Category Control of Solar Energy Systems

Fresnel Collectors

Linear Fresnel reflectors use various thin mirror strips to concentrate sunlight onto tubes where fluid is circulating (Fig. 1.11). Higher concentration can be obtained

Подпись: Fig. 1.11 Fresnel linear reflector at the School of Engineers of Seville University
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and these mirrors are cheaper than parabolic mirrors, but a more complex tracking mechanism is needed. Absorbers are located at the focal point of the mirrors and consist of an inverted air cavity with a glass cover enclosing insulated tubes. In some cases there are multiple absorbers to improve system efficiency.

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Parabolic Troughs

Parabolic-trough systems are the most used CSP technology. A parabolic trough consists of a linear parabolic mirror that reflects and concentrates the received solar energy onto a tube (receiver) positioned along the focal line. The parabolic mirror follows the Sun by tracking along a single axis. The heat transfer fluid (HTF), typ­ically synthetic oil or water, is pumped through the receiver tube and picks up the heat transferred through the receiver tube walls...

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Thermosolar Plants/Concentrating Solar Thermal Systems

Concentrating solar thermal (CST) systems use optical devices (usually mirrors) and Sun tracking systems to concentrate a large area of sunlight onto a smaller receiving area. The concentrated solar energy is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists. The main concentrating concepts are: (a) parabolic troughs, (b) solar dishes, (c) linear Fresnels and (d) solar power towers. The main purpose of concentrating solar energy is to produce high temperatures and, therefore, high thermodynamic efficiencies.

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Electricity Generation

1.3.1.1 Photovoltaic Plants

The direct generation of electricity from solar energy is based on the photovoltaic effect which refers to the fact that photons of light hitting certain materials will knock electrons into a higher state of energy producing an electrical current.

Although the first PV cells were used to generate electrical power for spacecrafts, there are many PV power generation systems for more normal applications such as

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Fig. 1.8 Aerial view of the PSA facilities (courtesy of PSA, www. psa. es)

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fixed Solar tracking 1 axis Solar tracking 2 axis

Fig. 1.9 Types of photovoltaic plant

houses isolated from the grid, pumps for water extraction, electric cars, roadside emergency telephones, remote sensing and cathodic protection of pipelines...

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Technology Classification

Solar energy has been used throughout time, mainly for heating and lighting but also for many other purposes such as refrigeration [413], detoxification [55], de­salination [131, 415] and primarily for the generation of electricity. Solar powered electrical generation can be achieved either directly, by the use of photovoltaic (PV) cells, or indirectly, by collecting and concentrating solar power (CSP) to produce steam which is then used to drive a turbine to provide electrical power [139]. In [254] various advanced solar thermal electricity technologies are reviewed with an emphasis on new technology and new market approaches.

Figure 1.7 shows a diagram of basic solar energy conversion systems [359], able to convert the solar resource into a useful form of energy...

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Calculating the Horizontal Coordinates

The horizontal coordinates (hs, as) are calculated using spherical trigonometry based on the time coordinates, related by the equations

sin(hs) = sin(^) sin(5s) + cos^) cos(8s) cos(as) sin(hs) cos(as) = sin(ф) cos(8s) + cos(as) – cos(ф) sin(5s) (1.7)

sin(hs) sin(as) = cos(ф) sin(as)

From these equations the maximum solar elevation for a determined day and latitude can be found. At noon the hour angle and the solar azimuth angle are zero, as = 0° and as = 0°, corresponding to the maximum elevation (hs, max). Using these values in Eq. (1.7) gives

hs, max = 90° — ф + $s [°] (1.8)

In [342], also meteorological, topographic and astronomic effects are studied.

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Calculating the Time Coordinates

The horizontal coordinates of the Sun vary depending on the time of day, day of year and latitude of the location. On the contrary, the time coordinates are easier to find, as the declination depends only on the day of the year and the hour angle at the time. The time coordinates are determined as a preliminary step for calculating the horizontal coordinates. Most solar calculations require solar time but our clocks

Подпись: Fig. 1.6 Horizontal celestial coordinates
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show the local time. There is a convention for using time, setting the civilian time in different cities with different solar times. The solar time (ts) is calculated by

ts = local standard time + 4(Lref – Lioc) + At [min] (1.3)

where Lref is the geographic longitude of the reference meridian [°] and Lloc the longitude of the local meridian [°]...

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Geometry of the Sun’s Movement

From the point of view of an observer on the Earth’s surface, the Sun seems to describe an arc from sunrise to sunset. The local meridian plane by definition is lo­cated in the middle of this path at solar noon. The vertical direction of the observer’s position on the Earth’s surface intersects the sky vault at a point called the zenith. The Earth’s axis forms an angle equal to the latitude of the location (ф) with the observer’s horizontal plane.

The latitude angle ф is the angle between a line drawn from a point on the Earth’s surface to the center of the Earth and the Earth’s equatorial plane. The intersection of the equatorial plane with the surface of the Earth forms the equator and is desig­nated as 0° latitude...

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