What are solar water heating systems (SWHS) for industrial processes?

Components of SWHS and types of SWHS

What is a solar water heating systems (SWHS)

Flat plate collector (FPC)
Evacuated tube collector (ETC) with 60 glass tubes
Horizontal storage tank with insulated tubes containing the heat transfer fluid

Solar water heating systems are using sunlight to heat water by converting the solar irradiation directly into heat within a solar collector.

There are two main types of solar collectors on the market:

Flat plate collectors (FPC)
are designed as flat box usually made from aluminium, with a glass pane on the top and heat insulation at the back. A metal sheet, called absorber, is located in the collector box. It is absorbing the sunlight and converting it into heat. To maximise the absorption, the absorber is black or blue coated. In small metal tubes, welded on the absorber, a heat transfer fluid is flowing and removing the solar heat.
Evacuated tube collectors (ETC)
are usually constructed out of two glass tubes which are put into each other. The interspace between the two glass tubes, in which a vacuum is applied, is sealed off. The inner tube is coated with an absorption layer. If the sun is shining on this absorber, the inner tube becomes warm, while the outer tube remains cold due to the vacuum. Within the inner tube either the heat transfer fluid is flowing or metal sheets with welded tubes are inserted in which the heat transfer fluid is flowing.

The solar heat is transported by the heat transfer fluid through insulated pipes either to a storage tank or directly to the point of use. If there is no risk of freezing, usually water is used as heat transfer fluid. To avoid corrosion in the collectors, mostly reverse osmosis water (RO) is used. In regions with cold winters, usually a mixture of glycol and water is used to avoid freezing of the fluid and destroying the collectors. Sometimes air is used as heat transfer fluid too, but then the collectors and the system is designed differently.

A SWHS is called non-pressurized, if no pressure is applied in addition to the static pressure. This means that the system of collectors, pipes and storage tank is open to the ambient atmosphere to compensate the volume expansion of hot water.

A SWHS is called pressurized, if the system is closed to the ambient atmosphere and an additional pressure, e.g. 3 bars is applied to the system. To compensate the variations in the fluid volume caused by temperature changes, an expansion vessel must be installed.

Types of SWHS

Small thermosiphon SWHS
Solar space heating system with air collectors in Ladakh
SWHS for industrial processes

Domestic hot water heating (DHW)

DHW systems heating water in residential homes, hotels, hospitals, etc. for domestic washing and cleaning purposes. Small DHW systems with for example 2 square meter collector area are used to provide hot water for one household. Large DHW systems supply multi family homes, hospitals, hotels and other buildings with hot water demand.

SWHS for DHW and space heating

In regions with space heating demand, SWHS can also deliver solar heat for space heating. A bigger number of collectors must be installed to generate sufficient heat in the heating season with usually lower irradiation intensity. However, some regions receive high irradiation also during the heating season, e.g. some mountain areas. In these regions solar space heating support is especially attractive. Beside SWHS, also solar air heating systems are used for space heating purposes.

SWHS for industrial processes

A significant share of hot water demand in the industry is at low temperatures, e.g. washing processes. To provide hot water up to 60°C and above SWHS are very suitable, especially due to the high solar irradiation in India. SWHS can also be used to preheat water, e.g. for steam generation within a steam boiler. Therefore, the potential of using SWHS to deliver hot water is very high in the industry.