Solar water heating

Solar hot water refers to water heated by solar energy.

Solar water heater on a rooftop in Jerusalem, Israel

Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and a reservoir or tank for heat storage and subsequent use. The systems may be used to heat domestic hot water, swimming pool water, or for space heating. The heat can also be used for industrial applications or as an energy input for other uses such as cooling equipment.

In many climates, a solar heating system can provide a very high percentage (50 to 75%) of domestic hot water energy. 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.

In the southern regions of Africa like Zimbabwe, solar water heaters have been gaining popularity, thanks to the Austrian and other EU funded projects that are promoting more environmentally friendly water heating solutions.

Residential solar thermal installations can be subdivided into two kinds of systems: compact and pumped systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting such as 50 °C. Hence, hot water is always available. The combination of solar hot water heating and using the heat from a wood stove chimney to heat water can enable a hot water system to work year round in northern climates without the supplemental heat requirement of a solar hot water system being met with fossil fuels or electricity.

Technique

In order to heat water using solar energy, a collector is fastened to the roof of a building, or on a wall facing the sun. In some cases, the collector may be free-standing. The working fluid 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 pipes and painted black, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In some cases, before the solar energy is absorbed, a parabolic mirror is used to concentrate sunlight on the tube.

A simple water heating system would pump cold water out to a collector to be heated, the heated water flows back to a collection tank. This type of collector can provide enough hot water for an entire family.

Heat is stored in a hot water tank. The volume of this tank will be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the absorber is lower than a typical immersion or combustion heater.

The working fluid for the absorber may be the hot water from the tank, but more commonly (at least in pumped 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 tubing within the tank.). 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 pressurised and is open to atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes empty by the time when freezing could occur.

When a solar hot water 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 be lower in the tank than the hotter one. It is important to remember, however, that the main need for central heating is at night when there is no sunlight 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.

The water from the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure and excess heat through a heat dump.

Economics, Energy and System Costs

In sunny, warm locations, where freeze protection is not necessary, a batch type solar hot water heater can be extremely cost effective. In higher latitudes, there are often additional design requirements for cold weather, which add to system complexity. This has the effect of increasing the initial cost (but not the life-cycle cost) of a solar hot water system, to a level much higher than a comparable hot water heater of the conventional type. When calculating the total cost to own and operate, a proper analysis will take into consideration that solar energy is free, thus greatly reducing the operating costs, whereas other energy sources, such as gas and electricity, can be quite expensive over time. Thus, when the initial costs of a solar system are properly financed and compared with energy costs, then, in many cases the total monthly cost of solar heat can be less than other more conventional types of hot water heaters (and also in conjunction with an existing hot water heater). In addition, federal and local incentives can be significant.

As an example, a 56ft² solar water heater can cost US $7,500, but that initial cost is reduced to just $3,300 in the US State of Oregon due to federal and state incentives. The system will save approximately US $230 per year, with a payback of 14 years. Lower payback periods are possible based on maximizing sun exposure. However, in more northerly locations, solar heating is less efficient. Useable amounts of domestic hot water are available in the summer months,on cloudless days, between April and October. During the winter and on cloudy days the output is poor. The installation costs in the UK are prohibitive, on average about £9,000. In view of this the pay back period can be up to 50 years.

Solar hot water systems

Solar hot water systems can be classified in different ways:

• The type of collector used (see below)
• The location of the collector - roof mount, ground mount, wall mount
• The location of the storage tank in relation to the collector
• The requirement for a pump - active vs passive
• The method of heat transfer - open-loop or closed-loop (via heat exchanger)

Compact systems (Passive systems)

A passive system also known as a monobloc(thermosiphon) system, a compact system consists of a tank for the heated water, a solar collector, and connecting pipes all pre-mounted in a frame. The water flows upward when heated in the panel. When this water enters the tank (positioned higher than the solar panel), it expels some cold water from inside so that the heat transfer takes place without the need for a pump. A typical system for a four-person home in a sunny region consists of a tank of 150 to 300 litres and three to four square metres of solar collector panels.

A special type of compact system is the Integrated Collector Storage (ICS, Batch Heater) where the tank acts as both storage and solar collector. They are simple and efficient but only suitable in moderate climates with good sunshine.

Direct ('open loop') compact systems are not suitable for cold climates if they are made of metals. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreezing liquid is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. ('closed loop'). However, direct ('open loop') systems are slightly cheaper and more efficient.

A compact system can save up to 4.5 tonnes annually of greenhouse gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosyphon principle).

Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter in colder climates but cost more than flat plate collector systems.

Pumped systems (Active systems)

How the solar water heating system is pumped and controlled determines whether it is a zero carbon or a low carbon system. Low carbon systems principally use electricity to circulate the fluid through the collector. The use of electricity typically reduces the carbon savings of a system by 10% to 20%.

Conventional low carbon system designs use a mains powered circulation pump whenever the hot water tank is positioned below the solar panels. Most systems in northern Europe are of this type. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts.

The electronic controllers used by some systems permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging.

Newer zero carbon solar water heating systems are powered by solar electric (photovoltaic or PV) pumps. These typically use a 5-20W PV panel which faces in the same direction as the main solar heating panel and a small, low flow diaphragm pump to pump the water.

The most commonly used solar collector is the insulated glazed flat panel. Less expensive panels, like polypropylene panels (for swimming pools) or higher-performing ones like evacuated tube collectors, are sometimes used.

Solar heating thermal collectors

See Solar thermal collector

There are three main kinds of solar thermal collectors in common use. In order of increasing cost they are: Formed Plastic Collectors, Flat Collectors, and Evacuated Tube Collectors. The efficiency of the system is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heating energy that can be usefully obtained from insulation). Heat losses are predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Efficiency decreases when either the ambient temperature falls or as the collector temperature increases. This decrease in efficiency can be mitigated by increasing the insulation of the unit by sealing the unit in glass e.g. flat collectors or providing a vacuum seal e.g. evacuated tube collector. The choice of collector is determined by the heating requirements and environmental conditions in which it is employed.

Formed Plastic Collectors (such as polypropylene, EPDM or PET plastics)

Consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. These are often used for extending the swimming season in swimming pools. In some countries heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated.

Flat plate collector

Consists of a thin absorber sheet (usually copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger or to some other device for using the heated fluid.

As an alternative to metal collectors, some new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one with extra heat exchangers.

Evacuated (or vacuum) tubes panel.

Evacuated tube collectors

Are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are disimilar metals creates no corrosion problems. The better quality systems use foam insulation in the manifold. low iron glass is used in the higher quality evacuated tubes manufacture.

Lower quality evacuated tube systems use the glass coated absorber. Due to the extreme temperature difference of the glass under stagnation temperatures, the glass sometimes shatters. The glass is a lower quality boron silicate material and the aluminum absorber and copper heat pipe are slid down inside the open top end of the tube.Moisture entering the manifold around the sheet metal casing is eventually absorbed by the glass fibre insulation and then finds its way down into the tubes. This leads to corrosion at the absorber/heat pipe interface area, also freeze ruptures of the tube itself if the tube fills sufficiently with water.

Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square metre than flat panels. However, they are more expensive and fragile than flat panels. The high stagnation temperatures can cause antifreeze to break down, so careful consideration must be used if selecting this type of system in temperate climates.

For a given absorber area, evacuated tubes can maintain their efficiency over a wide range of ambient temperatures and heating requirements. However, due to the design the absorber area only occupies about 50% of the collector panel. In most climates and for the majority of domestic hot water services, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. Unless employed in large arrays, the efficient but costly evacuated tube collectors have only marginal net benefit in winter and give little real advantage in the warmer months. Due to the vacuum, heavy frost or snow will not melt off under sunlight. They are most suited to extremely cold ambient temperatures or in situations of consistently low-light. They are also used in industrial applications, where high water temperatures or steam need to be generated.

Solar thermal cooling

Solar thermal cooling can be achieved via absorption refrigeration cycles, desiccant cycles and solar-mechanical processes.

The absorption cycle solar cooling system works like a refrigerator in that it uses hot water to compress a gas that, once expanded, will produce an endothermic reaction which cools the air. The main problem currently is that the absorber machine works with liquid at 90 °C, a fairly high temperature to be reached with pumped solar panels with no auxiliary power supply.

The same pumped solar thermal installation can be used for producing hot water for the whole year. It can also be used for cooling in the summer and partially heating the building in winter.

Usage

Flat-plate collectors for solar water heating were popular in Florida and Southern California in the 1920s. Due to the abundance of sunlight in Israel, solar water heaters were used by some 20% of the population by 1967. Following the energy crisis in the 1970s, the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes (except high towers with insufficient roof area). As a result Israel is now the world leader in the use of solar energy per capita (3% of the primary national energy consumption).

During this time, there was some resurgence of interest in solar heating in North America. Technical innovation has improved performance, life expectancy and ease of use of these systems. Installation of solar hot water heating has become the norm in countries with an abundance of solar radiation, like Israel and Greece, and in Japan and Austria, where there is less.

Solar hot water systems have become popular in China, where basic models start at around 1,500 yuan (US$190), much cheaper than in Western countries (around 80% cheaper for a given size of collector). It is claimed that at least 30 million Chinese households now have one, and that the popularity is due to the efficient evacuated tubes which allow the heaters to function even under gray skies and at temperatures well below freezing.

In 2005 Spain became the first country in the world to require the installation of photovoltaic electricity generation in new buildings, and the second in the world (after Israel) to require the installation of solar hot water systems .

Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology. The global solar thermal market is dominated by China, Europe, Japan and India.

See also

• Solar air conditioning
• Solar combisystem
• Solar energy
• Solar heating
• Renewable resource
• Renewable energy
• Sustainable design
• Solar thermal collector
• Seasonal thermal store
• Solar greenhouse (technical)
• Hot water heat recycling

External links

• Solar Hot Water & Heating at HeatingGuide
• Using Excel to Predict solar water heating, and heating of other objects
• 3E Strategies' Green + Solar with useful articles and studies
• Solar Water Heating System Complete Guide
• Solar Water Heating Explained