oday, people in urban areas consume 80% of global primary energy. Reducing energy consumption, improving energy efficiency, and moving towards renewable energy systems is crucial to reduce anthropogenic greenhouse gas emissions and ultimately limit global warming. Compared to other industry sectors, such as transportation, the building sector benefits from available and economically viable technology, materials, and knowledge to transform building stock towards low carbon emissions. The scarcity of space, high energy density, and complex interactions of urban areas make such a transformation significantly more difficult than in rural settings. Due to climate change, rising temperatures—especially in expanding urbanized regions near the equator—will result in a global energy demand for cooling that the International Energy Agency projects will double by 2040.

Prototype of Future Cities Lab Solar Facade. Photo by ETH Zurich / Arno Schlueter.

The Beauty of Urban Energy

Harvesting solar energy from the rooftops, skylights, and the facades of buildings termed “Building Integrated Photovoltaics (BIPV)” offers not only a source of on-site renewable energy generation, but also a number of economic advantages. For example, rather than utilizing additional urban space for infrastructure, integrating photovoltaics replaces conventional 'inactive' building elements and materials with an active resource. Three essential drivers recently boosted the potential of BIPV for large-scale application.

First, the mass production of photovoltaic (PV) cells has lowered their cost dramatically. In many countries, harvesting solar electricity on a building has become a competitive advantage equal to or even cheaper than the cost per kWh of electricity provided from traditional utility services. Globally, electricity generated using PV cells has reached a cost parity with fossil fuel generated electricity in many countries. The cost parity led to an exponential growth in installed PV capacity.

Second, new surface treatments have changed the aesthetic appeal of photovoltaics. Un-til recently, Building Integrated Photovoltaics lacked support for large-scale implementation from many architects, city planners, and officials who deemed the material visually unappealing. Recent advances in the treatment of the covering layers of photovoltaic cells unleashed almost limitless possibilities—from custom designed BIPV as high-standard glass facades to hidden active technologies behind thin layers of stone and, even, concrete. These developments allow architects and planners to treat BIPV like al-most any facade cladding material.

Third, new regulations beyond feed-in tariffs have made the joint exploration and self-consumption of on-site solar energy generation more economically attractive.

Now, less expensive and more energy efficient, BIPV technologies are becoming economically and ecologically viable also on surfaces that receive less solar radiation. This is especially relevant in dense urban settings where buildings are primarily vertical and their envelope offers large vertical surfaces areas. The challenge; however, is that vertical facades generally receive less solar radiation and are subject to shading from neighboring buildings. Optimization of urban design for renewable energy generation addresses both urban form, as well as function. Advances in integrated urban design could affect both the availability and solar energy generation potentials of surface facades, as well as, shape the energy loads and demands to maximize its utilization. On the energy systems level, it is just as important to consider complementary technologies such as storage and thermal systems, for example district heating and cooling networks, heat pumps, and chillers.

City Energy Analyst Tool. Image by ETH Zurich/Arno Schlueter.

Analytical Tools for Integrated Urban Design

Just considering the scale of individual buildings is too limited to depict present and future energy systems and networks that often span from just a few to hundreds of buildings. New methodologies and research enable the development and analysis of urban energy flows and systems. As part of our research at the Singapore-ETH Center’s Future Cities Laboratory, we developed the “City Energy Analyst”—an open-source digital modelling platform. The platform provides parametric urban design tools to generate prototypical urban blocks and districts, engineering models for buildings, and district energy supply and demand systems. It also enables a variety of occupancy models to depict the behaviors and activities of the urban population. Combined with location-specific weather data, these models, embedded in optimization routines, enhance the search for best-performing solutions within given targets and parameters, such as capital and operational expenditures, CO2 emissions, and primary energy.

Even though, urban design influences the lives of millions of people over multiple generations, urban designers and city planners rarely utilize analytical tools to make in-formed decisions toward low carbon, livable cities. In case studies such as the University District or “Hochschulquartier” in Zurich, Switzerland and the new Waterfront Tanjong Pagar in Singapore, researchers have investigated integrated processes and demonstrated the ability to generate knowledge that guides decision-making processes. These case studies illustrate that urban development and design, without the consideration of urban energy systems, makes it impossible to reach societal goals like the 2000Watt / 1t CO2 society or other emission reduction targets. Buildings have lifespans—from decades to centuries. Each building and each city that we develop without consideration of its impact and energy systems is lost in the fight against climate change.

Arno Schlueter
Professor Arno Schlueter is the chair of Architecture and Building Systems at ETH Zurich and a principal investigator at the Singapore-ETH Center’s Future Cities Laboratory.
The views presented in this article are the author’s own and do not necessarily represent the views of any other organization.