Combitest. A New Test method for Thermal Stores Used in Solar Combisystems

Abstract: This work deals with the testing of water based thermal buffer stores used for low temperature applications (<100C) and specifically those used in solar combisystems. Two different test methods are examined: a new method called Combitest, which was developed in this study; and the CEN prestandard ENV 12977-3. Combitest has two levels: Direct Characterisation (DC) where performance indicators are derived directly from measurements; and Annual Calculation (AC) with parameter identification and long term system simulations, as in ENV 12977-3. Five relatively complex buffer stores designed for solar heating systems have been tested according to ENV 12977-3. The stores included a large variety of features found in buffer stores used in solar combisystems that are currently on the market. Three of the models were used in 10 store variations to create simulated test data used in the development of the Combitest DC test sequence. A further 10 models of store variations were used for evaluating the test sequence and the resulting performance indicators. Two stores were tested in the test stand with the developed DC test sequence and results were compared to detailed simulations using the models derived from ENV 12977-3. A major advantage of Combitest is that it can be applied when the store cannot be modelled sufficiently accurately. Performance indicators are split into those relating to only the store and those relating to the store in the context of a system. The results show that the main system performance indicators of the DC level, fractional energy savings and final energy use, can be used to predict annual system performance of the studied stores with reasonable accuracy using a correction factor. However, the heat loss indicators cannot be used to compare the heat losses of different stores. The evaluation of the AC level, applied to two stores with measured data, shows that the results are similar to those achieved using ENV 12977-3 if the DC test sequence is supplemented with two test sequences from ENV 12977-3. An alternative method of extrapolating the DC system performance indicators to arbitrary boundary conditions was studied in less detail. The results showed that the method is possible but that there are significant uncertainties in the extrapolation process. It is suggested that Combitest be transformed into a system test method that can also be applied to stores using fixed boundary conditions for the boiler, collector and controllers. The study also showed that for some stores it was not easy to fit good parameter sets for the applied store model using ENV 12977-3. Guidelines are required in the choice of the model, choice of fixed values for parameters not identified and for interpretation of results from the identification program and verification sequences.