5 ms·

Why would the RTE of this salt solution be so much lower than the molten salts demonstrated in the DOE Solar Two project? According to the technical report at h

Why would the RTE of this salt solution be so much lower than the molten salts demonstrated in the DOE Solar Two project? According to the technical report at https://www.osti.gov/biblio/791898 https://www.osti.gov/biblio/791898 they achieved round-trip energy storage efficiencies of 97%.

thereisnospork 5y ago
That is probably quoting it on a heat-basis. Only 3% of heat might be lost from storage, but from the 97% remaining you can only ever convert ~60%[0] of that thermal energy into useful electricity. [0]Rough #. Carnot efficiency would define your max theoretical for a heat-engine process.
- londons_explore 5y ago
  I think you might be misunderstanding Carnot... The heat engine is reversible with 100% (theoretical) efficiency...
  - The_rationalist 5y ago
    How? (without a demon's maxwell)
  - thereisnospork 5y ago
    I was presuming resistive heating - a heat pump does make more sense. That said a practical heat pump with even a 1-way efficiency of 97% seems unlikely.
  - dredmorbius 5y ago
    Sorry, what?
    - londons_explore 5y ago
      If you have an ideal Carnot heat pump, it is fully reversible. For example, I can use 10 watts of electricity to remove 10 watts of heat from a cold place and add 20 watts of heat to a hot place. Later, I can apply the same in reverse. I can move 20 watts of heat from the same hot place, heating the cold place by 10 watts, while generating 10 watts of electricity. The use of the phrase "efficiency" while discussing ideal heat pumps is misleading - no energy is ever wasted or lost, the law merely limits the ratio of electrical Vs heat energy you get in/out.
      - dredmorbius 5y ago
        So ... yes, the heat pump can be run in either direction, and is "reversible" in the sense that the heat flow can be reversed through the pump. The process is not thermodynamically reversible in the sense that there's a net energy expenditure either way. No free lunches. What a heat pump can do is to move a greater amount of energy than it uses to move it. Measurements of this include COP (coefficient of performance), EER (energy efficiency ratio), and SEER or ESEER ((European) seasonal energy ratio). The COP is a direct measure of energy moved divided by energy input, and ranges from about 2--4, with typical ground-loop heat pumps achieving scores around 3--3.5. That is they move three times the heat energy that they run under. It's as if a furnace output three times more heat energy than fuel input. In your example, we'd first want to correct power (watts) to energy (watt-hours or joules). But 10 watt-hours of electricity would move about 30 watt-hours worth of heat, for a typical heat pump. If you're heating, you'd dump the additional 10 watt-hours into the heated space, if you were cooling, you'd want the heat pump's own heat to be directed to the external environment. Discussion of heat gained/wasted with heat pumps is ... complicated. Note that effectiveness in moving heat decreases as the differential being moved against increases. That is, if you're trying to cool a space in an environment that's already warm, you're pumping heat "uphill". Similarly, if you're trying to warm a space from a cold environment, there's not much external heat to extract. Ground-loop heat pumps benefit by the fact that ground temperatures tend to be more moderate than outdoor ambient air temperatures, so the temperature gradients are more favourable and predictable.
- dredmorbius 5y ago
  Power-cycle Carnot efficiencies are far more likely in the 20--40% range. Efficiencies above 50% are claimed by some dual-cycle plants (gas turbines with steam generation), and with cogeneration utilising waste heat, higher yet, but the electrical generation component remains in the 40--50% range.
  - pfdietz 5y ago
    Efficiency of gas-fired combined cycle plants has reached 63% (I believe LHV). https://www.powermag.com/another-world-record-for-combined-cycle-efficiency/ https://www.powermag.com/another-world-record-for-combined-c...
    - dredmorbius 5y ago
      Hrm, higher than I'd thought. I'll note it's a manufacturer claim (e.g., "value never to be exceeded" --- most optimal possibly conditions likely). What's the significance of LHV in this context?
      - pfdietz 5y ago
        Lower Heating Value, which ignores the latent heat of vaporization of the water produced by the combustion when computing the energy content of the fuel. Higher Heating Value, which includes that, would give a lower efficiency.
        dredmorbius 5y ago
        Would LHV calculations discount, say, condensing gas furnaces, which recapture most of the latent heat of fusion of water vapour in exhaust? Otherwise, what's a typical percentage difference between LHV and HHV efficiencies?
        pfdietz 5y ago
        Right, condensing furnaces recover the heat of evaporation (not fusion). https://en.wikipedia.org/wiki/Heat_of_combustion https://en.wikipedia.org/wiki/Heat_of_combustion The difference for natural gas is about 10%.
zardo 5y ago
That is what the report says. I think there must be some weasley definition of roundtrip efficiency happening as that exceeds the theoretical maximum given their hot and cold reservoir temps.

thereisnospork 5y ago

That is probably quoting it on a heat-basis. Only 3% of heat might be lost from storage, but from the 97% remaining you can only ever convert ~60%[0] of that thermal energy into useful electricity. [0]Rough #. Carnot efficiency would define your max theoretical for a heat-engine process.

londons_explore 5y ago
I think you might be misunderstanding Carnot... The heat engine is reversible with 100% (theoretical) efficiency...
- The_rationalist 5y ago
  How? (without a demon's maxwell)
- thereisnospork 5y ago
  I was presuming resistive heating - a heat pump does make more sense. That said a practical heat pump with even a 1-way efficiency of 97% seems unlikely.
- dredmorbius 5y ago
  Sorry, what?
  - londons_explore 5y ago
    If you have an ideal Carnot heat pump, it is fully reversible. For example, I can use 10 watts of electricity to remove 10 watts of heat from a cold place and add 20 watts of heat to a hot place. Later, I can apply the same in reverse. I can move 20 watts of heat from the same hot place, heating the cold place by 10 watts, while generating 10 watts of electricity. The use of the phrase "efficiency" while discussing ideal heat pumps is misleading - no energy is ever wasted or lost, the law merely limits the ratio of electrical Vs heat energy you get in/out.
    - dredmorbius 5y ago
      So ... yes, the heat pump can be run in either direction, and is "reversible" in the sense that the heat flow can be reversed through the pump. The process is not thermodynamically reversible in the sense that there's a net energy expenditure either way. No free lunches. What a heat pump can do is to move a greater amount of energy than it uses to move it. Measurements of this include COP (coefficient of performance), EER (energy efficiency ratio), and SEER or ESEER ((European) seasonal energy ratio). The COP is a direct measure of energy moved divided by energy input, and ranges from about 2--4, with typical ground-loop heat pumps achieving scores around 3--3.5. That is they move three times the heat energy that they run under. It's as if a furnace output three times more heat energy than fuel input. In your example, we'd first want to correct power (watts) to energy (watt-hours or joules). But 10 watt-hours of electricity would move about 30 watt-hours worth of heat, for a typical heat pump. If you're heating, you'd dump the additional 10 watt-hours into the heated space, if you were cooling, you'd want the heat pump's own heat to be directed to the external environment. Discussion of heat gained/wasted with heat pumps is ... complicated. Note that effectiveness in moving heat decreases as the differential being moved against increases. That is, if you're trying to cool a space in an environment that's already warm, you're pumping heat "uphill". Similarly, if you're trying to warm a space from a cold environment, there's not much external heat to extract. Ground-loop heat pumps benefit by the fact that ground temperatures tend to be more moderate than outdoor ambient air temperatures, so the temperature gradients are more favourable and predictable.
dredmorbius 5y ago
Power-cycle Carnot efficiencies are far more likely in the 20--40% range. Efficiencies above 50% are claimed by some dual-cycle plants (gas turbines with steam generation), and with cogeneration utilising waste heat, higher yet, but the electrical generation component remains in the 40--50% range.
- pfdietz 5y ago
  Efficiency of gas-fired combined cycle plants has reached 63% (I believe LHV). https://www.powermag.com/another-world-record-for-combined-cycle-efficiency/ https://www.powermag.com/another-world-record-for-combined-c...
  - dredmorbius 5y ago
    Hrm, higher than I'd thought. I'll note it's a manufacturer claim (e.g., "value never to be exceeded" --- most optimal possibly conditions likely). What's the significance of LHV in this context?
    - pfdietz 5y ago
      Lower Heating Value, which ignores the latent heat of vaporization of the water produced by the combustion when computing the energy content of the fuel. Higher Heating Value, which includes that, would give a lower efficiency.
      - dredmorbius 5y ago
        Would LHV calculations discount, say, condensing gas furnaces, which recapture most of the latent heat of fusion of water vapour in exhaust? Otherwise, what's a typical percentage difference between LHV and HHV efficiencies?
        pfdietz 5y ago
        Right, condensing furnaces recover the heat of evaporation (not fusion). https://en.wikipedia.org/wiki/Heat_of_combustion https://en.wikipedia.org/wiki/Heat_of_combustion The difference for natural gas is about 10%.

zardo 5y ago

That is what the report says. I think there must be some weasley definition of roundtrip efficiency happening as that exceeds the theoretical maximum given their hot and cold reservoir temps.