Reducing environmental impact of refrigeration technology

Assessing environmental impact of refrigeration systems

Since ozone depleting refrigerants being replaced worldwide, the environmental concerns shifted towards high contribution of refrigeration systems to global warming. Several environmental metrics are used to assess the contribution of a refrigerants and refrigeration systems to global warming. GWP is perhaps the most commonly used environmental metric. GWP is the index, which compares the global warming impact of an emission of a greenhouse gas in relation to the impact from the emission of similar amount of CO₂. GWP is an easy metric to use: the smaller the GWP, the lower contribution of a substance to the global warming.

In addition to the effects accounted by GWP, any refrigeration system indirectly affects the environment. The indirect impact is originated from CO₂ emissions from the energy production processes. In order to indicate the overall environmental impact from a refrigeration system during it operation, another environmental indicator, named TEWI, is used. TEWI accounts for the direct effect of refrigerant released during the lifetime of the equipment and the indirect impact of CO₂ emissions from fossil fuels used to generate energy to operate the equipment throughout its lifetime.

TEWI metric is more indicative than the GWP, but it is not taking in account all the relevant indirect emissions involved into refrigerant life cycle such as emissions related to the manufacture and transportation of the system and refrigerant. Hence, another indicator – LCCP - is used to account for all GWP related to the refrigeration system operation, including environmental impact of substances emitted during the process of refrigerant production and transportation.

An LCCP analysis has been performed to estimate the total environmental performance of an air/water heat pump system capable of covering the heating demand of a residential multifamily building located at various European climates. Several refrigerants have been considered in order to identify those with lowest environmental impact. The analyses has been performed for three different locations that represent European climates: Athens (warmer climate, 3590 heating hours), Strasbourg (average, 4910 h) and Helsinki (colder, 6446 h). Figure 1 presents the varying ambient temperatures and heating hours amount for these climates.

Since heat pump faces different operation conditions, its seasonal performance is varying depending on climate. For instance, refrigerant R152a is clearly more efficient in warmer climates, where it leads to a system that under the greater part of the heating season operated with better COP that alternative refrigerants. Similarly hydrocarbons reach similar or better seasonal efficiency at colder climates (see Figure 2, Table 1).

Table 1 - SCOP values for different refrigerants

The calculated values of life cycle climate performance are presented on Figures 3-5. The LCCP value of heat pump operating in colder climates is greater than LCCP values of heat pump operating in warmer climates. This can be explained with the longer heat pump annual operating time at colder climates and higher temperature lift of heat pump system. When comparing different refrigerants within specific climates, the best low GWP refrigerant is the one with highest SCOP values. This is due to the very small direct emissions of low GWP refrigerant.

The result of the study highlight how environmental impact of the system is dependent not only on the choice of refrigerant, but also on the choice of the location of a single system with identical refrigerant. This is particularly important when attempting to judge in the environmental performance of a system thought to be placed on European market by LCCP analysis at only one single climate/location.

For more details on the assumptions please refer to the original work [1].

New refrigerants for future heat pump systems

Selection of environmentally friendly refrigerant for heat pump systems continued in another work [2]. A number of lower GWP replacements have been considered as replacements to R410A in a domestic heat pump unit with 10 kW rated heating capacity. The performance of the unit was tested using R410A and modelled with other refrigerants (R32, R152a, R290, R1270, R1234ze(E) and R1234yf) with an aid of validated heat pump unit model.

As a result, only one of the analyzed refrigerants, R32, have greater than R410A performance and capacity (Figure 6). However, it is flammable and characterized by the very high compressor discharge temperatures that are significantly higher than that of R410A and can overcome safety limits at very low ambient temperatures that occur at colder European climates (Figure 7). Nevertheless, it is possible to handle the constrains of R32 as significant number of R32 based systems occurs recently on the market.

Contrary, R1234yf and R1234ze(E) have lowest capacity and COP compared to the R410A baseline. These refrigerants are the only non flammable alternatives analysed, however their use will require cycle improvements and components modification in order to reach comparable to R410A performance.

Hydrocarbons are great refrigerants for heat pump system as they are natural refrigerants that lead to significant global warming reductions at comparable COP. At the negative side is slight reduction in capacity and, of course, high flammability. It is therefore the attention should be given to safety of the equipment and its use.

New refrigerants for future supermarket systems

According to the requirements of the European regulation on fluorinated greenhouse gases, refrigeration equipment that contain high Global Warming Potential (GWP) refrigerants that have GWP equal or greater than 2500, with some exceptions, will be prohibited from placing on the European market from 1st January 2020. Moreover, the use of such fluorinated greenhouse gases to service or maintain refrigeration equipment with a charge size of 40 tonnes of CO₂ equivalent or more will be prohibited as well.

The greatest impact of the Regulation in the nearest time will be therefore on users of R404A refrigerant, which is the most popular refrigerant for supermarket refrigeration systems. Equipment users and manufacturers are therefore looking for the alternative refrigerants to replace R404A in new and existing equipment. While cascade refrigeration systems and CO₂ transcritical systems are feasible alternatives to R404A systems [3], R448A and R449A are potential replacements to R404A refrigerant in existing systems. R448A and R449A are non-toxic and non-flammable refrigerant blends that closely match the properties of R404A and have GWPs significantly lower than R404A. KTH, in collaboration with a number of industrial partners, performs a key study of drop in replacement of R404A in existing supermarket systems that currently operate with R404A. While the results of this study are yet to be published, the analytical results show that new lower GWP replacements R448A and R449A could be appropriate replacements to R448A. They bring better energy efficiency, comparable cooling capacity, at expense of the increased compressor discharge temperature, temperature glide, change in mass- and volumetric refrigerant flows and other related considerations.

The considerations presented above are based on the low temperature rig calculations obtained using the model of a real system validated with R404A measured data. Compression efficiency was obtained from compressor manufacturer for a new model of compressor rated for both refrigerants. The compressor efficiency data, presented on Figure 8 as a function of pressure ratio, indicate that there should be an energy saving of compressing the retrofit refrigerants at pressure ratios lower than 4.5 for the R448A and lower than 5 for the R449A, compared to the baseline R404A.

The results of the analysis show that an overall cooling capacity decrease for the both retrofit refrigerants (Figure 9). Where a decline between approximately 12% and 2% compared to the R404A baseline was noted for both R449A and R449A.

As for the energy performance, an increase in performance was noted on a wide range of evaporating temperatures, except for its lowest levels. The gain was noted for evaporation temperatures above −26 °C for the R448A and −28.5 °C for the R449A (Figure 10).

For additional results of the study please refer to the original work [4]. The results of an experimental work will be published in coming future both here, as well as on the project web page at bit.ly/lowGWPrefrigerants.

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