From Event: SPIE Optical Engineering + Applications, 2016
One path towards low electricity cost is the use of ever higher concentration values, since that, in turn, will provide less thermal losses at higher temperatures and high temperature operation means higher thermodynamic efficiency in the conversion of heat into electricity.
However concentration has an added value, since it is associated with larger primaries (see below) and thus with a reduction of collector rows in any given collector field. That, in turn, will reduce receiver length, connecting pipe lengths, number of components, thermal losses in pipes, heat transfer fluid mass, pumping power required (thus less parasitics), OM necessary, and all of that will contribute towards a lower electricity production cost.
Conventional PT and LFR concentrators are, essentially, focusing optics solutions and thus very far from the concentration limits set by Non Imaging Optics. However if a conventional PT optics is designed to accommodate a second stage concentrator (or, even better, if a parabolic like primary is designed in an optimal way with a secondary concentrator for a given receiver) the result will have a much higher concentration, but also, as a consequence, a much larger size, since available evacuated tubular receivers come in basically one (standard) size : 70mm diameter. Thus from a typical aperture size of ~6m and a concentration value of ~26, to double the concentration value with n.i.o., would bring the aperture close to ~12m, a value which is not practical for manufacture, transportation, field installation and operation (think about wind loads, for instance) .
But with LFR technology this size limitation is not there at all, and low concentration values can now be substituted by much higher ones, and primaries between 20 and 30 m can be produced for the same tube. Some LFRs on the market do have second stage concentration and offer primaries of about 12m total mirror width when designed for those evacuated tubes. These correspond to a CPC type second stage combined with the conventional primary.
But is possible to go much further in concentration ( or better yet, to go much further in CAP value – CAP= C*sinθ) by adopting Advanced LFR configurations which achieve the highest concentration possible for any given θ and do so by simultaneously conserve etendue as much as possible.
This talk will present and some of these solutions and discuss their merits for the application in view.
It will show that all things considered, Advanced LFR solutions, with Molten Salts operating at 565°C , have a much higher final solar to electricity conversion efficiency than the conventional solutions and thus LFR technology seems to have a future market potential (given its inherently low cost) much beyond its present very low market share.
Manuel Collares-Pereira, "Advanced linear Fresnel optics for solar thermal energy (STE) production
(Conference Presentation)," Proc. SPIE 9955, Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XIII—Commemorating the 50th Anniversary of Nonimaging Optics, 99550M (Presented at SPIE Optical Engineering + Applications: August 29, 2016; Published: 2 November 2016); https://doi.org/10.1117/12.2238665.5170791508001.
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