5 ms·

If a steam turbine could operate at those higher temperatures it would be more efficient. But it cannot do so under any reasonable condition. If you want the hi

If a steam turbine could operate at those higher temperatures it would be more efficient. But it cannot do so under any reasonable condition. If you want the higher efficiency from storing high grade heat energy, it's not feasible to use a steam turbine. Think of the steam turbine as a baseline. Like rating a vehicle in horse power.

cormacrelf 5y ago
Also I wonder if, using similar principles to a heat pump's operation, you could still get juice out of stored heat at lower temperatures. Surely you can have this hot graphite sitting at under 2000°C, heat some fluid/gas to say 1000°C, and then compress the gas to increase its temperature? Surely that would be the ideal solution anyway, since you don't want your hot graphite to become a chunk of useless heat simply for dropping below temp briefly. On the topic of heat pumps, you could also use a TPV for geothermal power. Since there are no moving parts and presumably no huge steam engine installation, it would be more feasible to have one of these in your back yard. The grid powers a heat pump, you compress the fluid till it hits 2000°C, and your TPV extracts power. The heat pump itself is >100% efficient, so overall you can steal a fair bit of electricity from the ground. Right?
- ok_computer 5y ago
  I just want to point out that compressing gas takes a non-zero amount of work. Compressing gas to increase temperature to use temperature to make energy would be better spent using the compressing work directly to make electricity.
- Szpadel 5y ago
  I'm not sure if there is any way to make heat pump operating in this scale of temperatures, and keep in mind that current heat pumps can only "pump" over about 50°C max until it's no longer profitable.
  - elil17 5y ago
    That’s not really true at all. Home heat pumps are stuck there because of the refrigerants used. Ammonia heat pumps can go way hotter (though still nowhere close to those temps - more like 150c iirc), it’s just that ammonia requires safety controls that are impractical on a small scale.
- qsdf38100 5y ago
  The best theoretical efficiency of a heat pump is 1/CarnotEfficency. So the best you can do is to have a round trip efficiency of 1. See Carnot batteries. So using a heat pump to increase the temperature so that a thermal engine is more efficient can’t possibly increase the overall efficiency.
  - mcbishop 5y ago
    Heat pumps are different than heat engines... I don't think Carnot efficiency applies. A heat pump can move several units of heat energy per input unit of electrical energy (thanks to awesome compressor / refrigerant / phase-change technology). ...They're >100% efficient.
    - hinkley 5y ago
      > The best theoretical efficiency of a heat pump is 1/CarnotEfficency. The facts have already been stated that refute your what if. Whether you missed it or not, you're arguing for a perpetual motion machine. The Layman's version of the 3 Laws of Thermodynamics are: 1) You can't win 2) You can't cheat 3) You can't even choose not to play
    - qsdf38100 5y ago
      Heat pumps really are reversed heat engines. For instance, take a Brayton thermodynamic cycle. It's motor cycle, but you can literally just reverse the arrows on this cycle, it becomes a reversed Brayton cycle, which corresponds to a heat pump (or a refrigeration machine, depending on which inlet of the machine you're interested in). In practice, the reversing of the arrow means the reversing of the fluid flow, which means compression processes are now expansion processes and vice versa. Carnot theory does still apply. The COP (Coefficient Of Performance) of a Carnot heat pump is 1/n, where n is the usual Carnot efficiency of a heat engine. With n=1/3, you get a COP of 1/1/3 = 3 = 300% Carnot efficiencies consider ideal machines. So ideal Carnot batteries have a n * 1/n = 100% efficiencies. It means you can't increase the efficiency of a heat engine by putting a heat pump in front of it. In an ideal case (ignoring all losses and inefficiencies), you would get the same efficiency. In practice, you can only be worse.
- bumby 5y ago
  >The heat pump itself is >100% efficient Are you sure you aren't confusing the coefficient of performance with the efficiency? If a heat pump was >100% efficient, I could cool my house by leaving my refrigerator door open.
  - hnuser123456 5y ago
    If you pipe the heat from the compressor directly outside instead of venting it into the same room, that would work.
    - bumby 5y ago
      Correct, that's what refrigerated air conditioners do. But that's really just playing a trick with the system boundary in the problem; it doesn't mean the global efficiency has exceeded 100%.
      - cormacrelf 5y ago
        Yes, it's an imprecise term.
        bumby 5y ago
        To be clear, I don't think the term is imprecise. Efficiency is always (work out)/(work in). The numbers get wonky when we aren't fully accounting for the "work in". In the case of refrigeration, we have to account for the work it takes to compress the fluid and the resulting waste heat.
brador 5y ago
What is high grade heat energy?
- neltnerb 5y ago
  High temperature, though I'm confused myself about how a 40% efficient conversion done twice can match a battery. Probably will need to read the actual paper, I thought batteries were on the order of 95%. Maybe it is more energy dense or something.
  - panicpanicpanic 5y ago
    It might be about profitability, long-term energy storage and much less maintenance (since no moving parts). Ref: https://www.sciencedirect.com/science/article/abs/pii/S0306261919305082 https://www.sciencedirect.com/science/article/abs/pii/S03062... "Evaluated herein is one E-TES concept, called Firebrick Resistance-Heated Energy Storage (FIRES), that stores electricity as sensible high-temperature heat (1000–1700 °C) in ceramic firebrick, and discharges it as a hot airstream to either (1) heat industrial plants in place of fossil fuels, or (2) regenerate electricity in a power plant. … We report that systems of 100–1000 s MWh may be cycled daily, and discharged at a constant heat rate typically for 70–90% of the storage capacity. Traditional insulation can reasonably limit heat leakage to less than 3% per day. Preliminary cost estimates indicate a system cost near $10/kWh, substantially less expensive than batteries."
HPsquared 5y ago
Not a steam turbine, a combined gas turbine (for the high temperatures) - steam turbine (for the low temperatures). The waste heat from the gas turbine would power the steam turbine. Something like a combined cycle gas turbine, but with a closed loop for the gas turbine (say, helium) rather than combustion.
hdersch 5y ago
Combined cycle power plants take heat at 2000°C using a combination of gas and steam turbines. Their conversion efficiency to electric power is 52%, see Wikipedia. This is proven technology, at least 30 years old, and quite a few of those exist out there. Steam turbines alone can be operated at higher temperatures by using mercury instead of water. Some plants using this approach were built in the 1920/30s.

cormacrelf 5y ago

Also I wonder if, using similar principles to a heat pump's operation, you could still get juice out of stored heat at lower temperatures. Surely you can have this hot graphite sitting at under 2000°C, heat some fluid/gas to say 1000°C, and then compress the gas to increase its temperature? Surely that would be the ideal solution anyway, since you don't want your hot graphite to become a chunk of useless heat simply for dropping below temp briefly. On the topic of heat pumps, you could also use a TPV for geothermal power. Since there are no moving parts and presumably no huge steam engine installation, it would be more feasible to have one of these in your back yard. The grid powers a heat pump, you compress the fluid till it hits 2000°C, and your TPV extracts power. The heat pump itself is >100% efficient, so overall you can steal a fair bit of electricity from the ground. Right?

ok_computer 5y ago
I just want to point out that compressing gas takes a non-zero amount of work. Compressing gas to increase temperature to use temperature to make energy would be better spent using the compressing work directly to make electricity.
Szpadel 5y ago
I'm not sure if there is any way to make heat pump operating in this scale of temperatures, and keep in mind that current heat pumps can only "pump" over about 50°C max until it's no longer profitable.
- elil17 5y ago
  That’s not really true at all. Home heat pumps are stuck there because of the refrigerants used. Ammonia heat pumps can go way hotter (though still nowhere close to those temps - more like 150c iirc), it’s just that ammonia requires safety controls that are impractical on a small scale.
qsdf38100 5y ago
The best theoretical efficiency of a heat pump is 1/CarnotEfficency. So the best you can do is to have a round trip efficiency of 1. See Carnot batteries. So using a heat pump to increase the temperature so that a thermal engine is more efficient can’t possibly increase the overall efficiency.
- mcbishop 5y ago
  Heat pumps are different than heat engines... I don't think Carnot efficiency applies. A heat pump can move several units of heat energy per input unit of electrical energy (thanks to awesome compressor / refrigerant / phase-change technology). ...They're >100% efficient.
  - hinkley 5y ago
    > The best theoretical efficiency of a heat pump is 1/CarnotEfficency. The facts have already been stated that refute your what if. Whether you missed it or not, you're arguing for a perpetual motion machine. The Layman's version of the 3 Laws of Thermodynamics are: 1) You can't win 2) You can't cheat 3) You can't even choose not to play
  - qsdf38100 5y ago
    Heat pumps really are reversed heat engines. For instance, take a Brayton thermodynamic cycle. It's motor cycle, but you can literally just reverse the arrows on this cycle, it becomes a reversed Brayton cycle, which corresponds to a heat pump (or a refrigeration machine, depending on which inlet of the machine you're interested in). In practice, the reversing of the arrow means the reversing of the fluid flow, which means compression processes are now expansion processes and vice versa. Carnot theory does still apply. The COP (Coefficient Of Performance) of a Carnot heat pump is 1/n, where n is the usual Carnot efficiency of a heat engine. With n=1/3, you get a COP of 1/1/3 = 3 = 300% Carnot efficiencies consider ideal machines. So ideal Carnot batteries have a n * 1/n = 100% efficiencies. It means you can't increase the efficiency of a heat engine by putting a heat pump in front of it. In an ideal case (ignoring all losses and inefficiencies), you would get the same efficiency. In practice, you can only be worse.
bumby 5y ago
>The heat pump itself is >100% efficient Are you sure you aren't confusing the coefficient of performance with the efficiency? If a heat pump was >100% efficient, I could cool my house by leaving my refrigerator door open.
- hnuser123456 5y ago
  If you pipe the heat from the compressor directly outside instead of venting it into the same room, that would work.
  - bumby 5y ago
    Correct, that's what refrigerated air conditioners do. But that's really just playing a trick with the system boundary in the problem; it doesn't mean the global efficiency has exceeded 100%.
    - cormacrelf 5y ago
      Yes, it's an imprecise term.
      - bumby 5y ago
        To be clear, I don't think the term is imprecise. Efficiency is always (work out)/(work in). The numbers get wonky when we aren't fully accounting for the "work in". In the case of refrigeration, we have to account for the work it takes to compress the fluid and the resulting waste heat.

brador 5y ago

What is high grade heat energy?

neltnerb 5y ago
High temperature, though I'm confused myself about how a 40% efficient conversion done twice can match a battery. Probably will need to read the actual paper, I thought batteries were on the order of 95%. Maybe it is more energy dense or something.
- panicpanicpanic 5y ago
  It might be about profitability, long-term energy storage and much less maintenance (since no moving parts). Ref: https://www.sciencedirect.com/science/article/abs/pii/S0306261919305082 https://www.sciencedirect.com/science/article/abs/pii/S03062... "Evaluated herein is one E-TES concept, called Firebrick Resistance-Heated Energy Storage (FIRES), that stores electricity as sensible high-temperature heat (1000–1700 °C) in ceramic firebrick, and discharges it as a hot airstream to either (1) heat industrial plants in place of fossil fuels, or (2) regenerate electricity in a power plant. … We report that systems of 100–1000 s MWh may be cycled daily, and discharged at a constant heat rate typically for 70–90% of the storage capacity. Traditional insulation can reasonably limit heat leakage to less than 3% per day. Preliminary cost estimates indicate a system cost near $10/kWh, substantially less expensive than batteries."

HPsquared 5y ago

Not a steam turbine, a combined gas turbine (for the high temperatures) - steam turbine (for the low temperatures). The waste heat from the gas turbine would power the steam turbine. Something like a combined cycle gas turbine, but with a closed loop for the gas turbine (say, helium) rather than combustion.

hdersch 5y ago

Combined cycle power plants take heat at 2000°C using a combination of gas and steam turbines. Their conversion efficiency to electric power is 52%, see Wikipedia. This is proven technology, at least 30 years old, and quite a few of those exist out there. Steam turbines alone can be operated at higher temperatures by using mercury instead of water. Some plants using this approach were built in the 1920/30s.