Talk:Energy tower (downdraft)

capillary action?

It is good to see that you took the effort to improve this section. However I have some concerns about the discussion about capillary action. Specifically, it says that "Capillary action is how trees evaporate water sucked from the ground." AFAIK that is not true. One of the ways trees move water up is by the use of osmotic pressure: Living cells, including cells in the roots of trees, maintain electrochemical gradients across their cell membranes, and they have to spend biochemical energy to do that, see Peter Mitchell (chemist). That way ion concentrations, and hence osmotic pressure, is maintained higher inside the cell than outside, and that difference in osmotic pressure causes water to move across the cell membrane into the cell, and -in the case of trees- up the tree, see Osmosis. Evaporation higher up in the tree keeps the water flowing as long as the difference in osmotic pressure across the cell membranes is maintained. Capillary action alone wouldn't get you very far, and is not sufficient to move water up trees, let alone move it up a 1 km high tower. (Zimmermann U, Schneider H, Wegner LH, Wagner HJ, Szimtenings M, Haase A, Bentrup FW. What are the driving forces for water lifting in the xylem conduit? Physiol Plant. 2002 Mar;114(3):327-335 PMID 12060254 )

Perhaps it would be possible to mimic what trees do, which is: set up a salt gradient across a semipermeable membrane, and have the water move up the tower by osmosis. That would require a difference in osmotic pressure of 100 atm or so; let me think now, that would require a 4M salt concentration (230 g/l or so); that is a lot of salt right there, a saturated solution even? But even if you get it up the tower that way then it has to evaporate up there; you can't spray it since that would cause you to loose salt, which would have to be replenished to keep the system going. The question then becomes whether you can move enough water by passive evaporation alone; spraying water is a far more efficient to evaporate a lot of water.

Anyway, there is no free lunch here. AFAIK semipermeable membranes are not easy to maintain, and the high salt concentration needed would cause evaporation to slow down. Perhaps a pumping system would be cheaper/easier to maintain after all? JDH 10:58 AM, 27 January 2006

I have to admit I'm no expert when it comes to tree biology, I just remembered being taught back in the day that capillary action is a very important part of how plants and trees function. I also knew the lower the diameter the higher the capillary height, but I guess there is a limit on how thin you can go. It was false to assume that trees use only capillary action, what you wrote it teaches that trees invest some extra chemical energy obtained by photosynthesis to 'pump' their water up. They do this via osmosis gradients, but basically it comes down to spending extra energy to getting the water up there, it won't just climb on its own to then evaporate on top, that mechanism functions only for very small height capillaries. Trees do it with osmosis, but for us, mere mortals, simply pumping with a positive displacement pump could practically give you any height, with fairly good energy efficiency. Sillybilly 02:16, 28 January 2006 (UTC)

Wikifying - moved comments from article to here

I modified 1/3 of the article extensively for alternative energy at www.pickensplan.com Hopefully we can get some more ideas, but I do think most of the changes I made are an improvement. From Wikipedia, This article is about electricity power generation by downdraft created by evaporation of water sprayed at the top of a hollow cylinder, perhaps 100 meters tall. Sharav Sluice Energy Tower. An energy tower is a method for producing electrical power for consumer consumption, the brainchild of Dr. Phillip Carlson, which has been expanded upon by Professor Dan Zaslavsky from the Technion. An energy tower produces electricity by drawing the energy from the air around it. An energy tower is a tall hollow cylinder with a water spray system at the top. The water is pumped up to the top of the tower and then sprayed inside the tower which cools the warm air hovering at the top. The cooled air, being denser than the outside warmer air, falls to the bottom of the cylinder which causes a wind turbine at the bottom of the cylinder to spin. The turbine is connected to a generator which produces the electricity. The need for large quantities of water may be solved by choosing a location that is not too far from the coast. The tower should optimally be situated in a hot dry climate, which thus allows for the greatest extraction of energy from the air. It is a form of solar power that has the peculiar advantage of also working at night. However, power generation by the energy tower is affected by the weather: it slows down each time the ambient humidity increases (such as during a rainstorm), or the temperature falls. An alternative approach to this is the solar updraft tower, which would require huge (up to 7 or 8 kilometres in diameter) agricultural glass house collectors to capture the solar heated air, to produce 100 megawatts perhaps 5 hours per day. The down draft tower uses some energy (about 50% of the turbine output) to pump water to the top and pressurizing nozzles. Dry air is continuously drawn at the top from the surroundings. Some people say that making a tower to test the capacity can be much easier than an up draft tower. This is now being tested in the Netherlands. Projections made by Altmann[4] and by Czisch[5][6] about conversion efficiency and about Cost of Energy (cents/kWh) are based only on model calculations[7], no data on a working pilot plant have ever been collected. Actual measurements on the 50kW Manzanares pilot solar updraft tower found a conversion efficiency of 0.53%, possibly this could be increased to 1.3% in a large and improved 100MW unit.[8] This amounts to about 10% of the theoretical limit for the Carnot cycle. You can read more details at Wikipedia. The discussion is also extensive and good. In improvement I suggest is to put the wind turbines 10 or 20 meters above the desert, so ambient wind can carry the cool damp air far from the tower before it is heated by the hot desert sand. Otherwise the air at the top of the tower increases in humidity, thus producing less energy. The output is also reduced by the fact that water vapor has only about half the density of dry air. Fresh water is typically in short supply in the best locations but salt, or brackish water is often available, but this will corrode the wind turbines, unless the humidity is under 100% at the turbines. That may be possible using very fine spray of computer controlled amounts of water. Ccpoodle (talk) 17:08, 21 August 2008 (UTC) About half the energy produced by the wind turbine at the bottom of the tower is required to pump water to the top and to supply the pressure drop for the spray nozzles. An alternative approach, to eliminate the pressure drop energy loss, would be using capillary action/large surface area sponge grids. Once sufficient downdraft is established, the evaporation rate would be amplified. Capillary action is how trees evaporate water sucked from the ground. However, one has to be careful with assumptions, because vapor pressure drops dramatically with capillary action, based on meniscus curvature, otherwise a perpetuum mobile would be possible by a capillary pumping water to some height, then a turbine running on the water flowing down. One would still have to rely on noncapillary action to drive the moisture up into a reservoir on top, from which the spongers are fed, but at least, without spraying, it is ascertained that all evaporated moisture is directly cooling the air, and no water droplets make their way down, wasting gravitational energy, and the turbines and the tower on the bottom would be dry. Even if the droplets completely evaporate on the way down, any height they fall as liquid without being vaporized thereby cooling the air is a direct waste of pumping height. One might as well cover the insulated inner wall with wet sponges, all the way down, and only pump some of the water halfway up then, instead of all the way to the top.

There is a danger of recycling moist air, in a circular pattern around the tower: cool moist air falls down displacing the hot dry light air at the foot of the chimney, which then rises up toward the top. The cool moist air hitting the hot dry sand at the bottom heats up, thereby gets lighter and rises too, and reaches the top of the tower, full of humidity. Such humid air is less efficiently coolable, because its relative humidity is higher than that of the hot dry air that came up previously, and there is an ultimate limit on how much water one can put into a given volume of air. Diffusion effects of moisture to miles away, and especially horizontal wind would end up setting up a steady state energy production scenario. So ideally, one would have these towers where there is some gentle horizontal dry breeze constantly available (wind not too strong because the stress on the supertall tower becomes huge), to sweep away the moisture filled air from near the turbine. A horizontal breeze on the other hand, because of wind shear effect above the ground, would create a venturi tube effect, pulling an updraft through the chimney countering the downdraft, so one would have to put a huge diameter curved rotating elbow on top of the tower that is rotated so that it faces the horizontal breeze head on, and the generated wind pressure it actually adds to the downdraft. By the way a venturi tube effect would help an updraft tower.

Also, ultimately the moisture has to come out of the air, and this happens through moisture turning into clouds then precipitation. A lot of precipitation happens near sudden altitude rises, because temperature decreases with altitude, such as near the Kilimanjaro mountains in Africa, which has interesting vegetation layers, desert on bottom, livable temperate climate with lots of precipitation somewhat higher, and snow on the peaks. Similar situation arises in the rainforests near the Himalaya's in India. In fact, one could argue that such a tower may function even without moisture, because the air at the top is already colder, thus heavier, and the hot air near the ground wants to rise, while the cold air near the top wants to descend. Whether it's the hot air rising through the stack and the cold air coming down on the outside, or the other way around, depends on luck (and the Venturi tube effect making most unoperating idle smokestacks still vent up heavily, not down), but as soon as inertia is set up via some fans or an elbow facing the dominant wind direction on top, the circulation should be somewhat self sustaining, as opposed to the random turbulent and self-braking intermixing of hot air on bottom and cold air on top that happens naturally. Perhaps there are some geological formation where such experiments can be easily conducted out, near sudden cliffs such as in the Grand Canyon. One could also create artificial tunnels laid down flat on the sloping ground to go up a steep mountainside, and such a structure, even though longer thus more friction-lossy, might still be less costly, less susceptible to demise under a storm and easier to repair than a 1 km tower standing upright in a desert.

So the moisture cooling scheme tower is limited by dry air availability from a distance. But in fact an updraft tower is limited too, in the sense that there is a total volume of cold air up there and nearby that can be displaced, before simply the warm air recirculates. Also, a large 30 km collector would collect cold air during the night, which, without suitable wind to stir it and move it out of the neighbourhood because it's shielded by the glass house, would take that much longer to reheat during the day. Air is a good insulator, and without meteorological horizontal wind or diffusion effects, one would end up with hot air stuck in the upper atmosphere in one spot for month, and just accumulate, and fluctuating hot/cold air at the bottom, though it would seldom be hotter than the hot bulk air stuck on top. Basically everything comes down to the day/night temperature fluctuations of the surface, which get buffered by the nearby air. Sand has a lot of reflectivity, so using black rocks as black body radiators would amplify such solar heating and nighttime cooling. Note: Actually, now that I read the Earth's energy budget page, it seems most of the infrared radiation to outer space happens by the atmosphere, and not the ground surface, the 70% absorbed solar radiation split in a 64% air to 6% ground ration for infrared cooling. So the hot air on top does cool down, the atmosphere is the major "coolant" device of this planet, while the ground is the major heating device, in a 51% for the ground to 19 % for the atmosphere ratio. So the surface will tend to be hotter, while the air at high altitudes colder.

Because of the limited temperature differences of , the maximum thermodynamic efficiency in such heat engines is low, on the order of maximum 7% of absorbed energy if operating between 10 °C at high altitudes to 30 °C near the ground. A more thermally efficient (not necessarily financially efficient) way to using solar energy would be using concentrating solar panels that heat an inert molten salt reservoir up to 1000-1500 °C, and use the cold night 5 °C temperature, or even the 30 °C available during daytime, and attain 75-85% thermodynamic efficiency. Such molten salt thermal schemes should beat even photovoltaic efficiencies of 15%, whereby solar radiation is 80% efficiently absorbed by a black body, 80% efficiently converted to mechanical energy, which is 95% efficiently converted to electricity, giving an overall 60% efficiency, far above the 10-20% possible with silicon solar cells. On the other hand silicon cells can function in diffuse light or cloudy days, while tracking-concentrator based systems have moving elements that are prone to breakdown, plus materials wear quickly at 1500 °C temperature, and need frequent replacements.

One could try to combine all technologies: the reflector mirrors would be the solar panels themselves, with shiny aluminum backing/50 micron thin silicon layer cover for photovoltaic, reflecting to a few nearby molten salt towers. Cover the whole thing into a solar-forged glass-house, to protect it from hail or even blown fine sand etching the mirror surfaces, and use the glass house with an updraft tower, which helps generate some constant local wind used by the cold temperature reservoir heat exchangers of the wind. Tap most of the energy into a suitable energy storage medium that the world still has to figure out what it should be, liquid hydrogen just doesn't seem too nice, compressed hydrogen a big maybe. I'm betting on aluminum and silicon metals, or the less abundant but easier to deal with magnesium and zinc, or even carbon or hydrocarbon based means if we have fuel cell devices that capture the CO2 in an easily liquefiable form, then transport it to solar stations to regenerate hydrocarbons. There isn't much carbon in the desert, most of the stuff is sand which is silicon dioxide, aluminum silicates, calcium-magnesium-iron-titanium-sodium-potassium-silicates. There should be quite a few ppm of uranium and especially the insoluble/nonweatherable thorium in sand that might be possible to get as an important byproduct of sand processing. But keep converting some of the energy into more mirrors and solar cells, and nonreflective molten glass roof, to have these things replicate themselves locally - though be careful with self replicating systems, that they do not evolve into something intelligent, possibly more intelligent than us humans, that displace us.

Theoretically, such solar tower energy, together with wind energy, extracted directly from the air and converted into a long term storage form, such as aluminum metal, would directly oppose the global warming effect. In fact, too much energy extracted from the atosphere might toss the atmospheric heat balance into a global cooling problem, and then we would need to purposefully pump carbon dioxide and methane into the atmosphere to maintain temperature. Basically any energy extracted would behave as if we had a device operating from a single temperature reservoir, extracting heat and thereby cooling it, which is forbidden by the 2nd law of thermodynamics. However, in our case, within this large reservoir we constantly find smaller hot and cold reservoirs, gradients being sustained by solar radiation on one hand, and near 0K Universe accepting the heat radiation on the other hand. We would not be dealing with a single temperature reservoir as forbidden by the 2nd law, just its flow balance. Another way to fight global warming is by reradiating the solar radiation with a mirror, instead of allowing it to heat the ground. Picture the Sahara covered with mirrors, that bounce sunshine back into outer space, then during the night they switch over to a black body surface to radiate infrared into outer space, cooling Earth as an effect. But in fact, they don't need to radiate this energy into outer space, just to the top of a concentrating tower, which, if capable of converting such energy 50% efficiently into say metallic silicon or aluminum or titanium forms of energy that can be stored for a long time in underground caverns, or just used as building materials, then at least for that 50% part the solar radiation did not contribute to global warming.

Unless done on the scale of a whole Sahara desert, we are far from affecting the atmospheric energy balance significantly, by removing energy from the atmosphere: the Earth's energy budget is measured in petawatts, and out of this 30% is instantly reflected into outer space, and the Earth absorbs 70% of the 174 petawatt radiation energy received from the Sun during the day. In comparison, humans use about 13 terrawatt energy, including oil, gas, coal, nuclear and hydro. There is a long way to go before the 174,000 terrawatt energy balance is significantly affected by directly removing energy, even though it's currently significantly affected by other means, by changing the atmospheric CO2 and methane concentrations that block the nighttime infrared radiation cooling, keeping the heat in, causing global warming. Sillybilly 19:27, 29 January 2006 (UTC)

Solar tower diameter

Solar towers can be very large when measured across, but 30 kilometers seems a little excessive! I assume the author meant 30 meters? 69.221.13.64 08:09, 11 March 2006

The planned glass warehouse associated with the Solar Tower has indeed a diameter of 30 km. The tower itself has a diameter of 100m or so. To anyone who is not caught up in the hype of this Solar Tower project it is clear that this is totally ridiculous. The Solar Tower has one fundamental flaw, and that is that its yield is pathetic; 2% or so. As a consequence it needs those oversized collector areas. JdH 13:24, 14 May 2006 (UTC)

Discussion of Solar Tower vs Energy tower

From a purely financial point of view, Solar Towers may prove more attractive regardless, as land in hot, dry, areas of the world (such as deserts) is relatively cheap, and a glass collector would be a one-time up front cost, placing put the Solar Tower in a well understood asset class of high up-front cost investments that consume no inputs and generate constant revenue over time, such as bridges and toll roads. 58.178.0.120 04:49, 20 April 2006 (moved from article to here by JdH)

58.178.0.120 can you please identify yourself? This remark may very well come from someone who is associated with the competing Solar Tower project, and it is very biased, to say the very least. As of now that Solar Tower has not been realized either, despite massive publicity campaigns that inappropriately extents to the pages of Wikipedia. It is obvious that the Solar Tower is a commercial monstrosity, as the maintenance of those glass house collectors is going to be far too costly. JdH 13:24, 14 May 2006 (UTC)

Yeah but under the glass house you could house expensive silicon solar panels under clean conditions, and instead of having to maintain broken/scratched solar panels, you'd have to maintain the broken/scratched glass house glass above them. You'd have a constant and predictable cooling draft too (panels are more efficient when cold). Most solar panels incorporate a protective thick window - might as well put all those windows together and move them higher, and only keep a thinner protective layer right on the solar panel that doesn't inhibit cooling, even if it has a slight extra light reflection loss. That monstrosity of a solar tower might be viable when you consider what all you can put under it. Whatever way you harness solar energy, the maintenance of large area solar energy harnessing things is just as much of a monstrosity. You could call the glass house a monstrosity abstraction layer, or monstrosity concentration liability/insurance policy. Sillybilly 18:06, 5 June 2006 (UTC)

Well, all the EnviroMission folks need to do is hire all the homeless people they can find on the streets of Sydney, and have them wash all those glass windows. And after they are done have them start all over again.

But according to the article on Solar panels those are at least an order of magnitude more efficient in converting sunlight to electrical power than the Solar Tower is; hence you need to hire 10x less homeless people to keep washing those panels. JdH 23:11, 5 June 2006 (UTC)

For now it's hard to beat life (such as corn, or superfast growing plants) at harnessing solar power when it comes to profitability. All you need to do is toss the seed, and you're done (well, other than what farmers do, still a lot of very hard work going on.) The problem is that photosynthesis/life is only about 0.25% efficient in extracting solar energy, and the maximum recorded deep in lagunes is something like 2%, where pretty much every photon hits a green surface. Also, life won't grow in some sunny areas such as deserts. Out of the total 174,000 TW solar energy hitting the planet, only 100 TW is converted by life - the rest is reflected, turned into heat, wind, infrared cooling at night, water vapour/rain, etc. Right now humanity uses 13TW of primary energy (oil, coal, nuclear, hydro), the US containing 5% of the world population using 25% of energy and even this isn't enough. If the rest of the world consumed at the US rate, the consumption would be 0.25%*100%/5%=500%=5 times what it is today, or about 65 TW. That's a lot compared to the 100 TW total absorbed by life, because you have to leave some to the gazelles, buffalos, birs, sharks, polar bears, jaguars - man is not the only top predator, and as you go up the foodchain, some energy is consumed (turned into heat) to support each step. Corn is cheap, toss a seed, but pretty much all life would have to be submitted to supporting the human biomass, so that doesn't solve the problem (let's not even talk about ethanol, which is a big sugar energy loss to the fermenting bacteria, plus the distillation required heat is tremendous - if you are going with ethanol you should pretty much mandate all nuclear plants to distill ethanol instead of just dumping all that heat through cooling towers, but there are safety/terrorist issues there. Using natural gas methane heat to get ethanol from corn is an unbelievably wasteful thing. At least the brazilians burn the rest of the corn plant.) We have to let life be only food, and get most of the primary energy some other way. A person uses about 100W of energy resting (out of which 20W is used by the brain, which is 5% of bodymass, it's an energy-hog, and it dies in 5 minutes without oxygen), and about 700 W (1 hp) when running at 6 mph. Multiply that with 10 billion people using about 200W and that's only 2TW of human food, which should be doable with the 100 TW of life. So sooner or later, when all nuclear is gone, and you can't touch coal and fossils because of global warming, and everywhere you look there are windmills that only work when the wind blows and they don't get enough, sooner or later you have to get down to getting solar energy in a more efficient way than life can get to. You have huge area deserts going wasted with a ton of predictable sunshine, you could fill all that with people even if their dayjob is to wash windows, and when you run out of room, you have plenty of room in outer space to keep it going. Silicon can get solar energy at least 8% efficiently largescale, that's 32x better than life can do, even if it's 100x pricier for now than life (but prices are flexible, they will take on any value to satisfy demand.) "Cheap" (which is still expensive) amorphous silicon does 8% efficiency, very expensive crystalline silicon can do 15% and 20%. I don't think multilayer exotic GaAs/In/P/Sb stuff that can do 35-40% efficient is ever going to fly largescale other than for satellite applications, because while we have a ton of silicon on the planet (75% of dirt througout the world is silicon dioxide) we don't have enough exotic materials, at least not enough to fill outer space with it. So coming back, yes, solar towers, solar panels of any kind are expensive monstrosities for now, but the question is not what does it cost now, but can you afford living without them later, what will they cost in the long run. Unless you want to implement a global one child per family policy like China has (China can do it, the rest of the world might be doubtful), or even play russian rulette with each other saying hey there is too many of us, who's it gonna be, me or you, you spin the barrel, I pull the trigger, then we switch, or start a third world war, so unless you want to think along these lines, you have to assume more people on the planet, and more energy needed, more global warming, and when you think of all of this, a solar tower monstrosity is a pretty peaceful thing to do. Hey, the Egyptians built pyramids, the Chinese built the Great Wall, what was the profit in those? Why are we so incapable of building something like a solar satellite? It's better to direct your energy/efforts into building something peaceful even if it's nonsense, instead of turning to warring with each other, what's the overall profit in a war?

Unfortunately the biggest problem right now is finding a good way to store energy, not the harnessing of renewable energy, but how to store it. Lead-acid batteries just won't cut it, neither will a hydrogen economy because liquid hydrogen is 10x less dense than oil, plus it's very very cold.

the answer to the energy storage issue is obvious: Pumped-storage hydroelectricity. All you need to do is built your solar power plant next to Hoover Dam and you are in business. JdH 08:18, 7 June 2006 (UTC)

First of all you can't take pumped storage with you in a car. Also, there isn't enough pumped storage in the world because it's very low on energy density. Just look at the hydroelectric statistics (USA gets about 60Mton oil/yr worth of hydroelectric out of the total 2300, even nuclear gives 190 Mtons worth today (multiply by about 1.3 to get GW from Mton oil/year, so 80 GW is hydro out of the 3000 GW or 3TW the USA uses)), with pretty much everything dammable dammed up hydroelectric barely provides a little bit of our electrical need, which is only a fraction of our transportation energy needs. I posted this to the pumped storage page a while ago, it's been slowly eradicated from there since: Another issue is the relatively low energy density with pumped storage systems, even though huge volume artificial lakes are available behind dams. An m=1 kg of water pumped h=100 meter high in a gravitational field of g=9.8 m/s² represents an energy content of only E=mgh≈0.001 MJ/kg (3.6 MJ = 1 kWh), while 1 kg of uncombusted aluminum represents 31 MJ/kg of chemical energy, and this number is 25 for magnesium, 32 for metallic silicon, 43 for lithium, 5 for zinc, 7 for iron, 15 for wood, 17 for sugar, 32 for carbon, and 46 MJ/kg for gasoline. Basically 1 kg of gasoline is theoretically capable of moving 46 tons of water to a 100 m height. (Theoretically, because only about 35% of this energy is extractable in an internal combustion engine for gasoline, but with fuel cells the efficiency might climb to 50-85%. Aluminum as an electrode in a properly designed battery might also be 85% efficient in giving up its energy content.) For reference the scarce uranium-235 is 90 million MJ/kg (1 kg U-235 enough to pump 90 million tons of water), thorium is approximately the same, while fusion would be 300 million MJ/kg, based on plentiful hydrogen, if the technology were available, eliminating any need for energy storage. For now, chemical potential energy might prove a better and more stable longterm storage mechanism, but for now the evaporation-risky pumped storage is the most cost effective means of storing large amounts of electrical power generated from renewables such as grid tied solar (≈ 3 MJ/m²/day - 1 m² photovoltaic producing 75 g of gasoline worth daily) or wind power stations. Sillybilly 18:25, 10 June 2006 (UTC)

Life solved this storage problem too, with superefficient energy/mass/volume hydrocarbon fats that are pretty much like crude oil in energy density (40 MJ/kg), or less efficient, but very soluble sugars (20 MJ/kg). Are we going to recycle carbon, ship carbon dioxide to the desert, and get oil back? You can bet the price of such oil won't be anywhere near as cheap as the crude oil you get by punching a hole in the ground today. If you don't want to transport all that oxygen back and forth on railcars, (32 g O2 vs. 12 g C in 44 g of CO2), and the atmosphere contains very little CO2(.03%), what will you do? In case you do want to extract CO2, a tower structure might be needed anyway to process large quantities of air. Otherwise we could turn to the sand in the desert, and extract aluminum (or silicon if we're lucky) to use as mechanically rechargable rods in batteries on the other side of the globe, then dump the aluminum oxide/fine silica generated into asphalt, cement and building materials. They are very nontoxic. What's it gonna be? Definitely not lead acid, nor hydrogen, nor ethanol(not life-based ethanol at least, industrially extracted CO2+H2 based maybe.) Are we gonna end up duplicating life and competing for the CO2 in the deserts with it in deserts? At least we could regulate global warming by how much carbon we extract and store as largescale reserves. On the moon there is no CO2, so there it's pretty much having to deal with the dust to have an energy economy. There is also liquid ammonia as a hydrogen carrier that needs to be mentioned, based on plentiful nitrogen available in the desert, which could also be used as a fertilizer instead of fuel. Problem is liquid ammonia is also cold, somewhat toxic (more toxic than gasoline), and especially it doesn't make a good fuel. Sillybilly 16:44, 6 June 2006 (UTC)

that's quite a story :-). Basically I agree that we need to wean ourselves from the fossil fuel addiction. Unfortunately, the laws of economics seem to be at least as unyielding to our whims as are the laws of physics. If we only could dismiss the 2nd law of thermodynamics, wouldn't that make things much easier?

You want to find a way to break the first law, the law of conservation of energy, meaning you could generate some without having to consume anyting in the process, and get a perpetuum mobile that outputs a net energy profit and consumes nothing. The 2nd law is about entropy, friction, about a forbidding a perpetuum mobile devoid of small losses that uses energy internally 100% efficiently, and therefore moves forever. That would never solve our energy problems. Unfortunately the first law is pretty much set in stone, even Pauli concoted the neutrino by putting faith into the law of conservation of energy instead of applying Occam's Razor and discarding this law in face of experimental evidence. Sillybilly 19:18, 10 June 2006 (UTC)

Likewise, if we only could convince people to pay a premium for renewable energy over fossil energy, wouldn't make that things a lot easier as well? But that seems even more unlikely than breaking the 2nd law of thermodynamics. So, to make this work renewable energy needs to be competitive with fossil energy. With all the coal reserves that are still out there that's going to be a tall order.

For now let's go back to comparing the Solar Tower with its sister approaches. I take it that Southern California Edison (SCE) and Stirling Energy Systems have announced an agreement that could result in construction of the world’s largest solar facility, see http://thefraserdomain.typepad.com/energy/2005/08/about_dishengin.html http://www.edison.com/pressroom/pr.asp?id=5885 and many other places, including Solar power. That project is even larger in scope than that EnviroMission project, 500-megawatt vs 200-megawatt. However, whereas the EnviroMission project talks about a collector area with a diameter of 30 km, the SCE-SES project needs 4,500 acres; that translates into a collector area with a diameter of 'only' 4.8 km. Still large, but much more manageable than EnviroMission's 30km. That much smaller collector area can be explained by the fact that the SCE-SES folks claim a 30% conversion efficiency, whereas the Solar Tower has less than 2%.

Still, the SCE-SES folks need to keep washing those 20,000 mirrors. JdH 21:08, 6 June 2006 (UTC)

You are right about heat engines being more efficient in the desert than silicon. I think the current stirling technologies are still not there, there is room for improvement. You need to create a superhot heat engine: a black box made of tugsten holding an inert gas like xenon that doesn't diffuse much into the metal at 1500C, and have a graphite lubed shaft providing the power. Molten salts will corrode your engine away at 1500C, you'll have to figure out something very inert and nonreactive to be able to take the temperatures very high and get high Carnot-cycle efficiencies.

What about ceramic engines? http://www.ceramicrotaryengines.com/

There is a good reason why car engines are made out of metals instead of say, clay. A potter could make the carparts, but that wouldn't last. Ceramics are brittle and they crack, especially under thermal cycling. Insulation is important though, so you might use ceramic insulation tiles on top of the mechanical metal part, like with the space shuttle. I was thinking vacuum + reflective wall + magnetic coupling (lubed shafts leak, magnetics don't) with a superduper hightech million dollar engine as the superefficient heart that deals with the sunlight received from 100,000 suntracking mirrors, the engine locked inside a perfect radiative black box with a hole for all the sunshine to enter. You still need a fluid that goes from hot to cold regions in the heat engine, and it's hard to think of anything fluid and inert at such high temperatures, other than the noble gases, like Argon, Helium, Xenon. Because they are gases they are very light with very little heat capacity (depressingly little), so you may need to supercritically compress them, so you may be talking some heavy duty solid block 2500C tungsten body engine with argon/xenon mix operating at 10000 atm pressure. Your problem then becomes efficient heat exchange to penetrate through that thick block of metal, at both the room temperature cold side, and the 2500C high temperature side. The Sun's surface is 6000C, so theoretically you could concentrator-radiation-equilibrate a body up to 6000C with sunlight, but you couldn't do 7000C, because then your body would radiate heat at a higher rate than the Sun, and would instead heat the Sun, back through the mirrors. With such supercrazy design you might be able to get 60% Carnot-cycle efficiency, far beating any kind of photovoltaic system, very expensive engine, but squaremiles of tracking mirrors homing in to one central heart (you may need a very high tower and very accurate tracking systems for that). Also, the problem with PV/concentrator based systems is that you can only concentrate so much on a cell before it fries because of internal resistance and amperage, so you can't take 10,000 mirrors and shine them on a single PV surface, unlike with a heat engine, that's a fairly small size for even a few hundred horsepowers conventional type, I don't know about superhighpressure noble gas filled ones though. So you're limited to small focusing, something like 10x area, but the problem becomes that your system suddenly became a tracking system with moving parts that won't track accurately, and break down, compared to a single cell that just sits still, and needs no maintenance for 40 years, other than wiping off the dust, and it can even give you some juice off the cloud reflections, or during bright cloudy days, other than that it just churns out juice without needing much messing with. The only danger is hail and cracking. Also, most concentrator based systems are exotic material based, mainly GaAs, and there is only so much of that to go around, they are perfect for satellite/space applications, but down here we have a ton of silicon instead, which is the most abundant element in rocks. I also agree that the concept of an energy tower by itself is bologni, but don't discard the concept to build it on top of whatever you're already building in the desert, whether a thermal concentrator, or just silicon flat panels, it might just be a feasible structure, and then whatever energy you get by putting wind turbines at the chimney exit at the top comes as a plus. 00:30, 13 June 2006 (UTC)

Still, I think people won't be very energy conscious if all they have to do is pay for energy, they'll just moan, like they do at cigarette prices, but still smoke them. If everyone had roof installed solar panels and they'd see their annual incoming energy budget vs. what they spend, that'd be a lot different. You can bet nobody would buy 6 mpg Hummers then, no matter how 'cool' they are, or how much profit they generate to the car companies because they contain more steel, more glass, more rubber. It's like knowing how much grain you harvested vs. how much you ate. It would also provide some private energy independence and decentralization of power, because centralized energy generation which is the only way possible with stirling/concentrator based systems yields to very severe means of power abuse by government over its people. Sillybilly 19:18, 10 June 2006 (UTC)

Or concentrating photovoltaic (CPV) systems rather than heat engines? Those folks claim 40% conversion rates, and seem confident that there is room for further improvement, see e.g. http://www.nrel.gov/news/press/release.cfm/release_id=10 http://www.energyinnovations.com/sunflower250.html http://www.nrel.gov/ncpv/new_in_cpv.html http://www.pvresources.com/en/concentrator.php It is something that should be mentioned in Solar power; AFAIK it isn't even mentioned there. Anyway, the reason I engaged in this discussion is that when I first heard about the Solar Tower and the Energy Tower I thought those were pretty neat ideas. However, with conversion rates of 30 or 40% of these concentrating systems (be it thermal or photovoltaic) and improving while we speak I now have my doubts. It appears to me that while the Solar and/or Energy tower technologies were interesting 10 or 20 years ago that in view of recent innovations they are now obsolete. Would it be appropriate to mention that in the respective articles, or should we stay away from that? JdH 20:58, 10 June 2006 (UTC)

Longevity of a solar updraft tower

The huge greenhouse situated at the base of a solar tower looks very vulnerable to weather. What wind speeds could the system tolerate. Other potential problems I have in mind would be hailstorms, snowstorms(Canada), duststorms in deserts etc.. There can't be many suitable sites in the world where such problems do not exist? Patj 17:28, 5 August 2006 (UTC)Patj

Requested move

"Water Spray Convection Energy Tower" is a ridiculous misnomer. To begin with: This is a downdraft tower, not an updraft tower as is implicated by the word convection. It is wrong, plain and simple.
The new name is also against the Wikipedia convention to use common names, and not invent new names, and certainly not new names that are wrong to begin with.
Surfing around it turns out that there are several common names in use for Dan Zaslavsky's Energy Tower. Most commonly used is "Aero-electric power station" or "Aero-electric power plant", "Aero-electric power tower", "Aeroelectric tower" or "S.N.A.P. tower"; "S.N.A.P." stands for SNeh Aero-electric Power, Sneh is the Hebrew word for ‘Burning Bush’. Zaslavsky also uses the name "Sharav Sluice Energy Tower"
I would appreciate some input on this issue and reach a consensus, to avoid further misnomers like the present one. JdH 13:50, 2 March 2007 (UTC)

I would like to have a name that is easier to remember than the ones you listed and describes it more. Since the energy in the hot air got there because of the sun, it is really a solar power system. Because there is a solar updraft tower it seems natural to call this other one a "solar downdraft tower". I put in a redirect so that solar downdraft tower works. I fully support a move to this name (which would make a redirect from here to there) but in the meantime a redirect from that name to here lets me start using the potential new name. Vincecate 19:31, 3 March 2007 (UTC)

Please have a look at Wikipedia:Naming conventions. In particular, where possible we should avoid neologisms. Solar updraft tower was not a neologism when it was proposed, since that was already used in Jörg Schlaich's paper , but your proposed name certainly is a neologism. Furthermore, solar downdraft tower does not accurately reflect the fact that for it to work it needs hot, dry air. The required hot, dry weather can be found at the Horse latitudes or Subtropical High of the subtropical latitudes between 30 and 35 degrees both north and south, and is caused by descending air of Hadley cell circulation. The system would not work in the hot, humid climate of the Intertropical Convergence Zone, so solar radiation alone does not suffice.
Aeroelectric power goes all the way back to Carlson's original patent US patent 3,894,393, Carlson; Phillip R., "Power generation through controlled convection (aeroelectric power generation)", issued 1975-07-15 , so it certainly has a long track record, even longer than Energy tower introduced by Dan Zaslavsky. But it might not meet Wikipedia's convention to use common names where possible. So perhaps it would be best to revert to the old name Energy tower, even though that has the problem that it is not very precise. But that may be addressed by using disambiguation, such as Energy tower (aeroelectric) or Energy tower (downdraft). JdH 08:42, 4 March 2007 (UTC)

To me Aeroelectric power just sounds like wind power, so I don't like that. I think it is easier to remember names that are a bit more on target. I agree that Solar downdraft tower is a neologism, but a far better one than the Water Spray Convection Energy Tower neologism this article currently has, so at least an improvement. Google does not find that anywhere outside of Wikipedia. But I think the disambiguation Energy tower (downdraft) seems best, since we are not supposed to make any new names. It fits the Wikipedia way of doing things. So you have my agreement on that. Vincecate 11:16, 4 March 2007 (UTC)

Theoretical not grounds for deletion

I read over the deletion policy and don't see anyplace where it says articles can not talk about things that have not been built yet. So grounds for deletion seem invalid. Please discuss first. If there is no consensus for deletion then it does not go through. I don't consent. Vincecate 00:52, 9 March 2007 (UTC)

Promotional is grounds for deletion

This article first appeared with "brainchild of Professor Dan Zaslavsky" as the credit and was finally corrected by Krysith to give credit to Carlson for the original patent. It has been and still is an attempt at promoting Sharav Sluice's (partially owned by Dan Zaslavsky) promotion.

Theoretical was not the reason for AfD. Promotional while attempting to secure funding is reason for deletion. The original sock of the article did not give proper credit and it still proposes that there was an improvement by Dan Zaslavsky. I would think that if a pilot plant was built the article should stay, until then it's only purpose is to promote an attempt at securing funding. —The preceding unsigned comment was added by Rohar1 (talk • contribs) 01:11, 9 March 2007 (UTC).

If you think it should not say "improvement by Dan Zaslavsky" then say you question that and want a reference. But to say that is grounds for deletion does not sound reasonable. Your argument only justifies deleting those 4 words.

This article is describing a potentially important idea. It could generate a lot of power where the only pollution is a bit of salty air. There is more here than someone trying to get funding. It is an interesting idea. Vincecate 02:53, 9 March 2007 (UTC)

--Rohar1 03:18, 9 March 2007 (UTC) It's an interesting idea. I have been working on an open project www.energytower.org partially based on the convection tower concept. It has a wikipedia article Bi-directional_Energy_Tower that was AfD by JdH. His argument was that it was theoretical and didn't have supporting published scientific papers. The whole discussion is open for a long argument, because the energytower.org project is based on solar chillers, SEGS/CSP and geothermal technology and some of the ideas in convection towers. I agree with JdH that it is theoretical and new and that it shouldn't be included in wikipedia until there was a working prototype. Published theoretical papers don't constitute a feasible power plant. There is a long discussion on User_talk:Rohar1 and User_talk:JdH on the issue. The energytower.org project doesn't need the wikipedia advertising or promotion, if it's an issue with it being in development than I am fine with deleting the Bi-directional_Energy_Tower page. Again, I would like wikipedia to be correct.

By this same reasoning, the Solar_updraft_tower had a working (although not very efficient) prototype for several years and is something real, but the water spray downdraft tower was patented in 1975 and never acted on and then revived in what is a disputable patent and attempted to market again.

I really would rather have wikipedia be accurate and to correctly reflect the rules. What is Wikipedia and what is Wikipedia not? Wikipedia is not a publisher of original thought, Wikipedia is not a soapbox, Wikipedia is not a blog, and Wikipedia is not a crystal_ball

By this definition, this page should be deleted. It isn't any more relevant than the millions of other patented devices that were never feasible or brought to commercial success. If you want to create those million articles on unfeasible patented devices, then do so. Otherwise this page and the Bi-directional_Energy_Tower should go.

If you want an explanation of why the water spray idea will have extreme difficulty becoming commercial, check User_talk:Rohar1 and User_talk:JdH for the discussion. There is more explanation on the Background and Prior Art page.

Conversion Efficiency

The cat is out of the bag: I don't believe that the Energy tower or the Solar updraft tower are going anywhere; in fact, these proposals are obsolete and should be discarded and filed in the old round file cabinet: that's long overdue anyway. The basic reason I say that is that a Chimney is a very inefficient heat engine. btw: "Tower" is a misnomer that obfuscates its principle: people who are more interested in promotion than in technology don't like to call it "chimney" because of its association with pollution.
The issue of conversion efficiency has been discussed before, see: Talk:Solar updraft tower/Archive 2#Carnot engine. According to the eXtreme High Altitude Conditions Calculator the atmospheric pressure at 1,000 m altitude is 89% of what it is at ground level; at 1,200 m it is still at 87%. Assuming that the temperature differential across the chimney wall is constant with altitude (not unreasonable given adiabatic expansion as the air rises inside the chimney or out) it follows that the conversion efficiency for a 1 km high chimney can never be better than (100-89)% = 11% of the theoretical limit of a Carnot cycle(or 13% for a 1,200 m one).
Stirling engines achieve in practice something like 50% of the Carnot limit. In order to match that efficiency one would have to build a chimney high enough to reach 50% atmospheric pressure: i.e.: the chimney would have to be 5,486 m high: a whopping FIVE times higher than has been proposed for the Energy tower (or for the initial optimistic design of Solar Tower Buronga). Obviously, that high is ludicrous.
This reasoning leads to is the simple conclusion that a chimney of only 10 meters (or 50 meters, whatever you need to achieve efficient evaporation), and using Stirling engines instead to drive the electrical generators would be 5 times more efficient than the Sharav Sluice Energy Tower.
Now why on earth didn't I patent that idea first before giving it away? JdH 17:07, 11 March 2007 (UTC)

I think you might be right. But there seems kind of a big leap right before you say "it follows that". A few more details there could help your readers. Vincecate 18:02, 11 March 2007 (UTC)

Simple: The moment the warm air escapes from the top of the solar updraft tower you have lost it, and it does no longer do any work. Therefore, the fractional atmospheric pressure at the top of the chimney provides a direct measure of the fraction of the thermal energy that escapes from the top of the chimney without having done any work. Of course, the Energy tower works in reverse, but the principle remains the same. As said, this is based on the assumption that the temperature differential across the chimney wall is independent of altitude. For a chimney of only 1 km high that is not unreasonable; remember: the air inside the chimney cools as well as it flows up due to adiabatic expansion. To get more accurate estimates one would have to do actual measurements of the local temperature gradients with altitude. JdH 18:40, 11 March 2007 (UTC)

Cool. That is a nice simple important point. So I understand/believe that they are not very efficient. Now if we had to buy fuel at $2/gallon we would really care about efficiency. But the only thing we have to do here is pump water to the top. So the question is do we get out enough more energy than we put in pumping the water to justify the capital costs.

I have a pool like http://www.qualityinflatables.com/easyset18footpool.html that is material held up by the water inside. It has an inflated ring around the top and the sides slope in so that the water pushes up/out on them and holds them in place. In this Energy Tower we have cool heavier air inside. We could make something similar to my pool with some hydrogen holding the very top edge and the rest supported by the heavier air inside. So the capital costs might be very low. Vincecate 22:50, 11 March 2007 (UTC)

You have to pump tons of water to the very top of that tower. That required a pretty sturdy construction, else the thing would collapse

A pipe coming up the side is probably not hard at all. I think the hard part is making sure it is safe in high winds. But my pool is amazingly sturdy. I really think a similar design could work. Still, it is a lot of material and would cost more than a couple bucks... Vincecate 22:50, 11 March 2007 (UTC)

If a Stirling engine to handle this volume of air was 3 times as efficient but cost 10 times as much, it would be better to stick with the tower, even though it was not energy-efficient.

In a big desert with a breeze we have plenty of hot air. The question is capital costs and how much energy do we get out relative to the energy of pumping the water up. It is efficient use of money, not efficient use of hot air, that is important when hot air is free. —The preceding unsigned comment was added by Vincecate (talk • contribs) 21:14, 11 March 2007 (UTC).

Sure, in the end it is all about $$$s. But remember: You have to compete with technologies such as CSP and CVP that have a 100-fold higher conversion efficiency. In this case it is all about capital investments and operational costs; sun and dry air are both free. OK, those mirrors and solar cells and Stirling engines are not cheap, but building a 1 km high chimney is no small feat either. I had that discussion before, see Financial hurdles, and that led to the conclusion that the Solar updraft tower was 10 times more expensive than wind turbines, and 2 or 3 times more expensive than solar thermal energy. If we are to believe Dan Zaslavsky the Energy tower would be competitive with wind turbines. I have a hard time to believe that: wind is free, and all it takes is a turbine. The Energy tower needs both turbines and a 1 km high chimney, so how can that possibly be cheaper than a turbine alone? Zaslavsky has an obvious bias, and I would have to see independent calculations first. He takes all sorts of peripheral things into account, such as: desalination, fish ponds, agriculture; without that it would look a lot worse. JdH 22:14, 11 March 2007 (UTC)

It could be cheaper if higher pressure meant you did not need the huge propellers, so you saved that cost. But I agree, it does not look easy. On the other hand, if you had a desert where the wind was not very strong, maybe this could be the way to go. Vincecate 22:50, 11 March 2007 (UTC)

I think it would be worth summarizing this in the Problems section. Vincecate 01:28, 16 March 2007 (UTC)

We need sources. We probably can make some sort of statement about conversion efficiency; one of those papers should have at least something about that. $$$s is more difficult: All we have is Zaslavsky's; I haven't seen independent numbers. But as I mentioned: I do have concerns about his numbers. JdH 02:51, 16 March 2007 (UTC)

btw: You may want to have a look at Michael Zwirn's report (Energy Towers: Pros and Cons of the Arubot Sharav Alternative Energy Proposal). He comes up with a long list of concerns, mostly environmental & political ones. Apparently he lives close by the proposed building site in de Arava desert, and points out that the tower poses a threat to the ecosystem there. Noise & visual pollution, saline contamination, changing climate. Also, the Arava desert is one of the major bird migration routes from Eurasia to Africa, and he is concerned the tower might suck up large numbers of birds.

Yes, that is interesting. It is amazing that people put up with coal power-plants spewing incredible amounts of all kinds of crap into the air (real poisons) and environmental concerns may kill something where the worse side-effects are some salt and increased humidity in the desert. Vincecate 14:33, 16 March 2007 (UTC)

I was just checking whether we could make some sense of this Conversion Efficiency issue. What I found in Dan Zaslavsky's Energy Towers is the following claim for efficiency:

$\eta =C(1-{\frac {T_{C}}{T_{H}}}),$

in which C ~ 0.7 - 0.8. When you compare that with the Carnot Efficiency (Formula 3) it appears that Zaslavsky claims that his Energy Tower manages 70-80% of the Carnot limit. I don't believe for one minute that the Energy Tower is that efficient. Just the paragraph before I made the argument that is can't possibly be better than 13% of the Carnot limit. Schlaich's measurements on the Spanish prototype of the Solar updraft tower are in perfect agreement with that. Furthermore, if you take into account that about half of the energy gained has to be used to pump the water up the tower that number drops by another factor 2. What is means is that my estimate of conversion efficiency is more than a factor 10 lower than what Zaslavsky claims it is. All I can say about that is that I feel pretty confident that I am a lot closer to the truth than Zaslavsky is, see Schlaich's paper.
Be it as it may, if I am right then the claims Zaslavsky makes about $¢/kWh have to bumped up by a factor 10, i.e.: it just went from $¢ 4-6 to $¢ 40-60. Conclusion: You'd better built a prototype, and do some measurement first to find out the truth about this. Else you could end up with an engineering blunder of biblical proportions. JdH 22:44, 16 March 2007 (UTC)

Dry Downdraft Tower

There is also serious debate over the necessity of using water to cool the air at high altitudes at all. At higher altitudes air is naturally cooler - out of the 70% absorbed solar raditation the majority, i.e. 64% is radiated as infrared heat to outer space by the atmosphere and clouds, and only 6% by the ground itself. In an updraft tower hot air rises inside the tower and cold air descends on the outside, mostly due to venturi tube effects initiating up upward motion and inertia keeping it going. If really necessary, it should be possible to construct a dry downdraft tower with a specially designed wind-facing rotating elbow on top initiating a downdraft, and the same inertia keeping it going afterwards. Sillybilly posted to article and Vincecate moved to talk

What we need is for air in the tower to be heavier than air outside the tower to get a downdraft. Air currents are mixing the air outside. If air was unusually cold it would go down, etc. The problem with what you are saying is PV/T = PV/T. The pressure * volume / temperature for a mass of air up high will be the same when it gets down low. In practice what this means is that thin-cold-high air can become warm-dense-low air, or vise versa. While the momentum of the wind going into your "wind-facing elbow" could keep a breeze going down the shaft, it would have no more power than the wind already had. It is not like the Energy Tower (downdraft) where the air in the tower is cooler and heavier than the air outside, and so we have a force*distance thing making work. Anyway, I am not buying your claim and asking for a reference or some more convincing description before putting it back in the main article. Vincecate 01:14, 16 March 2007 (UTC)

The reason I inserted that label was that I had similar concerns: As the air moves down the chimney the ambient pressure increases, and hence the downdrafting air compresses and increases in temperature. But there is something else as well: Water vapor is lighter than air; hence, spraying water does not only cool the air, but it also makes it lighter, which counteracts the cooling effect! (that was mentioned in one of the references I put in; I believe it was this one:[1]. JdH 02:51, 16 March 2007 (UTC)

Lets say *partially* counteracts the cooling effect. But again, when energy is free, efficiency is not your biggest concern, only the things you pay for. Vincecate 06:03, 16 March 2007 (UTC)

The conversion rate is really very low. Also, almost 50% of the generated energy has to be used to pump water to the top of the tower. So the yield of the Energy Tower is going to be at least a factor 2 lower than the updraft tower, let's say around 0.5% That translates directly into higher capital investments. We really need an independent assessment of that, and I can't find it.
Apart from that: No investor in his right mind will be willing to invest $ 10⁹ in unproven technology; not even the government will do that. Perhaps a pilot plant of a few $ 10⁶, but not a utility-scale plant. The problem with that is that it is not scaleble: a small version will have a much lower conversion factor than a utility-scale power plant. That was what is bedeviling the solar updraft tower as well: the pilot plant that operated in Spain was 1000x smaller than the proposed Tower in Australia, so it is of limited use for making projections. JdH 10:35, 16 March 2007 (UTC)

I could see a sort of true reverse of the Solar Updraft Tower for part of the day if you had a reflective/insulating wall so that the air inside was not getting heated from the sun. The shaded air could be colder and so go down. I think the economics of this would be worse than the Solar Updraft Tower, which we are both not so fond of. Vincecate 01:20, 16 March 2007 (UTC)

I need to sit down and do some actual calculations one of these days. The workhorse for calculations is the psychrometric chart.
File:Psychrometric chart simplified.png
In the psychrometric chart we follow the wet bulb temperature curve. Suppose you start with a 60°F 10% humidity air on top of the tower, and apply a moisture cooling to it. You basically look for the 60 on the x axis, and the 10% on the right side marking the upward curving humidity curve (some numbers such as 90% humidity end up on top of the chart). Now you apply a "adiabatic cooling" to it, which is following the wet bulb saturation line, sloping upleft/northwest toward the 100% humidity curve. So suppose we start with 60F/10% point on the chart and can saturate it to 90% in the tower - on the graph we go up the wet bulb line and end up with something like 42°F when we hit the 90% humidity line. 100% humidity would be 40°F. (Real psychometric charts give you density too, which gives the actual gravity driving force, search google beautiful charts available, we need a better one on wikipedia - moist air is less dense, but not that much.)

You just invented yourself a Perpetuum Mobile, which is a different way of saying: I don't believe that will work. The basic issue, I think, is that you have to consider your invention to be a closed system: For every mass of cold air that moves down the chimney there has to be an equal mass of air on the outside that moves up. If that didn't happen you would end up with a situation that you create a vacuum at the top of the chimney, and clearly that is not what is happening. So consider the following closed loop: First a certain volume of air moves down the chimney, and compresses and warms up in the process. Arrived at the bottom of the chimney that volume of air leaves the chimney, perhaps passing through some turbines you installed there, and having arrived at the outside it moves back up, back to the top of the chimney where it started its journey. And in the process it expands & cools down. As those compressions and expansions are reversible that volume of air will arrive at the top of the tower at exactly the same pressure and temperature it had when it started going down the chimney in the first place. The loop is closed, and no net work has been performed. JdH 02:17, 17 March 2007 (UTC)

The real issue here is what kind of air is available at the top. If it is cloudy, about to rain, you can pretty much assume 100% humidity and further humidifying does nothing, no cooling, no density increase. I think it would be pretty unreasonable to assume 120°F/5% air on top that's possible to cool to about 70°F/100% humidity. The reason why clouds form in the first place is because humidity is already there (we need a better chart on wikipedia, showing kg moisture/kg dry air, which is the horizontal line, the dew point line.) As I said in the above posting, much of the cooling on Earth happens by the atmosphere radiating the heat, not the ground (you think of air as being pretty transparent, but if you could see in infrared, from outer space our globe would look like a glowing gas ball, without being able to see much of the surface (6% of the 70% radiated infrared "light" "glows" from the surface, the rest 64% from the atmosphere into outer space.) This glowing is the cooling mechanism of Earth, and it happens in the atmosphere, and the higher you go the colder it gets. That's the reason why clouds form - sunlight hits the sea surface and ground, water evaporates into the warm air which rises up because of lower density, then as this air sits up there it infrared cools (nonadiabatically, horizontal line on chart) into outer space until it hits its dewpoint when clouds form, and when clouds coalesce into bigger droplets you get rain.

Technically you can always get cooler air by humidifying, but if what you start with is a cloud, then you're way past the 90% humidity mark. One needs to find climates with very dry and very hot air very high (a tall tower is necessary so that you can develop a very heavy tall column of cold air in a tower, so that it develops a huge driving force able to drive multistage turbines at the bottom - driving force is ρgh, ρ is density, what you're after, g you can't change, gravitational constant, and the other design parameter is h. Width of the tower has almost nothing to do with the driving force, only fluid flow friction - a superfluid would be fine with a tower thin as a pin. If the air at say 1 km height is already 50F cold, then humidification can only take it to 40F, not much of a change, but where will you find very hot air very high, with even hotter air below? Deserts, but even in deserts, the higher up you go the colder it gets, and is it worth pumping water up there if it's already cold. I guess it's always worth it, any little extra juice you can get is helpful. Note that if your droplets fall halfway down the tower, and then evaporate, they only cool and densify the bottom half of the air, not the top. As far as droplets falling is concerned, in the droplet, liquid form the turbines at the bottom do feel the extra pressure, from the droplet filled air being denser as bulk density, but the energy thus extractable is always less than what it took to pump those droplets up in the liquid form to the top. The only reasonable use of water on top is to cool, in a gasified form and any height fallen in the liquid form is a waste.

There are serious technological and financial concerns with the updraft tower, and even more serious with the downdraft one. You basically generate wind, predictable and reliable wind, but there is usually a lot of wind blowing already in the world, even if unpredictable, like the weather.

The Europeans have dreamed up a solution for that: Create a supergrid of interconnected windfarms, ranging from Denmark across the southern North Sea to England and the Celtic Sea to Ireland, and south to France and Spain. The idea is that by the time the first low pressure area has moved away across Denmark to the Baltic Sea the next one appears off Ireland; i.e.: the wind may not blow everywhere all the time, but the wind will always be blowing somewhere. JdH 01:55, 17 March 2007 (UTC)

The thermodynamic efficiency is very bad (taking 6000C photons and creating a 30C-10C thermal engine.) I think there should be some more balance in the page with some heavy criticism and let's keep our feet on the real ground instead of hype about a 1 km high tower (tallest structure ever built), so that the reader understands what the tower is, such a page describing the downdraft tower idea does belong to wikipedia, instead of being deleted as someone recommended, but the reader should get an overall impression after reading the page that feels a lot less like having just read some junk science article. There are pages about the caloric theory and phlogiston theory on wikipedia, but they do not present themselves as ultimate solutions to a topic, and at least one of the references in this article sounds like this energy tower thing is the ultimate solution to energy issues, but I think there are much more competitive ones available, such as very high temperature engines, stirling engines or even silicon solar panels. That's how I feel, but I could be wrong, I'd love to see some numbers, or even a real built up thing prove me wrong. Sillybilly 23:26, 16 March 2007 (UTC)

It just occurred to me that the downdraft tower could very well have a negative yield, see Second Law of Thermodynamics Qualms. As mentioned in the article, up to 50% of gross energy yield has to be expended to pump seawater to the top of the tower. But that estimate is based upon Zaslavsky's numbers; if it turns out that the gross yield is less than half of what Zaslavsky thinks it is the net result would be a negative yield: More energy would have to be expended to pump water up the tower than is harvested from the downdraft. JdH 19:03, 31 July 2007 (UTC)

Would rain mess it up?

Just wondering whether it is known if rain would mess up such a tower, what it's effects would be (positive maybe? due to water hitting the turbine?). Would it flood at the bottom? does anyone have any ideas as to the effects on the operation of it? not that it rains often in the desert, just asking. (212.46.56.195 16:50, 23 March 2007 (UTC))

That would most certainly mess it up. These power plants work only in a hot, arid climate. The idea is you "harvest" hot dry air, and cool it down by allowing water to evaporate at the top of the tower in order to generate a downdraft of cool humid air. When it rains the ambient humidity goes way up, and as a result evaporation comes to a grinding halt. JdH 18:38, 23 March 2007 (UTC)

Powered by a solar resource but not solar power

Solar energy resources span from the heat and light of the sun over to wind, waves and the products of photosynthesis. These are resources but solar technology is something else. A wind or wave turbine is not a solar power technology. Burning wood is not a solar energy technology because fire is its own separate thing with no direct connection to the sun. Although the downdraft tower, in a similar way to evaporative coolers, clearly uses a solar resource I don't think either of these technologies are properly described as solar energy/solar power technologies. I've edited out the reference to solar power. Mrshaba (talk) 16:11, 20 March 2008 (UTC)

Principle of COnservation of Energy gone bust?

Hang on, wouldn't the energy required to pump all that water against gravity to cool the air at the top off set any electricity produced by the tower?? --Pavithran (talk) 08:46, 6 June 2008 (UTC)

No. The sun is heating up the air outside putting in energy. It is not a closed system. It really works.  :=) Vincecate (talk) 11:53, 6 June 2008 (UTC)

Actual output

“ Zaslavsky, et. al., estimate that depending on the site and financing costs, costs would be in the range of 1-4 cents per kwh, well below alternative energy sources other than hydro. Pumping the water requires about 50% of the turbine's output. ”

So then is the actual output power only half as much? I think this could be stated more clearly either way. — NRen2k5^(TALK), 04:47, 29 April 2009 (UTC)

I think you should assume that the margin of error is much larger than a factor of 2 in those estimates and just not take them too seriously. Vincecate (talk) 19:15, 1 May 2009 (UTC)

Proposed Concept summary upgrade

Concept summary

An energy tower (also known as a downdraft energy tower because the air flows down the tower) is a tall (1,000 meters) wide (400 meters) hollow cylinder with a water spray system at the top.

Alternatively, temperature decreases uniformly with positive changes in altitude at a rate of ~ 16° C per mile. Therefore, should it be possible to tether heat conducting cables up through the inside of a convection chimney to a sufficient altitude above the top, could enough heat be drawn out of the air in the chimney to sustain a sufficient downdraft to power turbines 24 / 7 ? A flexible base would permit the tower to tilt. Regular lift maintenance balloons would be released to rise up the cables to the top..

Or, in the other direction, as long as the thermal inversion properties of the ocean hold up shielding the lower, colder depths from the circular flow of using the surface as a heat sink, why not have a 3D lattice of carbon nano tube elements inside the Downdraft chimney to cool the air so it falls and place the units off shore? — Preceding unsigned comment added by William Hale (talk • contribs) 00:02, 28 July 2013 (UTC)

=========NH: Cable weight, balloon lift, thermal conductivity of cable, winds in upper atmosphere, etc... — Preceding unsigned comment added by William Hale (talk • contribs) 21:39, 27 July 2013 (UTC)

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