Solar thermal collectors and applications

Linear Fresnel reflector

LFR technology relies on an array of linear mirror strips which concentrate light on to a fixed receiver mounted on a linear tower. The LFR field can be imagined as a broken-up parabolic trough reflector (Fig. 8), but unlike parabolic troughs, it does not have to be of parabolic shape, large absorbers can be constructed and the absorber does not have to move. A representation of an element of an LFR collector field is shown in Fig. 9. The greatest advantage of this type of system is that it uses flat or elastically curved reflectors which are cheaper compared to parabolic glass reflectors. Additionally, these are mounted close to the ground, thus minimizing structural requirements.

The first to apply this principle was the great solar pioneer Giorgio Francia [85] who developed both linear

Fig. 9. Schematic diagram of a downward facing receiver illuminated from an LFR field.

and two-axis tracking Fresnel reflector systems at Genoa, Italy in the 60s. These systems showed that elevated temperatures could be reached using such systems but he moved on to two-axis tracking, possibly because advanced selective coatings and secondary optics were not available [86]. Two of the early published works on this area are given in Refs. [87,88], whereas some later papers are given in Refs. [89,90].

In 1979, the FMC Corporation produced a detailed project design study for 10 and 100 MWe LFR power plants for the Department of Energy (DOE) of the US. The larger plant would have used a 1.68 km linear cavity absorber mounted on 61 m towers. The project however was never put into practice as it ran out of DOE funding [86].

A latter effort to produce a tracking LFR was made by the Israeli Paz company in the early 90s by Feuermann and Gordon [91]. This used an efficient secondary CPC-like optics and an evacuated tube absorber.

One difficulty with the LFR technology is that avoidance of shading and blocking between adjacent reflectors leads to increased spacing between reflectors. Blocking can be reduced by increasing the height of the absorber towers, but this increases cost. Compact linear Fresnel reflector (CLFR) technology has been recently developed at Sydney University in Australia. This is in effect a second type of solution for the Fresnel reflector field problem which has been overlooked until recently. In this design adjacent linear elements can be interleaved to avoid shading. The classical LFR system has only one receiver, and there is no choice about the direction and orientation of a given reflector. However, if it is assumed that the size of the field will be large, as it must be in technology supplying electricity in the MW class, it is reasonable to assume that there will be many towers in the system. If they are close enough then individual reflectors have the option of directing reflected solar radiation to at least two towers. This additional variable in the reflector orientation provides the means for much more densely packed arrays, because patterns of alternating reflector orientation can be such that closely packed reflectors can be positioned without shading and blocking [86]. The interleaving of mirrors between two receiving towers is shown in Fig. 10. The arrangement minimizes beam blocking by adjacent reflectors and allows high reflector densities and low tower heights to be used. Close spacing of reflectors reduces land usage but this is in many cases not a serious issue

Fig. 10. Schematic diagram showing interleaving of mirrors in a CLFR with reduced shading between mirrors.

as in deserts. The avoidance of large reflector spacing and tower heights is an important cost issue when the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses and steam line cost are considered. If the technology is to be located in an area with limited land availability such as in urban areas or next to existing power plants, high array ground coverage can lead to maximum system output for a given ground area [86].

Solar thermal collectors and applications

Collector thermal efficiency

In reality the heat loss coefficient UL in Eqs (2) and (42) is not constant but is a function of collector inlet and ambient temperatures. Therefore: TOC o "1-5" h …

Global climate change

The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, …

Limitations of simulations

Simulations are powerful tools for process design offering a number of advantages as outlined in the previous sections. However, there are limits to their use. For example, it is easy …

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