The Materials Book

229 Energy use for cooling buildings has doubled between 2000 and 2018, led by a combination of warmer temperatures and population and economic growth, according to the International Energy Agency (IEA). 2 Owing to the rise of incomes and the wider use of appliances that generate heat, such as refrigerators and computers, buildings are increasingly adopting air-conditioning systems. Despite a wide range of climatic contexts, from extremely cold to extremely hot, air-conditioning units are installed in almost 90 percent of homes in the United States. 3 And this increase is also permeating regions where the use of active systems has not been common—for example, in India, whose population is four times that of the United States and where the need for cooling is twelve times as great. The IEA’s 2018 report states that the number of AC units worldwide is predicted to increase from 1.6 billion to 5.6 billion by the middle of the century. As a result, the greenhouse-gas emissions from coal and natural-gas plants generating electricity to power the units will nearly double, from 1.25 billion tons in 2016 to 2.28 billion tons in 2050. Additionally, hydrofluorocarbons— the primary chemical compounds in AC units, which are leaked during their manufacturing, installation, and disposal 4 —are far more potent greenhouse gases than carbon dioxide, trapping thousands of times more heat in the atmosphere. Various publications on natural ventilation have illustrated the important role of passive systems in reducing energy consumption in buildings. According to a study at Harvard University’s Center for Green Buildings and Cities, up to 78 percent of cooling-energy consumption in China can be potentially reduced by natural ventilation, depending on the local weather and air quality. 5 However, without a process to design buildings that promotes a system combining active and passive strategies to reduce energy use and carbon emissions, new and retrofitted buildings will continue to shift to air-conditioning to maintain indoor thermal comfort. Greenhouse-gas emissions from buildings can be reduced by taking a climate-responsive design approach: buildings in hot and dry climates should have different features than those in hot and humid or cold climates. Not all climates allow for passive strategies year round; however, in every climate natural ventilation in buildings can be used for a few weeks or months a year. To reduce emissions, a new way of thinking about building design is needed—a strategic, hybrid mode that permits both active and passive modes of operation. Implementing strategies such as natural ventilation or thermal mass to regulate heat requires an analytic approach during the early stages of building design. Christoph Reinhart and I have created a support system