Viewing a single comment thread. View all comments

Bunslow t1_j19h5gu wrote

Reply to comment by seven_tech in Why do we use phase change refrigerants? by samskiter

> We can input up to 3 times less electric energy for the same transfer of 'heat' energy in a very efficient heat pump.

[edited] how close do residential electric [heat pump] heating systems reach this number? in other words, how much of a waste is it to heat my place via use of [resistive] stove/oven rather than the central electrical [heat pump] heating?

1

NeverPlayF6 t1_j19nlsu wrote

If your residential electric heating is resistive heating, then there isn't much difference between a resistive stove (the ones with the heating elements) and a heater, if there is a difference at all. In real terms, though- heating your whole house from a single point is probably less efficient due to a lack of air flow and distribution of that heat.

4

Bunslow t1_j19yxij wrote

well by default i assumed it wasn't resistive, but it could be for all i know. but i assumed that since heat pumps are more efficient that it would be a heat pump

1

SufferingIdiots t1_j1a10db wrote

Do you have an outdoor compressor/heat exchanger? If not it may just be an electric furnace that heats with resistive elements.

2

NeverPlayF6 t1_j1dh202 wrote

A lot of residential buildings have resistive heating. Baseboard radiators, cable ceiling, regular forced air heating are all potentially resistive heating.

Regarding how close residential heat pumps can get to 3x the efficiency of resistive heating- that's about where they are right now. Depending on the temperature at the exchanger, a bit better than 3x is not uncommon. But they become less efficient outside of optimal working temps.

1

GenericUsername2056 t1_j19r3cj wrote

It's 100% efficient, that is all electricity is converted into heat, eventually. So a 1 kW oven running at maximum capacity will consume some 1 kW of electricity to produce the same amount of heat. So it generates heat from electricity. A heat pump on the other hand merely 'pumps heat' using electricity. This means at certain operating conditions (this is dependent on e.g. the outside and inside temperatures) it will use 1 kW of electricity to move 3 kW of heat from the cold outside into your warm home. This gives it a Coefficient Of Performance (COP) of 3 kW/1 kW = 3 at those operating conditions.

3

Bunslow t1_j19z1mz wrote

right, but do residential heat pumps actually reach 300%, or do they only actually reach 250% or 200% or whatever and 300% is only possible with industrial heat pumps?

0

SufferingIdiots t1_j1a1nrq wrote

This be will dependent on the temperature differential, the refrigerant and your specific equipment.

>At 8°C, the coefficient of performance (COP) of air-source heat pumps typically ranges from between 2.0 and 5.4. This means that, for units with a COP of 5, 5 kilowatt hours (kWh) of heat are transferred for every kWh of electricity supplied to the heat pump. As the outdoor air temperature drops, COPs are lower, as the heat pump must work across a greater temperature difference between the indoor and outdoor space. At –8°C, COPs can range from 1.1 to 3.7

4

Bunslow t1_j1ajws0 wrote

hmm, so if it's -10C or -20C outside, and inside i want it at the usual 21 or 22C, then my cop might drop as low as 2? for residential purposes

1

bluesam3 t1_j1b6s2k wrote

It will vary between models and setups, but yes, that seems like a reasonable number to expect, broadly speaking.

2

bluesam3 t1_j1b6pb7 wrote

It's very climate-dependent - the colder the outside is, the less efficient air-source heat pumps tend to be (partly due to inherent reasons, and partly due to having to do work to defrost the outside unit) - if you're somewhere with relatively mild winters, COPs above 3.0 are very achievable with domestic units. If you live somewhere with extremely cold winters, it's much less achievable.

2

quintus_horatius t1_j19w0bu wrote

> in other words, how much of a waste is it to heat my place via use of stove/oven rather than the central electrical heating?

Since the rest of the conversation is about heat pumps, do you mean a central heat pump when you say "central electrical heating"?

Resistive electrical heating, as others have stated, is 100% efficient: every joule of electricity is used to produce heat.

A heat pump, however, can move a lot more joules of energy than it consumes. The term is "coefficient of performance" rather than efficiency, but you can think of it the same way. Most heat pumps have a CoP of 3 (or more), which means they're effectively 300% efficient - they move three times more heat than the electricity they consume, or three times more efficient than resistive heating.

OP was wrong, btw. Heat pumps are available with CoPs of 4.0.

3

Bunslow t1_j19zc1y wrote

well i dont know what mine is, but i'd always assumed it was a heat pump exactly since it is indeed 2-4x more efficient than simply dumping a whole bunch of power into heat thru a resistor. but it could be resistive heating for all i know.

and if there are indeed heat pumps with 400% heating efficiency (or CoP or whatever we want to call it), then probably 300% is a very achievable number for even "merely" residential purposes, one would assume?

1

quintus_horatius t1_j1a6369 wrote

You only have a few options for heat, and only one that can exceed 100% efficiency - a heat pump.

A typical heat pump exceeds 2.5, a good heat pump exceeds 3.0, and a fantastic heat pump approaches 4.0.

The latter generally show up in highly specialized applications like geothermal, where you can tailor your working fluid to a narrow, predictable temperature range.

2

Bunslow t1_j1ajqfp wrote

cool, so 2.5-3 is totally achievable for residential/end consumer purposes. is that what residential air conditioners achieve as well?

1

seven_tech t1_j1assin wrote

Mine almost does.

It uses a maximum of 1.9kW of electricity to move up to 5.2kW of heat in best circumstances. That's a CoP of 2.74. And it's definitely not as efficient as they get.

3

samskiter OP t1_j1co3h0 wrote

There are residential heat pumps with SCOP (seasonal cop) of 5.

1

bluesam3 t1_j1b6eed wrote

Yes, 3.0 is a very achievable number for a residential heat pump in a mild-ish climate.

2

Chagrinnish t1_j1avx5z wrote

Here's a Goodman brand heat pump spec sheet (see p21) with COP numbers vs. ambient air temperature. They're giving a COP of 1.2 to 1.5 (120% to 150%) at -10F. It's going to be pretty cold before you'll want to use any resistive heat. The more important factor is that it can't put out as much heat (MBh in the chart) so it might not keep up.

Edit: Looking at price of Propane, Natural Gas, and electricity (in Iowa prices) you need a COP of 1.9 or 2.2, respectively, for the heat pump to be more cost effective. So that translates to the heat pump being more cost effiective around 5F and above vs propane or 15F vs natural gas. Unfortunately it's -6F right now :)

2

softdetail t1_j1bcp21 wrote

True but it's only that cold for a small portion of the season, so on average, you are still saving money

1

Bunslow t1_j1bfexu wrote

well i dont exactly have any way to burn fuel around here, so all i got are resistors or heat pumps. lol. im in IL, so not that far away. 0F and falling to the same -9F low. apparently we have the same low temperature from st louis to winnipeg, it's a massive blast of fairly homogenous air

1