There are different types of heat pumps that are actually even more capable than air-source devices. Instead of absorbing heat from the air, you can choose to collect heat from the ground or even from bodies of water. However, these systems are usually more pricey.
Patrick Wheeler, director Vito Energydescribes a recent installation that involved drilling a borehole in a customer’s driveway. A fluid-filled pipe runs from the borehole to the roof, where it passes under 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’ve ever installed,” he adds. “We’re hoping it will end up with an annual efficiency of around 6.” Time will tell – the system has only been fully operational for a month. And this approach isn’t for those on a tight budget. The installation cost £60,000, excluding the fresh underfloor heating system.
Using water as a heat source can be particularly capable, says Star Refrigeration, a supplier of industrial heating systems in Scotland. In one proposed design, recalls Dave Pearson, the group’s sustainability director, a heat pump would exploit 60-degree wastewater to raise the temperature of the fluid in the heating loop from 60 to 70 degrees. The system in question had a theoretical COP of more than 10. “But it wasn’t built,” says Pearson.
Kircher argues that there is a fundamental limit to the COP of a heat pump, known as the Carnot limit. In compact, this means that the theoretical maximum COP will always be confined in proportion to the difference between Your external heat source and the room temperature. The greater this difference, the lower the highest possible COP – and then there are the inevitable efficiency losses in the system itself. For example, a heat pump compressor will never be 100 percent capable.
Striving for high SCOPs and reaching the Carnot limit are all well and good—but obsessing over them can become distracting, Wheeler emphasizes. “That’s something that annoys me—everyone talks about SCOPs,” he says. “Should we be referring to what is the cost per square foot for certain types of homes?”
HeatPumpMonitor.org allows users to sort listed installations by operating costs, based on the different electricity tariffs available, which changes the picture slightly, moving Ritchie’s system down a few places, depending on the options selected.
Michael de Podesta, a retired physicist from England, has a house with solar panels, external insulation and a heat pump operating at a SCOP of 3.5. While that’s not as much 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 the heat pump and all the other utilities, is just £250. This is possible even if the internal temperature of the property is kept to 20 degrees Celsius.
Last year de Podesta wrote blog post titled “COP Jealousy Is Pointless” in which he explained that when a home is exceptionally well insulated — as his was — the heat pump’s SCOP will actually drop because it is largely used to heat heated water, particularly in spring and summer. Because people tend to heat heated water to 50 degrees Celsius or more, the heat pump has to work relatively strenuous, albeit for compact periods, and its efficiency can seem confined.
“There’s a funny effect,” de Podesta says. “If you reduce the amount of space heating, which gives you a better average SCOP, the overall SCOP goes down.” Wheeler calls this the “summer drag” effect, and it’s one reason focusing on SCOP values can be misleading.
But competition among installers for SCOP is not a waste of time, de Podesta says: “It’s healthy, good competition.”
Ritchie, for one, is pleased with the attention his capable heat pump has received. He has fielded inquiries from a range of interested parties, including publications other than WIRED. “I really don’t want to be a celebrity on this,” he insists, but he’s pleased to be raising the profile of these systems. “I would definitely wave a flag for heat pumps. I think they’re brilliant.”