Written on: June 1, 2023 by George Carey
With the current emphasis on electrification and decarbonization coming from local, State and Federal bureaucracies, there is a lot of talk about using heat pumps to displace fossil fuel appliances and low-efficiency standard AC condensers. Of course, heat pumps have been around for a long time in the HVAC industry. When discussing heat pumps, historically there have been three categories:
• Air-to-Air heat pumps
• Water-to-Air heat pumps
• Water-to-Water heat pumps
In general, heat pumps are devices that can convert low-temperature heat into higher-temperature heat. The low-temperature medium is referred to as the source. This is where the energy comes from to heat the building. The “source” can be the outdoor air or tubing buried in the ground. This is where the “free” heat comes from—the renewable energy source. The converted “higher temperature” energy is then released into the sink—the place that can absorb this energy. An example would be an air handler unit that is moving cooler return air across a coil (the heat exchanger) and delivering the warmer air into the living space.
Most are familiar with Air-to-Air heat pumps, especially the mini-split units. They extract heat from the outside air and are very common throughout our market. Air-to-Air heat pumps deliver the high-temperature heat through either a forced-air network of duct throughout a home, or individual cassettes mounted throughout various rooms in the house. This type of heat pump is classified as an air-source heat pump. There are other heat pumps that extract heat by using water that is circulated through tubing that is buried in the ground. The earth heats the water circulating through a “field” of tubing and this heat is then converted by the heat pump into a higher temperature medium. If it is air, they are known as Water-to-Air heat pumps and if the high-temperature medium is water, then they are referred to as Water-to-Water heat pumps.
There is one additional type of heat pump that is starting to gain attention in our industry called an Air-to-Water heat pump. This style is similar to wall-hung boilers and can produce water temperatures from around 85°F up to 130°F. Air-to-Water heat pumps do not burn fossil fuel, which is another reason they are gaining in popularity. This unit extracts heat from the ambient air outside the home and transfers it through refrigerant piping to a module. This module contains a refrigerant-to-water plate heat exchanger that heats water, which is then circulated through floor heating systems, fan coils and low-temperature radiators. Since it is a heat pump, the whole cycle can be reversed and provide chilled water (45°F) for cooling. Air-to-Water heat pumps have been gaining popularity in Europe for the last 10–15 years.
Why a Heat Pump?
Heat pumps are considered the most energy efficient, electrically-operated heating and cooling system on the market. These modern Air-to-Water heat pumps can deliver between 3–5kWh of usable heat for every 1kWh of electricity that it uses. This equates to a Coefficient of Performance (COP) of 3–5 or 300%–500% more efficient than typical electrical resistance heat. The heat pump uses the renewable energy source (air) and therefore has no localized CO2 emissions. The same system can be used for heating in the Winter and cooling in the Summer. Another benefit to this style of heat pump is that they use an inverter technology to operate the compressor and can vary the speed of the compressor to match the actual load that the system is currently experiencing. This can provide more comfort by matching the output to the load. Furthermore, cycle losses are reduced, which increases the compressor’s efficiency and reduces wear and tear on the compressor, thus extending its life cycle.
By using air instead of geothermal energy, one can eliminate the expenses associated with drilling a well field and installing the tubing (anywhere from $10,000–$30,000), consuming the necessary footprint to support the well field and the operating costs of pumping the well all year long.
Where Does the Heat Come From?
Generally speaking, the heat contained in the soil, ground water and air all started as solar energy. Basically, we are taking energy from the sun and using it to heat the water for the hydronic systems. During warmer weather, the ground and the air absorb this heat and, as the weather gets colder, some of this heat dissipates to the outside air. However, the heat absorbed into the soil can take a long time to transfer back to the atmosphere—so even in the middle of Winter, the soil temperature in the earth is much warmer than the outside air temperature. This condition generally favors water source heat pumps because their efficiency or thermal performance remains high due to the warmer-source water temperature, whereas an Air-to-Water heat pump’s output capacity drops as the outside air temperature gets very cold (0°F to 25°F). The manufacturers are continuing to try and improve the efficiency of the refrigerant cycle, enabling the units to extract heat from colder temperatures while maintaining their capacities.
How the Heat Pump Does What It Does
Refrigerant plays a major role in the successful transfer of energy from one place where it is available to another place that needs or wants it. The refrigerant is a chemical that has unique properties that allow it to absorb heat from low temperatures and transfer that energy to a medium operating at higher temperatures. For this to happen, the refrigerant has to change its state from liquid to vapor and then back to liquid—and in doing so, undergo some pressure changes.
This whole process can be referred to as the “Refrigeration Cycle” and is the starting point for the operation of all vapor-compression heat pumps. There are four components that play a major role in this cycle. Their respective names indicate their function and how they play their part in the process:
• Thermal expansion valve
First, a cold liquid refrigerant enters into the evaporator. The pressure of this cold liquid 410A refrigerant is low, and there is a direct relationship between the pressure that the refrigerant is at and what its corresponding temperature is, so that the lower the pressure, the lower the refrigerant’s temperature. Additionally, the refrigerant’s pressure/temperature will adjust as the “source” (air) temperature changes. As it gets colder outside, the temperature of the liquid has to become colder so that it can absorb heat from the relatively cold air outside. The key is, as it absorbs heat from the air outside, the refrigerant evaporates or changes state into a vapor or gas. Its temperature is still low, but warmer than when it was a liquid. This step is important because the next component in the system is the compressor and since liquids aren’t compressible, if the refrigerant didn’t evaporate into a vapor, the liquid entering the compressor would severely damage it.
The compressor’s job is just as it sounds: it compresses the low-temperature vapor. This creates a large increase in both its pressure as well as its corresponding temperature as the refrigerant leaves the compressor. Another factor to consider is that the electrical energy used to compress the refrigerant by the compressor is added to the refrigerant. Now we have a high-temperature vapor that contains a lot of energy ready to be utilized.
This high-pressure, high-temperature vapor then enters the condenser, and the cooler return water from the hydronic system is pumped across the exchanger. The refrigerant, being hotter than the water, transfers its energy to the cooler water, elevating the water temperature going back out to the heating system. This transfer of energy causes the refrigerant to change its state and condense back to a high-pressure liquid.
The last step in this process is for the high-pressure/high-temperature liquid refrigerant to flow through an expansion valve (either thermal or electronic). The expansion valve controls the flow of the liquid refrigerant through its orifice, drastically reducing its pressure and thus its temperature so that the refrigerant is back to the cold temperature it was at the beginning of this process. As Air-to-Water Heat Pumps become more popular, they will find a place in the renewable energy industry.