Although the coronavirus pandemic has dominated recent headlines, climate change hasn’t gone away. Many experts are calling for a “green” economic recovery that directs investments into low-carbon energy sources and technologies.
Buildings account for 40% of total energy use in the U.S., much more than the 32% for industry and 28% for transportation. States and cities with ambitious climate plans are working to reduce emissions from the building sector to zero. This would mean increasing energy efficiency as much as possible to reduce buildings’ energy use, and then supplying remaining energy needs with electricity generated by carbon-free sources.
My colleagues and I are studying the best ways to rapidly reduce carbon emissions from buildings. In recent years, construction designs have advanced greatly. Net-zero-energy buildings, which produce the energy they need on site from renewable sources, are increasingly the default choice. But to speed the transition to zero carbon emissions, the United States must think bigger and concentrate on designing or redeveloping entire communities that are zero energy.
Schemes to deal with energy use in buildings at the district level provide economies of scale. Architects can deploy large heat pumps and other equipment to serve multiple buildings on a staggered schedule throughout the day. Districts that bring homes, places of work, restaurants, recreation centers and other services together also significantly reduce the energy needed for transportation. In my view, this growing movement will play an increasingly important role in helping the U.S. and the world deal with climate change.
Ambient loops heat and cool
Heating and cooling systems are the biggest users of energy in buildings. District-design plans can deal with these loads more efficiently.
District heating has long been used in Europe, as well as on some U.S. college and other campuses. These systems typically have a central plant that burns natural gas to heat water, which then is circulated to other buildings.
To achieve zero carbon emissions, the latest scheme relies on a design known as an ambient temperature loop that simultaneously and efficiently both heats and cools buildings. This technology was first developed for the Whistler Olympic Village in British Columbia.
In a typical ambient loop system, a pump circulates water through an uninsulated pipe network buried below the frost line. At this depth, the soil temperature is near that of the yearly average air temperature for the location in question. As water moves through the pipe, it falls or rises in temperature until it nears this average.
Heat pumps at individual buildings or other points along ambient loops add or extract heat from the loop. They can also move heat between deep geothermal wells and circulating water.
The loop also circulates through a central plant that keeps it in an optimum temperature range for maximum heat-pump performance. The plant can use cooling towers or wastewater to remove heat. It can also add heat using renewable sources, such as solar thermal collectors, renewable fuel or heat pumps powered by renewable electricity.
Putting wastewater to use
One example of a possibly zero-energy district currently being developed, the National Western Center, is a multi-use campus now under construction in Denver to house the annual National Western Stock Show and other public events dealing with food and agriculture.
A 6-foot-diameter pipe carrying the city’s wastewater runs underground through the property before delivering water to a treatment plant. The water temperature stays within a narrow range of 61 to 77 degrees F throughout the year.
The wastewater pipe and a heat exchanger transfer heat to and from an ambient loop circulating water throughout the district. The system provides heat in winter and absorbs heat in the summer using heat-recovery chillers, which are heat pumps that can simultaneously provide heating and cooling. This system serves individual buildings at very high efficiency.
Electricity used to operate the heat pumps, lighting and other equipment will come from on-site photovoltaics and wind- and solar-generated electricity imported from off-site.
Low-energy housing in Austin
Another project meant to hold carbon emissions to a minimum is the Whisper Valley Community, under construction in Austin, Texas. This 2,000-acre multi-use development includes 7,500 all-electric houses, 2 million square feet of commercial space, two schools, and a 600-acre park. Its design has already received a green building award.
Whisper Valley will run on an integrated energy system that includes an extensive ambient loop network heated and cooled by heat pumps and geothermal wells at each house. Every homeowner has the option to include a 5-kilowatt rooftop solar-photovoltaic array to operate the heat pump and energy-efficient appliances, including heat-pump water heaters and inductive stovetops. According to the developer, Whisper Valley’s economy of scale allows for a median sale price $50,000 below that of typical Austin houses.
The future of zero-energy communities
The National Renewable Energy Laboratory, Lawrence Berkeley National Laboratory, and other project partners are developing an open-source software-development kit called URBANopt that provides models elements of zero-energy districts, such as building-efficiency and demand-flexibility schemes, rooftop-photovoltaic arrays, ambient loop-district thermal systems. The software can be integrated into other computer models to aid in the design of zero energy districts. NREL engineers have been working with high-performance district projects throughout the country, such as the National Western Center, to help guide the development of the URBANopt platform.
The projects I’ve described will come in the form of new construction. It’s harder to achieve net zero energy in existing buildings economically, but there are ways to do it. It makes sense to apply efficiency measures that are the cheapest to retrofit, convert building heating and cooling systems to run on electricity and provide thae electricity with solar photovoltaics.
Utilities are increasingly offering time-of-use rate schedules, which charge more for power use during high-demand periods. Emerging home-energy management systems will allow home owners to heat water, charge home batteries and electric vehicles and run other appliances at times when electricity prices are lowest. Whether we’re talking about new or existing buildings, I see zero-energy districts powered by renewable energy as the wave of the future.
Charles Kutscher is the director of the Buildings and Thermal Systems Center at the University of Colorado Boulder