Modeling and design of a solar thermal system for hybrid cooking application

This paper proposes a hybrid solar cooking system where the solar energy is brought to the kitchen. The energy source is a combination of the solar thermal energy and the Liquefied Petroleum Gas (LPG) that is in common use in kitchens. The solar thermal energy is transferred to the kitchen by means of a circulating fluid. The transfer of solar heat is a twofold process wherein the energy from the collector is transferred first to an intermediate energy storage buffer and the energy is subsequently transferred from the buffer to the cooking load. There are three parameters that are controlled in order to maximize the energy transfer from the collector to the load viz. the fluid flow rate from collector to buffer, fluid flow rate from buffer to load and the diameter of the pipes. This is a complex multi energy domain system comprising energy flow across several domains such as thermal, electrical and hydraulic. The entire system is modeled using the bond graph approach with seamless integration of the power flow in these domains. A method to estimate different parameters of the practical cooking system is also explained. Design and life cycle costing of the system is also discussed. The modeled system is simulated and the results are validated experimentally.     

Modelling of parabolic trough direct steam generation solar collectors

Solar electric generation systems (SEGS) currently in operation are based on parabolic trough solar collectors using synthetic oil heat transfer fluid in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs direct steam generation in the collector has been proposed. In this paper the efficiency of parabolic trough collectors is determined for operation with synthetic oil (current SEGS plants) and water (future proposal) as the working fluids. The thermal performance of a trough collector using Syltherm 800 oil as the working fluid has been measured at Sandia National Laboratory and is used in this study to develop a model of the thermal losses from the collector. The model is based on absorber wall temperature rather than fluid bulk temperature so it can be used to predict the performance of the collector with any working fluid. The effects of absorber emissivity and internal working fluid convection effects are evaluated. An efficiency equation for trough collectors is developed and used in a simulation model to evaluate the performance of direct steam generation collectors for different radiation conditions and different absorber tube sizes. Phase change in the direct steam generation collector is accounted for by separate analysis of the liquid, boiling and dry steam zones.     

Performance of suspended flat plate air heater

In this communication, a heat transfer model to predict the transient behaviour of a suspended flat plate solar collector with constant flow of fluid (air) above the absorber has been presented. A reflecting sheet with an air gap between the absorber plate and bottom insulation to reduce heat loss has been used. The effect, on performance of the air heater, of the parameters viz, spacing between cover and plate, heat capacity of air and absorber plate, flow rate of fluid and collector length have been studied. The effect of changing the averaging inlet temperature with varying collector length has also been studied.     

Solar cookers for outdoors and indoors

The solar cooking process has been investigated to develop safe, simple, portable, and reliable solar cookers. The concept of insulated and vapor-tight pots has been introduced and applied to oven, point-focus and heat-pipe cookers. A new, flat-plate cooker with heat pipes has been developed. It requires no tracking and allows cooking to be done in the shade or indoors. Also, a novel-portable cooker, the Mina Oven, featuring a vapor-tight pot and an integral collector with reflector flaps, has been constructed and tested. A second portable cooker that has been developed is the Arafa Cooker, which comprises a parabolic dish focused at a glazed and insulated receiver. Experiments indicated that all cookers yielded satisfactory performance, with cooking times of 25–45 min per kg of food per m² of solar collection area while operating from 10 a.m. to 4 p.m.     

Heat transfer coefficient for constant flow solar collector/storage systems

An earlier expression for the heat transfer coefficient between the heated surface and the flowing fluid used in the investigation of the performance of a few collector/storage systems leads to results which are physically impractical. In this communication we derive the correct expression for the heat transfer coefficient in terms of the parameters of the tube, i.e. its length and circumference. The effect of the modified form of the heat transfer coefficient on the performance of a typical system, namely a ground collector system, has been studied in detail.     

空气作工质的真空集热管内流动与换热分析

针对热水作工质时真空集热管内普遍存在的冻结、腐蚀等问题,本文采用空气作工质,推导了空气作工质时真空集热管的集热效率、热损系数和集热管出口温度的方程,分析了影响空气作工质时各项参数对集热器热性能的影响。通过改变集热管进口空气流量来获得尽可能高出口温度的热空气,从而降低热损系数,提高集热效率。此外,由于热空气可以直接送入干燥室干燥物料,不需要二次转换,节能效果好。由理论分析和试验结果可以预见,利用空气作工质的真空传热管具有潜在的应用价值和前景。     

Mathematical modeling of cooking pots’ thermal efficiency using a combined experimental and neural network method

A mathematical framework to model the heat transfer efficiency of cooking pots is proposed and exploited in this paper. The model consists of combining the experimental results and the statistical data of Residential Energy Consumption Survey (RECS) of Iran with a soft-computing concept such as neural network. Using neural network results, the variations of the efficiency with various parameters have been studied. It is shown that Group Method of Data Handling (GMDH)-type neural network can effectively model and predict thermal efficiency, as a function of important input parameters for a conventional cooking pot. Results show that efficiency increases with increasing diameter to flame ratio, bottom wall curvature, pot wall slope, and overall conductivity. With increasing edge radius and pot height to pot diameter ratio, efficiency decreases. Occupied volume percentage does not have a significant effect on efficiency.     

Performance of a flat-plate solar thermal collector using supercritical carbon dioxide as heat transfer fluid

Energetic and exergetic performance analyses of flat-plate solar collector using supercritical CO₂ have been done in this study. To take care of the sharp change in thermophysical properties in near critical region, the discretisation technique has been used. Effects of zonal ambient temperature and solar radiation, fluid mass flow rate and collector geometry on heat transfer rate, collector efficiency, heat removal factor, irreversibility and second law efficiency are presented. The optimum operating pressure correlation has been established to yield maximum heat transfer coefficient of CO₂ for a certain operating temperature. Effect of metrological condition on heat transfer rate and collector efficiency is significant and that on heat removal factor is negligible. Improvement of heat transfer rate is more predominant than increase in irreversibility by using CO₂. For the studied ranges, the maximum performance improvement of flat-plate solar collector by using CO₂ as the heat transfer fluid was evaluated as 18%.     

Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector

Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl₂·6H₂O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively.     

太阳能集热系统过热影响因素分析

通过分析太阳能供热系统集热、蓄热和用热各个子系统的动态热量热平衡关系,建立太阳能集热系统热量传递数学模型,并结合集热系统内部流体过热汽化原理,提出集热系统过热度评价指标,并对集热系统在不同流量、水箱容积以及不同连接方式等条件下的过热程度进行分析计算。结果表明,集热系统过热度随热媒流量、集热器倾角和水箱容积取值的增大而减小,并通过对上述3种因素进行敏感性分析得到集热器倾角对系统过热度的影响最大。     

A modular phase change heat exchanger for a solar oven

A modular energy storing heat exchanger designed to use pentaerythritol for thermal storage (solid-solid phase change at 182°C) is tested in an oven by circulating heat transfer oil which is electrically heated in a manner to simulate a concentrating solar collector. Three efficiencies for heating the system under controlled and measured power input are determined—the heat exchanger efficiency, the efficiency of the heater with distribution lines, and the total system efficiency. Thermal energy retention times and cooking extraction times are determined, and along with the efficiencies, are compared with the results previously reported for a nonmodular heat exchanger. The modular configuration provides a highly improved extraction rate for cooking due to its wrap-around character and its increased surface-to-volume ratio. A full scale glass model of the copper tubing of the system is described and flow observations reported demonstrating how uniformly the parallel pumping branches perform and how trapped air pockets affect pumping power. A technique for measuring pumping power is described and its application to our system is quantified to show that less than 1 watt is required to circulate the heat transfer oil even when the system includes the solar collector and its longer connecting tubes.     

Thermal performance of a solar cooker integrated vacuum-tube collector with heat pipes containing different refrigerants

A solar cooking system using vacuum-tube collectors with heat pipes containing a refrigerant as working fluid has been fabricated, and its performance has been analysed experimentally. The experiments were conducted during clear days in July and August of 2002 in Elazı , Turkey under similar meteorological conditions for three refrigerants and water. Detailed temperature distributions and their time dependences were measured. The maximum temperature obtained in a pot containing 7 l of edible oil was 175 °C. Also, the cooker was successfully used to cook several foods. The cooking processes were performed with the cooker in 27–70 min periods.     

Dynamic simulation and parametric sensitivity studies on a flat-plate solar collector

A dynamic simulation program for a flat-plate solar collector has been developed and used for a parametric sensitivity study of the collector. A computer simulation language, C.S.M.P., was used for the simultaneous solution of the differential equations describing the system. The program is developed from basic principles and defined parameters, so that the effect of each element of the collector on its efficiency and outlet fluid temperature can be analyzed. The dynamic simulation program was validated by comparing calculated with actual test results on a commercial flat-plate solar collector; the agreement between the results was found to be very good.Sensitivity studies were carried out to determine the effect of the following parameters on the performance of a single-glazed solar collector: heat transfer coefficient between absorber plate and fluid transport tubes; fluid flow rate through the collector; plate absorptivity; plate emissivity and insulation thickness. Sensitivity studies were also performed on a double-glazed solar collector but for only the last three of the above listed parameters. The results of these sensitivity studies are analyzed in the text.The transient behavior of the solar collector, viz. the response of the outlet fluid temperature to a step change in insolation rate, has also been studied.     

Development and performance analysis of compound parabolic solar concentrators with reduced gap losses—‘V’ groove reflector

A system has been developed to use compound parabolic concentrators to collect solar energy and to generate steam. A CPC reflector profile with a V groove at the bottom of the reflector to reduce the gap losses was designed with a half acceptance angle of 23.5° for a tubular absorber of OD 30 mm. Five troughs fabricated with fiberglass substrate pasted over with UV stabilized self-adhesive aluminized polyester foil having high specular reflectivity joined together side by side comprise the CPC module with an aperture area of 2.04 m². Copper tubes coated with NALSUN selective coatings and enclosed by borosilicate glass envelope act as absorbers. The reflector absorber assembly housed in a single glass wool insulated wooden box forms the CPC collector. Using water as the heat transfer fluid efficiency tests were carried out with different inlet temperatures. In situ steam generation testing and possible application to steam cooking were also carried out. A theoretical modeling was developed by setting up different heat balancing equations and a reasonable agreement between theoretical computed values and the experimental values was observed.     

Performance analysis of a cylindrical parabolic focusing collector and comparison with experimental results

The efficiencies for focusing collectors which consist of a cylindrical parabolic reflector and a collector tube surrounded by a transparent envelope and which heat a fluid flowing through the collector tube have been predicted using heat transfer analytical methods. The analysis considers visible radiation transfer, IR radiation exchange, conductive and convective losses and energy transferred to a fluid flowing through the collector tube. The collector may have a tilted north-south axis, an east-west axis or it may fully track the Sun and geometric parameters associated with tracking the Sun are considered. Predicted results are in excellent agreement with recent collector performances measured using Sandia Laboratories' collector test facility.     

Comparative study of fluid-in-metal manifolds for heat extraction from single ended evacuated glass tubular collectors

An investigation is reported of heat transfer between the glass absorber tubes of all-glass evacuated collectors and fluid-in-metal manifolds designed for heat extraction from the glass asborber tubes. The heat transfer is studied using a novel solar simulator which heats a panel of glass tubes electrically to simulate solar input to a collector panel. Measurement of the temperatures at various points on the glass tubes and on the manifolds gives a measure of the efficiency of heat transfer for each manifold under various operating conditions and allows calculation of the efficiencies η₀ of collectors incorporating the manifolds. The effect of fluid temperature, collector inclination and fluid flow rate has been investigated for four manifold designs of increasing simplicity. Experimental results for the manifolds are compared with calculations of heat transfer. Potential lifetime problems for the manifolds are also discussed. The simplest manifold design is shown to have good prospects for high-temperature (>100°C) heat extraction.     

System performance studies on a photovoltaic/thermal (PV/T) air heating collector

A computer simulation model is presented for the analysis of a solar photovoltaic/thermal (PV/T) hybrid collector with air as heat transfer fluid and algorithm for making quantitative prediction regarding the performance of the system is described. Thermal efficiency curves for the solar PV/T hybrid collectors corresponding to various type of absorbers have been derived. In order to appreciate the model, numerical calculations have been made for evaluating the system performance corresponding to typical climate of Delhi, India     

Performance of Cylindrical Parabolic Collector with Automated Tracking System

Parabolic solar collector collects the radiant energy emitted from the sun and focuses it at a point. Parabolic trough collectors are the low cost implementation of concentrated solar power technology that focuses incident sun light on to a tube filled with a heat transfer fluid. However, the basic problem with the cylindrical parabolic collector without tracking was the solar collector does not move with the orientation of sun. Development of automatic tracking system for cylindrical parabolic collectors will increase solar collection as well as efficiency of devices. The main aim of this paper is to design, fabricate and analyze the performance of parabolic collector with automated tracking system. The automated tracking mechanism is used to receive the maximum possible energy of solar radiation as it tracks the path of sun. The performance of the parabolic trough collector is experimentally investigated with the water circulated as heat transfer fluid. The collector efficiency will be noted.     

Liquid cooling of a microprocessor: experimentation and simulation of a sub-millimeter channel heat exchanger

Abstract

A heat exchanger dedicated to the cooling of a microprocessor has been designed and realized. It consists of a bottom wall in contact with the processor and a cover that has been dug to a depth of 200?μm on one side and 1?mm on the other. Thus, by turning the cover, the hydraulic diameter of the channel can be changed. Both hydraulic and thermal performances of this heat exchanger have been experimentally tested. Three-dimensional numerical simulations were simultaneously carried out and good agreement was obtained. The influence of the distributor and the collector on the distribution of fluid flow and heat fluxes is emphasized. A new concept of micro-heat exchanger is proposed for the cooling of electronics devices for which wall to fluid heat exchange quality and pumping effect are critical. The ability of a liquid heat exchanger involving a dynamic deformation of one of its walls to cool a microprocessor is investigated. Three-dimensional transient numerical simulations of fluid flow and conjugate heat transfer were performed using commercial software. Effect of geometrical and actuation parameters has been explored, demonstrating the ability of such heat exchanger to simultaneously pump the fluid and enhance the heat transfer.     

Computer simulation of solar cooled buildings in Khartoum

The transient state heat transfer formulation has been used to determine the air conditioning cooling load for two selected one room buildings in Khartoum: one made from brickwork and the other from wood. This formulation has been achieved by applying an explicit finite difference numerical techniques and adopting thermal network of electrical analogy to solve the transient conduction heat transfer equations. A computer program was developed to analyse the thermal network and to determine the cooling load for the wall, roof, window, ventilation and the total cooling load for each building for a typical hot summer day in Khartoum.A solar cooling system comprising mainly of a flat plate solar collector, a Lithium Bromide-Water absorption air conditioner, a storage and auxiliary tanks was integrated in the building and the energy equation for each component was obtained. A general simulation program for the solar cooled buildings has been developed and it was found that about 65% of the total cooling load demanded by the brick building could be supplied by energy from the solar collector, the other 35% portion had to be met by the auxiliary tank compared with 70% of the total cooling load demanded by the wood building which could be supplied by energy from the solar collector.