¿ Solar energy and photovoltaic a viable alternative as a sustainable energy in the future?

The sun is known as the essential axis of energy which has been used by men to meet the needs of humanity, thanks to science and technology in this last decade has advanced in an incredibly dizzying way with its progress in the future, which has had consequences in the humanity and culture of the different nations, offering a better level of life in remote areas with difficult access to electricity. Nowadays man depends on this energy and looks for new alternatives to get a better living that allows an environmental balance and minimizing the pollution caused due to the burning of fossil fuels, society demands ever greater quality in the comfort conditions. This demand responds to a new demand for sustainability of the urbanization processes, both in its environmental dimension, as well as socially and economically.

Solar energy: It is a renewable energy that has been obtained from the electromagnetic radiation of the sun, which reaches the earth and is used by the human being since ancient times through different technologies that have been evolving. Today, heat and sunlight can be exploited by different occupants such as photovoltaic cells, heliostats or thermal collectors, which can be transformed into electrical or thermal energy to collect energy.

It can be said with certainty thanks to science and technology that the earth receives 174 peta watts of solar radiation from the highest layer of the atmosphere. 30% returns to space while clouds, oceans and landmasses absorb the rest. The electromagnetic spectrum of sunlight on the earth's surface, mainly occupies visible light and infrared ranges with a small part of ultraviolet radiation.

Passive solar energy: Is the set of techniques aimed at the use of solar energy directly, without transforming it into another type of energy for immediate use or storage without the need for mechanics or external energy input.

This also makes it easier to have a passive system that contains the system with direct and indirect gain for the heating of spaces, the use of thermal mass, materials, which will help to smooth the oscillations of air temperatures, school kitchens, solar chimneys to improve the natural ventilation.
Solar thermal energy: it is based on the use of the sun's original energy to transfer it to a heat carrier medium, generally water or air, between the different applications of solar thermal energy there is the possibility of generating electrical energy. The current technology allows heating water with solar luminescence to produce fluid and then obtain electrical energy. This energy also makes it possible to take advantage of the sun's energy to produce heat that can be used to produce food or to produce hot water for domestic consumption, whether hot sanitary water, heating or to produce mechanical energy and from it electric power for add it and use it to feed a refrigeration machine.

•Low temperature collectors: Provide heat at temperatures below 65 ° C by means of metallic or non-metallic absolutes to be applied to pool heating or domestic warming of bath water, in general for all those industrial activities in which the heat proceeds to greater ° C for example pasteurization, textile washing, among others.

•Medium temperature collectors: Are those that concentrate on solar radiation to deliver useful heat and higher temperature between 100 and 300 ° C.

•High temperature collectors: They were invented by Frank Shuman and nowadays there are three different types: the parabolic plate collectors, the new parabolic generation and the central tower systems. These operate at temperatures above 500 ° C and this is used to generate electricity (solar thermal electricity) and transmit it to the electricity network.

Low-temperature solar thermal energy: A low-temperature solar thermal installation consists of solar collectors, a primary and secondary circuit, heat exchanger, accumulator, expansion vessel and pipes.
If the system works by thermosyphon, it will be the density difference due to the temperature change that will move the fluid.
The solar collectors are those that capture solar radiation and convert it into thermal energy. Also the collectors are known as flat plate, vacuum tube and absolder collectors without protection or isolation.

The solar collectors are composed of the following elements:

•Cover: It is transparent, it may be present or not, it is usually made of glass and it is also used as a plastic since it is less expensive and accessible, but it must be a special plastic. Its function is to minimize the losses by convection and radiation, which is why it must have a solar transmission as high as possible.

• Air channel: It is a space (empty or not) that separates the cover of the absorbent plate. Its usefulness is to calculate taking into account balancing the losses by convection and the high temperatures that can be produced.

• Absorbent plate: It is the element that absorbs solar energy and transmits to the liquid that circulates through the pipes. His main work is that the plates must have a great solar absorption and a reduced thermal emission.

• Pipes or ducts: The tubes are sometimes welded to the absorbing plate so that the exchange of energy is as great as possible, the tubes will circulate the liquid that will heat up and go to the accumulation tank

• Insulating layer: Its purpose is to isolate and coat the system to avoid and minimize losses. In order for the insulation to be as good as possible, the insulating material must have a low thermal conductivity

Solar thermal medium temperature: The medium temperature installations can use several designs, the most common are: pressure glycol, back drainage and batch systems newer low pressure systems tolerant to freezing use polymer pipes containing water with pumping photovoltaic European and international standards are being revised to include innovations in the design and operation of medium temperature collectors. These operational innovations include the operation of "permanently wet collectors". This technique reduces or even eliminates the occurrence of high temperature non-flow stresses known as stagnation, which reduce the expected life of these collectors.

High temperature solar thermal energy: Temperatures below 95 degrees Celsius are sufficient for space heating, in which case flat collectors of the non-concentrator type are generally used. Due to the relatively high heat losses through the glass, the flat collectors can not reach much higher than 200 ° C even when the transfer fluid is stagnant. Such temperatures are too low to be used in efficient conversion to electricity.

Accumulation and heat exchange: The accumulation of heat allows solar thermal power plants to produce electricity during daylight hours without sunlight or at night. This allows the use of solar energy in the generation of base load as well as the generation of peak power, with the potential to replace fossil fuel-fired power plants. Additionally, the use of accumulators reduces the cost of electricity generated with this type of solar power plant.

The heat is transferred to a thermal storage medium in an insulated tank during daylight hours and is recovered for electricity generation at night. Thermal storage media includes pressurized steam, mortar, a variety of phase change materials, and molten salts such as calcium, sodium and potassium nitrate.

Photovoltaic Solar Energy: Photovoltaic solar energy consists of obtaining electricity derived directly from solar radiation by means of a semiconductor device called a photovoltaic cell, or by a deposition of metals on a substrate called a thin-film solar cell, transforming sunlight into electricity through the use of technology based on the photovoltaic effect. When the sun's radiation contacts on one of the faces of a photoelectric cell (which accesses the panels) there is a difference of electric potential between both faces, this causes the electrons to jump from one place to another, thus generating electrical current.

Photovoltaic solar panels:
The group of photoelectric cells or photovoltaic cells is known as a photovoltaic panel. The photovoltaic panels consist of a network of cells connected as a series circuit to increase the output voltage to the desired value (usually 12V to 36V are used) while connecting several networks as a parallel circuit to increase the electrical current which is capable of providing the device.
The average conversion efficiency obtained by commercially available cells (produced from mono crystalline silicon) is around 16%. The average useful life at maximum performance is around 25 years, period from which the power delivered decreases. The type of electric current they provide is direct current, so that alternating current is needed or its voltage is increased, it will be necessary to add an inverter and / or a power converter.
Therefore a photovoltaic panel consists of an association of cells, encapsulated in two layers of EVA (ethylene-vinyl acetate), between a glass front sheet and a back layer of a thermoplastic polymer (usually tedlar). This set is framed in an aluminum structure with the aim of increasing the mechanical strength of the assembly and facilitating the anchoring of the module to the support structures.

The most commonly used cells in photovoltaic panels are silicon, and can be divided into three subcategories:
•Mono-crystalline silicon cells are made up of a single silicon crystal, usually manufactured using the Czochralski process. This type of cells presents a uniform dark blue color.

•Polycrystalline silicon cells (also called multi crystalline) are made up of a set of silicon crystals, which explains why their performance is somewhat lower than mono crystalline cells. They are characterized by a more intense blue color.

•Amorphous silicon cells They are less efficient than crystalline silicon cells but also less expensive. This type of cells is, for example, the one used in solar applications such as watches or calculators
The standardized parameter to classify the power of a photovoltaic panel is called peak power, and corresponds to the maximum power that the module can deliver under standardized conditions, which are:
•Radiation of 1000 W / m²

•Cell temperature of 25 ° C (not room temperature).

The typical yields of a poly crystalline silicon photovoltaic cell range between 14% -20%. For monocrystalline silicon cells, the values oscillate in 15% -21% .3738 The highest are achieved with thermal solar collectors at low temperature (which can reach a 70% efficiency in the transfer of solar energy to thermal) .
Photovoltaic solar panels do not produce heat that can be reused - although there are lines of research on hybrid panels that allow the generation of electrical and thermal energy simultaneously. However, they are very appropriate for rural electrification projects in areas that do not have an electricity network, simple rooftop installations and photovoltaic self-consumption.

Development of photovoltaic solar energy in the world

Due to the growing demand for renewable energy, the manufacture of solar cells and photovoltaic installations has advanced considerably in recent years. Photovoltaic solar energy was traditionally used since its popularization in the late 1970s to feed innumerable autonomous devices, to supply shelters or houses isolated from the electricity grid, but above all, increasingly in recent years to produce electricity on a large scale through distribution networks, either by injection into the grid or for domestic consumption.
The countries Germany, Japan, China and the United States, the photovoltaic industry is experiencing more vertiginous growth. By the end of 2015, around 230 GW of photovoltaic power had been installed around the world, making photovoltaics the third most important source of renewable energy in terms of global installed capacity, after hydroelectric and wind power, and already accounts for a significant fraction of the electricity mix in the European Union, covering on average 3.5% of electricity demand and reaching 7% in periods of higher production.43
The considerable installed capacity in Germany (38 GW in 2014) has made several records in recent years. In June 2014, it produced up to 50.6% of all the country's electricity demand during a single day, reaching an instantaneous power of over 24 GW which is equivalent to the power generation of almost 25 nuclear power plants working at full capacity.

Photovoltaic self-consumption and network parity
Photovoltaic self-consumption consists of the individual production of small-scale electricity for own consumption, through solar panels. This can be complemented with the net balance. This production scheme, which allows to compensate the electrical consumption by means of the generated by a photovoltaic installation in moments of lower consumption, has already been successfully implemented in many countries. It was proposed in Spain by the photovoltaic association ASIF to promote renewable electricity without additional financial support.
In order to encourage the development of technology with a view to achieving network parity - to match the price of obtaining energy to that of other more economical sources at the present time - there are production bonuses, which guarantee a fixed purchase price by the parties of the electrical network. This is the case of Germany, Italy or Spain. This incentive scheme has already paid off, making the costs of photovoltaic energy fall below the sale price of traditional electricity in a growing number of regions.