Utilization of Ambient Air and Shallow Geothermal Energy
1.Introduction:
Within a range of 0 - 400m of depth stored heat is categorized as geothermal energy close to the surface (shallow geothermal energy). The direct utilization of this heat is possible by combinations of heat pumps and subsurface heat exchangers, such as collectors, wells or piles.
Shallow Geothermal Energy is low temperature heat (compared to energy from soil, sandstone and underground water in thermostat layer) reserved under the surface layer formed under the combined effect of solar energy and heat energy from the earth?s core.
Figure 2:
Solar Energy Distribution.
Shallow geothermal energy is regenerative energy which mainly uses the indirect solar energy. 30% of solar energy is reflected by cloud layer and the earth?s surface.15% is absorbed by the cloud and the rest absorbed by land and sea. However, the absorbed solar energy is released to the atmosphere and space in different forms. The radiation is the major supply source of the shallow geothermal energy in underground 400 meters. Due to the great influence of heat energy of the earth?s core, there is temperature gradient under the thermostat layer. Temperature will increase by 3-4?C underground for every 100 meters in depth underground.
Because the ground transports heat slowly and has a high heat storage capacity, its temperature changes slowly?on the order of months or even years, depending on the depth of the measurement. As a consequence of this low thermal conductivity, the soil can transfer some heat from the cooling season to the heating season as presented in Figure 3; heat absorbed by the earth during the summer effectively gets used in the winter. This yearly, continuous cycle between the air and the soil temperature results in a thermal energy potential that can be harnessed to help heat or cool a building.
Figure 3: Figure 4:
Typical Average Monthly Temperature Typical Soil Temperature Variation
Another thermal characteristic of the ground is that a few meters of surface soil insulate the earth and groundwater below, minimizing the amplitude of the variation in soil temperature in comparison with the temperature in the air above the ground (see Figure 3 and Figure 4). This thermal resistivity fluctuations further helps in shifting the heating or cooling load to the season where it is needed. The earth is warmer than the ambient air in the winter and cooler than the ambient air in the summer.
Ambient air and shallow geothermal energy can be harnessed by a number of different technologies, methods and concepts. As the utilisable energy is normally generated at a low temperature level (mainly below 20 ?C), a device to increase the temperature is generally required in order to enable the technical utilisation of the heat (e.g. to heat a residential building). This means that a heat pump needs to be built into the system. Alternatively, the subsoil temperature level can be increased by storing additional heat (e. g. from solar energy using solar collectors or excess heat from industrial processes). This option has hardly been put into practice so far. In order to harness ambient air and shallow geothermal energy, additional externally supplied energy is always required (e. g. electricity from the public grid, natural gas or biogas, fuels).
Hence a system to supply useful or final energy through ambient air and shallow geothermal energy utilisation generally consists of three system elements:
This increase is obtained by using a heat pump.
1.1.What is a Heat Pump and How Does it Work?
A heat pump is an electrical device that extracts heat from one place and transfers it to another. The heat pump is not a new technology; it has been used around the world for decades. Refrigerators and air conditioners are both common examples of this technology.
Heat pumps transfer heat by circulating a substance called a refrigerant through a cycle of evaporation and condensation. A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings.
The refrigerant is then compressed en route to the other coil, where it condenses at high pressure. At this point, it releases the heat it absorbed earlier in the cycle.
Refrigerators and air conditioners are both examples of heat pumps operating only in the cooling mode. A refrigerator is essentially an insulated box with a heat pump system connected to it. The evaporator coil is located inside the box, usually in the freezer compartment. Heat is absorbed from this location and transferred outside, usually behind or underneath the unit where the condenser coil is located. Similarly, an air conditioner transfers heat from inside a house to the outdoors.
The heat pump cycle is fully reversible, and heat pumps can provide year-round climate control for your home ? heating in winter and cooling and dehumidifying in summer. Since the ground and air outside always contain some heat, a heat pump can supply heat to a house even on cold winter days. In fact, air at ?18?C contains about 85 percentof the heat it contained at 21?C.
An air-source heat pump absorbs heat from the outdoor air in winter and rejects heat into outdoor air in summer. It is the most common type of heat pump at this time. However, ground-source (also called earth-energy, geothermal, geoexchange) heat pumps, which draw heat from the ground or ground water, are becoming more widely used.
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