Solar hot water
From Wikipedia, the free encyclopedia
Solar hot water describes heated water achieved through the usage of solar energy.
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 to stock the heat for subsequent use. The systems may be used to heat domestic hot water or a swimming pool, or to provide heat for a heating circuit. The heat can also be used for industrial applications or as an energy input for other uses such as cooling equipment.[1]
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 are used to provide 15 to 25% of home heating energy.
Residential solar thermal installations can be subdivided in two kind 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.
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[edit] Technique
In order to heat water using solar energy, a collector is placed on - or forms - 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 convection (passive system) through it.
The collector could be made of a simple glass topped box with copper pipes in it, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. A parabolic mirror can also be added to concentrate the sun's light 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 a family, for very little or no monthly cost.
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 (a coil of copper tubing within the tank.)
If a central heating system is also present and heats water, then either the solar heat will 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.
[edit] Compact systems
A passive system also known as a monobloc system, a compact system consists of a tank for the heated water, a solar collector, and connecting pipes all pre-mounted in a frame. Based on the thermosiphon principle, 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 liters and three to four square meters of solar collector panels.
Direct 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. However, direct systems are slightly cheaper and more efficient.
A compact system can save up to 4.5 tonnes annually of 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 and cost about the same as flat collector systems.
[edit] Pumped 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 use mains electricity to circulate the fluid through the collector. The use of mains electricity typically negates 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 use 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.
[edit] Solar heating thermal collectors
There are three main kinds of solar thermal collectors in common use. 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. 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.
A flat 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.
Instead of 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, they are able to use plain water in them 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 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.
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 vaporizes as it is heated. The vapor 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 meter than flat panels. However, they are more expensive and fragile than flat panels.
[edit] Solar thermal cooling
There are some new applications of thermal hot water, such as air cooling, currently under development. The absorber machine works like a refrigerator — 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.
[edit] Usage
Flat-plate collectors for solar water heating were popular in Florida and Southern California in the 1920s. There was a flicker of resurgence of interest in them in North America in the 1970s. However, the main improvements now are occurring in other countries which have enabled effective use of solar heating in both private houses and large scale installations. Technical innovation has improved performance, life expectancy and ease of use of these systems, with Germany/Austria the clear leaders. Installation of solar hot water heating has become the norm in countries such as Israel and Greece, where there is an abundance of solar radiation, and Japan and Austria where there is considerably 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 more 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.[2]
Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology; however they will not work very well in temperate climates.[citation needed]
[edit] See also
- Solar power
- Solar heating
- Renewable resource
- Renewable energy
- Sustainable design
- Solar thermal collector
- Seasonal thermal store
- Solar greenhouse (technical)
[edit] References
- ^ http://www.iea-shc.org/task25/index.html
- ^ Energy-Hungry China Warms to Solar Water Heaters - discusses China Himin Solar Energy Group in Dezhou. - Reuters article, posted on Planet Ark site.
[edit] External links
- Using Excel to Predict solar water heating, and heating of other objects
- Solar Power for Free - How to make a Solar Shower from Junk.
- MB Solco - Manufacturer of Polyetylene Based SWH.
