Why aren't heat pumps placed indoors ? The big unit that is installed outside, I mean.
Could you do something where the heat pump could be placed in a stall (or in the garage) attached to the house. In the winter, close the outside stall door and open it up to the house, so the heat pump is inside the house. The pump would then pull already-warmed air from inside the house. All the heat generated would stay in the house.
In the summer, change it around so the pump is outside, since it creates heat that needs to be release outdoors.
You can't take air, remove heat energy from it, add the same energy back to the same volume of air (after loosing some due to inefficiencies), and expect to have warmer air. It's true, you are adding energy to the system by using electricity, but it's still not going to be a net gain until the backup heat kicks on - and, if you're going to do that, you might as well scrap the heat pump and just use the backup as your primary heat source.
Heat pumps basically MOVE heat from onew location to another. In the winter they gather what they can of heat in the cold and move it indoors. aboiut 30 degrees they become ineffective and turn on resistance heat, thats really expensive.
in the summer they take heat out of the indoors, and move it outside.
Modern systems are designed to be efficient. The sealed motor/compressor is insulated. The electrical energy is converted to work used to compress the working fluid (freon or whatever) which heats it and heat which also heats the gas. In winter the gas goes into your house where it is cooled by the air of your house causing it to and condense to a liquid, the liquid flows to the evaporator outside where it expands into the lower pressure in the evaporator forming a gas. This cools the outside air. Little would be gained by putting the system inside.
Modern heat pumps can deliver 2 to 2.5 times as much heat as electrical energy to run them using a seasonal efficiency rating. I believe less than
1/3 of the energy of combustion in an electrical power plant makes to your home. Thus a 90% efficient gas furnace will use less net energy. The colder the climate the better for the gas furnace. The most efficient heating is to use cogeneration where you have your own electrical generator running a heat pump and also waste heat to heat your building.
For those interested in what science tells us about this--
Thermodynamics tells us that efficiency of a heat engine is the (high temperature-low temperature) divided by the high temperature. You need to use a temperature scale where zero equals absolute zero. Assume your inside condenser is at 100F (310 Kelvin) and outside is 32F (273 Kelvin) then efficiency is (310-273)/273 or about 13%. This means that a heat engine can only convert 13% of the heat energy to work operating at these temperatures. But a heat pump is the opposite so an ideal heat pump could pump (100%/13%) or over 7 times as much heat as work (electrical energy consumed). Unfortunately we can't come anywhere near this number or we would all use heat pumps to heat our homes.
The above equation shows why we want to operate systems that convert heat to work at as high a temperature as possible to have the most efficiency. This why a diesel engine is more efficient than a gasoline engine.
heh... this is something that i had been thinking about. I've been looking for an efficient cogen gas appliance for the house, but when i got the grid tie requirements, it kinda went out the window.
So, I'm thinking to myself: What if i just converted the electrical output into heat, and treated the thing as emergency generator and boiler for domestic hot water and heating. It would get most of it's use in the winter, of course. I hadn't thought of using a heat pump since we have hydronic heat. (although I suppose they make heat pumps that use a liquid exchange instead of air...)
The space would eventually heat up without the backup, very slowly, but it would heat up. No change in tempeature that would imply a perfect, lossless system. If there were a drop in temperature then that would mean that energy was being destroyed.
That was my point, if the heat pump was running inside and the temperature dropped it would mean energy was decreasing inside the house without any being sent outside.
True, but only because the energy lost due to electrical and mechanical inefficiencies will be lost mostly in the form of heat, so in that way, I guess it would gradually heat the house. However, if you wanted to heat the house with electrical resistance, it would be a lot cheaper just to install electric heaters instead of using the motor windings as heating elements.
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