Last week I was at the ECOGEN 2010 exhibition and conference at the Sydney Exhibition and Convention Centre. One of the sessions that I attended on the third day was about solar thermal energy.
Dr David Harries from EMC Solar gave a talk about some research into solar thermochemical storage using Calcium Hydride (CaH2). Thermochemical energy storage can result with an energy storage density of about 10 times the amount that could be stored in molten salts.
In the process of storing heat, the Calcium hydride dissociates into hydrogen and calcium. The hydrogen is removed from the reactor (around 1000 degrees centigrade), cooled to ambient temperatures and stored. Once heat energy is removed from the reactor for use in a heat engine, the hydrogen is drawn back into the reactor and energy is released with the formation of calcium hydride. The processes are reversible. There is no phase change and there is a high temperature gradient through the reactor. A working proof-of-concept prototype was a reliable storage medium of thermal energy.
These thermochemical storage media are being designed to be used with concentrated solar thermal power plants. Sunlight would be collected in a dish, concentrated into a beam and then manipulated with mirrors so that the beam of solar energy would be directed into the energy storage reactor. The stored energy could be extracted at night to power conventional steam turbines or rankine engines. That is how the use of this technology is envisioned.
Listening to the presentation, I was thinking that this kind of energy storage might be good for use in energy relay satellites. One of these thermochemical energy storage reactors might be suitable for absorbing the energy of a laser fired from a distant solar power satellite. The stored thermal energy on an energy relay satellite could then be used as the source of energy for an onboard laser that could be fired at another energy relay satellite or to a thermochemical energy storage reactor on the ground. If energy could be efficiently transferred between satellites by lasers, then it would be feasible to set up space-based concentrated solar power stations. The potential of this kind of energy source is that it would completely overcome the energy crisis with an abundant supply and we could have all the energy we need without increasing the concentration of atmospheric carbon dioxide. It would still take some time for such a transition of energy systems to take place.
One problem for space-based solar power is the loss of energy through the atmosphere when transmitting that energy to the earth. It might seem to be an obvious solution to this problem, but perhaps one way around this to situate the ground-based thermochemical energy storage reactors in places with a high altitude – such as on mountains and in alpine regions. The lasers fired from geostationary space-based energy relay satellites would pass through less atmosphere before it is captured for use, compared to locations that are closer to sea-level. Another added advantage is that most alpine regions are sparsely populated and remote, so there would be less people in the vicinity of these ground-based thermochemical energy storage reactors. It might be a problem hauling the infrastructure for a thermal power stations into alpine regions, but the transmission technology to connect the power station to the grid shouldn’t be too difficult. Another advantage is that the temperature in alpine regions might increase the Carnot efficiency of a heat engine.
Thermochemical energy storage could be used for concentrated solar power stations. The stored heat would be used at times when solar energy is not available, such as during the night. There could be other uses of this kind of technology, and it might be suitable technology for transmitting energy wirelessly and over large distances. An energy infrastructure that sources much of its energy from space-based solar power stations could revolutionise energy systems. With abundant space-based solar power, we could even manufacture carbon-based fuels or liquid hydrogen/oxygen fuels with that solar energy. The first space-based energy systems might not seem that impressive compared with conventional power systems (coal, nuclear and even renewables) in terms of the amounts of power made available, but you might see something like a boot-strapping effect once these systems are working and in place.
