Integrating renewable power systems into a net zero energy building
Engineers should consider several factors to integrate renewable technologies into electrical systems. Key codes/standards drive the design of renewable power systems. Best practices for achieving net zero energy are illustrated.
- Understand the design strategies to achieve net zero energy.
- Outline the available renewable technologies that can be utilized to achieve net zero energy.
- Illustrate the integration of renewable power into the design of a net zero energy building through the use of a case study.
As more owners want to build facilities that have a minimal or even positive impact on the environment, engineers will encounter the challenge of designing net zero energy buildings (NZEB) with increasing frequency.
There are numerous definitions and meanings of the term "net zero energy," which has led to some confusion on what qualifies as a NZEB. For example, some owners purchase renewable energy credits to offset their own electricity usage. Other owners aim to generate enough on-site electricity to cover all their energy usage, include energy provided by combustibles like natural gas. The International Living Future Institute (ILFI) offers a Net Zero Energy Building Certification, which provides its own definition of net zero energy (NZE). Because the ILFI certification is currently the primary method of certifying NZEBs, their definition will be used throughout.
ILFI requires 100% of a building's energy needs on a net annual basis to be supplied by on-site renewable energy. Under this system, combustible energy sources are not allowed. The term "net annual basis" is an important distinction. This means that the building must produce more electricity than it consumes over a 12-month period, which allows a building to be tied to an electric utility grid rather than requiring on-site battery storage. It's acceptable for the building to sometimes consume more electricity than it produces provided that it demonstrates at the end of a 12-month period that the "net" result is a positive flow of energy back to the grid.
Strategies for reaching NZE
While some people immediately think of the selection and design of on-site renewables when they think of NZEB, the reality is that a number of steps need to be taken before reaching the point of designing a renewable power system. Most projects have limited roof and site areas for renewables. In addition, on-site renewables can be one of the most expensive systems in the building. Therefore, design teams should start the design process by analyzing strategies to reduce the electrical demand in the building.
Successful NZEBs typically have very low energy-use intensities (EUIs), which then makes it possible to design on-site renewables that are capable of offsetting that EUI. Figure 2 shows the overall energy strategy for the Chesapeake Bay Foundation Brock Environmental Center in Hampton Roads, Va. The SmithGroupJJR design team first focused on lowering the building's energy consumption as much as possible before designing renewable energy systems to offset that consumption.
These energy-reduction strategies can be organized into passive and active strategies. Passive strategies include optimizing the building's thermal envelope, building shape and orientation, daylighting, natural ventilation, and exterior shading.
Active strategies focus more on the engineering systems in the building and include high-efficiency mechanical systems, energy-efficient lighting, and controls. Because every kilowatt-hour of electricity consumed translates into additional on-site renewables, design teams should make every effort to reduce the electrical demand of the building.
An area that has gotten more attention recently, partly due to new energy-code requirements, is plug loads. The computer equipment an owner chooses to purchase for their employees can have a substantial impact on the power consumption in a building. Engineers modeled the relative power consumption of different computer workstation setups to help the owner understand the implications of these decisions —from desktop to laptops, with and without Energy Star ratings.
Another plug load concern is a "vampire load," which is equipment that consumes energy even when the device is not in use. Computers and cell phone chargers are common examples of this. In an NZEB, these incremental loads can require a substantial amount of money to be allocated to additional on-site renewables to offset their consumption. Cutting electricity to vampire loads after hours can reduce or eliminate this wasted energy.
Energy modeling plays a critical role in selecting the active and passive strategies that work best for the project and should be an iterative process.
Only after all of these energy-reducing steps are taken should the engineer proceed with the design of an on-site renewable power system. At this point in the process, the energy modeling should have produced an estimate of the annual electricity consumption for the building. This is the target kilowatt-hours that the on-site renewables should be designed to provide.
Selection of on-site renewables
The selection of the on-site renewables used to achieve NZE is dependent on the local climate as well as the building and site characteristics. Some technologies are more suited to large, utility-scale power generation while others are better suited to smaller-scale buildings. Keep in mind that under the ILFI definition of NZEB, no combustibles are permitted, so some renewable technologies such as biomass are not an option. The most common types of technologies used for on-site renewable power are photovoltaics (PV) and wind turbines. Other technologies such as tidal- and hydropower are better suited to utility-scale generation.
The wind turbines used for commercial buildings would be considered small-scale wind turbines. These turbines are categorized by the orientation of the axis of their turbine—horizontal-axis turbines look like "propellers" while vertical-axis turbines have more of an "eggbeater" appearance. Each of these technologies has its pros and cons; horizontal-axis turbines are generally more efficient at converting wind power into electricity. Wind speed can be affected by local terrain and obstructions, making it difficult to predict wind speed at a specific site.
Because it is important to be fairly accurate in predicting the output of renewables for an NZEB design, wind turbines can be somewhat of a risk unless there is accurate wind-speed data specific to the building site. For small-scale turbines to be feasible, the American Wind Energy Association (AWEA) recommends an average annual wind speed of at least 12 mph at the site, which may preclude the use of turbines at many sites. There also are regulatory hurdles to overcome in some areas due to air rights (i.e., possible aircraft interference). While the wind turbines do generate ac power, it is "wild" ac power where the frequency and voltage fluctuate as the turbine speed changes. To convert this into usable power for the building, the wild ac power is converted to dc power and then converted back to stable ac power.
Solar PV modules are more commonly used in NZEBs. With a range of efficiencies and module types, there are many choices available when selecting a PV system. The output of a PV system can be predicted more easily than a system that uses wind turbines through the use of solar-insolation data for the area where the site is located. While the wind speed can vary greatly from site to site, solar insolation is much more consistent. PV modules generate dc power and because most buildings operate on ac power (particularly if they are grid-tied), the dc output from the modules must be converted to ac power through inverters.
With both types of technologies, it is important to factor in the overall system efficiency when calculating the anticipated power output of renewable technologies. The conversion between dc and ac power results in efficiency losses, as well as wiring losses and degradation over time. While the NZEB certification only requires a single 12-mo period of metering, if an owner wants to remain net zero over the lifespan of the building, the renewable system should be designed to factor in the decrease in output as the system ages over time.