| Author: | Cheng, Chun |
| Title: | Electrochemical systems to recover low-grade waste heat for electrical energy generation |
| Advisors: | Ni, Meng (BRE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Energy harvesting Energy conversion Waste heat Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building and Real Estate |
| Pages: | xx, 166 pages : color illustrations |
| Language: | English |
| Abstract: | A considerable amount of waste heat from industrial facilities and solar radiation is discarded into the environment without being efficiently utilized every year. A large proportion of this waste heat is low-grade waste heat, commonly defined as heat with temperatures below 150°C. Therefore, numerous approaches to generate electricity from low-grade waste heat have been emerging to better take advantage of low-grade waste heat over the past few decades, most of which are liquid-based electrochemical approaches. Thermally regenerative electrochemical cycles (TREC) are one of the promising liquid-based electrochemical systems for converting low-grade heat to electrical power. In this thesis, the TREC with nickel hexacyanoferrate (NiHCF) cathode and Zn anode achieves a markedly high thermopower (α) of -1.575 mV K-1 and a heat-to-electricity efficiency of 2.41% at the temperature difference of 30℃ (equivalent to 25.15% of Carnot efficiency), surpassing all the existing TREC systems. For the first time, the mixed membranes with mixed pH electrolytes are introduced into the TREC systems to boost α to a record-high value of -2.270 mV K-1. The proposed thermodynamic framework advances the understanding of the origin of α and electrochemical potential, which will guide people to engineer TRECs. By looking more deeply into the optimization in both thermopower and device design, the resultant efficiency is excellent but the power density of TREC is still unsatisfactory. Additionally with the discontinuous operation through a TREC, practical application of TREC is impeded. To address these challenges, efforts should be made to develop novel approaches possessing high power density, reliable and flexible assembly, and long-standing discharging times. Considering the current primary energy generation systems also produce a significant amount of waste CO2 coupled with low-grade heat, this study also proposes a thermally regenerative CO2-induced pH-gradient cell (TRCPC) that simultaneously utilizes CO2 and low-grade heat for waste to electricity conversion. CO2 is absorbed in one side of the symmetric electrolyte and causes a change in the pH of the cell to induce voltage generation, achieving a peak power density of 0.578 W m-2 (~ 10 times that of TREC). After discharging, the system can be regenerated using low-grade heat while the CO2 can then be stored and transported. This research proposes a promising way for economic and environmental benefits to exploit CO2 and waste heat into electricity before further CO2 storage. However, the power density and discharging time of TRCPC are limited by the sluggish kinetics of electrodes and unoptimized cell design. By replacing the electrode with an H2/H+ catalytic electrode and rationally improving the cell design, we further investigate an advanced pH-sensitive thermally regenerative cell (pH-TRC) with circulating hydrogen to achieve both long discharging time and high-power output. Between the H2/H+ catalytic electrodes, we have flowing anolyte and catholyte with various pH values, which can be neutralized through discharging reactions and then thermally regenerated to reset the initial state. The underlying mechanism of pH-TRC, kinetics, open circuit voltage (OCV) generation, cell design, and parameter study are scrutinized both computationally and experimentally verified. With this new design and the intrinsically faster kinetics of H2/H+ catalytic electrodes, a favorable peak power density of 5.296 W m-2 (approximately 10 times TRCPC) is obtained. More importantly, an incredibly long discharging time of over 40 hours enables the powering of a smartphone in comparison to only hundreds-of-seconds discharging time of previous TRCPC. This work presents a comprehensive study of new methods for low-grade waste heat harvesting, particularly in practical industrial applications. In our first attempt to generate electricity in a TREC, the intrinsic energy conversion mechanism including the thermodynamics and kinetics processes are discussed in detail. In approaching the emerging issues of TREC, TRCPC and pH-TRC are subsequently proposed, where the design strategy and underlying mechanism are fully studied in this thesis, which helps to establish effective follow-up industrial implementations aimed at thermal energy conversion. |
| Rights: | All rights reserved |
| Access: | open access |
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