Determining the COP (Coefficient of Performance) of a heat pump system 1

[StevenMorrow]

Hello engineers!

I am investigating the factors affecting the COP value of a proposed heat pump system used to prepare air for use in an automotive spray booth. This is a very low lift application (typically taking the air from around 15 degrees up to the required 23.9) and thus, using the Carnot COP formula with a modification factor of 0.5 (as per

http://www.ochsner.com/funktion.htm),

I am getting unfeasibly large COP values (as COP tends to infinity as lift tends to 0).

Are there other factors to consider in low lift applications is the Carnot COP formula not valid in this range or does the 'fudge factor' have to change? Is there a definitive formula for calculating the COP as I have seen another method that is basically a straight line that hits a maximum value of about 8 for zero lift?

The information that I have consists of sheets of temperature and relative humidity data for specific hours and a required temperature and RH value (for the spray booth) and I have to find a way to get the corresponding COP for these conditions. I am performing this calculation in order to determine the cost saving caused by using a heat pump system rather than using LPG. I have an empirical formula for determining the enthalpy in kJ/kg for a given temp and RH.

Any help with this matter would be greatly appreciated.

Steven Morrow

 

[tombmech]

You may be finding a COP that tends toward infinity in certain calculations, but you're on the end of a losing proposition with all other factors. Yes, the "fudge factor" is a fudge factor, nothing more. Real equipment performance data is better.

However, the fallacy with the heat pump is that, in my opinion, it should never be used for primary heat purposes. They are best used as air conditioners, and using their super-efficiency heat capability as a nice, non-critical by-product. You will find that even in these circumstances, electric strip heaters are often used to supplement.

The numbers look unbelievable on paper. However, what really happens is that the typical heat pump compressor runs without end, and pull-down is almost non-existent. This results in huge maintenance costs over time - if your primary application is heat, and a process one, at that.

Also, the lack of temperature gradient leverage for pull down also means your productivity will suffer. You may wait many minutes or hours each day to bring the booth into acceptable temperatures.

Finally, humidity control will be next to non-existent. I'm making a lot of assumptions here, but one of them is a reciprocating-compressorized DX heat pump. Those will only dehumidify when the compressor is on for cooling, and there is generally no modulation at all. When pulling in 100% outside air (SOP for paint booths), your heat pump will only run at one speed, or OFF. This will result in dehumidification only if the temperature presents a load that is greater than, the heat pump's Sensible Heat capacity - typically 65% of the total capacity. Multiple stages can help, but those increase complication and maintenance almost exponentially.

Humidification in the case of heating does not exist without a separate system.

It depends on your specific application, the magnitude of CFM, etc., but I would opt for the LPG with separate A/C and separate humidification, depending on your RH requirements.

 

[StevenMorrow]

tombmech - thanks for the reply.

Sorry for taking a while to respond, but I did not have internet access for the weekend.

I think I should clarify a few points about the installation:

> I am based in Port Elizabeth, South Africa which is a subtropical coastal region. This means that the temperatures are usually fairly moderate, usually in a range of 5 to 30°C (41 to 86°F).

> The cost of LPG is roughly double that of electricity, as we do not have access to natural gas reserves.

> There is an existing system in place that is to be improved upon. It consists of 4 ARPs running solely on LPG, with variable speed drives on the compressors. This means that the system would not work in a stop-start manner. There is also a fast reacting humidifier already in place that allows the required humidity control.

> The heat output is 6 to 8MW (which is now to be supplied mainly from the heat pumps and supplemented by the LPG) and the combined volume of the four ARPs is 255m^3.

> The current method used to condition the air costs around $2 million a year, so the savings accrued by an improved process would mean a relatively short payback period and that the maintenance costs could be quite easily absorbed.

In light of these factors, would you still consider LPG as the best solution?

Steven Morrow

 

[tombmech]

No - it sounds as if you have the ideal solution:

* variable speed drives on the compressors,
* a fast-reacting humidifier already in place, and
* heat ouput ... supplemented by the LPG ...

those are the key items. If you have these, you should be able to adequately control the paint process environment.

The rest is up to your calculations. Real measurements would be best, but that usually means you have to go ahead and implement the change. If the compressors have been used for cooling in the past, perhaps you have some historical data about their existing COP. If not, keep in mind that your original reference for a fudge factor, ~ 0.5, is just that - a fudge.

In those scenarios, I usually adjust the fudge factor so that I don't present ridiculous numbers to management. It's always best to hedge your claims, anyway. If things turn out better than you claimed, that's still to your advantage.