SUMMARY OF PHD THESIS

Investeşte în oameni! FONDUL SOCIAL EUROPEAN Programul OperaŃional Sectorial Dezvoltarea Resurselor Umane 007 013 Axa prioritară: 1 EducaŃia şi formarea profesională în sprijinul creşterii economice şi dezvoltării societăńii bazate pe cunoaştere Domeniul major de intervenńie: 1.5 Programe doctorale si postdoctorale în sprijinul cercetării Titlul proiectului: Proiect de dezvoltare a studiilor de doctorat în tehnologii avansate- PRODOC Cod Contract: POSDRU 6/1.5/S/5 Beneficiar: Universitatea Tehnică din Cluj-Napoca FACULTY OF ELECTRICAL ENGINEERING Eng. Călin-Octavian GECAN SUMMARY OF PHD THESIS DC POWER SUPPLY FOR ELECTRICAL RECEIVERS AND CONSUMERS PhD. Advisor Prof.PhD.eng. Mircea CHINDRIŞ Evaluation commission of PhD thesis: President: Members: - Prof.PhD.eng. Radu CIUPA - decan, Facultatea de Inginerie Electrică,Universitatea Tehnică din Cluj-Napoca; - Prof.PhD.eng. Mircea CHINDRIŞ - conducător ştiinńific, Universitatea Tehnică din Cluj-Napoca; - Prof.PhD.eng. Nicolae GOLOVANOV - referent, Universitatea Politehnică din Bucureşti; - Prof.PhD.eng. Dorin SARCHIZ referent, Universitatea Petru Maior din Târgu Mureş; - As.Prof.Physician Eng. Andrei CZIKER - referent, Universitatea Tehnică din Cluj-Napoca

This thesis is divided into seven chapters, one of which (the seventh chapter) is devoted to conclusions and personal contributions. References (books, manuals, magazine articles, contracts, theses, standards and catalogs) are located at the end of the thesis. Also a section is reserved to 1 annexes. In the first chapter the Photovoltaic (PV) Modules and Fuel Cell (FC) Stacks are analyzed as possible electrical energy supplying sources for a DC system. Particular emphasis was given to the PV and FC cells modeling and how different parameters affect their performance. In the case of the PV cells and modules a new model was proposed: where: 3 SP G T I = N P J L(G0 ) s cell c1 p 0 0 cell G 0 T 0 V q N E (V) = N P N S E = I SP PV system current, A; V SP PV system voltage, V; 3 ( 1) ( T) N J (T ) s E 1(T) E ( T) e e qe g E 1(T) = k 1 SP S V c SP c (T) = 1+ α ( T) R c (T) = 1 β 1 T SH 0 kt 1 T + I c (T) + I SP ( T T0) ( T T ) 0 SP R N S P RS J 0 Diode saturation current density, A/cm ; J L Photon current density, A/cm ; G Nominal solar radiation, W/m ; G 0 Reference solar radiation, W/m ; s cell Cell area, [cm ]; T Nominal temperature, K; T 0 Reference temperature, K; α β E g Current temperature coefficient, 1/K; Voltage temperature coefficient, 1/K; Bang gap energy of the semiconductor, ev; R S Series resistance, Ω; R SH Shunt resistance, Ω; q Electron charge, 1.6x10-19 C; k N P N S Boltzmann constant, 1.38x10-3 J/K. Number of cells in parallel; Number of cells in series. E 3, (1.1) Most times the electrical output of photovoltaic cells is determined in reference environmental conditions: temperature, solar radiation. The proposed model has the advantage of allowing the determination of PV cell electrical characteristics in terms of nominal, real 1

operation conditions, based on information provided by manufacturers in reference environmental conditions: G = 1000W/m, 5ºC. The proposed PV system model was validated experimentally. In this purpose, measurements were made on an experimental PV module. The PV module is part of an experimental bench whose scheme is shown in Figure 1.1. Figure 1.1. Experimental bench and scheme Two sets of measurements were realized using an adjustable resistance as a load. The first set of measurements are characterized by a PV module average temperature of 5.8 C and a solar radiation intensity of 13 W/m and the second set of measurement by an average temperature of 4.5º C and a solar radiation of 4 W/m. Figure 1. shows the isothermal images of the PV module during the measurements. The I V and P V characteristics determined experimentally and using the proposed model are presented in Figure 1.3. a. first set of measurements b. second set of measurements Figure 1.. Isothermal images of the PV module Comparing the characteristics it can be seen that the proposed model describes the operation of the PV module. In respect of maximum power point, the errors are up to.5%. Using the proposed model for a PV system a software application was developed in C # programming environment. This software enables the user to attain I - V characteristic of a PV system in real operating conditions. The software interface is shown in Figure 1.4. The developed software can be used in the general case of a PV system but can also can be used for PV module or a single PV cell through a proper choice of cells numbers connected in series and parallel. The user enters data on the PV cell, the reference and real conditions of operation and the application automatically generates the I - V characteristic for real operating conditions.

a. first set of measurements b. second set of measurements Figure 1.3. The I V and P V characteristics of the PV module Figure 1.4. Software graphical interface Fuel Cells are treated in the second part of this first chapter. In addition to the operating operational issues a model that describes the operation of proton exchange membrane fuel cell is presented (1.). where: V bcc E V c =N act =V bcc 0 V ( E -V -V -V ) + ohm dif c RT F act P H ln P cc =me cc nicc ohm HOc RT Icc+I = ln accf I0cc V =I R P O ncc dif (1.) 3

V bcc FC generated voltage, V; N bcc Number of fuel cells; E c Thermodynamic potential of the cell, V; V act Activation overvoltage, V; V ohm Ohmic overvoltage, V; V dif Concentration overvoltage, V; V 0 Open circuit voltage, V; R Universal gas constant, 8.314 J/molK; F Faraday s constant, 96.485 C/mol; P H P O P H0c a cc Partial pressure of hydrogen inside cell anode, Pa; Partial pressure of oxygen inside cell cathode, Pa; Partial pressure of water vapor inside cell cathode, Pa; Constant associated with cell activation losses; I cc FC current density, A/cm ; I ncc FC internal current density, A/cm ; I 0cc FC exchange current density, A/cm. R cc Area specific resistance, kωcm ; m Constant,,11x10-5 V; n Constant, 8x10-3. The second chapter of the thesis, titled Equipment in a DC voltage system, establish the conditions in which the existing equipment can be used in DC voltage systems and summarizes the main functional characteristics of converters (rectifiers, inverters, DC converters) emphasis the products size range (power, voltage, efficiency). Different cable reconfiguration possibilities that enable cable use in a DC voltage system are presented. The use of the electrical circuit protection devices realized for AC voltage in a DC voltage system is also investigated and certain conditions and corrections are provided. The electrochemical and electrical energy storage techniques were summarized, focusing on presenting the electrical characteristics (specific energy, energy density, specific power, efficiency, self discharge rate, loading/unloading cycles, rated voltage) of conventional batteries, widely used today, and of the new technology represented by redox batteries. The lack of guidance and design standards realized specifically for DC systems was solved in the 3 rd Chapter of the thesis. A design methodology similar with the one existing for the low AC voltage system was developed. Based on this methodology a set of diagrams that enable the user to achieve a quick selection of the DC circuit elements (Annex - Annex 7) was realized. Figure 3.1 presents one of the diagrams, namely the conductor/cable cross section selection. This diagram enable the user to select the Copper/Aluminum conductor cross section based on the calculated power, on the DC voltage level, k cable insulation/location correction coefficient, on the DA intermittent/permanent regime coefficient. A software application that enables accurate sizing and selection of equipment needed in a DC circuit was developed in C#. The graphical interface of this application is shown in Figure 3.. The software interface contains a series of panels in which the user enters the data needed to size the dc circuit: power supply type, circuit characteristics, receptor related information, conductor/cable features, the way the circuit overload and short circuit protection is realized. In 4

the last case if a circuit breaker is chosen then the application automatically generates a new field where the k ajust dc circuit adjustment factor has to be declared. Figure 3.1. Cable/conductor cross section selection diagram Figure 3.. Graphical interface of the DC voltage design software aplication 5

The "Calculate" button generates in the panel located at the bottom of the application the selection results: the source characteristics, the functional characteristics of the power electronic (if necessary), protection devices nominal characteristics and the conductor/cable features. For the equipment selection the application links to database made in Microsoft Access. Chapters 4, as the name implies, deals with the problem of the DC voltage systems in residential buildings. The operation of the main load types (resistive, inductive and electronic equipment) was analyzed and, for the loads that can be supplied in DC voltage, the minimum operating voltage level was determined. Also, the loads that need AC voltage to operate were identified. The optimal DC voltage level was determined in two cases: new DC voltage network: in this case the ratio between the price needed for the DC circuit and the price needed for the AC circuit was calculated and presented in graphs for different voltage levels; existing AC voltage network reconfigured in DC: a office building electrical network was considered in a study case and the voltage and power losses were calculated; also the calculated current was determined, all for different voltage levels. Following the analysis conducted several conclusions could be established. The main conclusion is that the only viable solution is represented by the 30 V and 30 V, levels that satisfy both economic and the technical conditions (voltage drop, power losses and cable thermal limit). The DC distribution system feasibility was analyzed through five proposed solutions for the reconfiguration of the AC network into a DC network. As a result the proposed solutions enable significant energy savings compared with the AC network. In Chapter 5, entitled The Energy Supply of Customers Using DC Networks, the used conventional (LEA 0,4 kv) and unconventional (LEA 1 kv) costumers supply solutions are presented together with 4 proposed solutions that implies the use of DC voltage. Calculation assumptions and equations, DC voltage level (U max = 1500 V DC in unipolar system and U max = ± 750 V in the bipolar system) and the conditions in which existing cables can be used in DC are presented. More than that, the existing and proposed solutions were analyzed in a case study: the energy supply of 100 costumers of an isolated village. Considering a given power, cable length has been computed using a graphical method: the cable power capability as a function on distance. Several cables power capability was determined based on two constraints: cable thermal limit and allowable voltage drop. Higher lengths were obtained for the cable used in a DC voltage system. All solutions, existing and proposed, were analyzed financially and as a conclusion the DC systems involves higher investments. The behavior in a permanent regime of a DC unipolar system supplied from AC mains and supplying two loads (one AC and one DC), both having a power variation, was analyzed using Matlab/Simulink modeling environment. Chapter 6, called DC voltage microgrids presents the energy management algorithm of a PV and FC Hybrid DC microgrid (Figure 6.1.). A management system flowchart is presented in the thesis and on its basis a virtual instrument was developed in Matlab/Symulink. The virtual instrument interface and the results obtained in case of an office building is presented in Figure 6.. The load profile, the hourly variation of solar radiation and temperature are introduced in the application through three.xls files. The Load Data (.xls file)', 'Solar Insulation (.xls file)' and 'Temperature (.xls file)' buttons open windows that allow the user to select the appropriate.xls files. The PV module and FC Stack model were implemented in the proposed virtual instrument. 6

Figure 6.1. DC microgrid structure Figure 6.. Virtual instrument interface This instrument enables the user to enter the considered input data for PV system, FC stack, storage system capacity and converters nominal powers. Based on the load profile, hourly variation of solar radiation and temperature, the virtual instrument determines the PV system and FC stack produced power/energy, the storage system discharge and charged power/energy, the public network injected and absorbed power/energy, the load necessary energy and the power losses of converters. 7

Conclusions and personal conclusions (selection) The use of DC voltage systems for the electrical energy supply of proper loads and consumers represents a solution which implies higher investments costs but, at the same time, can offer some advantages comparing with the conventional solutions widely used today: - higher overall conversions efficiency; - smaller power losses; - smaller conductor cross sections; - higher cable power capabilities and lengths; - smaller voltage drops. A selection of the author s main contributions is presented below: - systematic approach of the DC distribution possibilities for buildings and in supplying consumers; - an improved PV system model, which describes cells/models performance in real operating conditions; - the experimentally validation of the proposed model; - the development of a general software which determines the I V characteristic of a PV cell, module or system; - the implementation of a diagram set that enable a quick predetermination of the DC circuit elements; - the development in C# programming environment of a software application that makes a precise calculation, and selects the DC circuit equipment from a Microsoft Access database; - optimal operating DC voltage level selection analyzing the financial and technical (calculated current, voltage drop and power losses) aspects; - proposed DC voltage supplying solutions for residential buildings; - proposed DC voltage supplying solutions for consumers; - the selection of the energy supply optimal solution analyzing the following technical and financial criteria: overall efficiency, cable lengths, cable thermal limit, power capability, optimal cross section, voltage and power losses; - the study of the DC voltage network behavior in Matlab/Symulink; - the accomplishment of a energy management system for a hybrid PV/FC DC microgrid; - the implementation of the proposed PV model in the energy management system; - the development of a virtual instrument dedicated to the DC microgrid energy management. The results obtained at some points in research process were presented at national and international conferences and in journals, in published paper. Out of these papers, a selection is presented bellow: Gecan C.O. and Pop F., Electricity distribution systems inside the consumer - case study, in proceedings of the 6 th International Energy Efficiency Symposium, Cluj-Napoca, pp.115-10, 008, ISBN 978-973-1758-41-1 (in romanian); Gecan C.O., Chindriş M., and Bindiu R., DC Voltage Lighting Systems, in proceedings of The 5th International Conference ILUMINAT 009, Cluj Napoca, Romania, pp.10-1 10-6, ISBN 978-973-713-3-1 and in Lighting Engineering 009; 11,1: 7-34, granted with the Award of the Ph.D. Student Best Paper; Gecan C.O., Chindriş M., and Pop G.V., Aspects regarding DC Distribution Systems, in proceedings of CIE 009, Oradea, pp.6-31, ISSN 14-161; Gecan C.O., Chindriş M., and Pop G.V., Design of Low Voltage DC Networks, in proceedings of 7 th International Conference SIELMEN 009, Iaşi, pp.77-8, Vol I, ISBN 978-606-50-618-; 8

Gecan C.O., Chindriş M, and Bindiu R., Power Capability in Low Voltage DC Distribution Systems, in proceedings of The 4th edition of the Interdisciplinary in Engineering International Conference INTER-ENG 009, Târgu Mures, pp.15-0, ISSN 1843-780X; Gecan C.O., Chindriş M., Bindiu R. and Pop G.V., A Generalized Photovoltaic Model for Real Operating Conditions, in proceedings of The 3th edition of the International Conference on Modern Power Systems MPS010, Acta Electrotehnica, Cluj-Napoca, pp.14-147, ISSN 1841-333; Gecan C.O., Chindriş M, Bindiu R., Pop G.V., Vasiliu R., and Gheorghe D., PEM Fuel Cells Modeling and their use in a DC Distribution System, Journal of Sustainable Energy, Vol. 1, No. 1, 010, I.S.S.N. 14 161; Gecan C.O., Pop G.V., Bindiu R., The Energy Supply of Rural Customers Using DC Networks, în Revista Energetica nr. 6/010, volum 58, pp.75-81, ISSN:1453 360 (in romanian); Gecan C.O. and Chindriş M, DC systems - opportunities for integration in Smart Grids, in the Electronic Edition of the Smart Grids Conference, Sibiu, 010, ISBN 978-973-0-09194- 6 (in romanian); Gecan C.O., Chindriş M., Bindiu R. and Pop G.V., The use of DC voltage systems for the electrical energy distribution in residential buildings, in proceedings of the 7 th International Energy Efficiency Symposium, Cluj-Napoca, pp.119-16, 010, ISBN 978-973-133-8-4 (in romanian); C.O., Gecan, M., Chindriş, R. Bindiu, G.V. Pop, Energy Management of a Hybrid PV/Fuel Cell DC Microgrid, in proceedings of The 4th edition of the International Conference on Modern Power Systems MPS011, Acta Electrotehnica, 17-0 May 011, Cluj-Napoca, Romania, pp.170-177, ISSN 1841-333; C.O., Gecan, M., Chindriş, R. Bindiu, G.V. Pop, Design of Low Voltage DC System Using Calculation Diagrams, in Proceedings of the Internal PRODOC Program Conference, 4-5 Iunie 011, Technical University of Cluj-Napoca, 011. 9