Theoretical and experimental analysis of an organic Rankine Cycle

Collings, Peter (2018) Theoretical and experimental analysis of an organic Rankine Cycle. PhD thesis, University of Glasgow.

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In order to reduce emissions of carbon dioxide from the energy and transportation sectors, while still providing a reliable and affordable service, innovation in the fields of power generation and energy efficiency is needed. There exists a wide variety of low-temperature heat sources, such as waste heat from industry and transportation, solar thermal, biomass and geothermal, which contain large amounts of energy, but do not have sufficient temperature to be economically viable using traditional power generation techniques. Several technologies have been proposed to utilise these promising resources, of which the Organic Rankine Cycle is widely considered to be the technology with the most potential for large-scale commercial deployment. However, the low driving temperature differential available to Organic Rankine Cycles using these heat sources means that they face several technological challenges, some of which are addressed in this thesis. Firstly, they experience low efficiencies, which means that small absolute changes in efficiency and cost can be proportionally very significant, this makes cycle optimisation to achieve marginal gains a worthwhile exercise. Secondly, there is a lack of suitable working fluids for the Organic Rankine Cycle, meaning that they often have to operate with a fluid that is not tailored for the specific application. Producing tailor-made working fluids to a given heat source and sink temperature could represent a significant field for optimising the performance of ORCs. Thirdly, there is a lack of experimental validation of many theoretical aspects of the Organic Rankine Cycle, particularly for low heat source temperatures and power outputs. This thesis aims to contribute to the body of research on ORC technology by developing an analytical model to design an experimental rig. This rig is used to validate several theoretical predictions, which are then expanded upon to develop a novel method of cycle optimisation in an application with variable heat sink temperatures.
Firstly, a thermodynamic model was developed in MATLAB to analyse a small-scale Organic Rankine Cycle. This model builds on well-established analytical modelling principles that frequently appear in the literature. This basic model was used as a tool to design a lab-scale experimental Organic Rankine Cycle rig, capable of addressing several gaps in the current literature, most notably the lack of research on the impact of a regenerator on the performance of an Organic Rankine Cycle, and the lack of experimental research on the performance of an Organic Rankine Cycle using a working fluid composed of a mixture of two working fluids, in this case r245fa and r134a. The model, its results and the design of the experimental rig are described in detail.
The results from this experimental rig showed an increase in cycle efficiency and cycle output power with increasing heat source temperature and increasing cycle pressure ratio. The use of a regenerative cycle resulted in an increased cycle efficiency, but the extra flow resistance caused by the additional heat exchanger caused the mass flow rate of the cycle to drop, reducing the output power at the same time as reducing the evaporator heat demand and thereby increasing cycle efficiency. The addition of more r134a, which has a lower boiling point, to the working fluid mixture, increased the condenser pressure and thereby reduced the cycle pressure ratio, reducing output power and efficiency. The maximum efficiency achieved was 11.3%, for a regenerative cycle with a heat source temperature of 95°C and a pressure ratio of 4.56:1.
Using the results from the experimental rig, and the model that they validate, the concept for the Dynamic Organic Rankine Cycle is presented. The Dynamic Organic Rankine Cycle was conceived as a solution to a problem identified in the literature, namely that an Organic Rankine Cycle using ambient air as the heat sink cannot fully utilise the driving temperature differential available to it during times of colder ambient temperature, as it must be designed to still function on the hottest day of the year. In order to address this, the Dynamic ORC Concept uses a variable working fluid composition, capable of shifting the composition between one working fluid component and the other by batch distillation in order to change the fluid’s bubble and dew points to match the heat sink temperature. The use of working fluid mixtures is in contrast to most current research, which has focused primarily on pure, single-component working fluids. A theoretical analysis of this cycle in MATLAB was carried out, and it was found that the cycle results in substantial increase in year-round power generation from the cycle, of the order of 8-10% for a heat source temperature of 150°C, increasing to 23% and higher for heat source temperatures of 100°C and below, while operating in a continental climate, such as that of Beijing, China. When operating in a climate with less temperature variation, the gains are lower, but still significant.
Structurally, this paper presents a review of the relevant literature to the Organic Rankine Cycle, identifying the knowledge gaps that justify the work carried out. It then reviews the theory of the ORC, and how this was used both to build a computer model for analysis of the dynamic ORC and design the 1kW experimental rig. The experimental results from the rig are then presented and discussed. Finally, the results of the theoretical analysis of the dynamic ORC are presented, and analysed with the aid of the REFPROP fluid properties program to explain the trends observed in the data. Finally, suggestions for further work are made.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic Rankine Cycle, zeotropic, waste heat, geothermal, solar.
Subjects: T Technology > TJ Mechanical engineering and machinery
Colleges/Schools: College of Science and Engineering > School of Engineering > Systems Power and Energy
Supervisor's Name: Yu, Dr. Zhibin
Date of Award: 2018
Depositing User: Mr. Peter Collings
Unique ID: glathesis:2018-30642
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2018 07:27
Last Modified: 16 Aug 2018 16:13
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