Pre-design of collective residential solar districts with seasonal thermal energy storage: Importance of level of detail

Solar districts with seasonal thermal energy storage show great potential in reducing the climate impact of buildings. However, many possible configurations or concepts with different technologies exist for these solar districts. As a result, detailed modelling and simulation of all possible configurations are highly time-consuming and therefore a higher level pre-design phase is often necessary. To limit calculation time, system simplifications are usually applied in this pre-design phase. This paper investigates the effect of two of these simplifications, i.e. fixed coefficient of performance (COP) values for heat pumps and fixed efficiency profiles for solar thermal collectors. In this study, two pre-design methods are developed and investigated, one with the aforementioned system simplifications and one without these simplifications. The methods preliminary size the most important system components and consequently determine the total cost of ownership and CO2 emissions of each concept. Both approaches are applied to a virtual solar district of 50 residential dwellings for which 4 configurations with a seasonal tank storage and two configurations with a borefield storage are identified. Results show that the method with system simplifications underestimates the total cost of ownership of the configurations with a tank storage by 56%, while it overestimates the total cost of ownership of the configurations with a borefield by 9.3% and the average cost of the borefield concepts changes from being 230 thousand euros higher to being 5.44 million euros lower than the average cost of the tank concepts when the simplifications are refined. Adopting the system simplifications thus results in a completely different selection of preferred configurations, and should therefore be avoided when assessing different concepts for solar districts.