There are various types of heat pumps that are, in principle, even more efficient than air source devices. Instead of absorbing heat from the air, you can choose to capture it from the ground or even from bodies of water. However, these systems are usually more expensive.
Patrick Wheeler, director of Vito Energydescribes a recent installation that required drilling a well in the driveway of a customer’s home. A liquid-filled pipe runs from the well to the roof, where it passes beneath solar panels to collect even more heat, all in “one big loop,” he says.
“The end result is the most efficient heat pump system we have ever installed,” he adds. “We expect it to end up with an annual coefficient of around 6.” Time will tell: the system has only been fully operational for about a month. And this approach is not for the cash-strapped. The installation cost £60,000, not including a new underfloor heating system.
Using water as a heat source can be especially efficient, notes Star Refrigeration, a supplier of industrial heating systems in Scotland. In one proposed design, recalls Dave Pearson, the group’s sustainable development director, a heat pump would have used wastewater at 60 degrees Celsius to help raise the temperature of the fluid in a heating circuit from 60 to 70 degrees. The proposed system had a theoretical COP of more than 10. “But it hasn’t been built yet,” says Pearson.
Kircher states that there is a fundamental limit to the COP of heat pumps, known as the Carnot limit. In short, it means that the theoretical maximum COP will always be limited in proportion to the difference between The outside heat source and the inside temperature. The greater the difference, the lower the highest possible COP, and then there are the inevitable efficiency losses in the system itself. The heat pump compressor is never going to be 100 percent efficient, for example.
It’s all very well aiming for high-rise SCOP buildings and getting close to the Carnot limit, but obsessing over this could become a distraction, Wheeler stresses. “It’s something that’s been bothering me – everyone’s talking about SCOP,” he says. “What we should be looking at is what the cost per square metre is for certain types of housing.”
HeatPumpMonitor.org allows users to sort listed facilities by cost of operation, based on various available electricity rates, and this changes the picture slightly, causing Ritchie’s system to drop a few spots, depending on the options you select.
Michael de Podesta, a retired physicist from England, has a house with solar panels, insulation in the external walls and a heat pump that runs at a SCOP of 3.5. While this is not as high as Ritchie and others, it doesn’t really matter, because the running costs of de Podesta’s system are minuscule: his annual electricity bill, for his heat pump and all the other appliances, is just £250. That is possible even if he keeps his property at an internal temperature of 20 degrees Celsius.
Last year, Podesta wrote A blog post titled “Police envy is pointless” In this article, he explains that when a house is very well insulated (like his), the SCOP of the heat pump actually goes down, because it is mainly used to heat hot water, especially in spring and summer. As people tend to heat hot water to 50 degrees Celsius or more, the heat pump has to work relatively hard, even if only briefly, and its efficiency may seem limited.
“There’s a curious effect,” says de Podesta. “If you reduce the amount of heating, which produces a better average SCOP, the overall SCOP goes down.” Wheeler refers to this as the “summer drag” effect, and it’s one reason looking at SCOP values can be misleading.
But rivalry among installers for SCOPs is not a waste of time, says de Podesta: “It’s good, healthy competition.”
Ritchie, for his part, is enjoying the attention his high-performance heat pump has generated. He has fielded queries from many interested parties, including publications other than WIRED. “I don’t really want to be a celebrity for this,” he insists, but he is happy to spread the word about these systems. “I would certainly support heat pumps. I think they are great.”