surface of a typical car (~8 m sq. [2]) is only 8 kW, i.e. 10.5 bhp. Compare this to the power of an average gasoline engine (~110 bhp) and you realize that even if we capture 100% of the incident energy, we can't power a car purely from sunlight falling on it. To make a solar powered car, we need to concentrate the Sun's energy into a more dense form. We can store the energy into a battery, or make bio-diesel, or generate hydrogen from water, etc. Now fossil fuels have done this concentration naturally over the course of many million years, but renewable sources do not have that long! Since this extra time is not being priced by the market, all such technologies seem more expensive.

In some sense we have been spoiled by oil. Our requirements of energy density are just way too high and unfair. May be we need to reduce standards by going for efficiency. Smart grids, lighter cars, smaller cars, etc. are all needed before renewables can take off.


[1] http://en.wikipedia.org/wiki/Solar_energy
[2] http://www.usatoday.com/money/autos/2007-07-15-little-big-cars_N.htm