Transcript Solar Basics - Access Florida Tech

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? 
Happy New Yorkers out for a Stroll!
Are they
having
fun?
Why did
this
happen?
FOXnews 8/15/2003
080820
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
US RE Resources Differ Widely
081001
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
Solar Estimate from FSEC in Cocoa FL
• The “Sunshine State” has as much sunshine as Wyoming
081001
PV System Engineering Decomposition into
Functional Components
Collect & Distribute
Energy
Start
Collect Energy
Regulate Energy
Store Energy
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
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
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
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
Florida Energy Use Varies with
the Time of Day (Daily Living)
• Daily load peaking (1 a.m. to midnight graph)
megawatts vs. hours
http:
3 - 7 p.m.
7 a.m.
7 - 9 p.m.
http://www.dep.state.fl.us/energy/fla_energy/files/energy_plan_final.pdf
081001
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
PV Response Characteristics
http://www.chuck-wright.com/SolarSprintPV/SolarSprintPV.html
MPP
•
•
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 resistance
to achieve maximum power for use
081001
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 SUN
energy varies from <3.0 to 7.0 kWh/(m2 - day)
My house uses about
23 - 40 kilowatt-hours/day
Latitude Angle
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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
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http://solardat.uoregon.edu/
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
World Price for Photovoltaic Modules
1973-98
90.0
80.0
1997 Dollars Per Watt
•
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
1970
1975
1980
1985
1990
1995
2000
Compiled by Worldwatch Institute
PV prices are falling, though still
relatively expensive compared to
wind or fossil utility power
081001
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
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
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
081001
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
DeepCycle
Battery
AP-27
12 Vdc,
115 A-hr
20-hour
rate
081001
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
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 lowvoltage directly at higher
efficiency with special lamps
Trace
Legend
4 kilowatt
Inverter
081001
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: $21.30
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 $4.68
or six for $7.00!
081001
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
Energy Load Assessment
• Site: Classroom
Load
Power, W
No.
Daily Use, hr
40
2*16 = 32
8
PC & Monitor
200
1
24
Projector
600
1
4
2.4
Laptop
Computer
60
1
2
0.12
Vacuum
Cleaner
1560
1
0.023
Peak Power
1560
17.597
kWh/day
Simultaneous
Power
2460
535.6 kWh/mo
6427 kWh/year
Fluorescent
Lamp
Area = 25ft*
30ft = 750 ft2
8766 hr/avg
mo
730.5 hr/avg
mo
Energy,
kWh/day
10.24
4.80
0.037
Energy Density
= 23
Wh/day/ft2
30.4375 avg.
day / avg mo
081001
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
081001
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
Cost Analysis Spreadsheet
PV System Homework
Renewable Energy Class
PV Design for Cabin
Prof. Frank R. Leslie
10/1/2008
Loads
1
1
1
1
Type
CFL
CFL
CFL
Radio
Power (W) Time (h)
Energy (Wh) Comments
13
3
39.0 Daily use
13
0.5
6.5
19
2
38.0
15
3
45.0
Total
60 max watts
128.5 Wh Total
Margin
50%
Margined Load
90 W max
192.75 Wh/day Energy
Nominal wire amps
9.5 A
(Step 1)
Sun-hours per day
5.0 sun-hours
December average
For approximately
192.75 Wh, the Dec.
5.0
sun hours requires PV to yield
38.55 watts PV
Cabin Use
2 days per week
Adjusted average energy
55.1 Wh
38.55 W module suggests you use a 40.0 W
Battery
12 V
Indicated Wh
Indicated Ah
Battery size
(Discharging only some
Inverter Size
Cost Estimates
$5
$1
$192.75
$80.31
$112.50
$77.11
$462.68
PV
Battery
Inverter
Balance of system
Total System Cost
Line Cost
$
5,000 /mile
Break-even length
Better to use solar?
192.75
16.1
80.3
20%
$
Discharge Allowed 20%
Wh
Ah
Ah
963.75 Wh
extends the life of the battery.)
25% Margin
1.26 NEC code
112.50 W including margin
11.8 A
per watt PV
$1 per watt a.c. out
per Ah
Step 2a
Step 2b
$385.56 subtotal
Step 2c
20% add-on for BOS
1.00 mile to cabin
5,000 estimated cost for utility line to cabin
0.093 miles
489 feet
Yes, the utility line is too costly!
081001
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/storyimages/solar.jpeg
Photo of
FPL’s
Cape
Canaveral
Plant by
F. Leslie,
2001
081001
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 fossilfuel costs will soon surpass them
• At that time, PV will compete
with wind energy, which is
currently competitive with fossil
fuels
081001
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
Is a Solar Roof Practical?
Sun intensity at surface ~1000 watt / square meter
PV 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
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
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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