> Multi tower solar array (MTSA) technology

Since Francia (1968) developed the first operating solar tower system, solar tower (sometimes called 'central receiver' or 'power tower') technology has been developed much further (Winter et al, 1991; Kolb et al, 1991; Grasse, 1991). A typical solar tower power system consists of a field of two-axis tracking reflectors, called heliostats, which focus the direct solar radiation onto a tower-mounted receiver. The heliostat field is a special kind of Fresnel reflector which has performance limitations due to mutual blocking and shading of the heliostats. These limitations of a heliostat field were first derived by Riaz (1976).

Figure 1. Visualisation of appearance of one of the MTSA towers (by Schramek).

The Multi Tower Solar Array (MTSA) is a new concept of a point focussing two-axis tracking concentrating solar power plant (Fig. 1). The MTSA consists of several tower-mounted receivers which stand so close to each other that the heliostat fields of the towers partly overlap. Therefore, in some regions of the total heliostat field the heliostats are alternately directed to different aiming points on different towers. Thus the MTSA uses radiation which would usually remain unused by a conventional solar tower system due to mutual blocking of the heliostats.

The configuration of the heliostat field of an MTSA can be optimised to get a high annual efficiency for using the available beam energy which would otherwise strike the ground or roof below. In the regions close to the towers, where the shading effect predominates, all heliostats are directed to the nearest tower. In regions further away from the towers, the heliostats are alternately directed to two, three, or four aiming points on different towers.The MTSA approach reduces the losses occurred by mutual blocking of the heliostats more distant from the towers.

A heliostat field with a high density of heliostats can transfer radiation onto a tower mounted receiver with almost the theoretically maximum performance. However, this means a low utilisation of the installed reflector area due to blocking and shading. Therefore, the heliostats close to the tower of a conventional solar tower system are spaced apart to avoid mutual shading. Heliostats more distant from the tower are spaced apart, so that they do not block each other. Direct solar radiation falls onto the ground, instead of being reflected from costly reflector area uselessly upon the backside of the reflector of another heliostat.

Figure 2: An array of MTSA towers.

The proposed Multi Tower Solar Array (MTSA) consists of a group of solar towers where the heliostat fields of the towers partly overlap, similar to the operation of the linear CLFR (Fig. 2). For an MTSA, where the heliostat fields of up to four towers can overlap, the situation becomes more complex, because in some regions the neighbouring heliostats might be alternately directed to the aiming points on more than two different towers. In some regions, especially close to one of the towers, all heliostats are directed to only one aiming point, in other regions the heliostats are alternately directed to two, three or four aiming points on different towers.

The overlapping heliostat fields of a Multi Tower Solar Array increase the Annual Ground Area Efficiency and the Annual Reflector Area Efficiency substantially in contrast to conventional solar tower systems with a single central receiver, which means a more efficient usage of the solar radiation falling on a given ground area. On the one hand, this allows to set up a solar power plant for a specific output on a smaller ground area, or on the other hand to set up a solar power plant with a higher output on a given ground area. This consequently means savings in construction and land costs per installed capacity of the solar power plant. Additionally an MTSA with small towers and small heliostats can be set up in an urban environment over large parking lots or on flat roofs of big buildings, since urban applications need to use the given ground or roof area efficiently. Over 90% of the annual beam radiation falling on the ground or roof can be used with a practical array. In this way the advantages of concentrating solar power plants can be used in the urban environment.

COMBINED HEAT AND POWER and BEAM SPLITTING

The MTSA is a highly space efficient concept with several possible market niches, but we wish in particular to address possible generation of concentrating solar power in or near urban environments. It is ideal for restricted roof or over parking lot spaces. An initial thought was to develop this using separate high efficiency PV, thermal, and methane/hydrogen (reforming) receivers but it is now pssible to design beam splitting panels of negligible optical loss to separate the incoming beam into two spectral portions, one of which is suitable for PV and the other for thermal purposes. Beam splitting is useful because a PV receiver uses only photons above a certain energy. Within this range the PV can potentially be more efficient than any likely heat engine. The remaining photons can be used by a thermal receiver without sacrificing its thermodynamic potential. The thermal receiver is sensitive to total energy supplied and can make full use of the split-off lower energy photons. In combination, very high conversion efficiencies are possible (>30%). However, only high optical concentration can be used, because concentrating PV receivers are very expensive and because the thermal receivers are more thermally efficient under high concentration. This suggests that high concentration systems are likely to prevail in the long run because overall electrical output efficiency can be much higher than in any low or non-concentration system. However, access to beam splitting technology is essential to access this option.

Both dishes and tower systems like the MTSA could use this approach and achieve similar efficiencies of conversion. However, dish arrays are less space inefficient than MTSA. The MTSA can use also larger heat engines than dishes and these can be fixed in place, a significant practical advantage. As for the size of the systems, there is no advantage to constructing large PV receivers but there are strong size restrictions on the size commercially available heat engines. For small gas turbines under development, resulting tower size for initial urban applications is likely to be below 10 metres. With hydrogen receivers, efficiency grows with reformer receiver size but 10 metres will result in a reformer receiver size much larger than the current CSIRO unit. An international cooperation between Australia, Italy, Germany and Israel is forming at this time to develop this approach.

Figure 3: Impression of conditions in a parking lot topped by an MTSA solar array.

REFERENCES

Francia, G.(1968) Pilot Plants of Solar Steam Generation Systems. Solar Energy 12, 51-64 Grasse, W. (1991) PHOEBUS - international 30MWe solar tower plant. Solar Energy Materials 24, 82-95.

Karni J. and Ries H. (1994) Concepts for High Concentration Primary Reflectors in Central Receiver Systems. In Proceedings of the 7th International Symposium on solar Thermal Concentrating Technologies, Vol.4, pp. 796-801, Moscow, Russia.

Kolb, G.J., Alpert,D.J. and Lopez, C.W.(1991) Insights from the operation of Solar One and their implications for future Central Receivers Plants. Solar Energy 47, pp.39-47.

Mills D.R. and Morrison G.L. (2000) Compact Linear Fresnel Reflector solar thermal powerplants. Solar Energy 68, 263-283.

Riaz, M.R. (1976) A Theory of Concentrators of Solar Energy on a Central Receiver for Electric Power Generation. ASME Journal of Engineering for Power 98, 375-384.