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District geothermal loops allow property owners to install geothermal heat pumps when they cannot create geothermal loops on their property.



Air conditioning during hot spells can add as much as 1 degree Celsius to the air temperatures at night in urban environments (the effect is less pronounced during the day because the solar input dominates the temperature increase). This creates a feedback loop requiring more air conditioning which further raises the temperature, and so on. Using geothermal systems for cooling (and heating during winter) would reduce this waste heat input by putting the heat underground where it dissipates much more slowly into the air (another potential use for the waste heat in summer is to concentrate it for heating water for commercial or municipal use).

However, many property owners in Cambridge cannot create geothermal loops on their own property due to space constraints, prohibitive costs and other issues. One potential solution is for the city to create district geothermal loops under the roadways and other public right of ways, allowing property owners to connect heat pumps to these loops for a monthly fee.

Successful projects like this have been completed in other cities and towns in the US and the world. For example, this article describes a project in Iowa.

Specific hurdles in Cambridge would include the ability to add geothermal loops to the already crowded infrastructure below the streets, and doing the education and marketing to the populace to attract enough customers to make it worthwhile. These questions can easily be explored and settled up front through a feasibility study by a qualified engineering firm regarding the infrastructure question and by polling residents and businesses regarding the market interest question.

Category of the action


Who will take these actions?

The city would authorize the construction of geothermal loops in areas undergoing major road repair and replacement or other construction activities. Operation and maintenance of the loops could be outsourced to a management company or done by the city itself, similar to the water treatment facility being owned and operated by the city.

The city could issue bonds to finance these projects, and collect revenue from property owners who connect to the system through payment of a monthly fee. This fee could be set lower than the expected equivalent monthly cost of conventional heating and cooling via natural gas/electricity of a building/home, yet high enough to offset the costs of construction, maintenance and operation.

To reduce the risk of underutilization, the city would survey property owners in the area prior to construction, and would solicit customers to sign up prior to financing the project so that a sufficiently large customer base would be established prior to undertaking the project.

District geothermal would reduce the UHI effect by reducing the amount of waste heat that is dumped into the air from central or window mounted air conditioning units. With a geothermal heat pump, the waste heat is instead dumped deep into the ground, where it should not have a major impact on the ambient temperature in the city at all.

What are other key benefits?

In addition to immediately reducing the UHI effect, district geothermal heating and cooling would provide the following additional benefits:

1. Reduced fossil fuel consumption by displacing natural gas and oil heating during the winter with geothermal heating instead.

2. Reduced dependence on natural gas for heating and grid electricity for cooling would improve price stability of heating and cooling for property owners.

3. Air conditioning can be provided without increasing future climate change by relying on the underground heat sink in summer and re-using it for heating in winter.

4. The city's GHG emissions profile can be lowered without reducing economic activity and while increasing comfort for occupants and workers.

What are the proposal’s costs?

Geothermal systems cost about $5,000 per ton of heating/cooling. The exact price will depend on the size of the loops, because economy of scale reduces the per ton costs of large systems. The costs of the HVAC unit and pumps inside the building would be born by the property owner, further reducing the cost of construction of the loops itself. Suppose therefore that we estimate the cost of installing district geothermal loops at $2,000 per ton, by subtracting estimated costs of the HVAC system and pumps. Suppose that we estimate the required tonnage at 1 ton per housing unit, or commercial office space equivalent. In fact any tonnage is helpful in reducing dependence on conventional fuels, and as long as there are fewer connection points than potential customers the system cannot easily be over-built. If we pick a street in Cambridge with 100 housing units (or commercial equivalent) abutting it, and the city built enough geothermal capacity for 100 tons, that would be a cost of $200,000. If each housing unit pays $100/month in connection fees, that means $10,000/month in revenues, paying back the construction costs in less than 2 years. If cost of capital is taken into account it would take slightly more than 2 years to pay back the construction costs. This means monthly fees could be lower, though one also has to take into account operating and maintenance costs of the system. Even so, the system could be revenue neutral, or even revenue generating for the city, while providing additional benefits.

Time line

During the first 3 years of this program, the city would study and document the feasibility of district geothermal through its city planning department (CDD). Assuming feasibility, geothermal districts would be incorporated into the city master plan and a roadmap would be created for when to construct which geothermal district loop based on road maintenance schedules which are already carefully planned out in Cambridge. DPW would be staffed up to handle construction, maintenance and repairs of district geothermal systems in anticipation of construction. In preparation for the first district, abutters would be approached to sign long term contracts with the city in exchange for geothermal heating/cooling capacity. The system would be designed based on available space, customer demand (current and anticipated) and financing. Once constructed, customers would be connected to the system. Customers would be responsible for the HVAC and pumps internal to their buildings, but this could be financed as part of the project for residential properties that would otherwise not participate. Potentially PACE financing could be used as well, and local banks could be approached to finance such purchases. Districts would be designed, constructed and operated on a rolling basis as road repair and construction proceeds. In the long term (50 - 100 years) it is possible that the earliest loops would have to be replaced or extensively repaired. This could be seen as basically new construction of those loops, as technology will have improved considerably by then, customer contracts will have expired, and the systems' costs would have long since been paid off anyway.

One opportunity coming up in FY15 is the Fulkerson/Thorndike Str. area in East Cambridge, which has several advantages as a potential pilot project, including: high density, the nearby Cambridge Str. business district, slightly higher elevation and several community centers including a public school and library branch.

Related proposals


References above. Geothermal heating and cooling technology is common place and well understood and already in use at several municipal buildings in Cambridge (e.g. City Hall Annex) and commercial and residential properties (Sue Butler).

District geothermal is a more recent concept but already practiced in the US and abroad, and not dissimilar to district steam systems which are also in operation in Cambridge (Veolia and MIT).

DPW interactive map road and sidewalk 5 year plan:

DPW 5 year road and sidewalk construction plan:

PACE legislation updates:

City of Cambridge letter regarding PACE legislation in the previous session: