Impact of utilizing reflector, single-axis and two-axis sun trackers on the performance of an evacuated tube solar collector

ABSTRACT

In this paper, a novel evacuated tube solar collector (ETSC) is first designed and built. Then, the impact of adding reflector, reflector plus single-axis sun tracker and reflector plus two-axis sun tracker to the built ETSC on the thermal efficiency of the ETSC is evaluated both theoretically and experimentally. In this regard, four identical versions of the proposed ETSC have been built and utilized in four collectors built and presented in this research work. The first collector is the same proposed built ETSC, the second collector is a parabolic trough solar collector comprising one built ETSC and a reflector (ETSC+R), the third collector is composed of one built ETSC, a reflector and a single-axis sun tracker all built in this study (ETSC+R+ ST), and the fourth collector consists of one built ETSC, a reflector and a two-axis sun tracker all built in this study (ETSC+R+ DT). Theoretical basis and concepts of the four collectors are formulated and analyzed in separate subsections. Theoretical results are outlined and highlighted at the end of each subsection. Experimental measurements and data obtained from the operation of the four collectors in the four seasons are presented that point by point verify theoretical results obtained in this study. To provide a comprehensive view, a techno-economic numerical comparison is performed between the four collectors. The following points, which are also the novelty and contributions of this work, are deduced from theoretical concepts, experimental data, and comparison provided in this study:

•There is no technical and economic justification for adding a reflector to an ETSC that results in forming a parabolic trough solar collector (ETSC+R) without any sun tracker.

•There is no economic justification for adding a single-axis sun tracker to a parabolic trough solar collector (ETSC+R).

•There is no economic justification for adding a two-axis sun tracker to a parabolic trough solar collector (ETSC+R).

•Comparing between a two-axis sun tracker and a single-axis sun tracker, adding the single-axis type to a parabolic trough solar collector (ETSC+R) is more advantageous.

KEYWORDS:

• Evacuated tube solar collector
• thermal efficiency
• reflector
• single-axis sun tracker
• two-axis sun tracker

Nomenclature

$A_{egt}$	=	Aperture area of the ETSC (${\mathrm{m}}^2$).
$C_{wat}$	=	Specific heat of water flowing inside the manifold ($\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$).
$D_{tpct-inn}$	=	Diameter of the inner surface of the two-phase closed thermosiphon (TPCT) heat pipe (m).
$D_{tpct-out}$	=	Diameter of the external surface of the TPCT heat pipe (m).
$G$	=	Solar radiation ($\mathrm{W}{\mathrm{m}}^{-2}$).
$L_{tpct-e}$	=	Length of the evaporation section of the TPCT heat pipe (m).
$L_v$	=	Vaporization latent heat coefficient of the working fluid ($\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}$).
$L_{ref}$	=	Length of the reflector’s aperture (m).
$m_{m-wat}$	=	Mass flow rate of the water flowing inside the manifold ($\mathrm{k}\mathrm{g}{\mathrm{s}}^{-1}$).
$m_v$	=	Mass rate of change from liquid to vapor ($\mathrm{k}\mathrm{g}{\mathrm{s}}^{-1}$).
$Q_{ab-m-wat}$	=	Heat power absorbed by water flowing inside the manifold ($\mathrm{W}$).
$Q_{ab-wf}$	=	Heat power absorbed by the working fluid available inside the TPCT heat pipe (W).
$Q_{ab-in}$	=	Heat power available on the inner surface of the evaporation section of the TPCT heat pipe (W).
$Q_{ab-ex}$	=	Heat power absorbed by the external surface of the evaporation section of the TPCT heat pipe (W).
$Q_{egt}$	=	Heat power received by the evacuated glass tube in presence of the reflector (W).
$Q_{in-ev}$	=	Solar power received by the evacuated glass tube (W).
$Q_{ref}$	=	Solar power received by the reflector (W).
$Q_{ref-ST}$	=	Solar power received by the reflector installed on the single-axis sun tracker (W).
$Q_{ref-DT}$	=	Solar power received by the reflector installed on the two-axis sun tracker (W).
$T_{m-wat-in}$	=	Temperature of cold water entering the manifold ($\mathrm{K}$).
$T_{m-wat-out}$	=	Temperature of hot water leaving the manifold ($\mathrm{K}$).
$T_i$	=	Ambient temperature ($\mathrm{K}$).
$T_{tpct-out}$	=	Temperature of the external surface of the TPCT heat pipe ($\mathrm{K}$).
$T_{tpct-inn}$	=	Temperature of the inner surface of the TPCT heat pipe ($\mathrm{K}$).
$T_{wf}$	=	Temperature of the working fluid available inside the TPCT heat pipe ($\mathrm{K}$).
$W_{ref}$	=	Width of the reflector’s aperture (m).
$\alpha$	=	Altitude angle of the sun position (degree).
${\alpha }_r$	=	Altitude angle of the reflector’s aperture plate (degree).
${\alpha }_{ave}$	=	Average altitude angle of the sun position over the year (degree).
${\alpha }_{a-ex}$	=	Absorptance of the external surface of the evaporation section of the TPCT heat pipe.
$\beta$	=	Azimuth angle of the sun position (degree).
${\beta }_r$	=	Azimuth angle of the reflector’s aperture plate (degree).
${\beta }_{inn}$	=	Heat transfer coefficient between the inner surface and working fluid of the evaporation section of the TPCT heat pipe ($\mathrm{W}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$).
${\lambda }_{tpct}$	=	Thermal conductivity of the TPCT heat pipe ($\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$).
${\tau }_{egt}$	=	Transmittance of the evacuated glass tube of the TPCT heat pipe.
${\eta }_{ref}$	=	Efficiency of the reflector.
${\gamma }_{tpct}$	=	Thermal efficiency of the ETSC.
${\gamma }_{tpct-R}$	=	Thermal efficiency of the parabolic trough solar collector formed by adding the reflector to the ETSC.
${\gamma }_{tpct-SR}$	=	Thermal efficiency of the parabolic trough solar collector installed on the single-axis sun tracker.
${\gamma }_{tpct-DR}$	=	Thermal efficiency of the parabolic trough solar collector installed on the two-axis sun tracker.