Solar thermal energy

SOLAR THERMAL ENERGY

A solar thermal system exploits the sun's radiation to transform it directly into heat. 

Three types of solar thermal energy are distinguished:

• The “active” solar technology,
• The "passive" solar technology,
• The concentrated solar technology or "thermodynamic".

Solar thermal technology at low temperature: 

•  The “active” solar technology: Traditionally, this term refers to applications at low and medium temperature. Solar thermal collectors are installed on the roofs of buildings. A solar thermal collector is a device designed to collect energy from the sun and transmit it to a heat-carrier fluid. The heat is then used to produce domestic hot water or also, space heating.
• The "passive" solar technology: The passive solar installations also work at low or medium temperatures. In contrast to previous applications, they do not require components so-called active (the collectors). These applications are based on concepts of civil engineering and climatic involving a suitable architecture and the use of special materials. The passive use of the sun's energy can heat, light or refresh spaces.

Solar thermal technology at high temperature:

• The concentrated solar technology or "thermodynamic": This method provides the high temperature heat (250 to 1000°C) by concentration of solar radiation. The calorific is used to actuate a steam or gas turbine to produce electricity. The solar thermodynamic energy will be processed in another part.

Technical operation:
The types of thermal solar panels differ according to the nature of the heat-carrier fluid which carries the heat: water or air.

• The water solar collectors are used for heating and / or to produce Domestic Hot Water (DHW).
• In the air thermal collectors, the air is heated by contact with the absorbers. It is then blown into habitats for heating.

Solar collectors can also differ by their structure:

• Unglazed solar Collectors: Their structure is quite simple: they are composed of a plastic black tubing coil where heat carrier fluid circulates. They are mainly used for heating the swimming pool water, in summer.

• Flat plate glazed collectors: The heat-carrier fluid, usually water mixed with antifreeze, passes through a tubing coil placed behind a glass.

• Evacuated tube Collectors: The heat-carrier fluid circulates inside a double tube with air vacuum. The principle is the same as for the flat glazed collectors, insulation is ensured simply by the absence of air molecules.

Unglazed collector

Under vacuum collector

Glazed collector

 

 

The active solar technology:

Some applications of the technical operating of active solar energy:

The solar water heater: 

Solar water heater

 A solar water heater works on a simple principle. The energy of solar radiation is absorbed by flat plate collectors operating on the principle of the greenhouse effect. The stored heat is then transported in a "primary" circuit containing a mixture of water and antifreeze (heat-carrier). Through a heat exchanger, the heat carrier fluid contained in the primary circuit transmits its thermal energy to heat domestic water in the tank of a second isolated circuit, so-called "secondary". The cooled heat-carrier sets out again towards the collectors to be reheated. When the heated water, in the tank, is used, it is replaced by cold water from the water network, warmed by the same principle.

The Solar floor:  

Solar floor

The solar floor uses the same extraction principle of thermal energy that the solar water heater. The heat-carrier, heated to 25-30 degrees is here directly injected into a tubing coil installed in the building floor. This proven technique saves up to half the energy needs for heating a home. To provide a relatively constant heating throughout the day despite the intermittency of solar radiation, the solar floor uses thermal inertia (the ability to temporary energy storage) of the heat accumulator concrete slab.


The solar cooling:

Solar cooling

Paradoxically, the heat from the solar radiation can also be used to cool a building. The most common technique is to use solar collectors to supply heat, which is directed towards an absorption machine. This machine dissociates, by boiling, a solution of water and lithium bromide. After cooling, the recombination of the two components produces cold by heat absorption. Cold is then distributed identically to that of the conventional air conditioner.

The passive solar technology:

The heating and cooling of building are the principal energy expenditure items in Europe.
It represented, in 2009, more than 35% of total energy consumed.
Follow the rules of "solar architecture" (also called "bioclimatic architecture") substantially reduces the need for heating and cooling and thus to achieve substantial energy savings. The use of materials having high thermal efficiency goes in this direction.
A building shall be considered as a huge solar collector of which it is possible to improve performance. We must take into consideration the specificities of the local environment (sunlight, prevailing winds...) to maximize solar energy intakes, to store them and distribute them.

The « Trombe » Wall:

Trombe wall

 
The Trombe wall is an example of existing techniques allowing to use "passively" solar energy.
This type of wall, sometimes used in the building was designed by Professor Félix Trombe and architect Jacques Michel in the 60s, incorporating a patented concept of the late nineteenth century. This is a "heat collector" wall composed of a block of material with high inertia (concrete, stone, etc...) that accumulates the solar radiation during the day and gives it back at night.
A windowpane is placed in front of the wall to create a greenhouse in order to heat the air.

Energy Stakes:

The benefits:

• Solar thermal energy is renewable, freely available and can be used in areas with average sunning.
• The production process of thermal energy has no impact on the environment. There are no polluting emissions or waste.
• The solar heating technologies are simple and relatively inexpensive.
• It is possible to temporarily store the heat created and to give it back later, during the night for example.

The limits:

• Heat production is dependent on seasons and climates. In addition, auxiliary heaters are needed.

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