With the consideration of hydraulic dissipated energy, the influence of collector aspect ratio on the modified collector efficiency of sheet-and-tube solar wafer heaters has been investigated theoretically. As the collector area and the distance between tubes is fixed, the collector efficiency increases with increasing collector aspect ratio (i.e., either increasing tube length or decreasing tube number). However, based on the same hydraulic dissipated energy, decreasing the collector aspect ratio enhances the performance. (C) 2003 Elsevier Science Ltd. All rights reserved.