Targeting the Maximum Outlet Temperature of Solar Collectors

Abstract

The aim of this work is to show the construction and application of design curves for solar collector networks of the flat plate type. The design curves display two important design parameters: delivery temperature and the minimum number of solar collectors in series to achieve that target temperature. Given the complex nature in which the various ambient and operating parameters interact to determine the thermal performance, it is justifiable to develop quick and reliable design methodologies to estimate the collector surface area required for a given industrial application. Since a solar thermal plant operates under intermittent conditions, it recognised that it must exhibit a flexible design to allow it to deliver the required thermal duty (load and temperature) under any set of ambient conditions. When establishing the number of collectors required for a given duty, the question to answer is if the system should be sized based on the most favourable ambient conditions or on the least favourable. This problem becomes an optimization problem but before such an approach is undertaken, some design and operating criteria must be established. This paper seeks to establish these criteria based on the concept of maximum outlet temperature attainable with the minimum number of solar collectors. A graphical representation of these two parameters is presented and a design approach of solar collector networks taking advantage of the manipulation of inlet temperature is demonstrated. The plots are derived for design parameters such as solar radiation, mass flow rate and inlet temperature using an experimentally validated thermal model. The approach is demonstrated on flat plate collector technology, but it can be extended to any other type of low temperature solar technology.