Showing papers in "Solar Energy in 2009"

On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/ power correlations

Abstract: A brief discussion is presented regarding the operating temperature of one-sun commercial grade silicon-based solar cells/modules and its effect upon the electrical performance of photovoltaic installations. Suitable tabulations are given for most of the known algebraic forms which express the temperature dependence of solar electrical efficiency and, equivalently, solar power. Finally, the thermal aspects of the major power/energy rating methods are briefly discussed.

Online short-term solar power forecasting

Abstract: This paper describes a new approach to online forecasting of power production from PV systems. The method is suited to online forecasting in many applications and in this paper it is used to predict hourly values of solar power for horizons of up to 36 h. The data used is 15-min observations of solar power from 21 PV systems located on rooftops in a small village in Denmark. The suggested method is a two-stage method where first a statistical normalization of the solar power is obtained using a clear sky model. The clear sky model is found using statistical smoothing techniques. Then forecasts of the normalized solar power are calculated using adaptive linear time series models. Both autoregressive (AR) and AR with exogenous input (ARX) models are evaluated, where the latter takes numerical weather predictions (NWPs) as input. The results indicate that for forecasts up to 2 h ahead the most important input is the available observations of solar power, while for longer horizons NWPs are the most important input. A root mean square error improvement of around 35% is achieved by the ARX model compared to a proposed reference model.

Predicting solar radiation at high resolutions: A comparison of time series forecasts

Abstract: The increasing use of solar power as a source of electricity has led to increased interest in forecasting radiation over short time horizons. The relevant horizons for generation and transmission can range from as little as 5 minutes to as long as several hours. Forecasting experiments are run using six data sets, at resolutions of 5, 15, 30, and 60 min, using the global horizontal component. The data exhibits nonlinear variability, due to variations in weather and cloud cover. Nevertheless, the dominance of the 24-h cycle makes it straightforward to build predictive models. Forecasting tests are run using regressions in logs, Autoregressive Integrated Moving Average (ARIMA), and Unobserved Components models. Transfer functions, neural networks, and hybrid models are also evaluated. All the tests use true out-of-sample forecasts: The models are estimated over history prior to the start of the forecast horizon, the data is forecasted, and the predicted values are compared with the actuals. In nearly all the tests, the best results are obtained using the ARIMA in logs, with time-varying coefficients. There are some exceptions. At high resolutions, a transfer function using cloud cover is found to improve over the ARIMA. In a few cases, the neural net or hybrid models can improve at very high resolutions, on the order of 5 min. The success of the ARIMA is attributable mainly to its ability to capture the diurnal cycle more effectively than other methods.

A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins

Abstract: An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature–time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.

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

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

Estimating the manufacturing cost of purely organic solar cells

Abstract: In this paper we estimate the manufacturing cost of purely organic solar cells. We find a very large range since the technology is still very young. We estimate that the manufacturing cost for purely organic solar cells will range between $50 and $140/m 2 . Under the assumption of 5% efficiency, this leads to a module cost of between $1.00 and $2.83/W p . Under the assumption of a 5-year lifetime, this leads to a levelized cost of electricity (LEC) of between 49¢ and 85¢/kWh. In order to achieve a more competitive COE of about 7¢/kWh, we would need to increase efficiency to 15% and lifetime to between 15–20 years.

Optical properties of liquids for direct absorption solar thermal energy systems

Abstract: A method for experimentally determining the extinction index of four liquids (water, ethylene glycol, propylene glycol, and Therminol VP-1) commonly used in solar thermal energy applications was developed. In addition to the extinction index, we report the refractive indices available within the literature for these four fluids. The final value reported is the solar-weighted absorption coefficient for the fluids demonstrating each fluid’s baseline capacity for absorbing solar energy. Water is shown to be the best absorber of solar energy of the four fluids, but it is still a weak absorber, only absorbing 13% of the energy. These values represent the baseline potential for a fluid to be utilized in a direct absorption solar thermal collector.

Microencapsulated n-octacosane as phase change material for thermal energy storage

Abstract: This study deals with preparation and characterization of polymethylmetracrylate (PMMA) microcapsules containing n-octacosane as phase change material for thermal energy storage. The surface morphology, particle size and particle size distribution (PSD) were studied by scanning electron microscopy (SEM). The chemical characterization of PMMA/octacosane microcapsules was made by FT-IR spectroscopy method. Thermal properties and thermal stability of microencapsulated octacosane were determined using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The melting and freezing temperatures and the latent heats of the microencapsulated octacosane as PCM were measured as 50.6 and 53.2 °C, 86.4 and −88.5 J/g, respectively, by DSC analysis. TGA analysis indicated that the microencapsulated octacosane degrade in two steps and had good chemical stability. Thermal cycling test shows that the microcapsules have good thermal reliability with respect to the accelerated thermal cycling. Based on the results, it can be considered that the microencapsulated octacosane have good energy storage potential.

Capric–myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage

Abstract: Phase change materials (PCMs) can be incorporated with building materials to obtain novel form-stable composite PCM which has effective energy storage performance in latent heat thermal energy storage (LHTES) systems. In this study, capric acid (CA)–myristic acid (MA) eutectic mixture/vermiculite (VMT) composite was prepared as a novel form-stable PCM using vacuum impregnation method. The composite PCM was characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. Thermal properties and thermal reliability of the composite PCM were determined by differential scanning calorimetry (DSC) analysis. The CA–MA eutectic mixture could be retained by 20 wt% into pores of the VMT without melted PCM seepage from the composite and therefore, this mixture was described as form-stable composite PCM. Thermal cycling test showed that the form-stable composite PCM has good thermal reliability and chemical stability although it was subjected to 3000 melting/freezing cycling. Thermal conductivity of the form-stable CA–MA/VMT composite PCM was increased by about 85% by introducing 2 wt% expanded graphite (EG) into the composite. The increase in thermal conductivity was confirmed by comparison of the melting and freezing times of the CA–MA/VMT composite with that of CA–MA/VMT/EG composite. The form-stable PCM including EG can be used as energy absorbing building material such as lightweight aggregate for plaster, concrete compounds, fire stop mortar, and component of interior fill for wallboards or hollow bricks because of its good thermal properties, thermal and chemical reliability and thermal conductivity.

Development and testing of thermochromic coatings for buildings and urban structures

Abstract: The present study reports the development and comparative testing of thermochromic coating to be used in buildings and urban structures. Experimental results from an extensive comparative analysis of the thermal and physical behaviour of thermochromic, highly reflective (cool), and common coatings are reported and analyzed. The surface temperature was monitored on 24 h basis from August to mid-September 2007. It was revealing that the temperature of thermochromic coatings was lower than cool and common coatings. Measurements of spectral reflectance indicated that the thermochromic coatings at the colored phase (below the transition temperature of 30 °C) are energy-absorbing while at the colorless phase (above the transition temperature of 30 °C) are energy-reflecting. The data obtained was used for the calculation of solar reflectance. The results showed that the solar reflectance of the thermochromic samples was significally higher compared to the cool and common ones. A 10-day period test was also performed showing the impact of solar radiation on thermochromism. The comparative results demonstrate that the use of thermochromic coatings can both contribute to energy savings in buildings, providing a thermally comfortable indoor environment, while can contribute highly to improve the urban microclimate.

TiO2 thin films - Influence of annealing temperature on structural, optical and photocatalytic properties

Abstract: Nanostructured TiO 2 thin films were deposited on glass substrates by sol–gel dip coating technique. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and UV–vis transmittance spectroscopy. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phase was initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 600 °C was about 20 nm. The lattice parameters for the films annealed at 600 °C were a = 3.7862 Ǻ and c = 9.5172 Ǻ, which is close to the reported values of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and was found to decrease with the annealing temperature. At 550 nm the refractive index of the films annealed at 600 °C was 2.11, which is low compared to a pore free anatase TiO 2 . The room temperature electrical resistivity in the dark was of the order of 4.45 × 10 6 ohm-cm. Photocatalytic activity of the TiO 2 films were studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 400 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.

Thermal performance, economic and environmental life cycle analysis of thermosiphon solar water heaters

Abstract: In this paper, the environmental benefits or renewable energy systems are initially presented followed by a study of the thermal performance, economics and environmental protection offered by thermosiphon solar water heating systems. The system investigated is of the domestic size, suitable to satisfy most of the hot water needs of a family of four persons. The results presented in this paper show that considerable percentage of the hot water needs of the family are covered with solar energy. This is expressed as the solar contribution and its annual value is 79%. Additionally, the system investigated give positive and very promising financial characteristics with payback time of 2.7 years and life cycle savings of 2240 € with electricity backup and payback time of 4.5 years and life cycle savings of 1056 € with diesel backup. From the results it can also be shown that by using solar energy considerable amounts of greenhouse polluting gasses are avoided. The saving, compared to a conventional system, is about 70% for electricity or diesel backup. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 13 months, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 3.2 years according to the fuel and the particular pollutant considered. It can therefore be concluded that thermosiphon solar water hearting systems offer significant protection to the environment and should be employed whenever possible in order to achieve a sustainable future.

Performance of water-in-glass evacuated tube solar water heaters

Abstract: The performance of water-in-glass evacuated tube solar water heaters is evaluated using experimental measurements of optical and heat loss characteristics and a simulation model of the thermosyphon circulation in single-ended tubes. The performance of water-in-glass evacuated tube solar collector systems are compared with flat plate solar collectors in a range of locations. The performance of a typical 30 tube evacuated tube array was found to be lower than a typical 2 panel flat plate array for domestic water heating in Sydney.

Analysis of zeotropic mixtures used in low-temperature solar Rankine cycles for power generation

Abstract: This paper presents the analysis of low-temperature solar Rankine cycles for power generation using zeotropic mixtures. Three typical mass fractions 0.9/0.1 (Ma) 0.65/0.35 (Mb), 0.45/0.55 (Mc) of R245fa/R152a are chosen. In the proposed temperature range from 25 °C to 85 °C, the three zeotropic mixtures are investigated as the working fluids of the low-temperature solar Rankine cycle. Because there is an obvious temperature glide during phase change for zeotropic mixtures, an internal heat exchanger (IHE) is introduced to the Rankine cycle. Investigation shows that different from the pure fluids, among the proposed zeotropic mixtures, the isentropic working fluid Mb possesses the lowest Rankine cycle efficiency. For zeotropic mixtures a significant increase of thermal efficiencies can be gained when superheating is combined with IHE. It is also indicated that utilizing zeotropic mixtures can extend the range of choosing working fluids for low-temperature solar Rankine cycles.

Direct and indirect uncertainties in the prediction of tilted irradiance for solar engineering applications

Abstract: Global radiation measured on fixed-tilt, south-facing planes (40° and vertical) and a 2-axis tracker at NREL’s Solar Radiation Research Lab. in Golden, CO is compared to predictions from ten transposition models, in combination with either optimal or suboptimal input data of horizontal irradiance. Suboptimal inputs are typically used in everyday engineering calculations, for which the necessary data are usually unavailable for the site under scrutiny, and must be estimated in some way. The performance of the transposition models is first evaluated for ideal conditions when optimal data are used. In this specific case, it is found that the Gueymard and Perez models provide the best estimates of global tilted irradiance under clear skies in particular. The performance of four direct/diffuse separation models is also evaluated. Their predictions of direct and diffuse radiation appear biased in most cases, with a model-dependent magnitude. Finally, the performance of the resulting combinations of separation and transposition models is analyzed in a variety of situations. When only global irradiance is known, the accuracy of the tilted irradiance predictions degrades significantly, and is mainly conditioned by the local performance of the direct/diffuse separation method. For the south-facing vertical surface, inaccuracies in the ground reflection calculations becomes another key factor and significantly increase the prediction error. The Reindl transposition algorithm appears to perform best in this case. When using suboptimal input data for the prediction of plane-of-array irradiance on a moderately tilted plane (40°S) or a 2-axis tracking plane, the Hay, Reindl and Skartveit models are less penalized than others and tend to perform better. It is concluded that further research should be conducted to improve the overall process of predicting irradiance on tilted planes in realistic situations where no local high-quality irradiance or albedo measurements are available.

Modeling and simulation of a grid-connected pv generation system for electromagnetic transient analysis

Abstract: This paper addresses modeling and simulation of a grid-connected photovoltaic system (GCPS) to analyze its grid interface behavior and control performance in the system design. A simple circuit model of the solar array is used to easily simulate its inherent characteristics with the basic specification data. Detailed power and protection control of the GCPS as well as its electrical circuits have been represented by user-defined and built-in components to take into account transients in normal and fault conditions, which are dominated by its power electronic controller. The model has been described with the reference to and implemented in PSCAD/EMTDC, a power system transient software package. Extensive simulation results are presented and analyzed to validate that the proposed simulation model is effective for control and protection performance evaluation of the GCPS in terms of electromagnetic transient analysis.

Investigations on heat losses from a solar cavity receiver

Abstract: Thermal as well as optical losses affect the performance of a solar parabolic dish-cavity receiver system. Convective and radiative heat losses form the major constituents of the thermal losses. In this paper, an experimental and numerical study of the steady state convective losses occurring from a downward facing cylindrical cavity receiver of length 0.5 m, internal diameter of 0.3 m and a wind skirt diameter of 0.5 m is carried out. The experiments are conducted for fluid inlet temperatures between 50 °C and 75 °C and for receiver inclination angles of 0° (side ways facing cavity), 30°, 45°, 60° and 90° (vertically downward facing receiver). The numerical study is performed for fluid inlet temperatures between 50 °C and 300 °C and receiver inclinations of 0°, 45° and 90° using the Fluent CFD software. The experimental and the numerical convective loss estimations agree reasonably well with a maximum deviation of about 14%. It is found that the convective loss increases with mean receiver temperature and decreases with increase in receiver inclination. Nusselt number correlations are proposed for two receiver fluid inlet temperature ranges, 50–75 °C and 100–300 °C, based on the experimental and predicted data respectively. Besides no-wind tests, investigations are also carried out to study the effects of external wind at two different velocities in two directions (head-on and side-on). The wind induced convective losses are generally higher than the no-wind convective loss (varying between 22% and 75% for 1 m/s wind speed and between 30% and 140% for the 3 m/s wind speed) at all receiver inclination angles, the only exception being the loss due to side-on wind at 0° receiver inclination angle. This is because the wind acts as a barrier at the aperture preventing the hot air to flow out of the receiver. The head-on wind causes higher convective loss than the side-on wind. Nusselt number correlations proposed in this work are compared with the existing correlations in the literature. It is found that the correlations available in literature under-predict the convective losses at mean receiver temperatures between 100 °C and 300 °C. This is due to the fact that the correlations are developed for certain receiver geometries having the ratio of aperture diameter to receiver diameter equal to or lesser than one.

Methods to determine stratification efficiency of thermal energy storage processes – Review and theoretical comparison

Abstract: This paper reviews different methods that have been proposed to characterize thermal stratification in energy storages from a theoretical point of view. Specifically, this paper focuses on the methods that can be used to determine the ability of a storage to promote and maintain stratification during charging, storing and discharging, and represent this ability with a single numerical value in terms of a stratification efficiency for a given experiment or under given boundary conditions. Existing methods for calculating stratification efficiencies have been applied to hypothetical storage processes of charging, discharging and storing, and compared with the rate of entropy production caused by mixing calculated for the same experiments. The results depict that only one of the applied methods is in qualitative agreement with the rate of entropy production, however, none of the applied methods is in agreement with the rate of entropy production and also able to distinguish between the entropy production caused by mixing and the entropy changes due to heat losses.

Comparative analysis of different supporting measures for the production of electrical energy by solar PV and Wind systems: Four representative European cases

Abstract: In the 9th of March 2007, the European Council decided a fixing goal of 20% contribution of the renewable energy sources (RES) on the total European electric energy production in 2020. In order to reach such an ambitious goal, all the European countries are adopting different support policies for encouraging the installations of RES-based generation systems. In this paper, after a brief review on the main support policies for RES in Europe, the specific situations of four representative countries (France, Germany, Italy and Spain) are examined, with the purpose of putting into evidence the main differences in the support policies adopted for Photovoltaic (PV) and Wind systems. In particular, a comparison based on the calculation of the pay-back-period (PBP), the net present value (NPV) and the internal rate of return (IRR), for different sized PV and Wind systems, shows that in some situations a support policy can be not convenient for the owner of the RES-based generation system and that, in many cases, the differences between the way of implementation of the same support policy in different countries, can give place to significantly different results.

Robust maximum power point tracking method for photovoltaic cells: A sliding mode control approach

Abstract: Due to nonlinear I-V characteristics of photovoltaic cells, an maximum power point tracking algorithm is adopted to maximize the output power. In this paper, an approach for peak power tracking using the sliding mode control is proposed. The proposed controller is robust to environment changes and load variations. The stability and robustness of the controller are addressed. The performance of the controller is verified through simulations and experiments. It demonstrated that the proposed approach can be implemented effectively and economically.

General formula for on-axis sun-tracking system and its application in improving tracking accuracy of solar collector

Abstract: Azimuth-elevation and tilt-roll tracking mechanism are among the most commonly used sun-tracking methods for aiming the solar collector towards the sun at all times. It has been many decades that each of these two sun-tracking methods has its own specific sun-tracking formula and they are not interrelated. In this paper, the most general form of sun-tracking formula that embraces all the possible on-axis tracking methods is presented. The general sun-tracking formula not only can provide a general mathematical solution, but more significantly it can improve the sun-tracking accuracy by tackling the installation error of the solar collector.

Performance of a building integrated photovoltaic/thermal (BIPVT) solar collector

Abstract: The idea of combining photovoltaic and solar thermal collectors (PVT collectors) to provide electrical and heat energy is an area that has, until recently, received only limited attention. Although PVTs are not as prevalent as solar thermal systems, the integration of photovoltaic and solar thermal collectors into the walls or roofing structure of a building could provide greater opportunity for the use of renewable solar energy technologies. In this study, the design of a novel building integrated photovoltaic/thermal (BIPVT) solar collector was theoretically analysed through the use of a modified Hottel–Whillier model and was validated with experimental data from testing on a prototype BIPVT collector. The results showed that key design parameters such as the fin efficiency, the thermal conductivity between the PV cells and their supporting structure, and the lamination method had a significant influence on both the electrical and thermal efficiency of the BIPVT. Furthermore, it was shown that the BIPVT could be made of lower cost materials, such as pre-coated colour steel, without significant decreases in efficiency. Finally, it was shown that by integrating the BIPVT into the building rather than onto the building could result in a lower cost system. This was illustrated by the finding that insulating the rear of the BIPVT may be unnecessary when it is integrated into a roof above an enclosed air filled attic, as this air space acts as a passive insulating barrier.

Analysis of PV/T flat plate water collectors connected in series

Abstract: Photovoltaic–thermal (PV/T) technology refers to the integration of a PV and a conventional solar thermal collector in a single piece of equipment. In this paper we evaluate the performance of partially covered flat plate water collectors connected in series using theoretical modeling. PV is used to run the DC motor, which circulates the water in a forced mode. Analytical expressions for N collectors connected in series are derived by using basic energy balance equations and computer based thermal models. This paper shows the detailed analysis of thermal energy, exergy and electrical energy yield by varying the number of collectors by considering four weather conditions (a, b, c and d type) for five different cities (New Delhi, Bangalore, Mumbai, Srinagar, and Jodhpur) of India. Annual thermal and electrical energy yield is also evaluated for four different series and parallel combination of collectors for comparison purpose considering New Delhi conditions. This paper also gives the total carbon credit earned by the hybrid PV/T water heater investigated as per norms of Kyoto Protocol for New Delhi climatic conditions. Cost analysis has also been carried out. It is observed that the collectors partially covered by PV module combines the production of hot water and electricity generation and it is beneficial for the users whose primary requirement is hot water production and collectors fully covered by PV is beneficial for the users whose primary requirement is electricity generation. We have also found that if this type of system is installed only in 10% of the total residential houses in Delhi then the total carbon credit earned by PV/T water heaters in terms of thermal energy is USD $144.5 millions per annum and in terms of exergy is USD $14.3 millions per annum, respectively.

Optimal design of a forced circulation solar water heating system for a residential unit in cold climate using TRNSYS

Abstract: An indirect forced circulation solar water heating systems using a flat-plate collector is modeled for domestic hot water requirements of a single-family residential unit in Montreal, Canada. All necessary design parameters are studied and the optimum values are determined using TRNSYS simulation program. The solar fraction of the entire system is used as the optimization parameter. Design parameters of both the system and the collector were optimized that include collector area, fluid type, collector mass flow rate, storage tank volume and height, heat exchanger effectiveness, size and length of connecting pipes, absorber plate material and thickness, number and size of the riser tubes, tube spacing, and the collector’s aspect ratio. The results show that by utilizing solar energy, the designed system could provide 83–97% and 30–62% of the hot water demands in summer and winter, respectively. It is also determined that even a locally made non-selective-coated collector can supply about 54% of the annual water heating energy requirement by solar energy.

Improved photovoltaic energy output for cloudy conditions with a solar tracking system

Abstract: This work describes measurements of the solar irradiance made during cloudy periods in order to improve the amount of solar energy captured during such periods. It is well-known that 2-axis tracking, in which solar modules are pointed at the sun, improves the overall capture of solar energy by a given area of modules by 30–50% versus modules with a fixed tilt. On sunny days the direct sunshine accounts for up to 90% of the total solar energy, with the other 10% from diffuse (scattered) solar energy. However, during overcast conditions nearly all of the solar irradiance is diffuse radiation that is isotropically-distributed over the whole sky. An analysis of our data shows that during overcast conditions, tilting a solar module or sensor away from the zenith reduces the irradiance relative to a horizontal configuration, in which the sensor or module is pointed toward the zenith (horizontal module tilt), and thus receives the highest amount of this isotropically-distributed sky radiation. This observation led to an improved tracking algorithm in which a solar array would track the sun during cloud-free periods using 2-axis tracking, when the solar disk is visible, but go to a horizontal configuration when the sky becomes overcast. During cloudy periods we show that a horizontal module orientation increases the solar energy capture by nearly 50% compared to 2-axis solar tracking during the same period. Improving the harvesting of solar energy on cloudy days is important to using solar energy on a daily basis for fueling fuel-cell electric vehicles or charging extended-range electric vehicles because it improves the energy capture on the days with the lowest hydrogen generation, which in turn reduces the system size and cost.

Performance of a direct steam generation solar thermal power plant for electricity production as a function of the solar multiple

Abstract: This paper describes the influence of the solar multiple on the annual performance of parabolic trough solar thermal power plants with direct steam generation (DSG). The reference system selected is a 50 MW e DSG power plant, with thermal storage and auxiliary natural gas-fired boiler. It is considered that both systems are necessary for an optimum coupling to the electricity grid. Although thermal storage is an opening issue for DSG technology, it gives an additional degree of freedom for plant performance optimization. Fossil hybridization is also a key element if a reliable electricity production must be guaranteed for a defined time span. Once the yearly parameters of the solar power plant are calculated, the economic analysis is performed, assessing the effect of the solar multiple in the levelized cost of electricity, as well as in the annual natural gas consumption.

Experimental and simulated performance of a PV-ventilated solar greenhouse dryer for drying of peeled longan and banana

Abstract: This paper presents experimental and simulated performance of a PV-ventilated solar greenhouse dryer for drying of peeled longan and banana. The dryer consists of a parabolic roof structure covered with polycarbonate plates on a concrete floor. Three fans powered by a 50-W PV module ventilate the dryer. To investigate the experimental performances of the solar greenhouse dryer for drying of peeled longan and banana, 10 full scale experimental runs were conducted. Of which five experimental runs were conducted for drying of peeled longan and another five experimental runs were conducted for drying of banana. The drying air temperature varied from 31 °C to 58 °C during drying of peeled longan while it varied from 30 °C to 60 °C during drying of banana. The drying time of peeled longan in the solar greenhouse dryer was 3 days, whereas 5–6 days are required for natural sun drying under similar conditions. The drying time of banana in the solar greenhouse dryer was 4 days, while it took 5–6 days for natural sun drying under similar conditions. The quality of solar dried products in terms of colour and taste was high-quality dried products. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan and banana in the solar greenhouse dryer was developed and this system of non-linear partial differential equations was solved numerically using the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results reasonably agreed with the experimental data for solar drying of peeled longan and banana. This model can be used to provide the design data and is also essential for optimal design of the dryer.

Grid parity analysis of solar photovoltaic systems in Germany using experience curves

Abstract: The paper starts with experience curve analysis in order to find out the future prices of solar photovoltaic (PV) modules. Experience curves for 75–90% progress ratio are extrapolated with the help of estimated future growth rate for PV installation worldwide and current module price data until year 2060. A kWh PV electricity generation cost has been calculated for coming decades with the help of local market parameters and module prices data from extrapolated experience curve. Two different prices for grid electricity – wholesale electricity price and end user electricity price – are separately analyzed. Household electricity consumption profile and PV electricity generation profile for Cologne, Germany, have been analyzed to find out the possibility for PV electricity consumption at the time of its generation. This result is used to calculate the real grid parity year – which lies somewhere between grid parity years calculated for wholesale electricity price and end user electricity price.

Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating

Abstract: The continuous increase in the level of greenhouse gas emissions and the rise in fuel prices are the main driving forces behind the efforts for more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. In this study, the thermal performance of a phase change thermal storage unit is analyzed and discussed. The storage unit is a component of ten pieced solar air collectors heating system being developed for space heating of a greenhouse and charging of PCM. CaCl26H2O was used as PCM in thermal energy storage with a melting temperature of 29 °C. Hot air delivered by ten pieced solar air collector is passed through the PCM to charge the storage unit. The stored heat is utilized to heat ambient air before being admitted to a greenhouse. This study is based on experimental results of the PCM employed to analyze the transient thermal behavior of the storage unit during the charge and discharge periods. The proposed size of collectors integrated PCM provided about 18–23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the conventional heating device.

Thermodynamic assessment of photovoltaic systems

Abstract: In this paper, an attempt is made to investigate the performance characteristics of a photovoltaic (PV) and photovoltaic-thermal (PV/T) system based on energy and exergy efficiencies, respectively. The PV system converts solar energy into DC electrical energy where as, the PV/T system also utilizes the thermal energy of the solar radiation along with electrical energy generation. Exergy efficiency for PV and PV/T systems is developed that is useful in studying the PV and PV/T performance and possible improvements. Exergy analysis is applied to a PV system and its components, in order to evaluate the exergy flow, losses and various efficiencies namely energy, exergy and power conversion efficiency. Energy efficiency of the system is calculated based on the first law of thermodynamics and the exergy efficiency, which incorporates the second law of thermodynamics and solar irradiation exergy values, is also calculated and found that the latter is lower for the electricity generation using the considered PV system. The values of “fill factor” are also determined for the system and the effect of the fill factor on the efficiencies is also evaluated. The experimental data for a typical day of March (27th March 2006) for New Delhi are used for the calculation of the energy and exergy efficiencies of the PV and PV/T systems. It is found that the energy efficiency varies from a minimum of 33% to a maximum of 45% respectively, the corresponding exergy efficiency (PV/T) varies from a minimum of 11.3% to a maximum of 16% and exergy efficiency (PV) varies from a minimum of 7.8% to a maximum of 13.8%, respectively.