The anaerobic digestion of cow manure has a higher destruction of pathogens and weed seeds under thermophilic conditions compared to mesophilic conditions. To maintain such conditions, solar energy can be used. In this research, the consequences of the use of solar energy under Egyptian conditions are evaluated. In this study, experiments are combined with modelling. In the experimental part, anaerobic digestion on laboratory scale is studied in two continuously stirred tank reactors at 50 and 60 degreesC. Daily temperature fluctuations in the tank caused a decrease in methane production rate of only 12 and 20% at 50 and 60 degreesC, respectively. The results are used in a model for the thermal energy demand. In the model the net thermal energy production as a function of reactor volumes, thermal insulation and additional pre-heating of the influent is evaluated. The model results show that for continuously stirred tank reactors, additional pre-heater is not advised since it decreases the efficiency. The results also show that a maximum overall heat transfer coefficient of 1 W m(-2) K-1 is needed for reaching at least 50% of energy efficiency. Furthermore, adding a solar energy system improves the efficiency for large reactors only slightly, while for small reactors a large improvement is achieved. An energy efficiency of 90% can be reached. (C) 2003 Silsoe Research Institute. All rights reserved.