PRACTICAL APPLICATIONS OF USING SOLAR COLLECTORS FOR DRYING PURPOSES IN AGRICULTURE

The main application for solar collectors in agriculture are drying of agricultural crops. Different types of active air type solar collectors have been examlned In small scale experiments and in full scale trials. This report deals with practical applications OF solar collectors for dryers. The investigations have shown that solar collectors can replace conventional heating units based on 011, electricity etc. for the heating. The most economical and practical solution is received lf the collector is integrated in the roof or a wall of a farm building. The report gives recommendations for the construction and For the choice of materials. Design dates and operation schedules are also presented.     

Design of a small solar-powered desalination system

A design is presented for a solar/thermal system configured to power a reverse osmosis (RO) desalination unit to produce 7000 gallons of fresh water in an eight hour period. A field of line-focus tracking solar collectors is used to heat a high pressure liquid-vapor water storage tank supplying two compound reciprocating steam engines, one direct-connected to the RO high-pressure pump and the other to an electric generator for auxiliary power. An auxiliary heating loop with an oil-fired boiler is also used to supply the steam engines.The system operates in either all-solar, all-oil, or mixed solar/oil modes. Primary operating mode is assumed to be a mixed solar/oil mode in which the oil-fired boiler is used only to prevent shutdown of the RO system during the course of a partly sunny day. In this mode, the RO system does not come on line in the morning until the solar collector field has brought the high-pressure storage tank to a point near maximum operating pressure. Thereafter, the oil-fired boiler comes on automatically whenever the storage tank is drawn down to a pressure near minimum full-power operation (due to inadequate or intermittent insolation) and remains on, supplying the steam engines, until the solar collectors have again brought the storage tank to the high-pressure cutoff.In the all-solar mode, the system continues to operate at reduced power as storage tank pressure drops below the point at which the oil-fired boiler would otherwise come on. A portion of the RO system is shut down to maintain pressure in the remainder.The all-oil mode is used whenever fresh water is required during non-sunny periods, or to increase fresh water production in sunny periods.     

Water disinfection by solar photocatalysis using compound parabolic collectors

TiO₂ solar photocatalysis has been proven to be a degradation process for aqueous organic contaminant leading to total mineralisation of a large number of compounds. Furthermore, the interest in using this technique for water disinfection has grown in the last decade. Recent publications have reported photokilling of bacteria and viruses by TiO₂ photocatalysis. Therefore, solar photocatalysis disinfection seems to be a very promising process, which could help to improve public health in rural areas of developing countries.

The objective of this work was to assess the feasibility of using TiO₂ solar photocatalysis to disinfect water supplies for future applications in developing countries. This article reviews the viability of solar photocatalysis for disinfection in low cost compound parabolic collectors, using sunlight and titanium dioxide semiconductor, both applied as slurry and supported. We report on the bactericidal action of TiO₂ on a pure culture of Escherichia coli with a low cost photoreactor based on compound parabolic collectors. The influence of different experimental set-ups and parameters are also analysed.

The results and potential application of the solar photocatalysis technology to water disinfection are studied within the frame of two research EU projects whose objective consist on the development of a fully autonomous solar reactor system to purify drinking water in remote locations of developing countries.     

Polymeric Materials for Solar Energy Utilization: A Comparative Experimental Study and Environmental Aspects

Full-scale metal solar collectors and solar collectors fabricated from polymeric materials are studied in present research. Honeycomb multichannel plates made from polycarbonate were chosen to create a polymeric solar collector. Polymeric collector is 67.8% lighter than metal solar collector. It was experimentally shown that the efficiency of a polymeric collector is 7–14% lower than a traditional collector. An ecologically based Life Cycle Assessment showed the advantages of the application of polymeric materials in the construction of solar collectors.     

Thesis Summary: Theoretical and Experimental Investigation of a Solar-Biomass Hybrid Air Heating System for Drying Applications

The overall objective of this research was to design, develop, and investigate the performance of a renewable energy-based (solar-biomass) hybrid air heating system. The system consists of an unglazed transpired solar collector (UTC), a rock bed thermal storage, and a biomass gasifier stove with heat exchanger, to supply hot air at a required temperature and flow rate for a daily load fraction exceeding 90%. From a review of literature, an air heating system aimed at reducing the weather dependency and improving the temperature and flow rate stability without a conventional back-up heater was designed. Among the various types of solar collectors, thermal storage, biomass stoves, and heat exchangers that are generally used, specific designs were chosen and analyzed. Based on the analyses, individual components of the air heating system were designed and developed. The components were coupled, and detailed experimentation was carried out on the integrated system.     

Energy,Thermal, and Economic Analysis of Air Heat Pumps for Hot Water

Operation tests of an air heat pump used for domestic hot water (DHW) heating were performed. The influence of the supply air temperature in the evaporator cycle and the variable heat load of the condenser on the heating power of the heat pump and the coefficient of performance (COP) was evaluated and discussed. In order to ensure conditions for efficient compressor operation, it is advisable to gradate the heat load of the heat pump using a circulating pump with smooth capacity regulation. The economic analysis demonstrates that the usage of air heat pumps for DHW in comparison with installations equipped with solar collectors provides no measurable economic benefits.     

Solar barometric distillation for seawater desalting Part II: Analyses of one-stage and two-stage distillation technologies

The report concerns design aspects for the recently proposed solar barometric distillation technology for seawater desalting (SW–SBD) with an underground barometric layout. Two types of SW–SBD desalting plants are analysed. The first plant utilizes a single-stage distillation process with one distillation heat exchanger (operative condensation temperature 50°C). The second plant utilizes a two-stage distillation process with two distillation heat exchangers connected in series (operative condensation temperatures 40 and 60°C). Vacuum solar collectors of simple design and construction, utilizing glass or suitable glass/polymer blends as transparent material, are proposed for the SW–SBD plants. The present analyses suggest that SW–SBD desalting technology may have a promising technico-economic potential. Field research on SW–SBD prototype plants is necessary to bring SW–SBD desalting technology to its full technological development.     

Janus三角纳米片和"三明治"三角纳米片消光特性的数值研究

基于离散偶极近似法(DDA)计算了不同组分的Janus三角纳米片和“三明治”三角纳米片的消光特性,两种纳米片在紫外-可见光波段均出现了明显的吸收峰。当纳米片较小时,消光特性主要以吸收为主,当纳米片逐渐增大,散射作用开始明显。当纳米片增大时,吸收峰向长波方向移动并且峰会变宽。二氧化硅与银的组合能在较宽的波段内激发表面等离激元效应,因此波峰比金银组合纳米片的波峰宽,但是吸收峰值较后者低。增加纳米片的层数,或者添加纳米片的组分,可以在一定范围内对纳米片的消光特性进行调节,从而提高太阳能光热转换效率。     

离子液体在传热及相变储热中的应用研究进展

离子液体具有与传统的传热、储热材料相当,甚至更加优越的性质,如蒸气压低,储热密度高,物理和化学稳定性好,热传导性好,熔点低和可设计性等。因此,离子液体在太阳能集热、建筑节能、电力谷峰调控、低品位余热存储、吸附式热泵等领域具有良好的应用潜力。综述了离子液体在传热和储热中的应用研究进展,包括作为热传导液用于太阳能集热,作为吸附介质应用于制冷(制热),以及作为相变储热材料等。最后,指出离子液体的一些性质,如腐蚀性、毒性和长期稳定性等,也是离子液体在储热和传热应用中需要考察的问题。     

Energy–exergy–ANN analyses of solar-assisted fluidized bed dryer

In this study, a temperature-controlled solar air collector was designed and tested for drying. Solar drying systems have two disadvantages. First one is the lack of ability to store energy and the second one is the lack of temperature control. This study presents the experimental analysis of an air collector that is able to keep the drying air temperature at 40°C even in cases where the level of solar radiation received by the collectors changes. Most of the tests were performed at a solar radiation level ranging from 500 to 900?W/m² and at an air flow of 3 to 5?m/s. The system tested for drying three different crops separately performed 21?h of a total of 27-h drying period at or above the temperature set of 40°C. The thermodynamic analysis of the relationship between solar radiation, air temperature, flow, and the produced energy was performed. The relationship between productivity, energy produced, and set temperature was analyzed using distribution charts. Moreover, an artificial neural network model was used to estimate outlet air temperature from the solar collectors based on air flow, solar radiation, and outside air temperature.     

Flow rate distribution in evaporating parallel pipes—modeling and experimental

The motivation for the present work is related to the use of parabolic trough solar power technology for direct steam generation (DSG). So far, commercial plants use liquid oil to absorb heat from the collectors, and steam is produced in heat exchangers. A more efficient process of direct steam generation in the collectors is not used due to possible uneven flow rate distribution and other instabilities, related to liquid-vapor flow in parallel pipes.In this work we present an analysis and experimental results for the flow rate distribution of water flowing and evaporating in two parallel pipes with common inlet and outlet manifolds.A new simplified model that yields the pressure drop versus the flow rate in a single pipe is developed. This model is the basis for the calculation of steady-state solutions, stability analysis and transient simulations in two parallel pipes.Multiple steady-state solutions may be obtained depending on the flow rate and the heating power. Linear stability analysis and transient simulation allow to differentiate between the stable steady states and the unstable ones. It is also shown that when few stable state solutions are possible the one that takes place depends on the history of the inlet flow rate. Experimental results compare well with the theoretical solutions.     

The Coloration of Metals

The traditional recipes for metal coloration are briefly mentioned and the problems associated with these processes are discussed. The important developments since 1945 are reviewed; particular attention is paid to the industrial processes that have become established for the coloration of aluminium, stainless steel, zinc and gold. Processes for the blackening of metals for use as solar collectors are also discussed.     

石墨烯纳米片-乙二醇纳米流体光热转化特性研究

纳米流体应用于太阳能集热器是太阳能光热转化的重要突破，石墨烯纳米材料在可见光和近红外区域具有较好的吸收特性，实验基于Hummer法制备了石墨烯纳米片材料，对其进行表征。并配制了不同质量分数石墨烯纳米片-乙二醇纳米流体，将其在太阳能模拟器下进行闷晒实验，计算石墨烯纳米片的光热转化效率，并以基液作对比分析其光热转化特性。结果表明纳米流体溶液的光热转化效率随着其浓度的增加而提高，在达到临界值后光热转化效率不再提高反而降低。其中浓度为0.0007%(质量)时的石墨烯纳米片纳米流体溶液温度增加最高，为65.56℃，光热转化效率达到最高约为76.35%，较乙二醇效率升高49.65%。表明石墨烯纳米片具有良好的光学特性，在太阳能集热器中具有较好的应用前景。     

Economics of small solar-assisted multiple-effect stack distillation plants

The objective of this paper is to compare the economics of using solar energy to operate small, multiple-effect seawater distillation systems in remote areas with the conventional method of using fossil fuels. The particular multiple-effect system used is an advanced horizontal-tube, falling-film system called “multiple-effect stack” (MES) in which the pumping energy requirement is relatively low compared with the horizontal in-line system. Three system configurations were investigated: (1) a conventional system using a steam generator to provide steam for the MES evaporator and a diesel generator to provide pumping power, (2) a solar-assisted system which uses solar thermal collectors to provide hot water (instead of steam) for the evaporator and a diesel generator for pumping power, and (3) a solar stand-alone system which uses solar thermal collectors for the evaporator heat requirement and a solar PV array to provide electrical energy for pumping. At the present time, solar energy cannot compete favorably with fossil energy, particularly under the present international market prices of crude oil. However, in many remote sunny areas of the world where the real cost of fossil energy can be very high, the use of solar energy can be an attractive alternative. Two important cost parameters affect the relative economics of solar energy vis-à-vis conventional (fossil) energy: the collector cost in dollars per square meter and the cost of diesel oil in dollars per giga Joule. Solar energy becomes more competitive as the local cost of procuring conventional fuel increases and as the collector cost decreases. The water cost from a solar thermal-diesel-MES system (configuration #2) can be seen to approach the water cost from a steam generator-diesel-MES system (configuration #1) when the collector cost drops to $200/m² and diesel oil cost at the remote site reaches $50/GJ. Using a 100% solar system (configuration #3) with solar thermal and solar PV collectors, the economics was seen to improve in favor of the solar system. Even when diesel fuel can be procured at $10/GJ at the remote site, the cost of water from the solar system can be seen to approach that from a conventional plant when thermal collectors costing $200/m² are used. The cost of water from the solar system was shown to be always less than that from a conventional system which uses diesel oil procured at the high price of $50/GJ, but always higher than water produced from a conventional system using diesel oil at the low price of $10/GJ.     

Comparison of solar RO and solar thermal desalination systems

In this paper the unit water costs from 100m³/day solar RO and solar distillation plants are compared. RO plants powered by photovoltaic cells are already available, and distillation plants operated from solar collectors are being constructed. For the size chosen and these options it is concluded that distillation is cheaper than RO for salinities above about 6,000ppm.Solar ponds can provide both thermal and electrical energy more cheaply than the above options, but they do not yet have the market penetration of the more accepted systems. If solar ponds are used then distillation is cheaper than RO only for salinities above about 10,000ppm.     

新型太阳能/空气能直膨式热泵与空气源热泵供热性能对比

针对传统空气源热泵制热时极易出现室外机结霜的现象，提出了可同时吸收太阳能和空气能的新型太阳能/空气能直膨式热泵机组，把太阳能集热器和热泵蒸发器合二为一，使室外机结霜得到有效缓解。为了验证新型太阳能/空气能直膨式热泵机组性能优劣，分别搭建新型太阳能/空气能直膨式热泵系统和空气源热泵系统，在2月26日—3月2日期间，对邯郸某一农村建筑的地板辐射采暖用户进行5天实际测试，对比分析了两种系统的制热性能、耗电量和COP变化情况。通过测试发现室外平均温度为10℃，太阳辐射达到峰值571.5 W·m^-2时太阳能/空气能直膨式热泵的制热量较空气源热泵提高大约70%，全天总制热量较空气源热泵提高大约12%。0～8℃的低温状态时，COP值仍可达到3.46，基本满足建筑采暖需求。并在此基础上对太阳能-空气能直膨式热泵提出进一步的优化措施，逐步推广其在寒冷地区的实际应用。     

SOLAR GRAIN DRYING IN HUMID AND HOT-TEMPERATE CLIMATES

A natural solar drying system has been tested with wheat. Simple solar collectors give the small temperature rise needed for low temperature drying. The system, with a capacity of 35 tomes, is placed in a region of humid hot-temperate climate. It has been designed to dry all the locally produced grain (wheat, corn, soybean, sunflower, sorghum, etc.), without a supplemental heat source.

Two different types of solar collectors of the same effective area were tested, with one a storage-collector and the other a bare plate collector

Data from four field tests performed between 1981 and 1983 are given and discussed. The mean mass of water removed from the wheat per hour was of 6.54 kg, under an average air flow of 4.3 m3 /min. tonne     

FUNDAMENTALS OF LIQUID SPRAY TECHNIQUES

Two twin forced convection dryers of 1.5 m³ were built in Majorca (Spain). They are of a mixed kind, with solar air collectors and a green house type chamber. A wooden frame supports polycarbonate walls. After two years operation they have been proven weather resistant. Six solar air collectors 2.12 × 1.05 m were used in each dryer.

Apricots were processed in both dryers and at open sun. Three different tray heights were tested 5, 9 and 12 cm. The best results were obtained with 12 cm trays. Recycling part of the exhaust air improves the efficiency of the dryer. Blanching the fruits makes no difference to the dehydration rate. The rate of SO₂ loss during the drying process is higher within the chamber.     

SOLAR DRYING OF FRUITS IN A MEDITERRANEAN CLIMATE

Two twin forced convection dryers of 1.5 m³ were built in Majorca (Spain). They are of a mixed kind, with solar air collectors and a green house type chamber. A wooden frame supports polycarbonate walls. After two years operation they have been proven weather resistant. Six solar air collectors 2.12 × 1.05 m were used in each dryer.

Apricots were processed in both dryers and at open sun. Three different tray heights were tested 5, 9 and 12 cm. The best results were obtained with 12 cm trays. Recycling part of the exhaust air improves the efficiency of the dryer. Blanching the fruits makes no difference to the dehydration rate. The rate of SO₂ loss during the drying process is higher within the chamber.     

SOLAR GRAIN DRYING IN HUMID AND HOT-TEMPERATE CLIMATES

A natural solar drying system has been tested with wheat. Simple solar collectors give the small temperature rise needed for low temperature drying. The system, with a capacity of 35 tomes, is placed in a region of humid hot-temperate climate. It has been designed to dry all the locally produced grain (wheat, corn, soybean, sunflower, sorghum, etc.), without a supplemental heat source.

Two different types of solar collectors of the same effective area were tested, with one a storage-collector and the other a bare plate collector

Data from four field tests performed between 1981 and 1983 are given and discussed. The mean mass of water removed from the wheat per hour was of 6.54 kg, under an average air flow of 4.3 m3 /min. tonne