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Solar Air Heating Efficiency Versus Temperature Rise

When designing any solar air heating system, one must be aware that as solar collector efficiency increases, actual temperature rise of the heated air decreases. This is due to a basic law of physics - more heat is lost as the temperature differential increases. With this in mind, what should a designer consider when designing a SolarWall® heating system?

The first consideration is the purpose of the heated air; is it for space heating or pre-heating large amounts of ventilation air or for a process application? Space heating generally requires air to feel warm and be heated to a higher level, in the 40+°C (105+°F) range whereas ventilation or fresh air only has to be heated to room temperature typically 21°C (70°F).

SolarWall single stage system efficiency & temp charts (high air flows & low temp rise)

1 - Thermal Efficiency for SolarWall Single Stage at Various Wind Speeds
1 - Thermal Efficiency for SolarWall Single Stage at Various Wind Speeds

2 - Temperature Performance for SolarWall Single Stage with Wind Variance
2 - Temperature Performance for SolarWall Single Stage with Wind Variance

SolarWall 2-stage system efficiency & temp charts (for low air flows & high temp rise)

3 - Thermal Efficiency for SolarWall 2-Stage at Various Wind Speeds
3 - Thermal Efficiency for SolarWall 2-Stage at Various Wind Speeds

4 - Temperature Performance for SolarWall 2-Stage with Wind Variance
4 - Temperature Performance for SolarWall 2-Stage with Wind Variance

The Efficiency charts and Temperature Rise charts taken from Solar-A-Mark Certificates illustrate the relationship between efficiency and performance. For example, a flow rate of 1 cfm/ft2 (2-stage SolarWall) delivers a high temperature rise over ambient of 45°C (80°F) with an efficiency in 35% range; a medium flow rate of 4 cfm/ft2 has a temperature rise of 20°C (35°F) and at an efficiency of 60%; while at 7 cfm/ft2 solar efficiency reaches 80%, with a temperature rise of 15°C (27°F).

A limiting factor that will determine the temperature rise is the available surface area for SolarWall in relation to the volume of air to be heated. There may be insufficient wall space to accommodate a large collector array for high temperatures or the opposite may occur, there may only be a small wall surface available but the factory has a huge fresh air requirement.  

The main point to realize is that a designer should not be too concerned about collector efficiency, but rather focus on the end result. Most clients considering solar energy tend to maximize the savings from renewable energy and with SolarWall, it is possible to heat both the ventilation air as well as a building’s space heating component. With a high temperature rise system, hot water heating via Conserval’s air to water heat exchanger is another possibility for utilizing solar energy during the summer months.  

If you are considering LEED points, SolarWall can heat both the ventilation air and displace some of the space heating needs for up to seven LEED points. By designing the solar heater to handle 30% increased ventilation, another LEED point is possible. Renewable energy can play an important role in LEED buildings with more points for providing more heating. The bottom line, utilize as much wall surface as possible to achieve the maximum savings and highest number of LEED points.