
This page reflects my initial analysis. On October 9th 2003, I had the opportunity to repeat an efficiency test that I'd performed a month earlier, so I was interested to see if I obtained the
same results. The specific nature of the test was to see if the hot water
heating efficiency of my geothermal heat pump was within manufacturer specified limits.
In my home, all the hot water for the radiant floor, and for domestic uses
(shower, washing etc.) is generated by a single Ground
Source Heat Pump. See the diagram below.
To perform the heating analysis, I did the following:
Here's a picture of what happened. You can also download a spreadsheet
of this data. The test started at 10:25 and ended at 12:02 (duration 97 minutes). The temperature values were recorded at 1 minute intervals, and are plotted against the left hand Y Axis. The Current (Amps) readings were transcribed off the chart recording at 5 minute intervals, intermediate values were interpolated, and converted to KW. These values are plotted (in yellow) against the right hand Y axis. Certain trends can be observed from this chart.
Analysis: Without being able to measure water flow rates, it's hard to determine the actual heat transfer rates just from the static temperatures. However, based on the rates of change of the Tank water temp, it is possible to calculate the amount and rate of heat being transferred. Follow my chain of logic: (Apologies to my metric brethren, all these calculations are done in USA units, which never seem to sound right)
BTUH can be converted to KW by dividing by 3413. So 23,880 BTUH equates to an electrical equivalent of 7.0 KW. What does this mean? Well, if we had a 100% efficient electrical water heater which generated the same amount of heat as our heat pump, it would consume 7.0 KW of electrical energy. This number can be compared to the actual energy consumption of the heat pump to determine efficiency. Since the efficiency of a heat pump depends on the relative temperatures of the source and load, I've graphed my calculated COP (Coefficient of Performance) against ELT (Entering Load Temperature). I feel OK doing this as my Entering Source fluid Temperature (EST) was a pretty constant 53 °F during the test.
From this chart you can see that the COP, or efficiency, of the system (shown in blue) starts out at 306% and eventually drops to about 174%. To someone in the traditional HVAC world this may seem fantastic. But I was somewhat disappointed by the much lower efficiency at the high temperature end. Since I use my Hot Water for domestic purposes, I keep the tank temperature up at the 120 °F level all the time, so a constant 174% efficiency wasn't floating my boat. So I pulled out the manufacturer specs to see if if I was justified. The relevant except is shown below. I've only included data for EST values of 50 °F since this is where my unit was operating, but I've included a wide range of source flow rates as I'm not sure what my actual flow rates are. The worst case numbers seem to be for slow flow rates, so I'll assume the worst and use the Source 5.0 GPM section. For each ELT/EST combination, there are three rows if numbers. These correspond to 3 different load flow rates, so I've shown them for completeness. Once again, the worst case numbers are on the third row, so I'll use these in my discussions. There are three very interesting numbers in these tables. KW, HE and HC. KW is the actual electrical energy being used by the heat pump. This energy is used to extract heat from the source fluid, but it also has a side effect of indirectly heating the load fluid. That's what the other two numbers represent. HE is the heat extracted from the load fluid, and HC is the total Heating capacity which adds the thermal equivalent of the KW number to the Heat Extracted. We can see this in the green section. KW is 1.84. We know that if we multiply KW by 3413 we get BTUH (1.84 * 3413 = 6.3 KBTUH). Then if we add this to the listed HE number (24.3) we get the HC value (6.3 + 24.3 = 30.6). Unfortunately my numbers aren't anywhere near what the manufacturer says I should be getting:
So not only am I not generating enough heat, my power consumption seems too
high as well.. What's could be up?
I guess I need to call Water Furnace to get some suggestions regarding the
low BTUH number, and my expected Power Factor.
This site is all about building a cool, energy efficient house, that makes maximum use of earth sheltered design, passive solar heating and cooling, geothermal exchange energy management, and right sizing of the house for it's designated use. The home's placement is on a southfacing hillside in Deep Creek Lake, Maryland. This site describes the design process, the technologies used and the expected results. We also have a comprehensive Links Page for anyone who is also interested in designing a similar project. 