As the legislative push toward decarbonization continues and federal, state and local governments adopt restrictions on carbon emissions from buildings, the need for heat pumps is growing. With that need comes the demand for technologies that make heat pumps more efficient and effective, particularly in colder climates.
What is a heat pump?
In simple terms, it’s a device that transfers heat from one energy source to another one, usually through a vapor compression system and transferring the heat through the refrigerant. But there are many types of heat pumps based on where we’re pulling that energy source from. It can be from ambient air, water or the ground.
The term ‘heat pumps’ can be used in a broad sense to talk about many different applications. We are most familiar with the residential arena, with air-to-air reversible air conditioning in the summer and heating in the winter. You can do that with commercial split systems, packaged systems or larger rooftop units. All of these are reversible systems.
Over the last 10 years, we can see a 4-5% compounded annual growth rate in heat pumps and now we are seeing almost double that, 8-9%. We can already see there’s a higher growth rate for heat pumps than there is for central air conditioning systems. We’re already in a full market transition.
Across the U.S., there are different temperature zones. In colder climates, some sort of gas-fired furnace or boiler is needed during for lower ambient temperatures. This allows us to use a smaller heat pump, thereby reducing energy costs and creating a fuel cost optimization at lower temperatures.
Challenges in Electrification
The challenge with this transition is the increased demand on the electrical grid. The peak power consumption in the U.S. is typically in the summer months. We try to use capacity modulation in a heat pump system where we can use variable speed compressors, tandems and trios, where we can lower the capacity of the compressors. Under higher ambient conditions we can move to a lower load on the compressor.
In the winter months, particularly in the northern regions of the U.S., as temperatures drop, power consumption goes up. This creates a multitude of challenges to meeting the increased energy demands. One solution is capacity modulation, where in variable speed or tandem and trio compressors, you can ramp up the speeds at lower ambient conditions to operate at a higher capacity with lower energy use. When outside temperatures moderate, you can throttle down the speed of that compressor and as a result, lower energy consumption.
The design of dedicated heat pump compressors, using liquid injection and economized ports, or economized functions or cascade solutions, are all different options for balancing energy consumption and reducing the cost to the homeowners or the building owners.
Over the last decade, Danfoss has invested in developing some of the most efficient variable speed compressors on the market that meet a wide range of applications. We also have dedicated compressors for heat pumps for that higher compression ratio. Our PSH compressor has a liquid injection valve where you can unload the compressor a bit to provide liquid injection, giving you the ability to operate at lower evaporator temperatures. With these dedicated compressors, you can operate some at lower ambient conditions and others at much higher condensing temperatures, allowing for higher heat rejection or higher water heating.
From a system standpoint, you can implement cascade systems where you have two stages of compressor very similar to supermarkets where you use one on a medium temperature setting and one at a higher setting. This enables higher heat rejection to reach increased water temperatures. You can add economizer functions or assemblies where you’re able to increase the subcooling or decrease the load on the compressor.
In order to increase adoption of heat pumps and meet federal, state and local decarbonization goals, we need heat pump technologies that allow for effective operation at a wide range of ambient temperatures. Technology will also need to evolve to meet the larger challenges of electrification and load management. By developing these technologies, we can reduce carbon emissions and energy usage while maintaining occupant comfort in homes and buildings.
To read the ACEEE’s latest research on heat pumps, visit https://www.aceee.org/research-report/b2205
