Solar water heating

As of 2017, global solar hot water (SHW) thermal capacity is 472 GW and the market is dominated by China, the United States and Turkey.

Israel, Cyprus and Greece are the per capita leaders in the use of solar water heating systems supporting 30%–40% of homes.

Following the energy crisis in the 1970s, in 1980 Israel required the installation of solar water heaters in all new homes (except high towers with insufficient roof area).

[14][15][16] Solar water heating systems are popular in China, where basic models start at around 1,500 yuan (US$235), around 80% less than in Western countries for a given collector size.

This approach is common in climates where freezing temperatures do not occur often but can be less reliable than an automatic system as it relies on an operator.

When no hot water has been used for a day or two, the fluid in the collectors and storage can reach high temperatures in all non-"drainback" systems.

This is most effective in direct or thermal store plumbing and is virtually ineffective in systems that use evacuated tube collectors, due to their superior insulation.

Simple designs include a simple glass-topped insulated box with a flat solar absorber made of dark-colored sheet metal, attached to copper heat exchanger pipes, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder.

Active systems have controllers with features such as interaction with a backup electric or gas-driven water heater, calculation and logging of the energy saved, safety functions, remote access and informative displays.

They are simple and less costly than plate and tube collectors, but they may require bracing if installed on a roof (to support 400–700 lb (180–320 kg) lbs of water), suffer from significant heat loss at night since the side facing the sun is largely uninsulated and are only suitable in moderate climates.

Since the panels are located below the storage tank, heat loss does not cause convection, as the cold water stays at the lowest part of the system.

The collector piping is not pressurized and includes an open drainback reservoir that is contained in conditioned or semi-conditioned space.

The water passes through the thermally insulated header only, and in climates with moderate frosts, no draindown system is required.

However, the energy output of flat plate collectors is reduced slightly more than ETCs in cloudy or extremely cold conditions.

[30] Most ETCs are made out of annealed glass, which is susceptible to hail, failing given roughly golf ball -sized particles.

ETCs can gather energy from the sun all day long at low angles due to their tubular shape.

In a bubble pump system, the closed HTF circuit is under reduced pressure, which causes the liquid to boil at low temperature as the sun heats it.

ETCs higher operational temperature range (up to 200 °C (392 °F)) makes them suitable for industrial applications such as steam generation, heat engine and solar drying.

Pool covering systems, whether solid sheets or floating disks, act as insulation and reduce heat loss.

In cold or windy environments evacuated tubes or flat plates in an indirect configuration are used in conjunction with a heat exchanger.

A fairly simple differential temperature controller is used to direct the water to the panels or heat exchanger either by turning a valve or operating the pump.

With most solar water heating systems, the energy output scales linearly with the collector surface area.

Some methods of comparison[49] calculate the efficiency of evacuated tube collectors based on the actual absorber area and not on the space occupied as has been done in the above table.

In sunny, warm locations, where freeze protection is not necessary, an ICS (batch type) solar water heater can be cost effective.

The biggest single consideration is therefore the large initial financial outlay of solar water heating systems.

At higher latitudes, solar heaters may be less effective due to lower insolation, possibly requiring larger and/or dual-heating systems.

LCA considers the financial and environmental costs of acquisition of raw materials, manufacturing, transport, using, servicing and disposal of the equipment.

Elements include: In terms of energy consumption, some 60% goes into the tank, with 30% towards the collector[71] (thermosiphon flat plate in this case).

This figure was for a direct system, retrofitted to an existing water store, PV pumped, freeze tolerant and of 2.8 sqm aperture.

[citation needed] A test system in Italy produced about 700 kg of CO2, considering all the components of manufacture, use and disposal.