5 ms·

You have to do some “energy accounting” sleight of hand to get 63%, though. That number assumes that some of the low-grade waste heat can be used for, e.g., dis

You have to do some “energy accounting” sleight of hand to get 63%, though. That number assumes that some of the low-grade waste heat can be used for, e.g., district heating. It’s also a peak steady-state number, only achievable under optimal load conditions.

londons_explore 3y ago
I believe that figure is electric efficiency only. For combined heat and power plants, the efficiency routinely goes over 80%, and for heat-only plants, 95+% is common. However, there is still sleight of hand. The efficiency quoted is when the equipment is new and clean. Fouling and wear both take single digit percentages off. Also, they use the lower-heating-value for the gas energy supply. That, in my view, is dishonest - the correct energy measure for gas is the higher-heating-value, which is 10.7% more. The difference comes from how you account for the heat in the steam produced by burning gas. In my view, the energy from that steam should be accounted for when considering efficiency - in GE's view, it shouldn't.
- AtlasBarfed 3y ago
  And of course carbon removal from burning the natural gas is totally out of the equation.
  - lazide 3y ago
    Since no one is requiring it at scale, and since there is no commonly accepted way of doing it - pretty hard to add it to an equation no?
  - est31 3y ago
    > And of course carbon removal from burning the natural gas is totally out of the equation. Sorry for being nitpicky, but burning natural gas does not remove carbon, it adds new CO2 to the atmosphere (if you release the burn products which is usually done). You probably mean that if you released that natural gas directly into the atmosphere instead, it would have a larger greenhouse effect than the CO2 released by the burning process. This is true, and one should absolutely choose "burn it" when given the choice of releasing only vs burning and then releasing. But expressing it in the way of carbon removal is misleading. The number of carbon atoms in the atmosphere is the same in both scenarios, they are just bound in a less greenhouse-y form (also, natural gas decays into CO2 plus water eventually, but it's a very slow process). The number of CO2 equivalents goes up in both scenarios as well, just way less if you burn it before. Maybe some people refer to those through "carbon", idk.
    - AtlasBarfed 3y ago
      Ah, hacker news is out in force defending petroleum companies right to pollute the world. As always to economics, if you can't measure it well, it doesn't exist!
    - philwelch 3y ago
      > if you release the burn products which is usually done It’s also possible to capture the CO2 rather than releasing it into the atmosphere.
- adrian_b 3y ago
  I do not think that using the lower heating value is really dishonest, especially in the context of this discussion. The lower heating value corresponds to the heat actually produced during burning. The higher value is based on the fact that the exhaust gases contain water as a gas, and if that water were condensed into liquid water, an additional quantity of heat would be produced, the latent heat. Due to the low temperature of condensation, that latent heat cannot be easily recovered, except if it is used for hot water production or for home heating or for home cooling (e.g. with absorption chillers). If instead of using combustion gases, the same heat engine would use heat from an external source, like also the thermophotovoltaic devices discussed here, the heat engine would function in the same way and with the same efficiency, when receiving the amount of heat corresponding to the lower heating value (assuming no heat losses during heat transfer).
gumby 3y ago
> It’s also a peak steady-state number, only achievable under optimal load conditions. For base load generation at least, these run continuously under “optimal conditions”. The big ones are designed to be switched on and run for decades. A turbine is a tuned system (its shape is designed for its operating conditions). For base load generation it’s important to fix those operating conditions thus getting the most output from the least input. (This is quite different for a peaker plant that needs to spin up and down relatively wuickly in response to demand, much less, say, the turbine on an aircraft or locomotive, which go up and down depending on load. They can never be anywhere close to theoretical efficiency). Just a long winded way of saying that “optimal load” is not as uncommon as one might think. Its no spherical cow.
- hwillis 3y ago
  The nationwide average capacity factor for CCGT was 57% in 2020: https://www.eia.gov/todayinenergy/detail.php?id=48036 https://www.eia.gov/todayinenergy/detail.php?id=48036
  - mikeyouse 3y ago
    But that's a naive number that also includes periods when the CCGTs were turned off due to "market conditions" where other energy sources were cheaper, so is mostly unrelated to the technical ability for them to reach much higher capacity factors.
adrian_b 3y ago
No, by using both a combustion-gas turbine and three steam turbines to recover the waste heat you get 63% efficiency for electrical energy production. By using the residual heat for hot water and for heating in winter and cooling in summer, the global efficiency typically becomes well above 80%. Even the best Diesel generators may reach around 55% efficiency, while working at much lower maximum temperatures. For a few seconds I thought that this thermophotovoltaic technique is great, until I have seen that the emitter must have a temperature above 2000 Celsius degrees. For such a great temperature it is very easy to make heat engines with much better efficiency and which might even be less expensive, because these multijunction III-V photovoltaic cells are many times more expensive than normal solar panels. If these were used for solar power, the concentrators would also be very expensive. Already the concentrators that produce temperatures around 1000 Celsius degrees, which is more than enough for easily reaching 40% efficiency with closed-cycle heat engines, are much more expensive than the concentrators that reach only lower temperatures, like 650 Celsius degrees, which would still be good enough for a steam turbine or for a closed-cycle supercritical carbon dioxide engine. The only real advantage is that these should need less maintenance than turbo-generators, which may be essential outside Earth or in remote locations, but less important than cost and efficiency otherwise.
- megaman821 3y ago
  Different use cases. This is for taking intermittent electricity production and storing it as heat. It will mainly be used for industrial heat; the TPV electricity production adds a little extra utility to the system.
  - adrian_b 3y ago
    Thermal storage at so high temperatures is even less practical than any other application. Thermal storage at low temperatures is cheap and easy with molten salts. Thermal storage at over 2000 Celsius degrees will be extremely difficult, due to the difficulty of preventing heat losses. The best would be for the hot body to be stored in argon, because in vacuum it would evaporate and heavier inert gases are expensive. The storage vessel would be very expensive in any case, being made from multiple layers with high temperature resistance, an external surface with high reflectance in red and infrared and other layers with low thermal conductivity, so it is hard to imagine that it could have a size large enough to store much energy. Another obstacle is that the available power is determined by the emitting surface, not by the volume of the hot body, which is another obstacle for scaling to large amounts of stored energy. Another obstacle to scaling is that when the hot body cools down the conversion efficiency drops extremely quickly (fourth power), which means that it could store energy only e.g. by being heated and cooled between 2400 and 2100 Celsius degrees. So only a very small fraction of the thermal capacitance of the hot body can be used, many times lower than when the heat stored in that body would be used to power a closed-cycle heat engine. So no, these devices may have some useful applications, but energy storage is certainly not one of them, because they are much worse than almost any alternative. Even storing compressed air in a pressure vessel is much more practical.

londons_explore 3y ago

I believe that figure is electric efficiency only. For combined heat and power plants, the efficiency routinely goes over 80%, and for heat-only plants, 95+% is common. However, there is still sleight of hand. The efficiency quoted is when the equipment is new and clean. Fouling and wear both take single digit percentages off. Also, they use the lower-heating-value for the gas energy supply. That, in my view, is dishonest - the correct energy measure for gas is the higher-heating-value, which is 10.7% more. The difference comes from how you account for the heat in the steam produced by burning gas. In my view, the energy from that steam should be accounted for when considering efficiency - in GE's view, it shouldn't.

AtlasBarfed 3y ago
And of course carbon removal from burning the natural gas is totally out of the equation.
- lazide 3y ago
  Since no one is requiring it at scale, and since there is no commonly accepted way of doing it - pretty hard to add it to an equation no?
- est31 3y ago
  > And of course carbon removal from burning the natural gas is totally out of the equation. Sorry for being nitpicky, but burning natural gas does not remove carbon, it adds new CO2 to the atmosphere (if you release the burn products which is usually done). You probably mean that if you released that natural gas directly into the atmosphere instead, it would have a larger greenhouse effect than the CO2 released by the burning process. This is true, and one should absolutely choose "burn it" when given the choice of releasing only vs burning and then releasing. But expressing it in the way of carbon removal is misleading. The number of carbon atoms in the atmosphere is the same in both scenarios, they are just bound in a less greenhouse-y form (also, natural gas decays into CO2 plus water eventually, but it's a very slow process). The number of CO2 equivalents goes up in both scenarios as well, just way less if you burn it before. Maybe some people refer to those through "carbon", idk.
  - AtlasBarfed 3y ago
    Ah, hacker news is out in force defending petroleum companies right to pollute the world. As always to economics, if you can't measure it well, it doesn't exist!
  - philwelch 3y ago
    > if you release the burn products which is usually done It’s also possible to capture the CO2 rather than releasing it into the atmosphere.
adrian_b 3y ago
I do not think that using the lower heating value is really dishonest, especially in the context of this discussion. The lower heating value corresponds to the heat actually produced during burning. The higher value is based on the fact that the exhaust gases contain water as a gas, and if that water were condensed into liquid water, an additional quantity of heat would be produced, the latent heat. Due to the low temperature of condensation, that latent heat cannot be easily recovered, except if it is used for hot water production or for home heating or for home cooling (e.g. with absorption chillers). If instead of using combustion gases, the same heat engine would use heat from an external source, like also the thermophotovoltaic devices discussed here, the heat engine would function in the same way and with the same efficiency, when receiving the amount of heat corresponding to the lower heating value (assuming no heat losses during heat transfer).

gumby 3y ago

> It’s also a peak steady-state number, only achievable under optimal load conditions. For base load generation at least, these run continuously under “optimal conditions”. The big ones are designed to be switched on and run for decades. A turbine is a tuned system (its shape is designed for its operating conditions). For base load generation it’s important to fix those operating conditions thus getting the most output from the least input. (This is quite different for a peaker plant that needs to spin up and down relatively wuickly in response to demand, much less, say, the turbine on an aircraft or locomotive, which go up and down depending on load. They can never be anywhere close to theoretical efficiency). Just a long winded way of saying that “optimal load” is not as uncommon as one might think. Its no spherical cow.

hwillis 3y ago
The nationwide average capacity factor for CCGT was 57% in 2020: https://www.eia.gov/todayinenergy/detail.php?id=48036 https://www.eia.gov/todayinenergy/detail.php?id=48036
- mikeyouse 3y ago
  But that's a naive number that also includes periods when the CCGTs were turned off due to "market conditions" where other energy sources were cheaper, so is mostly unrelated to the technical ability for them to reach much higher capacity factors.

adrian_b 3y ago

No, by using both a combustion-gas turbine and three steam turbines to recover the waste heat you get 63% efficiency for electrical energy production. By using the residual heat for hot water and for heating in winter and cooling in summer, the global efficiency typically becomes well above 80%. Even the best Diesel generators may reach around 55% efficiency, while working at much lower maximum temperatures. For a few seconds I thought that this thermophotovoltaic technique is great, until I have seen that the emitter must have a temperature above 2000 Celsius degrees. For such a great temperature it is very easy to make heat engines with much better efficiency and which might even be less expensive, because these multijunction III-V photovoltaic cells are many times more expensive than normal solar panels. If these were used for solar power, the concentrators would also be very expensive. Already the concentrators that produce temperatures around 1000 Celsius degrees, which is more than enough for easily reaching 40% efficiency with closed-cycle heat engines, are much more expensive than the concentrators that reach only lower temperatures, like 650 Celsius degrees, which would still be good enough for a steam turbine or for a closed-cycle supercritical carbon dioxide engine. The only real advantage is that these should need less maintenance than turbo-generators, which may be essential outside Earth or in remote locations, but less important than cost and efficiency otherwise.

megaman821 3y ago
Different use cases. This is for taking intermittent electricity production and storing it as heat. It will mainly be used for industrial heat; the TPV electricity production adds a little extra utility to the system.
- adrian_b 3y ago
  Thermal storage at so high temperatures is even less practical than any other application. Thermal storage at low temperatures is cheap and easy with molten salts. Thermal storage at over 2000 Celsius degrees will be extremely difficult, due to the difficulty of preventing heat losses. The best would be for the hot body to be stored in argon, because in vacuum it would evaporate and heavier inert gases are expensive. The storage vessel would be very expensive in any case, being made from multiple layers with high temperature resistance, an external surface with high reflectance in red and infrared and other layers with low thermal conductivity, so it is hard to imagine that it could have a size large enough to store much energy. Another obstacle is that the available power is determined by the emitting surface, not by the volume of the hot body, which is another obstacle for scaling to large amounts of stored energy. Another obstacle to scaling is that when the hot body cools down the conversion efficiency drops extremely quickly (fourth power), which means that it could store energy only e.g. by being heated and cooled between 2400 and 2100 Celsius degrees. So only a very small fraction of the thermal capacitance of the hot body can be used, many times lower than when the heat stored in that body would be used to power a closed-cycle heat engine. So no, these devices may have some useful applications, but energy storage is certainly not one of them, because they are much worse than almost any alternative. Even storing compressed air in a pressure vessel is much more practical.