Zero Energy Buildings: An Exploration
Upon hearing the terms "zero energy" or "net zero", one usually assumes that the amount of energy used by a house/building is equal to or less than the amount of energy produced by that house/building.
However, it is not quite that simple. Is that the amount of energy used at the site or does it include the power plant? Does the figure only refer to electricity or does it include other fuel sources as well? Is the label reassessed every year or is it a permanent title? Must the renewable energy be generated within the building's footprint or within the entire property?
These nuanced questions certainly make it difficult to pin down the scope of zero energy buildings. However, four simple definitions help elucidate these answers and further categorize homes and commercial buildings alike:
1. Net zero source energy building
2. Net zero site energy building
3. Net zero energy cost building
4. Net zero energy emissions building
Let's delve into the basics of each definition and its applications:
A net zero source energy building, according to the Department of Energy (DOE) and the National Renewable Energy Lab (NREL), refers to a building that produces as much energy as it consumes in relation to the total energy content from the energy source. As the most robust definition, it includes all energy used by the building, the transmission system, the power plant, and the amount of energy required to bring the fuel source to the power plant. While all-encompassing, this type of zero energy building is difficult to define due to challenges from "site-to-source" conversions. It also relies on the purchasing and buying power of the utility, reducing the importance of the energy efficiency at the site.
Unlike a net zero source energy building, a net zero site energy building produces as much energy as consumed only at the site. Due to the simplicity of this definition, it is the most widely used and can be measured easily with meters and other devices. By omitting the complexities of the grid, it allows engineers and architects to focus on energy-efficient design and technologies at the site.
The most valuable definition to home and commercial property owners who care about the bottom line is a net zero energy cost building. As you probably guessed, this refers to a building that returns energy to the grid for the same amount that it costs to purchase the energy at the site. Although this can be easily verified on utility bills, it can be hard to achieve given utility rate structures. Most rate structures allow for energy to be returned to the grid but not in excess of that building's use -- making it nearly impossible to cover fixed and demand charges.
The final definition, a net zero emissions building, refers to a building that produces zero emissions from the consumption of energy. Like the net zero source definition, this can be extremely difficult to measure due to the uncertainties of off-site emissions.
Having outlined the different types of zero energy buildings, one might wonder: how do we get there? Typically, for both residential and commercial buildings, engineers begin with technologies that decrease demand and limit inefficiencies. These can include: cutting edge lighting fixtures, geothermal heating and cooling, photovoltaics (PV), ground source heat pumps, daylighting, wind (in larger sites), and natural ventilation techniques. PV seems to be the preferred renewable energy source given its flexibility and low demand for space. Conversely, wind turbines are limited by site size, and biofuels, like wood chips, must be brought to the site.
Many homes and commercial buildings across the country have embraced the zero energy building concept. As technologies improve and costs for renewables decrease, it is becoming easier and easier to accomplish. The Department of Energy (DOE) has set a goal for all new construction commercial buildings to be zero net energy by 2030 and all existing commercial buildings to be zero net energy by 2050. Although this transformation has proved to be difficult for larger buildings (given smaller roof size for PV), we have already seen an increase among commercial buildings including, a few highlighted by the DOE. Residential zero energy homes have popped up in bundles, and the National Institute of Standards and Technology (NIST) has even designed a simulation home (video!) to suggest that zero energy is both technologically and economically feasible.