PREFA Energy Roof
Motivation, aims, method
The new development of Prefa Aluminium Products, as opposed to other conventional solar systems available on the market, uses the entire roof surface as a solar collector in order to provide energy for heating and hot water for the household. The low temperature heat generated by water drops spread evenly on the roof surface exposed to sunshine is collected in an underground rain water tank. The heat is extracted from this storage via a heat exchanger and used by a heat pump connected to the heating system of the house. The latent heat of the water in the storage can be used through ice formation on the heat exchanger surface.
Drawing 1 illustrates the components of the heat and hot water energy supply circuit. Water from the tank is pumped to the water pipe on the rooftop. The roof evenly covered with water gets warmed up through natural radiation and it transfers part of this heat to the flowing water. Afterwards the heated water is collected by a gutter and is transported to the tank via a pipe system, where a heat exchanger which supplies the heat pump is located. The tank also serves as an ice storage (the heat exchanger ices up) and as a result the latent heat of ice can be used.
In order to establish collector characteristics measurements were carried out by Prefa. The parameters relevant for efficiency identification were measured over several months and average values were calculated. These data resulted in efficiency characteristic of the collector (aluminium roof sprinkled with water). A simulation model (TRNSED) for an economical analysis was developed by the Institute of Thermal Engineering at Graz University of Technology.
This model was developed using the simulation environment TRNSYS16 and includes a model for calculation of the ice storage developed at the Institute of Thermal Engineering. Excel tools were produced for the evaluation of simulation data and for defining input parameters. The simulations supported by measurements prove that the Prefa energy roof in the presented concept is sufficient for the supply of household's heating and hot water when the system is adequately designed. The whole system is currently in the development stage. Further measurements for establishing collector characteristics are being processed and the expertise will be deepened within the pilot projects. Those measurements should also prove the efficiency of the developed system and will be used for the validation of the provided simulation model.
After two years of intensive research there is no doubt that there should be use for the roof-generated energy and various field studies show the range of applications of the system.
Further steps can be taken after the 2006/2007 heating season is evaluated. There is no doubt however that under appropriate conditions e.g. roof inclination or heat demand, the system can provide enough energy to heat a building. It is also worth mentioning that the total investment in case of the energy roof equals approximately the cost of conventional heating systems and the whole system can be easily set up by a trained installer.
Ing. Harald Neumüller / PREFA Aluminiumprodukte GmbH
DI Dr. W. Streicher
Institute for Heat Engineereing
University of Technology Graz
Inffeldgasse 25/B, A-8010 GRAZ