Solar dryer with thermal energy storage systems for drying agricultural food products: A review

Developing efficient and cost effective solar dryer with thermal energy storage system for continuous drying of agricultural food products at steady state and moderate temperature (40–75 °C) has become potentially a viable substitute for fossil fuel in much of the developing world. Solar energy storage can reduce the time between energy supply and energy demand, thereby playing a vital role in energy conservation. The rural and urban populations, depend mainly, on non-commercial fuels to meet their energy needs. Solar drying is one possible solution but its acceptance has been limited partially due to some barriers. A great deal of experimental work over the last few decades has already demonstrated that agricultural products can be satisfactorily dehydrated using solar energy. Various designs of small-scale solar dryers having thermal energy storage have been developed in the recent past, mainly for drying agricultural food products. Therefore, in this review paper, an attempt has been taken to summarize the past and current research in the field of thermal energy storage technology in materials as sensible and latent heat in solar dryers for drying of agricultural food products. With the storage unit, agricultural food materials can be dried at late evening, while late evening drying was not possible with a normal solar dryer. So that, solar dryer with storage unit is very beneficial for the humans and as well as for the energy conservation.     

Review of solar assisted heat pump drying systems for agricultural and marine products

Combining solar energy and heat pump technology is a very attractive concept. It is able to eliminate some difficulties and disadvantages of using solar dryer systems or solely using heat pump drying separately. Solar assisted heat pump drying systems have been studied and applied since the last decades in order to increase the quality of products where low temperature and well-controlled drying conditions are needed. This paper reviewed studies on the advances in solar heat pump drying systems. Results and observation from the studies of solar assisted heat pump dryer systems indicated that for heat sensitive materials; improved quality control, reduced energy consumption, high coefficient of performance and high thermal efficiency of the dryer were achieved. The way forward and future directions in R&D in this field are further research regarding theoretical and experimental analysis as well as for the replacement of conventional solar dryer or heat pump dryer with solar assisted heat pump drying systems and solar assisted chemical and ground source heat pump dryers which should present energy efficient applications of the technologies.     

Experimental and simulation studies on the no-load performance assessment of a domestic solar dryer

Solar dryers have been tested with various simple and complex design modifications for better efficiencies. This article attempts to investigate the effect of a very simple design modification on the no-load performance of a natural convection domestic solar dryer (NCDSD). A direct-type domestic solar dryer has been developed with an air cavity surrounding the drying chamber. To compare the effect of air cavity, a domestic solar dryer without air cavity has also been developed and both the dryers were tested simultaneously under the climate of Hisar, India. The values of thermal efficiency were calculated along with convective heat transfer coefficient from absorber plate to the drying air. Both the dryers were also analyzed by developing finite element models in COMSOL multiphysics computer software. The no-load thermal efficiency for the domestic solar dryer without and with air cavity was found to be 22.68% and 34.08%, respectively. The values of coefficient of correlation for modeled and actual drying tray temperatures for dryer without and with air cavity were 0.980466 and 0.9833917, respectively. The proposed finite element model would be helpful in the design and development of NCDSDs.     

Design and testing of solar dryers for processing food wastes

This paper investigates the potential of using two solar dryers manufactured from locally available materials under Jordanian climatic conditions for drying food wastes for utilization as animal feed. The first dryer is a radiative–convective type while the second is a solar boiler dryer. Tests were also conducted to investigate the nutritious values of the dried products and their suitability as animal feed. It was found from tests that the solar boiler dryer is more efficient than the radiative–convective dryer for producing animal feed in terms of both quality and quantity. The nutritious values of the end products from the dryers were found to be within the international recommended values used for feeding chickens.     

Solar dryers in the Asia-Pacific region

The traditional method of solar drying in the Asia-Pacific region is by open-air drying where the product to be dried is exposed directly to the sun. The disadvantages of this traditional form of drying are many. Dirt, dust, and wind-blown debris, insect infestation, human and animal interference, will result in contamination of the product. Speed of drying and quality of dried product will be reduced due to over/under drying, intermittent sunshine, interruption and wetting by rain. Mechanical solar drying using natural convection or forced circulation would minimise these disadvantages. The author of this paper visited a number of countries and studied the numerous designs available for solar drying in the region. Three types of solar dryers considered by the author as having the best potential for development in the Asia-Pacific Region are discussed. These are the natural convection cabinet-type solar dryer, the forced convection indirect solar dryer, and the greenhouse-type solar dryer.     

Artificial breeding of an optimized solar tunnel dryer using genetic algorithms

Studies were carried out to artificially breed an optimized solar tunnel dryer using genetic algorithms (GAs). The energy harnessed by the dryer was simulated in Visual Basic Script (Microsoft Visual Basic Script 2010^TM) and the model was used to optimize the dryer by executing the Goal GA. The optimized dryer was developed and tested for energy harnessing against an existing solar tunnel dryer. The results of the analysis showed an 18–113% increase in plenum chamber temperature for the two dryers. Further, a two-way analysis of variance demonstrated the existence of a highly significant difference between plenum chamber temperatures for the two dryers (F=16.37, F _crit, 0.99=2.89). Furthermore, regression analysis and Student's t-test established the performance of the optimized dryer to be superior to that of the existing dryer. Finally, this study showed the effectiveness of Goal GA in artificial breeding of an optimized solar tunnel dryer.     

A review of solar drying technologies

Agricultural products such as coffee, tobacco, tea, fruit, cocoa beans, rice, nuts, and timber generally require drying through a consistent application of relatively low heat. Traditionally, crop drying has been accomplished by burning wood and fossil fuels in ovens or open air drying under screened sunlight. These methods, however, have their shortcomings. The former is expensive and damages the environment and the latter is susceptible to the variety and unpredictability of the weather. Solar crop drying is a happy medium between these two methods and it dries crops with more efficiency, uniformity, and less expense. A solar crop drying system does not solely depend on solar energy to function; it combines fuel burning with the energy of the sun, thus reducing fossil fuel consumption. In this paper, the status of solar drying technologies in developing countries is presented. The various designs of solar dryers, its types and performance analysis are reviewed. Special attention is given to the solar drying technologies that facilitate drying of crops in off-sunshine hours. The solar dryers specifically designed or tested using specific crops like the vegetable dryer, fruit dryer, grain dryer, grape dryer, and so on are also reviewed with details about the specifications and the results. In short, the state of art technologies and development of solar dryers are presented in this paper.     

Testing method for thermal performance based rating of various solar dryer designs

A generalized methodology is developed for thermal testing of various solar dryer designs operated for natural and forced air flow conditions. The steady state mathematical model based on heat balance concept of solar dryer without load is applied to identify the dimensionless parameter called no-load performance index (NLPI). Laboratory models of direct (cabinet), indirect and mixed mode solar dryer are designed and constructed to perform steady state thermal tests for natural and forced air circulation. The dryers with no-load are operated with air passage between absorber plate and glass cover for the range of 300–800 W/m² and 0.009–0.026 kg/s of absorbed thermal energy and air mass flow rate respectively under indoor simulation conditions. The present study reveals that the forced convection operated dryer provides higher NLPI in contrast to that of natural convection. The comparative performance analysis of dryers indicates that the mixed mode dryer exhibits maximum value of NLPI followed by indirect and cabinet ones for both natural and forced air circulation. It is also found that for any dryer operating at given air flow condition, almost invariable NLPI values have been obtained for a wide range of absorbed energy and ambient air temperature data, thus facilitating performance comparison between different dryer designs on equitable basis. The results of statistical analysis showing low standard errors of mean further demonstrate good consistency in NLPI values for various dryer designs. The uncertainty in NLPI due to error in measurement of several parameters by instruments ranges from 0.79 to 1.96% for various dryer designs operated under different conditions.     

正交各向异性相变材料的无网格法传热模型及应用

利用无网格迦辽金(EFG)法建立正交各向异性相变材料的传热计算模型,基于该模型编程完成各向异性材料太阳能相变蓄热水箱和管壳式相变蓄热单元的相变传热分析,并探讨热导率因子和材料方向角对复合材料相变传热特性的影响.研究表明:在相同节点布置下EFG法的温度场和相界面计算精度均高于有限元法,EFG法在动态相界面追踪方面具有明显...     

Optimization of mixed-mode and indirect-mode natural convection solar dryers

This paper presents a comparison of optimized mixed-mode and indirect-mode natural convection solar dryers for maize. The mixed-mode and indirect-mode solar drying simulation models were validated against results from a laboratory solar dryer with experiments carried out under a solar simulator at the University of Newcastle upon Tyne, UK. The models are now run under variable solar conditions in order to optimize the dryers and compare their performance. The inputs to the simulation models were Zambian weather conditions and materials. The solar drying simulations are combined with the cost of the dryer materials and a search technique that finds the dryer dimensions at the minimum drying cost. Optimization gave a shorter collector length for the mixed-mode solar dryer (1.8 m) than for the indirect-mode dryer (3.34 m) of the same grain capacity (90 kg). The drying cost, annual cost and initial cost of the mixed-mode dryer are lower than those of the indirect-mode although the quantity of dry grain obtained from the mixed-mode for the whole year is less than for the indirect-mode; the drying costs are 12.76 and 16.05 US$/ton for mixed-mode and indirect-mode dryers, respectively.     

A review on thermal energy storage systems in solar air heaters

The drying needs of agricultural, industrial process heat requirements and for space heating, solar energy is one of the prime sources which is renewable and pollution free. As the solar energy is inconsistent and nature dependent, more often there is a mismatch between the solar thermal energy availability and requirement. This drawback could be addressed to an extent with the help of thermal energy storage systems combined with solar air heaters. This review article focuses on solar air heaters with integrated and separate thermal energy storage systems as well as greenhouses with thermal storage units. A comprehensive study was carried out in solar thermal storage units consisting of sensible heat storage materials and latent heat storage materials. As the phase change heat storage materials offer many advantages over the sensible heat storage materials, the researchers are more interested in this system. The charging and discharging characteristics of thermal storage materials with various operational parameters have been reported. All the possible solar air heater applications with storage units have also been discussed.     

Review of solar dryers for agricultural and marine products

Drying for agricultural and marine products are one of the most attractive and cost-effective application of solar energy. Numerous types of solar dryers have been designed and developed in various parts of the world, yielding varying degrees of technical performance. Basically, there are four types of solar dryers; (1) direct solar dryers, (2) indirect solar dryers, (3) mixed-mode dryers and (4) hybrid solar dryers. This paper is a review of these types of solar dryers with aspect to the product being dried, technical and economical aspects. The technical directions in the development of solar-assisted drying systems for agricultural produce are compact collector design, high efficiency, integrated storage, and long-life drying system. Air-based solar collectors are not the only available systems. Water-based collectors can also be used whereby water to air heat exchanger can be used. The hot air for drying of agricultural produce can be forced to flow in the water to air heat exchanger. The hot water tank acts as heat storage of the solar drying system.     

强化太阳能相变蓄热技术的研究进展

为解决太阳能的间歇性问题,常将其与相变蓄热技术进行结合。与传统显热蓄热相比,相变蓄热可将蓄热能量提高数倍以上,具有巨大的研究和应用价值。本文总结分析了相变蓄热的传热机制及在强化太阳能相变蓄热技术上的研究手段,如变换蓄热结构、添加肋片、使用相变胶囊、充注多相变材料、蓄热材料中添加高导热物质等。分析结果显示,相变传热机制中,融化过程主要考虑对流换热,凝固过程热传导占主导;使用肋片、相变胶囊等,主要增大相变材料接触面与蓄热体的比值,进而改善传热;蓄热材料添加高导热物质,可以改善相变材料的团聚、结核及使用寿命,从而提高导热性能,其中添加泡沫金属效果最为显著。     

太阳池水合盐相变贮热的探讨

太阳池水合盐相变贮热能形成梯度层，既显热贮热又相变贮热，具有跨季大容量单一体系热贮存的特点，全年热循环仅３－５次。可以采用溶解盐粒子等办法使相变材料贮热能力再生，讨论了几种水合盐的相律和溶解度，为选择适用于太阳池的相变材料提供参考。     

Development of a multi-purpose solar tunnel dryer for use in humid tropics

Particularly in tropical countries, a high percentage of crops are spoiled during the drying period due to permanently high relative humidity and sudden rainfalls. Open air sun drying and smoke drying, which are traditional drying methods in these regions, lead to insufficient product quality. In order to be marketable, the produce meant for sale on the world market has to meet high quality standards. Otherwise the price will decline resulting in low profits for the exporting country and the producing farmer. To produce high quality products for the world market as well as to reduce losses, the development of suitable dryers for tropical regions is urgently needed. A multi-purpose solar tunnel dryer, orginally developed for the use in arid zones, was modified to enable operation under tropical weather conditions. This type of dryer consists of a small centrifugal blower, a collector and a tunnel drying chamber. To prevent penetration of water into the construction and subsequent flooding, the solar dryer was installed on a wooden substructure. To heat the drying air during cloudy and rainy days, particularly in the rainy season, a biomass furnace with heat exchanger was integrated into the solar drying system. The construction consists exclusively of materials available in Java. Only low energy requirements are necessary to run the blower. Results showed that compared to natural sun drying, the drying time of cocoa, coffee and coconut could be reduced up to 40%. Solar drying improves the quality of the products in terms of colour, flavour and appearance, reduces the risk of microorganism growth, prevents insect infestation and contamination by foreign matters and mycotoxins. The products dried with the solar drying system meet the national and ingternational market standards. Investigations further showed that even during the rainy season it was possible to dry the products to the final moisture content which is needed for storage and marketing. The modular system allows adaptation to different farm sizes as well as cooperative use.     

Performance analysis of double-pass oscillating bed solar dryer for drying of non-parboiled paddy grains

Drying of agricultural produce like paddy grains is necessary for reducing moisture content in them to the required level towards diminishing the deterioration and enhancing the storage time. Conventionally, open-space sun drying is widely used to reduce the moisture content of the most of the agricultural produce.. Now-a-day, so many solar dryers are used for drying agricultural produce. In the present work, a double-pass oscillating bed with a double-pass flat plate collector has been used for drying of non-parboiled paddy grains. The bed oscillated at a frequency of 2.75 Hz. The oscillating bed solar dryer can dry 45 kg of non-parboiled paddy grains in just a single day to the required level of moisture content, and the average moisture present in the dried grains was 13.55%. The average dryer thermal efficiency and pick-up efficiency in non-parboiled paddy were 28.79% and 52.17%, respectively.     

Parametric study of a reverse flat plate absorber cabinet dryer: A new concept

In this communication, the concept of reverse flat plate collector has been used as a heating medium of air for the drying of agricultural products in a cabinet dryer. The reverse flat plate absorber is a non-concentrating collector which can collect solar heat at high temperature unlike conventional nonconcentrating collectors. The thermal performance of the proposed dryer is analyzed by solving the various energy balance equations. An attempt has been made to optimize the vent area of the dryer for speedy flow of humid air from the drying chamber to the atmosphere. In order to have parametric studies, numerical computations have been carried out for a typical day in June for Delhi climatic conditions. The performance of this system is compared with that of conventional cabinet dryers. It is found that the reverse flat plate absorber dryer gives the better performance.     

固-固相变蓄热材料在太阳能高温空气集热系统中的应用研究

为利用太阳能获得稳定持续的高温空气工质,除了有效集热外,还需要解决因太阳辐射强度变化导致输出工质温度波动的问题。在性能优良的太阳能集热系统中采用蓄热技术是解决此问题的有效途径。根据给定的设计目标,研究将固-固相变蓄热材料季戊四醇应用到太阳能集热蓄热一体化的实验装置中。实验结果表明:按集热蓄热一体化思路设计的实验装置,集热单元能够输出最高温度超过220℃的高温空气,蓄热单元能够将高温空气的温度稳定在蓄热材料的相变温度附近。并且随着蓄热管级数的增加,空气出口温度稳定的时间就越长,为利用太阳能获得稳定持续的高温热媒工质奠定了基础。