Heat pumps do not generate heat (well, only as a side effect of moving parts). Heat pumps compress gas. By using a compressor, they cycle gas between a low pres

Heat pumps do not generate heat (well, only as a side effect of moving parts). Heat pumps compress gas. By using a compressor, they cycle gas between a low pressure chamber and a high pressure chamber. The low pressure chamber is outside, absorbing heat from the surrounding air. It is pumped into the high pressure chamber using the compressor, turning a large volume of cool gas into a small volume of hot gas (conserving energy). This hot gas warms your water or air. The now cooler, but still high pressure gas is leaked into the low pressure chamber, where it expands turning a small volume of warm gas into a large volume of cold gas (conserving energy), and the cycle continues. This is how you spend 100 joules of electrical energy to move 300 joules of heat energy from outside to inside and 300% heating efficiency. The thermo-acoustic ones are using sound waves instead of a compressor, but the same principle.