Transient thermal modelling of evacuated U-tube solar collectors: a case study for carbon-dioxide

In recent years, the use of renewable energy sources has attracted more attention due to reasons such as the decrease in fossil fuel reserves, environmental pollution, and global warming. In this respect, solar energy, being clean, abundant, and free, is seen as one of the most dependable renewable energy resources. Evacuated U-tube solar collectors operate at higher temperatures and more efficiently compared to flat plate collectors due to their selective surface coating and vacuum structure. Therefore, it is widely employed in solar energy applications. In addition, it is very important to use natural working fluids to protect the ozone layer and prevent global warming. CO2, which is one of the natural fluids, is an inert gas, non-explosive, non-flammable, non-toxic, and non-corrosive. It has also become very popular for use in power cycles that utilize energy from low/medium heat sources owing to its low critical properties. The objective of the current research is to conduct transient modelling of the CO2-based evacuated U-tube solar collectors. The evacuated U-tube solar collector system consists of 15 sets of collectors, each with 13 evacuated tubes. The transient performance of the solar collectors was calculated by developing a time-dependent mathematical model in the MATLAB computer program. The model has been validated with the experimental results of an actual system constructed at Doshisha University in Kyoto, Japan. The time-dependent temperature variations of each layer of the evacuated U-tube solar collectors and the fluid exiting the collectors were investigated. As a result of the study, taking into account the meteorological data of a specific date, an average deviation of 18% was achieved.