How is it that a machine (like a heat pump) can consume 1kw of power and produce an amount greater than that of heat? What am I misunderstanding?
In: 320
ELI5: You can grow the food for your lunch, or you can reach over to your friend’s desk and steal their lunch. Both lunches have a similar energy input, but stealing your friend’s lunch costs a lot less of *your* energy.
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Efficiency is useful energy output (usable heat energy) divided by energy input. When using a heat pump, the energy inputs include:
(a) the energy from the natural environment that previously created the heat, and
(b) the energy we apply (electricity) to move the heat where we want it.
A heat pump has a high efficiency (say 400%) because we’re only counting the energy inputs *we* applied. The energy from the natural environment is not a cost to us.
By excluding some of the inputs that don’t matter to us from the equation, it’s no longer really efficiency. That’s why we have other terms like coefficient of performance (COP). COP of 4 can be thought of as an efficiency of 400%.
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Some further commentary:
Heat pumps don’t create heat, unlike many of heating systems we use. They take heat from somewhere else and move it to where we want it. Heat wants to naturally flow from places of higher heat to lower heat. Using the refrigeration cycle, heat pumps allow us to move heat more effectively and in the direction we want (such as from places of lower heat to higher heat). Pressure and temperature of a substance are interconnected, by controlling these pressures we can adjust temperatures and create a system to pump heat in ways it wouldn’t flow naturally. Moving heat takes much less energy than creating heat. We get to take heat from somewhere that has already had energy sunk into it to make it.
For comparison:
1. **Electric Resistance Heating:** Examples include electric baseboards. Creates heat via the electrical resistance that occurs when running electricity through a material. This process is essentially 100% efficient as there are no losses.
2. **Combustion Furnaces:** Such as a natural gas furnace in many homes. Creates heat by combusting a fuel. These range in efficiency from 80% to 95% (questionable if you’ll reliably meet that higher end of that efficiency range though).
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