POWER ELECTRONICS AND PHOTOVOLTAIC POWER SYSTEM LABORATORY http ece colorado edu ecen4517 ECEN 4517 ECEN 5517 Photovoltaic power systems Power conversion and control electronics Prerequisite ECEN 4797 or ECEN 5797 Instructor Prof Bob Erickson TAs Mingyang Wang Tuesday and Wednesday and Mark Norris Thursday DC loads PV Panel 85 W Charge control DC DC converter for maximum power point tracking and battery charge profile Battery Deepdischarge lead acid 12 V 56 A hr Inverter AC loads 120 V 60 Hz 300 W true sinewave Digital control Power Electronics Laboratory 1 Lecture 1 The solar power business Approximate cost of solar power today Capital cost of cells 4 Wpk Capital cost of grid connected system 8 Wpk Cost of energy 20 30 kWh New installations 1460 MW in 2005 Significant growth underway Future growth in thin film technologies expected to lead to substantial reduction in capital cost and growth in industry size DOE NREL cost goal of 50 m2 or less than 0 50 Wpk Need a similar reduction in cost of balance of system Power conversion electronics Interconnections building integration installation protection Current cost of balance of system is approx 4 Wpk 1 Photovoltaic Cell Technologies Wafer based silicon Single crystalline Most of production to date High efficiency but high cost Cut from ingot with saw Needs 20 kg Si per 1 kWpk Multi crystalline Cut from cast polysilicon ingots Cheaper than single crystalline a few percent less efficient Both are normally p n devices 2 Photovoltaic Cell Technologies Thin film Amorphous silicon p i n devices active layer is i Degradation issues low deposition rates CIS Copper Indium diSelenide Cu In Se2 CIGS Copper Indium GalliumdiSelenide CdTe Cadmium Telluride The three above are p type materials A p n diode is constructed with a thin n layer such as CdS Gallium allows tweaking of bandgap to optimize efficiency Cost of Indium Toxicity of Cadmium Thin layers 2 m lead to low cost Deposition on flexible substrate such as stainless steel or polymer sheets 3 Mass production of rolls of thin film PV material Shah Torres Tscharmer Wyrsch and Kepner Photovoltaic Technology The Case for Thin Film Solar Cells Science vol 285 pp 692 698 July 30 1999 Evolution of record PV cell efficiency for various technologies CIGS record efficiency now at 19 6 Major producers pursuing this approach now include Sharp a Si Mitsubishi a Si Schott solar a Si Shell solar CIS Honda solar CIGS PESWiki com lists 15 other manufacturers of thin film PV now shipping 4 Variations in Solar Irradiance What the pyranometer sees total irradiance Cells that are not in direct sun can still experience substantial irradiance from clouds reflections diffusion T Stoffel 20 years of solar measurements the Solar Radiation Research Laboratory at NREL National Renewable Energy Laboratory SRRL 4 13 05 7 Plugging in numbers panel current panel voltage solar irradiance efficiency panel area If solar irradiance insolation 1 kW m2 Efficiency 10 Panel area 1 m x 1 m 1 m2 Voltage at peak power point 10 V Then current at peak power point is 1000 W m2 10 1 m2 10 V 10 A and Isc is a little greater than 10 A Panel output power at peak power point is 100 W Q how much power can you get out of this panel indoors using ambient lighting 8 Development of Electrical Model of the Photovoltaic Cell slide 1 Photogeneration Semiconductor material absorbs photons and converts into hole electron pairs if Photon energy h Egap Energy in excess of Egap is converted to heat Photo generated current I0 is proportional to number of absorbed photons satisfying photon Charge separation Electric field created by diode structure separates holes and electrons Open circuit voltage Voc depends on diode characteristic Voc Egap q Power Electronics Laboratory 6 Lecture 1 Development of Electrical Model of the Photovoltaic Cell slide 2 Current source I0 models photo generated current I0 is proportional to the solar irradiance also called the insolation I0 k solar irradiance Solar irradiance is measured in W m 2 Power Electronics Laboratory 7 Lecture 1 Development of Electrical Model of the Photovoltaic Cell slide 3 Diode models p n junction Diode i v characteristic follows classical exponential diode equation Id Idss e Vd 1 The diode current Id causes the terminal current Ipv to be less than or equal to the photo generated current I0 Power Electronics Laboratory 8 Lecture 1 Development of Electrical Model of the Photovoltaic Cell slide 4 Modeling nonidealities R1 defects and other leakage current mechanisms R2 contact resistance and other series resistances Power Electronics Laboratory 9 Lecture 1 Cell characteristic Cell output power is Ppv IpvVpv At the maximum power point MPP Vpv Vmp Ipv Imp At the short circuit point Ipv Isc I0 Ppv 0 At the open circuit point Vpv Voc Ppv 0 Power Electronics Laboratory 10 Lecture 1 Series String of PV Cells to increase voltage To increase the voltage cells are connected in series on panels and panels are connected in series into series strings All series connected elements conduct the same current Problems when cells irradiance is not uniform Power Electronics Laboratory 13 Lecture 1 Bypass Diodes Bypass diodes Limit the voltage drop across reversebiased cells or strings of cells Reduce the power consumption of reverse biased cells Power Electronics Laboratory 14 Lecture 1 Direct Energy Transfer Power Electronics Laboratory 11 Lecture 1 Maximum Power Point Tracking MPPT MPPT adjusts DC DC converter conversion ratio M D Vbatt Vpv such that the PV panel operates at its maximum power point The converter can step down the voltage and step up the current Battery is charged with the maximum power available from the PV panel Power Electronics Laboratory 12 Lecture 1 Deep Discharge Lead Acid Batteries Theory and modeling of batteries Don t overcharge this causes outgassing and can quickly ruin the battery Don t discharge below 50 SOC this reduces battery life 56 Ampere hour Battery state of charge SOC vs terminal voltage 100 SOC 12 80 volts or greater 75 SOC 12 55 volts 50 SOC 12 20 volts 25 SOC 11 75 volts 0 SOC 10 50 volts 16 Experiment 1 Direct Energy Transfer System Model PV panel Investigate direct energy transfer system behavior Investigate effects of shading Observe behavior of lead acid battery Power Electronics Laboratory 2 Lecture 1 Experiments 2 and 3 Maximum Power Point Tracking Design and construct dc dc converter Employ microcontroller to achieve maximum power point tracking MPPT and battery
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