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Pro and Con: Solar Air Heating Systems

| February 23, 2015 | 0 Comments

by Charles Hopkins

Charles Hopkins

Charles Hopkins

When most think of solar energy, photovoltaics and solar thermal water heating come to mind. These are technologies that most large-scale projects consider at one point or another and are easily understood by project stakeholders. However, what are the alternative options projects can investigate to cut energy bills and reduce our impact on the environment?

You may have already heard of solar air heating systems, also known as transpired solar collectors, a type of solar thermal technology that allows projects to preheat incoming air by installing a perforated metal cladding on the building skin. The tiny perforations allow air to be drawn in and heated via solar radiation before being directed to the building’s ventilation system. The dynamicity of the tech lies in its application flexibility, meaning that heated air can be brought in or used in a variety of ways. Most scenarios will see the air preheated and then ducted directly into the building or sent to a rooftop unit to assist in heat recovery systems, although more basic instances can allow for the air to be passed directly through the building envelope to heat adjacent spaces.

Other benefits include shading: Cladding will shade exterior walls, reducing cooling load on hot summer days. There is also minimal maintenance, requiring little mechanical support aside from placing exterior wall and ventilation systems appropriately in order to draw in preheated air. From an occupant standpoint, it allows for increased air changes, leading to better indoor air quality and, thus, happier occupants. Operationally, typical transpired collectors function at 60% to 80% efficiency, creating a significant difference in terms of payback. Installed cost is relatively low ($14 to $17 per sf)  offset from state incentives, depending on location.1 In Massachusetts, projects can qualify for the 30% federal solar tax credit and/or a 15% state tax exemption. Typical payback is estimated to be four to seven years.

As with most solar applications, project type, orientation, and location play a key role in optimizing efficiency. Envelopes dominated by southern-facing exposures with significant surface area stand to gain the most from transpired collectors, whereas smaller commercial or residential applications with limited envelope area and/or increased glazing may not be able to realize significant energy savings. As such, building orientation is paramount. Project design will have to take into account not only building massing, but envelope as well — large vertical cladding systems do not offer much recourse for glazing and envelope openings, thereby reducing daylighting and access to views. Thought should also be given to general programming within the building, as longer duct runs to occupied spaces will see increased heat loss, resulting in an overall drop in efficiency.

Lastly, the macro-massing of the site is critical, yet often overlooked. We recently reviewed the potential of solar preheating of air for a high-profile building in the Seaport area, but based upon the master plan, upcoming adjacent buildings would negate the solar potential. Additionally, detailed review of the cavity design, building massing to address snow drifts is required to ensure that outside air is always available to the building’s HVAC system during inclement weather.

One of the major hurdles transpired collectors face outside of general massing requirements is location — not all climates are ideal for transpired collectors, narrowing the potential market to those with colder climates located in higher latitudes  in locations that experience colder months and sufficient sunlight, as outdoor air heating load is greater in these climates.2 Location aside, there are also architectural design considerations — collectors require a large amount of surface area and a darker surface to more efficiently capture solar radiation, both factors potentially inviting concerns over aesthetics from owners.

The bottom line? Transpired solar collectors still seem to be re-emerging technology, bound to specific climates and building types. Regardless, solar air collection is a simple, cost effective technology with great potential in the Northeast. Our longer, colder winters offer ample opportunity to reduce a typical heating bill using a transpired collector — even glare from snow can help increase efficiency.

That being said, not all building types can successfully implement solar air collectors. Top case studies  highlighted buildings with larger masses: High-rise residential buildings, hospitals, warehouse/distribution centers, and manufacturing centers are a few instances of projects that successfully implemented solar air collectors. With some forethought, solar air collectors stand to offer significant benefits and straightforward integration with larger-mass buildings interested in reaching their energy goals.

1 Solar Air Heating. Product description. Atlas International. 2013.(http://www.atas.com/products/walls/inspire-solar-air-heating/inspire-wall#ProductInfo).
2 Kozubal et al, “Evaluating the Performance and Economics of Transpired Solar Collectors for Commercial Applications.” American Council for an Energy-Efficient Economy. 2008 (https://www.aceee.org/files/proceedings/2008/data/papers/3_107.pdf).

Charles Hopkins, LEED AP BD+C, is an energy engineer at Vanderweil Engineers.

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