# An Innovative Receiver Design for a Parabolic Trough Solar Collector Using Overlapped and Reverse Flow: An Experimental Study

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## Abstract

A new technique was proposed and implemented through an experimental model to improve the absorber tube of a parabolic trough collector, which was connected to a twisted tube and resulted in a heat transfer enhancement. The enhancement increased the capacity of the tube to absorb solar energy by improving the convective transfer between the inner surface of the absorber tubes and the heat transfer fluid. The new design was dependent on overlapped and reverse flow, which was validated through five receiver tube \((R_\mathrm{t})\) models, each one consisting of two types of overlapped flows. The first was a flow through inner twisted tubes (single (STT), dual (DTT), and triple (TTT) twisted tubes), with different hydraulic diameters. The second was a reverse flow through an outer round tube, with both concentric tubes joined at one side by a bayonet end cap. For each \(R_\mathrm{t}\), the inlet flow from the central or peripheral tubes was assessed. The results showed a remarkable increase in heat transfer enhancement for most STT models and a greater heat transfer in TTT that was overlapped than for a plain tube collector. The enhancement was accompanied by an increase in the pressure difference that was higher for TTT than STT. For each model, the efficiency of the collector and pressure losses was estimated, and the Nusselt number (*Nu*) and an efficiency evaluation criterion were calculated.

## Keywords

Concentrated solar energy Parabolic trough collector Twisted tube Heat transfer enhancement Friction factor Collector efficiency## List of symbols

- \(A_\mathrm{a} \)
Aperture area \((\hbox {m}^{2})\)

- \(A_{\mathrm{r}}\)
Receiver area \((\hbox {m}^{2})\)

- \(A_{\mathrm{w0}} \)
Outside surface area of the outer tube of the receiver \((\hbox {m}^{2})\)

- \(A_{\mathrm{wi}} \)
Inside surface area of the outer tube of the receiver \((\hbox {m}^{2})\)

- \(C_{\mathrm{o}}= A_{\mathrm{a}} / A_{\mathrm{r}}\)
Concentration ratio, (–)

- \(C_{\mathrm{d}}\)
Collector depth (m)

- \(c_\mathrm{p} \)
Specific heat capacity (J/kg/k)

- \(D_\mathrm{i} \)
Inside diameter of the outer tube of the receiver (m)

- \(D_\mathrm{o} \)
Outside diameter of the outer tube of the receiver (m)

- \(D_{\mathrm{hi}} \)
Inner hydraulic diameter of the inner tube of receiver (m)

*f*Focal length (m)

- \(h_\mathrm{wi} \)
Average convective heat transfer coefficient (\(\mathrm{W/m}^{2}\,^{\circ }\hbox {C}\))

- \(I_\mathrm{d} \)
Direct solar radiation (\(\hbox {W/m}^{2}\))

- \(k_\mathrm{w} \)
Thermal conductivity of the outer tube of the receiver (\(\mathrm{W/m}\,^{\circ }\hbox {C}\))

- \(L_{\mathrm{p}}\)
Twisted pitch length of the twisted tube (mm)

*l*Length (m)

- \(P_{{x}}\)
Flow Path from the inner tube to outer tube

- \(P_{{y}}\)
Flow Path from the outer tube to inner tube

- \(\hbox {Q}\)
Heat transfer rate (\(\hbox {Q})\) in the receiver tube \(R_\mathrm{t}\) which absorbed from the HTF (kW)

- \(\dot{m}_{{\text {HTF}}}\)
Mass flowrate of the water (kg/s)

- \(T_{{\text {out}}} \)
Temperature at the outlet of the receiver tube (\(^{\circ }\hbox {C}\))

- \(T_{{\text {in}}} \)
Temperature at the inlet of the receiver tube (\(^{\circ }\hbox {C}\))

- \(T_\mathrm{m} \)
Bulk mean temperature of fluid in the receiver tube \(R_\mathrm{t} \left( {\frac{T_{\mathrm{in}} +T_{\mathrm{out}} }{2}} \right) \, (^{\circ }\hbox {C})\)

- \(T_{\mathrm{wo}}\)
Average wall surface temperature outside of the outer tube of the receiver \((^{\circ }\hbox {C})\)

- \(U_{\mathrm{wo}}\)
Overall outside heat transfer coefficient for the outer tube of the receiver \((\hbox {W/m}^{2}\, \hbox {K})\)

- \(W_{\mathrm{a}}\)
Aperture width (m)

- \(\Delta p\)
pressure drop (mbar)

## Greek letters

- \(\theta _{\mathrm{R}}\)
Rim angle (\(^{\circ }\))

## Abbreviations

- STT
Single twisted tube overlapped

- DTT
Double twisted tube overlapped

- TTT
Triple twisted tube overlapped

- HTF
Heat transfer fluid

- PTC
Parabolic trough solar collector

- \(T_{\mathrm{u}}\)
Main tube holding the receiver tubes

- \(R_{\mathrm{t}}\)
Solar receiver tube of the parabolic trough solar collector

- \(R_{\mathrm{D}}\)
Overlapped diameter ratio, calculated by \((D_{\mathrm{hi}} /D_\mathrm{i})\)

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