This paper describes design, simulation, construction and measured initial performance of a solar water heating system (360 Evacuated Heat-Pipe Collector tubes, 54 m2 gross area, 36 m2 net absorber area) installed at the top of the hot water recirculation loop in the Social Security Mid-Atlantic Center in Philadelphia. Water returning to the hot water storage tank is heated by the solar array when solar energy is available. This new approach, as opposed to the more conventional approach of preheating incoming water, is made possible by the thermal diode effect of heat pipes and low heat loss from evacuated tube solar collectors. The simplicity of this approach and its low installation costs makes the deployment of solar energy in existing commercial buildings more attractive, especially where the roof is far removed from the water heating system, which is often in the basement. Initial observed performance of the system is reported. Hourly simulation estimates annual energy delivery of 111 GJ/year of solar heat and that the annual efficiency (based on the 54 m2 gross area) of the solar collectors is 41%, and that of the entire system including parasitic pump power, heat loss due to freeze protection, and heat loss from connecting piping is 34%. Annual average collector efficiency based on a net aperture area of 36 m2 is 61.5% according to the hourly simulation.

This paper describes the design, simulation, construction and measured initial performance of a solar water heating system (a 360-tube evacuated-tube heat- pipe solar collector, 54 m 2 in gross area, 36 m 2 in net absorber area) installed at the top of the hot water recirculation loop in the Social Security Administration's (SSA) Mid-Atlantic Center in Philadelphia. When solar energy is available, water returning to the hot water storage tank is heated by the solar array. This new approach, in contrast to the more conventional approach of preheating incoming water, is made possible by the thermal diode effect of heat pipes and low heat loss from evacuated-tube solar collectors. The simplicity of this approach and its low installation costs make the deployment of solar energy in existing commercial buildings more attractive, especially where the roof is some distance away from the water heating system, which is often in the basement. Initial performance measurements of the system are reported. Hourly simulation results estimate an annual energy delivery of 111 GJ/year of solar heat and an annual efficiency (based on the 54 m 2 gross area) of the solar collectors of 41%. For the entire system, including parasitic pump power, heat loss due to freeze protection and heat loss from connecting piping is estimated to be 34%. Annual average collector efficiency, based on a net aperture area of 36 m 2 , is 61.5%, according to the hourly