Net-zero-energy buildings or zero-carbon energy systems? How best to decarbonize Germany's thermally inefficient 1950s-1970s-era apartments
Ray Galvin
Abstract
A popular idea for reducing CO2 emissions from existing buildings is to renovate them to “net-zero-energy” standard. In Europe this usually involves increasing the energy-efficiency of the building envelope, replacing fossil fuel boilers with heat pumps, and installing photovoltaics to generate as much energy as the building uses over the course of a year. However, net-zero-energy consumption does not necessarily imply zero carbon emissions, since the carbon intensity of grid-based electricity is substantially higher in winter, when net-zero-energy buildings are consuming electricity from the grid, than in summer, when their on-site photovoltaic systems are feeding electricity into the grid. An alternative, emerging concept is that of “zero carbon energy systems” where a building is seen as part of a wider energy system, in this case the electricity grid, which aims to be carbon-neutral overall. This paper applies this concept to a typical case-study apartment from Germany's highly energy-inefficient 1950s-1970s-era apartment buildings. Using fine-grained data on national electricity generation and household-level consumption, it investigates costs and residual carbon impacts of a range of photovoltaic system sizes that would make the apartment “net-zero-energy” if the building envelope has been retrofitted to a high standard and an air-source heat pump installed. The study finds that (a) achieving net-zero-energy requires a 40% larger photovoltaic system than is technically optimal for the household; (b) achieving net-zero-energy fails to achieve net-zero-carbon by some 0.252 tCO2/y; (c) achieving net-zero-carbon would require a 60% larger than optimal photovoltaic system; and (d) it would be more economical to invest in remote wind power than in excess photovoltaic capacity. This strategy would accelerate decarbonization at the level of the energy system, i.e., the national electricity grid.