Solar panels can occupy a maximum of 34 square kilometers of roof spaces in Singapore. With optimal technological development, this will generate more than 20% of Singapore’s total electricity demand by 2050. In a country where solar remains the most viable low-carbon, clean energy, the full utilization of all potential space especially rooftop seems only logical. But rooftop is no longer a wasted space in land scarce Singapore. Notably, it can be–and have already been- developed into green roof for many good reasons. So which should it be: blue roof or green roof?
Solar roof is designed to fulfill at least part of a building’s energy demand with minimal negative impact on the environment. Since energy will always be on demand, it can be argued that the source of this energy is key to sustainability. But it is not the only key. Even if more energy can be generated cleanly through solar panels and other means, sustainability will not be achieved if energy demand continues shooting through the roof. That is, alternative energy generation must be accompanied with efficient energy use.
Green rooftop has the potential to do just that. It is estimated that a carefully designed and maintained green roof reduces energy demand of a medium-rise building by 15%. This reduction comes largely from savings in artificial cooling systems which can account for more than 50% of energy use in Singapore households. Green roof moderates temperature by capturing water in the soil and releasing heat during photosynthesis and transpiration. Conversely, the materials and design of solar panels may –if poorly designed- enhance urban heating (known as urban heat island effect), thus making the building less efficient. Often, energy saved from a passive green rooftop is comparable to energy generated by solar panels.
Just as plants do more than just lower ambient temperature, a roof covered with them also perform many other functions that solar roof cannot. By capturing water, the likelihood of flood during heavy rain is reduced and the rainwater that flows out of green roof system tends to be of higher quality as well. On top of filtering water, green roof filters the air from pollutants and even reduce noise pollution. All these contribute to the variety of ways that green roof can be seen as more effective in making a city liveable than blue roof.
Liveability is strongly associated with roof greening because it may also provide space for community activities and social interactions. Green roof can be likened to an elevated park. The rooftop of Marina Barrage illustrates this very well as it has become a popular place to have a picnic, fly kites and enjoy the city landscape. Alternatively, roof can be greened with edible plants as a community garden that contributes, albeit in a small way, to food sufficiency in Singapore.
Nevertheless, each green roof is different and will not always have the abovementioned functions and benefits. Some plants are more effective at cooling down the building and others require less maintenance. The choice of soil medium also affects the quality of water that comes out of the roof drainage. For example, coconut husks are able to filter excess nutrients from water and is preferred for being lightweight and durable. However, poor choice of medium can have the opposite effect, polluting rainwater runoff. Green roof designed for public use can also be much more costly to maintain than solar roof. At the same time, the energy saving effect of green roof in high-rise buildings may not be very significant.
There is no clear-cut way to compare green and blue roofs. In fact, there may not even be a need to choose one over the other as both types of roof have been seen on a single building such as in Temasek Polytechnic, the CDL Green Gallery and even Marina Barrage. In a sense, green roof and solar roof perform a complimentary role that overall makes energy consumption more sustainable. Still, it is important to consider the different ways that each may function better than the other for different types of buildings. In the end both green and blue roofs are innovative ways to make better use of spaces to build a more sustainable urban environment.
- Urban Heat Island Effect: https://en.wikipedia.org/wiki/Urban_heat_island