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.     

The drying of grain with heat pumps in South Africa: A techno-economic analysis

The economic viability of air heating for grain drying with the aid of heat pumps and the viability of replacing existing heating methods (i.e. direct electrical heaters and diesel burners) with heat pumps are investigated. The energy costs of different types of heating apparatus to dry grain are calculated and, taking the different capital costs into account, the life cycle costs of the heating methods can be predicted. On comparing the life cycle costs of the different heating methods, it is concluded that heat pumps are more economical than other methods of heating provided that the apparatus is used for more than a minimum period per year. Drying of grain is usually done for periods shorter than this minimum; the result is that the use of heat pumps cannot be economically justified for the drying of grain only.     

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.     

Solar drying vs. open sun drying: A framework for financial evaluation

Initiatives towards large-scale dissemination of solar dryers for drying of agri-produce face severe competition from the largely prevalent practice of open sun drying in most of the developing countries. Therefore, solar drying systems must offer exceptionally attractive financial gains to enhance their acceptance among the potential users. A modest attempt to develop a simple framework to facilitate a comparison of the financial feasibility of solar drying as against open sun drying has been made in the present work. Results of some exemplifying calculations are presented and briefly discussed.     

Technical feasibility assessment of a solar chimney for food drying

Solar dryers use free and renewable energy sources, reduce drying losses (as compared to sun drying) and show lower operational costs than the artificial drying, thus presenting an interesting alternative to conventional dryers. This work proposes to study the feasibility of a solar chimney to dry agricultural products. To assess the technical feasibility of this drying device, a prototype solar chimney, in which the air velocity, temperature and humidity parameters were monitored as a function of the solar incident radiation, was built. Drying tests of food, based on theoretical and experimental studies, assure the technical feasibility of solar chimneys used as solar dryers for agricultural products. The constructed chimney generates a hot airflow with a yearly average rise in temperature (compared to the ambient air temperature) of 13 ± 1 °C. In the prototype, the yearly average mass flow was found to be 1.40 ± 0.08 kg/s, which allowed a drying capacity of approximately 440 kg.     

Design and development of a simple solar dehydrator for crop drying

In this paper, a simple solar collector cum crop drying system is described. The categorical objectives of this paper are (1) to identify a suitable medium capacity solar drying system and (2) to study analytically and experimentally the performance characteristics of this newly fabricated solar drying system. The drying ratio, rehydration ratio, culinary and organoleptic characteristics, of Methi and Bhendi crops were studied and presented in this paper.     

A solar assisted heat pump drying system for grain in-store drying

For grain in-store drying, a solar assisted drying process has been developed, which consists of a set including a solar-assisted heat pump, a ventilation system, a grain stirrer, etc. In this way, low power consumption, short cycle time and water content uniformity can be achieved in comparison with the conventional method. A solar-assisted heat pump drying system has been designed and manufactured for a practical granary, and the energy consumption performance of the unit is analyzed. The analysis result shows that the solar fraction of the unit is higher than 20%, the coefficient of performance about system (COPS) is 5.19, and the specific moisture extraction rate (SMER) can reach 3.05 kg/kWh.     

Design of a new solar dryer system with swirling flow for drying seeded grape

In this study solar energy supported, swirling flow new drying system is designed and artificial drying of grapes grown around Elazığ/Turkey is investigated. With the developed swirling flow dryer with airy solar collector it is examined that drying occurs homogenously and lower moisture values are obtained in when compared with classical drying system. Also it is found that with an increase in the drying air velocity decreases drying time. When air directing elements are placed inside drying chamber and rotating element to the entrance, it is examined that drying time gets shorter compared to that of natural drying. Thus, drying time which is 200 h in natural conditions decrease to 80 h with an air velocity of 1.5 m/s with the developed solar energy supported swirling flow dryer.     

Solar—drier—desiccator system evaluation

This paper discusses the design of solar drying systems which include a desiccator as part of the cycle. The influence of the desiccator investigated and the performance of a counter flow unit is discussed.     

中密度纤维板纤维干燥系统热能应用分析

综述了当前中密度纤维板(MDF)生产过程中纤维干燥工序热能供应的工况参数,重点分析了基于热能中心能源供应模式下纤维干燥工序热能的优化应用,提出了MDF生产工艺进一步节能的设计思路.     

A study of the drying effect on lemon slices using a closed-type solar dryer

An experimental closed-type dryer associated with a photovoltaic system (PV) was developed. The transparent drying cabinet was designed with high transmittance glass to decrease the reflection of direct sunlight and to offer extra direct solar heating on the raw material during drying. Parallel wiring with a local electrical grid was necessary for switching purposes if there is insufficient battery backup during peak operation. Lemon slices were dried using the closed-type solar dryer and results were compared with hot air drying at 60 °C. The results indicate that the dried lemon slices using a closed-type solar dryer has better general levels of quality in terms of sensory parameters.     

上海市竹园污泥干化焚烧系统的能量平衡分析

结合上海市竹园污泥处理工程,根据竹园污泥干化焚烧工程的设计方案,研究了污泥干化焚烧系统的能量平衡模型。根据所建立模型的计算结果,上海竹园污泥处理工程焚烧系统可产生47×106kJ的能量,而干化机所需的能量约为57.2×106kJ,焚烧系统产生的能量不足以提供污泥干化系统所需的全部能量。     

Solar crop drying in A low humidity environment

Comprehensive field performance data are reported for solar cabinet dryers, with and without crop load, under the low humidity conditions of the local harmattan season. The field data collected provide a frame of reference for similar data on dryer performance gathered under the normal local conditions of high relative humidity. The relative humidity, it is found, plays a most decisive role in the entire drying process.     

Optimised model for community-based hybrid energy system

Hybrid energy system is an excellent solution for electrification of remote rural areas where the grid extension is difficult and not economical. Such system incorporates a combination of one or several renewable energy sources such as solar photovoltaic, wind energy, micro-hydro and may be conventional generators for backup. This paper discusses different system components of hybrid energy system and develops a general model to find an optimal combination of energy components for a typical rural community minimizing the life cycle cost.The developed model will help in sizing hybrid energy system hardware and in selecting the operating options. Micro-hydro-wind systems are found to be the optimal combination for the electrification of the rural villages in Western Ghats (Kerala) India, based on the case study. The optimal operation shows a unit cost of Rs. 6.5/kW h with the selected hybrid energy system with 100% renewable energy contribution eliminating the need for conventional diesel generator.     

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.     

Mechanism and mathematical model of heat and mass transfer during convective drying of porous materials

Based on classical analysis and conclusive comments about various kinds of drying models, a rigorously formulated and comprehensive theoretical model is established to describe heat and mass transfer during constant rate and falling rate periods in convective drying of porous materials. The concept of iterative correction is introduced, and a corresponding numerical method is developed for the moving boundary problem in numerical simulations of drying processes. The calculation results for the drying of bricks show that the model presented is more precise than other models. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 337–351, 1999     

Effects of grain boundaries in polycrystalline silicon thin-film solar cells based on the two-dimensional model

The two-dimensional calculation for polycrystalline Si thin-film solar cells was performed. Two models, “stripe structure” and “columnar structure”, were applied for the solar cells composed of grains. For the stripe structure of 20 μm active layer, to keep the efficiency distribution within 5% for individual unit cells, the stripe width requires more than 500 μm for a minority-carrier lifetime of 1×10⁻⁵ s and recombination velocity at the grain boundary of 1×10⁴ cm/s. For the columnar structure of 10 μm active layer, to keep the efficiency independent of grain size, the recombination velocity should be kept less than 1×10³ cm/s. If imperfect passivation of a grain boundary is given, the way of decreasing carrier concentration to 10¹⁴ cm⁻³ in an active layer may realize insusceptible output. An appropriate device modeling is needed in the two-dimensional calculation for polycrystalline Si thin films with an electron diffusion length close to or more than grain size and with a poorly passivated grain boundary. The calculated efficiency using bad model will include an error of about 1% as overestimation.     

Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices

A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed. The dryer is a bin type with a rectangular perforated floor. The bin has a dimension of 1.0 m×2.0 m×0.7 m. Hot air is supplied to the dryer from fiberglass-covered solar collectors, which also function as the roof of a farmhouse. The total area of the solar collectors is 72 m². To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002–2003. The dryer can be used to dry 200 kg of rosella flowers and lemon-grasses within 4 and 3 days, respectively. The products being dried in the dryer were completely protected from rains and insects and the dried products are of high quality. The solar air heater has an average daily efficiency of 35% and it performs well both as a solar collector and a roof of a farmhouse.