Comparison of the energy yield predictions of stationary and dynamic solar collector models and the models' accuracy in the description of a vacuum tube collector

One stationary and five dynamic models for solar collectors are analyzed in different ways. (1) The models are fitted to a set of measured data and the resulting collector parameters are compared. (2) Step responses of the models are compared in order to illustrate their different reactions to transient conditions. (3) The total daily energy yield predicted by the models for three days of different weather conditions is calculated using a reference collector parameter set. It turns out that the stationary model overpredicts the collector output by up to 15% in the case of one-minute averages of input data, while all dynamic models yield similar results. In the case of hourly input data, the error of the stationary model nearly vanishes.     

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.     

Design of mixed-mode natural convection solar crop dryers: Application of principles and rules of thumb

A mixed-mode natural convection solar crop dryer (MNCSCD) designed and used for drying cassava and other crops in an enclosed structure is presented. A prototype of the dryer was constructed to specification and used in experimental drying tests. This paper outlines the systematic combination of the application of basic design concepts, and rules of thumb resulting from numerous and several years of experimental studies used and presents the results of calculations of the design parameters. A batch of cassava 160 kg by mass, having an initial moisture content of 67% wet basis from which 100 kg of water is required to be removed to have it dried to a desired moisture content of 17% wet basis, is used as the drying load in designing the dryer. A drying time of 30–36 h is assumed for the anticipated test location (Kumasi; 6.7°N,1.6°W) with an expected average solar irradiance of 400 W/m² and ambient conditions of 25 °C and 77.8% relative humidity. A minimum of 42.4 m² of solar collection area, according to the design, is required for an expected drying efficiency of 12.5%. Under average ambient conditions of 28.2 °C and 72.1% relative humidity with solar irradiance of 340.4 W/m², a drying time of 35.5 h was realised and the drying efficiency was evaluated as 12.3% when tested under full designed load signifying that the design procedure proposed is sufficiently reliable.     

Integral-type solar-assisted heat pump water heater

An integral-type solar-assisted heat pump water heater (ISAHP) is designed and tested in the present study. The storage tank and the Rankine cycle unit are integrated together to make a more compact size. A thermosyphon loop is used to transfer the heat from the condenser to the water storage tank. The highest COP obtained in the tests is 3.83.     

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.     

Performance analysis of solar-assisted chemical heat-pump dryer

A solar-assisted chemical heat-pump dryer has been designed, fabricated and tested. The performance of the system has been studied under the meteorological conditions of Malaysia. The system consists of four main components: solar collector (evacuated tubes type), storage tank, solid-gas chemical heat pump unit and dryer chamber. A solid-gas chemical heat pump unit consists of reactor, condenser and evaporator. The reaction used in this study (CaCl2-NH₃). A simulation has been developed, and the predicted results are compared with those obtained from experiments. The maximum efficiency for evacuated tubes solar collector of 80% has been predicted against the maximum experiment of 74%. The maximum values of solar fraction from the simulation and experiment are 0.795 and 0.713, respectively, whereas the coefficient of performance of chemical heat pump (COP^h) maximum values 2.2 and 2 are obtained from simulation and experiments, respectively. The results show that any reduction of energy at condenser as a result of the decrease in solar radiation will decrease the coefficient of performance of chemical heat pump as well as decrease the efficiency of drying.     

Heat-pipe enhanced solar-assisted heat pump water heater

A heat-pipe enhanced solar-assisted heat pump water heater (HPSAHP) is studied. HPSAHP is a heat pump with dual heat sources that combines the performance of conventional heat pump and solar heat pipe collector. HPSAHP operates in heat-pump mode when solar radiation is low and in heat-pipe mode without electricity consumption when solar radiation is high. HPSAHP can thus achieve high energy efficiency. A prototype was designed and built in the present study. An outdoor test for a HPSAHP in the present study has shown that COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as compared to the heat-pump mode COP (2.58).     

Development of design charts for tunnel dryers

A dryer simulation model, comprising a material model and an equipment model, has been developed for the drying of food products in a tunnel dryer. The material model, which deals with the drying kinetics of the product, is based on the receding evaporation front phenomenon while the equipment model describes the dynamics of the dryer. The overall model takes into account the shrinkage of the product. Simulation results of the transient moisture content show good agreement with experimental results for two dryers of different drying capacity. We present a design chart relating air temperature and humidity to the required energy for drying and dryer length, which shows an appreciable effect of setting low humidity for low temperature drying. A reduction in dryer energy consumption and chamber size is obtained under this condition. Multi-stage drying is introduced for situations where saturation of the air is likely to occur in the drying chamber. © 1997 by John Wiley & Sons, Ltd.     

Long-term performance of solar-assisted heat pump water heater

A long-term reliability test of an integral-type solar-assisted heat pump water heater (ISAHP) was carried out. The prototype has been running continuously for more than 13,000 h with total running time >20,000 h during the past 5 yr. The measured energy consumption is 0.019 kWh/l of hot water at 57 ^oC that is much less than the backup electric energy consumption of the conventional solar water heater.     

热泵干燥装置的节能研究

对热泵干燥装置混合工质进行优选,建立典型热泵干燥装置结构数学模型,研究提出目标函数并开发出适合热泵干燥结构的程序算法。     

A hybrid solar-assisted adsorption cooling unit for vaccine storage

A concept of a hybrid adsorption cooling unit for vaccine storage utilizing solar energy as a main power supply and a gas burner as an alternative power supply has been developed. The components of the cooling unit have been designed to work under the weathering conditions of Burkina Faso, West coast of Africa according to the requirements of the World Health Organization. For the first adsorber, which is driven by a gas burner, zeolite-13X has been selected. For the second adsorber to be driven by solar energy selective water sorbent SWS-2L has been applied. Water is selected as a refrigerant for both adsorbents. Theoretical investigations of the expected performance of the designed cooling unit have shown a coefficient of performance (COP) of 0.28 for the solar-operated system based on the heat input to the adsorption unit, at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=122 °C. For the gas-heated system, also a COP of 0.28 has been estimated at the design conditions of T_evap=−5 °C, T_con=55 °C, T_ads=38 °C, T_des(max)=280 °C. The variations of COP, cooling capacity and the heating power required to operate both systems have been estimated for a broad range of desorption temperatures. It turns out that the SWS-2L/water system is much more sensitive to the operating conditions than the zeolite-13X/water system. The obtained results should serve in designing both control and heating components of the cooling unit.     

Thermodynamic and economic evaluation of a solar-assisted heat pump

A solar-assisted heat-pump system is proposed to supply heat for industrial processes in the range 100 to 130°C. The system is economically superior to electrical heating and solar-only systems, and is competitive with fuel burning systems.     

太阳能电池与热泵热水器联合运行系统性能分析

为了降低太阳能光伏电池的温度,同时提升热泵热水器的蒸发温度,利用循环水路冷却太阳能光伏电池,并将热量传递给热泵热水器的蒸发器,构成联合运行系统。针对杭州市的夏季和冬季气象条件,对该联合运行系统的性能进行了计算,分析了对应不同太阳能电池温度下的系统运行参数的变化情况,包括太阳能电池发电效率和所需换热量,热泵热水器的制热量以及热泵效率等。计算结果表明,该联合运行系统能够同时提高太阳能电池光伏转换效率和热泵效率。     

直接膨胀式太阳能热泵系统的理论分析

简述了直接膨胀式太阳能热泵的基本工作原理,从热力学理论出发,对直接膨胀式太阳能热泵的循环进行了理论热力分析,提出了系统各主要部件的能量平衡和火用平衡方程,分析了系统的性能系数COP和火用效率Eη。     

水循环式热泵干燥装置的分析

热泵干燥装置具有节约能源、环境友好、可低温干燥等特点。水循环式热泵干燥装置是指热泵和干燥部分通过水循环耦合而成的热泵干燥装置,与热泵和干燥部分直接耦合相比,具有不向环境排放热量、机组调控性好、对干燥过程的匹配性好、成本低等优势,是一种较适宜于中小型热泵干燥装置的结构型式。     

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.     

Experimental investigations of a small-scale solar-assisted absorption cooling system

The present study deals with a small-scale solar-assisted absorption cooling system having a cooling capacity of 3.52 kW and was investigated experimentally under the climatic conditions of Taxila, Pakistan. Initially, a mathematical model was developed for LiBr/H₂O vapor absorption system alongside flat-plate solar thermal collectors to achieve the required operating temperature range of 75°C. Following this, a parametric analysis of the whole system was performed, including various design and climate parameters, such as the working temperatures of the generator, evaporator, condenser, absorber, mass flow rate, and coefficient of performance (COP) of the system. An experimental setup was coupled with solar collectors and instruments to get hot water using solar energy and measurements of main parameters for real-time performance assessment. From the results obtained, it was revealed that the maximum average COP of the system achieved was 0.70, and the maximum outlet temperature from solar thermal collectors was 75°C. A sensitivity analysis was performed to validate the potential of the absorption machine in the seasonal cooling demand. An economic valuation was accomplished based on the current cost of conventional cooling systems. It was established that the solar cooling system is economical only when shared with domestic water heating.     

太阳能辅助热泵干燥粮食的数值模拟研究

粮食的干燥过程实质上是多孔介质热湿耦合传递的过程。基于多孔介质热质传递理论,通过数值模拟的方法,针对利用太阳能辅助热泵干燥粮食时热风随时间变化的情况,采用综合温度和空气绝对湿度作为瞬态边界条件来对干燥过程中粮食内部温度和水分的变化进行模拟研究。模拟结果显示小麦水分在干燥150h后达到安全水分13.6%(干基),而实验结果显示小麦水分在干燥135h后达到安全水分13.6%(干基),二者对比相差不大,并且模拟温度与试验温度吻合较好。     

Solar-assisted smokehouse for the drying of natural rubber on small-scale Indonesian farms

Natural rubber in Indonesia is mainly produced by smallholder farmers and—being the highest foreign currency generating agricultural commodity—is of great importance for the Indonesian economy. Nevertheless, due to the lack of appropriate dryers, more than 80% of the total production has to be sold as low-grade Standard Indonesian Rubber (SIR) for a relatively low price.In order to improve the product quality, a solar-heated rubber sheet dryer was developed. It consists of a flat-plate solar air heater connected to a drying chamber. Part-recirculation of the exhaust air leads to the desired drying air temperature of 45–60°C and results in a significant reduction of the drying time. To enable drying independent of weather conditions, a biomass furnace was incorporated underneath the drying chamber.The tests have shown that during the rainy season, 320 kg of sheets can be uniformly dried to the required moisture content of 0.5% within 5 days. Compared with conventional smokehouses, the firewood consumption could be considerably reduced, from 1.0–1.5 kg per kg dry rubber to 0.3 kg when producing Ribbed Smoked Sheets (RSS). A further reduction can be expected by optimization of the heating intervals. During favourable weather conditions Air Dried Sheets (ADS) can be produced within 6–7 days without supplemented heat. The quality of the RSS and ADS fulfills the international standards.The simple design of the solar-assisted smokehouse allows local production and—after further improvements—seems to be an economical alternative for farmer groups or nucleus estates.     

Polymer electrolytes based on modified natural rubber for use in rechargeable lithium batteries

Modified natural rubber (NR) polymer hosts having low transition glass temperatures have been investigated. Three types of modified NR, namely 25% epoxidised NR (ENR-25), 50% ENR (ENR-50) and polymethyl methacrylate grafted NR (MG-49) were employed. Results are reported for ionic conductivity and thermal properties for both unplasticised and plasticised polymer electrolyte systems. The samples were in the form of free standing films with the thickness 0.2–0.5 mm and mixtures of ethylene carbonate (EC) and propylene carbonate (PC) were used as plasticisers. Unplasticised modified NR based systems exhibit ionic conductivities in 10⁻⁶–10⁻⁵ S cm⁻¹ range at ambient temperatures. Incorporating 50–100% of EC/PC by weight to the systems yielded mechanically stable films and ionic conductivities in 10⁻⁴–10⁻³ S cm⁻¹ range at ambient temperature. The thermal event of the systems has displayed an increasing trend of transition glass temperature at elevated salt concentration whereas incorporation of EC and PC into the systems leads to marked reduction in their T_g values.