Environmentally Friendly Refrigerants for Heat Pumps

Water is an environmentally friendly natural refrigerant that is already being used in several heat pump or chiller processes. For efficient evaporation, the heat exchanger should be equipped with a large specific surface area.

Climate Friendly Refrigerants for Heat Pumps

Since the F-gas regulation entered into force, refrigerants that are harmful to the climate are slowly being phased out and climate-friendly refrigerants are on the advance worldwide. The lack of available market products and the search to find long-term solutions for climate friendly refrigerants has led system manufacturers to develop new products or make modifications.

Above all, synthetic refrigerants like the less known HFCs and HFOs as well as natural refrigerants like propane (R-290), propelyne (R-1270) and ammonia (R-717) are suitable as climate-friendly refrigerants for heat pumps.

Sources for more Information:

  • Climate-friendly refrigerants are compared in a well-respected study from McLinden et al. [1]

    From the refrigerants tested in this study, only a few are suitable for heat pumps. These are listed here:

    1. non-halogenated hydrocarbons (R-290, R-1270 and Dimethylether (R-E170))

    2. fluorinated and partially fluorinated hydrocarbons (R-32, R-161 and R-152a)

    3. fluorinated unsaturated hydrocarbons (R-1234yf, R-1132(E), R-1243zf, R-1252ye, R-1261ze, R-1132(Z))

    4. halogenated-oxygenated hydrocarbons (R-E143a)

    5. halogenated amins and thiols (3x n.n.)

    6. anorganic fluids (ammonia)

Challenges of Using Climate-friendly Refrigerants in Heat Pumps

The main challenge facing is that almost all of the climate-friendly refrigerants suitable for heat pumps are flammable or poisonous. Most of them are categorized in the safety groups A2L, A3 and B2 (R-717) which mandate strict requirements be fulfilled in the technical implementation.

The following aspects must be considered:

Implementation of Safety Concepts in Case of an Accident

The safety requirements for the use of refrigerants in heat pumps are defined mostly in EN 378 and EN 60335-2-40. Both standards harmonize, in part, with the machine regulations, which lays down the legal framework for the construction of heat pumps. These requirements include a variety of different safety installations for leak detection and avoidance as well as many other measures. However, some important measures affecting the safety are not included. For example, the massive reduction of the inner volume of components can play a large role in the system safety (by adhering to the charge limit prescribed in EN 378) and thus influence the appropriate location chosen for the installation.

Adaption and Optimization of Components and Operation Modes for Refrigerant Reduction

Most of the refrigerant is often located in the heat exchangers and the pipe system. Thus heat exchangers optimization to reduce the refrigerant charge can largely influence the total amount of refrigerant in the system. This can be done for  all types of heat exchangers like plate, fin-and-tube (with suitable tube diameters) as well as microchannel heat exchangers. The refrigerant distribution in the plate and microchannel heat exchangers still presents a scientific-technical challenge that is being addressed at several research institutes.

Choosing the Refrigerant

In choosing a suitable refrigerant, some aspects to be considered are: thermodynamic suitability, availability, climate friendliness, costs as well as to what extent the existing system concepts can be utilized.

Today synthetic refrigerants cost more than the natural refrigerants. On the one hand, this is due to the F-gas regulation but also to the higher manufacturing costs of the synthetic refrigerants. Therefore the market availability, costs and the system efficiency must be used as criteria when selecting an appropriate refrigerant.

As a result, refrigerant manufacturers are increasingly putting various mixtures on the market that have specific characteristics or functionalities, dependent on their composition. As a basis for the mixtures, established inexpensive A2L single-substance refrigerants like R-1234yf or R-32 are often used. Exact threshold values (GWP < 150) are set as targets and costly synthetic single-substances are mixed with these established refrigerants in order to reduce flammability, for example. The environmental compatibility and the long term stability of the mixture must be tested exactly on an individual basis.  

Natural refrigerants like R-290, R-1270, R-E170 also undergo technical tests. They are thermodynamically suitable, widely available, climate-neutral and low-priced. Their material properties allow their use at many operating conditions with only one compression step needed. As a result, the system costs are kept low and the systems compact due to the low volumetric cooling power and low viscosity with high heat transfer coefficients at the same time.

Sometimes open questions require further detailed investigations on the material and component levels. The technical, energetic and economical aspects are evaluated as needed on a case-by-case basis.

For example, compressors must be qualified and approved for operation with the given refrigerants or refrigerant mixtures.  leak tests on factory produced systems (meanwhile branch-wide standard) led to higher degree of leakproofness, making a fraction of the mixture or leaking less probable.

Research Activities at Fraunhofer ISE

Fraunhofer ISE is working on special solutions with propane, A2L liquids in heat pumps and the use of water as a refrigerant for adsorption heat pumps. In particular:

  • Design and layout for vapor compressor cycles using A3 and A2L refrigerants
  • Energetic optimization of vapor compressor and adsorption cooling cycles
  • Develop binary fluid distributors which reduce the amount of refrigerant required in microchannel evaporators
  • Test single and tandem compressor systems for energetic evaluation and also to design or analyze the oil management especially for an A2L refrigerant mixture based on the EN 13771
  • Develop safety concepts including sensors and the evaluate the leakproofness of systems and connections

The development of environmentally friendly and resource-saving heat pumps together with our customers is a central focus of our work at Fraunhofer ISE.  We carry out differential analyses on all types of climate-friendly refrigerants, which includes  natural refrigerants as well as refrigerants in the material class of the halogenated (fluoridated) olefins. Through our experience in projects with industrial customers, we have gathered many years of expertise in the development of both types of refrigerants.

For the design, evaluation and optimization of components and devices, Fraunhofer ISE has a specially equipped laboratory and experienced staff. The lab is set up to work on refrigerants of the safety groups A2L and A3. The laboratory is equipedto measure the velocity of flow at a single point or entire flow fields from 20µm up to 1000mm edge as cross-section with a resolution in the micrometer range. These measurements are carried out on components in a climate chamber with airways of 20 – 8000 m³/h where the humidity and thermal conditions can be set.

Fraunhofer ISE also maintains a variety of specialist equipment for performing experimental modal analysis, operational deflection shape analysis as well as recording events in heat pumps and in their actuators using condition monitoring as well as acoustic emission testing. We measure  standard parameters like velocity, volume flows, temperatures, humidity and pressure calibrated partly with primary and secondary standards or otherwise by the DAkks certified sensors.

R&D Services

  • Choosing and evaluating suitable refrigerants with simulations (IMST-ART, TIL-Suite in Modelica/Dymola and other software for the layout of the components).
  • Layout, characterization and optimization of operating concepts for heat pumps in normal and defrost operating modes
  • Design and characterization of thermal and acoustic (mechanical vibration) behaviour of heat exchangers with/without refrigerant flow
  • Evaluating the effect of the compressor oil on the heat transfer
  • Evaluating defrost detection, frost buildup and defrosting efficiency
  • Developing concepts to reduce the amount of refrigerant in heat exchangers and pipe systems. This is carried out using patented concepts as well as layout and qualification expertise for fluid distributors of circular pipe, fin and microchannel heat exchangers that are explicitly suited for evaporators with low refrigerant demand.
  • Measurements on single and compound compressor systems for energy evaluations
  • Design and analysis of the oil separators
  • Acoustic emission measurements as well as harmonic measurements on all relevant components
  • Creating concepts for testing components and systems for leakproofness, accelerated aging tests, test campaigns to evaluate and optimize operation
  • Measurement and characterization of components in the laboratory with integral safety concept by staff experts, certified according to the F-gas regulation, class 1
  • Device measurements carried out according to the standards EN 14511, EN 14825, EN 16147, EN 12102 (with the support of the VDE)

[1] McLinden, M. O. et al. Limited options for low-global-warming-potential refrigerants. Nat. Commun. 8, 14476 DOI:10.1038/ncomms14476 (2017).