Further market penetration by solar thermal systems needs a solution to overcome the mismatch between solar energy supply and energy demand. The implementation of thermal storage in a solar system is of great importance for effective use of the intermittent solar radiation. There are many possibilities to store solar energy, directly or indirectly, depending on the application. For space heating purposes with a high solar share, the integration of long-term (seasonal) storage is necessary because of the mismatch between solar radiation and heating demand during winter. New attractive applications for renewable energy heating technologies in Austria are combined solar-biomass heating systems, both individual systems as well as in combination with district heating. Biogenic fuel sources in the form of firewood, bark and wood chips from the forests and as remnants from the wood processing industry are an obvious form of 'natural storage' for solar energy, locally available, which can be stored, transported and grow again. Biomass is therefore an optimal form of seasonal storage for solar energy and an attractive auxiliary fuel for solar heating systems. (C) 2000 Elsevier Science Ltd. All rights reserved.