The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Solar Engineering Of Thermal Processes

Hybrid energy systems are being utilized for supplying electrical energy in urban, rural and remote areas to overcome the intermittence of solar and wind resources. A hybrid renewable energy system incorporates two or more electricity generation options based on renewable energy or fossil fuel unit. The techno-economic analysis of the hybrid system is essential for the efficient utilization of renewable energy resources. Due to multiple generation systems, hybrid system analysis, is quite complex and requires to be analyzed thoroughly. This requires software tools for the design, analysis, optimization, and economic viability of the systems. In this paper, 19 softwares with their main features and current status are presented. The softwares studied are HOMER, Hybrid2, RETScreen, iHOGA, INSEL, TRNSYS, iGRHYSO, HYBRIDS, RAPSIM, SOMES, SOLSTOR, HySim, HybSim, IPSYS, HySys, Dymola/Modelica, ARES, SOLSIM, and HYBRID DESIGNER. The research work related to hybrid systems carried out using these softwares at different locations worldwide is also reviewed. The main objective of the paper is to provide the current status of these softwares to provide basic insight for a researcher to identify and utilize suitable tool for research and development studies of hybrid systems. The capabilities of different softwares are also highlighted. The limitations, availability and areas of further research have also been identified.

Review of software tools for hybrid renewable energy systems

Public awareness of the need to reduce global warming and the significant increase in the prices of conventional energy sources have encouraged many countries to provide new energy policies that promote the renewable energy applications. Such renewable energy sources like wind, solar, hydro based energies, etc. are environment friendly and have potential to be more widely used. Combining these renewable energy sources with back-up units to form a hybrid system can provide a more economic, environment friendly and reliable supply of electricity in all load demand conditions compared to single-use of such systems. One of the most important issues in this type of hybrid system is to optimally size the hybrid system components as sufficient enough to meet all load requirements with possible minimum investment and operating costs. There are many studies about the optimization and sizing of hybrid renewable energy systems since the recent popular utilization of renewable energy sources. In this concept, this paper provides a detailed analysis of such optimum sizing approaches in the literature that can make significant contributions to wider renewable energy penetration by enhancing the system applicability in terms of economy.

Optimum design of hybrid renewable energy systems: Overview of different approaches

Renewable energy becomes a key contributor to our modern society, but their integration to power grid poses significant technical challenges. Power quality is an important aspect of renewable energy integration. The major power quality concerns are: 1) Voltage and frequency fluctuations, which are caused by noncontrollable variability of renewable energy resources. The intermittent nature of renewable energy resources due to ever-changing weather conditions leads to voltage and frequency fluctuations at the interconnected power grid. 2) Harmonics, which are introduced by power electronic devices utilized in renewable energy generation. When penetration level of renewable energy is high, the influence of harmonics could be significant. In this paper, an extensive literature review is conducted on emerging power quality challenges due to renewable energy integration. This paper consists of two sections: 1) Power quality problem definition. Wind turbines and solar photovoltaic systems and their power quality issues are summarized. 2) Existing approaches to improve power quality. Various methods are reviewed, and the control-technology-based power quality improvement is the major focus of this paper. The future research directions for emerging power quality challenges for renewable energy integration are recommended.

Emerging Power Quality Challenges Due to Integration of Renewable Energy Sources

This study presents the control scheme model of a micro-hydrokinetic turbine system equipped with a permanent magnet synchronous generator (PMSG) A power conversion system model is developed to allow a variable speed hydrokinetic turbine system to generate constant voltage and frequency at variable water speeds A DC-DC boosting chopper is used to maintain constant DC link voltage The DC current is regulated to follow the optimized reference current for maximum power point tracking (MPPT) operation of the turbine system The DC link voltage is controlled to feed the current into the load through the line-side pulse width modulation (PWM) inverter The proposed scheme is modelled and simulated using MATLAB/Simulink The results show a high quality power conversion solution for a variable speed hydrokinetic river system

Modelling and simulation of a power converter for variable speed hydrokinetic systems

Modelling Water Energy Recovery System Using MATLAB/Simulink Software

The current increases in energy demand is in effecting renewable resources of energy an attractive option. The steady progress in renewable energy technologies is treating new opportunities for the utilization of renewable energy resources. Photovoltaic (PV) systems are among one of the most popular renewable energy sources that may be integrated with an electricity grid. With the new feed-in tariff scheme implemented by a few of the municipalities in South Africa, a new opportunity opened for residential consumers with the ability to generate energy. There is a list of regulations and policies that should be executed before the municipality should allow the consumer to connect the grid. However, apart from infrastructure, government policy and other techno-economical aspects, residential application of solar PV systems remain promising. This paper will review the relevant literature on photovoltaic technology, under the time of use (TOU) tariff and feed-in tariff scheme, applicable to residential consumers of energy in South Africa. Key elements such as operation, design, application and previous studies of PV systems used in residential application, will be discussed.

Prospective Implementation of Grid-Interactive Photovoltaic Systems in the South African Residential Sector

In this work, an optimal scheduling model is developed to minimize the operation cost of solar photovoltaic-diesel-battery hybrid systems. Furthermore, two operation modes of the diesel generator, namely the "ON-OFF" and the "CONTINUOUS", are modeled and integrated in the hybrid system operation mode. The principal objective of these developed models is to minimize the hybrid system's daily fuel expenses while optimizing the power flow from the different power sources and from the storage system. The variable character of the load demand, the efficient operation zone of the diesel generator as well as the charging and discharging constraints have been taken into account in the development of the models. These models have been applied to two test examples in South Africa; and the results are analyzed and compared to the case where the diesel generator is the only alternative of providing the load with electricity. Sensitivity analyses have been conducted on the diesel generator's size, and the battery control setting as key parameters. The results show that using the developed models, the daily operation cost can be significantly reduced compared to the case where the diesel is the only alternative of providing the load.

Daily operation cost minimization of photovoltaic-diesel-battery hybrid systems using different control strategies

Pump theory is an interdisciplinary branch in water catchment/purification systems. The main determinant of the selling price of potable water to consumers is the amount of energy utilized when pumping the water from one area to another. These studies propose a water-pump configuration that decreases the energy consumption. The configuration involved an air-compressor that generates air-pressure; the air-pressure is used to supply a pneumatic actuator that converts chemical energy to mechanical energy. The mechanical force that exists at the pneumatic actuator piston is coupled to the piston of a hydraulic actuator to produce the mechanical stroke in the cylinder of the hydraulic actuator. Applying mechanical force at the hydraulic cylinder, the pumping of water from the water-catchment to a watertank/reservoir is achieved. Using their compress-air properties, high force realized at the actuator will allows for an optimal output of the system components. This paper highlights the benefits of fluid dynamics in water-pumping system configuration that range from better energy savings and a robust pumping arrangement.

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Kanzumba Kusakana

Papers

Modelling and simulation of a power converter for variable speed hydrokinetic systems

Modelling Water Energy Recovery System Using MATLAB/Simulink Software

Prospective Implementation of Grid-Interactive Photovoltaic Systems in the South African Residential Sector

Daily operation cost minimization of photovoltaic-diesel-battery hybrid systems using different control strategies

Energy savings configuration for a water-pumping system