Why heat pumps are key to decarbonizing HVACNick Ysidron | May 26, 2022
When you think about heating in commercial and residential buildings, electric heaters and natural gas furnaces probably come to mind. But what about air conditioners?
Energy naturally moves from areas of high temperature to areas of low temperature but, with the help of thermodynamics and a little bit of work, we can pump that heat from a low temperature to a high temperature efficiently and effectively. This key feature will play a pivotal role in decarbonizing HVAC. Let’s learn why.
How efficient is traditional heating?
As of a few years ago, natural gas furnaces and electric heaters were used in approximately 47% and 26% of U.S. households, respectively, according to the U.S. Energy Information Association. Electric heat pumps made up only 10% of those households with other fossil fuels making up another few percent.
With more than half of residential energy usage going to heating and cooling of the home, efficiency gains in that area can have tremendous benefits. As the push for decarbonization intensifies, it is important to look not only at the amount of carbon emitted but also the efficiency of the energy used by heating and cooling equipment.
Natural gas furnaces are the easy target for decarbonization. That makes sense considering they directly produce emissions like carbon dioxide during operation. While emissions aren’t a good thing, natural gas furnaces are popular for a reason. Piping natural gas is an effective way to move stored energy long distances. For many homes, the heating requirements would be too high for a reasonably sized electric heater, but can be easily managed by a natural gas burner.
In addition to being able to release a great amount of heat, natural gas burners have greatly increased in efficiency over the years. While furnaces are now required to be at least 80% efficient, furnaces on the market are capable of over 95% efficiency. While this is a great use of natural gas, fossil fuels are still being burned to generate this heating even if it’s being done more efficiently.
Because they generate heat by passing a current through a resistance, electric heaters do not directly contribute to emissions. Depending on how that electricity is generated will determine how clean the electric heaters really are, but it’s a step in the right direction. The biggest drawbacks for electric heaters are twofold: large capacities require large infrastructure and no leveraging of work input.
Electric heaters are easy choices when the heating requirement is small. Most homes have enough electrical power to be able to run an electric heater, especially if the air conditioner is not running. As one transitions to colder climates, the size of the compressor used for cooling goes down and the requirement for heating capacity goes up. This means more and more dedicated power is needed to be able to operate a sufficiently sized electric heater. Due to these requirements, natural gas burners begin to be the preferred choice for those regions.
Part of the reason electric heaters require so much power is because they don’t get any benefits from physics on leveraging work input. A 1 kW input of electrical power results in 1 kW of heat delivered to the space. While not a bad trade, it’s not particularly good either. This brings us to the concept of coefficient of performance (COP).
Coefficient of performance
The COP is simply the ratio of heat output to the amount of energy input. While traditionally used in cooling analysis, COP is what makes heat pumps key to decarbonizing HVAC.
In a cooling cycle, to move 3 kW of heat from the indoor space to the outside, the compressor might consume 1 kW of power. This would give the heat pump a cooling COP of 3. The same idea is true for a heating cycle. The COP varies based on the conditions that the heat pump is operating in but moving 3 kW of heat into the space might require that same 1 kW of electrical input. Whereas the natural gas burner has to burn a fossil fuel and for the electric heater what we put in is what we get out, the heat pump lets the user leverage the electrical input.
How do heat pumps heat and cool?
This leveraging of energy input is part of what makes heat pumps so special. Being able to cool and heat with the same equipment also makes heat pumps very versatile. Heat pumps can perform both heating and cooling cycles because of their ability to reverse the flow of refrigerant and the roles of the equipment’s components in the cycle. For example, during a cooling cycle the compressor compresses refrigerant and sends the hot vapor to a condenser outside to reject the heat to the ambient environment. The refrigerant cools to a liquid and is piped to an expansion valve. After the refrigerant goes through the expansion valve, it is a cooler, lower pressure mixed flow that goes into the evaporator. Air flowing over the evaporator draws heat into the evaporator, vaporizing the refrigerant and cooling the air. The refrigerant then returns to the compressor to begin the cycle again.
In a heat pump system, the heat pump can reverse this flow of refrigerant via solenoids, reversing valves and check valves. Switching the flow switches the function. The hot gas from the compressor now flows into the indoor unit where the cold air passing over the heat exchanger warms the air and condenses the refrigerant. The cooled refrigerant then travels to the outdoor coil where it passes through an expansion valve, cooling the refrigerant down. The cold refrigerant then extracts heat from the outdoor environment to keep the cycle going. Some heat pumps are able to operate down to -15° F, although the COP drops as the outdoor temperature gets exceptionally cold. It just becomes a lot harder to extract heat from really cold air.
Where do heat pumps need to improve?
Heat pumps work beautifully as long as there's enough heat in the cold outdoor air for extraction. As the temperatures get extreme and heating loads in the space are highest, heat pumps can struggle to keep up. Because of this, it is essential that HVAC systems in extremely cold environments have back up heating sources and that research into heat pump technology continues. The U.S. Department of Energy has even issued a Residential Cold Climate Heat Pump Technology Challenge to encourage innovation and foster development of even better heat pump designs.
With thousands of TWh projected to be needed for cooling and heating annually throughout the world in the near future, decarbonizing HVAC will be key to reducing emissions. Driving technology and innovation to produce better cold climate heat pumps is a great first step and allows the focus to be placed on decarbonizing the equipment producing the energy for the grid. Natural gas burners and electric heaters may become a thing of the past as heat pumps become the new norm. Either way, better technology that reduces carbon emission and energy usage is a step in the right direction.
About the author
Nick Ysidron is an engineer who specializes in product development and technical writing. He previously worked for an original equipment manufacturer (OEM), leading a team of engineers and designers in building custom equipment for the commercial and industrial HVAC sectors. Nick currently designs internet of things devices and lends his talents to DIY and maker type projects.