T. Matuska*, J. Metzger and V. Zmrhal

Czech Technical University, Faculty of Mechanical Engineering, Department of Environmental Engineering,

Technicka 4, 166 07 Prague 6, Czech Republic

* Corresponding Author, tomas. matuska@fs. cvut. cz

Abstract

Mathematical model and design software tool KOLEKTOR 2.2 with user-friendly interface for detailed modelling of solar thermal flat-plate collectors is presented. Mathematical model is based on internal and external energy balance of the absorber solved in iteration loops to determine the temperature distribution and heat transfer coefficients in main parts of solar collector. Mathematical model has been validated with experimentally obtained data for different solar flat-plate collector concepts (low and high performance atmospheric collectors, evacuated collector). The software tool KOLEKTOR 2.2 is applicable especially for design and virtual prototyping of new solar flat-plate collectors resulting in efficiency curve determination, for parametric analyses to obtain information on different parameters influence on collector performance and especially for investigation of thermal performance of advanced solar collectors (building integrated, evacuated collectors, etc.).

Keywords: solar collector, evacuated collector, performance modelling, experimental validation

1. Introduction

Computer modelling of solar thermal collectors is a principle approach for testing of new construction concepts and improvements in the development and design stage for developers and manufacturers. Virtual prototyping of solar collectors can save the investments into number of prototypes and foreseen the collector performance in advance. Analyses of individual construction parts and details parameters impact on the collector performance is needed to make decision on efficient solar collector concepts for given application, operation and climatic conditions with respect to economic parameters of construction.

A mathematical model is always a simplification of reality to certain extent. Too complex mathematical models and numerical programs require huge amount of computer time for calculations, too simplified models don’t take important influences of detailed collector parameters into account and result in considerable uncertainty in calculation. To find a good compromise between simplicity of the model and its accuracy is crucial for development of any design and simulation tool.

Although the theory of flat-plate solar collectors is well established and can be found in basic literature [1-3], there is a lack of user-friendly design programs for solar collector performance modelling considering the detailed geometrical and physical parameters of collector. Number of authors evolved simplified analytical models considering temperature independent solar collector overall heat loss coefficient (linear dependence of efficiency), neglecting the absorber temperature distribution or temperature difference between absorber surface and heat transfer fluid. Such

models are not comparable with physical experiments and cannot predict the real performance behaviour and evaluate efficiency characteristics of solar collectors.

Theoretical model of solar collector has been introduced in TRNSYS Type 73 [4] but with simplified calculation of collector heat loss coefficient U insufficient to cover wide range of parameters affecting the collector heat loss. More theoretical model with number of detailed input parameters and calculation of heat transfer coefficients in the individual parts of collector (in air gaps, inside pipes, at outer surfaces) has been evolved as a Type 103 [5].

A design program CoDePro [6] for energy performance calculation of solar flat-plate collectors has been developed with the Energy Equation Solver. It allows a very detailed specification of collector geometrical and material parameters. It covers large segment of solar collectors (unglazed, single and double glazed) and evaluates also optical properties of collector, e. g. incident angle modifier. On the other hand, the features of CoDePro program, analogous to TRNSYS Type 103, don’t allow energy performance modelling of advanced solar collectors, e. g. collectors integrated into building envelope, evacuated flat-plate collectors or solar collector with glazing made of transparent structures.

The presented model and design tool KOLEKTOR 2.2 has been developed to overcome the drawbacks of previous models. KOLEKTOR 2.2 is based on detailed calculation of heat transfer from the collector absorber to ambient and from the collector absorber to heat transfer fluid. The advantage of the design tool is its universality to wide-range of solar flat-plate collectors stock from evacuated to atmospheric, separately or building integrated, covered with different types of glazing (single glazing or transparent insulation structures), etc.