1 GEOTHERMAL SYSTEMS AND TECHNOLOGIES 5. SHALLOW GEOTHERMAL SYSTEMS
5. SHALLOW GEOTHERMAL SYSTEMS (SGS) 2 Shallow geothermal resources (< 400 m depth) are omnipresent. Below 15 - 20 m depth everything is geothermal: � the temp. field is governed by terrestrial heat flow and local ground thermal conductivity structure ± groundwater flow. Use of low to moderate temps. Direct use; Heating and cooling.
5. SHALLOW GEOTHERMAL SYSTEMS 3 The distinction between shallow and deep geothermal is not fixed. In North America, shallow geothermal technology is In North America, shallow geothermal technology is also known under the term “geoexchange” To use the constant, low temperatures of the ground, there are two options: � Ground Source Heat Pumps, GSHP, � Underground Thermal Energy Storage, UTES.
5.1.Introduction 4 GHPSs have changed the approach of geothermal energy use which until recently had been considered as economic potential only in areas where thermal water or steam is found concentrated at depths less than 3 km. GSHPs can be used basically everywhere and are not as site-specific as conventional geothermal resources. and are not as site-specific as conventional geothermal resources. GHPSs do not produce electricity, but they greatly reduce its consumption. In winter, GHPS draw thermal energy from the shallow ground, which ranges between 10° and 21°C depending on latitude. In summer, the process is reversed to a cooling mode, using the ground as a sink for the heat contained within the building. Consumption of electricity is reduced by 30% to 60%, with a payback period of the installation in 2 to 10 years.
5.1. Introduction 5 Geothermal heat pumps are systems with three main components: � the ground side to get heat out of or into the ground, � the heat pump to convert the heat to a suitable temperature level, and � the building side transferring the heat or cold into the space. Sources, type and output of GHP
5.1. Introduction 6 GSHPs are space conditioning units that use the refrigeration cycle to heat or cool a medium, using the earth as a heat source or sink. The refrigeration cycle is reversible, so these units can be used to heat or cool.
5.2. Working principle of geothermal heat pump 7 The most common type of heat pump is the compression heat pump (CHP). Refrigeration cycle of a CHP. WNA -heat delivery system VLH VLH -heating supply -heating supply RLH -heating return WQA -heat collection system VLO -collector supply RLO - collector return The thermodynamic principle behind a compression heat pump is the fact that a gas becomes warmer when compressed into a smaller volume.
5.2. Working principle of geothermal heat pump 8 In a heat pump, the refrigerant is evaporated by the ground heat, the resulting gas is compressed and thus heated, and then the hot gas supplies its heat to the heating system. An alternative is the absorption heat pump , where heat at higher temperature is An alternative is the absorption heat pump , where heat at higher temperature is used to drive a activate desorption-absorption cycle. used to drive a activate desorption-absorption cycle. In both cases, the amount of external energy input (electricity or heat), has to be kept as low as possible to make the heat pump ecologically and economically desirable. The measure for this efficiency is the COP. For an electric compression heat pump, it is defined as: useful heat = COP electric power input
5.2. Working principle of geothermal heat pump 9 The higher the COP, the lower the external energy input compared to the useful heat. COP is dependent on: � the heat pump itself & � the temperature difference between the low-temp. side and between the low-temp. side and the high-temp. side. COP can be given for the heat pump under defined temperature conditions, or as an average annual COP, also called SPF. COP versus space heating supply temperature
5.2. Working principle of geothermal heat pump 10 COP variation with temperatures Evaporator , Compressor, Condenser, Expansion valve COP 1 Heat distribution system supply/ return temp. “Water-to-Air heat pump” or “Water-to-Air heat pump” or 60/50 � C 60/50 � C Conventional radiators Conventional radiators 2.5 2.5 “Water-to-Water heat pump”. 35/30 � C Floor or wall heating 4.0 Refrigerants with ODP=0. 45/35 � C Modern radiators 3.5 R 134a, R 407C, R410A, R404A 48/38 � C and propane fulfill these Hydronic convectors 3.5 conditions. 1 Heat source 5 °C
5.3. Geothermal heat pump systems and application 11 GSHP offers very good conditions for achieving high COP. A ground source heat pump system can be used not only for heating, but also for cooling. The configurations manufactured are: The configurations manufactured are: � water-to-air, � water-to-water, and � water to air split type. Basic schematic of water-to water GSHP system
5.3. Geothermal heat pump systems and application 12 Direct expansion heat pumps do not have an intermediate heat exchanger on their source side. The compressor operation circulates the refrigerant operation circulates the refrigerant directly around the loop. There is no need for a source side circulation pump – the compressor undertakes this role. Refrigeration cycle–direct expansion. [33] WNA-heat delivery system VLH-heating supply RLH-heating return
5.3. Geothermal heat pump systems and application 13 The water to water geothermal heat pumps are usually grouped together in a mechanical space, and can be treated as a conventional heater/ chiller plant. chiller plant. The unit sizes range from 3 tons to 30 tons. The most common type of heat pump used with GSHP systems is a “water-to-air” unit ranging in size from 3.5 kW to 35 kW of cooling capacity. Basic schematic of water-to air GSHP system
5.3. Geothermal heat pump systems and application 14 Sizing the heat pump. The capacity of a heating system is defined according to the max. heat demand of a given building. The max. heat demand, called the heat load, is calculated according to specific weather conditions and indoor air temperature. The optimum economic size of the heat pump The optimum economic size of the heat pump design capacity is normally in the range of 30 to design capacity is normally in the range of 30 to 60% of the maximum heat load of the building. Such a heat pump can cover between 60 and 90% of the annual heat demand. HP capacity and building heat requirement (without DHW) in a heat duration diagram
5.4. Overview of ground systems for geothermal heat pump 15 The ground system links the geothermal heat pump to the underground and allows for extraction of heat from the ground or injection of heat into the ground. These systems can be classified generally as: � � open, or open, or � closed systems. To choose the right system for a specific installation, several factors have to be considered: � Geology and hydrogeology of the underground area and utilization on the surface, � Existence of potential heat sources like mines, and � The heating and cooling characteristics of the building(s).
5.4. Overview of ground systems for geothermal heat pump 16 The various shallow geothermal systems comprise: � horizontal ground heat exchangers 1.2 - 2.0 m depth (horizontal loops) � borehole heat exchangers 10 - 250 m depth (vertical loops) � energy piles � energy piles 5 - 45 m depth 5 - 45 m depth � ground water wells 4 - >50 m depth � water from mines and tunnels. Systems using a heat exchanger inside the ground are called “closed” systems, while the once producing water from the ground with a heat exchanger above ground are called “open” systems.
5.4.1. Closed vertical loop Closed 17 vertical loop This systems consist of one or several system boreholes in which BHE are installed. The boreholes may commonly be up to 200 m boreholes may commonly be up to 200 m deep. The two possible basic concepts of BHE are: � U-pipes � Coaxial (concentric) pipes.
5.4.1. Closed vertical loop 18 During the winter season the temp. of the fluid and the borehole surroundings will gradually reduce, as also the heat pump COP. In a correctly designed system the temp. will not be as low as making the heat pump to stop. This is will not be as low as making the heat pump to stop. This is a great advantage of GSHPs compared to air as heat source. In the summer, these systems may provide free cooling. Distance between the boreholes.
5.4.1. Closed loop horizontal systems 19 The shallowest system. Compared to vertical loops - less investment to construct; somewhat less efficient due to a lower working temperature of the fluid. lower working temperature of the fluid. The main thermal recharge for all horizontal systems is provided mainly by the solar radiation. Closed horizontal ground heat exchanger Trench ground heat exchanger
5.4.1. Closed loop horizontal systems 20 More compact horizontal is the so called “slinky” system. The best efficiency of horizontal systems is The best efficiency of horizontal systems is obtained in fine grained types of soil with a high content of water, such as clay and silt. A variation of the horizontal ground source heat pump is direct expansion system. Closed horizontal-Slinky loop system
5.4.1. Closed loop horizontal systems 21
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