Solar energy: solar thermal and photovoltaic
The environmental crisis is now at alert level, to seriously threaten the very survival of the planet. Italy has underestimated the importance of an energy-efficient and sustainable, economically and structurally.
One of the themes favored by advocates of environmental policy is that of the so-called "renewable energy", considered viable alternatives in view of decoupling the dependence on fossil fuels: oil, gas and coal.
This, state of the art, it is true only to a small extent, but the contribution that these forms of energy production can provide is important and deserves attention and study.
In this introductory article will illustrate the characteristics related to civil works (for housing) for the exploitation of solar energy.
I will try to illustrate the potential clearly, concentrating on the technical characteristics of the most common plants.
Solar energy can be used both to generate heat, is to transform the solar radiation into electrical energy. In the first case we speak of exploitation solar - thermal solar energy in the second case we are referring to photovoltaic systems.
Solar Thermal Technology
The solar radiation, hot intuitively, is capable of heating with ease the objects that are irradiated. The common experience offers dozens of examples of how much exposure to the sun has the power to heat any object in any season. Even in the coldest day inside a car in the sun heats up rapidly due to radiation on the body and through the glass.
If a dark plate of a material particularly sensitive to sunlight, inside a recess in which a fluid can flow, is placed directly under the influence of solar rays, will be able to transmit to the fluid a quantity of heat proportional to 'intensity of solar radiation.
Yeah, why must say that the sun does not radiate their energy in the same way in all the earth. It depends on the latitude, the seasons and weather conditions: in Milan in December and with the leaden sky and foggy, it is natural that the solar radiation is less than what we could be on a mountain in Sicily in June in a beautiful and breezy day.
The plate that allows the exchange of heat between the solar radiation and the fluid is called more correctly collector and the structural point of view is a bit 'more complex than intuition would imagine. It must in fact not only attract the solar radiation, but also protect from the atmosphere, which would tend to steal the heat generated immediately. It is therefore necessary that the plate is covered with transparent plates, which is thermally insulated and is protected by an atmospheric-proof agent. All this together is the fact the collector.
We could imagine at this point to use as a fluid inside the collector, for pure water, which when heated would be exploited to take a hot shower.
The problem is that, unless the collector is not as large as an entire roof of house, after a few seconds the hot water would end since the speed with which a collector heats the water is not comparable to that of an electric water heater.
There is therefore the problem of accumulating the hot water produced and keep warm until its use. Here it is necessary that a real system consisting of the collector, by a circuit that sends the cold water in the collector and that the channeling towards the use when it was heated, by a tank for collecting the water completely similar to a "boiler" able to keep the water at a temperature as high as possible.
As a tank can be isolated efficiently and will never be able to save a lot of hot water for a long time without giving a little 'heat. That is why the solar-thermal are suitable when the use of hot water does not require temperatures higher than 40, 50, at most 60 degrees. The ideal therefore for sanitary uses; almost unusable for heating housing that need to a circuit internal water at temperatures of at least 60, 70 degrees.
Some argue that with a heating system panels can actually heat the room by means of a solar - thermal fluid: I consider risky not be possible to heat the water of the circuit, at least in the more gray, with a standard gas boiler.
If in place of the fluid were to be conducted within the air manifolds, it would be possible to heat the environment through forced hot air, but the plant is still rather expensive and technologically not fully reliable. If it is difficult to imagine storing hot water than it is to keep hot air!
No solar thermal plant is still able to cover the full supply of water or hot air of a normal house. In the central areas of Italy it is estimated that a solar - thermal efficient and with good performance can not provide more than 65% of the average requirements. And it's not even worth to dramatically enlarge the size of the irradiated surface, adding collectors to collectors: the cost of expansion which nails, however, the overall yield no more than 60% on average. Therefore, anyone who wishes to acquire a solar thermal system will still have to provide to support to it a traditional system for the remaining 40% of demand required.
60% of the requirement is not, however little and solar-thermal deserve more widespread than it is to this day!
This article will not deal with the costs of air, which will be dedicated to the theme of a separate article. But I will limit myself to pointing out that, after subtracting the depreciation costs that usually die out in the first five years (on an average lifetime of the system fixed at twenty years) the savings can then leveling off at around 60/70% approximately. It is also fair to say that the costs of traditional fuels will grow and grow, while the technologies related with solar installations are slowly but surely evolving towards greater efficiency and economy.
Photovoltaic technology
Explain how it is possible that the sunlight can be converted into electrical energy is a bit 'more difficult, but not impossible.
The transformation is possible thanks to the characteristics of certain materials called semiconductors. Silicon is one of these, but research is developing new materials even more efficient and cost effective, which certainly represent the new generation of photovoltaic cells.
A photovoltaic cell is just a small structure, intuitively similar to a small solar collector-heat, but it does not have a plate that receives solar radiation nor a fluid with which exchange heat. The cell exposed to the sun directly of small components is silicon, which subjected to solar radiation, produce a tiny electrical energy.
More components connected together form a photovoltaic cell, more cells connected in series form the photovoltaic module. A photovoltaic module has a size of about half a square meter (there are also wider) and produces, under optimal conditions, about 40/50 watts of power. A system consists of two modules can, if hypothetically place in Sicily, in the winter to produce about 0.2 / 0.3 kilowatt hours, and in summer about 0.5 / 0.6 kilowatt hours.
Without going into technical details can be seen easily how much is underperforming solar electrical production. 0.5 kilowatt-hours would be able to barely run a refrigerator of the best in the energy classes.
However, you can tile the photovoltaic modules, reaching plants made up of 3, 6 or even 20 square meters of panels. In this way it might lead to the generation of electrical power from all over, similar if not higher than those necessary for an average home.
In Sicily, however! And in the summer! And only during the day and during the middle!
Yeah, because PV technology is still immature and is able to offer attractive returns only under optimal conditions. One installation in northern Italy, it makes much less than one in the south. It is also very complicated store electrical energy, so at noon you can count on the contribution of energy interesting, while at night, when lights are turned on, televisions, dishwashers and irons (during the day, alas it works!), A PV system contributes to the need for a very low percentage.
Some say that by acquiring low-voltage electrical appliances, light bulbs (highly recommended) and maybe sending clothes in Ironing you can count on substantial contribution from photovoltaics. But I fear that this can be done thinking of using the home only to sleep and ... sleep
The costs of PV systems are still quite high enough to make them, on balance, not particularly convenient. Each photovoltaic system must still be connected to the national grid, both to supply the needs of absorption where the plant does not arrive as power produced (often), and because the electricity authority may agree with the owner of the PV system so that you can entry into the national grid the energy produced but not used by the user, recognizing economically the sale of energy.
The real problem of photovoltaic systems is still made up of the cost of the plant, depreciable only after long periods of time (more than ten years) despite the fact that the average life of a photovoltaic cell does not exceed twenty years. This makes unenforceable a policy of wide dissemination of photovoltaic technology.
The difference between the solar-thermal technology and photovoltaics lies in the actual efficiency and convenience. The first is recommended, the second less.
In a future article I will analyze the issue from the economic point of view, taking into account government incentives for the diffusion of the two solar technologies.
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Translated via software
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Source:
Italian version of ReteIngegneri.it