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An Overview of Solar Ventilation Air Heating

Solar ventilation air preheating is a cost-effective application of solar energy thanks to an innovative transpired collector that is both inexpensive and high-performance.

Heating of ventilation air accounts for about 15% of the total heating load in an average commercial building, much more in buildings that require a lot of ventilation, like factories and laboratories. Preheating the air with solar energy before it is drawn into the space can save much of this energy.

Solar ventilation air preheating technology is simple, low-cost, extremely reliable (no moving parts except the fan), very low in maintenance requirements, and high in efficiency (up to 80%). There are no problems with freezing or fluid leaks, but there is also no practical way to store the heated ventilation air for nighttime use. Well over two million square feet of transpired collectors have been installed since 1990.

Transpired Collector Principle

The key to low cost and high performance is an elegant solar technology known as the transpired collector. A painted metal plate is perforated with small holes about 1 mm (0.04 in) in diameter and 3 mm (0.12 inch) apart. At this small scale, within 1 mm of the surface of the plate, flow within the laminar boundary layer is dominated by viscosity of the air, and heat transfer is dominated by conduction. This is in contrast to the air flow even a few more mm away from the plate where the flow is dominated by the momentum of the wind, and the heat transfer is dominated by convection. These two differences between the boundary layer of air within 1 mm of the plate and the air farther away are key to the operating principle of the transpired collector.

Sunlight strikes the black surface of the plate and is absorbed. Solar heat conducts from the surface to the thermal boundary layer of air 1 mm thick next to the plate. This boundary layer of air is drawn into a nearby hole before the heat can escape by convection, virtually eliminating heat loss off the surface of the plate. Since the plate operates at less than 20°C warmer than ambient air, heat loss by radiation is not overly consequential. There is no cover glass to reflect or absorb radiation.

To operate effectively, the fan-induced flow through the wall must be sufficient to continuously draw in the boundary layer. Consequently, efforts to increase the temperature of delivered air by reducing the flow rate will adversely affect performance. Don’t get greedy. They don’t call it ventilation preheating for nothing. On cold winter days, supplemental heating by gas or electricity will be required to ensure comfortable conditions.

The transpired collector is mounted about six inches away from the south wall of a building, forming a plenum between the wall and the collector. The collector is fastened to the wall, and the edges are sealed using standard metal building flashing techniques. A fan is installed in the wall to draw air from the plenum into the supply ductwork. The solar preheated air can be delivered to the air handler for the heater or directly into the space to be ventilated. The bypass damper could be thermostatically controlled, and fan operation will depend on the ventilation needs of the space.

The transpired collector makes an efficient sunlight-to-air heat exchanger that tempers the incoming fresh air. It is not possible to recirculate the room air back to the collector for reheating because the fact that it pulls air into the face of the wall is necessary to the operating principle. The amount of temperature increase that the air experiences coming through the collector depends on the air flow rate and on the incident solar radiation. The recommended air flow rate is about 4 CFM per square foot of collector area. At flow rates less than 2 CFM/SF, the boundary layer can blow away before it is sucked through a hole, and at flow rates higher than 8 CFM/SF, the required additional fan power begins to erode the cost savings.

Typical Applications for Solar Ventilation Air Preheating

The transpired collector technology is appropriate for preheating ventilation air in industrial and maintenance buildings, school and institutional buildings, apartment buildings, commercial buildings, and penthouse fans. Examples include factories, aircraft hangers, chemical storage buildings, and other facilities that require ventilation air. Industrial process uses for heated air, such as crop drying, can also be addressed with this technology.

Due to its metal construction, the transpired collector matches well with other metal construction, which is most common in industrial applications. The design of a new building is the best time to consider solar ventilation preheating, but it can be used in retrofit applications as well. It can even improve the appearance of a dilapidated façade. There must be sufficient south-facing vertical wall to mount the collector, and the wall must be largely unshaded by surrounding buildings, trees, hills, or other objects.

Design considerations for solar ventilation air preheating include some flexibility with wall orientation and color. A south-facing wall is best, but not absolutely necessary: +/- 20° of south gives 96%–100%, of heat delivery, while +/- 45° of south gives 80%–100% of the heat delivery of a south-facing wall. Black is best for absorbing solar radiation, but a wide choice of dark to medium colors may be used with efficiency loss of less than 10%, and about 20 colors are available standard from the supplier, with custom colors possible.

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