Havalı kollektörle ısıtma ve ekserji analizi
Heating with the solar air collector and its exergy analysis
- Tez No: 46435
- Danışmanlar: Y.DOÇ.DR. A. KORHAN BİNARK
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 41
Özet
ÖZET Bu çalışmanın amacı, havalı güneş kollektörleri için termodinamiğin I. Kanun analizini ve ekserji analizini yapmaktır, incelenen kollektörler açık sistemdir. Hava, bir ideal gaz olarak düşünülmüştür. Başlangıçta kütle ve enerji dengelerini gözönüne almak suretiyle sistemin I. Kanun analizi yapılmıştır. ikinci safhada, ekserji analizini yapabilmek için özellikle tesirlilik tanımı yapılmalıdır. Tesirlilik, birim zamanda çıkışta mevcut olan kullanılabilir enerjinin, girişte mevcut olan kullanılabilir enerjiye oranı olarak tarif edilir. Deney tesisatı için aynı boyutlarda üç model kollektör kullanılmıştır (630x315x100-116 mm). Saydam üst örtü olarak birinci kollektörde tek tabaka, diğerlerinde çift tabaka cam, kasa malzemesi olarak tahta ve kontraplak, yutucu plaka olarak mat siyah boyalı sac ve ısı yalıtım malzemesi olarak bir yüzü alüminyum folyo kaplı cam yünü kullanılmıştır. Birinci kollektörde, hava yutucu plaka ile alüminyum folyo arasındaki boş kanalda akmaktadır, ikinci ve üçüncü kollektörde bu kanalda sacdan labirentler vardır. Ayrıca bu iki kollektörde plaka üzerine yapıştırılmış, 40'ar adet siyah boyalı ping-pong topu çapraz olarak dizilmiştir. Topların içerisine sırası ile kalsiyumklorit hekzahidrat ve sodyum sülfat dekahidrat maddesi konmuştur. Kollektörler eğimi 41° olan bir deney standı üzerine, gölgeleme olmayacak şekilde kuzey-güney istikametinde yerleştirilmişlerdir. Işınım şiddeti solarametre ile ölçülmüştür. Kollektörlere ait sıcaklık farkı-zaman, birinci kanun verimi-zaman ve tesirlilik-zaman grafikleri çizilmiş ve sonuçlar değerlendirilmiştir.
Özet (Çeviri)
SUMMARY HEAT1NG W1TH THE SOLAR AIR COLLECTOR AND ITS EXERGY ANALYSIS Ali över in the world, new knowledge about the limitation and the depletion of the fossil fuel resources lead us renevved to the idea that solar energy should be used to cover our energy demands, if new reserves will not be discovered. it is well known that there are many applications in several fields of solar energy, mainly, collectors, passive solar systems and power generation. in this young field of solar technology ali developed and operating systems need and invariant measure to compare these systems with the conventional systems. The solar energy option has been listed as öne of the promising altemative energy sources for the future. The nature of this source, its magnitude and its characteristics have been described and a classification of the various methods -direct and indirect- of solar energy utilization has been given. in any collection device, the principle usually followed is to expose a dark surface to solar radiation so that the radiation is absorbed. A part of the absorbed radiation is then transferred to a fluid like air ör water. When no optical concentration is done, the device in which the collection is achieved is called a flat-plate collector. The flat-plate collector is the most important type of solar collector because it is simple in design, has no moving parts and requires liftle maintenance. it can be used for a variety of applications in which temperatures ranging from 40 °C to about 100 °C are required. As stated earlier it consists of an absorber plate on which solar radiation falls after coming through öne ör more transparent covers (usually made of glass). The absorbed radiation is partly transferred to a liquid flowing through tubes which are fixed to the absorber plate ör are integral with it. This energy transfer is the useful gain. The remaining part of the radiation absorbed in the absorber plate is lost by convection and re- radiation to the surroundings from the top surface, and by conduction through the back side and edges. The transparent covers help in reducing the losses by convection and re-radiation, while thermal insulation on the back side and the edges helps in reducing the conduction heat loss. The liquid most commonly used is water. A liquid flat-plate collector is usually held tilted in a fixed position on a supporting structure, facing south if located in the northern hemisphere. vî bWhen temperatures higher than 100 °C are required, it becomes necessary to concentrate the radiation. The concentrator is a mirror reflector having the shape of a cylindrical parabola. it focuses the sunlight onto its axis where it is absorbed on the surface of the absorber tube and transferred to the fluid flowing through it. A concentric glass cover around the absorber tube helps in reducing the convective and radiative losses to the surroundings. in order that the sun's rays should alvvays be focused onto the absorber tube, the concentrator has to be rotated. This movement is called tracking. in the case of cylindrical parabolle concentrators, rotation about a single axis is generally required. Fluid temperature upto around 300 °C can be achieved in cylindrical parabolic focusing collector systems. As stated earlier, öne of the majör problems associated with the utilization of solar energy is its variability. For this reason, most applications required some type of energy storage system. The purpose of such a system is to store energy when it is in excess of the requirement of an application and to make it available for extraction when the supply of solar energy is absent ör inadequate. Thermal energy can be stored as sensible heat ör as latent heat. Sensible heat storage is usually done in an insulated container containing a liquid like water ör a porous solid in the form of pebbles ör rocks. THERMAL APPLİCATİONS a)VVater Heating: Solar water heating is öne of the most attractive applications from an economic stand-point. in many countries of the world, it is already competing on equal terms with systems using other energy sources. The two main components of the system are the liquid flat-plate collector and the storage tank, the tank being located above the level of the collector. As water in the collector is heated by solar, it flows automatically to the top of the vvater tank and its place is taken by colder water from the bottom of the tank. Höt vvater for use is withdrawn from top of the tank. An auxiliary heating system is provided for use on cloudy ör rainy days. Typically, such systems have capacities ranging from 100 to 200 liters and adequately supply the needs of a family of four ör five persons. The temperature of the höt vvater delivered ranges from 50 °C to 70 °C. Solar vvater heating is a good example to illustrate öne of the assets of the direct use of solar energy which has not been mentioned so far. b)Space Heating: in a space heating system vvater is heated in the solar collectors and stored in the tank. Energy is transferred to the air circulating in the house by means of the water-to-air heat exchanger. Two pumps provide forced circulation betvveen the collectors and the tank, and betvveen the tank and viithe heat exchanger. Space heating is of particular relevance in colder countries, where significant amounts of energy are required for this purpose. in India, it is of importance mainly in the northem regions in winter. in contrast to the above methods, which are often called active methods, space heating giving a fair degree of comfort can also be done by adopting passive methods. A passive method is öne in which thermal energy flows through a living space by natural means without the help of a mechanical device like a pump ör a blower. c)Power Generation: The generation of electrical ör mechanical power is öne of the most important applications of an energy source. Many different ways are being tried ör are in the planning stages. Solar thermal power cycles can be broadly classifıed as low, medium and high temperature cycles. Low temperature cycles generally use flat-plate collectors so that maximum temperatures are limited to about 100 °C. Medium temperature cycles work at maximum temperatures ranging from 150 °C to 300 °C, while high temperature cycles work at maximum temperatures above 300 °C. For the low and medium temperature ranges, the thermodynamic cycle preferred is the Rankine Cycle. For the high temperature range apart from the Rankine Cycle, the Brayton and the Stirling Cycle are also being considered. d)Space Cooling and Refrigeration: Öne of the promising thermal applications of solar energy is for the purpose of obtaining cooling. Space cooling may be done with the objective of providing comfortable living conditions ör of keeping a food product cold. Since the energy of the sun is being received as heat, obvious choice is a system working on the absorption refrigeration cycle which requires most of its energy input as heat. Cooling is required most in summer. Hence, in this case, there is a seasonal matching betvveen the energy needs of the refrigeration system and the availability of solar radiation. e)Distillation: in many small communities, the natural supply of fresh water is inadequate, but brackish ör şaline water is available. Solar distillation can prove to be an effective way of supplying drinking water to such communities. The principle of solar distillation is simple. A slopping transparent cover is provided at the top. Solar radiation is transmitted through the cover and is absorbed in the black lining. it thus heats up the water by about 10 °C to 20 °C and causes it to evaporate. The resulting vapor rises, condenses as püre water on the underside of the cover and flows into condensate collection channels on the sides. An output of about 3 it. /m 2 can be obtained in a well designed stili on a good sunny day. viii bf)Drying: Öne of the traditional uses of solar energy has been for drying of agricultural products. The drying process removes moisture and helps in the preservation of the product. Traditionally, drying is done on öpen ground. The disadvantages associated with this are that the process is slow and that insects and dust get mixed with the product. The use of dryers helps to eliminate these disadvantages. Drying can be done faster and in a controlled fashion. in addition, a better quality product is obtained. g)Cooking: An important domestic, thermal application is that of cooking. Över the past 30 years, a number of designs of solar cookers have been developed. Solar cooker designs generally fail into öne of two categories. Öne category is the box type cooker which essentially consists of a rectangular enclosure insulated on the bottom and sides and having two glass covers on the top. Solar radiation enters through the top and heats up the enclosure in which the food to be cooked is placed in a shallovv vessels. A typical size available has an enclosure about 50 cm^ and 12 cm deep. Temperatures around 100 °C can be obtained in these cookers on sunny days and pulses, rise, vegetables, ete., can be readily cooked. The time taken for cooking depends upon the solar radiation and varies from 1/2 to 2- 1/2 hours. A single glass reflector whose inclination can be varied is sometimes attached to the box type cooker. The addition of the mirror helps in achieving enclosure temperatures which are higher by about 15 °C to 20 °C. As a result, the cooking time is reduced. The cookers with reflectors on ali four edges have also been built. Box type cookers with no reflector ör with öne reflector are simple to use and require liftle attention. The second category of solar cookers developed are those in which the radiation is concentrated by a paraboloid reflecting surface. The cooking vessel is placed at the focus of the paraboloid mirror and is thus directly heated. These cookers require some form of tracking. Various types of reflecting surfaces have been used. SOLAR AIR HEATERS A solar air heater is öne of the important means of utilizing solar energy and can find applications in many processes requiring low and moderate temperatures. A conventional solar air heater generally consists of an absorber plate with a parallel plate below forming a passage of high aspect ratio through which the air to be heated flows. As in the case of the liquid flat- plate collector, a transparent cover system is provided about the absorber ixplate, while a sheet metal container filled with insulation is provided on the bottom and sides. There are two majör limitations of conventional type solar air heaters. The first is that the heat transfer from a metallic plate to air is rather poor as the conductivity of air is low. The second problem is due to the combined effect of low heat transfer from the plate to air and the very small thickness of the absorber plate. This can be trackled by increasing heat transfer area through the use of fins, but the larger pressure drop in such structures may be a problem. Like a liquid flat-plate collector, a solar air heater is simple in design and requires little maintenance. in additîon, since the fluid does not freeze, the solar air heater has the advantage of not requiring any special attention at temperatures below O °C. Corrosion and leakage problems are also less severe. Hovvever, the value of the heat transfer coefficient betvveen the absorber plate and the air is low and this results in a lower efficiency. For this reason, the surfaces are sometimes roughened ör longitudinal fins are provided in the air flow passage. The face areas of most commercially available solar air heaters range from 1 to 2 m 2. Materials of construction and sizes are similar to those used with liquid flat-plate collectors. Thus, the absorber plate is a metal sheet 1 to 2 mm in thickness. Glass of thickness 3 to 4 mm is the most commonly used to cover material. Hovvever, plastics are being used in increasing numbers. THERMAL ENERGY STORAGE it has been noted earlier that the need for storage arises because in many situations there is a mismatch betvveen the availability of solar energy and the needs of the application for which it is being used. Thus, for example, a storage is invariably required in a water heating ör space heating application. There are^ree basic methods for storing thermal energy: a)Heating a liquid ör a solid which does not change phase. This is called sensible heat storage. The amount of energy stored is dependent on the temperature change of the material. b)Heating a material which under goes a phase change (usually melting). This is called latent heat storage. The amount of energy stored in this case depends upon the mass and the latent heat of fusion of the material. c)Using heat to produce a certain chemical reaction and than storing the products. The heat is released when the reverse reaction is made to occur. in this case also, the storage operates essentially isothermally xduring the chemical reactions. However, the temperatures at which the fonvard and reverse reactions occur are usually different. Of the above methods, sensible and latent heat storage systems are in use, while thermochemical storage systems are being proposed for use in the future for medium and high temperature applications. Sensible Heat Storage: in the case of sensible heat storage systems, energy is stored ör extracted by heating ör cooling a liquid ör a solid which does not change its phase during the process. A variety of substances have been used in such systems. These include liquids like water, heat transfer oils and certain inorganic molten salts, and solids like rocks, pebbles and refractors. in the case of solids, the material is invariably in porous form and heat is stored ör extracted by the flow of a gas ör a liquid through the pores ör voids. The choice of the substance used largely depends on the temperature level of the application, water being used for temperature below 100 °C and refractory bricks being used for temperatures around 1000 °C. Sensible heat storage systems are simpler in design than latent heat ör thermochemical storage systems. However, they suffer from the disadvantage of being bigger in size. A second disadvantage associated with sensible heat systems is that they can not store ör deliver energy at a constant temperature. Latent Heat Storage: The latent heat thermal energy storage systems will play an important role in the solution of the energy problems in future. The main advantageous of these systems seems to be storing the large amount of energy in relatively small volumes. But many problems arise at the design stages of these systems. Öne of them is the selection of the container vvhich includes the phase change material and identifıes the heat transfer between working fluid and phase change material. Hovvever, the choice of phase change material should be based on the fact that the substance should not be corrosive, toxic, flammable, and expensive. Additionally, it should suitable for the temperature ranges required and suffer no degradation. a) Solid- Solid Phase Change The solid-solid phase materials generally have lower latent heat than those used for other forms such as solid - liquid phase change. Therefore, they can be used in the form of pebble beds. xib) Solid- Liquid Phase Change Several inorganic, organic and eutectic compounds undergo a fusion process that provides a fairly high energy storage capacity for relatively low volume change. For example, ice was used for many countries as a storage material, but only around the 0 °C limit. Hydrates provide good storage properties because they are cheap and plentiful if the associated difficulties can be eliminated, that is, incomplete melting and precipitation of some of the solids and super cooling. Several such materials are mentioned in Table 2-3. It is of interest to note that the first phase change material studied for a solar application was sodium sulfate decahydrate ( Glauber's salt ) which undergoes a hydration -dehydration reaction at 32 °C. energy storage Na2SO4.10H2O Na2S04 + 10H2O energy extraction The main difficulty encountered with this chemical ( and with many other chemicals ) is that the thermal performance degrades with repeated cycling. This is partly due to the fact that at temperatures above the melting point, it separates into a liquid solution phase and a solid phase. The difficulties have been overcome by keeping the material in long thin containers and by mixing certain additives. c) Liquid - Vapor Phase Change This type of phase changing is associated with much higher storage capacity than the solid -liquid phase, but it has the disadvantage of requiring a larger volume. A good way of solving such a problem is to employ two tanks, so that as the vapor is generated in one tank, it can be absorbed by the other, and the heat of vaporization given off to the environment from the second tank. If the vapor pressure of the volatile liquid in the first tank is lowered owing to a temperature drop, then by connecting the two tanks for the reverse process, the liquid in the second tank is evaporated, taking its heat of vaporization from the environment, and is condensed in the first tank. This is similar to absorption refrigeration systems. An advantage associated with a latent heat storage system is that it is more compact than a sensible heat system. However, the heat exchange system for transferring energy between the working fluid ( which is usually a gas ) and the storage material is always more complex. This is due to the fact that during the heat extraction process, the storage material first solidifies at the heat transfer surface. Hence, the thermal resistance to the flow of heat keeps on increasing as heat is extracted from the storage. In order to prevent the thermal resistance from becoming too large, the storage material is placed in long thin containers, e.g., cylinders, and the gas is passed through narrow spaces between the tubes. Alternatively the XIIstorage material is in the spaces between the tubes and the gas is passed through the tubes. In this study, the first lawandexergy analysis for the solar air collectors are made and then applied to the total three model collectors manufactured in same dimensions (630x315x100-116 mm) in ITU. The solar air collector examined is the open system. Air is considered as an ideal gas. The collectors have been tested at the same time of the day (9:30-17:30), with same air speeds (1.0 and 2.0 m/s), at different dates and environment temperatures in ITU. Detailed information and figures are given in the case. XIII
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