The state of the art of hydraulics (collector and collector array hydraulics) and safety (including stagnation) aspects of the primary solar loop is presented and analysed in a theoretical as well as practical framework, also referring to examples of successfully implemented projects. It is based on international know-how collected by IEA networking activities, presented in a condensed form in this document.
Large-scale solar thermal plants (gross collector area of more than 500 m² resp. 0.35 MWth) provide a huge potential for reducing the consumption of fossil fuels and CO2 emissions. Especially in the context of district heating, industrial processes and thermal cooling, large-scale solar thermal plants are becoming more and more important. Numerous projects in Europe (especially in Denmark) but also internationally (China, Canada, Saudi Arabia, etc.) constitute powerful examples for this trend. The implementation of solar thermal energy has already proved to be technically and economically feasible and sustainable in the practical context. However, the potential is still far from being exhausted. This document focuses on the remaining practical challenges concerning the implementation of large-scale solar thermal plants. For this purpose, the state of the art of hydraulics (collector and collector array hydraulics) and safety (including stagnation) aspects of the primary solar loop is presented and analysed in a theoretical as well as practical framework, also referring to examples of successfully implemented projects. It is based on international know-how collected by IEA networking activities, presented in a condensed form in this document.
The efficiency of a solar collector is influenced by the solar collector fluid, flow rate and collector tilt. However, test institutes usually determine the collector efficiency for only one combination of fluid type, flow rate and tilt angle. This fact sheet describes investigations on the influence and importance of variations of solar collector fluid, flow rate and collector tilt on the efficiency and thermal performance of different solar collectors. Additionally, the effect of a fluorinated ethylene propylene foil used as convection barrier between glass cover and absorber is investigated.
In its basic form, a solar thermal collector is designed to intercept solar radiation, absorb that radiation to convert it into heat energy, and then deliver that heat to a heat transfer fluid. Therefore, the performance of a solar thermal collector is influenced by all variables that affect either the optical or the thermal properties of the collector. For example, the incidence angle of solar radiation onto the solar collector can affect the optical performance of the collector. While typically not a strong factor for solar thermal collectors, the changing spectral quality of sunlight with changing atmospheric conditions can influence the fraction of the incoming solar radiation that gets transmitted and absorbed by the collector. Tilt angle, especially for glazed flat plate collectors, affects internal and external convective heat transfer coefficients, and thus influences collector thermal performance. Heat transfer fluid flow rate and fluid thermal properties influence the heat transfer coefficient inside the fluid passages of the collector, and thus influence the collector efficiency.
Simulation is a very useful tool for design and sizing of a solar collector field. To get a good accuracy it is important to start with a load analysis and secondly to find accurate enough local weather data. Also a time resolution of at least hourly weather data is needed. The split into beam and diffuse radiation is also very important, to derive a good all-day simulation accuracy. Then of course the component models and accuracy of the input data is very important too. Below some hints are given to make a good collector field simulation.
This method for giving and checking annual output of collector fields takes into account that the weather and operating temperatures may vary from year to year. The method works with monthly average operation temperatures and hourly average weather data and will work for systems having approx. constant operating temperatures on a monthly basis – like e.g. solar district heating systems. The basic idea of the method is described in brief below.
A methodology for giving and checking the annual output of collector fields is described. The method takes into account that the weather and operating temperatures may vary from year to year. The method works with monthly average operation temperatures and hourly average weather data and will work for systems having approx. constant operating temperatures on a monthly basis – like e.g. solar district heating systems.
The performance guarantees described here relate to the power performance of a collector field and a heat exchanger under some restricted ("full load") operating conditions. The procedures described here do not pretend to give and check a guarantee on the annual output of the system.