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IN THIS ISSUE

### Research Papers

J. Sol. Energy Eng. 2016;139(2):021001-021001-6. doi:10.1115/1.4035059.

In general, architectural design is a loosely structured, open-ended activity that includes problem definition, representation, performance evaluation, and decision making. A number of approaches have been proposed in the literature to organize, guide, and facilitate the design process. The main objective of this paper is to seek a logical and rigorous means to aid in developing an optimized design that is acceptable to the customer or user of the product. The convention design approaches heavily involve decision making, which is integral to the architectural design process and is an important element in nearly all phases of design. There is a need to reframe the decision-making process to transform and improve the design process in order for finial building to achieve the performance goals. The first step in making an effective design decision is to understand the stakeholders' and team players' (architect, engineer, client, and consultant) different preferences based on their needs, experiences, and expectations of the project. In this paper, we first provide an overview about conventional decision-making method and process, identify the existing attributes that contribute to decision making in design, and outline the obstacles present in making optimized sustainable design decisions due to the uncertainty of different stakeholders' preferences. Then, we present one case study to identify and compare different preferences among engineering students, practicing architects, and the general public, and we analyze how the three groups attribute different weight to the major design attributes. This paper provides some novel insights into a value-driven sustainable design process, and it will be one of the building blocks for creating a framework to integrate game theory into the design decision-making process, considering multiple stakeholders' perspectives and preferences for building attributes as future research tasks.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021002-021002-9. doi:10.1115/1.4035066.

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.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021003-021003-7. doi:10.1115/1.4035024.

Reverse natural air convection (hot plate top) was experimentally investigated between two inclined parallel aluminum plates (1 m × 2 m × 3 mm) with a separation distance of 20 mm to 100 mm. The inclination ϑ to the horizontal was varied from 0 deg to 90 deg. The mean temperatures of the plates have been adjusted to 90 °C and 30 °C resulting in Rayleigh numbers Ra between 2.7 × 104 and 3.3 × 106. The experimental conditions correspond to the back side of an absorber in a typical solar flat-plate collector, where the conventional insulation has been removed. The upper hot plate simulates the absorber and was electrically heated by an area heater, while the temperature distribution over the plate was measured. The lower cold plate was held isothermally by integrated water tubes and a thermostat. The side walls of the rectangular cavity were thermally connected to the colder plate and had a distance of 10 mm to the hot plate, comparable to a typical collector casing. The experimentally obtained results for Nu (Ra,ϑ) were mathematically described and compared to rare reverse convection data of other authors, gained at smaller aspect ratios/flow lengths and for adiabatic side walls: The formula of Elsherbiny approximately (within 10%) describes solar flat-plate collectors between 0 deg and 60 deg inclination, while the relations of Arnold, Ozoe, and Inaba show large errors up to 50%. Additionally, we experimentally showed that pure air gap insulation (30–50 mm) has surprisingly acceptable loss coefficients between 1.3 and 2.5 W/m2K depending on collector slope. It can be used as a cheap insulation method for low temperature collector applications. Additionally, inserting an 25–50 μm thick aluminum film symmetrically between the plates, a new and efficient insulation method for the absorber of a solar flat-plate collector was experimentally investigated: At plate distances of 30–50 mm, temperatures below 100 °C and slopes below 45 deg, this compact and cheap film insulation was proven to be equivalent to dry mineral wool and avoids its disadvantage of worsening insulation properties due to humidity.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021004-021004-14. doi:10.1115/1.4034642.

The dynamic performance of a thermal energy storage tank containing phase change material (PCM) cylinders is investigated computationally. Water flowing along the length of the cylinders is used as the heat transfer fluid. A numerical model based on the enthalpy-porosity method is developed and validated against experimental data from the literature. The performance of this hybrid PCM/water system was assessed based on the gain in energy storage capacity compared to a sensible only system. Gains can reach as high as 179% by using 50% packing ratio and 10 °C operating temperature range in water tanks. Gains are highly affected by the choice of PCM module diameter; they are almost halved as diameter increases four times. They are also affected by the mass flow rate nonlinearly. A nondimensional analysis of the energy storage capacity gains as a function of the key nondimensional parameters (Stefan, Fourier, and Reynolds numbers) as well as PCM melting temperature was performed. The simulations covered ranges of 0.1 <  $Stẽ$  < 0.4, 0 < Fo < 600, 20 < Re < 4000, $0.2<(ρCP)*<0.8$, and $0.2<θm<0.8$.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021005-021005-8. doi:10.1115/1.4034910.

A thermal-to-acoustic energy converter (TAC) was developed and tested to produce sound waves in the kilohertz range directly from solar energy. The converter consisted of a glass window and a small amount of steel wool in the shape of a disk sealed in an aluminum housing. A Fresnel lens and a chopper wheel with 60 holes in it were employed to generate a pulsed sunbeam of approximately 200 sun intensity as the heat source of the TAC. Various designs and techniques were tested to improve the sound amplitude and signal-to-noise ratio of the converter at high frequencies. Reduction in air volume, better cooling, and improvement in air tightness were found to be effective in enhancing the sound amplitude. A shockproof mount commonly used in radio studios to reduce microphone vibration was essential in noise reduction for the TAC at high chopper wheel rotations. The sound amplitude was found to rapidly decrease with the increase in pulse frequency of the sunbeam at low frequencies. The relationship between the decibel value and frequency of the generated sound waves was changed to linear for sunbeam frequencies above 1 kHz. This is the frequency at which the penetration of surface temperature fluctuations into the aluminum housing becomes comparable with the aluminum housing thickness. At a given frequency, the sound amplitude increased almost exponentially with the increase in solar flux intensity. To the best of our knowledge, the 3 kHz sound frequency measured in our experiments is by far the highest frequency produced by a solar-to-acoustical energy converter.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021006-021006-7. doi:10.1115/1.4034928.

Quite a few computer programs have been developed to model power plant performance. These software codes are geared toward modeling steady-state operations, which are usually sufficient for conventional power plants. Solar thermal power plants undergo prolonged transient start-up and shut-down operations due to the periodic nature of solar radiation. Moreover, the large size of the solar field brings about large residence time that must be considered to accurately lag power generation. A novel scheme has been developed to fine-tune steady-state solar power generation models to accurately take account of the impact of those transient operations. The suggested new scheme is implemented by adjusting solar radiation data input to the model and has been shown to clearly improve modeling accuracy by moving modeled results closer to matching real operating data.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021007-021007-11. doi:10.1115/1.4034912.

In order to reach the targets on emissions set by the European Commission, both new and existing buildings must reduce their fossil fuel inputs. Solar thermal cooling supplying on-site renewable heating and cooling could potentially contribute toward this goal. In this paper, a novel concept for solar thermal cooling providing efficient coproduction of cooling and heating based on sorption integrated vacuum tube collectors is proposed. A prototype collector has been constructed and tested in a solar laboratory based on a method developed specifically for sorption integrated collectors. From the test results, the key performance parameters have been determined and used to calibrate a mathematical model for trnsys environment. System simulation has been conducted to optimize the collector and sorption module configuration by performing a parametric study where different vacuum tube center–center (C–C) distances and sorption module designs are tested for a generic hotel in Ankara, Turkey. The parametric study showed that the heating and cooling output per year can be as high as 1000 kWh/m2 for solar fractions above 50%, and that the output per sorption module compared to the prototype can more than double with an optimized design. Furthermore, cooling conversion efficiencies defined as total cooling output per total solar insolation can be as high as 26% while simultaneously converting 35–40% of the incident solar energy into useful hot water.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021008-021008-11. doi:10.1115/1.4035163.

The working principle of particle-based solar receivers is to utilize the absorptivity of a dispersed particle phase in an otherwise optically transparent carrier fluid. In comparison to their traditional counterparts, which use a solid surface for radiation absorption, particle-based receivers offer a number of opportunities for improved efficiency and heat transfer uniformity. The physical phenomena at the core of such receivers involve coupling between particle transport, fluid turbulence, and radiative heat transfer. Previous analyses of particle-based solar receivers ignored delicate aspects associated with this three-way coupling. Namely, these investigations considered the flow fields only in the mean sense and ignored turbulent fluctuations and the consequent particle preferential concentration. In the present work, we have performed three-dimensional direct numerical simulations of turbulent flows coupled with radiative heating and particle transport over a range of particle Stokes numbers. Our study demonstrates that the particle preferential concentration has strong implications on the heat transfer statistics. We demonstrate that “for a typical setting” the preferential concentration of particles reduces the effective heat transfer between particles and the gas by as much as 25%. Therefore, we conclude that a regime with Stokes number of order unity is the least preferred for heat transfer to the carrier fluid. We also provide a 1D model to capture the effect of particle spatial distribution in heat transfer.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021009-021009-5. doi:10.1115/1.4034908.

Natural dye extract of the saffron petal, purified by solid-phase extraction (SPE) technique, has been studied as a novel sensitizing dye to fabricate TiO2 nanoparticles-based dye-sensitized solar cells (DSSC). The extract was characterized using ultraviolet–visible (UV–Vis) and Fourier transform infrared (FTIR) spectroscopies to confirm the presence of anthocyanins in saffron petals. The typical current–voltage and the incident photon to current efficiency (IPCE) curves were also provided for the fabricated cell. The saffron petal extract exhibited an open-circuit voltage (Voc) of 0.397 V, short circuit current density (Jsc) of 2.32 mA/cm2, fill factor (FF) of 0.71, and conversion efficiency of 0.66%, which are fairly good in comparison with the other similar natural dye-sensitized solar cells. These are mainly due to the improved charge transfer between the dye extract of saffron petal and the TiO2 anode surface. Considering these results, it can be concluded that the use of saffron petal dye as a sensitizer in DSSC is a promising method for providing clean energy from performance, environmental friendliness, and cost points of view.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021010-021010-12. doi:10.1115/1.4034823.

Photovoltaic (PV) power forecasting has the potential to mitigate some of effects of resource variability caused by high solar power penetration into the electricity grid. Two main methods are currently used for PV power generation forecast: (i) a deterministic approach that uses physics-based models requiring detailed PV plant information and (ii) a data-driven approach based on statistical or stochastic machine learning techniques needing historical power measurements. The main goal of this work is to analyze the accuracy of these different approaches. Deterministic and stochastic models for day-ahead PV generation forecast were developed, and a detailed error analysis was performed. Four years of site measurements were used to train and test the models. Numerical weather prediction (NWP) data generated by the weather research and forecasting (WRF) model were used as input. Additionally, a new parameter, the clear sky performance index, is defined. This index is equivalent to the clear sky index for PV power generation forecast, and it is here used in conjunction to the stochastic and persistence models. The stochastic model not only was able to correct NWP bias errors but it also provided a better irradiance transposition on the PV plane. The deterministic and stochastic models yield day-ahead forecast skills with respect to persistence of 35% and 39%, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021011-021011-9. doi:10.1115/1.4035258.

Falling particle receivers are being evaluated as an alternative to conventional fluid-based solar receivers to enable higher temperatures and higher efficiency power cycles with direct storage for concentrating solar power (CSP) applications. This paper presents studies of the particle mass flow rate, velocity, particle-curtain opacity and density, and other characteristics of free-falling ceramic particles as a function of different discharge slot apertures. The methods to characterize the particle flow are described, and results are compared to theoretical and numerical models for unheated conditions. Results showed that the particle velocities within the first 2 m of release closely match predictions of free-falling particles without drag due to the significant amount of air entrained within the particle curtain, which reduced drag. The measured particle-curtain thickness (∼2 cm) was greater than numerical simulations, likely due to additional convective air currents or particle–particle interactions neglected in the model. The measured and predicted particle volume fraction in the curtain decreased rapidly from a theoretical value of 60% at the release point to less than 10% within 0.5 m of drop distance. Measured particle-curtain opacities (0.5–1) using a new photographic method that can capture the entire particle curtain were shown to match well with discrete measurements from a conventional lux meter.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021012-021012-7. doi:10.1115/1.4035328.

Within the solar energy technologies, the hybrid photovoltaic–thermal (PVT) systems offer an attractive option because the absorbed solar radiation is converted into thermal and electrical energies (the conversion can be done separately or simultaneously). In this study, an attempt has been made to evaluate the theoretical and practical performances and evaluation of a hybrid PVT collector based on a new integrated absorber configuration function of climatic and design parameters. Our objective is to obtain a more efficient use of solar energy by cheaper materials and simpler implementation. On the first hand, we considered two different configurations of hybrid collectors which are defined as PVT water with absorber in parallel vertical tubes (model I) and a PVT with absorber in an enclosure (model II). On the second hand, we presented a new integrated absorber configuration for hybrid collector; then we compared it to the two previous models. The last proposed design has the advantage of a simpler implementation and a lower cost compared to other configurations of PVT hybrid collectors. A computer simulation program has been developed in order to calculate the thermal and electrical parameters of the PVT–water collector. The obtained simulation results are found to be in good agreement with the experimental measurements. For a sample climatic, operating, and design parameter, the calculated thermal and electrical energies of the new configuration of PVT are about 125.36 W and 40 W, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021013-021013-12. doi:10.1115/1.4035152.

The modern concepts of sustainable cities and smart grids have caused an increase in the installation of solar systems in urban and suburban areas, where, due to the presence of many obstacles or design constraints, photovoltaic (PV) modules can operate in operating conditions that are very different from the optimal ones (e.g., standard test conditions, STC). Shading and reflection are the main phenomena that cause uneven distribution of irradiance on PV cells; in turn, they create a nonuniform distribution of PV cell temperatures. The latter problem can also be caused by different ventilation regimes in various parts of the PV array. On the other hand, due to the need to exploit different solar technologies (solar thermal and photovoltaic), problems related to the availability of a useful surface can arise. In this context, there is a technology that produces heat end electrical energy at the same time, such a technology is referred to as a solar hybrid photovoltaic/thermal (PV/T). Here, the uneven distribution of temperature is a design input and its effect depends on both path of the water flow and the PV cell connections. To study the electrical behavior of a PV array under mismatching conditions, a suitable matlab/simulink model has been developed. The model has been tested both numerically and experimentally. Finally, an application of this model in the electrical analysis of a PV/T module is reported, and the results are discussed.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021014-021014-7. doi:10.1115/1.4035329.

This paper describes a perturb-and-observe (P&O) control aiming to increase the heat exchange between solar heat stored underground and the ambient of a single conditioned room without any heat pump. This P&O control increases or decreases the water flow rate through an underground hosepipe heat exchanger. With this purpose, two power converters were used to activate, respectively, a low power water pump and a fan coil so as to keep the room within the limits of a reference temperature range (between 18 °C and 24 °C). Outside these limits, the P&O control searches for the best heat exchange between the ambient room and the underground soil and, when inside these limits, the water pump and fan coil are turned off. Two identical experimental rooms, referred in this study as “reference” and “test” rooms, had their temperatures measured every 1-min during winter and summer. For comparison purposes, the reference room was left at its natural conditions without any air conditioning. The experimental results show a remarkable improvement in the heat exchange and a considerable reduction in power demand when using the P&O control. As a result, it was obtained an energy saving of approximately 45% in one summer day and 22% in one winter day. It is important to point out that this paper refers, strictly, to the description of a P&O control for heat exchange systems involving solar heat stored underground in a single room.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;139(2):021015-021015-13. doi:10.1115/1.4035330.

This work reports a numerical investigation of the transient operation of a 100-kWth solar reactor for performing the high-temperature step of the Zn/ZnO thermochemical cycle. This two-step redox cycle comprises (1) the endothermal dissociation of ZnO to Zn and O2 above 2000 K using concentrated solar energy, and (2) the subsequent oxidation of Zn with H2O/CO2 to produce H2/CO. The performance of the 100-kWth solar reactor is investigated using a dynamic numerical model consisting of two coupled submodels. The first is a Monte Carlo (MC) ray-tracing model applied to compute the spatial distribution maps of incident solar flux absorbed on the reactor surfaces when subjected to concentrated solar irradiation delivered by the PROMES-CNRS MegaWatt Solar Furnace (MWSF). The second is a heat transfer and thermochemical model that uses the computed maps of absorbed solar flux as radiation boundary condition to simulate the coupled processes of chemical reaction and heat transfer by radiation, convection, and conduction. Experimental validation of the solar reactor model is accomplished by comparing solar radiative power input, temperatures, and ZnO dissociation rates with measured data acquired with the 100-kWth solar reactor at the MWSF. Experimentally obtained solar-to-chemical energy conversion efficiencies are reported and the various energy flows are quantified. The model shows the prominent influence of reaction kinetics on the attainable energy conversion efficiencies, revealing the potential of achieving ηsolar-to-chemical = 16% provided the mass transport limitations on the ZnO reaction interface were overcome.

Topics: Solar energy
Commentary by Dr. Valentin Fuster

### Discussion

J. Sol. Energy Eng. 2016;139(2):025501-025501-6. doi:10.1115/1.4034973.

The amount and quality of the energy converted by a photovoltaic system connected to the grid can be evaluated by experimental monitoring or computer simulation. The Solar Energy Laboratory at UFRGS developed a simulation software for analysis of grid connected photovoltaic systems (FVCONECT). In order to perform a reliable simulation, it is required for the implementation of suitable mathematical models that describe the behavior of each system component. The inverter is the equipment responsible for converting DC to AC. The manufacturers provide some technical parameters for the inverters. However, electrical and thermal characteristics require mathematical models which coefficients must be obtained from specific tests. This work presents a methodology for analysis of thermal behavior of inverters. Such analysis requires experimental determination of two thermal coefficients. Energy losses due to inverters overheating can be calculated through the proposed methodology, providing a more accurate simulation of a determined photovoltaic (PV) system. The proposed methodology has been tested in several inverters, providing good results.

Commentary by Dr. Valentin Fuster