Let us now consider two other technologies which exploit the solar radiation and are applied as CSP technologies for the generation of electrical energy, but they do not involve the concentration of solar beams. This raises the possibility, in the two case that we are going to discuss, of exploiting not only the direct radiation but also the indirect radiation which, in some seasons and in some countries, has a higher energy than direct radiation.

Solar chimneys, similar to solar ponds, are not characterized by other typical temperatures, whereas, the CSP technology is [7].

4.7.1 Solar chimneys/towers

Solar chimney plants allow producing electrical energy in a renewable way. They are made of a tower that is hollow inside and at the base it has a wide greenhouse, generally circular in shape that covers a notable ground surface. The greenhouse air, heated by the Sun, rises along the chimneys due to two physical phenomena (that function as the tower’s ‘motors’), namely:

• the air rises by floating (based on the phenomenon that hot air tends to rise high);

• the air rises due to the pressure difference between the base and the top of the tower (at the top of the chimney the pressure is lower and so the air is ‘backwashed’ towards the top).

Figure 111: Principle of the solar chimney.

As it rises in the chimney, the hot air accelerates until it reaches a speed of 70 km/h. This flow of air rotates a series of turbines placed at the internal base of the chimney to generate electricity: the turbines transform the kinetic energy and the air potential into electrical energy, as every Aeolian blade. The procedure is made easier from the absolute constancy both in direction and in intensity of the speed vector.

The heat collector in this case is the greenhouse. It can have plastic or glass cov­ers. From the pilot plant at Mazanares (Spain, Fig. 112) we can see that the glass is better because it is more resistant to bad weather. We also observed that if the height of the cover progressively improves towards the centre, the radial flow of the speed is enhanced. The performance directly depends on the chimney height. For this reason, in the current plans, they plan to build chimneys of 1,000 m height.

The main feature that makes the solar chimney/tower particularly interesting is its capacity to work without wind also, 24 hours, 7 days, generating a peak of energy during the hotter days of the year when there is a consumption peak.

Figure 113: Solar chimney.

The plant can also work at night, due to the ‘pressure gradient’ (i. e. the pressure differential) and, secondly, due to the ground covered by the greenhouse, which heats itself during the day and releases the stored heat during the night. We can easily improve the thermal capacity of the floor by putting a water layer in the greenhouse or using an appropriate arrangement containing water elements that store the heat and release it at night. Obviously, water must be contained and kept; it must not evaporate; otherwise, it consumes the thermal energy absorbed [7, 61, 63].

Among the most ambitious project in terms of dimensions is, without doubt, the solar chimney/tower that to be built in the county of Wentworth in New South Wales, Australia. Figure 114, where the greenhouse cover elements are considered the solar panels, shows the scheme for this project. The numbers of the initial project are as follows [60]:

• The greenhouse should cover an area of about 25,000 acres, which is equal to 5 km2.

• The central tower will be 3,280 feet high, corresponding to 1 km, which would make it the tallest building in the world.

• Inside the tower 32 turbines each of 6.25 MW are placed; every rising hot air motion is estimated to have a maximum speed of the order of 35 miles/hour (<60 km/h); the solar tower will have a total capacity of 200 MW, which is enough to feed almost 200,000 houses.

• The generation of 200 MW of power would allow saving, depending on estimates, between 750,000 and 900,000 t of CO2 per year.

Figure 114: Australian solar chimney scheme.

Currently, the project is in the final stage in terms of its feasibility, particularly regarding the economic aspects. In this step, the Guinness dimensions of the initial project have been reduced:

• The use of innovative materials has allowed reducing the height of the tower to 650 m without losing power.

• The power has been reduced to 50 MW.

• At the moment, it is not possible to know the final dimensions of the tower, but it is reasonable to assume that at such levels it should have a height of at least 450 m.

From the technical point of view, the project was already validated, because for 7 years (from 1981 to 1988) a pilot project of 50 kW power was operative at Mazanares. Conceptually, it is not a new technology, but at the moment of its birth, when an oil barrel cost 15 dollars, it did not provoke any particular interest, contrary to the situ­ation today. In fact, the present high price of crude oil and the necessity of reducing greenhouse gas emissions are pushing many countries towards more convenient and cleaner energy sources such as the solar chimney/tower [60].

The highest and most sophisticated solar chimney/tower (750 m) in Europe will be realized at Fuente del Fresno, in the Spanish region of Mancha. This colossal solar system will have a power of 30 MW. This plant will provide electrical energy that is equal to the requirements of 120,000 people and at the same time we will avoid putting into the atmosphere 78 t of CO2 that will be generated from 140,000 oil barrels that could produce the same energy in a year. The construction of this structure will start in 2007 and it will be finished in three years; it will cost 240,000,000 € and it will occupy 350 ha covered with a 3 km diameter crystal

panel. Exploiting the greenhouse effect principle, the overheated air will rise along the tower height, actuating 24 turbines that will produce electricity. A system of storage pipes filled with a gel keeps heat and allows the generators to produce energy even at night and during periods of scarce insulation. The tower has an estimated shelf life of 60 years [65].

4.7.2 Solar ponds

The term ‘solar ponds’ is used to describe a mass contained in a basin of water that also absorbs the solar incident energy and stores it in its interiors. To obtain this performance, three other basic kinds of solar lakes can be named, and they are identified by the terms: salinity gradient solar lake, gel pond and, finally, shallow solar pond. Among the three, the first is the one whose technique was realized for the totalities of the realizations and the management of the physical working studies. This kind of solar lake is realized putting in the bondage a solution of salt in water, e. g. sodium chloride, using filling techniques that allow establishing a growing salt concentration with the depth until the saturation at the bottom layer. Effectively, in the vertical section of the basin (see Fig. 115), which is generally deep 2-3 m, we can find three characteristically superimposed layers: the first layer is high and very slender, it is composed of water with a little quantity of salt (0-35 g/l); the central layer, where we can observe a linear salinity variation; and, finally, the homogeneous and salt saturated bottom layer (200-250 g/l).

Let us now analyse the difference between a normal water basin and a solar pond. In the first case, the solar energy heats water (exposed to the Sun), which,

however, tends to lose this heat. Indeed the water heated by the Sun expands and tends to move higher and higher as it becomes less dense. Convective motions are established and the superficial water is always hotter than the deep water; it rapidly evaporates cooling and giving heat to air. The cold water, which is heavier, moves towards the bottom. In this way, a water basin keeps a relatively low temperature in the deep bottoms and, as it is more radiated, it raises the circulation speed of the water and intensifies the evaporation. But if a system in which the mass of water has a layer shaped salinity is created, with the highest value at the bottom and the lowest value at the surface (solar pond), the convective motions are inhibited. In fact, the hot water specific gravity and high salinity are anyhow bigger than that of the modest salinity cold water, so heat is trapped at the bottom of the solar pond. The absence of convective motions inhibits the mixing of high salinity hot water with the superficial one. The superficial layers of salinity only increase diffusion and this happens over very long periods (years) and so bigger the solar pond spare part time that has to be fed to equalize the losses of evaporation.

When the solar radiation incident on the solar pond surface penetrates through the transparent solution mass, it is absorbed at the bottom and the produced heat transmits itself to the solution for convention. Following mass ascent and energy transfer that could lead to the dissipation of the heat at the surface, it finds a barrier in the interface with the salinity gradient layer and the heat is stored in the pickle at the bottom (where the temperature can also reach 100°C). In fact, the water in the salinity gradient area cannot rise because the water in this layer has a lower salinity content and so it is lighter; for the same reason, the water in the higher layers cannot go down because the water in the lower layer has a salinity content which is lower and heavier and even if its density wanes with the increase in tem­perature, it is always denser than the higher water layers. The intermediate layer acts as a transparent thermal isolator that allows the stored heat in the lower con­vective layer to be extracted with thermal exchange techniques and to be used for thermal purposes [66-68].

Solar ponds are mainly used as energy sources which are appropriate to feed the processes of [7, 66]:

• electrical energy production using organic fluid Rankine cycles; the electrical production yield of the system is very low, but the cost of the storage plant is contained;

• brackish water desalinization;

• agricultural greenhouses and habited environmental heating;

• vegetable drying.

Figure 117: Convective motion scheme in a solar pond.

A solar pond can be built using normal intervention techniques used by the building industry, such as digging the basin, covering the basin with an imper­meable membrane and building the structures for housing the devices used for extracting and producing heat. In this way, large heat collection surfaces can be realized, up to thousands of square metres in area with costs for unit area lower than the cost of every other methodology of solar energy exploitation. The big mass for collection and the thermal isolation capability characterize the solar pounds: they can preserve the thermal energy for long periods (seasons) without registering sensible brine temperature decreases.

The construction of a solar lake, in terms of the surface unit, can vary with the basin catchment area. The estimated unitary costs for building different size lakes are listed below:

• surface of 2,000 m2, cost: 150 €/m2;

• surface of 20,000 m2, cost: 95 €/m2;

• surface of 200,000 m2, cost: 70 €/m2.

Figure 119: Solar pond at El Paso (Texas).