Rooftop Wind Solutions
Introduction
Micro-wind turbines are used in micro-wind
generation and are much smaller in scale than those used in conventional wind
generation making
them more suitable for residential energy production. Micro-wind
generation is a method of micro
generation that
uses the flow of wind energy to produce electricity for a house or farm. Broadly
speaking, there are two types of wind turbines that can be installed: vertical axis wind turbines and horizontal axis wind turbines.
The installation of a micro-wind turbine usually consists of the turbine
and an inverter. Wind causes the blades of the wind turbine to rotate,
generating mechanical energy. The mechanical energy from the rotation is converted to direct current (DC) in the turbine and using the inverter, is converted to alternating current (AC). The inverter output is connected to a breaker panel where the
electricity can be shared among the electrical equipment in the home. Excess
electricity can be exported from the home to the electrical grid using a bidirectional meter and credits will be provided accordingly by
the retailer based on the electric current tariff for electricity.
The generation of electricity is mostly based on the rotational wind
speeds of the wind turbines. Certain geographic
locations are more suitable for producing electricity compared to others.
Depending on the amount of wind that can be obtained from a region, the
generation can vary. Another factor that can affect the performance of wind
turbines is the obstructions in the area of the installed turbine. Obstructions
from trees or other buildings will hinder the turbines from producing at its
optimum capacity.
Rated
Power
Wind turbines are advertised with a rated power. Small turbines,
like those you’d see on a roof, are generally rated at 400W to 1kW. The rated power of a turbine is a best-case
scenario. It’s a measure of how much power the turbine will generate at the
highest wind speed that the turbine can tolerate. To get a more accurate
estimate, look at the turbine’s power curve. Here’s a typical power curve for a
1 kW turbine:
The curve shows that the turbine starts generating power at around
3 m/s (6.7 mph) - the cut-in speed. Slower winds don’t have enough power to
make the rotor spin. As wind speed increases, there’s a rapid increase in
power, but the power output only hits 1 kW (the rated output) when the wind
speed is around 11 m/s (nearly 25 mph). To put that into perspective, if your
land had average wind speeds of 25 mph, all of your trees would be permanently
bent. It’s more likely that you’ll see winds in the 3 to 5 m/s range, which
means that a 1 kW turbine is usually producing less than one-tenth of its rated
value. The shut-down speed is the speed at which the turbine will apply a
braking mechanism to prevent damage. A typical shut-down speed is just a few
m/s higher than the rated speed, so the “sweet spot” - the range where the
turbine produces its rated power - is pretty narrow.
Sustained
Winds versus Turbulent Winds
To make wind power cost-effective, turbines need access to strong
and sustained winds. A residential rooftop offers neither. Wind speed increases
with altitude, and the top of a house is pretty close to ground level. Even
worse, all the obstacles - trees, other buildings, even the house itself -
cause turbulence in the wind. So instead of a fast steady breeze flowing in a
mostly constant direction, you get short choppy gusts of wind coming from
random directions. Turbulence not only decreases the turbine’s output, it also
causes mechanical stress that shortens the turbine’s life. The rule of thumb is
that the turbine should be at least 9 m (30 ft) higher than any obstacle within
150 m (500 ft): rooftop turbines should be mounted near the center of the roof
rather than along the perimeter, because turbulence is greater around the
outside of the roof than in the center. Fair enough, but that study only looked
at turbulence, not overall production. Following its recommendations means that
your turbine will last longer. That’s a good thing, because it could take a
very long time for the turbine to generate enough electricity to offset the
cost of the turbine itself. A better suggestion is to avoid putting wind
turbines on the roof.
Payback
Period
A 400 watt horizontal axis
wind turbine (HAWT) with a 10 m (33 ft) tower, since higher
altitudes are where you get the sustained winds, .with average wind speeds of
10 m is 3.6 m/s, or about 8 mph. With 3.6 m/s winds, the 400W turbine generates
50W. Assuming it runs 24/7/365, the turbine will generate 438 kwH per year. At
a utility cost of $0.12/kWh, the turbine saves the owner $52/year in
electricity cost. A typical 400W turbine costs about $400 - and that’s just the
turbine, not the tower. The least expensive tower kit that I could find sells
for just under $400, not including the concrete base. So at a minimum, it costs
upwards of $800 for one of these turbines and its tower. That’s more than a 15
year payback period, which wouldn’t be terrible for a long-term investment,
except that the turbine comes with a one-year warranty.
What
If the Grid Isn’t an Option?
It’s easy to say that these turbines aren’t worth the money when
compared to grid power, but what about remote locations where grid power isn’t
available? Let’s look at the 1 kW turbine and tower for $8800. We determined
that it produces 675 kWh per year. Is there a better renewable source of
energy? Solar?
In the same location, the average solar resource is 4.5 peak sun
hours (PSH) per day, with a worst case of 2.6 PSH in the winter. To generate
675 kWh/year, a photovoltaic (PV) array would need to produce 1.85 kWh/day. In
the winter, a 1 kW solar array would cover that, even if the overall system
were only 75% efficient. (85% is a more realistic number.) A small PV system
costs about $4/watt installed, so the total investment, including installation,
is about $4000. Even better, the PV system has no moving parts so it requires
no annual maintenance. PV seems the better option.
Wind:
Go Big or Don’t Bother
Wind power is a great source of
renewable energy - at the utility scale. Large
turbines are more efficient than small ones, and higher towers reach those
energy-rich high speed winds. If you’re considering a small turbine, think
again. Unless you’re in the Arctic region, you’re better off spending the money on solar panels.
Capacity Factor
The average capacity factor for rooftop wind turbines was 5%. This
poor performance is currently a major factor limiting the feasibility of small
urban wind turbines and is primarily due to inappropriate siting of the wind
turbines. An urban environment contains turbulence caused by buildings and
obstructions, and careful siting is required to avoid these less efficient
winds. With proper siting, higher capacity factors can be achieved, making
urban wind turbines more feasible. Capacity factors as high as 14% have been
achieved in urban settings. Greater capacity factors are also possible
considering that rural wind turbines see percentages in the 20-35% range and
higher. These reasonable capacity factors provide a more assuring outlook for
rooftop wind
Factors that inhabit Urban wind roof tops
The low capacity factor achieved in an urban environment is the main
hindrance in rooftop wind turbine feasibility; however, it is not the only
factor affecting it. The price of electricity and value of RECs also affect how
much income is generated from energy produced with a wind turbine. These three
factors have a significant impact on the economic feasibility of rooftop wind
turbines. Using future projections for these factors, and small increases in each of them could
significantly reduce the payback period of rooftop wind turbines, making them
more economically feasible.
Siting Factors
Certain areas and building types are more promising for rooftop wind turbines.
Buildings that meet the following criteria would be easier and more effective
to install rooftop wind turbines: Above
150 feet tall, and taller than buildings upwind; Roof area of at least 5,000
square feet; Supported by columns to which the turbine can be mounted
1 kW wall mounted wind turbine
Recently developed 1 kW turbines can be mounted
on a wall and does not need a structure for mounting. 1000 W power while only
1.50 m in diameter and 15 kg in weight, An integrated dump load and braking
system along with the following ;
· 2x patented aluminum rotor
· Nacelle incl. wind direction following system
· Grid inverter incl. on-board computer and display
· Dump load incl. ceramic screws and mount
· Automatic storm control system incl. sensor, actuator and relay
· Protective wax coating for corrosive sites
· Manual and detailed installation guide including pictures for
each step
The wind turbine can be plugged into the
next wall socket. The new inverter will synchronize with your local grid. You
start saving on your next electricity bill as soon as the wind starts blowing.
The inverter automatically adjusts to both 230 V
and 110 V AC power. The energy generated directly feeds your appliances, only
the missing power is bought from the grid. The power
generation ability of the turbine is presented as follows:
The complete turbine ready for installation is priced at Euro 1400 (US
$ 1620). This seems a more attractive solution which given better than average
wind conditions could be feasible.
Why Micro Wind Turbines don’t work.
Size
A simple equation gives the power of the wind. Power = 0.5 x
collection area x the wind speed cubed. What it tells us is that the power of a
turbine is related to two factors: the size of the turbine and the strength of
the wind. The area of the circle is
equal to the constant pi (3.14) times the radius of the circle squared. What
that means is that as you increase the length of a turbine blade, the
collection area increases disproportionately.
A micro turbine with Its blades 1.75m long, would give a collection area
of just under 10sq m. Tiny. Small turbines have disproportionately smaller
collection areas and therefore generate dramatically less power.
Wind speed
The key here is that cube function on the wind speed. The power
of the wind is related to the cube of the wind speed. So, at low wind speeds
you get virtually nothing. When it really blows it you get a lot of power. Double
the wind speed and you get eight times the power. Quadruple it and you get 64
times as much. Eight times the speed and we're talking more than 500 times the
power. Halve the wind speed to six meters per second (a moderate breeze) and -
thanks to that cube law - you now get just 120 Watts - that's two standard
incandescent light bulbs with average wind speeds likely to be between 4m and
5m per second the turbine would generate 25 Watts. That is barely enough for
two energy saving light bulbs.
Conclusion
Micro wind turbines are unable to generate significant amounts
of electricity, at this point in time, to be financially feasible. Wind solutions with small and micro turbines
that are to be mounted on walls, will cost less than the mast mounted turbines
but wall mounting will have disadvantages of lower wind speed. Work on micro
turbines is still in progress and in the future we may have designs that are
financially feasible. Small wind turbines in urban areas
today are only profitable under very favorable conditions. The coupling of the
small wind turbine to a storage system is crucial for cost-effectiveness.
