IEA Heat Pump Centre Newsletter: views on supermarket refrigeration

Using heat pumps for energy recovery in supermarket refrigeration systems, Vasile Minea, Hydro-Québec Research Institute, Canada

The article by Minea points out that waste heat recovery with heat pumps could be a key element of efficient supermarket operation, especially in cold and moderate climates. It is shown that two-stage heat recovery concepts with desuperheating coils and refrigerant-to-refrigerant heat pumps allow efficient heat recovery, while reducing the artificial increase of compressors’ head pressures by up to 35 %. Totally fluid secondary refrigeration systems allow for the efficient recovery of heat for space and/or domestic/process hot water heating.

More specifically, the article compares the basic heat recovery method in direct-expansion supermarket refrigeration systems with other concepts using heat pumps and direct heat recovery coils, including:

• Two-stage systems: The first stages of these systems contain refrigerant-to-refrigerant heat recovery coils (HRC), and the second stages include refrigerant-to-air heat pumps.

• Refrigeration systems with warm heat rejection loops: Heat recovery from such systems reduces the primary refrigerant charges and also offers energy saving opportunities in supermarkets.

• Totally secondary fluid systems: Concepts with secondary fluid (brine) loops reduce the required charge of primary refrigerants by more than 60%. The low-temperature zone (LT) contains a warm secondary fluid loop for heat recovery with heat pumps and heat rejection via an air-cooled liquid cooler. The brine-to-air heat pump units use the waste heat rejected by LT compressors as a heat source to heat supermarket areas such as entrances, merchandise reception and storage areas, and community rooms.

• CO₂ refrigeration systems: Both secondary loops with CO₂ evaporators and circulating pumps as well as CO₂ cascade systems offer opportunities to use secondary fluid (brine) rejection loops with air-cooled coolers. Depending on the brine temperature during the cold season, heat pumps can be installed upstream or downstream of the brine coolers. In both cases, three-way valves allow diverting the required brine flow through the heat pumps in order to heat the supermarket space and/or domestic/process hot water.

Modeling Supermarket Refrigeration with EnergyPlusTM, Therese K. Stovall and Van D. Baxter, Oak Ridge National Laboratory, USA

Developed by the U. S. Department of Energy (DOE), the EnergyPlus energy simulation software package is a tool that can be used to examine the energy use of any building, including supermarkets. Store refrigeration systems are modelled with a logic and system-driven approach, as opposed to the pipe and node approach used to model other building systems, thus simplifying input requirements for users and limiting the system configuration to a predefined set of options.

The article by Stover and Baxter discusses how the EnergyPlus software has been revised to enable modelling of newer system configurations, with one of the most significant changes being the expanded ability to transfer heat between and among multiple refrigeration systems. The following changes have been introduced to the model:

• Water-cooled and evaporative-cooled racks as heat-rejection options to the rack model.

• Separate models for the condensers and compressors, permitting a better examination of the energy use impact of variable condensing and evaporating temperatures, as well as increasing model opportunities for heat rejection and subcooler arrangements.

• A model for walk-in coolers exchanging energy with multiple conditioned zones.

• Capabilities to accommodate additional refrigeration systems and components, such as secondary loops (including low temperature CO₂), shared condensers, cascade condensers, and subcoolers.

In the future, new model features will include transcritical CO₂ compressors as well as additional compressor staging logic and a primary cycle liquid-over-feed arrangement. A model for a refrigeration system appropriate for use in a refrigerated warehouse is also under development.


Trends and perspectives in supermarket refrigeration, Michael Kauffeld, Karlsruhe University of Applied Sciences, Germany

In his paper, Kauffeld discusses different ways of implementing measures for the reduction of direct and indirect emissions.

• Reduced refrigerant charge: The refrigerant charge inside the heat exchangers can be reduced by up to 80 % using minichannel heat exchangers. Another possibility is the application of indirect refrigeration systems, very common in Sweden where refrigerant charge per system has been limited to some 30 or 40 kg for many years. The use of distributed systems is gaining considerable market share in the USA. They allow for 5 to 8 % lower energy consumption and about 30 to 50 % lower refrigerant charge than comparable R404A direct expansion (DX) systems.

• Refrigerants without, or with very low GWP: Looking at the Global Warming Potential (GWP) of refrigerants, Kauffeld discusses different systems with natural refrigerants, including R744 cascade systems or central multiplex systems with transcritical CO₂ that are gaining popularity in Europe and slowly in the rest of the world. He estimates that over 400 transcritical CO₂ stores have been built to date in Europe. Energy efficiency is usually better than for a comparable R404A system during outdoor temperatures below approximately 12 °C, equal to R404A between 12 and 26 °C and slightly lower at higher ambient temperatures.

Kauffeld then lists measures that could be employed during the design and construction phase of a supermarket refrigeration system in order to reduce energy consumption, including the use of glass doors or covers instead of open cabinets, hot gas defrost instead of electric defrost, flooded evaporators, free cooling and heat recovery. Finally, he points out the potential to use renewable energy in supermarkets as these have rather large roof areas. Indeed several supermarkets already use renewables in Germany and the UK.