The performance of a 2 × 500 kWhth thermal energy storage (TES) technology has been tested at the Masdar Institute Solar Platform (MISP) at temperatures up to 380 °C over a period of more than 20 months. The TES is based on a novel, modular storage system design, a new solid-state concrete-like storage medium, denoted HEATCRETE® vp1, - and has cast-in steel pipe heat exchangers. Measured data after specific intervals during various operation modes are analysed, and validation of system performance is done through direct comparison between measured values and numerically simulated performance. The demonstrated and measured long-term performance of the TES matches predictions based on performance simulations and proves the operational feasibility of the modular TES design. After accumulating close to 6 000 operational hours, inspection of extracted thermal elements prove that there is no degradation of the storage material, and no separation between steel pipes and storage material is observed. Measurements of core samples of the storage medium extracted from the TES confirms the material properties and stability. The thermal element design and storage material as demonstrated in the TES pilot has thus been proved to work in its final form with expected conditions and shows absolutely no sign of performance degradation. The modularity and simplicity of the TES design enables flexibility in scaling high temperature TES systems for among others industrial waste heat recovery, thermal power plants and concentrating solar power applications, thermal power plant.