Design of solar collector networks for industrial applications

Abstract

The design and selection of banks of solar energy collectors for thermal applications requires that a thermal and a hydraulic objective be met. On the thermal side, the working fluid must provide the heat load to the process at the specified temperature and on the hydraulic side the fluid must flow through the system experiencing a pressure drop that is within the specified limits. In the case of solar collectors, the working fluid used to transfer heat to the process either in an open circuit or in a closed circuit is water. A solar collector can be viewed as a particular type of heat exchanger and the set of solar collectors needed for a particular application, as a network of heat exchangers. In this work, the overall arrangement of solar collectors which form the total collector surface area is referred to as the network of solar collectors (NSC). An NSC is used in large scale heating applications in buildings or in production processes. Such a network of collectors can exhibit arrangements that go from series, parallel or any combination of these. Contrary to domestic applications where water flows through the exchanger in natural convection, in large scale applications the flow of water is forced through the use of a pumping system. This paper introduces the tools for the design and selection of the most appropriate network arrangement for a given application as a function of the specified pressure drop for fluid flow, the required temperature and heat duty. A thermo-hydraulic model for solar collector networks is presented and its solution is graphically displayed with the length of the exchanger plotted against the number of arrays in parallel. The thermal and the hydraulic models are solved separately so that two solution spaces are represented in the same plot. The thermal space represents the thermal length required to meet the specified heat load as a function of the number of parallel arrays. The hydraulic space, on the other hand, represents the hydraulic length that meets the specified pressure drop as a function of the number of parallel arrays. The point where the two spaces meet determines the network structure that fulfils the required heat duty with an acceptable pressure drop.