• Energy Research

The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface versus bulk properties. In this work, we show that the effect of chromium on photocatalytic performance of TiO2 depends on its elemental content and the related defect disorder that is determined by oxygen activity in the oxide lattice. At high oxygen activity, chromium leads to enhanced photocatalytic performance only for dilute solid solutions (up to 0.04–0.043 at.% Cr). Higher chromium content results in a decrease of photocatalytic activity below that for pure TiO2, despite the observed substantial decrease of the band gap. The photocatalytic performance of Cr-doped TiO2 annealed in reducing conditions is low within the entire studied range of compositions. The obtained results led to derivation of a theoretical model representing the mechanism of the light-induced reactivity of TiO2 with water and the related charge transfer. The photocatalytic performance is considered in terms of a competitive effect of several key performance-related properties. The performance is predominantly influenced by the concentration of titanium vacancies acting as reactive surface sites related to anodic charge transfer.

Abstract The recent climate change agreement in Paris highlights the imperative to aggressively decarbonize the energy economy and develop new technologies, especially for the generation of electrical energy that are environmentally clean. This challenge can only be addressed by a multi-pronged approach to research and education of the next generation of scientists and engineers as well as informed public discourse. Consequently this requires the introduction of new and comprehensive education programs on sustainable energy technologies for universities and, possibly, high schools. Among others, the new programs should provide in-depth knowledge in the development of new materials for more efficient energy conversion systems and devices. The enhanced level of education is also needed for properly assessing the competing technologies in terms of their economic and social benefits. The increasing recognition of the significance of clean and efficient energy conversion indicates the need for a comprehensive education program to be developed. The purpose of the present work is to consider the structure of both an education program and the related textbook where the energy-related fundamental and applied subjects are presented in a concentrated and uniform manner. Such a textbook could be an education aid for students of energy-related courses as well as the teachers involved in the formulation of the education programs. The textbook, which should be dedicated mainly for students at the undergraduate levels at universities, and possibly high schools, should include in-depth interdisciplinary sections dedicated to energy experts and graduate students. This paper considers the present international efforts in reducing the impact of climate change and the need to develop new technologies for clean energy generation. It is argued that progress in this area requires recognition of hydrogen as the main energy carrier of the future. This work also delineates the goals of the Sustainable Energy Network, SEN, involved in the UN program of Future Earth.

The current study presents a concept of a cogeneration system integrated with solar energy and solid oxide fuel cell technology to supply electrical and thermal energy in Malaysia. To appraise the performance, the system is analysed with two case studies considering three modes of operation. For the case-1, typical per day average electricity and hot water demand for a single family have been considered to be 10.3 kWh and 235 l, respectively. For the case 2, electricity and hot water demand are considered for the 100 family members. Energy cost, payback period, future economic feasibility and the environmental impact of the system are analysed for both cases using an analytical approach. The overall system along with individual component efficiency has been evaluated, and the maximum efficiency of the overall system is found to be 48.64 % at the fuel cell operation mode. In the present study, the proposed system shows 42.4 % cost effectiveness at higher load. Energy costs for case-1 and case-2 have been found to be approximately $0.158 and $0.091 kWh−1, respectively, at present. Energy costs are expected to be $0.112 and $0.045 kWh−1 for the case-1 and case-2, respectively, considering future (i.e. for the year 2020) component cost.

Abstract Global environmental concerns, increasing energy demands and developments in renewable energy technologies present a new possibility to implement renewable energy sources. Solar energy is the most prominent among renewable sources, as it is an inexhaustible resource and its exploitation has thus far been ecologically friendly. The potential amount of solar energy is considerably greater than current worldwide energy demands. Solar energy has been developing more rapidly than the other renewable energy sources for the last few decades. The best way to harvest the sun’s power is photovoltaic (PV) technology. This paper presents a study on solar energy in the form of a stand-alone and hybrid power generation system used to electrify off-grid locations. The stand-alone solar-PV system developed here is intended to be used to power a single house or a small community and it also functions as a mini-grid, generating power in places where adequate solar radiation is available throughout the year. However, many places throughout the world experience unsteady amounts of solar radiation and in those places, a hybrid solar-PV system is the most efficient solution for electrification. The main benefit of the hybrid system is that the weakness of one source is covered by the other source. This paper also presents some comparative case studies, project examples and demonstrations of stand-alone solar and hybrid solar systems implemented at various locations throughout the world over the last twelve years.

A tubular solid oxide fuel cell is designed to evaluate its current/voltage characteristics for validating an isothermal model. The model is divided into six subsystems. It can simulate performance based on mass/momentum transfer, diffusion through porous media, electrochemical reactions, polarization losses and heat generation inside the subsystems. The significance of this investigation involves the conversion of a macro-tubular solid oxide fuel cell into six connected micro-reactors in series. The model can successfully predict the dependence of current density on cell potential (observed experimentally). Thermal energy generation by means of fuel reactions as well as voltage irreversibility losses are simulated to account for efficiency losses using the experimental data. Increases in the efficiencies of electrical and thermal power generation by 50.11% and 47.54% are observed when the operating temperature rises from 923 to 1023 K. In addition, the effect of flow pressures and flow rates on solid oxide fuel cell performance is simulated and validated with the experimental results.

The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface versus bulk properties. This work considers the effect of oxygen activity on segregation-induced surface versus bulk composition for both polycrystalline and single-crystal specimens of Cr-doped TiO2. It has been documented that annealing of Cr-doped TiO2 at 1273 K in oxidising conditions results in an enrichment and depletion of the surface layer with chromium. It is shown that the segregation-induced enrichment factor for single crystal is substantially larger than that for polycrystalline specimen. The effect is considered in terms of a theoretical model showing that surface segregation of solute in polycrystalline specimen is encumbered by its segregation to grain boundaries. It is also shown that the segregation-induced enrichment is profoundly influenced by oxygen activity. The new insight of this work involves (i) the determination of well-defined chromium segregation in Cr-doped TiO2, including single-crystal and polycrystalline specimens, after annealing in the gas phase of controlled oxygen activity, and (ii) identification of the predominant driving force of segregation of chromium in Cr-doped TiO2 that is based on electrostatic interactions between the low-dimensional surface structure (LDSS) and electrically charge segregating species.

Abstract Having negative impacts on environment and the scarcity of resources of conventional fossil fuels, fuel cell technology draws more attention as an alternative for providing the electrical energy in parallel with thermal energy. In this study, a single tubular solid oxide fuel cell (SOFC) with an electrolyte of Yttria-Stabilized Zirconia 8 mol% ceramic powder was experimentally investigated. The investigation illustrated the effects of three different fuel flow-rates (175 ml/min, 250 ml/min and 325 ml/min) and two operating temperatures (650 °C and 750 °C) on the output electrical and thermal powers. The highest electrical voltage (open circuit) and overall output power of the cell were found to be 1.1 V and 5.30 W respectively for the fuel flow-rate of 250 ml/min at the operating temperature of 750 °C. The electrical power and efficiency were increased about 18.80% and 1.27% respectively for the increase of operating temperature from 650 °C to 750 °C for a constant fuel flow-rate of 250 ml/min, where; thermal power and efficiency were increased about 33.33% and 10.51% respectively for the same condition. The overall efficiencies of the fuel cell were obtained about 80.42%, 77.49% and 60.73% for the fuel flow-rates of 175 ml/min, 250 ml/min and 325 ml/min respectively for the operating temperatures of 650 °C. On the other hand, the overall efficiency of the cell was found to be 83.38% at the operating temperature of 750 °C and fuel flow-rate of 250 ml/min. The investigation recommends that for achieving higher efficiency, fuel flow-rate should be lower and operating temperature should be higher.

Abstract Due to the increasing future energy demands and global warming, the renewable alternative energy sources and the efficient power systems have been getting importance over the last few decades. Among the renewable energy technologies, the solar energy coupling with fuel cell technology will be the promising possibilities for the future green energy solutions. Fuel cell cogeneration is an auspicious technology that can potentially reduce the energy consumption and environmental impact associated with serving building electrical and thermal demands. In this study, performance assessment of a co-generation system is presented to deliver electrical and thermal energy using the solar energy and the reversible solid oxide fuel cell. A mathematical model of the co-generation system is developed. To illustrate the performance, the system is considered in three operation modes: a solar-solid oxide fuel cell (SOFC) mode, which is low solar radiation time when the solar photovoltaic (PV) and SOFC are used for electric and heat load supply; a solar-solid oxide steam electrolyzer (SOSE) mode, which is high solar radiation time when PV is used for power supply to the electrical load and to the steam electrolyzer to generate hydrogen (H 2 ); and a SOFC mode, which is the power and heat generation mode of reversible SOFC using the storage H 2 at night time. Also the effects of solar radiation on the system performances and the effects of temperature on RSOFC are analyzed. In this study, 100 kW electric loads are considered and analyzed for the power and heat generation in those three modes to evaluate the performances of the system. This study is also revealed the combined heat and power (CHP) efficiency of the system. The overall system efficiency achieved for the solar-SOFC mode is 23%, for the solar-SOSE mode is 20% and for the SOFC mode is 83.6%. Besides, the only electricity generation efficiency for the solar-SOFC mode is 15%, for the solar-SOSE mode is 14% and for the SOFC mode is 44.28%. An economic analysis is presented based on the annual electricity generation from the system and the system has shown the good economic viability in this study with a unit cost of energy (COE) about 0.068 $/kW h.

Energy is the lifeblood of the modern world. Because of the negative environmental impacts and limited sources of fossil fuels, researchers are increasingly focusing on renewable energy sources, particularly solar energy due to its cleanliness and natural availability. Along with photovoltaic systems, solar thermal energy has been used over the last few decades to meet the refrigeration needs for both domestic and industrial purposes. This study presents a review of different solar thermal refrigeration systems, with a specific focus on solar absorption refrigeration systems and solar adsorption refrigeration systems within various working fluids. The different working pairs are illustrated by considering their coefficients of performance, specific cooling power, cooling capacity and minimum and maximum working temperatures. By considering cooling efficiency and other problems, such as swelling and agglomeration, the study also explores solar thermal hybrid cooling systems with heterogeneous composite pairs.