The book presents an innovative design methodology for geothermal heat pump systems (GHP) with phase change storage (PCS) as well as the economic evaluation of the initial costs and the payback time of the plant. In particular the attention is addressed towards systems utilizing vertical U shape exchangers made in polyethylene (geothermal probes) with application for heat pumps. At the same time, the realization of a software for the design of the two PCS (one for heating and the other one for cooling) is fully presented, considering the predicted consumptions of a building based on a cluster of weather data lasting 15 years. To do this, the yearly performance of a tertiary building located in Chieri, Italy, is simulated with TRNSYS code and CLIMAWIN code. The output (hourly thermal loads for all the days of the year) has been the input for the design and optimal balance of the phase change storage size, the heat pump size and the number of vertical probes. The result obtained has been the input for the payback analysis, comparing this system with a conventional one. Payback has been found in 8.1 years.