Preface to Special Issue: Renewable Energy in South-Eastern Europe

South-Eastern Europe is sometimes identified with the Balkan Peninsula. The countries which are usually associated with this area are: Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Greece, FYR Macedonia, Rep. of Moldavia, Montenegro, Romania, Serbia, Slovenia, and Turkey. The region has good potential for renewable energy, including solar, wind, biomass, small hydro and geothermal. This special issue entitled "Renewable Energy in South-Eastern Europe" presents information about the status of renewable energy research and applications in this region.

The special issue was initiated as a joint collaboration between the Section of Technical Sciences of the Romanian Academy and the Section of Technical Sciences of the Serbian Academy. The Guest Editors are Viorel Badescu (Romanian Academy), Zoran V. Popovic (Serbian Academy), Laurentiu Fara (Academy of Romanian Scientists), and Milorad Bojic (University of Kragujevac). Original and review papers by authors working in this region are grouped in several sections: renewable energy potential, wind, solar thermal, photovoltaics, buildings, and energy policy. More details about selected papers follow.

A software application that allows one to establish the clear-sky solar radiation on a horizontal surface has been proposed in Ref. 1. It considers the shading effect of complex topography of the terrain and employs it to assess the amount of clear-sky irradiation over Romanian territory. A clear sky solar radiation model has been developed in accordance with the latest mathematical equations published in the European Solar Radiation Atlas.

A review of recent researches on linking the satellite image data to surface measured bright sunshine hours is presented in Ref. 2. First, some basics on estimation procedures of the daily global solar irradiation on a horizontal surface with the daily bright sunshine hours are introduced. Second, a satellite based model is presented briefly. Next, two recent combined models are reviewed and compared with the conventional models.

Wind energy resources in the Ionian-Adriatic coast of South-East Europe are analyzed in Ref. 3. Status of wind energy development in the countries of Greece, Albania, Montenegro, Bosnia-Herzegovina, and Croatia and regions of moderate and high wind potential at their west coasts are reviewed. The feasibility of the application of specific wind turbine generators with lower cut-in, cut-out, and rated speeds in moderate wind fields are investigated.

The article²⁰ presents a brief assessment of solar and wind resources in Serbia. Meteorological data from a network of meteorological stations covering a period of 30 years is used to obtain annual energy density distributions from these two climatic sources. This assessment indicates that southern and southeastern Serbia are very good regions for solar energy utilization. Southern Banat, mountain ranges of eastern and southeastern Serbia, and the lower Danube river valley are the most promising regions for wind power plant construction. The final objective of this article is to show that the problem of individual intermittency of wind and solar power potentials could be compensated in Serbia by their complementary character.

The study⁴ presents experimental research to study the natural soil temperature variations and to determine the soil thermal diffusivity based solely on temperature measurements. The research was conducted for two locations in Romania: Cluj-Napoca on bare clay soil and Reghin on grass covered clay soil. This manuscript shows that the design and methodology of the research can be successfully applied in any location. Based on a statistical analysis of the experimental data, a simple and precise mathematical model for natural temperature variation is identified for each location.

The territory of Bulgaria is rich in thermal water at temperatures in the range of 20–100 °C.¹⁵ Direct thermal water use nowadays is mainly for balneology, space heating and air-conditioning, greenhouse heating, geothermal ground source heat pumps, direct thermal water supply, and bottling of potable water and soft drinks. Despite the fact that geothermal energy used for heating of buildings and greenhouses is not a main concern in Bulgaria, several applications—balneology and geothermal ground source heat pumps, show promise for stable development

Power plant electricity production control is very much dependent on the type of power plant. Some types, such as storage hydro, have very good power control capabilities. On the other hand, electricity production of wind power plants is strongly dependent on the wind that is forecasted with some degree of uncertainty.

If wind power plant production misses the schedule submitted to the transmission system operator, the production of another power plant has to be altered in order to compensate mismatch of the scheduled power production. The paper⁵ shows why the Balkans region is very suitable for construction of large wind power plants.

In Serbia for heating of domestic hot water (DHW), it is customary to use electricity. As around 70% of electricity is produced by using low quality coal with high greenhouse gas emission, it is beneficial to the environment to use solar energy by flat-plate solar collectors for heating of DHW in a solar DHW system (SDHWS). The paper²¹ reports the investigation of the SDHWS with variable tilt flat-plate solar collectors placed in a north-south direction on the roofs of houses for their optimal operation in Belgrade, Serbia. The investigated collectors annually take 2, 4, and 12 tilts. The weather data used are from the meteorological station. These investigations use three computer codes: EnergyPlus and GenOpt, and the HOOKE JEEVES search algorithm. For the different solar collectors, the investigations reveal their optimum tilts that maximize the solar fraction by the SDHWS. Then, the solar fraction and avoided fossil energy by the SDHWS are maximized. In addition, the deficit in the solar fraction is estimated when the solar collectors are not at their optimum tilt.

A sea-shell stationary trough solar concentrator for heat production is studied in Ref. 6. It consists of a parabolic, cylindrical trough reflector, a flat double-sided absorber, and a flat aperture. The optical efficiency (the efficiency of solar ray transfer inside the concentrator) is investigated for different solar ray altitudes at the concentrator aperture and different kinds of reflector shells. For the concentrator installed in Kragujevac, Serbia, the highest optical efficiency is obtained for the solar ray altitudes between 45° and 69.75°.

The short term forecasting of PV system output is presented in the article.²² The approach has great significance for fast development of PV parks in South-East Europe. The approach is based on two methods: ARIMA (Autoregressive Integrated Moving Average) and ANN (Artificial Neural Network). The daily solar irradiation variability is analyzed and cloudiness changes are included. Decadal variations of global solar radiation are also considered for long term forecasting. The results are obtained using a database from the Bucharest/Afumati Meteorological Station. By using complete datasets and including meteorological parameters such as cloudiness, relative humidity, air temperature, atmospheric pressure, sunshine duration, etc., as input for the model, the forecasting errors are minimized and a more accurate forecast of the power output is obtained for the analyzed demo PV system.

The possibilities to decrease energy consumption of a residential building in Serbian conditions are analyzed in Ref. 16. The building uses electricity for the space heating system, heating of domestic hot water, lighting, and for other electric equipment. The electrical energy is generated by a photovoltaic (PV) system, and it may be consumed by the building or may be fed-in to the electricity grid. The major aim of the optimization of PV area is to determine the grid electricity that is avoided (avoided energy), and to minimize the consumption of primary energy.

A hybrid autonomous electricity supply system is proposed in Ref. 19 for monasteries as a specific type of load. This system is based on combined use of photovoltaic panels and diesel aggregates. Chilandar monastery (Mount Athos, Greece) is presented as a case study. Taking into consideration specific location, historical significance, present and future needs of the monastery, peculiar life style of its inhabitants, a new hybrid system for 400 kWh daily energy consumption and 80 kW peak load has been designed, proposed and discussed in this paper.

The numerical simulation of the physical parameters for dye sensitized solar cells (DSSCs) with black dyes is considered in Ref. 17 based on a new optimizing procedure. The influence of thickness and lifetime on the V-J characteristics parameters is analyzed. In this way, Jsc (short-circuit current density), Voc (open circuit voltage), FF (fill factor), and η (efficiency) are determined. A comparison between the classical ruthenium based solar cells and black dye cells is possible using a combined simulation and experimental approach.

A brief overview of research in the field of electrode materials for solar cells and supercapacitors is presented in Ref. 18. In addition to the standard electrode materials (graphite and silicon in supercapacitors with solar cells), different materials are examined, particularly oxides and sulfides of copper.

The paper⁷ presents an architectural project that simulates energy consumption and production using a model for an Energy Efficient House for Decentralized Energy (DE). The house model is designed for the area of Serbia, based on the needs of an average Serbian family. The model is designed to investigate opportunities for DE.

In a typical house, geometry with the advantages of passive solar architecture combined with active solar systems and heat pumps. Simulation of the model is performed for the climate conditions for the city of Nis.

The importance of the solar energy usage as a clean source of energy in Turkey is discussed in Ref. 8. Combined small capital usage along with State support is analyzed within various scenarios in order to make the sector attractive for investors at every level. In order for the results of the research to be usable, the technical infrastructure and cost analysis for a 10 MW solar energy system are considered. The costs of the solar parks selected worldwide are considered in the cost analysis. Although the research discusses setting up solar energy systems and management in Turkey, similar problems exist in many other places worldwide.

National implementation of the European Directive on the energy performance of buildings (EPBD) in Greece since 2010 is the first coordinated attempt to improve the energy performance of Hellenic buildings. In this framework, as of 2011, new buildings have an annual solar fraction of 60% for sanitary hot water production from solar thermal systems. Efforts are currently underway to comply with the EPBD recast so that by the end of the decade, all new buildings are nearly zero-energy buildings and that the very low amount of energy required should be covered to a very significant extent by energy from renewable energy sources (RESs). The paper⁹ reviews the relevant RES policies and status, with an emphasis on the Hellenic buildings.

The paper¹⁰ deals with the possibility for cross-cutting approaches to the agricultural and energy sectors, aiming toward better utilization of the available renewable energy sources. An integrated methodological framework is proposed to formulate the possible solutions to the core question, which is “who does what” to improve the utilization of renewable energy potential in the agricultural sector. The methodology includes mainly stakeholder analyses (focusing on importance, knowledge, position, and interests of each stakeholder), as well as problem and objective tree analyses. This methodology is applied for the case of solar and wind energy deployment in the Macedonian agricultural sector.

The Macedonian energy sector is the main emitter of greenhouse gases with about 70% of the total annual emissions. Also, 70%–75% of emissions are associated with the electricity generation due to the predominant role of the lignite fuelled power plants. Recently, the government has adopted a strategy for the use of RESs which identifies a target of 21% of final energy consumption from RES by 2020. In Ref. 11, analyses are conducted to investigate to what extent and in which way the absorption capacity of the power system for RES electricity can be improved.

Island communities require in-detail mapping of resources available for exploitation for energy purposes, since infrastructure and connections to the mainland present, in most cases, a weak point of the island energy supply. As present, the energy supply on Croatian islands relies mostly on fossil fuels and electricity from the mainland. It is obvious that exploitation of renewable energy sources is the only solution that leads towards self-sufficiency and sustainable development. In order to design a self-sufficient and sustainable island, three major technological changes are needed: integration of renewable energy sources alongside with energy savings and improvements in energy efficiency. Analyses for several other Croatian islands have been performed in Ref. 12 by using Renewislands/ADEG methodology in order to assess all possible outcomes.

The relationship between renewable energy consumption and economic efficiency is examined in Ref. 13. Conditional Data Envelopment Analysis estimators alongside with nonparametric regressions are applied in a sample of 25 European countries for the year 2010 with emphasis given to Eastern and Western European countries.

The main goal of Ref. 14 is to examine and quantify how the development of the energy system in Macedonia could be influenced by policies and programs that promote energy efficiency and renewable energy, in terms of energy security and diversification, economic competitiveness, and CO₂ mitigation. For this purpose, the MARKAL-Macedonia national energy planning model was used. These policies are explored by comparing a reference (business-as-usual) development of an energy system with three alternative policy scenarios: (1) Renewable Energy (RE) Scenario—introducing a renewable energy target by 2020 in line with that proposed by the Energy Community; (2) Energy Efficiency (EE) Scenario—exploring the range of energy efficiency measures that are the most cost-effective means to meet national targets aimed at reducing final energy consumption; and (3) Combined RE and EE Scenarios—a combination of supply-side and demand-side approaches that examine the resulting synergies of these policy goals.

This special issue of Journal of Renewable and Sustainable Energy illustrates the diversity of approaches and problems that are being considered by scientists and engineers working in the renewable energy sector in South-Eastern Europe.

We hope the issue will be of interest for researchers (engineers, physicists) involved or interested in renewable energy in general and in its application in South-Eastern Europe in particular. Also, the issue may be useful for industry developers interested in joining national or international renewable energy programs. Finally, it may be used for undergraduate, postgraduate and doctoral teaching in faculties of engineering and natural sciences.