The Roof Space Battle: PV vs Skylights and Other Technologies in Net-Zero Energy Capable Facilities

In a net zero energy (NZE) facility the roof is an important source for energy savings and production. A project can utilize passive solar design, reflective roof surfaces, a green roof, skylights, and/or renewable energy generation. In a NZE design, the roof must contribute either significant energy savings or energy generation. In particular, two uses of the roof space are directly tied to energy; skylights to reduce lighting energy and photo-voltaics (PV) to produce renewable energy. However, when it comes to implementing these technologies in energy efficient or NZE projects, competition for roof space occurs. This paper highlights the importance of proper calculations in choosing technology for best utilization of roof space in a NZE building and presents the main factors. First, this paper introduces the common energy efficiency technologies competing for roof space and their potential to contribute to NZE and conflict with one another. Next, the NZE path is chosen and a metric for NZE evaluation is introduced. Finally, sample scenarios for PV generation and skylight savings are analyzed and PV’s impact is favored. The findings demonstrate that PV produces more net energy than skylights save. Background In 2011, our team performed a NZE campus study of the Akron Zoo (Raffio et al. 2012). This campus, like most, has limited area on roof spaces and over parking lots to install PV. Our analysis concluded that all available south-facing and flat roof area must be utilized for PV generation if the campus were to be able to achieve NZE. Once the “PV Energy Budget” 1 was determined, the campus was incapable of achieving NZE without deep energy efficiency retrofits. Deep efficiency means efficiency measures that are typically not analyzed in traditional retrofit scenarios due to their long paybacks, such as skylights and expensive HVAC renovations to ultra-high efficiency systems. For example, in an administrative office area of the Akron Zoo with a flat roof, skylights offered lighting savings potential. However, the team concluded that in a NZE scenario, the skylights would reduce available roof area by more than their own foot-print and the potential electricity generation from solar panels would be reduced more than the skylight savings. This paper explores this topic further and to offer general guidelines for applying energy efficiency or generation technologies that reside on a roof.