Since the emergence of cities during the Agricultural Revolution, humans have struggled not to kill everything they love through bloody-minded exploitation of the environment. We've got two general options for preventing an environmental crisis: stopping doing the thing that's causing the problem, or doing it faster in the hope that we get rich enough that it doesn't matter any more. It's safe to say that the latter option usually wins out.
In China, where problems of air pollution and climate change are combining with the recognisable issues of urbanisation and industrialisation (inequality, poverty, disease) to damage quality of life, the big problems are being met with big solutions. A benefit of urban density is that it allows greater efficiencies in things like energy use, and part of the justification for building Sky City - the ever-delayed "tallest building in the world" in Changsha designed by a company that specialises in air conditioners - is that its offices, home, school, hospital and other amenities will make it something like an arcology from SimCity 2000, and a building that is a city unto itself.
Now, Dezeen reports that British architecture firm Chetwoods has drawn up a proposal for two incredibly huge towers - both of which would be taller than the Burj Khalifa, with the taller tower topping out at a kilometre - for Wuhan, in Central China, which will act as a giant air and water purification system for the city's more than ten million residents. The architects also say that "the use of a pair of towers reflects the dualist elements of Chinese culture in contrast to a more Western monolithic form", but that's just how architects like to talk. The important stuff is in the mechanics of the towers.
Located at the heart of a planned new development on one of Wuhan's lake islands - the city is famed for its numerous lakes - the project takes a Swiss Army Knife approach to environmental problems in China. Name a problem, and Phoenix Towers will deal with it. It has wind turbines and solar panels, biomass generators and vertical farms that can grow crops or insects. The bottom half of each tower will be occupied by the boring stuff of skyscrapers, offices and shops and the like, but the top half is where the party is at.
The towers act as giant solar chimneys, to clean the city's air and the water from the lake while also generating power. Solar chimneys have been around for decades, and while a clever way of turning solar energy into electricity without having to rely on silicon panels or any other direct conversion of light into power, they've not been particularly popular. They usually take the form of a tower surrounded by greenhouses, where the air is heated by sunlight. That warm air rises and slopes along the glass roofs to the tower, where turbines up the entire shaft convert the motion of the air into electricity. While it's an inefficient system compared to other solar power systems (like solar-thermal), it's also cheap in that the land is probably the most expensive component - and you can further economise, maybe by using the greenhouses for growing plants at the same time, or by sticking photovoltaic panels on the outside of the tower.
A prototype built in Manzanares, Spain in 1981 was 194.6m tall and 10m wide, with a circle of greenhouses with a diameter of 244m, and generated 50kW of power, which is roughly similar to the power needs of one of Tesla electric cars. To get a tower with an output of around 200MW - roughly a quarter the output of an average coal power plant, or a tenth the output of the Hoover Dam - there would need to be a tower a kilometre high (so on the scale of the Phoenix Towers) and 120m wide, with a 7km-diameter circle of land taken up by collector greenhouses.
These theoretical towers are assumed to be similar to existing industrial chimneys - this image gives a good idea of just how immense a 200MW one would be - and, in China, there has been a small, 200kW tower in operation in Wuhai City, Inner Mongolia since 2010. Phoenix Towers are slightly different, in that they don't have the greenhouses at the bottom. Instead, sunlight heats up the top of the tower, and as the hotter air at the top moves upwards it draws colder air up from below. The same principle works with water from the lake, and the towers therefore act as massive filters for both air and water. This likely makes them less powerful as generators than they would otherwise be, but the idea is the towers do as much as possible to justify becoming an eco-tourism destination and a centrepiece of a new government-led focus on environmental sustainability.
This is still a concept, and at least three years from any kind of construction getting underway, but it's an encouraging counterweight to much of the other environmental news that has emerged from China recently. Earlier this month it was revealed that, to make space for new towns and suburbs, engineers are literally slice the tops off of mountains and use them to fill the valleys in between. Chinese air pollution is now so bad it has been compared to a nuclear winter, damaging crops and threatening the food supply. If we must accept that large nations prefer to technologically escape the consequences of exploiting the environment, some kilometre-tall towers that act as giant filters are a considerable improvement on what we have already.