Since 1995, two large solar thermal systems (1200 and 2400 m(2)) have been realized in The Netherlands for industrial purposes. It is expected that large solar systems will be used more often in the coming years. For that reason and for the benefit of successful solar projects, important practical experience with these large systems is reported. In 1995/1996, a system was constructed in Lisse for the purpose of agricultural drying and conditioning, consisting of a 1200-m(2) (high performance) flat-plate collector array and a 1000-m(3) water storage tank. The tank has been constructed as the foundation of a warehouse building and is situated below groundwater level. The combination of these two conditions, together with the other storage parameters, has given grounds for many extra design considerations compared to other underground storage units. Special details had to be designed, for these specific conditions, concerning leakages (in/out of the tank), anti-floating measures and underwater insulation which is stable at high temperatures. The lessons learned in this project can be of value for those considering the construction of this kind of underground storage unit. In 1996/1997, a 2400-m(2) system was realized in Breda at a confectionery factory (van Melle), in which the drain-back concept was used on such a scale for the first time. The various design characteristics for drain-back systems, as were already known from two decades of usage in small domestic hot water systems, were upscaled to a 'super drain-back' system. The scale of the project further emphasized the advantages of this concept in cost (both operational and investment), performance and safety. However, the size of the system also brought some unexpected side effects which should be taken into account in future designs.