The University of Texas at Austin Solar Decathlon Houses

Michael Garrison
The School of Architecture


The U.S. Department of Energy, Solar Decathlon is an international intercollegiate competition that challenges each competing student team to design, build, and operate an 800-square foot, totally off the grid solar-powered house on the National Mall in Washington D.C. and to test the house over a twenty-day period of operation. The competition consists of ten contests focusing on ingenuity, energy production, energy-efficiency, design, and thermal comfort. The competition also calls for the design to appeal to the average lifestyle of the general public, with enough solar energy to support all the household power needs including heating, ventilation and air conditioning, lighting, cooking, cleaning, telecommunications, and a computer for home/office use. Into this mix, the competition also called for an electric “car” that can be charged from the home’s photovoltaic system. The Solar Decathlon is designed to stimulate research, industry and education to advance renewable energy technologies, with a specific focus on building-integrated photovoltaic systems. The projects and innovations of student teams serve as catalysts for change, leading the residential housing industry toward more sustainable practices while addressing the need for well-designed, appropriately diverse, economically viable, and environmentally responsible housing. Through use of solar power, the projects offer homeowners the means to directly participate in the energy economy, moving from energy consumers to become energy producers.

Solar Decathlon at the University of Texas at Austin

Interdisciplinary student teams from the University of Texas at Austin have completed in the Solar Decathlon competition three times, first in 2002 and then in 2005 and 2007. Not only has the competition enabled students and faculty to help advance research and practice in sustainable building design in the national spotlight, it has become an important part of UT- Austin’s sustainability curriculum, with increasingly recognized pedagogical value. The competition provides a rare opportunity for students to apply architectural theory through practical, hands-on experience, discovering how buildings really work as they are constructed and occupied.

Beyond Renewable Energy

In assessing the past contests, as process, pedagogy, and product, it becomes clear that the challenges and possibilities of the Solar Decathlon are not solely engaged in questions of renewable energy. In fact, logistics, material assemblies, systems, inhabitation and education become driving concerns that refocus the challenge of sustainability as one of collaborative, environmental, technical and social practice. These sustainability issues are critical to presenting the feasibility of solar energy technologies to the general public, and are closely inter-related. While there is no one single best sustainable building design, experience has proven that by blending design questions with logistics questions and performance questions with social questions, we ultimately turn an environmental challenge into environmental research based upon principles of collaboration—between a homeowner and the home, between a building and its environment, between ecology and economics.

Our investigations suggest that sustainable design technologies offer solutions to the serious emerging challenges of energy efficiency and sustainable development and thereby become a strong force in shaping design. These sustainable design technologies integrate photovoltaic systems, passive solar heating, solar induced ventilation, daylighting, water use efficiency, waste management, smart energy management systems, and other low- entropy open building systems that contribute to “green” architecture. Through the Solar Decathlon competition, the study of building systems also includes the principles, conventions, standards, applications and restrictions associated with the manufacture and use of existing and emerging construction materials and assemblies and their effect on the environment.

2002 UT/CMPBS Solar Decathlon House

photo of 2002 UT Solar Decathlon House in Washington D.C.
Fig. 01. 2002 UT/CMPBS Solar Decathlon House on the National Mall in Washington D.C.

Given the fact that the competition brief for the solar decathlon implied a portable and temporary structure, the 2002 Solar Decathlon House was designed, in collaboration with the Center for Maximum Potential Building Systems (CMPBS), as a “flat-box” kit of parts that was shipped to the site in Washington D.C. and assembled as an 800 square foot solar-powered house in just four days without the use of heavy equipment. The design features a flexible, modular, reusable kit of parts that sits lightly on the land and forms the superstructure around a mobile utility environment. The core of the house is a modified Airstream trailer, which contains a team built prefabricated kitchen, bathroom and laundry facility. The idea of isolating the heat and moisture gain from cooking, showering, washing clothes and dishes, etc, allows the design to compartmentalize these sources and reduce utility runs. In order to provide the remaining functions of the house, a modular and flexible building system was erected around the Airstream trailer in a dogtrot configuration. The Airstream aesthetic is carried throughout the building where, B-Line, structural steel eight-foot grid sections function as the platform and superstructure for open building systems. The basic idea of an open building system is a strategy of designing and producing built environments in which the parts making up the whole are given freedom for layout, construction and adaptation so building components, such as walls and windows, can be added on to as well as subtracted from in order to accommodate the needs of the household.

In order to make the solar decathlon project work effectively with the sun as its sole power source several state of the art sustainable design features were integrated into the kit of parts design. These features included shading, solar induced ventilation, daylighting, Energy Star appliances, equipment and lighting and the use of an evacuated tube solar hot water heating system with a 3.6kW roof mounted photovoltaic (PV) powered system. The University of Texas 2002 Solar Decathlon House is designed to be energy efficient and is a stand- alone system, which does not use electric utility power. PV’s provides direct DC power when sunlight is available. If power is needed when sunlight is not available, batteries will store power for the times when the sun is not shining. These systems work in conjunction with a Hydrosolar hydronic ductless HVAC system that is equipped with an ice battery and tuned to keep the interior at the required competition energy balance comfort zone of between 72˚-76˚F and 45-50% RH during the competition. The 202 University of Texas Solar Decathlon team chose to think beyond the competition requirements of solar power and energy efficiency by embracing the full spectrum of sustainable design. That strategy included resource efficiency and the use of recyclable, recycled, reused, and local underutilized materials.

The 2002 UT/CMPBS solar decathlon house performed well it maintained interior thermal balance comfort conditions during the competition and the team won third place in the design and livability contest and finished eighth overall in the competition against eighteen finalists. Post occupancy analysis on the house found significant infiltration problems with the many joints in the kit of parts assemblage and the 3.6 kW PV system size, although more affordable, was less than half the size of the University of Colorado’s winning entry.

After the 2002 Solar Decathlon Competition the University of Texas at Austin Solar Decathlon House was returned to Austin and re-constructed at the site of the CMPBS, a non-profit sustainable design Center. The Solar Decathlon house serves as learning center, office and guest quarters for the CMPBS. In 2003 a Vital Signs Tool day was held at the Texas Solar Decathlon house at the CMPBS. This Tool Day allowed participants to become familiar with the Solar
Decathlon House and gain hands-on experience measuring aspects of its performance such as photovoltaic power, ice battery cooling, daylighting, thermal comfort, visual comfort, and air movement. The solar house still hosts open house tours and remains a viable learning center for the design and operation of solar powered houses in Austin.

2005 UT Solar Decathlon House

photo of 2005 UT Solar Decathlon House in Washington D.C.
Fig. 02. 2005 UT Solar Decathlon House on the National Mall in Washington D.C.

The 2005 UT Solar Decathlon team combined a 7.8 kW PV power array with a design for ease of transportation and assembly called the SNAP house. The house features four pre-fabricated modules that can be snapped together in order to form a house that would appear more flexible and spacious than typical trailer homes. The team invented an innovative foundation system of rails and rollers that allowed each eight and one half foot wide module to be lowered off a truck using a crane and set onto the rails. The modules were then rolled into place and snapped together with other modules to form the house. By only having four large modules to put together on site the design was developed to minimize on site labor over a kit of parts scheme. The eight and one half foot wide module was sized for the width of a normal truck trailer to provide for ease of transportation and the elimination of special wide load travel permits.

The University of Texas 2005 Solar Decathlon House was limited in size by rules of the competition, which, required that the roof “foot-print” must be less than 800 square feet. With a dual use prescription for both exhibition and inhabitation the solar house design was modified for the 2005 competition to place added emphasis in meeting the livability and space needs of the typical American family. To meet the size limits while providing the modest 1-bedroom/1- bath house with a spacious feel, The University of Texas 2005 Solar Decathlon House utilized three design techniques,
1) The spaces are multi-use, so the bathroom is also the laundry room; the bedroom has a foldaway bed so it can function as an office/ study during the day; and a central main room contains the kitchen, dining and living room in an open plan configuration
2) The main room connects to a patio deck space along the entire south elevation to expand the interior space to the outdoor deck space and
3) The combination of a cathedral ceiling and generous amounts of daylighting enhance the sense of spaciousness and flow in the “not so big” house.

The Energy saving features of the house include a 7.8 kW photovoltaic solar power system, evacuated tube “heat pipe” solar water collectors, a high-efficient ductless HVAC system, and an energy conserving design that achieves a ratio of one ton of air conditioning per 933 square feet of conditioned space. The HVAC system combines a variable-speed inverter compressor mini-split heat pump with an energy recovery ventilator, a separate refrigeration whole-home dehumidifier, and a horizontal direct- drive chilled water DHW/Air Coil heat exchanger. The four components work together to assure a narrow interior comfort zone between 72˚F and 76˚F and a humidity range of between 45 and 50% relative humidity. To control the components the team developed a computer controlled smart building technology that allows the building to be controlled on the National Mall from Austin, Texas. Again in 2005 the house emphasized the use of green building materials.

The 2005 UT Solar Decathlon House performed well during the competition and maintained interior balance comfort conditions using the solar systems despite weeks of continuous rain. The team won a special citation from the National Association of Home Builders, won the BP solar award and finished sixth overall in competition against twenty finalists. Post occupancy analysis revealed that there were still unsatisfactory infiltration and onsite labor requirements associated with the modular snap construction.

After the competition the University transferred the 2005 solar house to the Blackland Community Design Center in the Blackland neighborhood. The Blackland neighborhood is a tri-ethnic neighborhood in East Austin adjacent to the main campus of the University of Texas at Austin. The Blackland CDC, a group that provides housing to low-income families, attached the SNAP house to an existing house in a duplex configuration that allows excess power from the SNAP house to share power with an existing house. The roof of the 670-square-foot SNAP house is covered with 46 photovoltaic panels, a 7.8-kilowatt system that is used to fuel both the solar house and the front bungalow, known as the Harden house in honor of the family that once lived there. One student team member was Stephanie Perrone-Freeborg, a graduate student in the UT School of Architecture. She and two other students organized a group to encourage fellow architecture students to volunteer once a month with nonprofit affordable-housing organizations to connect designers and builders to show how housing impacts a community. Stephanie and her husband, Adam Freeborg, were the solar SNAP unit’s first residents. During the last few years, Stephanie has recorded the duplex’s daily energy and water consumption and tinkered with the energy production systems to make the house as efficient as possible.

2007 UT Solar Decathlon House

photo of 2007 UT Solar Decathlon House in Washington D.C.
Fig. 03. 2007 UT Solar Decathlon House on the National Mall in Washington D.C.

The rules for the 2007 Solar Decathlon competition added a “Market Viability” contest to the contest. In addition to the normal rules and other contests involved in the competition new requirements allowed jurors to assess how well and easily the house could be brought to market. The jurors had to assess whether the house had market appeal, was livable, had a flexible layout and was affordable. To respond to this requirement for a broad-based market appeal the 2007 solar decathlon team adopted an interdisciplinary approach. The 2007 Solar Decathlon team under the direction of the School of Architecture included a diverse partnership of “Faculty Leads” from different disciplines from Engineering to Marketing to Interiors, and with an Advisory Council of business and industry professionals. In collaboration the team developed a 7.9 kW PV array with a skin based design strategy that responds to orientation, climate and culture. The streamlined singlewide trailer envelope design, called the “BLOOMhouse,” is a 14 wide x 50 -foot long open plan interior with moment frames, modular trailer that was determined by shipping dimensions. The house was first, entirely prefabricated in only two months in Austin, and then shipped by truck to Washington D.C. as a single wide load transport and unloaded on the National Mall using a system of hydraulic jacks. On site labor was reduced in this design scheme to the installation of the solar collectors and landscape decking, making the house quite affordable to construct. Because the house is made using lightweight, metal-faced R-30 Structural Insulated Panels (SIPs) as a single envelope, the infiltration load was reduced to less than one half of an air change per hour and an Energy Recovery Ventilator (ERV) was incorporated into the house to insure adequate amounts of fresh air.

The Energy saving features of the house integrate strategically placed shading devices, cross ventilation, daylighting, advanced Energy Star appliances, and fiber-optic and LED lighting prototypes, in collaboration with the use of an evacuated tube solar hot water heating system with a hydronic floor heating system and a 7.9 kW roof mounted photovoltaic (PV) powered system. The excess energy from the PV system is used to charge an electric vehicle to meet the needs of commuters than put as many as 12,000 miles a year on their vehicles. The 2007 Solar Decathlon House performed extremely well during the competition maintaining climatic balance comfort conditions and wining second place overall in engineering design and winning the BP Solar award.

After the competition, in 2008 the BLOOMhouse team traveled to McDonald Observatory in far west Texas in the Davis Mountains and reconstructed the house on the Mount Locke site of the decommissioned Millimeter Wave Telescope. It is now being used for observatory staff housing, and for scientific experiments to improve the home’s energy efficiency. Because of the surplus energy created from the 7.9 kW solar panels, which cover no more space than the roof of this 550 square foot house, it was decided to hook the neighboring house up to the BLOOMhouse as well as one other structure to help balance things out because the house captured so much excess power. This is the best immediate solution, because that the electrical utility grid in this remote region isn’t set up for uploading yet and the house produces way too much electricity to store in the batteries. Information about the house is on display at the Observatory’s Frank N. Bash Visitors Center, as visitors both tour the “Our Star, the Sun” exhibit and safely view the Sun live from a theater in the Frank N. Bash Visitors Center. And in the “Live, from McDonald Observatory” program, kids across Texas learn about the Sun through interactive videoconferences between classrooms and the observatory. The installation of the BLOOMhouse forges one more link in the chain connecting McDonald Observatory to the stars.

Pedagogical Implications

Although the competition is designed to enhance the performance of sustainable building systems including building integrated photovoltaic systems, perhaps the most significant benefit of the competition are the pedagogical implications. The competition requires the students to link architectural theory to practice, the hands on experience of the solar decathlon gave architecture students proper grounding in action and immediate experience and argues in favor of experiential knowledge learning.

This experience allowed students to develop the knowledge of how to apply and test out their ideas and theories on sustainable design. This kind of knowledge is rooted in the realm of values. And these kinds of values and consequences are acquired through the actual building experience. In this way the students are able to evaluate the performance of design decisions. Hands-on learning seeks to re-establish the continuity and interrelationship between the processes of conceiving, making, and using buildings. In architect Samuel Mockbee’s words, “it’s the importance of making and thinking at the same time.”

The “hands-on” process fosters a pedagogical approach that encourages faculty and students to discover how buildings really work as they are constructed and occupied. Through observation, simulation, and data gained by designing and then building the design, students discover lessons on the success and failure of different design approaches. Analysis of the material observed in the field, along with comparisons to values derived by model studies, computer simulation and calculations, gives students an opportunity to assess whether the stated design intent has been achieved and to understand and describe the variety of ways occupants actually experience a building.

This level of understanding involves both disciplinary and interdisciplinary learning. It is this area that the solar decathlon experience is especially potent as a forum in which disciplinary knowledge and interdisciplinary understanding take place.