On Clean Cooling Systems for Wind Turbine Nacelle operating in Hot Climate

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International Conférence en Clean Cooling Technologiesin the ME NA Regions ICT3_MENA 201 Bou Smail, W.

MAHDI Laboratoire de Génie Mécanique et Développement Ecole Nationale Polytechnique B.P. 182 El-Harrach, Algiers, 16200, ALGERIA arezki.smaili@g.enp.edu.

The wind turbines that would be installed in the Sahara, namely the electromechanical equipment located inside the nacelle, are subjected to high and fluctuating temperature gradients during the day

1 International Conférence en Clean Cooling Technologiesin the ME NA Regions ICT3_MENA 201 Bou Smail, W. Tipaza, 5-6 October 2015 On Clean Cooling Systems for Wind Turbine Nacelle operating in Hot Climate Arezki SMAILI, Mohammed A. MAHDI Laboratoire de Génie Mécanique et Développement Ecole Nationale Polytechnique B.P. 182 El-Harrach, Algiers, 16200, ALGERIA Abstract The climate of the Algerian Sahara is characterized by extreme thermal conditions. The wind turbines that would be installed in the Sahara, namely the electromechanical equipment located inside the nacelle, are subjected to high and fluctuating temperature gradients during the day and the seasons. This reality might generate inconsistent design stress and can lead to some difficulties of conception. In order to maintain acceptable temperature values of the air inside the nacelle (i.e. acceptable average temperature of the electromechanical equipment), the heat generated by the generator should be rejected to the atmosphere, and the heat exchange between the air inside the nacelle and the surrounding air should be controlled properly. In this work, a hypothetical wind-turbine-nacelle, represented as rectangular cavity, has been considered. For this purpose, the air flow fields within the nacelle have been described using the Navier-Stokes equations. The energy equation has been used to account for heat transfer effects. A CFD method has been employed to solve the resulting governing equations. Mainly, an overview of clean cooling techniques is presented and their potential use for wind-turbine-nacelle operating in Algerian Saharan climate is discussed and presented. Keywords: Wind turbine nacelle; Thermal analysis; Numerical simulation, Clean cooling systems, Hot climate Nomenclature A c p f k L c m Nu p q C q " T u i R S j surface area per deep unit heat capacity of the air cycle frequency of magnetic cooling device air thermal conductivity characteristic length defining the normal distance between the hot and cold plates mass of magnetic refrigerant Nusselt number air pressure cooling capacity j th heat flux air temerature i th air flow velocity component specific air constant entropy change of magnetic material air density shear stress tensor 1. Introduction Wind turbines that would be installed in Algerian Saharan climate should operate under severe weather conditions and fluctuating temperature during the day and seasons. Particularly, the electric/electronic equipments located within the nacelle may be subjected to extremely high temperature variations, thus leading to inconsistent design stresses. In order to maintain acceptable temperature levels inside the nacelle and to manage efficiently the thermal effect, the heat released by the electrical and mechanical components as results of various power dissipations (due to Joule

2 effect, friction losses; e.g. electrical generator, gearbox, bearings ) should be rejected to the atmosphere, and the heat exchange between the air inside the nacelle and the surrounding air should be controlled properly. Therefore, cooling systems must be employed to ensure safe operation and to prevent failure of the turbine, especially under high ambient temperature conditions (e.g. Saharan climate). However, while the turbine benefits high cooling efficiency, it also suffers lower reliability and higher cost for adding such a complex cooling system. The main challenge for electronic equipments in a wind turbine nacelle is that they must withstand a wide range of ambient temperature, usually from -40 C to +55 C. A numerical method to assess the effect of environment temperature on wind turbines-nacelle operating in Canadian Nordic climate has been developed previously [1]. In this method, the air flow is described by Reynolds averaged Navier-Stokes equations. The energy equation is used to account for the heat transfer effects. To solve the resulting governing equations, a Control-Volume Finite Element Method (CVFEM) has been employed [2]. In a recent work [3], a numerical method has been proposed to investigate the nacelle thermal behaviour operating under extreme Saharan hot weather conditions. This study is a first attempt to assess the cooling capacity needed to ensure acceptable temperature levels inside the nacelle for given extreme weather conditions and moderate wind velocities, by considering a hypothetical thermal nacelle problem. The present paper deals with (i) a numerical approach to determine the average temperature inside the nacelle and to investigate its variation with the required cooling capacity and (ii) investigations of appropriate clean cooling systems. 2. Physical problem and governing equations Consider a hypothetical wind-turbine-nacelle, represented as rectangular cavity. The flow field in the vicinity of nacelle (i.e. ignoring the effects of the tower and the ground) immersed in a uniform incoming flow parallel to the turbine's axis of rotation is assumed to be 2D. As a preliminary study, it is instructive to consider such a simplified case problem. Since it allows to efficiently appreciate the key parameters that play prominent role during thermal design and development steps of nacelle, especially those to be installed in hot climate (e.g. Saharan region). The following considerations and simplifying assumptions have been thus adopted. The flow field around the nacelle is neglected. The gravity (the buoyancy force) impact is considered. Thus, the effect free convection takes place within the nacelle. The heat transfer by radiation is neglected. The heat generation (considered to result mainly from electrical generator) is idealised as an isothermal condition, represented by a hot plate (i.e. generator wall) in the computational domain. The cooling system is idealised as an isothermal condition, represented by a cold plate (i.e. nacelle internal wall) in the computational domain. Taking account of the symmetry of the problem, we consider only the left-side of the hypothetical nacelle. For steady flow conditions, continuity, Navier-Stokes and energy equations written in Cartesian tensor form are given by (ρu ) x = 0, (1) ρu u x = p x + τ x, (2) ρu (c T) x = q " u + τ x, (3) x 2

3 where, u is the i-th air flow velocity component, Tis the air temperature, pis the air pressure, ρand c are respectively density and heat capacity of the air.

Numerical method The complete set of fluid equations, expressed in the two dimensional coordinate system, (x, y), consists of the continuity equation, two momentum equations for transport of

3 3 where, u is the i-th air flow velocity component, Tis the air temperature, pis the air pressure, ρand c are respectively density and heat capacity of the air. To take into account the temperature dependence of the air density, the ideal gas law has been used: p ρ = R T, (4) where, R is the specific air constant,τ is the shear stress tensor. 3. Numerical method The complete set of fluid equations, expressed in the two dimensional coordinate system, (x, y), consists of the continuity equation, two momentum equations for transport of velocity, and the energy equation for heat transfer effects. The solution of the resulting mathematical model is accomplished by employing the CVFEM formulation mentioned above Computational domain and boundary conditions The computational domain consists of a vertical cavity. Figure 1 shows the computational domain. The domain is discretized into unstructured meshes composed of triangular elements (Fig.2). As shown in Figure 1, the following boundary conditions have been adopted. For the temperature field, the isothermal conditions have been prescribed for nacelle internal wall (i.e. cold plate at T ) and for the electrical generator wall (i.e. hot plate at T ), as well as adiabatic boundary conditions for symmetry line of the nacelle. The non-slip conditions are prescribed for velocity fields. Fig. 1: Computational domain. Fig. 2: Grid topology of the domain Numerical procedure for calculating the resulting heat flux To determine the resulting heat flux, q", within the nacelle, the proposed numerical procedure is presented as follows. First, the local Nusselt number near the generator wall is calculated according to the relationship:

4 Nu(s) = L T T (s), (5) δ T T where, s denotes the curvilinear coordinate, T (s) is the temperature at the first grid node, and δ is the resulting distance between the wall and this node, L is a characteristic length defining the normal distance between the hot and cold plates. Next, the average Nusselt number is given by Nu = 1 Nu(s)ds, C C (6) where, C denotes the curvilinear path describing the generator wall. Finally, the resulting heat flux is obtained and given by the following relationship q" = k T Nu, (7) L where, T = T T is the temperature span between the two plates, k is the fluid thermal conductivity assumed to be constant and calculated at the average temperature (T + T )/2. 4. Results and discussions To assess properly the thermal behaviour of the hypothetical nacelle, first, the knowledge of the required cooling capacity and the resulting temperature-level inside the nacelle constitutes the key parameters. Next, the potential use of an appropriate clean cooling system is presented and discussed Required cooling capacity and average air temperature inside nacelle Since the heat exchange with the external environment has been neglected in this study, the required cooling capacity would be equal to the heat rate inside the nacelle, i.e. q". Thus, the energy balance equation is reduced to: q = q (8) where, q denotes the heat rate generated by the hot plate. It is given by : q = q" A, (9) where, A denotes the surface area of the hot plate per deep unit, and q" is computed using Eq. (7). The temperature level inside the nacelle has been determined by considering the average air temperature within the nacelle, T, which can be calculated as : T = 1 T(x, y)dv, (10) V where, T(x, y) is the resulting temperature distribution inside the nacelle, and V is the interior volume of the nacelle. 4

5 Fig. 3: Evolution of the average Nusselt number as a function of the temperature span, T. Figure 3 shows the evolution of the resulting Nusselt number as a function of the temperature span.

Figure 4 shows the results of the required cooling capacity and the average temperature within the nacelle as a function of temperature span, with and without the gravity impact.

We observe also that the buoyancy impact on the nacelle thermal behaviour is intensifying as the temperature span increases.

5 5 Fig. 3: Evolution of the average Nusselt number as a function of the temperature span, T. Figure 3 shows the evolution of the resulting Nusselt number as a function of the temperature span. These results are obtained by performing simulations for temperature spans varying from 20 C to 100 C. Figure 4 shows the results of the required cooling capacity and the average temperature within the nacelle as a function of temperature span, with and without the gravity impact. As expected, the cooling process requires more capacity rate in the presence of the heat transfer by natural convection. This is again a qualitative validation of the numerical method. We observe also that the buoyancy impact on the nacelle thermal behaviour is intensifying as the temperature span increases. It is also interesting to notice that the linear variation trends exhibited by both the curves: q and T versus T. However, as the temperature span increases, the linear trend seems not to be describing well the behaviour of the average temperature curve. Fig. 4: Evolution of the cooling capacity and the average air temperature inside the nacelle as a function of the temperature span, with and without gravity impact Appropriate clean cooling system Among clean cooling technique, magnetic refrigeration systems appear to be the most suitable and non-ozone depleting technology, because of their potential to operate with higher thermodynamic efficiency and they don t make use of CFC/HFC refrigerants, in comparison to conventional gas refrigeration devices. Hence, in this paper, such a cooling system is considered to investigate its

6 feasibility to cool the wind turbine nacelle operating in hot climate conditions. The concept of magnetic cooling is based on the principle of the magnetocaloric effect of certain magnetic materials (i.e. solid magnetic refrigerants), in which the entropy can change (i.e. S) when the material is subjected to changes in external magnetic field ([4], [5]). Based on the resulting cooling capacity values, q C, and average air temperature, T av, presented above, as function of temperature span, T (Figure 4). For given external air temperature, T, consider a magnetic cooling system that would operate, within temperature range [T av, T ] (where, T and T av represent the hot source temperature and cold source temperature of magnetic refrigerator respectively), on an Ericsson-like magnetic cycle. The resulting (rate of) entropy change, S, of a refrigerant material, is key parameter which can be calculated according to the relationship: S = q T (11) For extremely high atmospheric temperature, T = 55 C, being typical severe Saharan weather conditions (e.g. Adrar region); for a fixed temperature of the hot plate, T H = 100 C (being limited temperature for safe operation of electronic equipment), it can be found that (from Figure 4) the cold-plate temperature T C = 0 C and cooling capacity q C = 93.7 W. Thus, the resulting rate of entropy change can be estimated, S = 0.34 W/K. To design and size the cooling system, one should determine the required amount of magnetic refrigerant. Consider a refrigerant based on Gadolinium (Gd) material [4], in which specific entropy change s 1 J/kg.K, within the operating temperature range, is obtained under magnetic field changes of 1 Tesla and is given by the relationship: 2fm s = S (12) where, f is the cycle frequency and m is the mass of magnetic refrigerant. For a reel (irreversible) magnetic cycle (Ericsson cycle), the cycle thermal efficiency, coefficient of performance, COP, is given by COP = T T T (1 I) (13) where, I denotes the irreversibility of the thermodynamic cycle. For given typical values of cycle frequency f = 1Hz and the cycle irreversibility I = 30% (note that practical lower values up to 15% can also be used), the mass of the magnetic refrigerant (which constitutes the heart of cooling system) and the resulting cycle performance have been calculated to be m = 170 g and COP = 3.67 respectively. With the density of Gd is kg/m 3, the resulting volume of the refrigerant is cm 3. From these results, it can be concluded that the use of such a magnetic cooling system is highly promising; because of higher COP value (70% of Carnot cycle) and highly reduced volume (i.e. compact device: refrigerant being solid), in comparison with conventional gas refrigeration systems (in which thermal efficiency can never exceed 40% of Carnot cycle). 5. Conclusion This paper deals with numerical investigations of potential use of a magnetic refrigeration device (i.e. clean cooling system) to cool wind turbine nacelle that would operate in hot climate (e.g. Adrar region). For this purpose, a hypothetical nacelle has been considered. The heat generation and the appropriate cooling system idealised as isothermal conditions have been 6

7 7 represented respectively by hot-plate temperature and cold-plate temperature in the computational domain. First, to determine the average air temperature within the nacelle and the required cooling capacity, a CFD method has been used. Simulations have been carried out for different temperature spans. The simulation results including the average temperature inside nacelle and the required cooling capacity as a function of temperature span have been obtained. Next, based on the obtained cooling capacity and average temperature of air inside the nacelle, for given external air temperature, an appropriate magnetic refrigeration device has been proposed. It has been found that the potential use of such a device is highly promising, in comparison with conventional gas methods. References [1] Smaïli A., Masson C., Taleb S. R. and Lamarche L., Numerical Study of the Thermal Behaviour of Wind Turbine Nacelle Operating in Nordic Climate. Numerical Heat Transfer Part B: Fundamentals, (2006),Vol. 50, no2, pp [2] Tran L. D., Masson C. and Smaïli A., A Stable Second-Order Mass-Weighted Upwind Scheme for Unstructured Meshes, International Journal for Numerical Methods in Fluids, (2006), Vol. 51, pp [3] Smaïli A., Tahi A. and Masson C., Thermal Analysis of Wind Turbine Nacelle Operating in Algerian Saharan Climate, Energy procedia, 18, (2012), [4] Smaïli A., R. Chahine, Composite materials for Ericsson-like magnetic refrigeration cycle. J. Appl. Phys. 81(2), pp , 1997 [5] Gschneidner K. A, Jr., V.K. Pecharsky, Thirty years of near room temperature magnetic cooling_where we are today and future prospects, International Journal of Refrigeration, 25 January 2008.

Chapter 1. Introduction

Chapter 1. Introduction Chapter 1 Introduction Refrigeration: Our society is highly dependent on cooling technology. The cooling is used for preservation of foods, for different types of surgeries, for air conditioning, for different