In his comments, Paul Weisz concludes that our best hope is for solar cells and advanced nuclear energy. He dismisses wind energy with the assertion that “energy losses due to transmission, supply, and demand fluctuation or conversion to other energies will reduce the actual contribution” from his estimated maximum potential of 3–22 quads of energy per year, which is much less than the 100 Q required to sustain the US lifestyle. I find Weisz’s statement illogical because the solar cells and nuclear sources will also require transmission, supply-to-load matching, storage, and conversion. More wind farms are being built than any other electricity-generation facilities because they are now the lowest-cost option. In its last quarterly report, Florida Power and Light noted that its profits from wind energy are enough to cover its losses from nuclear energy. Given that 2% of all solar energy reaching Earth is converted to wind energy, 1 the maximum potential at 30% conversion efficiency is 22 000 Q/yr.
Weisz also dismisses agricultural fuel production on the ground that agriculture currently provides barely more energy than it consumes. However, present agriculture is not trying to be sustainable but to maximize profits given cheap fossil fuel. Agriculture provided sustainable fuels—dung, oils, and wood—for millennia until the energy revolution. We should try to devise innovations to make biomass production sustainable again when fossil fuels are no longer cheap.
Consider urea as a fuel made from air, water, and electricity alone. Produced artificially on a scale of 120 million tonnes per year and also produced biologically, urea is noncorrosive, nonexplosive, essentially non-toxic, and almost nonflammable. Unfortunately, its energy per unit mass is not as great as some authorities require. The Bush administration’s FreedomCAR Fuel Initiative set the target at 3.0 kWh/kg. One can do some engineering to recover waste heat from the fuel cell or the combustion engine to drive endothermic reactions needed to extract hydrogen from urea. Still, careful analysis shows that such recovery could provide only about 2.5 kWh/kg.
The key to sustainable energy is to develop a practical fuel system. I assert that the best sustainable fuel is guanidine, CN3H5, which provides 4 kWh/kg, or mixtures of guanidine and urea. I propose a combination wherein wind provides the majority of the energy and agriculture and aquaculture provide the carbon, hydrogen, nitrogen, and heat needed to package the energy as guanidine. If guanidine proves to be a practical fuel, then its relatively simple transportation would also solve the transmission, supply-to-load matching, and storage problems for any solar cell or advanced nuclear sources that do arise.
Although a molecule of guanidine contains only five hydrogen atoms, it can effectively store nine by extracting hydrogen from the water recovered from the exhaust. Guanidine is not as safe as urea because it easily reacts with water to form ammonia, but it ships in green containers, an indication that it is in the safest category for transportation. Economic processes for its mass production appear simple. 2
To get the energy, consider wind. The best sites for wind farms are mostly over water and far from large consumption sites. However, for guanidine production, the wind farms can be located in the best sites.
The hydrogen for the fuel likely will come from water by electrolysis. This is efficient if the water is at high temperature and in the supercritical state. One will want to convert H2 promptly to NH3 via the high-temperature Haber process, then to urea and then to guanidine by moderate pressure–temperature processes. 2 Wind generators (and solar cells) do not produce much heat. The fuel-producing unit that converts electrical energy in excess of what can be sold immediately into guanidine will need hot N2, hot CO2, and hot H2O.
A simple way to supply these hot gases is to burn organic material. It seems obvious that practical production of guanidine–urea fuels will find symbiosis with agricultural and aquacultural production of biomass fuel.
