Eco-building Club members

Project “Eco-Building International Club for advanced European sustainable energy technology dissemination in Europe and China ”

is funded by the EC DG TREN within ENERGIE Programme

Solar Thermal

Solar thermal heating is a renewable energy technology that is well proven and readily available and has considerable potential for many different applications for building sector. A variety of types of systems are available and suitable for many applications.

The sun's heat can be collected in a variety of different ways, for example, low-temperature unglazed systems can heat swimming pools and associated hot tubs or spas, saving money on conventional heating or extending the swimming season. In mild climates, passive systems without pumps or electronic controllers can provide low-maintenance hot water for facilities with limited or expensive utility service. High-temperature parabolic-trough systems can economically provide hot water to jails, hospitals, and other facilities in areas with good solar resources that consistently use large volumes of hot water. And active flat-plate systems can service any facility in any area with electric or otherwise expensive conventional water heating.

High-temperature systems

High temperature systems are especially used in certain industrial applications where over-heated water, steam or high quantities of power are needed.

Solar Parabolic Troughs consist of curved mirrors which form troughs that focus the sun's energy on a pipe. A fluid, typically oil, is circulated through the pipes which is used to drive a conventional generator to create electricity.

Solar Parabolic Dish systems consist of a parabolic-shaped concentrator (similar in shape to a satellite dish) that reflects solar radiation onto a receiver mounted at the focal point at the center. The collected heat is utilized directly by a heat engine mounted on the receiver which generates electricity. Solar Central Receivers or " Power Towers " consist of a tower surrounded by a large array of heliostats. Heliostats are mirrors that track the sun and reflect its rays onto the receiver, which absorbs the heat energy that is then utilized in driving a turbine electric generator.

Solar water heaters and solar space heaters are constructed of solar collectors, and all systems have some kind of storage, except solar pool heaters and some industrial systems that use energy "immediately." The systems collect the sun's energy to heat air or a fluid. The air or fluid then transfers solar heat directly to a building, water, or pool.

Residential Solar Water Heating and Solar Pool Heating

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Regarding residential uses, the most common system to use the sun irradiation is to collect the heat trough hot water collectors whom are capable to transfer the sun heat to the water. The heated water can be used for different applications such Hot Domestic Water and Solar Pool Heating.

Water heating is one of the most cost-effective uses of solar energy, providing hot water for showers, dishwashers and clothes washers. Every year, several thousands of new solar water heaters are installed worldwide.

A solar water heater reduces the amount of fuel you need to heat water because it captures the sun's renewable energy. Many solar water heaters use a small solar electric (photovoltaic) module to power the pump needed to circulate the heat transfer fluid through the collectors. The use of such module allows the solar water heater to operate even during a power outage.

Solar pool systems are a relatively simple arrangement that use the existing pool pump to circulate the pools water through large, flexible, black polymer collectors. A solid state controller operates a motorized divertor valve to control when heat is needed. Since swimming pools don't require high temperatures, pool systems are most efficient if designed to heat large amounts of water to moderate temperatures. Total collector panel square footage should typically equal between 60-110% of pools surface square footage.

Benefits

Although solar water-heating systems all use the same basic method for capturing and transferring solar energy, they do so with such a wide variety of specific technologies that one almost needs to learn a whole language of terms for distinguishing different collectors and systems. The distinctions are important though, because various water-heating needs in various locations are best served by certain types of collectors and systems.

Each solar collector on the market must be accompanied by its yield curve showed below, defined by a certified laboratory in which:

•  ordinate shows the collector instantaneous performance rate ?, as defined:

•  abscissa shows the parameter x, given by:

Flat-plate collectors

A flat-plate collector consists of an absorber, a transparent cover, a frame, and insulation. Usually an iron-poor solar safety glass is used as a transparent cover, as it transmits a great amount of the short-wave light spectrum. Simultaneously, only very little of the heat emitted by the absorber escapes the cover (greenhouse effect).

In addition, the transparent cover prevents wind and breezes from carrying the collected heat away (convection). Together with the frame, the cover protects the absorber from adverse weather conditions. Typical frame materials include aluminum and galvanized steel; sometimes fiberglass-reinforced plastic is used.

The insulation on the back of the absorber and on the side walls lessens the heat loss through conduction. Insulation is usually of polyurethane foam or mineral wool, though sometimes mineral fiber insulating materials like glass wool, rock wool, glass fiber are used.

Flat collectors demonstrate a good price-performance ratio, as well as a broad range of mounting possibilities (on the roof, in the roof itself, or unattached).

In order to reduce heat loss within the frame by convection, the air can be pumped out of the collector tubes. Such collectors then can be called evacuated-tube collectors. They must be re-evacuated once every one to three years.

Evacuated-tube collectors

In this type of vacuum collector, the absorber strip is located in an evacuated and pressure proof glass tube. The heat transfer fluid flows through the absorber directly in a U-tube or in countercurrent in a tube-in-tube system. Several single tubes, serially interconnected, or tubes connected to each other via manifold, make up the solar collector. A heat pipe collector incorporates a special fluid which begins to vaporize even at low temperatures. The steam rises in the individual heat pipes and warms up the carrier fluid in the main pipe by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe.

The pipes must be angled at a specific degree above horizontal so that the process of vaporizing and condensing functions. There are two types of collector connection to the solar circulation system. Either the heat exchanger extends directly into the manifold ("wet connection") or it is connected to the manifold by a heat-conducting material ("dry connection"). A "dry connection" allows to exchange individual tubes without emptying the entire system of its fluid. Evacuated tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation. Higher temperatures also may be obtained for applications such as hot water heating, steam production, and air conditioning.

Systems of solar collectors

A pumped system uses a pump to circulate the water, so the tank can be positioned independently of the collector location. This system requires external energy to run the pump (though this can be solar, since the water should only be circulated when there is incident sunlight). It also requires control electronics to measure the temperature gradient across the collector and modulate the pump accordingly. Systems using solar electric pumping and controls are known as zero carbon solar while those using mains electricity are known as low carbon, since they typically have a 10-20% carbon drawback.

For solar heating of domestic hot water, two common system types are thermo-siphon and pumped.

In the thermo- siphon system, a storage tank is placed above the collector. As the water in the collector is heated, it will rise and naturally start to circulate around the tank. This draws in colder water from the bottom of the tank. This system is self-regulating and requires no moving parts or external energy, so is very attractive. Its main drawback is the need for the tank to be placed at a level higher than the collector, which may prove to be physically difficult.

( document prepared by ISNOVA )