A central solar heating plant with seasonal ground storage is analysed by dynamic system simulations. A reference system, involving a collector area, water buffer storage and ground duct storage, is defined for typical Swiss conditions and simulated for several types of heat load. A methodology is established for the optimisation of the main system parameters. The thermal behaviour of such a system is highlighted. The short-term heat requirements are covered by the buffer unit, whereas the seasonal heat requirements are covered by the ground duct storage. As a consequence, a system such as this is intended to supply a large solar fraction ( > 50%). Optimal ratios between the main system parameters are sought for an annual solar fraction of 70%. An optimal buffer volume of 110 to 130 l per m(2) of collector area is obtained. The optimal duct storage volume and collector area vary respectively from 4 to 13 m(3) per m(2) of collector area and from 2 to 4 m(2) per MWh (3.6 GJ) of annual heal demand. They depend mainly on the specific heat losses from the duct storage unit. A large annual heat demand ( > 3600 GJ or 1000 MWh) and/or low temperatures in the heat distribution are essential for satisfactory system thermal performance. The spacing of the boreholes which form the ground heat exchanger of the duct store is fairly constant and is found to be about 2.5 m for a ground thermal conductivity of 2.5 Wm(-1) K-1. Some improvements of the system control are also investigated to assess the influence on the overall thermal performances of the system. They indicate that the system thermal performances are only slightly improved in contrast to the improvement brought by a simple but optimised system control. (C) 2000 Elsevier Science Ltd. All rights reserved.