The power of zero: How to build a real-world Net Zero office

zero net energy building DPR construction Phoenix
DPR Construction
An exterior view of DPR Construction's Phoenix office, which is certified Zero Net Energy.

The following is an excerpt from "The Power of Zero: Learning from the World's Leading Net Zero Energy Buildings," published by the International Living Future Institute.

DPR Construction’s Phoenix Regional Headquarters creates a space that exemplifies deep sustainability in the hot dry climate, on a site that provides revitalization and adaptive reuse.

The office is ILFI Net Zero Energy and LEED Platinum certified. DPR Construction is a technical general contractor building throughout the United States, including the Southwest and California, and is providing an example of world-class caliber net zero leadership through its offices.

In addition to Phoenix, DPR has also built regional headquarters in San Diego and San Francisco that are seeking Net Zero Energy certification.

DPR’s goal for the Phoenix office was to not only produce more energy on-site than it consumed, but to reimagine the open office environment for over 40 employees with passive and active cooling scenarios, including 87 operable windows, a 87-foot long, 13 feet high, zinc clad solar chimney and a 79 kWh PV solar array on the adjacent parking lot.

The project had a tight timeline of just 10 months to complete the office, which opened in October 2011 at a total cost of $4,571,280, not including soft costs or the site and existing building. This building is an outstanding example of thoughtful, constructor-driven pragmatism that resulted in a highly cost effective, very compelling building with a nuanced approach to achieving net zero energy usage.

Context and climate

The site was first developed in 1964 as a paint store, which later became a windowless adult bookstore. The property was vacant for a period of time before it was purchased by DPR.

While much of the space was built as a parking lot, some of that area was converted to host native plants in a green-screened courtyard. The climate of Phoenix is hot and arid, with little rainfall and temperatures exceeding 110 degrees Fahrenheit through the summer and dropping to freezing in the cooler months.

Sunshine and warmth are abundant but must also be moderated to keep spaces comfortable at the higher ranges.

An overview of the financing for DPR Construction's Net Zero Energy office.

Rather than fight the harsh climate, DPR Construction aimed to embrace the desert challenges and create accommodating functional design. Obviously, the primary energy use of the DPR Phoenix building is cooling. Phoenix has the hottest average temperature of any major city in the United States.

Rather than simply throwing a large mechanical system at the building, the DPR team took a more subtle approach that recognized different needs of the building during different seasons and time of day.

The result is a design that combines passive, simple mechanical and more complex mechanical systems that stage appropriately.

In particular, the building uses both stack effect and evaporative cooling, and fast air movement to regulate internal temperature and comfort. This approach also is more durable — the simpler systems will have a longer useful life, whereas a large heat pump driven system may last for only 15-20 years.

Design elements

Exterior: Retaining the original structure, the previous building’s concrete, wood and steel frame were left intact, and over 93 percent of the original framing was retained.

Window and door openings in the existing shell were created, with west and south facing walls having no windows, and the east- and north-facing walls featuring 87 operable windows and three roll-up doors to maximize lighting and minimize solar gain.

The concrete exterior walls are insulated with 6 inches of batt and additional gypsum board. The plywood roof features several inches of foam insulation with both metal and wood framing in the building’s interior.

Greenery, indoor-outdoor space and energy-saving "evaporative cooling" towers at DPR Construction's Phoenix office.

Heating and cooling: The space retains cool temperatures during workhours by utilizing a combination of active and passive cooling solutions.

The first stage of cooling, used when outside temperatures are in the mid-80s, combines a number of features. Temperature actuated windows open and close depending on outdoor temperatures. During cool Phoenix nights, windows open automatically to capture this air and store it until daytime. The changing rhythms of the windows are an interesting feature for the occupants because they exemplify a building model that constantly adapts to light, temperature and conditions.

An elevated plinth that runs the length of the building, sitting atop the roof, provides enough vertical separation to allow stack effect ventilation, which draws air up and out through the building. Twelve oversize, large fans strategically placed throughout the office facilitate air movement as a significant and additional aspect of the cooling strategy for the open office.

The fans also play a vital role in the night flush of air, part of the passive cooling strategies in the building. Finally, the building uses its innovative "shower towers" to further cool the building through very simple mechanical means. Active mechanical cooling in the form of high efficiency compressor-based air conditioners do provide peak cooling, but because the building uses very low energy use elements to cool the building in moderately hot periods, the overall cooling load is dramatically reduced.

Lighting: Daylight in the sunny southwest is an abundant and free resource that DPR Construction sought to harness for their facility. With ample daylight also comes significant potential solar thermal gain, so moderating that gain was crucial to maintaining a comfortable space.

Solatube skylights are placed throughout the building, in addition to the operable windows that facilitate changing light conditions through the day and the seasons. The Solatube skylights use a fresnel lens-based design that refracts light coming into the tube on the roof as well as distributing inside the space.

High efficiency, artificial LED lighting placed within the building is used as a backup light source for work spaces. In practice, lighting is rarely used, except for task lights — a noted characteristic of most well daylit buildings.

Plug loads: Plug loads are a significant factor in daily building electricity use, and several measures were taken to encourage energy conservation.

A "vampire shut-off switch" is connected to about 95 percent of the noncritical systems, or "phantom" outlets, which at end-of-day is controlled by the occupants. This "vampire" switch (a large red button) is popular with the staff, and has resulted in a 37 percent reduction in plug load energy consumption.

A look at Energy Use Instensity (EUI) at DPR's new office.

A Lucid Building Dashboard monitors the energy generation and usage in real time for the building, providing ongoing feedback to the workers about their net zero energy performance.

Renewable energy: The office has a 78.9 kW-DC photovoltaic array and solar thermal hot water system. The photovoltaic array is connected to the grid via the local utility, Arizona Public Service, and is composed of 366 Kyocera modules. Arizona Public Service provided net metering services, paying DPR with excess credit for power generated over the site’s use.

The PV array is mounted on the parking lot canopies, with the first configuration of panels in a cascading array and the second in continuous rows. Both configurations are at a 10 degree tilt and the arrays are positioned on the north and east facing parking lots, clear of shading from the nearby building.

DPR made use of the parking canopies to provide shade to the parking lot and to utilize a a large swath of space. This also freed up room for solar tube skylights on the actual building.