Is energy efficiency more important than low GWP?
We are all aware of the F-Gas legislation and the drive to low GWP refrigerants but this is only a small part of the overall solution to reduce climate changing emissions. Choosing a low GWP refrigerant does not guarantee a better energy efficiency and in some cases, it may even lead to an increase in the lifetime total emissions from the system if a lower efficiency option is selected. As well as refrigerant choice, there are other actions that can be taken to maintain or improve energy efficiency, reduce total emissions and in most cases they will even pay for themselves over the lifetime of the equipment.
Good Maintenance and Correct Control Settings
This is probably the easiest strategy to employ but often end users do not see the benefits of spending money on a system that appears to have nothing wrong with it. Simply cleaning the heat exchangers, using reliable leak detection methods, ensuring fans are operating correctly and checking the system operating parameters can have a significant impact on system emissions.
Even moderate fouling of a condenser will lead to an increase in condensing temperature of the system. Equally if the system has a high/condenser pressure controller, checking the setting is not causing the system to operate at excessively high conditions or is set correctly for the refrigerant used (for example after a retrofit) can have a big impact.
Figure 1 shows the impact of increasing condensing temperatures with R-449A at low and medium temperature conditions. Even a very small increase in the condensing temperature can lead to significant decreases in energy efficiency, for example, an increase from 25°C to 27°C condensing temperature can lead to a 5-8% loss of compressor energy efficiency.
It doesn’t take much fouling of the condenser coil to cause a small pressure increase and it is easily prevented by regular use of cleaning products such as Frionett condenser coil cleaner from Climalife, which will prevent build-up of debris that can block the air flow through the coils, maintaining the optimum energy efficiency.
In situations where condenser fans have either failed or are malfunctioning, energy efficiencies could easily drop by 25% or more.
Table 1 – Pressure differences of R-404A alternatives
Systems which have been retrofitted and have high pressure controllers can often lose some of the energy efficiency benefits available if the controllers are not correctly adjusted. For example, if a R-404A system is retrofitted to a lower GWP alternative, it is very likely the alternative product has a lower pressure temperature relationship (Table 1). If the pressure controller is not lowered to match the new refrigerant, then the energy efficiency of the system could be 5-10% lower than for the optimum setting.
Different refrigerants do have different energy efficiencies when used in the same equipment. In practice there are many potential variables that can affect this, but if it’s thermodynamically proven then it should be possible to achieve improved energy efficiency if the system is set up correctly.
Table 2 – Theoretical cycle calculation comparisons
For example, using theoretical cycle calculations for medium (-8°C) and low temperature (-35°C) refrigeration conditions it can be seen in table 2 that most of the lower GWP alternatives for R-404A have a higher energy efficiency. It can also be clearly seen in table 2 that low GWP does not necessarily mean good energy efficiency. The lowest GWP refrigerant in table 2 is R-744 (GWP=1), but the energy efficiency of a basic booster system compared to other refrigerants in basic direct expansion (DX) systems, delivering the same cooling capacity, is much lower which will lead to higher total emissions and higher energy costs. The energy efficiency of R-744 systems is often enhanced by using internal heat exchangers, parallel compression, and ejectors, but the energy efficiency of normal DX systems can also be improved by using internal heat exchangers or mechanical subcooling. This however, is often not considered for these types of systems even though the cost is likely to still be less than that for a basic R-744 system.
This is a hugely underutilised technique has the potential to greatly reduce energy usage by re-using the waste heat produced from the refrigeration system, or indeed any process that produces excess heat, that in the past has been expelled to the environment. A white paper published by Chemours in July 2021 considered heat recovery from a supermarket of approximately 2300m2 sales floor area using either R-454C or R-744 and compared the results to a basic R-404A refrigeration system with either a gas boiler or air source heat pump (AS HP) supplying all the heating requirements. Numerous (62) heat recovery scenarios were considered and the results for a typical UK climate (Leicester) demonstrated the huge potential for lowering total system emissions and life cycle costs. The majority of savings are achieved by reducing energy consumption, with the top 4 scenarios (all using R-454C) reducing emissions by 66-70% compared to the gas boiler baseline and 44-50% compared to the AS HP baseline.
It is clear from the results of this study that heat recovery will greatly decrease the energy consumption with any refrigerant but also clearly shows that the lowest GWP refrigerant does not necessarily give the best result.
With governments setting targets for reduced emissions and energy usage, it is clear the refrigeration industry has an important role to play. To achieve the maximum reductions available, it is important to thoroughly investigate all the options and not just make decisions on a single parameter, such as refrigerant GWP. As shown in this article, low GWP does not necessarily mean high energy efficiency and a poor choice of refrigerant technology may even lead to an increase in lifetime emissions, energy usage and operating cost. Advice on which solutions best fit your requirements is readily available and the team at Climalife have the tools and knowledge to lead you to an eco-efficient future.
* Theoretical calculations made for a 4kW LT, 10kW MT cooling load, comparing a basic flash gas bypass R-744 (i.e. no internal heat exchangers, parallel compression or ejectors) system with 2 individual systems for the other refrigerants. Superheat (12K), subcool (2K) and compressor efficiency (0.65 MT and 0.64 LT) values were the same for all refrigerants.