Geothermal Energy Systems
Building Blocks of Sustainability
Engineers' Corner
Geothermal loop design
Geothermal loop design
Direct Exchange of geothermal energy
Direct exchange geothermal heat pumps are the oldest and conceptually easiest geothermal system to understand. Unlike most installed systems, which have two heat exchange loops in series on the ground side, the direct exchange system has a single-loop of refrigerant in direct thermal contact with the ground. The refrigerant leaves the heat pump appliance cabinet, circulates through a loop of tubing buried underground, and exchanges heat with the ground before returning to the pump. The name "direct exchange" refers to heat transfer between the refrigerant and the ground without the use of an intermediate fluid. There is no direct interaction between the fluid and the earth; only heat transfer across the pipe.
Direct exchange systems are 20-25% more efficient and have potentially lower installation costs than water systems. While they require much more refrigerant, they require 1/3 to 1/2 the length of tubing, half the diameter of drilled holes, and therefore lower drilling or excavation costs. Higher joint quality is needed in the tubing to prevent the refrigerant gas from leaking. Copper loops should not be used due to the danger of corrosion in acidic soil and the consequent danger of refrigerant leaking. Direct exchange heat pumps are usually excluded by the terms "water-source heat pumps" or "water loop heat pumps" since there is no water in the ground loop.
Closed loop
Most ground-source heat pump system have two loops on the ground side: the primary refrigerant loop is contained in the appliance cabinet where it exchanges heat with a secondary water loop that is buried underground. In a closed loop system, the secondary loop is typically made of high-density polyethylene pipe and contains a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the internal heat exchanger, the water flows through the secondary loop outside the building to exchange heat with the ground before returning. The secondary loop is placed below the frost line where the temperature is more stable or preferably submerged in a body of water if available. Systems in wet ground or in water are generally more efficient than dryer ground loops since it is less work to move heat in and out of water than solids in sand or soil.
Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells (see below). The size of the loop field depends on the soil type and moisture content, the average ground temperature and the heat loss and or gain characteristics of the building being conditioned.
Vertical loops
A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically, 20 to 150 plus metres deep. Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a good thermal connection to the surrounding soil or rock to maximize the heat transfer. Grout also protects the ground water from contamination, and prevents artesian wells from flooding the property. Vertical loop fields are typically used when there is a limited area of land available. Boreholes are spaced 5–6 m apart and the depth depends on ground and building characteristics. For illustration, a detached house needing 10kW (3 ton) of heating capacity might need 3 boreholes 80 to 110 m (270 to 350 feet) deep. (A ton of heat is 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts.) During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.
Horizontal loops
A horizontal closed loop field is composed of pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped coils are placed horizontally inside the same trench. Horizontal loop fields are very common and economical if there is adequate land available. For illustration, a detached house needing 10kW (3 ton) of heating capacity might need 3 loops 120 to 180 m (400 to 600 feet) long of 3/4 inch (19mm) or 1.25 inch inside diameter polyethylene tubing at a depth of 1 to 2 m (3 to 6 feet).
Pond loop
A closed pond loop is not common because it depends on proximity to a body of water. A pond loop consists of coils of pipe attached to a frame and located at the bottom of an appropriately sized pond or water source.
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