A Study Advocates for a Utility-scale Approach to Replace Existing Gas Lines with Ground-source Heat Pumps

In 2019, about 97 percent of U.S. natural gas pipelines were made of plastic or steel. Cast- and wrought-iron pipes were the remaining few and among the oldest gas lines in the country, according to the U.S. Department of Transportation’s Pipeline and Hazardous Safety Administration. Nearly half of all iron pipes are concentrated in four states, including Massachusetts, New Jersey, New York and Pennsylvania. If cast-iron (and bare-steel pipelines) are left in the ground beyond their service life, utilities have to spend millions of dollars repairing leaks on old pipelines. When left unattended, leaking gas pipes release methane, which is a highly combustible gas that poses significant health, public safety and environmental risks.

In September 2018, a natural-gas leak resulted in 131 fires and three explosions in three communities in Merrimack Valley, Mass. Thousands of people from the city of Lawrence and towns of Andover and North
Andover were forced to evacuate their homes. About 25 people were injured and one person died in the incident. In the winter months that followed, more than 8,000 customers had no access to heat, hot water and gas. After a yearlong investigation, the National Safety Transportation Board determined there was inadequate planning and management by Columbia Gas of Massachusetts, which led to over pressurization in the valley’s aging cast-iron gas pipes. (Read a synopsis of NTSB’s report.)

Massachusetts Rethinks Gas Infrastructure

Massachusetts has the second-oldest gas infrastructure system in the country with 6,000 miles of aging and leak-prone pipes, which make up 26 percent of the state’s gas system. In recent years, natural-gas companies have faced a great deal of backlash caused by pipeline failures and methane
leaks from aging pipelines. Gas companies are therefore actively replacing aging pipelines with corrosion-resistant high-density polyethylene plastic pipelines. It is estimated that repairing and replacing the state’s leaking gas system can cost taxpayers more than $9 billion during the next 20 years. This does not include the price of lost gas that customers and businesses already pay in their monthly bills, which is estimated at $90 million per year in the greater Boston area alone. (Learn more from a research article published by the Proceedings of the National Academy of Sciences of the United States of America.)

Currently, nearly 70 percent of Massachusetts’ electricity and 50 percent of heating energy loads are met by natural gas, all of which is imported from other regions in the U.S. and overseas. During peak winter season, the state faces massive gas shortages, increasing the wholesale price of gas, as well as the cost of power production. To address this energy-security problem and overarching climate issues, the state has required electric utilities to use more renewable energy each year and has heavily invested in energy-efficiency programs. At the current pace, clean-energy alternatives are poised to undercut gas prices in coming years, per Rocky Mountain Institute.

The decreasing cost of renewables will likely accelerate the pace of renewable deployment and building electrification, which will decrease gas consumption. If gas consumption declines, the utilization of gas pipelines will drop, inadvertently increasing the financial burden of maintaining the pipelines on a shrinking pool of gas customers. Although it is imperative to maintain gas lines for public safety, continually replacing leaking pipelines with new ones will not only cost taxpayers billions of dollars but also lock some gas customers into another 40 to 70 years of fossil-fuel dependency. The macroeconomics have forced Massachusetts policymakers and industry leaders to rethink future expansion of gas infrastructure.

Following the aftermath of Merrimack Valley, the Home Energy Efficiency Team (HEET), a local environmental non-profit, began exploring potential substitutes for gas infrastructure. Long known for its work on energy-efficiency retrofits and methane-leak studies in Massachusetts, HEET wanted to explore an alternative business model where customers could purchase renewable thermal energy, instead of natural gas. In early 2019, HEET selected Buro Happold to lead a feasibility study for replacing aging natural-gas infrastructure with a network of ground-source heat pumps (GSHPs).

Viability of GeoMicroDistricts

During the development of the study, the term “GeoMicroDistrict” was coined to refer to the GSHP network that serves a street segment— the length of street between two intersections or an intersection and a dead end. The GeoMicroDistrict would contain a shared loop running through an array of boreholes installed within public rights-of-way. The ambient water circulating in the shared loop would serve as the heat-exchanging medium between the ground and heating and/or cooling distribution systems housed within individual buildings. In theory, the concept is to incrementally replace leaking gas pipes with GeoMicroDistricts that are eventually interconnected to create a district-scale energy system.

District-scale GSHP systems are not a novel idea. In the U.S., universities like Colorado Mesa, Ball State and Furman, as well as many large-scale residential development projects have successfully installed and operated district-scale GSHP systems for years. However, strategic replacement of natural-gas pipelines with GSHP systems is certainly a unique proposition. For successful execution of this concept, various risks pertaining to site suitability, building compatibility, high upfront capital costs and operational reliability (energy load and capacity management) need to be mitigated.

To address these constraints, the GeoMicroDistrict Feasibility Study examined the engineering and economic viability of installing shared loops in four prototypical street segments (PSS). The PSS represent distinct building and land-use typologies prevalent in Massachusetts. Each PSS is composed of two contiguous lines of residential and/or commercial properties on either side of a 40-foot-wide, 500-footlong public right-of-way.

To avoid site conflicts with underground infrastructure systems (potable water, telecom, data cables, sewer, stormwater systems and the like), borehole installation area is limited to the 2-foot-wide gas utility corridor. The engineering layout is informed by the state’s geological and seasonal parameters and designed to operate in balanced condition so as to prevent ground overheating or overcooling. The heating and cooling demand profiles of each PSS are then compared with the ground’s thermal capacity to assess annual thermal loads met by interconnected GeoMicroDistricts.