A2L low GWP HFO based refrigerant solutions in heat pump applications to drive energy efficiency, safety, and sustainability even in building restoration

Driven by energy efficiency requirements and legal restrictions like F-Gas Regulation Heat Pump OEM supplier are looking for alternative refrigerant solutions. HFO based low GWP refrigerants can help increasing the system efficiency, expanding the operating temperature range while reducing CO2 emissions. For building restoration in particular, high flow temperatures (for existing hydronic heating system) are essential for convenient and costand energy-efficient replacements of fossil fuel boilers. A2L classified refrigerants provide extended application range at highest safety level versus A3 classified refrigerants, but R-32 still carries a Global Warming Potential (GWP) of 675 making it a transitional solution. This paper compares different relevant parameters and points out the advantages provided by HFO A2L low GWP refrigerants (like R-454B and R-454C) for a wide capacity range of air to water and brine to water heat pumps. A special focus is put on the application of building restoration.


INTRODUCTION
As part of the European Green Deal, the European Commission proposed in September 2020 to raise the 2030 greenhouse gas emission reduction target, including emissions and removals, to at least 55 % compared to 1990. Considering that almost 50 % of EU final energy consumption is used for heating and cooling, of which 80 % is used in buildings. Therefore, the Energy Performance of Buildings Directive (EPBD) (Directive 2018/844) encourages creating a roadmap for the energy renovation of existing buildings and defines a strategy which includes the heating and cooling for reaching Nearly Zero-Energy Buildings.
Several studies have positioned heat pumps as a major enabler for energy consumption reduction in new buildings: Simon Harvey: "Industrial HPs have an important role to play!" in: The role of heat pumps for decarbonization of industrial processes, 12 th IEA Heat Pump Conference 2017, Rotterdam [8] European Commission: "Identifying/estimating current heating and cooling supply by technology: (among others) -heat pumps,…" in: Annexes to the Commission Recommendation on the content of the comprehensive assessment of the potential for efficient heating and cooling under Article 14 of Directive 2012/27/EU, C(2019) 6625 final, ANNEXES 1 to 7, Brussels, 25.9.2019 [3] European Commission: "…the integration of renewable and surplus energy into buildings. … photovoltaic solar panels on the roofs, thermal storage and heat pumps" in: Communication from the Commission to the European Economic and Social Committee and the Committee of the Regions, A Renovation Wave for Europe -greening our buildings, creating jobs, improving lives, COM(2020) 662 final, Brussels, 14.10.2020 [4] Thomas Nowak (EHPA): "100 % Renewable Energy with Heat Pumps is feasible today." in: Heat Pumps; Integrating technologies to decarbonize heating and cooling, European Copper Institute, Autumn 2018 [12] When it comes to Heat Pumps, the refrigerant choice has an impact on energy consumption. While the F-gas Regulation (Regulation (EU) No 517/2014) and the Kigali amendment to the Montreal Protocol drive the adoption of new refrigerants from the Heat Pump manufacturers with the objective of reducing their Global Warming Potential, the energy efficiency still plays the major role in the selection of refrigerants and Heat Pumps system design.

Heating Capacity Needs
For a typical Central-European new building, there is a heating system requirement for a relative heating capacity of 35 W/m², while for existing buildings -depending on the thermal insulation level, a relative heating capacity of 100 to 150 W/m² is estimated (https://www.haustechnikdialog.de/SHKwissen/1410/Spezifische-Heizlast). This leads to absolute heating capacity requirements of typically 5 to 8 kW for heating system in new buildings including sanitary hot water needs and of 10 to 15 kW for heating systems in existing buildings including sanitary hot water needs. As an example, for Germany, the quantity of new buildings compared to the renovation buildings is just about one third (31 % in 2019) as per figure 1.

Temperature Levels Needs
For residential applications / new buildings, the heat pump is operating at 35 °C for underfloor heating and 60 °C for sanitary hot water. For the renovation of buildings higher temperatures are needed to replace oil or gas boilers with 55 °C for radiator heating and 60 °C for sanitary hot water. The target is to provide monovalent solutions to avoid a drop in efficiency due to usage of direct electrical heaters and to avoid an increase in CO2 emissions due to the adoption of additional gas boiler at low outside temperatures. This requires a high temperature lift at low environmental temperatures, e.g. -25 °C to + 60 °C. This high temperature lift requirement cannot be provided by all refrigerants.

PERFORMANCE COMPARISION OF REFRIGERANTS
A calculation tool has been developed to compare the performance of different refrigerants for Heat Pump applications. The calculation tool, developed in Excel®, uses Refprop 10.0 (Reference Fluid Thermodynamic and Transport Properties) for the fluid properties. It is based on the basic thermodynamic equations linking pressure, temperature, enthalpy, and entropy to determine the capacity and coefficient of performance (COP) of a thermodynamic cycle. Figure 2 shows the capacity calculation results from the mentioned tool for the same compressor displacement. Except for R-454C the capacities are close to R-410A. For R-454C a bigger compressor would be required to achieve the same heating capacity, compared to R-410A.   Figure 3 shows the COP calculation results from the mentioned tool. For R-454B and R-452B the COP at higher ambient temperature is above that of R-410A, while at lower ambient temperatures it is slightly below that of R-410A. For R-32 the COP drops considerably from high ambient temperature conditions to lower ambient temperatures. The only refrigerant performing above the values of R-410A for all operating conditions is R-454C which is also the one offering the lowest GWP (148).

Figure 3: COP of different refrigerants at A/W conditions for renovation buildings (W55)
Drop-in tests performed on A/W (R-410A) and B/W (R-407C) Heat Pumps by Fraunhofer ISE Institute [6,7] show that just changing refrigerants without any modification to the heat pump system generally does not lead to an improvement. ). This is very beneficial for the application in buildings renovation, as often the existing hydronic system of radiators requests rather high flow temperatures. For sanitary hot water production, the additional legionella controls are not necessary anymore as a condensing temperature of +60 °C can still be achieved directly at -30 °C ambient temperature.

EMISSIONS IMPACT IN VARIOUS COUNTRIES
Although there is no combustion in a vapor compression cycle Heat Pumps, there can be direct emissions caused by leaking refrigerant with certain GWP from the system. Indirect emissions are always given due to the electrical power consumption of the Heat Pump. Both sources of emissions are further investigated in the following chapter.

Emission Factor in EU Countries (indirect emissions)
Indirect emissions from Heat Pumps are generated by the process of generating electrical power requested to make the Heat Pump operating. Here, the kind of power generation and the efficiency of this process determines the resulting emission factor of electrical power generation. The European Environment Agency (EEA) has published the evolution of emission factor or CO2 emission intensity for EU countries until 2016 as shown in figure 4 [5].

Total Emissions
The total emissions sum up from direct emissions due to refrigerant leakage and from indirect emissions caused by production of electrical energy as shown in figures 6 and 7, calculated with a SCOP of 3,5.

WAYS FOR REACHING THE THEORETICAL EFFICIENCIES
Several theoretical calculations, comparing the performance of different refrigerant solutions, have shown that there is a huge potential for improving system efficiency with low GWP A2L refrigerants. Several drop-in tests with the same refrigerants have shown that the theoretical calculated efficiencies cannot easily be achieved by dropping the high-performance refrigerant into an existing Heat Pump system. The conclusion is very clear: The heat pump system should be designed "around the refrigerant"! As many low GWP A2L refrigerant blends are zeotropic mixtures, they present a temperature glide which must be considered when designing and sizing heat exchangers (evaporators and condensers). This is not a novelty, but has it been addressed successfully e.g. when using R-407C? Ignoring this refrigerant behavior would lead to non-optimal solutions. For evaporators this can be used in a beneficial way to increase capacity and efficiency. To obtain the same performance, condensers must be dimensioned differently, with a slightly bigger size compared to those adapted to azeotropic refrigerants. When operating with zeotropic refrigerant blends, the implementation of dedicated refrigerant circuit design might be beneficial for increasing system capacity and efficiency.
Using a compressor which has been designed for a "similar" refrigerant, but not optimized for low GWP A2L refrigerants will work in general in a new system, but the maximum performance -capacity and efficiency wise -will not be achieved. A dedicated compressor shall be used to enable maximum system efficiency.
There are several examples of existing compressors in the market showing that the performance and the operating temperature range strongly depend on the choice and design of the compressor.

CONCLUSIONS
Building's renovation requires an extended operating temperature range compared to new buildings. This is due to the existing hydronic system consisting of heating radiators requiring higher water flow temperature to provide the heating capacity needs. Due to often missing or less efficient thermal insulation of old buildings, usually the required heating capacity is higher as well.
With existing refrigerant technologies -namely R-410A -these needs cannot be satisfied across the entire year, as at very low ambient temperatures, additional electrical heaters in the buffer tank or separate oil or gas boilers can provide the necessary heating capacity. Both solutions are characterized by much lower efficiency and contribute to much higher system emissions at those conditions. With the low GWP (148) refrigerant R-454C, the system efficiency can be improved compared to R-410A over the whole operating range and all temperature conditions. In addition, a flow temperature of 75 °C can be easily achieved without additional technical complications, like vapor or liquid injection. This way, the system cost can also be reduced. It has been shown that the direct emissions contribution from R-454C represents less than 1 % of total emissions over the complete lifetime of a Heat Pump. This leads to the conclusion that system efficiency is the main parameter influencing system emissions through its contribution to reducing the operational power consumption.