Rooftop gardens that create urban cool
Every summer in the heart of Manhattan, a tower block erupts with organic vegetables and herbs. The tomatoes, peppers, cucumbers and sweet potato and rosemary, basil and sage are not the produce of a private roof garden but a demonstration by the Earth Pledge Foundation of the potential for city-based food production.
A love of vegetables, however, is not why city authorities are starting to pay attention to organisations such as Earth Pledge. With energy savings and improved waste water management in view, they hope green roofs can cut costs as well as helping to save the environment.
North American cities such as Chicago, Toronto and Portland, Oregon, are starting to green their rooftops. Such roofs are also included in plans for prominent projects such as the expansion of the Vancouver Convention and Exhibition Centre, where a six-acre “living roof” will feature a variety of native plants.
Part of the reason is awareness of the “urban heat island” effect. With many large surfaces that absorb solar radiation and return it to the air as heat, urban areas are becoming hotter relative to the countryside.
“ It is a well-known phenomenon,” says Stuart Gaffin, research leader on green roofs at the Centre for Climate Systems Research at Columbia University. “There can be up to 6 degrees Fahrenheit [about 12 degrees centigrade] difference in the urban versus suburban temperature.”
A green roof is typically made of a specialised growing medium and plantings such as seedhams and grasses. It acts as insulation and the vegetation provides shade and returns moisture to the atmosphere, preventing solar energy being absorbed by the roof.
The effects are limited to the top storeys of any individual structure, so the key for high-rise cities is getting green-roof technology implemented on a large scale. “To the extent that you can get enough green roofs in an area to cool the city down, you can reduce the overall energy consumption in all the buildings and reduce the temperature in the city,” says Steven Peck, executive director of Green Roofs for Healthy Cities, based in Toronto. A recent study by the City of Toronto and Environment Canada suggested that relatively modest amounts of green roof 5 per cent of the total land use in Toronto, or half all roofs (assuming that roofs account for 10 per cent of urban land use) would create a 1 to 2 degree centigrade reduction in urban heat island.
Even when considering the effects for individual buildings, a further benefit emerges when the climate simulation models are applied to a single hot day, as the green roof can moderate peaks. The annual energy savings projected for an eight-storey building with a green roof in Madrid, Spain, were about 6.4 per cent, says Brad Bass, a researcher with Environment Canada and professor at the University of Toronto Institute for Environmental Studies. But when the simulation was run for one hot day, the savings were nearer to 10 per cent for the whole building.
Prof Bass says this has implications for power-generation companies. “The real beneficiaries of this peak-demand knowledge are the utilities,” he says. “If you are promoting green roofs, you tell the utilities that here is a simple way of easing the demand on the grid during a heat wave.”
While research in this area is in its early stages, there are signs that the role of utilities is being recognised. In Quebec, the province's Energy Efficiency Fund which creates energy efficiency programmes and activities for natural gas customers is offering clients a $C1 (80 US cents) per square foot incentive to implement green roof systems.
Leslie Hoffman, executive director of the Earth Pledge Foundation, a non-profit organisation, believes that for city authorities looking to cut costs, it is the capacity of green roofs to soak up storm water that will ultimately persuade officials to invest in the technology.
Earth Pledge has been commissioned by New York City's water board to create models for Lower Manhattan storm water flows. “What they are interested in is the ability to quantify the value of the benefit,” says Ms Hoffman.
By acting like a giant sponge, a green roof delays the run-off of storm water that, for cities with combined storm water and sewerage systems, can create problems. “There are drainage basins in New York City, for example, where as little as one-twentieth of an inch of rain could create an overflow,” says Ms Hoffman.
As well as the expense of managing the overflows, they pollute waterways. “The EPA [US Environmental Protection Agency] has been putting pressure on a number of cities because they have been contaminating waterways every time there is a rainstorm,” says Mr Peck.
Ms Hoffman says green-roof implementation could help manage both problems. Because of the tangible costs of storm water management, she believes this is the issue most likely to attract authorities to the technology.
“ With urban heat island effect, it's much more difficult to figure out who spends money,” she says. “Whereas sewerage is pretty clear we know what we spend, and we can measure the gallons.” 屋顶花园为城市“降温”
每年夏天，曼哈顿中心区的一座高楼上都会长出大片有机蔬菜和草本植物，一派郁郁葱葱。这些西红柿、辣椒、黄瓜、甘薯、迷迭香、罗勒和鼠尾草并非某私人屋顶花园的作物，而是美国地球信托基金会 (Earth Pledge Foundation) 的演示项目，旨在说明城市的食品生产潜力。
然而，市政当局之所以开始关注地球信托基金会这类组织，并非出于对蔬菜的喜爱。考虑到节约能源和改善废水处理，他们希望绿色屋顶在有助于拯救环境的同时，还能削减费用。
目前，芝加哥、多伦多、波特兰和俄勒冈等北美城市已经开始着手绿化其城市屋顶。绿色屋顶甚至已被纳入到一些重大项目的规划中，如温哥华会展中心 (Vancouver Convention and Exhibition Centre) 的扩建计划，届时，该中心面积达 6 英亩的“活力屋顶”将展示多种本地植物。
开展绿色屋顶计划的部分原因，在于人们已经意识到“城市热岛”效应。由于很多大面积建筑物的表面在吸收太阳辐射后，会将热量释放到大气中，因此，相对于乡村而言，城市地区的温度正在上升。
“这是一种广为人知的现象，”哥伦比亚大学气候系统研究中心 (Centre for Climate Systems Research) 绿色屋顶研究负责人斯图亚特?加芬 (Stuart Gaffin) 表示。“城市与郊区温差可达到 6 华氏度 ( 约 3.3 摄氏度 ) 。”
绿色屋顶通常由一种专用的培养基以及 seedhams 和青草等植物组成，可以起到隔热和遮阳作用。植物可以将湿气返回空气中，防止屋顶吸收太阳能。
此种效果局限于个别高楼的最高几层，因此，对高层建筑物密集的城市来说，大规模实施绿色屋顶技术是关键。“只要能够在某个区域拥有足够多的绿色屋顶来降低城市温度，那么就可以减少所有建筑物的整体能源消耗，并降低整个城市温度。”多伦多“绿色屋顶造就健康城市” (Green Roofs for Healthy Cities) 中心执行主任史蒂芬?派克 (Steven Peck) 说。最近，多伦多市和加拿大环保署 (Environment Canada) 联合开展的一项研究表明，建设相对适度数量的绿色屋顶，即占多伦多市总用地面积的 5% ，也就是将所有屋顶中的一半改造为绿色屋顶（假设屋顶面积占城市总用地面积的 10% ），将使城市热岛温度降低 1 到 2 摄氏度。
即便只考虑绿色屋顶对个别建筑产生的效果，如果我们把一些气候模拟模型应用到某一个高温夏日，就会发现另一个好处，即绿色屋顶能够缓解一天中的热度峰值。加拿大环保署研究员兼多伦多大学环境研究院 (Institute for Environmental Studies) 教授布拉德?巴斯 (Brad Bass) 说，西班牙马德里一栋 8 层高，带有绿顶的建筑物每年预计可节省约 6.4% 的能源。而当将该模拟模型应用于一个高温夏日时，整栋建筑物可节约近 10% 的能源。
巴斯教授说，这会给发电企业带来有关影响。“这种热门技术的真正受益人是电力公司，”他说。“你在推广绿色屋顶时，就等于告诉电力公司：这里有一个缓解热浪袭击期供电需求的简单方式。”
尽管这一领域的研究尚处于初期阶段，但已有迹象表明，电力公司的角色正在得到认可。在加拿大魁北克省，能源效率基金 (Energy Efficiency Fund) 是帮助天然气客户开展能源效率计划和活动的机构，它目前正在为其客户推广 1 加元 (80 美分 )/ 平方英尺的激励计划，用以实施绿色屋顶系统。
作为一家非营利组织，地球信托基金执行主任莱斯利?霍夫曼 (Leslie Hoffman) 认为，对希望降低开支的城市当局而言，绿色屋顶吸收暴风雨雨水的能力，才是最终说服官员们投资此项技术的因素。
目前，纽约市水务局已经委托地球信托基金为下曼哈顿地区设计暴风雨水流模型。“他们感兴趣的是我们有能力量化收益的价值，”霍夫曼女士说。
绿色屋顶像一个巨大的海绵，可以延缓雨水排流；雨水排流对用污水系统处理暴雨的城市会造成问题。“比如，纽约市有容易积水的低洼地区，只要二十分之一英寸的降水就可能造成雨水溢流，”霍夫曼女士说。
除了治理雨水溢流需要费用外，水道还会受到污染。“ EPA( 美国环境保护署 ) 已经对很多城市施加压力，因为每次发生暴风雨，水道就会受到污染。”派克先生说。
霍夫曼女士说，采用绿色屋顶可能有助于同时治理两种问题。由于暴雨雨水管理成本是可见的，她认为这是最有可能吸引城市当局采用该技术的原因。
“至于城市热岛效应，决定由谁出钱要困难很多，”她说。“而对排水而言，我们知道钱花在了什么地方，而且排水量是以加仑计算的。”
