Project Credits

Project NexusHaus 
Location On display at Solar Decathlon 2015, Irvine, Calif./Installed on site in Austin, Texas, after competition
Designer/Builder Students from University of Texas at Austin/Technische Universitat Munchen, Munich, Germany 
Size 840 square feet
Top water-saving technologies Aquaponic garden, rainwater reuse, real time water monitoring

Although the Department of Energy's Solar Decathlon competition is focused on renewable energy, one of the teams chosen to build a house for this fall's event will take resource conservation a step further.

The University of Texas at Austin, partnering with Germany's Technische Universitat Munchen, will construct a home that is net zero usage for energy as well as water. The team's NexusHaus, which will be on display during the competition held Oct. 8 to 18 in Irvine, Calif., will be one of the country's first water-independent dwellings, and its design could have major ramifications for home builders in drought-prone areas and beyond. Its concepts have takeaways for builders across the country as more jurisdictions allow measures such as potable rainwater reuse and graywater recycling.

NexusHaus: A Modular Design Addressing Density

The student-led team was adamant that water conservation be integral to the design of the house, as Central Texas' drought could soon reach the same level of urgency as the one in California, fueled in part by an influx of new residents. The state's population will double over the next 50 years but available water supply is projected to decline by 10%, according to the Texas Water Development Board.

Because of the looming water crisis, the first step for the 70-plus-member team was to ensure that water in the home be used as efficiently as possible, with a goal of 25 gallons per person per day. (A traditional home uses about 100 gallons per person each day.) The team speced the most efficient fixtures and appliances they could find, including a Bloomberg dishwasher that uses 4 gallons of water per cycle and Equator clothes washer that uses 9.5. "We knew that just as in energy where you have to be as efficient as you can before you get to net-zero, we had to be as water efficient as possible before we could think about being zero water," says co-captain Charles Upshaw.

Water from the home's bathroom sink, shower, and clothes washer will be filtered and re-used in a drip irrigation system, providing up to 75 gallons a day of graywater for maintaining the landscaping. Not even condensate from the HVAC system will go to waste; it will be used to provide the small amount of water required to top off the home's unique aquaponic grow beds, which will sustain vegetable gardens and edible fish such as tilapia (see diagram below).

The aquaponic system built with local firm Ten Acre Organics encompasses two 16-foot-long plant beds and a tank for fish such as tilapia and catfish. Harmless bacteria help keep the interconnected system clean with very little maintenance. This approach uses 90% less water than traditional food production, says co-captain Charles Upshaw.
The aquaponic system built with local firm Ten Acre Organics encompasses two 16-foot-long plant beds and a tank for fish such as tilapia and catfish. Harmless bacteria help keep the interconnected system clean with very little maintenance. This approach uses 90% less water than traditional food production, says co-captain Charles Upshaw.


The home will capture enough rainwater to supply all of its potable water needs, although it will be connected to the municipal water supply for backup during long dry spells. Rainwater will hit the 2,000-square-foot canopy between the home's two modules and flow down a system of gutters into under-deck bladder tanks that can hold up to 5,000 gallons. Before being used in the house for drinking, bathing, and clothes and dish washing, the rainwater will be run through a two-state filtration system: a carbon filter for particulates and a UV light filter to treat it to the National Sanitation Foundation's Standard 61 for potable water.

Rainwater will also fill a corrugated metal storage tank on the side of the house that in summer is used as a thermal storage unit for the hydronic cooling system. It shifts cooling from daytime to low-demand nighttime hours, reducing the load on energy systems, a common method for commercial buildings. Upshaw estimates that the technology will reduce peak energy consumption by about 80%.

A key element in the fight against water waste is to show Americans how much of the resource they consume. Utility bills provide a monthly snapshot of water usage, but the NexusHaus team wanted residents to be able to access detailed data in real time. To that end, a student-designed monitoring system (made in conjunction with Austin-based Silicon Labs and local research nonprofit Pecan Street) will provide minute-by-minute feedback on the amount of water used.

Water is just one set of data that a homeowner can access from the three floor-to-ceiling touchscreens the team calls Sense Bars (see graphic at right). Other metrics include indoor temperature, humidity control, and energy usage. Project organizers are hoping to use the customized system to monitor the home's operation for two years post-occupancy, says University of Texas/Austin faculty co-adviser Petra Liedl.

The team's innovative approach is not a one-off; in fact, it's designed to be replicated many times over, says Upshaw. Once the competition is finished and the house arrives back in Texas, the students envision it spawning ideas for Austin's alley flat initiative that provides affordable accessory dwelling units in the city's backyards.