I’m curious why, technically, a heat pump system is supposed to be more energy efficient than electric baseboard heat.
To me, running a current through a coil would seem to be very efficient. Where does the extra energy go, if not into convective or radiant heat? How could that be less efficient than running a blower, a compressor and a fan?
In general terms, for some conditions it takes less energy to MOVE heat than it takes to CREATE heat.
A heat pump works by using electricity to power a physical system that absorbs heat from one area (source) and then releases the heat into another area (sink).
The physical work has the added bonus of being able to grab heat from what we perceive to be a cool environment.
BubbaDog is on the right track … running a current through that coil, no matter the efficiency, is still bunches of current, bunches of energy … the heat pump draws all that heat energy out of the ground, no current and no electric energy … ergo we’re just moving the heat energy which uses less electricity … this is more “efficient” in terms of ºF per cubic foot per kilowatt-hour …
My understanding is that electric baseboard heaters aren’t all that efficient … not unless we run a fan to keep the air circulating … otherwise we’ll have our ceiling toasty warm and still be freezing water on our floor …
To expand on the point a little.
So long as the temperature difference is not too great it is easy to move heat from one place to another. It is always important to remember that temperature is only usefully measured in degrees absolute. So freezing is 273K and room temperature a shade under 300K. That means the difference in temperature from the freezing outdoors in winter to a nice warm indoors is only about 10%.
Moving heat around to make a 10% difference in temperature is pretty easy. Your air-conditioner does it all the time. Heating your house is exactly running the air-conditioner backwards. Which is why they are called heat pumps.
In general you have a heat engine. Car engines are heat engines, it is just that you get energy out of them when you let the heat move from the hot to the cold, rather than putting energy in to move energy from the cold to the hot. Steam engines, Stirling engines, turbines - even jet engines. All the same problem.
There is an absolute bound on the efficiency you can achieve with heat engines. If you are moving heat around - the lower the temperature difference the smaller the amount of energy you need to do it. But there is no free lunch. As it turns out, heating a house is quite efficient, as the temperature difference is small. Your efficiency can never be better heating than Temp[sub]cold[/sub]/(Temp[sub]hot[/sub]-Temp[sub]cold[/sub]). It is going to be worse than this due to other inefficiencies. But you can see that 273/(300-273) is roughly ten to one. So even if you only get a third of that (which is what a poor system gets) you are still three times ahead a simple resistive heater.
A heat pump is basically pumping calories uphill, from a colder environment to a warmer one. Your refrigerator (or air conditioner) basically does the same thing. It’s most efficient when the heat differential is small, because there are more calories to pump out in the cold side. Usually, these devices rely on the cold side boiling the coolant medium (due to low pressure, and right formula coolant) so the warmer the cold side, the easier for the coolant to absorb enough heat to go from liquid to gas. Then it is compressed on the hot side, where it condenses and sheds that heat. The work is compressing the gas so it condenses.
Do heat pumps still have resistive heating strips in the duct work to help them out?
It’s an option, for cold-winter areas.
My original heat pump had them, but the SEER-16 unit I replaced it with 7 years ago is so much more efficient that it doesn’t need them. Of course, it very rarely gets below 30°F out here. On the rare winter nights when it gets into the 20’s, the heat pump easily generates enough heat to keep the house warm, but it does blow chilly air when it goes into it’s defrost cycle. Still, I have no complaints. The new unit cut my bills by almost 1/3.
Cool.
Did you replace gas or electric heat?
If you dump X joules of energy into your electric baseboard (resistive) heaters, you get X joules of heat into your house.
If you dump X joules of energy into your heat pump to move Y joules of heat in from the great outdoors, you get X+Y joules of heat into your house. X+Y > X, therefore a heat pump gives you more bang for your operating dollar than resistive heating.
As Francis Vaughan says, when the great outdoors is too cold, Y << X, and the heat pump system doesn’t do better than resistive heat by a large enough margin to make up for its larger initial purchase cost.
In really cold climates, you need so much heat energy to keep your house warm that resistive systems and heat pump systems become prohibitively expensive to operate. The problem is that the utility company is burning fuel at the powerplant, and turning half of the fuel’s heat energy into electricity for you; you have to pay for all of the fuel they’re using, but you only heat your house with half of the energy from it. When big numbers are involved, it becomes a better deal to buy the fuel and burn it in a furnace at your house, where you can harvest >90% of the energy for home heating.
It was an ancient heat pump. One of those early over-sold technologies. It worked OK as an AC, but couldn’t keep the house warm when the outside temps went below 40°F.
thanks for the excellent answers, refreshing my long dormant memory of thermodynamics in college. Good job, all. I have two levels of resistance heaters in my heat pump/air conditioner unit. One is supposedly burned out, and the cost of replacing it I think far exceeds the need for it. In fact, even if the other unit craps out, I’ll just use a space heater to augment the heat pump. We rarely get below 20F, but once a year or so.
I’m working on a project right now in Cincinnati where they’re using variable refrigerant flow heat pumps since there’s no gas available and air conditioning is needed as well. With these particular units, even with an outdoor temperature of 0ºF they still have a CoP of 2, i.e. they’re still 2x as efficient as straight up electric resistance heaters. They also don’t have any auxiliary heating coils for that reason, but they do blow some cold air during defrost. This is more of a commercial system, and it’s not cheap, but the air it blows is quite hot, like 120º.
I’ve also worked on another project with a more standard but still high-efficiency residential heat pump, and instead of electric auxiliary heat they have normal high-efficiency gas furnaces. Because the heat pump coil is after the furnace, they never run concurrently except (I think) when the heat pump is defrosting. That gives the benefit of the high efficiency heat pump for mild weather, but also the quick recovery time of gas and the ability to switch over when the heat pump’s efficiency drops during colder weather.
Google “Effiency Maine”. There is some information there about people using an air-sourced heat pump which works down to -15°F, although at reduced efficiency!
I knew someone who had a cabin on a lake up north. They tossed a huge coil of piping for the heat pump into the lake, far enough offshore that it was fairly deep. Even though the lake freezes to 2 feet or more, the water under the ice is 4C (39F?) all the time. This made for a fairly efficient heat pump even in -20 weather.
I LOVE my heat pump. When it’s about freezing outside (34F) its electricity consumption is 660 Watts but it puts out 2200 Watts of heating into my house (I have a nice real-time heat meter to see that). My house takes about 30 kilowatt-hours worth of heating per day at that temperature but the electricity consumption of the heat pump is only 10 kilowatt-hours per day.