A new thermal storage system, a heat pipe heat exchanger with latent heat storage, is reported. The new system may operate in three basic different operation modes, the charging only, the discharging only and the simultaneous charging/discharging modes, which makes the system suitable for various time and/or weather dependent energy systems. In this part of the paper, the basic structure, the working principle and the design concept are briefly introduced. Extensive experimental results are presented of the charging only and discharging only operations, and the effects of the inlet temperature and the flow rate of the cold/hot water were also investigated. The results show that the heat exchanger performs the designed functions very well and can both store and release the thermal energy efficiently. (c) 2005 Elsevier Ltd. All rights reserved.