Photovoltaic thermal solar air heater under external recycle: A performance study

ABSTRACT

The photovoltaic thermal (PV/T) systems are promising techniques for cooling photovoltaic modules and performance improvement. The heat-exchanging fluid removes the heat from the PV module, which can be utilized in the low-temperature application, including space heating. In the present study, a photovoltaic thermal system based on a double pass air collector under recycling is theoretically investigated for its overall performance and analysis. An analytical model explaining the different temperatures and heat transfer characteristics of the proposed PV/T system is developed and employed to study the effects of different recycling ratios at varying mass flow rates, the area covered by the PV module (packing factor), solar irradiation, and varying duct depth on thermal, electrical, and combined efficiency. The analytical model utilizes an iterative solution to work out the governing energy balance equations describing the complex heat and mass exchanges. The results show that the new design collector encloses photovoltaic cells under recycling, reduces the PV surface temperature from 97°C (without cooling) to 45°C (with cooling) at 0.15 kg/sec mass flow rate of air and recycle ratio of 1.8 and, thereby improves the electrical efficiency from 12% to 16%, which is 13% more increment than a non-recycling case. The influences of the identified parameters, which may vary with the proposed design performance, are presented in detail.

KEYWORDS:

• Photovoltaic thermal system
• combined efficiency
• recycle ratio
• packing factor
• double-pass solar collectors

Nomenclature

${\mathrm{A}}_{\mathrm{c}}$	=	Area of PV collector plate (m2)
	=	CpSpecific heat of the air (kJ/kg.K)
${\mathrm{d}}_{\mathrm{h}1}$	=	The characteristic length of the upper duct (m)
${\mathrm{d}}_{\mathrm{h}2}$	=	The characteristic length of the lower duct (m)
$\mathrm{G}$	=	Recycle ratio
${\mathrm{h}}_{\mathrm{r}}$	=	Radiation heat transfer coefficient (W/m2K)
${\mathrm{h}}_{\mathrm{c}}$	=	Convection heat transfer coefficient (W/m2K)
H1	=	Height of duct between glass and collector plate, i.e., upper duct (m)
H2	=	Height of duct between the backplate and collector plate, i.e., lower duct (m)
I	=	Incident radiation (W/m2)
${\mathrm{k}}_{\mathrm{f}}$	=	The conductivity of the fluid (W/m.K)
K	=	Conductivity (W/m.K)
L	=	Length of collector plate (m)
$\dot{m}$	=	Mass flow rate of air (kg/s)
$\mathrm{N}{\mathrm{u}}_1$	=	Nusselt number for upper duct
$\mathrm{N}{\mathrm{u}}_2$	=	Nusselt number for lower duct
P	=	Packing factor
PE	=	Electrical power (W)
$\mathrm{R}{\mathrm{e}}_1$	=	Reynolds number for upper duct
$\mathrm{R}{\mathrm{e}}_2$	=	Reynolds number for lower duct
T	=	Temperature (oC)
${\mathrm{T}}_{\mathrm{P}\mathrm{V}\mathrm{m}}$	=	Mean temperature of PV collector plate (oC)
${\mathrm{T}}_{\mathrm{r}\mathrm{e}\mathrm{f}}$	=	Reference temperature (oC)
$u$	=	Wind speed over glass surface (m/s)
U	=	Overall heat transfer coefficient (W/m2K)
$\mathrm{W}$	=	Width of collector plate (m)
A, B, C, M, N, Q, R, S, c Algebraic manipulation constant	=	Greek Letters
$\alpha$	=	Absorptivity
$\epsilon$	=	Emissivity
$\sigma$	=	Stefan’s Boltzmann constant (W/m2K4)
$\eta$	=	Efficiency
${\eta }_{\mathrm{t}\mathrm{h}}$	=	Thermal efficiency
${\eta }_{\mathrm{o}\mathrm{p}}$	=	Nominal efficiency
${\eta }_{\mathrm{P}\mathrm{V}/\mathrm{T}}$	=	Photovoltaic thermal efficiency
$\sigma$	=	Stefan’s Boltzmann constant
$\tau$	=	Transmissivity
$\mu$	=	Viscosity of air (kg/m-s)
$\lambda$	=	Algebraic manipulation constant

Subscript

$a$	=	Ambient air
$bp$	=	Backplate
$bp{f}_2$	=	Between backplate and fluid at a lower duct
$F_o$	=	Fluid at outlet
$F_i$	=	Fluid at inlet
$f_1$	=	Fluid at upper duct
$f_2$	=	Fluid at lower duct
$g$	=	Glass cover Between glass and sky
$gw$	=	Between upper thin glass and wind
$g{f}_1$	=	Glass and fluid at upper duct
$gpv$	=	Between thin upper glass and collector plate
$pv$	=	Collector plate containing PV cells
$pv{f}_1$	=	Between collector plate and fluid at upper duct
$p{f}_2$	=	Between collector plate and fluid at lower duct
$pvbp$	=	Between collector plate and backplate
$s$	=	Sky