A High Pressure/High Temperature (HPHT) Thermal Energy Storage (TES) system based on inexpensive pebbles as the heat transport medium will be discussed after reviewing low pressure TES systems under development and undergoing field tests. The particle-bed TES design developed at City College of New York in 2011 used 8 mm diameter alumina particles as the heat storage medium contained in a 10â long, 8â diameter stainless-steel pipe and heated up to 600 oC at 30 bar. The thermal storage capacity was, however, limited due to the small internal volume of the stainless-steel pipe that could be adequate for small-scale use. To scale up this design to become practical for large scale Concentrated Solar Power plants that can produce high pressure air or superheated steam from a solar receiver, a large-scale TES system needs to be designed without raising the capital cost. For CSP plants equipped with Direct Steam Generation (DSG) solar receivers, for example, a 5 MWth CSP plant built and operated in Turkey, the latent and sensible heats of high pressure, superheated steam or sensible heat of a high pressure, high temperature gas needs to be stored in the pebble-bed TES system at pressures above 60 bar and temperatures above 600 oC. Considerations of design features needed to reduce the capital cost but can meet the TES requirements will be discussed.