Design

A SCAF is the adaptation of different, proven technologies, into one dual-purpose structure.

A solar tower is a gigantic greenhouse with a chimney in the middle to let the hot air escape. It uses a huge glass roof without exterior walls as a solar collector. The sun heats the air under the glass roof and the air rises to the middle of the collector and goes out by a high chimney. The solar tower uses an almost flat roof, (the one of the prototype constructed in Spain is 46'000 m2 and two meters off the ground) Increasing the height of the chimney will augment exponentially its draw and the force of the wind created inside the structure. A solar tower uses a big windmill inside the chimney, or several smaller ones around the base of the chimney, to generate electricity. Due to their large size, a solar tower generates high temperatures and strong currents of air in the interior. It is nonetheless important to note that a solar tower works on a pressure and temperatures difference, interior compared exterior. A solar tower has an effectiveness of about 2%. Where is the other 98% of the energy? Lost to gravity when the air rises, absorbed by the structure, used in the heating and expansion of the air and lost by réfraction. Therefore the energy produced by a solar tower = 2% of the solar flux x the square meter of the collecting surface of the tower.

A SCAF tower works on the same principle as a solar tower but on a smaller scale and it also performs air filtration.

A SCAF is a lot smaller than a solar tower therefore the roof climbs higher to increase the air volume in the structure.

A solar tower is opened at the exterior of the roof/collector but a SCAF is closed by an outer wall with filters situated in the base of the wall, the filters take at least 75% of the wall area. The movement of the air is therefore a lot slower in a SCAF compared to a solar tower, but the temperature and pressure of the interior may climb more quickly. Under slight pressure, the heated air takes the way of least resistance and rises up the chimney. The area of the chimney is 5% that of the filters. The pressure difference between the air inside and the air at the top of the chimney causes a natural draft, and the air rises up out through the chimney, pulling cooler air in from the exterior through the filters. The air that enters is cooler than the air already present and therefore denser. This fresh cooler air buoys the warmer air and more hot air goes out of the chimney and the cycle continues.

An idea of wind speed in the chimney can be had by observing the principle of Bernouli, A1V1=A2V2, one can see that if the speed at the filters is 0.5 m/s, then with the current tower configuration, it will be 10 m/s in the chimney. To find the wind speed at the filtres the following calculation from "Kittas C, Boulard T, Papadakis G. Natural ventilation of a greenhouse with ridge and side openings: Sensitivity to temperature and wind effects. Trans ASAE. 1997;40(2):415-425".is used;

The underlying principle of the SCAF allows the triple usage of the solar energy, the first of which is filtration. The preliminary design uses particle filtres with a 60% efficiency and activated charcoal filtres with an efficiency of 15%. All calculations for pollution removal are made with these efficiencies but testing may prove increases possible, especially in activated carbon filtre efficiency. The filtres used will be of low efficiency to reduce resistance and an activated carbon filtre is included to trap gases. The dust filtres should also be electrostatic to help capture small particles. Two other possible CO2 capture systems are currently under consideration. The first is a mist of NaOH solution which could be sprayed within short chimneys or short horizontal tubes within the structure connected to the filtre entries. As it falls the solution would absorb CO2 from the entering air and the resulting Na2CO3 solution could be collected and sent to have the CO2 removed and sent for sequestration. (for some info on the chemical process see here, the mist application would have to be tested for efficiency) The second possibility is a plastic developed by Professor Klaus Lackner of Columbia university, the plastic is coated with a resin which absorbs CO2 and then they simplywash it with humid air to knock the CO2 off and the plastic can be reused. The humid CO2 rich air is captured and the CO2 removed. Sheets of this plastic could be arranged at the filtre entries. Both methods would help to improve the tower's efficiency by absorbing CO2 without impeding the flow of air and both are proven CO2 capturing materials with high efficiency rates.

The second use is to heat the air in the structure and turn a windmill, placed just underneath where the roof joins the base of the chimney. The turbine should be fitted with wide rotor blades similar to a cieling fan to maximize the wind sweep area. At the moment the blades' area should be equal to 10% of the filtre area. Another possible placement for the turbine is on the ground below the chimney. The rotor blades could be arrayed in a cone form pointing at the centre of the chimney. This would reduce the windspeed acting on the rotors but greatly increase the windsweep area. The amount of energy the wind turbine could produce can be found using the results from the previous calculation in the following equation;

P=Rho*A*Cp*V3*Ng*Nb

Where:

P = Power (in Watts)

Rho = Air Density

A = Rotor sweep area (m2) exposed to wind

Cp= Coefficient of performanc (.59 is the betz limit and .35 for a good design)

V = Wind Speed (m/s)

Ng = Generator efficiency (80 for a good one)

Nb = Gearbox/Bearing efficiency <> 95%

The SCAF possesses another distinctive difference from a solar tower, the third manner to use the solar energy. (Northern hemisphere) The Northern half of the tower is covered in DSC panels to produce additional energy. As the roof of the SCAF is inclined, the Northern half will not receive a lot of direct sunlight, and so it is not then necessary that it be completely transparent. DSCs have the happy capacity of functioning by taking light from all incidence, from the front as well as from behind because of their semi-transparence. Also, DSCs don't lose effectiveness with increased temperatures. Standard silicium panels lose 0.05% efficiency for every degree Celsius.

The tower's energy efficiency may be improved with the inclusion of one or more Stirling engines, depending on the structure size. The Stirling(s) would be placed in the tower below the wind turbine and cooling pipes run to a three sided finned grill, like a car radiator. The air heated by the Stirling's action would be pumped through the radiator grill and the grills placed around filtres. As new air enters the structure it passes through the radiator simultaneously cooling the Stirling heated air and being preheated as it enters the structure. The hot part of the Stirling would be heated by one or more parabolic mirrors, the excess heat would rise off the engine and help turn the turbine like the heat from a candle turns the little rotor of wooden Christmas toys. By building the cooling radiator like a short 'holey' chimney more direct draw may be created through the filtre. The cooling pipe could also run through water containers in the base of the SCAF before arriving at the radiator. The inclusion of Stirling engines would make a fourth use of the solar energy.

The SCAF recovers what it loses in size through its versatility. It is conceived to be built in a network using the previously wasted spaces in the middle of traffic throughout a city or road network, on round-abouts and large traffic islands or highway centre strips for example. In this way it is well situated to trap the pollution at the source of one of its largest contributers, traffic. You can see proposed SCAF sites on Google Earth by downloading this placemark. (go to the Downloads page first if you don't have Google Earth) And you can see what a SCAF network could do in your city with this spreadsheet.

All the glass on the SCAF will be anti-reflective. This has several benefits: Less solar energy is reflected away and therefore more can pass into the structure to heat the air inside. The DSCs lose less solar energy, and the drivers on the road are not dazzled by reflections of the sun. The first SCAF should have more place for filters than the foreseen proportions to see how the SCAF reacts with different proportions of air entry/exit.

Advertising billboards of about 1.5 metres high will be erected around the top of the exterior wall on the northern side of the structure. This advertising space can be rented out to companies for advertising. The backs of the boards are to be painted gloss white to reflect light back onto the DSCs.The management of the advertising space will be the responsibility of the city. As the SCAF project and SPA are NPO's, all proceeds from the advertising will be the property of the city.

There are also many adaptations that can be made to the basic SCAF design to make it more efficient depending on its site.

How a SCAF works