What is a zero energy building?
What does it really mean for an engineer to design a zero energy building?
The current use of the term “zero energy building” does not mean that the building literally has zero energy consumption. In addition to on-site renewable sources, a zero energy building by definition can use energy sources originating from outside the boundary of the building site.
The energy provider in the area will most likely have nonrenewable fuel sources. The times that the on-site renewables cannot match the demand of the ZEB, electricity must be used from the grid. In addition to keeping the electricity load of the building as low as possible, understanding the mix of renewables in the local energy provider’s portfolio is an important piece of the puzzle to keeping CO2 emissions attributable to the ZEB as low as possible.
According to the U.S. Department of Energy, the specific definition of a zero energy building is: “An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.”
If we look at the on-site renewable energy production used for the ZEBs in their entirety, a distributed generation network model emerges. The magnitude of electricity produced by the individual renewable energy technologies located on the zero energy building site will all vary individually based on building demand, solar intensity, wind speed/direction, etc.
When all of these individual sources of electricity production are fed back into the grid, portions of the demand on the large generation and peaker plants are reduced. By decreasing a small portion of the electrical demand on the fossil fuel driven power plants, the zero energy building owner benefits in two ways: lower electricity bills and reduced CO2 emissions.
Without the ability to conduct two-way, coordinated communication between the customer and the utility, exporting electricity to the grid poses some challenges. If looked at in aggregate, when the renewable energy systems that support zero energy buildings generate electricity, there are times when the grid doesn’t need electricity and, conversely, there are times when the electrical demand of the building exceeds the capacity of the renewable energy source, requiring electricity generated by fossil fuel sources.
There are times where it is not productive for the on-site equipment to export electricity to the grid, especially during off-peak times. For example, if the demand on the overall grid is low, exporting electricity can cause reliability problems with the grid.
Conversely, there are operational challenges when on-site production resources can no longer meet the demand of the ZEB. This is most common when photovoltaics are used for electricity production. For example, as the sun begins to disappear below the horizon, the utility experiences a spike in electricity demand. Similarly, wind speed and direction are not 100% predictable, resulting in uneven electricity output. As an example, if excess electricity from wind turbines is exported to the grid during the morning hours, the demand at that time is very low and excessive electricity will be exported to the grid, which can stress the grid.
Clearly there are utility-scale PV and wind generation facilities around the world that are producing electricity and have been for decades. The operational difficulties mentioned above are basic examples of scenarios that might become more pronounced as renewable energy technology becomes more widespread.
Also, these points demonstrate the importance of the smart grid to the continued success and proliferation of renewable energy. In any event, it is clear that the design for a zero energy building does not end at the property line. To have continued success in ZEB development, there needs to be a broader dialog with the utilities, municipalities and building owners.