Investigation of a new solar greenhouse drying system for peppers

Solar drying is the oldest preservation technique of agricultural products using several types of solar crop dryers based mostly on solar energy, which is abundant, renewable and sustainable. This study aimed to modeling a new solar greenhouse drying system (SGDS) for the drying of red peppers. The proposed mixed-mode (SGDS) consists of two main parts, namely a flat plate solar air collector and an experimental greenhouse. A mathematical model is developed using the TRNSYS simulation program to predict the change in the drying kinetics during the drying process under our proposed (SGDS). The experimental part consisted in testing the solar air collector to investigate its performance. The test showed that this solar air collector has a good performance; its efficiency varies between 0, 5 and 0, 65. The model was validated with the observed data and showed good agreement with experimental values. The influence of the area of the product to be dried, airflow rate and collector area, on moisture content changes, air temperature and humidity inside the greenhouse was studied. For the case study of this SGDS, the results obtained from simulation showed that the optimum values of area of the product to be dried, the exhaust airflow rate and the collector area were found to be 40 m², 250 kg/h and 2 m², respectively.     

Optimization of a solar-assisted drying system for drying bananas

This paper presents a mathematical model for optimal design of a solar-assisted drying system for drying bananas. The optimization model consists of a simulation model of a solar-assisted drying system combined with an economic model. The simulation model is composed of two systems of differential equations: one for the collector and other for the drying cabinet. These systems of the differential equation were solved using the finite difference method. Values of the model parameters were determined experimentally. A computer program in FORTRAN was developed to simulate the performance of the drying system. The model was validated by comparing the simulation results with the experimental results and they were in good agreement. This simulation model was used for the optimization of the solar-assisted drying system. An economic model was formulated to calculate the annual drying cost. The optimization problem was defined as the optimization of the geometry and operational parameters of the drying system so as to minimize the drying cost per unit of dried product. Currently used collector area and the air recycle factor were considered as the parameters for basic mode of operation of the drying system. The adaptive pattern search technique was adopted to find the optimum values of the solar collector area and the recycle factor. The optimum values of the collector area and the recycle factor were found to be 26 m² and 90%, respectively. The computer program developed in this study can be used to optimize similar drying systems.     

Development of advanced solar assisted drying systems

The Solar Energy Research Group in the Universiti Kebangsaan Malaysia has been set-up more than two decades ago. One of the activities is in the field of solar thermal process, particularly in development of solar assisted drying systems. Solar drying systems technical development can proceed in two directions. Firstly simple, low power, short life, and comparatively low efficiency-drying system. Secondly, the development of high efficiency, high power, long life expensive solar drying system. The group has developed four solar assisted drying systems namely (a) the V-groove solar collector, (b) the double-pass solar collector with integrated storage system, (c) the solar assisted dehumidification system for medicinal herbs and (d) the photovoltaic thermal (PVT) collector system. The common problems associated with the intermittent nature of solar radiation and the low intensities of solar radiation in solar thermal systems can be remedied using these types of solar drying systems. These drying systems have the advantages of heat storage, auxiliary energy source, integrated structure control system and can be use for a wide range of agricultural produce.     

Techno-economic model of a solar still coupled with a solar flat-plate collector

In this paper, a techno-economic model has been developed for a solar still coupled with a flat-plate collector through a heat-exchanger. The model is based on the establishment of periodic steady-state conditions. The economic criterion used in the model is based on the least cost of a unit mass of distilled water evaluated from the life-cycle costing of the system. To evaluate the model, numerical calculations have been made corresponding to the climate of Delhi (India). It is concluded that the addition of a solar collector enhances the distillate yield; however, this is not always economical. This system therefore needs careful and economic system design.     

A review of solar dryers developed for grape drying

This paper attempts to review various solar dryers developed exclusively for grape drying on a normal scale. Many popular varieties of solar dryers, certain typical models as well as traditional methods practiced for drying grapes are presented in this paper. Technical and economical results have proved that solar drying of grapes is quite feasible. Commercialization of solar drying of grapes has not gained momentum as expected, may be due to high initial investment and low capacity of the dryers. Even, the farmer’s acceptance of solar dryers developed is not encouraging. Exhaustive research and development work has to be carried out in order to make solar drying of grapes economical and user friendly. There has been a remarkable achievement in solar drying of grapes due to sustained research and development associated with the adoption of advanced technologies. A review of various solar drying models for grapes is thus necessitated.     

J-CHART METHOD IN DESIGNING A PARTIALLY SOLAR HEATED DRYER

A new chart of simulation method, named J-chart, has been developed for predicting the long-term average performance of solar drying systems. The result of many simulations allows to develop a simple graphical method, represented by 3 charts and their polynomial correlations, to obtain a general design procedure for a partially solar heated dryer. The first one is the drying time, the second one is the fraction of heating load supplied by solar energy and the third one is the fraction of economized energy. These charts and the correlations are used in establishing relationships between the collector area and the weight of the produce to be dried or the dried produce. This method is developed by using the monthly average values for a moderate climate (Perpignan, Φ = 42.41^°) and with the assumption that the dryer is used daily over a year and the duration of drying operation is assumed to be less than 24 hours.     

Performance of a double pass solar air collector

Double pass counter flow solar air collector with porous material in the second air passage is one of the important and attractive design improvement that has been proposed to improve the thermal performance. This paper presents theoretical and experimental analysis of double pass solar air collector with and without porous material. A mathematical model has been developed based on volumetric heat transfer coefficient. Effects of various parameters on the thermal performance and pressure drop characteristics have been discussed. Comparison of results reveals that the thermal efficiency of double pass solar air collector with porous absorbing material is 20-25% and 30-35% higher than that of double pass solar air collector without porous absorbing material and single pass collector respectively.     

Technoeconomic optimization of a hybrid solar air-heating system

This paper presents a simple techno-economic model for a hybrid solar air-heating system based on water as the storage medium. The configuration of the system consists of a conventional solar air-heater, water tank for thermal storage, a unit which adjusts the higher air temperature (during peak sunshine hours) to the required limit (by mixing fresh air) and an arrangement for providing auxiliary energy if and when required. A thermostatically controlled electric heater is assumed to be the source of auxiliary energy, in the present calculations. In order to evaluate the performance of the system using the developed model numerical calculations have been made corresponding to the climate of Delhi, India. The calculations have been extended to obtain the optimized values of collector area and storage mass which correspond to the minimum value of useful energy. Numerical results show that the cost of useful energy obtained for optimized values of collector area and storage mass is much less than the cost of electrical heating.     

Solar chimney power plants for high latitudes

A solar chimney system for power production at high latitudes has been designed and its performance has been evaluated. A mathematical model and a code on MATLAB platform have been developed based on monthly average meteorological data and thermodynamic cycle. The thermal performance of a 5 MW nominal power production plant at three locations in Canada, namely Ottawa, Winnipeg and Edmonton, is studied. The sloped collector field is built at suitable mountain hills, which also functions as a chimney. Then a short vertical chimney is added to install the vertical axis air turbine. The results showed that solar chimney power plants at high latitudes may have satisfactory thermal performance and produce as much as 85% of the same plants in southern locations with horizontal collector field. The overall thermal performance of these plants is a little less than 0.5%.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

A mathematical model of thermal performance of a solar air heater with slats

A mathematical model for computing the thermal performance of a single pass flat-plate solar air collector is presented. Air channels were formed by providing metal slats running along the circulated air passage linking the absorber plate by the bottom one in an endeavor to enhance the thermal efficiency of the solar air collector. A mathematical model, therefore, is developed by which the influence of the addition of the metal slats on the efficiency of the solar collector is studied. A computer code that employs an iterative solution procedure is constructed to solve for the governing energy equations to estimate the mean temperatures of the collector. The effect of volume airflow rate, collector length, and spacing between the absorber and bottom plates on the thermal performance of the present solar air heater was investigated. Furthermore, a numerical comparison of the present design with the most common type of solar air heaters is conducted. The results of the comparison have indicated that better thermal performance was obtained by the modified system.     

Design concept and mathematical model of a bi-fluid photovoltaic/thermal (PV/T) solar collector

This paper presents an improved design of a photovoltaic/thermal (PV/T) solar collector integrating a PV panel with a serpentine-shaped copper tube as the water heating component and a single pass air channel as the air heating component. In addition to the electricity generated, this type of collector enables the production of both hot air and water, increasing the total efficiency per unit area compared to the conventional PV/T solar collector. The use of both fluids (bi-fluid) also creates a greater range of thermal applications and offers options in which hot and/or cold air and/or water can be utilized depending on the energy needs and applications. In this paper, the design concept of the bi-fluid PV/T solar collector is emphasized with 2D steady state energy balance equations for the bi-fluid configuration are developed, validated and used to predict the performance of the bi-fluid solar collector for a range of mass flow rates of air and water. The performance of the collector is then compared when the fluids are operated independently and simultaneously. The simulations indicate that when both fluids are operated independently the overall thermal and electrical performance of the solar collector is considered as satisfactory and when operated simultaneously the overall performance is higher. The bi-fluid PV/T solar collector discussed in this paper will add insights to the new knowledge of optimizing the utilization of solar energy by a PV/T solar collector and has potential applications in various fields.     

Energy and exergy analysis of an indirect solar cabinet dryer based on mathematical modeling results

In the present study, using a previously developed dynamic mathematical model for performance analysis of an indirect cabinet solar dryer [1], a microscopic energy and exergy analysis for an indirect solar cabinet dryer is carried out. To this end, appropriate energy and exergy models are developed and using the predicted values for temperature and enthalpy of gas stream and the temperature, enthalpy and moisture content of the drying solid, the energy and exergy efficiencies are estimated. The validity of the model for predicting variations in gas and solid characteristics along the time and the length of the solar collector and/or dryer length was examined against some existing experimental data. The results show that in spite of high energy efficiency, the indirect solar cabinet dryer has relatively low exergy efficiency. Results show that the maximum exergy losses are in midday. Also the minimums of total exergy efficiency are 32.3% and 47.2% on the first and second days, respectively. Furthermore, the effect of some operating parameters, including length of the collector, its surface, and air flow rate was investigated on the exergy destruction and efficiency.     

Simulation of a pilot solar chimney thermal power generating equipment

A pilot experimental solar chimney thermal power generating equipment was set up in China. A simulation study was carried out to investigate the performance of the power generating system based on a developed mathematical model. The simulated power outputs in steady state were obtained for different global solar radiation intensity, collector area and chimney height. By intercomparison, it is found that the simulated power outputs are basically in agreement with the results calculated with the measurements, which validates the mathematical model of the solar chimney thermal power generating system. Furthermore, based on the simulation and the specific construction costs at a specific site, the optimum combination of chimney and collector dimensions can be selected for a required electric power output.     

Optimization of tank and flat-plate collector of solar water heating system for single-family households to assure economic efficiency through the TRNSYS program

Solar water heating systems are widely used in Brazil for domestic purposes in single-family households. The exploitation of the potential energy of the water from the upper tank and the thermosyphon phenomena for hot water circulation constitutes the absolute majority of the residential solar water heating systems in the country. But, these water heating systems are usually sized according to tables provided by the manufacturers, which show the number of plates required based on the size of the family and the number of hot water outlets. This sizing is based much more on intuition rather than on scientific data. For that reason, this work has developed an optimization model for water heating systems design parameters, using a numerical simulation routine, in a long-term transient regime. The optimized design gives the slope and area of the flat plate collector, which results in the minimum cost over the equipment life cycle. The computing procedure was executed considering specific characteristics of the project. A thermosyphon solar water heating system with flat-plate collector for Sao Paulo's climate was simulated. The practice of Brazilian designers and manufacturers is to recommend the maximization of the energetic gain for the winter. This paper has analyzed in economic terms if it is more attractive to increase the gain of solar energy in the winter period, with the consequence of reduction of the solar energy gain along the year, or to adopt the adequate slope, which improves the yearly solar energy gain.     

Numerical 3-D heat flux simulations on flat plate solar collectors

A transient 3-D mathematical model for solar flat plate collectors has been developed. The model is based on setting mass and energy balances on finite volumes. The model allows the comparison of different configurations: parallel tubes collectors (PTC), serpentine tube collectors (STC), two parallel plate collectors (TPPC), and other non-usual possibilities like the use of absorbent fluids with semitransparent or transparent plates. Transparent honeycomb insulation between plate and cover can also be modelled. The effect of temperature on the thermal properties of the materials has also been considered. The model has been validated experimentally with a commercial PTC. The model is a useful tool to improve the design of plate solar collectors and to compare different configurations. In order to show the capabilities of the model, the performance of a PTC collector with non-uniformity flow is analysed and compared with experimental data from literature with good agreement.     

Analysis of solar-dehumidification drying

A mathematical model for describing solar-dehumidification drying is presented. Lumped dynamical models have been applied to the moist material and to the solar collector. The other units of the drying system, drying chamber and dehumidifying heatpump were considered to be in a quasi-steady state. The simulation of the weather conditions was carried out by a dynamical weather model. Drying operations under humidistat and temperature control with and without the solar collector were simulated. The analysis of the numerical study shows that although the drying time is not necessarily shorter in the case of solar-dehumidification drying than in that of simple dehumidification drying, the running time of the dehumidifier can be considerably reduced.     

Solar heat pump drying and water heating in the tropics

In this study, the performance of a solar assisted heat pump dryer and water heater has been investigated. A simulation program has been developed. The predicted results are compared with those obtained from experiments under the meteorological conditions of Singapore. A coefficient of performance (COP) value of 7.0 for a compressor speed of 1800 rpm was observed. Maximum collector efficiencies of 0.86 and 0.7 have been found for evaporator–collector and air collector, respectively. A value of the specific moisture extraction rate (SMER) of 0.65 has been obtained for a load of 20 kg and a compressor speed of 1200 rpm. Results suggest that the total drying time of the product decreases with the increase in drying potential. Drying potential is directly proportional to the air flow rate, drying air temperature and inversely proportional to the air relative humidity. Three important parameters that affect the system performance are solar radiation, compressor speed and the total load placed in the drying chamber. Both SMER and COP decrease with increase in compressor speed.     

Simulation and optimisation of the ventilation in a chimney-dependent solar crop dryer

As published earlier on the performance of a chimney-dependent solar crop dryer (CDSCD) designed by the authors, the solar chimney can be combined with an appropriately inclined roof of drying chamber for ventilation improvement in the dryer. Mathematical models and a computer code are now developed to simulate the ventilation in relation to the design of the CDSCD. This is done for situations without any crop (no-load) in the dryer, to relate the ventilation to the external dimensions. The pressure-loss and bulk-fluid-temperature coefficients are deduced empirically from trials on the physical model. The simulation code predicts the ventilation to within 5% and the temperatures to within 1.5% of observed data, confirming the validity of the code as an effective design tool for the CDSCD. Results of parametric studies performed with the code indicate that, maximum airflow can be achieved when the inlet-exit area ratio is around 4:1, above which the system then approaches saturation without any real variation. The drying-chamber roof inclination and the chimney height are critical for the design in the geographical regions far from the equator, whereas the decisive parameters in the regions close to the equator are the drying chamber height and the area ratio of the dryer floor to chimney cross section. A high drying chamber with a short solar chimney is generally favoured at locations close to the equator, whereas a short drying chamber with a high solar chimney is suitable for regions far away from the equator.     

Mathematical modelling of thin layer drying process of long green pepper in solar dryer and under open sun

An experimental study was performed to determine the thin layer drying characteristics in a solar dryer with forced convection and under open sun with natural convection of long green pepper. An indirect forced convection solar dryer consisting of a solar air collector and drying cabinet was used in the experiments. Natural sun drying experiments were conducted for comparison at the same time. The constant rate period is absent from the drying curves. The drying process took place in the falling rate period. The drying data were fitted to 13 different mathematical models. Among the models, the logarithmic model for forced solar drying and the Midilli and Kucuk model for natural sun drying were found best to explain the thin layer drying behaviour of long green peppers. The performance of these models was investigated by comparing the coefficient of determination (R), reduced chi-square (χ²) and root mean square error (RMSE) between the observed and predicted moisture ratios.