Jack Sweet 0000-00-00 00:00:00
HydroSci installs a solar ground source heat pump system Converging technologies are the name of the game in today’s plumbing world. We’re all familiar with your basic ground source heat pumps. Every now and again you run into a ground source heat pump using solar. But what about a solar/ground source heat pump system that decouples the solar load from the ground load so it can use the warmest water temperature available?That’s pretty cool. “This was one of the coolest projects we’ve had the opportunity to be involved with. Most of them are just typical radiant installations,” said Greg Rehn, the owner of HydroSci in Mead, Wash. The company may be best known for its radiant hydronic work, but a recent project at the 16,000 acre Turnbull National Wildlife Refuge near Cheney, Wash., is a little out of the ordinary. “The building is a vehicle maintenance building,” Rehn said, adding the first step of the project was the removal of the previous propane heating system. “It’s like a big truck shop. They maintain lots of vehicles, fire equipment, that kind of thing. It has a couple of overhead cranes. It’s a pretty good-sized shop— 6,000 square feet.” Energy efficiency led the Department of Fish & Wildlife to decide on a ground source heat pump system for the facility’s forced-air heating needs. Vertical drilling was out of the question on this project, so a horizontally trenched closed-loop ground heat Exchanger, or GHEX, was laid out installed. This configuration presented a problem in that it would not allow as much summer solar energy to be stored and used because of its shallower depths. All of the solar energy harvested in the fall, spring and winter can be used.“We decided to add a solar thermal array to augment the ground load,” Rehn said, noting the addition of the solar thermal array did allow for a 50 percent decrease in the size of the GHEX. “The solar thermal system would minimize the depression of ground temperatures during the heating season. This addition would increase the EWT, in turn increasing the coefficient of performance.” [COP = energy out divided by energy in.—Ed.]. Key system details include 15 4-foot by 10-foot Heliodyne flat plate solar panels, six heat pumps with 30 tons total capacity (five water-to-air units and one water-to-water unit). Add in an 80-gallon storage tank, variable speed solar and ground source pumps and an 80-gallon DHW buffer tank: “The buffer tank was to give a time delay for the thermal energy entering the ground source system,” Rehn said. “Because it was attached to HDPE we wanted to ensure we could never damage it with solar water that’s too hot. This tank gives the controls and safety features more time to react.” Decoupling the solar load from the ground load so the water can use the warmest temperature is an interesting twist Rehn pulled out of his personal Magic Hat of Knowledge. “I had done quite a bit of research on the possibility of doing a system like this. I looked through a bunch of the systems that I found on the Internet and have never found anyone who actually decoupled the two,” he said. “Everybody was just running the solar energy directly into the ground first and then pulling it back out of the ground into the heat pumps. We have a full-blown decoupling header that allows us to prioritize and gets the warmest entering water temperature going into those heat pumps.” Conventionally ground source heat pump systems are designed with a calculated minimum entering water temperature between 30- and 35 degrees Fahrenheit. This results in a COP around 3.34. “[That’s] pretty good compared to any system out there, but we found room for improvement, lots of room,” Rehn said. “By adding the solar array we were able to increase the daily average ground temperature by 10 F. This brings the COP to a minimum of 3.77 or a 12 percent increase in efficiency. “ He said there is also the ability to control the higher temperature limit so the incoming water isn’t too hot to the heat pumps. This ability to prioritize and mix ground and solar energy allows for extremely high solar efficiencies and significantly higher COP’s at the heat pumps. The system also incorporates variable speed drives on the solar pump and the ground heat exchanger pump; this greatly reduces parasitic pumping loads. “When the sun is shining and the building does not need heat, the solar energy is rejected back to the to the GHEX to be recaptured that evening,” Rehn said. “If the building is calling for heat and the sun is out, the system diverts all solar energy to the EWT of the heat pump. This increases the EWT and results in the highest possible COP. Another advantage is a lower demand on the ground source field allowing it more time to regenerate.” The heat pumps have the ability to run completely off of the solar array, or by mixing the solar water with the ground source water to create the warmest possible EWT, he explained. This configuration is sending the coldest water to the solar array and the warmest to the heat pump. When the sun is out and the building is under a partial load he said he often sees the system performing with the ground source field turned off and only solar energy being used. COPs between 4.5 and 5 are common during these times, he said. Rehn said the company didn’t have any similar systems to model from, so the engineering and design on this project was extensive.The system is performing better than anticipated. Rehn said a recent letter from the Fish & Wildlife Department’s contracting officer for the project said the building has posted a 34 percent energy use reduction since the installation. “I would love to claim that I am a hydronics genius, but I’m certainly not,” Rehn said. “I’m just studious. I’ve studied all the magazine articles, modern hydronic books and all of the standard concepts and incorporated those concepts using a decoupling chamber— basically an oversized pipe and some closely spaced tees and some different pumping concepts and we split up the loads that way. Basically water is being pulled from the ground and we have the ability to inject warm water right in front of it as it goes into the header to mix water temperatures there. If it gets too hot, we can throw a mixing valve and cool it down. We can run these heat pumps just off of the solar panels if there’s enough thermal energy available. We can also run off of the ground, or a combination of both. It’s really working great.”
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