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Climate change caused by humans is a threat to flora, fauna and our entire civilization. We must therefore reduce net greenhouse gas emissions to a sustainable amount. Possible future solutions in the building sector include zero-carbon / zero-emissions buildings, which are already required in some areas. This paper discusses how much sense this approach makes and, based on that discussion, suggests a targeted system for evaluating buildings.
In the scientific community, there is almost unanimous agreement that greenhouse gas emissions must be stopped in the mid-term in order to stabilize the global climate and limit the negative effects of climate change on civilization and the environment. The “zero carbon” requirement for buildings is therefore a welcome idea in principle, although the postulate that tends to be behind this idea is somewhat questionable.
The common definition for zero-carbon / zero-emission / zero-energy / plus-energy buildings is generally as follows: demand minus production is less than or equal to zero. This simple definition does not clarify which demand is being used or whether primary, final or useful energy is being considered. Before we can make headway with these questions, we must first point out the systematic problem in the “zero emission = sustainable” postulate for buildings.
Even carbon-free (and true carbon-poor) energy sources are finite – every kilowatt-hour of renewable energy can be used only once. Once we have reached the limits of availability, we will have to return to fossil (non-sustainable and nonrenewable) energy sources.
For example, imagine a village with 100 homes that are to be supplied with heat in a sustainable way. Each year, 200 solid cubic meters of wood can be cut for this purpose. No other renewable energy sources are available (in this example), and the houses have an annual energy demand of 20 solid cubic meters of wood each (which is within the normal range for a single-family home in a cool-moderate climate).
The available 200 solid cubic meters of wood can supply ten buildings, which are therefore zero-carbon buildings in terms of heat supply. They are not, however, sustainable, since the other 90 houses still need fossil fuels for heating. This method does not achieve the goal of sustainably heating the village as a whole.
With this example, we can see that the “zero carbon” approach is much too short-sighted, since it does not account for resource limits. The heat supply for this example village would only be sustainable if all buildings could be heated with the available wood. Even though only two solid cubic meters of wood is available for each building in the village, however, there is still a solution. That amount of wood is certainly sufficient to heat a single-family Passive House.
The definition “demand minus production is less than or equal to zero” is therefore correct, but must also be applied beyond the limits of the single building. Indeed, it must also apply beyond the limits of the village, since the neighboring village could have less energy at its disposal. Trying to set a limit eventually leads us to consider the whole earth, although varying climates lead to very different requirements in different regions. Various energy sources are also available in different amounts in various parts of the world. In addition, these energy sources must be kept reasonable in terms of transport and the resulting dependencies and potential for conflict. The most sensible approach therefore seems to be to consider similar climates and similar renewable resource situations that are to be largely energy independent.
Meanwhile, calls for general (in terms of overall balance) energy independence for individual buildings are ill advised for other reasons. Even for an only moderately efficient, free-standing single-family home (with sufficient power storage capacity), it is not difficult to achieve semi-independence in terms of energy, since it has a large amount of roof space available for solar power production relative to its treated floor area. On the other hand, even in central Europe, it is difficult for even a highly efficient apartment building to achieve energy independence because of the small amount of roof space relative to the treated floor area. It therefore makes sense to assess the energy produced by a building relative to the building's floor or roof area. To complete the balance, this weighted energy production must be compared to energy demand.
In light of the scarcity of renewable resources, energy production as part of a building is desirable, especially since it increases users' identification of the building as part of the necessary changes in overall energy supply. However, the primary consideration must be how to realistically achieve the goal of energy efficiency.
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[Feist (ed.) 2012] Proceedings of the 16th International Passive House Conference in Hanover, Passive House Institute Darmstadt, 2012
[Fouad, Nabil (ed.) 2007] Fouad, Nabil (Ed.): Bauphysikkalender 2007; Ernst&Sohn, Berlin 2007
[GEMIS 4.7] GEMIS Version 4.7: Ökoinstitut, 2011 (www.gemis.de)
[Schnieders 2012a] Schnieders, Jürgen: “Passive Houses in various climate zones – technical and economic aspects.” [Feist (ed.) 2012]