Impact of Small Weather Data Time Steps on the Simulation of Solar and Heat Pump Systems

Abstract

For the simulation of solar thermal systems mainly hourly based weather data are used today. The question arises whether short term fluctuations of e.g. solar irradiation that may be present in measured weather data with smaller time resolution has an impact on the result of solar thermal systems simulation in general, and on solar and heat pump systems in particular. To analyse this impact different solar and heat pump systems were simulated in TRNSYS17 with six minute and hourly weather data from Zurich (Switzerland). The simulations were performed using the boundary conditions that were defined in IEA SHC Task 44 / HPP Annex 38. Starting with a reference solar and heat pump system, single parameters of the system were varied, e.g. collector area, thermal capacity of the collector loop, specific mass flow rates through the collector loop. The most influencing factors were the solar coverage, the specific mass flow rate through the collector and the thermal capacity of the collector loop. A large influence was also observed when the heat exchanger of the solar loop was changed from an internal storage heat exchanger to an external plate heat exchanger with a stratifying charging pipe in the storage. In the reference system the solar collector yield was 0.06% and the consumption of electricity 1 was 0.28% less when simulating with hourly weather data than with six minute weather data 2 . In realistic systems the difference of seasonal performance factor, electricity consumption and solar collector yields were about 1.2% at maximum when hourly averaged weather data were used instead of six minute profiles. In solar thermal systems with an unrealistically small thermal capacity of the collector loop these differences reached up to 2%.