Heat Pump Real-World Performance

[russ_watters]

For those who don't know, I'm an HVAC engineer. From time to time, the issue of heat pump efficiency comes up. When picking a system, obviously efficiency (cost) is important, so we need to decide between, usually, heat pumps (air source) and gas for a small split system. The government has a rating system for seasonal energy efficiency, and a heat pump can be rated according to coefficient of performance, but as COP changes with temperature, that doesn't really tell you very much. So my boss and I decided to do a real-world test of a heat pump and attempt to get it published. And so the next time someone asks us "heat pump or gas furnace?" we'll have a meaningful answer.

There are a few potential issues with heat pumps:
-As said, COP drops with outside temperature.
-Capacity drops with outside temperature - so heat pumps require another heat source as a backup.
-Depending on the weather, a heat pump may collect frost. A lot of frost. This has to be melted by switching the heat pump back to air conditioning mode. Yeah, that's right, heat pumps will start air conditioning your house in the middle of winter. That, of course, needs to be offset with supplimental heat.

So, what is the real-world performance?

So we selected the lowest capacity Carrier window heat pump and installed it in the window of my walk-out basement (ZQA101RB: http://www.docs.hvacpartners.com/idc/groups/public/documents/marketing/02-racbr07-25.pdf ). It has a listed COP of 2.8 and the marketing literature says "ZQ Heat Pumps use 1/2 to 1/3 less electricity than an electric heater (to 45F). That implies to me that a communications major who doesn't understand engineering didn't understand what a 2-3 COP means (actually, maybe it was written by an engineer with bad grammar, but whatever). But anyway, 2.8 is still pretty good. Sounds promising.

Now as it turns out, my basement is big, but doesn't have a lot of load. It's a walk-out with two walls exposed to the air, that I insulated pretty well. I tried to match the heat pump size to the load so we'd get a high usage factor (and keep costs down). We attached temperature, voltage, and amperage sensors, and started-it up.

As it turns out, the marketing info is actually trying to tell you something and hoping you aren't listening. That 45F isn't just a rating point, it's the point at which the unit cuts from heat pump mode and switches to electric resistance heating mode. It's just a coincidence, but the night I installed it was an extremely humid, somewhat warm winter night, of about 45F, with 100% humidity. And the unit didn't frost-over. We had a sensor attached to the reversing valve, and it never de-energized to switch to a/c mode. I can only conclude that the unit was designed with frost avoidance in mind. This makes sense, because despite still having a good COP of 2.5 at 45F, the low capacity and constraints of the electrical service mean you can't afford to switch to air conditioning mode to defrost it.

The next few days were that record warmpth of a few weeks ago, so I got to see pretty well the performance profile (I haven't finished reducing the data yet). But anyway, in Pennsylvania, running as a heat pump only down to 45F just doesn't cut it. And we wanted to know how a heat pump really performs when it is cold out. So we're terminating the study of this unit (anyone want to buy a heat pump?). I'm still going to do the full analysis, though, and we did get about all we need for it.

Fortunately, one of my coworkers has a heat pump at his house that we're going to study. Hopefully, we have enough winter left to get good data. I'll keep you guys aprised of how it's going.

 

[FredGarvin]

I love it. Very cool (groan). Sorry about that.

 

[Q_Goest]

Hi Russ. Interesting set up you have. So you've turned your house into a lab? Hope they pay you for it!

I recently had to determine the heat leak for a (cryogenic) vacuum jacketed pipe. I'm going to do something similar to what you're doing. I had an 18" long, 2" diam pipe made up with end caps welded on and a hole drilled to put water and a thermocouple in. I then filled it with cold water and instrumented the water temp, wrapped that in insulation with a pipe surface temp under the insulation, and another temp for ambient. Knowing the weight of pipe and water, and using values for specific heat, I obtained a heat leak through the insulation. Ends were heavily insulated so that the 1" thickness of insulation could be assumed to be the only conductive path. Using this "calibrated" insulation, I can now install it on the pipe with similar instrumentation and see what kind of heat leak there is.

I was wondering if you might do something similar to test heat pumps. What if you put the evaporator and condenser in large, insulated tubs of water and monitored temperature of each water bath? From the water temperature and assuming isothermal water conditions (put a stirring device in) you could watch the temperature of each rise and fall.

Knowing the specific heat of water and vessel, and making some allowance for heat transfer through the insulation, I'd think you could get a fairly accurate calculation of a heat pump's efficiency. You'd have heat in/out of the water equals heat in/out of condensor/evaporator and this would correspond to some power draw. The heat in/out would be calculated from the change in water temperature, and the rate of water temperature change would be dictated by the total thermal mass you're trying to heat or cool. Make sense?

One problem might be that the water has a much higher convective heat transfer coefficient than air, so you may also want to instrument the refrigerant lines and then see what dT you have in real life. In other words, your dT may be very small between refrigerant and water but may be 10 F or more with air, so rather than assuming the efficiency of the heat pump at some water temp is the same as the efficiency at the same air temp, the efficiency of the heat pump is actually the same when the refrigerant temps downstream of the condensor & evaporator are the same between the water and air.

ex: water bath the condensor is sitting in is at 78 F, refrigerant temp out of the bath is 80 F. This corresponds to some efficiency. But when operating in air, air temp has to be as low as 70 F to get refrigerant out of condensor at 80 F. So the heat pump would have the same efficiency when air temp was actually at 70 F, not at 78 F since the condensor temp have to correspond to efficiency. I haven't thought all this through. I suppose you may also need to instrument temperatures in different locations. Anyway, just a thought.