Analysis of an Active Charge/Passive Discharge Solar Space Conditioning System

Recently builders have shown considerable interest in combining the features of active and passive solar space heating systems into a hybrid system employing active collection and some form of passive discharge/storage device. We examined the performance and economics of a hybrid, air-based, residential system using 18.5 m2 of roof-mounted air collectors and a hollow-core slab that dischargesdirectly to the space by radiation and convection. In addition to space heating, the system provides domestic hot water heating and summer cooling by night venting of the slab. The system is assumed to be installed in a fairly 'tight' new house with a floor area of 140 m2 and 7 m2 of south-facing glazing. Preliminary slab design parameter sensitivities were examined by simulating the slab understeady-state conditions using a three-dimensional multi-node model. These preliminary findings were employed in developing an annual simulation of the entire system and house using the TRNSYS computer code. A limited number of sensitivity analyses were conducted with TRNSYS. These included sensitivities to climate, collector parameters, slab design, heat transfer parameters, heating loads, andcontrols, as well as an economic and performance comparison with an air-based active solar system using rockbed storage. The hybrid system heating performance and economics under the base case conditions were comparable to those of the active/rockbed system. The hybrid system collector outlet temperatures were lower than those of the active system, yet the inlet temperatures were higher,yielding comparable collector efficiency.