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The Sun

Although

Encyclopedia of the Solar System (Third Edition)

2014, Pages 235-259

Chapter 11 - The Sun

Author links open overlay panelMarkus J.Aschwanden

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https://doi.org/10.1016/B978-0-12-415845-0.00011-6Get rights and content

Abstract

The Sun, our nearest star, is the gravity center and ultimate energy source for many processes in our solar system. We give a physical description of the Sun, from inward to outward, starting with the solar interior, and continuing to the photospheric surface, the chromosphere, and to the corona, which displays eruptive phenomena such as flares, and coronal mass ejections, a rich realm of high-temperature plasma physics, magneto-hydrodynamics, and high-energy particle acceleration kinematics.

The Future of Solar Energy

Solar energy has started to grow but until the costs are substantially reduced it will remain a very small component of the world and the U.S. energy picture. Even though the cost has been substantially reduced over the past 50 years, it still remains a very expensive energy source. The PV effect has been known since 1839 and, despite extensive research efforts since then, solar power is still very expensive. It does not seem likely that further research efforts will make the breakthroughs that will lead to the commercialization of solar power in the foreseeable future. There is a lot of solar power waiting to be harnessed if such a breakthrough could be made; however, even if there was a way to make solar power viable, it is to be expected that this cannot be done without having a significant impact on the environment.

Solar Power

Paul Breeze, in Power Generation Technologies (Third Edition), 2019

The Solar Energy Resource

Solar energy is generated by nuclear reactions within the body of the sun. This energy reaches the surface of the earth in the form of electromagnetic radiation. The composition of this radiation as it travels through space towards the earth is around 56% infrared, 36% visible radiation and 7% ultraviolet with the remainder belonging to regions of the electromagnetic spectrum outside the energy ranges covered by these three.

Not all this radiation reaches the surface of the earth. Some is scattered by dust and molecules in the atmosphere. This scattering is a random process, sending the radiation in all directions so that much goes directly back into space. The remainder reaches the surface, but as diffuse, indirect radiation. Clouds act to reflect more sunlight back into space and they play an important role in regulating the temperature on the surface of the earth.

Another part of the radiation is absorbed by molecules such as water, carbon dioxide, ozone and oxygen within the atmosphere. Water and carbon dioxide absorb energy from the infrared region, while oxygen and ozone absorb from the ultraviolet. All these interactions reduce the solar energy flux by around 40% while at the same time changing its composition so that the sunlight which reaches the earth's surface comprises 50% visible radiation and 47% infrared.

The amount of energy carried by solar radiation is normally expressed in terms of the solar constant which measures the quantity of solar energy passing through one square metre of space perpendicular to the direction of travel of the radiation at the average distance of the earth from the sun. According to the World Energy Council, the value of this constant is 1367 W/m2 (Table 13.2). When absorption and scattering are taken into account, the total solar flux reaching the surface of the earth is estimated to be 1.08×108 GW and the total amount of energy reaching the surface of the earth each year is 3,400,000 EJ. This is between 7000 and 8000 times annual global primary energy consumption. If 0.1% of this energy was converted into electricity with 10% efficiency, it would provide an equivalent of around 10,000 GW of generating capacity: the global total is around 6000 GW.

Table 13.2. Solar Energy and the Earth

Solar constant at the distance of the earth from the sun1367 W/m2Total solar energy reaching the earth in a year3,400,000 EJTotal solar flux reaching earth1.08×108 GWAverage solar energy density at earth's surface170 W/m2

Source: World Energy Council.

To put this into a more practical perspective, take the example of a group of solar thermal power plants that were built in California in the late 1980s and early 1990s. These plants were designed on the basis of a solar input of 2725 kWh/m2/y or 22.75 GWh/y for each hectare. Assuming this energy can be converted with 10% efficiency into electricity, ten million hectares (100,000 km2) would be sufficient to supply the entire United States.

The solar input is a key parameter when planning a solar thermal power plant. This is determined by the thermal energy density at the earth's surface, a factor which varies with position on the earth. The average (which takes account of the fact that any point on the earth's surface only receives sunlight for about half the time, the other half being hours of darkness) is 170 W/m2, while the largest, near the Red Sea, is 300 W/m2. (The latter is just under one quarter the value of the solar constant.) Over a year, the average incident energy over 1 m2 is equivalent to a barrel of oil or 200 kg of coal.

The sunlight that reaches the earth's surface is of two types, direct radiation and diffuse radiation. The latter is the result of various scattering and absorption processes that take place as the sunlight passes through the atmosphere. Vegetation is able to absorb both types of radiation, and so can solar cells. However, a solar thermal plant requires direct radiation to operate effectively. This limits the applicability of this technology to regions where there is low average annual cloud cover. With no cloud cover, and whatever the location, between 80% and 90% of the sunlight reaching the surface will be direct radiation.

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Solar energy

Lee Phillips, in Managing Global Warming, 2019

9.5 The potential of solar energy to reduce greenhouse gas emissions

Solar power, despite its recent rapid growth in adoption, and the enthusiastic support it receives both from the scientific establishment and energy investors, remains a very small proportion of total energy consumption (see Fig. 9.2). Its potential to drive a significant reduction in the emissions of greenhouse gasses lies in a potential future. If the exponential trend of the last decade continues, solar power will replace a sufficient quantity of fossil-fuel combustion to play a large role in reaching humanity's climate goals [35].

In fact, if the doubling time in the proportion of solar energy to total energy consumption of 2.26 years (see Fig. 9.2) remains the same, then solar energy will reach 100% of the total in approximately the year 2032, as illustrated in Fig. 9.5, where we have adopted the use of a log scale, where the exponential function appears as a straight line. The figure simply extends the exponential fit to the data into the future, assuming a constant growth rate.

Fig. 9.5. Naive projection of solar energy adoption.

Obviously, in this case, the effect of solar energy on the global climate problem would be profound, as it would have completely replaced greenhouse-gas-emitting combustion of fossil fuels in another 15 years. Although such a naive extrapolation obviously needs to be tempered by a serious encounter with reality, such thinking appears to be taken seriously in some quarters [36]. The first obvious problem with this projection is that it predicts that solar energy will not only have replaced fossil fuels, but other forms of renewable energy as well—and there is no reason to expect solar power to supplant all sources of hydropower, wind power, etc.

The observation of exponential growth leads naturally to the supposition of a model that can be expressed by the differential equation

dSdt=bS

where, in this case, S is the solar energy fraction and b is a growth rate. The meaning of this equation is that the time rate of change of S is proportional to S itself. This has an obvious application in the modeling of populations, where the number of new organisms created every generation is proportional to the number of reproducing organisms at any time. If an equation of this form tells us anything about the mechanism underlying the observed exponential growth of solar adoption, it suggests that we should look for reasons that the rate of growth should be proportional to the installed base at any particular time. Since solar panels do not reproduce themselves, we need, if we are to proceed along this line of speculation, to surmise the cause for dS/dt to be proportional to S. One plausible mechanism may be a kind of reassurance created by the appearance of rooftop solar installations in a neighborhood: a homeowner may not seriously consider investing in solar panels until he is encouraged by their appearance on his neighbors' houses, and motivated by their recounting of a good experience with them.

No matter the underlying reasons for the exponential growth, however, it is clear that it cannot continue indefinitely. As in biological populations, there will be factors that limit growth. In the case of solar energy, these factors, in addition to the political pressure applied by industry lobbying groups mentioned earlier, include rooftop market saturation, where the limited amount of rooftop area situated in desirable weather regions becomes already occupied by panels; cheap, "green" energy supplied to the grid from other renewable sources; reduction in retail electricity prices due to a temporary high supply of natural gas or other fossil fuel, diluting the motivation for investing in solar panels; and any number of unforeseeable factors.

In order to include growth-limiting factors, the differential equation may be modified with an additional term:

dSdt=bL−SLS

The new factor L represents the finite capacity of the environment to support (in this case) solar power installations. Note that when S is small, during the early stages of growth, then L − S ≈ L and the increase is exponential, but the growth rate decreases and goes to zero as S approaches L.

Although it is certain that L is finite, because of the multitudinous, unforeseeable factors that contribute to its effective value, it is impossible to predict for how long the recently established exponential growth of solar energy adoption will continue, or at what value it will level off. In other words, the future of solar energy adoption is likely to resemble Fig. 9.6, which plots the solution to our second differential equation (sometimes called the logistic equation) with L arbitrarily chosen to result in saturation at about 25%. Once we enter the phase of saturation, or slowing down of solar energy adoption, it will be possible to estimate L and make an informed prediction of the future; but as we are, according to the data, in the early, exponential phase of solar energy growth, where S is much smaller than L, it is at present impossible to predict the level at which saturation will occur. Consequently, since the equilibrium level of solar energy deployment is impossible to estimate, a quantitative estimate of the amount of greenhouse gas emissions that will ultimately by replaced by solar energy is also not possible, at this still early phase of adoption. It seems clear, however, that solar energy will be a significant portion of the mix of renewables in the future, even if naive predictions [36] of complete dominance in 15 years can be discounted.

Fig. 9.6. Projection of solar energy adoption based on the logistic equation, with an assumed saturation level of 25%.

Partly because of the suppression efforts by the fossil-fuel industry mentioned, partly due to market saturation in certain regions, and also partly due to the faltering of several manufacturers of solar panels, the exponential growth in solar power consumption may already be showing some initial signs of slowing down [24]. Nevertheless, in 2016 the United States added 15 GW of solar capacity, an increase greater than that from any other source of energy, including fossil fuels.

According to at least one study [37], the world is already on an inexorable path to complete replacement of fossil and nuclear fuels by renewable energy. Solar energy, in its various forms, has a guaranteed place at this table. Solar energy technology has matured to the point where humankind can, if it recognizes the need, extract any part or all of its energy requirements from our star, because, as it is written [38] by the scientists of the Los Alamos National Laboratory, "Sunlight is abundant beyond the energy needs of the entire human race and completely free."

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Solar Power

S.P. Deolalkar, in Designing Green Cement Plants, 2016

8.1

General

Solar power is conversion of sunlight into electricity. There are two ways of doing this.

Concentrated Solar Power (CSP), in which sunlight is focused on an area containing water which is converted into steam and is used to generate power, as in a thermal power plant.

CSP produces concentrated solar beam irradiation to heat liquid, solid or gas as in a regular TPS. The best sites for CSP are in equatorial belt cloud-free regions.

Lenses or mirrors are used to focus rays onto a solar tower. Mirrors track the sun. CSP is more cost effective. These are observations in literature available on the system. One reason could be that – generation of electricity does not come to stand still when sunlight is not available because of its inherent thermal mass as it happens in case of PV cells. It is more efficient and has longer life.

PV cells, in which light is converted into electricity using photovoltaic cells (PV). Solar cells produce DC power, which fluctuates according to the intensity of irradiated light. This requires an inverter to produce power at the desired voltage frequency and phase. PV Systems are connected to the grid. They need batteries for backup.

Fig. 6.8.1 shows a circuit diagram of a PV-cell-based solar power plant.

Figure 6.8.1. Circuit diagram of a PV solar power plant.

(Report IISc-DCCC 11 RE 1).

PV cell electricity production globally in 2012 was ~ 55,000 MW; it is expected to increase by 2015 to 70,000 MW. India's share will be 5000 MW or more.

8.2

Solar power special features

Solar power is not available at night. The ability to store generated power is an important aspect of solar power. Available outputs must be obtained when available and either stored for later use or transmitted to where it can be used.

Because of its inherent thermal mass, fluctuations in CSP systems are of less intensity.

8.2.1

Solar energy can be stored at high temperatures using molten salts.

8.2.2

Concentrated PV systems use concentrated light on PV cells. Many types of concentrators are available.

8.2.3

An inverter is required to convert it into AC at the desired voltage, frequency and phase.

8.2.4

Solar power does not have any solid, liquid, or gaseous byproducts. Its social & environmental impact is insignificant compared to a TPS.

8.3

Options for solar power

Basically there are several options for solar power installations.

Centralized

Distributed

On grid

Off grid

8.3.1

In grid-connected systems, excess power can be fed to the grid, with the grid serving as storage.

Inverters are needed to convert DC power to AC.

Off-grid systems have opportunities in areas of developing countries without electricity.

8.3.2

Distributed systems supply power to grid-connected consumers.

Centralized systems are common grids: they are not for any particular consumer.

8.4

Solar power in cement plants

Solar power can be used in any of the following ways:

solar PV cells

solar thermal

solar PV hybrid diesel.

Fig. 6.8.2 shows the three most common types of PV solar power systems.

8.4.1

Captive solar power systems in cement plants

There are three principal options:

standalone

grid tied

grid interactive with battery backup

Grid-tied systems feed power to the grid (whole or surplus); when power generation is off (sunlight not available) power can be bought from the grid.

This seems to be the most practical way of installing solar power. There is one more option, that of a hybrid system with wind power or a diesel generator (dg) set.

Figure 6.8.2. Common types of PV solar power systems.

(Science & technology of photovoltaics by P. Jayarama Reddy).

8.5

Layout of solar power station

Layouts differ radically depending on the system installed, that is,

concentrated solar power (CSP)

PV cells.

Plate 6.8.1 shows a CSP power plant.

8.5.1

A PV cell system requires considerable ground space, spread over hundreds of hectares.

Homes and offices may use solar power by installing PV cells on the rooftop or by integrating them in the architecture of the building.

8.5.2

In one installation of solar power for a cement plant, the footprint of the PV cells is 2650 sq. meters and is comprised of four arrays of PV cells, with each array containing 22 sets of 24 panels each.

Plate 6.8.1. Concentrated solar power project.

(Solar power).

8.6

Availability of solar power

Solar power, like wind power, is an intermittent source of power. All available power must be taken when available or sent where it can be used.

Neither sunshine nor its intensity is evenly spread on the surface of the earth.

Solar power thus cannot be used by itself on the large scale required by industry everywhere.

Tropical countries are more fortunate in that they have a maximum number of sunny days with high intensity of sunlight.

Metereological laboratories of various countries maintain data on solar radiation and daily duration of sunlight throughout the year.

In India, for example, the average number of sunny days is 250 to 300 in a year.

Annual global radiation varies between 1600 and 2000 kwh/sq. km. Equivalent energy potential is around 6000 gwh per year.

8.7

Capital costs

A 25-MW solar power plant is estimated to cost $37.5 million, that is ≃$1.5 million per MW.

Capital costs of solar power are high because substantial investment is required in inverters to convert DC power to AC.

Payback periods are between eight to twelve years currently. They are expected to come down considerably as costs of PV cells come down significantly due to advances in technology.

8.8

Costs of production

The cost of production of solar power is presently around 22 to 26 US cents per unit (Rs 12 to 14.5), compared to 7 to 9 US cents (Rs 4 to 5) per unit for grid power.

Solar power is cheaper on a lifecycle basis. The life of PV cells is 25 years or longer.

Maintenance and operational costs are much less.

Power from solar plants may be available in the future for 2.2 US cents (1.23 Rs), compared to grid power at 9 US cents (Rs 5).

8.8.1

Table 6.8.1 furnishes the current status of solar power systems by way of investment costs, operation and maintenance costs, capacity factor, life and LCoE.

Table 6.8.1. Performance Data on Solar Power

Sr no.TypeTypical Size (mw)Investment cost ($/kw)O & M Cost (USD/kwh)Capacity Factor (%)Design life time (years)LCoE @ 10% discount (USc/kwh)PV Industrial fixed tilt0.5-1002700-520014-6915-2120-3016-52

$ = US dollar; Usc = US cent; LCoE = levellised cost of energy.

Source: Annex III, Recent Renewable Energy Cost & Performance Parameters. Report by University of Cambridge.

8.9

Compared to wind power, solar power has a long way to go to become an alternative viable source of renewable energy in cement plants.

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Solar Energy

Paul Denholm, ... Mark Mehos, in Generating Electricity in a Carbon-Constrained World, 2010

10.2.1 The Solar Resource

The mean annual solar energy resource is frequently expressed as the amount of radiant energy received on a given surface area per unit time (kWh/m2/day). This incident solar energy is shown globally in Figure 10.1.

Figure 10.1. Global mean solar resource (kWh/m2/day).

Source: Adapted from the NASA/SSE satellite dataset.2

Solar energy contains a direct component, which is light from the solar beam, and a diffuse component, which is light that has been scattered by the atmosphere. This distinction is important because only the direct solar component can be effectively focused by mirrors or lenses. The direct component typically accounts for 60–80 percent of the total solar insolation in clear sky conditions and decreases with increasing humidity, cloud cover, and atmospheric aerosols such as dust or pollution plumes. Technologies that rely on the direct solar component such as CSP plants work best in areas with high direct normal irradiance, which generally limits their application to arid regions, as seen in Figure 10.2. Nonconcentrated solar technologies such as PV panels can use both the direct and diffuse solar components and are not as geographically limited in their application.

Figure 10.2. Direct normal solar insolation for a two-axis tracking concentrator in the United States.

Source: NREL.

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Solar energy

Nikolay Belyakov, in Sustainable Power Generation, 2019

17.1 Solar energy potential and conversion

The amount of the Sun energy that reaches the Earth and can potentially be captured is significant and ranges from as low as 15750 EJ to 49837 EJ [1] (which is around 4375–13843 PWh per year) as discussed in Chapter 2. The average amount of solar energy received at the border of the Earth's atmosphere is around 342 W m−2, of which about 30% is scattered or reflected back to space, leaving around 70% or approximately 239 W m−2 available for harvesting and capture [2]. Solar energy is by far the largest energy resource on the Earth that is readily available on the surface.

Unfortunately, this potential is not entirely exploitable. Solar is the intermittent source of energy as it is subject to the rotation of the Earth (so that the Sun light hits the Earth only during the day period) and its rotation around the Sun, providing seasonality of the solar energy.

The promising fact is that solar energy is the intrinsic source of other forms of renewable energy like wind, bioenergy, ocean, and is the initiator for major cycles and fossil fuels. Directly, solar energy has been used by humankind since centuries for heating and cooking purposes. The idea to convert solar energy into other forms, and especially into electricity, has been of vital importance among the scientists and engineers, and today we have two major options for solar energy use based on the conversion chains discussed in Chapter 1. These are [3,2]:

Passive solar technologies involve the accumulation of solar energy without transforming thermal or light energy into any other form. This is mostly used, for instance, for collecting, storing, and distributing solar energy for heating purposes.

Active solar technologies collect solar radiant energy and use special equipment to convert it into other forms of energy, e.g., heat or electricity. These technologies can be further grouped into two major categories:

2.1.

Solar thermal technology that collects and concentrates solar energy by special devices and further converts it into electricity through other forms, and

2.2.

Photovoltaic technology that enables the direct conversion of solar energy using semiconductor devices.

On the industrial scale, both active solar technology options — photovoltaic and solar thermal — are implemented. The intense research efforts of energy scientists with regard to solar options have helped to yield an improved efficiency of photovoltaic technology [2], which enabled increasing the speed of solar photovoltaic deployment for industrial electricity generation scale. Similarly, solar thermal technologies have also been economically feasible for large electricity generation. This is achieved through the concentrating solar power technology — an approach that allows collecting solar radiation and using its energy to convert liquid into steam and employ steam turbine cycle for electricity generation.

Fig. 17.1 shows the development of the installed capacities of photovoltaic (PV) and thermal solar (CSP) technologies for the last decade. For the last five years, Asia eclipsed all other markets for PV technology, accounting for about two-thirds of global additions. The top markets — China, the United States, Japan, India, and the United Kingdom — accounted for about 85% of PV additions in 2016. For the cumulative PV capacity, the top countries included China, Japan, Germany, and the United States [4]. While China continued to dominate both the use and manufacturing of solar PV, emerging markets on all continents have begun to contribute significantly to global growth. By the end of 2016, every continent had installed at least 1 GW of PV capacities, at least 24 countries had 1 GW or more of PV capacity, and at least 114 countries had more than 10 MW [4].

Figure 17.1. Solar energy installed capacity.

Solar energy installed capacity growth shows that PV technology dominates the market due to its massive use both in private and industrial electricity generating applications. Solar thermal CSP technology is much smaller but is utility scale only.

Source: Based on data from [5].

Contrary to PV, solar thermal technology has been showing moderate growth during the last decade. In 2016 South Africa led the market in new additions, becoming the second developing country to do so after Morocco in 2015 [4]. CSP market growth continued to be driven outside of the traditional markets of Spain and the United States, which have been among the top drivers of CSP installations and still have the largest share.

The two most economically and technically feasible solar energy options — photovoltaic and solar thermal — are the viable option for sustainable future energy mix. Though the installed capacities seem large, their estimated share in the global electricity generation mix varies from less than 1% to around 1.5%, depending on the data source. Still, as solar energy is the abundant source of power, these technologies prove to be an important contribution to the generation mix and will definitely express growth and technological advancement in the coming years.

Further on, we consider these technologies in detail and discuss their economics, benefits, and potential drawbacks from the sustainability point of view.

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Solar energy

Ahteshamul Haque, in Electric Renewable Energy Systems, 2016

3.1 Introduction

Human beings have tried to harness solar energy in the past for their convenience. In the fifth century BC, passive solar systems were designed by the Greeks to utilize solar energy for heating their houses during the winter season. This invention was further improved with the advancement of mica and glass, which prevent the escape of solar heat during the daytime. Another invention was made in USA to use solar energy to heat water. The first commercial solar water heater was sold in the 1890s. In the nineteenth century scientists in Europe constructed the first solar powered steam engine [1].

In the 1950s scientists working at Bell Labs developed the first commercial photovoltaic (PV) cells. These PV cells were capable of converting sunlight into electrical energy to power electric equipment. These PV cells began to be used in space programs, that is, to power satellites, etc. Further advancement in the technology reduced the price of solar PV and it began to be used for household applications [2].

Currently, global demand for electricity is increasing [3]. The limited reservoirs of fossil fuels and emission of greenhouse gases have led to serious concerns regarding energy crises and climate threats. These concerns led researchers to look for alternative sources of energy, and solar energy is considered as the most acceptable source among all renewable energy sources. Solar energy is available in abundance and free of cost all across the globe. It is reported that Earth receives energy from the Sun, which is 10,000 times more than the total energy demand of the planet [4].

The conversion from solar energy to electrical energy is done by using solar PV. Solar PV has a nonlinear characteristic and its output varies with ambient conditions like solar irradiation, ambient temperatures, etc. [5].

In this chapter passive and active solar energy conversion is discussed. PV modeling, its operation, module, integration, and evaluation parameters are described. Finally, practical problems are given.

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Solar Power

Wilfrid Francis, Martin C. Peters, in Fuels and Fuel Technology (Second Edition), 1980

Publisher Summary

This chapter provides an overview of solar power, and its advantages and disadvantages. Solar power has the general advantage of being a daily renewable source of power but the disadvantages of being erratic because of cloud cover, discontinuous because of the night and the seasons, and diffuse. It is collected in a number of ways. Flat-plate collectors are black absorbers through which a cooling medium (usually water) flows. For low-temperature operations (swimming-pool heaters), the absorber is usually bare, but for higher temperatures (domestic hot water or water distillation), the absorber is usually covered in glass at the front end and with insulation underneath. The energy absorbing is given by the Hottel–Whillier equation. Flat-plate absorbers are normally made of black plastic, for low-temperature use, or of copper or aluminum for high-temperature use. The latter are usually covered with a single layer of glass (double glazing is marginal because of the greater reflection) and backed with a weather-proof plastic. The front surface is usually painted with a special coating.

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SOLAR POWER

J.R. Williams, in Solar Energy Conversion, 1979

Publisher Summary

This chapter discusses the basic concepts related to solar power. Energy from the sun can be collected and utilized in a variety of different ways as follows: (1) direct thermal applications involving collection of sunlight by solar thermal collectors for heating and cooling of buildings, heating water, distillation, or providing industrial and agricultural process heat; (2) solar electric applications in which energy from the sun is transformed into electricity by solar-thermal-electric, photovoltaic, wind, or ocean-thermal conversion systems; and (3) fuels from biomass, involving the production of fuels such as wood, methane, alcohols, or hydrogen from vegetation. The chapter describes solar-thermal-electric technologies, that is, the development of equipment for the collection of solar radiation and conversion of this energy into heat that, in turn, is utilized to generate electric power. These technologies are divided into two categories: (1) central receiver systems, which use a field of mirrors to concentrate sunlight onto the heat receiver to generate fairly large amounts of power, and (2) dispersed systems. The chapter provides an overview of solar concentrator concepts. It also discusses linear single axis tracking concentrators.

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Solar Power

Paul Breeze, in Power Generation Technologies (Second Edition), 2014

Solar power generation technologies

As already outlined, there are two ways of turning the energy contained in sunlight into electricity. The first, called solar thermal power generation, involves using the sun simply as a source of heat. This heat is captured, concentrated, and used to drive a heat engine. The heat engine may be a conventional steam turbine, in which case the heat will be used to generate steam, but it could also be a closed-cycle turbine system using an organic thermodynamic fluid, a gas turbine, or a Sterling engine.

The second way of capturing solar energy and converting it into electricity involves use of the photovoltaic or solar cell. The solar cell is a solid-state device like a transistor or microchip. It uses the physical characteristics of a semiconductor such as silicon to turn the sunlight directly into electricity. The simplicity and durability of the solar cell makes it an extremely attractive method of generating electrical power.

As with several other renewable technologies, solar energy is intermittent; it is only available during hours of daylight. In many parts of the world where there is a good solar resource, high levels of sunlight often coincide with a peak in demand for air conditioning, so solar power, particularly in the form of rooftop solar panels, can provide synchronized peak power. In addition, some solar thermal power plants can incorporate thermal energy storage, which will allow them to operate round-the-clock, depending on the size of the energy store. Otherwise, solar power is generated when the sun shines and must be fed into the grid immediately. This means that under normal circumstances, the solar power must be dispatched when it is available, while other generating plants must be ready to provide an alternative source of power when solar power is not available.

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