SIMULATION AND APPLICATION OF A DC ENERGY DISTRIBUTION SYSTEM FOR HYBRID RENEWABLE ENERGY SYSTEMS

1 SIMULATION AND APPLICATION OF A DC ENERGY DISTRIBUTION SYSTEM FOR HYBRID RENEWABLE ENERGY SYSTEMS Engin Cetin a* Ahmet Yilanci a Harun Kemal Ozturk a Mahmut Hekim a Ismail Kasikci b Metin Colak c Siddik Icli c a Energy Research&Application Center, Pamukkale University, Kinikli, 20070, Denizli-Turkey b Electroctechnic Department, Biberach University of Applied Sciences, Biberach Germany c Solar Energy Institute, Ege University, Bornova, 35100, Izmir Turkey ABSTRACT Hybrid renewable energy systems are the systems where the renewable energy sources as wind, hydrogen, solar etc. have been utilized simultaneously. In the recent years, the increase in the petroleum prices, the rapid run out tendency of fossil fuel reserves and their inavoidable harmful effects and political instabilities in the regions of energy sources have been accelerated the studies on the renewable energy systems. The electrical energy produced by renewable energy systems like photovoltaic panels and fuel cells is in the form of the direct current (DC) electrical energy. In effect, despite of the fact that the electrical energy produced by the wind turbines is in the form of alternative current (AC) in proportion to the wind speed, this AC energy is converted into the DC energy by the internal converters in the low-scale wind turbines used in the buildings and then the network is supplied by this DC energy. Thus, the DC energy produced by photovoltaic panels, fuel cells and wind turbines has to be converted into AC energy due to the fact that the consumers need generally AC electricity. Such a DC/AC conversion brings some disadvantages such as the need of a DC/AC converter, the involvement of some harmonics, the loss of energy, the increase in dimension and cost, and some degradation of the quality of the energy. Transmission of electrical energy from production place to consumption place faces with the problem of energy loss. The energy produced by photovoltaic panels, fuel cells and wind turbines is in the form of DC and thus we can abstain from the above mentioned problems if we use DC loads without a DC/AC conversion. In this study, firstly, a Matlab simulation software is created for DC hybrid system and then, its application, in which the DC energy produced by RHESs is consumed by the DC loads (44 compact flurescent lamps, circulation pump, freezer/refrigerator, vacuum cleaner, TV and 2 fans) without being converted into the AC form, has been presented. For this purpose, a DC energy distribution unit is established for the photovoltaicwind-fuel cell hybrid energy system existing at Pamukkale University, Denizli, Turkey. Keywords: DC Energy Distribution, Matlab, Photovoltaic, Fuel Cell, Wind Turbine. 1. INTRODUCTION Conventional fossil fuel sources like coal, oil, natural gas have recently entered into a quick decreasing tendency. In addition, with the use of such fossil fuel sources, it is inevitable that they contaminate the nature, cause global warming by forming greenhouse effect and therefore take our world into an undesirable disaster. Therefore alternative and renewable energy sources have importance more than before in the human history (Oner et al., 2009). That the present energy production sources enter quickly into the exhaustion tendency, that raw material prices increase, their negative effects on the environment and human health, some difficulties in the usage have increased the studies made on new and renewable energy sources (Cetin et al., 2001). *Corresponding author, tel.: 0090-258-2963747, e-mail: engincetin@pau.edu.tr

2 Distribution of electrical energy from production location to another consumption location also brings some energy losses. In power distribution systems, Alternative-Current (AC) distribution technology is used generally. Distribution of electrical energy produced by Hybrid Renewable Energy Systems (HRES) with minimum losses is very crucial since these systems are costly and their intermittent production nature. Power factor exists in AC energy distribution and affects the distributed active power unfavorably. In Direct Current (DC) energy distribution, there is no energy losses like above since power factor is 1. For the same conductor, DC voltage distributes more power than AC voltage. Voltage control in AC distribution has some problems if line capacity and inductive voltage drop are considered. In AC energy distribution, there are some problems which exist in DC energy distribution less than AC energy distribution such as voltage oscillation, active losses, stability problems, limitations in current carriage capacity, line interactions, frequency effects, voltage drops, power plant expenses etc. Nowadays, although AC energy has these problems, it is widely used because conventional power stations like hydroelectric, natural gas have alternators and available appropriate AC devices for usage. However, energy produced by photovoltaics, fuel cell systems and wind turbines (with internal DC/DC converter) is DC, and also in case of DC loads usage, DC/AC conversion is not needed. This eliminates the problems mentioned above (Cetin et al., 2010a). In this study, it is aimed to distribute electricy in DC form and also to be planned to carry out R&D activities by using in loads which have DC characteristics. For this purpose, photovoltaics-wind turbine-fuel cell system, DC loads, and DC distribution mechanism were examined experimentally in Pamukkale University Clean Energy House. 2. RENEWABLE HYBRID ENERGY SYSTEM The photovoltaic wind turbine fuel cell hybrid energy system established at Clean Energy House (CEH) includes 5 kw photovoltaic panels group, two 1.2 kw fuel cells and two 400W wind turbines (Cetin et al., 2010b). The aim was to design a unique, integrated system for the house, using only solar energy to meet all energy needs (with no inputs from fossil based energy sources) and to provide an environmentally benign design and operation (Cetin et al., 2009). The photovoltaic panel group constitutes the primary energy supplier of the system, while fuel cells and wind turbines are the secondary suppliers since the area is not windy through the year except for in the winters. Therefore, wind turbines assist in charging the batteries during the period when the solar energy is not enough (Cetin et al., 2010b). In Fig. 1, the CEH is illustrated (Cetin et al., 2010a). Each of the photovoltaic panels is Kyocera 125GHT-2: the output power is 125 W, the material is polycrystalline, the maximum output voltage is 17.4 V, and the maximum output current is 7.2 A, the open-circuit voltage is 21.7 V, the short-circuit current is 8.0A (Kyocera, 2009). Fig. 2 illustrates the fixed and mobile photovoltaic panel groups (Cetin et al., 2010b). The fuel cells shown in Fig. 3 are used to produce electrical energy from the hydrogen stored in the system. The PEM-type fuel cells are adopted due to their simplicity, compactness and easiness to-maintenance (Cetin et al., 2010b). Each of the Nexa TM fuel cell modules produces maximum 1.2 kw DC energy and has a 22 50 V DC nominal output voltage. The Nexa TM power module provides up to 1.2 kw of unregulated DC power at a nominal output voltage of 26 V DC. The fuel cell stack consists of 47 sandwiched fuel cell elements (Nexa, 2003).

3 Fig. 1 Clean Energy House (CEH). Fig. 2 The fixed and mobile photovoltaic panel groups. Fig. 3 Fuel cell units. Fig. 4 Wind turbine. The system also includes 2x400 Wp wind turbines which are chosen in accordance with the 48 V DC system as shown in Fig. 4. The Air-x TM model turbine is of 1.17 m rotor diameter and its power is 400 W. The range of wind speed for the turbine to be able to produce electrical energy is between 3.58 m/s (cut-in speed) and 12.5 m/s (cut-out speed) (Air-x, 2008).

4 3. MATLAB-SIMULINK MODEL OF DC SYSTEM Before the contruction of the DC system, Matlab simulation was examined. In Fig. 5, Matlab-Simulink model of DC system is illustrated (Cetin, 2010). Fig. 5 Matlab-Simulink model of DC system. In simulation, PV modules, wind turbine and fuel cell are simulated seperately. Some sections of simulation model (i.e. wind turbine, DC-to-DC converters and load units) are created. The other equipments were studied from some papers (Colorado, 2010; Wang and Nehrir, 2009, Wang et al., 2005, Nehrir et al., 2006). In PV model, four PV modules are connected in serial form to achieve 48 V DC system. There are ten parallel branches in Simulink model. Solar radiation for PV modules is supplied from port In1 (Fig. 5).

5 To create wind turbine model, datas from wind turbine manual were used. Energy production limits are regulated between 8 m/s and 13 m/s wind speeds. For fuel cell model, some parameters like current, voltage, and anode pressure are entered by an operator. Voltage, current, efficieny, power, and other electrical curves can be achieved from Simulink model. In Fig. 6 and 7, DC/DC converter voltage current and efficiency curves are shown respectively. Fig. 6 DC/DC converter output voltage waveform from Matlab-Simulink model. Fig. 7 DC/DC converter efficiency curve from Matlab-Simulink model. 4. CONSTRUCTION OF DC SYSTEM In DC system, there are 12 V (44 fluorescent lamps, TV, and vacuum-cleaner) and 24 V (freezer-refrigerator, circulation pump, and 2 fans) DC loads (Fig. 8). The total load power is 735 W. Electrical properties of load units are shown in Tab. 1 (Cetin et al., 2010b). Tab. 1 Devices used in the experimental study and their electrical properties.

6 Fig. 8 DC load units. In Fig. 9, a DC distribution power box unit, which is composed of five main units as described below, is illustrated: Unit 1 has connectors and distribution bars (Fig. 9, sections A and B respectively) for 12 V and 24 V DC power distribution and DC surge protector (Fig. 9, section C). Unit 2 has 12 V and 24 V DC/DC converter devices (Fig. 9, section D). Unit 3 has DC circuit breakers, DC insulation monitoring device against faults between positive/negative line and earth protection line (Fig. 9, section E). Unit 4 has PLC CPU unit, its analog modules for voltage and current data acquisition (Fig. 9, section G) and PLC power supply module (Fig. 9, section F). Unit 5 has voltage signal converters and shunt devices to convert analog voltage and current signals to digital form (Fig. 9, section H) (Cetin et al., 2010b). Fig. 9 DC power distribution box.

7 In the system, circuit breakers are used for protection. Having 12 V and 24 V DC voltage levels of energy distribution is itself a matter of safety for the habitants and the devices, for the critical voltage level for human is 120 V DC (Kasikci, 2008). The system has also a RIR48N Contrel Elettronica insulation monitoring device. This device protects the system by switching off the circuit in the case of a grounding fault for instance positive pole-ground or negative poleground contact (Cetin et al., 2010b; Contrel, 2009). In the main power distribution panel of the system, a DC surge protector is employed to prevent the possible damages of the voltage pulses stemming from PV/wind turbine group or fuel cell. The surge protector can operate up to 60 V DC, and its maximum operating voltage is 130 V, its nominal current is 26 A, its short-circuit switch-off capacity is nominally 2.5 ka and maximum 6.5 ka (Cetin et al., 2010b; Siemens, 2009). 5. EXPERIMENTAL STUDY For the analysis of the DC distribution system, a Fluke-43B power quality analyzer is used. Fluke-43B is a device that can analyze AC and DC power, voltage, current and energy. For DC current measurements, the Hall-effect 100 A DC probe of the device is used (Cetin et al., 2010b). Fluke-43B can make the measurements on any points on the DC distribution panel. Fig. 10 shows voltage and current signals from the input of 12 V converter when all 12 V loads (CFLs, TV and vacuum cleaner) in charge are switched on. As can be seen from the figure, the input voltage of the converter is 47.81 V and the input current is 14.31A. Fig. 11 shows the voltage and current signals read from the output of the 12 V converter when all of the 12V loads (CFLs, TV and vacuum cleaner) are switched on. As can be seen from the figure, the output voltage of the converter is 12.29 V and the output current is 46.91A (Cetin et al., 2010a). Fig. 10 Voltage and current signals from input of 12 V converter when all loads in charge. Fig. 11. Voltage and current signals from output of 12 V converter when all loads in charge. Fig. 12 shows the voltage and current signals read from the input of the 24 V converter when circulation pump and 2 fans are switched on. As can be seen from the figure, the input voltage of the converter is 47.53V and the output current is 2.560 A. Fig. 13 shows the voltage and

8 current signals read from the output of the 24 V converter when circulation pump and 2 fans are switched on. As can be seen from the figure, the output voltage of the converter is 24.31 V and the output current is 3.586 A (Cetin et al., 2010a). Fig. 12 Voltage and current signals for 24V converter input when pump and fans are switched on. Figure 13. Voltage and current signals from 24V converter output when the fans and the circulation pump are activated. For fuel cell applications, an emergency situation is described. In an emergency situation only selected loads (e.g. 20 DC lamps, TV, 2 fans, freezer/refrigerator) are supplied by fuel cells. These situations are actualized in no power situation from PV module and/or wind turbine. Figure 14 shows the voltage and current signals from the output of the fuel cell when all loads in charge are switched on. As can be seen from the figure, the output voltage of the fuel cell is 30.63 V and the output current is 16.05 A (Cetin et al., 2010c). Figure 15 shows the power values with voltage and current signals read from the output of the fuel cell when all loads are switched on. As can be seen from the figure, the active output power of the fuel cell is 492 W, reactive power is 13 VAR and apparent power is 492 VA (Cetin et al., 2010c). Fig. 14 Voltage and current signals from output of the fuel cell when all loads in charge. Fig. 15 Power values with voltage and current signals from output of the fuel cell when all loads in charge.

9 6. CONCLUSION In this study, a DC power distribution system with a hybrid renewable energy system in Denizli, Turkey, in terms of DC power distribution, DC loads, system simulation, and etc., is performed. In addition, some measurements such as voltage, current, and power are measured to obtain the electrical characteristics of the system. Photovoltaic panel, wind turbine and fuel cell systems produce Direct Current (DC) electrical energy. Therefore, DC electrical energy produced by photovoltaics, fuel cell systems and wind turbines has to be converted to Alternative Current (AC) due to AC demands of consumer. This DC/AC conversion causes cost increase (additional DC/AC inverter), energy losses, increase in system size and deterioration of energy quality. In this study, a photovoltaic-wind turbine-fuel cell hybrid energy system was examined. A micro DC power distribution system which has 12 V / 565 W DC loads and 24 V / 170 W DC loads was constructed. On this system, several electrical experiments were examined. Experimental outputs and Matlab simulations were compared. From the obtained electrical construction of whole system, it can be said that the DC power distribution is suitable for hybrid renewable energy systems. In the residence, 12 V and 24 V voltages have been used for safer conditions i.e. not only for inhabitants but also the devices in the residence. The need for AC conversion is overcome by distributing and consuming the DC energy in a DC manner. The DC distribution eliminates the cost of conversion, the electrical losses during conversion and also the need for some space required by the inverter. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support provided by the Scientific Research Projects Council of Pamukkale University (project no: 2010KKP119), Turkish State Planning Organization (DPT), the Scientific and Technological Research Council of Turkey (TUBITAK), Bereket Energy Inc. (Turkey), Kaelsa Heating Systems Industry (Turkey), Siemens (Turkey) and Contrel Elettronica (Italy). REFERENCES Air-x Owner s Manuel PDF Version, 2008, http://www.windenergy.com. Cetin, E., 2010, Design, application and analysis of a direct-current distribution grid for a photovoltaic-wind-fuel cell hybrid energy system, Ph. D. Thesis, Ege University Solar Energy Institute, 183 p. Cetin, E., Yilanci, A., Ozturk, H. K., Hekim, M., Kasikci, I., Colak, M., and Icli, S., 2010a, Investigation of DC and AC power distribution systems for renewable energy applications, 5 th International Ege Energy Symposium & Exhibition, June 27-30, Denizli-TR. Cetin, E., Yilanci, A., Ozturk, H. K., Colak, M., Kasikci, I., and Iplikci, S., 2010b, A micro DC power distribution system for a residential application energized by photovoltaic-wind/fuel cell hybrid energy systems, Energy&Buildings, 42 (8), 1344-1352.

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