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A rendered model of the team's home for the 2023 Build Challenge.

University of Colorado Boulder

Team Name: Spruce Canopy

Build Location: Boulder, Colorado

The University of Colorado Boulder is building a highly efficient and resilient 670 ft2, carriage-style home for an affordable housing community in north Boulder.

Project Summary

The University of Colorado Boulder’s “The Canopy”, located in Boulder, CO, is an affordable and energy independent home design with an effort to move faster to decarbonize our buildings past Net-Zero Energy. The project started with a partnership with Habitat for Humanity and the City of Boulder to take their base design and make it Net-Zero Energy without breaking the budget. Through the process we focused on a high-performance envelope that was low in embodied carbon, easy to construct with volunteer labor, and repeatable.

With the increasing energy inequality especially for low-income families, we wanted to build a home with diversified energy storage that was resilient to changes in energy prices, extreme weather events, and government policies. The home will be one of the first grid-connected homes to produce, store, and use “Green Hydrogen” to power the home energy demand without the need for an electric grid or intensive Li-Ion Batteries. In doing so the home will save approximately 4.5 metric tons of carbon dioxide, the equivalency of removing a 24-mpg car off the road each year.

Design Philosophy

As we transition our energy needs away from fuel towards electricity, a global shift of how we interact and use energy is needed. At the same time, energy volatility, extreme weather events, and general unrest are broadening the gap between affordable housing and green energy equality.

At the crux of the problem is the mismatch between affordable construction and net-zero energy with the building and homeowner having different incentives. Although practical, traditional net-zero energy relies on the grid as an infinite battery.

Consolidating our energy needs to electricity, combined with the intermittent of renewables, requires significant upgrades and solutions and has adverse effects on grid operators. They must ramp up and slow down power generation on the grid suddenly to accommodate the mismatch between supply and demand.

Our solution to this challenge is energy storage in the form of Green Hydrogen and Thermal Storage. During the day, when the sun is shining, a rooftop solar array provides power to meet the home's energy demand. When there is excess energy, instead of sending it to the grid or storing it in a battery, our energy management system can produce green hydrogen, store it in our thermal battery for later use in heating water, or run our heat pump to charge PCM (Phase Change Material).

To meet the electrical needs of a home, we use a Hydrogen Energy System. This system converts a small amount of water into hydrogen and oxygen through electrolysis. The hydrogen is then safely stored in low compression tanks outside the home, while the oxygen is released harmlessly into the atmosphere.

At night, when there is no power from the solar system, the Hydrogen Energy System provides power to the home through a fuel cell. The fuel cell combines hydrogen with oxygen from the air to create water vapor, which in turn generates electrical energy. Hydrogen provides a unique benefit in that it allows up to ~56kWh of seasonal energy storage, which can help dampen the excess energy needed in the winter. This equates to approximately 5 days of the home's energy demand.

To meet the demand for hot water, we use a PCM Thermal Battery, which stores heat instead of electricity. Excess energy from our solar system can be used to melt PCM into a liquid, storing compact latent heat that can be used later to heat water on demand. The thermal battery isone-third the size of its equivalent water tank, which allows it to be placed within the living space next to where it is needed, reducing the time, water, and energy typically wasted. Furthermore, its compact size provides high energy density, allowing us to store excess energy for when there is no sun.

Finally, we store around ~17kwh or 7 hours of our average heating load within our walls in the form of PCM. Metallic sheets of aqueous salt PCM is placed behind our drywall. The PCM helps to shift the heating and cooling load and dampen the peaks when there are drastic temperature changes. Rather than just resisting heat flux through the wall, it prevents and stores that energy to be used later. We wanted to build a home that pushed the bounds of what it meant to be sustainable, what it means to be affordable, and past net zero energy towards energy independence.


    Market Analysis

    This project is related to the redevelopment of the Ponderosa Mobile Home Park. The community was first settled in the 1950s. Since the 1990s, the city has been trying to purchase the park to prevent the development of luxury developments. In 2017 the park was purchased, and the city has invested more than $3 million dollars to upgrade the utility infrastructure. Through a partnership with Habitat for Humanity, the community of 48 mobile homes will be redeveloped and kept affordable.

    One of the main differences of the projects is financing, in which the city is planning to front more than $3 million to pay for zero interest loans. This new model is something that the city sees being expanded across the city to help manage the affordable housing crisis.

    The ponderosa community houses are some of the last affordable locations within Boulder. According to a survey conducted by CU Boulder students, most of the residents fall in the federal government’s definition of “extremely low-income”, earning less than $26,000. The new homes will be deed-restricted towards families earning 60% AMI (Area Median Income) or about $53,000 for an individual. In order keep the cost of the home affordable, the city plans for residents to only pay 30% of their income on the home.

    Key Features:

    • Flood resilient
    • Hydrogen home energy system
    • Replicable sustainable carriage style design
    • Prefabricated wall systems.