Zero-energy buildings in cities with different climates and urban densities: energy demand, renewable energy harvest on-site and off-site and total land use for different renewable technologies

Zero-energy buildings (ZEBs) have no fossil energy consumption; this is achieved by optimizing the building and balancing the remaining energy needs by renewables. If this energy can be harvested onsite, on the building’s envelope and its estate, a net-ZEB is reached. If supplementary renewable energy has to be produced off-site on compensating land, the ZEB can be reached with such compensating measures (ZEB_CM). Climate and urban density determine how far a ZEB is possible. Temperatures out of comfort range, lack of daylight and overheating by solar radiation may cause energy demand while high insolation or wind speed delivers good preconditions to produce renewable energy on less land. A high urban density avoids urban sprawl and saves land outside of the cities that can be used for other purposes (agriculture and energy production, among others). But, at a certain density, net-ZEB cannot be realized furthermore, and compensating land is necessary. The paper investigates these effects for 15 selected cities around the globe, covering all main climatic conditions. Based on design rules out of literature and own experiences, a prototypical optimized building is derived for each location, and its energy demand is simulated. Standard assumptions for the efficiency of renewable energy systems are used to determine the need of land to cover it. For different urban densities, it can be concluded how far net-ZEB is possible; if necessary, the need for compensating land is calculated. The results show that for cities with moderate climates, the total land use (city plus compensating land) can decrease with increasing urban density if the technology used off-site has high efficiency (like PV). On the other hand, the total land use may increase remarkably with increasing urban density if the used technology off-site has a low efficiency (like the wind for electricity and especially wood pellets for heating). The final understanding is that cities should meet the energy needs on-site by optimized buildings and structures plus renewable energy production (PV on the building’s roofs, geothermal systems, etc.).