Sunny Side Up: Important Considerations for the Installation of Rooftop Photovoltaic Arrays
Michael Ratcliff, Ph.D., P.E., Rowan Williams Davies and Irwin, Inc.
Rooftop photovoltaic (PV) installations are becoming more common with a rise in incentives and requirements for the design of greener buildings. However, building rooftops are often already congested with mechanical equipment, and laboratories are no exception. Siting PV arrays to maximize gains, minimize losses, and minimize negative interaction with other rooftop equipment can therefore present a challenge to building designers.
These PV arrays are expensive and operate at relatively low efficiency, making it important to maximize solar gains and minimize losses. Furthermore, shading of solar panels cells can cause considerable efficiency loss, and can be affected by a number of factors, including:
- Row spacing and tilt (self-shading).
- Proximity to adjacent buildings.
- Proximity to penthouses, stair towers, or other rooftop massing.
- Proximity to rooftop exhaust stacks.
Soiling also affects solar panel performance. Dust, sand, exhaust particulate, bird droppings, and snow can all deposit on solar panels over time, reducing the amount of solar radiation that can penetrate to the PV cells, thus reducing overall efficiency. This type of performance loss is more common in extreme environments such as the Middle East, but nevertheless is important to consider even in North America. Periodic cleaning of PV arrays can be required to counter the effects of soiling, and a cleaning strategy needs to be considered as part of the design process.
Efficiency losses are the major concern with rooftop PV installations; however, other considerations such as wind pressure effects on solar panels and how solar panels can affect the rooftop wind environment are also important to understand. On short buildings, wind pressure is less of a concern; however, on tall buildings, structural re-enforcement may be needed to account for increased wind loads. PV arrays can also affect rooftop wake zone formation, which can lead to concerns for re-entrainment of hazardous building exhausts discharged at roof level, particularly if stacks are kept short to avoid shading effects on the panels.
During this presentation, the speakers will provide an overview of many important design considerations for the use of roof-mounted PV arrays as summarized above. The speakers will present examples that highlight the effects of shading and wind loads and the effects that PV installations can have on rooftop wake formation and the resultant re-entrainment of hazardous exhausts.
Biography:
Michael Ratcliff is a technical director with RWDI and has been with RWDI for 13 years. Dr. Ratcliff has 27 years experience working with building exhaust, re-entrainment, and other aspects of air flow around buildings. Dr. Ratcliff has a Ph.D. in civil engineering from Colorado State University, has been a member of ASHRAE since 1991, and is a registered professional engineer.