ABSTRACT
Wind from the Sun is a new technology for obtaining power
from the sun and wind. This hybrid system turns the sun's light into heat, then
uses that heat to create a wind within a horizontal large-diameter pipe. The wind inside the pipe is converted
into electric power using a series of wind turbines.
1.
The
Collector: A large area of land is covered with a material
with low reflectivity (dark in color). This material converts the sun's light into
heat.
2.
Heating
the air: The collector increases in temperature in the
sunlight, causing the air above the collector to also increase in temperature.
The heated air increases in volume and decreases in pressure.
3.
Hot
Air Rises: The hot air over the collector also rises. Cooler
air moves in from the land around the collector to replace the rising hot air.
The collector continually heats the air. Thus, the air pressure over the
collector remains lower than the air pressure over the surrounding land.
4.
Air
Channel: A very large diameter pipe (or "air
channel") connects the area of low air pressure over the collector to an
area of higher air pressure away from the heat of the collector. Air moves
through the air channel from high pressure to low pressure, creating wind.
5.
Wind
Turbines: A series of very large, pressure-staged wind
turbines within the air channel turn the wind into electricity with a high
degree of efficiency. Each air channel can contain several turbines and a large
collector may support more than one air channel.
This technology is
very new, untested, and unproven. The aim of the Wind From The Sun project is to prove that the technology
works and to determine how much power can be derived from such a system.
CONTENTS
Ø
Introduction.
Ø
The solar collector.
Ø
Air movement.
Ø
Air channel.
Ø
Wind velocity.
Ø
Wind turbine.
Ø
Alternate Design Probability.
Ø
Conclusion.
Ø
Reference.
INTRODUCTION
In nature it is solar power that creates wind power.
Consider the example of the sea breeze. The sun heats both land and sea, but
the land heats up more quickly and reaches a higher temperature than the sea.
The air over the land becomes hotter than the air over the sea and the hot air
rises, creating an area of lower air pressure(close to the surface).Air moves
from the area of higher pressure over the sea to the area of lower pressure
over the land. The cool sea air heats up as it moves over the land and so it
rises, creating a cycle. The result of this cycle is a steady wind moving from
the sea to the land .In this example from nature, the land is acting like a
solar collector, changing sunlight into heat. The heated land heats the air and
creates a wind. Wind turbines can
harvest this wind energy.
Wind from the Sun is a new technology for obtaining power from the sun and wind. This
hybrid system turns the sun's light into heat, then uses that heat to create a
wind within a horizontal large-diameter pipe. The wind inside the
pipe is converted into electric power using a series of wind turbines A wind
from the sun power plant would imitate this same type of system that occurs in
nature, but with a greater degree of control and predictability. This will
results in amore reliable wind with a higher average wind speed.
2. THE SOLAR COLLECTOR
2.1
Materials
A large area of land is covered with a material with low
reflectivity (dark in color). This material collects the sun's energy in the
form of heat, and is therefore called the collector. The area of land covered
is circular.
More
of the sun's light turns to heat when it strikes a dark material. If the
material were a light color, such as white, then much of the sun's light would
be reflected. Black is the best color to use for this purpose.
The collector should be black ceramic
gravel. Tiles of black ceramic would work, but it might be time-consuming to
layout all of the tiles. Gravel will allow water to pass through into the
ground below whenever it rains. Perhaps the best material to use for the
collector would be black ceramic gravel. This material should be easy to spread
over the ground and will allow rain to pass through to the ground.
The solar collector converts the energy
from sunlight into heat. The heat from the collector causes the air above the
collector to also increase in temperature. The purpose of the solar collector
is to heat the air so that it will rise, creating a wind. The energy in the
wind is converted to electricity using wind turbines.
Any matter
black color will absorb, rather than reflect, visible light. However, in order
to convert as much of the sun’s energy to heat as possible, the solar collector
must also absorb light well in the ultraviolet and infrared wavelengths. The
energy from sunlight is approximately 5% ultraviolet light, 44% visible light
and 51% infrared light. A carbon-based pigment (known commercially as “carbon
black”) will absorb greater than 90% of the energy from light across the
spectrum of ultraviolet, visible, and infrared wavelengths. Carbon black is a
common pigment used in industrial paints.
Any one of a large number
of different materials could be covered with carbon black pigment for use in
the solar collector. The solar collector material does not need to have high
thermal mass. Since the goal is to transfer the heat from the collector to the
air, the collector does not need to retain the heat. Black ceramic gravel is
heavy enough not to blow away in the wind, will allow rainfall to pass through
to the ground beneath, and can be spread over a large area of land by
machinery, requiring much less manual labor. It is also less expensive to
manufacture than many other materials. Black ceramic gravel is one of the
better materials for the solar collector. The black color would have to come
from carbon black pigment, either painted on the ceramic after firing, or fired
into the clay itself. A 100 Megawatt solar chimney power plant is projected to
have an increase in air temperature of 35.7 degrees Centigrade . This
comparison suggests that the small scale tests showed a high enough increase in
temperature to drive a Wind From The Sun power plant. In other words, an
area of ceramic heated by the sun gets hotter if it is surrounded by more hot
ceramic. The surrounding hot ceramic keeps the ceramic within from losing much
of its heat. A very large area of land covered with black ceramic gravel should
theoretically increase in temperature to a much greater extent than a small
area of land.
2.2 Size
The
optimum size of the collector depends on a number of factors. Since a Wind
from the Sun power plant has not yet been built, the optimum size can only
be estimated at this point in time. A comparison with a similar technology, the
Solar Chimney power plant, will give us a reasonable starting point for such an
estimate.
The
Solar Chimney power plant operates on similar principles to the Wind from
the Sun power plant.
Both
use a solar collector to heat air. Both generate wind from the rising of the
heated air. According to Schlaich, a Solar Chimney power plant with a solar
collector of 4000 meters in diameter and a 1500 meter tall chimney will produce
600 GWh/y. The area of such a collector would be approximately 12.566 million
square meters (Лr2). A Solar Chimney power plant is planned for Mildura , Australia
The
efficiency is calculated by dividing power output (600 and 500 GWh/y,
respectively) by power input (28,900 GWh/y). For the Wind from the Sun power
plant, the efficiency has not yet been determined.
But, even
with a low overall efficiency, a sufficient amount of power might be obtained
from such a system. Increasing the area of the solar
2.3 Temperature
The increase in
temperature of the solar collector is what drives the entire system. The higher
the temperature of the collector, the higher the temperature of the air, and
the more wind
How hot will the
collector get? In small scale tests, an area of 36 square feet was covered with
black ceramic. The ceramic in direct sunlight increased in temperature as much
as 40 degrees Centigrade (72 degrees Farenheight) above ambient temperature (26
degrees Centigrade; 80 degrees Farenheight). The test took place at 42 degrees
latitude in August. Estimated solar radiation for that place and time is 5 to 6
kWh/m2 per day. The amount of solar radiation would be significantly
higher in the southwestern U.S.
A 100 Megawatt solar
chimney power plant is projected to have an increase in air temperature of 35.7
degrees Centigrade (The Solar Chimney, Jorg Schlaich, Edition Axel Menges, p.
37). This comparison suggests that the small scale tests showed a high enough
increase in temperature to drive a Wind From The Sun power plant.
A large-scale collector should give
an even greater increase in temperature than 40 degrees Centigrade. A small
collector loses some heat to its surrounding perimeter. A large collector has
less perimeter per unit area and so loses less heat. The collector will be
hotter towards its center
The solar collector will tend to be
hotter towards its center and cooler towards its perimeter. A smaller solar
collector loses some heat to its perimeter. A larger collector has fewer
perimeters per unit area and so loses less heat to its perimeter, making the
center of the collector hotter than the perimeter. The center of a very large
solar collector will reach a significantly higher temperature than the outer
edges of the collector. However, the crucial temperature difference is in the
air.
3.1 Air Movement
Hot
Air Rises:
A number of
factors make the rising column of air above the collector significantly
narrower than the collector's diameter:
(1)
Hot air rises. The hotter the air, relative to the temperature of the
surrounding air mass, the faster the air rises. The collector is hotter
in its center, making the air in the center also hotter. The air
over the center of the collector rises faster, because it is hotter. The hotter
air towards the center of the collector rises faster than the air over the
perimeter, causing the air over the perimeter to curve inwards as well as
upwards. This effect narrows the rising column of air over the collector.
(2) As the cooler air moves inward from the perimeter
towards the center of the collector, it gradually increases in temperature as
it spends more time over the hot collector. The hotter the air gets, the faster
it rises. The cooler air, which moves mostly horizontally while at the
perimeter, moves more and more vertically as it moves towards the center and
increases in temperature. The result is that the path of the air curves upwards
as it moves inwards (see diagram below), and so the rising column of air is
much narrower than the collector itself.
Thus there are two factors which
make the air hotter over the center of the collector, first, that the center of
the collector itself is hotter and, second, that the air in the center has
spent more time over the collector.
(3)
The rising air in the center meets with less resistance as it rises, because it
is surrounded by air that is also rising. This factor causes the air towards
the center to raise faster, drawing in air from the perimeter, and again
narrowing the rising column of air.
(4)
As the hot air over the collector rises, new cooler air must move in from the
sides around the collector to replace the rising air. The cooler air at the
perimeter moves inward from 360 degrees around the collector. This results in a
wind moving inwards towards the center of the collector. Air movement over the
perimeter of the collector is more horizontal than vertical. This wind pushes
the rising column of air inwards, again making the column of air narrower than
the collector itself.
For the above reasons, most of the
updraft over the collector occurs around the center of the collector.
Therefore, the air pressure will also be lowest towards the center of the collector.
Air moves from an area of high
pressure to an area of low pressure. The rising hot air significantly lowers
the air pressure over the collector relative to the surrounding land. The air
from the land surrounding the collector moves in towards the center of the
collector and also rises upwards. In this way, the solar collector creates
wind. This wind moves from 360 degrees around the collector in towards its
center and upwards. The updraft created should be quite strong, since it is the
result of air moving in from 360 degrees.
The effect is
such that the air over the center of the collector rises just as if it were
confined within a chimney. A solar chimney-like result is achieved without the
expense of a tall vertical structure. This virtual chimney effect depends upon
the diameter of the collector and the difference in temperature from the center
to the edges of the collector. The effect is greater with a larger collector
and a larger increase in temperature towards the center. The hotter the air,
the faster it rises; the faster it rises, the more the air from the perimeter
is drawn in towards the center. The updraft created by this effect produces an
area of low air pressure in the center of the collector.
3.3 Air Pressure
In a closed container, when we
increase temperature, volume remains the same, so pressure must increase to
balance the system. However, in the case of a solar collector in the open air,
the collector increases the air temperature, causing air volume to increase and
pressure to decrease. The decrease in air pressure is what drives the Wind
from the Sun system. This same decrease in air pressure is seen in nature
in the case of a sea breeze.
Calculating the change in
pressure in a system open to the atmosphere is complex. The pressure is
affected by the changing temperature of the solar collector, by changes in
atmospheric pressure, surface winds, and humidity. In addition, the solar collector
causes a strong updraft, which contributes to the decrease in pressure over the
collector. Future experiments are needed to quantify the solar collector’s
effect on the air pressure over the collector. What is clear, though, even at
this point in time, is that the solar collector will decrease the air pressure
over the collector.
4) Air
Channel:
A very large
diameter pipe or air channel connects the area of low air pressure over the
solar collector to an area of higher air pressure away from the heat of the
collector. Air moves from high pressure to low pressure, creating a wind within
the pipe.
The
amount of wind power generated by this system depends on the air pressure
difference from the center of the collector to the surrounding land. An air
pressure difference of about 400 Pascals should generate a wind speed of 15
meters per second (m/s). An air pressure difference of about 700 Pascals should
generate a wind speed of 20 m/s.
The
diagram below shows 4 pipes, each of which is 2000 meters in length and 175
meters in diameter. The solar collector has a diameter of 4000 meters. The
pipes extend 1500 meters into the collector because the inner portion of the
collector has the lowest air pressure. The pipes extend only 500 meters away
from the collector. Cool air is constantly moving in towards the collector, so
perhaps the pipes could extend an even shorter distance away from the
collector.
The pipes
shown in the diagram above are circular in cross-section. However, they could
be rectangular in cross-section
.This shape may be less expensive and easier to build. It should also take
better advantage of the heat from the collector, since it will be closer to
ground level. To obtain the same cross-sectional area, these rectangular air
channels (instead of pipes) would need to be about 60 meters high by 400 meters
wide. Pipes of the same cross-sectional area would have a diameter of about 175
meters.
Air flows
into the air channel (blue rectangles below) from the perimeter of the
collector. Air flows out of the air channels near the center of the collector
and rises upward. The air rises for two reasons. First, a strong updraft occurs
in the center of the collector. The updraft results from the solar collector
heating the large volume of air which does not travel through the air channels.
Second, the air which does travel through the air channels is heated by the
collector as it travels through the air channels and after it exits the air
channels.
The hot
air rising from the center of the collector creates and maintains a low
pressure area in the center of the collector. The pipes have a constant
difference in air pressure from one end to the other. The air pressure is
higher away from the collector and lower towards its center. This difference in
air pressure moves air through the pipes.
Each pipe
has pressure-staged wind turbines which present a kind of obstacle to the air
movement. The wind turbines remove mechanical energy from the wind in the pipes
and convert it to electricity. Even so, air continues to move through the pipes
because the rising hot air in the center of the collector maintains a
significantly lower air pressure at one end of the pipe.
The wind speed can be controlled by
increasing or decreasing the size of the collector. Such a change can even be
made after the power plant has been built, by covering or uncovering part of
the collector's surface with a white material. In summer, the collector will
tend to reach a higher temperature and produce a higher wind speed. It is
possible to reduce the wind speed by covering part of the collector, thus
reducing the effective size of the collector. In winter, the collector will not
reach as high a temperature and so the collector can be completely uncovered to
obtain maximum wind power.
5 Wind Velocity
The wind velocity (V) through the
air channel depends mainly on the total pressure difference (ΔPtot)
from one end of the air channel to the other. Air density (D) is a much less
significant factor. In Equation (5), velocity is multiplied by air density, so
that a greater air density would seem to increase power. However, Equation (6)
below shows that the square root of the air density affects the velocity. Since
that velocity is cubed in Equation (5), overall, a greater air density provides
less power. Higher air temperature generally results in lower air density,
which, in this system, will provide higher air velocity and greater power.
…….. (6)
Equation (6) can be solved for the total pressure difference (Ptot),
giving us Equation (7) below
ΔPtot = V2*D*3/2…….. (7)
This equation assumes that the wind
turbine extracts the theoretical maximum power by reducing pressure across the
turbine (ΔPs) by 2/3rds of the total pressure difference (ΔPtot).
Table (1) below compares total air pressure difference (ΔPtot) and
air velocity (V) through the air channel, to power available in the wind per
square meter (P1), to swept area of the wind turbines (A) and total
power output (P2). Air density is assumed to be 1.165kg/m3 (The
value for 300C and standard atmospheric pressure). Note that a 50%
increase in velocity increases power output by 3.375 times. Total power output
(P2) is simply the product of the power of the wind per square meter
and the total swept area (A) of the turbines. Note that total power output is
the theoretical maximum, actual power output will be less than is theoretically
possible.
Air velocity will increase towards
solar noon, when solar radiation is greatest, and decrease as sunset
approaches. Maximum air velocity will also increase as summer approaches and
decrease as winter approaches. Since power is based on the cube of air
velocity, more power will be generated in the middle of the day and the middle
of the year. However, locations closer to the Equator will have less of a
difference in power production between summer and winter.
6 Wind Turbines:
A series of very
large wind turbines within the pipe turn the wind into electricity with a high
degree of efficiency. Each pipe can contain several turbines and each collector
can support several pipes.
For a power plant with a collector diameter
of 4000 meters, the interior diameter of the pipes would be 175 meters and the
cross-sectional area would be about 24,050 square meters. A pipe of that size
could support multiple wind turbines in a variety of configurations. One
possible configuration is shown below. Seven large turbines, each with a
diameter of about 55 meters, can fit within a pipe of this size.
Of course, the pipes do not have to be round
in cross-section. They can certainly be rectangular, as shown below. Again,
seven large turbines, of about 55 meters in diameter each, can fit within a
rectagular pipe with the same cross-sectional area as the round pipe.
The swept area of a wind turbine with a
diameter of 55 meters is about 2375 square meters. If a 175-meter pipe has 7
wind turbines, the swept area is 16625 square meters. But if the same 175-meter
pipe had 1 large wind turbine with a diameter of 173 meters, the swept area
would be 23500 square meters. This represents an increase in swept area of over
40%. Since swept area is roughly proportional to power output, the output would
be increased by 40% by using one large wind turbine, instead of 7 smaller ones.
The air velocity (V) or wind in the air
channel depends on the total pressure difference from one end of the air
channel to the other (Ptot) and on the air density (D):
Ptot = V2 * D * 3/2
The power (P) available in the wind follows
the formula:
P = 0.5 . D . V3
A 15 m/s wind will generate about 1965 Watts
per square meter. So, if the total swept area of the wind turbines is 25,000
square meters, the power generated will be about 49 Megawatts. A 20 m/s wind
will generate about 4660 Watts per square meter. So, if the total swept area of
the wind turbines is 25,000 square meters, the power generated will be about
115 Megawatts.
The larger the solar collector, the greater
the air pressure difference, the greater the power produced. A larger solar
collector may also be able to support more than one air channel, providing a
greater total swept area and more total power produced.
7. Alternate Design Possibilities
The above design for a Wind from
the Sun power plant requires a large area of land to be covered with black
ceramic gravel. What would be the effect on the environment of covering
thousands of acres of land with gravel? And if the power plant one day had to
be dismantled, what would be the cost to remove the gravel? The large solar
collector required for a Wind from the Sun power plant is a significant
concern within this type of solar power. An alternate design is possible, which
either omits the solar collector, or uses a smaller collector. The collector’s
purpose is to produce an area of lower air pressure relative to the surrounding
land. Such pressure differences also occur naturally, as seen in the example of
the sea breeze. If the location of the Wind from the Sun power plant
were chosen astutely, the natural air pressure difference might be sufficient
to produce enough power to operate the plant economically. Most sea breezes are
between 0.3 and 1.0 kilometers in depth, yet the wind velocity can exceed 12
m/s. An air channel of less than one kilometer, connecting the air over a large
body of water to the air over an adjacent land mass could find a sufficient
natural air pressure difference to produce a significant amount of power. And
the cost of building the power plant would be reduced significantly because the
amount of land required would be lessened, and the expense of a large quantity
of black ceramic gravel would be eliminated. Another possible location with a
natural air pressure difference would be at the border between two different
types of land topography. The cause of the differing air pressures would
generally be the difference in reflectivity of the two areas and differences in
how quickly each area is heated by the sun.
If the natural difference in air
pressure is not sufficient, a solar collector (reduced in size from the power
plant described in 1 – 5 above) could augment the natural pressure difference.
To increase efficiency, place air channel so that one end is over a large body
of water. The water will be much cooler than the collector and will increase
the air pressure difference along the air channel.
8. Conclusions
At this point in
time, two conclusions are clear. First, this type of system will produce some
power. The sun will heat the collector, which will heat the air. The higher air
temperature will expand the air, reducing air pressure. Air must move from high
to low pressure, through the air channel and past the wind turbines, producing
power. A sea breeze works much the same way and produces significant wind. Some
power can certainly be produced in this way. Second, the system has not been
built and tested to a large enough scale to determine how much power will be
produced. Will the air pressure difference be large enough to produce
significant air velocity through the air channel? Will the system produce
enough power to be economical? What are the possible ecological effects of such
a large solar collector? Further study is needed to answer these and other
questions.
8. REFERENCES
WEBSITES
1.
http://www.google.com
2.
http://www.wind-energie.del/englischer-teil/english.htm
3.
http://www.visionengineer.com
4.
http://www.howstuffworks.com
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