Solar thermal collectors and applications

History of solar energy

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 umbrella­like 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 photo - voltaics 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 mm as compared to 100 mm 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.

Solar thermal collectors and applications

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