Evaluation of efficiency of hybrid geothermal basket/air heat pump on a case study winery based on experimental data
Introduction
Ground Source Heat Pumps (GSHPs) are a combination of a heat pump and a number of Ground Heat Exchangers (GHEs), which exploit/inject heat from/into the ground, through the circulation of a working fluid in a closed loop circuit [1]. The GHEs use the insulation potential from weather conditions of the ground, increasing along depth, to provide significant heat transfer between the soil layers and the working fluid [2].
Several possible configurations of GHE exist, depending on the shape, geometry and material of the pipes and on the consequent installation technique into the ground. As regarding the shallowest GHE, which offset the thermal influence of ambient conditions with reduced installation costs with respect to deeper ones, the common configurations are the horizontal collectors [3], the slinky coils [4], the geothermal baskets [5] and the helical heat exchangers [6]. They are installed through excavations and in some cases by dry auger drilling; they can even be inserted into the foundation structures of buildings [7] and infrastructures [8]. The penetration depth into the ground does not generally exceeds 10 m. Deep GHEs are the Borehole Heat Exchangers (BHEs), which are installed through drilling, with the common add-on of drilling fluids (air or water, depending on the soil and rock types and groundwater conditions [9]).
BHEs are universally considered more efficient than shallow alternatives in exploiting/injecting heat from/into the ground [10]. The reason lies in the fact that they can relatively easily be placed everywhere and then reach relevant depths because of their vertical geometry, up to many hundreds of meters; therefore:
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if no drilling restrictions persist, the GSHP designers can decide for each borehole the optimum width of the heat exchange side area, according also to the effective available space at the surface for the installation of the geothermal field;
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heat exchangers can reach the so-called “neutral zone” where temperature is not influenced by weather conditions but it is constant over the year [11]; the “neutral zone” can usually be found among 15 and 30 m, varying according to the soil thermal properties [12]. Descending, geothermal gradient, with temperature increasing with depth, becomes relevant, affecting the BHE performance. Deep BHEs are generally used in GSHP projects with prominent heating needs;
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thermal behaviour of circulating fluid within the BHE can be finally affected by advection phenomena related to the groundwater presence. The possibility of descending to the confined aquifers increases the rate of heat exchange and speeds up the recovery period between two heat pulses, improving the overall efficiency of GSHP.
Among all possible configurations of BHE, the most common on the market are 80–100 m deep and 127–152 mm width, with single or double PE pipe. They currently represent an effective compromise between an efficient fulfilment of prominent heating needs and the containment of the available space at the surface and the drilling costs.
Because of these peculiarities, the BHE configuration is particularly efficient in GSHP projects when energy needs are prominently dependent on weather conditions. This is the typical case of heating and cooling of buildings. In general, the typical BHE is usually able to exploit from 4 to 5 kW of thermal power, without causing significant underground energy depletion on the long period.
With respect to all other alternatives, the main barrier limiting the choice of BHEs concerns the drilling, since it needs qualified professionals and high quality standards, to ensure safety and environmental protection. This leads to high initial investments in GSHP projects, with percentages varying per labour cost in different countries. In many soil and rocky conditions, the installation of BHEs can face high drilling costs, compared to the investment costs for fossil fuel burners, making unattractive to many end-users the selection of GSHP. In most countries, state and local incentives are provided to favour the choice of GSHP with respect to fossil fuels alternatives, and so encouraging the use of low environmental impact and energy saving technologies.
The shallow GHEs fall in the surface and shallow zones of underground, affected by ambient weather (daily and seasonal temperature variations, irradiation, wind speed, rainfall, snowfall, etc…). The shallow GHEs increase the heat exchange surface thanks to the spiral geometry exploiting more energy per meter of excavation with respect to vertical configuration.
The installation of shallow GHEs, through simple excavation, is generally economic with respect to BHEs, but their convenience is strongly reduced due to the following main issues:
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to cover heating and cooling needs the efficiency of GSHP is lower than the solution with BHE because of the weather influence on the underground. This affects the energy costs related to the working of the compressor of heat pump;
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each GHE, because of the limited depth, exploits a very limited total power from underground (basically 1–2 kW, varying according to the external conditions). Therefore, a larger number of shallow GHE is needed with respect to BHE configuration. The costs of horizontal connections partially reduce the advantage with respect to BHE;
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GHEs are practically suitable only for very low power demands, since they need a large area for the installation. With some few exceptions, they are almost adopted by single or double family houses in the countryside, where availability of free space is not a problem.
Although these considerations are true for final users governed by heating and cooling needs, a different matter relates to other types of final users, where thermal energy needs are partially affected by weather. A good example where shallow GHEs can have economic convenience with respect to BHEs is the agricultural sector and in particular the wine-making sector. The main issues and common problems are all tackled below:
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during wine fermentation and grape drying (September-November in the Northern Hemisphere), weather does not play a prominent role in the energy consumption. These processes require the highest quantities of cooling and/or heating in autumn. For these cases, the use of shallow GHE, whose performance is affected by weather, can lead to an efficiency increase with respect to BHE;
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even with improved efficiency in mid-seasons, the exploitable thermal power by a shallow GHE is still lower than a BHE; therefore, the issue of the horizontal connections costs persists. Nonetheless, the processes of agricultural sectors, especially for small family-owned farms, are not energy intensive so the peak powers are generally limited. Moreover, large part of the excavation costs for the installation of shallow GHE and horizontal connections can be covered by the farm itself, which usually disposes all the necessary equipment for excavation. With respect to installation of BHE, where drilling is a highly-specialised activity, the investment cost can be substantially lowered and simply included in the usual work of the farm;
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in agricultural sector, large areas are suitable for the GHE field; in fact the GHE field can be installed directly in agricultural used zones, specifically under rural roads, lawns and, for some types of crops, under cultivated fields without affecting the harvesting phase. Regarding influence of shallow GHE on qualities of products, further investigations should be conducted.
This paper presents a simulation of a shallow GHE field to address the energy needs in a case study wine farm in Northern Italy, specifically the wine farm “Branchini” in Toscanella di Dozza (BO). Previous studies allowed the authors to gather data about production, energy and water consumptions, weather, indoor and ground temperatures.
Moreover, a homemade cylindrical geothermal basket was installed in the courtyard of the farm and a Thermal Response Test campaign was carried out. Based on the results of the experimental campaign, a complete assessment of the operation of a geothermal basket field connected to a heat pump was performed. Then, a system optimisation, through the integration of an Air Source Heat Pump (ASHP) in hybrid mode, was conducted. The energy needs of the farm, calculated on the basis of on-site data survey, were used as inputs of the simulation.
Section snippets
Thermal response test campaign
The standard Thermal Response Test (TRT) procedure on BHE consists in providing a constant heat injection for a period of about 3 days, and then in measuring the inlet and outlet temperatures of the circulating fluid [13]. The Infinite Line Source (ILS) theory is the most common analysis method; it allows getting, traditionally by inverse analysis based on logarithm regression on data set, the ground thermal conductivity and the borehole thermal resistance of a BHE [14].
Variations from this
Application to the case study
The energy input data of the wine farm with the 2 energy scenarios AGR.min.mec and PUG.law.man were used to perform the simulation. Data on rainfall for the specific area for year 2013 (the same year of energy simulation) were also used.
The design tool defines the working temperature and consequent efficiency of GSHP and ASHP on a daily basis. For each heating or cooling day, efficiencies of GSHP and ASHP are compared and the best option is chosen for any situation. For each day, the
Results
Table 6 (for the configuration AGR.min.mec) and Table 7 (for the configuration PUG.law.man) illustrate the main results of simulation in the following three alternatives: hybrid GSHP/ASHP, hybrid GSHP/ASHP with irrigation, ASHP alone. Since minimum thresholds of efficiency are set at 2.5 for both heating and cooling, the maximum number of 20 geothermal baskets is set up in all conditions, after that ASHP intervenes. The differences in results between dry and wet conditions are appreciable by
Discussion
For the specific case study, Thermal Response Test was performed on a geothermal basket installed 2 m deep in low moist clay in countryside, which was supposed to partially cover the energy demand of a wine farm. Because of these starting conditions, the general performance of such a system could not satisfy alone the energy needs, and the contribution of the ASHP became fundamental in order not to thermally depleting underground, and then not to compromising the functioning of the system. Two
Conclusions
The paper presented the results of an experimental campaign − Thermal Response Test −on the working of a homemade geothermal basket in real environment and related heat flux in the surroundings. The results of the TRT campaign were used to simulate the behaviour of a geothermal basket field, in hybrid mode with an Air Source Heat Pump, to cover heating, cooling and process loads of a winery in proximity of the experimental set up. The software simulating the process was realised in the VBA®
Acknowledgements
Authors wish to thank farm winery “Branchini 1858”, Toscanella di Dozza (BO), Italy, for the availability of sharing data and the collaboration in the research concerning the phases of experimental campaign performed.
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