Buro Happold’s study concluded that interconnected GeoMicroDistricts can provide nearly 100 percent of the annual thermal loads in lowto medium-density residential and mixed-use commercial areas. High-density neighborhoods would require supplemental heating and cooling energy. The study strongly advocates for a utility-scale approach wherein existing gas companies install, operate and maintain the GeoMicroDistricts. It is assumed that any costs associated with systems outside the building are borne by the gas company. As the customer base increases, the load diversity, efficiency and economies of scale would improve. Furthermore, the utility also would be responsible for scaling and managing system capacity while integrating thermal energy storage and backup energy systems, as needed.
At this conceptual stage, it is assumed that customers would pay for monthly thermal energy consumption and any additional costs associated with heat-pump installation or improvements needed inside the buildings with financial support from the state. The performance and cost for retrofitting existing buildings can therefore vary significantly based on their type, size, height, and age of the building and the original space-conditioning system. Installing heat pumps in new buildings is relatively easier than renovating existing buildings. GSHPs in existing buildings are generally more compatible with forcedair and hydronic systems because the distribution system can be reused with minor adjustments in the ducts, pipes and controls. However, buildings utilizing steam distribution systems and electrical baseboards are not well suited to GSHPs and may need a gut rehab. Furthermore, gas-powered appliances, like domestic hotwater heaters, stoves, ovens and clothes dryers, etc., would need to be replaced with electric appliances. It is also critical that existing buildings are made as efficient as possible prior to or during the conversion. This would allow for the installation of smaller and less expensive systems, reducing upfront costs and energy bills for the customer.
Safety First
GeoMicroDistricts represent a viable “safety first” alternative to natural-gas heating along with a host of additional benefits, including climate-change mitigation, infrastructure resilience and air-quality improvements. The concept promotes equitable transition to a renewable thermal system that can be deployed in any neighborhood, city or region. Allowing gas utilities to operate GeoMicroDistricts will help them retain their current organization purpose, structure and workforce. Installing GeoMicroDistricts will not be an easy or inexpensive feat and will require significant coordination with public utilities, policymakers and communities. It is safe to say that investing in heat pumps and building electrification is certainly a better utilization of billions of taxpayer dollars that are currently being used for funding an obsolete energy system.
A FUTURE FOR RENEWABLE ENERGY
Following the public release of the GeoMicroDistrict Feasibility Study,
Home Energy Efficiency Team (HEET), a Massachusetts environmental non-profit, and its supporting partners filed an act for utility transition to using renewable energy. Known as the FUTURE Act, the act was developed with legislative leaders to pass laws to improve regulations for gas distribution systems and to accelerate the transition to renewable energy. The FUTURE Act will grant utilities the permission to bill customers for renewable thermal credits (or Btus) instead of gas and proposes a renewable thermal credit market for the gas industry. The act calls for flexible regulations, allowing municipalities to choose energy alternatives, and requests for funding and financial incentives to encourage gas companies to distribute thermal renewable energy instead of gas. The bill was supported by 13 municipalities at the Telecommunications, Utilities and Energy Committee hearing last November. HEET is currently supporting Eversource Gas, a local utility, to pilot the GeoMicroDistrict concept in 2021.
UNDERSTANDING GSHP SYSTEMS
A ground-source heat pump (GSHP) is a heat-exchanging device that transfers heat to or from the ground, groundwater or surface water to provide space heating and cooling. These devices are some of the most energy-efficient and low-carbon spaceconditioning technologies available today. The GeoMicroDistrict Feasibility Study discussed in this article focuses on a subset of GSHPs called vertical ground-coupled heat pumps that exchange heat with the ground.
As compared to ambient air, the temperature of the ground remains relatively constant. Vertical ground-coupled systems utilize the temperature difference between the ground and interior spaces to transfer heat. The system consists of one or more heat pumps that are connected to a series of pipes, typically made of high-density polyethylene. These pipes form a closed loop that is buried in vertical boreholes. The boreholes can range from 4 to 6 inches in diameter and 100 to 500 feet in depth, though greater depths and larger diameters are possible.
A heat-transfer fluid, typically water or water with a nontoxic antifreeze solution, is circulated through the closed-loop to exchange thermal energy with the ground. The heat pumps regulate the fluid flow to transfer thermal energy to the building’s HVAC system. In winter months, the circulating fluid extracts heat from the ground to heat indoor spaces and, in summer, heat is transferred from indoor spaces and rejected to the ground to provide cooling.
Multiple factors must be considered prior to sizing a GSHP system, including building heating and cooling requirements, available land area, and the geological and hydrogeological characteristics of the ground. For optimal performance, it is very critical to maintain stable ground temperature over the long-term. This can be achieved by balancing heat extracted and rejected into the ground (maintaining thermal balance).
The ground can withstand a 10 to 15 percent difference between heating and cooling loads but beyond that the ground may become gradually warmer or colder, reducing the operating efficiency of the GSHP system. To design a GSHP system that meets 100 percent of a building’s heating and cooling demands, the system must have enough capacity to perform during extreme winter and summer days. Because heating and cooling demands are rarely identical, supplemental heating or cooling may be needed to meet peak demands while maintaining thermal balance.
PHOTO: COURTESY INTERNATIONAL DISTRICT ENERGY ASSOCIATION, ENERGY SAVING TRUST