For single-pass open-loop solar thermal energy systems, in which the flow rate can vary with radiation but is constrained to have a given annual average, an upper bound is derived for the yearly energy delivery. Compared to the corresponding constant flow rate, single-pass open-loop system, the upper bound for additional yearly delivery is found to be negligible for solar systems with practical design parameters. This implies that the sophistication and expense involved in searching for, and implementing, optimal variable (radiation-dependent) collector flow rate strategies for these systems may not be worth the effort. The sensitivity of this upper bound to climatic and collector characteristics is determined analytically, and is illustrated by several numerical examples. The results derived are general in that they apply to all collector types.

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