Solar Updraft Towers: Variations and Research

Tom Bosschaert
October 07, 2008 | 16 Comments

The idea of using solar radiation to generate air convection that can subsequently be converted to an energy source has been around since the start of the 20th century, when a Spanish Colonel called Isidoro Cabanyes proposed it in a scientific magazine. Solar Updraft towers, also called solar wind or solar chimney plants, provide a very simple method for renewable electricity generation, with a constant and reliable output. Other renewable energy sources such as wind turbines and solar arrays suffer from high diurnal and seasonal fluctuations, or unpredictable patterns of output.

Due to the large initial investment required, unfamiliarity with the system and the solar updraft tower's relatively low capture efficiency, only one prototype was ever built, in the 1980's in Spain. This prototype however, performed above and beyond expectations, and continued to operate for almost 7 years after its designed life span of 3.

The solar updraft tower has been left on the shelf due to its perceived low efficiency, which is to a large degree undeserved. Most studies on this elegant and simple renewable energy producer consider the land occupied by the tower and its collector as one of the largest resource inputs required for this process. However, Except Architecture & Consultancy is investigating the possibility of more creative applications of the system, which would combine the tower with other land uses.

The Classic Solar Updraft Tower Scenario

The small experimental solar updraft tower plant, built in Manzanares, Spain in 1982 by Schlaich Bergermann, can be considered the classic example of the system. The design calls for a large, unused plot of land to house a collector between 500 m and 10 km in diameter, with a centrally located chimney ranging from 100 m to 1 km in height. The collector is a transparent membrane suspended several meters off the ground, which can be made of glass or a strong transparent polymer. (See pilot plant image left, as well as lead image, above.)

Sunlight penetrates this membrane, and the solar radiation is converted to heat upon hitting the ground. The air underneath the membrane quickly increases in temperature due to the greenhouse effect and flows towards the chimney, which, through the stack effect, becomes the lowest point of pressure in the system. This continuous airflow spins a turbine located at the base of the chimney. The nighttime difference in temperature between the ground and the air allows this effect to continue. Thermal storage devices can be used to smooth out the differences in intensity between night and day temperature differentials. As with other solar technologies, a higher latitude placement translates into a lower energy output. (See Figure 1, below.)

Figure 1: Classic Solar Updraft Tower Diagram

The majority of the cost associated with solar updraft towers is the initial investment required. By contrast, operations and maintenance costs are very low. The system is simple and very reliable. The parts that require maintenance are close to the ground and rely on elementary, proven technologies. One of the largest expenses is the cost of land, which means that the interest rate for financing that purchase is a major factor in determining the cost of the electricity produced. This makes the solar updraft tower a very suitable energy plant for remote, arid areas near the equator with inexpensive land.

Estimates for the cost of electricity produced range from €0.05 per kilowatt-hour (kWh) up to €0.25 [US $0.07 to 0.34 per kWh], depending on the cost of land and the financing scenario. By comparison, a normal gas operated power plant can produce electricity for as little as €0.05/kWh. Except's investigation in combined land use may increase the financial viability of this renewable energy technology dramatically, allowing it to be applied beyond its current potential as an affordable developing world technology to a high tech source of first world power generation.

Expansion of Potential: Agricultural Use

An important side effect of placing a large transparent membrane over an area of land is the capture of evaporated ground water and its return back to the topsoil. This localized increase in land moisture can make the soil underneath the collector suitable for agricultural uses, through the effective creation of a partial greenhouse. In some cases, the land under the collector would not have been agriculturally viable without the presence of membrane. This means that certain barren lands could be reclaimed for productive use, making this energy generation strategy more economically favorable while also building agricultural capital. (See Figure 2, below)

Figure 2: Agricultural Solar Updraft Diagram

The clearance height underneath the collector can easily accommodate farm equipment, and the supports for the collector can be far enough apart to allow the working of the land. Different kinds of crops can be planted depending on the local soil and moisture conditions, bearing in mind that the area near the center will have airflow too strong to allow plant growth. If the vegetation is very substantial, it may impact the output of the tower due to wind drag. However, increasing the distance of the membrane from the ground can mitigate this.

Using agriculture in combination with the updraft tower this system can be applied as both a sustainable economic strategy or for community restoration in developing nations.

Expansion of Potential: Co-generation Use

Other solar technologies, such as collectors that use solar radiation to convert water to steam or photovoltaic (PV) arrays, generate substantial excess heat. In the case of PV, high temperatures diminish their power generation capacity. Using the solar updraft tower in combination with solar collectors or PV arrays can improve the efficiency of both systems. The constant wind flow can air-cool the collectors while increase the energetic output per area of land used, making the solar updraft tower a more efficient proposition. Things to consider in terms of the efficiency of combining these systems include the loss of some direct solar radiation as a result of its deflection by the membrane, and the amount of cooling that can ideally be achieved. (See Figure 3, below)

Figure 3: Co-generation Solar Updraft Tower

This strategy can also be combined with agricultural uses, where solar collectors or PV arrays would be placed at the center of the collector, where the winds are too strong for plant growth.

Expansion of Potential: Urban Use

A completely different alternative use is the urban application of solar updraft towers, which is not necessarily conceived of primarily for its energy generating capacity. A tower in a city could hardly be the size of one in a field, so its generative power would be diminished. However, it could provide three distinct advantages alongside electricity generation.

First, the tower could be fitted with particulate, carbon and other air filters. The air rushing through the chimney would thus be cleaned resulting in urban air quality improvement, while at the same time generating some electricity. Systems like these would be very suitable for highly polluted cities. (See Figure 4, below)

Figure 4: Urban Solar Updraft Tower

Second, in cold climates, the heat dissipation of the urban environment can be buffered. The trapped heat will aid the power generation, but will also reduce the temperature gradient between interiors and exteriors, resulting in energy savings for building heating and higher insulation efficiencies.

Third, in hot climates, a second layer with a semi-transparent PV membrane could be installed. This would partially block out the sun, causing the temperature gradient to drop. There will then be two layers generating convection, possibly increasing the efficiency of the tower. The top layer would ensure the heat is not trapped in the bottom layer, thus preventing the heating up of the city. The constant air pull of the solar updraft tower will partially combat the heat island effect.

Other Uses

Solar updraft towers do not need to be placed on any specific land type and they are flexible in the height of the collection membrane. The efficiency of the tower might change depending on the resistance of the ground, but the advantage of using land for a productive purpose while at the same time using it to generate electricity is a very attractive prospect. The Solar updraft tower is one of the simplest and most straightforward of all renewable energy systems, and if designed appropriately, it can be applied in a great variety of circumstances. It lends itself well to the redevelopment of economically depleted areas, and for the generation of electricity in remote areas without the need for large maintenance crews.

Except is currently investigating several other uses of the solar updraft tower, both for remote electricity generation and for urban use.

Tom Bosschaert is founder of Except Architecture & Consultancy, designing and researching our built environment, based in Rotterdam, the Netherlands.

16 Comments

Mary Saunders
August 1, 2010

Underground water storage has potential for adjusting temperature in the greenhouse as well. With on-site power, it would be possible to have two storage areas, one for warm water and a second for cool water. Using wood, cob, rock, concrete (broken concrete is urbanite in permaculture-speak) or whatever is around, benches can be heated or cooled to make micro-climates for particular crops that are desired. If storing water at grade is desired, aquaculture could be added to the mix.

MAHMOUD ALMADHOUN
August 1, 2010

Garfield: I have been involved in a pre-feasibility study of incorporating water desalination in solar chimneys including thermodynamic modeling for the design so I would be happy to share my thoughts on that. Thermoelectric materials is now my research field and I like your ideas of using hi-tech materials to improve the efficiency of classical yet promising concepts in solar updraft towers.

Robert Valdez
May 29, 2009

Good-Day,
I would like to be reconized for giving a suggested purposal that could be incorporated into the towers design to condense water for the green house that lies below the canopy.
This design could utilize photo voltaic cells to coltrol a battery charging system that during controlled conditions could be utilized to condense water from the towers interior of rising air, and store the water in a under ground reservior for the greenhouse.
The condensation system in the tower would consist of hundreds if not thousands of Peltier Cooler Thermoelectric panels that would have to be computer controlled in order to control the voltage, and current to keep the temperature at the optimum condesation dew point.
This purposal would quite possibly eliminate the need to provide the lower greenhouse with an long range water source, hence it might make it possible to provide water, food and power to areas that are in extremely remote and inhospitable areas of the globe.
Best Regards Robert Valdez

Joe Craft
October 23, 2008

James, I'm not entirely sure you've accounted for certain needs in your plan. As I understand it, creating bio-diesel from algae requires a closed or at least controlled airflow. With all the wind whipping past the ponds in which it is fermenting, wouldn't the diesel evaporate into the air?
also, though they are useful, the additions you proposed would add significantly to the cost of building the structure. I think the best way of augmenting the technology would be either with agricultural development on the land below AND adding PV at the outer edges of the structure. The only danger to adding development below the installation is the increased activity WILL result in accidents. Combines don't stop or turn on a dime. they never have had to before, and probably never will, so avoiding the supports would be crucial. But collecting run off from rain could not only be used to water crops below, but also clean the darn thing. If you think your wind shield gets dirty in the dustier parts of the south, just think of how this will build up particulate matter. This will in turn reduce efficiency, especially if the structure is built in the cone shape with mildly angled sides. this could more or less be avoided by a flat top, but this, I believe, reduces efficiency.
like any technology there are trade offs, but lets not try to do too much at once.
As a final note, James, on the site you noted, you proposed building several thousand of these towers. WHERE?!

JoECoat@gmail.com

Tom Bosschaert
October 12, 2008

First, thank you for all the great comments and suggestions. I'll try to address some of the comments here.

Ben:
While some of the moisture would be dragged along, condensation on the membrane would drip down back down and be heavier than would be dragged along. The pilot plant in Manzanares had plant life spontaneously erupt while outside of the plant's membrane it remained barren, so this is supported by empirical research.

Again, thanks for all the comments
Adrian:
That is indeed something we were looking at, not ready for the article at time of writing. What algae plants under the membrane would also achieve is a certain amount of thermal storage to smooth out the diurnal shift. However, it would require a substantial water source nearby, and higher maintenance (algae would tend to want to grow on the moist membrane as well).

Anthony:
A plant on a slope facing south would improve its operation. A vertical axis mill at the top will reduce wind flow and reduce the efficiency of the main turbine. Passing wind already pulls air out of the tower due to the venturi effect (blow over a bottle with water in it, it'll come out).

Igor:
As said in the article the idea is from 1903.

John:
Rainwater collection would be possible through the membrane supports.

Roger:
A dome would null the Venturi effect, I suspect, and in parts also the stack effect, reducing its efficiency, but it'll still work due to thermal rising, but to lesser degree.

Philip:
Great additions, Thanks

James:
There are many ways to apply the towers, no need to focus on just one application.

Fireofenergy:
Land usage to energy capacity is hardly a factor. It's much more important that updraft towers are more consistent and thus more predictable than turbines.
The article itself already mentions diurnal usage

Mary:
Funky original stuff, I'd say. Go for it. Although you'd need to piece the membrane to allow vines to grow up the tower, which I wouldn't recommend.

Mary Saunders
October 8, 2008

For continuing production of most plants, you will need to replenish certain minerals, the usual suspects being nitrogen, phosphorus, and potassium.

Also, plants need carbon dioxide, possibly meaning humans or animals to consume oxygen and exhale CO2, as noted above.

Some plants survive high turbulence and their resiliency and pest-resistance are actually improved by it.

Timber bamboo is available in some locales for supports, and it can be bent so as to deflect wind rather than to sail with it. Pole beans will be good candidates for growing up the supports, if so desired. Beans set nitrogen, which would lessen or end a need for imported nitrogen, over time.

You could also use towers as trellises, growing hops, grapes, and other useful vines up the towers if you want. Providing hand holds will allow your rock-gym enthusiasts to double as grape harvesters, if they choose. Of course, you will charge them for the privilege, if they are wealthy, and make them sign waivers, though presumably if they land on PET, they will bounce.

I think of the acres of greenhouses near Vancouver, B.C. Adding towers to this already-built environment would be interesting in the season when they need cooling.

I appreciate this article and the thread of comments it generated.

I have a friend who grows bamboo under her rabbit cages. The bamboo grows up through the cage for the rabbits to eat. They then excrete useful minerals for the bamboo. It's pretty tidy. So much is possible when we perceive a need to put our brains and hearts in gear.

James Bowey
October 8, 2008

You're on the right track, except for a few things:

You need to focus on algae cultivation, recapture of water vapor and positioning of aquaculture and real estate at the outer rim of the greenhouse where the aquaculture ponds can double as recreational areas for real estate, and where the CO2 and other heat from human habitation can be recaptured by inflowing fresh air.

http://www.geocities.com/jim_bowery/sutabs.html

Brief Proforma for a Solar Updraft Tower Algae Biosphere

by James A. Bowery
July 8, 2006

There is a great need to radically decrease the per person ecological foot-print of developed economies, world-wide -- a need to radically increase carrying capacity while reducing impact on high biodiversity ecosystems such as the Brazilian rainforests or continental shelf fisheries, and reduce concerns about global warming. There may be an economic option that uses sea water pumped to desert areas powered by the fact that ground level temperatures are much higher than temperatures at high altitudes. Indeed, it would dump greenhouse heat to space for its power while producing biodiesel, electricity, fish, fresh water, salt and real estate -- all in quantities demanded by developed-world populations -- without adding to, and possibly even sequestering, greenhouse gases.

Proposals for solar updraft towers have typically assumed that they would be single use structures: solar to electricity via heat differentials between high altitude air and ground level greenhouse-enclosed air. The resulting system has marginal economic value.

Something which would radically enhance the value of the solar updraft tower power structure is to use the greenhouse area for algae ponds to add biodiesel, water, fish and salt production to the production of electricity normally envisioned.

Philip Anderson
October 8, 2008

Some possibilities to wring more services out of the physical plant while generating income to reduce electricity cost: 1. apply solar selective surface on the upper portion of the tower to warm the air more there, thus increasing the solar chimney effect and driving convection within the tower itself. 2. rent the tower for mounting electronic devices for communications (cell phone and HAM radio repeaters), weather service sensors, defense devices. 3. food drying facility (including crops grown there during the growing season) or grass-drying or adobe brick drying or green brick production beneath the membrane (drying by heat and wind evaporation; this will also provide thermal mass to enhance night-time /24 hour convection) 4. mount wind turbines on swivel rings for swinging them to the leeward side of the column, starting 30 feet above the ground, 3 turbines per ring /all facing into the wind 5. where a suitable aquifer is present beneath the tower, wind turbines could pump water from it high up the tower for gravity flow to irrigation pipes mounted on the membrane structure for watering crops beneath 6. use the membrane as a watershed to collect rainwater for irrigation of adjacent crops (if not needed for sub-membrane crops) --the membrane watershed will assure normal crop-watering of an equal area of cropland during a 50% drought (where half the needed rain falls in a growing season), or during a 66% drought (where 1/3 needed rain falls) for a cropland area half the membrane area, etc.

Roger Monger
October 8, 2008

Wouldn't this concept work as well using the top half of one of Buckminster Fuller's Geodesic domes? Then practically none of the surface area under the dome would be subject to high winds and thus usable for agriculture, solar arrays, or buildings. There would be no vertical supports to interfere with agriculture equipment. We all know that geodesic domes are more stable in high exterior winds than the type of construction shown here.

Lee Fellows
October 8, 2008

The solar chimney concept has apparently been around for more than a century. I conceived an application for it at Grand Canyon National Park in 1978 in an energy proposal. The basic idea has languished over the years, never implemented in a significant way, although mentioned periodically in science and energy related magazines.

Though typically pictured as a rural plant requiring hectares of costly land, the idea has great merit in an urban setting where it can benefit city inhabitants by utilizing the "heat Island" effect to good use and lowering incidental heat load on the community at large.

Picture a high-rise office building surrounded by city streets and parking lots that soak up solar heat. Imagine a vertical chimney in the center of the tower structure, and the inner exposed walls of the chimney covered with the heat condensers for the air-conditioning plant for the building. The basement parking garage constructed like the spokes of a wagon wheel, with the support walls creating wind tunnels that radiate outward from the base of the central chimney. The building heat load is used to good effect to drive the convection engine, without having sequestered hectares of land. The updraft through the chimney cools the building and the parking garage, creating ventilation forces that carry away polluting exhaust fumes from the car park, etc.

In essence, modern skyscrapers (office buildings) become solar chimneys that cool the city, carry away polluting fumes from lower street levels, and operate independent turbines that generate power. The combination of effects reduces the overall cost of power generation and the air-conditioning and lighting heat load (power demand) that the building normally creates. The distributed power generation increases community security, an important thing in these terrorist times, and output from individual buildings can be joined together in a common city power grid.

Lee Fellows
Fellows Research Group, Inc.

Jerry Griffin
October 8, 2008

Could someone give us the data on efficiency and resulting requirements for land area?

John Pisciotta
October 8, 2008

What effect will rain and large storms have on ersosion around the margins? The system could easily be adapted for fresh rain water collection. Storm force winds may be a problem.

John M. Pisciotta

Igor Gjurovski
October 8, 2008

The idea is not new, I was reading similar text for the use in Australian deserts, but the construction was more a cone than flat top. I thing the cone will improve the air flow. In my country (Macedonia) we use a black cover on the earth and we produce strawberries on it. I thing if we place black sheets of nylon on the land and grow strawberries on it, we will get more warm air and we will have faster crops.

Anthony McCarthy
October 8, 2008

This looks like a very economical solution, though a couple of modifications might help.
1) If the tower was placed at the highest point on the land, could the tower be a lot shorter? Perhaps placing the membrane on a downslope. This might be ideal for Vineyards?
2) If you added a vertical axis wind turbine at the top of the tower you could create additional updraft
3) Perhaps the top of the tower could be angled and swivel and thus any passing wind will drag the air out of the tower?
I like the addition of the fish tanks, positioned at the outer rim, feeding condensation to the plants. Could a waved membrane trap the water vapour into drips before it reaches the tower?
Anyone planning on building these anywhere yet?

anthonycmccarthy@gmail.com

Adrian Akau
October 7, 2008

The article is very good. I am wondering if algae and fish could be grown in shallow warm water ponds aside from regular agricultural crops. The water could be heated up without too much of a cooling effect on the air if the water is not deep.. The algae and fish might also grow at a faster rate; fish grow much faster in the tropics in warm water as long as the water is oxygenated. The ponds might be located at the greatest distance from the tower with crops inside and then PV modules close to the center.

A complete self-sustaining system might be developed in a desert environment.

adrianakau2aol.com

Ben Gatti
October 7, 2008

This technology can be compared to ocean thermal energy conversion.

The quickest way to evaluate RE is energy per mass, or specific energy density. This approach is low specific energy density, It is a very large and massive structure for relatively less energy than a Wind Turbine. Not a great start.

The stack is in effect a steam cylinder and operates efficiently in proportion to its thermal isolation from the atmosphere - so it can't be made lighter without consequence.

A greenhouse would create a warm and moist atmosphere which would promote ag; however, the updrafting would evacuate this atmosphere and send both the warmth and the moisture up the stack. If it could be recaptured, that might not be so bad, but recapture would require cooling, cooling would slow down the updraft, and in the end, you'd be right back to a static greenhouse. These ends create a conflict of physics.

Anyone else see an issue there?