Integrating renewable energy into building electrical systems

Electrical engineers should determine whether a building will be connected to the local utility or be independent of the electrical grid.

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

A net zero energy building (NZEB) can potentially be connected to the electric grid, rather than be totally independent of the grid, to take advantage of the benefits of being grid-tied. Buildings that are independent of the grid must rely on battery storage to collect excess power generation for times when the electricity consumption exceeds what the renewables are producing—for example, at night when photovoltaic (PV) modules are not generating power. Battery storage can be quite expensive and requires space to be set aside in the building for the batteries. Battery systems also reduce the overall system efficiency as you introduce a battery charger into the system. The efficiency can be 5% to 10% less. On top of that, an off-grid building needs to design for autonomy, meaning you have to survive your worst months (winter for solar in the Northwest, for example). This will increase your connected solar (or other renewable) system size and/or battery size just to make it through the worst months.

This could be a feasible solution in remote areas that lack nearby utility power. In cases where utility power is available in proximity to the building site, it is generally more cost-effective to design a grid-tied building. In this scenario, when the production of the on-site renewables exceeds the electrical demand of the building, the excess power will flow from the building back into the grid. When the building’s electrical demand exceeds the production of the renewables, the building will draw power from the utility grid. This allows the electrical distribution in the building to be fairly similar to a conventional system, with a few exceptions.

It is important that the utility meter be designed for net metering (also known as bidirectional meters). Standard meters can only advance forward and can’t effectively "spin backward" in instances where power is being sent back to the grid. Also, breakers used to connect a renewable system’s output into the electrical distribution system will experience power flowing "backward" to the utility source. These breakers need to be suitable for backfeed applications and listed for the application.

Since the success of a NZEB design is only really known once the building is in operation for at least 12 months, it is helpful to have the ability to troubleshoot causes of excess or unexpected power consumption. Beyond the required net meter from the utility, it is helpful to have additional submetering within the building to help ensure that the building is operating as efficiently as possible post-occupancy. There should be close coordination between the project’s mechanical and electrical engineers in the design of a submetering system, because the building automation system (BAS) typically has the ability to meter the output and run time of mechanical and plumbing equipment that it is controlling.

In this case, the electrical-submetering strategy can focus mainly on lighting, receptacles, and other miscellaneous equipment. The more detailed the submetering system can be, the more detailed the troubleshooting capabilities will be. However, the cost of submetering may require the engineering team to be more selective about what they meter. If the panelboards are segregated by load so that the lighting, mechanical, plumbing, and receptacles are all fed from separate panelboards, it is possible to design submetering at the panelboard level rather than the branch-circuit level.

Code considerations

Solar PV and wind electric systems are covered in NFPA 70: National Electrical Code (NEC) Articles 690 and 694, respectively. Article 705 covers scenarios where sources of electric power are operating in parallel with the utility, which applies to any installation with on-site renewables. The article contains specific requirements for interconnecting the power-generation systems familiar to many engineers and designers. This article is especially important if renewables are being connected on the load size of the distribution board, which they were in the Chesapeake Bay Foundation Brock Environmental Center in Hampton Roads, Va.

Based on the requirements in Article 705.12, upsizing the bus rating for the main distribution board for the building may be required. Article 690 on PV systems has content that has rapidly changed over recent versions of the code (reference NEC 2014 section 690.12 for new requirements on rapid shutdown). Engineers should study this section thoroughly to understand these special requirements and ensure that they are designing to the version of the code that applies to their local jurisdictions. Key sections to note: System Component and Equipment Listing Requirements (690.4), Ground Fault Protection (690.5), Conductor and Overcurrent Sizing (690.8), and Arc Fault Protection (690.11). 

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.