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Ground Source Heat Pumps / Geothermal

Ground Source Heat Pumps / Geothermal


Ground source/Geothermal heat pumps could help decarbonize hospital buildings by eliminating natural gas use on space heating.


Executive Summary

Ground Source Heat Pump (GSHP) systems, often called Geothermal Heat Pumps because they store energy in and use energy from the earth, are similar in many ways to Heat Recovery Chillers. Refer to that chapter for background information on the concept of recovering building heat through a chilled water system and repurposing it for building heat and domestic hot water.

The primary feature of GSHP systems is that they store excess heat from a building deep in the ground, then use that energy to heat the building when it is needed. They use a series of holes bored deep (300-500 ft) into the earth, each with a U-shaped plastic pipe inserted into it. A fluid, either water or a mixture of water and antifreeze, circulates through the tubes. When water temperature is greater than the surrounding earth, the water is cooled by this interaction, as heat transfers to the earth. If the water temperature is lower than the earth, heat is extracted from the earth to heat the water. There are systems that use open wells to pump groundwater through heat exchangers on the surface, then push the water back into the ground. Due to potential groundwater contamination, the authors do not recommend these open systems and will not discuss them further here.

There are two major forms of GSHP system:

Distributed GSHP, in which water from the ground loop is pumped to small, packaged water-source heat pumps located around the building. Each heat pump interacts directly with the ground loop water, either extracting heat from the water or rejecting heat to it, as needed for the space the unit serves.

Central Plant GSHP, in which chillers and heat pumps interact with the ground loop water in a central location. The system produces chilled water and hot water, much as a traditional central plant, to be distributed to loads around the building. From the perspective of building operations, these systems are pretty much the same as a boiler/chiller based plant, except that it has heat pumps rather than gas-fired boilers. Inside the plant, a bank of heat pumps and chillers provide both heating and cooling.

In either system type, there may also be supplemental cooling towers or evaporative coolers. In climates where the annualized cooling load significantly exceeds the annualized heating load, towers are used to reject excess heat to help balance the annual load on the borefield. This is often called a hybrid GSHP system.

Regardless of the type of GSHP system, these systems can provide 100% of the heating and cooling and domestic hot water for a hospital.

GSHP systems help to decarbonize by eliminating Scope 1 emissions for heating, cooling, and domestic hot water. All energy consumption shifts to electric, so carbon emissions at a given location depend on the local grid emissions and improve as the grid improves. More than that, though, GSHP systems function the same way as heat recovery chillers, in that they transfer heat, rather than using fuel to make “new” heat. Operating with an effective heating COP of 5 (meaning five units of heat output for each unit of energy input), means they use far less energy than electric resistance heat.

Construction of the borefield is a significant aspect of implementing GSHP. Boreholes are typically drilled 200-500 feet deep, sometimes deeper. Capacity varies with geology, but a rough rule of thumb is that the system needs about 200 feet of borehole for each ton of cooling capacity. In a cooling-dominated building such as a hospital, the borefield is most economically designed for the heating requirement, reducing the size of the borefield and using cooling towers to reject excess heat. In effect, the borefield becomes a large-scale thermal energy storage device that stores excess heat from the cooling process for use when supplemental heat is needed.

Time and expense of drilling the borefield is dependent on geology as well. A common misconception is that rock formations make drilling unfeasible. While rock does make drilling more expensive, drilling through rock is common and rock makes a good conductor of heat, which is beneficial to the system.

Ground Source Heat Pump

Technical Description

Refer to the chapter on Heat Pumps and Heat Recovery Chillers for background technical information. Ground source heat pumps (GSHP) are a specialized form of heat recovery chiller.

Design considerations

The GSHP projects we have done have been traditional – cooling and heating. What we found, of course, is that hospitals are exothermic (maybe you know this already…), which has a major impact on the borefield. Unless the excess heat is managed, the borefield temp gradually increases and makes cooling impractical. Actually, with some types of heat pumps, high temps are a problem even for heating. None of this is new information – but we were still surprised by how out of balance the loads were. Of course, this is in Georgia, so experience in SF will be different. But we found that the hospital didn’t go out of net cooling mode until about 25 degF. That means that we are essentially operating in heat recovery chiller mode at all temps above that. I don’t think that is climate dependent.

We are on the verge of designing a major expansion of one of those facilities and we intend to decouple heating and cooling, using the highest-efficiency chillers and the lowest temperature condenser water we can to reduce to cooling power load. We will use the heat pumps in heat recovery mode only to the extent that we need heat, then use the borefield when there isn’t enough heat to recover. We will have to use something to “charge” the borefield with heat – probably condenser water during warm weather, but maybe use the heat pumps to make and store heat.

All of this means that the borefield will be converted from GSHP to what is called Borefield Thermal Energy Storage (BTES). Same boreholes and piping, but a different way to think about it. One advantage we anticipate is that it will take less land area. In GSHP, the boreholes are spaced to reduce thermal overlap. In BTES, thermal overlap is fine, as long as we engage enough earth to store the heat we need.

We haven’t done this design yet, but I’m guessing that instead of spacing boreholes on a 25 ft grid, they may be spaced 15 ft. Guessing – lots of computer simulations and calculations will be needed for actual design.

We have a total of 156 boreholes, 500 ft deep, to get a cooling capacity of about 800 tons peak. I think our peak heating load was estimated at about 9600 MBH, including domestic hot water, but I’m not certain of that and whatever we estimated was probably high. You could say that this means 61 MBH per borehole, but that isn’t really the way to think about these. It’s about storage as well as peak capacity, and about how long you need to be able to carry a load. Think in terms of total BTUs, rather than BTU/hr, or kWh (like a battery), rather than kW.

How does this decarbonize?

Finding a heat source to replace the reliability of natural gas is key to decarbonizing buildings. Ground source heat pumps do exactly that, by enabling the earth as a heat source for buildings in all climates, including very cold northern climates. GSHPs use electricity and will only be as clean as the grid.


Barriers: Codes

Installation cost is high due to the requirement of excavation and boring of site. Environment side effects due to the abundance of greenhouse gases below the surface of the earth. The effective of earthquake event may affect the stability of the site.

Development in new technologies to bore holes in the ground more efficiently, to help reduce the installation cost. The high installation cost can have a quick rate of return on investment from the energy savings obtained.

Institute incentive plans to offset initial high installation cost. This technology is endorsed by the EPA (environmental protection agency) and DOE (department of energy) to be energy efficient and environmental friendly.

Barriers: Location

The climate around San Francisco, CA is very very different from northern Georgia. Not as cold, not as hot, lots more in between. I think that the relatively small number of hours less than 30 should make a borefield smaller, with heat recovery doing the job above 30.

It will be very helpful if they have a sense of when – at what outdoor temperature - they run out of heat in heat recovery mode. And does the plant heating load correlate to outdoor temperature in a quantifiable way? We could then look at weather data and get a rough estimate of how many hours the borefield would have to carry. This technology uses 25% to 50% less electricity than conventional heating or cooling systems

Case Study: Great River Medical Center

A good case study for geothermal heat pump is Great River Medical Center in West Burlington, Iowa. Constructed in the year 2000.


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