Design of combi systems

Investigations have shown that the thermal performance of Danish combi systems is a subject of large variations from system to system. Some systems are well performing, however, more systems have a poor performance. [Ellehauge K et al (2000)]. Most of the combined systems that are installed in Denmark correspond to the system illustrated in Figure1. The control system operates the three-way valve in the solar collector circuit so solar heat is supplied either to the storage tank or to the heat exchanger between the collector loop and the space-heating loop. [Ellehauge K, ShahL.J. (2000)] This paper addresses an experimentally and theoretically investigation of three different marketed combi systems. Two different combi systems, based on the above mentioned system design, with units where the auxiliary energy supply system is built into the same cabinet as the hot-water tank and one combi system based on a tank in tank heat storage are investigated experimentally in a laboratory test facility. One of the units is based on a mantle tank the other is based on a tank with a built-in heat exchanger spiral. The thermal performances of the systems in the laboratory test facility are measured with constant daily hot water consumption, consumption patterns and space heating demand for all days, and the results are used to validate TrnSys models. Based on simulation models of the combi systems, the thermal behavior is simulated and the thermal performance and the solar fraction of the systems is determined. The calculations are based on the simulation program TrnSys [Klein S.A et al. (1996)] and weather data from the Danish Design Reference Year, DRY. The paper will present and compare measured and calculated thermal performances and solar fractions of different combi systems and the main reasons for the different thermal performances and solar fractions for the investigated systems will be given. Further, the paper will present the results of an optimisation of the combi systems in question. The optimisation of the systems is based on experience from the measurements and calculations with the validated simulation models where a number of different design-, control- and consumption parameters are varied.