Analytic optical design of linear Fresnel collectors with variable widths and shifts of mirrors

Linear Fresnel collectors still present a large margin to improve efficiency. Solar fields of this kind installed until current time, both prototypes and commercial plants, are designed with widths and shifts of mirrors that are constant across the solar field. However, the physical processes that limit the width of the mirrors depend on their relative locations to the receiver; the same applies to shading and blocking effects, that oblige to have a minimum shift between mirrors. In this work such phenomena are studied analytically in order to obtain a coherent design, able to improve the efficiency with no increase in cost. A ray tracing simulation along one year has been carried out for a given design, obtaining a moderate increase in radiation collecting efficiency in comparison to conventional designs. Moreover, this analytic theory can guide future designs aiming at fully optimizing linear Fresnel collectors' performance

Alpha decay perturbations by atomic effects at extreme conditions

The alpha tunneling effect in the presence of electron screening is calculated within the Debye model. Calculations show that very small effects are predicted by cooling the metal to low temperatures. However the alpha lifetime decay may be reduced by about 15% if solid samples of the alpha emitters are cooled and compressed to relatively high densities. These conditions can be achieved at high pressures by using existing diamond anvil cells (DACs). Even so, practical consequences for speeding-up the decay of actinides (from the nuclear waste) seem to be negligible. Keywords: Alpha decay; Lifetime reduction; Extreme atomic condition

New generation Line-Focusing Solar Power Plants with Molten Salts and Supercritical Carbon Dioxide Joule-Brayton Cycles

Nowadays there is no dominant technology for the concentrated solar power plants that means there is still a way to go. Within this context, new concepts for solar fields and power cycles are being studied. One of them is the proposed on this paper: the integration of line-focusing solar field, with parabolic trough or linear Fresnel solar collectors, with molten salts as heat transfer fluid and supercritical carbon dioxide Joule-Brayton power cycles. This concept works as a feasible design solution to increase efficiency and reduce final energy cost in solar electricity production. In this work, four Joule-Brayton cycles configurations were assessed and compared with the considered reference, a concentrated solar power plant with direct steam generation in the solar field and a Rankine power cycle. The studied Joule-Brayton cycles are: simple cycle, recompression cycle, partial cooling with recompression cycle and recompression with main compression intercooling cycle. The common operation conditions for all the configurations are that at design-point the high pressure turbine inlet temperature value is 550ºC, this limit was established considering maximum temperature allowed by selective coating material in linear receivers. Also is analyzed the hypothetical scenario of increasing the turbine inlet temperature to 650ºC, extrapolating the receivers heat losses regressions. The innovative configurations of solar field and supercritical carbon dioxide power cycles increase plant efficiency, for recompression cycle configuration, up to 46.84% (550ºC turbine inlet) and 50.85% (650ºC turbine inlet), and reduces required solar field effective aperture area and land area for a fixed plant power output. Proposed configurations, parabolic trough collector and linear Fresnel coupled with a Joule-Brayton cycle decreases the solar field required for the same net power. Relating to power block, the supercritical carbon dioxide higher density in comparison with water steam, reduces turbines and compressors dimensions, footprint and final cost, but is a technology nowadays under industrial development and final turbo machines cost could not be assessed in this study. Another important keystone in JouleBrayton cycle costs are the heavy duty heat exchangers required. Printed circuit heat exchangers are the most advisable solution proposed for supercritical carbon dioxide recuperators, mainly due to higher compactness and better heat transfer coefficient inside channels. However, in this paper it is demonstrated how common shell & tube heat exchangers, with AISI 347 (austenitic) stainless steels, are competitive and feasible solutions for the primary and reheating molten salts – carbon dioxide heat exchangers

Supercritical Steam power cycle for Line-Focus Solar Power Plants

The supercritical Rankine power cycle offers a net improvement in plant efficiency compared with a subcritical Rankine cycle. For fossil power plants the minimum supercritical steam turbine size is about 450MW. A recent study between Sandia National Laboratories and Siemens Energy, Inc., published on March 2013, confirmed the feasibility of adapting the Siemens turbine SST-900 for supercritical steam in concentrated solar power plants, with a live steam conditions 230-260 bar and output range between 140-200 MWe. In this context, this analysis is focused on integrating a line-focus solar field with a supercritical Rankine power cycle. For this purpose two heat transfer fluids were assessed: direct steam generation and molten salt Hitec XL. To isolate solar field from high pressure supercritical water power cycle, an intermediate heat exchanger was installed between linear solar collectors and balance of plant. Due to receiver selective coating temperature limitations, turbine inlet temperature was fixed 550ºC. The design-point conditions were 550ºC and 260 bar at turbine inlet, and 165 MWe Gross power output. Plant performance was assessed at design-point in the supercritical power plant (between 43-45% net plant efficiency depending on balance of plantconfiguration), and in the subcritical plant configuration (~40% net plant efficiency). Regarding the balance of plant configuration, direct reheating was adopted as the optimum solution to avoid any intermediate heat exchanger. One direct reheating stage between high pressure turbine and intermediate pressure turbine is the common practice; however, General Electric ultrasupercritical(350 bar) fossil power plants also considered doubled-reheat applications. In this study were analyzed heat balances with single-reheat, double-reheat and even three reheating stages. In all cases were adopted the proper reheating solar field configurations to limit solar collectors pressure drops. As main conclusion, it was confirmed net plant efficiency improvements in supercritical Rankine line-focus (parabolic or linear Fresnel) solar plant configurations are mainly due to the following two reasons: higher number of feed-water preheaters (up to seven)delivering hotter water at solar field inlet, and two or even three direct reheating stages (550ºC reheating temperature) in high or intermediate pressure turbines. However, the turbine manufacturer should confirm the equipment constrains regarding reheating stages and number of steam extractions to feed-water heaters

Comparison between s-CO2 and other supercritical working Fluids (s-Ethane, s-SF6, s-Xe, s-CH4, s-N2) in Line-Focusing Solar Power Plants with supercritical Brayton power cycles

Thermosolar power plants with linear solar collectors and Rankine or Brayton power cycles are maturing as a competitive solution for reducing CO2 emissions in power plants as an alternative to traditional fossil and nuclear fuels. In this context, nowadays a great effort is being invested in supercritical Carbon Dioxide Brayton (s-CO2) power cycles for optimizing the line-focusing solar plants performance and reducing the cost of renewable energy. However, there are other working fluids with similar properties as s-CO2 near critical point. This researching study was focused on assessing the solar plants performance with alternative supercritical working fluids in the Balance Of Plant (BOP): Ethane, Sulfur Hexafluoride, Xenon, Methane and Nitrogen, see [1, 2, 3]. The integration between linear solar collectors (Parabolic or Fresnel), Direct Moten Salt (MS) as Heat Transfer Fluids (HTF) and a Simple Brayton cycle with Recuperation and ReHeating were studied in this paper. Main innovation in this researching study is the Brayton power cycle parameters optimization at Design-Point via the Subplex algorithm as proposed in John Dyreby Thesis [4]. After obtaining the optimum reheating pressure, compressor inlet pressure, recompression fraction, and other optimized variables, the solar power plants performances were simulated and detail designed with Thermoflow software [5], providing a first approach about the Solar Fields (SF) effective areas and investment costs. As main conclusion, we deducted the importance of heat exchangers conductance (UA) for increasing the Brayton power plants efficiency and reducing the SF effective area and investment cost. The pinch point at recuperators exit is the main constrain for increasing the UA in s-CO2 cycles. This limitation is overcome with the other working fluids proposed in this study providing higher plant efficiency but requiring higher UA in the recuperators. In future studies the heat exchangers detailed design constitute a great challenge for increasing the UA and optimizing these equipments cost. The material corrosion and equipments dimensions and cost is another key issue discussed for selecting the optimum energy transfer fluid in Brayton power cycles

Dual Loop Line-Focusing Solar Power Plants with Supercritical Brayton Power cycles

Most of the deployed commercial line-focusing solar power plants with Parabolic Troughs (PTC) or Linear Fresnel (LF) solar collectors and Rankine power cycles use a Single Loop Solar Field (SF), Configuration 1 illustrated in Fig. 2, with synthetic oil as Heat Transfer Fluid (HTF) [1, 2]. However, thermal oils maximum operating temperature should be below ~400ºC for assuring no oil degradation, hence limiting the power cycle gross efficiency up to ~38%. For overcoming this limitation Molten Salts (MS) as HTF in linear solar collectors (PTC and LF) were recently experimented in pilot facilities [3, 4]. Direct MS main drawbacks are the equipments and components material corrosion and the salts freezing temperature, requiring heat tracing to avoid any sald solidification, hence increasing the Solar Field (SF) capital investment cost and parasitic energy looses. Concentrated Solar Power plants (CSP) with Dual Loop SF are being studied since 2012 [5] for gaining the synergies between thermal oils and MS properties. In the Dual Loop SF the HTF in the primary loop is thermal oil (Dowtherm A) [6] for heating the Balance Of Plant (BOP) working fluid from ~300ºC up to ~400ºC, and a secondary loop with Solar Salt (60% NaNO3, 40% KNO3) as HTF, for boosting the working fluid temperature from ~400ºC up to 550ºC [7, 8, 9]. The CSP Dual Loop state of the art technology includes Rankine power cycles, the main innovation of this paper is the integration between Dual Loop SF and the supercritical Carbon Dioxide (s-CO2) Brayton power cycles [10], see Configurations 2 and 3 illustrated in Fig. 3a, Fig 3b. A secondary innovation studied in this paper is the integration between thermal oil HTF (Dowtherm A) in linear solar collectors, a widely validated and mature technology, with the s-CO2 Brayton power cycles. This technical solution is very cost competitive with carbon steel receiver pipes, low SF operating pressure, and no requiring any heat tracing. Two main conclusions are deducted from this researching study. Firstly we demonstrated the higher gross plant efficiency ~44.4%, with 550ºC Turbine Inlet Temperature (TIT), provided by the Dual Loop with the Simple recuperated s-CO2 Brayton cycle with reheating, in comparison with 41.8% obtained from the Dual Loop SF and subcritical water Rankine power cycle. And finally the second conclusion obtained is the selection of the most cost competitive plant configuration with a Single loop SF with Dowtherma A and a s-CO2 Brayton power cycle due to the receiver material low cost and no heat tracing for the thermal oil

Thermodynamic optimisation of supercritical CO2 Brayton power cycles coupled to Direct Steam Generation Line-Focusing solar fields

In this paper a new generation line-focusing solar plants coupled to a s-CO2 Brayton power cycles are studied. These innovative CSP will increase the plant energy efficiency, and subsequently optimizing the SF effective aperture area and SF investment cost for a fixed power output. Two SF configurations were assessed: the Configuration 1 with a condenser between the SF and the Balance Of Plant (BOP), for Turbine Inlet Temperatures (TIT) up to 400oC, and the Configuration 2, for higher TIT up to 550oC, with steam compressors in SF for pressure drop compensation. Both alternatives are interchangeable in the same CSP, and boosting with a backing boiler to warranty the plant performance. In relation to the BOP three configurations were studied the Recompression cycle (RC), the Partial Cooling with Recompression cycle (PCRC), and the Recompression with Main Compression Intercooling cycle (RCMCI), all these solutions without ReHeating. The methodology considered the thesis developed by Dyreby [1] as starting point, fixing the Brayton cycles recuperator conductance (UA), and optimizing the power cycles performance by means of the SUBPLEX [2] algorithm. The cycles optimal operating parameters were calculated with a “Windows” desktop application, called Supercritical_CSP (SCSP), calling the supercritical fluids properties database REFPROP, developed in C#, calling Fortran compiled dynamic linked libraries. The results obtained from the Brayton cycles optimizations were exported to Thermoflow [3] for SF simulation and design. The mathematical algorithms UOBYQA [4] and NEWOUA [5] were also integrated in the SCSP tool, for validating the SUBPLEX results. The HTF studied was Direct Steam Generation (DSG) in the SF, and the solar collectors simulated were PTC and LF. The plant net power output, the net efficiency, the SF effective aperture, were computed at DesignPoint. As main conclusion obtained it is confirmed minimum Pinch Point in heat exchangers is the main constrain, reaching a threshold in the net plant efficiency, when increasing the Low Temperatura Recuperator (LTR) and High Temperature Recuperator (HT) conductances UA. The shell-tubes heat exchanger types are the most suitable solution to couple the Balance Of Plant (BOP) and the SF. The target of future works will be aligned with the analysis of innovative linear solar collectors, as the Norwich Technologies company solution, for getting higher TIT as provided by Central Tower CSP. The s-CO2 BOP equipments detail design and detailed cost estimation are pending items under industrial development. Finally, the annual plant performance calculation, considering the variable ambient temperature and Direct Normal Irradiance (DNI), and the TES integration, are future researching works for calculating the Levelized Cost Of Energy (LCOE) in this new generation line-focusing solar power plants

Solar multiple optimization for a solar-only thermal power plant, using oil as heat transfer fluid in the parabolic trough collectors

Usual size of parabolic trough solar thermal plants being built at present is approximately 50 M We. Most of these plants do not have a thermal storage system for maintaining the power block performance at nominal conditions during long non-insolation periods. Because of that, a proper solar field size, with respect to the electric nominal power, is a fundamental choice. A too large field will be partially useless under high solar irradiance values whereas a small field will mainly make the power block to work at part-load conditions. This paper presents an economic optimization of the solar multiple for a solar-only parabolic trough plant, using neither hybridization nor thermal storage. Five parabolic trough plants have been considered, with the same parameters in the power block but different solar field sizes. Thermal performance for each solar power plant has been featured, both at nominal and part-load conditions. This characterization has been applied to perform a simulation in order to calculate the annual electricity produced by each of these plants. Once annual electric energy generation is known, levelized cost of energy (LCOE) for each plant is calculated, yielding a minimum LCOE value for a certain solar multiple value within the range considered

Hybrid reactors: nuclear breeding or energy production?

After reviewing the long-standing tradition on hybrid research, an assessment model is presented in order to characterize the hybrid performance under different objectives. In hybrids, neutron multiplication in the subcritical blanket plays a major role, not only for energy production and nuclear breeding, but also for tritium breeding, which is fundamental requirement in fusion–fission hybrids. All three objectives are better achieved with high values of the neutron multiplication factor (k-eff) with the obvious and fundamental limitation that it cannot reach criticality under any event, particularly, in the case of a loss of coolant accident. This limitation will be very important in the selection of the coolant. Some general considerations will be proposed, as guidelines for assessing the hybrid potential in a given scenario. Those guidelines point out that hybrids can be of great interest for the future of nuclear energy in a framework of Sustainable Development, because they can contribute to the efficient exploitation of nuclear fuels, with very high safety features. Additionally, a proposal is presented on a blanket specially suited for fusion–fission hybrids, although this reactor concept is still under review, and new work is needed for identifying the most suitable blanket composition, which can vary depending on the main objective of the hybrid

A comparative analysis of configurations of linear Fresnel collectors for concentrating solar power

Linear Fresnel collector arrays present some relevant advantages in the domain of concentrating solar power because of their simplicity, robustness and low capital cost. However, they also present important drawbacks and limitations, notably their average concentration ratio, which seems to limit significantly the performance of these systems. First, the paper addresses the problem of characterizing the mirror field configuration assuming hourly data of a typical year, in reference to a configuration similar to that of Fresdemo. For a proper comparative study, it is necessary to define a comparison criterion. In that sense, a new variable is defined, the useful energy efficiency, which only accounts for the radiation that impinges on the receiver with intensities above a reference value. As a second step, a comparative study between central linear Fresnel reflectors and compact linear Fresnel reflectors is carried out. This analysis shows that compact linear Fresnel reflectors minimize blocking and shading losses compared to a central configuration. However this minimization is not enough to overcome other negative effects of the compact Fresnel collectors, as the greater dispersion of the rays reaching the receiver, caused by the fact that mirrors must be located farther from the receiver, which yields to lower efficiencies