̽��ֱ�� of Cambridge - energy-efficient cities

Wrong trees in the wrong place can make cities hotter at night, study reveals

ta385 — Tue, 10 Dec 2024 09:45:00 +0000

Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure.

Some cities have already started implementing mitigation strategies, with tree planting prominent among them. But a ̽��ֱ�� of Cambridge-led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.

̽��ֱ��study, published today in Communications Earth & Environment found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83% of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.

“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” said Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at Cambridge's Dept. of Architecture.

“Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide.”

Previous research on the cooling effects of urban trees has focused on specific climates or regions, and considered case studies in a fragmented way, leaving major gaps in our knowledge about unique tree cooling mechanisms and how these interact with diverse urban features.

To overcome this, the authors of this study analysed the findings of 182 studies – concerning 17 climates in 110 global cities or regions – published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.

During the day, trees cool cities in three ways: by blocking solar radiation; through evaporation of water via pores in their leaves; and by foliage aerodynamically changing airflow. At night, however, tree canopies can trap longwave radiation from the ground surface, due to aerodynamic resistance and ‘stomatal closure’ – the closing of microscopic pores on the surface of leaves partly in response to heat and drought stress.

Variation by climate type

̽��ֱ��study found that urban trees generally cool cities more in hot and dry climates, and less in hot humid climates.

In the ‘tropical wet and dry or savanna’ climate, trees can cool cities by as much as 12 °C, as recorded in Nigeria. However, it was in this same climate that trees also warmed cities most at night, by up to 0.8°C.

Trees performed well in arid climates, cooling cities by just over 9°C and warming them at night by 0.4 °C.

In tropical rainforest climates, where humidity is higher, the daytime cooling effect dropped to approximately 2°C while the nighttime heating effect was 0.8 °C.

In temperate climates, trees can cool cities by up to 6°C and warm them by 1.5°C.

Using trees more strategically

̽��ֱ��study points out that cities which have more open urban layouts are more likely to feature a mix of evergreen and deciduous trees of varying sizes. This, the researchers found, tends to result in greater cooling in temperate, continental and tropical climates.

̽��ֱ��combined use of trees in these climates generally results in 0.5 °C more cooling than in cities where only deciduous or evergreen trees feature. This is because mixed trees can balance seasonal shading and sunlight, providing three-dimensional cooling at various heights.

In arid climates, however, the researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts such as Cairo in Egypt, or Dubai in UAE.

In general, trees cooled more effectively in open and low-rise cities in dry climates. In open urban layouts, cooling can be improved by about 0.4 °C because their larger green spaces allow for more and larger tree canopies and a greater mix of tree species.

“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” Dr Ronita Bardhan said.

“Our results emphasize that urban planners not only need to give cities more green spaces, they need to plant the right mix of trees in optimal positions to maximize cooling benefits.”

“Urban planners should plan for future warmer climates by choosing resilient species which will continue to thrive and maintain cooling benefits,” said Dr Bardhan, a Fellow of Selwyn College, Cambridge.

Matching trees to urban forms

̽��ֱ��study goes further, arguing that species selection and placement needs to be compatible with urban forms. ̽��ֱ��orientation of the ‘street canyon’, local climate zones, aspect ratio, visible sky ratio and other urban features that influence the effects of trees all need to be carefully considered.

Although a higher degree of tree canopy cover in street canyons generally results in more cooling effects, excessively high cover may trap heat at the pedestrian level, especially in compact urban zones in high temperature climates. In such locations, narrow species and sparse planting strategies are recommended.

̽��ֱ��researchers emphasise that we cannot rely entirely on trees to cool cities, and that solutions such as solar shading and reflective materials will continue to play an important role.

̽��ֱ��researchers have developed an interactive database and map to enable users to estimate the cooling efficacy of strategies based on data from cities with similar climates and urban structures.

Reference

H Li et al., ‘Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait’, Communications Earth & Environment (2024). DOI: 10.1038/s43247-024-01908-4

While trees can cool some cities significantly during the day, new research shows that tree canopies can also trap heat and raise temperatures at night. ̽��ֱ��study aims to help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.

Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide

Ronita Bardhan

hannahisabelnic via Flikr (Public domain)

Trees in an Indian city street. Photo: hannahisabelnic via Flikr (Public domain)

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Cambridge and Nanjing break ground on 'smart cities' Centre

plc32 — Tue, 10 Sep 2019 04:03:44 +0000

Cambridge Vice-Chancellor Professor Stephen J Toope joined Zhang Jinghua, Party Secretary of Nanjing City Party Committee and Nanjing Deputy Mayor Jiang Yuejian to turn the first soil at the site where the Centre's dedicated building will rise in Nanjing's Jiangbei New Area.

̽��ֱ��Cambridge ̽��ֱ��-Nanjing Centre of Technology and Innovation will establish a home for joint research and innovation in collaboration with the Chinese government, industry and China's global research universities that is dedicated to the future of creating 'smart' cities.

"Here in Nanjing, an ancient city and former imperial capital, we are embarking on a unique enterprise," Vice-Chancellor Toope said at the groundbreaking ceremony. " ̽��ֱ��innovations emerging from this Centre will enable the development of 'smart' cities in which sensors can enable sustainable lifestyles, improve healthcare, limit pollution and make efficient use of energy."

Cambridge and its Chinese partners will share revenue derived from the commercialisation of Intellectual Property (IP) developed at the Centre. It is the ̽��ֱ��’s first overseas enterprise at this scale.

Funded by the Nanjing Municipality for its first five years, the project will have its own dedicated building as a pilot urban development based on high levels of technological innovation.

At the heart of the new Centre’s activities will be research into technologies that support a modern 21st century city with integrated IT, health care and building management. Innovations emerging from the Centre will enable the development of 'smart' cities in which sensors – applied at the individual level and all the way through to the level of large infrastructure – will enable sustainable lifestyles.

As well as supporting health and wellbeing in new cities, the new Centre will help deliver efficient energy use through its academic and entrepreneurial activities.

̽��ֱ��agreement between Cambridge and Nanjing will fund positions in Nanjing, both academic and management, and will allow Cambridge-based academics to engage with specific, long-term projects in Nanjing. It will also support the establishment of a professorship, based in Cambridge, with responsibility as the Centre’s Academic Director.

̽��ֱ��project has been driven by Cambridge’s Department of Engineering, although it is hoped that there will be opportunities to widen participation to other departments and Schools. IP generated by research funded through the Centre will be licensed for commercialisation by Cambridge ̽��ֱ��’s innovation branch, Cambridge Enterprise.

̽��ֱ��Centre will seek to demonstrate the power of collaboration with China’s universities, industry, government and other partners to conduct the kind of academic research of excellence today that will make life better for the city dwellers of tomorrow.

One of the two initial projects already approved is to create a high resolution scanner that can provide a low-cost easily accessible method for examining difficult areas of the body, such as bent spines, without using large and expensive CT scans.

That project will be led by Cambridge Engineering Department Professor Richard Prager, in collaboration with China's Southeast ̽��ֱ�� and established local ultrasonic manufacturer Vinno.

A second identified project led by Principal Investigator Professor Toni Vidal-Puig from Cambridge’s Clinical Biochemistry Department, will study the associated complications of increased obesity in China.

Both themes are closely linked to the focus area of local partner, NIHA (Nanjing International Healthcare Area).

̽��ֱ��Vice-Chancellor was joined at the groundbreaking ceremony by representatives from partners Nanjing ̽��ֱ��, Southeast ̽��ֱ��, Peking ̽��ֱ��. Tsinghua ̽��ֱ��, Fudan ̽��ֱ�� and Zheijiang ̽��ֱ�� as well as the Academic Director of Cambridge ̽��ֱ��-Nanjing Centre, Professor Daping Chu of Cambridge's Electrical Engineering Department, and Pro-Vice-Chancellor for International Relations Eilis Ferran.

̽��ֱ�� ̽��ֱ�� of Cambridge and the Nanjing Municipal Government have broken ground on the Cambridge ̽��ֱ��-Nanjing Centre of Technology and Innovation.

Here in Nanjing, an ancient city and former imperial capital, we are embarking on a unique enterprise.

Vice-Chancellor Stephen J Toope

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Cambridge and Nanjing launch strategic collaboration

ag236 — Tue, 27 Mar 2018 08:00:00 +0000

Professor Stephen Toope, the ̽��ֱ�� of Cambridge’s Vice-Chancellor, today signed an agreement to formalise a strategic partnership with the Nanjing Municipal Government.

̽��ֱ��creation of the Cambridge ̽��ֱ��-Nanjing Centre of Technology and Innovation will entail the establishment of a joint research centre and the sharing of revenue derived from the commercialisation of intellectual property. It is the ̽��ֱ��’s first overseas enterprise at this scale.

Funded by Nanjing Municipality for five years in the first instance, the project will have its own dedicated building in Nanjing’s Jiangbei New Area – a pilot urban development based on high levels of technological innovation.

At the heart of the new Centre’s activities will be research into technologies that support a modern 21^st century city with integrated IT, health care and building management. Innovations emerging from the Centre will enable the development of 'smart' cities in which sensors – applied at the individual level and all the way through to the level of large infrastructure – will enable sustainable lifestyles.

As well as supporting health and wellbeing in new cities, the new Centre will help deliver efficient energy use through its academic and entrepreneurial activities.

̽��ֱ��agreement will fund positions in Nanjing, both academic and management, and will allow Cambridge-based academics to engage with specific, long-term projects in Nanjing. It will also support the establishment of a professorship, based in Cambridge, with responsibility as the Centre’s Academic Director.

̽��ֱ��project has been driven by Cambridge’s Department of Engineering, although it is hoped that there will be opportunities to widen participation to other departments and Schools. IP generated by research funded through the Centre will be licensed for commercialisation by the ̽��ֱ��’s innovation branch, Cambridge Enterprise.

Speaking just before the official signing of the agreement, held at the British embassy in Beijing, Professor Toope said: “This is only the most recent example of our collaboration with Chinese partners – but it is by far the most ambitious to date. And it is very exciting indeed.”

“We see it as an essential part of Cambridge’s contribution to society to tackle some of the great world problems. But we cannot do this on our own. There is a proverb: ‘You cannot clap with just one hand’. To me this means that we can only accomplish great things by working together – which is what we will be doing with Nanjing.”

Mr. Luo Qun, a member of the Standing Committee of Nanjing's Municipal Party Committee, and Deputy Party Secretary of the Party Committee of Jiangbei New Area, added: "We sincerely hope that both sides will rely on this new Centre to push the world's technological frontiers and to promote the integration of science, technology, industry and financial innovation."

̽��ֱ��Vice-Chancellor was joined by Professor Sir Mark Welland, Head of the ̽��ֱ��'s Electrical Engineering Division and Master of St Catharine's College. ̽��ֱ��signing of the agreement was witnessed by H.E. Dame Barbara Woodward, the United Kingdom's ambassador to China.

Knowledge and development

̽��ֱ��launch of the Cambridge ̽��ֱ��-Nanjing Centre of Technology and Innovation came only a few days after the Vice-Chancellor addressed the annual China Development Forum, in Beijing.

Speaking on the subject of 'Knowledge Capital and development for all', Professor Toope said: "Of all the intangible assets that underpin our knowledge capital, the most precious is people. It is people who generate the new ideas; it is people who ask the searching questions, and collect the relevant data to answer them; it is people who make the discoveries; it is people who bring those discoveries to the market, and create the intellectual property. ̽��ֱ��conclusion I draw from this is that, for countries and institutions wishing to expand their knowledge capital, the single most important investment is in their human capital."

He singled out equality and diversity as essential to the sustainability of knowledge-based capital, before concluding: “'Knowledge itself is power' is a famous line attributed to one of Cambridge’s most famous graduates – 17th century philosopher Francis Bacon. ̽��ֱ��question before us – particularly those of us in universities – is how we build and deploy and share all that knowledge for the greater good."

̽��ֱ��new joint centre will support innovative research into smart cities and fully integrated urban environments.

We see it as an essential part of Cambridge’s contribution to society to tackle some of the great world problems. But we cannot do this on our own. We can only accomplish great things by working together – which is what we will be doing with Nanjing.

Prof Stephen Toope

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Back to the future of skyscraper design

sjr81 — Wed, 01 Mar 2017 09:09:44 +0000

Newly-published, ̽��ֱ��Recovery of Natural Environments in Architecture by Professor Alan Short is the culmination of 30 years’ research and award-winning green building design by Short and colleagues in Architecture, Engineering, Applied Maths and Earth Sciences at the ̽��ֱ�� of Cambridge.

“ ̽��ֱ��crisis in building design is already here,” said Short. “Policy makers think you can solve energy and building problems with gadgets. You can’t. As global temperatures continue to rise, we are going to continue to squander more and more energy on keeping our buildings mechanically cool until we have run out of capacity.”

Short is calling for a sweeping reinvention of how skyscrapers and major public architecture are designed – to end the reliance on sealed buildings which exist solely via the ‘life support’ system of vast air conditioning units.

Instead, he shows it is entirely possible to accommodate natural ventilation and cooling in large buildings by looking into the past, before the widespread introduction of air conditioning systems which were ‘relentlessly and aggressively promoted’ by inventor Willis Carrier and rival entrepreneurs.

“ ̽��ֱ��majority of contemporary buildings have absolutely no resilience to climate at all,” he added. “To make them habitable, you have to seal them and air condition them. ̽��ֱ��energy use and carbon emissions this generates is spectacular and to a large extent unnecessary. Buildings in the West count for 40-50% of electricity usage, generating substantial carbon emissions. ̽��ֱ��rest of the world is catching up at a frightening rate, China at 31% and rising in 2017.

“Modern buildings cannot survive unless hard-wired to a life support machine, yet this fetish for glass, steel and air-conditioned skyscrapers continues; they are symbols of status around the world on an increasingly vast scale.”

Short’s book highlights a developing and sophisticated art and science of ventilating buildings through the 19th and earlier 20th centuries, including the two chambers of the Houses of Parliament, and the design of ingeniously ventilated hospitals. Of particular interest were those built under the aegis of John Shaw Billings, designer of the first Johns Hopkins Hospital in Baltimore (1873-1889).

“We spent three years digitally modelling Billings' final designs and a brilliant alternative design,” added Short. “We put pathogens in the airstreams, modelled for someone with TB coughing in the wards and we found the ventilation systems in the room would have kept patients safe from harm.

“We discovered that nineteenth century hospital wards could generate up to 24 air changes an hour– that’s similar to the performance of a modern-day, computer-controlled operating theatre. We believe you could build wards based on these principles for the NHS now. Single rooms are not appropriate for all patients. Communal wards appropriate for certain patients – older people with dementia, for example – would work just as well in today’s hospitals, at a fraction of the energy cost.”

Professor Short contends the mindset and skill-sets behind these designs have been completely lost, lamenting the disappearance of expertly designed theatres, opera houses, and other public buildings where up to half the volume of the building was given over to ensuring everyone got fresh air. Early twentieth century climate determinists like Ellsworth Huntington at Yale inadvertently promoted the export of cool, temperate climates around the world and explicitly condemned the inhabitants of hot climates as uncivilised and backward.

Much of the ingenuity present in 19th century hospital and building design was driven by a panicked public clamouring for buildings that could protect against what was thought to be the pernicious threat of miasmas – toxic air that spread disease.

Bad, malodourous air was considered lethal by huge swathes of the populace. Miasmas and other quasi-mystical phenomena were feared as the principal agents of disease and epidemics for centuries, and were used to explain the spread of infection from the Middle Ages, right through to the cholera outbreaks in London and Paris during the 1850s. Miasma theory attracted the attention of luminaries such as Florence Nightingale who believed that foul air, rather than germs, was the main driver of 'hospital fever' leading to disease and frequent death. ̽��ֱ��prosperous steered clear of hospitals.

While miasma theory has been long since disproved, Short has for the last thirty years advocated a return to some of the building design principles produced in its wake.

“ ̽��ֱ��air conditioning industry has persuaded us that you can’t do this naturally any more and that it would defy progress to do so. Huge amounts of a building’s space and construction cost are today given over to air-conditioning instead.

“But I have designed and built a series of buildings over the past three decades which have tried to reinvent some of these ideas and then measure what happens – publishing what works as well as what doesn’t.

“To go forward into our new low energy, low carbon future, we would be well advised to look back at design before our high-energy high-carbon present appeared. What is surprising is what a rich legacy we have abandoned. There is an analogy with the widespread introduction of affordable antibiotics and the relaxation in the ferocious cleanliness regimes in hospitals and the frightening consequences emerging now.”

Successful examples of Short’s approach include the iconic Queen’s Building at De Montfort ̽��ֱ�� in Leicester. Containing as many as 2,000 staff and students, the entire building is naturally ventilated, passively cooled and naturally lit, including the two largest auditoria each seating more than 150 people.

Conventional wisdom in the ventilation and heating industry was that this omission of mechanical and electrical equipment was impossible. Confounding its critics, the building was awarded the Green Building of the Year and RIBA’s Education Building of the Year in 1995 and was at the time the largest naturally ventilated building in Europe, influencing guidance in Europe and the USA. ̽��ֱ��building uses a fraction of the electricity of comparable buildings in the UK.

Following success there, Short and industry associates have also experimented with theatre design, including the Contact Theatre in Manchester, which uses the abundant heat sources of theatre lights and audience to drive air flows around the building and the passive downdraught cooled School of Slavonic and East European Studies in Bloomsbury, epicentre of the London Heat Island.

Short contends that glass skyscrapers in London and around the world will become a liability over the next twenty or thirty years’ time if climate modelling predictions and energy price rises come to pass as expected. He points to the perfect storm of the skyscraper boom in China, where huge high-rise, all-glass metropolises expand at an exponential rate. Meanwhile, 550 million people south of the Qin-Huai line in that country are not allowed to centrally heat or cool their own homes because of the energy that would demand and consume.

Short is convinced that sufficiently cooled skyscrapers using the natural environment can be produced in almost any climate, pointing to his research work on cooling an 11-storey tower at Addenbrooke’s Hospital in Cambridge. He and his team have also worked on hybrid buildings in the harsh climates of Beijing and Chicago – built with natural ventilation assisted by back-up air-conditioning – which, surprisingly perhaps, can be switched off more than half the time on milder days and during the spring and autumn.

“I think you can upscale these designs,” he added. “As you go higher, airspeeds increase and it becomes easier to control the climate within tall buildings.

“My book is a recipe book which looks at the past, how we got to where we are now, and how we might reimagine the cities, offices and homes of the future. There are compelling reasons to do this. ̽��ֱ��Department of Health says new hospitals should be naturally ventilated, but they are not. Maybe it’s time we changed our outlook.”

̽��ֱ��Recovery of Natural Environments in Architecture: Air, Comfort and Climate, published by Routledge, is out now.

Answers to the problem of crippling electricity use by skyscrapers and large public buildings could be ‘exhumed’ from ingenious but forgotten architectural designs of the 19th and early 20th century – according to a world authority on climate and building design.

̽��ֱ��air conditioning industry has persuaded us that you can’t naturally ventilate buildings any more.

Alan Short

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Post-war advertisement for air conditioning by Carrier

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