This paper presents an innovative high solar fraction plant, operating since December 2011, which provides heating and cooling in an office building (426.6m2) in Athens. The design solar fraction accounts for 80%. The plant design includes solar thermal collectors (149.5m2), a cylindrical underground thermal energy storage (58m3), an absorption cooling machine (35kW), a cooling tower and a heat pump. The overall control strategy is presented focusing on the charging of the thermal storage and on the solar cooling operation. As solar collectors operate throughout the whole year and is the main heat source, an energy efficient control has to be implemented. In this plant the concept of critical radiation is adapted accompanied by variable mass flow rate. Regarding the absorption chiller the control strategy is based on the characteristic equation ΔΔt. The chiller operation will be controlled by the inlet cooling water temperature to the chiller. Initial measurements of the storage charging and heating operation are presented. The successful integration of the heat pump to the solar system is also indicated by the initial measurements.

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