Hybrid Photovoltaic-Fuel Cell Power Plant

1 Hybrid Photovoltaic-Fuel Cell Power Plant Józef PASKA, Piotr BICZEL Abstract--Nowadays, in many countries the increase of generating capacity takes place in small units of so-called distributed power industry (distributed generation). In the paper are presented: the experiences from exploitation of hybrid solarwind power plant; concept of solar power plant with fuel cell. This last solution enables optimal utilization of primary energy sources and increases the level of supply reliability. Authors have worked for several years on stand alone hybrid solar-wind power plant for supply of telecommunication equipment. The main problem in such installations is how to guarantee power supply all year without interruptions. Weather conditions in Poland provide to breaks in winter and autumn. The paper shows proposal of a new power plant with fuel cell and solar panels. The idea is to generate energy from PV panels as long as it is possible. When there is no Sun energy will be produced by fuel cell. Because of the system will operate rather far from service centers it has to work as long as it is possible without refueling. Power summation and control algorithm is explained, power electronics converters and control system are described. II. HYBRID SOLAR WIND POWER PLANT FOR TELECOMMUNICATIONS EQUIPMENT The team from Warsaw University of Technology has built a hybrid solar and wind power plant described in [4]. That was an answer to order of one of Polish telecommunication companies. The power plant has supplied the telecom equipment. The company wanted to have clean energy source. Something what could replace Diesel generators. Particularly in installations placed far from public grid. The power plant had to produce energy all time without any breaks. Fig. 1 shows a general view and Fig. 2 shows a block diagram of the hybrid power plant. Index Terms--Distributed generation, fuel cells, renewable energy, solar photovoltaic panels, supply reliability. C I. INTRODUCTION URRENTLY we can observe very fast development of new electrical power sources called renewable sources. These sources are environment friendly and use primary energy carriers like solar, wind and water flow, biogas, biomass etc. The sources mentioned above can be split into two groups: controlled sources and uncontrolled sources. As controlled sources authors mean primary energy sources giving possibility to control electrical power production, for example coal. It is obvious that power production of uncontrolled sources is unpredictable and human independent. Solar and wind power plants are uncontrolled sources. On the other hands, electrical power should be produced exactly at the same time when it is needed. Sun and wind do not meet this requirement. So, special kind of power plants should be built to avoid shortages of power and to utilize all of available sun or wind power. These are at least two ways to achieve that aims: electrical energy storage or power plants using two primary sources with additional control systems. One of the sources must be a controlled power source. Fig. 1. View of solar panels and wind turbine of the hybrid power plant The plant has been tested for several years. Among other things, the nature of produced power has been particularly observed. Problems, connected with cooperation between power network and unstable sources, have been studied. An example of daily power production is shown in Fig. 3. J. Paska, D.Sc., Ph.D., is with Institute of Electric Power Engineering of the Warsaw University of Technology, Warsaw, POLAND (e-mail: Jozef.Paska@ien.pw.edu.pl). P. Biczel, Ph.D., M.Sc., is with Institute of Electric Power Engineering of the Warsaw University of Technology, Warsaw, POLAND (e-mail: biczelp@ee.pw.edu.pl). Fig. 2. Block diagram of the hybrid solar-wind power plant

2 Fig. 3. An example of daily power production of solar-wind power plant A heart of the system was a chemical battery. The battery has been charged by solar panels and wind turbine. The main idea was to use only solar panels but there were no enough sunny days in Poland. Solar panels could produce enough energy from May to September. But in winter breaks were very often. Fig. 4 shows power production during all year from the power plant (P obc means required power demand). So the wind turbine was added. But in Poland when there is no Sun also there is often no wind. P/Pobc. 1,60 1,40 1,20 1,00 0,80 0,60 0,40 0,20 0,00 January February March April May Fig. 4. The power produced by the solar-wind power plant during a year June III. SOME PROBLEMS CAUSED BY UNCONTROLLED POWER SOURCES IN POWER GRID As a result of research authors have noticed some problems connected to uncontrolled power production and cooperation with power grid. Among other the most important problems, in authors opinion, are: July August September October November December rapid and unpredictable changes of power production, sudden disappearances of power generation, bad usage of primary carriers. It has to be stressed that time constants of the phenomenon are much smaller then in classic power plants. Uncontrolled sources power production depends mainly on Sun irradiation or wind speed. The power versus time curve, called production profile, follows the primary carrier versus time curve. In fact, the changes are extremely rapid (Fig. 3). In case many similar power plants are installed, source power changes cause necessity to increase power hot reserve in power grid. The reserve has to be able to cover load demand in case of wind or sun production fall. The additional power reserve is necessary in grids with relatively high power in sources like wind turbines. German experiences show that more then 10% power in unstable sources causes significant drop in power quality. The sudden disappearance of power production was observed in the power plant. In the aftermath of that, it could be a large power shortage in power grid. The shortage has to be immediately filled up by other sources. The problem is that turbine sets or diesel sets cannot be speeded up enough quickly. The problem could not be solved by increasing of hot reserve in power production. The reason is bad turbine sets dynamics. Thus, other methods have to be applied. One of them could be to apply an energy storage system or a new fast enough controlled power source. There is almost impossible to produce power from both source in plant shown in Fig. 2. This is due to DC link nature. Power converters (DC/DC and AC/DC2) had diodes in output circuit. Those diodes were necessary to protect solar panels from opposite polarization or it is a consequence of converters topology. So, there were two parallel connected diodes in

3 sources connection. In consequence only one of two sources could supply load at the same time. If Sun has given e.g. 60% of load needs and wind 40%, it was necessary to supply load from chemical battery. Although sources together could produce enough power to meet needs. But none of them could not to meet load alone. The best way, in authors opinion, to solve problems described in this chapter is to build the power plant as controlled source. It will be possible if the additional controlled source is used. There are several possible additional sources and different possible schemes of connection with wind power turbines. But it is sure that new hybrid power plant has to be renewable or at least green source. So Diesel generator is excluded. Therefore, authors suggest fuel cells [1], especially fuelled by hydrogen for this purpose [8]. IV. HYBRID SOLAR FUEL CELL POWER PLANT Hydrogen fuelled fuel cells are new, efficient and clean DC power sources. They have also very good dynamic properties. Authors have tested PEM fuel cell Nexa [3], produced by Ballard. As it was told above, the additional source has to be very fast. PEM fuel cell meets this condition. Fig. 5 shows an example of time response for sudden load increase (from 0 to 30% of I max ), measured by authors. Authors have developed photovoltaic and fuel cell hybrid system. The system s block diagram is shown in Fig. 6. There are photovoltaic panels and PEM fuel cell as power sources. The fuel cell is fuelled by hydrogen. A heart of the control system is a special microprocessor control unit. The unit controls power electronic converters transferring power from sources to load. The unit allows maximizing of the usage of renewable uncontrolled source. Then sources will able to supply load together. So, even if photovoltaic panels are not able to cover power demand, they could be used. The lack of power is filled up with fuel cell power. The most important advantage of the proposed system is maximizing solar panels working time and minimizing fuel demand. Control system can keep output power fixed and Sun irradiation independent. V. EXPERIMENT RESULTS Authors have prepared some simulations and physical model to confirm advantages of proposed hybrid power plant. First, mathematical model using Simulink was prepared. It contained PV array model (using some concepts from [5]-[7]), simple PEMFC model and power electronics converters and control unit models. The model allowed to recognize problems with sources cooperation and to match parameters with power converters regulators. Solar irradiation and ambient temperature were given as input parameters. Load power and current and sources currents were output. An example of simulation results is shown in Fig. 7. It could be observed that output current (I total ) was fixed and solar irradiation independent. Solar current has changed due to irradiation. Hence, fuel cell current has fallen when solar current has risen and inverse it has risen when solar current has fallen. Fig. 5. Nexa s time response for sudden load increase Fig. 7. Simulation results - currents flow in the hybrid system Fig. 6. Authors hybrid solar-fuel cell power plant block diagram Very good simulation results have encouraged authors to prepare small physical model of power plant. One GPV 110ME solar module and Heliocentris NP50 fuel cell were used. Authors have performed power converters and control unit. The plant capacity was about 20 W. View of the plant is shown in Fig. 8 and Fig 9. Some experiments were done. Currents were measured

4 during sunset and sunrise periods (Fig. 10 and Fig. 11). Simulation results have been confirmed in practice. So, because solar panels do not need fuel, using both sources permits to maximise refuelling period. 0.40 A B C D 0.35 0.30 current [A] 0.25 0.20 0.15 solar current fuel cell current load current 0.10 0.05 18:10:05 18:17:17 18:24:29 18:31:41 18:38:53 18:46:05 18:53:17 19:00:29 0.00 Fig. 10. Currents in the plant sunset, 9 th September 2003 Fig. 8. GPV 110ME solar panel used in experimental power plant Current [A] 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 06:14:24 06:28:48 06:43:12 load current fuel cell current solar current 06:57:36 07:12:00 07:26:24 Fig. 11. Currents in the plant sunrise, 22 nd September 2003 07:40:48 07:55:12 In comparison to solar power plant energy production, described hybrid installation will give power as it is shown in Fig. 12. 1,8 1,6 1,4 fuel cell sun P/Po 1,2 1 0,8 0,6 Fig. 9. NP 50 PEM fuel cell and power converters VI. PROS AND CONS OF NEW HYBRID POWER PLANT The main reason to build described system was to supply stand alone telecom system using renewable energy sources. So, the power plant has to produce energy independently from any external (weather) fluctuations. That could be obtained by using two sources: weather dependent solar panels and weather independent fuel cell. Such installation can give energy all time and do not produce any pollutants. On the other hand, problem with the fuel cell is limited hydrogen tank capacity. 0,4 0,2 0 January February March April May June July August September Fig. 12. Hybrid solar and fuel cell power plant production In addition, cost of produced energy has decreased despite equipment costs increase. It was possible because total number of PV working hours in year was almost doubled using hybrid system. October November December

5 Now the question is how much we can earn having such hybrid system. At first sight hybrid system seems much more expensive. Authors have discussed this problem in [2], [8]. The most important thing is to determine total PV working time in a year with load power. It could be done using meteorological data and some statistical information about weather in place where the plant is installed. Authors have performed such calculation using PVSIM simulation tool [7]. Two variants were analyzed. Variant 1 is a power plant without special control unit and variant 2 with such system. Sources cannot work together in variant 1. So load can be powered only by sun power or fuel cell power. Control unit allows sources to supply load together in variant 2 (Fig. 10 and Fig. 11). In variant 1 it was obtained working time about 1200 hours per year with load power out of PVSIM simulation for location in Warsaw (Poland). The working time was only taken into consideration when PV array has produced more power then load needs. The plant construction did not allow to use sun power when produced power level is lower in variant 1. Then, next total power cost per 1 kwh was calculated using UNIPEDE method. The cost at level 5.5 /kwh was received. In variant 2 it was possible to supply load by sun power even if production was smaller then needs. Then it was obtained working time at level 2200 hours per year with load power. The cost approximately 4.8 /kwh was received. So, cost reduction at level 10% was reached. VII. CONCLUSIONS Very good simulation and experimental results have given authors possibility to say that proposed hybrid power plant has worked as it was planned. So, the main conclusion is that it is possible to build hybrid solar and fuel cell power plant which allows optimal utilization of renewable uncontrolled primary energy carrier. It is possible with using of dynamic controlled back up source, i.e. PEM fuel cell. VIII. REFERENCES [1] Z. Barisic, Challenge to the Development of PEM Fuel Cell Systems for Stationary Power, The Fuel Cell World, Lucerne, Switzerland, 1-5 July 2002. [2] P. Biczel, Optimal usage of primary energy carriers on example of hybrid solar and fuel cell power plant, Ph.D. dissertation, Electrical Engineering Faculty, Warsaw University of Technology, Warsaw, Poland, 2003. [3] K. Bonhoff, The NEXATM 1200 Watt Compact Power Supply, The Fuel Cell World, Lucerne, Switzerland, 1-5 July 2002. [4] A. Dmowski, P. Biczel, B. Kras, Stand-alone telecom power system supplied by PEM fuel cell and renewable sources, International Fuel Cell Workshop 2001, Kofu, Japan, 12-13 November 2001. [5] A. D. Hansen, P. Sørensen, L.H. Hansen, H. Binder, Models for a Stand-Alone PV System, Risø National Laboratory., Roskilde, December 2000. [6] A. Hoque, K. A. Wahid, New Mathematical Model of a Photovoltaic Generator (PVG), Journal of Electrical Engineering, The Institute of Engineers, Bangladesh, vol. EE 28, No. 1, June 2000. [7] D. L. King, J. K. Dudley, W. E. Boyson, PVSIM: A. Simulation Program for Photovoltaic Cells, Modules, and Arrays, 25th IEEE PVSC, Washington DC, May 13-17, 1996. [8] J. Paska, P. Biczel, Hybrid photovoltaic power plant with fuel cell as an example of optimal utilization of primary energy sources in distributed power industry (in Polish), Elektroenergetyka Technika, Ekonomia, Organizacja, Nr 4, 2003. IX. BIOGRAPHIES Józef Paska (D.Sc., Ph.D., MEE) was born in Poland. He received the M.Sc. degree in electrical engineering from Warsaw University of Technology in 1974, Ph.D. in 1982 and D.Sc. (habilitation) in 2002 (both in electric power engineering). Presently, he is Associate Professor at Institute of Electric Power Eng. and Vice-Dean of the Electrical Engineering Faculty. Member of Committee of Power Engineering Problems of the Polish Academy of Sciences and editorial boards of newspapers Electrician and Energy Market. His areas of interest include power system reliability, electricity generation technologies, power engineering economics, distributed generation, renewable energy. Author of over 140 papers and 3 academic textbooks on power system reliability, electricity generation, renewable energy sources. He is a member of the Polish Society of Theoretical and Applied Electrical Engineering, the Polish Nuclear Society and World Scientific and Engineering Academy and Society. Piotr Biczel (Ph.D., M.Sc.) was born in Poland. He received the M.Sc. degree in automation and robotics from Warsaw University of Technology in 1999 and Ph.D. in 2003 (in electric power engineering). Presently, he is Assistant Professor at Institute of Electric Power Eng. of the Warsaw University of Technology. His areas of interest include power electronics, distributed generation, renewable energy. Author of over 20 papers on power electronics, renewable energy sources and distributed generation.