Towards carbon free cities: Interplay between urban density and energy demand

Compensating area, which refers to off-site land being used if the energy demand cannot be met due to the urban arrangement of buildings, is required in a carbon-free city because the energy demand, including thermal energy (heating, cooling and hot water), power (ventilation and artificial light) in buildings and transport, need to be covered by the renewables gained on site or in the surrounding area outside of the town. This paper aims to develop a method to explore the urban density that could deliver an energy saving, land saving, and human-scaled urban situation. Various scenarios of urban densities in the cities in different climate zones were created to emphasize the comparison and the relative difference in the required compensating area. It is found that, although transportation energy consumption can be reduced by increasing number of storeys, the rate of decrease slows down as the number of storeys increases. Also, building energy consumption increases with the number of storeys because the artificial light will reach saturation (100% of hours of use) with the increased number of storey. In terms of the comparison between climate zones, the optimal scenario would be 4 to 6 storeys in cold or moderate climates. And the optimal choice would be 6 to 8 storeys in the hot and humid climates. With regard to the consideration of human scale, not only do these optimal ranges of the number of storeys provide good daylight access, but they also fall into the range of human scale.