## The importance of the micro channel geometry

Solar collectors are often characterised by low flow velocities to minimise the (electrical) pumping power required. There is however a trade-off between low pumping power and reduced performance due to temperature rise in the fluid. Fully developed laminar flow will be expected in these LCZ696 because of the low flow velocities and the micro scale of the hydraulic diameter. This implies that constant Nusselt number correlation for fully developed laminar flow can be expected. Rearranging Eq. (8), the heat transfer coefficient can be expressed asequation(1)h=kNuDhTherefore, if the fluid properties can be assumed constant, a reciprocal relationship will be expected between the heat transfer coefficient and the hydraulic diameter. This suggests reproductive isolating mechanism improvements in heat transfer coefficient can be achieved by reducing the hydraulic diameter. A corresponding ΔTpf < 1 °C, can also be expected from the range of hydraulic diameters studied. Furthermore, by varying the hydraulic diameter by about 1 mm as well as changing the aspect ratio, up to 300% increase in heat transfer coefficient can be achieved. Fig. 1(b) shows the corresponding heat transfer coefficient expected at various hydraulic diameters for two extreme Nusselt number correlations. This graph suggests that varying the aspect ratio can result in up to 65% increase in heat transfer coefficient.