Mediterranean Green Buildings & Renewable Energy pp 445-464 | Cite as

# Bi-fluid Photovoltaic/Thermal PV/T Solar Collector with Three Modes of Operation: Experimental Validation of a Theoretical Model

## Abstract

This chapter discusses theoretical and indoor experimental studies of a bi-fluid type photovoltaic/thermal (PV/T) solar collector. Two-dimensional steady-state analysis was developed and computer simulations were performed using MATLAB. Experiments were conducted for steady-state analysis under the solar simulator at Solar Energy Research Lab UiTM, Perlis, Malaysia, and under real sky conditions of Northern Peninsular Malaysia, to validate the model. The solar collector and the test rig facilities were fabricated to be suitable for mathematical validation purposes in both indoor and outdoor testing. For indoor collector testing, at an average wind speed of 1 m/s and average solar radiation of 700 W/m^{2}, the air and water mass flow rate was varied from 0.0074 to 0.09 kg/s and 0.0017 to 0.0265 kg/s respectively. The thermal efficiency increased as the mass flow rate increased. At a mass flow rate of 0.0262 and 0.0066 kg/s for air and water respectively, the thermal efficiency curves tended to plateau, which marked the optimum point of the fluid flow. For the simultaneous mode of fluid operation, testing was conducted with air and water fixed at a flow rate of 0.0262 and 0.0066 kg/s respectively, while the fluids’ mass flow rate was varied according to the range used during the independent mode. The range of the computed efficiencies for the simultaneous mode were higher than for the independent mode. In this study, collector outdoor testing was conducted for each mode of operation on a typical day in January in Perlis, Northern Peninsular Malaysia. Based on the outdoor monitoring analysis for simultaneous mode, the performance of the collector was also higher overall than the independent mode. The test was conducted by monitoring the performance of the collector with the air mass flow rate and water mass flow rate fixed at 0.0262 and 0.0066 kg/s respectively. Theoretical analysis was also performed and then validated against the experimental results by a direct comparison of the plotted curves and using mean absolute percentage error (MAPE) analysis. For the air, water and simultaneous modes, by taking into account both indoor and outdoor collector testing, the theoretical and experimental curves were found to be in good agreement, and the computed MAPE values for the fluids’ output temperature were less than 2 %. Thus, the two-dimensional mathematical model was proven valid. The PV/T collector designed in this study has a variety of applications because it can be operated in three different modes of fluid operation, and the theoretical model is useful in modelling all three modes without further modification.

## Keywords

Photovoltaic/thermal 2D analysis Bi-fluid Indoor Outdoor## Nomenclature

*A*_{PV}Collector aperture area’s subsegment, which is equal to the area of the PV module’s subsegment (m

^{2})*C*_{f}Conversion power factor

*C*_{pf1}Specific heat capacity of air

*C*_{pf2}Specific heat capacity of water

*D*_{i}Inner pipe diameter

*D*_{o}Outer pipe diameter

*f*A General subscript to represent a fluid

*f*_{r}A Friction factor in a fluid’s channel

*G*Global radiation

*h*Heat transfer coefficient (W/m

^{2}K)*h*_{cvbs f1}Convection heat transfer coefficient from back surface of Tedlar to air flow (W/m

^{2}K)*h*_{cvbs f2}Convection heat transfer coefficient from back surface of Tedlar to water flow (W/m

^{2}K)*h*_{cvfin f1}Convection heat transfer coefficient from surface of a fin to flowing air (W/m

^{2}K)*h*_{cvw}Wind convection heat transfer coefficient (W/m

^{2}K)*h*_{i}Generalised notation for heat transfer coefficient of linear equations derived from developed energy balance equations for general heat transfer coefficient

*i*(W/m^{2}K)*h*_{rbsbp}Radiation heat transfer coefficient between inner surfaces of collector (W/m

^{2}K)*h*_{rpvsky}Radiation heat transfer coefficient from PV cells to sky (W/m

^{2}K)*k*_{fin}Thermal conductivity of fin (W/mK)

*k*_{f1}Thermal conductivity of air (W/mK)

*k*_{f2}Thermal conductivity of water (W/mK)

*k*_{PV}Thermal conductivity of photovoltaic cells (W/mK)

*L*_{fin}Length of fin (m)

*m*Subsegment for each temperature node

- \( {\overset{.}{m}}_{f1} \)
Air mass flow rate (kg/s)

- \( {\overset{.}{m}}_{f2} \)
Water mass flow rate (kg/s)

*N*_{fin}Total number of fins

*q*Rate of heat flux (W/m

^{2})*q*_{uf1, m}Rate of heat transfer per unit area for air nodes (W/m)

*q*_{uf2, m}Rate of heat transfer per unit area for air nodes (W/m)

*Re*Reynolds number

*S*_{PV}Total rate of solar energy absorbed by solar cells of PV module

*S*_{T}Total energy absorbed by Tedlar

*T*Temperature (K)

*T*_{a}Ambient temperature (K)

*T*_{bp, m}Temperature nodes of surface of back plate (K)

*T*_{bs, m}Temperature nodes of rear surface of Tedlar (K)

*T*_{bp}Mean temperature of surface of back plate with fins (K)

*T*_{bs}Mean temperature of rear surface of Tedlar (K)

*T*_{PV}Mean temperature of cell (K)

*T*_{f1, m}Temperature nodes of air flow in channel (K)

*T*_{f2, m}Temperature nodes of water flow in copper pipe (K)

*T*_{fin, m}Mean temperature of fin at subsegment

*m*(K)*T*_{PV, m}Temperature nodes of PV cells (at centre of cells) (K)

*T*_{ref}Temperature at reference point

*T*_{sky}Sky temperature (K)

*T*_{tin}Mean inner wall temperature of copper pipe (K)

*U*_{t, m}Overall top heat loss coefficient for subsegment

*m**v*_{f1}Maximum velocity of air (m/s)

*v*_{w}Wind speed (m/s)

*W*Tube spacing (m)

*α*_{c}Absorptance of PV cells

*β*_{c}PV module packing factor

*β*_{ref}Temperature coefficient at reference temperature

*δ*_{PV}Thickness of PV module

*δ*_{si}Thickness of Si cells

*δ*_{T}Thickness of Tedlar layer

*δ*_{fin}Fin thickness

- Δ
*y*_{PV}Δ*x* Area of subsegment of PV module

- Δ
*x* Distance between temperature nodes (\( \Delta x=1\ \mathrm{cm} \))

*ε*_{sky}Emissivity of sky

*ε*_{g}Emissivity of glass cover

*γ*_{bsbp, m}Area correction factor for heat transfer from back surface of Tedlar to surface of back plate with fins

*γ*_{bs f1, m}Area correction factor for heat transfer from back surface of Tedlar to flowing air

*γ*_{bs f2, m}Area correction factor for heat transfer from back surface of Tedlar to flowing water

*γ*_{fin f1, m}Area correction factor for heat transfer from fin surfaces to flowing air

*γ*_{bp f1, m}Area correction factor for heat transfer from surface of back plate with fins to flowing air

*γ*_{bpa, m}Area correction factor for heat transfer from surface of back plate with fins to ambient

*τ*_{g}Transmittance factor of front cover glass of PV module

*τ*_{gg}Transmittance factor due to double glazing (glass covers)

*η*Efficiency

*η*_{c}Electrical efficiency of a PV cell

*η*_{ele}Electrical efficiency

*η*_{eth}Electrical efficiency converted to equivalent thermal efficiency

*η*_{fin}Fin efficiency

*η*_{p}Fin effectiveness

*η*_{Tref}Electrical efficiency at reference temperature

*η*_{th f1}Thermal efficiency of air

*η*_{th f2}Thermal efficiency of water

- \( \sum {\eta}_{\mathrm{th}} \)
Total thermal efficiency of solar collector

- \( \sum {\eta}_{\mathrm{PVT}} \)
Primary energy saving or equivalent thermal efficiency

- \( \sum {\eta}_{\mathrm{th},\mathrm{inst}} \)
Instantaneous total thermal efficiency of solar collector

## Notes

### Acknowledgement

This work was funded by the Malaysian Fundamental Research Group (FRGS) 600-RMI/ST/FRGS 5/3/Fst (160/2010) and Solar Energy Research Lab, Universiti Teknologi Mara (UiTM), Perlis, Malaysia

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