Numerical and Experimental ......(22)

Numerical and Experimental Investigation of Building-Integrated Photovoltaic-Thermal Systems

 NUMERICAL MODELING OF BIPV SYSTEM绿色建筑博客D^e+|OR;`O

3.1 Problem Statement

As depicted in Chapter 1, the BIPV systems are tested at Concordia University in two different configurations (see figure 1.1). Configuration 1 with one simple cavity flow is modeled with numerical simulation. Configuration 2 can be considered as two separate cavities similar to Configuration 1 and so the results from the numerical modeling can be also used to develop the convective heat transfer coefficients for Configuration 2. In this thesis, if Configuration 2 is not specified, the discussions are all for Configuration 1.绿色建筑博客 e.L|eE-t%y0L
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_N2\Aa^;@v0Based on experimental measurements, we know that our problem is mainly 2-D. Measurements showed negligible temperature or velocity variation along the width of cavity. Therefore, the 2-D CFD model is used and these two dimensional are along the gap and height of the cavity.绿色建筑博客/t/h%f%VqD5{%M
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-H[Tti G j5HH0Therefore this BIPV thermal system is simplified into two-dimensional air flow and heat transfer problem and is shown in Figure 3.1. The outside air is drawn from the bottom inlet of the cavity and the air intake temperature is measured specifically for the boundary condition setting. The flow is assumed to be quasi-steady, since the experiments show the temperatures and air flow speed barely change in a period of 2 hours around solar noon. The PV panel, located at the left side of the cavity, is heated by the solar radiation and cooled by the air flow in the cavity so its surface temperature appears a function of the height. This temperature gradient also proves that the convective heat transfer coefficient should be different along the flow path. The right hand surface in Figure 3.1 is assumed to be adiabatic (thermal insulation) but exchanges heat with the PV through longwave radiation. The air cavity as shown in Figure 3.1 has width, L=0.092 m and height, H=1 m.绿色建筑博客8hj8b:p`8W^dI

The air intake temperature is measured near the intake damper and is set as the inlet thermal boundary condition. Since the inlet velocity distribution is hard to measure and sensitive to the final simulation results, the pressure difference condition is used for the inlet/outlet velocity boundary conditions. The pressure difference is pre-calculated using the pressure loss calculation according to the respective average flow speed. The outlet condition is set to the outflow condition so that only the background temperature is needed to give for the longwave radiation heat transfer calculation from the background. The left PV panel is the most active component in the whole system, because it participates in various kinds of heat transfer: solar radiation to the outer surface, convection between the outer surface and the ambient air, background radiation between the outer surface and background, convection between the inner surface and flowing air inside the cavity, long-wave radiation between the inner surface and other surfaces inside the cavity, and possible conduction to the attached framing and other structures. For this complexity, the uniform temperature or uniform heat flux is not proper for this boundary setting, the real experimental measurements are taken and a temperature boundary profile curve is generated according to the regression of the experimental data. A CFD software FLUENT is employed to discretize and solve the governing equations.