| Author: | Aman Zaharil, Hafiz Bin |
| Title: | Novel design, methods and development of a next-generation solar-driven cogeneration system |
| Advisors: | Yang, Hongxing (BEEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxvii, 231 pages : color illustrations |
| Language: | English |
| Abstract: | Concentrated solar power (CSP) is currently limited in its thermal efficiency and thus economic feasibility relative to other technologies such as solar photovoltaics (PV) and fossil fuels. The current consensus roadmap for CSP's transition to 3rd generation, includes higher temperature working fluids, efficient thermal storage, advanced power cycles and, efficient waste heat utilisation from the system. Currently, the widespread use of thermal oil in CSP systems is constrained by its thermal stability limit of around 400°C, which restricts the overall efficiency of the power cycle. Molten salts offer a higher operational temperature of up to 565°C and improved thermal storage capability, but their high freezing point (~220°C) introduces significant challenges in system design, especially in preventing solidification during operation and downtime. Alternative fluids such as gas, which overcome some of the disadvantages mentioned above, have been theorised, but their low heat capacity and density, and high viscosity limit amplify the challenges. To overcome these limitations, supercritical fluids have emerged as promising candidates for next-generation CSP systems. Among them, supercritical carbon dioxide (sCO₂) is particularly attractive due to its ability to operate at higher temperatures, thereby enhancing thermodynamic efficiency. Compared to gases, sCO₂ has a much higher density and lower viscosity, which enables more compact turbomachinery, leading to lower land footprint and reduced compression work, thus increasing efficiency. Currently, research on the supercritical carbon dioxide (sCO₂) Brayton cycle has been largely focused on nuclear applications, with limited exploration in the solar energy sector. In particular, its integration with solar thermal systems has underutilised the substantial waste heat produced, especially near the fluid's critical point, where the cooler rejects heat due to the high specific heat capacity of sCO₂. Moreover, the promising concept of operating in a direct configuration, without intermediate heat exchangers, remains underexplored, despite its potential advantages such as higher operating temperatures and reduced exergy losses, particularly in its design methodology, where many assumptions were made and detailed steps are absent. To address these gaps, this study proposes an innovative direct integration of a parabolic trough solar collector (PTSC) with an sCO₂ Brayton cycle, an Organic Rankine Cycle (ORC), and a direct contact membrane distillation (DCMD) unit, along with some innovative methodologies, aiming to enhance overall energy utilisation and system efficiency Firstly, a validated mathematical model from each of the proposed components in the system is developed and validated with experimental data from previous articles in the literature and reports from renowned national laboratories such as Argonne National Laboratory. The mathematical model is based on the foundations of heat transfer and thermodynamics, which lay the foundation for comprehensive energetic and exergetic analyses throughout the thesis. Additionally, typical economic models of levelised cost of electricity (LCOE), levelised cost of water (LCOW) and Net Present value (NPV) are developed for detailed economic analyses. Secondly, a foundational thermodynamic analysis was conducted on a direct system integrating a PTSC with a recompression sCO₂ Brayton cycle and DCMD units. The study examined the system's energetic and exergetic performance under various pressure ratios, solar irradiance levels, and thermal boundary conditions. Results showed that both thermal and exergetic efficiencies decreased with lower solar input, with the highest exergetic efficiency of 38.3% observed at an optimal pressure ratio of approximately 3.1–3.2. Simultaneously, water production reached up to 96.6 m³/h under high feed temperatures and low permeate temperatures. The analysis also revealed that higher feed temperatures enhanced water output but increased sensitivity to solar fluctuations. These findings contribute to the design of more efficient solar thermal systems capable of simultaneously generating power and producing clean water. Thirdly, this research presents a comprehensive thermodynamic and techno-economic analysis of a novel third-generation CSP system integrating a direct PTSC, sCO₂ Brayton cycle, DCMD, and a bottoming ORC. The primary goal was to understand the performance of this integrated system across key operational parameters, and to analyse and optimise ORC working fluids used in the bottoming cycle. Under baseline conditions, neopentane and isobutane demonstrated slightly higher net power outputs, whereas toluene provided superior water production due to its lower condenser inlet temperature. At an ambient temperature of 10°C, cyclohexane, n-octane, n-nonane, and n-heptane achieved the highest thermal efficiencies, reaching up to 36.52%. Exergy analysis identified significant losses in the DCMD unit, especially at the membrane, and in the second-stage compression and expansion processes of the sCO₂ cycle at high DNI. Nevertheless, the integrated configuration showed improvements in exergetic efficiency of 0.12–0.35% compared to single-cycle systems. Techno-economic evaluation further highlighted n-octane as having the highest net present value under colder conditions, while cyclohexane consistently performed well across different ambient temperatures. Additionally, this research establishes a discretisation-based modelling foundation enabling detailed design of PTSC and power cycles in direct configuration and the introduction of a ‘duality method’, which comprises a comprehensive methodological steps and utilisation of both pinch point and approach temperature to achieve maximal waste heat utilisation. Fourthly, this research introduces a discretised 1-D modelling approach for CSP systems, providing a detailed methodology previously lacking in literature, specifically applied to a direct-configuration PTSC integrated with sCO₂ Brayton cycle. By employing this discretisation technique, the system’s performance was explored across essential parameters such as inlet and outlet temperatures, mass flow rates, collector length, and solar irradiance. This method revealed significantly different optimal conditions compared to prior approaches, offering more precise design and operational insights. The results demonstrated distinct optimal inlet conditions for various targeted outlet temperatures and lengths, highlighting novel results missed by traditional optimisation in the literature. Thus, the discretised method serves as a foundational advancement, enabling detailed system designs and superior optimisation of future configuration CSP systems. Lastly, this study presents a comprehensive thermo-economic mapping of the integrated PTSC-sCO₂-ORC-DCMD system, incorporating projected future cost reductions of sCO₂ Brayton cycle technology, which is anticipated to become commercially viable by 2030. For the first time, a detailed evaluation was conducted using component-level thermodynamic and economic correlations across varying power outputs, turbine inlet temperatures, and the effectiveness of the high-temperature (HTR) and low-temperature recuperators (LTR), parameters widely recognised as the most influential in power generation systems. Additionally, a duality method that considers both pinch point and approach temperature of a bottoming ORC is utilised, and the results are compared to a typical energy balance method, which is substantial. A 25-year life cycle assessment was performed using levelised cost of electricity (LCOE) and levelised cost of water (LCOW) as evaluation metrics. Results revealed that while larger power outputs significantly reduce LCOE, higher turbine inlet temperatures do not always translate to lower LCOE despite improving thermal performance. The LCOE of the system ranged from approximately 0.115 USD/kWh to 0.025 USD/kWh, and LCOW from 0.70 USD/m³ to 0.40 USD/m³, demonstrating the potential of this system to meet the targeted 0.05 USD/kWh for third-generation CSP and compete with the current cost range of reverse osmosis desalination (0.35–1.00 USD/m³). This thesis introduces a novel integrated system comprising four distinct components that collectively utilise waste heat from an sCO₂ Brayton cycle with high efficiency. Furthermore, it proposes, for the first time in the literature, a direct integration methodology supported by a discretised PTSC 1-D model, which leads to significantly novel results compared to previous studies. In addition, a duality approach, combining pinch point and approach temperature analyses, is developed to improve heat recovery between cycles more effectively than current methods reported in the literature. Finally, the thesis presents a comprehensive and detailed thermo-economic analysis and framework, incorporating component-level thermodynamic and economic correlations across varying power outputs, turbine inlet temperatures, and the effectiveness of HTR and LTR, which, to the best of current knowledge, is novel in the literature. |
| Rights: | All rights reserved |
| Access: | open access |
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