Page 196 - Geothermal Energy Renewable Energy and The Environment

10 Low Temperature Geothermal

Resources: Ground Source

Heat Pumps

Heat pump technology is one of the most sophisticated engineering accomplishments to come out
of the twentieth century. Heat pumps are simple devices that operate at the highest efficiency levels
accomplished by heat-transporting systems. They are, in essence, Carnot cycles made manifest.
They transfer heat in ways that can accomplish both heating and cooling, while consuming a small
fraction of the amount of energy they move. Their great advantage comes from the fact that they
move heat that already exists, using basic thermodynamic principles, and thus do not require that
heat be generated. In this sense they provide the ideal means to satisfy energy demands for heating
and cooling buildings and spaces. This chapter considers the principles that need to be addressed
when coupling heat pumps to the Earth’s near surface heat reservoir, design concepts for such sys-
tems, and issues that need to be considered to successfully complete such an application

basIc heaT pUmp prIncIples

Heat pumps that utilize the Earth’s heat energy are fluid-mediated mechanical devices that transfer
heat from one location to another, relying on the thermodynamics of fluid systems. The first known
use of a heat pump was a device invented by Peter Ritter von Rittenberg in the mid-1850s in Austria
that employed exhaust steam for heat in salt mines. But it wasn’t until the early 1900s that heat
pumps, mainly as the heart of household refrigerators, became commonplace.
In a refrigerator, a heat pump is used to remove heat from the interior air of the refrigerator (heat
source) into the air of the room in which the refrigerator is located (the heat sink). This process is
exactly analogous to a typical Carnot cycle, with the air of the refrigerator interior being the heat
source used in the initial isothermal expansion phase of the cycle, and the room air being the heat
sink into which heat is expelled from the cycle during the isothermal compression phase, as depicted
in Figure 10.1. For heating, ventilation, and air conditioning (HVAC) purposes in which geothermal
energy is utilized, the heat transfer principles and processes are the same, the only difference being
the heat source and sink.
Ground-coupled or ground source heat pumps take advantage of the immense thermal mass of
the Earth and its moderate temperature. In regions where summer cooling is required, exterior day-
time temperatures generally are above 26°C (80°F). Where winter heating is required, exterior tem-
peratures generally are below 10°C (50°F). At shallow depths in the Earth, a consistent temperature
of about 10°C to 13°C (50°F to 55°F) is maintained. This intermediate temperature value between
summer highs and winter lows makes the Earth an excellent potential heat sink in the summer and
heat source in the winter, provided efficient heat transfer between the Earth and a building can be
accomplished.
The basic configuration of an Earth-building couple is depicted in Figure 10.2. The design shown
is that of a vertical loop of pipe that continuously circulates a working fluid between a heat pump
inside the building and the Earth. Such a system is called a closed loop because the working fluid
perpetually flows within the loop of pipe. Numerous other designs have also been employed for
closed loop systems, including single and multiple horizontal closed loops that are emplaced in

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