You are not logged in.
Louis' original topic was about a crushed rock, sensible heat thermal storage system. The advantage it has is in being a really simple system, using victorian era technology to raise steam and constructed from cheap materials. Crushed rock and alloy steels. These systems are conceptually easy to build.
The downside is poor cycle efficiency. Despite what the proponents of this system claim, realistic round trip efficiency will be 30 - 40%. But that is efficiency of electrical energy recovery. The other 60 - 70% is going to be low grade heat, with a temperature of around 30°C. But the heat is by no means useless. If we can integrate this system for combined heat and power, its efficiency will be improved. We can calculate what is known as exergy efficiency. Whereas energy is always conserved, exergy is degraded by entropy. We can think of exergy as the work potential of energy. The work potential of electricity is 1, or 100%. The ideal work potential of heat can be calculated from the carnot equation:
W = (Th - Tc)/Th = (303 - 283)/303 = 0.066
In reality, a real heat engine gets half to two thirds of ideal efficiency. So W = 0.033 - 0.044.
Adding this to a 30 - 40% electricity recovery, gives an effective exergy recovery of 33.3 - 44.4%. Actual energy recovery may be close to 100%. The exergy recovery is better than it sounds if the alternative is generate heat by resistance heaters.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline
A hot rock energy storage system could be scaled down to levels useful for offgrid applications.
A tank would containing rock and gravel bonded by concrete. It would be insulated with rockwool and sand and heated to around 300°C using resistance heaters. Steam pipes would run through it. During power recovery, water would boil in the tubes, producing wet steam which wouod be dried yielding dry steam and condensate, which woukd return to the feed pump charge vessel. Dry steam would generate power using a small turbine. The condenser water would have a temperature of about 60°C.
For each unit of electricity used to charge the system, some 25% would be recovered as mechanical power by the turbine and 75% as hot water, which can be used as washwater.
Concrete has a density of about 2000kg/m3 and a specific heat of 1KJ/kg.K. A cubic metre of concrete heated from 20 to 320°C, would store some 600MJ of heat, some 150MJ (41.7kWh) of which is recovered electrical energy. That is sufficient electrical energy for 2 -4 days of power for a household and sufficient heat for all washwater and much of the space heating needed over the same period.
The overall exergy performance of this concept isn't great, but has a heat equivelent of about 1.46, whereas a pure resistance heater would be 1.0. How did I calculate that?
Some 75% of the initial electrical energy is recovered as heat, with a temperature of 60°C. The remainder is recovered as electricity. Assume that the 25% electricity is used to run a heat pump extracting heat from an ambient 10°C temperature, and outputting water at 60°C. Ideal COP = 283/50 = 5.66.
N = 5.66 x 0.25 + 0.75 = 2.115. But real heat pumps might get only half the carnot efficiency. So adjusting for this gives an exergy equivelant of 1.46. Compared to a resistance heater providing pure heat for direct use, steam recovery gives us more value because of the high work potential of that 25% of recovered electricity.
Last edited by Calliban (2024-02-16 06:36:11)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
Offline