The principle of solar chimney power plant (SCPP) is based on harvesting the thermal spectrum of solar radiation and converting it to mechanical energy by the means of a collector, a wind turbine, and a chimney. In this work, a number of experiments were performed on a modified model made up of one-third of the circular collector area. Field data from selected clear, sunny days were recorded and studied. The analysis focused on time-temperature relations for ambient, near chimney entry point and the collector periphery, in addition to hourly solar radiation intensity and air velocity inside the chimney. The results show that for this geometry arrangement, the maximum temperature of the air entering the chimney is achieved before the ambient temperature reaches its peak value. Air velocity inside the chimney depends on the intensity of solar radiation and the temperature difference between the air temperature entering the chimney and the ambient temperature. Solar intensity directly affects the temperature of air beneath the collector, and a part of this energy is stored in the ground. Later, when the solar radiation is impaired, the stored energy can be utilized. Air velocity of 2.1 m/s is obtained after the solar noon, when the solar intensity is 737 W/m 2 and the maximum temperature difference is 11.2 °C. Due to the unique geometrical shape of the rig, a minor temperature loss of up to 1.3 °C occurs for the air near the center of the chimney.

The work presented in this paper used rigorous 3D flow-field analysis combined with multi-objective constrained shape design optimization for the design of complete blade + bladelet configurations for a three-blade horizontal-axis wind turbine. The fluid flow analysis in this work was performed using Open foam software. The 3D, steady, incompressible, turbulent flow Reynolds-Averaged Navier–Stokes equations were solved in the rotating frame of reference for each combination of wind turbine blade and bladelet geometry. The free stream uniform wind speed in all cases was assumed to be 9 m s −1 . The three simultaneous design optimization objectives were as follows: (a) maximize the coefficient of power, (b) minimize the coefficient of thrust force, and (c) minimize twisting moment around the blade axis. The bladelet geometry was fully defined by using a small number of parameters. The optimization was carried out by creating a multidimensional response surface for each of the simultaneous objectives. The response surfaces were based on radial basis functions, where the support points were designs analyzed using the high-fidelity computational fluid dynamics (CFD) analysis of the full blade + bladelet geometry. The response surfaces were then coupled to an optimization algorithm in mode frontier software. The predicted values of the objective functions for the optimum designs were then again validated using Open foam high-fidelity analysis code. Results for a Pareto-optimized bladelet on a given blade indicate that more than 4% increase in the coefficient of power at minimal thrust force penalty is possible at off-design conditions compared to the same wind turbine rotor blade without a bladelet.

Energy storage helps in waste management, environmental protection, saving of fossil fuels, cost effectiveness, and sustainable growth. Phase change material (PCM) is a substance which undergoes simultaneous melting and solidification at certain temperature and pressure and can thereby absorb and release thermal energy. Phase change materials are also called thermal batteries which have the ability to store large amount of heat at fixed temperature. Effective integration of the latent heat thermal energy storage system with solar thermal collectors depends on heat storage materials and heat exchangers. The practical limitation of the latent heat thermal energy system for successful implementation in various applications is mainly from its low thermal conductivity. Low thermal conductivity leads to low heat transfer coefficient, and thereby, the phase change process is prolonged which signifies the requirement of heat transfer enhancement techniques. Typically, for salt hydrates and organic PCMs, the thermal conductivity range varies between 0.4–0.7 W/m K and 0.15–0.3 W/m K which increases the thermal resistance within phase change materials during operation, seriously affecting efficiency and thermal response. This paper reviews the different geometry of commercial heat exchangers that can be used to address the problem of low thermal conductivity, like use of fins, additives with high thermal conductivity materials like metal strips, microencapsulated PCM, composite PCM, porous metals, porous metal foam matrix, carbon nanofibers and nanotubes, etc. Finally, different solar thermal applications and potential PCMs for low-temperature thermal energy storage were also discussed.

This paper presents the modeling theory and results of an innovative thermal energy storage (TES) facility, ideated, realized, and tested by ENEA (Italy). This prototype enabled the thermocline storage with molten salts in a novel geometry ideated for small-medium scale decentralized solutions, which includes two vertical channels to force the circulation through two heat exchangers, respectively, and realized for charging and discharging phases (in a single tank). A thermophysical model was built and tested properly for this particular geometry in order to analyze the temperature distribution along the radius. The numerical results well reproduced the experimental values. Furthermore, the analytical solution provided a short-cut methodology able to evaluate the thermocline distribution (along the vertical axis) depending on both the time and the radius values. Hence, the influence of the radial position (r) on the thermocline degradation was studied finding that, at the edges (r → 1), the thermocline remains unchanged for longer (around ten times more) than at the center of the tank (r → 0). The obtained numerical modeling and the analytical correlation can be useful for the process analysis to scale-up the thermal storage system and to evaluate the system reliability for industrial plants.

This study presents an innovative idea to augment heat transfer to an air heater using helicoidal finned arrangement. A parametric analysis of the helicoidal shaped fin geometry is considered with helicoidal pitch ratio of 0.1666–0.3, fin diameter ratio of 1.75–2. For the placement of the fin beneath the absorber plate, longitudinal pitch ratio ranging from 0.0416 to 0.1666 are used. The flow Reynolds number used for the study ranges from 4800 to 25,000. The effects of helicoidal pitch ratio, wire diameter ratio and longitudinal pitch ratio on Nusselt number and friction factor have been discussed. It is seen from the analysis that there is a significant improvement in Nusselt number for the case of helicoidal fin of wire diameter ratio of 1 when compared to base model as well as straight fin model for the operating range of Reynolds number. It is also observed from the analysis that for the helicoidal fin configuration of helicoidal pitch ratio of 0.2333, friction factor appears to be moderate. Flow and roughness parameters for roughened solar air heater have been optimized using thermal-hydraulic enhancement factor (THEF). The study reveals that by the use of helicoidal fins, maximum enhancement in the Nusselt number is found to be 2.21 times when compared to the base model for longitudinal pitch ratio of 0.0416, helicoidal pitch ratio of 0.166 for a fixed wire diameter. The improvement obtained in performance corresponding to increased Nusselt number establishes the efficacy the helicoidal fin design for the absorber plate.

The accurate prediction of the direct and diffuse solar radiation is of foremost importance for deployment of photovoltaic (PV) systems. A number of solar radiation forecasting techniques have been developed for longer and shorter forecasting times. Numerical weather prediction (NWP) models provide the best results for the longer forecasting times (4–6 h), required by utility companies. However, NWP methods are usually developed for clear-sky and open areas. These methods cannot be directly applied to urban areas with shading, trees, multisurface reflection, and other sources of solar radiation losses. To overcome these issues, improvement to the existing prediction tools are required. In this study, we develop an automated radiation forecasting tool for urban areas. This tool combines a NWP model (Weather Research and Forecasting (WRF) model) and a solar calculator (developed in the numerical toolbox O pen FOAM ) to compute shading, reflection, and other losses in the urban canopy. An algorithm for extraction of building outlines and heights (if they are publicly available) is also developed as a part of the tool. Finally, the coupled solar power estimator can be applied to past, present, or future solar power predictions. Initial results obtained using the developed tool are demonstrated for an urban neighborhood in Singapore.

Solar-to-thermal energy conversion technologies are an important and increasingly promising segment of our renewable energy technology future. Today, concentrated solar power (CSP) plants provide a method to efficiently store and distribute solar energy. Current industrial solar-to-thermal energy technologies employ selective solar absorber coatings to collect solar radiation, which suffer from low solar-to-thermal efficiencies at high temperatures due to increased thermal emission from selective absorbers. Solar absorbing nanofluids (a heat transfer fluid (HTF) seeded with nanoparticles), which can be volumetrically heated, are one method to improve solar-to-thermal energy conversion at high temperatures. To date, radiative analyses of nanofluids via the radiative transfer equation (RTE) have been conducted for low temperature applications and for flow conditions and geometries that are not representative of the technologies used in the field. In this work, we present the first comprehensive analysis of nanofluids for CSP plants in a parabolic trough configuration. This geometry was chosen because parabolic troughs are the most prevalent CSP technologies. We demonstrate that the solar-to-thermal energy conversion efficiency can be optimized by tuning the nanoparticle volume fraction, the temperature of the nanofluid, and the incident solar concentration. Moreover, we demonstrate that direct solar absorption receivers have a unique advantage over current surface-based solar coatings at large tube diameters. This is because of a nanofluid's tunability, which allows for high solar-to-thermal efficiencies across all tube diameters enabling small pressure drops to pump the HTF at large tube diameters.

The parabolic trough collector (PTC) is one of the most widely deployed concentrating solar power technology in the world. This study aims at improving the operational efficiency of the commercially available LS-2 solar collector by increasing the convective heat transfer coefficient inside the receiver tube. The two main factors affecting this parameter are the properties of the working fluid and the inner geometry of the receiver tube. An investigation was carried out on six different working fluids: pressurized water, supercritical CO 2 , Therminol VP-1, and the addition of CuO, Fe 3 O 4 , and Al 2 O 3 nanoparticles to Therminol VP-1. Furthermore, the influence of a converging-diverging tube with sine geometry is investigated because this geometry increases the heat transfer surface and enhances turbulent flow within the receiver. The results showed that of all the fluids investigated, the Al 2 O 3 /Oil nanofluid provides the best improvement of 0.22% to thermal efficiency, while the modified geometry accounted for a 1.13% increase in efficiency. Other parameters investigated include the exergy efficiency, heat transfer coefficient, outlet temperatures, and pressure drop. The analysis and modeling of a parabolic trough receiver are implemented in engineering equation solver (EES).

A method for inverse design of horizontal axis wind turbines (HAWTs) is presented in this paper. The direct solver for aerodynamic analysis solves the Reynolds-averaged Navier–Stokes (RANS) equations, where the effect of the turbine rotor is modeled as momentum sources using the actuator disk model (ADM); this approach is referred to as RANS/ADM. The inverse problem is posed as follows: for a given selection of airfoils, the objective is to find the blade geometry (described as blade twist and chord distributions) which realizes the desired turbine aerodynamic performance at the design point; the desired performance is prescribed as angle of attack ( α ) and axial induction factor (a) distributions along the blade. An iterative approach is used. An initial estimate of blade geometry is used with the direct solver (RANS/ADM) to obtain α and a. The differences between the calculated and desired values of α and a are computed and a new estimate for the blade geometry (chord and twist) is obtained via nonlinear least squares regression using the trust-region-reflective (TRF) method. This procedure is continued until the difference between the calculated and the desired values is within acceptable tolerance. The method is demonstrated for conventional, single-rotor HAWTs and then extended to multirotor, specifically dual-rotor wind turbines (DRWT). The TRF method is also compared with the multidimensional Newton iteration method and found to provide better convergence when constraints are imposed in blade design, although faster convergence is obtained with the Newton method for unconstrained optimization.

Designing blade geometry as a multidisciplinary optimization presents important challenges due to the increment in the number of design variables and computational cost of calculating the constraints and objective function. Blades have an important impact on loads because they capture the kinetic energy in wind and transfer it to the rest of the wind turbine components. Thus, consideration of the fatigue response is necessary in the optimization problem. However, the calculation of the damage equivalent loads (DELs) implies time-consuming simulations that restrict the number of design variables due to the increment of the search space. This article proposes a frequency domain method to estimate the fatigue response, which produces an advantage in terms of computational cost. The method is based on wind turbine model linearization by means of an aero-elastic code and the subsequent calculation of a frequency response function (FRF), which serves to estimate the response of the wind turbine. The Dirlik method is then applied to infer the damage equivalent loads. This process, which is useful for variables that have a stochastic nature, provides rapid approximate prediction of the fatigue response. An alternative estimation is proposed for loads subjected to an important periodic component. The results show that the method is useful in the initial stages of design.

The computer program called Solar_PVHFC has been modified to model a compound parabolic concentrator (CPC) that uses photovoltaic cells to produce electrical energy. This program was used to study the effects of concentration ratio, truncation height ratio, and photovoltaic cell efficiency on electrical power output and relative levelized cost of energy (LCE) of a fixed CPC photovoltaic device. Comparisons are made to fixed, conventional flat photovoltaic panels. This study indicates that CPCs can reduce the levelized cost of electrical energy produced by high efficiency, high cost photovoltaic cells, but provides no advantages for lower efficiency, lower price photovoltaic cells.

Concentrated solar power (CSP) plants have the potential to reduce the consumption of nonrenewable resources and greenhouse gas emissions in electricity production. In CSP systems, a field of heliostats focuses solar radiation on a central receiver, and energy is then transferred to a thermal power plant at high temperature. However, maximum receiver surface fluxes are low (30–100 W cm −2 ) with high thermal losses, which has contributed to the limited market penetration of CSP systems. Recently, small (∼4 cm 2 ), laminated micro pin-fin devices have shown potential to achieve concentrated surface fluxes over 100 W cm −2 using supercritical CO 2 as the working fluid. The present study explores the feasibility of using these microscale unit cells as building blocks for a megawatt-scale (250 MW thermal) open solar receiver through a numbering-up approach, where multiple microscale unit cell devices are connected in parallel. A multiscale model of the full-scale central receiver is developed. The model consists of interconnected unit cell and module level (i.e., multiple unit cells in parallel) submodels which predict local performance of the central receiver. Each full-scale receiver consists of 3000 micro pin-fin unit cells divided into 250 modules. The performance of three different full-scale receivers is simulated under representative operating conditions. The results show that the microscale unit cells have the potential to be numbered up to megawatt applications while providing high heat flux and thermal efficiency. At the design incident flux and surface emissivity, a global receiver efficiency of approximately 90% when heating sCO 2 from 550 °C to 650 °C at an average incident flux of 110 W cm −2 can be achieved.

Solar combi-storages are used in many countries for storing solar heat for space heating and domestic hot water (DHW) in one device. When a combi-storage is used in combination with a heat pump, the temperature stratification efficiency of the storage is a decisive factor for the overall efficiency and thus, for the consumed end-energy of the system. In particular, fluid that is entering the storage with a high velocity may cause considerable mixing, thus, destroying stratification and leading to poor system performance. This work presents computational fluid dynamics (CFD) simulations of direct horizontal inlets at midheight of a typical solar combi-storage of about 800 L volume. Different inlet diffusor designs were simulated, and laboratory measurements were used to validate CFD experiments. For the given tank geometry, mass flow rates, and inlet position, it is found for a fluid inlet temperature of 30 °C that fluid velocities should be below 0.1 m/s and Reynolds numbers below 3000–5000 at the outlet of the diffusor in order to avoid the disturbance of a hotter 50 °C zone above the inlet. Furthermore, the fluid path within the diffusor must exceed a minimum length that corresponds to three to four times the hydraulic diameter of the diffusor.

This paper presents assessments of the sensitivity of the performance of high flux solar simulators to the key variables of conical secondary concentrators for metal halide lamps, which offer complementary benefits compared with xenon arc lamps. The assessment is performed for both a single-lamp configuration and a seven-lamp array, each lamp close-coupled with its own elliptical reflector, and then aligned with a common conical secondary concentrator. The simulation of heat flux from both the single- and the seven-lamp systems was performed with the Monte Carlo ray-tracing code, which was validated with the experimental results from the single-lamp system. The calculated heat flux at the focal plane agrees with the measured peak flux to within 5% and to within 13% of the measured half width. Calculated results also show that the addition of the secondary concentrator to the single-lamp system can increase the peak flux by 294% and the average flux by up to 93% within a target of 100 mm in diameter, with a corresponding reduction in total power by 15%. The conical secondary concentrator is less effective for a seven-lamp system, increasing the peak and average fluxes by 87.3% and 100%, respectively, within 100 mm diameter focal plane, with a corresponding reduction in total power by 48%. The model was then used to assess the sensitivity of the geometry of the secondary concentrators for both the single- and seven-lamp systems. The results show that the average heat flux is sensitive to the surface reflectance of the secondary concentrator, with the average flux decreasing almost linearly with the surface reflectance. The presence of the secondary cone greatly reduces the sensitivity of the concentrated heat flux to misalignment of the tilting angle of the elliptical reflector relative to the arc.

A new symmetric nonimaging parametric trough collector (PmTC) for an evacuated circular receiver is proposed having an absorber diameter of 70 mm similar to the LS3/Eurotrough absorber one, and a circular secondary. The optical design method is explained and a sensitivity analysis is conducted with respect to the diameter of the secondary and to the acceptance angle. In the design process, transmission, absorption, and reflection losses are neglected. A secondary mirror radius of 145 mm and an acceptance angle of ±14 mrad were chosen as realistic values. For these values a concentrator geometry having 8.7 m gross aperture and 100% intercept factor for rays impinging on the primary within the referred acceptance angle is obtained producing a net concentration ratio relative to the thermodynamic ideal limit of 0.52 compared to 0.31 for traditional parabolic trough collectors. The new concentrator is composed of a primary discontinuous reflector with two symmetrical portions with parametric geometry, and a central parabolic portion located between the other two. The ensemble secondary receiver can be dressed up with the secondary concentrator manufactured by partially mirroring a diameter adapted glass tube—either internally or externally—or alternatively by means of a commercial evacuated receiver and an independent 145 mm radius arc of circumference external secondary reflector. Monte Carlo ray-tracing results show that only 15% of the rays undergo secondary reflection before arriving to the absorber and a sharp angle transmission curve. The new concentrator is proposed for application in solar thermal energy (STE) trough power plants.

Today most commercial parabolic trough collector (PTC) solar power plants make use of the well-known LS3/Eurotrough optics. The PTC has a concentration ratio relative to the maximum thermodynamic limit equal to 0.31. In order to improve the competiveness of PTC technology, two well differentiated R&D strategies have been undertaken: (i) developing larger parabolic troughs, which places a higher demand in tracking accuracy and lower tolerances with respect to wind loads, quality of mirrors, control and assembly imprecisions, and (ii) developing secondary concentrators with the aim of bringing the concentration ratio relative to the maximum one as close to 1 as possible. In this paper, a parametric trough collector (PmTC) for a flat receiver designed with the simultaneous multiple surface (SMS) method is proposed. The method assumes zero transmission, absorption, and reflection optical losses and allows for both reflective primary and secondary surfaces (XX-reflective plus reflective) to be simultaneously designed, guaranteeing Etendue matching. The proposed PmTC geometry increases the referred ratio up to 0.59 with a rim angle greater than 100 deg and with the same effective acceptance angle as the PTC. The flat absorber can be replaced with a multitube receiver for application in direct steam generation (DSG).

In this study, the parabolic trough collector's (PTC) performance is analyzed. In order to achieve this goal, the adopted procedure comprises two main steps. In the first step, the concentrated solar heat flux densities in the solar concentrator focal zone are calculated by soltrace software. In the second step, computational fluid dynamics (CFD) simulations are carried out to analyze and to optimize the thermal performance of the tube receiver. The calculated heat flux densities by soltrace software are used as wall heat flux boundary conditions for the receiver tube. The effect of the receiver tube diameter variation on the PTC thermal performance is studied. A new type of receiver tube is tested. This latter is covered with a metallic thickness. The performance of tube receiver covered with a metallic layer for different diameters is compared to those of the same diameters without the addition of metallic thickness. It has been found that increasing tube metallic thickness enhances the performance of PTC system comparing to the tubes of the same diameter and crossed by the same flow rates.

We present a systematic approach to the design of a set of high-flux solar simulators (HFSSs) for solar thermal, thermochemical, and materials research. The generic simulator concept consists of an array of identical radiation modules arranged in concentric rows. Each module consists of a short-arc lamp coupled to a truncated ellipsoidal specular reflector. The positions of the radiation modules are obtained based on the rim angle, the number of concentric rows, the number of radiation modules in each row, the reflector radius, and a reflector spacing parameter. For a fixed array of radiation modules, the reflector shape is optimized with respect to the source-to-target radiation transfer efficiency. The resulting radiative flux distribution is analyzed on flat and hemispherical target surfaces using the Monte Carlo ray-tracing technique. An example design consists of 18 radiation modules arranged in two concentric rows. On a 60-mm dia. flat target area at the focal plane, the predicted radiative power and flux are 10.6 kW and 3.8 MW m−2, respectively, and the predicted peak flux is 9.5 MW m−2.

The technical performance of a nontracking hybrid PV/T concept that uses a wavelength selective film is modeled. The wavelength selective film is coupled with a compound parabolic concentrator (CPC) to reflect and concentrate the infrared portion of the solar spectrum onto a tubular absorber while transmitting the visible portion of the spectrum to an underlying thin-film photovoltaic module. The optical performance of the CPC/selective film is obtained through Monte Carlo ray tracing (MCRT). The CPC geometry is optimized for maximum total energy generation for a roof-top application. Applied to a roof-top in Phoenix, AZ, the hybrid PV/T provides 20% more energy compared with a system of the same area with independent side-by-side solar thermal and PV modules, but the increase is achieved at the expense of a decrease in the electrical efficiency from 8.8% to 5.8%.

The fixed mirror solar concentrator (FMSC) possesses a geometry that can produce thermal energy in medium temperature range. Due to its static reflector, the FMSC has several advantages when compared to other designs, such as being one of the best adapted for integration onto building roofs. An optical ray-tracing analysis of its geometry was presented in a previous paper (Pujol Nadal and Martínez Moll, 2012, “Optical Analysis of the Fixed Mirror Solar Concentrator by Forward Ray-Tracing Procedure,” Trans ASME J. Solar Energy Eng., 134(3), pp. 031009-1-14). The optical results were obtained in function of three design parameters: the number of mirrors N, the ratio of focal length and reflector width F/W, and the intercept factor γ (in order to represent different receiver widths). In this communication, the integrated thermal output of the same parameter combinations has been determined in order to find optimal values of the design parameters at a working temperature of 200 °C. The results were obtained for three different climates and two orientations (North-South and East-West). The results show that FMSC can produce heat at 200 °C with an annual thermal efficiency of 39, 44, and 48%, dependent of the location considered (Munich, Palma de Mallorca, and Cairo). The best FMSC geometries in function of the design parameters are exhibited for medium range applications.