Transmissive microfluidic active cooling for concentrator photovoltaics

We present the design, fabrication, characterization, and field testing of transmissive active cooling for use in a point-focus spectrum-splitting hybrid concentrator photovoltaics/thermal (CPV/T) system. Seven parallel-path 100 mu m thick microchannels are made using polydimethylsiloxane and attached to a CPV module containing a 6 x 6 array of 5.5 mm transmissive CPV cells on a sapphire substrate. Water is flowed through the micro-channels to actively cool the CPV cells. The total transmittance of the CPV module reduces by 5.2% with the addition of the active cooling microchannels, relative to the module transmission with no microchannels. The peak cell temperature is measured as 69 degrees C with a thermal resistance of 9.351 K/W at 157 suns, well below the 110 degrees C maximum allowed temperature. A maximum flowrate of 16.7 g/s is achieved from a 13 psi pressure drop across the microchannels and manifold assembly. The flow characteristics within each microfluidic channel show maximum fluid velocity of 4.3 m/s (Re = 953) with a calculated convection coefficient of 1.7 x 10(4) W/m(2)K (Nu = 5.36). The CPV/T module and cooling system performance was validated during week-long outdoor tests under varying solar conditions up to 250 suns using a 2.7 m(2) parabolic dish collector mounted to a two-axis tracking system.