  Concentrated solar power (CSP) plants utilize two-tank, sensible-heat thermal
energy storage (TES) for uninterrupted electricity generation. However, the
cost for the design and operation of TES is expensive. Therefore, researchers
are focusing on implementing single-tank storage. Additional cutbacks can be
made by utilizing pump-less thermosyphon charging for the TES. But prior
thermosyphon researches for TES are related to domestic water-heating systems
of small-capacity (<100 liters) and low-temperature (<100 {\deg}C). Thus,
investigations into thermosyphon charging for high-temperature storage are
desired. This study focuses on thermosyphon-charging and storing of a
single-medium stratified TES. The experiments were conducted on a 370 liters
cylindrical storage (aspect ratio 4:1) with a heat-pipe system (3-liter volume)
acting as a collector. Dowtherm-A oil was used as the heat transfer fluid
(HTF), and the thermal expansion of HTF was accommodated in an expansion tank
via two different designs (top and bottom connections from storage tank to
expansion tank). Moreover, continuous and pulsatile charging are investigated
for low (150 {\deg}C) and high (250 and 300 {\deg}C) temperatures. The results
indicate that the maximum HTF temperature coming out of the heating pipes is
~25 {\deg}C more for the bottom-expansion design. Furthermore, it results in
higher charging efficiency than the top-expansion setup for high-temperature
studies. Finally, it is revealed that under design conditions, there are limits
on the degree of thermal stratification achieved in the charging cycle and the
maximum layover time allowable for interrupted charging. These results provide
insights into the operational strategy of thermosyphon-charging stratified
storage for CSP applications.
