3 ms·

Gas turbine efficiency targets for combined cycle are targeting 65%, so loss is at least 35%. To me I the loss sucks, but if it can be cheap to build and mainta

Gas turbine efficiency targets for combined cycle are targeting 65%, so loss is at least 35%. To me I the loss sucks, but if it can be cheap to build and maintain it matters less. If solar is the energy source and that gets driven cheap enough then that loss could be acceptable.

NavinF 3y ago
No way you'll get 65%. This sand won't be stored at the same temperature as a natgas generator's combustion chamber. Efficiency depends on delta T. I'd guess 20%
- regularfry 3y ago
  They're estimating over 50%, apparently. The sand is stored at over 1000 degrees. Not combustion temperatures, true, but also nothing to sniff at.
- adrian_b 3y ago
  Nope. The report says that the sand will be heated up to 1200 Celsius degrees. This is much higher than the maximum temperature for steam turbines and equal to the temperature of the gas in the best gas turbines. Therefore they will use a combined cycle, first a gas turbine will use hot air passed through the sand and the exhaust from the gas turbine will produce steam for a chain of steam turbines with decreasing working temperatures. There should have been no problem in reaching a 65% efficiency for the conversion from heat to electricity, except that between hot sand and a gas burner there is the same difference as between an electric capacitor and a battery, while heat is extracted from the sand, it cools down. Presumably, when the sand becomes too cold, the gas turbine is bypassed and the hot air just produces steam. When it becomes even colder, I suppose that the first steam turbine is also bypassed and only the low-temperature steam turbines are used. This will lower the average efficiency, probably to around 50%. If the residual heat (after the steam turbines) had been used for heating or for cooling (i.e. heat-powered air conditioning), the efficiency could have been higher, e.g. over 80% in the beginning, while the sand is still very hot.
  - credit_guy 3y ago
    I did not read the report, but one way you can improve the round trip efficiency is to not let the sand cool below, let's say 1000 °C. You pay a one-time penalty of heating the sand to this temperature and then you operate the storage only between 1000 and 1200 °C. This reduces the capacity of the "sand battery", roughly by a factor of 6. But sand is (reasonably) cheap. I have seen another proposal where instead of sand the material would be graphite, which can be heated to more than 2000 °C. Graphite also has a huge heat conductivity. But it's more expensive.
    - zizee 3y ago
      Sand has the added benefit over graphite of being nonflammable.
      - datameta 3y ago
        Sand will also benefit from greater insulation due to lower packing efficiency and therefore more air.
ajb 3y ago
IIUC there is a physical limit from thermodynamics in converting back, based on the temperature. So when efficiencies are quoted, it could be either the proportion of input energy retrieved, or the proportion of the theoretical max efficiency achieved. I'm guessing you're quoting the former?

NavinF 3y ago

No way you'll get 65%. This sand won't be stored at the same temperature as a natgas generator's combustion chamber. Efficiency depends on delta T. I'd guess 20%

regularfry 3y ago
They're estimating over 50%, apparently. The sand is stored at over 1000 degrees. Not combustion temperatures, true, but also nothing to sniff at.
adrian_b 3y ago
Nope. The report says that the sand will be heated up to 1200 Celsius degrees. This is much higher than the maximum temperature for steam turbines and equal to the temperature of the gas in the best gas turbines. Therefore they will use a combined cycle, first a gas turbine will use hot air passed through the sand and the exhaust from the gas turbine will produce steam for a chain of steam turbines with decreasing working temperatures. There should have been no problem in reaching a 65% efficiency for the conversion from heat to electricity, except that between hot sand and a gas burner there is the same difference as between an electric capacitor and a battery, while heat is extracted from the sand, it cools down. Presumably, when the sand becomes too cold, the gas turbine is bypassed and the hot air just produces steam. When it becomes even colder, I suppose that the first steam turbine is also bypassed and only the low-temperature steam turbines are used. This will lower the average efficiency, probably to around 50%. If the residual heat (after the steam turbines) had been used for heating or for cooling (i.e. heat-powered air conditioning), the efficiency could have been higher, e.g. over 80% in the beginning, while the sand is still very hot.
- credit_guy 3y ago
  I did not read the report, but one way you can improve the round trip efficiency is to not let the sand cool below, let's say 1000 °C. You pay a one-time penalty of heating the sand to this temperature and then you operate the storage only between 1000 and 1200 °C. This reduces the capacity of the "sand battery", roughly by a factor of 6. But sand is (reasonably) cheap. I have seen another proposal where instead of sand the material would be graphite, which can be heated to more than 2000 °C. Graphite also has a huge heat conductivity. But it's more expensive.
  - zizee 3y ago
    Sand has the added benefit over graphite of being nonflammable.
    - datameta 3y ago
      Sand will also benefit from greater insulation due to lower packing efficiency and therefore more air.

ajb 3y ago

IIUC there is a physical limit from thermodynamics in converting back, based on the temperature. So when efficiencies are quoted, it could be either the proportion of input energy retrieved, or the proportion of the theoretical max efficiency achieved. I'm guessing you're quoting the former?