Design procedure for cooling ducts to minimise efficiency loss due to temperature rise in PV arrays

The principal variable to be fixed in the design of a PV cooling duct is its depth, and hence the hydraulic diameter of its cross-section D. Analysis of the flow and heat transfer in the duct under still-air (buoyant flow) conditions, when the temperature rise is greatest, is validated by measurements on a full-scale test rig. It is shown that there is an optimum value of this design variable, such that for an array of length L the minimum temperature occurs when the ratio L/D is about 20. The optimum value is not affected much by other quantities, including the slope of the array. In practical situations, the flow is obstructed by devices across the duct inlet and outlet to exclude insects, birds and rain, and by structural support members crossing the duct interior. It is shown that the latter are no cause for concern, since the effect of the reduction in the flow-rate due to their presence is more than offset by an increase in heat transfer through additional turbulent mixing. It is also shown that array temperatures are strongly reduced by wind effects, which increase both the heat lost from the front surface of the array and by enhancement of the flow in the duct. Though the trends are clear, limitations are encountered in the present state of knowledge in both areas.

[1]  B. J. Brinkworth,et al.  Estimation of flow and heat transfer for the design of PV cooling ducts , 2000 .

[2]  B. J. Brinkworth,et al.  A procedure for the routine calculation of laminar free and mixed convection in inclined ducts , 2000 .

[3]  Symmetric vs asymmetric periodic disturbances at the walls of a heated flow passage , 1984 .

[4]  E. Sparrow,et al.  Turbulent Heat Transfer in a Symmetrically or Asymmetrically Heated Flat Rectangular Duct With Flow Separation at Inlet , 1982 .

[5]  W. Beckman,et al.  Solar Engineering of Thermal Processes , 1985 .

[6]  Mats Sandberg,et al.  A validated procedure for determining the buoyancy-induced flow in ducts , 2005 .

[7]  Jenn-Jiang Hwang,et al.  Simulation and measurement of enhanced turbulent heat transfer in a channel with periodic ribs on one principal wall , 1993 .

[8]  V. C. Patel,et al.  Large-eddy simulation of turbulent flow in a channel with rib roughness , 2003 .

[9]  Mats Sandberg,et al.  Buoyancy-induced air flow in photovoltaic facades: Effect of geometry of the air gap and location of solar cell modules , 2002 .

[10]  Je-Chin Han,et al.  Heat transfer and friction behaviors in rectangular channels with varying number of ribbed walls , 2003 .

[11]  A. Hatton,et al.  Turbulent Flow and Heat Transfer in the Entrance Region of a Parallel Wall Passage , 1969 .

[12]  Hongxing Yang,et al.  Thermal regulation of photovoltaic cladding , 1997 .

[13]  Jenn-Jiang Hwang,et al.  Heat transfer-friction characteristic comparison in rectangular ducts with slit and solid ribs , 1998 .

[14]  A. Bejan.  Convection Heat Transfer , 1984 .

[15]  B. J. Brinkworth,et al.  Coupling of Convective and Radiative Heat Transfer in PV Cooling Ducts , 2002 .

[16]  M. Pinar Mengüç,et al.  Thermal Radiation Heat Transfer , 2020 .

[17]  Steve Sharples,et al.  Full-scale measurements of convective energy losses from exterior building surfaces , 1984 .

[18]  W. Kays,et al.  Heat transfer in annular passages—hydrodynamically developed turbulent flow with arbitrarily prescribed heat flux , 1963 .

[19]  Je-Chin Han,et al.  Heat Transfer and Friction in Rectangular Channels With Ribbed or Ribbed-Grooved Walls , 1994 .

[20]  B. N. Prasad,et al.  Effect of artificial roughness on heat transfer and friction factor in a solar air heater , 1988 .

[21]  Jie Han,et al.  Heat transfer and friction characteristics in rectangular channels with rib turbulators , 1988 .

[22]  J. Whitelaw,et al.  Convective heat and mass transfer , 1966 .

[23]  H. M. Tan,et al.  Effect of thermal entrance region on turbulent forced-convective heat transfer for an asymmetrically heated rectangular duct with uniform heat flux , 1969 .

[24]  E. Sparrow,et al.  Radiation Heat Transfer , 1978 .

[25]  Je-Chin Han,et al.  Heat Transfer and Friction in Channels With Two Opposite Rib-Roughened Walls , 1984 .

[26]  D. Dipprey,et al.  An experimental investigation of heat and momentum transfer in smooth and rough tubes at various Prandtl numbers , 1961 .

[27]  Steve Sharples,et al.  Full-scale measurements of wind-induced convective heat transfer from a roof-mounted flat plate solar collector , 1998 .

[28]  B. M. Cross.  Development, testing and first installations of an integrated solar roof system , 1994, Proceedings of 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion - WCPEC (A Joint Conference of PVSC, PVSEC and PSEC).

[29]  E. Sparrow,et al.  Numerical and experimental turbulent heat transfer results for a one-sided heated rectangular duct , 1986 .

[30]  Zahari Ibarahim,et al.  A validated model of naturally ventilated PV cladding , 2000 .

[31]  Natale Arcuri,et al.  Experimental investigation on thermal radiation exchange of horizontal outdoor surfaces , 2003 .