What's the most effective way of keeping a hot city cool?

Nantes during last summer's heat wave. Image: Getty.

The recent spate of heatwaves through eastern Australia has reminded us we’re in an Australian summer. On top of another record hot year globally, and as heatwaves become more frequent and intense, our cities are making us even hotter.

This is the urban heat island, where city temperatures can be significantly warmer than the surrounding rural regions. The question, then, is what we can do to keep our cities cooler.

Why are cities hotter?

The temperature difference is caused by a range of factors, including dense building materials absorbing more of the sun’s energy, fewer trees to provide shade, and less soil to cool by evaporation.

Buildings can also act like the hairs on a husky, reducing wind speeds and blocking thermal radiation up to the night sky. On top of that, waste heat from car engines, air-conditioners and other energy use adds to overall air temperatures.

Why does this matter? Even a small increase in air temperature pushes up overall energy demand, and about 25 per cent of our energy bills are for only 40 hours per year when the grid is most heavily used.

The most extreme heat events can buckle train lines, cause rolling blackouts and cost billions in lost productivity. And it’s not just bad for our wallets.

Heat stress can damage organs or exacerbate existing illnesses. Since 1900, extreme heat events have killed more Australians than bushfires, cyclones, earthquakes, floods and severe storms combined.

So, what can we do?

There are a number of things individuals can do to reduce the impact of heat in their homes, such as installing light coloured roofing material, insulation or an air-conditioner.

But it gets more complicated when considering the city as a whole, and how these small actions interact with each other and with the climate.

Air-conditioners

In heatwaves, air-conditioners save lives, allowing stressed bodies time to cool. But our homes can only be made cooler by blowing heat outside, along with the extra energy to run the system.

As well as increasing outside air temperatures in the short term, the fossil fuels burned add to global warming. A world cooled by air-conditioning probably isn’t the answer.

Trees and parks

Trees provide shade, but also cool the air, because evaporating water from leaves takes energy, reducing peak temperatures by 1-5° C.

Most city planners agree on the broad benefits of urban vegetation, with some metropolitan councils developing urban greening strategies.

However, urban trees can be a vexed issue for some councils; they use water, can be costly to maintain, can damage utilities and property, and can worsen air quality instead of improving it. Larger cities are often made up of dozens of councils; getting them to agree is a major challenge.

White roofs

We know that black surfaces get hotter in the sun, but demand for dark roof tiles still far outweighs demand for light colours. More reflective roofs can reduce a household’s energy bill, as well as the overall temperature of a city.

White roofs are most effective in warmer climates, because in cold climates, the cost savings in summer must be balanced with additional heating costs in winter.

Green roofs and walls

Green roofs and walls are building structures with integrated vegetation. They provide cooling benefits by shading buildings and through evaporation from leaves. They generally show less cooling benefit than white roofs, cost more to install and maintain, and use additional water and energy.

But they do look nice, improve biodiversity and make people happier.


Pavement watering

Prior to an extreme heatwave, it may be possible to reduce temperatures by wetting down building and road surfaces. It’s a traditional practice in Japan, and is now being considered in major cities like Paris.

But temperature and humidity are important factors in heat stress, so pavement watering should only be undertaken if the extra humidity does not increase heat stress.

Large scale rooftop solar

Solar panels convert energy from the sun into electricity, so less energy is required from the network overall. If enough roofs were covered with solar panels, could that lower air temperatures?

Probably a little. Other benefits include a reduction in the energy required for cooling (because the roofs are shaded by panels), and a stable, lower cost, decentralised renewable energy system.

Building density

A building with lots of thermal mass (think sturdy, double-brick home) can be an effective way to keep inside temperatures more stable. Heat is absorbed during the day and released at night. The same idea can work for an entire city.

An urban cool island can form in high-density cities like Hong Kong because tall buildings provide extra heat capacity and shade.

For similar reasons, the tight street layout of traditional Arabian and Mediterranean cities keep those streets cooler.

Shading structures

Installing light shading structures over streets, pavements and roofs can reduce the surface temperature of materials, and reduce the heat absorbed and radiated back into streets. Shading structures need to be designed so that they do not limit airflow, trapping heat and air pollution in streets.

Which is best?

To figure out what works best, we need to be able to model the physics of different strategies, in different types of cities and in different climates. We can then assess the economic and health impacts and decide on appropriate and plans that give us the biggest bang for our buck.

Here we have focused on heat in cities, but there are other important concerns like air quality or flooding.

In colder cities, an urban heat island could actually be a good thing. Each city is different; each requires a tailored and integrated plan developed over the entire metropolitan region, and then implemented locally by councils, businesses and households.The Conversation

Mathew Lipson is a PhD Candidate, and Melissa Hart the graduate director of the ARC Centre of Excellence for Climate System Science, at UNSW Australia.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

The mountain in North Wales that tried to stop the UK’s blackout

Elidir Fawr, the mountain in question. Image: Jem Collins.

Last Friday, the UK’s National Grid turned to mush. Not the official term perhaps, but an accurate one after nearly one million people were left without power across the country, with hundreds more stranded at train stations – or even on trains (which isn’t nearly as fun as it might immediately sound). 

Traffic lights stopped working, back-up power failed in hospitals, and business secretary Andrea Leadsom launched an investigation into exactly what happened. So far though, the long and short of it is that a gas-fired power station in Bedfordshire failed just before 5 o’clock, followed just two minutes later by Hornsea offshore wind farm. 

However, amid the resulting chaos and inevitable search to find someone to blame for the outage, a set of mountains (yes, mountains) in North Wales were working extremely hard to keep the lights on.

From the outside, Elidir Fawr, doesn’t scream power generation. Sitting across from the slightly better known Mount Snowdon, it actually seems quite passive. After all, it is a mountain, and the last slate quarry in the area closed in 1969.

At a push, you’d probably guess the buildings at the base of the mountain were something to do with the area’s industrial past, mostly thanks to the blasting scars on its side, as I did when I first walked past last Saturday. 

But, buried deep into Elidir Fawr is the ability to generate an astounding 1,728 megawatts of electricity – enough to power 2.5 million homes, more than the entire population of the Liverpool region. And the plant is capable of running for five hours.

Dubbed by locals at the ‘Electric Mountain’, Dinorwig Power Station, is made up of 16km of underground tunnels (complete with their own traffic light system), in an excavation which could easily house St Paul’s Cathedral.

Instead, it’s home to six reversible pumps/turbines which are capable of reaching full capacity in just 16 seconds. Which is probably best, as Londoners would miss the view.

‘A Back-Up Facility for The National Grid’

And, just as it often is, the Electric Mountain was called into action on Friday. A spokesperson for First Hydro Company, which owns the generators at Dinorwig, and the slightly smaller Ffestiniog, both in Snowdonia, confirmed that last Friday they’d been asked to start generating by the National Grid.

But just how does a mountain help to ease the effects of a blackout? Or as it’s more regularly used, when there’s a surge in demand for electricity – most commonly when we all pop the kettle on at half-time during the World Cup, scientifically known as TV pick-up.

The answer lies in the lakes at both the top and bottom of Elidir Fawr. Marchlyn Mawr, at the top of the mountain, houses an incredible 7 million tonnes of water, which can be fed down through the mountain to the lake at the bottom, Llyn Peris, generating electricity as it goes.


“Pumped storage technology enables dynamic response electricity production – ofering a critical back-up facility during periods of mismatched supply and demand on the national grid system,” First Hydro Company explains.

The tech works essentially the same way as conventional hydro power – or if you want to be retro, a spruced up waterwheel. When the plant releases water from the upper reservoir, as well as having gravity on their side (the lakes are half a kilometre apart vertically) the water shafts become smaller and smaller, further ramping up the pressure. 

This, in turn, spins the turbines which are linked to the generators, with valves regulating the water flow. Unlike traditional UK power stations, which can take hours to get to full capacity, at Dinorwig it’s a matter of 16 seconds from a cold start, or as little as five if the plant is on standby.

And, designed with the UK’s 50hz frequency in mind, the generator is also built to shut off quickly and avoid overloading the network. Despite the immense water pressure, the valves are able to close off the supply within just 20 seconds. 

At night, the same thing simply happens in reverse, as low-cost, surplus energy from the grid is used to pump the water back up to where it came from, ready for another day of hectic TV scheduling. Or blackouts, take your pick.

Completed in 1984, the power station was the product of a decade of work, and the largest civil engineering project commissioned at the time – and it remains one of Europe’s largest manmade caverns. Not that you’d know it from the outside. And really, if we’ve learned anything from this, it’s that looks can be deceiving, and that mountains can actually be really damn good at making electricity. 

Jem Collins is a digital journalist and editor whose work focuses on human rights, rural stories and careers. She’s the founder and editor of Journo Resources, and you can also find her tweeting @Jem_Collins.