This guide will summarize and present some scientific research to assess why the CO₂-based R744 refrigerant is being touted as the solution to heat pump water heating in colder climates. Using multiple third-party research reports, this article begins by assessing the performance of two of the most common refrigerants in heat pump water heaters today (R134a and R410a). Highlighting the challenges faced by these two refrigerants in heat pump water heaters, the article explores how R744 mitigates their challenges. By doing so, the guide attempts to provide a technically sound response to industry-wide fears over the effectiveness of heat pump water heaters in colder climates like the American Midwest region. Continue reading to find out if the adoption of R744 along with other technical improvements have made electric heat pump water heaters to be as efficient as traditional forms of water heating.
You can access the full white paper containing all the information in this article as well as the complete set of results and citations for sources used in this piece by filling the form below.
With several states banning the use of natural gas in commercial properties, there has been a serious push towards the mass electrification of water heating, with heat pump water heaters taking center stage due to their promise of higher efficiency. Some industry experts have even claimed the advent of heat pump water heaters to be a product of California Green Tech’s lack of understanding of differing geographies. While it is crucial to note that any disruptive technology takes time before it reaches a level of perfection required for mass-scale adoption, water heating is far too crucial for businesses and commercial properties to accept any change that can affect the function’s effectiveness. Hence, it is completely understandable that businesses and HVAC professionals in regions like the American Midwest are hesitant to embrace heat pumps as their go-to power source for water heating.
While heat pumps promise heightened efficiency and cost-savings, such claims have frequently not stood up in harsh, colder climates. A comprehensive study by Willem et al in 2017, titled “Review of Energy Efficiency and System Performance of Residential Water Heaters” began by highlighting the failure of mass-scale heat pump water heater adoption in the 1980s. Introduced with a high promise of heightened efficiency and cost-savings by as many as 17 manufacturing companies, the earlier models “did not achieve the initial goal of balancing between cost and performance to meet market needs while addressing the issues plaguing the HPWH systems from earlier period.” Some common issues associated with these models involved refrigerant leaks, component wear and tear and control system failures, among several others.
Most heat pump water heaters available currently use R134a as their refrigerant, mainly because of its continued use as an established product. On the other hand, several models have begun to utilize R410a. R134a and R410a are hydrofluorocarbons with zero Ozone Depleting Potential and Global Warming Potentials of over 1000.
However, R134a has shown to exhibit limited heating capacities and higher costs due to the incorporation of design elements to prevent refrigerant leakage. On the other hand, R410a could potentially mitigate these problems but needs to operate at higher pressure, causing increased vibration and noise issues.
Moreover, with a recent push towards sustainable technologies, reduction of greenhouse gas emissions has become a top priority. Since the R134a and R410a refrigerants are hydrofluorocarbon-based, they are far from perfect at controlling greenhouse gas emissions despite having zero Ozone Depleting Potential. To summarize:
| R134a | R410a | |
|---|---|---|
|
Characteristics |
Ozone Depleting Potential: 0 Global Warming Potential: 1430* |
Ozone Depleting Potential: 0 Global Warming Potential: 2088⁺ |
|
Pros |
- Zero Ozone Depleting Potential - Can operate at a lower pressure than R410a, leading to lower noise issues |
- Zero Ozone Depleting Potential - Better heating capacity in comparison to R134a |
|
Cons |
- Limited Heating Capacity compared to R410a
- High Global Warming Potential leading to it being potential phased out by the government , which would cause shortages and increased pricing - Prone to leakage and needs special anti-leak design elements which increases costs |
- Needs to operate at a higher pressure than R134a leading to vibration and noise issues
- High Global Warming Potential leading to it being potentially phased out by the government, which would cause shortages and increased pricing |
*https://www.infraserv.com/en/services/facility-management/expertise/f-gas/refrigerant/specific-refrigerant/r-134a.html
⁺https://www.infraserv.com/en/services/facility-management/expertise/f-gas/refrigerant/specific-refrigerant/r-410a.html
Despite the above-noted disadvantages, heat pump water heaters with either of the two refrigerants have shown to provide significant savings on electricity bills compared to water heaters that use traditional, electric resistance heating elements.
The above conclusion was drawn by a study conducted by Kate Hudson, Bethany Sparn, and Dane Christensen for the National Renewable Energy Laboratory, titled “Heat Pump Water Heater Technology Assessment Based on Laboratory Research and Energy Simulation Models” in 2012. We will be summarizing the results in more detail below. You can also read the research paper using the button provided:
APA Citation:
Hudson, K., Sparn, B. and Christensen, D. (2012) “Heat Pump Water Heater Technology Assessment Based on Laboratory Research and Energy Simulation Models,” in Papers Presented at the 2012 ASHRAE Winter Conference in Chicago, Illinois: January 21-25, 2012. Atlanta, Ga: American Soc. of Heating, Refrigerating and Air-Conditioning Engineers. Available at: https://www.nrel.gov/docs/fy12osti/51433.pdf.
Hudson et al. made use of the TRNSYS HPWH Energy Simulation software to simulate the performance of various heat pump water heaters (with R134a/R410a refrigerant). The TRNSYS simulation software, developed at the University of Wisconsin, is an industry-wide accepted simulation program that can closely match the performance of various built systems in different geographies. The paper’s authors simulated the performance of five different heat pump water heaters (three with R134a refrigerant and two with R410a) in six locations across the United States (Atlanta, GA; Chicago, IL; Houston, TX; Seattle, WA; Los Angeles, CA; and Phoenix, AZ) while also dividing each location’s simulation for a conditioned and an unconditioned space. Here are the average savings recorded:
Conditioned Space: An average savings of 50.98% was recorded in comparison to electric resistance water heaters.
Unconditioned Space: An average savings of 45.98% was recorded in comparison to electric resistance water heaters.
To access the full set of results, specific to each location with more details, please fill the short form at the end of this blog to access the complete whitepaper.
While the above results seem impressive at first, there are several caveats associated with the findings that are in line with the challenges associated with the performance of heat pump water heaters in colder climates:
Firstly, it was found that the savings were significantly lesser in colder climates. For instance, energy savings in a conditioned space were 40.21% and 38.99% in Chicago, IL and Seattle, WA respectively. On the other hand, the savings in an unconditioned space were 30.82% and 42.1% in Chicago, IL and Seattle, WA respectively.
However, when compared to gas-powered water heaters, heat pump water heaters with R134a/R410a were proven to have minimal effects on energy consumption savings. In the case of colder climates, the heat pump water heaters displayed much higher consumption of energy compared to their gas counterparts with the difference in Chicago, IL and Seattle, WA (unconditioned space) being -39.7% and -16.47% respectively.
Secondly, since the above results were based on the TRNSYS energy simulation software, frosting or evaporator icing was not considered. Hence, the unimpressive results recorded for colder climates could be even worse if frosting or evaporator icing is taken into account: a phenomenon that is a real possibility when operating air source heat pumps in climates with a low ambient temperature.
Willem et al. summarized the effects of frosting on air source heat pump water heaters as follows:
“Heat pumps that operate under low ambient air temperature (below 40 °F) will have frost build-up on the coil surface. Frost accumulation and ice formation increase thermal resistance and pressure drop across the heat exchanger. This process restricts airflow and reduces heat transfer across the evaporator, causing a drop in system heating capacity, potential mechanical problems, and system shutdown. Frost conditions will eventually lead to failure to meet load demand, frequent activation of resistance element, and reduction of overall efficiency.”
Heat pumps that operate under low ambient air temperature (below 40 °F) will have frost build-up on the coil surface. Frost accumulation and ice formation increase thermal resistance and pressure drop across the heat exchanger. This process restricts airflow and reduces heat transfer across the evaporator, causing a drop in system heating capacity, potential mechanical problems, and system shutdown. Frost conditions will eventually lead to failure to meet load demand, frequent activation of resistance element, and reduction of overall efficiency.
Thirdly, the heat pump water heater units studied were observed to have far lower recovery rates than their electric/gas counterparts. This indicates that current heat pump water heaters that use R134a or R410a refrigerant do not perform well in providing adequate hot water at the set temperature when the storage tank has been completely utilized. A decrease in recovery rate between 41% and 46% was noted for all the heat pump water heater units studied. Since the units studied were tank-styled, the low recovery rate can be theoretically improved by a larger storage tank. However, this would indicate higher space requirements, in conjunction to additional space needed by the heat pumps.
Furthermore, several air source heat pump water heaters might have standard electric resistance heating elements as backups. When recovery is needed, the air source heat pump water heater might utilize its electric resistance heating elements based on the system’s control logic. In such a scenario, the energy-saving effects of heat pumps would be mitigated in high-demand settings.
To summarize, the current problems associated with using R134a and R410a heat pump water heaters are:
Given the previously mentioned roadblocks associated with R134a and R410a refrigerants, the CO₂-based R744 refrigerant has garnered strong attention in recent years. Not being hydrofluorocarbon-based and having a Global Warming Potential of 1 (which is more than 1000 times lower than R134a and R410a), R744 comfortably passes the test of environmental sustainability.
In a 2011 research paper, titled “Performance Improvements in Commercial Heat Pump Water Heaters Using Carbon Dioxide,” Bowers C.D., Elbel S., Petersen M., and Hrnjak P.S. noted that one of the earliest uses of the R744 refrigerant for heat pump water heaters was in Japan in the mid-1990s. Termed “EcoCute,” these R744-based heat pump water heaters have been documented to perform at a COP between 4.1 and 4.8. The mass-scale adoption of Eco-Cute water heaters in Japan was accelerated by fiscal government incentives. Moreover, they have been reported to have impressive performance even in colder climates, with temperatures during a Japanese winter usually falling within the range of 30 to 45 degrees Fahrenheit.
When it comes to the American context, there has been an indication of a push towards electrification with generous government rebates in various locations which could speed up the adoption of R744-based air source heat pump water heaters (also called CO₂-based air source heat pump water heaters). However, the concern of performance reliability in colder regions (with several areas in the American Northwest and Northeast being much colder than Japan) is still valid.
Additionally, another area of concern is the application of air source heat pump water heaters in large commercial applications. As mentioned earlier, R134a and R410a-based air source heat pump water heaters performed poorly in high demand settings. Can an R744-based air source heat pump water heater replace traditional water heaters in multi-family buildings and other commercial applications?
Several studies have been conducted to assess the performance of R744 refrigerants in colder climates. These studies are summarized concisely by researchers, Rajib Uddin Rony, Huojun Yang, Sumathy Krishnan and Jongchul Song in a 2019 paper titled “Recent Advances in Transcritical CO₂ (R744) Heat Pump System: A Review.” They point out that CO₂-based air source heat pump water heaters can achieve a COP of 3.1 for an outdoor ambient temperature of -4 degrees Fahrenheit. Moreover, they also note that CO₂-based heat pump water heaters can reduce energy use by 75%.
Additionally, the Minnesota Department of Commerce also conducted a study in 2015 using existing field research and applying it to Minnesota’s hot usage patterns and geographical conditions. While the refrigerant used was not mentioned, the study concluded that heat pump water heaters can be an attractive option for Minnesota homes with as much as $215 of energy savings each year in a single-family home. If we assume that the study used an R134a/R410a refrigerant, it can be safely concluded that such savings would be significantly higher for an R744 refrigerant-based heat pump water heater given their heightened performance results summarized by Rajib et al.
What justifies the increased efficiency of the R744 refrigerant is the temperature glide in CO₂ exothermic process that produces high heating capacity. As interpreted by Willem et al., “in other words, it could heat up water to a relatively high temperature in a short period of time.”
In other words, it could heat up water to a relatively high temperature in a short period of time.
Such studies have started to further prove that R744 could be the antidote when it comes to mitigating reduced performance of R134a and R410a refrigerants in cold climates. The studies prove that R744 can safely mitigate the first 4 challenges of R134a and R410a refrigerants outlined in this document.
However, there is still significant scope for improvement. Moreover, the challenges of frosting and low recovery rate still remain. However, these can be mitigated by innovation in engineering design. Such mitigation would typically be dependent on each heat pump water heater’s build, regardless of the usage of R744.
For instance, it has been suggested by Bowers et al. that an internal heat exchanger could increase energy savings by 10%. Furthermore, the 2022 paper, “Field Measurement of Central CO₂ Heat Pump Water Heater for Multifamily Retrofit” by Adria Banks, Colin Grist, Jonathan Heller, and Hyunwoo Lim concludes that a single pass heat exchange technology can help deliver reliably hot water more consistently as opposed to a multi-pass system.
Moreover, Willem et al.’s study also summarizes that advanced thermal storage and schedule optimization could further increase an air source heat pump water heater’s operation efficiency. Plus, they also suggest that a heat exchanger could assist with defrosting. If such design changes are to be implemented, R744 or CO₂-based air source heat pump water heaters could comfortably reduce the built environment’s reliance on traditional forms of water heating.
Most of the research presented in this paper is relatively new. As of today, not many air source heat pump water heaters have been able to address all the challenges associated with the revolutionary technology.
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