Loop Types
A ground loop functions as a heat exchanger, extracting or adding heat to the ground. It's important to note that the ground is not an ideal heat sink or source, as the energy introduced to the ground by the loop can influence its temperature over time. The underlying principles of this interaction are universal across all loop types and will be elaborated upon here. Geothermal systems are available in various configurations, each with distinct strengths and weaknesses, which will be explored below:
Open-loop systems draw groundwater directly into the building to heat or cool the heat pumps. Sufficient groundwater is essential to meet the building's needs, but groundwater often contains minerals and contaminants that can adversely affect equipment.
Alternatively, open-loop systems utilizing lake water are available but should incorporate filtration equipment or secondary heat exchangers to address contaminants. It's crucial to use lake water in climates where the entering water temperature is above 40 degrees F to avoid freezing risks. The ground must have the capacity to accommodate the discharge from an open-loop system.
Open-loop systems face challenges below 40°F, and additional considerations are necessary to account for the increased pump head from the lake/groundwater level to the heat pumps. Notably, open-loop systems are uncommon in commercial and institutional applications and will not be discussed here.
On the other hand, closed-loop systems feature a dedicated fluid loop circulated through the ground or pond for energy exchange. The ground/pond water and loop water remain separate. Closed-loop systems are further categorized into different loop types.
Open Loop
A horizontal loop runs piping parallel and close to the surface. The undisturbed ground temperature often changes seasonally depending upon where the loops are installed. Horizontal loops are easier to install but require significantly more area (approximately 2500 ft2/ton) than other loop types.
Horizontal Loop
Vertical loops involve piping that runs perpendicular to the surface, and the holes can extend several hundred feet deep. At such depths, the undisturbed ground temperature remains constant throughout the year. Vertical loops only necessitate about 250 to 300 ft²/ton.
Verticle Loop
Surface water or pond loops utilize a body of water as the heat sink. Heat dissipates from the water primarily through surface evaporation, closely tying the process to pond temperature and ambient wet bulb conditions. During winter, when the pond might be frozen, heat transfer is primarily governed by the interaction between the loops, the bottom water, and the soil surface at the pond's bottom.
Surface Water Loop
