At national level and most European countries, the air conditioning of buildings accounts for 30% of the total primary energy demand. To reduce this energy consumption, there are essentially two solutions:
Reduce the needs of the building, mainly by acting on the envelope (better insulation).
Use highly efficient systems, if possible emplying renewable energy resources.
From an economic, performance, and environmental point of view, heat pumps are the most advantageous solution to this problem. In particular, geothermal heat pumps are systems for the air conditioning of buildings coupled with the ground through a system of ground heat exchangers (borehole heat exchangers when vertical), which allow very high energy performance to be reached. In Europe, it is estimated that 80,000 geothermal heat pumps are installed every year (data referring to the years 2016 and 2017), for a total of approximately one million plants currently operating in Europe (approximately 2 million in the USA and Canada).
With reference to the environmental sustainability, the geothermal heat pump allows equivalent emissions of CO2 equal to approximately one-third of the ones related to latest-generation natural gas boilers. This fraction is moving to zero, as the electricity production fraction is moving towards 100%. Concerning the emissions of CO2 related to the production of electricity in Italy, the table below shows the CO2 equivalent emissions per thermal kWh for the various technologies:
0.315 kg / kWh for an electric resistance heater
0.26 kg / kWh for a gas boiler
0.072 kg / kWh for a ground source heat pump with COP = 4
Below the earth's surface, the ground temperature increases approximately linearly with depth, according to a geothermal gradient generally in the 0.02-0.03 K / m range. Quite rare "geothermal anomalies" (due to surface magma pockets) and the presence of deep water saturated soils are the exceptions to the above rule. The strategies to exploit the heat available in the ground can be different and 3 conditions can be identified based on the depth and temperature of extraction:
Heating with heat pumps: 0 - 1 km (10 ° C - 30 ° C)
Direct heating without heat pumps: 1 - 3.5 km (30 ° C - 100 ° C)
Production of thermal and electrical energy: 3.5 - 6 km (100 ° C - 200 ° C)
Low-temperature geothermal energy (10-30 ° C), therefore at low depths (hundreds of meters up to 1 km), uses the technology of heat pumps consisting in the traditional reverse thermodynamic cycle with vapor compression (and more rarely with absorption ). The evaporator of the heat pump is thermally connected to the ground through a system of ground closed-circuit exchangers in which a vector fluid evolves (typically water possibly with additives). The most frequently used ground heat exchangers are the vertical ones (BHE, Borehole Heat Exchangers). During heating operation (winter mode), the heat is taken from the lower source (the ground) and, by supplying the mechanical energy necessary for the compressor, the useful heat is made available to the upper source (the building). During operations in cooling mode (summer mode) the geothermal fluid flows in the condenser of the heat pump system and releaseds heat to ground, possibly to be partially recovered during the following winter operations.
Principle of operation of a heat pump
As it is well known, the COP (coefficient of performance) of a heat pump increases as the evaporator and condenser temperatures get closer to each other. The ground, therefore, constitutes a thermodynamically very advantageous solution because the average temperature is higher during the winter and lower during the summer season with respect to ambient air temperature on which Air Souice heat pumps work with. Furthermore, unlike the air heat pump, which is affected by changes in the temperature of the outside air, the geothermal system works in fairly stable conditions since the ground temperature is very constant over time starting from 10-20 meters of depth and it is not affected by daily and seasonal temperature fluctuations. Worth noticing, building side, high COP values (in winter operations of the order of 4) can be obtained with proper condenser temperatures, say with hot water distribution systems working at low temperatures (30-40 ° C), such as radiant floors and fan coils.
Functional and system diagram of a geothermal heat pump (GCHP, Ground Coupled Heat Pump)
The correct sizing of the borehole heat exchangers has the greatest impact on the performance of the plant and, especially considering the high cost of drilling the BHE field, also on its economic sustainability.
The ground heat exchangers are peculiar ones, since no steady state operating condition is possible. This is related to ground huge thermal inertia and borefield large extension. Furthermore the building itself and the heat pump are always working in dynamic mode. For these reasons all methods for simulating the thermal behaviour of a ground heat exchanger field rely on the solution of the Transient Fourier Equation, with proper superposition techniques to be applied, with Fourier number horizons ranging from hours to decades.
If the heat pump system, and in particular the borehole field, is correctly sized (not excessive intensity of heat extraction Q '[W / m]), the temperature of the ground surrounding the heat exchanger sets few degrees less (winter mode) or more (summer mode) than the undisturbed ground temperature. In such a condition the performance of the system remains high in time; on the contrary, if the borefield overall length L is "not enough" , the ground temperatures in the borefield can slowly reach temperature too low or to high with respect to the design COP.
The knowledge of the thermophysical properties and of the thermal response of the ground in which the BHE field operates is of fundamental importance. The modeling of the heat transfer rate exchanged to the ground is necessarily time-varying, as a function of the variable heat load of the building.
The borehole heat exchangers can be vertical or horizontal. Vertical type heat exchangers (borehole heat exchangers, BHEs) have the advantage of being minimally invasive on land and are installed by making a perforation (usually 100-150 m deep, with a diameter of about 0.15 m) and lowering the pipes inside it (single U-pipe, double U-pipe, or coaxial pipes). The volume between the U-pipes and the wall of the perforation is filled with cementitious filling material (grout) which remains plastic over time and guarantees good thermal contact between the pipes and the ground, with thermal conductivity of up to 1.5-2 W / mK.
U-shaped pipes represent the most frequently adopted solution since they can be installed into the borehole more easily than coaxial heat exchangers, which are generally more rigid and non-deformable and present considerable difficulties associated with tolerance problems at the time of installation. Unlike the U-shaped exchanger, in fact, the coaxial exchanger is already in contact with the ground without the need for grout. The pipes are inserted into the drilled hole with the aid of a weight of about 20 kg hooked to the head of the BHE which counteracts the buoyancy thrusts and reduces the risk of jamming against the lateral walls of the drilling itself.
U-shaped pipes are generally in polyethylene (PN10, PN16 depending on the nominal operating pressures in bar) or, less frequently, in reinforced polyethylene called PE-XA which guarantees better mechanical properties at high temperatures (up to about 60 ° C) .
The pipes are preferably inserted into the perforations equipped with special pre-molded plastic spacers so that they are kept in contact with the surrounding ground as much as possible and the effects of thermal short circuiting, harmful to the performance of the system, are reduced.
The Thermal Response Test (TRT)
The value of the ground thermal conductivity kgr has a considerable influence on the performance of the borehole heat exchangers. Even more than the density and specific heat, the ground thermal conductivity represents a fundamental parameter when sizing the plant and the geothermal borefield. Unfortunately, while the density ρ and the specific heat c of the ground assume a rather defined range of values (2200≤ρ≤2900 [kg / m^3], c≃850 [J / kgK]), the thermal conductivity kgr presents a wide variability (1 ÷ 5 W / mK) due to the rock type. Therefore an in situ measurement of the ground thermal conductivity is necessary. Usually, the thermal conductivity is experimentally estimated through a procedure called Thermal Response Test (TRT), for the first time proposed and implemented by the Swedish engineer Palne Mogensen in 1983.
The test consists in circulating, in a pilot heat exchanger buried on site, a carrier fluid flow rate. The inlet and outlet fluid temperatures are measured and the average temperature calculated, denoting a time-varying trend T f, ave (τ).
During the test, a constant heat transfer rate over time is conferred to the carrier fluid above ground by means of an electric heater. It is assumed that this heat transfer rate is then uniformly distributed in the ground with depth. The same principle can be applied by extracting heat transfer rate from the carrier fluid and the ground (“TRT in cold”). In this case, the measuring equipment becomes more complex, especially from the point of view of regulation.
The first hours of the experiment are dedicated to the circulation of the carrier fluid in the absence of the heat transfer rate provided by the external heater. This first phase must have a sufficient duration (2 or more hours, depending on flow rate and BHE depth) for the carrier fluid to mix and reach the depth average undisturbed ground temperature T gr, ∞ .
Subsequently, the heat transfer rate begins to be injected through the electric heater; the physics of the phenomenon related to the time-varying thermal diffusion in the ground within the typical time window of the TRT (about a hundred hours) suggests that the temperature field in the ground can be considered one-dimensional. For this reason, the analytical solution provided by the Infinite Line Source (ILS) model is chosen as the reference interpretative model of the temperature measurements taken during the test.
During the first part of the thermal transient (10 hours as an indicative value, according to the Fourier number related to the BHE volume) the thermal resistance of the borehole Rbhe plays an important role, which is actually related to its thermal inertia associated with the mass of material constituting the grout, the pipes, the carrier fluid.
According to the ILS model, when the thermal transient associated with R bhe is ended, the average temperature of the fluid T f, ave (τ) assumes a linear trend as a function of time on a semilogarithmic scale. From the slope of the regression line of the T f, ave (τ) experimental points it is possible to deduce the value of the ground thermal conductivity k gr .
Finally, once the ground thermal conductivity k gr has been inferred, the knowledge of T gr, ∞ allows estimating the thermal resistance of the borehole R bhe .
In order to be able to suitably apply the ILS interpretative model, the stability over time of the heat power supplied to the carrier fluid must be ensured during the TRT test. This aspect usually constitutes a criticality that can compromise the reliability of the experimental measurements and their elaboration. The research group coordinated by Prof. Fossa has designed, built and patented (in collaboration with the Erde Company of Acqui Terme and with the fundamental help by Francesco Berti, Imola) a Thermal Response Test machine (TRT machine Dime,  ) capable of solving this problem, by controlling constantly the heat transfer rate supplied to the fluid through appropriate electronic instruments onboard the machine.
This innovative TRT machine is available for any collaborations in order to carry out measurements of the ground thermophysical properties also in the "pulsated TRT" mode, allowed by the machine and the measurement interpretation algorithms .
TRT machine developed at DIME.
PID control over three-phase TRIAC is able to stabilize the heat transfer rate even during the daily oscillations of grid voltage and to keep constant the flow rate as fluid viscosity changes due to the temperature evolution. Control is also available on 3G transmission
The Distributed Thermal Response Test (DTRT)
The classic TRT procedure is performed by measuring the temperatures of the above-ground fluid. However, there is also the possibility of carrying out this measurement campaign by detecting the fluid and the pipe temperatures at different ground depths during the so-called Distributed Thermal Response Test, DTRT. This type of application requires robust, reliable and, if possible, low cost sensors.
In this context, for example, optical fiber sensors (Optical Fiber T sensors) are used. These sensors however have the disadvantage of requiring very expensive equipment; The Reader is addressed to to the work by Acuña  for an in-depth analysis of technical issues, sensor installation methods and the analysis of DTRT-type temperature measurements.
The operating principle underlying temperature measurements using optical fiber sensors consists of Back Scattering (Raman Scattering) obtained through the use of a pulsed laser (a pulse of monochromatic light) which is sent and runs through the entire optical fiber. By acquiring the signal and therefore knowing the time that elapses between the sending and the return of the signal itself, it is possible to go back to the depth in which the temperature was recorded. By also measuring the temperature difference between two successive points along the pipeline, it is possible to estimate the heat transfer rate locally transferred by the carrier fluid in the pipeline section.
Local measurements with optical fibers require specific instrumentation of high cost (40k € typical) and are subject to systematic errors related to the connections between different parts of the fiber. These two reasons are the basis of new DTRT techniques based on the use of digital temperature sensors that are discussed on this website in section Innovative TRT.
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