Appreciate the analysis, however I believe you made a fairly significant error with the solar capacity percentage. Unlike other solar power, the cool thing about this structure is that it doesn't specifically require constant sunshine to work effectively being that it works off the temperature differential through the tower, rather than pulling energy strictly from solar panels. Because of this, I would imagine capacity to be much nearer to 100% than the opposite.

"for every hundred metres you go up from the surface, the ambient temperature drops by about 1 degree. The greater the temperature differential, the harder the tower sucks up that hot air at the bottom - and the more energy you can generate through the turbines... Because the heat of the day warms the ground up so much, it continues working at night;"

As for how the difference in temperature between the surface and 100s of meters in the air changes throughout the day/night, I don't know.

So according to your equation, at 80% capacity, it could cover 128k homes. At 93.3% capacity it'd reach 150k.

It still needs a temperature differential to run, which will not be there at night, so the best you could get is about 50%. This is the number that Enviro Mission uses.

The article claims, "Because the heat of the day warms the ground up so much, it continues working at night"

It also means less power generation in the morning. The thermal inertia in the base means it lags behind the air temperature, so there will be times when the base and top are the same temperature.

Not necessarily. Lets say the lows at the top of the tower are maybe 40 degrees F. They're heating up the base to 180 degrees F during the heat of the day. Whether the thermal gradient ever inverts depends on whether the ambient nighttime temperatures are able to bring that 180 degree thermal mass (concrete? dirt?) down to 40 degrees in the time available before the sun comes back up. You'd have to do the math.

What you say is correct. If there's energy production at night using stored heat then it's pulling out power from the heat reserve and cooling it down. If, say, the system was 10% efficient at producing power directly and put 25% into heat reserves which are 80% efficient at releasing that heat and turing it into power, then yes, the bottom would, excepting rare cases (strange weather?), be warmer than the top.

I don't believe that is the case for two hand-waving reasons. The desert in northern New Mexico (where I lived) gets a thermal inversions. You can see that smoke from morning fires rises, hits the inversion layer, and goes horizontal. I believe this is common in deserts, at least those with mountains. From http://www.srh.noaa.gov/media/abq/LocalStudies/ABQthermalinv... there's an 5.8C difference for shallow (~155m thick) inversions, which occurs throughout the year, and there were "130 inversion cases" in that year. Thus, the "1 degree per 100 meters" rule of thumb only applies during the day. A power system would have to work against the inversion.

Second, power extraction is more efficient with higher differentials. It would, I believe, be better to extract more power during the day (when the difference is high and demand for cooling is also high) than to store it for energy production at night.

The previous comment proposed the "capacity to be much nearer to 100% than the opposite." I just don't see that as being likely.

For working against the inversion, a simple solution would be to implement some venting along the sides and vent at the height of minimum temperature for a maximum differential, assuming that if the inversion was strong enough and the minimum temperature was lower in altitude than the max height of the tower. Yeah, you're gonna lose efficiency, but you'd still be able to generate some power, but I think even with that amount of thermal mass you could still maintain a decent differential I think. Also, southwestern and northeastern Arizona are pretty flat and the southwestern part is much much lower in altitude than Albuquerque.

Also, where from NM are you? I'm from ABQ and Farmington.

But there isn't much power there, and we aren't good at extracting power efficiently from lower temperature differentials. I read that wind power goes as the cube of the wind speed, so you really want to maximize this system to extract power during the day time (when the temperature difference and hence wind speed is most), and not the night. Even if it can support 1/2 the wind speed, that's only 1/8th of the power production.

Yeah, ABQ's in the valley so it's probably more prone to thermal inversions. I get the idea (eg, from http://www.arizonensis.org/news/sonorandesertedition/news03_... "PHOENIX, Az. ... The perfume is most noticeable after dark when temperature inversions trap it close to the ground.") that mountains aren't required, and that it's a common feature of deserts, but I wasn't able to track down numbers.

I was in Santa Fe for 8 years. I come back about once a year for my green chile fix. ;)

The most exciting claim to me was how efficient and robust this method of harnessing solar was. In particular:

- "Because it works on temperature differential, not absolute temperature, it works in any weather"

- "Because the heat of the day warms the ground up so much, it continues working at night"

- "It requires virtually no maintenance - apart from a bit of turbine servicing now and then, the tower "just works" once it's going, and lasts as long as its structure stays standing"

If these claims are true, then it would make sense for this plant to have a much higher capacity factor than most solar plants given that the major factors lowering CF is mitigated in this design (works at night and no shutdown for maintenance). Maybe I'm stuck in CS land where we think by factors of 10 but the numbers in this article doesn't seem too far off the beaten path.

IMO the most significant oversight efficiency of around 60% figure which is ridiculously high. Solar towers are vary low efficiency heat engines and have trouble reaching 1% efficiency (yes 1% efficiency ). The 38 km² collecting area is expected to extract about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar power that falls upon it. http://en.wikipedia.org/wiki/Solar_updraft_tower

I read their use of the term "60% efficiency" as what would more correctly be described as a Capacity Factor, in the sense used by Spyro above.

Whether they can actually average 120MW in operation, I don't know.

They are not using solar panels, they are creating a giant heat pump. They are therefore relying on there being a heat differential between the top and bottom of the tower.

Greenhouse gets hot, creates a pressure difference between inside the greenhouse and outside of the tower at the top, air flows up through turbines. It is very simple.

There are very few moving parts and it is always going to be colder at the top of the tower than at the bottom.

Note that it isn't pure desert floor, but a man made structure that will purposefully trap heat. Imagine how hot it would get if you sat in your car with the windows rolled up in the middle of the desert. Then think about how cold it would be at 2x the height of the empire state building.

It will be colder just because of the air pressure at that altitude. Even so, the ground cools at a much different rate than air does, so I'm sure night time generation is bolstered somewhat by that.

It will have a very high capacity factor because the only thing that you should ever have to 'fix' is the turbine, which should last quite a long time.

It seems likely that cf will be much higher than even that of nuclear.

It might continue to generate during the night, but I would be very surprised if its nighttime capacity was even a quarter what it generated during the day. Also, the glass will need to be cleaned periodically.

From Enviro's Web site: "A single 200MW Solar Tower power station will provide enough electricity to power around 100,000 households, similar to the number of homes in a city the size of Burbank (California, USA) or Palm Bay (Florida, USA)."

They're saying, 100k, not the 150 from the article. That seems more realistic.

From the Phoenix Business Journal: " . . . air is funneled up the tower at 35 mph past 32 wind turbines at its base . . . " http://www.bizjournals.com/phoenix/print-edition/2010/10/29/... That's a pretty impressive air velocity.

Um, regarding "MWh = MWx365x24xCF", isn't an hour 3600 seconds, so isn't the factor 3600? You seem to be converting using days in year and hours in day, yielding a factor-of-ten error.

With the numbers supplied the calculation is correct. As written it is ambiguous though.

Think of it as: MWh/year = MWh/h x 365 x 24 x CF

I could be wrong, what why is it called a solar tower when it's mainly based on Bernoulli?