METER ENVIRONMENT Ground Source Heat Exchange Systems Instructions
- June 4, 2024
- METER ENVIRONMENT
Table of Contents
Instructions
IMPROVING THE EFFICIENCY OF GROUND-SOURCE HEAT EXCHANGE SYSTEMS
In an effort to find sustainable energy solutions for heating and cooling
buildings, many homeowners, companies, and university campuses are turning to
ground source heat exchange systems (GSHE) to reduce energy usage and
greenhouse gas emissions. GSHE systems are designed to take advantage of the
moderate and nearly constant temperatures in the ground as the exchange medium
for space heating and cooling and to heat water for domestic use.
In these systems, water or specially formulated geothermal fluid is circulated
through plastic pipes (i.e., ground loops) installed in vertical boreholes. In
the winter, geothermal loops tap heat from the ground, while in the summer,
heat from the surface is transferred to the ground. Currently, the application
of ground source heat exchange systems reduces overall carbon emissions by up
to 50%, and according to the U.S. Department of Energy, they are up to 4 times
more efficient than gas furnaces.
ARE GSHE SYSTEMS AS EFFICIENT AS THEY CLAIM TO BE?
The answer, according to researchers at the University of Illinois at Urbana- Champaign (UIUC), is that it depends. Drs. Yu-Feng Forrest Lin and Andrew Stumpf and their associates at the Illinois State Geological Survey (a division of the Prairie Research Institute) at the UIUC and their collaborator, Dr. James Tinjum from the University of Wisconsin–Madison (UWM), is working on a project funded by the UIUC Student Sustainability Committee (SSC) to improve the efficiency of GSHE systems. They also hope to show that ground-source heat exchange systems could be included in the University’s multifaceted sustainability plan to reduce carbon emissions on campus to zero by 2050. Members of their research team are trying to determine whether GSHE systems would be feasible for heating and cooling buildings on campus with the existing subsurface geologic conditions.
A TEMPOS thermal properties analyzer will provide a better understanding of
how thermal energy is stored and transported in the subsurface.
The UIUC is not the first university to explore the development of GSHE
systems. For example, Ball State University recently replaced its coal-powered
heating and cooling system on campus with a large district-scale GSHE system.
Other universities with similar systems include the Missouri Institute of
Science and Technology and the University of Notre Dame. These ground-source
heat exchange systems are specifically designed to meet future energy needs.
However, as Dr. Stumpf notes, “Historically, quite a few large district-scale
systems have not achieved their projected efficiencies. Some systems have even
overheated the ground, forcing them to go offline. We’re trying to come up
with a way to make borehole fields more efficient and prevent these hazards
from occurring.”
SOME SYSTEMS DO NOT MEET THEIR EFFICIENCY TARGETS
Dr. Stumpf explains that many times, the contractors that install ground-
source heat exchange systems do a single conductivity measurement in the
borehole. Or they run a thermal response test (TRT) and then use these
calculations to determine the conductivity of the geologic materials at the
proposed site. In many cases, however, especially for district-scale GSHE
systems with multiple large-bore fields and complex geology, this information
does not adequately characterize the site conditions. He states, “Because only
limited measurements are taken, many systems have developed problems and are
unable to keep up with the thermal demands.”
To assist contractors and other groups involved in designing and installing
ground-source heat exchange systems, the UIUC research team is studying the
thermal conditions in a shallow geo-exchange system and collecting data from
geologic samples from a 100-m-deep borehole located on the UIUC Energy Farm. A
fiber-optic distributed temperature sensing (FO-DTS) system is being used to
collect detailed temperature measurements in this borehole during and after a
TRT. The FO-DTS system is an emerging technology that utilizes laser light to
measure temperature along the entire length of a standard telecommunications
fiber-optic cable. By analyzing the laser’s backscattered energy, the team can
estimate temperatures along the entire sensor cable as a continuous profile.
The ground temperature can be measured every 15 seconds, in every meter along
the cable, with a resolution from 0.1 to 0.01 °C (depending on the measurement
integration time). These data can be integrated with the TRT results,
ultimately providing a better understanding of the subsurface thermal profile,
which will lead to increasing the efficiency of the GSHE system.
Continuous core collected from the 100-m borehole was subsampled to measure
the thermal properties of the subsurface geologic units, and testing was
performed at the UWM with a TEMPOS thermal properties
analyzer. The resulting information will provide a better
understanding of how thermal energy is stored and transported in the
subsurface.
Geologic and geophysical logs from the borehole at the UIUC Energy Farm
EVERY SITE NEEDS UNIQUE DESIGN CONSIDERATION
Dr. Stumpf states that the ground under the UIUC Energy Farm includes various
geologic materials that conduct heat differently and require some additional
design considerations. He explains, “The upper 60 m of the borehole was
drilled into glacial sediment, including till, outwash (sand and gravel), and
lake sediment (silt and clay), which have different thermal conductivities.
Flowing groundwater in the sand and gravel units also increases thermal
transport. Conversely, the bottom 40 m of the borehole penetrated
Pennsylvanian-age bedrock, mostly shale and siltstone, which included layers
of coal. Unlike the other lithologies, coal has a very low thermal
conductivity and is therefore not optimal for a GSHE system. The most
efficient GSHE systems avoid low-conductivity geologic units and are optimized
to take advantage of flowing groundwater.
To learn more about this research project, visit the UIUC sustainability
project site or the ISGS
blog.
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