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(a) (b) FIGURE 2. (a) Photograph of a cross-sectioned cylindrical heat exchanger element showing the steel tubes cast in HEATCRETE®. (b) Illustration of EnergyNest thermal energy storage pilot, with insulation surrounding the modules. Storage Medium Performance The unique HEATCRETE® concrete recipe used in the TES is developed in collaboration with HeidelbergCement and differs significantly from regular structural concrete; it has a higher thermal conductivity compared to regular concrete for effective heat transfer, at the same time being chemically stable for operation up to 450 °C, and with sufficient strength to withstand repeated thermal stress cycles. All parameters relevant to the performance of the material have been thoroughly tested and characterized in parallel with the construction of the TES. The mechanical strength (compressive strength at elevated temperatures) and thermal properties (TPS method [3]) of the concrete were measured by independent lab facilities at NTNU in Trondheim, Norway and The Fire Research lab at SP Technical Research Institute of Sweden. Normal concretes have rather poor thermal conductivity; in fact, in most applications such as in buildings one prefers concrete to be as thermally insulating as possible. Comparing the thermal performance to earlier demonstrations of concrete-based TES by DLR [4] (Fig. 3a & b) HEATCRETE® has both significantly higher thermal conductivity and heat capacity. The experience from the test lab is an accuracy of 2-5% for thermal conductivity and 5-10 % for diffusivity up to 500 °C given that the thermal contact between sample and material is good. High heat capacity is desirable since it reduces the storage volume, and high thermal conductivity enhances the heat transfer dynamics in the system [4]. The results shown in Figure 3a & 3b clearly show that for a given thermal energy storage capacity, less concrete will be required using the current HEATCRETE® than by previously available concretes. (a) (b) FIGURE 3. Measured thermal conductivity (a) and heat capacity (b) of HEATCRETE® after thermal conditioning over a temperature range from 20 to 450 °C (compared to DLR demonstrated concrete [4]). When the HEATCRETE® is first heated, free water and some chemically bound water will evaporate, diffuse and escape. Losing water by evaporation does not affect porosity, however, as the temperature increases, dehydration creates additional pore space and shrinkage. In the case of HEATCRETE® the water-cement ratio is very low by design, and hence the porosity remains very low ensuring good heat transfer capability and suitability for thermal energy storage applications. During the thermal conditioning process (first start-up), the TES is heated in a slow and 080011-3PDF Image | EnergyNest thermal energy storage (TES) technology
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