In a startup period for a solar thermal electricity generating system, a non-solar source of steam heats a downstream receiver (for example, a superheating receiver) prior to insolation being available. Insolation, once available, heats an upstream receiver (for example, an evaporator). The upstream receiver can be arranged in a recirculation loop with a steam separation drum, which may be bypassed during the initial heating of the upstream receiver by insolation. Once sufficient temperature and pressure have been reached, steam from the upstream receiver is directed to the downstream receiver by way of the steam separation drum to replace the non-solar source of steam. Heating of the downstream receiver using steam from the upstream receiver continues until a threshold temperature and pressure are reached. Insolation is then directed at both the upstream and downstream receivers to generate steam for electricity production by a turbine.