Networked ground source heat pumps offer an intriguing vision that speaks to several of the challenges in electrification. While the vision may not pencil out, it definitely merits study. The Massachusetts Clean Energy Center is currently supporting experimentation.
Although ground source heat pumps are prohibitively difficult to install on most urban lots, they are a well-understood and widely used technology. They use the ground, instead of the air, as a heat sink when cooling or as a heat source when heating. (Remember that heat pumps always move heat from a cooler thermal mass to a warmer thermal mass — one doesn’t need a heat pump to make heat flow in the opposite (natural) direction, from warmer to cooler. More basics here.) Instead of blowing air over coils outdoors, as does an air source heat pump, a ground source heat pump circulates water (possibly mixed with safe anti-freeze) over the heat pump coils and out into a network of pipes in the ground which is extensive enough to spread out the heat or cold picked up from the pump.
Because the ground below the surface has a stable temperature of 55F (varies by region), ground-source heat pumps don’t have to work as hard as air-source heat pumps (which are trying either to sink heat in hot air well over 55F or draw heat from cold air well under 55F). As a result, they have higher coefficients of performance and will draw less power in the winter power utilization peaks.
The intriguing “networked” application of ground source heat pumps developed by Heet is as follows:
- Natural gas utilities would go into a new business: They would drill deep circulation wells in the street and connect buildings to those wells; they would supply buildings with water at 55 degrees (close to ground temperature) instead of supplying them with gas. (The water would cycle back to the wells after it lost or gained heat from the building’s heat pumps.)
- The natural gas utilities would not be able to reuse their physical gas infrastructure for the circulation system, but they could preserve the jobs of their workforce, and reuse some equipment, expertise, and corporate systems.
- In addition to providing water at a fixed temperature from the circulation system, the gas utilities would interconnect buildings so that buildings could interchange heat when they have opposite needs (for example a building housing a lot of computer equipment might have cooling needs in winter when other buildings need heat).
- Ideally, the electrification of homes in a particular neighborhood would be done at more or less the same time. Electric utilities would upgrade as needed to support the increased load from the new heat pumps (but the upgrades would be more modest than for air source heat pumps). The gas utility would put in the networked circulation system and forego any improvements to its natural gas lines — shifting its workforce from gas line maintenance to groundwater drilling.
- Some of the homeowners who use hot water heat in baseboards or radiators might be able to continue to reuse those “hydronic” distribution systems — unlike air source heat pumps, ground source heat pumps can be directly connected to some hot water heating systems. This approach might make whole-home heat pumps attractive to many more people.
- Many homeowners would still face most of the same challenges when installing heat pumps, but the ground source pumps would be more efficient and could be smaller and somewhat less expensive. They might also be able to operate in a home with lower-quality insulation.
- The natural gas utilities would preserve a broader customer base, facilitating a more equitable transition.
Massachusetts is currently piloting a networked geothermal installation. It will be carefully monitored.
There are many engineering feasibility questions and it is clear that not all sites will support a network geothermal installation. High-density neighborhoods may pose the greatest challenges, but local geology can also create challenges.
And then there are the economic challenges: Networked geothermal at scale is a vastly expensive project. Networked geothermal at scale has to cost far more than fixing our existing leaky gas infrastructure: Fixing leaks mostly means replacing main lines and connections in existing trenches. Networked geothermal means new main lines, but also a new double connection (in/out) with each building, together with new deep circulation wells. That has to mean a much greater investment and it is not clear whether the incremental cost is justified by the lowered heat pump installation and operating costs and the reduced need for electric grid upgrades — the current straight electrification strategy using air-source heat pumps may be cheaper and more feasible.