Building-integrated unglazed solar collectors are cost effective solar devices that are suitable for various low temperature applications. In this article we present the design and the parametric analyses of the efficiency of a large-panel unglazed roof-integrated liquid solar collector and an economic evaluation of a large-panel solar-heating system for a swimming pool that is installed at a tourist facility on the Adriatic coast. The design of the solar collector is based on standard metal roofing; it takes into account the technological limitations of prefabricated panels, which makes the serial manufacturing of solar collectors possible. The parametric analyses of the large-panel solar collector's efficiency were made using the finite-volume numerical method. The numerical model was verified with outdoor measurements, according to the ISO 9806 standard. The efficiency was analysed for the most important parameters: the fin length; the absorber material and thickness; the water mass flow rate; and the wind speed. A solar absorptance of 0.85 was considered in the analyses because this is the value that corresponds to the most frequently used roofing colour. The results show that the efficiency of the analysed panels at T-in = T-a is in the range between 0.26 and 0.74, and the heat loss factors are 2.9-7.9 under no-wind conditions. An economic evaluation using the payback period method was used to select the optimum design for the developed panels on the basis of equal solar gains and the known or estimated initial and operating costs. It was found that for the optimum panel design the payback period is between 1.5 and 2.7 years, based on the current price for non-renewable energy sources. The payback period for presented solar systems is up to four times shorter than for a glazed solar collector system. (C) 2003 Elsevier Ltd. All rights reserved.