Güneş enerjili kendinden pompalı sıcak su sisteminin analiz ve simülasyonu
Analysis and simulation of a two phase self pumping solar water heater
- Tez No: 21977
- Danışmanlar: PROF. DR. A. NİLÜFER EĞRİCAN
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 138
Özet
ÖZET Bu çalışmada güneş enerjisiyle çalışan kendinden pompalı sıcak su sisteminin Türkiye iklim şartlarında verimi incelenmiştir. Sistemde konvansiyonel sistemlerin tersine soğutucu akışkan kullanılmıştır. Böylece kısın donma tehlikesi elimine edilmiştir. Sistem istenildiğin de mekanik pompayla da çalışabilmektedir. Teorik inceleme sisteme termodinamiğin birinci kanununun uygulanmasından oluşmaktadır. Sistem, basınçlandırma fazı, pompalama fazı ve ısıtma fazı olmak Üzere Uç fazdan oluşmaktadır. Her faz için ayrı matematiksel model kullanılmıştır. Modeller izotermal kabul edilmiş tir. Matematiksel modelin sayısal çözümü bir bilgisayar programı yapılarak elde edilmiştir. Programda kullanılan soğutucu akışkan özellikeri ile ilgili bağıntılar Du Pont de Neumours üretici firma tarafından sağlanmıştır. Program 4 alt programdan oluşmaktadır. SYS alt programı sistemin ısıl özelliklerini belirleyen katsayıları hesap lamaktadır. PROPS alt programı soğutucu akışkan özelliklerini, HX alt programı kondenserden geçen ısı miktarını, ISINIM alt programı ise Türkiyede herhangi bir enlemde herhangi bir yükseklikte ve herhangi bir saatte anlık güneş ışınımını hesaplamaktadır. Programda saat 12 deki anlık güneş ışınımı enerjisi kullanılmıştır ve program sonuna kadar sabit kabul edilmiştir. Sistem çevrim halinde çalışmaktadır. Bir çevrim yukarıda bahsi geçen üç fazdan oluşmaktadır. Simulasyon programı sistemin ve rimini hesaplamaktadır. Sonuçlardan da görüleceği gibi sistemin verimi güneş enerjisi miktarına ve çevre sıcaklığına büyük ölçüde bağlıdır.
Özet (Çeviri)
SUMMARY ANALYSIS AND SIMULATION OF A TWO PHASE SELF PUMPING SOLAR WATER HEATER After reviewing passive systems for downward trans port of heat, this thesis presents an aproximate mathema tical model for one particular system in which refrige rant that is evaporated in a solar collector transports heat to a condenser located some arbitrary elevation below the collector. The vapor pressure lifts the liquid from lower accumulator to an upper accumulator above the collector, from which it flows into the collector. Passive systems technologies in the use of solar energy are often favoured because of their Intrinsic reliability and, once installed, can be forgotten. Al though passive technologies are mostly related to archi tectural features, i.e. for living comfort, also solar hot water passive systems are always more frequently offered on the market. In these cases, the hot water reservoir is placed just above the collector, or is integrated with it, and the heat is transported from the collector to the water reservoir by a short thermosyphon loop. Obvious disadvantages are the weight of the system when it is full of water, and the heat losses at night, particularly in the cold season, and the danger of freezing. A reliable passive system for downward heat trans port allows the choice of a better place for the hot water reservoir, letting the collectors in their optimum placeCe.g. on the roof 3. There are many other circums tances in which a passive system would be highly desii - able, e.g. places where no electrical or mechanical energy is available for running the pumps, remote regions or those where technical asistance is scarce, projects for the heating of swimming pools or for heating the soil up to a certain depth C seasonal storage of solar energy). Because of the reasons presented above, it is attractive to transport heat from a solar collector to thermal storage by evaporating a fluid in the collector and condensing it within the thermal storage unit. The evaporation causes part or all of the solar absorber to operate at a single temperature, which may increase collector efficiency by effectively increasing the heat removal factor. The relatively large latent heat permits a lower rate of mass circulation than would be required by circulation of liquid only. The vapor pressure will drive the vapor from collector to condenser and can also x-be utilized to force the return of the liquid to the collector. Although it is useful for heating of service water, vapor transport is particularly attractive for passive space heating systems becauseCin principle) any number of physically separate thermal storage and delivery units can be connected in parallel. The vapor will preferentially condense at the unit of lowest temperature» thereby tending to maintain all units at the same temperature. Therefore, a passive vapor transport system has the potential for providing a uniform temperature throughout a building. Furthermore, such a system would permit simple occupant control by a manual shutoff valve when heat is not needed. If the collector is located below the elevation of storage, the condensed liquid will return to the collector by gravity. Such systems, called two-phase thermosyphons, are commercially available for heating of domestic water and are not the subject of the present investigation. A number of different system types have been conceived to take advantage of the improved heat transfer characteristics and self-circulating ability of a two- phase fluid. These include thermosyphons, active (mecha nically pumped) systems, bubble lift systems, self -pump ing systems. The use of a two phase fluid greatly improves the efficiency of thermosyphon loops C wherein the condensate is returned to the evaporator by gravity) over those using a single phase fluid. Thermal transport efficien cies Cheat delivered/heat in) greater than 60 percent are reported for a thermosyphon system charged with n-butane Per İman and Chen report that passive thermosyphons provided a solar fraction of 48 percent in a side-by- side test of six refrigerant charged active and thermosyphon! ng systems. Experiments by Soin, seen to show that sensible thermsyphons perform better than two- phase thermosyphons by as much as 18 percent However, Soin point out that this poor performance is perhaps due to noncondensible gases in the condenser. Price et.al. performed the analysis required to employ a two- phase thermosyphon loop in a transient system simulation CTRNSYS), thus enabling designers to predict the performance of thermosyphon systems prior to const ruction In a domestic hot water system, thermosyphon systems are sever ly limited by the requirement that the condenser be located at an elevation above that of the evaporator. Wachtell described a number of concepts for passively Cwithout external power) effecting heat transfer from an upper elevation to a lower one. The bubble lift device described by Wachtell uses the buo yancy of bubbles to drive sensible heat convection -xi-Wachtell also included liquid pumping by vapor expansion in his survey. In these so called self-pumping or passive downward heat transport systems the vapor pressure of a fluid boiling in the collector drives both the flow of vapor to the condenser and the return of the condensate to the evaporator. Systems using a single accumulator return the con densate directly from the condenser to an accumulator positioned above the evaporator. Bohanon and Tacchi hold patents for systems relying on this concept Both are controlled by the position of f loatactuated valves within the accumulator. Researchers in Ispra built an indoor prototype single-accumulator system which pressurizes an inert gas in the accumulator located at the condenser exit. As the boiler dries out, the boiler pressure decreases and the inert gas expands, pushing the condensate back up to the boiler Hedstrom and Neeper at Los Alamos National Labora tory have experimented with a variety of refrigerant- charged system types, including percolation systems, thermosyphons, active systems, and single and two accu mulator self pumping systems. In a comparison of passive solar systems, they showed that two phase systems could perform better than other passive systems because they allow less heat loss at night The single accumulator design has thermodynamic draw backs. As the condenser fills with condensate, the area available for heat transfer diminishes and the tempera ture difference across the condenser increases. This causes a corresponding increase in vapor pressure which forces the return of the liquid to the single accumulator positioned above the evaporator. Since increased pressu re in the condenser is achieved by partially flooding it with liquid, the area available for heat transfer and the heat exchanger effectiveness are consequently decreased. Low pressure in the accumulator is achieved by heat loss to the ambient which decreases collection efficiency. A two accumulator design alleviates many of the problems encountered with the single accumulator systems. The upper accumulator is at all times connected to the condenser so that heat need not be deliberately lost to the ambient in order to return condensate to the collector loop. The lower accumulator provides for storage of the condensate between pump cycles so that the condenser is not flooded. The primary disadvantage of this design is the additional hardware required, namely another tank and additional piping to connect the collector and the lower accumulator. Knecht patented a simpler system controlled by a float-actuated valve in the lower accumulator. Experimental results from tests with an indoor prototype at Los Alamos confirm that the -xlitwo accumulator design was superior to that with one accumulator This thesis consists of a theoretical study of a two phase self pumping solar water heating system. The ob jectives of the study are to quatify the operating cha racteristics and thermal efficiency of the system, iden tify design and operating modifications which may enhance the viability of the self pumping system in the domestic water heating market. Theoretical study results in a system cycle simulation in which the equations governing system temperature and energy flow rates are derived from the principle of conservation of energy. These equations are solved by finite difference approximation. Output of the simulation includes operating temperatures and pres sures, energy flow rates at each time step and cycle thermal efficiency. A QUICKBASIC program CPASSDOWN) was written to execute this procedure on a digital computer. The analysis is based on conservation of mass and the first law of thermodynamics. The only empirical values used in the analysis are materials properties and solar collector parameters. The condenser is modelled using an NTU effectiveness method. Thermal losses from pipes are treated as conduction coefficient to the ambient. The analysis neglects fluid friction in the pipes. Superheating and subcooling of the refrigerant are neglected. As a result each model is assumed to be isothermal. In Section 3.1 of this thesis, a combination of models which suitably characterize the physical system is identified. A mass transfer analysis is presented in Section 3.2. The objective of energy analysis is to derive the equations governing energy flow rates and define cycle efficiency. The numerical solution of the governing equations is the subject of Section 3.4. System operation is cyclic, progressing through a run or heat collection phase, a pressurizing phse, and a pump phase. During the run phase, liquid refrigerant is gravity fed to the collector from the upper accumulator. Vapor travels downward to the condenser and liquid refrigerant collects in lower accumulator tank. After a preset quantity of refrigerant (determined by the setting of the lower float switch in the upper accumulator) con denses, solenoid valve No.l closes, solenoid valve No. 2 opens, and the pressurizing phase begins. Since vapor is prevented from entering the condenser, pressures and temperatures in the collector and lower accumulator increase. When the pressure in the lower accumulator exceeds the pressure in the upper accumulator by an amount equivalent to the hydrostatic pressure of the liquid column refrigerant is forced from the lower to upper accumulator. As soon as the upper float switch is tripped, solenoid valve No.l reopens and solenoid valve - xili -No. 2 closes. This reinitiates the run phase and system temperature drops as pressurized vapor vents to the con denser. The conservation of energy analysis is simplified by the assumption that each system model is isothermal. This assumption is justified by the behavior of the two phase heat transfer fluid which ideally transfers heat by its latent heat rather than by a sensible change in its temperature. The refrigerant is thus always saturated and no subcooling or superheating is considered. Free movememt of vapor along pipes facilitates heat transfer throughout the system and supports the validity of the isothermal assumption. The system boundary is chosen to minimize the error introduced by the isothermal assumption. In all three models the system boundary is assumed to be a surface located inf initesimally exterior to the inside surface of the insulation. A surface located just below the liquid level in the lower accumulator specifies the system boundary within that component for the pressurizing and pump mode models. Closed valves which do not allow mass transfer also constitute system boundaries. Thus, all three mode models are closed systems with only energy interactions and no mass transfer across the system boundary. The system boundary is in a fixed position for the run mode and pressurizing mode model but moves with the surface of the liquid in the lower accumulator for the pump mode model. The thermal capacitances of the pipes, tanks, and other components are included in all system models because they are in close thermal communi cation with the working fluid. The insulation is model led only a resistance to heat loss from the system to its surroundings. Low thermal conductivity causes tempera ture gradients within the insulation which are not compa tible with the isothermal assumption. Excluding the insulation from the model introduces some error in that the effects of the thermal capacitance of the insulation are neglected. The data required by PASSDOWN is divided into four input files. The input file named AMB contains the ambient conditons, INIT contains the initial conditions, CONS contains the refrigerant properties correlation constants and SYSTEM contains the system dimensions and material properties. The individual variables of each input file are included in Appendix A. The QUICKBASIC code which executes algorithm followed in the solution of the energy equation is included as the program PASSDOWN in Appendix B. The main body of the program reads the input files, iterates temperature until the energy equation balances at each time step, increments time and integrates energy flow quantities, and outputs results at each time step and at -xiv-the end of cycle simulation. There are three subroutines in program PASSDOWN. Subroutine SYS calculates the thermal parameters CUA, MC, and V) of the system from the physical dimensions and material properties of the system. Subroutine HX calculates the heat across the condenser from the refrigerant temperature, storage water temperature, and the dimensions and material properties of the condenser. Subroutine PROPS calculates the thermodynamic properties of the refrigerant as functions of temperature. The properties are related to temperature by empirical correlations provided by the manufacturer of the refrigerant. Subroutine ISINIM calculates the insolation held by the collector from latitude, slope, month, height, insolation duration, and angle hour. The angle hour is taken to be 12.00 which is held constant through the procedure. -xv-
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