Theoretical Investigation on a Control - Based Approach to Avoid Stagnation of Solar Heating Systems
Abstract
During the periods of high sun insolation and minimal hot water consumption is the potential that a solar heating system goes into stagnation: the thermal store is full of hot water and the exceed heat cannot be removed from the collector any more. If exposed to high temperatures, the heat transfer fluid in the collector loop may rapidly degrade and excessive pressure may be produced that may damage the solar system components. The stagnation problem is even more harmful in climates with potential freezing periods where the propylene-glycol/water mixtures are typically used as the working fluid in the collector loop. Such mixtures are subject to deterioration at temperatures above 120°C and may become corrosive, resulting in damages to the solar system components. It is a common practice to shut down the pump in the collector loop when the temperature in the thermal store is too high. This helps to prevent damage of the store but accelerates the degradation of the working fluid in the collector loop. Sometimes stagnation can also happen on the sunny days in spring and fall if the mass flow in the collector loop is designed to be too low. There are a plenty of approaches how to minimize the stagnation time of the solar heating systems. Most of them fall in one of the following three categories, see (Morhart, 2010):