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The interest in storing latent energy in phase change materials has risen over the last years as the need grows for more energy efficient systems. By storing energy, free chilling and heat can be saved for later use during high load hours. Thus the gap between supply and load can be overcome. It is an efficient way to provide both cooling and heating to buildings using phase change material (PCM) as they take up much less volume compared to a corresponding water-cistern with the same amount of stored energy. Low thermal conductivity of most of the PCMs can be compensated with advanced heat transfer design, however impact of different heat transfer mechanisms is not explicitly studied. In this work, a heat transfer study has been made on a finned cylindrical PCM heat exchanger with focus on determining the heat transfer effect of convection in non-gelled PCMs and the different ways to model such a system in a two dimensional axis-symmetric plane. The first and simpler numerical model of the two was built using Matlab, where the convection effect was simulated using an enhanced-conduction factor based on empirical equations. The other model was built in a CFD environment and simulates the convection with more complexity and more realistic behavior. The results show that the convection may contribute to 65% of the total heat transfer in non-gelled PCMs at a certain time and that using empirical equations for simulating convection is a fast and easy way to estimate the heat transfer, though not a recommended method for high accuracy results. The study also showed that because of the gravity-induced convection, the angle of the cylindrical finned heat exchanger affects the heat transfer and that more fins, while increasing the overall heat transfer rate, inhibits the effect of convection in a vertically positioned heat exchanger.

The objective of this research is to evaluate the potential of thermal energy storage in supermarkets with CO2 refrigeration systems. Suitable energy storage techniques are investigated and the seasonal storage technology of boreholes is chosen to be the focus of the study. The calculations are done for five supermarket refrigeration systems with different combinations of heating systems and borehole thermal energy storage control strategies. The two heating systems analyzed are the ground source heat pump and the heat recovery from the supermarket’s refrigeration system. The simulation results show that the introduction of thermal energy storage in the scenarios with heat pump can reduce the annual total energy by 6.3%. It is also shown that increasing the number of boreholes can decrease the life cycle cost of the system. Moreover, it is established that a supermarket system with heat recovery consumes 8.1% less energy than the one using heat pump and adding thermal energy storage on the heat recovery system further improves the energy consumption by 3.7% but may become costly.

In deregulated electricity markets, profit driven electricity retailers compete to supply cheap reliable electricity to electricity consumers, and the electricity consumers have free will to switch between the electricity retailers. The need to maximize the profits of the electricity retailers while minimizing the electricity costs of the electricity consumers has therefore seen a drastic increase in the research of electricity markets. One of the factors that affect the profits of the electricity retailers and the energy cost of the consumers in electricity retail markets is the supply and demand. During high-supply and low-demand periods, the excess electricity if not managed, is wasted. During low-supply high-demand periods, the deficit supply can lead to electricity blackouts or costly electricity because of the volatile electricity wholesale spot market prices. Research studies have shown that electricity retailers can achieve significant profits and reduced electricity costs for their electricity consumers by minimizing the excess electricity and deficit electricity. Existing studies developed load forecasting models that aimed to match electricity supply and electricity demand. These models reached excellent accuracy levels, however due to the high volatility character of load demand and the rise of new electricity consumers, load forecasting alone is unable to mitigate excess and deficit electricity. In other studies, researchers proposed charging the electricity consumers’ batteries with excess electricity during high-supply low-demand periods and supplying their deficit electricity during low-supply high-demand periods. Electricity consumers’ incorporating batteries resulted in minimized excess and deficit electricity, in turn, maximizing the profits for the electricity retailers and minimizing the electricity costs for the electricity consumers. However, the batteries are consumer centric and only provide battery energy for the battery-owned consumer. Electricity consumers without battery energy during low-supply highdemand periods have electricity blackouts or require costly electricity from the electricity wholesale spot market. The peer-to-peer (P2P) energy sharing framework which allows electricity consumers to share their energy resources with one another is a viable solution to allow electricity consumers to share their battery energy. P2P energy sharing is a hot topic in research because of its potential to maximize the electricity retailers’ profits and minimize the electricity consumers’ electricity costs. Due to the increased profits for the electricity retailer and reduced electricity costs for the electricity consumers from implementing battery charging and P2P energy sharing, this dissertation proposes a day-ahead electricity retail market structure in which the electricity retailer supplies consumers’ batteries with excess electricity during high-supply low-demand periods, and during low-supply highdemand periods the electricity retailer discharges the consumers’ batteries to supply their deficit supply or supply their peers’ deficit supply. The electricity retailer aims to maximize its profits and minimize the electricity cost of the electricity consumers in its electricity retail market, by minimizing the excess and deficit electricity. The problem is formulated as a non-linear optimization model and solved using game theory. This dissertation compares the profits of the electricity retailer and electricity costs of the consumers that charge their batteries with excess electricity, discharge their batteries and purchase electricity from their peers to supply their deficit supply, with consumers that only charge their batteries with excess electricity but do not share their battery energy with their peers, consumers that only purchase electricity from their peers to supply their deficit supply but do not employ a battery, and consumers that neither employ a battery nor purchase electricity from their peers to supply their deficit supply. The results show that the consumers that charge their batteries with excess electricity, discharge their batteries and purchase electricity from their peers to supply their deficit supply achieved the lowest electricity cost and highest profits for the electricity retailer. / Dissertation (MEng)--University of Pretoria, 2020. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

Philosophiae Doctor - PhD / Energy storage systems represent a viable option to integrate renewable energy sources into the grid network. Multiple energy storage technologies are available such as mechanical, electrical, thermal, and electrochemical storage technologies. Battery Energy Storage Systems are considered as an accepted solution for energy storage with advantages such as, sustained power delivery, geographical independence and, fast response capability. This thesis describes the development of rechargeable bipolar Nickel-Iron batteries as potential candidates for cost effective energy storage solutions. The first objective of this work was to design a bipolar electrode comprising an Iron (Fe)-based anode, a Nickel (Ni)-based cathode and a flexible bipolar plate and to optimise its production process in order to attain high performance in terms of capacity and efficiency. Research questions to be answered included;

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Aqueous zinc ion storage system has been deemed as one of the most promising alternatives due to its high capacity of zinc metal anode, low cost, and high safety characteristics. Recently, significant attempts have been made to produce highperformance aqueous Zn batteries. (AZBs) and great progress has been achieved. Yet there are a lot of issues still exist and need to be further optimized. In this thesis, we proposed several strategies to tackle these challenges and finally optimize the overall battery performance, including metal anode protection, cathode structural engineering, and rational electrolyte design. In the present thesis, we first developed the ZnF2 layer coated Zn metal anode via a simple plasma treatment method. The plasma treated Zn anode leads to dendrite-free Zn electrodeposition with lower overpotential. Density function theory calculation results demonstrate that the Zn diffusion energy barrier can be greatly reduced on the ZnF2 surface. Benefiting from these merits, the symmetric cell and full cell exhibited much improved electrolchemical performance and stability. Afterthen, We synthesised a layered Mg2+-intercalated V2O5 as the cathode material for AZBs. The large interlayer spacing reachs up to 13.4 A, allowing for efficient Zn2+ (de)insertion. As a result, the porous Mg0.34V2O5・nH2O cathodes can provide high capacities as well as long-term durability. We then recongnized that most of the parasitic side reactions are related to the aqueous electrolyte. We therefore further designed a hybrid electrolyte to realize the anode-free Zn metal batteries. It is demonstrated that in the presence of propylene carbonate, triflate anions are involved in the Zn2+ solvation sheath structure. The unique solvation structure results in the reduction of anions, thus forming a hydrophobic solid electrolyte interphase. Consequently, in the hybrid electrolyte, both Zn anodes and cathodes show excellent stability and reversibility. More importantly, we design an anode-free Zn metal battery, which exhibits good cycling stability (80% capacity retention after 275 cycles at 0.5 mA cm–2).

The world is facing a major challenge when it comes to proper energy utilization. The increasing energy demand, the depleting fossil fuel resources and the growing environmental and ecological concerns are factors that drive the need for creative solutions. Renewable energy resources such as solar sit in the center of these solutions. Due to their intermittent nature, development of energy storage systems is crucial. This dissertation focused on the latent thermal energy storage systems that incorporate phase change materials (PCM). The main goal was to enhance the heat transfer rates in these systems to address the low melting (energy storage stage) and solidification (recovery stage) rates that are caused by the PCMs’ low thermal conductivity values. The application of multiple PCMs (m-PCMs) with varying melting temperatures in several arrangements was investigated. The effects of applying m-PCMs on the conduction heat transfer and on the natural convection heat transfer in both horizontally and vertically oriented heat exchangers were studied. This was followed by an optimization study of the PCMs’ melting temperatures and the working fluid flow rate. Further heat transfer enhancement using metal fins was also investigated. Numerical models were developed and validated. Results are reported and discussed. Significant enhancement in both complete melting time and energy storage capacity was obtained by the m-PCMs in series arrangement. This enhancement is more pronounced in the vertically oriented system. The working fluid flow rate was found to have a limited effect during the melting stage. However, it seems to be crucial in the solidification stage.

Energy storage is an important key for future energy systems. A most common form of energy storage is the battery. However, producing a battery is not very efficient nor sustainable. Therefore, every part and every machine in the manufacturing process must be measured and analysed. The next step is to find solutions of how to make each part more effective. The purpose of the thesis was to analyse the power consumption of a battery cycling machine and log the temperature changes. The quality of a battery cell is tested by charging and discharging the cell to different state of charge in this machine. The results showed a lower efficiency during standby state, which is a state when the machine is not used yet is still running. The efficiency increased during charge and discharge of the cells. Moreover, with enough cells discharging at the same time, the machine could produce electricity. This would also mean that the cells charge at the same time and lead to a volatile load profile. The temperature increased slightly during charge and discharge but not above the upper limit. In summary, by scheming the usage of the machines adapted to the number of cells, some machines can be turned off instead of being in standby state. All the machines should be connected to each other in order to exchange excess electricity between them. These solutions can lower the power consumption and make the process more efficient.

With a growing demand for electrical energy storage materials, lignin-derived carbon materials have received increasing attention in recent years. As a highly abundant renewable carbon source, lignin can be converted to a variety of advanced carbon materials with tailorable chemical, structural, mechanical and electrochemical properties through thermochemical conversion (e.g. pyrolysis). However, the non-uniformity in lignin structure, composition, inter-unit linkages and reactivity of diverse lignin sources greatly influence lignin fractionation from plant biomass, the pyrolysis chemistry, and property of the resulting carbon materials. To introduce a better use of lignocellulosic biomass to biofuels and co-products, it is necessary to find novel ways to fractionate lignin and cellulose from the feedstock at high efficacy and low cost. Deep eutectic solvent (DES) was used to extract lignin from high lignin-content walnut and peach endocarps. Over 90% sugar yields were achieved during enzymatic hydrolysis of DES pretreated peach and walnut endocarps while lignins were extracted at high yields and purity. The molecular weights of the extracted lignin from DES pretreated endocarp biomass were significantly reduced. The native endocarp lignins were SGH type lignins with dominant G-unit. DES pretreatment decreased the S and H-unit which led to an increase in condensed G-units, which may contribute to a higher thermal stability of the isolated lignin. Lignin slow pyrolysis was investigated using a commercial pyrolysis–GC/MS system for the first time to link pyrolysis chemistry and carbon material properties. The overall product distributions, including volatiles and solid product were tracked at different heating rates (2, 20, 40 ℃/min) and different temperature regions (100-200, 200-300 and 300-600 ℃). Results demonstrate that changes in reaction chemistry as a factor of pyrolysis conditions led to changes in yield and properties of the resulting carbon materials. Physical and chemical properties of the resulting carbon material, such as porosity, chemical composition and surface functional groups were greatly affected by lignin slow pyrolysis temperature and heating rate. Lignin-derived activated carbons (AC) were synthesized from three different lignin sources: poplar, pine derived alkaline lignin and commercial kraft lignin under identical conditions. The poplar lignin-derived ACs exhibited a larger surface area and total mesopore volume than softwood lignin-derived AC, which contribute to a larger electrochemical capacitance over a range of scan rates. The presence of oxygen-containing functional groups in all lignin-derived ACs, which participated in redox reaction and thus contributed to an additional pseudo-capacitance. By delineating the carbonization and activation parameters, results from this study suggest that lignin structure and composition are important factors determining the pore structure and electrochemical properties of the derived carbon materials. A 3-dimensional, interconnected carbon/silicon nanoparticles composite synthesized from kraft lignin (KL) and silicon nanoparticles (Si NPs) is shown to have a high starting specific capacity of 2932 mAh/g and a retaining capacity of 1760 mAh/g after 100 cycles at 0.72 A/g as negative electrode in a half-cell lithium-ion battery (LIB) test. It was found the elemental Si and C of the C/Si NPs were most likely linked via Si-O-C rather than direct Si-C bond, a feature that helps to alleviate the mechanical degradation from Si volume change and assure a sound electronic and ionic conductivity for enhanced electrochemical performance. EGA-MS and HC-GC/MS analyses suggest that the interaction of the Si, O and C can be tailored by controlling pyrolysis conditions. This study systematically investigated the interconnecting aspects among lignin source, pyrolysis chemistry, characteristics of the derived carbon materials and electrochemical performance. Such knowledge on the processing-structure-function relationships serves as a basis for designing lignin-based carbon materials for electrochemical energy storage applications.