Energy efficiency measures - Commercial - Supermarket - Secondary loop - Heat recovery / Canada
Introduction
A supermarket chain is building a new store and wants to innovate by installing a completely secondary loop refrigeration system for distributing cold to the display cases and implementing extensive heat recovery strategies. The RETScreen Software Energy Efficient Arena & Supermarket Project Model (Version 3.1) has been used to compare, in terms of green house gas (GHG) reduction and energy savings, the conventional refrigeration system design with the proposed package of improvements.
Building information
The new supermarket will be located in the Montreal (QC) region. The projected building has a floor area of 10,680 m², of which around 8,000 m² will be for sales. The store will be open 14 hours a day, 7 days per week, and accommodate up to 500 people at the busiest times of the year, although typically occupants will number roughly a quarter of that.
The building will be well insulated and have 8 metre high ceilings. At full occupancy, fresh ventilation air of 0.6 L/s (2.16 m³/h) per m² of floor area will be introduced into the building; this will be modulated depending on the building occupancy. The building will be kept at 21ºC during heating season and during cooling season it will be kept at 22ºC and a maximum of 45% relative humidity.
Around 350 kW of lighting will be used during occupied hours. During unoccupied hours, lighting levels will be reduced to a quarter of this.
Approximately 10,000 L/d of hot water will be required for washing, domestic use, and kitchens.
During occupied hours, the bakery's ovens, various grills and appliances, and IT equipment will together consume around 310 kW of electricity and dissipate 2/3 of this energy as heat within the building. At other times, these loads consume and dissipate as heat about 50 kW.
Display cases
Within the store there will be 193 m of medium temperature (MT) display cases and 145 m of low temperature (LT) display cases. The design parameters of the display cases are summarized in the following table:
Introduction
A supermarket chain is building a new store and wants to innovate by installing a completely secondary loop refrigeration system for distributing cold to the display cases and implementing extensive heat recovery strategies. The RETScreen Software Energy Efficient Arena & Supermarket Project Model (Version 3.1) has been used to compare, in terms of green house gas (GHG) reduction and energy savings, the conventional refrigeration system design with the proposed package of improvements.
Building information
The new supermarket will be located in the Montreal (QC) region. The projected building has a floor area of 10,680 m², of which around 8,000 m² will be for sales. The store will be open 14 hours a day, 7 days per week, and accommodate up to 500 people at the busiest times of the year, although typically occupants will number roughly a quarter of that.
The building will be well insulated and have 8 metre high ceilings. At full occupancy, fresh ventilation air of 0.6 L/s (2.16 m³/h) per m² of floor area will be introduced into the building; this will be modulated depending on the building occupancy. The building will be kept at 21ºC during heating season and during cooling season it will be kept at 22ºC and a maximum of 45% relative humidity.
Around 350 kW of lighting will be used during occupied hours. During unoccupied hours, lighting levels will be reduced to a quarter of this.
Approximately 10,000 L/d of hot water will be required for washing, domestic use, and kitchens.
During occupied hours, the bakery's ovens, various grills and appliances, and IT equipment will together consume around 310 kW of electricity and dissipate 2/3 of this energy as heat within the building. At other times, these loads consume and dissipate as heat about 50 kW.
Display cases
Within the store there will be 193 m of medium temperature (MT) display cases and 145 m of low temperature (LT) display cases. The design parameters of the display cases are summarized in the following table:
Anti sweat heaters will be used on average 15% of the time during the eight winter months and 100% of the time during the remaining four summer months.
Base case refrigeration, heating and air conditioning systems
The base case refrigeration system that is considered to fulfill the supermarket refrigeration requirements is a typical direct expansion (DX) system using HFC-507A for the MT and LT compressor racks, meaning that the refrigerant flows from the mechanical room to the refrigerated and frozen product display cases. The MT compressor rack has a combined capacity of 117 RT, and a design coefficient of performance (COP) of 2.5. The LT compressors rack has a combined capacity of 56 RT and a design COP of 2.0.
The design condensing temperature of both MT and LT systems is 40ºC, much higher than maximum air temperatures, which would rarely exceed 35ºC in the Montreal area. The design refrigerant evaporating temperature is -9.5ºC for the MT and -32.2ºC for the LT.
A total refrigerant charge of 2,500 kg is required (1,675 kg for the MT system and 825 kg for the LT system). The long refrigerant lines leading to and from the display cases, with their many brazed connections, would tend to leak around 15% of the charge each year.
The superheat energy contained in the LT compressor discharge gas is recovered to meet the supermarket domestic hot water needs. The remaining heat is rejected out of the building with a condenser.
The equivalent of 1,000 kW of natural gas heating systems (a 440 kW rooftop mounted natural gas fired heater for the sales area and several smaller units distributed across the building) need to be installed to provide for the building space heating. A 610 kW conventional vapour compression cycle air-conditioning system with a COP of 5.5, provides cooling when outside air temperatures are too high to permit sufficient free cooling.
Proposed improvements
A package of improvements to the base case refrigeration system is proposed as an avenue for reducing energy consumption, GHG emissions and operating costs.
The improvements include:
- 1. Implementation of secondary loops for cold distribution: It is proposed that the DX cooling of the display cases be supplanted by secondary loops, one each for the low temperature and the medium temperature cases. Secondary loop systems allow a significant reduction in synthetic refrigerant charges in comparison to conventional DX systems, and consequently offer a very promising alternative for the reduction of GHG emissions. In the case of the studied supermarket, eliminating the refrigerant lines to the display cases would permit a refrigerant charge of only 700 kg (470 kg for the MT system and 230 kg for the LT system), and reduce losses to around 5% per year.
For the MT compressor rack, the refrigerant evaporating temperature would have, however, to be lowered to -12ºC to accommodate the addition of a heat exchanger, and the secondary fluid would leave the evaporator at -9.5ºC. For the LT compressor rack, the refrigerant evaporating temperature would be lowered to -34.7ºC and the secondary fluid would leave the evaporator at -32.2ºC. This decrease in the refrigerant design evaporating temperature will result in a slight decrease in the system COP.
Secondary loop systems are usually used not only for the benefits of refrigerant charge reduction, but also for enhanced product quality and decreased system maintenance.
- 2. Condensing heat recovery: It is proposed to recover the heat rejected by both MT and LT compressor racks, and to transfer this heat to secondary fluid loops by the means of a heat recovery condenser. The MT heat recovery loop would provide heat to the air coil for the sales area. The LT heat recovery loop would provide heat to a bank of heat pumps that would raise the temperature as required in order to provide heat to storage areas and other non-sales spaces. The heat pumps would have a combined capacity of 240 kW and a seasonal COP of 3.5. Excess heat in both heat recovery loops would be rejected outside by the means of rooftop fluid coolers. The design parameters of the heat recovery loops are summarized in the following table:
- This measure aims at eliminating the need for the natural gas fired heater. However, Electric heating systems would have to be installed as back up when recovered heat can not meet the supermarket space heating load.
- 3. Floating head pressure: It is proposed to design the refrigeration system as to permit the condensing temperature to float in response to the outdoor air temperature and the building heating requirement. The condensing temperature would be permitted to fall as low as 25ºC for the MT and 10ºC for the LT system.
- 4. Subcooling of the LT compressor rack: It is proposed that the warm liquid refrigerant at the outlet of the LT compressor rack condenser be cooled below its condensing temperature by the MT compressor rack. This involves the use of a 15 RT subcooler. This unit is a heat exchanger with the MT evaporating refrigerant on one side, and the LT condensed refrigerant on the other side. Subcooling the LT refrigeration system increases its performance and its refrigeration capacity.
Financial information
Implementation costs of improvements
The implementation of the proposed secondary loop systems (on the cold side and hot side) vs the base case DX system has an impact on the cost of the display cases and the refrigeration system. The incremental cost for the display cases is associated with the installation of more expensive coils. The incremental cost of the refrigeration system (MT and LT compressor racks) is mainly associated with additional heat exchangers on the cold and hot side (evaporator, heat recovery condenser and subcooler), additional auxiliary equipments (piping and pumps, etc.), and the installation of a rooftop fluid cooler instead of a rooftop condenser.
A breakdown of the incremental costs is given in the following table:
Elimination of the natural gas heating systems would save around $60,000. However, the cost of installing back up electric systems for space heating is around $5,000.
Operating costs
Annual and periodic operation and maintenance costs would likely be lower due to the proposed measures, but can be ignored in this prefeasibility study.
The supermarket uses the following energy costs for the financial analysis:
- Electricity: $0.065 /kWh
- Natural gas: $0.35/m³
Other financial information
Inflation is expected to average 2% over the twenty year life of this project, but energy costs are expected to escalate at around 2.5%.
The supermarket owner would borrow half the funds for this project at an interest rate of 7%, paid back over at 15 year term. The company applies a 10% discount rate to projects of this nature.
The company's effective income tax rate is 25%. The capital costs of the project could be depreciated according to a declining balance method at 10% per year.
A local utility has a program to support innovative refrigeration technologies that would contribute $100,000 to the cost of this project.
Solution
Click here to download the worked-out solution (174 KB).
Analysis of RETScreen results
By all measures, this is a financially attractive project. With an after-tax IRR on equity of over 80% and an after-tax IRR on assets exceeding 25%, this would be a highly profitable project with or without the leverage of debt financing. The debt service coverage ratio in excess of 4 would reassure any potential lenders that the project generated sufficient revenues at all times to cover the debt payments. The simple payback of 3.1 years and the equity payback of 1.8 years are short enough to satisfy all but the most cash-strapped business. The reader can verify that the project would be profitable even without the $100,000 subsidy, and this despite the relatively low price of electricity.
It is important to note that the financial analysis excludes some cost savings associated with the proposed system, simply because they are hard to quantify. Hence, the implementation of secondary loop for cold distribution to the display case should allow a better control of the temperature in the display cases and consequently more stable food temperature. This results in enhanced product quality and shelf life improvements.
Notes on parameter selection
- Hours of lighting per week: 115.5 hours has been entered to take into account 350 kW of lighting during occupied hours and a quarter of this during unoccupied hours.
- Miscellaneous heating source: 141.4 has been entered to take into account the portion of dissipated energy as heat within the building of the ovens, appliances and IT equipment being operated during operating hours and 50 kW for the remainder of the time.
- Display cases anti-sweat usage factor: 43% usage factor was entered as an average over one year.
- LT Superheat recovery rate: the superheat recovery rate was set such that the heating energy required by the domestic water heating system becomes zero, for the base case and proposed case.
- Lighting & miscellaneous electricity consumption: A value of 1,767 MWh has been entered to take into account 310 kW of ovens, appliances and IT equipment being operated during operating hours and 50 kW of loads for the remainder of the time.
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