R Thermodynamics: Heat Pump Efficiency Comparison

[the_dialogue]

I'm looking for information regarding household uses of an electric heat pump, direct electric resistance heater, and a gas-fired furnace. I'm trying to produce a comparison between the three, to determine the optimal means to maintain a constant room temperature in a house.

Any websites containing useful data would be appreciated.

Thanks.

 

[russ_watters]

I'm not sure what you mean - are you looking to compare energy costs? Obviously, any of those means can "maintain a constant room temperature in a house."

All the information necessary to evaluate energy costs can be found via Google - I've done such research myself for my work (I'm an HVAC engineer).

 

[cesiumfrog]

The heat pump is higher efficiency than the resistance heater, but the gas will be cheapest (since converting heat to electricity first is inefficient).

 

[the_dialogue]

Yes. I'm looking to compare energy costs, which would include information such as efficiency, energy produced, etc.

I've tried looking through google, but can't seem to sort through all the junk to find anything useful.

Thanks again.

 

[russ_watters]

Here's a scratch-pad of calculations and data from a report I did a while back. There are a lot of variables to consider and the deeper you go, the deeper the problem gets. But here's the basics, from the sheet labeled "calcs"

Assumptions:
$1.91/CCF gas
90% efficient furnace
$.14/kWh for first 600 kWh
$.07/kWh for additional kWh
2.0 heat pump COP

Gas cost:
48,413 BTU/$

Electric cost
48,653 BTU/$ for first 600 kWh
99,504 BTU/$ for additional kWh

Perhaps the biggest complication is the COP of the heat pump. The colder it is, the lower the COP is, so as the temperature drops, gas becomes more attractive. But it depends on your local rates, your house's energy use, your climate, etc.

Surprisingly enough, gas isn't as attractive as it was just a few years ago: the price of gas has gone up faster than the price of electricity.

I just checked my electric company's tariff (google the name of your electric company and the word "tariff") and right now there is no ratchet for residential electric in the winter: it is a flat $.1428 / kWh. The gas may even be a little higher by now, and that makes it more than double what it was about 5 years ago.

 

[Andrew Mason]

The heat pump is higher efficiency than the resistance heater, but the gas will be cheapest (since converting heat to electricity first is inefficient).

Whether gas furnace or heat pump uses less energy depends on the Coefficient of Performance of the heat pump (COP). That depends on the temperature of the house and of the reservoir you are pumping from. If the efficiency in converting thermal energy to electricity in the power plant is 30%, and the conversion of electricity to thermodynamic work in the heat pump is 80% then you need a COP of over 3 in order to be as energy efficient as the furnace (you have to take into account that 80% of the electricity used in heat pump produces 3 x its energy value in heat and 20% of the electricity produces 1 x its energy value as heat). Heat pumps can have COPs of more than 4.

AM

 

[russ_watters]

Heat pumps can have COPs of more than 4.

Just to clarify my logic in using a 2.0 COP, first I assumed an air source heat pump (which most residential heat pumps are). You'll be unlikely to find an air source heat pump with an advertised COP of over 3.0 (ie: http://uk.carrier.com/news/aqua_puron.htm ). Realistically, these often don't adequately take into account very cold weather performance and the occasional need for a defrost cycle, which is why I used 2.0. I could see an argument for up to about 2.5, though.

 

[Andrew Mason]

Just to clarify my logic in using a 2.0 COP, first I assumed an air source heat pump (which most residential heat pumps are). You'll be unlikely to find an air source heat pump with an advertised COP of over 3.0 (ie: http://uk.carrier.com/news/aqua_puron.htm ). Realistically, these often don't adequately take into account very cold weather performance and the occasional need for a defrost cycle, which is why I used 2.0. I could see an argument for up to about 2.5, though.

I see you are using an air-air heat pump. I was assuming that the cold reservoir for the heat pump is the earth. It would not make much sense (at least where I live in Saskatchewan) to use the outside air as the cold reservoir. In Canada, heat pumps must have a COP of 3.0 or higher.

If you drill down several hundred feet you can get even better COP. I have a friend who put such system in.

AM