Deliverables Determination of the thermal characteristics of a saturated sand layer The thermal characteristics (borehole resistance, thermal conductivity) of an underground type are strongly influenced by local circumstances such as homogeneity, sand/silt/clay content, water saturation, and groundwater level. Within this project the characteristics of a homogeneous water saturated sand layer are measured in-situ by logging temperatures and flows while supplying heat to a vertical heat exchanger. This so called TRT-test (Thermal Response Test) also provides information on the borehole refilling material as three tests on three different boreholes with three different fillings were performed. In the past, the thermal characteristics of the underground were estimated based on experiences and lab tests. This led to a wide range with a recommended value, e.g. water-saturated sand has a conductivity between 1,6 and 5,2 with 2,5 (W/(mK)) as a recommendation. It's clear that significant differences can occur between the best and worst circumstances and that in one case the value is overestimated while another case leads to an underestimation. Lab tests can indeed contribute to a better estimation but an in-situ measurement takes in account all influencing parameters. In water saturated sand layers, little experience with TRT-tests was gained in the past. It was also a surprise that borehole filling with drilled out sand provided the best results compared to a filling with a sorted sand mixture. These conclusions gave better insight in creating optimal borehole storage design. The thermal characteristics are not only of importance in huge borehole energy storage projects. Ground coupled heat pump systems also make use of vertical heat exchangers. There is a significant growth in the yearly installed heat pump systems all over the world. Installation designers do not know the performance of the source side of a heat pump system very well. In the past, this led to installation problems or even failures that had a negative impact on the heat pump market growth. The accurate determination of thermal conductivity and borehole resistance leads to an optimal system design that brings on two main advantages: a reliable and efficient system operation and high cost-effectiveness. Designers of underground thermal energy storage and large geothermal heat pump systems can benefit from a better insight on the thermal characteristics of the underground. Nowadays, they rely on values from literature and they neglect the importance of the borehole resistance. Thermal enhanced grouting can be developed in order to reduce this resistance value. This can contribute to a higher system power capacity, in many cases the major bottleneck in the design of an energy storage or heat pump system. At residential heat pump systems, drilling companies often advise on the heat source system. A careful preparation by examining the thermal characteristics is also for them of great importance in order to guarantee long-term system reliability. Analysis of the heat transfer and flow in saturated sand The TESSAS installation is, as a demonstration plant, equipped with a unique underground temperature monitoring system. In total, 66 temperature sensors were placed at various places on different depths inside, below and outside the energy storage field. Due to the continuous logging of ground and flow temperatures as well as flow rate, a complete view of the energy transfer to and temperature spread in the underground could be obtained. These efforts allowed an analysis of the heat transfer in the water saturated sand layer. Many influencing parameters on this heat transfer and distribution can be determined as there are ground water flow, hydraulic conductivity and heat convection. Heat transfer in water saturated sand layers is thereby much more complicated in comparison with heat storage in clay or solid rock. Due to the storage of heat at a high temperature level, the described effects are even more significant. The obtained results are very valuable in understanding the heat transfer process in the underground. At the moment, some heat transfer processes arent described very well. Especially the heat convection flow becomes very important at high temperatures, especially for sediments with a significant hydraulic conductivity. This led to increased heat losses through the top of the storage field. This analysis is valuable for researchers dealing with underground thermal energy storage in order to optimize borehole storage design. In future, adaptations on the simulation tools are necessary in order to include all relevant parameters (convection flow, ground water flow). Integration of a high temperature storage field in an existing (building or process) heating system The integration of renewable / high efficiency energy systems in existing installations led to numerous problems in the past. Those installations were designed according to traditional rules and uses. The change of heat provider can result in bad performance. The typical problem regards the (too high) return temperature from the heating system of the building. Many condensing boilers do not condensate because of this reason. The higher investment doesn't pay off in that case, as boiler efficiency doesn't improve. The execution of the TESSAS project also made clear that special attention to efficient system integration is of main importance. Even if individual underground storage performance is superb if the building system doesn't match with the storage system, total system performance can be disastrous. The same arguments can be stated for process heating systems. In order to meet with these needs, the design of an underground storage or heat pump system is performed with a simulation tool that also has extensive building simulation tools (TRNSYS software). Due to it's modular structure, the set up possibilities are various and all kinds of innovative energy systems can be evaluated. Short and long term system dynamics can be analyzed. In order to improve system performance, installation adaptations may be necessary. A combination of expertise on underground, innovative and traditional energy systems is required in order to cope with these challenges. Specialists on every field need to be gathered for a successful result. Development of a system for storing large quantities of high temperature heat underground in a saturated sand environment Availability and use of thermal energy doesn't always match with each other. This leads to the destruction of large amounts of energy, day in day out. The development of a huge underground energy storage system provides a solution for storing the heat on a long-term base (several months). Traditional tank storage systems are far to limited in size for this application. Especially for high temperature heat this solution becomes interesting as it can be recuperated for direct use (without the use of heat pumps). The design and construction of such a system is significantly different to the traditional heat extraction by heat pumps. This system leads to large energy and cost savings which has a significant contribution to greenhouse gas emission reduction. For large storage systems (>1GWh of stored heat) very interesting economical perspectives can be reached. There are numerous applications for this technology. All kinds of heat residues can be stored in the underground : waste heat from power plants, heat from industrial processes, condenser heat from chillers, heat from incinerators, solar heat, heat from a combined heat and power plant and even environmental heat during summer. All kinds of end-users can benefit from this technology: industry, large building owners, office buildings, schools, hospitals, (in short, every one who has "waste" heat or a solar heat installation or a CHP plant). Solar heat applications often use only a fraction of the heat that can be captured with the system, as heat demand during summer time often is limited to sanitary hot water. Combined heat and power on the other hand, can't operate during summer time due to the same reason. Storing the solar or CHP heat during summer time with recuperation during winter also saves on capacity of the required heating system. Searching for OpenAIRE data... 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