Elsevier

Energy and Buildings

Volume 56, January 2013, Pages 210-220
Energy and Buildings

Electricity production and cooling energy savings from installation of a building-integrated photovoltaic roof on an office building

https://doi.org/10.1016/j.enbuild.2012.06.032Get rights and content

Abstract

Reflective roofs can reduce demand for air conditioning and warming of the atmosphere. Roofs can also host photovoltaic (PV) modules that convert sunlight to electricity. In this study we assess the effects of installing a building integrated photovoltaic (BIPV) roof on an office building in Yuma, AZ. The system consists of thin film PV laminated to a white membrane, which lies above a layer of insulation. The solar absorptance of the roof decreased to 0.38 from 0.75 after installation of the BIPV, lowering summertime daily mean roof upper surface temperatures by about 5 °C. Summertime daily heat influx through the roof deck fell to ±0.1 kWh/m2 from 0.3–1.0 kWh/m2. However, summertime daily heat flux from the ventilated attic into the conditioned space was minimally affected by the BIPV, suggesting that the roof was decoupled from the conditioned space. Daily PV energy production was about 25% of building electrical energy use in the summer. For this building the primary benefit of the BIPV appeared to be its capacity to generate electricity and not its ability to reduce heat flows into the building. Building energy simulations were used to estimate the cooling energy savings and heating energy penalties for more typical buildings.

Highlights

► We measured effects of installing a building integrated photovoltaic roof (BIPV) on a building. ► BIPV contained thin film solar PV laminated to white membrane, above a layer of insulation. ► Roof solar absorptance decreased to 0.38 from 0.75, lowering roof temperatures by 5 °C. ► Primary benefit of BIPV was electricity generation and not reduced heat flows into the building. ► Building energy simulations predicted energy savings for more typical buildings.

Introduction

Roofs are one of the major interfaces between buildings and the atmosphere. Solar energy absorbed by a roof is transferred either into the building below or to the atmosphere above. Roofs have been traditionally dark in color, thus absorbing a large fraction of incoming sunlight. However, roofs can also be made of highly reflective materials that stay cooler in the sun.

Past monitoring studies have found that increasing the solar reflectance (fraction of incident solar energy that is reflected) of a roof to 0.60 from about 0.10–0.20 can decrease cooling energy use by 10–20% or more [1], [2], [3], [4], [5], [6], [7]. Cooling energy savings can vary with building construction, roof insulation, shade cover, and climate. Widespread use of reflective roofs in urban areas can lower air temperatures, helping mitigate the urban heat island effect [8], [9], [10], [11], and improve air quality [2], [8], [9]. Installation of reflective roofs and pavements has also been proposed as a method to counter the climate warming effects of greenhouse gases [12], [13], [14].

Roofs can also be used to site photovoltaic (PV) modules that convert a modest fraction of incoming sunlight to electricity. Crystalline silicon modules account for approximately 85–90% of the current global PV market [15]. Though crystalline silicon modules usually offer the highest conversion efficiencies (up to around 20% for commercially available products), thin film PVs using cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon, and other chemistries have recently expanded in the marketplace, in part due to their lower cost per watt at the module level. Current panel conversion efficiencies for thin film PV range from about 6 to 12% [15].

Companies now offer a wide range of building integrated photovoltaic (BIPV) systems that integrate PV panels with common building envelope components such as roofs [16], [17]. BIPV systems can ease installation and improve aesthetics by seamlessly integrating PV panels into the building envelope [17]. For example, some BIPV products can be used in place of asphalt shingles on residential roofs. BIPV systems are currently used in both new construction and retrofits of buildings [15].

In this study we assess the effects of retrofitting an office building in Yuma, AZ with a BIPV roof system. The original low-slope, built-up roof had a gray cap sheet surface with a solar reflectance (ρ) of 0.25. The BIPV system (Fig. 1) consists of PV (ρ = 0.27) that is laminated to a white membrane (ρ = 0.77), which in turn lies above a 3.8 cm layer of insulation. The PV panels cover only 36% of the white membrane surface; thus, the area weighted spatial average solar reflectance of the BIPV system (ρ = 0.59) is appreciably higher than that of the original gray roof (ρ = 0.25).

The BIPV system was installed in early June 2009 on top of the original gray built-up roof. Measurements were carried out from 1 May 2009 to 25 September 2011. We report changes due to the installation of the BIPV system in (a) temperatures of the roof surface, roof underside, attic air, and indoor air in the conditioned space; (b) heat fluxes through the roof and ceiling; and (c) HVAC, plug load, and building energy uses. We also report PV energy production and system conversion efficiency.

Section snippets

Description of building

The office building (see electronic supplementary material, Fig. A1) is located at the Marine Corps Air Station (MCAS) in Yuma, AZ and is designated as Building 228 (32°39′39.50″N, 114°35′10.79″W). The office building was built in 1943 and is aligned nearly along an east–west axis. The roof area is 861 m2 (Table 1). The solar reflectance of the cap sheet on the original built-up roof was measured to be 0.25 (Table 2); this reflectance was measured in December 2008 using a Kipp & Zonen CMP3

Results

We first provide an overview of the pre- and post-retrofit periods by presenting time series over the entire measurement period. Next, we evaluate the effect of the BIPV system retrofit by accounting for differing weather conditions for the pre- and post-retrofit periods. We consider weather primarily by plotting building temperatures, heat fluxes, HVAC energy use, and PV energy production versus CDD18C. Finally, we report an analysis of energy production and efficiency of the PV system.

Building temperatures and heat fluxes

Installation of the BIPV system decreased the roof's spatial average solar absorptance by 0.37 (49%) and increased its thermal resistance by 0.47 m2 K/W. The reduction in solar absorptance caused marked decreases in daily mean spatial average roof surface temperatures. Decreases in surface temperature of the white membrane accounted for the majority of BIPV roof surface temperature reductions; surface temperatures of the PV were nearly the same as surface temperatures of the pre-retrofit roof due

Summary

In this study we assess the effects of retrofitting an office building in Yuma, AZ with a building integrated photovoltaic (BIPV) roof system. The BIPV system consists of thin film PV that is laminated to a white membrane, which in turn lies above a 3.8 cm layer of insulation. The PV covered 36% of the white membrane surface. The solar absorptance of the roof decreased to 0.38 from 0.75 after installation of the BIPV system, lowering daily mean roof surface temperatures by about 5 °C. Roof

Acknowledgments

This work was supported by the Department of Defense Environmental Security Technology Certification Program under Project No. SI-200813. It was also supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, State, and Community Programs, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We wish to thank Luke O’Dea of Unisolar for technical information on the PV; Michael Boyd and Ronald Durfey of MCAS Yuma for technical

References (25)

  • S. Konopacki et al.

    Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin

    (2001)
  • D.S. Parker et al.

    Measured cooling energy savings from reflective roofing systems in Florida: field and laboratory research results

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