The Efficiency Tango: A Deeper Look at Geothermal Heat Pump Efficiency
Tom Konrad CFA
|Geothermal heat pump diagram via Bigstock|
A couple weeks ago, I compared the efficiency of the two most advanced geothermal heat pumps (GHPs) recently launched by Waterfurnace Renewable Energy (TSX:WFI, OTC:WFIFF) and Climatemaster, as division of LSB Industries (NYSE:LXU). Like most things in life, it turns out that heat pump efficiency is a lot more complicated than just comparing a couple numbers.
Since I concluded that Waterfurnace’s 7 Series heat pumps were slightly more efficient than Climatemaster’s Trilogy 40 pumps, one of Climatemaster’s district managers pointed me to third party efficiency ratings conducted according to standards set by the Air Conditioning, Heating, and Refrigeration Institute (AHRI). He compared Waterfurnace’s 4 ton unit (the most efficient 7 Series) to Climatemaster’s 2.5 ton unit (the most efficient Trilogy 40), noting that the former had a 41 EER at ground loop conditions, while the latter had a 42.1 EER, according to AHRI.
He concluded that the Trilogy 40 had a slightly higher cooling efficiency than the 7 Series.
The Efficiency Tango
Had I got it wrong?
I checked with the pros. Scott Lankhorst, President of geothermal and solar thermal installer Synergy Systems in Kingston, NY said it was “an apples to oranges comparison” between 4 ton and 2.5 ton GHPs.
Lloyd Hamilton, a Certified Geoexchange Designer at Verdae, LLC in Rhinebeck, NY, called this normal marketing. He says that the only reliable way to compare units is to look at the operational performance data for the designed condition. The AHRI-compliant EER and COP numbers allow comparison of two units so long as they are at the same capacity, but it does not demonstrate actual performance, “like MPG for cars. … COP, SEER, and EER become worthless when comparing different types of equipment” such as air source and ground source heat pumps, because the testing criteria are different. He calls the act of picking an choosing GHP models and operating conditions to make your company’s GHP look more efficient the “Efficiency Tango.”
Both agree that the contractor can mess up the rated efficiency of a GHP, or even make it perform above specification, with the wrong (or right) system design and installation.
I don’t have the performance data a geoexchange designer would use, but there are a lot more publicly available efficiency numbers than I used in my last article. I put them together in a pair of bubble charts:
There are three 7 Series models and two Trilogy 40 models, each of which was tested at full load and part load, under two types of conditions. The “ground water” series are when the ground water is pumped up out of the ground for heat exchange; the liquid water helps heat transmission and results in a higher rating. The “ground loop” series is representative of the much more common installation, when an antifreeze fluid (usually propylene glycol) is pumped through the geothermal loop, which results in relatively lower efficiency (although still much higher than other types of heating and cooling equipment.) Even in ground loop conditions, different heat exchange fluids will result in different effective inefficiencies. The partial-load results are the sets of two or three smaller bubbles to the right (and a little below) sets of larger bubbles of the same color.
Looking at the charts holistically, I reach the following conclusions:
- The 7 Series is generally more efficient than the Trilogy 40 for heating.
- The Trilogy 40 is generally more efficient than the 7 Series for cooling.
- These units operate at dramatically (about 50%) higher efficiency under partial load. Two-stage heat pumps show only modest (5% to 15%) efficiency gains at partial load. This is likely to lead to higher overall efficiency of these GHPs in practice than the numbers alone might lead you to believe.
- The Trilogy 40 typically operates at lower fluid flow rates than the 7 Series, which should produce some energy savings from pumping.
Hence, I revise my earlier conclusion to say that, based solely
on efficiency, the Climatemaster Trilogy 40 will have a definite
edge over the Waterfurnace 7 Series in cooling climates,
while the 7 Series has an efficiency edge in
Efficiency Isn’t Everything
That said, for most installations, factors other than efficiency will probably dominate the decision. As noted above, Waterfurnace expects exclusivity from its dealers, and I expect Climatemaster and its other major competitors often do the same. This will make it nearly impossible for a residential customer to compare the two without having to weigh other factors such as their confidence in the installer who, as noted above, can make or break a geothermal installation.
Then there is the Trilogy 40′s Q-Mode. As Dan Ellis, president of Climatemaster told me in an interview, the potential savings from using geothermal to generate hot water year round from the Trilogy’s Q-Mode are likely to dwarf the savings from a point or two of EER or a fraction of a point of COP. In fact, Climatemaster designed the Trilogy 40 with the whole system energy savings in mind, partially at the expense of efficiency ratings. In a residential setting, Q-Mode (which is patent-pending to Climatemaster) is likely to make the financial returns decisively favor the Trilogy 40 in a head-to-head comparison.
In commercial settings, which typically have year-round cooling requirements, Q-Mode is unlikely to be important. Furthermore, the two largest 7 Series heat pumps have higher capacity than the larger of the two Climatemaster Trilogy 40 models. This should also give Waterfurnace an advantage in commercial settings, which typically have larger cooling loads than residential settings.
Ellis promised to send me some data to help quantify the overall energy savings from Q-Mode, which I plan to return to in a future article.
For residential customers in warm climates, Climatemaster’s Trilogy 40 seems like it will be the better GHP value when it becomes commercially available. In other cases, the comparison is not as clear cut, and a customer should probably focus on finding a contractor who can deliver the best system design and installation possible. That is the only way to capture the full benefit from either of these incredibly efficient geothermal heat pumps.
Disclosure: Long LXU, WFI
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