Concentrated Solar Power (CSP) is one of the most promising ways to generate electricity from solar thermal sources. In this situation, large tracking mirrors focus sunlight on a receiver and provide energy input to a heat engine. Inside the receiver the temperature can be well above 1000°C, and molten salts or oils are typically used as heat transfer fluid (HTF). However, since the sun does not shine at night, a remaining concern is how to store thermal energy to avoid the use of fossil fuels to provide baseline electricity demand, especially in the late evenings when electricity demand peaks. In this study, a new method will be investigated to store thermal energy underground using a borehole energy storage system. Numerical simulations are undertaken to assess the suitability and design constraints of such systems using both molten salt as HTF.