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| Fig. 1: Photovoltaic cells on window shades for a net zero energy building in Singapore. (Source: Wikimedia Commons) |
The energy footprint of a building derives primarily from: the resources and energy used in mining, extraction and processing of building materials; transportation of materials and building processes; daily energy use from occupants as well as heating and cooling; and lastly, the energy costs of decommissioning. [1] Combined, the impact of these processes can be quite substantial. The IPCC has estimated that buildings account for around 33 % of annual greenhouse gas emissions, which puts efforts to reduce the carbon footprint of buildings at the forefront of tactics to combat a changing climate. [2]
Zero energy buildings aim to either eliminate the need for a building to be connected to the electricity grid, or provide ways for buildings that are connected to the energy infrastructure to draw net zero energy by supplying the energy they use back to the grid over a longer timescale. These tend to be referred to as Zero Energy Buildings (ZEB) and Net Zero Energy Buildings (NZEB) respectively. [1] Strategies to reduce energy usage of buildings include energy efficiency optimizations of: building envelopes (i.e. insulation, window glazing etc.), internal conditions (i.e. lighting equipment, appliances etc.), and building service systems (i.e. HVAC, escalators, electrical services etc.). [1] For the remaining energy usage of the building, or to offset the energy cost of construction and material use, ZEBs and NZEBs use either on-site or off-site renewables. [3] An example of on-site renewables can be seen in Fig. 1, from an NZEB in Singapore that uses photovoltaics installed in the external window shades.
One preliminary challenge in the development of ZEBs is the lack of formal or agreed upon definitions. For example, many designs for net zero energy buildings assume a constant connection to a distributed energy grid, such that the building can draw extra energy during high-load periods, and supply energy back to the grid through on-site renewable production. This definition of net zero energy may not be completely accurate, as there could be times when excess energy cannot be used by the grid, and supplied energy from on-site renewables would simply be wasted. [4]
This perspective could paint NZEBs in a different light: as a branding strategy rather than a true solution to high energy use buildings. However, as pointed out my Marszal and Heiselberg, NZEBs and ZEBs are often built as a long-term cost saving venture by the building's owner, and depending on the host country's energy policy, there can be a large incentive to minimize the total amount of energy drawn from the grid. This leads to implementing strategies that maximize energy use efficiency and minimize dependence on both renewables and grid supplied energy. [3,5]
Zero Energy Buildings have the potential to represent a large shift in how we use electricity in both residential and commercial spaces. As with many emerging energy efficient technologies, the true impact of ZEBs demands further study and concrete criteria need to be developed that ensure the implementation of zero energy buildings is having the intended effect. Primarily, the issue of on-site or off-site energy storage can play a large role in the effectiveness of a ZEB and consequently, the label "Zero Energy Building" does not always reflect the full energy effect of a building. [4]
© Scott Malley. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. H. W. Li, L. Yang, and J. C. Lam, "Zero Energy Buildings and Sustainable Development Implications - A Review," Energy 54, 1 (2013).
[2] G. J. Levermore, "A Review of the IPCC Assessment Report Four, Part 1: The IPCC Process and Greenhouse Gas Emission Trends from Buildings Worldwide," Building Serv. Eng. Res. Technol. 29, 349 (2008).
[3] P. Hernandez and P. Kenny, "From Net Energy to Zero Energy Buildings: Defining Life Cycle Zero Energy Buildings (LC-ZEB)," Energy Buildings 42, 815 (2010).
[4] I. Sartori, A. Napolitano, and K. Vos, "Net Zero Energy Buildings: A Consistent Definition Framework," Energy Buildings 48, 220 (2012).
[5] A. J. Marszal and P. Heiselberg, "Life Cycle Cost Analysis of a Multi-Storey Residential Net Zero Energy Building in Denmark," Energy 36, 5600 (2011).
