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> the Energy Department calculates that a 20-meter steam turbine would shrink down to one meter if replaced with an sCO2 turbine. Where do these improvements

> the Energy Department calculates that a 20-meter steam turbine would shrink down to one meter if replaced with an sCO2 turbine. Where do these improvements come from [1]. > CO2 has a relatively low critical pressure of 7.4 megapascal (MPa) and a critical temperature of 31C ... A consequence of this is that it can be compressed directly to supercritical pressures and readily heated to a supercritical state before expansion. In a heat engine, this can facilitate obtaining a good thermal match with the heat source. The critical temperature is also sufficiently high for ready heat rejection from the cycle at terrestrial ambient temperatures. Therefore, the system has a great potential for high efficiency since a large temperature difference is available ... CO2 near its critical point becomes more incompressible and hence, the compression work can be substantially decreased leading to high cycle efficiency. > The high density and volumetric heat capacity of sCO2 with respect to other working fluids make it more energy dense, meaning that the size of most system components such as turbine and pump can be considerably reduced, which leads to a smaller plant footprint and possibly lower capital costs. [1] https://www.powermag.com/what-are-supercritical-co2-power-cycles/ https://www.powermag.com/what-are-supercritical-co2-power-cy...

adrian_b 3y ago
Due to the high pressure in the closed circuit, the density of supercritical CO2 is similar to that of a liquid, and this allows very small turbines and pumps for a given power. Nevertheless, the circuit must also include heat exchangers and those are not reduced in size as much as the turbines and the pumps. Therefore the very small sCO2 turbine will be accompanied by much larger heat exchangers, so the reduction in size of a complete system is not so impressive as the shrinking of the turbine. Even so, the sCO2 system has another advantage over the traditional steam turbines, when used for the recovery of the waste heat from a gas turbine, or when using any other high-temperature heat source, like a nuclear reactor. Because the steam generation happens at a constant temperature, which must be relatively low for reasonable pressures, the heat transfer from a high-temperature source is inefficient. Due to this, for the recovery of the waste heat of a gas turbine are used typically 3 steam turbines whose steam generators work at different temperatures and pressures, not a single turbine. Supecritical CO2 can be heated with a heat exchanger having a gradient of temperature along it, which allows a single sCO2 turbine to replace 3 steam turbines in a combined-cycle power plant.
- perihelions 3y ago
  - " or when using any other high-temperature heat source, like a nuclear reactor. Because the steam generation happens at a constant temperature, which must be relatively low for reasonable pressures, the heat transfer from a high-temperature source is inefficient" Nuclear (fission) reactors really aren't high-temperature heat sources. The working fluid inside reactors is itself liquid water, which doubles as a neutron moderator. That severely limits their temperature range (<374° C), so, there's no downside to limiting yourself to steam on the energy conversion side as well. The linked articles make brief mention of a nuclear/sCO2 combination, but, to be clear, they're talking about radically different types of nuclear reactors. Not types that are currently commercialized/mature technology. Types where you replace the working fluid on the nuclear side with higher-temperature compatible substances—molten metals, molten fluoride salts, or inert gases like CO2 or helium. edit: Also applies to nuclear fusion, I guess. IIRC, the proposed working fluids for those are molten lead/lithium, or molten lithium fluoride—both match with the sCO2 temperature range. (Lithium is the common factor, because the overriding concern of the working fluid is to transmute lithium into tritium, using the fusion reactor's neutron flux, to hopefully allow a sustainable fuel cycle).
  - amluto 3y ago
    > The working fluid inside reactors is itself liquid water, which doubles as a neutron moderator. There are fission reactor designs with different working fluids that operate at much higher temperatures.
  - adrian_b 3y ago
    Most existing nuclear reactors are cooled with water as you say, and they use steam turbines. Nevertheless, for the future reactors it is desirable to use higher temperatures in order to increase their energy efficiency. This requires the use of other cooling fluids, as you also mention, and for such high-temperature nuclear reactors sCO2 turbines are preferable to steam turbines.
- abdullahkhalids 3y ago
  Thanks for the perspective. The reduced size is also dwarfed by however the CO2 is being heated. If by concentrated solar field of many acres, than the size of the turbine doesn't matter on its own. What matters is if the reduced size leads to reduced complexity/cost/maintenance burden.
moffkalast 3y ago
So does that also mean that SMRs could be made even more compact? Or that one could increase reactor size and run a 20-meter sCO2 turbine generating as much as a 400-meter turbine?
KennyBlanken 3y ago
I don't see how reducing turbine size is a good thing. Power plants aren't just a source of power; they're a source of mechanical inertia for the grid. I suppose you can compensate by incorporating energy storage, which also has the side benefit of allowing the plant to be self-sufficient for cold-start.

adrian_b 3y ago

Due to the high pressure in the closed circuit, the density of supercritical CO2 is similar to that of a liquid, and this allows very small turbines and pumps for a given power. Nevertheless, the circuit must also include heat exchangers and those are not reduced in size as much as the turbines and the pumps. Therefore the very small sCO2 turbine will be accompanied by much larger heat exchangers, so the reduction in size of a complete system is not so impressive as the shrinking of the turbine. Even so, the sCO2 system has another advantage over the traditional steam turbines, when used for the recovery of the waste heat from a gas turbine, or when using any other high-temperature heat source, like a nuclear reactor. Because the steam generation happens at a constant temperature, which must be relatively low for reasonable pressures, the heat transfer from a high-temperature source is inefficient. Due to this, for the recovery of the waste heat of a gas turbine are used typically 3 steam turbines whose steam generators work at different temperatures and pressures, not a single turbine. Supecritical CO2 can be heated with a heat exchanger having a gradient of temperature along it, which allows a single sCO2 turbine to replace 3 steam turbines in a combined-cycle power plant.

perihelions 3y ago
- " or when using any other high-temperature heat source, like a nuclear reactor. Because the steam generation happens at a constant temperature, which must be relatively low for reasonable pressures, the heat transfer from a high-temperature source is inefficient" Nuclear (fission) reactors really aren't high-temperature heat sources. The working fluid inside reactors is itself liquid water, which doubles as a neutron moderator. That severely limits their temperature range (<374° C), so, there's no downside to limiting yourself to steam on the energy conversion side as well. The linked articles make brief mention of a nuclear/sCO2 combination, but, to be clear, they're talking about radically different types of nuclear reactors. Not types that are currently commercialized/mature technology. Types where you replace the working fluid on the nuclear side with higher-temperature compatible substances—molten metals, molten fluoride salts, or inert gases like CO2 or helium. edit: Also applies to nuclear fusion, I guess. IIRC, the proposed working fluids for those are molten lead/lithium, or molten lithium fluoride—both match with the sCO2 temperature range. (Lithium is the common factor, because the overriding concern of the working fluid is to transmute lithium into tritium, using the fusion reactor's neutron flux, to hopefully allow a sustainable fuel cycle).
- amluto 3y ago
  > The working fluid inside reactors is itself liquid water, which doubles as a neutron moderator. There are fission reactor designs with different working fluids that operate at much higher temperatures.
- adrian_b 3y ago
  Most existing nuclear reactors are cooled with water as you say, and they use steam turbines. Nevertheless, for the future reactors it is desirable to use higher temperatures in order to increase their energy efficiency. This requires the use of other cooling fluids, as you also mention, and for such high-temperature nuclear reactors sCO2 turbines are preferable to steam turbines.
abdullahkhalids 3y ago
Thanks for the perspective. The reduced size is also dwarfed by however the CO2 is being heated. If by concentrated solar field of many acres, than the size of the turbine doesn't matter on its own. What matters is if the reduced size leads to reduced complexity/cost/maintenance burden.

moffkalast 3y ago

So does that also mean that SMRs could be made even more compact? Or that one could increase reactor size and run a 20-meter sCO2 turbine generating as much as a 400-meter turbine?

KennyBlanken 3y ago

I don't see how reducing turbine size is a good thing. Power plants aren't just a source of power; they're a source of mechanical inertia for the grid. I suppose you can compensate by incorporating energy storage, which also has the side benefit of allowing the plant to be self-sufficient for cold-start.