Modelling and system analysis of new photovoltaic thermal solar collectors

This project report is a deliverable within the scope of WenSDak project, which is being carried out by a consortium of a number of photovoltaic-thermal (PVT) panel manufacturers and knowledge institutes. This project is financed by RVO (Rijksdienst voor Ondernemend Nederland) – project number TEZG113008.
WenSDak project aims to generate knowledge about the performance and application of both PVT liquid collectors and PVT air collectors, in the built environment. Eindhoven University of Technology (TU/e) is a project partner in WenSDak project, offering expertise in lab testing of the panels and PVT collector modelling. Solar Energy Application Centre (SEAC) is another partner in the project, offering expertise in field testing of the panels and system analysis. This report comprises of work performed at TU/e and at SEAC, documenting the results of lab testing, PVT collector modelling and system analysis.
In the context of this project, a PVT collector is a roof-mounted or a roof-integrated panel in which a photovoltaic (PV) module and a thermal absorber are coupled together to make a single hybrid panel, for the purpose of harnessing solar energy to produce heat and electricity from the same area. The absorber has tubes or channels containing circulating liquid to capture heat while the PV module can be connected to the grid or to a battery.
The first goal of the project was to build a test setup at TU/e lab to carry out performance testing of PVT panels. A test setup was built to measure thermal and electrical efficiencies of PVT panels under controlled lab conditions. This setup consists of a solar simulator to imitate the solar spectrum, a thermostatic bath to regulate the liquid temperature at the inlet of the panel, and a flow meter to regulate the flow of liquid, among other components. The electrical test circuit has a LEM module to measure current, a voltage bridge to measure voltage and a micro-inverter to condition the electrical output to feed into the grid. With this test setup thermal and electrical efficiencies can be measured with relative standard uncertainties of 2.5 – 10 % and 2.5 %, respectively.
Four unglazed PVT panels from two different manufacturers were tested using this setup. The goal of the measurement was to test each panel in accordance with the solar thermal testing norm NEN-EN ISO 9806, and draw the thermal efficiency curves for each panel, which are standard in solar thermal industry. For each tested panel it was observed that as the panel temperatures go up, it loses more heat to the surroundings. Therefore, the thermal efficiency of the panel goes down. Furthermore, the thermal efficiency goes up slightly with increasing liquid flow rate. This is due to the fact that at higher flow rates, the panel temperatures are lower and hence less heat loss to the surroundings.
First three panels tested on this setup are identical with different heat conduction media between the aluminum absorber and the liquid carrying tubes partially in contact with the absorber. The first panel had air, a very poor heat conductor, in between the absorber and the liquid-carrying tubes. The second and the third panel had heat conducting epoxy and silicon paste respectively. It was observed that with increasing conductivity of the media, the thermal output of the panel increased significantly. The zero loss thermal efficiency (corresponding to average liquid temperature inside the panel equal to the ambient temperature) of the three panels was measured to be 23 %, 28 % and 31 % respectively, at 75 kg/hr flow rate.
The fourth PVT panel tested on this setup has different thermal absorber principle than the previously tested panels, which leads to enhanced heat conduction from the PV to the liquid carrying channels. As expected it exhibited much better performance during the tests. Zero loss efficiency for this panel was tested to be 56 % at 75 kg/hr flow rate which is 80 % better when compared to the panel-3.
The second objective of the project was to develop numerical models for the tested panels. A steady-state numerical model was developed by adapting the previously developed model for glazed PVT collectors, to the unglazed collectors. A new radiation heat exchange model for the top panel surface and a heat conduction model for the metal absorber have been added to this adapted numerical model. This numerical model was validated with the measurements carried out in the lab. To carry out the model validation, temperatures were measured at two different locations on different surfaces inside the panel. Then, these measured values were compared with the temperatures predicted by the numerical model. It was found that the model is a fairly accurate representation of the tested panels within the accuracy of the measurement.
The validated model was used for design optimization of the tested PVT. It was shown that for panel-1, improving the thermal contact between the PV and the metal absorber can improve the zero loss efficiency by up to 43 %, while improving the thermal contact between the absorber and the liquid-carrying tubes can improve the zero loss efficiency by up to 25 %. Panel-2 and panel-3 are only an improved version of the first panel with better thermal contact between the absorber and the liquid- carrying tubes. Combined, these two improvements can improve the zero loss efficiency by 94 %.
Panel-4 is already a much better concept with very good thermal heat conduction between the absorber and the liquid carrying channels. Hence, there is virtually no potential for improvement. However, the thermal contact between the PV and the absorber can be improved which can lead to up to 27 % increase in zero loss efficiency.
The numerical model was further used to compare all the PVT panels on the basis of annual yield for a fixed flow rate of 75 kg/hr and a fixed liquid inlet temperature of 10 °C. It was estimated that panel-4 has 90 % higher annual yield when compared with panel-1.
The third and the fourth goal of this project were to design an optimized PVT-based system for supplying space heat and domestic hot water to a single family house; and to carry out a techno-economic analysis of this system, respectively. To accomplish goal-3, a detailed literature study of various users and different heating services was done. Based on this analysis the focus was narrowed down to space heating and domestic hot water (DHW) in residences, space heating in offices, swimming pool heating, and manure drying. Finally, a single family house with space heating and DHW services was selected for further investigation and for designing a PVT-based heating system.
The next step was to identify the most suitable system for this application. To do so, a number of market-vailable and novel system concepts were studied. Based on the type of PVTs involved in WenSDak project and in consultation with project partners, a PVT heat pump system was shortlisted for system simulations and system optimization. This system consists of a PVT and a heat pump which supply heat to the storage separately. The storage supplies both space heating as well as domestic hot water. The source of heat pump is the PVT collector itself, coupled directly to the evaporator of the heat pump. The heat pump is used in times of low irradiations and when the storage is not able to supply space heating. To supply domestic hot water the output from the storage is boosted to the desired temperature by using an electric heating element.
Once the user, the heat demand and the system was finalized, a TRNSYS® model was created. For this simulation, an energy efficient reference house with annual space heating demand of 10.8 GJ and an annual domestic hot water heating demand of 9 GJ was selected. The space heat delivery system is assumed to be low temperature radiators. This model is currently under development. In future, the designed system will be optimized by varying the component sizes, and control and stratification scheme to obtain an optimized design which leads to highest seasonal performance. Finally, a techno-economic analysis will be carried out by comparing this optimized design with a conventional gas boiler or an air heat pump system, to make a business case for the designed system by estimating the energy savings as well as the financial savings achieved by this system, when compared with the conventional system.