When it comes to environmentally friendly heating, heat pumps are the first choice. These are devices that work on the vapor compression refrigeration cycle and convert low-grade waste heat to high-grade useful heat. This can be used for various purposes, such as water heating, space heating or industrial processes.

There are four basic components of the heat pump: evaporator, compressor, condenser and expansion device. The low-temperature low-pressure liquid refrigerant in the evaporator absorbs heat from its surrounding fluid and evaporates to become a low-pressure low-temperature vapor. This refrigerant vapor is then compressed into high temperature, high-pressure gas. In the condenser, this gas rejects heat to its surrounding fluid and condenses to form a high-pressure high-temperature liquid. The fluid to which it rejects heat is heated and can be used for various useful purposes. The refrigerant liquid is then expanded in the expansion device, such as a capillary tube or thermostatic expansion valve, to form a low-temperature low-pressure liquid (actually a liquid-vapor mixture) which then goes to the evaporator and the cycle repeats.

Types of heat pumps

Figure 1: Temperature variation with respect to the depth below the ground level. Source: John Wiley & Sons

There are three broad categories of heat pumps depending upon the heat source at the evaporator. Heat pumps are called air-source pumps the most popular of the types, if the source is atmospheric air. They are known as water source heat pumps if the source is water, such as a river or pond. If the heat pump absorbs heat from the ground, they are called geothermal or ground source heat pumps.

The temperature of the Earth at shallow depths remains almost constant, irrespective of the ambient air temperature, which varies daily as well as seasonally. As shown in the graph, at shallow depths the atmospheric temperature fluctuations strongly affect the temperature of the Earth up to 5 meters. Below this depth up to 10 meters, the Earth’s temperature is approximately equal to the annual air temperature. Going deeper below ground, the Earth’s temperature rises at an average rate of about 3° C per 100 meters.

Ground source heat pump utilizes this low-grade ground heat and converts it to high-grade useful heat to provide comfort conditions in winter. If the heat pump is reversible, the source side and the sink side are interchanged and the same unit can provide cooling in summer, rejecting heat to the ground which is at a relatively lower temperature than air. Theoretically, in the vapor compression cycle, COP is high when the condensing pressure is lower and the evaporating pressure is higher. Since the ground temperature is always closer to the desired space temperature (for example for the comfort air conditioning application), either for heating or cooling, the ground source heat pumps are efficient than their air source heat pump counterparts.

Types of ground source heat pumps based on installation

Figure 2: Horizontal ground source heat pump.

There are two basic construction types when considering ground source heat pumps: horizontal installation and vertical borehole type installation. As against the air source heat pumps, the ground-source heat pumps generally involve a secondary loop of high-density polyethylene pipes. These pipes carry an environmentally friendly antifreeze/water solution, which is circulated through the polyethylene pipes by a pump and absorbs or rejects heat to the ground.

Horizontal ground source heat pump consists of multiple parallel pipes laid out horizontally in the trenches or horizontal boreholes, at shallow depths of 1 to 2 meters as shown in Figure 2.

They are either laid out in a slinky manner or a straight configuration if there is enough room horizontally. A horizontal arrangement is more suitable when there is a large land area available. They are the preferred choice since the installation costs are lower than their vertical counterpart.Figure 3: Vertical ground source heat pump. No heavy drilling machinery is required, and the installation takes place at shallow depths. But this is also a challenge since the shallow ground is susceptible to outside disturbances such as weather and temperature fluctuations. Thus, it makes designing such a system more challenging than a vertical type of arrangement.

In the vertical ground source heat pumps, the U-type plastic pipes are installed parallelly in the boreholes drilled to the depth ranging from 50 meters to 100 meters as shown in Figure 3. These pipes are then connected to horizontal manifolds that circulate the secondary fluid to and from the heat pump. The vertical arrangement is most common with large commercial buildings, such as schools and hospitals, since large land areas are usually not available. This arrangement may also need to be chosen when the soil is too shallow for horizontal pipework.

Summary

Ground source heat pumps are green technology when it comes to the sustainable heating and cooling. No doubt the green buildings are considering it an option for the overall carbon reduction targets. Even though this technology offers high energy efficiency, rigorous research of the particular location needs to be performed at the early design stage to be sure that the system performs as per the requirements. This is mainly because a constant Earth temperature does not guarantee high efficiency.

Many factors contribute to the effectiveness of the heat exchange underground, such as type of soil, water content and water flow, sediments and rock content. The heat transfer depends upon the thermal conductivity and thermal diffusivity of the soil. Knowledge of these parameters plays a vital role in the geothermal system design.