Heat pumps are devices that are used to transfer heat from one location to another. A refrigerant is used to absorb heat from a lower temperature source and release it at a higher temperature. The process is more efficient than gas combustion and eliminates on-site greenhouse gas emission. Conventional heat pumps use synthetic refrigerants, such as R134a, which has a Global Warming Potential (GWP) of 1432, or R410a, which has a GWP of 2088. GWP is a measure of the greenhouse effect of a substance relative to carbon dioxide. Carbon Dioxide (CO2) can itself be used as a refrigerant, identified as R-744. As a naturally-occurring compound, it is a non-toxic and non-flammable refrigerant with 0 ozone depleting potential (ODP) and very low GWP, a value of 1. This makes it a more environmentally friendly option. CO2 heat pumps have the potential to provide hot water up to 190°F, which is typically higher than heat pumps that use other HFC refrigerants, allowing use in existing buildings designed for higher heating water temperatures and storage-type domestic water heaters.

Contributors: Philip Leduhovsky, Jim Crabb

Technical Description

Background

Domestic hot water (DHW) heating systems provide hot water to sinks, showers, and other water fixtures in the building. Traditionally, gas combustion equipment generates DHW, either directly through gas-fired water heaters or indirectly with steam-fired water heaters. Either one can generate hot water on-demand with instantaneous heaters, or may be coupled to storage tanks. The hot water is stored at high temperatures in the tanks to ensure the demand for hot water can always be met and to prevent bacteria growth. Steam boilers may have an efficiency of 75-80%, while gas-fired water heaters may have efficiencies in the 90s.

Heat pumps, however, use electricity to power the compressor and move heat, rather than burning fossil fuels such as natural gas to generate heat directly. Heat pumps have a high coefficient of performance (COP), which means that they can transfer 3-4 units of heat for every 1 unit of energy input, compared to combustion equipment that generates less than one unit of heat for each unit of fuel burned.

Heat pumps can use a variety of refrigerants with different benefits and limitations. R-134a, R-410a, propane, and CO2 are just a few of the options available. One of the critical performance aspects of a domestic hot water heater is the ability to produce hot water at 140 degrees or higher. To prevent legionella growth, a harmful pathogenic water-borne bacteria that if ingested can cause the disease Legionnaires, domestic hot water stored in a tank must be at 140 degrees or higher. Therefore, if a system can’t make 140 degree hot water, the use for domestic hot water is limited.

CO2 heat pumps are similar to conventional heat pumps, except that they use R-744 (CO2) as the refrigerant, which is a natural, safe, and non-toxic alternative to synthetic refrigerants such as R-410a or R-134a. Compared to synthetic refrigerants, CO2 heat pumps operate at higher output temperatures and lower source temperatures.

CO2 heat pumps operate on the same refrigeration cycle as synthetic-refrigerant heat pumps, using a compressor to raise the temperature of the source to transfer heat to water. The source can be ambient indoor air (limited to single-family residential scale units), outdoor air, or a water source, such as return chilled water, wastewater, condenser water, or a ground source heat pump loop. Figure 1 below shows the refrigeration cycle that the heat pump uses.

Figure 1: Heat Pump Refrigeration Cycle

The performance of CO2 heat pumps varies with design, but air-source commercial units are available that can produce 170°F at ambient air temperatures as low as -4°F or source water temperatures as low as 18°F.

The performance of heat pump water heaters also varies with temperature. The greater the difference between the source temperature and the output temperature (known as the “lift” because the compressor is used to “lift” the heat to a higher temperature), the lower the COP, or the more energy input required to produce the same heat output. Because of its thermodynamic properties, CO2 maintains capacity and uses less energy at higher lift than heat pumps using synthetic refrigerants. Figure 2 below is a chart indicating measured performance of two heat pumps, one operating with CO2 refrigerant and one operating with R-134a. There are other differences between these products, but the performance of the CO2 heat pump is clearly superior to the one with synthetic refrigerant. It should be mentioned that these two devices are small systems intended for residential applications and not at a hospital scale. The source was used to show the contrast between the two refrigerants.

Figure 2: Weekly COP vs Outside Air Temp

Design considerations

Choosing the best heat source: Water or Air

Water sources heat pumps typically benefit from higher performance. When a water source is used, typically the chilled water system returns water, there is a cooling effect on the system, which is an added benefit and results in a much higher effective COP. It is recommended to find a source that is always available. In cold climates, chilled water systems are typically turned off during winter months. In mild climates, when temperatures are lower, airside economizers may be used instead of running the chilled water system. Other sources include waste heat from kitchen refrigeration equipment, IT equipment coolers, which are both year-round constant loads, or ground-source heat. Many heat pump manufacturers specify acceptable operation at entering water or glycol temperatures as low as 18°F.

Air source systems are simpler in the sense that integrating the heat pump into an existing water system is not necessary. Air source heat pumps can be effective in outdoor temperatures as low as -4°F, but with a reduced COP. The system efficiency is lower due to greater temperature differences and lower heat transfer coefficient.

In new facilities, space requirements must be identified early so that they can be incorporated into the design, however this may make retrofit projects challenging if existing mechanical space is not adequate.

How to size a system for your facility

Determine the hot water demand using the same method that is used for gas-fired water heaters. However, it should be noted that traditional methods result in oversized water heaters in hospitals, so additional “safety factors” are not needed.

Heat pump water heaters have a slower recovery rate than gas-fired boilers, so the system will require a larger storage tank to satisfy the typical hot water demand. Consult with the system manufacturer to determine recovery requirements and system sizing guidelines.

Best practices

Heat pumps are not a one-size fits all solution. Here are some best practices to note when designing a system for your facility:

1. Water source in cold climate: In ASHRAE climate zones 5, 6, and 7, where outdoor temperatures often drop below 0°F, air source heat pumps will not be an effective solution; it is recommended to use a water source system with a consistent year-round load if available.
2. Air source in warm climates: In ASHRAE climate zone 1, 2, and 3, air-source heat pumps are a viable option, but a water source system with the added benefit of a cooling effect can boost efficiencies of the system overall.
3. Mild climate: In ASHRAE climate zone 4, designers will need to assess the local weather data, capabilities of the system, and potential water-based sources to determine whether air- or water-source systems will be more effective.
4. Due to the high operating pressure of the refrigerant and the presence of oil in the heat exchanger, double-wall heat exchangers are recommended to protect the water supply should a leak develop. UPC 603.5.4 requires double-wall heat exchangers.
5. Load shifting: Many cities are starting load shifting and demand response programs to shift electric water heating away from on-peak rate times. These optional programs can be leveraged to provide a cost savings on top of the energy efficiency benefits.

Available Equipment

Lync by Watts offers both water-source and air-source CO2 heat pump water heaters, designed and marketed for commercial applications. Mitsubishi/Trane offers a modular air-source CO2 heat pump water heater, announced for the US market in late 2021. Sanden manufactures residential and light-commercial air-source CO2 heat pump systems. These units are split systems with a compressor unit and a separate hot water storage tank. A single compressor size can connect to different storage tanks, and multiple compressor units can connect to larger tanks.

How does this decarbonize?

Heat pumps run on electricity and use a refrigeration cycle to move heat from one place to another. Heat pumps operate with efficiency of 350% (COP 3.5) or higher, depending on the conditions, compared to 80-90% for gas-fired boilers. When connected to a chilled water system, the cooling effect also offsets chiller load, improving the effective COP. Shifting water heating from natural gas to electricity eliminates on-site greenhouse emissions. Heat pumps use less than a third of the energy of electric resistance heaters, reducing emissions from electricity generation and substantially reducing operating cost. Heat pumps can also leverage available renewable energy to eliminate emissions altogether.

Implementation

Barriers: Cost and technology

Heat pump water heaters have a higher first cost when compared with gas-fired water heaters. They also have a larger footprint, potentially increasing construction cost.

Operating cost of heat pumps varies considerably across the US, due to the range of electricity rates. Throughout the US, the cost of natural gas is much cheaper than electricity.

With higher first costs and operating costs, simple financial analyses typically do not pencil out. Reduced carbon emissions must be factored into the equation with costs of offsets.

Barriers: Codes

In some jurisdictions, hospital building codes require emergency power systems with onsite fuel storage to operate domestic water heaters. With this requirement, the first cost of a heat pump system will include any additional equipment added to provide adequate emergency power capacity.

Barriers: Culture

Gas-fired boilers have long been the preferred choice for domestic hot water production by engineers, architects, and building owners. With cheap first costs, cheap operating costs, and unmatched reliability, many engineers are reluctant to design buildings with alternative technologies such as heat pumps in fear that costs will be higher or the system will not be reliable. Early heat pump technology brought with it a variety of problems including failing equipment, poor energy performance, controls issues, and lack of operator education. Researchers and manufacturers of newer heat pump technology must address these past shortcomings and ensure this technology can match or exceed its fossil-fuel consuming rival.

Currently, not many systems are operational in the United States. Europe and Asia have more systems in place.

Strategy

If the goal of a facility is decarbonization and electrification, CO2 heat pump water heaters are the best alternative for fossil fuel combustion-free domestic hot water. The high temperature output capability and low-GWP refrigerant makes CO2 heat pumps the most efficient and environmentally friendly option. Heat pump water heaters should be considered for all new buildings and major retrofits and for equipment replacement in existing buildings.

Many of the barriers to implementation involve costs. Incentives can lessen first cost burdens. Both the Inflation Reduction Act and Infrastructure Jobs Act have buckets of money to fund decarbonization technologies such as heat pump water heaters. More details…

Financial analysis and business case

Typical cost of these new systems would be approximately $50,000 with the potential to provide significant annual utility bill savings and payback over time.

The operating cost and energy cost savings between different options varies depending on the type of system selected. As an example, we can assume an electricity rate of $0.15/kWh and methane rate of $0.95/therm. For an air source system, assuming a 70°F water temperature rise and a COP of 3.7, the cost to heat 1,000 gallons of water will be 46 kWh times the cost of electricity. At $0.15/kWh, it will cost $6.90. For the same conditions, a gas-fired water heater with 90% efficiency will cost 6.5 times the cost per therm of natural gas. At $0.95/therm, it will cost $6.16. Operating cost, then, is roughly similar and gas-systems cost about 10% less to operate.

In water source system, where heat is recovered from the chilled water system and reused, heating is essentially a byproduct of cooling, but it is more energy-intensive to raise condenser water to 140°F than to 90°F. The extra energy works out to about 0.06 kWh per kBtu. Using the same conditions as above, the added cost of heating 1000 gallons of water would be 35 times the cost of electricity, or $5.26 for 1000 gallons at $0.15 per kWh.

2017 ORNL study