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 (321) my.fit.edu/~fleslie

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

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

US RE Resources Differ Widely

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!”

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

PV System Engineering Decomposition into Functional Components Collect & Distribute Energy Store EnergyRegulate EnergyCollect Energy Use EnergyDistribute EnergyControl Energy Store EnergyRegulate Energy Start Each function drives a part of the design, while the interfaces between them will be defined and agreed upon to ensure follow-on upgrades

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

Solar Energy Intensity Energy from our sun (~1372 W/m 2 ) 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

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

http: Daily load peaking (1 a.m. to midnight graph) megawatts vs. hours Florida Energy Use Varies with the Time of Day (Daily Living) p.m. 7 a.m p.m

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

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 resistance to achieve maximum power for use MPP

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/(m 2 - day); 4.7 equivalent sun hours Throughout the Contiguous United States, daily solar energy varies from <3.0 to 7.0 kWh/(m 2 - day) SUN Latitude Angle My house uses about kilowatt-hours/day

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

Solar Path for Florida Tech 2/21/anyyear

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

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

Roof-top Solar Array Computations Find the south-facing roof area; say 20 ft * 40 ft = 800 ft 2 Assume 120 Wp solar modules are 26 inches by 52 inches; 9.4 ft 2 /120 watt; W/ft 2 Assume 90% of area can be covered, 720 ft 2, ~ 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

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

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

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 Battery AP Vdc, 115 A-hr 20-hour rate

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

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

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!

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

Energy Load Assessment Site: Classroom LoadPower, WNo.Daily Use, hrEnergy, kWh/day Fluorescent Lamp 402*16 = PC & Monitor Projector Laptop Computer Vacuum Cleaner Peak Power kWh/day Simultaneous Power kWh/mo 6427 kWh/year Area = 25ft* 30ft = 750 ft 2 Energy Density = 23 Wh/day/ft hr/avg mo hr/avg mo 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

Some Important Electrical Information P = E I = E 2 /R = I 2 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 ohms per 1000 ft –#12 AWG has a resistance of ohms per 1000 ft –#9 AWG has a resistance of ohms per 1000 ft –#6 AWG has a resistance of ohms per 1000 ft –#3 AWG has a resistance of ohms per 1000 ft

Cost Analysis Spreadsheet

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.: pictures/general/ windfarm/index.asp?i=2 Ref.: education/story/story- images/solar.jpeg Photo of FPL’s Cape Canaveral Plant by F. Leslie,

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

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

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, , TJ807.9.U6B76, ’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.

References: Internet on geothermal energy Site devoted to the decline of energy and effects upon population Federal Energy Regulatory Commission PV Array Cost analysis Energy analysis Renewable energy