9 - Active solar water heating systems

Abstract

Active solar water heating (SWH) systems comprise five main elements: a collector or collectors that capture solar radiation, a pump to activate working fluid circulation, a storage system for the hot water, an auxiliary or back-up water heating system for use when sufficient hot water cannot be supplied by the solar system, and a set of controls to regulate the operation of the total system. When sunlight heats one or more collectors sufficient for heating, sensors and a controller activate the pump to circulate either water from the storage tank or antifreeze solution. When antifreeze solution is used, the solar heat is transferred to water in the storage tank through a heat exchanger (HX). The hot fluid can be used directly for hot water needs or space-heating/cooling needs. The heated energy can also be stored in a proper storage tank for use in the hours when sunlight is not available. It is essential that the storage tank and the piping connections from the collector to the tank are well insulated to prevent heat loss. The amount of solar heat produced depends on the collector design, the surface area of collector tubes, the type of working fluid, and the local climate and site conditions. SWH systems are designed to meet a particular load by utilizing a combination of properly sized collector-storage units and auxiliary energy source. It is desired to deliver hot water for most of the year. Thus during the winter when freezing temperatures occur, antifreeze chemicals are used as well as auxiliary energy to meet sufficient hot water needs. In addition, auxiliary energy can be supplied either by a gas or electric booster within the water tank or at the water exit of the tank. On the other hand, during summer, if no energy is withdrawn over a sufficiently long period of time, overheating could occur. In such conditions, a control system is essential to turn off the circulating pump. SWH has gained wide applications in the building sector all over the world [1]. This chapter covers a detailed overview on various types of solar-assisted water heating systems and their market potential.