Engineering 1 : Photovoltaic System Design What do you need to learn about? Gil Masters I. Very quick electricity review Terman 390 … but leaving town tonight feel free to email me anytime: [email protected] II. Photovoltaic systems III. PV technology IV. The solar resource V. Batteries VI. Load analysis VII. PV Sizing I’m here to help... VIII. Battery Sizing … all in one class !! ?? !! December 2, 2003 I. BASIC ELECTRICAL QUANTITIES Energy (W,joules) q (Coulombs) POWER Watts = Power is a RATE !! Time (sec) Electric Charge 1 electron = 1.602 x10-19 C dW dW dq P = = ⋅ dt dq dt P = v i dq watts Current …is the flow of charges i = charge/time = current dt energy/charge =volts e- 1 Coulomb i (Amps) = second i + ENERGY ENERGY = POWER X TIME (watt-hrs, kilowatt-hours) Voltage “the push” Watt hours = volts x amps x hours = volts x (amp-hours) energy W 1 Joule V = 1 Volt = Batteries ! charge q Coulomb 1 II. PV SYSTEM TYPES: 2. A FULL-BLOWN HYBRID STAND-ALONE SYSTEM WITH BACKUP 1. GRID-CONNECTED PV SYSTEMS: ENGINE-GENERATOR (“Gen-Set”) ….Not what you will design • Simple, reliable, no batteries (usually), 2 • ≈ $ 15,000 (less tax credits), A=200 ft for efficient house DC DC DC loads DC Batteries DC Fuse ..may want all DC, • Sell electricity to the grid during the day (meter runs backwards), buy it Charge Controller Box all AC, back at night. DC or mix of AC/DC * Sizing is simple… how much can you afford? Charger Inverter AC AC loads PVs Fuse AC AC to DC DC to AC AC • But compete with “cheap” 10¢/kWh utility grid power Generator Box AC DC Power Utility Inverter/Charger Conditioning Grid DC-to-AC to run AC loads Unit some can do AC-to-DC to charge batteries PVs AC Complex, expensive, requires maintenance, tricky to design But… competes against $10,000/mile grid extension to your house or …NOT what you are going to design 40¢/kWh noisy, balky, fuel-dependent on-site generator TRADE-OFF BETWEEN DC AND AC SYSTEMS: 3. SIMPLER STAND-ALONE SYSTEMS: 3000 Wh/d + DC ALL AC = 3530 Wh/ d ALL DC 0.85 Example: DC Loads - - + h=0.85 Battery DC DC DC AC 3000 Wh/d AC Charge Batteries Inverter Controller (including a 1200 Wh/d AC fridge) DC DC AC Batteries Inverter INVERTER FOR AC AC/DC OR…Buy a more expensive, very efficient DC refrigerator that uses say 800 Wh/d DC DC DC AC CHARGE CONTROLLER TO instead of the 1200 Wh/d for an AC fridge Charge Batteries Inverter Controller PROTECT BATTERIES 1800 Wh / d + 800 Wh/ d = 2920 Wh/d 0.85 0.85 DC AC DC DC DC AC SOME AC, SOME DC (e.g. fridge) DC DC 1800 Wh/d AC Charge Batteries Inverter Charge Batteries Inverter Controller ..avoids some inverter losses Controller DC DC ..smaller inverter saves $ DC Loads 800 Wh/d DC ..but more $ for DC fridge ..but need AC and DC wiring, $ More expensive fridge ..YOU’LL DESIGN ONE OF THESE ! Cheaper PVs, battery, inverter More expensive wiring 2 MOST SOLAR CELLS ARE MADE FROM SILICON.. III. PHOTOVOLTAIC TECHNOLOGY SO, HOW DO WE COLLECT SOLAR ENERGY? VALENCEvalence ELECTRONSelectrons +14+14 +4+4 (a) actualactual (b)(b) simplified QUARTZ TO SILICON… CZOCHRALSKI METHOD OF FORMING CRYSTALLINE WAFERS.. Si is ≈ 20% of the earth’s crust, usually as SiO 2 MELT the pure Si (1400 C) in a quartz crucible.. ..an energy intensive process using an arc furnace DIP, then withdraw, a “seed crystal” turning continuously so that each converts it to pure silicon atom freezes in place in the crystal.. GET a cylindrical ingot (perhaps 1 m long, 20 cm diameter) Rock-like hunks of 99.9999% pure silicon.. SLICE the cylinder into wafers…. (same as integrated circuits) 3 IF A PHOTON HAS “ENOUGH” ENERGY, IT CAN BUMP AN CRYSTALLINE SILICON FORMS A TETRAHEDRAL ELECTRON INTO THE CONDUCTION BAND..leaving a positively STRUCTURE… charged “hole” behind Hole Photon silicon nucleus + +4 +4 +4 Free shared valence electron electrons +4 +4 +4 +4 Si (a) tetrahedral a) Tetrahedral (b) 2-D version b) 2-D version Max efficiency ≈ 50% Photons with TOO MUCH energy (l < 1.11 mm) waste 30.2% Photons with TOO LITTLE energy (l > 1.11 mm) lose another 20.2% SEPARATE HOLES AND ELECTRONS USING CELLS, MODULES AND ARRAYS... THE ELECTRIC FIELD CREATED IN A p-n JUNCTION ARRAY wired in series and parallel for voltage and power Produces Direct Current (dc) Single CELL ≈ 0.5 V Electrical contacts electrons on top - - - - - p-n junction n-type creates an E V Load electric field 5” - 8” diameter p-type + + + + + MODULE, typically “12-V or 24V” Bottom contact Rated by peakwatts (e.g. 53 W) (≈ 1 m x 0.5 m) 36 cells wired in series Current I Trim edges 4 LOTS OF PHOTOVOLTAIC TECHNOLOGIES…. Manufacturer specification of photovoltaic module: PHOTOVOLTAICS Short circuit current, Isc Thick Si Thin films Open circuit voltage, Voc 200 - 500 mm 1 - 10 mm Current at “rated conditions” IR Voltage at maximum power point (rated voltage) VR Heterojunction Homojunction Rated power (@1 kW/m2 solar insolation, 25oC cell temperature, at max pwr pt) P Single-crystal Si R Multicrystalline Si CdTe Example: AstroPower 7105: P = 75W, I =4.4A, V = 17.0V, Isc = 4.8A, Voc = 21.0V Czochralski CIS R R R CZ 30% Polycrystalline thin-film Si 4 Ribbon 1 kW/m2 insolation (“1-sun”) Amorphous Si 20% GaAs 4.8A InP Maximum power point Flat-plate 3 Concentrator 4.4A 4.4A x 17V = 75W 50% Multijunction, Tandem cells 2 0.5 kW/m2 insolation 1/2 sun 2.4A I (amps) 1 12.7% 6.3% 0 0 10 17.0 21.020 V V (volts) IV. THE SOLAR RESOURCE…. kWh/ m2-day of insOlation THE KEY TRICK TO INTERPRETING INSOLATION DATA… * Location * Orientation of modules (due south generally best for U.S.) “mid-day, clear day, normal to rays” * Tilt angle * Fixed orientation vs 1-axis tracking vs 2-axis tracking “1-SUN” OF INSOLATION IS DEFINED TO BE 1 kW/m2 Summer AVERAGE DAILY INSOLATION EXPRESSED IN (KWh/m2-day) Spring, Fall CAN BE INTERPRETED TO MEAN HOURS OF FULL SUN Winter e.g. Boulder, CO in June, collector tilt = latitude sees 6.1 kWh/m2 of insolation tilt south “that’s like 6.1 hours/day of 1 kW/m2 “full sun” Tilt = Latitude gives perpendicular angle to sun at equinoxes at noon 5 GOOD SOURCE OF REAL DATA… SINGLE-AXIS TRACKER (East to West) kWh/m2-day 1-axis tracker, tilt = lat Zomeworks: Passive single-axis tracker… on the roof since 1977 … IS IT WORTH THE EXTRA COST? INSOLATION IN BOULDER, CO.. 9 1-Axis Tracking 8 (Annual 7.2 kWh/m2-d) 7 Lat - 15 (5.4 kWh) 6 Lat (5.5 kWh) 5 Lat + 15 (5.3 kWh) 4 3 INSOLATION (kWh/m2-day) 2 1 0 JAN FEB MAR APR MAY JUN JLY AUG SEP OCT NOV DEC 30% extra insolation with 1-axis tracker (annual) 2-AXIS TRACKER….. Not much better than 1-axis 6 V. ENERGY STORAGE… Basic lead-acid battery... One Cell ≈ nominal 2 V BATTERIES COMPRESSED AIR 6-cells, “12-V battery” HYDROGEN FLYWHEELS, etc + CHARGED - + DISCHARGED - FOR NOW.. BATTERIES ARE IT.. Lead-Acid car batteries (SLI =Starter, Lighting, Ignition system)_ H + PbO Pb PbSO PbSO4 2 + 4 Designed for high current (400-600A), shallow discharge (20%), not so good for PV H H O Lead-Acid “golf-cart” deep-cycle batteries… often used due to low cost, satisfactory performance = 2 SO 4 True Deep-Cycle Lead-Acid batteries… very good, but expensive Nickel-Cadmium batteries… very expensive, great for very cold, harsh conditions, can take abuse Figure 9.40 A lead-acid battery in its charged and discharged states. Max Depth Energy Density Cycle life Calendar life Efficiencies Cost Battery Discharge Wh/kg cycles years Ah % Wh % $/kWh As battery discharges: Lead-acid, SLI 20% 50 500 1-2 90 75 50 Lead-acid, golf cart 80% 45 1000 3-5 90 75 60 * Specific gravity of electrolyte drops (gives indication of state-of-charge) Lead-acid, deep-cycle 80% 35 2000 7-10 90 75 100 Nickel-cadmium 100% 20 1000-2000 10-15 70 60 1000 o o Nickel-metal hydride 100% 50 1000-2000 8-10 70 65 1200 * More vulnerable to freezing…(charged freeze at -57 C; discharged at -8 C) Rough comparison of battery characteristics * Plates coated with PbSO4 yields higher internal resistance, cell voltage drops STATE OF CHARGE (SOC) Hydrometer to measure specific gravity (but electrolyte may stratify with H2SO4 on BATTERY RATINGS…. bottom) * Voltage… for lead-acid about 2 V per cell. Typical “nominal Voltage can be used… (but battery needs to have been “at rest” for several hours to be voltage” for battery is 6 V or 12 V (3 or 6 cells per battery) accurate) 13.0 1.30 * Voltage depends on state of charge and whether you are charging the 12.8 1.28 battery or discharging it. Voltage can range from about 11 to 15 V 12.6 1.26 SPECIFIC 12.4 1.24 V * Energy stored = Volts x Amps x Hours = Watt-hours 12.2 1.22 12.0 1.20 GRAVITY SG 11.8 1.18 * With voltage varying all over the map, how do you describe the energy VOLTAGE (V) 11.6 1.16 stored in a battery? 11.4 1.14 11.2 1.12 Ans. Use “Amp-hours” @ nominal battery voltage as the measure of energy 11.0 1.10 stored in battery ! 100 80 60 40 20 0 STATE OF CHARGE (%) A 12-V battery that reads only 12 V (at rest) is almost completely discharged… 7 BATTERY STORAGE IS DESCRIBED USING A NOMINAL VOLTAGE (2-V/cell) AND.