Roof Integrated Solar Absorbers: The Measured Performance of “Invisible” Solar Collectors: Preprint

The Florida Solar Energy Center (FSEC), with the support of the National Renewable Energy Laboratory (NREL), has investigated the thermal performance of solar absorbers which are an integral yet indistinguishable part of a building’s roof. The first roof-integrated solar absorber (RISA) system was retrofitted into FSEC’S Flexible Roof Facility in Cocoa, Florida in September 1998. This “proof-of-concept” system uses the asphalt shingle roof surface and the plywood decking under the shingles as an unglazed solar absorber. The absorbed solar heat is then transferred to water that is circulated from a storage tank through polymer tubing attached to the underside of the roof decking. Data collected on this direct 3.9 m (42 ft) solar system for a period of 12 months indicates that it was able to provide an average of 3.4 kWh per day of hot water energy to the storage tank under a 242 liters (64 gal) per day load. The RISA system’s average annual solar conversion efficiency was also determined to be 8 percent, with daily efficiencies reaching a maximum of 13 percent. In addition, a thermal performance equation has been determined to characterize the Phase 1 RISA system’s year-long efficiency under various ambient temperature, insolation, and wind speed conditions. As a follow-on to the proof-of-concept phase, two prototypes of approximately 4.5 m (48 ft) surface area were constructed and submitted for FSEC thermal performance testing. These Phase 2 RISA prototypes differ in both roof construction and the position of the polymer tubing. One prototype is similar to the “proof-ofconcept” RISA system as it employs an asphalt shingle roof surface and has the tubing mounted on the underside of the plywood decking. The second RISA prototype uses metal roofing panels over a plywood substrate and places the polymer tubing between the plywood decking and the metal roofing. Both prototypes were tested according to ASHRAE Standard 93 for determining the thermal performance of solar collectors. From performance data measured both outdoors and indoors using a solar simulator, FR(ταe)’s were determined to be approximately 18% and 33% for the asphalt shingle and metal roof RISA prototypes, respectively. In addition, the coefficients of linear and second-order efficiency equations were also determined at various wind speeds. Finally, an FSEC thermal performance rating was calculated at the low and intermediate temperature levels. In summary, this paper is a first look at the thermal performance results for these “invisible” solar absorbers that use the actual roof surface of a building for solar heat collection. NOMENCLATURE Ar = absorber area of collector, m (ft) FR = solar collector heat removal factor, dimensionless Gt = global solar irradiance incident upon the aperture plane of collector, W/m2 (Btu/h • ft) tc = collector temperature (average of the fluid inlet and outlet temperature,°C (°F) tf,e = temperature of the transfer fluid leaving the collector, °C (°F) tf,e,initial = temperature leaving collector at the beginning of time constant period, °C (°F) tf,e,T = temperature of the heat transfer fluid leaving the collector at specified time, °C (°F) tf,i = temperature of the transfer fluid entering the collector, °C (°F) α = absorptance of the collector absorber surface for solar radiation, dimensionless η = collector efficiency (actual useful energy collected divided by solar energy intercepted by absorber area) τ = transmittance of the solar collector cover plate, dimensionless UL = solar collector heat transfer loss coefficient W/(m • °C) (Btu/(h • ft •°F)) (τα)e = effective transmittance-absorptance product, dimensionless