Direct carbon fuel cells are interesting in that they have potential for very high efficiency. But, I am beginning to think of biochar a too valuable to burn because its micro structures seem to be quite good at buffering nitrogen. Buffering nitrogen seems to me to be key to land use changes that can help to restore the carbon sequestration potential of coastal waters while at the same time sequestering carbon in soil. I also feel that while one can get some parasitic energy production from making biochar, the inefficiency of rooted plants at collecting solar energy makes solar PV and wind power look much more attractive as energy sources. The electrification of transportation leads to about 0.5 days of storage total power consumption if batteries are used because transportation grade batteries are of such high quality that they are still useful after they have partially degraded. If the compressed air method of transportation becomes popular, then the air tanks and engines might also see a second life as stationary storage with the heat from compressing air being used for heating water and the cooling from use of the engines having application in refridgeration. Potentially, this could lead to even more storage than going with batteries and even greater overall system efficiency in the aftermarket application. So, just from converting transportation we will see substantial storage which should allow renewable penetration up to 70% or more.

The big advantage of standard hydro is that it can cover for the emergency shut down of a nuclear power plant because it is both large and responsive. This advantage seems to be disappearing in some places as decadal scale changes in precipitation reduce the availabilty of hydro power and the flow rate of rivers used to cool nuclear power plants. Further, I've been thinking in the last week about the sustainability of using biochar for sequestration, worrying about the ability of terra preta to regenerate itself. Could there be a problem with soils becoming carbon sinks that will continue to reduce the atmospheric carbon dioxide concentration below 280 ppm because, once started, they continue to accumulate carbon on their own? We may want to make estimates of how much terra preta surface area should be created. In that context, your picture of retaining biochar as energy storage for decades seems a little like Joseph's interpretation of Pharaoh's dream. I wonder if there is a place for a strategic biochar reserve at some point in the future. This would clearly be a distributed reserve rather than centralized. I think, though, that as wind and solar begin to become our dominant energy sources, it may turn out that liquid hydrocarbon reserves will be produced using carbon dioxide and water as feedstocks wih less disturbance of natural ecosystems just because we are substantially more efficient than rooted plants at doing this. With Lake Mead projected to go dry within 14 years or so, rethinking the usefulness of large scale hydro as energy storage is probably in order.

Chris