Solar energy

Parameters of solar radiation

First of all, it is necessary to evaluate the potential energy possibilities of solar radiation. Here is the most important is the total specific power at the Earth's surface and the distribution of this power for various ranges of radiation.
The power of solar radiation

Surface solar radiation power that is at the zenith, is estimated as approximately 1350 Vt/m2. A simple calculation shows that for a 10 kW solar radiation must be collected only from the area of 7.5 m2. But this is - on a clear afternoon in the tropics. As soon as the sun begins to descend to the horizon, the path of rays through the atmosphere has increased, respectively, and increased losses on the road. However, in the middle band in the summer afternoon on per square meter, oriented perpendicular to the sun, accounting for more than 1 kW of solar energy.

And, of course, even cloudy dramatically reduces the energy reaching the surface, especially in the infrared (heat) range. In the middle band, with strong cloud at noon, the power of solar radiation is estimated at about 100 Vt/m2 and only in very rare cases may drop below this figure.
Energy distribution in the solar spectrum

Solar spectrum is almost continuous in extremely wide frequency range - from low-frequency radio wave and microwave X-ray and gamma radiation. It is obvious that it is very difficult to effectively capture the same way as different types of radiation (which can only be done with the help of theoretical «absolutely ideal black body»). But it is not necessary - first, the sun in different frequency ranges radiate with different force, and secondly, not all that emitted the Sun reaches the Earth's surface - some parts of the spectrum is largely absorbed by the atmosphere - mainly ozone, pairs water and carbon dioxide.

Therefore, we should identify the frequency bands in which we have the greatest flow of solar energy at the Earth's surface, and to use them. Traditionally, solar and cosmic radiation are not divided by frequency, and the wavelength (this is linked too exponent for frequency of this radiation, which is very inconvenient - visible light in hertz corresponds to the 14 th order). Let's see the same dependence of the energy distribution of the wavelength for solar radiation.
The energy spectrum of the Sun.
The intensity incident on the Earth from solar radiation (H1), depending on the wavelength. Shaded areas correspond to parts of the spectrum, observed at sea level due to absorption of these components of the atmosphere. 1 - solar radiation abroad atmosphere, 2 - Solar radiation at sea level, 3 - absolutely black body radiation at 5900 K. (Handbook of geophysics and space. Ed. SL Valleya and MakGrou-Hill, New York , 1965). Source: «The evolution of biological mechanisms saving energy» (caution, it seems, is the site broken and full of all rubbish!).

Range of visible light is considered to be part of wavelengths from 380 nm (deep violet) to 760 nm (deep red). Everything that has a smaller wavelength, high energy photons and is divided into ultraviolet, X-ray and gamma-radiation ranges. Despite the high energy photons of the photons in these bands is not too much, so the total energy input of the plot range is very small. Everything that has a longer wavelength, is lower than the energy of visible light photons and is divided into infrared (heat radiation) and various radio stations. From the schedule shows that the infrared sun emits almost as much energy as in the visible (levels lower, but wider range), but in the radio range of the energy of radiation is very small.

Thus, from the energy point of view, we should confine itself to visible and infrared frequency bands, as well as near UV (200 nm, a short ultraviolet hard almost completely absorbed by the ozone layer). A lion's share of solar energy reaching the Earth's surface, is concentrated in the wavelength range from 300 to 1800 nm.
Restrictions on the use of solar energy

The main limitation of the use of solar energy are clear - it is the volatility: the solar installation will not work at night, and ineffective in cloudy weather. This is usually obvious to virtually all.

But there is another fact which is particularly pertinent to our northern latitudes enough - this is the seasonal differences in the length of the day. If the tropical and equatorial zones of the length of day and night weakly dependent on time of year, it is at the latitude of Moscow is less than the shortest day of the longest in nearly 2.5 times more! About the Polar area, I will not say ... As a result, if the summer day, solar installation near Moscow could produce energy, not less than at the equator, the winter, when demand for energy is high, the development of an objective decrease in the times. In addition, the rays of the low winter sun even at noon, should be much thicker layer of atmosphere, and on the way to lose much more power than in summer when the sun is high (the expression «the cold winter sun» is the most direct physical meaning). But this does not mean that the solar installation in the median strip and into more northern areas are useless - even in winter they do little good, for the period of long days, as minimum six months between spring and autumn equinox, it is quite effective.

A particularly interesting application of solar installations for the operation of the increasingly widespread, but rather «gluttonous» conditioners. Indeed, the stronger the sun, the hotter, the air conditioner needful, and (just!) So more energy is able to develop a solar installation, and this energy will be used by an air conditioner is «here and now»! This is not necessarily to convert energy into electric form - heat absorption machines use heat directly, which means that instead of photovoltaic cells can be used more efficient solar collectors.
Traditional types of plants to use solar energy

From time to time, messages appear on the construction of another solar power station (SES) or the desalination plant. Throughout the world all the more common thermal solar collectors and photovoltaic solar cells. These methods are the use of solar energy will not develop for decades, he devoted a lot of sites on the Internet. Therefore consider them in the most general terms.
Solar panels
Solar panel.

In general, under the «solar cell» can be understood as any set of identical modules perceived solar radiation and combined in a single device, including a purely thermal, but traditionally, the term is established for the panel photovoltaic cells. Therefore the term «cell battery» almost always means a photovoltaic device that directly convert solar radiation into electrical current. The technology developed from the middle of XX century. A great impetus to its development was the exploitation of outer space, where competition for solar power production and the duration of work can now make only a small nuclear power sources. During this time, the conversion efficiency of solar cells increased from one to two per cent to 20% or more in mass relatively cheap models and over 42% in experimental samples. Significantly increased lifespan and reliability.
Advantages of solar panels

The main advantage of solar panels - they limit the constructive simplicity and complete lack of moving parts. As a consequence, unpretentiousness maximum ease of installation and minimal service requirements during the operation (usually need only wipe dirt from the working surface). Introducing a flat design, they are very well placed to appeal to the sun, slope of a roof or wall of the house, almost no changes for themselves any additional building or separate bulky designs. The only condition - nothing should obscure them for as much time as possible.

Finally, solar cells can produce energy from dawn to dusk, even in cloudy weather, when the thermal solar collectors have a temperature only slightly different from the surrounding temperature. Of course, compared with a clear sunny day their productivity drops many times, but better something than nothing at all! In this regard, particularly interesting  cells with a maximum conversion of energy in frequency ranges where clouds do not absorb solar radiation.
Disadvantages of solar panels

Of course, the shortcomings of the solar panels a lot. In addition, depending on weather and time of day, you can note the following.

    1. Low efficiency. The same solar collector at the correct choice of form and material capable of absorbing a surface of nearly all fell on his little solar radiation across the spectrum of frequencies, which have the visible energy - from the far infrared to ultraviolet range. Solar panels convert the energy is selectively - to work the excitation of atoms requires a photon energy (radiation frequency), so in some bands transformation is very effective, but other frequency ranges for them are useless. In addition, the energy captured by the photon is a quantum - it «surplus», exceeding the required level, are harmful in this case, heating the material photo. In many ways it is this, and due to their low efficiency.
       By the way, badly chosen material safety glass, you can greatly reduce the efficiency of the battery. The matter is compounded by the fact that ordinary glass is fairly well absorbed high-energy ultraviolet portion of the range, and for most types of PV is very important it is that the range - the energy of infrared photons is too small for them.
    2. Sensitivity to high temperature. With increasing temperature the efficiency of solar cells, as well as most other semiconductor devices is decreasing. At temperatures above 100 .. 150 ° C, they can temporarily become dysfunctional, and even greater heat can lead to irreversible damage. It is therefore necessary to take all measures to reduce heat, inevitable under the scorching direct sunlight.
       Additionally complicating the situation is that the sensitive surface is quite fragile photocells often closes the protective glass or transparent plastic. The result is a kind of «greenhouse», aggravating overheating. However, increasing the distance between the protective glass and the surface of photo detector and connecting the top and bottom of this cavity with the atmosphere, it is possible to organize a convective air flow, naturally cooling photocells. However, in bright sun and the high temperature of the outside air may not be enough. Therefore, solar cell is not even a very large size may require special cooling system. For the sake of justice it is necessary to note that such systems are usually easy and completely automated, and drive the fan or pump requires only a small fraction of generated energy. In the absence of bright sun is not so much heating and cooling is not required, so that the energy saved in the drive of the cooling system can be used for other purposes.
    3. The relatively limited lifespan. Semiconductor wafers, which are usually composed of solar cells over time to degrade and lose their properties as a result of the already not very high efficiency solar panels gradually decreases even more. Long-term effects of high temperature accelerates the process. Nevertheless, today's photo are able to maintain its effectiveness over many years. It is believed that, on average, for 25 years, the efficiency of solar cells is reduced by 10%.
    4. Sensitivity to pollution. Even a relatively thin layer of dust on the surface of photocells or safety glass can absorb a significant portion of sunlight, and markedly reduce the energy. In the dusty town that may require very frequent cleaning of the solar panels installed horizontally or obliquely. Of course, the same procedure can not be avoided, and after every snowfall, and after the dust storm ...

Finally, last but one of the most important obstacles to a broad and spread of solar panels - to their relatively high price. The cost of solar cells are currently at least $ 1 / W (1 kW - $ 1000), and it is inefficient for the options without the cost of the converter provided by low voltage DC to domestic or industrial standard. In most cases, the real cost estimates, these figures should be multiplied by 3-5 times.
Solar thermal collectors
Solar collector.

Name «solar collectors» fixed for devices using the direct heating of the solar heat - as a single, and extensible (modular). The simplest model of the solar heat collector - above the tank on the roof of the summer shower (by the way, the efficiency of heating water in a summer shower can be markedly improved, built around a mini-tank least of polyethylene film, preferably between the film and the walls of the tank remained gap 2-3 cm above and 4-5 cm laterally).

However, today's collectors are like a little tank. Usually they are a flat design of thin  tubes packed in a lattice or Snake. The tube is attached to the same sheet tray to catch the sun and heat, falling in the intervals between the tribes of the tube. To reduce heat losses and more efficient heating of the reservoir on top can be closed with a sheet of glass or transparent polycarbonate honeycomb, while the bottom layer of insulation keeps the heat - is a kind of «greenhouse». As the tube moves heated water or other coolant, which can be collected in the thermally insulated storage tank. Movement occurs by the action of the coolant pump or by gravity. In the latter case, more or less effective circulation requires a careful calculation of slopes and cross-sections of pipes and placement of the reservoir as possible. Usually the collector is placed in the same locations, where solar cell - solar wall or on the sunny slope of the roof, the truth is somewhere further need to host and storage tank. Without such a tank with intensive handling of heat (say, if you need to fill the bath or shower) capacity reservoir might not be enough, and after a small time from the tap go slightly warm water

In addition to the above liquid solar collector, there are also other interesting types of structures: air (coolant - air), thermal tubes, «solar pond», etc. Unfortunately, most research and development of solar collectors is devoted to models of liquid, so the alternative series are produced and almost no information about them is not too much.

Here I am considering solar collectors only in the most general terms. For those who want to look into this matter in more detail, I recommend to visit the site on the solar heating.
Advantages of solar collectors
The second very important advantage is that, unlike solar panels, the collector is able to capture and convert to heat more than 90% got it from solar radiation. Even with light cloud his performance exceeds the efficiency of solar cells.
Disadvantages of solar collectors

Solar collectors are more sensitive to weather than the solar panels. In overcast windy weather is the use of virtually no reservoir, and solar cell at least a little bit of energy, so producing. Even in bright sun fresh breeze (but not stormy) can significantly reduce the efficiency of heat exchanger open. The protective glass, of course, reduces the heat loss from wind, heavy cloud cover but it is powerless.

Among other shortcomings of solar collectors primarily highlight of their season. Even the spring and autumn night frosts can result in the formation of ice in the pipes and heater burst. Of course, it can be deleted, heating cold nights with the source of heat, but in this case, the overall energy efficiency of the collector can easily become negative! Another option - double-header with antifreeze in an external circuit - do not require such an expenditure of energy, but it would be very difficult as in the manufacture or during operation. Air construction in principle can not be frozen, but there is another problem - the low specific heat of air.

But perhaps the main disadvantage of solar collector is that it is the heating, and obtained in the temperature rarely exceeds 60 .. 80 ° C (although industrially made samples of this type in the absence of random heat can heat the coolant up to 190 .. 200 ° C). Therefore, use heat to develop in any significant amounts of mechanical work or power in most cases very difficult. Even for most of the low-temperature steam-water turbine (like the one at the time described the VA Zysin) must be overheated water at least up to 110 ° C! And specifically in the form of heat energy, as is well known, long stored, and at a temperature below 100 ° C it can usually be used only in hot water and heating homes. However, given the low cost and ease of manufacture it may be sufficient cause for having own solar collectors.

For the sake of justice it should be noted that the «normal» working cycle heat engine can be arranged, and at temperatures below 100 ° C - or if you lower the boiling point, reducing the pressure in the evaporator by means of pumping out steam, or use a liquid, the boiling point of which lies between the temperature heating of the solar collector and the ambient temperature (optimum - 50 .. 60 ° C). However, I can think of only one is not exotic and relatively safe liquid, more or less meet these conditions - this is ethanol, in its normal boiling at 78 ° C. Obviously, in this case necessarily will have to organize a vicious cycle, solving many related problems. In some situtsiyah perspective may be the use of engines with external heating
The concentration of solar energy

Improving the efficiency of solar collector first and foremost is the steady rise in temperature of water heated above the boiling point. To do this usually applies to the concentration of solar energy collector with the help of mirrors. It is this principle is the basis of most solar power plants, the differences lie only in the quantity, configuration and placement of mirrors and collector, as well as management mirrors. As a result, the focus point it is possible to achieve the temperature even in the hundreds and thousands of degrees - at that temperature is already going on direct thermal decomposition of water into hydrogen and oxygen (hydrogen can be burned at night and on cloudy days)!

Unfortunately, the efficiency of such installations is impossible without a fairly complex control system mirrors-concentrators, which must track the constantly changing position of the Sun in the sky. Otherwise, just a few minutes, the focus leaves the reservoir (which is in such systems is often relatively small size), and heating the body stops working. Even the use of mirror-parabola  solves the problem only partially - if not periodically move followed by the Sun, then a few hours it will no longer appear in the bowl or it will cover only the region - is the use of this will be a little bit.

The easiest way of solar energy in the «home» environment - it is horizontal to the mirror near the reservoir, so that most of the day «bunny» from the mirror falls on the collector. Creating a system of vertical mirrors hubs - undertaking more efficient, but much more troublesome, though in some cases it is justified it may be easy to install large mirrors on the wall adjacent to the manifold - it all depends on the configuration and location of the building and deployment of the reservoir.

The concentration of solar radiation through the mirrors can increase electricity generation and solar cell. But at the same time increasing its heat, as he is known, can bring the battery down. Therefore, in this case is usually only a relatively small gain (a few tens of percent, but not at times), and the need to carefully control the temperature of the battery, especially in warm clear days!
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