Research Papers

J. Sol. Energy Eng. 2017;139(4):041001-041001-6. doi:10.1115/1.4036412.

This study presents a numerical approach to calculate the optimum photovoltaic (PV) tilt angle by considering the three different PV technologies (monocrystalline, polycrystalline, and thin film). This analysis focuses on determination of optimum tilt angle considering seasonal and yearly solar radiation on a plane (Wh/m2) and seasonal and yearly energy production (Wh) of PVs. The angle at maximum global radiation and maximum energy output is considered as the optimum tilt angle. It is found that optimum tilt angles obtained by total radiation and total energy output are different from each other considering seasonal and yearly base. Total radiation-based tilt angle results show that the optimum tilt angle is 13 deg in spring, 9 deg in summer, 17 deg in autumn, 12 deg in winter, and 12 deg as yearly. Energy production-based optimum tilt angles vary from 5 deg to 13 deg for monocrystalline, from 11 deg to 15 deg for polycrystalline, and from 12 deg to 25 deg for thin film technology according to seasonal and yearly tilt angle results.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041002-041002-9. doi:10.1115/1.4036331.

Compared with recirculation and injection modes, once-through direct steam generation (DSG) parabolic troughs are simpler to construct and require the lowest investment. However, the heat transfer fluid (HTF) in once-through DSG parabolic trough systems has the most complicated dynamic behavior, particularly during periods of moving shadows caused by small clouds and jet contrails. In this paper, a nonlinear distributed parameter dynamic model (NDPDM) is proposed to model the dynamic behavior of once-through DSG parabolic trough solar collector row under moving shadow conditions. Compared with state-of-the-art models, the proposed NDPDM possesses three characteristics: (a) adopting real-time local values of the heat transfer and friction resistance coefficients, (b) simulating the whole collector row, including the boiler and the superheated sections, and (c) modeling the disturbance of direct normal irradiance (DNI) level on DSG parabolic trough solar collector row under moving shadow conditions. Validated using experimental data, the NDPDM accurately predicts the dynamic characteristics of HTF during periods of partial and moving DNI disturbance. The fundamental and specific dynamic process of fluid parameters for a DSG parabolic trough solar collector row is provided in this paper. The results show the following: (a) Moving shadows have a significant impact on the outlet temperature and mass flow rate, and the impact lasts up to 1000 s even after the shadows completely leave the collector row. (b) The time for outlet steam temperature to reach a steady-state value for the first time is independent of the shadow width, speed, and moving direction. (c) High-frequency chattering of the outlet mass flow rate can be observed under moving DNI disturbance and will have a longer duration if the shadow width is larger or the shadow speed is slower. Compared with cases in which the whole system is shaded, partially shading cases have shown a longer duration of high-frequency chattering. (d) Both wider widths and slower speeds of shadow will cause a larger amplitude of responses in the outlet temperature and mass flow rate. When the shadow speed is low, there is a longer delay time of response in the mass flow rate of the outlet fluid. (e) The amplitude of response in the outlet temperature does not depend on the direction of clouds movement. However, if the DNI disturbance starts at the inlet of the collector row, there will be significant delay times in both outlet temperature and mass flow rate, and a larger amplitude of response in outlet mass flow rate.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041003-041003-11. doi:10.1115/1.4036497.

The possibility of a wind turbine entering vortex ring state (VRS) during pitching oscillations is explored in this paper. The work first validated the employed computational fluid dynamics (CFD) method, and continued with computations at fixed yaw of the NREL phase VI wind turbine. The aerodynamic performance of the rotor was computed using the helicopter multiblock (HMB) flow solver. This code solves the Navier–Stokes equations in integral form using the arbitrary Lagrangian–Eulerian formulation for time-dependent domains with moving boundaries. With confidence on the established method, yawing and pitching oscillations were performed suggesting partial vortex ring state during pitching motion. The results also show the strong effect of the frequency and amplitude of oscillations on the wind turbine performance.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041004-041004-9. doi:10.1115/1.4036414.

A high-temperature, high-pressure solar receiver was designed as part of the advanced thermal energy storage project carried out in collaboration with Abengoa Solar NT at CSIRO Energy Centre in Newcastle, Australia, with support through the Australian Renewable Energy Agency (ARENA). The cavity-type receiver with tubular absorbers was successfully installed and commissioned, using concentrated solar energy to raise the temperature of CO2 gas to 750 °C at 700 kPa in a pressurized, closed loop system. Stand-alone solar receiver tests were carried out to investigate the thermal characteristics of the 250 kWt solar receiver. The on-sun full-load test successfully achieved an outlet gas temperature of 750 °C while operating below the maximum allowable tube temperature limit (1050 °C) and with a maximum pressure drop of 22 kPa. The corresponding estimated receiver thermal efficiency values at full flow rate were 75% estimated based on measured receiver temperatures and heat losses calculations for both single aim-point and multiple aim-point heliostat control strategies. The use of a quartz glass window affixed to the receiver cavity aperture was tried as a means for improving the receiver efficiency by reducing convective heat losses from the receiver aperture. However, while it did appear to significantly reduce convective losses, a more effective metal support frame design is necessary to avoid damage to the window caused by stresses introduced as a result of distortion of the supports due to heating by the spillage of rays from the heliostat field.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041005-041005-11. doi:10.1115/1.4036545.

Buildings in major metropolitan centers face increased peak electrical load during the warm season, especially during extreme heat events. City-wide, the increased demand for space cooling can stress the grid, increasing generation costs. It is therefore imperative to better understand building energy consumption profiles at the city scale. This understanding is not only paramount for users to avoid peak demand charges but also for utilities to improve load management. This study aims to develop a city-scale energy demand forecasting tool using high resolution weather data interfaced with a single building energy model. The forecasting tool was tested in New York City (NYC) due to the availability of building morphology data. We identified 51 building archetypes, based on the building function (residential, educational, or office), the age of the building, and the land use type. The single building simulation software used is energyplus which was coupled to an urbanized weather research and forecasting (uWRF) model for weather forecast input. Individual buildings were linked to the archetypes and scaled using the building total floor area. The single building energy model is coupled to the weather model resulting in energy maps of the city. These maps provide an energy end-use profile for NYC for total and individual components including lighting, equipment and heating, ventilation, and air-conditioning (HVAC). The methodology was validated with single building energy data for a particular location, and with city-scale electric load archives, showing good agreements in both cases.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041006-041006-6. doi:10.1115/1.4036415.

In solar power plants, both in photovoltaic (PV) and concentrated ones, the electrical output is a key parameter for the development of solar energy. To ensure relevant predictability of electrical output, the durability of photovoltaic panels or concentrating systems has to be warranted. The assessment of the optical performance durability of the front glass throughout the lifetime of the solar power plant involves using a nondestructive method in the field without disrupting the energy generation of such systems. The aim of this work is to experiment a new accurate nondestructive method to evaluate the aging impact of glass used in solar energy conversion systems. The results bring out a correlation between the apparent emissivity, used as an aging indicator, in a spectral bandwidth of 8–12 μm and the integrated transmittance in the visible range, i.e., 400–800 nm for a float glass of 2 mm thickness aged under damp heat (DH). The optical characterizations of the soda-lime glass exposed to the DH test highlight the relevance of apparent emissivity used like a nondestructive aging indicator. The sensitivity coefficient of apparent emissivity, which is defined as the ratio of partial derivative of integrated transmittance (δT) to the partial derivative of apparent emissivity (δε), reaches 3.83, meaning that the apparent emissivity is three times more sensitive than the integrated transmittance for the case study.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041007-041007-8. doi:10.1115/1.4036635.

The thermal performance of an array of pressurized-air solar receiver modules integrated to a gas turbine power cycle is analyzed for a simple Brayton cycle (BC), recuperated Brayton cycle (RC), and combined Brayton–Rankine cycle (CC). While the solar receiver's solar-to-heat efficiency decreases at higher operating temperatures and pressures, the opposite is true for the power cycle's heat-to-work efficiency. The optimal operating conditions are achieved with a preheat stage for a solar receiver outlet air temperature of 1300 °C and an air cycle pressure ratio of 9, yielding a peak solar-to-electricity efficiency—defined as the ratio of the net cycle work output divided by the solar radiative power input through the receiver's aperture—of 39.3% for the combined cycle configuration.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041008-041008-10. doi:10.1115/1.4036636.

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.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041009-041009-12. doi:10.1115/1.4036726.

Recently, linear Fresnel reflectors (LFR) arouse an increasing interest by the scientific and industrial community and have had a really fast development in the domain of concentrated solar power (CSP). LFR is considered as a promising technology which could produce an optical performance lower than those of parabolic trough collector, but its component simplicity would allow high cost reductions in its manufacturing compared to high investment costs of parabolic troughs. The purpose of this paper is to analyze the optical performances of an LFR prototype developed in the framework of CHAMS project, Morocco. The development of this prototype comes to supply industrial applications needing heat at small to medium temperature levels. To achieve this objective, an optical code based on the Monte Carlo (MC) ray tracing technique was developed for optical optimization purposes. The developed code identifies geometrical parameters that have a greater influence on optical efficiency of the LFR system as the mirror spacing arrangement, the receiver height, the receiver geometrical configuration taking into account the secondary reflector shape, and the absorber tube diameter. An analysis is conducted to identify the contribution of each mode of optical losses (blocking, shading, cosine…) in the optical efficiency of the system. Then, an optimization procedure is applied to enhance the optical performances of the prototype.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041010-041010-12. doi:10.1115/1.4036773.

The quality of the stability of the nonconvective zone of a salinity-gradient solar pond (SGSP) is investigated for an operating protocol in which the flushing procedure exactly compensates for evaporation losses from the solar pond and its associated evaporation pond. The mathematical model of the pond uses simplified, but accurate, constitutive expressions for the physical properties of aqueous sodium chloride. Also, realistic boundary conditions are used for the behaviors of the upper and lower convective zones (LCZs). The performance of a salinity-gradient solar pond is investigated in the context of the weather conditions at Makkah, Saudi Arabia, for several thickness of upper convective zone (UCZ) and operating temperature of the storage zone. Spectral collocation based on Chebyshev polynomials is used to assess the quality of the stability of the pond throughout the year in terms of the time scale for the restoration of disturbances in temperature, salinity, and fluid velocity underlying the critical eigenstate. The critical eigenvalue is found to be real and negative at all times of year indicating that the steady-state configuration of the pond is always stable, and suggesting that stationary instability would be the anticipated mechanism of instability. Annual profiles of surface temperature, salinity, and heat extraction are constructed for various combinations for the thickness of the upper convective zone and storage zone temperature.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041011-041011-12. doi:10.1115/1.4036854.

In this research, an inclined three-dimensional nanofluid-based tube-on-sheet flat plate solar collector (FPSC) working under laminar conjugated mixed convection heat transfer is numerically modeled. The working fluid is selected to be alumina/water (Al2O3/water) and results from heat transfer, entropy generation, and pressure drop points of view are being presented for various prominent parameters, namely volume fraction, nanoparticles diameter, Richardson and Reynolds numbers. According to the simulations, Nusselt number decreases as the Richardson number or volume fraction of the nanofluid rises, whereas heat transfer coefficient experiences an augmentation when volume concentration and the Richardson number surge. Also, data reveal that total entropy generation rate of the system declines when the alumina/water nanofluid is utilized inside the system as the volume fraction or the Richardson number increases. Additionally, it is found that increasing the nanoparticle volume concentration or the Richardson number diminishes the pressure drop considerably, whereas friction factor substantially proliferates as the Richardson number or volume fraction rises. Eventually, employment of larger alumina nanoparticles mean diameter eventuates in providing lower Nusselt number and apparent friction factor while it increases the pressure drop and heat transfer coefficient. Finally, comparing the efficiency of the presented FPSC design with those available in the literature shows a superior performance by the present design with its maximum occurring at 2 vol %.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041012-041012-9. doi:10.1115/1.4036774.

In this paper, a theoretical model of solar chimney power plants (SCPPs) is presented based on compressible ideal gas law assumptions. The theoretical optimal turbine pressure drop factors (TPDFs) for constant and nonconstant densities (CD and NCD) are studied, and the effects of flow area parameters examined. Results show that the theoretical optimal TPDF for CD is equal to 2/3 and is independent of the flow area parameters. Results also show that the theoretical optimal TPDF for NCD is close to 1 and is affected by the flow area parameters. However, the theoretical maximum fluid power (MFP) obtained for NCD is never attained in real life. For the actual states, the theoretical optimal TPDF for NCD is still effectively high enough. The TPDF and the fluid power for NCD increase with the reduction of the collector inlet area, and more precisely with the reduction of the chimney inlet area. The TPDF and the fluid power definitely increase with larger chimney flow area. The increase in the fluid power due to shape optimization of the SCPP is limited compared to that due to higher input heat flux of collector. Divergent-top and upward slanting roof shapes are recommended for the solar chimney and the solar collector, respectively, for better SCPP performance. Additionally, locations exposed to strong solar radiation are preferred for SCPPs.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041013-041013-9. doi:10.1115/1.4036775.

The fluid flow characteristics and heat transfer in triangular duct solar air heater (SAH) have been studied experimentally and numerically for Reynolds number range from 4000 to 18,000. In the present paper, three different models of triangular duct solar air heater were considered, namely, model 1 with simple triangular duct, model 2 with rounded corner on one side of the triangle with fixed radius of curvature of 0.39 times the duct height as flow passage, and model 3 with rounded corner on one side of the triangular duct with roughness on the absorber plate of SAH. The absorber plate and apex angle values are assumed as constant in all the three models of SAH, i.e., 160 mm and 60 deg, respectively. The three-dimensional numerical simulations were performed by discretization of computational domain using finite volume method (FVM) and are analyzed with the help of computational fluid dynamics (CFD) code. Experiments were performed to validate numerical results by comparing absorber plate temperature along the length of the SAH. A detailed analysis of different models of solar air heater was carried out by solving flow governing equations numerically on ansys fluent 12.1. A close match has been observed between the simulated and experimental results of SAH with maximum percentage deviation of approximately ±5% in absorber plate temperature. The rounded apex improves velocity distribution near the corner region and helps in improving heat transfer. In the three studied models of solar air heater, the best performance is observed in the case of model 3.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):041014-041014-7. doi:10.1115/1.4036934.

A new anidolic parametric trough solar collector (PmTC) having 8.12 m net width aperture has been recently proposed for a commercial evacuated receiver tube with an absorber diameter of 70 mm. Since the collector was designed ignoring transmission, absorption, and reflection optical losses, calculations of the optical efficiency and the incidence angle modifier (IAM) by means of Monte Carlo spectral raytracing simulations using real slope errors distributions and taking into account Fresnel reflection losses were done. Comparison with an Eurotrough parabolic trough collector (PTC) shows an optical penalization of 5.1% due to the reflectivity and additional soiling of the secondary mirror, to an increase in the end losses and to the Fresnel reflection losses. The National Renewable Energy Laboratory (NREL) system advisor model (SAM) was used to perform annual simulations of two commercial 50 MWe oil power plants without thermal energy storage located in Seville. A PTC solar field consisting of 90 loops, each one having four Eurotrough solar collector assemblies (SCA) with 150 m length was first modeled resulting in a gross production of 386 kWh/(m2 yr). A PmTC solar field with the same module length and similar SCA net aperture area was also simulated. A final configuration of 94 loops and four SCAs with 100 m length per loop yields a gross production of 379 kWh/(m2 yr) showing no improvement compared to the reference PTC plant. The present study allows to advance in the understanding of the potential of the anidolic optic to produce optical geometries able to effectively improve the PTC technology in the short-term projecting results at a commercial plant level.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2017;139(4):044501-044501-4. doi:10.1115/1.4036637.

This study compares the potential annual energy absorption of a flat-plate solar collector at different tilt angles in Poland. Optimal tilt angles were tested in three variants: over the course of the year, in fall/winter and in spring/summer. The results were compared with automatically tracked collectors where the active surface is perpendicular to the angle at which solar radiation reaches the collector. The results were simulated based on the meteorological data. A comparison of the energy outputs of solar collectors in optimization variants 1, 2, and 3 indicates that variant 1 produces the highest energy output.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(4):044502-044502-4. doi:10.1115/1.4036725.

Parabolic trough solar concentrating technology is a new and clean way to replace the conventional fossil fuel technology to generate steam for heavy oil recovery in oilfield. A computational model was constructed with simulated direct normal irradiance from nearby similar climate locations. Different system configurations were analyzed with the model, such as with single- and dual-loop, with and without heat storage system. Finally, several solar field configurations with different collector field layouts were compared by the cost of unit generated steam. Results show that using heat storage can effectively improve the stability of steam production, and in a certain oilfield, an optimum steam production amount and optimum heat storage time (HST) exist for lowest steam cost. The methods and results in the paper provide useful suggestions for the implementation of a solar thermal oilfield steam production system.

Commentary by Dr. Valentin Fuster

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