Research Papers

J. Sol. Energy Eng. 2017;140(2):021001-021001-8. doi:10.1115/1.4038591.

According to modern grid codes (GCs), high penetration of photovoltaic power plants (PVPPs) to the utility grid requires a reliable PV generation system by achieving fault ride-through (FRT) requirements. In order to meet these requirements, there are two major issues that should be addressed to keep the inverter connected during grid fault. The two issues are the ac over-current and dc-link over-voltage that may cause disconnection or damage to the grid inverter. In this paper, the control of single-stage PVPP inverters is developed to address these issues and enhance FRT capability. The proposed control scheme introduces the dc brake chopper circuit and current limiter to protect the inverter and ride through the fault smoothly with no perceptible overcompensation. A 1.5 MW PVPP connected into the Malaysian grid and modeled in simulink is utilized to explain the proposed control scheme. The simulation results presented demonstrate the effectiveness of the overall proposed control strategy to ride through different types of faults and to help to ensure the safety of the system equipment.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;140(2):021002-021002-10. doi:10.1115/1.4038621.

Photovoltaic/thermal (PV/T) collector is a novel collector which incorporates photovoltaic power generation and low-temperature heat utilization of solar energy. In this paper, a three-dimensional (3D) physical model of flat-box PV/T collector is established in the cfd software. The effects of different tube heights, flow rates, inlet temperature, wind speed, and ambient temperature were tested. By analyzing and comparing the simulated and experimental results, the relative errors of the thermal efficiency between the simulated and experimental values are smaller than those of the electric efficiency. According to the experiment, when the water flow is 210 L/h, the average outlet temperature is 37.598 °C, and the thermal and electric efficiencies are 52.524% and 10.064%, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021003-021003-11. doi:10.1115/1.4038811.

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.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021004-021004-6. doi:10.1115/1.4038786.

The potential applicability of a developed recycled textile material, based on acrylic spinning waste, as thermal insulation is conducted. The prepared acrylic spinning waste (AS) is thermo-physically characterized in terms of density, air permeability, and thermal conductivity. The results show that the density and air permeability are 10.583 kg/m3 and 1100 L/m2/s, respectively. In addition, the thermal conductivity is found to be 38.27 mW/(m K). The developed thermal insulator is then tested in a thermally controlled reduced scale cavity. Two walls of the cavity are outfitted with AS at two different locations and compared to the walls without AS. The comparison is made based on the wall surface temperature and heat flux. A reduction in surface temperature is observed in the walls outfitted with AS, compared to wall without AS. Indeed, compared to a control wall, the peak heat fluxes are reduced by 27.23% and 18.67%, respectively, related to the walls with AS at location 1 and location 2. The obtained results show that the AS is a competitive thermal insulation material and can increase the thermal performance of the building walls.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021005-021005-5. doi:10.1115/1.4038787.

The increase of operating temperature on a photovoltaic (PV) cell degrades its electrical efficiency. This paper is organized to describe our latest design of an aluminum substrate—based photovoltaic/thermal (PV/T) system. The electrical efficiency of the proposed PV/T can be increased by ∼ 20% in comparison with a conventional glass substrate-based PV. The work will benefit hybrid utilization of solar energy in development of building integrated photovoltaic systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021006-021006-9. doi:10.1115/1.4038849.

This research demonstrates scale-up studies with the development of concentrating and nonconcentrating solar reactors employing suspended and supported TiO2 for the degradation of herbicide isoproturon (IPU) with total working volume of 6 L. Novel cement beads were used as support material for fixing the catalyst particles. In the case of nonconcentrating slurry reactor, 85% degradation of IPU was achieved after 3 h of treatment with four number of catalyst recycling, whereas nonconcentrating fixed-bed reactor using TiO2 immobilized cement beads took relatively more time (10 h) for the degradation of IPU (65%) due to mass transfer limitations, but it overcame the implication of catalyst filtration post-treatment. The immobilized catalyst was successfully recycled for ten times boosting its commercial applications. High photon flux with concentrating parabolic trough collector (PTC) using fixed catalysis approach with same immobilized catalyst substantially reduced the treatment time to 4 h for achieving 91% degradation of IPU. Working and execution of pilot-scale reactors are very fruitful to extend these results for a technology development with the present leads.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021007-021007-14. doi:10.1115/1.4038620.

This work describes a new simple and effective method to extract the loss parameters of solar panels (solar cells) and able to accurately represent their electrical behavior. This approach allows the extraction of the parameters of the single diode model using only the information provided by the manufacturer's data sheet. The proposed method presents a computational procedure of low complexity, which makes it possible to estimate the five parameters of any photovoltaic generator. Using the complete equation of the single diode model, the number of parameters to be calculated is reduced only to two parameters by an equation exclusively connecting the series resistance and the diode current. Suitable validations on important case studies are presented; an experimental data from multicrystalline MSX120 and thin film NA-F135 solar panels were used to test the single diode model with the extracted parameters. The experimental data are first collected at the same temperature at two different irradiances levels and at low irradiance level at a fixed temperature for MSX120. In the second stage, variations in temperature are considered at different irradiance level for NA-F135. The extraction results show that the I–V curves accurately fit the entire range of the experimental data. In addition, the results of the proposed procedure are compared to the most recent proposed techniques in literature. Furthermore, the results obtained show a highly accurate; in particular, at maximum power point (MPP), the error is always less than 0.005%, which is quite far of the authorized error of 1%.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021008-021008-9. doi:10.1115/1.4038961.

Variation in direct solar radiation is one of the main disturbances that any solar system must handle to maintain efficiency at acceptable levels. As known, solar radiation profiles change due to earth's movements. Even though this change is not manipulable, its behavior is predictable. However, at ground level, direct solar radiation mainly varies due to the effect of clouds, which is a complex phenomenon not easily predictable. In this paper, dynamic solar radiation time series in a two-dimensional (2D) spatial domain are obtained using a biomimetic cloud-shading model. The model is tuned and compared against available measurement time series. The procedure uses an objective function based on statistical indexes that allow extracting the most important characteristics of an actual set of curves. Then, a multi-objective optimization algorithm finds the tuning parameters of the model that better fit data. The results showed that it is possible to obtain responses similar to real direct solar radiation transients using the biomimetic model, which is useful for other studies such as testing control strategies in solar thermal plants.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021009-021009-8. doi:10.1115/1.4039077.

Photovoltaic (PV) hybrid systems and their optimal energy management are still being actively studied. PV systems must be positioned at optimal tilt and azimuth angles to obtain maximum system performance. This paper presents the effect of energy management of a grid-connected PV-battery-load system on optimal PV placement using a linear programing (LP) method. The optimal placement of PV arrays aims to minimize the electricity cost of a fixed PV system for investors by considering monthly average daily global radiation, ambient temperature, wind speed, demand management constraints, and electricity tariffs in Turkey. The analysis is extended to consider 68 locations across Turkey. The analysis reveals that the optimal placement (optimal tilt and azimuth angle) can be different from that of previous studies because of multiple objective considerations. The optimal tilt angle results are lower than the local latitude in the range of 0 deg–8 deg in all regions. Furthermore, the optimal azimuth angle results are in the range of 6 deg–24 deg westbound throughout Turkey. These results can play a significant role for investments in the design of grid-connected PV-battery-load systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021010-021010-9. doi:10.1115/1.4039094.

The performance of photovoltaic (PV) systems is highly influenced by the tilt angle of PV modules and the incidence of global solar irradiance, which may change the solar to electrical conversion efficiency. Some authors have addressed these uncertainties arising from PV solar generation by using mechanisms and methods in which solar tracking systems are integrated to PV systems. Since the advent of the internet of things (IoT), this solar tracking strategy has yet to meet the requirements of scalable distributed power systems that can seamlessly support the PV solar generation, mainly for remote monitoring and control. In this context, this paper aims at developing a prospective study devoted to examine fundamental concepts to implement solar tracking algorithms based on local solar time by using embedded technology from the IoT platform. Preliminary results evidenced an improvement of up to 38% in power generation performance for algorithm-driven PV modules compared to fixed PV modules.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021011-021011-8. doi:10.1115/1.4038983.

A direct methodology for intra-day forecasts (1–6 h ahead) of power output (PO) from photovoltaic (PV) solar plants is proposed. The forecasting methodology uses publicly available images from geosynchronous satellites to predict PO directly without resorting to intermediate irradiance (resource) forecasting. Forecasts are evaluated using four years (January 2012–December 2015) of hourly PO data from 2 nontracking, 1 MWp PV plants in California. For both sites, the proposed methodology achieves forecasting skills ranging from 24% to 69% relative to reference persistence model results, with root-mean-square error (RMSE) values ranging from 90 to 136 kW across the studied horizons. Additionally, we consider the performance of the proposed methodology when applied to imagery from the next generation of geosynchronous satellites, e.g., Himawari-8 and geostationary operational environmental satellite (GOES-R).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):021012-021012-8. doi:10.1115/1.4039024.

The primary objective of this study was to develop a fiber-optic hybrid day-lighting system for mobile application such as military shelters in order to cut energy use and the use of fossil fuels. The scope included the design, development, and testing of a hybrid lighting system that is capable of producing about 16,000 lm output with design challenges including light-weight, compactness, and optics that can tolerate a high tracking misalignment. The designed system is comprised of two subsystems: the solar collector and the solar hybrid lighting fixture (SHLF). The solar collector, consists of a housing, a structural stand (tripod), a dual axis tracking system, Fresnel lenses, secondary optics, and fiber-optic cables. The collector is a telescoping aluminum box that holds eight 10-in diameter Fresnel lenses, which focus sunlight onto eight secondary optics and deliver uniform light to the fiber-optic cables. The secondary optics have filters to block UV/IR. The optics has been designed to have a high half-acceptance angle of 1.75 deg and can accommodate a tracking accuracy of 1.50 deg or better. This novel SHLF consists of two components: a solar fiber-optic system and a light emitting diode (LED) system. The fiber-optic cable is coupled to an acrylic light diffusing rod that delivers the sunlight into the room. During sunny periods, the solar fiber-optic lighting could provide full illumination level. In order to keep the same level of lighting during cloudy periods, the LED portion of the light fixture can supplement the output of the SHLF. A compact, light-weight prototype system was built and tested. The results showed that the system's output per lens for the 20 ft cable was about 1750±50 lm at a global solar illuminance of 115,000 lx. The total system was capable of delivering 14,000 lm of sunlight.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2018;140(2):024501-024501-6. doi:10.1115/1.4038788.

This study addresses the method of adding heat to a salt gradient solar pond (SGSP) from external sources and investigates the thermal performance of the pond. In this case, the external heat source is solar heat collected by evacuated tube solar collectors (ETSC), and collected heat is transferred to the lower-convective zone (LCZ) of the SGSP by circulating fluid from the LCZ. Results show that heat addition from the external source enhances the thermal performance of the SGSP in terms of heat recovery and thermal efficiency but with certain constraints. The heat addition efficiency reduces with increase in aperture area of the ETSC. Also with increasing heat addition, the heat removal from the SGSP has to be increased; otherwise, the SGSP efficiency reduces rapidly. Heat removal from SGSP has to be performed keeping in mind the heat demand and the quality of heat. The latter reduces with an increase of heat extraction beyond a certain limit. Hence, optimizing the range of parameters in case of adding heat from external sources is very important for the best performance of a SGSP.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):024502-024502-7. doi:10.1115/1.4038962.

This technical brief develops a theoretical model of all the pressure losses in the solar chimney power plant (SCPP, also called solar updraft power plant) and analyzes the pressure losses for different chimney internal stiffening appurtenance (SA) structures, different roof heights, and different collector support parameters. Results show that the exit dynamic pressure drop (EDPD) accounts for the majority of the total pressure loss (TPL), while other losses constitute only small proportions of the TPL, and the collector inlet loss is negligible. Pressure losses are strongly related to the mass flow rate, while reasonable mass flow rates excluding too low flow rates have little influence on the pressure loss ratios (PLRs, defined as the ratios of the pressure losses to the TPL) and the total effective pressure loss coefficient (TEPLC). Designing of the SA structure in view of reducing the drag, for example, using the ring stiffeners without wire spoked instead of the spoked bracing wheels (SBWs), reducing the width of the chimney internal rims of SAs, or reducing the number of SAs results in large reduction of the SA PLR and the TPL. Lower roof leading to higher velocity inside the collector, larger supports, or shorter intersupport distance leads to the increase in the support PLR. This technical brief lays a solid foundation for optimization of SCPPs in future.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(2):024503-024503-6. doi:10.1115/1.4039097.

The world energy consumption has exhibited high growth over the last several decades. Alternative energy sources like photovoltaic (PV) systems generate electricity, reduce pollution air, and have little environmental impact. The commonly used fixed-tilt solar panels, however, have low efficiency and high production cost. Thus, it takes a long time to obtain a return on the investment. Solar trackers increase the efficiency of PV systems and are therefore more attractive from a financial point of view. In order to design tracking systems that will be efficient, it is necessary to analyze the results during various periods during the year and over their lifespan. Thus, we performed a comparative study between fixed-tilt panels and the tracking system installed in Lodz, Poland. Fixed-tilt panels are at normal to the Earth's surface (90 deg from horizontal plane) and are attached to a building façade, azimuth 180 deg (S direction) with 15 cm ventilation gap so slight efficiency drop may be presumed. We performed short- and long-term analyses of the solar tracking and fixed-tilt systems, which allowed us to conclude that the panels tracking the sun had an additional gain of energy during the year as compared to the fixed-tilt panels. During some months, however, the solar tracking system did not produce as much energy as the fixed-tilt, vertically positioned panels. These results might be useful in designing and constructing solar tracking PV systems.

Commentary by Dr. Valentin Fuster

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