Quote:
Originally Posted by wewantutopia
My thought of preheating the ground loop would be for "emergency heat" only insead of the resistance heater.
I undersand they have a COP of 5 or higher. If I understand correctly, if you add an additional 1kw to raise the temp of the water you would get 4kw of heat out of the pump. I you use 1kw resistance heater you only get 1kw heat.
Again, this is only me pondering these questions and looking for feedback.
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The sentence describing the resistance heat is correct. With resistive heating, the effect is an increase of sensible heat from the energized device. A piece of wire, when energized, experiences electric "friction" and the temperature rises. You cannot get more energy radiated from the wire than the current that runs through it. Watts = volts x amps. The end.
The heat pump is a whole different animal. It doesn't generate most of its heat. The input energy moves the refrigerant around a loop with a compressor. If the compressor shell isn't insulated, a decent portion of the generated electric heat leaks out of the shell.
With a hermetic electric compressor, most of the energy consumed is turned into rotational mechanical kinetic energy, which spins a scroll, piston or drum to move refrigerant gas from low to high pressure. As the gas is pressurized, it naturally heats up, following the ideal gas law. Its condensation temperature (dewpoint) also rises due to the pressure increase. Yes, some of the electric resistive heat ends up in the gas, but as the gas pressure increases, the heat flow is pinched off by the natural increase in gas temperature.
At some point, the compressor pushes so much gas into the high side of the loop that it cannot exist as a gas below the temperature of the plumbing that contains it. At this temperature and pressure, the gas drastically reduces in size, condensing from a gas to a liquid and heat is released. Latent heat. Lots of it. The heat moves through the walls of the plumbing wherever it can and is released into the environment on the other side of the walls.
In reality, the "plumbing" is mostly a high-efficiency heat exchanger (condenser), designed to aid this transition of gas to a liquid. What happens is that the heat generated by compressing this gas is harvested as it condenses. The gas keeps on giving up heat ,at its raised transition temperature, until it is all a liquid.
Without a method to escape, this high pressure would build up, and the plumbing would eventually burst. Heat pumps are designed with a metering device at the liquid end of the condenser that keeps this from happening. As the high pressure liquid travels through the metering device, it experiences a massive pressure drop. At the exit end, the liquid is above its boiling point.
The liquid quickly begins absorbing massive amounts of latent energy, because it cannot exist as a liquid above its boiling temperature. The resulting temperature drop is harvested in another heat exchanger (evaporator), and heat flows into the refrigerant to complete the boiling process. The refrigerant leaves the heat exchanger as a low-pressure gas, saturated with latent heat absorbed in the boiling process.
The latent heat I describe is the "magic" in the system. Just like water, it takes much more heat to boil the refrigerant than it does to change its temperature. If you put a glass of water in a vacuum chamber, it will boil off as the pressure drops below its reduced boiling point. As the water boils, its temperature will drop to its ever reducing boiling point until it is all vapor or it freezes. At a certain low pressure, the ice will "sublimate", skipping the boiling process entirely and evaporating spontaneously like dry ice.
If this leaves you with confusion, the subject is covered in the sticky at the top of this forum.