A new TRNSYS collector model,for evacuated tubular collectors with tubular absorbers is developed. Traditional flat plate collector performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbor tube. An all glass tubular collector with tubular absorbers with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility Performance calculations with the new model are compared with measured results and a good degree of similarity between the measured and calculated results is found. Further it is illustrated how the model can be used for geometrical parameter studies both for constant collector mean fluid temperatures and for varying temperature conditions in a solar heating plant. These investigations art performed for two climates: Copenhagen (Denmark) and Uummannaq (Greenland).