Solar Cooling
• Consider a refrigeration system with no moving parts.
– Heat the coolant (say ammonia gas dissolved in water) and force it via a
generator into an evaporator chamber where it expands into a gas and cools.
Move it to a condenser and cool it back to a liquid and repeat the process.
• These systems actually have existed for a number of years, refrigerators in
the 1950s were sold with this technology (gas powered and there was/is a
danger of CO emissions).
• Energy to heat the coolant and drive it through the system comes from
burning fuel or a solar cell to provide electricity to do the heating.
– Need what is called a concentrating collector (lens or other system to
concentrate more light on the solar cell).
• Ideally, you could do this with a flat plate collector system, though you do
not obtain as much cooling.
• Devices are not widely used, due to the intermittency of sunlight
Other renewable energy sources
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Hydropower
Wind energy
Ocean Thermal
Biomass
Geothermal
Tidal
Hydropower
• Well established electrical generation technology, about 100 years
old
• Known for over 2000 years that the force of moving water on a
water wheel could save human labor
• 13th century-hammers in iron works in Europe were operated with
waterwheels.
• 16th century –primary source of industrial power in Europe
• In the US, mills were established at sites with reliable water flow
and dams were constructed to regulate water flow.
– Cave mill here in BG. Several hydro powered mills for corn, flour and
sawing in the 19th century existed on this site at different times.
• With the advent of electricity, water wheels were used to drive
electricity generation.
• About 7% of US energy generation is from hydroelectric plants
The physics of Hydropower
• Gravitational potential energy in the water at a height h above the wheel
is converted to kinetic energy of the wheel which drives a turbine and
generates electricity.
• So each mass element of water, m, falls a distance h and attain a velocity v.
So its initial potential energy is mgh, where g is the acceleration due to
gravity (9.8m/s2) and the kinetic energy is 1/2mv2.
• This tells us the amount of potential energy available to be converted to
kinetic energy is 9.8 joules per kilogram of water per meter of height
above the wheel.
• h is often called the head.
• Efficiencies of 80-90% can be achieved.
• Power = (Height of Dam (distance the water falls)) x (River Flow) x
(Efficiency) / 11.8
where the height is in feet, river flow is in cubic feet per second ,
efficiency is what you expect and 11.8 converts from feet and seconds to
killowatts
Plant operation
Hydro Turbine
Net exploitable hydropower resources
Advantages
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No Pollution
No waste heat
High efficiency
Plants have decades long lifetimes and low
maintenance costs
• Good response to changing electricity demands
• Damming of rivers can serve other purposes:
flood control, irrigation, drinking water supply
Limitations
• About 50% of the US capacity for Hydro is developed
• Limited lifetimes for certain reservoirs-as the fill with silt,
they become less useful for water storage. But the dam
must be maintained long term, if it fails, communities
downstream are in danger from the tremendous volume of
silt that would be released.
• Loss of free flowing streams due to damming and the loss
of the lands flooded by damming a river-environmental
impacts
– Salmon population in the Nothwest has been impacted
• Flood risk due to dam failures
– Currently hundreds thousands of people in danger if dam
failures occur
Fish Ladders
• Solution to the salmon problem - have not
been very effective
Wind power
• Not subject to day night cycles
• Direct result of solar heating of the Earth’s
atmosphere
• Use of wind for energy first noticed by sailors the old sailing ships could extract the equivalent
of 10,000 hp from the wind!
• Windmills were prevalent in Europe in the 19th
century
• Several million were pumping water in the US in
the early 1900s
WHAT YOU’RE PROBABLY THINKING OF….
Power in a windmill
• The power in the wind can be calculated by P/m2 =6.1 X
10-4v3
• This gives the power in kilowatts per meter squared, where
the cross sectional area is oriented perpendicular to the
wind direction.
• This is the total power, of course not all of it can be
extracted. According to Betz’s Law, developed in 1919 by
German physicist Albert Betz, no turbine can capture more
than 59.3 percent of the potential energy in wind.
• However, the total amount of economically extractable
power available from the wind is considerably more than
present human power use from all sources!
Extracting the energy: The turbine
• The world's first
automatically operated
wind turbine was built
in Cleveland in 1888 by
Charles F. Brush. It was
60 feet tall, weighed
four tons and had 12kW
turbine.
Turbine types:
• 2 types, based on the
direction of the axis that
the turbine rotates about.
• Horizontal axis wind
turbines (HAWT) -the
turbine rotates around an
axis that is horizontal.
• Vertical Axis Wind
Turbines (VAWT) –the
turbine rotates around a
vertical axis
HAWT
• Horizontal Axis Wind Turbines
• main rotor shaft and electrical generator are
locate at at the top of a tower, and must be
pointed into the wind.
• Small turbines are pointed by a simple wind
vane, while large turbines generally use a wind
sensor coupled with a servo motor.
• Most have a gearbox, which turns the slow
rotation of the blades into a quicker rotation that
is more suitable to drive an electrical generator.
HAWT
• the turbine is usually pointed upwind of the
tower since it creates turbulence behind it.
• Turbine blades are made stiff to prevent the
blades from being pushed into the tower by
high winds.
• The blades are placed a considerable distance
in front of the tower and are sometimes tilted
up a small amount.
HAWT Advantages
• Variable blade pitch so the turbine collects the
maximum amount of wind energy for the time of
day and season.
• The tall tower base allows access to stronger
wind in sites with wind shear. In some wind shear
sites, every ten meters up, the wind speed can
increase by 20% and the power output by 34%.
• High efficiency, since the blades always move
perpendicularly to the wind, receiving power
through the whole rotation.
HAWT Disadvantages
• The tall towers and blades up to 90 meters long are difficult to transport.
Transportation can be 20% of equipment costs.
• Tall HAWTs are difficult to install, needing very tall and expensive cranes
and skilled operators.
• Massive tower construction is required to support the heavy blades,
gearbox, and generator.
• Reflections from tall HAWTs may affect side lobes of radar installations
creating signal clutter, although filtering can suppress it.
• Their height makes them obtrusively visible across large areas, disrupting
the appearance of the landscape and sometimes creating local opposition.
• Downwind variants suffer from fatigue and structural failure caused by
turbulence when a blade passes through the tower's wind shadow (for
this reason, the majority of HAWTs use an upwind design, with the rotor
facing the wind in front of the tower).
• HAWTs require an additional yaw control mechanism to turn the blades
toward the wind.
VAWT
• Vertical Axis Wind Turbines
• have the main rotor shaft arranged vertically.
• turbine does not need to be pointed into the
wind to be effective. This is an advantage on
sites where the wind direction is highly
variable.
• VAWTs can utilize winds from varying
directions.
Types of VAWT
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Darrieus wind turbine
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"Eggbeater" turbines. They have good efficiency,
but poor reliability. Also, they generally require
some external power source, or an additional
Savonius rotor, to start turning.
Giromill
–
A subtype of Darrieus turbine with straight, as
opposed to curved, blades. The cycloturbine
variety has variable pitch and is self-starting.
– more efficient operation in turbulent winds; and a
lower blade speed ratio which lowers blade
bending stresses. Straight, V, or curved blades
may be used.
Savonius wind turbine
–
These are drag-type devices with two (or more)
scoops that are used in anemometers, Flettner
vents (commonly seen on bus and van roofs), and
in some high-reliability low-efficiency power
turbines. They are always self-starting if there are
at least three scoops. They sometimes have long
helical scoops to give a smooth torque.
VAWT Advantages
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A massive tower structure is less frequently used, as VAWTs are
more frequently mounted with the lower bearing mounted near
the ground.
A VAWT can be located nearer the ground, making it easier to
maintain the moving parts.
VAWTs have lower wind startup speeds than HAWTs. Typically, they
start creating electricity at 6 m.p.h. (10 km/h).
VAWTs may be built at locations where taller structures are
prohibited.
VAWTs situated close to the ground can take advantage of locations
where mesas, hilltops, ridgelines, and passes funnel the wind and
increase wind velocity.
VAWTs may have a lower noise signature.
VAWT disadvantages
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Most VAWTs produce energy at only 50% of the efficiency of HAWTs
A VAWT that uses guy-wires to hold it in place puts stress on the bottom
bearing as all the weight of the rotor is on the bearing. Guy wires attached
to the top bearing increase downward thrust in wind gusts. Solving this
problem requires a superstructure to hold a top bearing in place to
eliminate the downward thrusts of gust events in guy wired models.
• VAWTs' parts are located under the weight of the structure above it, which
can make changing out parts nearly impossible without dismantling the
structure if not designed properly.
• Because VAWTs are not commonly deployed due mainly to the serious
disadvantages mentioned above, they appear novel to those not familiar
with the wind industry. This has often made them the subject of wild
claims and investment scams over the last 50 years.
Efficiencies based on blade type