Solar Electric Energy Basics:System Design Considerations

Frank R. Leslie B. S. E. E., M. S. Space Technology, LS IEEEAdjunct Professor, Florida Tech, COE, DMES

10/1/2008, Rev. 1.3fleslie @fit.edu; (321) 674-7377

my.fit.edu/~fleslie

Are they having fun?

Why did thishappen?

Does Energy Affect our Lives?

FOXnews 8/15/2003

Happy New Yorkers out for a Stroll!

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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

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US RE Resources Differ Widely

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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!”

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Solar Estimate from FSEC in Cocoa FL

• The “Sunshine State” has as much sunshine as Wyoming

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PV System Engineering Decomposition intoFunctional Components

Collect & Distribute Energy

Store EnergyRegulate EnergyCollect Energy

Use EnergyDistribute EnergyControl Energy

Store EnergyRegulate EnergyStart

Each function drives a part of the design, while the interfaces between them will be defined and agreed upon to ensure follow-on upgrades

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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 receivedhttp://www.fsec.ucf.edu/PVT/Projects/fpl/kev/main.htm#TOP 081001

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

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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

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http://www.dep.state.fl.us/energy/fla_energy/files/energy_plan_final.pdf

http:

• Daily load peaking (1 a.m. to midnight graph)megawatts vs. hours

Florida Energy Use Varies with the Time of Day (Daily Living)

3 - 7 p.m. 7 a.m. 7 - 9 p.m.

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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

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PV Response Characteristics

• 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

http://www.chuck-wright.com/SolarSprintPV/SolarSprintPV.html

MPP

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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)

SUN

Latitude Angle

My house uses about 23 - 40 kilowatt-hours/day

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PV Systems• 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

This solar intensity plot for Cocoa FL shows the cloud effect on what otherwise would have been a cosine effect Ref.: FSEC

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Solar Path for Florida Tech 2/21/anyyear

http://solardat.uoregon.edu/

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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

World Price for Photovoltaic Modules1973-98

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Compiled by Worldwatch Institute

1997

Dol

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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

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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 SM110Maximum power rating, 110 WMinimum power rating, 100 WRated current. 6.3 ARated voltage, 17.9 VShort circuit current, 6.9 AOpen circuit voltage, 21.7 V

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Battery Charge Controller• 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

Soltek Mark IV 20 Amp

Regulator

“Big as a breadbox” for a 4 kW inverter

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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

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Storage Batteries• 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

Soltek Deep-Cycle

BatteryAP-27

12 Vdc,115 A-hr 20-hour rate

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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

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Inverter

• 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

Trace Legend

4 kilowatt Inverter

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Loads• 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

27 watt (100 W

equivalent)Compact

Fluorescent Lamp (CFL)

CFL Costs without replacement labor: $21.30Incandescent Costs with replacement labor: $39.98

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CFL Costs with replacement labor: $23.30Incandescent Costs with replacement labor: $56.54Hint: You can buy a CFL at a large local

discount store for $4.68or six for $7.00!

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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

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Energy Load Assessment• Site: Classroom

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Load Power, W No. Daily Use, hr Energy, kWh/day

Fluorescent Lamp

40 2*16 = 32 8 10.24

PC & Monitor 200 1 24 4.80Projector 600 1 4 2.4Laptop Computer

60 1 2 0.12

Vacuum Cleaner

1560 1 0.023 0.037

Peak Power 1560 17.597 kWh/day

Simultaneous Power

2460 535.6 kWh/mo6427 kWh/year

Area = 25ft* 30ft = 750 ft2

Energy Density = 23 Wh/day/ft2

8766 hr/avg mo

730.5 hr/avg mo

30.4375 avg. day / avg mo

Load Analysis for a Yacht

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

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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

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Cost Analysis Spreadsheet

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PV System HomeworkRenewable Energy Class

PV Design for CabinProf. Frank R. Leslie

10/1/2008

Loads Type Power (W) Time (h) Energy (Wh) Comments1 CFL 13 3 39.0 Daily use1 CFL 13 0.5 6.51 CFL 19 2 38.01 Radio 15 3 45.0

Total 60 max watts 128.5 Wh Total

Margin 50%Margined Load 90 W max 192.75 Wh/day EnergyNominal wire amps 9.5 A (Step 1)Sun-hours per day 5.0 sun-hours December averageFor approximately 192.75 Wh, the Dec. 5.0 sun hours requires PV to yield

38.55 watts PVCabin Use 2 days per weekAdjusted average energy 55.1 Wh

38.55 W module suggests you use a 40.0 W

Battery 12 V Discharge Allowed 20%Indicated Wh 192.75 WhIndicated Ah 16.1 AhBattery size 80.3 Ah 963.75 Wh (Discharging only some 20% extends the life of the battery.)Inverter Size 25% Margin 1.26 NEC code

112.50 W including margin 11.8 ACost Estimates $5 per watt PV $1 per watt a.c. out

$1 per AhPV $192.75 Step 2aBattery $80.31 Step 2bInverter $112.50 $385.56 subtotal Step 2cBalance of system $77.11 20% add-on for BOSTotal System Cost $462.68

Line Cost 1.00 mile to cabin5,000$ /mile 5,000$ estimated cost for utility line to cabin

Break-even length 0.093 miles 489 feet

Better to use solar? Yes, the utility line is too costly!

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

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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

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Thank you!

Questions? ? ?My website: my.fit.edu/~fleslie

for presentationsRoberts Hall weather and energy data:

my.fit.edu/wx_fit/roberts/RH.htmDMES Meteorology Webpage: my.fit.edu/wx_fit/?q=obs/realtime/roberts

Is a Solar Roof Practical?

Sun intensity at surface ~1000 watt / square meterPV cells about 15% efficient = ~150 watt / square meterRoof might be about 20 x 40 feet = 800 square feet; 90% coverage = 720 square feetA 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 powerOptimally, roof array could yield 9202 watts for 5.5 hours/average day = 51 kWh each day on average; average house might need 30 kWhStorage would provide energy at night and during cloudy weather, but increases the costCurrent cost estimates are about $5/W & $0.06 to $0.20 per kWh vs. $0.07 from utilityUtility line extension costs about $18,000 to $50,000 per mile

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

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