The cogen facility - about the size of a tractor trailer - will save BMC about $1.5 million in heat and energy costs annually. Traditional power plants, which release excess heat into the atmosphere, operate at about 35 percent efficiency. Cogen technology, which instead traps and reuses the heat, operates at 70 percent efficiency.

"BMC can make a strong case for being the greenest and most resilient hospital in Boston," said hospital CEO and President Kate Walsh. "For us, increasing efficiency and resiliency makes financial and operational sense. Cogen will save about $1.5 million in energy and heating costs, which are resources we can spend on patient care, instead of utility bills."

BMC is the only major teaching hospital in Massachusetts that currently has "black start" capability, meaning that if the electric grid goes down, the hospital can restart the cogen plant and heat and power its inpatient units on an "island" for months at a time, as long as it has a supply of natural gas. The cogen plant, which became operational this spring, is also located on the roof of the Yawkey Ambulatory Care Center, high above any potential floodwaters.

"We've learned lessons from Hurricane Katrina and Sandy, which devastated the health care infrastructure in their communities," said Bob Biggio, Senior Vice President for Facilities and Support Services. "Hospitals that had cogen were able to stay open and care for patients, while those without cogen were forced to evacuate. As the largest safety-net hospital and biggest trauma center in New England, we have an obligation to protect our patients in a natural disaster. Cogen gives us the ability to continue to care for the most vulnerable population in our city, even when the electric grid goes down."

The $15 million cogen plant was made possible by a $3.7 million grant from the Massachusetts Department of Energy Resources' Community Clean Energy Resiliency Initiative. The Boston Public Health Commission partnered with BMC in securing the grant. In addition to powering and heating much of the hospital, the cogen facility will also serve as a backup power source for city and state emergency communications.

"Projects like Boston Medical Center's new combined heat and power plant are critical to ensuring that Massachusetts has a clean and reliable energy future," said Department of Energy Resources Commissioner Judith Judson. "We are proud to partner with BMC and the City of Boston to ensure that their facilities have the resiliency needed to provide the highest level of care to their patients regardless of extreme weather."

"Boston is the most energy-efficient city in the country, and it's through partners like BMC that we lead the way in environmental and resiliency goals," said Boston Mayor Martin J. Walsh. "I congratulate BMC on this accomplishment, which makes their hospital and our city greener and safer."

Eversource provided BMC with financial incentives to bring cogen to the hospital campus. In the last four years, Eversource and BMC have partnered on more than 30 efficiency projects, achieving more than 8 million kilowatt hours in annual energy savings, or roughly 5 percent of the hospital's total electricity usage. The hospital has also avoided more than 5,600 metric tons of carbon dioxide emissions each year through its comprehensive energy strategy with Eversource, which is the equivalent of removing 1,200 cars from the road.

"Boston Medical Center has truly gone above and beyond, implementing a significant number of measures to lower their energy use and drive down costs," said Jim Hunt, Senior Vice President of Regulatory Affairs & Chief Communications Officer at Eversource. "Those are savings that can go directly back into the incredible patient care the hospital provides every day to Boston and the surrounding area."

The cogen plant, manufactured by Caterpillar and designed by Engineered Solutions, will supply more than 41 percent of the hospital's electricity and has the capacity to meet 25 percent of peak electricity demand at any given time.

By reusing the trapped heat for hot water, humidification and room temperature, the hospital will lower costs by reducing its need for steam heat by an estimated 44,000 pounds annually. Cogen will also reduce electric grid consumption by more than 16 million kilowatt hours a year.

The hospital has taken other important steps toward resiliency: installing new generators on floors of patient care buildings; creating cross-building critical infrastructure connections, so when one patient building loses power, it can connect to the generator in an adjacent building; and programming elevators to stop short of the lower floors to maintain patient care in the event of a flood.

BMC, which was already cutting emissions by 50 percent as part of a clinical campus redesign, also announced a solar power purchase agreement last fall with MIT and the Post Office Square Redevelopment Corporation. The solar purchase is equivalent to 100 percent of BMC's projected electricity consumption, making BMC the greenest hospital in Boston and on pace to become the first carbon-neutral hospital in New England upon completion of the campus redesign in 2018.

Becker's Hospital Review has named BMC one of the 50 greenest hospitals in America, and this week, BMC received three prestigious awards from Practice Greenhealth, the Top 25 Environmental Excellence Award, the highest honor Practice Greenhealth bestows on hospitals, as well as the Greening the OR Recognition Award and the Circle of Excellence award in the energy category.