solar electric system design

8/3/2019 Solar Electric System Design

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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%3DU


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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%3DU


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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

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