Ground heat exchanger concepts

Heat pumps provide wintertime heating by extracting heat from a source and exhausting it to the building. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground-source heat pump uses the shallow ground as a source of heat, thus taking advantage of its seasonally moderate temperatures.
In the summer, the process can be reversed so the heat pump extracts heat from the building and exhausts it to the ground. Exhausting heat to a cooler sink is more efficient, so the air-conditioning efficiency of the heat pump again benefits from the moderate ground temperatures.
Ground-source heat pumps must have a heat exchanger in contact with the ground or groundwater to extract or exhaust heat. Several major design options are available for these.

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Direct exchange

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 copper tube 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 and their tubing is more expensive per foot, 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. The copper loop must be protected from corrosion in acidic soil through the use of a sacrificial anode. 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 system

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.

As compared to direct exchange systems, closed loop systems need an additional heat exchanger between the refrigerant loop and the water loop, as well as an extra water pump. Some manufacturers have a separate ground loop fluid pump pack, while some integrate the pumping and valving within the heat pump. Expansion tanks and pressure relief valves may be installed on the heated fluid side. The lower efficiency of closed loop systems requires longer and larger pipe to be placed in the ground, increasing excavation costs. ASHRAE defines the term ground-coupled heat pump to encompass closed loop and direct exchange systems, while excluding open loops.
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. A rough approximation of the initial soil temperature is the average daily temperature for the region.

Vertical

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Ins Bohrloch Eingebaute und Verpresste Erdwärmesonde.

A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically, 75 to 500 plus feet 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. Bore holes 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.[5] (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

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 or slinky 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).
A slinky (also called coiled) closed loop field is a type of horizontal closed loop where the pipes overlay each other. The easiest way of picturing a slinky field is to imagine holding a slinky on the top and bottom with your hands and then move your hands in opposite directions. A slinky loop field is used if there is not adequate room for a true horizontal system, but it still allows for an easy installation. Rather than using straight pipe, slinky coils, use overlapped loops of piping laid out horizontally along the bottom of a wide trench. Depending on soil, climate and your heat pumps’ run fraction, slinky coil trenches can be anywhere from one third to two thirds shorter than traditional horizontal loop trenches. Slinky coil ground loops are essentially a more economic and space efficient version of a horizontal ground loop.

Open loop system

In an open loop system, (also called a groundwater heat pump,) the secondary loop pumps natural water from a well or body of water into a heat exchanger inside the heat pump. ASHRAE calls open loop systems groundwater heat pumps or surface water heat pumps, depending on the source. Heat is either extracted or added by the primary refrigerant loop, and the water is returned to a separate injection well, irrigation trench, or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. Since the water chemistry is not controlled, the appliance must be protected from corrosion by using different metals in the heat exchanger and pump. Limescale may foul the system over time and require periodic acid cleaning. Also, as fouling decreases the flow of natural water, it becomes difficult for the heat pump to exchange building heat with the groundwater. For this reason a mechanical or digital water coil freeze stat is used to protect the freon to groundwater heat exchanger from damage. If the water contains high levels of salt, minerals or hydrogen sulfide, a closed loop system is usually preferable.

Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually more efficient than closed systems because they are better coupled with ground temperatures. Closed loop systems, in comparison, have to transfer heat across extra layers of pipe wall and dirt.
Some types of open-loop systems are illegal in Ontario, after the Walkerton Tragedy, and other jurisdictions may not allow some of these systems which may drain aquifers or possibly contaminate wells.

A standing column well system is a specialized type of open loop system. Water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well, where traveling downwards it exchanges heat with the surrounding bedrock.The choice of a standing column well system is often dictated where there is near-surface bedrock and limited surface area is available. A standing column is typically not suitable in locations where the geology is comprised of mostly clay, silt, or sand. If bedrock is deeper than 200 feet (61 m) from the surface, the cost of casing to seal off the overburden may become prohibitive.
A multiple standing column well system can support a large structure in an urban or rural application. The standing column well method is also popular in residential and small commercial applications. There are many successful applications of varying sizes and well quantities in the many boroughs of New York City, and is also the most common application in the New England states. This type of ground source system has some heat storage benefits, where heat is rejected from the building and the temperature of the well is raised, within reason, during the Summer cooling months which can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system. As with closed loop systems, sizing of the standing column system is critical in reference to the heat loss and gain of the existing building. As the heat exchange is actually with the bedrock, using water as the transfer medium, a large amount of production capacity (water flow from the well) is not required for a standing column system to work. However, if there is adequate water production, then the thermal capacity of the well system can be enhanced by periodic discharge during the peak Summer and Winter months.
Since this is essentially a water pumping system, standing column well design requires critical considerations to obtain peak operating efficiency. Should a standing column well design be misapplied, leaving out critical shut-off valves for example, the result could be an extreme loss in efficiency and thereby cause operational cost to be higher than anticipated. From Wikipedia, the free encyclopedia

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