Thermal performance of a dual function air-water heater with a cover cum reflector mirror

ABSTRACT

In the present work a dual function integrated solar air-water heater which contains reflector cover has been proposed. The reflector can be reversed back over the glass cover to serve as an insulated cover for heater during no sunshine hours. The proposed heater’s mathematical model has been formed and experimentally verified. The average monthly optimum tilt angle of reflector has been determined for climate of Raipur, Chhattisgarh. The average optimum tilt angle is maximum 54.5° for the month of June and minimum 23.4° for the month of December. The average optimum angle of tilt for winters is 26.1° whereas for summer the average optimum tilt angle is 48.5^o. The use of reflector enhances the output, by using reflector the maximum water and air temperature achieved is 16.4% and 12% more than without reflector. The performance of the heater is influenced by the flow rate of air flowing in the upper compartment. Less heating of the water and air occurs with higher mass flow rates. For a typical winter day in Raipur, the maximum air and water temperatures reached are 45.5°C and 70.2°C at a flow rate of 0.010 kg/s, respectively. At a flow rate of 0.030 kg/s it is 34.6°C and 61.8°C. The maximum air temperature reached is 12% more without reflector. For three consecutive days of operation the maximum water and air temperature achieved on the third day is 92.4°C and 54.1°C, respectively.

KEYWORDS:

• dual function air-water heater
• mirror reflector
• insulated cover
• optimum tilt angle
• water temperature

Acknowledgement

The authors like to express their gratitude to Prof. Dr. M.S Sodha.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Nomenclature

$A_c$	=	Area of cross section of upper duct $m^2$
$A_p$	=	Area of absorber plate $m^2$
$A_r$	=	Reflecting surface area $m^2$
$A_s$	=	Side area of water tank $m^2$
$b$	=	Breadth of heater $m$
$c_a$	=	Specific heat of air $J/kgK$
$c_w$	=	Specific of water $J/kgK$
$d_e$	=	Hydraulic diameter $m$
$h_a$	=	Heat transfer coefficient due to convection between absorber plate and air $W/m^{2}K$
$h_b$	=	Overall heat transfer coefficient from water to air through bottom$W/m^{2}K$
$h_w$	=	Convective heat transfer between absorber plate and water $W/m^{2}K$
$h_1$	=	Overall heat transfer coefficient from air to ambient through glass cover $W/m^{2}K$
$K$	=	Thermal conductivity of air $W/mK$
$K_i$	=	Thermal conductivity of insulation material $W/mK$
$L$	=	Length of heater $m$
${\dot{m}}_a$	=	Air Flow rate of through heater $kg/s$
$M_w$	=	Mass of water $kg$
$n$	=	Number of days
$S(t)$	=	Solar radiation incident on heater $W/m^2$
$S^{\prime }(t)$	=	Solar radiation incident on reflector $W/m^2$
$t$	=	Time interval $s$
$t_i$	=	Thickness of insulation $m$
$T_{amb}$	=	Ambient air temperature ${\hspace{0.17em}}^{o}C$
$T_a$	=	Temperature of air inside heater ${\hspace{0.17em}}^{o}C$
$T_{ao}$	=	Temperature of air inside heater at $x=L$${\hspace{0.17em}}^{o}C$
${\bar{T}}_a$	=	Average temperature of air inside heater ${\hspace{0.17em}}^{o}C$
$T_f$	=	Film temperature ${\hspace{0.17em}}^{o}C$
$T_p$	=	Temperature of absorber plate ${\hspace{0.17em}}^{o}C$
$T_w$	=	Temperature of water stored in heater ${\hspace{0.17em}}^{o}C$
$v$	=	Velocity of air inside duct $m/s$
$W_p$	=	Wetted perimeter of air duct $m$

Greek letters

$\alpha \tau$	=	Effective transmittance-absorptance product
$\rho$	=	Reflectivity of reflecting surface
${\rho }_f$	=	Density of air $kg/m^3$
$\mu$	=	Dynamic viscosity of air $Ns/m^2$
$\psi$	=	Inclination angle of reflector
$\varphi$	=	Latitude of given place
$\beta$	=	Inclination angle of heater
$\delta$	=	Declination angle

Dimensionless numbers

Nu: Nusselt number

Re: Reynold’s number