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.
Fraunhofer Institute for Solar Energy Systems ISE