Researchers have evaluated how supermarkets can reduce emissions by using recovered heat from refrigerators to supply heat to stores.
In a paper published in Applied Energy, scientists analysed existing commercial refrigeration systems to evaluate which set up has the greatest potential for reducing emissions and primary energy usage, by using recovered heat to replace gas boilers. The team compared results from five Refrigeration Integrated Heating and Cooling (RIHC) systems, each with slightly different modifications. The paper demonstrates for the first time in UK supermarkets that RIHC systems with a thermal storage unit are able to provide significant emissions reductions and have the potential to replace gas heating boiler systems.
The team of researchers from Imperial College London and the Polytechnic University of Valencia used a reference baseline scenario and five different case studies for comparison to find out which refrigeration configuration provided the greatest reduction in emissions. With the support of a major food retailer, the researchers collected energy consumption and weather data over the course of a year from a supermarket in the North of England, which they used to evaluate energy costs, emissions and energy consumption.
The aim of the study was to understand the energy savings and emissions reduction that could be achieved by recovering the low-grade heat from commercial refrigeration systems that usually goes unutilised because it is released into the environment.
They found that an RIHC system with a thermal storage unit was the most effective, producing a reduction in supermarket emissions and energy consumption by 13% and 18% respectively. The team say that these systems have the potential to replace the gas heating boiler systems used for space heating and enable the displacement of fossil fuelled boilers without compromising business operations.
Lead author Professor Emilio José Sarabia Escriva said: “This study provides evidence that it’s feasible to use excess heat from refrigeration systems to provide heating in supermarkets and replace gas boilers. This could provide a ‘quick win’ for food retailers looking to reduce their emissions in line with the UK’s target to be carbon neutral by 2020”.
Emissions in UK supermarkets
Reducing fossil fuel consumption is pivotal to tackling climate change. Food retail is responsible for approximately 3% of electricity consumption and 1% of greenhouse gas emissions in the UK. They often inhabit large spaces which means managing significant cooling and heating demands to maintain a balance between the freshness of food and comfort for customers.
Refrigerators account for around 50% of the total electricity consumed in supermarkets as they are constantly running to preserve food. This cooling process produces a lot of heat which is released to the environment through the condenser systems, whilst at the same time gas boilers are being used to provide space heating to buildings.
Previous research has suggested that using heat recovered from refrigerators could cover 40% of space heating for supermarkets in Northern England, however real-life applications have been limited in supermarkets and testing thus far has been limited mostly to computer simulations and has not considered the impact of using thermal storage to recover heat until it is required. The research team set out to test its theories using real-world conditions, in an operational supermarket.
Calculating cooling and heating demands
The researchers built a model to simulate different operating strategies in redirecting heat from the cooling cycle to satisfy space heating demands, calculating the amount of heat that can be recovered from a fridge system and understanding the periods of excess or shortfall to meet the heating demand of the supermarket.
A controlled study was used as a baseline to benchmark conventional performance, where food cabinets and space heating were provided by the two separate systems of fridges and a gas boiler. Five additional simulations were undertaken, each with different specifications. The first used a gas boiler system to supplement heat recovered from a fridge when demand was higher than recovery, another tested an RHIC system which exclusively used recovered heat for space heating. The final three all used RIHC systems with thermal storage, but with different operating constraints for each.
The results showed that in terms of costs savings, the model using the gas boiler to supplement heat recovery saved the most money but its impact on emissions was limited.
The case study using a standard thermal storage unit with no modifications was the second most cost effective, but mostly importantly it was the most effective in terms of emissions and energy reduction. In addition to this, the ability of the unit to store heat for when it is needed increases the ratio of useful to recoverable heat by 11-12%.
Opportunity in policy change
Industry will play a major role in helping the UK reach its legal target to bring greenhouse gas emissions by 2050 and decarbonising all sectors of the UK economy through the 2020s. This research is important because it provides industry with practical examples of how they can help meet these commitments.
These research findings are relevant because it’s the first time computer models have evaluated heat recovery systems using data from real-world settings in the UK. It provides evidence that eliminating the use of natural gas boilers in supermarkets is possible, which can reduce overall emissions and energy usage. Crucially, it can be done relatively using refrigeration systems that are already in place or will be in place soon.
The analysis was carried out on CO2 booster refrigeration systems, which will become increasingly commonplace in commercial outlets due to the phasing out of hydrofluorocarbon (HFC) refrigerants. HFC are known to be a thousand times more potent in their global warming potential (GWP) than carbon dioxide. Recent EU policy banning the use of HFC in new equipment where less harmful alternatives are available, means that HFC refrigerants will eventually become obsolete and replaced with more environmentally neutral CO2 booster units.
This research is meaningful because it provides valuable insights into a viable alternative of how commercial businesses might significantly reduce their emissions. When upgrading their refrigeration systems, building stakeholders need to carefully consider adapting them for heat recovery with thermal storage units, if they are seeking to eliminate the use of natural gas in their premises.
Professor Nilay Shah, Principle Investigator on the project and Head of Chemical Engineering and principle investigator on the project said: “This is an excellent example of engineering innovation with a real-world application which can support retail businesses to achieve net zero emissions.”
The next stage for this research will involve exploring ways to maximise the impact and scalability of heat recovery systems with thermal storage. This includes looking at optimal materials, configuration and sizing of thermal storage vessels, and using models to predict control of heat recovery strategies.
To learn more about the research programme supporting undertaking this work, please visit the Imperial-Sainsbury's Decarbonisation Partnership website.
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