DESIGN AND IMPLEMENTATION OF A PARABOLIC TROUGH COLLECTOR SYSTEM (M.S. Thesis, Egemen Orhan Bilgin)

With the effect of global warming on weather, most countries are interested in renewable energy sources and solar energy is the most promising one among others with an endless source. With this awareness, a lab scale test setup with an existing parabolic trough solar collector was designed and implemented with the aim of measuring inlet and outlet temperatures from the collector to calculate the thermal efficiency, useful heat gain, heat loss, and optical efficiency. A boiler was used as thermal storage with its two coils inside. Also, a solar tracking system was put into practice to follow the movement of the Sun to decrease the angle of incidence. The experiments were performed at Boğaziçi University Renewable Energy Technologies Laboratory on Sarıtepe Campus. As a result of experiments, maximum temperature and maximum thermal efficiency are found to be 65.25°C at 3.20 P.M. and 14.5% at 3:30 P.M. Moreover, the transition and turbulent flow are observed in the receiver at around 60°C.

THERMODYNAMIC AND ECONOMIC ANALYSIS OF ORC WITH OPTIMIZATION OF SYSTEM COMPONENTS (M.S. Thesis, Hasan Eren Bekiloğlu)

Organic Rankine cycles (ORCs) are used to generate power from low temperature heat sources. There are two main chapters in this study. In both parts, 28 different working fluids are used for three different heat source temperatures (90, 120 and 150 °C). In the first part, a preliminary radial-inflow turbine (RIT) design with loss calculations is incorporated into the model to add dynamic turbine efficiency. The turbine model is run together with the ORC, and real gas properties are used. A genetic multi-objective optimization algorithm (NSGA-II) is used to obtain an ORC and design of system components according to minimum thermal conductance per net power output and maximum performance factor (PF). The decision variables are specific speed, condensation pressure, pressure ratio through turbine, radius ratio at the nozzle, degree of superheating and pinch point temperature difference (PPTD) in the evaporator. Pareto frontiers are obtained as a result, and a decision-making method (TOPSIS) is used to select an optimum solution for each working fluid. R1234yf, R1234ze(e) and isobutane are found to be the optimum working fluids for 90, 120 and 150 °C respectively. For each solution, turbine geometry, fluid velocities and rotor rotational speed are calculated. In the second part, a heat transfer model is added into the model used in the first part for the evaporator preferred as a plate heat exchanger (PHE). The first objective function is redefined, and evaporator heat transfer area is used instead of minimum thermal conductance. The length, the width of the plates and the spacing between the plates in PHE are added as new decision variables to optimize evaporator geometry. R1234yf is found to be the optimum working fluid for 90 °C source temperature, and butane is obtained to be ideal for both 120 and 150 °C unlike the first chapter.

DESIGN, PRODUCTION AND TESTING OF A LABORATORY SCALE ORGANIC RANKINE CYCLE SYSTEM (M.S. Thesis, Onur Kardaş)

This study presents the thermodynamic design, implementation and commissioning of a small scale organic Rankine cycle test bed. The objective is to design a flexible organic Rankine cycle test bed to study performance of different working fluids and cycle components. The test bed is instrumented with pressure, temperature and flowmeter sensors to collect data from cycle operation. These data will be utilized to validate various thermodynamic models. During the design process environmental, safety issues and physical properties of 13 different fluids are studied  to obtain suitable working fluids.  Four fluids namely R134a, R141b, R245ca and R245fa are taken into consideration for further detailed analyses. An organic Rankine cycle system which is capable of operating with all these working fluids is designed and built.

3D RADIAL INFLOW TURBINE DESIGN AND ANALYSIS FOR A LABORATORY SCALE ORC APPLICATION (M.S. Thesis, Emre Sezerkan)

In this study, the aim is to design and analyze a single stage subsonic radial inflow turbine for the Organic Rankine Cycle system established in Boğaziçi University Renewable Energy Technologies Laboratory, by using R245fa and R134a as the working fluids. Complete design process of the ORC turbine, from preliminary design to 3D blade design stage, is presented. Design point of the turbine is determined according to the cycle limitations and by comparing the maximum Mach number, rotational speed and mass flow rate parameters as a result of detailed preliminary turbine design and basic cycle analysis. Results of the streamline and CFD analysis of the designed turbine are compared and discussed from the point of Mach number distribution, turbine efficiency and power output. The comparison shows that the two analyses give similar efficiencies, power outputs and Mach number distributions. Finally, the performance charts of the designed turbine are generated for both R245fa and R134a. Streamline analyses results show that the maximum total to static isentropic turbine efficiency and power output are 87.5 % and 3.97 kW respectively.

1D COMPRESSION-IGNITION ENGINE SIMULATION AND COMPARISON OF SOOT EMISSION MODELS (M.S. Thesis, Alpay Asma)

The aim of this paper is to develop an engine model that can predict the performance and soot emission trends of a selected engine under full load. With this aim PUMA GLOBAL 2.2 liter 125 PS CI engine having 4-cylinders with turbochargers has been modeled numerically. The work can be divided into two stages including validation of thermodynamic & performance based results of the selected engine, and correlation of selected phenomenological soot models with measured data at all 35 speed modes ranging from 1000 to 4400 rpm. In order to predict the heat release rate, and calculate the in-cylinder pressure and temperature values with good accuracy, multi-zonal combustion model called as Vibe 2-Zone is used.

EFFECT OF AIR FLOW, WATER SPRAY AND NOZZLE INJECTION CHARACTERISTIC ON DIRECT EVAPORATIVE COOLING (M.S. Thesis, Hamed Shafiei)

Evaporative cooling  occurs through evaporation of water droplets in air. It works on the  principle  of  injecting  water  spray  directly into  the  air flow to reduce  its  temperature  by  a  conversion  of  sensible  heat  of  the air flow to latent heat of water. Incomplete evaporation and non-uniform temperature distribution are the main concerns of water spray cooling systems. Incomplete evaporation of water droplets may cause corrosion in the heat exchanger and increases operational cost due to water consumption. Moreover, non-uniform temperature distribution due to gravitational effect can decrease the efficiency of the cooling system. In this study the effect of different parameters (air velocity, relative humidity, droplet velocity, nozzle cone angle, droplet size distribution, turbulence intensity, and nozzle injection angle) on dry bulb temperature, the evaporated water fraction, and spray cooling efficiency are investigated. Contact surface area between water droplets and air and the residence time are two main characteristics which affect spray cooling efficiency. Increasing spray dispersion and residence time are main focus in our study. The results show that as D, the mean of the Rosin-Rammler distribution, is reduced from 60 to 30 μm, the cooling performance of the system is improved by more than 40%. Also, for a given values of the inlet air velocity, as the inlet air velocity decrease from 3.5 m/s to 1.5 m/s, the spray cooling efficiency of the system improves by more than 29.4%. Moreover, it can be concluded that moving toward the end of duct length where the coolant flow become more turbulent and wide enough we could reach a better water evaporation, more temperature drop for hot air flow, and consequently, higher spray cooling efficiency.

THERMODYNAMIC MODELLING AND EXPERIMENTAL VALIDATION OF A LABORATORY SCALE ORC SCROLL EXPANDER (M.S. Thesis, Doruk Can Ogan)

The detailed geometrical and thermodynamic modelling and experimental validation of scroll expander is presented in this thesis. Firstly, a fully mathematical model is developed for moving and fixed scrolls  inside the scroll expander, benefiting from an arc of spiral shape. Scroll expander contains two scroll structures which are interlaced so that one of the scrolls can move within the boundary of the other scroll. Once the scroll spirals are defined mathematically, the chamber volumes between moving and fixed scrolls are calculated. However, the scroll wall thickness is assumed to be constant throughout the study. Besides, a thermodynamic modelling is conducted in the chambers by means of first law of thermodynamic and ideal gas equations. The working fluid is assumed to be an ideal gas and all of the leakages and heat transfer between chambers or between scroll expander and outside are neglected. The dynamic process of scroll expander comprises of three phases which are charging, expansion and exhaust. Thus, temperature, pressure and volume variation charts for every chamber are given. The motion analysis of moving scroll is covered and torque calculation which results in the rotation speed of scroll expander is given. The mathematical model and simulation process are implemented in Matlab. Then, the simulated model is compared and validated by a reference model. In the experiment chapter, the test rig is introduced in detail at first and the scroll expander is run with compressed air. Test results are presented with two different supply pressure values and also validated by the simulation results with compressed air. Finally, a simulation study of scroll expander running with HFC-134a is also performed and the results are presented.

MODELLING OF UNSTEADY PROCESSES OF ADSORPTION REFRIGERATION SYSTEMS (M.S. Thesis, Mehmet Emre Berk)

Energy savings on refrigeration systems are highly desired due to the awareness of limited energy sources in recent years. In this regard, new technologies have been developed in addition to the conventional refrigeration systems, which utilizes fossil fuels as an energy source, in order to utilize renewable energy sources and waste heat. Adsorption refrigeration systems, which are operated by the renewable low-grade thermal energy sources, have drawn attention because of the advantages in aspects of energy saving and being environment friendly. However, adsorption refrigeration systems have many problems in terms of long cycle time and low system performance yet to be overcome. In this project, adsorption refrigeration systems with non-uniform temperature and non-uniform pressure profile within the adsorbent bed have been investigated by comparing the effect of different operating conditions and working pairs such as silica gel-water and activated carbon-methanol on various system parameters in order to analyze the poor heat and mass transfer process within the adsorbent bed. Moreover, heat and mass recovery operations have been applied to the basic cycle in order to improve system performance by demonstrating their effect on coefficient of performance (COP). The mathematical model has been performed in only radial direction and numerically solved with the help of finite difference method and Newton-Rhapson method by using pre-defined initial and boundary conditions, and discretized partial differential equations implemented in Matlab. The results indicate that the analysis carried out in this thesis shows good agreement with the studies in the literature.

DYNAMIC MODELING OF AN ORGANIC RANKINE CYCLE (M.S. Thesis, Hasan Börke Birgin)

A numerical model of an ORC (Organic Rankine Cycle) system based on MATLAB environment is constructed in this study. The main components of the ORC are pump, evaporator, turbine and condenser. The organic working uid (r134a) with low saturation temperature is employed. The low saturation temperature enables organic fluid to change it’s phase with low heat input, thus system gains high amount of enthalpy without having excessive heat source. Waste heat and solar collectors are some popular heat sources for ORC’s. Energy systems are considered to operate in a steady state condition. System design and component selections are made according to this steady state operating condition. However, when a disturbance appears, one can not calculate the response of the system based on the steady state model. Hence, it is important to utilize a dynamic model of the energy system. Apart from disturbances, start-up and shut-down phases of the system have also transient outcomes. To define those transients in a thermodynamic cycle is a developing topic. The transient model is employed for the heat exchangers of the system. The turbine and the pump are modeled in the steady state, because of their comperatively short response time. The effect of variable pressure on the fluid density is considered as instantly. Discretization of the heat exchangers is carried out according to the backward Euler formulation, and implicit scheme of time integration is employed in the model. Between time steps, an iterative solution method is introduced for guessing the spatial distribution of the mass flow rate. Different correlations are presented for the heat transfer in a plate heat exchanger. The examination of the dynamic model is made with constructed steady state heat exchanger model and with the data acquired from the experiment, which is conducted in BURET Laboratory at the Bogazici University Kilyos Campus. The dynamic model can be considered as a reliable predictor of outcomes, when system has disturbances or when system is in a transition, in consequence of the change of inputs.

MODELING AND SIMULATION OF A SMALL SCALE SOLAR ORGANIC RANKINE CYCLE SYSTEM FOR POWER GENERATION (M.S. Thesis, Umut Soysal)

In this study, a numerical model of a small scale concentrated solar power system is designed for Istanbul conditions and the annual performance of the system is evaluated. A thermal model of parabolic through collectors are modeled and the heat output of the collector system is compared with experimental data. For the power block, an organic Rankine cycle (ORC) system is used. For the working fluid, R245fa is chosen and whole system is modeled in MATLAB environment.  Single tank thermocline storage model is implemented to observe the performance increase of the system with a suitable thermal storage. Using meteorological data, the annual performance of the designed system is estimated. Optimum values for mass flow rates, cycle pressures and required collector area is studied.