[Daumic (OP)]

The air pollution in big cities is frequently created by a temperature inversion (1). Normally the air cools with altitude. But in the case of a temperature inversion, a layer of hot air in altitude blocks the cold air on the ground. This layer of cold air plated on the ground has a thickness of a few hundred meters. It concentrates all the pollutants immobilized by the absence of wind.

It seems to me that the concept of solar chimney developed by Jorg Schlaich (2) could fight these episodes of air pollution. The Schlaich solar chimney functions in the following way:
-   solar energy heats the air under an horizontal glass roof: the collector,
-   the chimney is set up in the center of the glass roof; the hot air goes up in the chimney whereas the cold air goes under the collector along its external perimeter,
-   at the base of the chimney a turbine converts the kinetic energy of the ascending hot air into electric power.

To fight the air pollution blocked by the temperature inversion, I propose to establish this solar chimney in urban environment. For this purpose, the collector and the chimney are gathered in a single element: a modified chimney equipped with a transparent sunny face and a dark coating on the interior of the opposite face. 

The dark coating absorbs the solar radiation and heats the air contained in the chimney. The reduction of its density generates the rise of air locked up in the chimney. This movement creates an aspiration of air at the base of the chimney. 

The Wikipedia website describes two mathematical formulas (3) which permit to calculate the air flow and the aspiration according to the temperature difference of the air between the inside and the outside of a chimney. I used these formulas in an algorithm on Excel software to simulate the operation of a solar chimney. If my calculations are exact, the air flow could be great.

A numerical example:
- chimney dimensions (rectangular section): height 400 meters, width 50 meters, depth 5 meters,
- solar radiation: 500 W/m2,
- simulation results: temperature rise in the chimney 3 K, aspiration 50 Pa, air flow 1600 m3/s.

In one hour of daylight, this example of chimney could propel more than 5 millions m3 of air in altitude.

Which would be the effect of a solar chimney whereas the gas effluents which leave the traditional chimneys remain blocked under the layer of a temperature inversion? The temperature inversion blocks the air which leaves the traditional chimneys because in this case the heat source is at the base of the chimney. The air which emerges from these factory chimneys is already cooled all along the conduit and thus does not have much any more of upward force. In a solar chimney, the air is heated all along the conduit and thus has a maximum upward force at the exit of the conduit. The air which leaves a solar chimney could have a sufficient speed and a temperature to bore the layer of inversion. A solar chimney could thus disperse the air pollution.

In addition to this function of ventilation, the solar chimneys could also produce electricity. Place a turbine in the conduit as the original idea of Jorg Schlaich would not be a good solution because the conversion output of solar energy in kinetic energy of the air does not exceed a few percent. Moreover, the presence of the turbine in the conduit could reduce the air velocity and thus harm the effectiveness of ventilation. The best solution to produce electricity would be to replace the dark coating in the chimney by photovoltaic panels. These panels would convert 10 to 20 % of solar energy into electricity and would dissipate the remainder in heat.

The solar chimneys could have a beneficial effect against urban pollution while producing electricity.

(1)   [https://en.wikipedia.org/wiki/Inversion_(meteorology)]
(2)   [https://www.math.purdue.edu/~lucier/The_Solar_Chimney.pdf]
(3)   [https://en.wikipedia.org/wiki/Flue-gas_stack]

[chris]

Interesting idea; does the equation stack up when the carbon / pollution cost of the construction and maintenance is also factored in?

[alancalverd]

Consider an urban environment with black tarmac roads and tall buildings with glazed windows on either side. Looks like an array of chimneys to me! Which is why cities have their own microclimate.

In the days of London smog, my father was tasked to fly over the city and photograph the concentrations of smog. The consensus was that coalfired power stations were causing all the problem. It turned out that the only ground features visible from 10,000 ft were the power stations, whose exhaust velocity and temperature propelled the effluent into the stratosphere and prevented fog nucleation - it was domestic coalburning that caused the smog! Hence the Clean Air Acts and Smokeless Zones.

[chris]

That's quite a story, Alan! What was your Dad's job?

[Bored chemist]

The air pollution in big cities is frequently created by a temperature inversion (1).

No it isn't.
The pollution is largely made by burning fossil fuels.
It may be trapped by a temperature inversion, or the  wind may dissipate it or whatever.

But the best thing to do is to reduce the production of pollutants.

[Daumic (OP)]

The pollution is largely made by burning fossil fuels.
It may be trapped by a temperature inversion, or the  wind may dissipate it or whatever.
But the best thing to do is to reduce the production of pollutants.

I agree well. The air pollution is the consequence of the combustion of fossil fuels. The episodes of temperature inversion concentrate this pollution on the ground. The best solution to reduce the pollution is of course to do without fossil fuels but that will take a long time. 

Many cities fight this air pollution by measures of toll, of prohibition of the most polluting vehicles, of circulation alternated during the episodes of pollution. These measures are restrictions of circulation and thus harm the economic activity of the cities where they are applied. Fight the air pollution by the construction of solar chimneys seems to me on the contrary beneficial to the economic activity: new construction site and new electric production.

[alancalverd]

That's quite a story, Alan! What was your Dad's job?

Electrical engineer in Scientific Services of the Central Electricity Generating Board.

The photographic mission, as I recall, took off from Northolt but ended in Prestwick as every other airfield in the country was fogged in. He returned by train: a one-hour flight having taken three days.

[alancalverd]

Incidentally, termites have been building solar chimneys for several million years.

[RjMaan]

I think yes, solar chimneys does help us in fight against air pollution.

[noplaneta4]

The solar chimney power plant has a promising future in the world. Α new design of solar chimney is offered including both PV panels with solar chimney plant for electricity generation. Two experimental models of a hybrid solar chimney were built and designed (systems A&B). System (A) had a collector glass roof cover and a PV panel as an absorber with a chimney of 2 m height while system (B) is similar to system (A) but with PV panel as collector roof cover and plywood as an absorber in the base of the chimney.

[daniellee5]

I never heard about such a theory before, very interesting!