A comprehensive review of the solar dryer

ABSTRACT

Drying of agricultural products is an energy-demanding process. High costs, scarcities in fossil fuels and environmental threats attract the use of solar energy as an alternate source, especially in developing countries. Increasing environmental concerns led to the use of clean and green energy resources in the field of drying agricultural products. A solar dryer integrated with a thermal energy storage system is reasonably efficient for continuous and uniform drying of agricultural produce in the temperature range of 40–75°C which is the requisite for drying most of the products. Such dryers have become a natural choice to replace drying systems based on fossil fuels. Many studies have been performed till date by a number of researchers for the last few decades for drying agriculture and food products by a solar dryer. This review paper primarily presents an appraisal of the significant contributions made so far in the field of solar drying systems, with the latest updates in drying technology. Solar dryers which offer drying during off-sunshine hours have been specially mentioned. Some novel ideas like use of phase change materials and desiccant materials which enhance the performance and effectiveness of the dryer have also been attempted.     

Design, construction and performance testing of a new system for energy saving in rural buildings

Drying fruit, heating residential buildings and providing a hot water supply in villages all consume energy. Using fossil fuel for these purposes creates pollution and costs too much. In contrast, the use of solar energy in these applications leads to a noticeable decrease both in pollution and investment costs. In this study, a new solar system was designed and tested in order to reduce energy usage in rural residential buildings and the food drying industry. As the peaks of energy consumption in the proposed system are not simultaneous, this new system is very effective in reducing energy consumption, controlling energy peaks and reducing environmental pollution. This system has the ability to provide the required energy in both summer and winter modes. In the summer mode, the energy supply is used for providing hot water and drying agricultural products, while in winter mode it is used for rural residences heating and hot water supply. Drying time has been varied between 51.23 and 42.45 h according to type of application, and average temperature difference between room and ambient is almost nearly 10 °C with different air heaters. The system includes energy supply and storage equipment, solar dryers, water collectors and rectangular, triangular, trapezoidal and double-pass with longitudinal fins air heaters. The system was tested in Iran for drying apricots, heating rural residential buildings and supplying hot water for domestic use, meanwhile, the energetic and exergetic efficiency of the system was calculated 37.3-61.3 and 3.2-9.7 respectively for different types of installations.     

Potential for Sewage Sludge Co-Combustion in the UK Based on German Experience

Increasingly strict legislation, escalating costs and external pressures are causing the water industry to look for alternative strategies to agricultural recycling for the disposal of biosolids. Although the development of the 'safe sludge matrix'has encouraged agricultural recycling, negative press has hampered this outlet and public perception is seen as a major threat. Accordingly, many water companies are looking towards thermal-reduction technologies and have invested in thermal dryers and sludge incinerators; however, such technology has high capital costs and complex maintenance and operator requirements. An alternative method, i.e. co-combustion, is becoming more popular in mainland Europe. Unlike incineration/drying, the capital expenditure is low and operator/maintenance risks are minimised.
This paper (i) discusses a case study in which 40 000 dry tonnes of sludge per annum have been successfully combusted since 1996, and (ii) explores how this experience could be applied in a UK situation.     

燃气红外线辐射器在污泥处理中的应用

介绍了一种以燃气作为干燥能源的污泥干化中试成套设备,对炉窑中使用的红外线辐射器数量、进泥含水率以及排放量等影响干化效果的主要因素进行了分析。该套设备干化含水量为82．5％的污泥,其成本为910元／t,生成干泥成本约为4000元／t。     

污泥干化成套设备研制及其性能分析

污泥热干燥处理是实现污泥减量化、无害化、稳定化的一种重要手段。介绍了一种自行研制的以煤气作为干燥能源的污泥干化中试成套设备,燃气红外干化炉窑是核心部分,在污泥干化过程中的热效率≥36．8％,并对影响其工作性能的几个主要因素进行了分析。     

太阳能烟囱技术应用研究

太阳能烟囱技术典型的三类用途:室内通风、发电、农产品干燥。本文概括介绍了三类用途的特点、研究现状,并探讨了其发展方向。     

Auswirkungen der Baurestfeuchte auf Innenklima und Holzfeuchte bei Holz‐Mischbauweisen

Effects of the build‐in‐moisture on indoor climate and wood moisture of timber mixed construction Within the scope of the competence center “wood technology” the Holzforschung Austria deals with the combination of prefabricated timber elements in the building shell with concrete construction parts for high rise buildings. In addition to the design of construction details a one‐zone model has been created – in close collaboration with the Technical University of Vienna – to analyze the effects of residual humidity on the internal climate and on the moisture content of the timber elements. The simulated drying properties of the concrete elements are consistent with the results of the analysis of the Verein der Österreichischen Zementfabrikanten in matters of drying time and change of moisture content. In addition to the variation of the beginning and progress of construction work and the behavior of air conditioning three different exterior wall constructions – one timber frame construction with OSB and one with PE as vapour barrier and a construction with solid timber boards without vapour barrier have been analysed.     

砖坯干燥原理及应用

介绍砖坯水分存在形式和干燥原理,总结生产实践中干燥工艺特点,给出了新建中大断面隧道窑应在干燥工艺设计方面应注意的要点。     

链板式二次码烧快速干燥窑的研究开发及实际应用效果分析(一)

在调查国内外干燥技术的基础上,结合陶瓷挂板单层码放、多层快速干燥流程,在我国砖瓦行业首次研发制造了链板式二次码烧快速干燥窑,并成功用于砖厂技术改造,取得了可喜的研发应用效果。     

小截面长隧道窑生产潜力的认识

详细介绍了小截面直通道一次码烧长干燥焙烧隧道窑的结构特点及优点。     

An Energy-Efficient Combustion System for High-Moisture Organic Wastes and Biomasses

A special combustion system comprising a drier, combustion unit and heat-recovery unit, designed to produce low-grade heat from high-moisture fuels, was investigated using sewage sludge, animal manure and energy crops as the feed material. Part of the investigation dealt with the development of a new method of combined wet pelletizing and surface drying of sludges to facilitate simple and efficient combustion as well as easy handling of the dried product. The investigation showed that (a) drying at temperatures up to at least 950° is feasible, and (b) low-temperature heat may be produced for space and water-heating purposes at high thermal efficiency.     

Some engineering properties of stabilized clayey soils incorporating natural pozzolans and industrial wastes

Clayey soils are stabilized with various dosages of cement kiln dust, volcanic ash and their combinations. The influence of stabilizers is evaluated through Atterberg limits, standard Proctor compaction, unconfined compressive strength, splitting tensile strength, modulus of elasticity and California bearing ratio (CBR) tests. The durability properties of 14 stabilized soil mixtures are also investigated by studying the influence of water immersion on strength, water sorptivity and drying shrinkage. Correlations between strength, modulus of elasticity and CBR are also established. Developed stabilized soil mixtures have shown satisfactory strength and durability characteristics and can be used for low-cost construction to build houses and road infrastructures. The use of stabilized soils with locally available soils, volcanic ash and cement kiln dust can provide sustainability to the local construction industry.     

The use of a heavy internal wall with a ventilated air gap to store solar energy and improve summer comfort in timber frame houses

This paper considers supplementary heating and cooling within timber frame houses. The transmission of solar energy to an internal concrete cavity wall by air is analyzed. The objective of this work was initially to study the dynamic insulation in timber frame houses. The initial studies showed that it is more efficient to recover solar energy rather than heat losses, which is the principle of dynamic insulation. Clearly, the thermal regulations lead to lower heat losses through walls by conduction. Due to these factors we have decided to study a wall with an integrated solar air collector and a heavy ventilated internal wall. This internal wall, which is used to store solar energy will allow the reduction of heat demand in winter and will improve thermal comfort in summer because thermal mass increases and ventilation during the night will cool the internal wall. We have selected a closed loop air circulation system because, with an air to air heat exchanger, it can be proved to be more effective and the risk of unhealthy air pollution is reduced because the flow of fresh air will not pass through the ventilated air gap. We are constructing an integrated air collector prototype.     

Analysis of feasibility on heating single family houses in rural areas by using sun and wind energy

Households in Lithuania consume about 1/3 of total final consumption of fuel energy. In order to reduce imports of fossil fuel and emissions of dangerous pollutants, solar energy could be used for the above-mentioned needs. That would require large collector areas and volumes for seasonal heat storage. In wintertime the wind speed velocity is much higher than in summertime in Lithuania. Therefore, it is advisable to study meeting the thermal needs of single family houses by combining use of wind and solar energy. To this end analytical research has been made by using deterministic method. The analysis has been carried out for the case when 1 m² of heated room area requires 0.25 m² of solar collector area and 0.5 m² working area of wind turbine rotor. Heat storage is planned for 24 h. By using such a hybrid system during the heating season 42.6–56.2% of heating needs for space and domestic hot water are satisfied. However, for individual days (especially from May to October) a surplus of generated heat is formed and it reaches about 53.6% of space heating needs per year. This relative surplus of energy could be used for transmitting wind power-plant energy to the electric network or in a household and thermal energy can be used for drying agricultural produce, heating greenhouses, open swimming pools and satisfying other needs.     

自然劣化对文物建筑木质构件热性能的影响

为探讨自然劣化对文物建筑木质构件热性能的影响,采用傅立叶变换红外光谱（FTIR）与差式扫描量热分析法（DSC）对4种文物建筑木质构件样品与云南松现代材样品进行官能基团和热稳定性能测定,讨论了自然劣化对其的影响。结果显示,较软木松而言,硬木松即使在自然劣化后仍然有着较好的耐燃性能。同时,木质构件在自然劣化过程中主要发生了木质素与半纤维素的分解。清代劣化材由于木质素含量减少,干燥阶段与固相燃烧阶段所需温度都有大幅度提前,更易热解。另外,虫害情况越严重,老化材纤维素含量越少,放热量随之降低,同时木构件化学结构越不稳定,劣化材干燥阶段所需温度随之降低,热解反应越剧烈。     

利用湖泊淤泥掺粉煤灰生产平瓦

介绍湖泊淤泥掺配粉煤灰平瓦生产过程中原料的处理、混合料的掺配与陈化、坯体成型与干燥、码窑与焙烧等工艺及操作技术.     

Timber-framed partition walls and their restraining effect on warp in built-in wall studs – A model for spring

The shape of timber changes due to variations in moisture content (MC). A poor way of designing and/or erecting a structure may result in excessive deformation. An analytical model, which explains spring in a partition wall, is presented. The model is used for parametric studies of various properties such as longitudinal shrinkage coefficient, modulus of elasticity and cross-sectional size of the timber studs and stiffness of the cladding. With respect to spring, a problem is likely to occur when erecting a wall with single-sided cladding in a drying climate and the structure will benefit from relatively stiff studs and weak cladding.     

浙江杭嘉湖平原区农业地球化学区划的初步研究

根据杭嘉湖平原区区域农业环境地球化学调查资料及在此基础上开展的区域农业、专题农业地球化学研究结果，对本区进行了农业地球化学区划的初步研究。研究结果将本区分为泻湖湖沼粮油桑地球化学区、水网平原粮桑油地球化学区、滨海平原麻棉菜地球化学区和低山丘陵菜竹林地球化学区，提出了本区１０种营养元素施肥规划，阐述了每个地球化学区的作物布局、施肥方法，预测了据此结果可能给本区农业生产增加的经济效益。     

Experimental study and simulation of a solar dryer for spearmint leaves (Mentha spicata)

An indirect forced convection solar dryer consisting of an air flat plate collector and a drying cabin was designed and fabricated to investigate its performance under the climate of Algiers. Drying experiments have been performed for spearmint leaves at different air flow rates in order to determine the drying velocity, the characteristic drying curve, the effective diffusion coefficient and the activation energy. A mathematical model based on thermal and mass balances over the component of the solar collector and the cabinet dryer was developed. Simulations are carried out for meteorological data of Algiers (Algeria). We analyse the effect of air mass flow rate, air temperature, products mass, collector area, air recycling rate on the drying time, the solar fraction and the efficiency of the dryer.     

Numerical model of natural convective heat transfer within a solar dryer using an indirect double pass collector

The double pass collector is one of the most efficient means of drying crops because heating of air takes place from both sides of the absorber plate. To investigate the performance, a double pass natural convection solar dryer was fabricated for experimentation; the results were collected and used to develop a validated numerical model using MATLAB. The experiments were undertaken during February–March 2016 in Salem Tamil Nadu, India over 10 sunshine days. The simulation was carried using the developed model under the measured ambient conditions. The results showed that the mathematical model could predict the performance of the double pass collector for a particular ambient condition to an uncertainty of ±5%. This model could be used to optimise the performance of the collector so that we can predict and investigate the actual performance of the collector with the various optimised designs before fabricating the dryer.