“I couldn’t find a small, inexpensive ground source heat pump,” said Logan Bryce of Belt, Montana. So he decided to create his own to heat his 850 sq. ft. shop.
I’ve known Logan and Jenni Bryce for several years since interviewing Jenni for a wind power article. Their home is a veritable playground for an alternative energy enthusiast because they’re always experimenting with something new. So, it didn’t surprise me too much when Logan said they were building a “redneck geotherm system” about three years ago.
They overcame the cost and size obstacles by turning a mini split air conditioner heat pump into a more efficient unit.
HOW IT WORKS
As a layman, it was hard for me to wrap my head around how this system worked until Logan told me it’s like an air conditioner, which pulls ambient air into a condenser where refrigerant gas that has been compressed into a high pressure liquid is cooled. It is then pushed through an expansion valve where it changes to a low-pressure liquid and enters into the evaporator coil. The refrigerant liquid is cold so air across the evaporator is cooled and pushed into the room. Conversely, hot air is blown out the exterior side from the condenser. The refrigerant is changed back to a low pressure gas and returns to the compressor.
When an air-to-air heat pump is used to heat a room, the direction of the refrigerant in the system is reversed. But the heat pump has its limitations. When the winter temperatures drop below zero, it can’t keep up with the heating requirements due to the lack of latent heat available in the air.
“So in the redneck setup, we cool or heat the air before it passes through the condenser by adding a water-to-air exchanger to the air intake on the outdoor unit,” said Logan. “In the summer that heat is carried into the ground by the water flowing in the ground loop, and the unit works better as an air conditioner blowing cold air out of the evaporator into the shop. In the winter when the heat pump is used in the heating mode, the air is warmed by the water flowing in from the ground loop before it goes into the outside heat exchanger. That makes the unit produce more heat on the inside heat exchanger.”
Logan modified his air-to-air unit by reconfiguring it with a water-to-air heat exchanger, which works like a radiator. This allows the fluid pulled from the earth to pre-warm the air before it goes through the outdoor unit. Instead of the ambient air being the heat sink source, the warmth of the earth acts as the thermal sink.
The water-to-air heat exchanger is most often found on wood burning furnaces. In these units, the water is heated with wood instead of being pulled from the ground. It can then be transferred to a forced-air system in the house through the water-to-air exchanger. You can find water-to-air heat exchangers for around $200.
The fan on the heat pump outside unit pulls ambient air through the water-to-air heat exchanger, which extracts the heat from the system’s fluid as it comes out of the ground. “The water is the same temperature as whatever the ground is,” he said.
The temperature from the earth typically ranges between 45 to 65 degrees F. By utilizing this natural thermal source, the optimal operating capacity of the system is boosted. When the fluid returns to the ground in the loop system, it is significantly cooler. (Logan has monitors on the unit that constantly record the ingoing, outgoing temperatures on the fluid when the system is running. To learn more, check out pineridgeproducts.com/PRPSite/DAQ.html.
“When it was -20 the heat pump was still cranking out good heat and did not suffer from coil freeze up,” he said.
And the beauty is that the system didn’t cost much. The Bryces found the heat-pump unit on eBay for around $600 and the air-to-water exchanger cost them about $200. The nice part about the entire geothermal system is that the parts are readily available. All that’s needed can be found at Home Depot or on eBay.
BURYING THE PIPE
What makes any geothermal system different is that it utilizes the heat from the earth. This is accomplished by running lines with fluid deep enough (well below the frost line) to warm the fluid and bring it back to the heat pump.
The Bryces have their own backhoe. Renting or hiring one will be more cost effective for most homeowners. But, it’s important to note, not everyone is a candidate for a geothermal system because of space considerations, particularly since homeowners typically aren’t set up to drill hundreds of feet for a vertical system.
“I dug a trench 500 feet long and 10 feet deep. In it we laid 1,000 feet of standard ¾-inch black plastic irrigation pipe,” he said. (If you are digging a trench, be sure to hire a reputable contractor or use adequate safety measures to prevent a trench collapse.)
A small, inexpensive pump draws the fluid from the pipe and through the water-to-air heat exchanger to warm (or cool, depending on the season) the air before it goes through the unit. The system isn’t pressurized and the pump is wired into the heat pump’s outdoor unit’s fan motor.
WINDSHIELD WIPER FLUID
Since the Bryce’s have a closed loop system, which means they use the same fluid over again versus pumping water into the system, they needed something that wouldn’t freeze. Logan wanted one that wouldn’t thicken in the cold, wasn’t toxic and was relatively inexpensive. “It’s -30 windshield washer fluid,” said Logan.
He continued, “The fluid comes out of the ground and into the water-to-air heat exchanger; then it dumps into a reservoir made from 5-inch white PVC pipe. From there the pump pulls the fluid into the intake side of the pump, gravity feeding the suction side of the pump. The water is then pumped back through the ground line from the pressure side of the pump.”
As the fluid travels through the ground loop it extracts the thermal energy in the ground. The fluid goes through the water-to-air exchanger mounted to the intake side of the heat pump’s outdoor unit. The ambient air is warmed or cooled depending on the incoming air temperature as it is pulled through the ground loop’s exchanger and enters the heat pump. This thermal energy is transferred to the heat pump’s refrigerant and travels to the interior unit where it’s blown into the shop as hot or cold air depending on the operating mode.
“A copper ‘line set’ connects the indoor and outdoor units. This is filled with refrigeration gas or liquid depending on whether the unit is heating or cooling,” Logan said.
It’s not an absolute necessity, but one additional feature the Bryces added to their geothermal system was a method of adding water around the line.
“When you get it good and wet it helps transfer the heat better,” he said. “To help the heat transfer, we inject water into the trench.”
We don’t often think of how well water conducts heat, particularly in a home heating situation. Remember when you last grabbed a hot pan with a damp oven mitt. Didn’t that heat go through the material quickly?
By dampening the soil around the line, the fluid is able to pull more thermal energy from the earth. If you live in a region of the country that receives lots of rain (or you situate your loop in an irrigated field), it’s probably not an issue. But in arid regions adding water improves the performance.
Logan used ½-inch commercial-grade drip line that has built-in emitters every 24 inches. He placed it directly over the pipe in the trench. The drip line is attached to a hose bib, and he turns it on periodically. The drip line has the ability to back flush to keep it from plugging over time.
Pump up the heat—or the cool—with a low cost, highly efficient system from Mother...
by Donal B. Lloyd / Feb 28, 2013