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Parisio A.,Linnaeus Center | Rikos E.,Center for Renewable Energy Sources, Greece | Glielmo L.,Universitadegli Studi Del Sannio
IEEE Transactions on Control Systems Technology | Year: 2014

Microgrids are subsystems of the distribution grid, which comprises generation capacities, storage devices, and controllable loads, operating as a single controllable system either connected or isolated from the utility grid. In this paper, we present a study on applying a model predictive control approach to the problem of efficiently optimizing microgrid operations while satisfying a time-varying request and operation constraints. The overall problem is formulated using mixed-integer linear programming (MILP), which can be solved in an efficient way by using commercial solvers without resorting to complex heuristics or decompositions techniques. Then, the MILP formulation leads to significant improvements in solution quality and computational burden. A case study of a microgrid is employed to assess the performance of the online optimization-based control strategy and the simulation results are discussed. The method is applied to an experimental microgrid located in Athens, Greece. The experimental results show the feasibility and the effectiveness of the proposed approach. © 1993-2012 IEEE.

Kontogianni A.,University of Western Macedonia | Tourkolias C.,Center for Renewable Energy Sources, Greece | Skourtos M.,Agricultural University of Athens | Damigos D.,National Technical University of Athens
Renewable Energy | Year: 2014

The evidence is compelling that extended use and production of energy are globally responsible for the serious deterioration of physical environment and climate change. The further penetration of renewable energy sources (RES) emerges as a crucial factor contributing to the mitigation of global warming. Within this framework wind energy is expected to have a leading role for Greece's compliance with the EU environmental targets of 2020. However, the installation of wind parks in specific regions with high wind potential is seriously constrained by the reaction of local communities. Using a survey the present research dissects public acceptance for existing and proposed wind farms in the region of Southern Evia, Greece. Results indicate an overall support for wind energy and confirm the growing inconvenience with NIMBYism, especially in areas with existing wind parks in operation, as a theoretical framework explaining resistance to planned wind energy investments. By contrasting self-reported ex ante- and ex post-perceptions of impacts and benefits we highlight the role of experience in community acceptance of wind energy installations. Our statistical models prescribe the profile of those most probably in favour of existing installations, new installations in other parts of Evia or new installations elsewhere in Greece. Finally, we introduce the 'Not-In-My-Front-Yard' (NIMFY) syndrome suggesting that the impact of visibility on public acceptance is far from being a simple concept as it is linked to both a physical landscape context and socio-economic parameters. © 2013 Elsevier Ltd.

Sakellaridis N.G.,Center for Renewable Energy Sources, Greece | Karystianos M.E.,Hellenic Transmission System Operator SA | Vournas C.D.,National Technical University of Athens
International Journal of Electrical Power and Energy Systems | Year: 2011

This paper presents a number of local and one global (homoclinic loop) bifurcation observed in a small power system involving a load tap changer transformer and an induction motor. Besides the well-known generic local bifurcations (saddle-node and generic Hopf), the paper presents examples of higher codimension bifurcations, such as fold with double zero eigenvalues, degenerate Hopf, and swallowtail bifurcation. © 2011 Elsevier Ltd. All rights reserved.

Zegada-Lizarazu W.,University of Bologna | Elbersen H.W.,Wageningen UR Food and Biobased Research | Cosentino S.L.,University of Catania | Zatta A.,University of Bologna | And 2 more authors.
Biofuels, Bioproducts and Biorefining | Year: 2010

The recent policies enacted by the EU foresee an increased interest in the cultivation of energy crops. Hence systematized information on new energy crops and cropping strategies is necessary to optimize their production quantitatively and qualitatively and to integrate them into traditional production systems. This kind of information will offer farmers new perspectives and options to diversify their farming activities. Some of these crops, however, may compete for land and resources with existing food crops, while others could be grown in marginal/degraded lands with consequent beneficial effects on the environment. Therefore choosing the appropriate management components and species should be site specific and oriented to minimize inputs and maximize yields. In some cases, traditional food crops are used as dedicated energy crops with the advantage that their management practices are well known. On the other hand, the management of new dedicated energy crops, such as perennial herbaceous crops, often demands a range of structural features and tactical management approaches that are different to those commonly used for traditional food crops. Most of these crops are largely undomesticated and are at their early stages of development and improvement. In this work, state-of-the-art research and development of agronomic management and the production of a wide range of multipurpose future energy crop species are reviewed and where possible examples of appropriate crop management practices that would enhance energy yields are provided. Interesting lines of investigation are also suggested. © 2010 Society of Chemical Industry and John Wiley & Sons, Ltd.

Karytsas C.,Center for Renewable Energy Sources, Greece
Advances in Building Energy Research | Year: 2012

This paper summarizes the current state of the art of geothermal heat pumps as applies to buildings. Geothermal or ground source heat pump (GSHP) systems provide heating, cooling as well as domestic hot water, by the use of the underground or bodies of surface water as heat source or sink. More specifically, a GSHP system comprises a heat pump (usually waterto- water) and a ground source system (ground heat exchanger or groundwater well) in order to provide heating and cooling to the building through a low-temperature heating system and domestic hot water as well. The main concept of a GSHP system is the maximization of the heat pump efficiency i.e. minimization of electricity consumption mostly due to underground temperature which is almost invariant throughout the year. So, it is clear that GSHP systems have contribution to the environmental protection and reduction of greenhouse gas emissions. GSHPs are a mature industry, with a continuous trend of development and due to their increasing energy efficiency due to recent advances of the technology (described in the following chapters) are attractive and are more and more considered as an excellent substitute of conventional heating/cooling sources including air source heat pumps and variable refrigerant volume. Due to these advances in several cases the high efficiency of GSHP compared to the initial capital cost minimizes the need for subsidies. © 2012 Taylor & Francis.

Bottger D.,University of Leipzig | Gotz M.,University of Leipzig | Theofilidi M.,Center for Renewable Energy Sources, Greece | Bruckner T.,University of Leipzig
Energy | Year: 2015

Control power for the reliability of electricity supply is currently mainly provided by conventional power plants. In the situation of low residual power demand, the provision of negative control power may lead to must-run generation through base load power plants. Due to low wholesale electricity prices during low residual power demand, the must-run power plants that provide control power are confronted with high opportunity costs. Alternatively to base load power plants, electric boilers in district heating grids could provide relatively cheap negative secondary control power. The effects of the availability of electric boilers in the German control power market on overall system-wide costs and CO2-emissions of the power supply are assessed using a model-based analysis for 2012 and 2025. Power-to-heat plants are able to dissolve the conflict of must-run generation of base load power plants for control power provision. Therefore, they can enhance the integration of fluctuating RES (renewable energy sources), which in turn can reduce overall CO2-emissions of the power supply. The growing share of RES in Germany as well as cost reductions of electric boilers that can be expected in the next years could lead to a situation where the overall cost savings exceed the necessary investment costs. © 2015 Elsevier Ltd.

Drosou V.N.,Center for Renewable Energy Sources, Greece | Tsekouras P.D.,National Technical University of Athens | Oikonomou T.I.,Center for Renewable Energy Sources, Greece | Kosmopoulos P.I.,Democritus University of Thrace | Karytsas C.S.,Center for Renewable Energy Sources, Greece
Renewable and Sustainable Energy Reviews | Year: 2014

The scope of the HIGH-COMBI project is the development of high solar fraction systems by innovative combination of optimized solar heating, cooling and storage technologies as well as control strategies, in order to contribute and assist the further deployment of the solar energy market. Within this project, six demonstration plants were installed in four European countries (Greece, Italy, Spain and Austria). The purpose of this article is to assess the result achieved in the technical field of the project and to present the technical aspects of the six innovative demonstration systems realised during the project period. © 2013 Elsevier Ltd.

Kyriakarakos G.,Center for Renewable Energy Sources, Greece | Patlitzianas K.,Center for Renewable Energy Sources, Greece | Damasiotis M.,Center for Renewable Energy Sources, Greece | Papastefanakis D.,Center for Renewable Energy Sources, Greece
Renewable and Sustainable Energy Reviews | Year: 2014

Initiatives as the Covenant of Mayors and the European Union (EU) binding targets of 20-20-20 are bringing Regional Planning of Renewable Energy Sources (RES) at the center of attention nowadays. This situation creates the need for simplified and straight forward decision support systems for local governance officers. This paper presents the design and implementation of a fuzzy cognitive maps (FCM) decision support toolkit (DST) for local RES planning. DST provides the decision maker with an overall qualitative evaluation of the examined investment promptly with minimum effort. All the related parameters (legal/regulative/administrative, financial, technical, social and environmental) that affect the evaluation of RES investment in a local community are investigated. A tool based on fuzzy cognitive maps is designed and implemented on a web platform. The DST has been tested and validated successfully through application in real investments οn Crete Island and comparison to the evaluation results reached by a panel of experts. © 2014 Elsevier Ltd.

Prospathopoulos J.M.,Center for Renewable Energy Sources, Greece | Politis E.S.,Center for Renewable Energy Sources, Greece | Chaviaropoulos P.K.,Center for Renewable Energy Sources, Greece
Journal of Wind Engineering and Industrial Aerodynamics | Year: 2012

A 3D Reynolds-averaged Navier-Stokes solver, CRES-flowNS, is used in this paper for the flow simulation of four flow cases established during the Bolund hill experiment. CRES-flowNS, which has been developed at CRES, solves the steady state, incompressible Navier-Stokes equations on a curvilinear grid, employing k-ω turbulence closure. By generating alternative meshes, grid independent results are achieved that are compared to measured wind speed and turbulence characteristics in the four flow cases. From a methodological point of view, the scope of the present paper is to illustrate the appropriate numerical steps in order to assure reliable atmospheric flow predictions in complex terrain. © 2012 Elsevier Ltd.

Prospathopoulos J.M.,Center for Renewable Energy Sources, Greece | Politis E.S.,Center for Renewable Energy Sources, Greece | Politis E.S.,Center for Renewable Energy Sources CRES | Rados K.G.,National Technical University of Athens | Chaviaropoulos P.K.,Center for Renewable Energy Sources, Greece
Wind Energy | Year: 2011

The modelling of wind turbine wakes is investigated in this paper using a Navier-Stokes solver employing the k-Ï turbulence model appropriately modified for atmospheric flows. It is common knowledge that even single-wind turbine wake predictions with computational fluid dynamic methods underestimate the near wake deficit, directly contributing to the overestimation of the power of the downstream turbines. For a single-wind turbine, alternative modelling enhancements under neutral and stable atmospheric conditions are tested in this paper to account for and eventually correct the turbulence overestimation that is responsible for the faster flow recovery that appears in the numerical predictions. Their effect on the power predictions is evaluated with comparison with existing wake measurements. A second issue addressed in this paper concerns multi-wake predictions in wind farms, where the estimation of the reference wind speed that is required for the thrust calculation of a turbine located in the wake(s) of other turbines is not obvious. This is overcome by utilizing an induction factor-based concept: According to it, the definition of the induction factor and its relationship with the thrust coefficient are employed to provide an average wind speed value across the rotor disk for the estimation of the axial force. Application is made on the case of five wind turbines in a row. © 2010 John Wiley & Sons, Ltd.

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