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This research presents a unified approach to power limits in power producing and power consuming systems, in particular those using renewable resources. As a benchmark system which generates or consumes power, a well-known standardized arrangement is considered, in which two different reservoirs are separated by an engine or a heat pump. Either of these units is located between a resource fluid (‘upper’ fluid 1) and the environmental fluid (‘lower’ fluid, 2). Power yield or power consumption is determined in terms of conductivities, reservoir temperatures and internal irreversibility coefficient, F. While bulk temperatures Ti of reservoirs’ are the only necessary state coordinates describing purely thermal units, in chemical (electrochemical) engines, heat pumps or separators it is necessary to use both temperatures and chemical potentials mk. Methods of mathematical programming and dynamic optimization are applied to determine limits on power yield or power consumption in various energy systems, such as thermal engines, heat pumps, solar dryers, electrolysers, fuel cells, etc. Methodological similarities when treating power limits in engines, separators, and heat pumps are shown. Numerical approaches to multistage systems are based on methods of dynamic programming (DP) or on Pontryagin’s maximum principle. The first method searches for properties of optimal work and is limited to systems with low dimensionality of state vector, whereas the second investigates properties of differential (canonical) equations derived from the process Hamiltonian. A relatively unknown symmetry in behaviour of power producers (engines) and power consumers is enunciated in this paper. An approximate evaluation shows that, at least ¼ of power dissipated in the natural transfer process must be added to a separator or a heat pump in order to assure a required process rate. Applications focus on drying systems which, by nature, require a large amount of thermal or solar energy. We search for minimum power consumed in one-stage and multi-stage operation of fluidized drying. This multi-stage system is supported by heat pumps. We outline the related dynamic programming procedure, and also point out a link between the present irreversible approach and the classical problem of minimum reversible work driving the system.

This paper presents the analysis of the current state of grain dryers and storages in the Republic of Srpska in terms of their age structure, installed capacities, working order and geographical distribution. The current state of grain dryers and storages which have storage capacities over 1000 t was determined through a survey conducted in all centres in the Republic of Serbia. The survey results showed that the total installed capacities for storing grain in the Republic of Srpska are 317,917 t of wheat, while the total installed capacities of grain dryers are 207.4 t/h.

El principal problema que tiene un secador solar híbrido (solar asistido con gas o electricidad) son sus costos de operación por consumo de energía. Para ello se determinó el espesor óptimo de aire confinado en la cubierta plástica el cual actúa como aislamiento térmico, se calculó el coeficiente global de transferencia de calor 'U', se estimó la potencia de la fuente externa la cual fue de 1,8 kW y se encontró que el consumo de energía es aproximadamente 5,35 veces menos que la potencia del secador híbrido con cubierta sencilla. El concepto es aplicable a cualquier tipo de secador solar, solar-asistido o cualquier invernadero para el sector agroindustrial.0 principal problema con um secador solar híbrido (solar-gás assistido ou com electricidade) são os custos operacionais para o consumo de energia. Para este fim a pesquisa determinou a espessura ideal de ar confinado na tampa de plástico (duas carnadas) que atua como isolamento térmico, foi calculada a taxa global de transferencia de calor 'U' e estimada pela alimentação externa de 1,8 kW e descobriu que o consumo de energía é de cerca de 5,35 vezes menor do que a potência do secador revestido híbrido simples. 0 conceito é aplicável a qualquer tipo de secador solar, solar assistida ou qualquer tampa plástica no seitor agrícola.In a solar hybrid dryer (solar assisted gas or electricity) its main problem is to minimize its operational costs due to energy consumption. For this, was determined the optimum thickness of air confined in the plastic cover as thermal isolation, was calculated overall coefficient of heat transfer 'U', and was estimated the power of the external source which was of 1,8 kW and found that energy consumption is approximately 5,35 times less than the power of the hybrid dryer with single plastic deck. The concept is applicable to any type of solar dryer, solar-assisted or any greenhouse in agroindustrial area.

This work presented a review of energy requirement for drying sliced agricultural crops in order to produce high quality and shelf-stable products. Work on the estimation of energy requirement for drying different sliced crops such as potato, carrot, garlic, onions, mango, banana, apple, tomato etc. and factors affecting their energy requirement were reported. From the review, programming models for drying sliced fruits, as well as different empirical equations adopted by some researchers for estimating the energy requirements of different sliced crops at specific drying parameters were reported. Obtained results showed that crop functional characteristics, initial and desired moisture contents, slice thickness, air temperatures, specific heat capacity, relative humidity, and air velocity are the major parameters affecting sliced crop drying energy requirement. Generally, the minimum energy requirement for drying moisture-laden sliced crops like tomatoes, apples, carrot, mango, cucumber etc. were found to be between 4.22 and 24.99 MJ/kg water removed. Field test results by other researchers for different drying systems and crops were also presented. Prospects for future work were suggested. [ABSTRACT FROM AUTHOR]

A theoretical study on the maximum evaporation rate obtainable in a small-scale indirect solar dryer is presented, considering evaporation of free water. A mathematical model of the evolution of the temperature and the specific humidity of the airflow along the drying chamber is presented. Based on the results, some simplifications are proposed and justified in order to calculate the maximum evaporation rate as a function of a reduced number of parameters, to study their effect. The results show that the effect of the air mass flow rate on the maximum evaporation rate depends on the aspect ratio of the drying chamber, defined as the ratio of the total drying area to the cross section in the drying chamber. Design and operation criteria can be extracted from the results. As a global result, for the typical range of dimensions and air mass flow rates employed in solar dryers, the drying chamber aspect ratio should be between 200 and 300 to obtain a proper evaporation rate. Within that range, doubling the air mass flow rate the maximum evaporation rate obtainable increases around 20%. [ABSTRACT FROM AUTHOR]

A survey was carried out in Togo, Benin and Burkina Faso involving 140 respondents and three climatic zones. Ten typical types of dryers with batch operating and natural convection were inventoried. Analysis of the results showed that: (1) Traditional sun dryers were widely used everywhere from 1 to 4000 kg per cycle, costing €0 to €7.63 per m 2 drying area. (2) Low power solar dryers were mostly used in the Sahel and Sudan-Sahel tropical climate, from 5 to 1000 kg, costing from €9.11 to €238.55 per m 2 of tray. (3) Gas dryers were used in all climatic zones at around 100 kg per cycle, with a high drying power, costing from €107.52 to €181.75 per m 2 of tray. The acquisition of dryers depended mostly on the distance between provider and user and on the means of dissemination. A thermo-economic analysis was carried out to determine which dryers were most successfully adopted. These dryers will be re-employed by the users when the investment and maintenance costs enable profit to be made from the drying operation. Two types of dryers met those conditions: the traditional simple sun dryer used by families and groups: a study of an application on maize showed costs of €0.01/kg of evaporated water equal to 7% of daily capital gain; the Atesta gas dryer used by small and medium-sized enterprises: applied to pineapple, cost €0.07/kg of evaporated water equal to 9% of daily capital gain. New criteria had to be added to all those listed from the literature in order to choose a dryer properly: characteristics of the product to be dried, product added value, local energy availability and cost, labour skills and availability, local material availability and user investment capacity. [ABSTRACT FROM AUTHOR]

Abstract: In this synthesizing research methods of mathematical programming and dynamic optimization are applied to determine limits on power yield or power consumption in various energy systems, such as thermal engines, heat pumps, solar dryers, electrolysers, and fuel cells. Methodological similarities are enunciated when treating power limits in engines, separators, and heat pumps. Numerical approaches to multistage systems are based on the methods of dynamic programming (DP) or Pontryagin’s maximum principle. The first method searches for properties of optimal work and is limited to systems with low dimensionality of state vector, whereas the second investigates properties of differential (canonical) equations derived from the process Hamiltonian. In this paper a relatively unknown symmetry in behaviour of power producers (engines) and power consumers is shown. An approximate evaluation shows that, at least 1/4 of power dissipated in the natural transfer process must be added to a separator or heat pump in order to assure a required process rate. Applications include drying systems which, by nature, require a large amount of thermal or solar energy. We search for minimum power consumed in one-stage and multistage operation of fluidized drying. This multistage system is supported by heat pumps. We outline the related dynamic programming procedure, and also point out a link between the present irreversible approach and the classical problem of minimum reversible work driving the system. [Copyright &y& Elsevier]

This research presents a unified approach to power limits in power producing and power consuming systems, in particular those using renewable resources. As a benchmark system which generates or consumes power, a well-known standardized arrangement is considered, in which two different reservoirs are separated by an engine or a heat pump. Either of these units is located between a resource fluid ('upper' fluid 1) and the environmental fluid ('lower' fluid, 2). Power yield or power consumption is determined in terms of conductivities, reservoir temperatures and internal irreversibility coefficient, F While bulk temperatures Ti of reservoirs' are the only necessary state coordinates describing purely thermal units, in chemical (electrochemical) engines, heat pumps or separators it is necessary to use both temperatures and chemical potentials ? k. Methods of mathematical programming and dynamic optimization are applied to determine limits on power yield or power consumption in various energy systems, such as thermal engines, heat pumps, solar dryers, electrolysers, fuel cells, etc. Methodological similarities when treating power limits in engines, separators, and heat pumps are shown. Numerical approaches to multistage systems are based on methods of dynamic programming (DP) or on Pontryagin's maximum principle. The first method searches for properties of optimal work and is limited to systems with low dimensionality of state vector, whereas the second investigates properties of differential (canonical) equations derived from the process Hamiltonian. A relatively unknown symmetry in behaviour of power producers (engines) and power consumers is enunciated in this paper. An approximate evaluation shows that, at least 1/4 of power dissipated in the natural transfer process must be added to a separator or a heat pump in order to assure a required process rate. Applications focus on drying systems which, by nature, require a large amount of thermal or solar energy. We search for minimum power consumed in one-stage and multi-stage operation of fluidized drying. This multi-stage system is supported by heat pumps. We outline the related dynamic programming procedure, and also point out a link between the present irreversible approach and the classical problem of minimum reversible work driving the system. [ABSTRACT FROM AUTHOR]

The paper presents the design and operating characteristics of a model of reversible axial fan with only one impeller, whose reversibility is achieved by changing the direction of rotation. The fan is designed for the purpose of providing alternating air circulation in wood dryers in order to reduce the consumption of electricity for the fan and increase energy efficiency of the entire dryer. To satisfy the reversibility of flow, the shape of the blade profile is symmetrical along the longitudinal and transversal axes of the profile. The fan is designed with equal specific work of all elementary stages, using the method of lift forces. The impeller blades have straight mean line profiles. The shape of the blade profile was adopted after the numerical simulations were carried out and high efficiency was achieved. Based on the calculation and conducted numerical simulations, a physical model of the fan was created and tested on a standard test rig, with air loading at the suction side of the fan. The operating characteristics are shown for different blade angles. The obtained maximum efficiency was around 0.65, which represents a rather high value for axial fans with straight profile blades. [ABSTRACT FROM AUTHOR]

Abstract: 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 42.45h 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. [Copyright &y& Elsevier]

In a solar hybrid dryer (solar assisted gas or electricity) its main problem is to minimize its operational costs due to energy consumption. For this, was determined the optimum thickness of air confined in the plastic cover as thermal isolation, was calculated overall coefficient of heat transfer "U", and was estimated the power of the external source which was of 1, kW and found that energy consumption is approximately 5,35 times less than the power of the hybrid dryer with single plastic deck. The concept is appficable to any type of solar dryer, solar-assisted or any greenhouse in agroindustrial area. [ABSTRACT FROM AUTHOR]

Abstract: Drying is a major and challenging step in the pre-treatment of biomass for production of second generation synfuels for transport. The biomass feedstocks are mostly wet and need to be dried from 30 to 60 wt% moisture content to about 10–15 wt%. The present survey aims to define and evaluate a few of the most promising optimised concepts for biomass pre-treatment scheme in the production of second generation synfuels for transport. The most promising commercially available drying processes were reviewed, focusing on the applications, operational factors and emissions of dryers. The most common dryers applied now for biomass in bio-energy plants are direct rotary dryers, but the use of steam drying techniques is increasing. Steam drying systems enable the integration of the dryer to existing energy sources. In addition to integration, emissions and fire or explosion risks have to be considered when selecting a dryer for the plant. In steam drying there will be no gaseous emissions, but the aqueous effluents need often treatment. Concepts for biomass pre-treatment were defined for two different cases including a large-scale wood-based gasification synfuel production and a small-scale pyrolysis process based on wood chips and miscanthus bundles. For the first case a pneumatic conveying steam dryer was suggested. In the second case the flue gas will be used as drying medium in a direct or indirect rotary dryer. [Copyright &y& Elsevier]

In recent years, computational fluid dynamics (CFD) has been used increasingly to improve process design capabilities in many industrial applications, including industrial drying processes. Drying of food and beverage products, industrial and municipal wastewater sludge, and other manufacturing and environmental products is done regularly in order to enhance the quality and life span of these products and to facilitate their use, storage, and transportation. With recent advancements in mathematical techniques and computer hardware, CFD has been found to be successful in predicting the drying phenomenon in various types of industrial dryers, which utilize all forms of drying operations including spray, freeze, and thermal drying techniques. The CFD solutions are being used to optimize and develop equipment and processing strategies in the drying industry, replacing expensive and time-consuming experimentations. However, a comprehensive review on the application of CFD for the design, study, and evaluation of industrial dryers is not yet available. A comprehensive review of the current literature on the use of CFD models in both industrial and lab-scale drying applications is presented in this article. The use of Eulerian-Eulerian and Eulerian-Lagrangian models in the study of the drying kinetics for gas-solid multiphase flow systems is fully discussed. Merits and disadvantages of using various CFD models in the design of industrial dryers are illustrated and the scope of their applicability is also discussed. [ABSTRACT FROM AUTHOR]