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Heating - Solar air heater - Classroom / Canada
Case study assignment
You are the energy manager for a large school board and you have been asked to investigate problems arising with many of the portable classrooms. These problems include high fuel costs, lack of proper ventilation, mold growth, complaints from students and teachers, and the resulting bad publicity.
To solve these problems, a heated fresh air ventilation system will have to be installed in each portable classroom. Adding proper ventilation will dramatically increase the heating costs for the classrooms so you wish to evaluate whether solar heating of the fresh air is viable. The School Board has said that the portables are temporary and therefore the payback from energy savings must be quick; the board admits, however, that in reality the portable classrooms tend to have extended tenures.
Site information
The portable classrooms are located in Toronto, Ontario, Canada. Air-conditioning handbooks suggest that for a portable classroom with 30 students, fresh air of approximately 200 L/s will be required. South-facing wall area of 20 m² is available for a solar collector. The classrooms are occupied for about 9 hours per day. Their walls have an RSI value of about 1.5 (m²-ºC)/W. When the heat from the solar collector is insufficient, additional heat will be provided by electrical heaters.
Financial information
The project costs can be paid out of a maintenance budget if the payback period is short. An incentive of 25% of the initial costs of the solar project is available from the federal governement. The school assumes that inflation will be 2% and that fuel costs will escalate at 3%. Their discount rate is 9%. They assume that the portable classroom will only be in use for 10 years. The school board does not pay taxes.
The retail price of electricity is approximately $0.08/kWh.
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
Real project
Results
The Toronto School Board's portable classrooms require high fresh air ventilation rates in order to avoid air quality problems. The fresh air can be heated electrically, but this results in very high operating costs. The School Board has experimented with an expensive heat recovery ventilation system, but this also increased operating costs, utilised a 200 watt fan due to the pressure drop through the heat exchanger, and required maintenance for its filters.
Natural Resources Canada supplied a Photovoltaic/solar air heating system (PV/SAH) to the School Board so that they could evaluate its performance. The system uses a metal collector for SAH purposes and 2 PV modules to power the ventilation fans. The system was monitored by Ontario Hydro. The School Board was responsible for installing and operating the system.
Solar air heating is ideally suited to the portable classroom as heated ventilation air is required only during daytime. The ratio of 16 m2 of collector surface to 680 m³/h of ventilation air, as used in the system, is an ideal ratio for the collector. Ontario Hydro also reported that the system reduced the utility's electric load at the time of its mid-day peak.
The monitoring data indicated that the fans would turn on each morning at approximately 9 a.m. and turn off at around 5 p.m. On sunny days, the temperature rise of the incoming air was approximately 30 ºC over ambient; even on cloudy days the temperature rise was in the range of 5 - 8 ºC. The system was self regulating, substantially raising the temperature of the incoming air under most conditions. The hybrid solar collector delivered in excess of 30 kWh of heat on most days of the test period and the daily profiles of heating demand and supply were strongly correlated in time. The monitoring also showed that even on very cold days, on average, the only electrical energy consumption was for lighting, with the SAH system being able to displace not only the ventilation heating needs but also the space heating needs.
System description
Sixteen square meters of solar collectors were installed on the south wall of the portable classroom. Two PV-powered fans, each capable of delivering 340 m³/h of air, were installed. Inside the classroom, two distribution ducts, each 150 mm in diameter, were mounted under the ceiling. These run the length of the classroom and distribute the solar heated fresh air to all of the students.
The fans were powered by two 60 watt PV modules. This simplified system provided sufficient power to run the fans even under cloudy conditions. The considerable expense of connecting the fans to the grid was avoided.
The incremental costs for the SAH system were approximately $1,500 and the predicted annual energy savings were over $500, giving a payback of approximately 3 years.
Lessons learned
Photo
Portable classroom - Solar air heater, Ontario, Canada
References
Case study assignment
You are the energy manager for a large school board and you have been asked to investigate problems arising with many of the portable classrooms. These problems include high fuel costs, lack of proper ventilation, mold growth, complaints from students and teachers, and the resulting bad publicity.
To solve these problems, a heated fresh air ventilation system will have to be installed in each portable classroom. Adding proper ventilation will dramatically increase the heating costs for the classrooms so you wish to evaluate whether solar heating of the fresh air is viable. The School Board has said that the portables are temporary and therefore the payback from energy savings must be quick; the board admits, however, that in reality the portable classrooms tend to have extended tenures.
Site information
The portable classrooms are located in Toronto, Ontario, Canada. Air-conditioning handbooks suggest that for a portable classroom with 30 students, fresh air of approximately 200 L/s will be required. South-facing wall area of 20 m² is available for a solar collector. The classrooms are occupied for about 9 hours per day. Their walls have an RSI value of about 1.5 (m²-ºC)/W. When the heat from the solar collector is insufficient, additional heat will be provided by electrical heaters.
Financial information
The project costs can be paid out of a maintenance budget if the payback period is short. An incentive of 25% of the initial costs of the solar project is available from the federal governement. The school assumes that inflation will be 2% and that fuel costs will escalate at 3%. Their discount rate is 9%. They assume that the portable classroom will only be in use for 10 years. The school board does not pay taxes.
The retail price of electricity is approximately $0.08/kWh.
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
- The solution proposed in the RETScreen spreadsheets consists of a solar collector on the south wall feeding a rooftop ventilation fan. The fan is powered by two 20 watt photovoltaic modules, also located on the wall. The fan rate is thus related to the amount of solar energy available, such that the air exiting the solar collector will always have a relatively high temperature gain. A bypass damper with thermostat permits unheated fresh air to be used when the portable classroom has no heating requirement.
- On very cloudy days, the photovoltaic power will drop and the ventilation rate will be less than recommended. It should be noted, however, that fan flow rates are related to the cube of the fan power: on a very cloudy day, with 1/8th the solar energy of a sunny day available, airflow will be roughly halved.
- This solution is a win-win situation (i.e., keeping ventilation and heating costs low while providing good air quality).
- The velocity of the wind hitting the solar collector has a significant influence on the estimated energy delivered by the solar air heating system. Obstacles such as other buildings, trees and fences will typically attenuate, but in some cases even intensify, the wind speed hitting the solar collector. In this case, a 0.70 wind sheltering coefficient is assumed (this was done by multiplying each of the wind speed values by 0.70 in the climate data section of the Start worksheet).
- The low feasibility and engineering costs are appropriate for a standard kit that could be installed on all portable classrooms.
- The cladding material and labour credit is given assuming that this is a kit installed on new portable classrooms. Retrofits would not benefit from this credit.
- Ducting costs are low because the ventilation fan can be located on the roof, arbitrarily close to the outlet of the solar collector.
- A project life of 10 years has been assumed to reflect the school board's view that the portable classroom is a temporary structure. In reality, the portable classroom and its solar collector will almost certainly last longer than this.
Real project
Results
The Toronto School Board's portable classrooms require high fresh air ventilation rates in order to avoid air quality problems. The fresh air can be heated electrically, but this results in very high operating costs. The School Board has experimented with an expensive heat recovery ventilation system, but this also increased operating costs, utilised a 200 watt fan due to the pressure drop through the heat exchanger, and required maintenance for its filters.
Natural Resources Canada supplied a Photovoltaic/solar air heating system (PV/SAH) to the School Board so that they could evaluate its performance. The system uses a metal collector for SAH purposes and 2 PV modules to power the ventilation fans. The system was monitored by Ontario Hydro. The School Board was responsible for installing and operating the system.
Solar air heating is ideally suited to the portable classroom as heated ventilation air is required only during daytime. The ratio of 16 m2 of collector surface to 680 m³/h of ventilation air, as used in the system, is an ideal ratio for the collector. Ontario Hydro also reported that the system reduced the utility's electric load at the time of its mid-day peak.
The monitoring data indicated that the fans would turn on each morning at approximately 9 a.m. and turn off at around 5 p.m. On sunny days, the temperature rise of the incoming air was approximately 30 ºC over ambient; even on cloudy days the temperature rise was in the range of 5 - 8 ºC. The system was self regulating, substantially raising the temperature of the incoming air under most conditions. The hybrid solar collector delivered in excess of 30 kWh of heat on most days of the test period and the daily profiles of heating demand and supply were strongly correlated in time. The monitoring also showed that even on very cold days, on average, the only electrical energy consumption was for lighting, with the SAH system being able to displace not only the ventilation heating needs but also the space heating needs.
System description
Sixteen square meters of solar collectors were installed on the south wall of the portable classroom. Two PV-powered fans, each capable of delivering 340 m³/h of air, were installed. Inside the classroom, two distribution ducts, each 150 mm in diameter, were mounted under the ceiling. These run the length of the classroom and distribute the solar heated fresh air to all of the students.
The fans were powered by two 60 watt PV modules. This simplified system provided sufficient power to run the fans even under cloudy conditions. The considerable expense of connecting the fans to the grid was avoided.
The incremental costs for the SAH system were approximately $1,500 and the predicted annual energy savings were over $500, giving a payback of approximately 3 years.
Lessons learned
- SAH systems can be used to solve the indoor air quality problems associated with portable classrooms.
- The size of the PV modules could be reduced significantly. For 2 fans, a total of 40 to 60 watts of PV capacity are required. Using smaller modules would reduce the total cost of the system.
- The SAH system can operate on weekends and displace the space heating requirements.
- The combined solar thermal and solar electrical system is an ideal learning tool for schools interested in solar energy. The children were involved in the project by taking daily readings of the temperatures in the solar system.
- The general comments received from the students were that the classroom air was significantly fresher and the room was warmer with the SAH system.
Photo
Portable classroom - Solar air heater, Ontario, Canada
References
- Nikiforov, Vladimir, "Personal communication," Conserval Engineering Inc., 2000.
