PV Energy Utilization

Transcription

1 EEL5285 & EEL 4930 All Sections (Spring 2017) PV Energy Utilization Professor Osama A. Mohammed Department of Electrical and Computer Engineering

2 Photovoltaics (PV) Photovoltaic definition- a material or device that is capable of converting the energy contained in photons of light into an electrical voltage and current "Sojourner" exploring Mars, Solar House Rooftop PV modules on a village health center in West Bengal, India

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5 Loads- Household Consumption Source: EIA 2008 Annual Energy Review

6 Example: Daily Variation for CA

7 Example: Weekly Variation

8 Example: Annual System Load MW Load Hour of Year

9 DEMAND (MW) Load Duration Curve A very common way of representing the annual load is to sort the one hour values, from highest to lowest. This representation is known as a load duration curve HRS Load duration curve tells how much generation is needed

10 Ball park Energy Costs Source:

11 Support for Renewable Energy The White House issued report about how the stimulus is going. Renewable energy projects were very much included. For example the largest solar PV installation in US, 579 MW Solo Star Site in California. Cost of residential solar is projected to decrease from $0.21 per kwh in 2009 to $0.10 in 2015 and $0.06 by 2030; wholesale parity is $

12 Largest Solar Plants in the US Solar Star (I and II) United States N W Topaz Solar Farm United States N W Desert Sunlight Solar Farm United States N W Copper Mountain Solar Facility United States N W

13 Efficiency and Home Energy Use Whole-house energy efficiency approach find out which parts consume the most energy

14 PV History Edmund Becquerel (1839) Adams and Day (1876) Albert Einstein (1904) Czochralski (1940s) Vanguard I satellite (1958) Today Cost/Capacity Analysis (Wp is peak Watt)

15 PV System Overview Solar cell is a diode Photopower converted to DC Shadows & defects convert generating areas to loads DC is converted to AC by an inverter Loads are unpredictable Storage helps match generation to load Shadows

16 Some General Issues in PV The device Efficiency, cost, manufacturability automation, testing Encapsulation Cost, weight, strength, yellowing, etc. Accelerated lifetime testing 30 year outdoor test is difficult Damp heat, light soak, etc. Inverter & system design Micro-inverters, blocking diodes, reliability

17 Current n-type p-type What are Solar Cells? Load - + Solar cells are diodes Light (photons) generate free carriers (electrons and holes) which are collected by the electric field of the diode junction The output current is a fraction of this photocurrent The output voltage is a fraction of the diode builtin voltage Short-circuit current Voltage Maximum Power Point Opencircuit voltage

18 Photocurrent source Diode Shunt resistance (maximize) Load Standard Equivalent Circuit Model Where does the power go? Series resistance (minimize)

19 Photons Photons are characterized by their wavelength (frequency) and their energy E c v hc hv v (8.1) (8.2) Table 8.2 Band Gap and Cut-off Wavelength Above Which Electron Excitation Doesn t Occur Quantity Si GaAs CdTe InP Band gap (ev) Cut-off wavelength (μm)

20 Energy-band Diagrams Silicon band gap energy is 1.12 ev Conduction band top band, here electrons contribute to current flow, empty at absolute zero for semiconductors An electron must acquire the band gap energy to jump across to the conduction band, measured in electron-volts ev Electrons create holes when they jump to the conduction band Photons with enough energy create hole-electron pairs in a semiconductor mmons/c/c7/isolator-metal.svg

21 Silicon Solar Cell Max Efficiency Upper bound on the efficiency of a silicon solar cell: Band gap: 1.12 ev, Wavelength: 1.11 μm This means that photons with wavelengths longer than 1.11 μm cannot send an electron to the conduction band. Photons with a shorter wavelength but more energy than 1.12 ev dissipate the extra energy as heat

22 Silicon Solar Cell Max Efficiency For an Air Mass Ratio of 1.5, 49.6% is the maximum possible fraction of the sun s energy that can be collected with a silicon solar cell

23 Review of Diodes Two regions: n-type which donate electrons and p-type which accept electrons p-n junction- diffusion of electrons and holes, current will flow readily in one direction (forward biased) but not in the other (reverse biased), this is the diode

24 The p-n Junction Diode Voltage-Current (VI) characteristics for a diode Id Id I e / 0 ( qv d kt -1) (8.3) I e 38.9V 0 ( d -1) (at 25 C) Figure 8.15

25 Circuit Model I + I + PV + - V Load I SC I d V Load - - The current going to the load is the short-circuit current minus diode current qv / kt I I I ( e -1) (8.8) SC Setting I to zero, the open circuit voltage is V OC kt q 0 I SC ln 1 (8.9) I0

26 PV Equivalent Circuit Add impact of parallel leakage resistance R P (want R P to be high) V I ( ISC Id ) (8.12) R P V d Figure PV Cell with parallel resistance Add impact of series resistance R S (want R S to be small) due to contact between cell and wires and some from resistance of semiconductor V V I R d S (8.14)

27 Photocurrent source Diode (maximize) Load Series and Shunt Resistance Effects: Considering both R s and R p : 1 I ISC I 0 V I R 38.9( V IR ) e s C (8.18) RP Series resistance drops some voltage (reduces output voltage) Equivalent Circuit R s I Shunt resistance drops some current (reduces output current) I SC I d R P (minimize) Voltage & Current are coupled

28 Series and Shunt Resistance Effects Parallel (R P ) current drops by ΔI=V/R P Figure 8.23 Series (R S ) voltage drops by ΔV=IR S Figure 8.25

29 Fill Factor and Cell Efficiency Fill Factor (FF) = V R I R I sc V oc AM 1.5 Incident Solar Power ~100 mw/cm 2 J SC Cell Efficiency (h) = I sc V oc FF Incident Power = I max V max Incident Power

30 Intensity (mw/m 2 -mm) Top Cell (1.9 ev gap) Cell #2 (1.4 ev gap) Cell #3 (1.0 ev gap) Cell #4 (0.6 ev gap) n-type p-type n-type p-type n-type p-type n-type p-type Multijunction Cells Load Problem: Single junction loses all of the photon energy above the gap energy Solution: Use a series of cells of different gaps. Each cell captures the light transmitted from above. Energy (ev) But can t just use a graded gap. Allows electrons to escape to low gap region Need separate cells Wavelength (nm)

31 Record laboratory thin film cell efficiencies

32 Cells to Modules to Arrays Rain, hail, ice, snow Local failures, shadows Heat, humidity, corrosive gases Modules A 12V module has 36 cells wired in series, each cell ~ 0.5 V 72-cell modules are also common Arrays combination of modules connected in series and/or in parallel

33 Interconnect Schemes Soldered (standard for Si cells) Monolithic interconnects Encapsulation

34 Impact of Temperature 1 sun of irradiance = 1 kw/m 2 for AM = 1.5 V OC decreases by ~0.37% per C for crystalline silicon cells I SC increases by about 0.05% per C NOCT Normal Operating Temperature NOCT 20 C Tcell Tamb S (8.24) 0.8 Figure 8.36

35 Impact of Shading Shading causes PV to act as a resistor instead of a current source, can reduce output power by > 50% Figure 8.38 Figure 8.37

36 Blocking Diodes -- protection from low voltage strings Cell arrays including a weak cell; the array performs at the level of the worst cell. Two good cells One good cell, one weak cell Diodes can be used to prevent current from strong arrays flowing through weak array segments. However, they reduce output voltage. Front contact Back contact Monolithically-interconnected Cell Array Cell 1 Cell 2 For a full discussion, see H.S. Rauschenbach, Solar Cell Array Design Handbook (Van Nostrand Reinhold, New York, 1980)

37 Bypass Diodes -- protection from low current strings Bypass diodes are used in modules to route current around shadowed or defective strings (series connected strings must maintain constant current throughout) Used typically every cells

38 Silicon Solar Modules Crystalline silicon Single crystals Cast polycrystals Well understood Cost analysis easy Source material is expensive Material sensitive to impurities & defects The major limitation to Si technology is the availability of electronic-grade Si most manufactured technology

39 Silicon Solar Modules Steps to make a Si module: Growth of the Si bulk crystal (ingot) Cutting of the wafers from the grown ingot Diffusion of dopants to form the junction Interconnection Packaging Energy Figure Systems courtesy Research R. Birkmire, Laboratory, Univ. of Delaware FIU

40 Czochralsky Si Bulk single crystal boules grown by this method. Growth rate mm/min Typical PV boule diameter: mm length: cm Some issues: Low throughput High energy use Requires skilled operator Photographs courtesy MEMC Electronic Materials Inc, St. Peters MO.

41 Cast Monocrystalline Silicon Figures Courtesy: Seed crystal

42 Crystalline Si by Casting Bulk Polycrystals grown by this method Wire Saw Mold Cast Polycrystal Features: Large grains possible Batch process is fast Low technology, cost Relatively fast Lower quality crystals Slow cooling

43 PV Systems Four configurations 1. Grid-connected systems

44 PV Systems Four Configurations 2. Stand-alone systems with directly-connected loads

45 PV Powered Water Pumping Professor O. A. Mohammed, EEL5285 Lecture Notes, Spring 2017

46 DC Motor I-V Curve DC motors have an I-V curve similar to a resistor e = kω is back emf, R a is armature resistance V IR k (9.3) a

47 DC Motor I-V Curve Linear Current Booster (LCB) helps the motor be able to start in low sunlight Figure 9.10 Figure 9.9

48 PV Systems Four Configurations 3. Stand-alone systems which charge batteries

49 PV Systems Four Configurations 4. Microgrids A small electric grid with several generation sources The microgrid can be configured to operate either connected to the main grid or standalone The military is a key proponent of microgrids, since they would like the ability to operate bases independent of any grid system for long periods of time Renewable generation by be quite attractive because it decreases the need to store large amounts of fossil fuel Time magazine reported in Nov 2009 that average US solider in Afghanistan requires 22 gallons of fuel per day at an average costs of $45 per gallon

50 PV Implementation, Examples and Assignments 1- Practical PV System component Grid-connected PV systems without battery storage Grid-connected PV systems with battery storage Grid-connected PV systems with battery storage 2- Modeling the PV Array PV equivalent Circuit PV Simulink Model Impact of changing irradiance on Voc & load current 3- Simulation of PV VI & VP characteristics Modeling Variable Load Study the VI & VP characteristics of the PV panel 4- Maximum Power Point Tracking Modeling MPPT algorithm Modeling DC-DC boost converter Case study for MPPT performance 5- Modeling Grid Connected PV systems Model Description Voltage source Inverter (VSC) Model Voltage source inverter (VSC) control Case Study

51 Practical PV System component Irradiance W/m^2 PV Array Cable DC DC Boost Converter DC VSI AC TL Transformer G Grid To filter the ripple on the output voltage of the PV arrays Maximum Power Point Tracking is implemented here Reactive power compensation is needed to connect to the grid PV supplies active power To convert the voltage from DC to AC The AC voltage has to be synchronized with the grid voltage

52 Why we need Power Conditioning in PV Systems? 1. The output voltage of PV systems is DC, while most of the loads are AC 2. The characteristics of PV arrays are nonlinear hence power conditioning units are needed to track their maximum power point

53 Grid-connected PV systems without battery storage MPPT Phase-locked loop has to be used to synchronize the power with the grid MPPT is implemented in the boos converter controller Note: During power deficiency (e.g. at night), the power is received from the grid

54 Grid-connected PV systems with battery storage Battery Charger MPPT PV Array Battery Distribution Panel Inverter AC Loads Utility Grid In this system, there are two options for covering power deficiencies: the grid and the battery. This can be used for energy management.

55 Stand alone PV systems with battery storage This system is applicable for rural residential areas PV Array Battery Charger Battery Optional Backup Generation AC Loads Inverter The capacity of the battery is an essential parameter in stand-alone systems.

56 Modeling the PV Array Objective 1. Building simulation model that represent the relation between the PV output( voltage, current) and environmental parameters. 2. Utilizing the simulation model to study the impact of changing irradiance on the PV output voltage and current. 3. Simulating the performance of different PV modules consisting of series and parallel connected cells.

57 Modeling the PV Array Objective Building mathematical models for PV array to represent the PV voltage and current output as a function of irradiance input. Modeling a PV array is achieved by implementing the equations relating the PV output voltage and power the its parameters: photo-generated current I L, diode saturation current I D, parallel resistance R SH and series resistance R s

58 PV equivalent Circuit

59 PV Simulink Model Irradiance Pattern Irradiance /Voc Characteristic Isc Vt Irradiance

60 Model Setting

61 Impact of changing irradiance on Voc Irradiance Diode current Output current Output Voltage Output Voltage Irradiance

62 Simulation Example Assignment #1 Modify the Irradiance pattern in the Simulink Model EX_IR_CHAR to represent actual summer and winter Day In Miami Area. Change the number of series Module to two modules and observe the effect of the irradiance change. Change the number of Parallel Module to two modules and observe the effect of the irradiance change.

63 Impact of changing irradiance on Load Connect 20 Ohm resistive Load to the PV Model. Apply variable Irradiance pattern from 0 to 1000 W/m2 Observe the effect of the Irradiance change at the load current current

64 Simulation Result Irradiance Diode current Output current Output Voltage Load current Irradiance Time

65 Homework Assignment #2 Modify the Irradiance pattern in the Simulink Model EX_IR_I_CHAR to represent actual summer and winter Day In Miami Area. Modify the model to calculate the output power and efficacy Change the number of series Module to two modules and observe the change in the load current. Change the number of Parallel Module to two the change in the load current.

66 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

67 Resistive Load I-V Curve V IR 1 I 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 R m V (9.2) I m m V Figure 9.5 (9.1)

68 Maximum power point (MPP) should occur when the load resistance R = V R /I R under 1-sun 25 C, AM 1.5 conditions Maximum power transfer A MPP tracker maintains PV system s highest efficiency as the amount of insolation changes.

69 Maximum Power Point Trackers Maximum Power Point Trackers (MPPTs) are often a standard part of PV systems, especially gridconnected 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

70 Hourly I-V Curves Current at any voltage is proportional to insolation V OC drops as insolation decreases Can just adjust the 1-sun I-V curve by shifting it up or down

71 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

72 Components of Grid-Connected PV Principal components in a grid connected PV system using a single inverter

73 Individual Inverter Concept Easily allow expansion Connections to house distribution panel are simple Less need for expensive DC cabling

74 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 efficiencies above 90%

75 Losses from Mismatched Modules Illustrates the impact of slight variations in module I-V curves Only 330 W is possible instead of 360 W

76 Peak-Hours Approach 1-sun is 1 kw/m 2 We can say that 5.6 kwh/(m 2 -day) is 5.6 hours of peak sun PVUSA test conditions (PTC) 1-sun, 20 C ambient temperature, wind-speed of 1 m/s P ac(ptc) = AC output of an array under PTC If we know P ac, 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) kw h/day of "peak sun" (9.14) P ac

77 Capacity Factor of PV Energy kwh/yr kw CF 8760 h/yr (9.15) P ac h/day of "peak sun" CF (9.16) 24 h/day Figure 9.28 PV Capacity Factors for US cities

78 Stand-Alone PV Systems When the grid is not 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

79 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

80 Batteries and PV Systems Batteries in PV systems provide storage, help meet surge current requirements, and provide a constant output voltage. Lots of interest in battery research, primarily driven by the potential of pluggable hybrid electric vehicles $2.4 billion awarded in August 2009 There are many different types of batteries, and which one is best is very much dependent on the situation Cost, weight, number and depth of discharges, efficiency, temperature performance, discharge rate, recharging rates

81 Battery I-V Curves Energy is stored in batteries for most off-grid applications An ideal battery is a voltage source V B A real battery has internal resistance R i V V R I (9.4) B i

82 Battery I-V Curves Charging I-V line tilts right with a slope of 1/R i, applied voltage must be greater than V B Discharging battery- I-V line tilts to the left with slope 1/R i, terminal voltage is less than V B Figure 9.12

83 Lead Acid Batteries Most common battery for larger-scale storage applications Invented in 1859 There are three main types: 1) SLI (Starting, Lighting and Ignition) : optimized for starting cars in which they are practically always close to fully charged, 2) golf cart : used for running golf carts with fuller discharge, and 3) deep-cycle, allow much more repeated charge/discharge such as in a solar application

84 Basics of Lead-Acid Batteries Positive Plate: PbO + 4H + SO + 2e PbSO 2H O (9.21) Negative Plate: PbO + SO PbSO 2e (9.22)

85 Basics of Lead-Acid Batteries During discharge, voltage drops and specific gravity drop Sulfate adheres to the plates during discharge and comes back off when charging, but some of it becomes permanently attached

86 Battery Storage Battery capacity has tended to be specified in amphours (Ah) as opposed to an energy value; multiply by average voltage to get watt-hours Value tells how many amps battery can deliver over a specified period of time. Amount of Ah a battery can delivery depends on its discharge rate; slower is better Figure shows how capacity degrades with temperature and rate

87 Battery Technologies Type Lead-acid, deep cycle Nickel-metal hydride Density, Wh/kG Cost $/kwh Cycles Charge time, hours Power W/kg Lithium Ion The above values are just approximate; battery technology is rapidly changing, and there are many different types within each category. For stationary applications lead-acid is hard to beat because of its low cost. It has about a 75% efficiency. For electric cars lithium ion batteries appear to be the current front runner

88 Estimating Storage Needs

89 Stand-Alone PV Systems Design Summary Battery Sizing How many days of storage needed? Generator Sizing System Costs