Solar Thermal

11.1 Solar Thermal Energy
Frank R. Leslie,
B. S. E. E., M. S. Space Technology, LS IEEE
2/12/2010, Rev. 2.0
fleslie; (321) 674-7377
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 MWp solar thermal plant at KSC
11.1.1 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
11.1.1 About This Presentation
 11.1.2 US Solar Energy Distribution
 11.1.3 Flat Collector Types
 Water Heaters
 11.1.4 Reflectors
 11.1.5 Solar Thermal Electric Power
 11.1.6 Balance of System
 11.1.7 Food Processing and Cookers
 11.1.8 Advantages and Disadvantages
 11.2 Conclusion
11.1.2 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
Arizona has clearer skies than Florida. Ref.: Innovative Power Systems
From 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
 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)
090211 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
Sustainable Living and Renewable Energy Fair
at Ft. Collins CO, 2003
Photo by F. Leslie, 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
090211 Typical Water Heater
 A small pump
moves colder
water into the
whenever a
controller senses
that the panel is
hotter than the
 Electrical heating
is used if there is
no sunshine
 The system shown
adapts a
conventional tank
050210 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
090211 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.
090211 Solar Ponds
 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
This El Paso solar pond is run by the Univ. of Texas
080205 Axicon Conical Reflector
 A polished mirror
cone reflects the
sun onto a waterfilled pipe and
can boil the water
 The surface of
polished metal or
foil is cheap to
make and form
 First used in the
090211 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
 Cylindrical reflector
can be on one half
of the vacuum tube
and approximates
the parabolic shape
Y = 3/16 * X2 Solar Focusing Systems: Paraboloidal
 The focus at one
spot is achieved
by the parabolic
 Flat mirror
segments may be
used to
approximate the
curve at much
lower expense
050210 Solar Electricity Generation System (SEGS)
 SEGS are a
well developed
design, and
thus the
systems may
be reproduced
and installed
 FPL owns
plants in the
 There are ten
SEGS in all
090211 SEGS (Solar Electricity Generation System)
 Luz International,
an Israeli
company, built
these nine large
systems in
 Luz went
bankrupt in 1991,
but the systems
are still supplying
 Sunray Energy,
Inc is an owner
 FPL Energy
Group is the
owner of SEGS
III through IX
090211 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
 In the background
is the control and
engineering building
090211 Parabolic Trough System Schematic
 Natural gas is used to increase the temperature of the
steam 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
050210 Eurotrough Field Layout
 Each pair of tubes
forms a loop
from the central
 The power block
is centrally
located to
minimize thermal
 Oil is heated to
050210 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
090211 Solar Power Towers: Sandia CRTF
 Flat mirrors are aimed to focus sun at the receiver target to melt salt
030215 Solar Power Towers: Sandia CRTF
5 MW power
Flux to 280 W/cm2
Each heliostat is separately
driven to focus its beam on
the receiver
Missile nosecones have
been tested in the hot spot
 The Receiver Tower has multiple target areas
030214 Solar Power Towers: Sandia CRTF
 The large tank
stores energy to
use during cloud
passage or at
 The output power
is extracted at a
constant rate
090211 Solar Power Towers: Barstow CA
 Flat mirrors are aimed to melt salt at the receiver target
030207 Solar Power Mirrors
 Disks of thin aluminized mylar are pulled into rings
under slight vacuum to form a paraboloidal surface
050210 renewable/solar/ 75 kW Solar Thermal Test Facility
 These Sandia trackers have a paraboloidal dish surface
similar to a radar antenna
050210 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
11.1.6 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
090211 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
060205 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
050210 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!)
090211 A Traditional Sun Cooker
This cooker at the
2003 Ft. Collins CO
Sustainable Living
Fair was in too
shady a location to
The polished aluminum wings provide more reflected light
energy into the box to increase the temperature to about
270 ºF
090211 A Paraboloidal Sun Cooker
The black pot suspended at
the focus can be moved on
and off for loading and
The mount allows vertical
shifts while the entire unit is
turned to track the sun
The mirror is made of
segments to approximate a
090211 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!
from two
that are
sections of
11.1.8 Advantages and Disadvantages
 Solar thermal systems work efficiently at lower
temperatures since cheap flat collectors can work
 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 MWe, while a typical
utility plant is 500-1500 MWe
 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
References: Books
 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
 Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351
pp. ISBN 0-8493-1605-7, TK1541.P38 1999, 621.31’2136
 Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press,
2000, 911 pp. ISBN 0-12-656152-4.
References: Websites, etc. Excellent site!
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