Güneş enerjisi destekli zeolit iklimlendirme sistemi

TÜBİTAK MAG01.01.2009Güneş enerjisi ekonomik, temiz ve yenilenebilir bir enerji türüdür. Çevresel faktörler ve artmakta olan enerji talepleri dikkate alındığında bu enerji türünün kullanımı büyük önem kazanmaktadır. Güneş enerjili adsorplanmalı soğutma (GEAS) sistemleri gelecekte yaygın olarak kullanılması düşünülen ve ticarileşmesi için yoğun bir şekilde çaba sarf edilen termal enerji destekli soğutma sistemlerindendir. Bu sistemlerin başarılı bir şekilde çalışması büyük ölçüde adsorban- adsorplanan çiftinin doğru seçimine bağlıdır. Zeolit-su, zeolit-organik soğutucular, silika jel-su ve activated karbon-methanol katı adsorplanmalı güneş enerjili soğutma sistemlerinde kulanılabilecek uygun adsorban- adsorplanan çiftlerinden bazılarıdır. Adsorban malzemenin adsorplama kapasitesi, x=f (P, T), adsorpsiyon basıncı ve adsorban malzemenin sıcaklığı ile değişmektedir ve bu değişim çoğunlukla eş sıcaklık eğrileri çizilerek ifade edilmektedir. Bu çalışmada ilk olarak, zeolit-su çalışma çiftine ait eş sıcaklık eğrilerini elde etmek için adsorpsiyon deney düzeneği tasarlanmış ve imal edilmiştir. Daha sonra, bu düzenek üzerinde bir takım deneyler yapılmıştır. Bu deneyler sonucunda, zeolit-su çalışma çiftine ait eş sıcaklık eğrileri, adsorpsiyon ve desorpsiyon prosesleri için farklı adsorban o sıcaklıkları(40-150 C)veadsorpsiyonbasınçları(0.87-7.38kPa)altındadeneyselolarakelde edilmiştir. Ayrıca, adsorpsiyon ve desorpsiyon prosesleri için elde edilen eş sıcaklık eğrilerine Dubinin–Astakhov denklemi uyarlanmıştır. Đkinci olarak, adsorpsiyon deneylerinden elde edilen sonuçlar çerçevesinde, zeolit-su çalışma çiftini kullanan güneş enerjisi destekli adsorpsiyonlu örnek soğutma grubu tasarlanmış ve imalatı gerçekleştirilmiştir. Daha sonra, adsorpsiyonlu örnek soğutma grubunun termal performansı farklı buharlaştırıcı sıcaklıklarında deneysel olarak incelenmiştir. Desorpsiyon prosesi sırasında zeolit yatağının yenilenmesi için gerekli enerji, güneş enerjisi yerine elektrik enerjisiyle ısıtılan ısı transfer sıvısının gövde ile adsorban yatak arasında dolaştırılmasıyla sağlanmıştır. Diğer bir ifade ile güneş enerjisi elektrik enerjisi kullanılarak benzeştirilmiştir. Deneyler sonucunda, soğutma grubunun COP değeri ortalama 0, 25 olarak bulunmuştur. Son olarak, GEAS sistemlerini başarısı çalışma çiftinin doğru seçiminin yanı sıra sistemde kullanılacak güneş toplaçlarının performansına ve ekonomik maliyetinede büyük ölçüde bağlıdır. Dolayısıyla bu çalışmada, soğutma sisteminin güneş enerjisi destekli olması sebebiyle bir yıl boyunca düzlemsel ve vakumlu boru tipi güneş toplaçlarının performansı Ankara ili iklim koşulları altında deneysel olarak incelenmiştir. Ayrıca, güneş enerjisi destekli zeolit-su çalışma çiftini kullanan xi adsorpsiyonlu soğutma sisteminin yıllık analizlerini yapmak üzere TRNSYS programı yardımıyla sayısal bir model tasarlanmıştır.Solar energy is economical, clean and renewable. The use of solar energy is gaining more attention due to environmental factors and increasing demand for energy. Solar powered adsorption cooling (SPAC) systems are considered to have a large potential for use in the future and extensive efforts have been expended to make the thermal powered adsorption cooling systems commercial. The successful operation of these systems mostly depends on the correct choice of the adsorbent-adsorbate working pair. Zeolite-water, zeolite-organic refrigerants, silica gel-water and activated carbon-methanol are some of the suitable adsorbent-adsorbate pairs that can be used in solar powered adsorption cooling systems. Adsorption capacity of the adsorbent material, x=f (P, T), varies with the adsorption pressure and adsorbent temperature and this variation is commonly represented by isotherms of the adsorbent investigated. In this study, firstly, an adsorption experimental set-up was designed and constructed to obtain the isotherms of a natural zeolite-water working pair. After that, a set of experiments was conducted using this set-up. As a result of these experiments, isotherms of the zeolite-water pair were obtained experimentally under various adsorbent o temperatures (40-150 C) and adsorption pressures (0.87-7.38 kPa). In addition, the Dubinin– Astakhov equation was used to fit the isotherm data that were obtained for adsorption and desorption processes. Secondly, within the framework of the results of the adsorption experiments, a prototype of the solar powered adsorption cooling system using the natural zeolite-water working pair was designed and constructed. Afterwards, the thermal performance of the prototype was investigated experimentally under different evaporation temperatures. In these experiments, during the desorption process, the solar energy that is needed to dry out the saturated adsorbent was simulated by using electrical energy. As a result, the average COP value of the prototype studied was found as 0,25. Finally, in addition to the correct choice of the adsorbent-adsorbate pair, the success of the (SPAC) systems depends largely on the thermal performance and cost of the solar collectors. Therefore, in this study, the performance of flat plate and evacuated tube collectors were investigated under the climatic conditions of Ankara for one year. Additionally, a numerical model of the solar powered adsorption cooling system using zeolite-water pair was developed with the help of the TRNSYS program to analyze the annual performance of this system

Adsorpsiyonlu soğutma sisteminin performansı üzerinde deneysel bir çalışma ve sistemin adsorban yatağının sayısal analizi.

In this thesis, firstly, the equilibrium adsorption capacity of water on a natural zeolite at several zeolite temperatures and water vapor pressures has been experimentally determined for adsorption and desorption processes. Additionally, the modified Dubinin-Astakhov adsorption equilibrium model has been fitted to experimental data and separate correlations are obtained for adsorption and desorption processes as well as a single correlation to model both processes. Experimental results show that the maximum adsorption capacity of natural zeolite is nearly 0.12 kgw/kgad for zeolite temperatures and water vapor pressures in the range 40-150 C and 0.87-7.38 kPa. Secondly, a thermally driven adsorption cooling prototype using natural zeolite-water as working pair has been built and its performance investigated experimentally at various evaporator temperatures. Under the experimental conditions of 45 C adsorption, 150 C desorption, 30 C condenser and 22:5 C, 15 C and 10 C evaporator temperatures, the COP of the adsorption cooling unit is approximately 0.25 and the maximum average volumetric specific cooling power density (SCPv) and mass specific cooling power density (SCP) of the cooling unit are 5.2 kWm-3 and 7 Wkg-1, respectively. Thirdly, in order to investigate the dynamic heat and mass transfer behavior of the adsorbent bed of an adsorption cooling unit, a transient local thermal non equilibrium model that accounts for both internal and external mass transfer resistances has been developed using the local volume averaging method. Finally, the influence of several design parameters on the transient distributions of temperature, pressure and amount adsorbed inside the cylindrical adsorbent bed of an adsorption cooling unit using silica-gel/water have been numerically investigated for the one and two dimensional computational domains. Moreover, validity of the thermal equilibrium model assumption has been shown under the given boundary and design conditions. Generally, for the conditions investigated, the validity of the local thermal equilibrium and spatially isobaric bed assumptions have been confirmed. To improve the performance of the bed considered, e orts should be focused on reducing heat transfer resistances and intra-particle mass transfer resistances but not inter-particle mass transfer resistances.Ph.D. - Doctoral Progra

Nemlendirme ve nem alma tekniğine dayalı güneş enerjili damıtma sisteminin performansının deneysel ve teorik olarak incelenmesi.

In this thesis, experimental and numerical studies have been carried out to investigate the performance of a solar desalination system working on humidification-dehumidification principle under the climatological conditions of Ankara, Turkey. The desalination unit was configured mainly by a double-pass flat plate solar air heater with two glass covers, pad humidifier, storage tank and dehumidifying exchanger. The system used in this work is based on the idea of closed water and open air cycles. A computer simulation program based on the mathematical model was developed by means of MATLAB software to study the effect of different environmental, design, and operational parameters on the desalination system productivity. In this simulation program, the fourth order Runge-Kutta method was used to solve the energy balance equations simultaneously and numerically. In order to compare the obtained theoretical results with experimental ones and validate of the developed mathematical model of the system, an experimental study has been carried out. For that, an experimental set-up was designed, constructed and tested at the solar house of the Mechanical Engineering Department of METU. In addition, the existing solar desalination system was integrated with an evacuated tubular solar water heater unit (closed water circulation) and performance of the system has been studied experimentally.M.S. - Master of Scienc

Transient behavior of a cylindrical adsorbent bed during the adsorption process

A transient two dimensional local thermal non-equilibrium model is developed to investigate the influences of heat transfer and operating parameters on the dynamic behavior of a cylindrical adsorbent bed during the adsorption process. Local volume averaging method is used to drive the macro scale governing conservation equations from the micro scale ones. In the model, linear driving force model and Darcy's equation are considered to account for the resistances to internal and external mass transfer, respectively. Silica gel-water pair widely used in the adsorption cooling systems is chosen to be an adsorbent-adsorbate working pair. The parameters of interest are convective heat transfer coefficient, solid phase thermal conductivity, bed thickness, evaporator pressure, condenser pressure, driving heat source temperature and cooling source temperature. It is found that amount of refrigerant circulated through the system increases with increasing evaporator pressure-driving heat source temperature and decreasing condenser pressure-cooling source temperature. The duration of adsorption process is more sensitive to heat transfer resistances than to mass transfer resistances. The conductive and convective resistances need to be reduced to reach the equilibrium conditions in a short period of time and hence to have a better specific cooling power

Physical properties and adsorption kinetics of silica-gel/water for adsorption chillers

The choice of a suitable adsorbate/adsorbent pair is critical for an adsorption cooling cycle. The surface characteristics and thermo-physical properties of the adsorbent, and the adsorption rate of adsorbate are key parameters in making the choice. Through literature review, it is found that there are disagreements among the experimental measurements and various equations/models used to describe adsorption isotherms and surface diffusivity of water in silica-gel. In this work, an experimental set-up is built to measure the isotherms and kinetics of vapor adsorption for any working pair. Using the newly measured data, those from the manufacturers and from the literature, these inconsistencies are eliminated by utilizing the Dubinin-Astakhov (D-A) model to fit the entire adsorption isotherm curve. The Brunauer-Emmett-Teller (BET) method is used to calculate the surface area, pore volume and pore diameter of two different types of silica-gel. Based on the adsorption rate and the adsorbent temperature measured simultaneously, a new approach is proposed to measure the surface diffusivity in the temperature and pressure ranges typical of those during the operating conditions of adsorption cooling systems. Analysis of the results indicates that the surface diffusivity follows the Arrhenius-form equation. The calculated activation energy at different adsorption conditions varies from 40.0 to 41.2 kJ/mol and the pre-exponential factor varies from 2.5 × 10−4 to 2.8 × 10−4 m2/s. These values are close to those previously reported in the literature. Thus, the proposed approach can be used to measure the surface diffusivity in nanoporous materials

Revisiting the adsorption equilibrium equations of silica-gel/water for adsorption cooling applications

This paper addresses the discrepancies among the common adsorption isotherms of silica-gel/water provided in the literature. It is reported that the Freundlich model and Tòth equation cannot be used to estimate the uptake at relative pressure less than 0.15. In addition, inconsistencies are found among the various models and equations used to describe the uptake of water vapor onto silica-gel. New coefficients for the Dubinin–Astakhov (D–A) model are proposed to eliminate these disagreements. Due to the limited experimental measurements, an experimental setup is designed and built to measure the sorption kinetics and equilibrium uptake of any working pairs. Experimental measurements show that the maximum uptakes of silica gel RD-2060 and Type-RD are 0.38 kg kg−1 and 0.48 kg kg−1, respectively. Apparent capillary condensation is observed at a relative pressure of 0.4 and 0.35 for silica-gel RD and RD-2060, respectively. Also, it is found that the D–A model can fit the adsorption isotherms of silica-gels appropriately for the entire range of relative pressure when the characteristic energy is set as a function of relative pressure instead of assuming constant values