Boston explores sea- and river-source heat pumps for big buildings

The waters of Boston Harbor, most famous for the crates of tea hurled rebelliously into them more than 250 years ago, could soon be the site of a whole new kind of disruption.

“We’re launching a thermal revolution,” said Zeyneb Magavi, executive director of the Home Energy Efficiency Team, or HEET, a Boston-based nonprofit that advocates for the creation of thermal networks as an alternative to fossil fuel–based energy sources.

Boston and neighboring Cambridge are home to three projects that plan to tap the city’s rivers and harbor to provide emissions-free heating and cooling to some of the area’s largest institutions.

The concept of the heat pump — a device that pulls heat from one place and deposits it in another to warm or cool a space — is over 150 years old. More recently, however, the technology has become a major part of the clean energy conversation. Air-source heat pumps are hailed as an efficient way to shift homes off natural gas and heating oil, and sales have soared in the past several years. Many states have also started looking at the possibilities for geothermal networks that draw thermal energy from the earth and send it through pipes to homes and businesses along the system, much in the way natural gas is delivered.

The Boston area’s forays into river-source and sea-source heat pumps could open up a whole new way to decarbonize building operations, supporters say. Conventional geothermal projects can be challenging to build on a large scale in Boston, said Lindsey Butler, executive director of the Boston Green Ribbon Commission, a public-private climate partnership.

“We’re a very old city, we’re dense, we have a very crowded underground, and a large portion of our city is built on fill,” she said. Geothermal is ​“impossible in some places and in others just very expensive.”

Water has other advantages as well. Water temperatures vary less than air temperatures — they stay cooler in the summer and are generally warmer in the winter — making Boston Harbor and the Charles River more efficient sources of thermal energy.

And thermal energy is abundant in the water, thanks in large part to the climate-warming activities of humans. In Boston Harbor, rising water temperatures since 1920 mean there is enough thermal energy — what Magavi calls ​“anthrothermal” — to heat an estimated 2.4 million households annually, according to calculations done by HEET researchers. Ocean currents replenish the harbor’s heat every two to 10 days, making it essentially impossible to extract all the thermal energy, said Archana Dayalu, a scientist who worked on the analysis.

“Really, this is a near-infinite source of energy,” Dayalu said. ​“It is a long-term sustainable solution.”

The newest plan to take advantage of this aquatic abundance is the Boston Area Thermal Energy Network Project, or BosTEN. In late July, the Boston Green Ribbon Commission held a kickoff event launching an effort to determine how best to provide heating and cooling to major institutions using the energy stored in Boston Harbor, the Fort Point Channel, the Charles and Mystic rivers, and the bedrock beneath the rivers. The commission has hired a technical team to investigate how much thermal energy can reliably be extracted, how it can best be transported, where the most likely customers are, and what permitting and regulatory issues might crop up.

The commission intends to have final recommendations available in spring or summer of 2027, and to get building as soon as possible after that. 

“We really want to know where we can put a shovel in the ground at the end of this,” Butler said. ​“We’re really trying to emphasize the era of implementation.”

The feasibility project’s kickoff event was held at the University of Massachusetts Boston, a nod to the school’s long-standing use of seawater for climate control. Since the 1970s, the coastal campus has relied on water for cooling, drawing heat away from the facilities and transferring it into the harbor. Now, the school is in the process of adding equipment that will allow it to do the reverse: draw energy from the seawater to warm classrooms, laboratories, and dorms across 12 different buildings.

The university project is in the design phase, and currently going out to bid. When completed, the new system will meet the majority of the campus’s heating needs and reduce the use of natural gas by 82%. A later phase will add more equipment, getting that number nearly to 100%, said James O’Day, the university’s director of utilities, energy management, and laboratories.

“From an environmental standpoint, it’s definitely a win,” O’Day said. ​“It’s also much more energy-efficient.”

At the same time, energy company Vicinity Energy is in the midst of overhauling its systems to use thermal energy from the Charles River to produce the steam that delivers heat to its network of large institutional customers in Boston and Cambridge. The project’s 35-megawatt heat pump will be the largest in North America, at least for a little while, said Matt O’Malley, Vicinity’s chief sustainability officer.

This shift to thermal energy was inspired by similar systems across the Atlantic, O’Malley said. He points to Glasgow, Scotland, where a district heating network carries thermal energy to and from the River Clyde, and Malmö, Sweden, which draws heat from the wastewater discharged from a sewage treatment plant.

“We are not trying to prove something,” O’Malley said. ​“We are shamelessly emulating what has worked in leading European sustainability cities.”

The trio of Boston-area projects could promote more widespread adoption of such thermal networks. Advocates are optimistic that the support of universities, state and city governments, hospitals, and environmental nonprofits bodes well for the future of these systems.

Magavi envisions a thermal loop laid on the seafloor in Boston Harbor, serving the first round of large institutions, then expanding to add more loops and more customers over time.