Conventional Roofs as Collectors in a Solar-assisted Heat Pump System

Abstract

The performance of a series configuration, solar-assisted heat pump system with sensible heat storage in water is evaluated. The system is installed in an occupied house near Basingstoke, U.K., and the solar assistance is provided by the conventional tiled roof of the dwelling. The results of system monitoring over one heating season are presented, and the saving in the requirement of purchased energy is estimated to be about 24% over a resistance heating system. A discussion of store modifications indicates that savings of up to 40% would be possible. From further monitoring, empirical relations are obtained which characterise the in situ performance, in the U.K. climate, of the installed air source heat pump. These permit calculation of the heating capacity and the heating mode coefficient of performance from a knowledge of the psychrometric state of the source air. An analytical model is presented of an air-heating solar collector, which incorporates the finite thickness and thermal conductivity of the absorber plate. Laboratory tests show the model to be applicable to a conventional tiled roof when used as an air heater; similar testing of corrugated metal panels is described. Further experiments show that wind over an un-torched tiled roof increases the outlet air temperature and useful energy gain. A modified model is proposed to explain these effects. Conventional tiled and metal roofs are suitable pre-heaters for air source heat pumps. Assessments, for the monitored installation, of no roof, tile roof and steel roof pre-heating give corresponding seasonal COP(H) values of 2.44, 2.50 and 2.57 (for the same fan power requirements). The analytical models for roofs as air heaters, together with the performance relations for the heat pump, may be used in computer modelling applications to predict the performance of both newly designed and retrofitted installations.