Solar Electric Energy Basics Frank R. Leslie, BSEE, MS Space Technology 9/19/2001 (321)

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

Energy Considerations for 2050 Fossil-fuel energy will deplete in the future; millions of years to create that much fuel US oil production peaked about 1974; world energy will peak about 2009 or so Renewable energy will become mandatory, and our lifestyles may change Transition to renewable energy must occur well before a crisis occurs

PV Cell Basics Semiconductor of transparent positive silicon and negative silicon backing Incoming light (photons) cause energized electrons to move to the n-silicon and out the connector Nominal voltage of 0.55 V requires series connections to get useful voltage, 16 V Short circuit current is proportional to light intensity Maximum output when normal to cell is pointed at light (cosine of sun offset angle) Ref.: FSEC

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

PV System 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

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

Charge controller Limits charge current to protect battery Disconnects battery if voltage falls too low (10.6 V is typical) Removes charge current if voltage rises too high (14V is typical) Shifts output of source to a load (water heater or electric furnace) if battery is fully charged Soltek Mark IV 20 Amp Regulator “Big as a breadbox” for a 4 kW inverter

Energy Storage Energy may be stored chemically, in water behind high dams, electrically, flywheels, and compressed air caverns. Hydrogen (like electricity) is an energy carrier, not a direct fuel source, but compressed H 2 can also be stored Hydrogen can be made by electrolysis of water or “cracking” methane World's largest storage dam, Uganda's Owen Falls Dam. The hydroelectric station at the dam supplies most of the electricity requirements of Uganda, and parts of Kenya. (Photo:Faculty of Engineering, Kasetsart University, Thailand)

Storage Battery Lead-acid (car) batteries most economical; but should be deep-cycle Critical rating is 20-hour value Charge cycle is ~60-70% efficient -- rather wasteful Requires maintenance to ensure long life A home might have ten of these batteries Soltek Deep-Cycle Battery AP Vdc, 115 A-hr 20-hour rate

Inverter Inverter converts low voltage direct current to 120 Vac Loads can use low-voltage directly at higher efficiency 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! 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!

Energy Transmission Energy transmission lines may lose 5% en route Only three electrical grids for all Contiguous US (CONUS) Mine-head coal plants avoid coal trains and the power to drive them Hydrogen pipelines could transport gaseous energy cheaply and parallel natural gas lines Guyed transmission tower with multiple conductor spreaders (note at left) to increase current capacity or reduce loss

Energy in Transportation Air and ground transportation require energy-dense fuels (liquids) Fixed natural gas plants compete with CNG for cars and trucks Research is on-going with a Lear jet fueled with hydrogen from two large high-pressure vessels running lengthwise over the passenger compartment -- a dubious location Compressed natural gas car at FSEC

Generic Trades in Energy Energy trade-offs 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, 2001

Conclusion Solar electric energy is best applied where the cost justifies; remote from the grid 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

Questions & Answers

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