Energy efficiency measures - Institutional - Arena - hockey & skating - Heat recovery / Canada
Introduction
An arena in the region of Montreal (QC) implemented, a few years ago, heat recovery measures aiming at meeting part of the building space and water heating loads. The RETScreen Software Energy Efficient Arena & Supermarket Project Model (Version 3.1) has been used to evaluate the possibility of intensifying the heat recovery rate and reducing the refrigeration system energy consumption.
Site information
Description of the facility
The arena is located in a municipality on the south shore of the St. Lawrence River, just east of Montreal. It operates from mid-August through mid-April, 16 hours a day on weekdays and 18 hours a day on weekends.
The building contains a single rink, measuring 200 ft by 85 ft. The ceiling is 25 ft above the ice surface. On either side of the rink there are bleachers, together occupying 7,218 ft². There is room for 800 spectators in the bleachers, but the average number of spectators is around 10% of the total seating capacity. The temperature of the stands is kept at 64.4°F (18.1°C) with 50% relative humidity. The entrance lobby, snack bar, store, public toilets, and ticket office occupy in total 10,219 ft² as surface area and have roughly 10 ft high ceilings. The dressing rooms occupy 2,960 ft² and the mechanical room and the ice resurfacer storage room occupy 850 ft².
The building was built in the mid-70s. The walls are poorly insulated, but the roof is reasonably well insulated. The rink has its original, high emissivity ceiling.
The ice is resurfaced 10 to 12 times a day. A single resurfacing requires 420 L of water at 140°F (60°C). The hockey players take around 500 showers per week. Around 200 L/d of hot water is consumed at the arena for domestic usages.
Description of the mechanical system
The 120 tons refrigeration system contains five compressors and uses Freon (R-22) as refrigerant. The system is over 25 years old and has a coefficient of performance (COP) of 2.0 under design conditions, a refrigerant condensing temperature of 95°F (35°C), and a refrigerant evaporating temperature of 5°F (-15°C). The entering air temperature at the condenser is 85°F (29.4°C) and the leaving secondary fluid temperature at evaporator is 15°F (-9.4°C). The condenser head pressure is fixed.
The ice surface is cooled by a calcium chloride brine solution. Heat is transferred from the brine to the primary refrigerant in a direct expansion tube & shell evaporator. A 40 HP pump circulates the brine in a two-pass circuit. The 3 inches ice is kept at 19°F (7.2°C).
It is estimated that the refrigeration system looses around 15% of its refrigerant charge each year, which is equivalent to 27 kg of Freon that need to be replaced annually in the system.
The refrigeration system was retrofitted a few years ago to recover the heat released by two compressors at two temperature levels:
Description of the heating system
Originally, two diesel oil-fired heaters provided the necessary heat each of the two banks of bleachers, but with the retrofit project conducted a few years ago, coils for preheating the fresh air with the refrigerant condensing heat have been added. The total design fresh airflow is 8,000 cfm, but in reality the system generally uses 1,360 cfm of fresh air. Now, the fuel fired furnaces are rarely needed, and provide back-up supplemental heat (equivalent to approximately 10% of the heating energy required) during only the coldest periods of the year. All other spaces in the arena are heated by electricity. Although the space that the dressing rooms occupy is fairly small, it is believed to consume around 75% of the electricity used for heating, probably due to high fresh ventilation air requirements. There is no air conditioning during the summer.
A 242,000 Btu/h diesel oil boiler, is used to heat the domestic water supply and to provide supplemental heat for the resurfacing water. There are two large hot water storage tanks, storing well over 3,000 L.
Description of the lighting system
Thirty-six 400 W metal halide lamps illuminate the rink. In addition, there are 4.5 kW of lighting for the bleachers, 10 kW for the dressing rooms, mechanical room, and corridors, and 6 kW for the lobby, snack bar, and offices. The rink lights are turned off at night. The other lighting is less carefully controlled, and tends to stay on all the time.
Proposed improvements
The following improvements are proposed:
1. Conversion of the rink brine circuit: It is proposed to retrofit the two-pass circuit for a four-pass configuration. This measure enables to reduce the flow rate necessary for cooling the ice and consequently the energy consumption of the refrigeration system.
2. Increase the heat recovery rate:
Financial information
The estimated implementation costs of the proposed measures are as follows:
Introduction
An arena in the region of Montreal (QC) implemented, a few years ago, heat recovery measures aiming at meeting part of the building space and water heating loads. The RETScreen Software Energy Efficient Arena & Supermarket Project Model (Version 3.1) has been used to evaluate the possibility of intensifying the heat recovery rate and reducing the refrigeration system energy consumption.
Site information
Description of the facility
The arena is located in a municipality on the south shore of the St. Lawrence River, just east of Montreal. It operates from mid-August through mid-April, 16 hours a day on weekdays and 18 hours a day on weekends.
The building contains a single rink, measuring 200 ft by 85 ft. The ceiling is 25 ft above the ice surface. On either side of the rink there are bleachers, together occupying 7,218 ft². There is room for 800 spectators in the bleachers, but the average number of spectators is around 10% of the total seating capacity. The temperature of the stands is kept at 64.4°F (18.1°C) with 50% relative humidity. The entrance lobby, snack bar, store, public toilets, and ticket office occupy in total 10,219 ft² as surface area and have roughly 10 ft high ceilings. The dressing rooms occupy 2,960 ft² and the mechanical room and the ice resurfacer storage room occupy 850 ft².
The building was built in the mid-70s. The walls are poorly insulated, but the roof is reasonably well insulated. The rink has its original, high emissivity ceiling.
The ice is resurfaced 10 to 12 times a day. A single resurfacing requires 420 L of water at 140°F (60°C). The hockey players take around 500 showers per week. Around 200 L/d of hot water is consumed at the arena for domestic usages.
Description of the mechanical system
The 120 tons refrigeration system contains five compressors and uses Freon (R-22) as refrigerant. The system is over 25 years old and has a coefficient of performance (COP) of 2.0 under design conditions, a refrigerant condensing temperature of 95°F (35°C), and a refrigerant evaporating temperature of 5°F (-15°C). The entering air temperature at the condenser is 85°F (29.4°C) and the leaving secondary fluid temperature at evaporator is 15°F (-9.4°C). The condenser head pressure is fixed.
The ice surface is cooled by a calcium chloride brine solution. Heat is transferred from the brine to the primary refrigerant in a direct expansion tube & shell evaporator. A 40 HP pump circulates the brine in a two-pass circuit. The 3 inches ice is kept at 19°F (7.2°C).
It is estimated that the refrigeration system looses around 15% of its refrigerant charge each year, which is equivalent to 27 kg of Freon that need to be replaced annually in the system.
The refrigeration system was retrofitted a few years ago to recover the heat released by two compressors at two temperature levels:
- The energy contained in the refrigerant superheat (hot gas sensible energy), enabling to fulfill almost 95% of the water heating needs for ice resurfacing purposes;
- The refrigerant condensing heat to preheat the fresh air for the stands.
Description of the heating system
Originally, two diesel oil-fired heaters provided the necessary heat each of the two banks of bleachers, but with the retrofit project conducted a few years ago, coils for preheating the fresh air with the refrigerant condensing heat have been added. The total design fresh airflow is 8,000 cfm, but in reality the system generally uses 1,360 cfm of fresh air. Now, the fuel fired furnaces are rarely needed, and provide back-up supplemental heat (equivalent to approximately 10% of the heating energy required) during only the coldest periods of the year. All other spaces in the arena are heated by electricity. Although the space that the dressing rooms occupy is fairly small, it is believed to consume around 75% of the electricity used for heating, probably due to high fresh ventilation air requirements. There is no air conditioning during the summer.
A 242,000 Btu/h diesel oil boiler, is used to heat the domestic water supply and to provide supplemental heat for the resurfacing water. There are two large hot water storage tanks, storing well over 3,000 L.
Description of the lighting system
Thirty-six 400 W metal halide lamps illuminate the rink. In addition, there are 4.5 kW of lighting for the bleachers, 10 kW for the dressing rooms, mechanical room, and corridors, and 6 kW for the lobby, snack bar, and offices. The rink lights are turned off at night. The other lighting is less carefully controlled, and tends to stay on all the time.
Proposed improvements
The following improvements are proposed:
1. Conversion of the rink brine circuit: It is proposed to retrofit the two-pass circuit for a four-pass configuration. This measure enables to reduce the flow rate necessary for cooling the ice and consequently the energy consumption of the refrigeration system.
2. Increase the heat recovery rate:
- Increase the capacity of recovering the refrigerant sensible heat by adding a desuperheater on no.3 compressor discharge. This measure should, enable to minimize the use of the oil boiler for water heating loads;
- Increase the capacity of recovering the refrigerant condensing heat such as to fulfill the dressing room heating loads. This measure requires adding a heat recovery condenser on compressor no.3 circuit, a heat recovery loop and heat recovery air coils in the dressing rooms. The electric heating systems will remain for back-up and is expected to fulfill 20% of the dressing room space heating load.
Financial information
The estimated implementation costs of the proposed measures are as follows:
The arena pays $0.065 per kWh of electrical energy used, a price that is not expected to rise more than 2% a year. The price of diesel fuel varies widely from year to year, but the arena uses an average value of $0.41/L in their planning and budgeting. This price is expecting to rise at around 3% per year.
R-22 refrigerant is very damaging to the ozone layer and is being phased out under the Montréal Protocol. By 2020 it will no longer be produced. Presently it costs around $8 per kg, but driven by demand from existing R-22 systems, its price is expected to rise by around 10% a year as production tails off.
The arena would pay for the proposed measures out of a municipal budget; upgrades should, considered together, have a simple payback of 6 years or quicker and a return on investment greater than 8% over a 20 year planning horizon. Utilities and various levels of government have incentive programs to encourage energy efficiency in ice arenas. It is believed that the proposed system would qualify for a total of $25,000 in grants.
Solution
Click here to download the worked-out solution (712 KB).
Analysis of RETScreen results
The project has a very good return on investment, but does not quite manage to meet the simple payback target. This demonstrates the dangers of relying on the simple payback as the only measure of profitability. Simple payback favours a quick return over a good return and can discourage investment in financially attractive projects.
Greenhouse gas emissions reductions are minimal, since they are mainly resulting from the reduction of electricity consumption and hydropower provides the majority of the electricity used by this arena. Note that this does not mean that the arena does not have significant GHG emissions: only that there is not much reduction going from the base case to the proposed case. Indeed, through R-22 refrigerant leakage this arena contributes to 49 t eq. of CO2 emissions annually.
Notes on parameter selection
- Lighting load: The rink lighting is turned off at night, but the other lighting is not. Since RETScreen does not permit different lighting schedules for rink lights and other lights, the lighting schedule of the rink has been selected, and 50% added to the other lighting loads, such that their energy consumption over the day will match reality.
- Inflation rate of R-22: the price of R-22 refrigerant is expected to rise at 10%, well above the 2.5% rate of general inflation that is applied by RETScreen. To account for this in a most rudimentary fashion, the current price of refrigerant has been raised from $8 to $12.
- Space heating equipment: energy required is met entirely by electricity, even though in reality, there is a diesel fired heater that provides back-up heating to the stands. This assumption is reasonable since 1) with condensing heat recovery, the diesel fired heater meets less than 10% of the stands heating required, and 2) the space heating system for the stands remains unchanged in the proposed analysis, and consequently the difference between the costs for base case and proposed case will be unaffected.
- Superheat recovery rate - base case: To determine the superheat recovery rate in the base case the following steps were undertaken:
1. Superheat recovery was turned off and the resurfacing and domestic hot water heating energy required were noted.
2. Superheat recovery rate for the base case was raised to the level that the reduction in the total hot water heating required was equal to 95% of the resurfacing water with no superheat recovery.
- Superheat recovery rate - proposed case: The superheat recovery rate for the proposed case was raised until the desuperheaters were meeting 80% of the total hot water heating required. This 80% level was chosen because without longer term storage, it may be difficult to meet the entire hot water heating required with the heat available from the desuperheaters.
- Condensing heat recovery rate - base case: To determine the condensing heat recovery rate for the base case the following steps were undertaken:
1. The space heating energy required in the base case with no condensing heat recovery was noted.
2. To isolate the part of this energy required that is met by electric heaters, the stands temperature operating strategy was set to no heating/no cooling.
3. The difference between these two numbers is the heating energy required for the stands.
4. The stands temperature operating strategy was set back to heating/no cooling, condensing heat recovery enabled, and the base case condensing heat recovery rate set to the level that reduced the space heating energy required by an amount equal to 90% of the heating energy required for the stands.
- Condensing heat recovery rate - proposed case: For the proposed case, the condensing heat recovery rate was set to recover 80% of the space heating energy required for the dressing room - the latter representing 75% of the base case space heating energy required.
References
All data describing the arena and the proposed improvement are coming from a feasibility study conducted by the engineering firm Dessau Soprin for a municipality located on Montreal South shore. In this study, Dessau studied several other scenarios that are not presented in this RETScreen case study.
