What is Net Zero Energy Design?

“Net zero energy” means the total amount of energy used yearly by a building is equal to or less than the amount of energy created onsite through the use of innovative technologies and renewable power generation. Achieving this goal will allow for the reduction of energy, waste, and water* consumption to almost zero and greatly reduce the carbon footprint, with the intent of putting excess power back on the grid.

There are four industry-recognized definitions of net zero buildings. The most rigorous is known as “net zero site energy,” which requires a building to capture as much energy within the footprint of the project site as it uses onsite over the course of a year. This is the definition adopted by the AGU project design team.

Net Zero Energy Building (NZEB) Certification is awarded by the International Living Future Institute and is based around one central requirement: 100 percent of the project’s energy needs must be supplied by on-site renewable energy on a net annual basis, without the use of on-site combustion. Buildings must also meet an additional list of rigorous performance standards over a minimum of 12 months of continuous occupancy. Net zero energy buildings consequently contribute less greenhouse gas to the atmosphere than non-NZE buildings.

The District of Columbia’s 2012 sustainability plan calls to reduce greenhouse gas emissions by 50 percent and to plan for climate change impacts by retrofitting 100 percent of existing commercial and multifamily buildings to achieve net zero energy standards by 2032.

*Due to stringent federal and local water regulations, this project will not be pursuing net zero water strategies although the proper technology for on-site potable water treatment now exists.

Want to know how design strategies could work together and be applied to engineering principles to create a net zero energy effect? So did we.

Project Design Features

To achieve net zero energy, AGU’s design focused on four key engineering principles—reduction, reclamation, absorption, and generation. Within these four principles, more than 50 different strategies were individually researched and tested, with the project team ultimately selecting the key 24 strategies that would take us to net zero energy. Here are a few of the most unique strategies that will be a part of our net zero energy building:

Solar Photovoltaic Array

Rendering of the rooftop solar PV array

Photovoltaics (PV) is the name of the method that converts solar energy into direct current electricity. A solar PV array generates electricity from the sun at the time of day when energy is most expensive to purchase and is at peak demand. In turn, it decreases the carbon footprint of the building and increases the revenue stream by lowering utility bills. PV arrays tied to the electrical grid are now a standard design feature for net zero energy construction.

After reviewing all options for on-site renewable energy, including vertical wind turbines and solar concentrators, solar PV energy made the most sense and was the strategy chosen for the concept design.

To make the most of the available sunlight, our building will have more than 700 PV panels laid out horizontally above the penthouse roof and 24 south-facing panels installed along a vertical surface. Altogether, there will be 13,200 square feet of PV panels elevated above the 12,130-square-foot roof. The 250-kilowatt PV arrays could generate up to 100 percent of the building’s total energy in combination with other net zero design strategies—the equivalent of 20 percent of the current building’s total energy.

Example of a radiant ceiling system.Radiant Cooling System

By circulating chilled water through a network of pipes installed in the ceiling’s 2-by-2 or 2-by-4 panels, the hydronic cooling system will help maintain spaces at even, comfortable temperatures using less energy than a traditional forced-air system. The cooled ceiling surface panels will evenly absorb heat energy transferred from people, lights, and equipment. Heat and ventilation air will be provided by a dedicated outdoor air system (DOAS) that will work in conjunction with the radiant cooling system for ventilation, pressurization, and humidity control.

Example of a green wall.Hydroponic Phytoremediation Wall System, or Green Wall

A green wall works with a building’s HVAC system to reduce energy loads and improve indoor air quality. Typically, buildings are ventilated with air sourced from outside and must be heated in the winter and cooled in the summer, which can represent more than 30 percent of the energy consumed by a building.

The green wall is an engineered system which will allow indoor air that is already at the right temperature and humidity to be circulated through the root system of live plants where it will be cleaned and filtered of carbon dioxide before passing back into the building, providing a large energy cost savings. Because the plants’ roots are submerged in water, instead of being buried in soil, the plants’ air-cleaning capacity increases by 200 to 300 percent and is extremely effective at removing large particles and oxygenating the air. Sampling of the outside air quality will help determine what types of plants will need to be selected to address the air pollutants found.

Example of a direct current electrified grid in the ceiling.Direct Current Electrified Grid

Electricity is primarily transmitted from utility companies to homes and businesses by alternating current (AC) as it is the best way for electricity to travel long distances. However, many devices—LED light bulbs, computers, printers, phone chargers, and kitchen appliances—run on direct current (DC). This requires a converter at the end of the electrical cord to bring the higher AC voltage down to DC. This conversion accounts for an energy efficiency loss of nearly 20 percent.

By installing 2-by-2 DC electrified grids in the ceiling, the conversion process will become obsolete allowing the building to be more energy efficient and use DC power generated by the solar PV array. A controller mounted in the ceiling will constantly monitor the power supply to determine if there is an adequate supply of DC power being generated by the solar PV array. On cloudy days, and during evening hours, the power supply to the ceiling grid will automatically shift to the city’s AC power supply as necessary. This technology will also allow the building to have limited functionality during a catastrophic failure of the city’s electric grid.

Water Reclamation Cistern

Schematic of the Storm Water Capture & Reuse DiagramWater reclamation is an effective way to reduce water and sewer utility bills by reusing rainwater before it is discharged to the combined storm and sanitary sewer system. In addition, changes to the District’s water codes now require new construction to capture the first 1.2 inches of rain on-site before it runs off to help control the peak flow during a storm, and to protect the Anacostia and Potomac Rivers.

A water reclamation cistern will collect rainwater from the roof, as well as condensate water from the dedicated outdoor air system, and will filter, chemically treat, and condition the water. Through this process, the building will be able to produce all the water needed for flushing toilets and irrigating onsite.

Municipal Sewer Heat Exchange System

- Rendering of the Huber Heat Exchanger for the Sewer Heat Exchange System.

A municipal sewer heat exchange system is an innovative way to recover thermal energy from wastewater. Close beneath the street on Florida Avenue runs a large combined storm and sanitary sewer line built in the 1890s. The municipal sewer heat exchange system will tap into the sewer line and divert wastewater to a settling tank located outside of the building. Water from the settling tank will then be circulated inside the building to an exchange system that will extract energy from the water for heating and cooling before the water is returned to the sanitary sewer system.

During periods of the year when the sewer temperatures are cool, the building’s radiant cooling system will operate in “free cooling” mode using the water from the sewer heat exchange system allowing the building’s chiller to be turned off. This eliminates the need for a cooling tower on the roof and a substantial quantity of fresh potable water is saved. The noise and unsightly plumes usually associated with cooling towers is also eliminated.