The Solar-Campus Julich is an area of 14 ha in the north of the existing University of Applied Sciences. Three independent partners are constructing low energy buildings on this site (heating demand similar to 144 MJ m(-2) a(-1)). To date (June 2000) an auditorium with a library has been completed as well as an additional laboratory building. The Students' Association Aachen has erected 23 houses with accommodation for 136 students. The houses are arranged in five rows, each of them demonstrating different kinds of modern energy-saving technologies for heating and ventilation. The energy utility in Julich intends to build industrial buildings and move their complete organisation from southern Julich to the Solar-Campus. A solar district heating concept, with seasonal storage, is planned to cover about 50-60% of the heating demand of all the buildings. The paper gives details of the design of the pyramidal seasonal storage tank including cost analysis. The top cover of the 2500 m(3) tank will be constructed of several insulated floating pontoons, which are connected to each other, so that it is possible to walk on it. The 1200 m(2) of collectors are distributed over the different buildings and coupled to the underground storage tank by either a 2-, 3- or 4-pipe distribution network. Due to the low energy demand, the overall energy consumption will be low (2124 GJ a(-1)). The heating cost consequently will be high: similar to0.17 DM/kWh with a conventional gas-based system, and 0.54 DM/kWh with the solar system including seasonal storage. (C) 2000 Elsevier Science Ltd. All rights reserved.