Solar thermal collectors and applications

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Abstract

In this paper a survey of the various types of solar thermal collectors and applications is presented. Initially, an analysis of the environmental problems related to the use of conventional sources of energy is presented and the benefits offered by renewable energy systems are outlined. A historical introduction into the uses of solar energy is attempted followed by a description of the various types of collectors including flat-plate, compound parabolic, evacuated tube, parabolic trough, Fresnel lens, parabolic dish and heliostat field collectors. This is followed by an optical, thermal and thermodynamic analysis of the collectors and a description of the methods used to evaluate their performance. Typical applications of the various types of collectors are presented in order to show to the reader the extent of their applicability. These include solar water heating, which comprise thermosyphon, integrated collector storage, direct and indirect systems and air systems, space heating and cooling, which comprise, space heating and service hot water, air and water systems and heat pumps, refrigeration, industrial process heat, which comprise air and water systems and steam generation systems, desalination, thermal power systems, which comprise the parabolic trough, power tower and dish systems, solar furnaces, and chemistry applications. As can be seen solar energy systems can be used for a wide range of applications and provide significant benefits, therefore, they should be used whenever possible.

Introduction

The sun is a sphere of intensely hot gaseous matter with a diameter of 1.39×109 m. The solar energy strikes our planet a mere 8 min and 20 s after leaving the giant furnace, the sun which is 1.5×1011 m away. The sun has an effective blackbody temperature of 5762 K [1]. The temperature in the central region is much higher and it is estimated at 8×106 to 40×106 K. In effect the sun is a continuous fusion reactor in which hydrogen is turned into helium. The sun's total energy output is 3.8×1020 MW which is equal to 63 MW/m2 of the sun's surface. This energy radiates outwards in all directions. Only a tiny fraction, 1.7×1014 kW, of the total radiation emitted is intercepted by the earth [1]. However, even with this small fraction it is estimated that 30 min of solar radiation falling on earth is equal to the world energy demand for one year.

Man realised that a good use of solar energy is in his benefit, from the prehistoric times. The Greek historian Xenophon in his ‘memorabilia’ records some of the teachings of the Greek Philosopher Socrates (470–399 BC) regarding the correct orientation of dwellings in order to have houses which were cool in summer and warm in winter.

Since prehistory, the sun has dried and preserved man's food. It has also evaporated sea water to yield salt. Since man began to reason, he has recognised the sun as a motive power behind every natural phenomenon. This is why many of the prehistoric tribes considered Sun as ‘God’. Many scripts of ancient Egypt say that the Great Pyramid, one of the man's greatest engineering achievements, was built as a stairway to the sun [2].

Basically, all the forms of energy in the world as we know it are solar in origin. Oil, coal, natural gas and woods were originally produced by photosynthetic processes, followed by complex chemical reactions in which decaying vegetation was subjected to very high temperatures and pressures over a long period of time [1]. Even the wind and tide energy have a solar origin since they are caused by differences in temperature in various regions of the earth.

The greatest advantage of solar energy as compared with other forms of energy is that it is clean and can be supplied without any environmental pollution. Over the past century fossil fuels have provided most of our energy because these are much cheaper and more convenient than energy from alternative energy sources, and until recently environmental pollution has been of little concern.

Twelve winter days of 1973 changed the economic relation of fuel and energy when the Egyptian army stormed across the Suez Canal on October the 12th provoking an international crisis and for the first time, involved as part of Arab strategy, the threat of the ‘oil weapon’. Both the price and the political weapon issues quickly came to a head when the six Gulf members of the Organisations of Petroleum Exporting Countries (OPEC), met in Kuwait and quickly abandoned the idea of holding any more price consultations with the oil companies, announcing that they were raising the price of their crude oil by 70%.

The reason for the rapid increase in oil demand occurred mainly because increasing quantities of oil, produced at very low cost, became available during the 50s and 60s from the Middle East and North Africa. For the consuming countries imported oil was cheap compared with indigenously produced energy from solid fuels.

But the main problem is that proved reserves of oil and gas, at current rates of consumption, would be adequate to meet demand for another 40 and 60 years, respectively. The reserves for coal are in better situation as they would be adequate for at least the next 250 years.

If we try to see the implications of these limited reserves we will be faced with a situation in which the price of fuels will be accelerating as the reserves are decreased. Considering that the price of oil has become firmly established as the price leader for all fuel prices then the conclusion is that energy prices will increase over the next decades at something greater than the rate of inflation or even more. In addition to this is also the concern about the environmental pollution caused by the burning of the fossil fuels. This issue is examined in Section 1.1.

In addition to the thousands of ways in which the sun's energy has been used by both nature and man through time, to grow food or dry clothes, it has also been deliberately harnessed to perform a number of other jobs. Solar energy is used to heat and cool buildings (both active and passive), to heat water for domestic and industrial uses, to heat swimming pools, to power refrigerators, to operate engines and pumps, to desalinate water for drinking purposes, to generate electricity, for chemistry applications, and many more. The objective of this paper is to present the various types of collectors used to harness solar energy, their thermal analysis and performance, and a review of applications.

There are many alternative energy sources which can be used instead of fossil fuels. The decision as to what type of energy source should be utilised must, in each case, be made on the basis of economic, environmental and safety considerations. Because of the desirable environmental and safety aspects it is widely believed that solar energy should be utilised instead of other alternative energy forms, even when the costs involved are slightly higher.

Energy is considered a prime agent in the generation of wealth and a significant factor in economic development. The importance of energy in economic development is recognised universally and historical data verify that there is a strong relationship between the availability of energy and economic activity. Although at the early 70s, after the oil crisis, the concern was on the cost of energy, during the past two decades, the risk and reality of environmental degradation have become more apparent. The growing evidence of environmental problems is due to a combination of several factors since the environmental impact of human activities has grown dramatically. This is due to the increase of the world population, energy consumption and industrial activities. Achieving solutions to environmental problems that humanity faces today requires long-term potential actions for sustainable development. In this respect, renewable energy resources appear to be one of the most efficient and effective solutions.

A few years ago, most environmental analysis and legal control instruments concentrated on conventional pollutants such as sulphur dioxide (SO2), nitrogen oxides (NOx), particulates, and carbon monoxide (CO). Recently however, environmental concern has extended to the control of hazardous air pollutants, which are usually toxic chemical substances which are harmful even in small doses, as well as to other globally significant pollutants such as carbon dioxide (CO2). Additionally, developments in industrial processes and structures have led to new environmental problems. A detailed description of these gaseous and particulate pollutants and their impacts on the environment and human life is presented by Dincer [3], [4].

One of the most widely accepted definitions of sustainable development is: “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. There are many factors that can help to achieve sustainable development. Today, one of the main factors that must be considered is energy and one of the most important issues is the requirement for a supply of energy that is fully sustainable [5], [6]. A secure supply of energy is generally agreed to be a necessary, but not a sufficient requirement for development within a society. Furthermore, for a sustainable development within a society it is required that a sustainable supply of energy and effective and efficient utilization of energy resources are secured. Such a supply in the long-term should be readily available at reasonable cost, be sustainable and be able to be utilized for all the required tasks without causing negative societal impacts. This is why there is a close connection between renewable sources of energy and sustainable development.

Pollution depends on energy consumption. Today the world daily oil consumption is 76 million barrels. Despite the well-known consequences of fossil fuel combustion on the environment, this is expected to increase to 123 million barrels per day by the year 2025 [7]. There are a large number of factors which are significant in the determination of the future level of the energy consumption and production. Such factors include population growth, economic performance, consumer tastes and technological developments. Furthermore, governmental policies concerning energy and developments in the world energy markets will certainly play a key role in the future level and pattern of energy production and consumption [8].

Another parameter to be considered is the world population. This is expected to double by the middle of this century and as economic development will certainly continue to grow, the global demand for energy is expected to increase. Today much evidence exists, which suggests that the future of our planet and of the generations to come will be negatively impacted if humans keep degrading the environment. Currently, three environmental problems are internationally known; these are the acid precipitation, the stratospheric ozone depletion, and the global climate change. These are analysed in more detail below.

This is a form of pollution depletion in which SO2 and NOx produced by the combustion of fossil fuels are transported over great distances through the atmosphere and deposited via precipitation on the earth, causing damage to ecosystems that are exceedingly vulnerable to excessive acidity. Therefore, it is obvious that the solution to the issue of acid rain deposition requires an appropriate control of SO2 and NOx pollutants. These pollutants cause both regional and transboundary problems of acid precipitation.

Recently, attention is also given to other substances such as volatile organic compounds (VOCs), chlorides, ozone and trace metals that may participate in a complex set of chemical transformations in the atmosphere resulting in acid precipitation and the formation of other regional air pollutants. A number of evidences that show the damages of acid precipitation are reported by Dincer and Rosen [6].

It is well known that some energy-related activities are the major sources of acid precipitation. Additionally, VOCs are generated by a variety of sources and comprise a large number of diverse compounds. Obviously, the more energy we spend the more we contribute to acid precipitation; therefore, the easiest way to reduce acid precipitation is by reducing energy consumption.

The ozone present in the stratosphere, at altitudes between 12 and 25 km, plays a natural equilibrium-maintaining role for the earth, through absorption of ultraviolet (UV) radiation (240–320 nm) and absorption of infrared radiation [3]. A global environmental problem is the depletion of the stratospheric ozone layer which is caused by the emissions of CFCs, halons (chlorinated and brominated organic compounds) and NOx. Ozone depletion can lead to increased levels of damaging UV radiation reaching the ground, causing increased rates of skin cancer and eye damage to humans and is harmful to many biological species. It should be noted that energy related activities are only partially (directly or indirectly) responsible for the emissions which lead to stratospheric ozone depletion. The most significant role in ozone depletion have the CFCs, which are mainly used in air conditioning and refrigerating equipment as refrigerants, and NOx emissions which are produced by the fossil fuel and biomass combustion processes, the natural denitrification and nitrogen fertilizers.

In 1998 the size of the ozone hole over Antarctica was 25 million km2. It was about 3 million km2 in 1993 [7]. Researchers expect the Antarctic ozone hole to remain severe in the next 10–20 years, followed by a period of slow healing. Full recovery is predicted to occur in 2050; however, the rate of recovery is affected by the climate change [8].

The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, it has been increasingly associated with the contribution of CO2 which is estimated that contributes about 50% to the anthropogenic greenhouse effect. Additionally, several other gasses such as CH4, CFCs, halons, N2O, ozone and peroxyacetylnitrate (also called greenhouse gasses) produced by the industrial and domestic activities can also contribute to this effect, resulting in a rise of the earth's temperature. Increasing atmospheric concentrations of greenhouse gasses increase the amount of heat trapped (or decrease the heat radiated from the earth's surface), thereby raising the surface temperature of the earth. According to Colonbo [9] the earth's surface temperature has increased by about 0.6 °C over the last century, and as a consequence the sea level is estimated to have risen by perhaps 20 cm. These changes can have a wide range of effects on human activities all over the world. The role of various greenhouse gasses is summarized in Ref. [6].

Humans contribute through many of their economic and other activities to the increase of the atmospheric concentrations of various greenhouse gasses. For example, CO2 releases from fossil fuel combustion, methane emissions from increased human activity and CFC releases all contribute to the greenhouse effect. Predictions show that if atmospheric concentrations of greenhouse gasses, mainly due to fossil fuels combustion, continue to increase at the present rates, the earth's temperature may increase by another 2–4 °C in the next century. If this prediction is realized, the sea level could rise by between 30 and 60 cm before the end of this century [9]. The impacts of such sea level increase could easily be understood and include flooding of coastal settlements, displacement of fertile zones for agriculture toward higher latitudes, and decrease the availability of fresh water for irrigation and other essential uses. Thus, such consequences could put in danger the survival of entire populations.

Renewable energy technologies produce marketable energy by converting natural phenomena into useful forms of energy These technologies use the sun's energy and its direct and indirect effects on the earth (solar radiation, wind, falling water and various plants, i.e. biomass), gravitational forces (tides), and the heat of the earth's core (geothermal) as the resources from which energy is produced. These resources have massive energy potential, however, they are generally diffused and not fully accessible, most of them are intermittent, and have distinct regional variabilities. These characteristics give rise to difficult, but solvable, technical and economical challenges. Nowadays, significant progress is made by improving the collection and conversion efficiencies, lowering the initial and maintenance costs, and increasing the reliability and applicability.

A worldwide research and development in the field of renewable energy resources and systems is carried out during the last two decades. Energy conversion systems that are based on renewable energy technologies appeared to be cost effective compared to the projected high cost of oil. Furthermore, renewable energy systems can have a beneficial impact on the environmental, economic, and political issues of the world. At the end of 2001 the total installed capacity of renewable energy systems was equivalent to 9% of the total electricity generation [10]. By applying a renewable energy intensive scenario the global consumption of renewable sources by 2050 would reach 318 exajoules [11].

The benefits arising from the installation and operation of renewable energy systems can be distinguished into three categories; energy saving, generation of new working posts and the decrease of environmental pollution.

The energy saving benefit derives from the reduction in consumption of the electricity and/or diesel which are used conventionally to provide energy. This benefit can be directly translated into monetary units according to the corresponding production or avoiding capital expenditure for the purchase of imported fossil fuels.

Another factor which is of considerable importance in many countries is the ability of renewable energy technologies to generate jobs. The penetration of a new technology leads to the development of new production activities contributing to the production, market distribution and operation of the pertinent equipment. Specifically in the case of solar energy collectors job creation mainly relates to the construction and installation of the collectors. The latter is a decentralised process since it requires the installation of equipment in every building or every individual consumer.

The most important benefit of renewable energy systems is the decrease of environmental pollution. This is achieved by the reduction of air emissions due to the substitution of electricity and conventional fuels. The most important effects of air pollutants on the human and natural environment are their impact on the public health, agriculture and on ecosystems. It is relatively simple to measure the financial impact of these effects when they relate to tradable goods such as the agricultural crops; however when it comes to non-tradable goods, like human health and ecosystems, things becomes more complicated. It should be noted that the level of the environmental impact and therefore the social pollution cost largely depends on the geographical location of the emission sources. Contrary to the conventional air pollutants, the social cost of CO2 does not vary with the geographical characteristics of the source as each unit of CO2 contributes equally to the climate change thread and the resulting cost.

In this paper emphasis is given to solar thermal systems. Solar thermal systems are non-polluting and offer significant protection of the environment. The reduction of greenhouse gasses pollution is the main advantage of utilising solar energy. Therefore, solar thermal systems should be employed whenever possible in order to achieve a sustainable future.

The idea of using solar energy collectors to harness the sun's power is recorded from the prehistoric times when at 212 BC the Greek scientist/physician Archimedes devised a method to burn the Roman fleet. Archimedes reputedly set the attacking Roman fleet afire by means of concave metallic mirror in the form of hundreds of polished shields; all reflecting on the same ship [2].

The Greek historian Plutarch (AD 46–120) referred to the incident saying that the Romans, seeing that indefinite mischief overwhelmed them from no visible means, began to think they were fighting with the gods. The basic question was whether or not Archimedes knew enough about the science of optics to device a simple way to concentrate sunlight to a point where ships could be burned from a distance. Archimedes had written a book “On burning Mirrors” but no copy has survived to give evidence [12].

Eighteen hundred years after Archimedes, Athanasius Kircher (1601–1680) carried out some experiments to set fire to a woodpile at a distance in order to see whether the story of Archimedes had any scientific validity but no report of his findings survived [12].

Amazingly, the very first applications of solar energy refer to the use of concentrating collectors, which are by their nature (accurate shape construction) and the requirement to follow the sun, more ‘difficult’ to apply. During the 18th century, solar furnaces capable of melting iron, copper and other metals were being constructed of polished-iron, glass lenses and mirrors. The furnaces were in use throughout Europe and the Middle East. One furnace designed by the French scientist Antoine Lavoisier, attained the remarkable temperature of 1750 °C. The furnace used a 1.32 m lens plus a secondary 0.2 m lens to obtain such temperature which turned out to be the maximum achieved by man for one hundred years.

During the 19th century the attempts to convert solar energy into other forms based upon the generation of low-pressure steam to operate steam engines. August Monchot pioneered this field by constructing and operating several solar-powered steam engines between the years 1864 and 1878 [12]. Evaluation of one built at Tours by the French government showed that it was too expensive to be considered feasible. Another one was set up in Algeria. In 1875, Mouchot made a notable advance in solar collector design by making one in the form of a truncated cone reflector. Mouchot's collector consisted of silver-plated metal plates and had a diameter of 5.4 m and a collecting area of 18.6 m2. The moving parts weighed 1400 kg.

Abel Pifre was a contemporary of Mouchot who also made solar engines [12], [13]. Pifre's solar collectors were parabolic reflectors made of very small mirrors. In shape they looked rather similar to Mouchot's truncated cones.

In 1901 A.G. Eneas installed a 10 m diameter focusing collector which powered a water pumping apparatus at a California farm. The device consisted of a large umbrella-like structure open and inverted at an angle to receive the full effect of sun's rays on the 1788 mirrors which lined the inside surface. The sun's rays were concentrated at a focal point where the boiler was located. Water within the boiler was heated to produce steam which in turn powered a conventional compound engine and centrifugal pump [1], [12].

In 1904 a Portuguese priest, Father Himalaya, constructed a large solar furnace. This was exhibited at the St Louis World's fair. This furnace appeared quite modern in structure, being a large, off-axis, parabolic horn collector [12].

In 1912 Shuman, in collaboration with C.V. Boys, undertook to build the world's largest pumping plant in Meadi, Egypt. The system was placed in operation in 1913 and it was using long parabolic cylinders to focus sunlight onto a long absorbing tube. Each cylinder was 62 m long, and the total area of the several banks of cylinders was 1200 m2. The solar engine developed as much as 37–45 kW continuously for a 5 h period [1], [12], [13]. Despite the plant's success, it was completely shut down in 1915 due to the onset of World War I and cheaper fuel prices.

During the last 50 years many variations were designed and constructed using focusing collectors as a means of heating the transfer or working fluid which powered mechanical equipment. The two primary solar technologies used are the central receivers and the distributed receivers employing various point and line-focus optics to concentrate sunlight. Central receiver systems use fields of heliostats (two-axis tracking mirrors) to focus the sun's radiant energy onto a single tower-mounted receiver [14]. Distributed receiver technology includes parabolic dishes, Fresnel lenses, parabolic troughs, and special bowls. Parabolic dishes track the sun in two axes and use mirrors to focus radiant energy onto a point-focus receiver. Troughs and bowls are line-focus tracking reflectors that concentrate sunlight onto receiver tubes along their focal lines. Receiver temperatures range from 100 °C in low-temperature troughs to close 1500 °C in dish and central receiver systems [14]. More details of the basic types of collectors are given in Section 2.

Another area of interest, the hot water and house heating appeared in the mid 1930s, but gained interest in the last half of the 40s. Until then millions of houses were heated by coal burn boilers. The idea was to heat water and fed it to the radiator system that was already installed.

The manufacture of solar water heaters (SWH) began in the early 60s. The industry of SWH expanded very quickly in many countries of the world. Typical SWH in many cases are of the thermosyphon type and consist of two flat-plate solar collectors having an absorber area between 3 and 4 m2, a storage tank with capacity between 150 and 180 l and a cold water storage tank, all installed on a suitable frame. An auxiliary electric immersion heater and/or a heat exchanger, for central heating assisted hot water production, are used in winter during periods of low solar insolation. Another important type of SWH is the force circulation type. In this system only the solar panels are visible on the roof, the hot water storage tank is located indoors in a plantroom and the system is completed with piping, pump and a differential thermostat. Obviously, this latter type is more appealing mainly due to architectural and aesthetic reasons, but also more expensive especially for small-size installations [15]. These together with a variety of other systems are described in Section 5.

Becquerel had discovered the photovoltaic effect in selenium in 1839. The conversion efficiency of the ‘new’ silicon cells developed in 1958 was 11% although the cost was prohibitively high ($1000/W) [12]. The first practical application of solar cells was in space where cost was not a barrier and no other source of power is available. Research in the 1960s, resulted in the discovery of other photovoltaic materials such as gallium arsenide (GaAS). These could operate at higher temperatures than silicon but were much more expensive. The global installed capacity of photovoltaics at the end of 2002 was near 2 GWp [16]. Photovoltaic (PV) cells are made of various semiconductors, which are materials that are only moderately good conductors of electricity. The materials most commonly used are silicon (Si) and compounds of cadmium sulphide (Cds), cuprous sulphide (Cu2S), and GaAs.

Amorphous silicon cells are composed of silicon atoms in a thin homogenous layer rather than a crystal structure. Amorphous silicon absorbs light more effectively than crystalline silicon, so the cells can be thinner. For this reason, amorphous silicon is also known as a ‘thin film’ PV technology. Amorphous silicon can be deposited on a wide range of substrates, both rigid and flexible, which makes it ideal for curved surfaces and ‘fold-away’ modules. Amorphous cells are, however, less efficient than crystalline based cells, with typical efficiencies of around 6%, but they are easier and therefore cheaper to produce. Their low cost makes them ideally suited for many applications where high efficiency is not required and low cost is important.

Amorphous silicon (a-Si) is a glassy alloy of silicon and hydrogen (about 10%). Several properties make it an attractive material for thin-film solar cells:

  • 1.

    Silicon is abundant and environmentally safe.

  • 2.

    Amorphous silicon absorbs sunlight extremely well, so that only a very thin active solar cell layer is required (about 1 μm as compared to 100 μm or so for crystalline solar cells), thus greatly reducing solar-cell material requirements.

  • 3.

    Thin films of a-Si can be deposited directly on inexpensive support materials such as glass, sheet steel, or plastic foil.

A number of other promising materials such as cadmium telluride and copper indium diselenide are now being used for PV modules. The attraction of these technologies is that they can be manufactured by relatively inexpensive industrial processes, in comparison to crystalline silicon technologies, yet they typically offer higher module efficiencies than amorphous silicon.

The PV cells are packed into modules which produce a specific voltage and current when illuminated. PV modules can be connected in series or in parallel to produce larger voltages or currents. Photovoltaic systems can be used independently or in conjunction with other electrical power sources. Applications powered by PV systems include communications (both on earth and in space), remote power, remote monitoring, lighting, water pumping and battery charging.

The two basic types of PV applications are the stand alone and the grid connected. Stand-alone PV systems are used in areas that are not easily accessible or have no access to mains electricity. A stand-alone system is independent of the electricity grid, with the energy produced normally being stored in batteries. A typical stand-alone system would consist of PV module or modules, batteries and charge controller. An inverter may also be included in the system to convert the direct current generated by the PV modules to the alternating current form (AC) required by normal appliances.

In the grid connected applications the PV system is connect to the local electricity network. This means that during the day, the electricity generated by the PV system can either be used immediately (which is normal for systems installed in offices and other commercial buildings), or can be sold to one of the electricity supply companies (which is more common for domestic systems where the occupier may be out during the day). In the evening, when the solar system is unable to provide the electricity required, power can be bought back from the network. In effect, the grid is acting as an energy storage system, which means the PV system does not need to include battery storage.

When PVs started to be used for large-scale commercial applications, about 20 years ago, their efficiency was well below 10%. Nowadays, their efficiency increased to about 15%. Laboratory or experimental units can give efficiencies of more than 30%, but these have not been commercialized yet. Although 20 years ago PVs were considered as a very expensive solar system the present cost is around 5000$ per kWe and there are good prospects for further reduction in the coming years. More details on photovoltaics are beyond the scope of this paper.

The lack of water was always a problem to humanity. Therefore among the first attempts to harness solar energy were the development of equipment suitable for the desalination of sea-water. Solar distillation has been in practice for a long time. According to Malik et al. [17], the earliest documented work is that of an Arab alchemist in the 15th century reported by Mouchot in 1869. Mouchot reported that the Arab alchemist had used polished Damascus mirrors for solar distillation.

The great French chemist Lavoisier (1862) used large glass lenses, mounted on elaborate supporting structures, to concentrate solar energy on the contents of distillation flasks [17]. The use of silver or aluminium coated glass reflectors to concentrate solar energy for distillation has also been described by Mouchot.

The use of solar concentrators in solar distillation has been reported by Pasteur (1928) [17] who used a concentrator to focus solar rays onto a copper boiler containing water. The steam generated from the boiler was piped to a conventional water cooled condenser in which distilled water was accumulated.

Solar stills are one of the simplest type of desalination equipment which uses the greenhouse effect to evaporate salty water. Solar stills were the first to be used on large-scale distilled water production. The first water distillation plant constructed was a system built at Las Salinas, Chile, in 1874 [12], [17]. The still covered 4700 m2 and produced up to 23 000 l of fresh water per day (4.9 l/m2), in clear sun. The still was operated for 40 years and was abandoned only after a fresh-water pipe was installed supplying water to the area from the mountains.

The renewal of interest on solar distillation occurred after the First World War at which time several new devices had been developed such as: roof type, tilted wick, inclined tray and inflated stills. Some more details on solar stills are given in Section 5.5. In this section it is also indicated how solar collectors can be used to power conventional desalination equipment. More information on solar desalination is given in Ref. [18].

Another application of solar energy is solar drying. Solar dryers have been used primarily by the agricultural industry. The objective in drying an agricultural product is to reduce its moisture contents to that level which prevents deterioration within a period of time regarded as the safe storage period. Drying is a dual process of heat transfer to the product from the heating source, and mass transfer of moisture from the interior of the product to its surface and from the surface to the surrounding air.

The objective of a dryer is to supply the product with more heat than is available under ambient conditions, increasing sufficiently the vapour pressure of the moisture held within the crop, thus enhancing moisture migration from within the crop and decreasing significantly the relative humidity of the drying air, thus increasing its moisture carrying capability and ensuring a sufficiently low equilibrium moisture content.

In solar drying, solar energy is used as either the sole source of the required heat or as a supplemental source, and the air flow can be generated by either forced or natural convection. The heating procedure could involve the passage of the pre-heated air through the product, by directly exposing the product to solar radiation or a combination of both. The major requirement is the transfer of heat to the moist product by convection and conduction from surrounding air mass at temperatures above that of the product, or by radiation mainly from the sun and to a little extent from surrounding hot surfaces, or conduction from heated surfaces in conduct with the product. Details of solar dryers are beyond the scope of this paper. More information on solar dryers can be found in Ref. [19].

Section 2 gives a brief description of several of the most common collectors available in the market.

Section snippets

Solar collectors

Solar energy collectors are special kind of heat exchangers that transform solar radiation energy to internal energy of the transport medium. The major component of any solar system is the solar collector. This is a device which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy thus collected is carried from the circulating fluid either directly to the hot water or space

Thermal analysis of collectors

In this section the thermal analysis of the collectors is presented. The two major types of collectors, i.e. flat-plate and concentrating are examined separately. The basic parameter to consider is the collector thermal efficiency. This is defined as the ratio of the useful energy delivered to the energy incident on the collector aperture. The incident solar flux consists of direct and diffuse radiation. While FPC can collect both, concentrating collectors can only utilise direct radiation if

Performance of solar collectors

ASHRAE Standard 93:1986 [108] for testing the thermal performance of collectors is undoubtedly the one most often used to evaluate the performance of flat-plate and concentrating solar collectors. The thermal performance of the solar collector is determined partly by obtaining values of instantaneous efficiency for different combinations of incident radiation, ambient temperature, and inlet fluid temperature. This requires experimental measurement of the rate of incident solar radiation falling

Solar collector applications

Solar collectors have been used in a variety of applications. These are described in this section. In Table 10 the most important technologies in use are listed together with the type of collector that can be used in each case.

Conclusions

Several of the most common types of solar collectors are presented in this paper. The various types of collectors described include flat-plate, compound parabolic, evacuated tube, parabolic trough, Fresnel lens, parabolic dish and Heliostat field collector (HFC). The optical, thermal and thermodynamic analysis of collectors is also presented as well as methods to evaluate their performance. Additionally, typical applications are described in order to show to the reader the extent of their

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