Hello everyone,
I have modeled a ground source heat pump within the building I have designed and have been getting results that seem almost too good to be true. I decided to compare my results with a typical high efficiency air side heat pump and discovered that the geothermal system is operating about 65-75% better than the air side heat pump in terms of cost. The results from the air side heat pump appear to be correct and are reasonable for my design.
Has anybody else ever gotten results for a geothermal system design that were much better than expected? If so, was there something incorrect within your model or have you deemed it as an internal flaw of eQUEST? This project is for my masters thesis, so I am trying to justify my results as much as possible. For my system I am using two 3.5 ton GSHPs with an EER of 21.8 and a COP of 4.4, which is rather efficient.
Thanks,
Stephen Parker
Two quick things to check would be the supplemental electric heat (for the air-source system) as well as your loop field design parameters. Your numbers could be correct if it?s in an extremely cold climate, you?ve oversized the loop field or your loop field ground thermal properties are unrealistic for the area in question.
Anthony Hardman
Are all zones satisfied without too many over or under-heated hours? eQuest
will not auto-size your ground loop for you, and if it's not large enough,
your energy use will be really low, but the people in the space will be
freezing or roasting, depending on the season.
Have you made the switch in detailed edit mode or in the wizard? If it's in
detailed edit mode, there are all kinds of things that can trip you up a
bit.
--
Karen
Stephen,
I did the energy modeling on a small office building that has 9 GSHPs
in it. The model showed a substantial savings over the baseline
(similar to what you report). The baseline building used air-cooled
rooftop units of standard efficiency. So far, the building is not
quite as efficient as the model. While less than the baseline, the
demand kW for the building is much higher than the demand predicted in
the model. On the other hand, there are some control issues not
relating to the GSHP units that may be causing the decreased efficiency.
One of the things I noticed about my model is that eQUEST predicted
that the aux electric heat would almost never run. I am not sure that
this is the case in reality, and if the changes to the controls do not
improve the building performance, that is the next thing I will check.
Charles Ladd, PE, AIA
Stephen -
I have seen significant savings on several GSHP models, but never quite as high as you're showing. I'd agree with the replies you've gotten so far, especially the point about supplemental heat on the air-source system...If your building is in a northern climate then your air-source system will be running on electrical resistance most of the heating season, unless you have a cold-climate heat pump and modify the lock-out temperature accordingly...
I'd also add, on your ground-source model (especially since this is research-level work), you should check that the default performance curves in eQUEST (which you can view in the detailed interface) reflect your equipment well. The EER and COP you mention are pretty high; as the fluid temperatures change in the model, is the heat pump efficiency reflecting levels similar to what the manufacturer had documented for the actual equipment you're trying to model? And last but certainly not least, I would turn on an hourly report of the heat pump entering water temperature, and check it to be sure that the loop field you're modeling is giving you reasonable temperatures back at your heat pumps (and that backup heat isn't being used at all for the GSHPs - unless you're planning a hybrid system).
Best of luck to you -
Scott Hackel