This work introduces the concept of flexible operation in the design of solar thermal utility systems for low temperature processes. The design objectives are: (a) The supply of the thermal needs of the process (heat duty and minimum required temperature), and (b) the maximization of the operating time during the day. The approach shows how the network structure is defined by adjusting the mass flow rate and the inlet temperature of the working fluid to achieve the smallest collector surface area. This work emphasizes the need to specify the solar network structure, which is comprised of two main elements: The number of lines in parallel and the number of collectors in series in each line. The former of these two design specifications is related to the heat load that the system will supply, while the latter is directly related to the delivery temperature. A stepwise design approach is demonstrated using two case studies where it is shown that the detailed design of the solar collector network structure is fundamental for a successful thermal integration with minimum investment. In this paper, the design methodology is based on flat-plate solar collectors, but it can be extended to any other type of low temperature solar technology.

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The aim of this paper is to present a design approach for the selection of the most cost-effective solar thermal plant composed of flat plate solar collectors for a given process. The approach seeks to provide the designer with the necessary elements for decision making, namely minimum total operating costs, minimum required plot area for installation, maximum daily operating time and determination of payback time. The optimisation methodology search uses a cost equation for flat plate solar collectors along with a thermal model linked in a MatLab and General Algebraic Modeling System (GAMS) platform. In stand-alone collector design, tube diameter, tube length and collector width are the variables that have a major impact on the cost; in the case of solar collector fields, the number of collectors in series is the variable with the strongest impact. It is shown that the optimised networks fulfil the thermal duty within the limitation of land area. The designs are carried out for the conditions that guarantee operation throughout the year and exhibit saving of up to 50% and payback times lower than six years. The approach is demonstrated using a case study.

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