Geothermal Heat Pump Systems: Integration with Precast Concrete

A brief primer on geothermal systems for engineers and architects.

By Robert Mancini, P.Eng.

Geothermal energy systems use the earth as a heat source, a heat sink and as an energy-storage device. Geothermal systems are also known as ground source heat pumps, geothermal heat pumps and earth energy systems or geo-exchange systems. Geothermal heat pump (GHP) systems represent a mature technology that has been in use for more than 50 years. GHP systems not only can reduce a building’s carbon footprint but also its total operating costs.

The GHP systems in North America were mostly installations for single-family residential buildings and typically were small in scale, with only two or three vertical boreholes into the ground. Today, however, the size of these geoexchange systems is limited only by the area available for the installation of the ground exchanger.

One of the largest GHP systems in the world is at Fort Polk, La., and provides heating, cooling and domestic hot water for 4,000 homes on the military base. It reduces electrical consumption by 33%, eliminates 260,000 therms (76 kWh) of natural gas, reduces peak demand by 43% and reduces CO2 emissions by 22,400 tons per year.

Some systems use water sources and aquifers, but the commercial and institutional building applications mostly use the ground as the thermal exchanger and have vertical closed-loop systems (see Figures 1a and 1b).

Ground exchangers are typically constructed using high-density polyethylene pipe similar to pipe used in the distribution of natural gas. The most common way to build a ground exchanger is to drill a vertical borehole 6 inches (150 mm) in diameter and anywhere from 150-ft to 700-ft (45-m to 210-m) deep. Once drilled, a pipe fashioned as a U-tube is inserted to the bottom of the empty space and the tube is tremie-grouted from the bottom up (Figure 2).

Innovation in precast pipe

An innovation by Renewable Resource Corp., located in Sudbury, Ontario, integrates a ground exchanger into precast concrete sewer pipe. This new system saves on the installation cost of GHPs by piggybacking on conventional site drainage and sewer infrastructure. The product, known as the @Source-Energy Pipe, can be built in any size from 6 inches (150 mm) in diameter and up. The precast pipe can be used in sanitary and storm lines. The sanitary line application results in higher energy capacity as energy is recovered from passing effluent.

Energy: reclaim, store and reuse

One major advantage of GHP systems is the ability to store thermal energy in the ground rather than reject it to the atmosphere as is the case with conventional heating and cooling systems. In essence, this rejected energy has already been paid for by the utility user. The ground exchanger size is calculated by the engineer or designer based on the building’s monthly energy requirements as well as its monthly peak energy needs. In addition, the following information is required to design a vertical ground exchanger:

• earth’s formation thermal conductivity and diffusivity;
• grout thermal conductivity;
• borehole thermal resistance;
• pipe arrangement within borehole or trenches;
• heat pump performance information;
• undisturbed ground temperature; and
• circulation fluid properties (specific heat; density and flow rate; design heat pump inlet temperatures in heating and cooling mode; and additional system power data including that for pumps and fluid cooler).

Renewable energy savings

The earth energy that GHP systems access is recognized as renewable energy by organizations such as the U.S. Environmental Protection Agency (EPA), Natural Resources Canada (NRCan) and the European (EU) Commission. In terms of energy savings, a new building with a well-designed and constructed geoexchange system with a vertical closed-loop geothermal exchanger should not consume more than 10 kWh/ft2/yr (108 kWh/m2/yr).

Operating cost savings

In general, the energy costs of buildings with geoexchange systems are in the range of $1/ft2/yr ($11/m2/yr). Conventional heating and cooling systems costs will typically run more than $1.50/ ft2/yr ($16/m2/yr). Geoexchange systems can also provide a hedge against rising fossil fuel costs. The maintenance costs of several buildings with geoexchange systems have been studied by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). The results showed maximum long-term maintenance costs of $0.135/ ft2/yr ($1.45/m2/yr) compared with $0.24 to $0.35/ ft2/yr ($2.58 to $3.77/m2/yr) for buildings with conventional systems.

Capital costs

Capital costs for geothermal heat pump systems range between $14/ft2 and $24/ft2 ($150/m2 and $258/m2) depending on location, type of building and local geological  conditions. The @Source-Energy Pipe could reduce the installed cost of the ground exchanger by approximately 25%. The cost of the ground exchanger should be amortized over its 50-year-plus service life.

What can a GHP system provide for your next project?

GHP systems can:

• Provide hot water for: panel heating, in-floor heating, snow melting and heating outdoor air for ventilation
• Provide chilled water for refrigeration
• Store heat energy in the ground by using energy rejected during the cooling cycle (stored in the earth and reused for winter heating) and using energy extracted from the ground during the winter heating cycle, which actually cools the ground during winter and, therefore, reduces energy consumption for summer cooling
• Provide heating or cooling any time of the year (inhabited areas can be as small as a single office or as large as required for commercial/industrial facilities)
• Reclaim thermal energy from industrial processes involving low-grade heat
• Store solar thermal energy in summer for winter heating

Big opportunity for precast pipe

The integration of alternative energy technologies and precast concrete has been generally overlooked by designers; the @Source-Energy Pipe is a first step in maximizing the benefits of these two proven technologies. The product not only recovers energy that would normally be lost but represents an even bigger opportunity for precast concrete pipe manufactures to reclaim the small-diameter pipe market from plastic pipe manufacturers.

It is certain that other integrated precast elements will follow and have a positive impact on the precast concrete industry’s share of energy-related product markets. Precast concrete is inherently suitable as a material for geothermal energy systems because of its thermal mass and conductivity properties. Moreover, significant tax credits – up to 30% of the total cost – are available for geothermal energy installations (U.S. tax incentives for geothermal systems are mentioned in Precast Inc. magazine, July-August 2010).

Robert Mancini, P. Eng., earned a mechanical engineering degree from the University of Toronto and is an internationally known expert in geothermal systems. He is president of R. Mancini and Associates Ltd., a geothermal consulting firm in Bolton, Ontario.