For a reliable dimensioning of the expansion vessel, knowledge of the steam volume (SV) emerging during the stagnation process is necessary. Unlike SPP, the steam volume is not a fixed parameter of the collector array, but is additionally dependent on the diameter and heat losses of the collector circuit pipework. The emerging steam volume during stagnation per collector aperture area vG can be calculated as follows:

At the moment of maximum SPP and SV, the specific steam volume in the collector array vG, coll has been determined to approx. 0.5 liters/m2 for all the types of collector arrays examined. The specific thermal heat losses of the collector circuit pipes (PHL) during stagnation Q* ; depend

on the steam temperature, the ambient temperature, the pipe diameter da and the thickness of pipe insulation dHI. A list of the calculated heat losses of the pipes is shown in Tab. 1.

Tab 1. Internal volume and heat losses (X = 0.05 W/mK) under stagnation conditions (AT = 115 K) per
meter of pipe. The number-combination of the pipe dimensions indicates the external diameter da and the
wall thickness sR in millimeters. dHI denotes the level of insulation thickness: 50% means, that the
insulation thickness is 0.5 times the external tube diameter.