Green Energy Course- Renewable Energy Systems Biên sọan: Nguyễn Hữu Phúc Khoa Điện- Điện Tử- Đại Học Bách Khoa TPHCM PV Systems How Fast is Solar PV Growing? The growth in total solar energy is slower (0.06 quad in 2001) versus 0.081 quad in 2007) partially due to solar thermal retirements PV Current-Voltage Variation with Insolation and Temperature Pat Chapman Solar Example • When Prof. Chapman built a new house in Urbana in 2007 he added some solar PV. • His system has 14 modules with 205 W each, for a total of 2870W. He has a 3300 W inverter.
• Total cost was about $27,000, but tax credits reduced it to $16,900. • He should be getting about 3700 kWh per year Source: www.htm ECE 333 Green Electric Energy Lecture 24 PV Systems Professor Tom Overbye Department of Electrical and Computer Engineering PV Systems – Three configurations Grid-connected systems PV Systems – Three configurations Stand-alone systems which charge batteries PV Systems – Three configurations Stand-alone systems with directly-connected loads Load I-V Curves • PV panels have I-V curves and so do loads • Use a combination of the two curves to tell where the system is actually operating • Operating point – the intersection point at which the PV and the load I-V curves are satisfied Resistive Load I-V Curve V IR 1 I V (9.1) R • Straight line with slope 1/R • As R increases, operating point moves to the right • Can use a potentiometer to plot the PV module’s IV curve • Resistance value that results in maximum power Vm Rm (9.5 Maximum power transfer • Maximum power point (MPP) should occur when the load resistance R = VR/IR under 1-sun 25˚C, AM 1.5 conditions • A MPP tracker maintains PV system’s highest efficiency as the amount of insolation changes DC Motor I-V Curve • DC motors have an I-V curve similar to a resistor V IRa k (9.3) • e = kω is back emf, Ra is armature resistance DC Motor I-V Curve Linear Current Booster (LCB) helps the motor be able to start in low sunlight Figure 9.9 Battery I-V Curves • Energy is stored in batteries for most off-grid applications • An ideal battery is a voltage source VB • A real battery has internal resistance Ri V VB Ri I (9.4) Battery I-V Curves • Charging– I-V line tilts right with a slope of 1/Ri, applied voltage must be greater than VB • Discharging battery- I-V line tilts to the left with slope 1/Ri, terminal voltage is less than VB Figure 9.12 Maximum Power Point Trackers • Maximum Power Point Trackers (MPPTs) are often a standard part of PV systems, especially grid-connected • Idea is to keep the operating point near the knee of the PV system’s I-V curve • Buck-boost converter – DC to DC converter, can either “buck” (lower) or “boost” (raise) the voltage • Varying the duty cycle of a buck-boost converter can be done such that the PV system will deliver the maximum power to the load MPPTs – Example 9.2 • A PV module has its maximum power point at Vm = 17 V and Im = 6A. • What duty cycle should its MPPT have if the module is delivering power to a 10Ω resistance? • Max power delivered by the PVs is 17V*6A = 102W VR 2 P VR 31.2 • The converter must boost the 17 V PV voltage to the desired 31.88 Vi 1 D 17 1 D • Solving gives D 0.65 Hourly I-V Curves • Current at any voltage is proportional to insolation • VOC drops as insolation decreases • Can just adjust the 1-sun I-V curve by shifting it up or down Grid-Connected Systems • Can have a combiner box and a single inverter or small inverters for each panel • Individual inverters make the system modular • Inverter sends AC power to utility service panel • Power conditioning unit (PCU) may include – MPPT – Ground-fault circuit interrupter (GFCI) – Circuitry to disconnect from grid if utility loses power – Battery bank to provide back-up power Components of Grid-Connected PV Individual Inverter Concept • Easily allow expansion • Connections to house distribution panel are simple • Less need for expensive DC cabling Interfacing with the Utility • Net metering – customer only pays for the amount of energy that the PV system is unable to supply • In the event of an outage, the PV system must quickly and automatically disconnect from the grid • A battery backup system can help provide power to the system’s owners during an outage • Good grid-connect inverters have http://www.php efficiencies above 90% DC and AC Rated Power • Estimate the AC output power under varying conditions Pac Pdc , STC (Conversion Efficiency) (9.10) • Pdc,STC - DC power of array from adding module ratings under standard test conditions (STC) (1-sun, AM 1.5, 25˚C) • Conversion efficiency – includes losses from inverter, dirty collectors, mismatched modules, and differences in ambient conditions • These losses can derate power output by 20-40%, even in full sun Losses from Mismatched Modules • Illustrates the impact of slight variations in module I-V curves • Only 330 W is possible instead of 360 W Losses due to Cell Temperature • As temperature increases, power decreases • PVUSA test conditions (PTC) – 1-sun insolation in plane of array, 20˚C ambient temperature, wind-speed of 1 m/s • Pac(PTC) AC output of an array under PTC test conditions is a better indicator of actual power delivered in full sun than the more commonly used Pdc(STC) • Describing a system based on Pdc(STC) without correcting for temperature and the inverter is misleading Impact of Temperature • VOC decreases by ~0.37% per ˚C for crystalline silicon cells • ISC increases by about 0.05% per ˚C • NOCT – Normal Operating Temperature Figure 8.36 NOCT 20C Tcell Tamb S (8.3 - PV Derating using PTC • A PV array has rating of 1 kW under standard test condtions (STC). Nominal operating temperature (NOCT) from Chapter 8 is 47˚C • DC power output drops by 0.5%/ ˚C above the STC temperature of 25˚C • Mismatched module loss= 3% • Dirt loss = 4% • Inverter efficiency = 90% • Estimate Pac(PTC), the AC output power under PVUSA test conditions (PTC) Ex.3 – “1 kW PV system” PTC Rated AC Power • The estimated cell temperature is NOCT 20C Tcell Tamb S (8.8 • With DC losses at 0.5%/ ˚C above 25˚C, Pdc ,( PTC ) 1 kW 1 0.856 kW • Including inefficiencies, estimated AC rated power at PTC is Pac ,( PTC ) 8.72 kW “Peak-Hours” Approach • 1-sun is 1 kW/m2 • We can say that 5.6 hours of “peak sun” • If we know Pac, computed for 1-sun, just multiply by hours of peak sun to get kWh • If we assume the average PV system efficiency over a day is the same as the efficiency at 1-sun, then Energy (kWh/day) Pac kW h/day of "peak sun" (9.14) Capacity Factor of PV Energy kWh/yr Pac kW CF 8760 h/yr (9.15) CF h/day of "peak sun" (9.28 PV Capacity Factors for US cities Stand-Alone PV Systems • When the grid isn’t nearby, the extra cost and complexity of a stand-alone power system can be worth the benefits • System may include batteries and a backup generator Stand-Alone PV - Considerations • PV System design begins with an estimate of the loads that need to be served by the PV system • Tradeoffs between more expensive, efficient appliances and size of PVs and battery system needed • Should you use more DC loads to avoid inverter inefficiencies or use more AC loads for convenience? • What fraction of the full load should the backup generator supply? • Power consumed while devices are off • Inrush current used to start major appliances Power Requirements of Typical Loads Table 9.10 – Power Requirements of some typical loads Note that these tables are useful for getting an idea of the average values, but the best data comes from actual measurements! Consumer Electronics as Loads • Consider the power when the device is actively used • Also consider the power consumed when device is in standby Table 9.10 – Power requirements of some consumer electronics Batteries and PV Systems • Batteries in PV systems provide storage, help meet surge current requirements, and provide a constant output voltage • Lead-acid batteries are still the most commonly-used batteries for PV systems • The lead-acid battery is an electrical storage device that uses a reversible chemical reaction to store energy.
• Lead-acid batteries date back to the 1860s http://img.com/photo/11244127/Lead_Acid_Batteries.jpg Basics of Lead-Acid Batteries Positive Plate: PbO2 + 4H + + SO42 + 2e PbSO4 2H 2 O (9.21) Negative Plate: PbO 2 + SO42 PbSO4 2e (9.22) Basics of Lead-Acid Batteries • During discharge, voltage drops and specific gravity drops • Sulfate adheres to the plates during discharge and comes back off when charging, but some of it becomes permanently attached Stand-Alone PV Systems – Design Summary • Analysis of load – Determine daily demands for power and energy – What fraction of the worst month “design month” should you cover with the PV system? How much should you cover with a backup generator? – What PV system voltage should you have? – Convert total DC load to amp hours @ system voltage • PV sizing – Pick a PV module based on insolation data for the site for the design month – Determine how many parallel strings of modules and how many modules in each string Stand-Alone PV Systems – Design Summary • Battery Sizing – How many days of storage needed? • Generator Sizing • System Costs http://www.au/How_a_Standalone_System_Works-28.htm ECE 333 •Lecture 25Green Electric Energy •PV Systems, Energy Storage •Prof. Tom Overbye •Department of Electrical and Computer Engineering Announcements • Homework 12 is 8. It should be done before the final but need not be turned in. • Reading: Chapters 8 and 9 • Final exam is on Friday December 11 from 8 to 11am.
Hall and Ceramics Building 218. – If your last name begins with “A” through “K” come to 106B8 Eng Hall; otherwise go to Ceramics Building 218. – Final is comprehensive, with more emphasis on solar (since it wasn’t on an earlier exam) – Same procedure except you can bring in one new notesheet and your two previous notesheets. In the News • On Monday US Environmental Protection Agency (EPA) said that greenhouse gases are a danger to public health and welfare • This is a necessary first step to allow the EPA to regulate greenhouse gas emissions – Some industries are concerned these regulations may be more restrictive than a legislative approach • Some in Congress have called on EPA to withdraw its proposal because of recent email releases that question the underlying science PV Systems – Four configurations 1.
Grid-connected systems PV Systems – Four Configurations 2. Stand-alone systems with directly-connected loads DC Motor I-V Curve • DC motors have an I-V curve similar to a resistor • e = kω is back emf, Ra is armature resistance V IRa k (9.3) PV Systems – Four Configurations 3. Stand-alone systems which charge batteries PV Systems – Four Configurations 47 4.