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INTRODUCTION
Geothermal energy has emerged as a game-changer in the pursuit of sustainable power solutions, steadily growing in the United States as a promising way to produce power and regulate building temperatures. Derived from the Earth's internal heat, geothermal takes advantage of the planet's steady internal temperature, for electricity generation or heat pump systems, often referred to as Geoexchange, to regulate indoor climates. Geoexchange systems use the ground as a renewable heat source in winter and a heat sink in summer to control building temperatures. These systems can be broadly categorized into closed loop and open loop systems.
WHAT IS CLOSED LOOP?
Closed loop systems use a closed circuit of pipes underground, circulating a heat transfer solution of treated water. This fluid absorbs or releases heat from the ground around the pipes through conduction, interacting indirectly with Earth’s temperature. Closed loop systems have different configurations depending on spatial constraints, including horizontal and vertical. These systems typically extract heat from the ground around the pipes, except in the case of a Darcy Solution, a company who developed a way to use underground aquifers to utilize convection for heat exchange.
WHAT IS OPEN LOOP?
Open loop systems, often referred to as groundwater systems, utilize groundwater through a “pump and dump” method. In this system, water is pumped from a well, circulated through a heat pump to extract the heat, and then discarded, often into another well or surface body water. This method relies on the constant temperature and availability of groundwater to regulate indoor temperatures.
WHICH IS BETTER?
Efficiency
When comparing open loop and closed loop geoexchange systems, efficiency is a critical factor indicating their performance. Open loop systems are typically more efficient, due to the use of groundwater. This is considered a direct heat exchange: the groundwater transfers heat with the building. In a closed loop system, there is the addition of the heat transfer fluid that flows through plastic pipes. The ground transfers heat to the fluid which in turn transfers heat to the building. This additional fluid reduces efficiency as energy is lost through each transfer. Additionally, closed loop systems rely on heat conduction through plastic pipes. This process is slow and the systems can require many loops of pipe to get the heat transfer fluid to the desired temperature.
While open loop systems are more efficient due to their direct heat transfer method, they are more sensitive to the environment around them and rely on stable external conditions. Open loops systems depend on groundwater availability in order to operate, posing challenges for the performance of the system depending on the quality and availability of the groundwater. The presence of impurities in the water, such as minerals or pollutants, can lead to clogging and corrosion of the system, reducing fluid circulation and causing damage to pipes, pumps and other components.
Environmental Impact
Closed loop systems operate independently of groundwater and minimize water usage by circulating the same fluid. There are environmental concerns with land use for closed loop systems, as digging horizontal trenches and vertical boreholes can have an impact on the surrounding ecosystem.
Open loop systems have several environmental concerns due to the use of groundwater. Groundwater contamination is a concern with open loops systems, not only from an efficiency standpoint, but the repeated extraction and discharging of polluted water. Special considerations must be made to where the system will extract and discharge water to. Additionally, repeated extraction and discharge of groundwater can influence aquifer conditions including groundwater levels, flow patterns, and disruptions of thermal gradients. Open loop systems can contribute water scarcity as they require large quantities of water at a time to operate and might over extract from water-stressed regions. The disruption of thermal gradients is also a concern as sudden temperature changes can impact aquatic habitats and influence flora and bacterial growth.
Cost
Open loop systems commonly have lower upfront costs since they use existing groundwater resources and don’t need extensive piping installations. However, depending on the location, open loop systems might require more test-bores and upfront research to ensure that there is a stable supply of groundwater available for the system.
While closed loop systems can be costly upfront due to digging trenches, boreholes and installing complex piping systems, their long-term cost can be lower than open loop systems. Closed loop systems are more versatile and can be installed in a variety of configurations (horizontal, vertical, spiral, pond, etc.) depending on the available land and surroundings for the system.
Additionally, closed loop systems have less long-term maintenance costs as the closed circuit is protected from external elements, recycles the same heat transfer fluid and does not have the same water quality concerns that open loop systems have. The water treatment costs, potential problems with water availability and contaminants of the water can make an open loop system more unpredictable and costly in the long run.
CONCLUSION
In summary, the distinction between open and closed loops in geoexchange systems lies in their approach to heat transfer. Open loops interact directly with groundwater, while closed loops operate in a self-contained circuit using a heat transfer fluid. Closed loops offer better reliability, and environmental compatibility, while open loop systems can be more efficient and less costly upfront yet are sensitive to water quality and availability. Regardless of the chosen system, geothermal solutions are an effective way to reduce carbon footprint and reach emissions goals. While installation of these systems can be complex, Circadia has a track record of managing challenging decarbonization projects to help campuses meet their net zero carbon emission targets.
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