4 ms·
> But converting it back to electricity is typically less than 50% efficient. It's generally much much worse than 50%. Say that you used a resistor to heat the
by rnhmjoj 4y ago
> But converting it back to electricity is typically less than 50% efficient.
It's generally much much worse than 50%. Say that you used a resistor to heat
the water in a (perfectly insulated) boiler and want to know how much useful
work you can extract from it, ideally. The answer is given by the exergy
efficiency, which can be computed by attaching an imaginary heat engine to it,
integrating the infinitesimal works δW = δQ⋅η_Carnot while the temperature
drops from the initial T_boiler to T_ambient and dividing by the internal
energy of the boiler.
The result is η_exergy = 1 - T_ambient/T_lmtd where
T_lmtd = (T_boiler - T_ambient)/log(T_boiler/T_ambient)
is the logarithmic mean temperature difference. Even heating water to 90°C
gives an efficiency of only 9%.
- jbay808 4y ago> Say that you used a resistor to heat the water If you're looking to maximize round trip efficiency, this is probably not a good starting point. Better to use a heat pump, as mentioned elsewhere.
- rnhmjoj 4y agoYes, if you're counting the heat output as part of the useful energy, but parent was specifically talking about converting electricity into heat and back again.
- jbay808 4y agoSo am I. If you're going from electricity to heat for storage and back to electricity, you'll get better efficiency by using a heat pump to generate that stored heat. In general, sticking to processes that are as close to reversible as possible will be the most efficient.
- rnhmjoj 4y agoOk, I see your point now.