Thermal Store Testing-Evaluation of Test Methods
Licentiate thesis, 2002
This work deals with the testing of water based thermal buffer stores for low temperature applications (<100°C). Two different test methods are examined: the preliminary CEN standard prEN129773 and a new method called Combitest, which was developed here. Both methods comprise identification of parameter values for a simulation model, in this work the Multiport store model Type 140, developed for the simulation program TRNSYS.
Three relatively complex buffer stores designed for solar heating systems have been tested according to prEN12977-3. The stores inelude a large variety of features found in buffer stores currently on the market.
The study shows that for some stores it was not easy to fit good parameter sets for the applied store model using prEN12977-3. This means that simplifications to the model and the amount of data used for parameter identification give varying degrees of increased inaccuracy dependent on the store and system model used. One parameter however, the effective vertical conductivity, proved to be insensitive in all variations and can easiiy be estimated. Heights could also be estimated with reasonable accuracy, although care should be taken for heights where the effective volume is only a small part of the whole store.
A major advantage of Combitest is that it can be applied when the store cannot be modelled sufficiently accurately. Combitest has two parts: a Direct Characterisation (DC) part where performance indicators can be derived directly from measurements, and an Annual Caiculation (AC) part with parameter identification and long term system simulations, The Combitest method was evaluated using synthetic measurement data that was created from the ouput of a simulation modeL The resuits show that the main performance indicator of the DC part, fractional energy savings, can be used to rank the 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 part, restricted to one store, shows that the resuits are very similar to those achieved using prEN12977-3. Further development of Combitest will be based on real measurements.
PrEN12977-3 requires detailed expert knowiedge and is rather time consuming. The full test procedure requires about 17 working days as weil as 25 days use of the test stand. Simplification of the procedure can save time in the test stand, as weil as up to two working days for an expert. Similar savings can be achieved using Combitest. However, if only the DC part of Combitest is to be appiied, only haif the work time wouid be required compared to prEN12977-3.