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This study is focused on proposing the combination of a Dual Loop solar field, with Dowtherm A and the Solar Salt as heat transfer fluids in parabolic or linear Fresnel solar collectors, coupled to supercritical Carbon Dioxide (s-CO 2 ) Brayton power cycle. The Dual-Loop justification relies on gaining the synergies provided by the different heat transfer fluids properties. The oils advantages are related with the operating experience accumulated in numerous solar power plants deployed around the World, assuring the commercial equipment availability. Also the pipes metal corrosion with oil is much lower than with molten salt. The pipes material cost saving is significant with the oil alternative. The thermal oil main constraint is imposed by the maximum operating temperature (around 400 °C) for avoiding chemical decomposition and degradation, stablishing the plant threshold efficiency 37% due to Carnot principle. On the other hand the Solar Salt mixture (60%NaNO 3 40%KNO 3 ) maximum operating temperature goes up to 550 °C, but the freezing point is stablished around 220 °C requiring pipes and equipment electrical heating for avoiding salts solidification at low temperature. Regarding the balance of plant, the s-CO 2 power cycle is the most promising alternative to the actual Rankine power cycle for increasing the plant energy efficiency, reducing the solar collector aperture area and minimizing the equipment dimensions and civil work. Three Brayton cycles configurations with reheating were assessed integrated with the line-focusing Dual-Loop solar field: the simple Brayton cycle (SB), the Recompression cycle (RC), the Partial Cooling with Recompression cycle (PCRC), and the Recompression with Main Compression Intercooling (RCMCI). The power cycle operating thermodynamic parameters (split flow, reheating pressure, mass flow and pressure ratio) were optimized with unconstrained multivariable algorithms: SUBPLEX, UOBYQA and NEWUOA. The main conclusion deducted is the significant efficiency improvement when adopting the s-CO 2 Brayton cycle in comparison with the Rankine legacy solution. The Dual-Loop solar field integrated with a Rankine cycle provides a gross efficiency around 41.8%, but when coupling to s-CO 2 Brayton RC or RCMCI the plant efficiency goes up to ≈50%. It was also demonstrated the beneficial effect of increasing the total heat exchangers (recuperators) conductance (UA) for optimizing the Brayton cycles efficiency and minimizing the solar field aperture area for a fixed power output, only limited by the minimum pinch point temperature in heat exchangers. [ABSTRACT FROM AUTHOR]

This study is focused on comparing four supercritical fluids: Ethane, Xenon, Methane and Nitrogen, as possible alternative to supercritical Carbon Dioxide (s-CO 2 ) in Brayton power cycles coupled to line- focusing solar power plants with Solar Salt (60% NaNO 3 ; 40% KNO 3 ) as heat transfer fluid. The Simple Brayton cycle with heat recuperation and reheating is the configuration selected in this paper, providing a balance of plant design with reduced number of equipment and cost. The gross plant efficiency is calculated fixing the recuperator conductance (UA) for different Turbine Inlet Temperatures (TIT), confirming the maximum plant gross efficiency is related with the minimum allowable recuperator pinch point temperature. The reheating pressure and compressor inlet temperature are optimized with the mathematical algorithms SUBPLEX, UOBYQA and NEWOUA. According to the REFPROP database ranges of applicability, the maximum TIT limits are established for the supercritical fluids (N 2 TIT = 550 °C, CO 2 TIT = 550 °C, C 2 H 6 TIT = 400 °C, Xe TIT = 450 °C and CH 4 TIT = 350 °C). The reference scenario considered for calculating the thermosolar plant energy balances and simulations is the wet-cooling system with a Compressor Inlet Temperature (CIT = 32 °C). The gross efficiency results with the wet-cooling system are: N 2 (45.8%), CO 2 (44.37%), C 2 H 6 (40.74%), Xe (39.88%), CH 4 (32.15%). The plant efficiency is also translated into solar field effective aperture area and estimated cost, for a fixed power output. For optimizing the solar collector aperture area and cost, the Primary Heat Exchanger (PHX) and the ReHeating Heat Exchanger (RHX) capacity ratio (C R ) are fixed (C R = 1). The dry-cooling system scenario (CIT = 47 °C) is alto estimated: N 2 (43.34%), CO 2 (42.42%), C 2 H 6 (37.34%), Xe (37.26%), CH 4 (29.53%). For predicting the recuperator heat exchanger dimensions for a fixed conductance (UA), the heat transfer coefficient (HTC) is calculated with the Dittus–Boelter correlation and compared with the CO 2 as reference. The C 2 H 6 , and CH 4 have relative higher HTC in relation with CO 2 . Also is calculated the recuperator pressure drop. The C 2 H 6 , CH 4 and N 2 pressure drop is lower in comparison with the CO 2 for the same operating conditions. The energy efficiency in solar power station coupled to Brayton cycle is very constrained by the ambient temperature variation, impacting directly in the dry-cooling system performance. For this reason a Compressor Inlet Temperature (CIT) sensing analysis is carried out ranging from 32 °C to 57 °C, and also varying TIT from 400 °C to 550 °C. A sensing analysis is also developed varying the Turbine Inlet Pressure (TIP) from 200 bar to 375 bar. The CO 2 improves the plant efficiency when increasing the TIP from 250 bar to 350 bar, however the rest of fluids (Ethane, Methane, Nitrogen and Xenon) nearly not suffered any impact in the plant efficiency when increasing the TIP. [ABSTRACT FROM AUTHOR]

Direct Steam Generation in Parabolic Troughs or Linear Fresnel solar collectors is a technology under development since beginning of nineties (1990's) for replacing thermal oils and molten salts as heat transfer fluids in concentrated solar power plants, avoiding environmental impacts. In parallel to the direct steam generation technology development, supercritical Carbon Dioxide Brayton power cycles are maturing as an alternative to traditional Rankine cycles for increasing net plant efficiency and reducing balance of plant equipments dimensions and cots. For gaining synergies between these two innovative technologies, in this paper, Direct Steam Generation and Brayton power cycles are integrated in line-focusing solar power plants. Four configurations are studied: Configuration 1 consists on installing a condenser between solar field and power cycle; condensing the heat transfer fluid (steam water) with the balance of plant working fluid (carbon dioxide). The condenser would be a shell & tubes type. Along tubes carbon dioxide flows, and steam water condensates at shell-side. Main advantage of the condenser equipment is the high heat transfer coefficient at water condensing-side, reducing condenser dimension and weight. The main disadvantage of this configuration is the high operating pressure required in solar field for condensing steam into liquid water. This pressure should be between 150 bar and 175 bar for obtaining 400 °C at turbine inlet. In the Configuration 2, the superheated steam delivered by solar collectors transfers the heat energy in a primary heat exchanger to the balance of plant working fluid. In this configuration the steam not condensate into liquid water, and only reduces the temperature from 550 °C–560 °C to 420 °C. The steam pressure drops in solar field along receivers, headers and heat exchangers are compensated by means of steam compressors. This second solution is compatible with higher turbine inlet temperatures, up to 550 °C. The keystones of this second configuration are the steam conditions at compressor inlet, pressure ∼175 bars and temperature ∼420 °C, for minimizing steam compressor electrical consumption. The third design solution (Configuration 3) includes a solar field with direct steam generation in solar collectors with boiling recirculation mode, but the balance of plant is integrated by two Brayton power cycles in cascade. The first power cycle operating at 550 °C turbine inlet, and the second cycle at 410 °C turbine inlet. Main advantage is the integration between a validated direct steam generation technology (recirculation boiling mode) with the Brayton power cycles avoiding steam compressors, a technology not yet commercially available, and main drawback of this design is the increasing number of balance of plant equipments. The Configuration 4 is very similar to the Configuration 2, with the same direct steam generation solar field with superheated steam without condensing, and a single reheating stage solar field with molten salt as heat transfer fluid. The Configuration 1 provides similar efficiency and net power output, for similar solar field effective aperture area, as obtained with molten salt solar collectors with supercritical carbon dioxide power cycle (recompression with main compression intercooling cycle provides 36.6% net efficiency, for a maximum 400 °C turbine inlet). The second design solution (Configuration 2) net efficiency is not very much impacted for steam compressor electrical consumption recompression cycle net efficiency is 43.6% with steam solar field, versus 45.16% with molten salt solar field, in both cases with 550 °C turbine inlet. The Configuration 3 performance is ∼39.7% with two cascade Brayton power cycles with recompression and main compression intercooling. Finally, the Configuration 4 optimum plant performance is obtained for the recompression cycle with a net efficiency ∼45.77%, and is constrained by the molten salt drawbacks (material corrosion, material cost, environmental impact, etc). [ABSTRACT FROM AUTHOR]

Highlights: [•] Greenhouse cover type and photoperiod affects the tomato lycopene content. [•] Tomatoes growing in March under double layer polyethylene had higher lycopene content. [•] The CaCO3 application on glass cover flat decreased the lycopene content. [•] Tomato fruits receiving greater amounts of light accumulated more lycopene. [ABSTRACT FROM AUTHOR]

Abstract: In this work the design of a continuous-time modulator for Gigabit Ethernet applications is presented. The input bandwidth and oversampling ratio are, respectively, 62.5MHz and 8, resulting in a clock frequency of 1GHz. It was designed and implemented in a standard 90nm CMOS technology. The active area of the modulator measures . It consists of a loop filter based on RC-opamp integrators and a 3-bit quantizer which includes a data weighted averaging scrambler. A digital tuning scheme to deal with process variations has also been included. System level simulations including several non-ideal effects have been carried out in order to determine in detail the performance of the converter. Experimental results show a resolution of 7.1 effective bits, and a power consumption of 10.8mW from a nominal power supply of 1V. [Copyright &y& Elsevier]