Wow! I just happend to notice the diameter of the proposed solar tower is 167 meters! See: <http://tinyurl.com/6ow84>. It says: "The fact the tower would be only six times higher than its diameter is the key to its strength and stability, says davey."
Not too much worry about insulation with that kind of diameter. That's 22,000 m^2 of chimney cross section. Running up a mountainside, assuming the flue had a 22 meter cross section height (72 ft) it would take up 1000 m (3280ft) of mountainside. It sure would provide a lot of advertising space! The 20 km^2 (2x10^7 m^2) greenhouse proposed at the base can at 100 percent efficiency provide only 20 MW of solar power. The mountainside flue noted above, assuming it were also 1 km high, could only capture 1 MW of power, so the solar capturing effect is of only nominal consideration to the design, assuming the Australian project actualy can produce the proposed 200 MW output. I now don't see why a solar base is necessary at all, unless it is of use to prime the flue. The vast majority of the hoped for 200 MW power then has to be from the bouyancy due to the difference in ambient temperature (and thus column bouyancy) between the base and the exit of the flue. This sounds like it needs some checking. In a "standard atmosphere" the density at sea level is 1.225 kg/m^2 at 288 K, and at 1000 m it is 1.112 kg/m^2 at 281K. We see the expected top to bottom average temperature difference is only 7 K. Also, the mass of air in a 1 km high 1 m^2 column outside the flue is roughly 1000*(1.225 kg +1.112 kg))/2 = 1,169 kg. The mass of the air inside the flue, assuming no heat loss through the sides of the tower, can be computed using the average temperature outside the flue, which is (288 K + 281 K)/2 = 284.5 K. A 1 m^2 column of air inside the flue is warmer, thus weighs (284.5 K)/(288 K) (1,169 kg) = 1,155 kg. The difference in weight of the two columns is 14 kg, so we have a bouyant force of 14 kg/m^2 in static conditions. The bouyant force of air across the (2x10^7 m^2) cross section area flue is thus on average about 2.8x10^8 kgf. We can only obtain this force if the flow is zero. As the flow rate increases the pressure differential drops. Assuming the tower produces air flow at 35 m/s as stated in the above article, the ideal power, assuming all the 2.8x10^8 kgf is available, is 9.61x10^4 MW. Assuming 30 percent of that can be harnessed that is 28,800 MW, about 144 times the 200 MW forecast for the tower. Unless I made a big mistake (not uncommon), the 200 MW number is even looking conservative. It would be very interesting to see the engineering work done earlier by Schlaich and now by Davey. The use of base solar collectors is now looking almost entirely irrelevant. It also is of interest that flow resistance should be fairly small in a pipe that has a diameter 1/6 the length. It sounds like Davey is playing all this very conservatively. One problem with the solar tower concept is that inversions might occur, or variations in the expected conditions. Even more varistions might be expected if the towers were built against mountainsides, thought they would likely be much cheaper. If built in a hot clear environment the towers would benfit marginally from solar collection, but the added benefit for the cost, compared to just building more towers, might be unjustified. All this brings up another point of interest, especially as related to solar towers in far northern climates. Since air density is higher, and actual solar collection is nearly moot, solar towers might do very well in even arctic conditions, because the air density is much higher. Often the temperature gradients may be higher, though inversions are also likely more common. The interesting thing, though, is the high wind that flows across the tops of mountain ranges. Appropriate ducting at the top of the flue could have dramatic effects on the air flow through the duct when the wind is crossing the ridge - and that air flow would be highly controllable using damping. There would not be the shut down problems associated with turbines, and no one would have to visit mountain tops or climb towers to do maintenance on turbines or generators. The "buisness end" would be the ground end. If necessary, in places like Alaska, the flues could be camoflauged to look like mountainside. In the winter snow would hide it well is the outside were slightly contoured. If I understand things right, then this is all incredibly cool stuff. Regards, Horace Heffner
