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SPECTRUM

Solar Power Exploitation by Collection and Transportation by fibre optics to Remote Utilisation Modules

Several types of solar concentrator have been developed in the last years, examining optical configurations with increased complexity from simple parabolic concentrators to lens mirror combinations. The use of optical fibres for transferring solar power to an utilisation system has been proposed some years ago; recently this technique has been suggested for surgery use and to transfer the power of a wide sun image by a fibre bundle.

In order to maximise the collected power the optical system, concentrating the solar power at the top of the optical fibre, must be optically designed for matching diameter and angular aperture of the fibre. Moreover there is the necessity of keeping an accurate orientation of the optics in the sun direction, which is commonly solved either tracking the sun with a large paraboloid or lighting a fixed paraboloid with one or several plane mirrors acting as heliostats. The first solution requires a big and heavy mechanical support due to the system dimensions. In the second solution the collecting surfaces are virtually represented by the heliostats and the collecting system must be placed quite far from the mirrors with consequent very large dimensions of the whole plant.

Summary:  In the framework of the "Joule European Project" we have studied a collecting system composed of several single optical systems, each of which is coupled to an optical fibre. The plant is formed by several small basic units, indicated as tiles, incorporating optics and fibres. Each tile is self-oriented in the sun direction by a couple of small power electric motors driven by a sun tracking system. A prototype of the solar collection system has been realised and tested in real conditions. A possible application of the solar system can be for domestic uses.

List of SPECTRUM Reports by INOA.

STUDY OF THE OPTICAL CONFIGURATIONS FOR THE SOLAR COLLECTOR

The solar system modular unit is composed of a concentrator coupled to a fibre for power transportation to the user. The choice of the optical elements started from the selection of the optical fibre in Silica with NA = 0.48 and core diameter = 0.6 mm. This selection has been agreed among the European partners and in particular with German fibre produced (Ceram Optec). Starting from the main requirement of minimising the diameter of the fibre bundle obtained for the final system, this Silica fibre measuring 0.6 mm has been judged to be the lower limit for realising the alignment and during the realisation we have confirmed this judgement. The numerical aperture of NA = 0.48, corresponding to a total field of view approaching 60°, has been considered sufficiently wide for the project purposes. Typically Silica fibre have NA between 0.22 and 0.43 and they are quite expensive with respect to glass fibres. NA = 0.57 can be obtained using particular procedures, which made these fibres very expensive. NA = 0.48 is the wider NA value realised in big fibre production, which means with acceptable costs.

The optical project of the sun light collector is a fundamental aspect of the study for the realisation of the solar system. The final purpose of this optical project is to obtain the maximum collected power within the fibre aperture NA and core diameter, while usually the main requirement for a lens is to give a good image.

Several optical projects of collectors have been designed and some of them realised; other collectors (aspherical plastic lenses) were commercially available. The six optical configurations studied for the sunlight collector are presented in Table 1 evidencing their main characteristics. The optical project of the CCM (Catadioptric Concentrator Monoblock) has been developed with the aim of optimising the optical characteristics of the collector but also its compactness.

Table 1

Fig. 1 reports the sun image for the six configurations matched with the selected fibre with diameter of 0.6 mm (indicated as dashed circle). In Fig. 1 the central yellow circle indicates the sun image, while the lateral dashed circles are the spots rms at marginal field of view (aberrations).

Fig. 1

The conic CCM (Catadioptric Concentrator Monoblock)

All optical configurations for the sunlight collector have been designed to maximise the light focused into the fibre with core of 0.6 mm and N.A. = 0.48. Fig. 2 presents the optical configuration of the conic CCM collector that is composed of a conic primary mirror and a spherical secondary mirror. The main optical data of the CCM are: focal length: 55 mm f/#: 0.98 field of view: 0.5 deg enter pupil diameter: 56 mm spectral range: 400 ÷ 1400 nm distance between M1 and M2: 23.8 mm thickness of Fused Silica lens: 23.8 mm M2 diameter: 22 mm

Fig. 2

COMPARISON OF THE SUN LIGHT COLLECTORS

The efficiency obtained by the collector coupled to a 0.6 mm fibre has been theoretically estimated and experimentally measured. This comparative study shows that the CCM, especially designed for our solar system, has very reduced thickness (25 mm), is easy to be alighted and mounted and performs good power collection. While the Mangin configuration represents the best trade-off between efficiency and cost. The mechanical alignment between concentrators and optical fibre is a critical aspect, especially for Mangin (A) and Parabolic (B) concentrators, where the secondary mirror is physically separated from the primary mirror (it is realised on the protective window of the tile, as shown in Fig. 5). This alignment difficulty increases if an array of concentrators is used.

For the choice of the collectors for the solar system prototype Table 2 summarises advantages and drawbacks of the three classes of collector.

Table 2

REALISATION OF THE COLLECTORS

The CCM has been realised in Silica and all CCM samples reported in the figures (Fig.s 4, 6, and 10) are made in Silica.

Some samples have also been realised in PMMA, with considerably reduced weight and costs, but keeping good collection performances. The realisation of aspherical lenses as plastic optical components could allow a massive production at low cost.

The collector of type Mangin A1 has been realised in Glass with enter pupil size 62 mm and focal length 60 mm (Mangin60). The enlarged diameter of Mangin60 has the aim of improving the collected energy with respect to Mangin40, corresponding to A1 in Table 1. Modified Mangin A2 and A3 are more complicated to be realised, but the same performances can been obtained by a standard Mangin of size 62 mm. Thus Mangin60 represents a trade-off between the modified Mangin A3, characterised by a diameter of 62 mm and the standard version of A1, which has a diameter of 40 mm and focal length 39.8 mm.

Fig. 3 A1 collector: Mangin60.	Fig. 4 CCM in Silica.

Taking into account the efficiency parameters, the realisation costs and the compactness of the optical system, we decided to test only Mangin60  (Fig. 3) and CCM (Fig. 4) in real conditions. Field tests have been repeated in all seasons of the year and at different hours during the day. The average output power measured at the end of a 5m fibre length is respectively 0.80-0.85 W for Mangin60 and 0.95-1.05 W for CCM in Silica. The CCM realised in PMMA provide an output power of 0.80-0.90 W, thus resulting slightly less efficient than the CCM in Silica.

TILES MOUNTING THE COLLECTORS

The Mangin and CCM concentrators are mounted on tiles of four collectors, obtaining a modular system. The base element is a tile of 14cm x 14cm holding 4 optical systems. This tile dimension has been chosen taking into account that the use of a small tile facilitates the alignment operations and improves the possibility of its massive reproduction.

Reduced size of the tile and system geometry makes it adaptable to the available space and to specific architectural requirements. Fig. 5 shows the tile with 4 Mangins. The housing for the single collector is a metallic support of diameter 64 mm, which also holds the mirror. The fibre holder with its focusing adjustment is placed in the centre of the metallic support. The secondary mirrors are realised by evaporated aluminium on the input window covering the four collectors. The tile holding 4 CCM is reported in Fig. 6. The fibre adjustment mechanism is placed in the rear part of the tile, behind the collector. Both tiles have the same external dimensions, so they are interchangeable if they are mounted in the same moving support.
	Fig. 5 Tile with 4 Mangins
	Fig. 6 Tile with 4 CCM

THE SUN POINTER MOUNTED ON THE TILE

The sun pointer, in Fig. 7, works as a pin-hole camera without lenses. The distance L from the pin-hole to the four quadrant detector determines the maximum field of view q and the resolution of the pointer. The size of the sun image and its intensity depend on the diameter d of the pin-hole.
	Fig. 7
Fig. 8	Fig. 9

Fig. 8 shows the sun pointer inserted in the tile by means of an adjustable tilter in order to simplify the alignment operations.

Fig. 9 presents the items composing the sun pointer: the camera; three pin-holes with different diameters; the four quadrant detector; the blocking screws.

The sensor driving the sun tracking system is visible at the centre of the tile in Fig 6.

THE TRACKING SYSTEM: FRAMES WITH 4 OR 36 LENSES

In order to test, in operative conditions a prototype of solar plant, two different moving frames have been experimented.

The main feature of these configurations is their self-alignment capability. The theoretical positioning of the frame requests the orientation of the hourly axis of an angle j = 47° with respect to the horizon in the North to South direction. Therefore the real alignment can be made within an error of ±1° since the sun tracking system will automatically correct the alignment errors.

SOFTWARE FOR SUN TRACKING:

azimuth and elevation of the sun calculated by SazEl

The sun tracking is realised by the SazEl software, especially developed for calculating azimuth and elevation of the sun and thus for driving the mechanical supports to orient the tile with the four collectors. The SazEl tracking system is schematically presented in Fig. 12, where it is also shown that in case of temporary cloud the system restarts as the sun reappears.

Fig.s 13 and 14 show two control panels of the SazEl software: the location is set by the input panel in Fig. 13, while the screen view in Fig. 14 shows the control of the sun image centering by the four quadrant sensor.

A SOLAR SYSTEM FOR DOMESTIC USES

A prototype of the solar system has been realised and tested for energy supply of a building. The modular unit of the solar plant is the tile including 4 collectors, each of which is coupled to a fibre. The fibres are then collected in bundles and through an optical switch the collected energy can be addressed to different uses. The applications are internal room illumination, water heating or supply for domestic devices. For this last use a photovoltaic panel provides the conversion into electric energy, which otherwise can be stored for successive employment.

The Optical Switch is presented in Fig. 15. The lighting cone at the output of the fibre bundle is directed towards 3 different utilisation systems by means of the optical switch (OS). The main components of OS are: a spherical mirror (SM) and a stepping motor equipped with a control electronic programmed to rotate SM of -10° on the left and in other two positions of 10° and 20° on the right (Fig. 15). The SM has a diameter of 66mm and a focal length of 35mm. The optical fibres are at a distance of 60mm from the mirror vertex. The light is utilised for three different applications: the first position directs the light beam on a photovoltaic cell, whose produced energy can supply domestic devices or can be stored for successive uses. The second position directs the light on a paint in order to verify the high quality of colours rendering. The third position focalise the light beam in a black body equipped with a temperature monitoring system simulating the application to water heating.

Fig. 15 The optical switch

The intensity distribution in the light cone is asymmetric because the SM works off axis, but considering the contribution of 4 fibres an increase of uniformity has been obtained.

Fig. 16 illustrates the possible uses and installation of the system in a house: the tiles are placed on the roof and fibres transport the collected energy into the house. The figure shows how the light collected by the tiles placed on the roof is used for illumination, collecting the fibres in bundles. Another application in house supply can be water eating but the light can also be converted into electric energy or stored for successive uses. The optical switch can be used to choose between "energy use" and "energy storage", as well as, to address the energy towards the different domestic applications.

Fig. 16 Different energy uses in the house

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