Learning Objectives - After this course, you should be able to:
- Discuss the four definitions of net-zero energy
- Understand the basic steps to achieving net-zero
- Explain the impact of climate and building types in net-zero energy buildings
- Discuss the basic strategies for making net-zero buildings affordable
The concept of net-zero buildings has set off a spark of interest and imagination in the green building sector. Policies such as California's ambitious goal of requiring new construction to be net-zero energy by 2020 are helping push the idea mainstream. Since the development of net-zero buildings is jumping from infancy to adolescence, several important questions arise: What's the best path for achieving zero energy, does it make sense for all buildings, and what are the costs?
Stephen Selkowitz, head of building technologies at Lawrence Berkeley Labs, uses the history of the refrigerator as analogous to the path to net-zero buildings. In the early 1970s, refrigerators used 80 percent more energy and cost more than they do today. What turned the tide for the antiquated appliance? The energy crisis, public awareness, and subsequent policy shifts mandating more efficiency set the stage. Eventually, designers and engineers found themselves having to think outside the (ice) box to find new designs and technology to save energy-and the fridge has never been the same.
Defining Net-Zero Energy
Most net-zero energy buildings are grid-connected and net-metered. When a net-metered building draws power from the grid, the meter runs forward; when the building generates power the meter runs in reverse. If the sum of the positive and negative flow of energy is equal over the course of a year the building is considered net-zero. DOE's National Renewable Energy Laboratory (NREL) lists four definitions:
- A net-zero site energy building is the most common definition. In a nutshell it means a building generates, on its site or building footprint, at least as much energy as it uses.Â Â
- A net zero source building produces as much energy as it consumes when compared to the energy used to generate and deliver the energy to the site from a power plant. This definition factors in aspects such as transmission loss and the energy used to get fuel to the power plant.Â Â
- Net-zero cost buildings is a definition close to the heart of owners. Here, the net-zero balance is based on the cost of energy purchased from a utility versus the income paid by the utility for energy exported to the grid. Utilities generally charge more than they pay for power.
- A net-zero emissions building generates at least as much renewable energy as it consumes from non-renewable sources.Â For example, if a building uses power from a non-renewable source such as a coal-fired power plant, it must produce or purchase at least as much power from renewable sources such as wind or hydro-electric.
Illustration: Simon Page