Experimental study of heat transfer enhancement in a flat-plate solar water collector with wire-coil inserts

This work presents an experimental study on the use of wire-coil inserts to enhance heat transfer in a typical flat-plate solar water heater with optical efficiency of 77% and heat loss coefficient of 5.76 W/m(2) K at a rated mass flow rate of 0.04 kg/s. The testing setup was fully designed following the requirements of the European Standard EN 12975-2 and was capable of measuring the performance of a standard and an enhanced collector under the same operating conditions (mass flow rate and inlet fluid temperature) and weather conditions. In order to expand the experimental range, both solar collectors were tested at five different values of mass flow rate, from 0.011 to 0.047 kg/s. Thermal efficiency results are provided and the differences between both collectors' performance are explained in terms of the changes that the wire-coil inserts produce inside the risers. In addition, to quantify the overall enhancement that the inserts promote in the standard collector, the experimental results have been processed to obtain a performance evaluation criterion (R3(m)) that computes the increase in useful power collected and pumping power. Wire-coil inserts promoted an average increase in thermal efficiency from 14% to 31% and an increase in the useful power collected with no additional pressure losses of up to 8-12%, for the values of mass flow rates investigated. The inserts have shown a lower potential of enhancing heat transfer for increasing mass flow rates. According to the present work, wire-coil devices can be successfully inserted within the flow tubes in solar water collectors for increasing its thermal efficiency. (C) 2013 Elsevier Ltd. All rights reserved.