Green Energy company in Boston

There are quite a few

When Veolia Energy North America recently agreed to buy surplus heat from a Cambridge power plant and use it to warm downtown Boston buildings, it was widely celebrated as a green solution to a problem of a company discharging hot, harmful water into the cool Charles River.

But the idea behind it is hardly new: For more than 80 years, a labyrinth of steam pipes under downtown Boston has silently warmed — and cooled — many of Boston’s tallest buildings. The steam system, which even heats the New England Aquarium fish tanks, is undergoing a major growth spurt in Boston as Veolia performs millions of dollars worth of upgrades and recently won or renewed long-term contracts for 19 buildings.

The only obvious outward sign of the system may be columns of steam wafting from manholes in cool weather when rain seeps underground, hits the hot pipes, and evaporates.

“Many people don’t even know these systems are there, ’’ said Robert Thornton of the International District Energy Association, a trade organization based in Westborough. “But it’s competitive, it comes in a usable form for buildings, and it’s growing’’ in popularity.

District energy systems, so named because they provide heating and/or cooling to a group of buildings in close proximity, produce heat in two main ways: through large boilers usually burning natural gas or by capturing and using surplus heat from existing power plants that generate electricity, also known as combined heat and power, or cogeneration.

Steam is distributed through underground pipes to buildings that use it for space heating, hot water, or other processes, such as humidification at museums or sterilization at hospitals. The steam can also be used to drive chillers in buildings for air conditioning or go to large chillers located in the same central plant that pipe cold water out to buildings in the network.

The country has some 2, 500 district energy systems, according to the International District Steam Association, and most large cities and college campuses use them. The Empire State Building and US Capitol are heated by district steam.

In recent years, as concerns about climate change have sparked greater calls for energy efficiency, cities, building owners, and energy companies have renewed efforts to capture and use the surplus heat from power plants in district heating networks rather than allowing it to disappear as waste. About 50 percent of the Boston system is currently heated with this “green steam, ’’ according to Veolia.

“The net result is that we can back down our Boston production plants and reduce the natural gas we use, ’’ said Stewart A. Wood, president and CEO of Veolia Energy North America. In recent years, he said, Veolia has invested close to $40 million in its Boston and Cambridge systems. “When you reduce the volume of fossil fuels consumed, you reduce the city’s carbon dioxide emissions.’’

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