Theoretical analysis to investigate thermal performance of co-axial heat pipe solar collector


The thermal performance of co-axial heat pipe solar collector which consist of a collector 15 co-axial heat pipes surrounded by a transparent envelope and which heat a fluid flowing through the condenser tubes have been predicted using heat transfer analytical methods. The analysis considers conductive and convective losses and energy transferred to a fluid flowing through the collector condenser tubes. The thermal performances of co-axial heat pipe solar collector is developed and are used to determine the collector efficiency, which is defined as the ratio of heat taken from the water flowing in the condenser tube and the solar radiation striking the collector absorber. The theoretical water outlet temperature and efficiency are compared with experimental results and it shows good agreement between them. The main advantage of this collector is that inclination of collector does not have influence on performance of co-axial heat pipe solar collector therefore it can be positioned at any angle from horizontal to vertical. In high building where the roof area is not enough the co-axial heat pipe solar collectors can be installed on the roof as well as wall of the building. The other advantage is each heat pipe can be topologically disconnected from the manifold.

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Fig. 7


A :

Surface area (m2)

A abs :

Absorber area (m2)

C p :

Specific heat capacity (J/kg K)

d :

Diameter (m)

F R :

Heat removal factor

g :

Gravitational constant (9.81) (m/s2)

h :

Heat transfer coefficient (W/m2 K)

k f :

Thermal conductance (W/m K)

I :

Solar insolation (W/m2)

L :

Length (m)

\( \dot{m} \) :

Mass flow rate (kg/s)

Nu :

Nusselt number

Pr :

Prandtl number

Q :

Heat flow (W)

Ra :

Rayleigh number

Re :

Reynolds number

t :

Thickness (m)

T :

Temperature (°C)

T avg :

Average temperature

T v :

Vapour temperature (°C)

ΔT lm :

Suitable mean temperature difference (°C)

U L :

Overall heat-transfer coefficient (W/m2 °C)

α :


β :

Volumetric thermal expansion (1/K)

ε :


μ :

Dynamic viscosity (kg/m s)

η :

Collector efficiency

τ :


λ :

Latent heat (J/kg)

ν :

Kinematic viscosity (m2/s)

ρ :

Density (kg/m3)

σ :

Stefan–Boltzman constant (5.67E−8) (W/m2 K4)




Inside condenser surface


Outside condenser surface


Inside evaporator surface


Outside evaporator surface


Glass envelope inner surface


Glass envelope outer surface


Surrounding air



abs :


g :


conv :


rad :


Loss :


Avg :


Sol :


l :


ν :


w :


f :





Outlet, outside


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Correspondence to E. Azad.

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Azad, E. Theoretical analysis to investigate thermal performance of co-axial heat pipe solar collector. Heat Mass Transfer 47, 1651 (2011).

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  • Heat Transfer
  • Nusselt Number
  • Heat Pipe
  • Radiation Heat Transfer
  • Solar Collector