1 Introduction
1.1 Project background
With the increasing concern on environmental problems, more and more renewable energy resources are being installed and big efforts are being put on their development.
Nowadays, in large stationary power generation scale, natural gas and coal fueled gas cycles and vapour cycles play a dominant role [1]. However, the renewable energy boom has led to the need of developing new technologies for power production, as vapour and gas cycles are not a technical nor an economical viable solution when the temperature available from the heat source is low, which may be the case for certain renewable energy sources such as biomass energy, solar energy or geothermal energy.
Among the listed renewable energy sources, geothermal energy shows a promising future, since it has advantages that none of the rest can provide. These advantages are related to its availability and stability, and to the fact that it does not depend on ambient conditions, offering the possibility of renewable energy base-load operation [1]. Being able to make the most of these advantages is one of the principal goals of new research studies, mainly focused on the investigation and development of Organic Rankine Cycles (ORCs), the primary technology that is used to produce power from low-temperature heat sources.
1.2 Motivation
During the last years, ORC technology has become a strong player in the market, showing a promising future in the renewable energy power production. Improving the performance and decreasing the cost of these systems has become one of the most demanding activities for engineers and researchers.
With this aim, from January to June (2017), I worked on a project related to the thermodynamic optimization of Rankine cycles, reaching results that included fluid selection, cycle layout choices or the influence of the pump and turbine efficiencies on
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the overall performance, among others. As a fruit of the labor, professor Lars O. Nord, PhD candidate Roberto Agromayor, and I, published a conference paper (see [2]).
After this project, I came to the conclusion that the thermodynamic assessment alone is not an exhaustive indicator of the optimal configuration of a Rankine cycle, mainly because every working fluid presents different properties, which have a great influence not only on the performance of the cycle, but also on its size and, therefore, on its cost [3].
In order to provide the right assessment of a Rankine cycle project, the optimization process must include both a thermodynamic and techno-economic analysis.
1.3 Objectives of the work
The objectives of this project are:
1. Perform a literature review on the design of Rankine cycles components, heat transfer correlations, cost correlations, working fluids and cycle layouts for a varying geothermal heat source.
2. Develop a MATLAB program able to execute the steady-state cycle optimization of the Specific Investment Cost and the second-law efficiency by computing all cost, heat transfer and pressure drop correlations, as well as all thermodynamic states of the cycle.
3. Validate the model.
4. Select a case study and execute the Specific Investment Cost cycle optimization for different heat exchanger configurations and working fluids.
5. Select the most suitable heat exchanger configuration and a set of working fluids, and compare the thermodynamic and techno-economic optimization results for different heat source and heat sink scenarios.
6. Generalize the results.
An assessment of the accomplishment of these objectives is presented in Section 11.3.
3 1.4 Risk Assessment
This work is purely theoretical and no risk assessment was required.
1.5 Organization
The project has been divided in 11 chapters:
The first two chapters constitute an introduction to Organic Rankine Cycles, which includes a brief description to their development through history and their current situation in the market, a technical overview of the different configurations and layouts that can be implemented, and a general description of the main components that are part of the cycle. After having introduced the theoretical basis, Chapter 4 presents the heat exchanger geometry parameters and design considerations that are required for reckoning these components. Chapter 5 includes the thermodynamic fundamentals that allow to compute all the thermodynamic states of the cycle and to define the thermodynamic objective function. The definition of all heat transfer coefficients and pressure drop correlations can be found in Chapter 6, while cost correlations are included in Chapter 7, digging into depth in the basis behind all calculations, and leading to the definition of the techno-economic objective function. Chapter 8 introduces the optimization process, including the methodology for the selection of degrees of freedom and constraints, the objective functions and the different algorithms that may be used in the simulations.
Chapter 9 presents the starting case study, accompanied by the assumptions and boundary conditions, and the implemented degrees of freedom, constraints, optimization algorithm, etcetera. Chapter 10 addresses the challenges related to the model validation, and includes the obtained results and their discussion, leading to the final conclusions and suggested further work presented in Chapter 11. Finally, some of the most important and complex concepts have been included in a glossary, and a final section of appendixes provides some extra information that might help to understand the theory behind this work and to support the presented results.
4 1.6 Limitations
The main limitation of this work was the impossibility of carrying out the model validation. In order to compute the cycle and to reach the desired results, many different cost, heat transfer coefficient and pressure drop correlations needed to be implemented in the model. This implied having to resort to a great amount of different literature sources and to combine correlations from different authors (as none of them covers all the aspects we did at the same time). For this reason, when trying to execute the validation process, no model including the same correlations as the ones we had chosen could be found.
Furthermore, authors do not provide all information nor data related to assumptions, design of heat exchangers nor boundary conditions for their developed models, making it not possible to compute the same cycle conditions and hence, to compare the obtained results. The validation challenges will be further treated in Section 10.1.
Although studying transcritical Rankine cycles would have been of interest, the field is still under research, and no accurate heat transfer coefficient correlations nor pressure drop formulas that could be applied to compute this kind of cycles could be found in open literature. The omission of transcritical cycles is then the second limitation of this work, and it will be further addressed in Section 3.3.2.
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