Hybrid Renewable Energy Systems for North-Eastern Poland

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1 Hybrid Renewable Energy Systems for North-Eastern Poland * Janusz PIECHOCKI, Piotr SOLOWIEJ, Maciej NEUGEBAUER Department of Electrical, Power, Electronic and Control Engineering, University of Warmia and Mazury in Olsztyn, POLAND jpt@uwm.edu.pl Abstract: Hybrid renewable energy systems pose an interesting alternative for farms, small plants, villages and other remote sites. Hybrid energy systems consist of wind generators, photovoltaic cells and pumped-storage hydroelectric power stations. Hybrid energy solutions can be effectively deployed in north-eastern Poland, an area referred as the Land of a Thousand Lakes on account of its multitude of lakes and other water reservoirs. Many reservoirs are situated in the proximity of hybrid power plants, and they are often located at different altitudes. For this reason, they can be used as lower and upper reservoirs in pumped-storage hydroelectric power stations. The output of wind power generators and photovoltaic cells is determined by wind speed and the availability of solar energy. Renewable power plants do not fully cater to the energy needs of remote sites. Therefore, electric energy is stored in upper reservoirs of pumped-storage hydroelectric power stations. A comprehensive power control system was designed for effective management of energy flow between the power source and storage elements. The system provides uninterrupted power supply for remote sites by adapting energy production to energy consumption based on stored energy levels. This study proposes a new power control strategy for optimized energy management in a hybrid system. The performance of the control strategy for optimal energy management in a hybrid system is discussed in view of power generation and load demand. Key words: Wind generator, photovoltaic cell, pumped-storage hydroelectric power station INTRODUCTION North-eastern Poland is characterized by low population density, a low level of industrialization and a well-developed farming sector. The region abounds in forests, lakes and areas of great natural beauty that receive statutory protection. Those areas have favorable conditions for renewable energy generation, in particular with the involvement of wind generators and photovoltaic cells. The generation capacity of renewable energy sources is limited. The above applies to the power capacity and the amount of energy that can be generated by photovoltaic cells during different times of the day and the year as well as the energy supplied by wind generators under different weather conditions. Those sites are located remotely from conventional power plants that cater to the baseload demand of energy users (Khan et al, 2004; Paska, 2005; Paska et al, 2005). The fluctuations in end users' demand for energy are taken into account in the national energy management system, where every 24 hour period is divided into periods of peak, off-peak and night-time demand. Conventional power plants, most of which are fired with hard and brown coal in Poland, are characterized by stable power capacity and guarantee stable supply of electricity, whereas the output of renewable power stations, such as wind farms and photovoltaic systems, may vary substantially. The generating capacity of photovoltaic panels differs subject to the time of day, season and sun exposure, which can be predicted with a certain degree of accuracy, but the variations in the power capacity and the amount of energy produced by wind generators are much greater. Winds with minimum speed at which the turbine will generate usable power may be absent over periods of several days. The energy generated from renewable sources has to be stored and transmitted to end users at the time of demand and in the amount corresponding to current demand. This problem can be resolved with the use of hybrid energy systems. Hybrid energy systems consist of renewable power stations, such as 178

2 wind generators and photovoltaic cells, as well as pumped-storage hydroelectric power plants that can directly supply energy to large farms, industrial plants, residents of housing estates and villages. Hybrid energy systems can be directly coupled with power grids supplying larger areas and groups of end users (Hadjipashalis et al, 2008; Paska et al, 2009). North-eastern Poland features several thousand natural water bodies of varied size. This part of the country is known as the Land of a Thousand Lakes, which implies that around one thousand water bodies are large enough to be classified as lakes that store sufficiently large amounts of water. Lakes situated in close proximity but at various altitudes can be used to develop pumped-storage hydroelectric power stations. The map of the part of Poland is shown at figure 1. Figure 1. The map of northern-eastern part of Poland. MATERIALS and METHOD Pumped-storage power stations The principle behind the operation of a pumpedstorage plant is generally known and frequently applied in practice. In Poland, there are several pumped-storage plants with various capacity. Most of them store electricity generated by conventional heat power plants during the night and release that energy during day-time periods of high electrical demand. At present, pumped-storage plants are the only fully-evolved industrial method of electricity storage. The steady increase in the utilization of renewable energy sources implies the need to compensate for the growing and intermittent output of renewable energy stations in shorter time intervals. Supply fluctuations from renewable energy sources have to be balanced by introducing power sources that can quickly and reliably respond to such changes. This requirement is met by pumped-storage plants, which can respond to the demand of the electric grid within several minutes by receiving and storing electricity in the form of potential energy of water or by releasing stored water to produce and transmit electricity via the grid, subject to demand. Hydroelectric storage is a much cheaper method of energy storage than compressed air, thermal 179

3 energy or chemical energy storage systems. Hydroelectric plants also tend to have longer working lives than other types of energy storage stations. There are numerous sites in Poland with favorable conditions for the development of pumped-storage hydroelectric power plants, in particular in the Region of Warmia and Mazury, where the water table in many reservoirs had been lowered in the past century. Those reservoirs can be now used for hydroelectric plants. The efficiency of electricity conversion to potential energy of water and the efficiency of water energy conversion to electricity transmitted over the power grid are high enough for those processes to be commonly applied in practice. In both cases, energy conversion efficiency is generally estimated as 70-80%. The amount of stored energy and system efficiency are determined by the difference in the altitude of reservoirs in the pumped-storage plant, the size of those reservoirs and the rate of water flow from one reservoir to the other. The amount of energy stored in the upper reservoir can be calculated with the use of the potential energy formula (1), (2): E = m g h [GJ] ( 1 ) m mass of water [kg] g acceleration due to gravity [9.81 m s -2 ] h difference in the altitude of two reservoirs [m] were the mass of water: m = V ρ [kg] ( 2 ) V water volume [m 3 ] ρ water density [kg m 3 ] Hybrid energy systems Most hybrid energy systems combine small wind farms and photovoltaic cells. Their effectiveness could be enhanced by incorporating hydroelectric power stations and fuel cells for energy storage. A typical hybrid system that relies solely on renewable energy sources consists of elements that generate energy from solar radiation, wind and water. The operating principle behind a hybrid energy system is presented in a figure 1 and hybrid energy systems in northeastern part of Poland is presented in a figure 2. The carrier of energy generated from wind or sun is electricity which powers pumps in a pumped-storage plant that stores electricity in a hybrid system (Hadjipashalis et al, 2008; Paska et al, 2009). Hybrid systems combine two or more renewable energy sources to compensate for their intermittent power output. A typical hybrid system may consist of solar panels and wind generators as sources of electricity and a pumped-storage plant for energy storage. Systems that rely on renewable sources of energy transform energy from the sun, wind and flowing water into electricity. The key disadvantage of renewable sources is that the amount of generated energy is highly dependent on the time of day, season and weather conditions, therefore, it is difficult to predict. Hybrid energy systems combining several types of electricity generation systems are used to compensate for intermittent power outputs. Hybrid solutions are small-scale systems incorporating energy generation units that rely on various sources of primary energy, mainly renewable, and contain energy storage systems (Hadjipashalis et al, 2008; Paska et al, 2009). 180

4 Figure 2. Scheme of hybrid energy system. Photovoltaic cells Pumpedstorage plant Energy users Wind generators Figure 3. Hybrid energy system investigated in north-eastern Poland. Energy storage systems in hybrid energy solutions Systems that rely on renewable energy sources, such as solar panels and wind turbines, are highly promising distributed generation systems. However, the generating capacity of those systems cannot be fully harnessed due to strong natural fluctuations in wind and solar power plants. This problem can be addressed by combining photovoltaic cells and wind generators with systems that compensate for the fluctuating output of intermittent energy sources. Energy storage systems are used to accumulate 181

5 excess energy generated by wind farms and photovoltaic cells and to deliver stored energy to the power grid when generation systems are unable to cater to the load demand. The use of energy storage solutions maximizes the generating capacity of solar and wind power plants. Energy storage units also effectively counteract momentary voltage sags or transients, and they store reserve power until it is needed. Pumped-storage plants have long been used in the power industry to store electricity. RESULTS It is a short overview paper containing some recommendations in area of activity for hybrid energy systems for specified part of Poland. The preliminary research shows that in area of north-eastern Poland there are hundreds of places where it is possible to use hybrid power systems using pumped storage hydropower stations as storage of electricity. Electricity storage options depend on the size of the lakes and the difference in levels. The potential for energy storage in hybrid electric power systems in this area are very large, fully meet the needs of customers. These studies will be continued. CONCLUSIONS Distributed generation systems that rely on renewable sources of energy are evolving rapidly, and one of the main goals of the developed solutions is to reduce the distance over which electricity is transmitted. The number of energy conversion processes will be limited, and all available forms and carriers of energy will be effectively managed. Hybrid power solutions that integrate various electricity generation systems make highly effective use of renewable and distributed carriers of primary energy. The development of medium- and low-voltage hybrid energy systems that utilize renewable and distributed sources of primary energy and are located near end users will contribute to a reduction in energy transmission and distribution costs. In addition to economic benefits, hybrid systems also deliver environmental advantages because the integrated energy sources produce little or no pollution. REFERENCES Hadjipashalis I., Poullikas A., Efthimiou V.; Overview of current and future energy storage technologies for electric power applications. Renewable & Sustainable Energy Reviews. September Khan M., M. Iqbal; Pre-feasibility study of stand-alone hybrid energy systems for applications in Newfoundland Memorial University of Newfoundland St. John s Canada June 2004: Elsevier Renewable Energy Issue ( ). Paska J.: Renewable Energy Sources, Distributed Generation and Hybrid Power Systems in Works of WUT Institute of Electrical Power Engineering. 8th' International Symposium "Höfler's Days". Portoroz-Slowenia, 6-8 November, Paska J., Biczel P.: Hybrid Photovoltaic - Fuel Cell Power Plant. IEEE St. Petersburg PowerTech'2005. St. Petersburg, Russia. June 27-30, Paska J., Biczel P., Klos M.; Hybrid power systems An effective way of utilizing primary energy sources. Renewable Energy, Vol. 34, No 11, November