EXPERIENCE WITH HYBRID POWER GENERATING SYSTEMS Józef PASKA, Piotr BICZEL, Mariusz KŁOS Warsaw University of Technology Institute of Electrical Power Engineering, Warsaw (Poland) Abstract - Nowadays, in many countries the increase of generating capacity takes place in small units of so-called distributed power industry (distributed generation), and among them in hybrid power (generating) systems (HPS). In the paper are presented: different concepts of hybrid power generating systems, the experiences from exploitation of hybrid solar-wind 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. Because of the system will operate rather far from service centers it has to work as long as it is possible without refueling. 1. INTRODUCTION Currently we can observe very fast development of new electrical power sources called renewable sources. These sources are environmentally friendly and use primary energy carriers like solar, wind and water flow, biogas, biomass etc. The sources mentioned above can be splitted 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 hand, 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 available sun or wind power. There 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. 2. HYBRID SOLAR-WIND POWER PLANT FOR TELECOMMUNICATION 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. 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.
Fig. 1. Viev of solar panels and wind turbine of the hybrid power plant Fig. 2. Block diagram of the hybrid solar-wind power plant Fig. 3. An example of daily 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 June Fig. 4. The power produced by the solar-wind power plant during a year 3. 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: 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 phenomena are much smaller then in classical 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 availability versus time curve. In fact, the changes are extremely rapid (Fig. 3). In case when 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 capacity in power sources like wind turbines. German experiences show that more then 10% of 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 July August September October November December 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 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 simultaneously from both sources 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 sources connection. In consequence only one from 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] for this purpose, especially fuelled by hydrogen [8]. 4. 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 (Fig. 5).
Fig. 5. General view of NEXA TM 1,2 kw PEM fuel cell As it was told above, the additional source has to be very fast. PEM fuel cell meets this criterion. Fig. 6 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. 7, and the equivalent electrical circuit in Fig. 8. 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. Fig. 6. Nexa s time response for sudden load increase Fig. 7. Authors hybrid solar-fuel cell power plant block diagram Fig. 8. Equivalent electrical circuit of the plant 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. 5. EXPERIMENTS 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. 9. 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. 11. NP 50 PEM fuel cell and power converters used in experiments Fig. 9. Simulation results - currents flow in the hybrid system 0.40 0.35 0.30 A B C D 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. 12. Currents in the plant sunset, 9 th September 2003 Current [A] 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 load current fuel cell current solar current 0.00 06:14:24 06:28:48 06:43:12 06:57:36 07:12:00 07:26:24 07:40:48 07:55:12 Fig. 10. GPV 110ME solar panel used in experimental power plant Then the expermmental plant was created (Fig. 10 and Fig. 11) and some experiments were done (Fig. 12 and Fig. 13). Fig. 13. Currents in the plant sunrise, 22 nd September 2003 6. 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. So, because solar panels do not need fuel, using both sources permits to maximise refuelling period. In comparison to solar power plant energy production, described hybrid installation will give power as it is shown in Fig. 14. P/Po 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 January February March April May June July August September fuel cell sun Fig. 14. 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. 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. 12 and Fig. 13). October November December 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. 7. INTEGRATING OF DG SOURCES USING DC MICROGRID Strategic aims of EU clearly describe the role of distributed generation (DG) and renewable energy resources (RES) in electricity generation. Experts are unanimous that DG participation in generation will be raising also due to economical reasons. Unfortunetly it will cause necessity of some changes in power system. Current centralized solutions have to be transformed. The transformation is forced by another EU aim power quality raising. From the other hand, another EU main goal is sustainable development. In case of power it means increasing of efficiency, RES utilization and local energy balancing. Local balancing means that almost all energy consumed in separated area needs to be produced within this area using local primary carriers. Such solution allows to minimize power loses connected with power transmission and power distribution and encorages investment in local small production units. Goals and development directions described above require transforamtion of current distribution networks of medium and low voltage. It is necessary to introduce possibility of local control of power flows. Protection systems need to work with bidirectional energy flows. There is no data communication subsystem necessary to control all components in current distribution network. In consequence the idea of microgrids was introduced. Microgrid means small, balanced
power subsystem which connect distributed power stations and consumers located on not big area. The principle of mcrogrid operation is balancing of production with consumption. In case of stand-alone microgrid system has to be balanced all time. In case of subsystem connected to power system has to be constructed in the way the power interchange should be plannned like in case of big power plants. Unfortunetly, a disadvantage of microgrids is problem of voltage and frequency control (reactive and active power flow). It is clear that there will also problems with voltage distortion due to wide utilization of power electronic converters. Authors propose to use DC microgrids as a solution which avoids most of problems desribed above. The solution allows to keep low costs in many cases and makes easier some issues connected with control of quality parameters. Problem of quality in DC systems is reduced to keep voltage or current in required range. Fig. 15. DC microgrid concept An idea, which is a base of DC microgrid conception, is realization of postulate of local balancing of production with consumption like in case of AC microgrid. In addition high quality parameters will be kept. Thanks to introduction of DC there will not be such problems like frequency, shape, reactive power control. Block diagram of the system proposed by authors is shown in Fig. 15. DC microgrid principle of operation is based on convertion of all kind of produced power into DC. In case of generators with variable frequency the convertion is always applied. Connection with power grid is also realized by retifier and inverter. All power balancing and control functions are performed in DC circuit. Consumers are connected by inverters. If connection needs to be bidirectional it has to be done by bidirectional converter or parallel connection of rectifier and inverter. Problem of power quality in the microgrid is reduced to voltage level control. It is obvious consumers need to be joined to the AC 50 Hz sources. So, there are inverters which produce 50 Hz sinusoidal voltage. Due to the power control is executed in DC circuit, if the DC voltage are kept higher then minimum threshold value, the output AC voltage will meet all requirements. It is particulary interesting that voltage in DC circuit does not have to be kept with very high precision. Modern inverters can keep stable sinusoidal waveform with relatively wide changes of DC input voltage.
Described solution can have a few variant depending on number of inverters and their power and localization. The border solutions are the one inveter supplying all consumers on one side and individual inverters for each consumer on the second side. The second possibility allows for instance to measure energy in DC circuit. It could be interesting approach in the light of current trade problems in Poland. The critical problem is data communication subsysem and proper control strategy. The strategy will have direct impact on power quality, power sources utilization, balancing level etc. In consequence it will have crucial influence on energy costs in the microgrid. 8. 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-fuel cell power plant which allows optimal utilization of renewable uncontrolled primary energy. It is possible with using of dynamic controlled back up source, i.e. PEM fuel cell. The important problem is integration of distributed generation and renewable energy sources into existing power networks, especially local power networks. Authors propose, as their own contribution, DC microgrid. The DC microgrid allows: avoid many difficulties with control of energy parameters, simplification of control strategy and control units, costs reduction, transmission loses reduction, developing of new method of energy measurement, introducing load active control methods. Technical realization of the DC microgrid is possible and quite simple at present time with relatively low costs. It is the result of sudden development of power electronic converters and data communication equipment and significant their costs reduction. 9. REFERENCES 1. Barisic Z.: Challenge to the Development of PEM Fuel Cell Systems for Stationary Power. The Fuel Cell World, Lucerne, Switzerland, 1-5 July 2002. 2. Biczel P.: 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. Bonhoff K.: The NEXA TM 1200 Watt Compact Power Supply. The Fuel Cell World, Lucerne, Switzerland, 1-5 July 2002. 4. Dmowski A., Biczel P., Kras B.: 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. Hansen A.D., Sørensen P., Hansen L.H., Binder H.: Models for a Stand-Alone PV System. Risø National Laboratory, Roskilde, December 2000. 6. Hoque A., Wahid K.A.: 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. King D.L., Dudley J.K., Boyson W.E.: PVSIM: A Simulation Program for Photovoltaic Cells, Modules, and Arrays. 25 th IEEE PVSC, Washington DC, May 13-17, 1996. 8. Paska J., Biczel P.: 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. 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 electrical power engineering). Presently, he is Associate Professor at Institute of Electrical 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 periodicals Electrician, Energy Market and International
Journal of Emerging Electric Power Systems. His areas of interest include power system reliability, electricity generation technologies, power engineering economics, distributed generation, renewable energy. Author of over 150 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. Mailing address: Józef Paska Warsaw University of Technology Instytut Elektroenergetyki Ul. Koszykowa 75, 00-662 Warszawa, POLAND Tel./Fax.: (48/22)6218646 E-mail: Jozef.Paska@ien.pw.edu.pl 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 electrical power engineering). Presently, he is Assistant Professor at Institute of Electrical 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. Mailing address: Piotr Biczel Warsaw University of Technology Instytut Elektroenergetyki Ul. Koszykowa 75, 00-662 Warszawa, POLAND Tel./Fax.: (48/22)6218646 E-mail: biczelp@ee.pw.edu.pl Mariusz Kłos (M.Sc.) was born in Poland. He received the M.Sc. degree in electrical engineering from Warsaw University of Technology in 2001. Presently, he is Ph.D. student at Institute of Electrical Power Eng. of the Warsaw University of Technology. His areas of interest include power electronics, distributed generation, renewable energy, energy storage systems. Mailing address: Mariusz Kłos Warsaw University of Technology Instytut Elektroenergetyki Ul. Koszykowa 75, 00-662 Warszawa, POLAND Tel./Fax.: (48/22)6218646 E-mail: mariusz_klos@wp.pl