The energy carried by sunrays as a consequence of nuclear reactions (hydrogen fusion) and transmitted to the Earth as electromagnetic radiation is called solar energy. Electromagnetic radiations are made of photons. A photon is a neutral particle that spreads into the air at a speed of 300,000 km/sec, with an energy that depends on its frequency and a mass that is considered as void when at rests (when it is not moving).
The definition passive solar systems refers to the systems in which solar energy is used directly. For example greenhouses are glass structures that allow the Sun to enter, but do not let the heat escape. In this way they are able to maintain temperatures inside the structure that are higher than those outside. Then there are the solar distillers, in which in a closed space, covered by transparent panels that are exposed to the Sun, sea water evaporates and condenses and forms water that has no salts, and therefore that can be reutilised.
In the active solar systems instead, solar energy is collected and transformed into thermal or electric energy before it is utilized. This type of system includes solar thermal power systems, solar concentrator systems used for the production of thermal energy and solar photovoltaic panels for the production of electric energy. The best technologies also enable the cogeneration of different types of energy, and it is possible to accumulate thermal energy in many ways and for different uses.
Thermal solar panels
Solar panels catch the energy of the sun and use it to produce hot water (up to 60/70°C) which collected in an ad hoc tank, can be used both for household (i.e. for household and water heating) and industrial purposes, as well as for the production of electric energy on a large scale through thermoelectric solar plants.
A solar panel system includes two elements: the actual solar panel and the accumulation tank. The first includes a solar heat accumulator, i.e. a steel or copper panel crossed by the pipes in which the fluid to be heated by the sun flows: generally antifreeze is added to the water in order to tolerate winter temperature.
Concentration collectors are thermal solar panels that use a mirror system that reflects the sunrays and makes them concentrate on a receiver. Collectors can be linear, when they concentrate sunrays on a segment of a straight line, or they can concentrate sunrays on a single point, heating the out-flowing fluid of the panel at more than 100°C.
Some thermal solar panels are called vacuum-pipe collectors as they are made of special glass vacuum pipes, covered by a layer that transforms the sun light into heat. In this case the heat absorber has a round shape and is hosted inside the pipe vacuum cavity.
Thermo-solar energy accumulators
Just like the other renewable sources, solar energy is not constantly available. As a consequence, accumulation systems are extremely important for the evolution and development of technologies.
Photovoltaic solar panels
Photovoltaic technology allows the direct transformation of solar energy into electric energy by exploiting the photovoltaic effect. The photovoltaic effect is based on the characteristics of certain semiconductor materials such as silicon which, after being ad hoc processed, generates electric energy after being hit by the solar radiation.
Concentration collectors include solar towers, which consist of a system of mirrors (called heliostats) that follows the movement of the Sun and that reflects the solar energy on a receptor located on the top of a central tower. The solar heat is collected by a fluid (a melted salt) that has the task to accumulate energy. With the heat accumulated on melted salts, vapour is produced (565°C), in order to make an electric turbo-generator turn.
Linear parabolic mirrors
Linear parabolic mirrors are called SEGS (Solar Electric Generating System): they are used to concentrate sunrays on a long receiving pipe positioned on the concentrator line. A heat-carrier, i.e. oil, pumped by receiving pipes, supplies a plant. The solar heat is transformed into vapour in order to activate an electric turbo-generator. The typical operational temperature is 390 °C. These installations today work with 30-80 electric megawatts and also burn a certain quantity of fossil fuel (sometimes natural gas) in order to produce energy when the solar energy is not sufficient.
The photovoltaic system includes different mechanical, electrical and electronic components that attract the solar energy, transform it into electric energy until the user can use it.
These systems are not connected to the national electric network and directly supply some equipment. They also have a battery system that guarantees the supply of power even during poor light hours or in the dark. These systems are technically and economically advantageous in those cases where the electric network is absent or difficult to reach.
Systems connected to a network
These systems are permanently connected to the national electric grid. When the photovoltaic generator is not able to produce the electric energy needed to satisfy the demand for electricity, the network provides the requested energy.
Installations integrated in buildings
They are among the most promising applications of photovoltaic systems. These systems are installed on civil or industrial buildings in order to be connected to the national electric network. The power generated by photovoltaic modules is supplied to the internal network of the user building and, at the same time, to the public distribution network.
Where to position a plant
In order to obtain the maximum energy production, when designing a plant, it is necessary to study the area irradiation and the sun exposure. In this way it will be possible to make decisions on the inclination and orientation of the receiving device.
Thermo-photovoltaic co-generation systems include thermo-solar technologies for the production of hot and cold water and thermo-solar systems for the production of electric energy. An example of these systems are those panels where a thermo-solar collectors and photovoltaic cells are integrated and where the primary energy is the direct solar energy.
Thermodynamic solar power
In the thermodynamic solar power process, electric power is obtained by using solar energy to heat water, which is then transformed into steam, which in turn moves the turbines analogously to the traditional thermoelectric power plants.