Solar Water Heating

Solar water heating is the heating of water through the use of solar energy.

Solar heating systems are generally composed of solar thermal collectors, a water storage tank or another point of usage, interconnecting pipes and a fluid system to move the heat from the collector to the tank.

This thermodynamic approach is distinct from semiconductor photovoltaic (PV) cells that generate electricity from light; solar water heating deals with the direct heating of liquids by the sun where no electricity is directly generated.

A solar water heating system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The water can be heated for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, underfloor heating or energy input for space heating or cooling are common examples of solar water heating. A solar water heating system can form part of a solar thermal cooling system, promoting efficient temperature control of buildings or parts thereof.

During cool conditions, the same system can provide hot water. Solar heating of buildings in temperate climates has a season-problem: In winter, when most heating is needed, least is available from the sun. This can often be solved by storing solar heat in the ground or in groundwater.

Passive Systems

A special type of passive system is the Integrated Collector Storage (ICS or Batch Heater) where the tank acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with glass in front of it generally in or on house wall or roof. They are seldom pressurised and usually depend on gravity flow to deliver their water. They are simple, efficient and less costly than plate and tube collectors but are only suitable in moderate climates with good sunshine.

A step up from the ICS is the Convection Heat Storage unit (CHS or thermosiphon). These are often plate type or evacuated tube collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from collector to tank. Neither pumps nor electricity are used to enforce circulation. It is more efficient than an ICS as the collector heats a small(er) amount of water that constantly rises back to the tank. It can be used in areas with less sunshine than the ICS. An CHS also known as a compact system or monobloc has a tank for the heated water and a solar collector mounted on the same chassis. Typically these systems will function by natural convection or heat pipes to transfer the heat energy from the collector to the tank.

Direct (‘open loop’) passive systems use water from the main household water supply to circulate between the collector and the storage tank. When the water in the collector becomes warm, convection causes it to rise and flow towards the water storage tank. They are often not suitable for cold climates since, at night, the water in the collector can freeze and damage the panels.

Indirect (‘closed loop’) passive systems use a non-toxic antifreeze heat transfer fluid (HTF) in the collector. When this fluid is heated, convection causes it to flow to the tank where a passive heat exchanger transfers the heat of the HCF to the water in the tank.

The attraction of passive solar water heating systems lies in their simplicity. There are no mechanical or electrical parts that can break or that require regular supervision or maintenance. Consequently the maintenance of a passive system is simple and cheap. The efficiency of a passive system is often somewhat lower than that of an active system and overheating is largely avoided by the inherent design of a passive system.


An integrated collector storage (ICS) system

Direct systems: (A) Passive CHS system with tank above collector.
(B) Active system with pump and controller driven by a photovoltaic panel
Source of info:

Other resources:
US Department of Energy:
The Renewable Energy Center: