Post by David B. Benson on Nov 29, 2017 21:56:31 GMT 9.5
A thermal store just stores heat for some time. Low grade heat might be used for space heating but high temperature heat has many industrial uses, including generating electricity. One way to store heat is in a phase change material. The Adelaide, South Australia, organization 1414 degrees proposes to do so with pure silicon, hence the name.
Since their website offers rather little technical information, one supposes they plan to use electrical resistance heaters to melt the silicon; the electricity to come from solar panels and wind turbines at times when there is little direct demand. Since 1414 °C is quite hot this is enough, I think, to run a supercritical carbon dioxide Brayton cycle turbine. Such is being prototyped by GE, based on the research provided by Sandia Laboratory. My understanding is that it is 50% thermally efficient.
Equally thermally efficient would be a Stirling cycle engine, but I don't know of any under development for that high a temperature.
In either case, the reject heat will be 400--600 °C according to their website. That sounds right to me. It is enough to generate steam for a Rankine cycle second stage with a thermal efficiency of about 30%. So if I have done my thermo sums properly, the two stages might give 65% thermal efficiency. Roughly the same as a combined cycle gas turbine under ideal conditions.
The company website claims 95% retrieval of heat from their thermal store. Assuming 90% for this stage and a more realistic 60% for the two stage electrical generators, one recovers 54% of the input, maybe a bit more.
I don't know enough to do the economics of this plan. My rough estimates suggest that this thermal store is too expensive for applications in the Pacific Northwest. But most parts of the world do not have our plentiful hydropower. Perhaps in at least South Australia this thermal store will prove economic while so-called carbon free.
Since their website offers rather little technical information, one supposes they plan to use electrical resistance heaters to melt the silicon; the electricity to come from solar panels and wind turbines at times when there is little direct demand. Since 1414 °C is quite hot this is enough, I think, to run a supercritical carbon dioxide Brayton cycle turbine. Such is being prototyped by GE, based on the research provided by Sandia Laboratory. My understanding is that it is 50% thermally efficient.
Equally thermally efficient would be a Stirling cycle engine, but I don't know of any under development for that high a temperature.
In either case, the reject heat will be 400--600 °C according to their website. That sounds right to me. It is enough to generate steam for a Rankine cycle second stage with a thermal efficiency of about 30%. So if I have done my thermo sums properly, the two stages might give 65% thermal efficiency. Roughly the same as a combined cycle gas turbine under ideal conditions.
The company website claims 95% retrieval of heat from their thermal store. Assuming 90% for this stage and a more realistic 60% for the two stage electrical generators, one recovers 54% of the input, maybe a bit more.
I don't know enough to do the economics of this plan. My rough estimates suggest that this thermal store is too expensive for applications in the Pacific Northwest. But most parts of the world do not have our plentiful hydropower. Perhaps in at least South Australia this thermal store will prove economic while so-called carbon free.
