GeoSensingDesign.org
Geothermal technology
Geothermal heat pumps are highly efficient air conditioning systems for buildings (here a video on Rainews, in Italian) and countless evidences from scientific literature identify them as the most effective system (in terms of energy savings and reductions in greenhouse gas emissions) for the air conditioning of buildings for civil and industrial use. At a national level, air conditioning of buildings accounts for 30% of total primary energy consumption, which is why energy efficiency in this sector is more important and beneficial than ever.
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The geothermal heat pumps are coupled to the ground through a system of ground heat exchangers (vertical ones are named Borehole Heat Exchangers), whose sizing requires a through knowledge of the thermal characteristics of the ground.
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This section is dedicated to the description of geothermal heat pump technology, to better understand their operation and the advantages deriving from their correct use.
All-in-one patent
Given the importance of ensuring correct sizing of the geothermal heat pump system, the present research group has proposed an innovative procedure, called EDDTRT (Electric Depth Distributed Thermal Response Test). The estimation of the thermal conductivity of the ground surrounding the BHE is increased in terms of reliability by introducing an appropriate instrumented heat exchanger proper of the EDDTRT innovative procedure. The proposed invention is related to the UniGE Italian Patent n. 102019000023082 ("Method and device for the measurement of geothermal parameters for sizing and subsequent monitoring of geothermal heat pumps").
A) U-tubes of the underground heat exchanger
B) Extruded tube rib, to contain the instrumented cable with digital temperature sensors
C) Waterproofed digital temperature sensors. They are placed every 0.5-2 m along the exchanger
D) Metal spacer that keeps the heater E in a central position, and acts as an elastic element between the pipes
E) Electric heater
The EDDTRT system allows continuous measurement of temperatures in defined positions along the entire BHE length. Thanks to these measurements and proper reverse problem techniques, it is possible to estimate the local (layer by layer) ground thermal conductivity and even the filling material (grout) one. The present invention also allows the measurement of the BHE operating parameters for monitoring and controlling the operation of the plant during the years. Temperature monitoring along the heat exchangers can be used to control the operation of the heat pump and assure the best conversion efficienciy (COP) avoiding abnormal or dangerous operating conditions (ground temperatures too high or too low). The monitoring of the temperatures in the ground also allows the detection of unexpected geological situations, such as the presence of groundwater movements in the different soil layers.
The present invention also involves a device for reading the measurements and the defintion of the suitable pipe geometry for realizing a of "all-in-one" heat exchangers equipped with a heating cable and temperature sensors. The patent and its industrial sustainability have been confirmed by an extensive experimental validation (the PoC Experiment) that confirmed the numerical simulations carried out in the patenting phase.
PoC experiment
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The experimental validation related to the UniGE Italian Patent n. 102019000023082 (“Method and device for measuring geothermal parameters for sizing and subsequent monitoring of geothermal heat pumps”) was financed by Liftt / Compagnia di San Paolo (PoC tender).
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As part of the project, an experimental scale model was created, consisting of a block of rock (slate) of suitable size that simulates the ground around a real heat exchanger for geothermal applications (at Fourier numbers related to TRT durations). The dimensions of the slate sample (80x80x40cm, about 700kg) were determined through an accurate preliminary evaluation based on dimensionless analyzes and numerical simulations (Comsol MultiPhysics). The prototype of the piping for the “all-in-one” system was then inserted inside the block of slate, assembled with spacers between the pipes, a heater and a system of temperature sensors. The pipes, suitably shaped, were created through the use of a small 3D printer.
Background Video: Credits to www.videvo.net
Ground Heat Exchanger Design ( BHEDesigner8 )
The correct sizing of the geothermal heat exchangers field (borehole heat exchangers) is essential for the correct operations of the heat pump system for building heating and cooling. The present research group has been active for over 15 years in the dynamic modeling of geothermal systems using vertical ground heat exchangers (BHE, borehole heat exchangers) which represent the most popular type in Europe. The dynamic thermal response of the soil is described by models that adopt suitable numerical techniques for superimposing the effects and using mathematical solutions of the Thermal Conduction equation known as Temperature Response Factors or g-functions. The present research group has developed a BHE field design algorithm named Ashrae / Tp8, further described in the BHEDesigner8 theory section. The algorithm is available here as a free calculation tool (a "world premiere") on a dedicated page. The inputs of the algorithm are the monthly thermal demand of the building, the thermophysical properties of the ground and the characteristics of the heat pump. The output of the program is the geometry of the BHE field for assigned expected temperatures of the geothermal fluid at 10 years, as required by the Ashrae Method and by the Italian Standard Uni 11466.
Various procedures, aimed at the correct design of the borefield, have been developed over the years and some of them are implemented in commercial softwares. A well known method is the one proposed by Kavanaugh and Rafferty (K&R), which constitutes the reference for the design of BHE fields adopted by the American Society of Heating Refrigerating and Air-Conditioning Engineers (ASHRAE).
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The research group of the University of Genoa (Unige, Department of Mechanical Engineering, Dime) has been active for over 15 years in the dynamic modeling of geothermal systems using ground vertical exchangers and has developed, starting from the original ASHRAE method, a BHE field design algorithm named ASHRAE / Tp8 , widely described and validated in a series of scientific papers in international journals [1] , [7] , [15] , [17] .
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This method, based on the work of the research team coordinated by Marco Fossa, is here made available as free of charge sizing tool (BHEDesigner8 web-app) . To date (2022), BHEDesigner8 is the only (and completely "free") web-app on the entire internet. It will be constantly developed and updated free of charge, as a tool aimed at technicians, engineers and researchers involved in GCHP applications for the realization of heating/cooling systems with minimum environmental impact.
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The inputs of the algorithm are the thermal loads of the building, the thermophysical properties of the soil and the characteristics of the heat pump in terms of expected fluid temperature for target COP. The output of the BHEDesigner8 web-app is the geometry of the BHE field (number of BHEs, depth, their arrangement) for assigned expected temperatures of the geothermal fluid at 10 years, as required by the ASHRAE method and by the Italian Standard Uni 11466.