Energy from different sources, non-renewable and renewable, in various forms, including electricity, heat and motion/position is used to deliver goods and services in modern society. Carbon-based non-renewable oil, gas and coal are the dominant global energy sources [1]. As these extractable resources are depleted, and the detrimental environmental consequences from burning fossil fuels accelerate, a global search for un-depletable energy sources is taking place. There is a tremendous interest in primary, inexhaustible energy resources derived from biomass, hydropower, geothermal, solar, wind, marine, wave and/or tidal energy. Nuclear fission energy is considered nearly inexhaustible and capable of replacing fossil fuels [2]. The coupling of renewable and electrification is an enticing path to simultaneously combat deleterious\anthropogenic greenhouse gas (GHG) emissions on climate change and particulate emissions on urban air quality. Renewable energy sources and advanced electricity storage/conversion systems are gaining popularity, as evidenced by significant investments and policies that favor renewable and clean energy technologies.
Some of the challenges associated with metal-based redox electrolytes may be alleviated by using metal-free redox electrolytes in RFBs. Expensive metal-based electrolytes can be replaced by cheaper metal-free electrolytes, less corrosive solvents, and abundant redox-active materials to realize a sustainable energy storage route [82]. The solubility of metal-free redox materials can be improved by attaching metal ligands, whereas the functional groups present in the aromatic ring of the organic molecules tune the redox potential [83, 84]. There are numerous advantages to metal-free electrolytes, such as increased solubility, higher electron transfer numbers and increased stability through molecular engineering strategies [85]. In one study, a materials engineering approach applied to organic molecules led to the synthesis of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)/phenazine combi-molecule for symmetric aqueous RFBs with a redox potential of 1.2 V [86]. Attaching hydroxyl, sulfonate or carboxylate groups to phenazine derivatives offered a clear separation of electron-dense and electron-deficient regions. This clear separation increased the solubility and widened the window of redox potential by more than 0.4 V [87]. The redox potentials and solubility of organic redox molecules can be engineered by attaching positively charged and negatively charged functional groups to the aromatic rings [88]. The position of side groups determined the redox potential and solubility in organic redox species [89]. The negatively charged redox materials offer reduced crossover due to the repulsion from the negatively charged ion-conducting membrane.
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The EES device for solid, liquid, gaseous and hybrid reactant types has evolved, but the anode, the cathode and the separator/membrane continue to form the fundamental components. The challenges associated with the solid form of electrodes are higher interfacial resistance, diffusion-controlled mass and charge transport and changes to the ideal electrode morphology over the prolonged/cyclic operation of the device. Liquid electrolytes have a lower capacity to store energy when compared to solid counterparts while providing flexibility for energy storage and safe handling [118]. Faster charge transfer at the surface and the bulk is possible in liquid and gaseous reactants. Gaseous reactants are considered candidates with low volumetric energy density. The following subsections describe the commonly used battery designs for single-phase reactants in conventional battery design and mixed-phase reactants in a hybrid design. Figure 4 provides examples of the best-performing candidate batteries gathered from recent literature.
The available advanced membrane-free designs are inferior to conventional EES devices in terms of performance. This is evident in the electrochemical performance, as presented in Table 2, where the alphabets correspond to the design shown in Fig. 6. These designs have a combination of diffusion-controlled and surface-area-controlled conversion and storage mechanisms. This approach is advantageous for practical application by avoiding membrane-related issues and mixed-potential losses. Porous electrodes can be employed for improved mass transport and reduced ohmic losses (for high concentration reactants). The membrane-less unconventional EES technology necessitates the reaction-selective electrodes development to improve electrochemical efficiency.
A single basin solar still made up of Copper sheet was designed for the basin size of 900 300 50 mm and 2 mm thick as shown in Figure 2. The Copper has higher thermal conductivity and the rate of heat transfer to water in the still is more. The basin is enclosed in a wooden box of inner cross section 1050 350 430 mm and thickness 15 mm. Plywood is used as outer cover to keep the still basin inside. The gap of 25 mm between the sides of the tray and the wooden box is filled with saw dust to prevent heat loss. Copper sheet was made in to a rectangular tray by sheet metal work of bending and cutting. The effective area of the solar still for saline water is 0.27 m2. The height of the solar still was taken 15 cm for lower end of glass. The glass of size 1175 320 mm was used as the roof for the still. The top of the basin is covered with a 5 mm thick transparent glass of mm. The cover is sealed tightly using silicon sealant to reduce the vapor leakage and the basin becomes air tight. The still is positioned in such a way that the sloping sides always face the sun. The whole experimental setup was kept in the North-South direction, with the inclination of 9. The solar stills were properly oriented, and directly exposed to the solar radiation. The measured parameters and quantities were: the solar radiation intensity (I), the glass temperature (Tg), the basin water temperature (Tw), and the ambient air temperature (Ta).
Figure 6 shows the comparison of convective heat transfer coefficient in summer and winter. The convective heat transfer coefficient is not varied drastically and it is dependent on wind velocity. The maximum convective heat transfer coefficients of 2.91 W/m2 K and 2.41 W/m2 K for the PSS-Cu, B were obtained for the summer and winter respectively. The convective heat transfer coefficient is PSS-Cu, B is lower due to the vacuum present inside the solar still. The temperature difference between (ΔT) evaporative surface (water) and condensing surface (glass inside surface), which plays significant role in raising the yield and in evaporative heat transfer coefficient as well.
Figure 7 shows the comparison of evaporative heat transfer coefficient in summer and winter. The evaporative heat transfer coefficient increases with time and reaches a maximum at 14:00 h during the summer and 15:00 h during the winter prior to decreasing with time. The maximum value of the evaporative heat transfer coefficient measured during the summer and the winter was 75.13 W/m2 K and 42.52 W/m2 K respectively. The evaporative heat transfer coefficient is much higher than the convective and radiative heat transfer coefficients. The evaporative heat transfer coefficient is temperature dependent and is higher in the summer due to higher water temperature in the basin and lower in the winter. This is due to the higher thermal conductivity of copper sheet.
Figure 8 shows the comparison of radiative heat transfer coefficient in summer and winter. The radiative heat transfer coefficient did not vary much compared to the evaporative heat transfer coefficient and it was independent of temperature under normal operating conditions (
Figure 9 shows the comparison of total heat transfer coefficient insummer and winter. The maximum total heat transfer coefficients were 110.62 W/m2 K and 65.43 W/m2 K in the summer and winter respectively, which can be attributed to the higher operating water temperature in the basin. The total heat transfer coefficient was much lower. The maximum values of total heat transfer coefficient 85.01 W/m2K and 52.10 W/m2K in the summer and winter respectively.
Figure 10 shows the hourly variation of the convective, radiative, evaporative and total heat transfer coefficients. It also shows that the evaporative and total heat transfer coefficients were much higher than the convective and radiative heat transfer coefficients. The evaporative heat transfer coefficient increased with time until 14:00 h and then its value decreased with time. The maximum values of the total and evaporative heat transfer coefficients were 58.6 W/m2K and 50.4 W/m2 K respectively. The higher value of the total and evaporative heat transfer coefficients are due to the increase in the basin water temperature. The convective heat transfer coefficient did not vary much and mainly depended on the wind velocity. 2ff7e9595c
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