Proceedings of the 4th ICMEM International Conference on Mechanical Engineering and Mechanics August 10-12, 2011, Suzhou, P. R. China Design of a Micro-Probe For Direct Measurement of Convection Heat Transfer on a Vertical Building Integrated Photovoltaic (BIPV) Madadnia, J., Dehestani, D., Mehta, A., Vakiloroaya, V., and Koosha, H. Faculty of Engineering & IT, University of Technology Sydney P.O. Box 123, Broadway, NSW 2007, Australia Abstract: A proximity probe with two k-type thermocouples, 1.5 mm apart, was designed, built to simultaneously measure local surface and air temperatures on the PV and to quantify local convention heat transfer coefficient. Experimental investigations of natural convection on a vertical photovoltaic (PV) panel exposed to solar radiations are presented. The variation of non-isothermal surface temperature of a PV is expressed with a second-order polynomial relation. In the absence of any correlation to predict the natural convection heat transfer coefficient on a PV, experimental results are presented in the form of variations of the local Nusselt numbers (Nuz), and the average Nusselt numbers (Nu), with Rayleigh number (Ra). The variations are best expressed with a power law correlation form of Nu=a*(Ra) b for the range 10 6 <Ra<10 8 where a and b are determined experimentally. The power-law correlations for photovoltaic were compared with a number of correlations developed from natural convection research in laboratories. The analysis showed that for a given Rayleigh number, the predicted value of Nusselt number by the PV correlations are within the range covered by others. However, the PV correlations overestimate the Nusselt number by 20% in Rayleigh number higher than 10 6 . The work is in progress to further extend the correlation to predict the combined radiation and convection on all PV configurations, as required in the efficient design of building integrated photovoltaic (BIPV) systems. Keywords: Natural Convection heat transfer, Solar energy, Energy Efficiency, Photovoltaic, Building Integrated Photovoltaic (BIPV), Experimental method, Empirical correlations 1 Introduction The natural convection heat transfer has been researched experimentally, numerically and analytically during past few decades. The natural convection from a flat surface with iso-flux, step-variations in flux, isothermal, and non- isothermal distributions with both the power and exponential forms is investigated considerably and refined over the years. A number of correlations are proposed for isothermal surfaces, isoflux surfaces and a surface with power-law temperature distributions in air, water and mercury. However, correlations to predict the natural convection heat transfer on a photovoltaic exposed to solar radiation are very limited. Saunders (1939) was the first in the previous century to study experimentally and theoretically natural convection in laminar and turbulent flows on an isothermal vertical plate in water and mercury. He expressed his results in a power-law form [1] . McAdams (1954) studied natural convection heat transfer for a vertical laminar flow in an isothermal surface. He covered the following range of Rayleigh number Ra for laminar flow 10 4 < Ra< 10 9 , and developed a correlation for air in the following power law form; Nu= 0.59 Ra 0.25[2] . Sparrow and Gregg (1956) also studied natural convection on a uniform heat flux(isoflux) vertical surface and derived an exact solution for Prandtl numbers in the range of 0.1 to 100 and presented an expression for Nusselt number as a function of Prandtl number and Rayleigh number [3] . Sparrow and Gregg (1958) studied the variable fluid-property problem for laminar free convection on an isothermal vertical flat plate [4] . For a number of specific cases, they solved conservation equations for the boundary layers with variable-property situations. They used “reference temperatures” to extend the results derived for constant- property fluids to variable-property situations. For gases, the constant-property heat transfer results are generalized to the variable property situation by replacing β (expansion coefficient) by 1/T ∞ and evaluating the other properties at the reference temperature of T r =T w -0.38(T w -T ∞ ). They observed that the film temperature can adequately serve as reference temperature (with β =1/T ∞ for gases) for most engineering applications. Authors have used T ∞ as the film temperature and works are in progress to generalize the natural convection heat transfer to incorporate radiation. Sparrow and Gregg (1958) studied natural convection in the laminar boundary layer over a vertical flat plate with two families of surface temperature variations, namely the power law and exponential. The power law distribution of T s -T ∝ = N.Z n . The exponential distribution of T s -T ∝ = M.e mz . They developed a correlation with 0.25 as the exponent for Gr or Pr. Their correlation for non-isothermal surfaces can be fitted into a second order parabola form for air. N u/(Gr/4) 0.25 =f (Pr, n)=