11.1 Solar Thermal Energy Frank R. Leslie, B. S. E. E., M. S. Space Technology, LS IEEE 2/12/2010, Rev. 2.0 (321)

In Other News... Senate passed the energy bill; now consolidation with House bill forms a new H.R.1 bill at $789B Some rail enhancements, some renewables, some RE job training In Florida, push to regulate tailpipe emissions starting with new models in 2013 (CAFÉ standards) that will lead to more electric and hybrid cars Push to approve FPSC recommendations to require utilities to produce 20% power from RE by 2020 and 7% by 2013 FPL decides to build 10 MW p solar thermal plant at KSC

Overview of Solar Thermal Systems Solar thermal systems convert sunlight to heat Categorized by reflector/collector types  None; uses lens; regular or Fresnel  Flat mirror (possibly many of them)  Parabolic/cylindrical: single axis forms “trough”  Paraboloidal: axis of revolution forms “dish”  Spherical: approximates paraboloid, but focus can move to track sun

About This Presentation US Solar Energy Distribution Flat Collector Types  Water Heaters Reflectors Solar Thermal Electric Power Balance of System Food Processing and Cookers Advantages and Disadvantages 11.2 Conclusion

US Solar Energy: Thermal Quality of heat; the value in temperature  Low-temperature extraction of heat from ground; ~70° F to 80° F  Water heating for home and business; ~90° F to 120° F  High-temperature process-heating water for industry; ~200° F to 400° F Solar thermal power plants; ~1000° F or higher From Arizona has clearer skies than Florida. Ref.: Innovative Power Systems

Flat-Plate Absorbers Perforated black metal sheet leaks air in through slots, and a fan pushes/pushes hot air to desired area Trombe Wall absorbs solar heat and stores it  Glass wall plus water barrels often used  Warmed air is slowly released into the building at night Water heater with roof solar collector (I like mine) Food dryers circulate warmed air over cut food to remove moisture and preserve it; dried fruit, etc. Wood dryers “cure” green wood to reduce shrinkage after it is cut to size (manufactured)

A Solar Wall Collector This perforated black sheet metal is fastened directly to a building exterior A small computer fan pulls outside air into the slots, it is warmed, and the air is pushed through a pipe to the interior of the building Photo by F. Leslie, 2003 Sustainable Living and Renewable Energy Fair at Ft. Collins CO, 2003

Trombe Wall Collector A Trombe wall has thermal mass placed behind a south wall of glass Sometimes barrels filled with water are stacked in this space as a thermal storage capacitor The air in this area is heated and then passes into the rest of the house At night, the thermal inertia of the water-filled drums keeps heating the air These areas are often used to grow plants in winter

Typical Water Heater A small pump moves colder water into the collector whenever a controller senses that the panel is hotter than the tank Electrical heating is used if there is no sunshine The system shown adapts a conventional tank

Solar Water Heaters Household Use  Rooftop; one or two panels about 3 ft x 10 ft  Rather standard appearance water tank, but has multiple water connections for collector in and out; more insulation Pipes are cold in, hot out, collector in, and collector out A temperature sensitive resistor is built in for control 10 kilohm thermistor  Ingenious designs allow a standard water heater tank to be used by tapping the safety thermostat port and the drain port Industrial-grade Parabolic Heaters  Multiples of collectors used to achieve more Btus Used at schools, prisons, or military bases

Rooftop Heaters Thermosyphon units place the storage tank above the panels, and heated water rises into the tank. Cooler return water flows from the tank to the bottom of the collectors to enter at the cold end. No pump is needed to circulate the water.

Solar Ponds This El Paso solar pond is run by the Univ. of Texas Fresh water floats on top of salt water Solar heating of the brine raises its temperature to 85°C (185°F) A Rankin Cycle engine turns a generator to drive the utility grid with up to 70 kW

Axicon Conical Reflector A polished mirror cone reflects the sun onto a water- filled pipe and can boil the water The surface of polished metal or foil is cheap to make and form First used in the 1800s

Solar Focusing Systems: Parabolic Focuses parallel rays to a line A black pipe is placed with its center at the focus Pipe can be in a vacuum or could have a glass cover tube to reduce convection Cylindrical reflector can be on one half of the vacuum tube and approximates the parabolic shape Y = 3/16 * X 2

Solar Focusing Systems: Paraboloidal The focus at one spot is achieved by the parabolic cross-section Flat mirror segments may be used to approximate the curve at much lower expense

Solar Electricity Generation System (SEGS) SEGS are a well developed design, and thus the systems may be reproduced and installed readily FPL owns California plants in the Mohave Desert There are ten SEGS in all

SEGS (Solar Electricity Generation System) Luz International, an Israeli company, built these nine large systems in California Luz went bankrupt in 1991, but the systems are still supplying power Sunray Energy, Inc is an owner FPL Energy Group is the owner of SEGS III through IX engnet.anu.edu.au

Parabolic Trough Collectors This large array shows the gaps needed to keep one row from shadowing the next row Note the flexible pipe at this end that circulates the thermal oil regardless of the tilt angle In the background is the control and engineering building

Natural gas is used to increase the temperature of the steam Parabolic Trough System Schematic

Eurotrough Reflector The focus pipe has flexible pipes on the ends to allow rotary motion to track the sun, while the absorber pipe is kept at the heat focus

Each pair of tubes forms a loop from the central manifolds The power block is centrally located to minimize thermal losses Oil is heated to ~500°C Eurotrough Field Layout

Solar Power Towers Solar power towers were developed at Sandia, south of Albuquerque, New Mexico  Salt was melted by the focused heat  If the sun was blocked, the salt could harden in the pipes, requiring torches to get it flowing again  The heliostat mirrors are about ten feet square and mounted on the azimuth-elevation mounts (like a radar antenna mount) about 10 feet off the ground The larger version with a full circle array, Solar One, was built near Barstow, CA  Water is used, but the system is being modified for molten salt

Solar Power Towers: Sandia CRTF Flat mirrors are aimed to focus sun at the receiver target to melt salt

Solar Power Towers: Sandia CRTF The Receiver Tower has multiple target areas MW power Flux to 280 W/cm 2 Each heliostat is separately driven to focus its beam on the receiver Missile nosecones have been tested in the hot spot

Solar Power Towers: Sandia CRTF The large tank stores energy to use during cloud passage or at dusk The output power is extracted at a constant rate

Solar Power Towers: Barstow CA Flat mirrors are aimed to melt salt at the receiver target

Solar Power Mirrors Disks of thin aluminized mylar are pulled into rings under slight vacuum to form a paraboloidal surface renewable/solar/

These Sandia trackers have a paraboloidal dish surface similar to a radar antenna kW Solar Thermal Test Facility

The Axicon Steam Generator Collins 1_29

Solar Path vs. Collector Weight and Size The Sun’s changing position requires moving the focus during the day  Solar noon elevation changes annually as the sun declination angle changes Massive structures require high power to move them  The STTF has 11-meter reflectors Lighter components can move sun’s image Structures have a lot of sail area, and high winds could damage the reflectors  Reflectors are moved to a vertical stow position that minimizes wind torques when weather warnings arise

Balance of System (BOS) You need all this other stuff to make it work!  Heat exchangers, boilers  Circulating pumps for the heat fluid  Heat storage tanks, special thermal oils  Automated controls  Natural gas peaking for steam high temperature  Rotary expanders (positive displacement steam motors) can substitute for a turbine  Wash truck and sprayer to clean the desert dust off the mirrors

Food Dryers Heated air passes over the food  drying the moisture and preserving it Separate trays allow keeping fruits or nuts of the same dryness together for processing The glass or plastic top allows sunlight in but reflects infrared energy back onto the food The food is in the thick end of the box

Solar Coffee Bean Dryers Coffee is normally dried on open tables in the sun These solar dryers were built to speed the process Heat retention increases the inside air temperature Materials are inexpensive and local workers can build the driers

Solar Coffee Bean Roaster The Solar Roast coffee company built and uses this segmented mirror to heat the receiver and roast the coffee (incredibly expensive) Roasting coffee beans sound like popcorn popping They will move from Oregon to Southern Colorado for more sun and year-around operation Find out more at or their ad in Home Power magazine  (not a paid announcement!)

A Traditional Sun Cooker The polished aluminum wings provide more reflected light energy into the box to increase the temperature to about 270 ºF This cooker at the 2003 Ft. Collins CO Sustainable Living Fair was in too shady a location to work

A Paraboloidal Sun Cooker The black pot suspended at the focus can be moved on and off for loading and serving The mount allows vertical shifts while the entire unit is turned to track the sun The mirror is made of segments to approximate a paraboloid

A Paraboloidal Solar Cooker This massive servotracker unit was originally developed at Sandia and later given to the local Renewable Energy Society An oven area at the back can cook three pans of cookies A twin unit was shown at the 2004 MREA Fair in WI Photo by F. Leslie, 2003 The segments approximate a paraboloidal

Making Solar Pancakes A whimsical way to make money at a European renewable energy fair! Solar pancakes from two reflectors that are offset sections of a paraboloid

Advantages and Disadvantages Solar thermal systems work efficiently at lower temperatures since cheap flat collectors can work economically Reflecting systems need broad acceptance angles so that the optics don’t cost too much  The beam need only strike the absorber, not form a picture image (noncoherent) Reflectors must be kept clean by frequent washing Reflectors wouldn’t be wise in hailstorm country!  Could be protected by a quickly erected tarp

11.1 Conclusion: Solar Thermal Solar thermal systems are cost effective at low temperatures for water heating or cooking Water heaters are energy savers, but initial cost dissuades many from using them Power tower electricity cost is at $6/W peak, too high to compete with PV power Massive power tower yields 10 MW e, while a typical utility plant is MW e Power towers aren’t likely to be economically practical for some time Solar dryers, cookers, and ovens are relatively inexpensive and available for remote cooking  Backpack versions are sold for camping

Olin Engineering Complex 4.7 kW Solar PV Roof Array Questions?

References: Books 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 Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351 pp. ISBN , TK1541.P , ’2136 Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press, 2000, 911 pp. ISBN

References: Websites, etc Excellent site! king/ ___________________________________________________________________ Site devoted to the decline of energy and effects upon population Federal Energy Regulatory Commission solstice.crest.org/ dataweb.usbr.gov/html/powerplant_selection.html

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