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Solar Electric Energy Basics:System Design Considerations
Frank R. LeslieB. 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
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Energy Considerations for 2050
Fossil-fuel energy willdeplete in the future;millions of years to createthat much cheap fuel
US oil production peakedabout 1974; world energywill peak about 2009 or so
The US imports about 10million barrels crude oil/day
Renewable energy willbecome mandatory, and ourlifestyles may change
Transition to renewableenergy must occur wellbefore 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 CO 2 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 & DistributeEnergy
Store Energy Regulate Energy Collect Energy
Use Energy Distribute Energy Control Energy
Store Energy Regulate Energy Start
Each function drives a part of the design, while the interfaces between them willbe 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
http://images.google.com/imgres?imgurl=www.eren.doe.gov/femp/techassist/images/07172.jpg&imgrefurl=http://www.eren.doe.gov/femp/techassist/sustainability.html&h=294&w=200&prev=/images%3Fq%3Dbuilding%2Broof%2Bphotovoltaic%26svnum%3D10%26hl%3Den%26lr%3D%26ie%3DU8/3/2019 Solar Electric System Design
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Solar Energy Intensity
Energy from our sun (~1372 W/m 2) is filtered throughthe atmosphere and is received at the surface at ~1000watts per square meter or less; average is 345 W/m^2
Air, clouds, rain, and haze reduce the received surfaceenergy
Capture is from heat (thermal energy) and byphotovoltaic cells yielding direct electrical energy
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http://images.google.com/imgres?imgurl=www.eren.doe.gov/femp/techassist/images/07172.jpg&imgrefurl=http://www.eren.doe.gov/femp/techassist/sustainability.html&h=294&w=200&prev=/images%3Fq%3Dbuilding%2Broof%2Bphotovoltaic%26svnum%3D10%26hl%3Den%26lr%3D%26ie%3DU8/3/2019 Solar Electric System Design
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Energy Usage & Conservation
The loads supported bythe system must beminimized to match theavailable energy
Load analysis shows thelargest concerns thatmight be reduced to cutcosts
Conservation by
enhanced buildinginsulation and reducedlighting loads
Increased efficiency of energy plants willconserve 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 withthe 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 siliconand negative silicon backing
Incoming light (photons)cause energized electrons tomove to the top n-siliconand out the connector
Nominal voltage of 0.55 Vrequires series connectionsto get useful voltage, 17 V
Short circuit current isproportional to light intensity
Maximum output occurs whennormal to cell is pointed atlight (cosine of sun offsetangle)
Ref.: FSEC
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PV Response Characteristics
As light intensity increases, the change in current is much greater than thechange 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 AffectPotential Capture
A flat-plate collector aimed normal to the sun (directly atit) will receive energy diminishing according to theamount of atmosphere along the path (overhead air
mass 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 radiationis 4.0 to 4.5 kWh/(m 2 - day); 4.7 equivalent sun hours
Throughout the Contiguous United States, daily solarenergy varies from
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PV Systems
PV modules of 120 W costabout $400
Mounting angles to matchsun --- fixed or tracking
Average module slopeangle is equal to latitude Zoning and regulations ---
Not In My Back Yard(NIMBYs) problem
Protection required forelectric line workers dueto islanding backfeed
This solar intensity plot for Cocoa FL showsthe cloud effect on what otherwise wouldhave been a cosine effect Ref.: FSEC
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Solar Path for Florida Tech 2/21/anyyear
http://solardat.uoregon.edu/
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Solar Energy: Photovoltaic Sunlight to Electricity
Photovoltaic cells typicallycan extract about 15-17%of incoming solar energy;theoretical is about 31%;$/W is the key(~$3.50/W, 2007)
Low voltage direct currentis produced at about 0.55volt per cell; clusters areseries-connected for ~17volts output for charging a12 volt system
Arrays of cells (modules)can be fixed or can track
the sun for greater energygain Storage is required unless
the energy is inverted to120 Vac to synchronouslydrive the utility grid
PV prices are falling, though stillrelatively expensive compared towind or fossil utility power
World Price for Photovoltaic Modules1973-98
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
1970 1975 1980 1985 1990 1995 2000
Compiled by Worldwatch Institute
1 9 9 7 D o l
l a r s
P e r
W a t
t
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Collector-Module Sizing
Most manufacturers modules now average about 120watts 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 thecover glass
After the first 10% decline, there is little change inpeak output
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Roof-top Solar Array Computations
Find the south-facing roofarea; say 20 ft * 40 ft = 800 ft 2 Assume 120 Wp solar
modules are 26 inches by 52inches; 9.4 ft 2/120 watt; 12.78W/ft2
Assume 90% of area can becovered, 720 ft 2, ~ 9202 W and that there are 5.5
effective hours of sun/day; 51kWh/day
The south-facing modules aretilted south to the latitudeangle
76 modules would fit the area,but 44 would provide anaverage home with 30kWh/day and cost ~$17600 formodules alone, ~one mile ofpowerline
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
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Battery Charge Controller
Limits charge current to protectbattery from overheating anddamage that shortens life
Disconnects battery loads if voltage falls too low (10.6 V istypical)
Removes charge current if voltage rises too high (14V istypical)
Regulates charge voltage toavoid battery water gassing
Diverts output of source to asecondary load (water heater orelectric furnace) if battery isfully charged Saves energy wisely
Soltek Mark IV 20 Amp
Regulator
Big as a breadbox for a 4 kW
inverter 081001
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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 aremost economical; but mustbe deep-cycle type
Critical rating is 20-hourvalue or Reserve Capacity(RC) in minutes at 25A load
Charge cycle is ~70%efficient -- rather wasteful
Requires maintenance toensure 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
Battery AP-2712 Vdc,
115 A-hr
20-hourrate
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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 theleft is the sizeof a car battery;the one on theright has muchmore capacity
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Inverter
The inverter converts lowvoltage (12V to 100s V)direct current to 120 Vac
Synchronous inverters maybe inter -tied with powerline to reduce billable energy
In net metering states, theenergy is metered at thesame rate going into and out
of the electrical grid --- nostorage required (except foroutages)!
Loads can use 12 volt low-voltage directly at higherefficiency with special lamps
TraceLegend
4 kilowatt
Inverter081001
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Loads
Household load analysisestimates the peak andaverage power and energyrequired
Some might be reduced or
time-shifted to decreasesystem costs
Incandescent lamps producefar more heat than light;CFLs provide ~100 W light
equivalent at 27 W load
27 watt(100 W
equivalent)Compact
FluorescentLamp (CFL)
CFL Costs without replacement labor: $21.30
Incandescent Costs with replacement labor: $39.98
____________________________________
CFL Costs with replacement labor: $23.30Incandescent Costs with replacement labor: $56.54
Hint: You can buy a CFL at a large localdiscount store for $4.68
or six for $7.00!
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Load Analysis Spreadsheet
A spreadsheet program like Excel will speed analysisof the various loads, their use time, peak power, andenergy required
Once done, modifications for other systems are easy
List the loads, enter the power, time per day, andcompute the rest
From total energy required and total power, one cancompute 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
FluorescentLamp
40 2*16 = 32 8 10.24
PC & Monitor 200 1 24 4.80
Projector 600 1 4 2.4
LaptopComputer
60 1 2 0.12
VacuumCleaner
1560 1 0.023 0.037
Peak Power 1560 17.597
kWh/daySimultaneousPower
2460 535.6 kWh/mo6427 kWh/year
Area = 25ft*30ft = 750 ft 2
Energy Density= 23Wh/day/ft 2
8766 hr/avgmo
730.5 hr/avgmo
30.4375 avg.day / avg mo
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Load Analysis for a Yacht
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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 resistanceloss 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 flyingmolten metal Special dc-rated fuses and circuit breakers are required
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Some Important Electrical Information P = E I = E 2 /R = I 2R,
where P is power (instantaneous), E is electromotive force, Iis 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 ft081001
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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(Discharg ing on ly some 20% extends the l ife of t he ba tt ery. )
Inverter Size 25% Margin 1.26 NEC code112.50 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!
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Generic Trades in Energy
Energy trade-offs arerequired to make rationaldecisions
PV is expensive ($5 per wattfor hardware + $5 per watt
for shipping and installation= $10 per watt)compared to wind energy($1.5 per watt forhardware + $5 per watt
for installation = $6 perwatt 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 FPLsCapeCanaveralPlant byF. Leslie,2001
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Conclusion
Solar electric energy is bestapplied where the cost justifies;remote from the grid or forindependent backup power
True costs of fossil-fuel pollutionand subsidies are not easilyfound -- controversies exist
PV costs are falling, but fossil-
fuel costs will soon surpass them At that time, PV will competewith wind energy, which iscurrently competitive with fossilfuels
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080710
Thank you!
Questions? ? ?My website: my.fit.edu/~fleslie
for presentationsRoberts 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
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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 = 720square feet
A 120 watt solar module is about 26 inches x 52 inches = ~ 9.4 sq. ft, thuspeak 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 = 51kWh each day on average; average house might need 30 kWh
Storage would provide energy at night and during cloudy weather, butincreases the cost
Current cost estimates are about $5/W & $0.06 to $0.20 per kWh vs. $0.07from utility
Utility line extension costs about $18,000 to $50,000 per mile
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References: Books, etc.
Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, 1992. 0-262-02349-0,TJ807.9.U6B76, 333.7940973. 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
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References: Internet
http://geothermal.marin.org/ on geothermal energy http://mailto:[email protected] 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