The tops of solar towers, also known as solar chimneys, should be
ringed with vertical layers of inverted airfoils. In windy
conditions, nearly always present at high altitudes in many
locations, these inverted airfoils about the periphery, with trailing
edges to the inside, have the effect of reducing air pressure at the
top of the chimney. They direct horizontal airflow upwards, thus
reducing air pressure in the chimney. This enhances the Bernoulli
effect already present for such chimneys. This pressure drop
increases airflow and thus turbine output at the base of the
chimney. Use of variable pitch airfoils permits controlled
feathering and continual operation in high winds. The airfoils
increase load on the structure and cost of the structure, but airfoil
pitch control may be of use in preventing resonant vibration buildup
in high wind conditions. The use of such airfoils increases the
optimal chimney aspect ratio to less than that which is optimal
without the airfoils. A typical (height to diameter) aspect ratio
for solar towers is currently 6.
http://en.wikipedia.org/wiki/Wind_power states
“The wind blows faster at higher altitudes because of the reduced
influence of drag of the surface (sea or land) and the reduced
viscosity of the air. The variation in velocity with altitude, called
wind shear, is most dramatic near the surface. Typically, the
variation follows the 1/7th power law, which predicts that wind speed
rises proportionally to the seventh root of altitude. Doubling the
altitude of a turbine, then, increases the expected wind speeds by
10% and the expected power by 34%.”
“Wind farms or wind parks often have many turbines installed. Since
each turbine extracts some of the energy of the wind, it is important
to provide adequate spacing between turbines to avoid excess energy
loss. Where land area is sufficient, turbines are spaced three to
five rotor diameters apart perpendicular to the prevailing wind, and
five to ten rotor diameters apart in the direction of the prevailing
wind, to minimize efficiency loss. The "wind park effect" loss can be
as low as 2% of the combined nameplate rating of the turbines.”
A non-economic wind power class 2 location at an altitude of 50 m has
average wind speed of 5.6 m/s and power density of 200 W/m^2.
Applying the 1/7th power law, a 1 km tower in that location would
experience an average wind speed of (1000m/50m)^(1/7) *(5.6 m/s) =
1.53*(5.6 m/s) = 8.54m/s. This turns a useless wind class 2
location, like the coast of Georgia, into a wind class 6 location,
with 600 W/m^2 wind power density. Suppose the effective area of
the tower with respect to wind power extraction is roughly the
diameter of the tower squared. A 1 km high solar tower would thus
have a useful wind cross section of (1000 m)^2. However, due to
pressure drop losses in the flue, and other inefficiencies, only
about 10 percent of that power can be extracted. The wind power
available is then (1000 m)^2 * (600 W/m^2) * 0.10 = 60 MW, but this
is 24 hours a day, not just through daylight, providing a 120 MW
solar equivalent enhancement to a 200 MW solar tower.
Use of wind power to enhance solar tower performance has the
advantage that wind power tends to be available when solar is not.
Coastal wind power is larger at dusk and dawn, while solar power
peaks around noon. Wind power also tends to be larger during
overcast or stormy conditions. Solar towers, being ducted with the
power concentrated, can be throttled so as to continue running in
high wind conditions.
Obtaining the wind power requires use of aerodynamic structures at
the tower top to reduce pressure in the chimney there. These can be
static structures - horizontal airfoils, or vortex creating vertical
slits.
One problem with this idea is that solar tower performance data must
necessarily already include any Bernoulli effect pressure drop
enhancement due to wind. The incremental performance gain due to
airfoil engineering may not be as much as expected.
Horace Heffner
