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Solar Electric Energy Basics: System Design Considerations

Solar Electric Energy Basics: System Design Considerations. Frank R. Leslie B. S. E. E., M. S. Space Technology, LS IEEE Adjunct Professor, Florida Tech, COE, DMES 10/1/2008, Rev. 1.3 fleslie @fit.edu; (321) 674-7377 my.fit.edu/~fleslie.  Does Energy Affect our Lives? .

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Solar Electric Energy Basics: System Design Considerations

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  1. Solar Electric Energy Basics:System Design Considerations Frank R. Leslie B. S. E. E., M. S. Space Technology, LS IEEE Adjunct Professor, Florida Tech, COE, DMES 10/1/2008, Rev. 1.3 fleslie @fit.edu; (321) 674-7377 my.fit.edu/~fleslie

  2.  Does Energy Affect our Lives?  Happy New Yorkers out for a Stroll! Are they having fun? Why did thishappen? FOXnews 8/15/2003 080820

  3. Energy Considerations for 2050 • Fossil-fuel energy will deplete in the future; millions of years to create that much cheap fuel • US oil production peaked about 1974; world energy will peak about 2009 or so • The US imports about 10 million barrels crude oil/day • Renewable energy will become mandatory, and our lifestyles may change • Transition to renewable energy must occur well before a crisis occurs 081001

  4. US RE Resources Differ Widely 081001

  5. Why use Solar Energy? • Far from utility power lines; costly to extend lines • Provide backup power during utility outages • Minor glitch backup might be only for two minutes • Hurricane line damage may need two weeks to repair • Cleaner energy with no CO2 emissions • Self-satisfaction of using some “free” energy (but it costs money to get it) • “Greener than thou” syndrome bragging rights • “I just want it!” 081001

  6. Solar Estimate from FSEC in Cocoa FL • The “Sunshine State” has as much sunshine as Wyoming 081001

  7. PV System Engineering Decomposition intoFunctional Components Collect & Distribute Energy Collect Energy Regulate Energy Regulate Energy Store Energy Store Energy Start Control Energy Distribute Energy Use Energy Each function drives a part of the design, while the interfaces between them will be defined and agreed upon to ensure follow-on upgrades 081001

  8. A Representative Grid-Intertie Solar Electric System • The energy flow is protected and metered • Grid interties vary with the regional restrictions • Multiple meters show energy generated and the utility energy supplied and received http://www.fsec.ucf.edu/PVT/Projects/fpl/kev/main.htm#TOP 081001

  9. Solar Energy Intensity • Energy from our sun (~1372 W/m2) is filtered through the atmosphere and is received at the surface at ~1000 watts per square meter or less; average is 345 W/m^2 • Air, clouds, rain, and haze reduce the received surface energy • Capture is from heat (thermal energy) and by photovoltaic cells yielding direct electrical energy 081001

  10. Energy Usage & Conservation • The loads supported by the system must be minimized to match the available energy • Load analysis shows the largest concerns that might be reduced to cut costs • Conservation by enhanced building insulation and reduced lighting loads • Increased efficiency of energy plants will conserve fossil fuels Arizona has clearer skies than Florida. Ref.: Innovative Power Systems 081001

  11. Florida Energy Use Varies with the Time of Day (Daily Living) • Daily load peaking (1 a.m. to midnight graph)megawatts vs. hours http: 7 a.m. 3 - 7 p.m. 7 - 9 p.m. http://www.dep.state.fl.us/energy/fla_energy/files/energy_plan_final.pdf 081001

  12. PV Cell Basics • Semiconductor of transparent positive silicon and negative silicon backing • Incoming light (photons) cause energized electrons to move to the top n-silicon and out the connector • Nominal voltage of 0.55 V requires series connections to get useful voltage, 17 V • Short circuit current is proportional to light intensity Maximum output occurs when normal to cell is pointed at light (cosine of sun offset angle) Ref.: FSEC 081001

  13. PV Response Characteristics http://www.chuck-wright.com/SolarSprintPV/SolarSprintPV.html • As light intensity increases, the change in current is much greater than the change in open-circuit voltage; a dim sun still produces voltage • The maximum power point (MPP) indicates the load resistanceto achieve maximum power for use MPP 081001

  14. SUN Latitude Angle Variations in Surface Energy Affect Potential Capture • A flat-plate collector aimed normal to the sun (directly at it) will receive energy diminishing according to the amount of atmosphere along the path (overhead air mass Ξ 1); (you can look at the sun at dawn or dusk) • The received energy varies around the World due to local weather; in Central Florida, direct normal radiation is 4.0 to 4.5 kWh/(m2 - day); 4.7 equivalent sun hours • Throughout the Contiguous United States, daily solar energy varies from <3.0 to 7.0 kWh/(m2 - day) My house uses about 23 - 40 kilowatt-hours/day 081001

  15. PV modules of 120 W cost about $400 Mounting angles to match sun --- fixed or tracking Average module slope angle is equal to latitude Zoning and regulations --- Not In My Back Yard (NIMBYs) problem Protection required for electric line workers due to “islanding” backfeed PV Systems This solar intensity plot for Cocoa FL shows the cloud effect on what otherwise would have been a cosine effect Ref.: FSEC 081001

  16. Solar Path for Florida Tech 2/21/anyyear 081001 http://solardat.uoregon.edu/

  17. Solar Energy: Photovoltaic Sunlight to Electricity • Photovoltaic cells typically can extract about 15-17% of incoming solar energy; theoretical is about 31%; $/W is the key (~$3.50/W, 2007) • Low voltage direct current is produced at about 0.55 volt per cell; clusters are series-connected for ~17 volts output for charging a 12 volt system • Arrays of cells (modules) can be fixed or can track the sun for greater energy gain • Storage is required unless the energy is inverted to 120 Vac to synchronously drive the utility grid PV prices are falling, though still relatively expensive compared to wind or fossil utility power 081001

  18. Collector-Module Sizing • Most manufacturers’ modules now average about 120 watts for ease of handling at installation • Larger 285 W modules are 4 ft by 6 ft, 107 pounds, and require two people to use great care in handling and positioning (our field trailer carries one of these) • Hardware must secure module to resist winds of ~130 mph based upon zoning codes • Module output should be ~10% larger than calculated to allow for aging and darkening of the cover glass • After the first 10% decline, there is little change in peak output 081001

  19. Roof-top Solar Array Computations • Find the south-facing roof area; say 20 ft * 40 ft = 800 ft2 • Assume 120 Wp solar modules are 26 inches by 52 inches; 9.4 ft2/120 watt; 12.78 W/ft2 • Assume 90% of area can be covered, 720 ft2, ~9202 W • and that there are 5.5 effective hours of sun/day; 51 kWh/day • The south-facing modules are tilted south to the latitude angle • 76 modules would fit the area, but 44 would provide an average home with 30 kWh/day and cost ~$17600 for modules alone, ~one mile of powerline Siemens Solar SM110 Maximum power rating, 110 W Minimum power rating, 100 W Rated current. 6.3 A Rated voltage, 17.9 V Short circuit current, 6.9 A Open circuit voltage, 21.7 V 081001

  20. Limits charge current to protect battery from overheating and damage that shortens life Disconnects battery loads if voltage falls too low (10.6 V is typical) Removes charge current if voltage rises too high (14V is typical) Regulates charge voltage to avoid battery water gassing Diverts output of source to a secondary load (water heater or electric furnace) if battery is fully charged Saves energy wisely Battery Charge Controller Soltek Mark IV 20 Amp Regulator “Big as a breadbox” for a 4 kW inverter 081001

  21. Power Line Outage Protection • Storage for utility power outages requires batteries • Two or three days with no sun is possible; design for it by adding more storage or array surface • Segregate important or critical loads • At least one light per room • Use a cable going to each room for a light and put on one 15A circuit breaker • Connect that breaker to a transfer switch to substitute inverter power when needed 081001

  22. Lead-acid (car) batteries are most economical; but must be deep-cycle type Critical rating is 20-hour value or Reserve Capacity (RC) in minutes at 25A load Charge cycle is ~70% efficient -- rather wasteful Requires maintenance to ensure long life A home might have ten of these batteries Need to know the length of time without full sun in days Inverter must match series battery voltage Storage Batteries Soltek Deep-Cycle BatteryAP-2712 Vdc,115 A-hr 20-hour rate 081001

  23. Energy Storage • Battery banks are current practice • Hydrogen gas from charging must be vented outside • Batteries should be kept warm (above 60°F) for full capacity • Charge controller needed for large systems to prevent overcharging • Deep discharge reduces expected life; ~5000 cycles • Float voltage maintains full charge without gassing • Low voltage disconnect switches are recommended The battery on the left is the size of a car battery; the one on the right has much more capacity 081001

  24. The inverter converts low voltage (12V to 100s V) direct current to 120 Vac Synchronous inverters may be “inter-tied” with power line to reduce billable energy In “net metering” states, the energy is metered at the same rate going into and out of the electrical grid --- no storage required (except for outages)! Loads can use 12 volt low-voltage directly at higher efficiency with special lamps Inverter Trace Legend 4 kilowatt Inverter 081001

  25. Household load analysis estimates the peak and average power and energy required Some might be reduced or time-shifted to decrease system costs Incandescent lamps produce far more heat than light; CFLs provide ~100 W light equivalent at 27 W load Loads 27 watt (100 W equivalent)Compact Fluorescent Lamp (CFL) CFL Costs without replacement labor: $21.30 Incandescent Costs with replacement labor: $39.98 ____________________________________ CFL Costs with replacement labor: $23.30 Incandescent Costs with replacement labor: $56.54 Hint: You can buy a CFL at a large local discount store for $4.68or six for $7.00! 081001

  26. Load Analysis Spreadsheet • A spreadsheet program like Excel will speed analysis of the various loads, their use time, peak power, and energy required • Once done, modifications for other systems are easy • List the loads, enter the power, time per day, and compute the rest • From total energy required and total power, one can compute the size of solar modules and batteries 081001

  27. Energy Load Assessment • Site: Classroom 081001

  28. Load Analysis for a Yacht

  29. Energy Transmission • Solar power is expensive, so design wires for 1% loss instead of usual 3 to 5% for utility power • Use higher voltage (120Vac for long lines) instead of 12 Vdc • Spend more on larger wire than normal to reduce resistance loss • Battery and inverter wires might be AWG #0 or 2 or larger • Inverter output is 120Vac, so AWG#12 and 14 are common for 20A and 15A home service • Danger with batteries is not shock but flash burns and flying molten metal • Special dc-rated fuses and circuit breakers are required 081001

  30. Some Important Electrical Information • P = E•I = E2/R = I2•R,where P is power (instantaneous), E is electromotive force, I is intensity or current, and R is resistance • Energy = P•t, where t is the time that power flows • V = I•R for a load or E = I•R for a source,where V is voltage drop across resistor • Wire size numbers roughly double the area and halve the resistance for every three size number changes • #18 AWG is used in ordinary lamp cord (zip cord) • #18 AWG has a resistance of 6.385 ohms per 1000 ft • #12 AWG has a resistance of 1.588 ohms per 1000 ft • #9 AWG has a resistance of 0.7921 ohms per 1000 ft • #6 AWG has a resistance of 0.3951 ohms per 1000 ft • #3 AWG has a resistance of 0.197 ohms per 1000 ft 081001

  31. Cost Analysis Spreadsheet 081001

  32. Generic Trades in Energy • Energy trade-offs are required to make rational decisions • PV is expensive ($5 per watt for hardware + $5 per watt for shipping and installation = $10 per watt) compared to wind energy ($1.5 per watt for hardware + $5 per watt for installation = $6 per watt total) • Are Compact Fluorescent Lamps (CFLs) better to use? Ref.: www.freefoto.com/pictures/general/ windfarm/index.asp?i=2 Ref.: http://www.energy.ca.gov/education/story/story-images/solar.jpeg Photo of FPL’s Cape Canaveral Plant by F. Leslie, 2001 081001

  33. Conclusion • Solar electric energy is best applied where the cost justifies; remote from the grid or for independent backup power • True costs of fossil-fuel pollution and subsidies are not easily found -- controversies exist • PV costs are falling, but fossil-fuel costs will soon surpass them • At that time, PV will compete with wind energy, which is currently competitive with fossil fuels 081001

  34. Thank you! Questions? ? ? My website: my.fit.edu/~fleslie for presentations Roberts Hall weather and energy data: my.fit.edu/wx_fit/roberts/RH.htm DMES Meteorology Webpage: my.fit.edu/wx_fit/?q=obs/realtime/roberts 080710

  35. Is a Solar Roof Practical? Sun intensity at surface ~1000 watt / square meterPV cells about 15% efficient = ~150 watt / square meter Roof might be about 20 x 40 feet = 800 square feet; 90% coverage = 720 square feet A 120 watt solar module is about 26 inches x 52 inches = ~ 9.4 sq. ft, thus peak power production is ~12.78 watt / square ft 720 square feet*(12.8 watt/square feet) = 9202 watts peak power Optimally, roof array could yield 9202 watts for 5.5 hours/average day = 51 kWh each day on average; average house might need 30 kWh Storage would provide energy at night and during cloudy weather, but increases the cost Current cost estimates are about $5/W & $0.06 to $0.20 per kWh vs. $0.07 from utility Utility line extension costs about $18,000 to $50,000 per mile

  36. References: Books, etc. • Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, 1992. 0-262-02349-0, TJ807.9.U6B76, 333.79’4’0973. • Duffie, John and William A. Beckman. Solar Engineering of Thermal Processes. NY: John Wiley & Sons, Inc., 920 pp., 1991 • Home Power magazine. Ashland OR. www.homepower.com

  37. References: Internet • http://geothermal.marin.org/ on geothermal energy • http://mailto:energyresources@egroups.com • http://www.dieoff.org. Site devoted to the decline of energy and effects upon population • http://www.ferc.gov/ Federal Energy Regulatory Commission • http://www.humboldt1.com/~michael.welch/extras/battvoltandsoc.pdf • http://www.siemenssolar.com/sm110_sm100.html PV Array • http://www.soltek.ca/products/solarmod.htm • http://www.soltek.ca/index.htm • http://www.ips-solar.com/yourproject/costanalysis.htm Cost analysis • http://www.ips-solar.com/yourproject/resource.htm Energy analysis • http://www.aep.com/Environmental/solar/power/ch5.htm Renewable energy • http://ens.lycos.com/ens/dec2000/2000L-12-01-01.html

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