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This paper is devoted to the analysis of different secondary reflector designs for linear Fresnel collectors. A number of authors have proposed alternative shapes of secondary reflectors in order to enhance the efficiency and the flux intensity at the absorber tube. In this work the primary mirrors layout has been maintained constant for all designs. Thus, only secondary concentrators that do not require a change in the primary mirrors field have been studied. In order to carry out the study a validated Monte Carlo ray trace code has been used, where three optical properties are considered: annual optical efficiency, mean flux intensity and circumferential flux intensity homogeneity. The maximum efficiency reached by each technology, for given optical flux specifications, is obtained. Finally, the optical performance of linear Fresnel collectors with secondary reflector is compared with that of state-of-the-art parabolic trough collectors. It is concluded that the efficiency of Fresnel collectors is around 23% than that of parabolic troughs, although with mean flux intensities around 46% higher and circumferential flux intensities notably more homogeneous (top to bottom flux intensity ratios five times higher).

A novel proposal to drive forward linear Fresnel technology within the solar market competitiveness is presented in this paper. It consists of two innovative storage strategies conceived for the management of a flexible three-tank thermal storage system of molten salts, which seeks the enhancement of part-load plant efficiency in alternative East-West oriented Fresnel solar plants. A consistent methodology has been developed in order to assess quantitatively this new proposal by means of annual performance simulations of a global model. This allows the comparison of the aforementioned approach of Fresnel-based plant with reference parabolic trough collectors (PTC) and linear Fresnel collectors (LFC), which are equipped with state-of-the-art two-tank thermal storage of molten salts and North-South solar field orientation. Results show an increase of more than 10% in annual plant efficiency compared to conventional North-South Fresnel plants based in high latitude locations, i.e. Almeria (Spain). This boost in LFC performance would imply narrowing the gap with PTCs, where expensive high optical quality receivers are used. Findings in this paper state that the proposed innovative strategies are proven to be more effective when low quality receivers are used and when the solar fields are located far from tropics.

The use of micro-scale centrifugal compressors in closed-loop thermodynamic cycles working with fluids different than air is gaining in importance in novel miniaturized energy systems. Miniaturization implies a decrease in the Reynolds number, which usually entails efficiency drops. The aim of this paper is to study the low-Reynolds-number effects on the performance of micro-scale centrifugal compressors under off-design conditions. Consequently, an analytical prediction method is formulated to characterize the efficiency variations using different working fluids and pressurization factors. A numerical model of a reference micro-scale compressor is developed to assess the degree of agreement between the results obtained by these two methods. The results show the Reynolds-number effects on the whole characteristic map of the reference compressor for air, carbon dioxide, propane and isobutane. These fluids were chosen according to their efficiency increase potential, which is measured by a proposed Reynolds-number-based estimator. Although some divergences have been identified, the agreement between analytical and numerical results can be considered to be globally satisfactory. Efficiency increases in the range of 2–6% points are obtained with slight pressurization factors using air or carbon dioxide. These variations could even achieve a rise of 9 pp in the case of propane and isobutane.

Peculiar thermodynamic properties of carbon dioxide (CO 2 ) when it is held at or above its critical condition (stated as supercritical CO 2 or sCO 2 ) have attracted the attention of many researchers. Its excellent thermophysical properties at medium-to-moderate temperature range have made it to be considered as the alternative working fluid for next power plant generation. Among those applications, future nuclear reactors, solar concentrated thermal energy or waste energy recovery have been shown as the most promising ones. In this paper, a recompression sCO 2 cycle for a solar central particles receiver application has been optimized, observing net cycle efficiency close to 50%. However, small changes on cycle parameters such as working temperatures, recuperators efficiencies or mass flow distribution between low and high temperature recuperators were found to drastically modify system overall efficiency. In order to mitigate these uncertainties, an optimization analysis based on recuperators effectiveness definition was performed observing that cycle efficiency could lie among 40%–50% for medium-to-moderate temperature range of the studied application (630 °C–680 °C). Due to the lack of maturity of current sCO 2 technologies and no power production scale demonstrators, cycle boundary conditions based on the solar application and a detailed literature review were chosen.

The aim of this paper is to study a new modular micro-scale trigeneration system based on a rotary Fresnel collector, named “SunDial”, under a restricted design space. This system is based on the integration of an organic Rankine cycle with a vapor compression heat pump using miniaturized turbomachinery. Four layouts areoed to maximize the energy supply according to different heat management strategies, as requested by potential end-users. In addition, a working fluid screening is carried out which has lead to the selection of two candidates: isobutane and propane. A consistent system performance metric has been defined in order to coherently asses the configurations conceived. Different degrees of integration between the power cycle and the heat pump are analyzed. The influence of power, heating and cooling supply on the global metrics are studied for each design. Finally, an energy flow sizing is presented for a 15 kWth solar thermal input SunDial module applied to different industrial end-users. Results show that isobutane is preferable when high shares of heating and cooling are required, whereas propane leads to higher power outputs.