A flat-plate solar collector for process steam production was developed. The operating temperatures are in the range between 100 and 150 degrees C. The boiling collector can be used for process heat supply in the industry and for solar cooling applications as well. It operates as a system with controlled influx of liquid. instabilities of the two-phase Bow in the internal evaporator have been successfully suppressed and the design of the system has been investigated. We constructed a prototype collector based on a commercially available evacuated flat-plate collector. To realise high thermal efficiencies at temperatures up to 150 degrees C, the thermal losses of the absorber have been drastically reduced using an ultra low emissive Selective absorber, a low pressure krypton filling (50 hPa) in the collector casing, and a highly reflecting aluminium foil between absorber and rear side. The prototype collector was dynamically tested at our outdoor test facility and showed very high efficiencies of more than 60% at 100 degrees C steam temperature and of 45% at 150 degrees C steam temperature (T-amb = 15 degrees C). The operation behaviour of the prototype was always stable and the steam mass quality showed excellent values of nearly 100%.