10 mai 2017

Acronym: Star-Foam


Future high-efficiency plants converting concentrated solar energy into electricity or solar fuels will require a heat receiver in which thermal transfers will be optimized to achieve maximum conversion efficiency and high temperatures (> 700 ° C). Among the technologies studied, porous ceramic receivers and exchangers have the advantage of resisting high temperatures and of increasing transfers between the heat transfer fluid and the solid matrix. In order to achieve high thermal efficiency and to improve the understanding of limiting phenomena, the optimization and intensification of heat transfer requires an accurate model of the conversion efficiency dependencies to the geometric, thermophysical and thermoradiative properties of the porous solid phase. The thesis aims to develop a reduced model of the porous medium whose parameters will be obtained through the analysis of the random paths used by the Monte Carlo Symbolic (MCS) statistical method to solve coupled thermal transfers. The model will then be used to optimize the receivers and the porous exchangers according to the operating conditions and their geometries. 

21 avril 2017

Novel utility-scale solar electricity generation strategy 

In the framework of a collaborative research project between Ben Gurion University and PROMES-CNRS, we offer 3 post-doctoral positions at Ben-Gurion University (1 year). One of these 3 post-doctoral position will be extended for a one-year supplementary contract at PROMES (Odeillo, France).

Motivation: The proposed project aims at a novel utility-scale solar electricity generation strategy, with day-night storage and unprecedented efficiency, via an innovative conflation of high-efficiency photovoltaics (for direct conversion of sunlight to electricity), high-temperature solar thermal (for conversion via Rankine cycles), and a new genre of innovative concentrator optics that facilitates these ambitious objectives.

The system design is predicated on new unprecedented 3D concentrator optics - to wit, aplanatic solar tower systems - where a completely static secondary mirror atop the tower can be used to actually increase flux concentration (in contrast to conventional Cassegrain beam-down solar tower optics), while simultaneously permitting a receiver (focus) at or below ground level, with prodigious practical advantages. This core concept can also be implementated as part of a multi-tower system, where a given heliostat can be aimed at more than one tower, depending on solar geometry, thereby substantially reducing shading and blocking losses, as well as markedly increasing system ground-cover ratio.

Multi-junction photovoltaics currently lack the affordable, efficient storage capability of solar thermal. Hence the paramount importance of storage skews the system optimization toward a limited fraction of photovoltaic direct conversion.

The importance of operating at high concentration (e.g., exceeding 1,000 suns) is multi-fold. First, solar cell efficiency can increase as the logarithm of concentration. Second, high concentration basically removes the per-cell cost of expensive multi-junction solar cells from the cost equation. Third, system heat loss need not exceed more than a few percent of the the collected solar beam radiation, even at these high temperatures, simply for geometric reasons, also obviating the need for selective coatings used extensively in line-focus solar thermal power systems. And fourth, the high collection temperature needed for high-efficiency turbines is readily achieved at high concentration.

The project divides into three linked scientific realms: photovoltaic materials, optics, and thermal design, as follows.