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By far the largest energy resource is the sun. Annually, 3.94024 J = 1.084018 kWh of solar energy reaches the surface of the Earth. This is about ten thousand times more than the annual global primary energy demand and much more than all available energy reserves on earth. In other words, using one-ten-thousandth part of the incoming sunlight would cover the whole energy demand of mankind. Figure 1.9 illustrates these values with energy cubes.

There is a distinction between direct and indirect solar energy. Technical systems using direct solar energy convert incoming solar radiation directly into useful energy, for instance electricity or heat. Wind, river water and plant growth are indirect forms of solar energy. Here, natural processes convert solar energy into other types of energy. Technical systems can use these indirect types of solar energy as well.

The theoretical foundations of solar radiation needed for all technical solar systems are reviewed in Chapter 2. The following sections briefly describe the different technologies using direct and indirect solar energy. Some of them are described in more detail in the following chapters.

Use of direct solar energy

The following technologies can utilize direct solar energy:

• solar thermal power plants

• photolysis systems for fuel production

• solar collectors for water heating

• passive solar heating systems

photovoltaics, solar cells for electricity generation.

Solar thermal power plants Solar thermal power (power derived from the thermal use of solar energy) can be used to generate electricity or to produce high-temperature steam. Solar thermal power plants used for electricity generation are:

• parabolic trough power plants

• solar thermal tower power plants

• solar furnace

• Dish-Stirling systems

• solar chimney power plants.

Parabolic trough power plants were the first type of solar thermal power plant technologies operating commercially. Nine large power plants called SEGS I to IX (Solar Electric Generation System) were commissioned in California between 1984 and 1991. These power plants have a nominal capacity of between 13.8 and 80 MW each, producing 354 MW in total.

The parabolic trough collector consists of large curved mirrors, which concentrate the sunlight by a factor of 80 or more to a focal line. A series of parallel collectors are lined up in rows 300-600 metres long. Multiple parallel rows form the solar collector field. The collectors moved on one axis in order to follow the movement of the sun; this is called tracking. A collector field can also be formed by long rows of parallel Fresnel collectors. In the focal line of the collectors is a metal absorber tube, which usually is embedded into an evacuated glass tube to reduce heat losses. A special selective coating that withstands high temperatures reduces radiation heat losses.

In the Californian systems, thermo oil flows through the absorber tubes. These tubes heat the oil to 400°C. A heat exchanger transfers the thermal energy from the oil to a water-steam cycle (also called the Rankine cycle). A pump pressurizes the water and an economizer, vaporizer and a superheater jointly produce superheated steam. This steam expands in a two-stage turbine; between the high- and low-pressure parts of this turbine is a reheater. The turbine itself drives an electrical generator that converts the mechanical energy into electrical energy; the condenser after the turbine condenses the steam back to water, which allows the closing of the cycle by feeding this water back into the initial pump.

Solar collectors can also produce superheated steam directly. This makes the thermo oil superfluous and reduces costs due to savings associated with not using the expensive thermo oil. Furthermore, heat exchangers are no longer needed. However, direct solar steam generation is still at the prototype stage.

Steam generator and superheater

Steam generator and superheater

Pump Pump Condenser Cooling tower

Figure 1.10 Principle of a Parabolic Trough Solar Power Plant

Pump Pump Condenser Cooling tower

Figure 1.10 Principle of a Parabolic Trough Solar Power Plant

One important advantage of solar thermal power plants is that they can operate with other means of water heating and thus a hybrid system can ensure security of supply. During periods of insufficient irradiance, a parallel burner can be used to produce steam. Climate-compatible fuels such as biomass or hydrogen produced by renewable energy can also fire this parallel burner. Figure 1.10 shows the principle of a parabolic trough solar power plant.

In solar thermal tower power plants, hundreds, or even thousands, of large two-axis tracked mirrors are installed around a tower (Figure 1.11). These slightly curved mirrors are called heliostats. A computer is used to calculate the ideal position for each of these and positioning motors ensure precise focus on the top of the tower. The absorber is located at the focus of the mirrors on top of the tower. The absorber will be heated to temperatures of typically 1000°C or more. Hot air or molten salt transports the heat from the absorber to a steam generator where superheated steam is produced. This steam drives a turbine and an electrical generator as described above for the trough power plants. Demonstration plants exist in the US, Spain and Israel. Commercial power plants are under construction in Spain.

Figure 1.11 Demonstration Solar Thermal Tower Power Plant in Spain

Another system using mirrors is the solar furnace. The solar furnace in Odeillo (France) consists of various heliostat mirrors that have been set up on sloping ground. These heliostats reflect the sunlight onto a concave mirror with a diameter of 54 m. At the focus of this mirror, temperatures up to 4000°C can be reached and used for scientific experiments or, in a commercial product, for industrial processes. Further solar furnaces exist in Almería (Spain) and Cologne (Germany).

So-called Dish-Stirling systems can be used to generate electricity in the kilowatt range. A parabolic concave mirror (the dish) concentrates sunlight. A two-axis tracked mirror tracks the sun with the required high degree of accuracy. This is necessary in order to achieve high efficiencies. The receiver at the focus is heated to 650°C. The heat absorbed drives a Stirling motor, which converts the thermal energy into mechanical energy that is used to drive a generator producing electricity. If sufficient sunlight is not available, combustion heat from either fossil fuels or bio-fuels can also drive the Stirling engine and generate electricity. The system efficiency of Dish-Stirling systems can reach 20 per cent or more. Some Dish-Stirling system prototypes have been tested successfully in a number of countries; however, the cost of electricity generation using these systems is much higher than that of trough or tower power plants. Large-scale production might achieve significant cost reductions for Dish-Stirling systems. Figure 1.12 shows the principle of a Dish-Stirling system.

A solar chimney power plant has a high chimney (tower), with a height of up to 1000 metres. This is surrounded by a large collector roof, up to 5000 metres in diameter, that consists of glass or clear plastic supported on a framework. Towards its centre, the roof curves upwards to join the chimney, creating a funnel. The sun heats up the ground and the air under the collector roof, and the hot air follows the upward slope of the roof until it reaches the chimney. There, it flows at high speed through the chimney and drives wind generators at the bottom. The ground under the collector roof acts as thermal storage and can even heat up the air for a significant time after sunset. The

Hot air flows with high velocity through the chimney

Hot air flows with high velocity through the chimney


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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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