1. Introduction
Buildings consume about one-third of world’s energy. In the US, buildings consume 39% of the energy and 68% of the electricity; they generate 38% of the carbon dioxide, 49% of the sulfur dioxide and 25% of the nitrogen oxides found in the air. Owing to the energy crisis, increased emissions of wastes and the depletion of fossil fuels, research and development in building technologies and integrated processes has attained greater and renewed interest among stakeholders worldwide. Such novel building technologies are the culmination of several decades of research, development and practice of building design, construction and materials technology.
Nevertheless, it is critical to assess a building and its sub-systems before it is put in place. One way of measuring building performance is using performance indices and definitions. One of the widely used definitions is “Net Zero Energy.”
Net Zero Energy definitions are still in the early phase of development as new knowledge is drawn upon to revise and classify buildings. NZE can be defined based on boundaries determined by energy flow and renewable supply options. While energy flow based NZE definitions are determined by means of segregating the boundaries of energy consumption and generation (e.g., at the site or source levels), and their quantification (i.e., energy quantity or energy costs), the renewable supply options based NZE definitions are established by way of demand-side location of onsite renewable capacities. These improvements can be derived from the buildings’ energy consumption and/or generation [1], they can be categorized as Net Zero Site Energy, Net Zero Source Energy, Net Zero Energy Costs and Net Zero Energy Emissions. On the other hand, demand-side renewable supply options based NZE definitions [2] such as “onsite supply options,” and “off-site supply options” offer definitions based on the location of the site of the renewable contributions.
The notion that raw materials for building construction are plentiful and can be extracted “at will” from Earth’s geobiosphere, and that these materials do not undergo any degradation or related deterioration in energy performance while in use is alarming and entirely inaccurate. It must be acknowledged that only a finite mass of material resource exists irrespective of the multitude of transformations needed to make a product, and that entropic degradation of such products is inevitable. For these reasons, a particular building, like an organism or an ecosystem must seek self-sustenance to prevail in competition with other building designs in a time with limited availability of energy and materials. Self-organization of systems to maximize useful power is the key to self-sustenance. To this extent, NZE buildings achieve a net annual operating energy balance. However, approaching an NZE building goal based on current definitions is flawed for the following reasons –
(a)
NZE definitions only deal with operating energy quantities and related emissions.
NZE definitions deal with operating energy quantities and related emissions and do not include all other energy inflows required for the particular building to exist, e.g., the energy required for building manufacturing, maintenance, etc., In current NZE practice, this vast quantity of energy is unaccounted for and ignored for simplification purposes and perhaps also because up to this time there has not been a way to efficiently and accurately quantify these requirements in a uniform manner. In addition, current definitions and calculations for Net Zero Energy do not include the energy flows from the sun, wind, rain, geological cycles and so-forth from the beginning and by including them using the emergy methodology, we demonstrate how a complete energy and material balance for buildings can be quantified.
(b)
NZE definitions do not establish an “energy threshold” which ensures that buildings are optimized for reduced consumption of resources before renewable systems are integrated to obtain an energy balance.
Current NZE definitions are at a level that is particularly generic and does not provide information on the desired “energy threshold” to optimize building energy consumption prior to renewable system integration. For example, a building can attain NZE status by way of surplus renewable energy generation without optimizing its building energy consumption as can be noted in several of the current NZE projects. Such an approach defeats the goal of NZE and may not fulfill the larger objective of energy efficiency.
More importantly, for a building design strategy that aims to contribute to the larger goal of global sustainability, it must be acknowledged that a building relies on inputs from and outputs to the geobiosphere for its very existence. Current definitions and calculations of net energy do not include the energy flows from the sun, wind, rain, geological cycles, and so-forth from the beginning. Therefore, using NZE defini