The solar collector represents the main aspect of the economic analysis that will decide the realization of a solar central plant and hence its cost and efficiency are of particular importance for the diffusion of the concentration of solar tech­nology. For this, ENEA has planned and realized, together with the industry, an original prototype of the linear parabolic collector with the double aim of improving the techno-economic parameters and putting the national industry in a situation to produce this in series, both for the Archimedes Project (see par. 4.5.3) and for making it available in the international market in a competitive manner.

The ENEA collector, shown in Figs 100-102 comprises:

• a structure that supports the mirrors, realizing the parabolic geometry and it allows orienting them to follow the motion of the Sun;

• a series of mirrors of appropriate geometrical design;

• a motion system that is capable of making the structure rotate with the accuracy of required pointing.

• a series of receiver pipes on which the solar rays are concentrated and in which the thermal energy is given to the vector fluid.

The collector was developed in its entirety and tested using a circuit test at the ENEA centre in Casaccia. The length of the prototype collector is equal to 50 m, but the combined length of the series is 100 m. The structure must contemporaneously assure rigidity, geometrical precision and low cost. The main aspect as regards the

Basement

Figure 100: Solar collector scheme used by ENEA.

dimensioning of the structure is the determination of the aerodynamic loads due to the wind action. The solution adopted by ENEA, apart from presenting structural resistance, is characterized by constructive economy and simplicity of assembly. Such a solution based on a central bearing pipe and lateral supports of a variable shape (Fig. 100), heavier than similar concurrent realizations, presents its trump card in terms of the constructive reasons and in the choice of the material, which make it less expensive, easily portable, with quick installation and simple registration, within the required distance from the optical concentrator system. Despite the considerable dimensions (total length of 100 m, width 6 m and height 3.5 m from the rotation axis), after assembly tolerances of 1 mm final can be easily realized [45, 53].

The mirrors are realized with several technologies with the aim of exploring a series of alternatives to achieve a lower final cost and better mechanical character­istics compared with the traditional linear parabolic collector, which uses a thick and hot bent glass mirror. Among the alternatives examined, the better solution is the one which is based on the use of a glass mirror which is sufficiently thin (850 pm) to be cold bent until it assumes the required parabolic shape and to be applied to a conveniently shaped support panel with a structural function (Fig. 103). The sup­port panel is made of an aluminium nucleus with a honeycomb structure, which is often 2.5 cm, and wrapped between two surface layers (leather), 1 mm thick, in composite material [45, 50, 53].

The handling system is made of an autonomous oleo dynamic unit that is capable of moving the whole 100 m collector on the basis of instructions sent to the central supervision system, ensuring that the movement the Sun is followed with a precision of 0.8 mrad.

The system is able to carry the collector safely (when faced with atmospheric events such as strong wind or hail) in the presence of winds with speeds up to 14 m/s; once placed safely, the collector can resist winds with speeds of up to 28 m/s [45].

The receiver pipes (4 m long) are welded to make a line that, in the position of reference during use, must be in axis with the focal line of the parabolic mirrors. The receiver pipeline is held in position by sustaining arms equipped at the extrem­ities with cylindrical hinges which allow the thermal expansion of the pipes when the plant is in use.

The function of the receiver pipes is to transform heat at high temperature and to transfer to the heat transformer fluid the largest quantity, reducing at least the losses of energy by irradiation towards the external environment.

Each receiver pipe (Fig. 106) is made of a stainless steel absorber on whose external surface is deposited, by sputtering technology, a selective spectral cover­ing (coating) made of composite cermet material (CERMET), which is character­ized by an elevated absorbance of the solar radiation and a low emissivity of heat in the infrared region. The stainless steel absorber is capped, vacuum at about 10-2 Pa, in an external borosilicate glass pipe that is coaxial with the receiver pipe; this external glass pipe protects the receiver pipe from the contact with air, reducing at least the thermal exchange for convention between the pipes.

On the surface of the glass pipes, an antiglare treatment is made to improve the transmittance of the solar radiation, reducing the reflected energy. The links between the glass and the steel pipes are realized with two stainless bellows (placed at the extremities of the glass pipe), which are able to compensate the differential between the two materials’ thermal dilatations. To create the vacuum it is necessary to insert in the cavity between the two pipes an appropriate quantity of getter material which is capable of absorbing the gas mixture that could form in the receiver pipe.

A second material absorber, which is very reactive with air (Barium getter), is deposited on the internal surface of the glass pipe, resulting in metal colour scrubs of some cm. When the vacuum is created in the pipe the soaking getter saturates, the scrub becomes white, indicating the loss of heat transmission efficiency to the heat transfer fluid. The receiver pipe is the most delicate element of the solar technology, because it has to grant in time a high energy absorbing coefficient which is concen­trated from the parabolic mirrors, limiting at maximum the losses by irradiation towards the environment. To achieve high reliability, there are two important characteristics:

• the capacity of CERMET to maintain almost unweathered the photo-thermal characteristics at the maximum working temperature of the coating (580°C);

• the capacity of the metal-glass junctions to resist the strains of thermo­mechanical fatigue which originate from the variability in the solar irradiation (the maximum temperature of reference in the proofs of the mechanical characterizations is 400°C).

These characteristics, peculiar to the ENEA project, have led to the development of new technological solutions because the receiver pipes which are available in the market are able to operate up to a maximum coating temperature of 400°C. In the ENEA laboratories in Portici, different CERMET made of metal and ceramic material are being planned, realized and undergoing spectrally selective character­ization until the chemical composition and the optimal physical characteristics to obtain the photo-thermal characteristics required by the ENEA project are attained [45, 53]. The reference parameters of the coating developed in Portici, determined by photo-thermal characterization at the same laboratories, are:

• high photo-thermal efficiency, which means high solar absorbance (>94%) and low emissivity (<14%) up to the temperature of 580°C;

• high chemical and structural stability up to the temperature of 580°C.