The feasibility of employing stand-alone solar energy systems to meet demand-side loads depends strongly on providing appropriate solar energy storage. The present paper presents an efficient and economical, underground, thermal storage design to store hot water at a temperature of around 180?C required for running a double effect absorption chiller to cool a zero-energy-house in a desert environment. The performance of the design is evaluated employing a specially developed efficient mathematical model, for simulating the steady state radiation, convection and conduction processes occurring within the storage unit. The model is presented and analyzed, and employed to investigate the effects of various design parameters on storage efficiency. It is demonstrated that high storage efficiency may be reached, providing that appropriate insulation materials are used. It is also revealed that the soil conductivity has little effect on storage efficiency.