Fluctuation in the demand for electrical power and the intermittent nature of the supply of energy from renewable sources like solar and wind have made the need for energy storage a dire necessity. Current storage technologies like batteries and supercapacitors fall short either in terms of power output or in their ability to store sufficient energy. Pseudocapacitors combine features of both and offer an alternative to stabilize the power supply. They possess high rates of charge and discharge and are capable of storing much more energy in comparison to a supercapacitor. In the quest for solutions that are economical and feasible, we have investigated Prussian Blue in aqueous electrolytes for its use as a pseudocapacitor. Two different active materials based on Prussian Blue were prepared; one that has just Prussian Blue and the other that contains a mixture of Prussian Blue and carbon nanotubes (CNTs). Four electrolytes differing in the valence of the cation were employed for the study. Cyclic voltammetry and galvanostatic charge-discharge were used to characterize the electrodes. Our experiments have shown specific capacitances of Prussian Blue electrodes in the range of 140-720 F/g and that of Prussian Blue-CNT electrodes in the range of ~52 F/g. The remarkable capacity of charge storage in Prussian Blue electrodes is attributed to its electrochemical activity ensuring surface redox and its tunnel-like structure allowing ease of entry and exit for ions like Potassium. Simple methods of synthesis have yielded specific capacitances of the order of hundreds of Farads per gram showing that Prussian Blue has promise as an electrode material for applications needing high rates of charge-discharge.

The growing renewable energy consumption has stimulated the rapid development of diverse energy storage systems (ESSs) in our electronic society. As a successful representative, lithium-ion batteries (LIBs) play a vital role in meeting today's energy storage demand. However, LIBs are plagued by intrinsic unsafety and detrimental environmental contamination. In this respect, rechargeable aqueous zinc-ion batteries (ZIBs) and supercapacitors (SCs) as potential alternatives have attracted considerable attention due to their characteristics such as innate safety, environmental friendliness, cost-effectiveness, competitive gravimetric energy density, and loose fabrication process. Inspired by these merits, massive efforts have been devoted to designing and exploring high-performance aqueous Zn-based energy storage devices. The key for advanced Zn-based energy storage devices is to exploit high-performance cathode materials. Covalent organic frameworks (COFs) are an emerging class of organic polymer with periodic skeletons showing attractive properties in structural tunability, well-defined porosity, functional versatility, and high chemical stability. The distinguishing features of COFs make them promising electrode materials for electrochemical energy storage applications. However, the electrochemical storage capability and charge storage mechanism of COF materials have been rarely investigated, and their potential applications have not been evaluated yet so far. In this thesis, COFs are proposed as cathode materials for rechargeable aqueous Zn-ion energy storage. Initially, a new phenanthroline COF (PA-COF) material was synthesized and used as an electrode for Zn-ion supercapatteries (ZISs) for the first time. The as-synthesized PA-COF shows abundant nucleophilic sites and suitable pore structure, demonstrating the efficient storage capability of Zn2+ and H+. Further, hexaazatriphenylene-based COF (HA-COF) material with and without precisely grafted quinone functional groups has been proposed to understand structure-activity relationships. In this chapter, the influence of quinone groups on the electrochemical performance of HA-COF has been systematically studied, disclosing an enhancement coordination capability of Zn ions against protons in the quinone-functionalized HA-COF. Lastly, we synthesized a radical benzobisthiazole COF (BBT-COF) and deeply investigated the electrochemical performance. As expected, this COF electrode shows an ultrastable cycling performance and demonstrates a radical reaction pathway.

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The major challenge associated with renewable-energy systems especially solar, is the supply intermittency. One effective solution is to incorporate thermal energy storage components utilizing phase change materials (PCMs). These materials have the potential to store large amounts of energy in relatively small volumes and within nearly an isothermal storage process. The primary drawback of today’s PCMs is that their low thermal conductivity values critically limit their energy storage applications. Also, this grossly reduces the melting/ solidification rates, thus making the system response time to be too long. So, the application of heat transfer enhancement is very important. To improve the PCM storage performance, an efficient performing containment vessel (triplex-tube) along with applications of various heat transfer enhancement techniques was investigated. The techniques were; (i) dispersion of solid nanoparticles, (ii) incorporation of metal foam with nanoparticle dispersion, and (iii) insertion of longitudinal fins with nanoparticle dispersion. Validated simulation models were developed to examine the effects of implementing these techniques on the PCM phase-change rate during the energy storage and recovery modes. The results are presented with detailed model description, analysis, and conclusions. Results show that the use of nanoparticles with metal foam or fins is more efficient than using nanoparticles alone within the same volume usage. Also, employing metal foam or fins alone results in much better improvement for the same system volume.

To achieve greater economic stability, CHP plant operators such as VEAB from Växjö are motivated to search for a new business model that are compatible with their existing facilities while also contribute to increasing the overall revenue of the company. These processes include hydrogen production and biochemical products such as biopolymer and biofuels. However, these processes also produce a substantial amount of heat that needs to be taken care of. Alternatively, the extra heat storage capacity could allow the plant to be more selective of when to produce those heat to maximize profit. Therefore, it is important to investigate different approaches to achieve that, both traditional approach (e,g, convective cooling) and alternative approaches (different large scale underground heat storages). Lake source cooling is also investigated to determine whether it can replace convective cooling as a method of cooling off waste heat from the plant. The technical analysis showed that the alternative approach is certainly promising albeit with more land use (BTES requiring 36 000 m2 against 750 m2 of convectional cooling system) with some limitations that must be addressed when deciding the appropriate approach. In addition, it is found that by increasing the scale of the BTES system, the amount of heat loss per heat capacity reduces while increasing the borehole depth decreases the overall heat loss of the system. The economic analysis showed that when used solely to deal with the waste heat, the alternative approach is costs magnitude more than convective cooling, the alternative costing almost 6 times more than the convective cooling. There are certainly opportunities in the future that can make the BTES system to be a much more feasible choice if additional utilization of the BTES system could be found or potential demand may make the BTES system a more attractive choice to deal with the excess heat that comes with expanding the business of a CHP plant operator.