Air source heat pumps: harvesting heat from cold air
While air source heat pumps (ASHPs) are gaining traction in milder regions of Alaska like Southeast, they aren’t powerful enough to be used as a primary heating system in the Interior because they only work down to about 0 degrees.
First, some background on air source heat pumps. Essentially, the appliances use electricity to absorb heat from the air and move it around. In cooling mode, the heat pump acts like a refrigerator, absorbing heat from the inside of a home (like a fridge absorbs heat from the objects inside it) and moving it to the outside. In heating mode, the process is reversed–heat is extracted from the outside air and transferred to the interior of a home.
Because they use electricity to power pumps, fans and the compressor, they eliminate the need for on-site storage of heating oil or propane. They can also reach efficiencies greater than 100 percent, which means the home receives more heat than is contained in the electricity that the ASHP uses.
The fact that ASHPs can provide heating and cooling makes them attractive for many regions of the world. In Fairbanks, however, most homes do not require cooling (though this is starting to change as temperatures warm). The severe winter temperatures are another obstacle in this climate, as ASHPs get less efficient as the weather gets colder. In other words, more electricity is required to absorb heat from colder air. And while more cold-climate models are entering the market, there is still a cutoff temperature at which they no longer work at full capacity. Most ASHP models can only efficiently produce heat at temperatures down to 0°F or -5°F, which Fairbanks typically sees early in the winter. This means that homes using ASHPs must have a back-up heating appliance for most of the heating season.
However, an ASHP may still be a good choice for buildings that require cooling year-round, such as a business with a server room. The system could be designed to cool one area of a building while providing heat to another at the same time.
For homeowners in Interior Alaska, a ground source heat pump may be a better option. It works in a similar way, using a refrigeration cycle to transfer heat, however the outdoor coils are buried underground. Because the ground is much warmer than the air temperature in the winter, ground source heat pumps don’t need a back-up system, and the constant source of heat allows them to function at a higher seasonal efficiency in a cold climate.
As with all technology, heat pumps are advancing quickly and we are likely to see more of them as Alaskans look for alternatives to fuel oil.