<abstract> Planet Earth is simultaneously approaching a number of ecological and resource limits. The resulting uncertainties will heavily impact future energy choices, both the level of primary energy used globally and the shares of fossil, renewable and nuclear fuels in the energy mix. This paper reviews the possible futures for the various types of renewable energy. To be viable, all potential energy sources must be assessed on their energy return on energy invested (energy return). Given that renewable energy RE growth is considered important for sustainability reasons, renewable energy must be assessed on its ecologically sustainable or 'green' energy return, which includes the energy costs of ecosystem maintenance as input energy costs. The green energy return is accordingly much lower than the conventional value, so that ecologically sustainable renewable energy is unlikely to deliver anything near existing global energy use. The paper further argues that such constraints on renewable energy growth rates mean it cannot be a timely response to global climate change. The paper concludes that energy reductions will be essential, mainly in high energy use countries. </abstract>

Electroluminescent (EL) and negative electroluminescent (NEL) devices are radiative thermoelectric energy converters that use electric power for refrigeration. For the EL system, we apply a forward bias to the emitter that we want to cool, whereas a reverse bias voltage is applied to the hot absorber for the NEL system. In this work, we derive the thermodynamic limits of the cooling power density and coefficient of performance (COP) of near-field EL and NEL refrigeration systems based on entropy analysis that considers near-field effects. We show numerically that operating the EL and NEL systems in the near-field regime could increase the cooling power density and the COP bounds to a certain extent. As the vacuum gap decrease from 1000 to 10 nm, the near-field effects improve the performance of the NEL system all the time, but the performance of the EL system increases to the optimal value and then decreases. In addition, the increase in temperature difference weakens the performance of both refrigeration systems greatly. Moreover, we also investigate the effects of the absence of sub-bandgap thermal radiation on the performance of the EL and NEL systems. Our work indicates significant opportunities for evaluating the performance of near-field radiative thermoelectric energy converters from the perspective of thermodynamic limits. Meanwhile, these results establish the targets for cooling power density and COP of the near-field EL and NEL systems.

&lt;abstract&gt; &lt;p&gt;The paper deals with small wind turbines for grid-independent or small smart grid wind turbine systems. Not all small turbine manufacturers worldwide have access to the engineering capacity for designing an efficient turbine. The objective of this work is to provide an easy-to-handle integrated design and performance prediction method for wind turbines and to show exemplary applications.&lt;/p&gt; &lt;p&gt;The underlying model for the design and performance prediction method is based on an advanced version of the well-established blade-element-momentum theory, encoded in MATLAB™. Results are (i) the full geometry of the aerodynamically profiled and twisted blades which are designed to yield maximum power output at a given wind speed and (ii) the non-dimensional performance characteristics of the turbine in terms of power, torque and thrust coefficient as a function of tip speed ratio. The non-dimensional performance characteristics are the basis for the dimensional characteristics and the synthesis of the rotor to the electric generator with its load.&lt;/p&gt; &lt;p&gt;Two parametric studies illustrate typical outcomes of the design and performance prediction method: A variation of the design tip speed ratio and a variation of the number of blades. The predicted impact of those parameters on the non-dimensional performance characteristics agrees well with common knowledge and experience.&lt;/p&gt; &lt;p&gt;Eventually, an interplay of various designed turbine rotors and the given drive train/battery charger is simulated. Criterions for selection of the rotor are the annual energy output, the rotor speed at design wind speed as well as high winds, and the axial thrust exerted on the rotor by the wind. The complete rotor/drive train//battery charger assembly is tested successfully in the University of Siegen wind tunnel.&lt;/p&gt; &lt;/abstract&gt;

Ethiopia is endowed with abundant renewable energy resources, which can meet the ambitions of nationwide electrification. However, in spite of all its available potentials the country energy sector is still in its infancy stage. The majority of Ethiopia population lives in the rural area without access to modern energy and relied solely on traditional biomass energy sources. Nowadays Ethiopia has one of the lowest electricity consumption per capita in Africa. Recognizing that energy access and security are a crucial factor to economic growth; Ethiopia needs to cope with key challenges related to energy security and diversification of energy supply. This paper provides a comprehensive and extensive review of renewable energy potentials in Ethiopia. Further, current state of renewable energy resources is described and existing energy policies are articulated. Various policies, that could possibly promote energy technology use in a rural Ethiopia, are proposed. Finally, this paper helps the researchers as well as the government officials to find the better renewable energy technology to meet rural community energy demand; by raising awareness of the country energy potentials and current state of renewable energy, along with proposing pragmatic recommendations.

An experimental investigation of the performance of a fresh air handling unit integrating indirect evaporative and vapor compression cooling is conducted. Temperature and relative humidity measurements at main points within the cooling unit were logged using a wireless data acquisition system. Experimental data downloaded from the acquisition system is used for linear regression analysis, and to calculate the wet-bulb thermal effectiveness, cooling capacity and coefficient of performance of the unit. The air conditioning unit is a patented system designed and assembled at the Gulf Organisation for Research and Development (GORD) in Qatar. The peak wet-bulb thermal effectiveness of the system was found to be 1.3, and the COP was 3.4. The results showed that the unit could save as nearly 60% of the sensible cooling load required by a conventional vapor compression cooling unit. In addition, the unit could reduce power consumption by 36% when utilizing the indirect evaporative cooling cycle. Depending on ambient conditions, the investigated unit generated enough condensate to meet the water requirements of the indirect evaporative cooling cycle, which made the air conditioning system sustainable.

Biodiesel is one potential solution to the challenge of sustainable energy development, especially in the industrial and transportation sectors. Since 2006, Indonesia has issued regulations regarding the use of biofuel as a substitute for fossil fuels. In 2018, the Indonesian government approved regulations regarding the use of biodiesel as a substitute for fossil fuel across all economic sectors. However, there are only a few studies that are focusing on the current status, future potential, barriers and opportunities of biodiesel development in Indonesia. The research reported here reviews the history of governmental policy relating to biodiesel, while also analyzing the current status of this fuel and the opportunities for its exploitation in Indonesia. This study finds that Indonesia has been promoting biodiesel commercialization using national strategy by make government mandate and providing financial incentives. Biodiesel distribution mechanism and related actors in the Indonesian biodiesel supply chain have been described in this study. The study concludes that the recent Indonesian government mandate to mix 20% biodiesel with 80% fossil fuel (B20) produced a positive effect on the economic, social and environment. However, this policy still represents a work-in-progress which can be brought to the optimum conclusion, despite the numerous challenges ahead. One such challenge constituting the focus of this research relates to the respective delivery points of the biodiesel and fossil fuel industry distribution networks remaining problematic. This situation can influence the mode of transportation employed by the storage refinery resulting in variations of either transportation or investment costs. This research provides recommendations regarding current policy and future research relating to the successful implementation of the biodiesel mandate in Indonesia.

In smart grid distribution systems, the applications of Shunt Capacitor Banks (SCBs) and Distributed Generations (DGs) are widely known and installed for system loss reduction, voltage stability improvement of system buses, and to free up the system loading. However, the installation of SCBs and DGs in distribution systems with distorted waveforms will increase the harmonic distortion level if they are not placed at appropriate locations. In this paper, a Multiobjective Grey Wolf Optimizer (MOGWO) is proposed to find the optimal size and locations of SCBs and DGs simultaneously with regard to the impact of ambient temperature on distorted distribution systems. Electric utilities will be able to reduce system active losses, enhance the voltage stability for system buses, release the section loading, and reduce the total harmonic distortion level. Fuzzy set theory is utilized to choose the optimum compromise solution from the Pareto front solutions of the MOGWO method. The proposed method is applied to IEEE 69-bus and real data taken from the Saudi Electricity Company (SEC). The results show a higher capability in finding optimum solutions for SCBs and DGs placement in distorted Radial Distribution Systems (RDSs) compared to conventional methods.

Phasor measurement unit (PMU) is among the most important measurement devices in modern power systems. It measures the voltage and current phasors which have both magnitude and phase angle using a common timing reference. These measurements are utilized in real time applications of electrical power systems. The main drawback of PMUs is its high cost so if PMUs are used to collect or send online real data from a system to a data centre or a decision maker, the used number of PMUs should be minimum with full observability of system measurements. This paper proposes a flower pollination algorithm (FPA) to find the optimal number and locations of PMUs in power systems. The optimization objectives of the work are the minimization of PMUs number, the achievement of complete observability of power system states, and the maximization of measurement redundancy. Existence of zero-injection buses in the system decreases the number of PMUs that is required to fulfil the complete observability of system states. Furthermore, additional constraints for remaining system full observable following failure of single PMU and single line outage are also included to increase the system reliability. The performance and efficiency of the proposed FPA is tested on IEEE standard networks such as 14-bus, 30-bus, 57-bus and 118-bus and new England 39-bus network. The obtained simulation results approve the ability of the proposed optimization method to find the optimal allocation of PMUs in different cases with fulfilling complete observability and reliability of electric power systems. The superiority of FPA is also verified by comparing its results of other optimization algorithms.

Superconducting Fault Current Limiters (SFCL) have been used in power systems for many years, however the best possible positioning of different SFCL still needs a lot of work. Resistive and Inductive are two types of SFCL used in power system. Function of both these current limiters is to control the fluctuations and abnormalities in the system by introducing a high impedance in it, through transforming itself from normal stage to superconducting stage. In this research, placement of resistive SFCL and inductive SFCL in AC and DC microgrids (MG) are analyses at two different positions; at the point of integration of conventional source and distributed generator (DG) and at the point where DG is added to the system. The AC and DC microgrids contains industrial and residential loads, while they use respective main wind power generation source and photo voltaic power source along with conventional power, supplied from main grid. The behavior of both types of SFCL in their grid models is analyzed through MatLab Simulations, in order to compare their effectiveness and determine best possible position to place them.

Resource complementarity carries significant benefit to the power grid due to its smoothing effect on variable renewable resource output. In this paper, we analyse literature data to understand the role of wind-solar complementarity in future energy systems by evaluating its impact on variable renewable energy penetration, corresponding curtailment, energy storage requirement and system reliability. Results show that wind-solar complementarity significantly increases grid penetration compared to stand-alone wind/solar systems without the need of energy storage. However, as capacity increases, the capability of complementarity to increase grid penetration approaches its limit due to the reduced matching of output to the load profile and pursuant increase in excess generation. Thus, achieving very high penetration requires appropriately designed energy storage and curtailment. Yet, even at higher grid penetration, complementarity carries significant multidimensional benefits to the power system. The most important observation was the achievement of very high grid penetration at reduced energy storage and balancing requirements compared to stand-alone systems. Researchers reported that using the same energy storage capacity, wind-solar complementarity led to significantly higher penetration of up to 20% of annual demand compared to stand-alone systems. In addition, by coupling to curtailment as an enabler, and related dispatch flexibility that comes with storage application, lower balancing capacity need was reported at higher penetration. Wind-solar complementarity was also found to reduce ramping need while contributing to improved system adequacy. Complementarity from other dispatchable renewable resources further reduces storage need and curtailment and improve system reliability, whereas power grid integration and relative cost changes allow for further optimisation while transitioning to 100% renewable energy.