6 ms·

Well now. We can have a heat pump with a COP of 3.5 or so. Let's say we put in 1kW of electricity, and take out 3.5kW of heat energy. Now let's take 2.5kW of th

Well now. We can have a heat pump with a COP of 3.5 or so. Let's say we put in 1kW of electricity, and take out 3.5kW of heat energy. Now let's take 2.5kW of that heat energy, push it through this gadget at 40% efficiency to get 1kW electricity out. Push that back into the heat pump and, violà, 1kW of free heat left over! What am I missing?

syntaxing 5y ago
Entropy. You cannot look at a system purely from an energy conservation stand point. Take Carnot efficiency for instance. That’s the extreme case where it teeters on the fully recoverable entropy. For your example, the entropy price has to be paid somewhere in the system (usually in the heat to work ratio).
audunw 5y ago
> What am I missing? Just guessing, but a heat pump that can actually output at least 1900°C of heat? If we had those kinds of heat pumps I guess we'd use them in all kinds of industrial processes
danans 5y ago
> We can have a heat pump with a COP of 3.5 or so. Let's say we put in 1kW of electricity, and take out 3.5kW of heat energy. A heat pump COP of 3.5 or higher or happens only at relatively low delta-T between the source and destination temps of the heat pump [1] - like the delta-T typical for space or water heating. The COP degrades exponentially with increasing delta T as it has to work ever harder to pump heat against an ever higher temperature/pressure (assuming thermal storage with a fixed volume with few losses). The refrigeration cycle can't raise temperatures even hundreds (much less thousands) of degrees C - otherwise we'd already have heat pump stoves and ovens. This is coincidentally also one reason why all else equal, heat pump clothes dryers (which are great) take a bit longer than conventional technologies to dry clothes: they only reach about 50C (vs 70C-75C for standard gas or electric resistance dryers). > Push that back into the heat pump and, violà, 1kW of free heat left over! Because the COP degrades with the delta-T, it bottoms out at 1 (an electrical resistance heater), and in your scenario, you end up with 1kWH in, and 400Wh out, so a theoretically 40% efficient battery. Minus the electrical generation, and at much lower temperatures, your scenario with a heat pump + thermal storage does however describe how the new domestic thermal heat batteries can work with heat pumps [2]. 1. https://www.engineeringtoolbox.com/heat-pump-efficiency-ratings-d_1117.html https://www.engineeringtoolbox.com/heat-pump-efficiency-rati... 2. https://sunamp.com/ https://sunamp.com/
BlueTemplar 5y ago
You are missing that any heat engine (including photovoltaics !) is limited by the Carnot efficiency. And a heat pump is just a reverse heat engine. And you are trying to couple their inputs and outputs in a loop. I'm not sure that this would even qualify as a perpetual motion machine of the second kind ? P.S.: A heat pump is usually able to "cheat" an "efficiency" (aka Coefficient of Performance) over 100% because (for a refrigerator) it's defined as the inverse of the usual Carnot efficiency. When you want heating, you artificially get up to +100% to the CoP from the spent "mechanical" power being 100% converted into heat. EDIT : Ugh, took a while to get this right !

syntaxing 5y ago

Entropy. You cannot look at a system purely from an energy conservation stand point. Take Carnot efficiency for instance. That’s the extreme case where it teeters on the fully recoverable entropy. For your example, the entropy price has to be paid somewhere in the system (usually in the heat to work ratio).

audunw 5y ago

> What am I missing? Just guessing, but a heat pump that can actually output at least 1900°C of heat? If we had those kinds of heat pumps I guess we'd use them in all kinds of industrial processes

danans 5y ago

> We can have a heat pump with a COP of 3.5 or so. Let's say we put in 1kW of electricity, and take out 3.5kW of heat energy. A heat pump COP of 3.5 or higher or happens only at relatively low delta-T between the source and destination temps of the heat pump [1] - like the delta-T typical for space or water heating. The COP degrades exponentially with increasing delta T as it has to work ever harder to pump heat against an ever higher temperature/pressure (assuming thermal storage with a fixed volume with few losses). The refrigeration cycle can't raise temperatures even hundreds (much less thousands) of degrees C - otherwise we'd already have heat pump stoves and ovens. This is coincidentally also one reason why all else equal, heat pump clothes dryers (which are great) take a bit longer than conventional technologies to dry clothes: they only reach about 50C (vs 70C-75C for standard gas or electric resistance dryers). > Push that back into the heat pump and, violà, 1kW of free heat left over! Because the COP degrades with the delta-T, it bottoms out at 1 (an electrical resistance heater), and in your scenario, you end up with 1kWH in, and 400Wh out, so a theoretically 40% efficient battery. Minus the electrical generation, and at much lower temperatures, your scenario with a heat pump + thermal storage does however describe how the new domestic thermal heat batteries can work with heat pumps [2]. 1. https://www.engineeringtoolbox.com/heat-pump-efficiency-ratings-d_1117.html https://www.engineeringtoolbox.com/heat-pump-efficiency-rati... 2. https://sunamp.com/ https://sunamp.com/

BlueTemplar 5y ago

You are missing that any heat engine (including photovoltaics !) is limited by the Carnot efficiency. And a heat pump is just a reverse heat engine. And you are trying to couple their inputs and outputs in a loop. I'm not sure that this would even qualify as a perpetual motion machine of the second kind ? P.S.: A heat pump is usually able to "cheat" an "efficiency" (aka Coefficient of Performance) over 100% because (for a refrigerator) it's defined as the inverse of the usual Carnot efficiency. When you want heating, you artificially get up to +100% to the CoP from the spent "mechanical" power being 100% converted into heat. EDIT : Ugh, took a while to get this right !