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.

By Sara Lappano, PE, LC, LEED AP BD+C, SmithGroupJJR, Washington, D.C. December 14, 2015

Learning objectives:

  • 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.

NZE in action at Chesapeake Bay Foundation

SmithGroupJJR partnered with the Chesapeake Bay Foundation to design its headquarters, the Philip Merrill Environmental Center, in Annapolis, Md., which went on to be the first U.S. Green Building Council LEED Platinum project in the world when it was completed in 2001. When the foundation decided to build a new 10,000-sq-ft educational and office facility in Hampton Roads, Va., they tasked SmithGroupJJR with designing the most sustainable building possible.

For the client and the design team, that meant pursuing Living Building Challenge certification. This certification requires the building to be net zero energy, water, and wastewater, as well as requiring the team to avoid the use of building materials that contain chemicals listed in the "red list" produced by the ILFI. The site’s location on the Chesapeake Bay helps to further the foundation’s educational mission. The building was completed in early 2015 and is currently in the 12-mo monitoring period to achieve the certification for net zero energy, water, and wastewater. The design team included architects and engineers who collaborated closely during the design process to ensure the building was designed to minimize energy consumption and meet all of the client’s sustainability goals.

A number of passive strategies were successfully implemented early on in the design process. Energy-modeling results of a building orientation study must be considered by the engineering team; this model is an example of how a passive strategy can reduce energy consumption. The natural ventilation strategy for the Brock Environmental Center, which had bidirectional wind patterns, is shown in Figure 3. The building was designed to take advantage of the natural breezes prevalent near the Chesapeake Bay, with windows and even walls designed to open up and take advantage of airflow (see Figure 4). Figure 5 illustrates how the building was designed to maximize diffused daylighting from the north while shielding the building interior from direct sunlight from the south. In all of these cases, extensive computer modeling was used to optimize each of these selections. Even the roof and wall construction were modeled to determine the optimal R-value.

An iterative design process was also used to determine the most energy-efficient active systems to implement on the project. The mechanical system uses a variable-refrigerant flow (VRF) system with geothermal wells. This system is well-suited to natural ventilation because it allows users to operate some spaces in natural-ventilation mode while other spaces can be in mechanical-conditioning mode. Electric lighting was modeled to ensure that target light levels would be provided and with no overlit spaces. In addition, a photosensor dimming-control system was used in almost every space to reduce the electric lighting when sufficient daylight is present. The daylighting optimization combined with the electric lighting controls has resulted in the electric lighting being mainly unused during daytime hours (see Figure 6).

Once the passive and active strategies were optimized, the design team worked to design on-site renewables to offset the anticipated annual electricity consumption of the building. Due to the building’s location, the annual wind speed of 12 mph was high enough to make the use of wind turbines a possibility. Also, the waterside location meant that there were fewer obstructions to slow down the wind speeds from the data the team obtained from the nearby Norfolk, Va., airport.

Solar PV was also of interest to the design team because of the number of established manufacturers and products available as well as the improved predictability of solar insolation at the site compared to wind speeds. After a number of studies and iterations, the design team settled on an on-site generation strategy that relied on wind turbines for one-third of the electricity generation and PV modules for two-thirds of the generation.

The wind-turbine system consists of two 10-kW turbines, each on a 70-ft pole. The turbines were located off the east and west ends of the building, as far away as possible from nearby trees. The sandy soil conditions at the site required a large concrete pad for each turbine, substantially adding to the cost of the wind turbine system.

The PV system designed for the building consists of 141 270-W modules for a total of 40 kWp (kilowatt peak). The design team originally selected a 19%-efficient module, but after working through some cost comparisons with the construction team, it was determined that 16%-efficient modules would result in substantial savings to the project, even with the module quantities being adjusted to account for this lower efficiency, which is a fairly typical module efficiency. Solar-insolation modeling was performed to determine the areas of the roof with the highest insolation values. The PV system was located on the sloped roof with additional space allocated for more modules to be added in the future. Shortly after construction was completed, the client team opted to install 6.5 kW of additional PV modules. Due to the early planning on both the roof system and the electrical system, they were able to be added fairly easily.

The wind turbines and the PV modules were all tied back into the main electrical distribution panel for the building. To aid in troubleshooting any power-consumption issues during occupancy, an extensive metering system was designed. The building automation system (BAS) tracks individual pieces of mechanical and plumbing equipment while electrical meters track the consumption of lighting and plug loads in the building. All of this data is reported back to the BAS, which then generates daily reports on both the consumption and the production of energy in the building. The Chesapeake Bay Foundation also has an online dashboard that provides a real-time view of this data. A schematic electrical riser diagram showing the renewables tied into the building electrical system, as well as the metering points on the electrical system, are illustrated in Figure 7.

On track to net zero

The Brock Center currently is in its seventh month for its net zero certification. As of the end of October, the building has produced 95.7% more electricity than it consumed. The energy modeling forecasted the summer months as being a time period where the building would likely consume more than it produced. Despite this forecast, the building has gone through the summer months being "net positive," which puts it in a strong position to achieve net zero energy certification in early 2016. This is partially due to the additional PV modules installed by the owner; but even before this addition, the PV system was outperforming the estimates made during the design of the building.

For the first several months of metering, the wind turbines produced less energy than forecasted, but it appears that the actual wind speeds early on in the monitoring period were lower than the historic data used by the design team. It is also possible that local conditions at the site cause wind speeds to be lower than the data the design team used from the nearby airport. Due to higher winds in recent months, the turbines have produced 5.4% more energy than predicted since metering started in April. The energy modeling used to predict the building’s power consumption has been very accurate. To date, the actual electrical consumption of the building has been within 2.7% of the predicted consumption.

While this points to a highly accurate energy model, the submetering of individual systems has shown some differences from what was anticipated. For example, the lighting has consumed far less energy than predicted while some mechanical fans have consumed more, as shown in Figure 8 for August. The benefit of this level of detailed metering is that the design team has been actively working with the owner to evaluate the causes behind some equipment exceeding its expected power consumption.

For a project to achieve net zero energy, all of the involved team members need to be actively engaged in reaching that goal. In the case of the Brock Environmental Center, an integrated design team combined with an owner and a construction team who were all committed to creating a project that achieved Living Building Challenge certification setup the project for success from the beginning. The Chesapeake Bay Foundation is already using the Brock Environmental Center as an important teaching tool for the school groups that visit their facility. As the project continues to near the end of its 12-mo monitoring period, both the client and the design team continue to monitor and learn from the project.

Sara Lappano is an electrical engineer and principal in the learning studio at SmithGroupJJR. Sustainability is also a priority for Lappano, and she brings this design sense into all projects with which she is involved, ranging from high-efficiency lighting design to the design of renewable power systems.