The simulation study is based on a single family house located at WUrzburg, Germany with a heated living area of 128 m2. The roof on which the collectors are mounted is facing south and is inclined by 45°. The building fulfils the requirements according to the German Energy Saving Directive (Energieeinsparverordung: EnEV) resulting in a space heating demand of 9090 kWh or 71 kWh/m2 respectively. The space heating loop is controlled according to the outdoor temperature and the maximum forward / return temperatures are 50/30 °C.

The heat demand for a hot water production of 200 litres per day at 45 °C equals 28 kWh/(m2a) or 3590 kWh/a respectively including the heat losses of a “typical” conventional domestic hot water store. Based on these assumptions the total heat demand for domestic hot water preparation and space heating comes up to 12680 kWh/a. If an oil or gas boiler with an average boiler efficiency of 85% is used, this results in an annual total energy demand of approximately 14900 kWh. The performance of the different systems is assessed on the basis of the fractional energy savings fsav. This quantity describes the percentage of energy that is saved by using a thermal solar system instead of a conventional, none solar heat generation system.

2 Results

By means of a simulation study the influence of different design parameters and store concepts on the fractional energy savings was determined for the reference conditions described above. The results for the so-called advanced storage concepts are shown in figure 1. Additionally two curves for a hot water store with a conventional thermal insulation are plotted as a reference in figure 1. An extensive simulation study about the influence of the hot water store volumes and different collector areas as well as collector types was already published in /1/.

Figure 1 shows, that by using latent or sorption heat stores with an effective store volume of approximately 1 m3 (plus 750 litres for the “conventional” combistore) and collector areas in the range of 45 m2 (flat plate collector) it is already possible to cover more than one half of the heat demand by solar energy. Due to the fact that the simulations were based on idealised assumptions, the store volumes may be twice as large in reality. One main reason for this is the volume of the necessary heat exchanger or condensate tank respectively.

At present latent and sorption heat stores for this kind of application are still in the scientific and industrial development phase. Therefore such stores are only offered by a few companies. However, regarding the long-term developments these technologies should not be ignored.

Compared with latent and sorption stores, hot water stores require larger store volumes. However, the main advantage of hot water stores is that they are based on a well known technology that is already introduced to the market. Due to this they also show at present the largest potential for cost reduction. With further improvements such as vacuum heat insulation an increase in thermal performance is still possible.