State of the art: Large solar thermal systems for different applications are gaining importance for the sustainable generation of heat, both on Austrian and international levels. The Austrian solar industry is aware of this development and plays a key role in the segment of plants with >500 m² of collector area. Numerous large installations with Austrian participation are a proof of this. Given the huge market potential and the good market position of Austrian companies, at the beginning of ParaSol it was essential to build up fundamental knowledge in this area.
Motivation and problem: One of key aspects in the design of large solar thermal plants is the hydraulic layout of the collectors and collector arrays. This starts with the hydraulics inside one collector and ends with the design of several layers of hydraulic connections in the collector array. For what concerns the hydraulics inside one collector, no reliable information was available in the scientific literature regarding the fluid dynamics of real-world T pieces like they occur in the parallel connection of absorber tubes; this hinders the development of optimized collector designs. Also on higher hydraulic levels (connection of collectors in parallel and in series to form groups and rows), at the beginning of ParaSol there existed no adequate means to comprehensively describe the crucial hydraulic conditions in a large collector array. For this reason, large solar thermal systems currently fall below their potential in both technical and economic terms (reduction of system cost and levelized cost of energy), which in turn reduces their competitiveness compared to conventional heating systems.
Objectives: Against this background, one of the aims of ParaSol was to develop a solid mathematical-physical model capable of detailed calculations of various hydraulic processes in solar thermal plants. This model had to be validated by means of experimental measurements in the laboratory and in the field. In particular, one objective was to carry out detailed calculations of flow and temperature distributions as well as friction and minor pressure losses in all hydraulic levels of the system. Based on these findings, technical key figures should be developed within ParaSol in order to facilitate the effective characterization and assessment of different hydraulic connections of large solar collector arrays.
Methods and results:
- Within ParaSol, detailed laboratory experiments on flow distributions and pressure loss behavior of T pieces could be carried out for the first time under boundary conditions that are relevant for solar thermal. These findings generate information that was not available up to now in the scientific literature. A total of 20,596 individual readings were recorded, each with straight and side pressure losses for dividing and combining flow. The following parameters were varied: Reynolds number (range 250 to 25,000), volume flow ratio absorber to manifold, intrusion depth absorber into manifold (-3mm to + 9mm) and four manifold pipe diameters. These pressure drop measurements were evaluated in MATLAB with the help of 16 Artificial Neural Networks and integrated in the calculation tool for solar thermal collectors and collector arrays by means of a .dll file. The calculation tool also comprises findings from detailed theoretical investigations as well as from experimental work in the laboratory of AEE INTEC concerning the transition region between laminar and turbulent flow and flow distribution in collectors.
- Next step in the project was the validation of the calculation tool: In the first instance, this validation was carried out in the laboratory of AEE INTEC on the basis of measurements at eight specially crafted harp-shaped tube registers. Subsequently, the calculation tool was validated with the aid of operating data from a large solar thermal plant in Graz (362 m² collector area). The level of detail of the measuring equipment of this system is unique in the world. The agreement between model and measurement is very good for what concerns temperature and flow distribution and good for what concerns pressure losses; considerable uncertainty comes from the material properties of the heat transfer fluid. With the outcomes of ParaSol, an extensive, solid and validated calculation methodology is available to calculate flow distributions, friction and minor pressure losses, temperature distributions as well as thermal and hydraulic efficiencies for solar thermal collectors and collector arrays.
- In the next part of the ParaSol project, a fundamentally novel approach for assessing collector arrays was developed: A set of 11 "characteristic key figures" allows a quick overview of the essential technical phenomena in collector arrays and forms a sound basis for economic and technical evaluation of different hydraulic concepts for solar thermal collector arrays. All key figures can be calculated already within the design phase of a solar thermal plant. In this sense, the characteristic key figures are an improvement of the detailed design process of solar thermal plants.
Benefits and outlook: Based on the work carried out, ParaSol is a significant contribution to understanding hydraulic-thermal and economic issues in large solar thermal systems. The project results provide new findings for the solar thermal industry, in particular for large solar thermal systems. ParaSol developed important fundamental knowledge, drew simplifications for the design process, identified specific cost-reduction potentials and increased planning reliability. The project results contribute to an improved profitability of large solar thermal plants and improve the chances of Austrian collector manufacturers, planners, constructors and operators at home and abroad.
In order to deepen and extend the knowledge gained from the Parasol project, as a next step it would now be important to link the project results with practical experience from the design and operation of large thermal plants. For this purpose, one way to go is to compare design aspects (such as hydraulic-thermal calculations of collector array layouts and the characteristic key figures) with measurement data of the plants. The aims are cost reduction and further standardization of the system design as well as strengthening the solar thermal industry.
MATLAB Toolbox to calculate pressure loss coefficients (zeta values) for T-pieces at low Reynolds numbers, based on a Neural Network model and experimental data, published under the Creative Commons Attribution Share-Alike 4.0 license, available for download at Zenodo: https://zenodo.org/record/383647#.WOIWw_IZK5R