GEOTHERMAL SYSTEMS AND TECHNOLOGIES 5. SHALLOW GEOTHERMAL SYSTEMS - - PowerPoint PPT Presentation
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
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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.
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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:
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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.
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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
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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.
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Refrigeration cycle of a CHP. WNA -heat delivery system VLH
The most common type of heat pump is the compression heat pump (CHP).
VLH
RLH
WQA -heat collection system VLO
RLO
The thermodynamic principle behind a compression heat pump is the fact that a gas becomes warmer when compressed into a smaller volume.
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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 used to drive a activate desorption-absorption cycle. An alternative is the absorption heat pump, where heat at higher temperature is 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:
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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
COP versus space heating supply temperature
between the low-temp. side and the high-temp. side. COP can be given for the heat pump under defined temperature conditions,
as an average annual COP, also called SPF.
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Heat distribution system supply/ return temp. COP1 Conventional radiators 60/50C 2.5
Evaporator, Compressor, Condenser, Expansion valve
COP variation with temperatures
“Water-to-Air heat pump” or
Conventional radiators 60/50C 2.5 Floor or wall heating 35/30C 4.0 Modern radiators 45/35C 3.5 Hydronic convectors 48/38C 3.5
1 Heat source 5 °C
“Water-to-Air heat pump” or “Water-to-Water heat pump”. Refrigerants with ODP=0. R 134a, R 407C, R410A, R404A and propane fulfill these conditions.
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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:
Basic schematic of water-to water GSHP system
The configurations manufactured are:
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Direct expansion heat pumps do not have an intermediate heat exchanger
Refrigeration cycle–direct expansion. [33] WNA-heat delivery system VLH-heating supply RLH-heating return
directly around the loop. There is no need for a source side circulation pump – the compressor undertakes this role.
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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.
Basic schematic of water-to air GSHP system
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.
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Sizing the heat pump. The capacity of a heating system is defined according to the
calculated according to specific weather conditions and indoor air temperature. The optimum economic size of the heat pump design capacity is normally in the range of 30 to The optimum economic size of the heat pump 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%
HP capacity and building heat requirement (without DHW) in a heat duration diagram
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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:
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).
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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 5 - 45 m depth energy piles 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.
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Closed vertical loop system
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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.
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The shallowest system. Compared to vertical loops - less investment to construct; somewhat less efficient due to a lower working temperature of the fluid.
Closed horizontal ground heat exchanger Trench ground heat exchanger
lower working temperature of the fluid. The main thermal recharge for all horizontal systems is provided mainly by the solar radiation.
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More compact horizontal is the so called “slinky” system. The best efficiency of horizontal systems is
Closed horizontal-Slinky loop system
The best efficiency of horizontal systems is
high content of water, such as clay and silt. A variation of the horizontal ground source heat pump is direct expansion system.
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If there is surface water available, the cheapest geothermal heat pipe system
Closed loop system submerged in surface water
cheapest geothermal heat pipe system could be build. Coils should be fully soaked in water in the depth of at least 2.4 m below the surface.
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Groundwater systems are more efficient than closed loop systems. The technology “normal” groundwater wells is used for energy extraction. energy extraction. The temperature
groundwater is practically constant all over the year and as such it is the best carrier of thermal energy. Open ground water loop system
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The limitations can be physical, such as climate and geological circumstances, but may also be connected to other site conditions. The other potential limitations could be of a social, cultural or political nature, but more often economical or legal. more often economical or legal.
For systems using the underground for seasonal storage of heat and cold, the source
waste heat from industrial process cooling waste cold from heat pump evaporators, technical limitations such as load, duration, temperatures, availability.
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The geological requirements differ according to what type of system is to be installed: Closed loop systems are in general applicable in all types of geology. Open systems require a geology containing one or several aquifers.
The hydro geological conditions in practice govern the design of any open loop system. For the design and realization of such systems essential are: type of aquifer, geometry, groundwater level and gradient, textural composition, hydraulic properties and boundaries. For closed loop systems these parameters are of less importance, but can in some cases constitute limiting conditions.
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Climate plays an important role in the application of GSHP systems. One essential condition is that the ambient temperature of the ground is reflected by the average temperature in the air. Another climate factor is the humidity. In hot climates with a high humidity, Another climate factor is the humidity. In hot climates with a high humidity, there will be temperature requirement for cooling that allows condensation.
GSHP energy systems will in general contribute to less global emission of carbon dioxide and other harmful environmental substances.
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