1,408 views Aug 10, 2018, 01:03am

The Rise Of The Urban Rooftop

Laurie Winkless - Contributor

Science

With space at a premium, cities are exploring new ways to make better use of their rooftops.

Our cities have never been denser, taller, or busier than they are now, and with that, comes the constant battle for land. Whether you’re a city dweller, developer, transport planner, or farmer, you’re forced to compete for dwindling amounts of available space. And with two-thirds of the world’s population predicted to live in cities by 2050, the stress on urban infrastructure looks set to outpace even the most carefully-laid plans. But if we look at aerial images of any city center, we can quickly spot plenty of unused space – the rooftops. Speaking to Scientific American, Steven Peck from a non-profit called Green Roofs for Healthy Cities, describes the roofscapes of our cities as “the last urban frontier – (representing) 15 to 35 % of the total land area.”

So what can we use this precious resource for? In a growing number of high-density cities, some of it is dedicated to recreation – everything from bars and pools, to soccer pitches and running tracks can now be found atop skyscrapers. Others host smog-eating roof tiles or questionable wind turbines, while in China, a large shopping mall has 25 villas on its roof. But when a rooftop offers access to sunlight, there are two more obvious candidates for its use – agriculture and solar power.

Green Cities

Green roofs have been growing in popularity for more than a decade, and in some cases, growing in scale too – atop a convention center in Manhattan sits the city’s largest, covering an area of 89,000 m2. Usually comprised of planted beds, or carpet-like tiles that encouraged the growth of low profile vegetation, green roofs can provide a habitat for birds and insects in an otherwise hostile environment. They also act as thermal insulation for the building, and reduce storm water runoff that can otherwise cause havoc in urban sewers.

Green roofs come with the added benefit of mitigating the dreaded urban heat island effect, whereby, as a result of heat-absorbing materials like asphalt and concrete, cities can be several degrees warmer than the surrounding countryside. In contrast, trees and green spaces can absorb shortwave radiation, and use it to evaporate water from their leaves – a kind of ‘double cooling’ effect. There are countless studies that back up this idea. One of the most interesting came from researchers at the University of Georgia. In 2015, they showed not only that ‘green’ cities are cool cities, but that networks of small urban green spaces, such as parks, gardens and green roofs, were more effective at reducing a city’s temperature than a singular park of the equivalent size.

In some European and US cities, councils now offer significant financial incentives to developers who install a green roof – in Hamburg, building owners can receive subsidies of 30–60% of its installation costs. And from 2020, green roofs will be considered compulsory for all new, large-scale builds in the city. (CONTINUED...)

As food security and urban nutrition creep ever-higher on the agenda for the United Nations, there’s also a worldwide movement of using green roofs for hyper-local food production. In regions with suitable climates, hundreds of different vegetables, fruits, herbs and salad leaves can be grown on rooftops. Beehives and chicken coops are also becoming commonplace amongst the high-rises. But not all rooftop farms are equal, as we’ll discuss.

Power Up

But first, what about solar power? With so many cities now divesting from fossil fuels, and the costs of solar panels dropping dramatically, photovoltaic (PV) systems have become the ‘go to’ option for generating distributed power in built-up areas. And, even with standard commercial panels, the energy gains are dramatic. The US National Renewable Energy Laboratory (NREL) have estimated that rooftop PV systems could generate almost 40% of electricity demands nationwide. NREL also developed a very cool visualization tool called PVWatts to help people discover if solar panels would work for them. In 2012, Dutch researchers calculated that building-integrated photovoltaics could deliver 840 TWh of electricity – that’s more than a fifth of the total annual demand for all of the EU-27 countries.

In 2011, a fascinating map was published by the City University of New York. Assembled from images taken by a LIDAR-enabled (Light Detection and Ranging) aircraft, it showed that at the time, 66.4% of New York’s buildingshad roof space suitable for commercial photovoltaic systems. Furthermore, they estimated that, even with NYC’s changeable weather, rooftop installations could meet close to 14% of the city’s annual electricity consumption.

And cities have certainly taken note of this data – in many cases, making such installations compulsory, as in Sao Paulo, where Brazil’s Ministry of Cities announced that future low-income housing developments should include rooftop photovoltaics.

Which one to choose?

There’s no doubt that in both cases, a network of ‘productive rooftops’ could benefit the local community by supplying a portion of a necessary resource – either food or electricity – while also reducing their environmental burden. But is one better than the other? (CONTINUED...)

This was the question posed by researchers from MIT and the University of Lisbon in a recent paper in the journal Cities. Focusing on the rooftops of a mixed-use neighborhood in Lisbon, they carried out a Cost-Benefit Analysis for four scenarios – 1. Open-air rooftop farming, 2. Rooftop farming in low-tech greenhouses, 3. ‘Controlled environment agriculture’ (farming in high-tech greenhouses) or 4. Solar PV energy generation. Starting with existing data on everything from installation costs and resources used, to carbon footprint and yield, they modelled the impact that each installation would have on the local community over a period of 50 years.

The researchers took a footprint of one square meter of roofing, considered a population of 17,500 residents, and for the farms, looked only at a single crop – tomatoes. The fruit is not only incredibly popular in the Portuguese diet – with an average of 10.4 kg eaten per year, per person – there’s also a large body of data available on their growing requirements and yields. For solar power, they considered standard single-crystalline silicon PV modules, arranged on flat or pitched roofs. They also assumed that the PVs installation would act as part of the grid – in other words, though generated locally, the electricity would be distributed via existing infrastructure.

Here are a few highlights from their study:

• The benefits of food production varied according to the supply chain, except for high-tech rooftop farms, which were predicted to be profitable regardless. Open-air and low-tech greenhouses were found to only profit when the crops are sold directly to consumers.

• If the demand for tomatoes in Lisbon was to be met through rooftop farming, you’d require 1.89 square meters for organic field cultivation (#1), 0.37 m2 for soil-less cultivation in unconditioned greenhouses (#2), or 0.15 m2 for controlled-environment agriculture (#3)

• A rooftops farm could potentially create five times as many local jobs as a rooftop PV system, but would involve much higher operation and maintenance costs

• The electricity yield for PVs installed on south-oriented unshaded rooftops was calculated to be approximately 300 kWh/m2/year.

So, both options seem pretty good. But it’s when we look at the bigger picture that the differences start to show. I’ve written about the questionable profitability of urban farming in the past, but that was only ever as measured on individual farmers, which – I say, with the benefit of hindsight – doesn’t make all that much sense in the context of a city.

This study, however, takes a broader, and much more sensible view – it looks at rooftop installations as part of an interconnected network. And it’s which measures the overall value – in terms of economics and the environment – to both the building owner, and the community around it. And by looking at it that way, they concluded that for Lisbon, the use of rooftops for food production could yield significantly higher local value than solar PV energy generation or standard green roofs. Of course, this is very site-specific – in another city, you might come to exactly the opposite conclusion, but that’s kind of the point. There is no one-size-fits-all solution to make cities more sustainable, and anyone who tells you otherwise is massively over-simplifying a complex issue.

There are lots of things that will help anywhere – e.g. moving from private cars to mass transit, making renewable energy the default option, being smarter with how we use water, reducing our reliance on concrete, minimizing our waste footprint, and reserving single-use plastics for very, very specific applications. But when it comes to ‘other’ questions, like should we use our rooftops for energy generation or farming, the answers are a lot less black-and-white. By adopting research like the study I’ve featured here, we can get much closer to making the right decision for our specific circumstances. The authors said that their study “…aims to provide decision-makers with a basis for systematic and integrated comparison of these productive uses of rooftops.” Now, all I hope is that some of those decision makers start using it.