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Heating - Solar water heater - Apartment building / Canada
Case study assignment
You have been hired by the engineering department of a municipal housing corporation to prepare a preliminary study for a potential solar water heating project. The building under consideration is typical of the small, flat roofed apartment buildings managed by the corporation. They would like you to consider an evacuated tube solar collector which is available locally.
Site information
The building is located near Montreal, Quebec, Canada; the nearest airport is at St. Hubert. The roof is flat and an area of 25 m by 18 m is available for the collector. The 9-unit apartment building has sufficient indoor space in the mechanical room (25 m²) for all the solar equipment (storage tanks, heat exchanger, piping, etc). There are 3 floors from the mechanical room to the collector and the horizontal distance from the mechanical room to the proposed solar collector is roughly 4 m. Domestic hot water is presently heated to 60°C by 2 electric water heaters. The occupation rate of the building is 100%. It is possible to run external piping between the mechanical room and the collector on the roof, thus making roof penetration unnecessary.
Financial information
Typical financial figures for the analysis are provided by the corporation (inflation of 2.5%, debt ratio of 80%, debt interest rate of 8%, discount rate of 9% and a debt term of 10 years). The housing corporation does not pay taxes.
The solar water heating system is assumed to last 25 years. It qualifies for a 25% government subsidy. Operation & maintenance costs are limited to an annual inspection (requiring about 1 hour) and replacement of the water/glycol mixture every 10 years.
The housing corporation pays about $0.05/kWh for electricity; this price will escalate at a rate of about 3% annually.
Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.
Solution
The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.
Teacher's notes
Results
In 1991 the Montreal Housing Corporation installed an evacuated tube solar water heating (SWH) system on an existing 9-unit apartment building owned by OMHM (Office municipal d'habitation de Montréal). This project was undertaken as a result of a feasibility study conducted for the provincial electric utility, Hydro-Québec, evaluating the potential of SWH systems for demand-side management. Specifically, the utility wished to evaluate 1) the benefits of using solar hot water storage with a programmable thermostat installed on the electric auxiliary hot water tank, and 2) the reliability of hot water supply at temperatures between 45 °C and 65 °C.
System performance was monitored from June 1993 to February 1994. During this period, the overall system efficiency was 31%. The monitoring also showed that system performance during the summer was reduced due to lower hot water usage and by a malfunction of the variable pump speed controller (a variable flow rate system was under evaluation during the first months of monitoring).
Despite good system performance, this application was not duplicated, for two reasons:
System description
The service hot water preheating system was retrofitted upstream from existing hot water tanks located in the mechanical room in the basement of the building. The south-facing 26 m² solar array includes 14 solar collectors rack-mounted at 45° on the flat roof. The supports are not anchored to the roof but rather secured together by wood beams and attached by cables to the walls surrounding the roof. In this way, the installation maintains the integrity of the roof membrane. Each collector consists of 14 heat pipe evacuated tubes (0,1 m² absorber area each) secured in a frame of anodised aluminium. There are no reflectors at the back of the tubes. The design provides a gap of about 3 cm between the tubes; this reduces wind loads and has been proven to eliminate snow accumulation on the solar collector. In winter, the rear absorber of the evacuated tubes benefits from the upward reflection of sunlight from the snow accumulated on the flat roof.
The system uses 3 series-connected standard 450-litre glass-lined tanks for solar preheat storage. The system also uses a heat exchanger, a drain-top fluid reservoir, a differential temperature controller, and 2 circulating pumps, one for water on the storage side and the other for the antifreeze mixture in the solar loop.
A freeze resistant water/propylene glycol mixture is used as transfer fluid from the collectors to the heat exchanger. After 10 years (1991-2000) of operation, the original mixture shows no chemical degradation, thanks to careful sizing of the system and the drain-top configuration that protects the mixture from overheating.
The cost of the installed solar system was $26,800, distributed as follows:
Case study assignment
You have been hired by the engineering department of a municipal housing corporation to prepare a preliminary study for a potential solar water heating project. The building under consideration is typical of the small, flat roofed apartment buildings managed by the corporation. They would like you to consider an evacuated tube solar collector which is available locally.
Site information
The building is located near Montreal, Quebec, Canada; the nearest airport is at St. Hubert. The roof is flat and an area of 25 m by 18 m is available for the collector. The 9-unit apartment building has sufficient indoor space in the mechanical room (25 m²) for all the solar equipment (storage tanks, heat exchanger, piping, etc). There are 3 floors from the mechanical room to the collector and the horizontal distance from the mechanical room to the proposed solar collector is roughly 4 m. Domestic hot water is presently heated to 60°C by 2 electric water heaters. The occupation rate of the building is 100%. It is possible to run external piping between the mechanical room and the collector on the roof, thus making roof penetration unnecessary.
Financial information
Typical financial figures for the analysis are provided by the corporation (inflation of 2.5%, debt ratio of 80%, debt interest rate of 8%, discount rate of 9% and a debt term of 10 years). The housing corporation does not pay taxes.
The solar water heating system is assumed to last 25 years. It qualifies for a 25% government subsidy. Operation & maintenance costs are limited to an annual inspection (requiring about 1 hour) and replacement of the water/glycol mixture every 10 years.
The housing corporation pays about $0.05/kWh for electricity; this price will escalate at a rate of about 3% annually.
Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.
Solution
The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.
Teacher's notes
- Losses due to snow and dirt are set to a relatively low 2% because the gap between the evacuated tubes will eliminate most snow accumulation.
- Solar collector costs fall below the range suggested by RETScreen for evacuated tube collectors. The price used in the spreadsheet was based on commercially available evacuated tube collectors manufactured in China, with the price multiplied by a factor of 1.7 to cover transport, import taxes, and distribution costs.
Results
In 1991 the Montreal Housing Corporation installed an evacuated tube solar water heating (SWH) system on an existing 9-unit apartment building owned by OMHM (Office municipal d'habitation de Montréal). This project was undertaken as a result of a feasibility study conducted for the provincial electric utility, Hydro-Québec, evaluating the potential of SWH systems for demand-side management. Specifically, the utility wished to evaluate 1) the benefits of using solar hot water storage with a programmable thermostat installed on the electric auxiliary hot water tank, and 2) the reliability of hot water supply at temperatures between 45 °C and 65 °C.
System performance was monitored from June 1993 to February 1994. During this period, the overall system efficiency was 31%. The monitoring also showed that system performance during the summer was reduced due to lower hot water usage and by a malfunction of the variable pump speed controller (a variable flow rate system was under evaluation during the first months of monitoring).
Despite good system performance, this application was not duplicated, for two reasons:
- Peak load management is not credited to the users as there are still no time-based rates at Hydro-Québec for residential and small commercial clients.
- There are concerns at Hydro-Québec about programmable hot water thermostats and low temperature storage tanks. Specifically, the utility is concerned about bacteriological contamination of the hot water (e.g. legionella).
System description
The service hot water preheating system was retrofitted upstream from existing hot water tanks located in the mechanical room in the basement of the building. The south-facing 26 m² solar array includes 14 solar collectors rack-mounted at 45° on the flat roof. The supports are not anchored to the roof but rather secured together by wood beams and attached by cables to the walls surrounding the roof. In this way, the installation maintains the integrity of the roof membrane. Each collector consists of 14 heat pipe evacuated tubes (0,1 m² absorber area each) secured in a frame of anodised aluminium. There are no reflectors at the back of the tubes. The design provides a gap of about 3 cm between the tubes; this reduces wind loads and has been proven to eliminate snow accumulation on the solar collector. In winter, the rear absorber of the evacuated tubes benefits from the upward reflection of sunlight from the snow accumulated on the flat roof.
The system uses 3 series-connected standard 450-litre glass-lined tanks for solar preheat storage. The system also uses a heat exchanger, a drain-top fluid reservoir, a differential temperature controller, and 2 circulating pumps, one for water on the storage side and the other for the antifreeze mixture in the solar loop.
A freeze resistant water/propylene glycol mixture is used as transfer fluid from the collectors to the heat exchanger. After 10 years (1991-2000) of operation, the original mixture shows no chemical degradation, thanks to careful sizing of the system and the drain-top configuration that protects the mixture from overheating.
The cost of the installed solar system was $26,800, distributed as follows:
Lessons learned
The big picture
Evacuated tube solar water heating systems are technically feasible in cold, snowy climates. When the price of the energy displaced by the solar heating is very low, e.g., the inexpensive electricity in this case study, the solar water heating system will have difficulty attaining strict financial feasibility. Some consumers may be willing to pay a premium for the green energy provided by the collector; alternatively, reduced electric loads during peak load periods may be a source of additional benefits, as investigated by the utility in this case study.
Photos
Apartment building - Solar water heater - Technical room, Quebec, Canada
Apartment building - Solar water heater, Quebec, Canada
References
- This system has operated for 10 years, demonstrating the reliability of solar technology and the ability of the evacuated tube technology to operate efficiently in winter, at low ambient temperatures, without snow accumulation.
- After 10 years of operation, the original water/glycol mixture is still in good condition. The system was designed to allow the circulating fluid to drain back into a drain-top reservoir when the solar pump is not running. This design protects the water/glycol mixture from chemical degradation during stagnation, which can occur in the collector modules.
- The reliability of advanced design strategies or components that are supposed to increase the system efficiency must be proven before being used in the field. Otherwise, they may adversely affect overall performance. On the water-side of the heat exchanger, a custom speed controller was installed on a DC pump in order to raise the temperature of the water leaving the heat exchanger. After 2 years of trouble-shooting this component, it was removed and replaced by a simple, low power AC circulator. Since that time, the system has operated reliably and with low maintenance costs.
The big picture
Evacuated tube solar water heating systems are technically feasible in cold, snowy climates. When the price of the energy displaced by the solar heating is very low, e.g., the inexpensive electricity in this case study, the solar water heating system will have difficulty attaining strict financial feasibility. Some consumers may be willing to pay a premium for the green energy provided by the collector; alternatively, reduced electric loads during peak load periods may be a source of additional benefits, as investigated by the utility in this case study.
Photos
Apartment building - Solar water heater - Technical room, Quebec, Canada
Apartment building - Solar water heater, Quebec, Canada
References
- Hosatte, Pierre, "Personal communication," TN Conseil, 2000.
- Queen's University, Performance Monitoring of a Multi-Unit Solar Domestic Hot Water System, DSS Contract n°23440-1-9612, Energy Technology Branch, CANMET, NRCan, Ottawa, Ontario, 1994, p. 34.
- TN conseil, Eau chaude solaire et réduction de la charge du réseau électrique au Québec, Projet de démonstration, DSS contract n°23440-1-9612, Hydro-Québec PO: HA309120, 1991-1993.
