What is the correct method to determine COP?

[pilko]

Hi everyone,
When I plot my AS heatpump COP (output power/input power) over time at a specific outdoor temp, as expected, it ranges from zero at cycle start to max after about 10 minutes (with no frost build-up). It also decreases as expected due to frost build up.
So -- when is the right time to determine the COP?
----Is it the peak for a specific outdoor temp?
----Is it the average over a cycle?
----Or,is it something else?

Regards

pilko.

[chemi-cool]

COP can only be calculated.

[pilko]

"COP can only be calculated."
Yes, I am calculating it continuously with a microprocessor, and plotting it on my computer.

pilko

[monkey spanners]

I would think its the average while its running.

[Bigfreeze]

COP is dependant in specific parameters. Your average COP is your SPF or SEER as you call it on that side of the pond.
So you'll need to pick specific parameters ie evap temp and condensing temp in order to calculate a COP for those particular temps.

[Mick13]

COP's are calculated at certain conditions, if u look up the COP of a certain unit u can also see what conditions it is calculated at, indoor temp, outdoor temp, wet bulb/dry bulb.

not sure as to when its calculated though, (over the cycle).

[MikeHolm]

For Pilko it is to compare with the ARI values in the States which are based on some ASHRAE numbers. I have them somewhere..........buried somewhere though.

[Bigfreeze]

The only real way to determine it is with a ultra sonic heat meter and and electrical meter, like this
http://kamstrup.com/14934/multical-402-energy-meter

[Greengrocer]

Hi Pilko.

As you probably know the heating COP (or cooling EER) of any system varies continuously with changes in outdoor & indoor temperatures which affects the system pressures which determines how hard the system has to work. i.e. as ambient temps get lower the work done by the system (power input) increases, so the COP gets lower.
However, the indoor coil temps are also changing as the system starts to heat the room up. When the system first starts up the TD across the indoor coil(s) is higher & the system is more efficient. However, as the room warms up the TD across the indoor coil decreases so the system efficiency drops back.

Most manufacturers publish combination ratings in tabular format for the heating / cooling capacities & power input of their systems at various conditions. These values can be converted into graphs in a simple XL spreadsheet for easier reading. Note that in heating mode the graph should kick down & have a noticable drop in capacity when ambient temps drop below 10c / 50f which is when the outdoor coil starts to frost over & defrost cycles start to occur. This should be noted in the manufacturers data as "integrated" as opposed to "instantaneous" capacity ratings. The integrated ratings allow for the defrost cycles where as the instantaneous ratings do not (another crafty way of making their numbers look good). Also, if your system has any electric heaters intalled on the indoor coils to temper the indoor air when the system is in defrost mode (cooling), then this additional power input should be taken into account when calculating the overall system COP.
Also note that many manufacturers ignore the indoor fan power input when calculating their published COP's.

Direct comparrison against the manufactures figures won't be possible because their figures will be taken at steady state / ideal conditions in a laboratory which you cannot simulate.

So in answer to your question, you can only really average the COP of a system over a period of time &, only when your outside / inside conditions match the manufactures tabular data can you make any direct comparrisons which will be an approximations at best.

Its a bit like trying to replicate the fuel consumption figures of your car against the manufacturers published data. Not easy because the manufactures figures are generated under lab conditions (so they can get the best results) which you cannot replicate in real life.

[pilko]

Thanks everyone --- great info.

pilko