Modeling of solar thermal heat pipe collectors with overheating prevention in system simulations

• A new TRNSYS type for the simulation of solar thermal heat pipe collectors with overheating prevention is presented. • The type functionality is successfully validated with a mean absolute deviation between simulation and measurement of less than 2 %. • Well-designed heat pipe limitation has only a small influence on the annual system yield. • Use of established discontinous efficiency curve approach leads to significant overestimation of the heat pipe limitation effect. Solar thermal collectors can provide renewable heat in an efficient way. They face however issues of overheating under certain circumstances, which can lead to thermal stress and steam formation. One solution to this issue consists in using solar thermal collectors with overheating prevention based on heat pipes. This paper presents the new TRNSYS type 839 that accurately reproduces the heat pipe overheating prevention effect. The model is validated using data from high accuracy solar tracker measurements as well as real solar thermal systems. A mean absolute deviation in the validation of less than 2 % shows the functionality and accuracy of type 839. The results were compared with simulations using the existing Type 832 (without heat pipe limitation) and the simpler 2-part efficiency curve approach adapted from practice-oriented planning tools. By comparison, type 832 shows a mean absolute deviation of 44.8 % using the same data. The use of the 2-part efficiency curve approach results in a mean absolute deviation of 49 %. With type 839 it is possible to design and evaluate solar thermal collectors with heat pipe limitation in TRNSYS and assess their use in a wide variety of systems. The new model is particularly recommended for applications with a relatively high and constant temperature level (e.g. process or district heating), for gross heat yield simulations or for the design of systems in order to correctly analyze the influence of the heat pipe limitation.