At the recent New York Solar Energy Society (NYSES) Solar Thermal training event in Malta, I presented a lecture on geothermal and air source heat pumps and their use in high performance homes. It was well received and I would like to thank NYSEA for giving me an opportunity to educate those involved in designing and installing solar thermal heating systems the advantage of using geothermal heat pumps in conjunction with solar thermal heating systems.
One part of the presentation was about the relative efficiency of geothermal and air source heat pumps. One important fact to understand when comparing these two types of heat pumps is that most of them use the same refrigerant, 410A, which means that entering fluid temperature for the evaporator and condenser is a critically important factor in operational efficiency. Please note that both air and water are fluids. In heating dominated climates, the most efficient air source heat pump uses twice as much electricity to operate as the most efficient geothermal heat pump under design conditions. They also use twice as much electricity in cooling under design conditions. In order to understand the actual operational efficiency of a heat pump it is critical to look at the expanded performance chart, rather than looking at the rated efficiency. A primary reason is that the ratings for air source heat pumps do not reflect actual operating conditions, rather they use 47°F for heating and 82°F for cooling, which is not representative of actual operational conditions. Geothermal heat pumps are rated at 30°F for heating and 77°F for cooling, which is very representative of the entering water temperature under design conditions.
My presentation ended with a discussion of how geothermal heat pumps are able to enhance solar thermal heating systems. The excess heat from solar domestic water heating can be injected into the ground loop and stored for winter heating, rather than letting the panels stagnate and overheat when the heat is not needed for water heating. For solar thermal heating systems, the geothermal heat pump can be used to concentrate the stored heat and eliminate periods of time when the system is not able to fully heat the building.
After my presentation, I was in the audience for John Siegenthaler’s excellent presentation on hydronics. He covered how to make energy efficient hydronic heating systems. I have to say that I know of no one who understands and teaches about hydronic systems better than John. If you want to know more about hydronics, look up John’s books and articles.