Numerical Simulation of Wind Effect on a Rooftop Solar Array

Transcription

1 J. Energy Power Sources Vol. 2, No. 8, 2015, pp Received: July 1, 2015, Published: August 30, 2015 Journal of Energy and Power Sources Numerical Simulation of Wind Effect on a Rooftop Solar Array Debdutta Ghosh 1, Siddharth Behera 2 and Achal Kr. Mittal 3 1. Research Scholar, Indian Institute of Technology Delhi, Delhi, India 2. Scientist, CSIR-Central Building Research Institute, Roorkee, India 3. Principal Scientist, CSIR-Central Building Research Institute, Roorkee, India Corresponding author: Debdutta Ghosh (debdutta001@gmail.com) Abstract: The recent demand of alternative energy sources increases the installation of Photovoltaic (PV) Solar arrays. The arrays are often vulnerable to wind loading. These systems are usually installed in two different locations: (1) mounted on building tops or (2) installed in the open ground. Guidelines are rarely available for the design of these solar arrays due to wind action. Some researchers have tried to analyse the system considering it as a mono-slope roof. Limited wind tunnel studies are preformed on this aspect worldwide. This paper investigates the wind action on a Roof-top Solar Array (RSA) using Computational Fluid Dynamics (CFD). A solar power generation facility of capacity 100 KW has been installed at CSIR-CBRI, Roorkee with roof mounted solar arrays. The wind action on a single solar array is considered in the study, where array is located at different location of the roof. Results of the CFD simulations are compared with Indian Standard (IS) 875 part 3 Codal provisions which may be used as an approximate estimate of wind load on a solar array. The results show that the solar array is subject to significant suction under wind loading which may cause lifting of the array if it is not properly designed. Keywords: Solar array, wind pressure, CFD, ANSYS FLUENT. 1. Introduction Rooftop Solar Arrays (RSA) is very sensitive to wind loads. It needs to be designed for surrounding wind conditions. Wind tunnel tests for ground and roof mounted solar arrays are performed by different researcher. Design parameters of RSA viz. wind direction (θ), building height (H), tilt (φ), building parameters, array spacing etc. are considered by Ref. [1-11]. Although wind tunnel testing is recommended for the wind pressure calculation on RSA, but with the help of CFD simulation the wind flow can be simulated numerically and pressure on RSA can be calculated with certain degree of accuracy. Numerical calculations of wind loads on solar photovoltaic collectors are performed by [12]. CFD simulations are carried out by [13] to estimate the wind loads for different wind directions on single and arrayed solar panels. CFD technique used by Ref. [14-15] to estimate the wind loads and observe the flow field around groundmounted solar panels. Wind flow characteristics around solar array are discussed by [16]. Detached Eddy Simulations (DES) is performed by [17] to observe the influence of ground clearance on solar panels. In the present study, CFD technique is used to simulate the wind characteristics around CBRI building and RSA situated at different locations of the roof (i.e., edge & centre). 3D Unsteady Reynolds Averaged Navier-Stokes (RANS) simulations are carried out to calculate the wind load on RSA. Building and RSA system is immersed in the Atmospheric Boundary Layer (ABL) using the k-ε turbulence closure. The solar arrays are considered as mono-slope roof. The general topography and aerial view of CBRI

2 318 Numerical Simulation of Wind Effect on a Rooftop Solar Array Solar CSIR-CBRII Building Fig. 1 CSIR-CBRI building G-earth image. Fig. 3 Geometry of CSIR-CBRI building. Fig. 2 Rooftop solar array. building is shown in Fig. 1 using Google Earth image. The typical locations of the solar arrays on CBRI rooftop is shown in Fig Numerical Modelling ANSYS FLUENT commercial software is used for the numerical modelling of wind flow. The details of the numerical modelling are described in the following sections. 2.1 Computational Domain The dimensions of the computational domain are decided according to the guidelines mentioned in [18-19]. A geometric scale of 1:40 is adopted for both building and solar array. Blockage ratio is kept 0.2% for the computational domain. The actual dimensions of computational domain are L D x W D x HD = 820 x 560 x 720 m 3 in full scale where model scale dimensions are 20.5 x 14 x 18 m 3. The dimension of the full-scale solar array and the building are depicted in Fig. 3. The dimension of CBRI building is x x 9.0 m 3 in Fig. 4 Details of computational domain. real scale measurement. A single solar array of 5.02 x 1.96 m 2 is mounted both at edge as well as at the centre of the roof. The solar array inclination is considered as 25º, which is a normal practise. The details about the computational domain are mentioned in Fig. 4. Computational domain is divided into control volumes to form mesh. An unstructured hexahedral mesh with various sizes cells is generated throughout the domain and approximately , cells are employed for this. Near wall treatment is performed at the bottom of the domain and around the building. The distance (y p ) between the centre point of the wall-adjacent cells and the ground surface i.e., first layer thickness is decided according to the Reynolds number and y + (30 < y + < 100) value and is fixed at m (simulation scale) which is equivalent to 0.2 m in full scale. A commercial CFD code, ANSYSS FLUENT is used to discretize Navier-Stokes equations using Finite Volume Method

3 Numerical Simulation of Wind Effect on a Rooftop Solar Array 319 and solved them over the domain. 2.2 Governing Equations The governing equations are as follows [20-21]: ( U j ) 0 (1) t x U i ( UU i j) t xj ' (2) P U U i j [ eff ( )] SM xi xj xj xj where S M is the sum of body forces, μ eff is the effective turbulent viscosity, and Pˈ is the modified pressure. The k-ε model is based on the eddy viscosity concept, so that: eff t (3) where μ t (or eddy) is the turbulence viscosity. In k-ε model, the turbulence viscosity is linked to the turbulence kinetic energy and dissipation via the relation: 2 k t C (4) where C μ is aconstant. The governing equations for k and ε are t k ( k) ( ku j ) [( ) ] t xj xj k xj (5) G G Y S k b M k t ( ) ( U j ) [( ) ] t xj xj xj 2 (6) CS 1 C2 C1 C3 Gb S k k where n k C1 max[0.43, ], ns, S 2SijSij (7) n 5 G k represents the generation of turbulence kinetic energy due to the mean velocity gradients, G b is the generation of turbulence kinetic energy due to buoyancy and Y M represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, C 2ε and C 1 are constants. σ k and σ ε are the turbulent Prandtl numbers for k (turbulence kinetic energy) and ε (dissipation rate). The other notations are having their usual meaning. The constants specified in FLUENT are C 1ε =1.44, C 2ε = 1.9, σ k = 1 and σ ε =1.2. S k and S ε are user defined source term [21]. The density of air is kg m Solver Settings The numerical procedure employs k-epsilon turbulence closures to predict wind pressure and least square cell based method is used to solve the equations. The solver modifies the continuity and momentum equation after incorporating the two variables, k (turbulent kinetic energy) and ε (turbulence eddy dissipation). Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm is used for the pressure-velocity coupling. First-order upwind discretization schemes are used for solving both convection and viscous terms of the governing equations. Computations repeated until desired small residuals are obtained (~1000 iterations). 2.4 Boundary Conditions Suitable boundary condition is adopted for the wind flow simulation. A velocity inlet is used at the upwind boundary. The sides and the top of the computational domain are specified as slip walls (zero normal velocity and zero normal gradients of all variables). At the downwind boundary, a pressure outlet boundary is used, with the relative pressure is specified at zero Pascal i.e., zero static pressure is specified at the outlet. The bottom wall of the domain is specified as no slip wall with specified roughness to simulate the effect of proper ground roughness. 2.5 Simulation of Atmospheric Boundary Layer (ABL) A User Defined Function (UDF) of velocity Vs height is used in the inlet of the domain to create boundary layer velocity profile. Turbulence energy and turbulence dissipation rate is also varied accordingly through UDF. In boundary layer simulation, a very accurate description near the ground surface is required. Roughness height is specified to simulate sand grain

4 320 Numerical Simulation of Wind Effect on a Rooftop Solar Array (a) (a) (b) Fig. 5 (a) Atmospheric boundary layer in inlet; (b) Velocity distribution throughout the domain. roughness is (scaled) or m in full scale, which is equivalent to equivalent sand grain roughness of a smooth floor. This value is smaller than the y p (0.005) in simulation scale. The velocity along the height at domain inlet is plotted for boundary layer depiction (Fig. 5a), where U 0 = 5.9 m sec -1. Velocity distribution throughout the numerical domain can be seen in Fig. 5b. (b) Fig. 6 (a) Solar array at the edge of CBRI building; (b) Solar array at the centre of CBRI building. 3. Results and Discussions 3.1 Pressure Contours The pressure contours on single solar array placed at different roof locations are shown in Fig. 6a-6b. It is evident that solar array situated at the centre is subjected to higher negative pressure than the array situated at the edge. In individual solar array the sides and corners of the solar array subjected to high negative pressure compared to centre of the solar array. Pressure contours on building front face and solar array are shown in Fig. 7. Pressure in the front face (along the wind flow direction) of the building is more Wind Flow Fig. 7 Pressure contour on building and solar array. than other faces of building. Solar array is subjected to lesser pressure than the building face towards the wind direction, but array may suffer lift due to high negative pressure created around the array. It is evident that the building roof subjects to negative pressure where the solar arrays are located. The windward edge of the roof is also subjected to significant negative pressure, so solar arrays situated at that locations will be subjected to highh suction pressure.

5 Numerical Simulation of Wind Effect on a Rooftop Solar Array 321 Elevation (a) Plan (b) Fig. 8 Velocity streamline around building in elevation and plan view. 3.2 Velocity Streamlines Velocity streamline around the building periphery and solar array is depicted in Fig. 8 (Plan and elevation view). Vortex is clearly visible in the leeward side of the building. The wind separation is observed from the windward edge of the building roof and vortex is also found at building front face. Solar array at leeward edge of the building subjected to less negative pressure (Fig. 6a) than centrally located solar array (Fig. 6b) because the windward face of the building, wind flow separates and it starts reattaching near the leeward edge (Fig. 8). So, a negative pressure zone is observed at the centre of the roof. 3.3 Pressure Coefficients of RSA The pressure coefficients (C p ) on RSA is calculated numerically and compared with the IS: (Part 3) results. Fig. 9 Pressure coefficient of RSA situated at different location. The C p can be calculated as following: P P0 C p 1 2 (8) u0 2 ρ is free stream density, u 0 is free stream velocity, P 0 is free stream static pressure, P is static pressure at the point of interest. The pressure coefficients along the width of the RSA are compared for solar arrays situated at different locations of building roof, are shown in Fig. 9. The results are also compared with the IS 875 Part-3 [22] results, where RSA is consideredd as a 25º mono-slope roof with solidity ratio of 1 (one). 4. Conclusions (1) Average pressure coefficient (C p ) on the solar array located centrally is approximately two times than the solar array located at leeward edge of the building (Fig. 9). It happens due to flow separation at windward edges negative pressure at the position of centrally located solar array; of the building and creation of significant (2) Pressure coefficient (C p ) on the solar array situated at leeward edge of the building varies from -0.6 to -5.6 while for centrally located solar array, C p varies from -6 to -10; (3) Most of the local C p values as per IS 875 Part-3, for a 25º mono-slope roof with solidity ratio of 1, coincide with the C p values obtained for an array situated at the edge of the CBRI building in our study (Fig. 9). Therefore, C p values specified in the IS Code can be used with reasonable confidence;

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