A life cycle assessment (LCA) of residential air-to-water heat pumps found that ammonia (R717)-based units are more environmentally friendly than those using synthetics or hydrocarbon refrigerants.

The assessment was conducted by the Institute for Energy Efficiency Buildings and Indoor Climate at RWTH Aachen University in Aachen, Germany.

Hannah Romberg, Research Associate, E.ON Energy Research Center and the Institute for Energy Efficiency Buildings and Indoor Climate at RWTH Aachen University, presented the findings at the International Energy Agency (IEA) Heat Pump Conference held in Chicago, Illinois, May 15–18.

Heat pumps offer “huge” greenhouse-gas-emission-reduction potential, depending on the refrigerant, Romberg said. “However, they could increase the impact on other environmental categories, which is why we need an LCA to avoid burden shifting from one environmental category towards others.”

Under the LCA definition, unit operation had the largest impact on environmental categories. “Therefore, the electrical demand has the highest influence on the environmental impact,” said Romberg. 

“Across all refrigerants, R717 causes the lowest environmental impacts in all categories due to high efficiencies,” the Institute says in a paper submitted for the IEA conference, despite the refrigerant being used more often in the commercial and industrial heat pump markets than the residential one.

“The system boundaries [of the LCA] include the production of refrigerant and heat pump, the operation of the heat pump, any leakage of refrigerant, and upstream processes, e.g., materials and energy,” says the paper. The LCA did not include unit or refrigerant recycling due to a lack of data.

The LCA found the environmental impact of refrigerant production and leakage was negligible for the hydrocarbons and ammonia refrigerants. There was a slight impact on climate change from HFC refrigerant leaks and a bigger effect on ozone depletion for synthetic refrigerant production due to leakages in the production chain, Romberg said.

In heat pump production, the largest environmental impact was in the human toxicity and resource consumption categories. “[However], it is also noticeable that the impact of ammonia heat pump production is comparatively low in these categories, which can be attributed to the absence of copper,” Romberg added. Copper is not approved for use with R717 since ammonia “attacks” copper and copper alloys.

Compared to gas heating, the Institute found heat pumps reduced environmental impacts in four of the 16 categories, significantly reducing greenhouse gas emissions. However, “in the remaining 12 environmental categories, a heat pump causes higher environmental impacts,” such as on resource and water consumption, the Institute said, describing this as “burden shifting.” Further research has to assess the relevance of the identified burden shifting, the Institute added. Increasing heat pump efficiency could further reduce environmental impacts.

“Across all refrigerants, R717 causes the lowest environmental impacts in all categories due to high efficiencies.”

The Institute for Energy Efficiency Buildings and Indoor Climate at RWTH Aachen University

Assessment scope

The assessment evaluated seven refrigerants in a “simple” air-to-water heat pump cycle to provide residential space heating in a Western German climate through a 20-year life span across 16 environmental impact areas. The operating ambient temperature profile ranged from roughly -10 to 30°C (14 to 86°F) with an indoor air temperature set at 21°C (70°F), the Institute said. 

The refrigerants assessed included two HFCs: R410a and R32; an HFO: R1234yf; three hydrocarbons: R290, R1270 and R600a; and ammonia. Not included in the assessment was the natural refrigerant CO₂ (R744).

For the natural refrigerants, primary resources for the LCA came from the commercial database ecoinvent, allowing direct modeling. Such data sets do not exist for synthetic refrigerants, so the modeling was based on the literature data outlined in the submitted paper, Romberg said.

Based on the data sheets, the LCA used a specific heat pump size of 18kg/kW (141.5lbs/TR). “We made the assumption of the specific weight being constant because we could not find any refrigerant-dependent data,” Romberg said.

 “Using a TEASER framework for the simulation of a two-story single-family house, we have a heat demand of about 20,000kWh with a maximum heat output of 7.5kW [2.1TR],” she added. The paper lists the specific refrigerant charges for the refrigerants as 0.3kg/kW (2.5lbs/TR) of R410a, 0.25kg/kW (2.0lbs/TR) of R32, 0.35kg/kW (2.8lbs/TR) of R1234yf, 0.15kg/kW (1.2lbs/TR) of R290, R1270 and R600a and 0.1kg/kW (0.8lbs/TR) of R717.

The heat pump model, having fluid-dependent efficiency, determined the optimal SCOP of each refrigerant by varying the pressure levels accordingly, Romberg said, giving R410a a SCOP of 3.71; R32 – 3.99; R1234yf – 3.82; R290 – 4.2; R1270 – 4.19; R600a – 3.81; and R717 – 4.27. The researchers calculated the electrical input required for each refrigerant based on the results.

With R410a as the reference refrigerant, ammonia gave up to a 20% lower impact on climate change, Romberg said. Ammonia also had the lowest impact across the remaining 15 environmental areas in all four system boundaries.

Grid influence

“If we want to reduce the operational emissions, we basically have two options,” Romberg said. “We could either increase the efficiency of the heat pump or have a look at the environmental impact of the grid electricity.”

The German electricity (GE) mix of 2018 was used for the heat pump LCA, Romberg said. Using available data from ecoinvent, the Institute looked at four other grid electricity possibilities: GE transitioning to sustainable development scenario (SDS), SDS, wind energy and photovoltaic (PV).

Using heat pumps with static SDS reduces nine out of the 16 environmental impact categories and GHG emissions by 79% over gas heating, the report said. Wind power reduces GHG emissions by 89% with reductions in 11 categories. PV reduces GHG emissions by 81% with reductions in 8 categories.

Using heat pumps reduced environmental impact in climate change, ozone depletion, photochemical ozone formation and energy resource consumption in all five electricity mixes, the report says. “However, eutrophication, freshwater and human toxicity; the environmental impacts of carcinogenicity; and resource and water consumption all increase when switching to a heat pump for all five electricity mixes.”

Future research

“This work is intended to provide a basis,” the Institute said, adding that future work should consider the following:

• other parts of the building energy system, such as the distribution system, the buffer tanks, or the building envelope based on each refrigerant’s volumetric refrigerant capacity;
• using dynamic LCA for improvements in the supply chains of materials and refrigerants;
• unit and refrigerant recycling; and
• analyzing the causes associated with increased environmental impact in the 12 affected categories due to burden shifting.