Learning to Live on Alternative Energy
Three landmark projects show us how to integrate renewable-energy strategies into architecture, without compromising design
One Bryant Park, New York City, 2008
Though already common in industrial applications, combined heat and power [CHP] technology is rarely used in buildings in the U.S., even though it can provide a more efficient and lower greenhouse-gas-emitting alternative to traditional grid-supplied power. But one project that is a CHP pioneer is under construction in Midtown Manhattan and is headed for completion later this year.
Designed by Cook+Fox Architects, and jointly owned by the its primary tenant, the Bank of America, and the developer, the Durst Organization, the 55-story One Bryant Park will have a 4.6-megawatt CHP system. The designers and owners say that the building will be the first high-rise commercial office tower in the country to use this technology at such a scale. The CHP plant will satisfy about one third of One Bryant Park’s peak power demands and will provide for almost 70 percent of its energy needs on an annual basis.
Also known as cogeneration, CHP involves simultaneous production of electricity and useful thermal energy (typically steam) from a single fuel source (often natural gas). At One Bryant Park, the heat produced by its natural-gas-fired turbines will be used to make steam, which in turn will be used to heat the building and the domestic water supply, and to operate an absorption chiller for cooling.
Relying on CHP for much of its energy needs should significantly reduce the carbon emissions of the tower compared to a conventional office building dependent solely on the grid. Part of these savings are due to its distributed energy strategy. The term “distributed energy” refers to a generation source that is an alternative or enhancement of traditional grid-supplied power, located in close proximity to the building it supplies. Such systems can be more efficient than centralized generation since electricity carried over the grid loses 7 to 8 percent of its power in transmission, according to some estimates. However, retaining this electricity is a relatively minor contributor to the efficiency of CHP, since a much larger portion (about two thirds) of the energy generated at traditional power plants escapes through smokestacks. “By preventing transmission loss, CHP does save something on an overall Btu basis,” says Don Winston, Durst director of technical services. “But it is the heat recovery that really makes the system work,” he says.
About 86 gigawatts of CHP capacity are currently operating in the U.S.; however, the vast majority of these facilities are located at industrial sites rather than in individual buildings, according to Richard Sweetser, president of Exergy Partners, a consulting firm based in Herndon, Virginia. Sources say a number of factors make cogeneration a good choice for industrial applications, including a relatively flat demand for energy over the course of the day and through the various seasons. But in buildings, this demand is generally more variable, creating challenges for making the most of a cogeneration system’s thermal output. “If you are sending steam to the roof, CHP doesn’t make [economic] sense,” says Vinnie Galatro, director of technical services for the Fulcrum Group, commissioning agent for the One Bryant Park project.
The heat produced as a by-product of electrical generation will be used to make steam for heating
the building and the domestic water supply, and to operate an absorption chiller for cooling (above).
A thermal energy storage system (below) will help reduce demand during peak hours.
Diagrams courtesy: Fulcrum Group (top); Doyle Partners for Cook+Fox (bottom).
In order to avoid wasting valuable thermal energy, One Bryant Park includes a thermal storage system that will produce ice at night from excess steam. Then, during peak daytime hours, the ice will be used for cooling, resulting in “a nice and even load profile 24 hours a day,” says Galatro. Other challenges with which the One Bryant Park team had to contend included routing natural gas lines through a densely occupied structure, and the isolation of the CHP equipment for noise and vibration. There were also permitting and regulatory hurdles, though New York City officials are working to reduce such barriers to achieve a goal of 800 megawatts of installed clean distributed energy by 2030.
But impediments aside, CHP proponents say that the technology is an economically and environmentally viable alternative to the construction of additional conventional centralized generation capacity. According to Scott Frank, partner at Jaros Baum & Bolles, the project’s mechanical engineer, “generating electricity on-site and using the waste heat just makes sense.” Joann Gonchar, AIA