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Improving Thermal Efficiency of horizontal ground heat exchangers

Periodic Reporting for period 1 - ITER (Improving Thermal Efficiency of horizontal ground heat exchangers)

Reporting period: 2015-11-01 to 2017-10-31

The overall aim of ITER is to ensure the sustainability of ground coupled heating-cooling systems and especially the horizontal ground heat exchangers systems.
Key challenges are:
• to enhance the heat transfer of the ground surrounding the pipes creating thermally enhanced backfilling material (TEBM) suitable for horizontal systems;
• to assess the performance and the environmental impacts of new promising technological solutions as helix systems with and without TEBM;
• to monitor the results over time through direct measurements and numerical simulation
Thermally enhanced material have been studied mainly for grouting of the vertical closed systems, where the effects of silica sand or graphite additives were tested over bentonite and cementitious grout, both in laboratory and in situ case studies. Therefore, focusing the research activities on (i) identifying new TEBM mixtures for horizontal loop, (ii) conceiving new solution for heat exchangers coupled with TEBM and (iii) monitoring the trend in a real in situ case study will lead to a significant advancement in improving the performance of the horizontal systems, with interesting relapses on the practical and economic point of view.
The grain size distribution (texture) of unconsolidated material provides very important basic information for the estimation of the very shallow geothermal potential (vSGP). Therefore, research efforts are focused in the range from sand to clay material, in order to analyse the pore size distribution (texture-characteristic water content) and mineral composition (texture-characteristic proportion of quartz) of the quaternary sediments, usually interested by horizontal heat exchangers.
Since heating and cooling demands constitute almost 50% of the final energy demand in Europe, the development of geothermal energy systems and especially shallow geothermal solutions, reveals a huge potential in providing thermal energy for residential and tertiary buildings, thanks also to its local availability, manageability and flexibility. Projects and ideas challenging a better performance of shallow geothermal plants as those promoted by ITER are expected to support the European economic growth and technological excellence, boosting competitiveness and job creation, taking into account also the individual and collective well-being of citizens, protecting the environment and, accordingly, the human health.
The work performed during the project consists mainly of three research phases:
1. laboratory activities: the physical-thermal properties of more than 15 soil mixtures [(i) natural soil, (ii) pure sand and (iii) mixtures of pure sand and clay additives] have been tested under different water content percentages and different consolidation degree. The collected data constitute a valuable reference to support modelling for very shallow geothermal applications and to validate heat and water transport model in soil and, due to the specific methodological approach selected, can be directly related to in situ field test measurements. In general, bulk density and moisture content affect both the thermal conductivity of soils, responsible for the transfer of heat to the helix system. On one hand, an increase of soil density for the same moisture content lead to an increase of the thermal conductivity; on the other, for the same soil density, an increase in thermal conductivity is related to an increase of the moisture content;
2. field test monitoring concerns (a) monthly measurement of thermal conductivity and moisture content on surface; (b) continuous recording of air and ground temperature (T); (c) continuous climatological and ground volumetric water content (VWC) data acquisition.
In all soils tested in situ, when the system is not running, the amplitude of Tground waves is already reduced at 0.60 m depth compared to Tair. When the heat pump is running, Tground variations induced by the helix are no longer noticed at 0.30 m from the top and 0.50 m from the lateral margins of the helix itself. In situ, a decrease of VWC with depth is registered (no groundwater flow is present), followed by a reduction of thermal conductivity with depth. The influence of precipitation on soil moisture content is limited to the first 20-30 cm depth below the ground level (bgl), while at 80 cm bgl the main changes are related to T variations induced by running the VSG system in heating mode. In detail, in coarse sand material a gradual decrease of moisture content implies a rapid decrease of thermal conductivity, while on bentonite mixtures or loamy sands, the reduction is more gradual. Then, these materials are promising for a better performance of the helix if initial adequate moisture conditions (> 12.5%) are provided and maintained over time.
3. numerical simulation (simplified 2D) for the helix has been set up using the finite element interactive modeling system code FEFLOW according to the laboratory and monitoring data. However, due to the complex geometry of the helix and the contribution of several factors to the heat transfer (i.e. conduction and latent heat…) the model can still be improved, so the research will continue in order to refine this topic. In fact, when T drops below 0 °C, the VWC is noticeably reduced and the soil is dried out. However, even if the thermal conductivity of the unfrozen soil decreases, the helix take advantage from the release of latent heat during the phase change process. The greater latent heat storage capacity belongs to the fine sand with 15% bentonite, that shows in situ always a volumetric water content equals or greater than 25%, corresponding to the ideal amount of water to reach the maximum thermal conductivity for a given material and the best heat transfer with the VGS system. Anyway, periods of not operating condition must be respected to allow soil to regenerate its thermal energy content and thermal properties.
The challenge of ITER is to develop the green entrepreneurial possibilities related to the sustainable use of shallow geothermal sources. In addition, the project promotes an interdisciplinary approach to the study of low-enthalpy geothermal basins. It involves several sectors of Earth Sciences, such as pedology, geology, applied geology, and the exchange of knowledge and expertise with engineers and other environmental scientists and professionals (e.g. chemists, biologists, installers), fostering a continuous dialogue and comparison between the different fields.
The ITER project results increase the knowledge of thermal behavior of soils, by direct measurements performed both in laboratory and in situ. These data are a great resource to support modelling for very shallow geothermal applications with real data and to validate heat and water transport model. In addition, the observation of helix behavior when the ground temperature falls below 0 °C, using the latent heat storage capacity of the soil, opens new interesting research perspective also for the industrial partners.
ITER test site in Eltersdorf_the five sectors under monitoring
ITER_detail of helix_shallow geothermal collector