Solar Water Heater

Sola water heaters provide hot water for commercial or domestic purposes using the heat of the sunlight.

Solar water heating (SWH) or solar hot water (SHW) systems comprise several innovations and many mature renewable energy technologies that have been well established for many years. SWH has been widely used in Australia, Austria, China, Cyprus, Greece, India, Israel, Japan, Jordan, Spain and Turkey. In a "close-coupled" SWH system the storage tank is horizontally mounted immediately above the solar collectors on the roof. No pumping is required as the hot water naturally rises into the tank through thermosiphon flow. In a "pump-circulated" system the storage tank is ground- or floor-mounted and is below the level of the collectors; a circulating pump moves water or heat transfer fluid between the tank and the collectors.

SWH systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be sufficient solar heat gain to deliver sufficient hot water. In this case a gas or electric booster is used to heat the water.


Water heated by the sun is used in many ways. While perhaps best known in a residential setting to provide domestic hot water, solar hot water also has industrial applications, e.g. to generate electricity. Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology for these places. The global solar thermal market is dominated by China, Europe, Japan and India.

In order to heat water using solar energy, a collector, often fastened to a roof or a wall facing the sun, heats working fluid that is either pumped (active system) or driven by natural convection (passive system) through it. The collector could be made of a simple glass-topped insulated box with a flat solar absorber made of sheet metal, attached to copper heat exchanger pipes and dark-colored, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In industrial cases a parabolic mirror can concentrate sunlight on the tube. Heat is stored in a hot water storage tank. The volume of this tank needs to be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the solar collector is lower than a typical immersion or combustion heater. The heat transfer fluid (HTF) for the absorber may be the hot water from the tank, but more commonly (at least in active systems) is a separate loop of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger (commonly a coil of copper heat exchanger tubing within the tank). Copper is an important component in solar thermal heating and cooling systems because of its high heat conductivity, resistance to atmospheric and water corrosion, sealing and joining by soldering, and mechanical strength. Copper is used both in receivers and primary circuits (pipes and heat exchangers for water tanks).

Another lower-maintenance concept is the 'drain-back': no anti-freeze is required; instead, all the piping is sloped to cause water to drain back to the tank. The tank is not pressurized and is open to atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes are empty before freezing could occur.

How a Solar Hot Water system works

Residential solar thermal installations fall into two groups: passive (sometimes called "compact") and active (sometimes called "pumped") systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) which is activated when the water in the tank falls below a minimum temperature setting such as 55 °C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.

When a solar water heating and hot-water central heating system are used in conjunction, solar heat will either be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will replace the lower heating element and the upper element will remain in place to provide for any heating that solar cannot provide. However, the primary need for central heating is at night and in winter when solar gain is lower. Therefore, solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched. In many climates, a solar hot water system can provide up to 85% of domestic hot water energy. This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.

What are the latest advances in solar water heaters?

While solar water heater devices have been around for over 100 years, the last 20 years have seen significant advances in absorber coating technologies, resulting in solar collectors that can reliably convert >50% of available sunlight for domestic hot water supply.

Low-profile collectors - which can fit more closely to the roof and have more concealed connective pipes and components, have less visual impact on the roof:

Drainback collectors - Caleffi’s new drainback flat-plate collector, the StarMax V, drains via internal pitched piping. The tilted headers drain to the middle of the header and then to an outlet in the center of the collector enclosure. Multiple collector center ports can be connected together, even with piping hidden beneath the roof, to drain an entire array. They can be installed parallel to roof lines instead of at an east-to-west slant, and with hidden piping, making them less obtrusive.

Solar Skies’ Flexible Water Storage Tank - a collapsible atmospheric heat storage tank, is a new twist on the old idea of providing large water storage without using pressurized vessels. While this type of tank may require more maintenance compared with pressurized tanks, it has the advantages of having a much lower cost per gallon of storage capacity, easy transportability, and modular features. Besides being able to ship a tank with a very large volume in a very small package, collapsible tanks can come in handy in situations where a large tank is needed, but there’s not enough room to get it through a doorway.

Multifunction storage tanks - Many pressurized storage tanks include electric heating elements and a thermostat, much like a conventional electric water heater tank. This is the easiest way to provide backup to your solar-heated water. The electric element is usually mounted near the top of the tank so that only the top is heated electrically, and the bottom remains cooler for better solar heat gain.

How much area does a solar water heater require for so many liters per day or hour?

For a 100 LPD system suitable for a small family of 2 to 3 persons, about ‘6 feet x 6 feet’ area is required. Add 6 feet lengthwise for additional 100 LPD capacities, generally speaking.

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