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4th International Conference on Power and Energy Systems Engineering, CPESE 2017, 25-29 2017, Berlin, Germany 4th International Conference September on Power and Energy Systems Engineering, CPESE 2017, 25-29 September 2017, Berlin, Germany
Numerical of the thermal a HAWT The investigations 15th International Symposium on Districtbehavior Heating andof Cooling Numerical investigations of the thermal behavior of a HAWT nacelle using ANSYS FLUENT Assessing the feasibility using the heat demand-outdoor nacelle usingofANSYS FLUENT M. a A.long-term Mahdi*, A. Smaili temperature function for district heat demand forecast M. A. Mahdi*, A. Smaili *, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Laboratoire de Génie Mécanique et Développement, a a b c c Ecole Nationale Polytechnique, El-Harrach, Algiers, 16200, Algeria Laboratoire de Génie Mécanique et Développement, Ecole Nationale Polytechnique, El-Harrach, Algiers, 16200, Algeria a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract
a,b,c
I. Andrić
Abstract The Algerian Sahara is characterized by severe climate conditions where temperature reaches high levels with large variations during day and season. Nacelles of by horizontal axis wind turbines (HAWT) operating in this to the The Algerian Sahara is characterized severe climate conditions where temperature reaches highregion levels are withsubjected large variations Abstract problem, in particular the electromechanical components becoming less effective as they heat up during use. In order overheating during day and season. Nacelles of horizontal axis wind turbines (HAWT) operating in this region are subjected to the to maintain problem, an appropriate temperature of the air within the nacelle, an activeless cooling system hasheat to be up use. to reject the overheating in particular the electromechanical components becoming effective as they up set during In order District heating networks are commonly addressed the in the literature one ofmust the most effective solutionsThe for decreasing the generated the external environment heatas be properly controlled. to maintainheat an towards appropriate temperature of the airand within resulting the nacelle, antransfer active cooling system has to be set up present to rejectwork the greenhousethe gasthermal emissions from the building sector. These systems requirewind high turbine. investments which are returned through the heat investigates behavior of the nacelle of a typical commercial The impact of radiation and turbulent generated heat towards the external environment and the resulting heat transfer must be properly controlled. The present work sales. convection Due to theheat changed climate conditions and building renovation policies, using heat demand in the future could decrease, natural within thenacelle nacelleof has investigated numerically ANSYS code, whereas the investigates the thermaltransfer behavior of the a been typical commercial wind turbine. The impactFLUENT of radiation and turbulent prolonging the investment return period. forced with the environment has been modeledusing usingANSYS a suitable correlation. natural convection convection heat heat exchange transfer within the surrounding nacelle has been investigated numerically FLUENT code,Temperature whereas the Thevelocity main scope of this paperthe is nacelle to assesshave the feasibility of using heat demand – outdoor function for heat demand and profiles been presented andthediscussed. Variations of temperature the averagecorrelation. temperature inside the forced convection heatwithin exchange with the surrounding environment has been modeled using a suitable Temperature forecast. The district of Alvalade, located in Lisbon and (Portugal), wasdiscussed. used as a case study. The district is consisted of 665 nacelle and the required cooling capacity are determined thoroughly and velocity profiles within the nacelle have been presented and discussed. Variations of the average temperature inside the vary Published in both construction period and typology. Three weather scenarios (low, medium, high) and three district ©buildings 2017 and Thethat Authors. Elsevier nacelle the required cooling by capacity areLtd. determined and thoroughly discussed. © 2017 The Authors. Published by Elsevier Ltd. intermediate, renovation scenarios were developed (shallow, deep). To estimate the error, obtained heat demand values were Peer-review responsibility of Elsevier the organizing © 2017 The under Authors. Published by Ltd. committee of CPESE 2017. Peer-review under responsibility of the scientific committee the 4th International Conference and Energy compared with results from a dynamic heat demand model,ofpreviously developed and validatedonbyPower the authors. Peer-review under responsibility of the organizing committee of CPESE 2017. Systems Engineering. The results showed that when weather change is Numerical considered, the margin of error could be acceptable for some applications Keywords: Thermal analysis; HAWTonly nacelle; Saharan climate; simulation (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: Thermal analysis; HAWT nacelle; Saharan climate; Numerical simulation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1. Introduction renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the The climate of the Algerian Sahara is characterized by extreme conditions. Wind turbines that would be installed coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and inimprove this are subjected to Sahara highestimations. and temperature during theturbines day andthat seasons. reality The region climate of the is fluctuating characterized by extremegradients conditions. Wind would This be installed the accuracy ofAlgerian heat demand
in this region are subjected to high and fluctuating temperature gradients during the day and seasons. This reality © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +213 (0) 666 92 8299; fax: +213 (0) 23 82 8529. Cooling.
address:author.
[email protected] * E-mail Corresponding Tel.: +213 (0) 666 92 8299; fax: +213 (0) 23 82 8529. E-mail address:
[email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review the organizing committee 1876-6102 ©under 2017responsibility The Authors. of Published by Elsevier Ltd. of CPESE 2017. Peer-review under responsibility of the organizing committee of CPESE 2017.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 4th International Conference on Power and Energy Systems Engineering. 10.1016/j.egypro.2017.11.049
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might generate contradictory constraints of design and can lead to some difficulties of conception. In order to maintain an appropriate temperature of the air inside the nacelle, a cooling system must be employed, so the heat released by the generator could be rejected towards the atmosphere. The resulting heat transfer must be controlled properly to ensure safe operation and to prevent failure of the wind turbine. Thus, a compromise should be found between the air temperature levels inside the nacelle on one hand, and the high cooling efficiency which leads to lower reliability and higher cost for such a complex cooling system, on the other hand. To assess the effect of the environment temperature on wind turbine nacelle operating in Canadian Nordic climate, a numerical method has been developed previously by Smaili et al. [1]. The air flow has been described by Reynolds averaged Navier-Stokes equations and the energy equation has been used to account for the heat transfer effects. The resulting governing equations have been solved by the Control-Volume Finite Element Method (CVFEM) [2]. In a recent work [3], a numerical method has been proposed to investigate the nacelle thermal behavior operating under extreme Saharan hot weather conditions. The main purpose of the present study is to assess the cooling capacity needed to ensure acceptable temperature levels inside the nacelle, by considering the turbulent natural convection and radiation effects within the nacelle. Detailed results including temperature and velocity fields within the nacelle have been presented and discussed. To assess properly the thermal behavior of the nacelle, average temperature within the nacelle, as well as the required cooling capacity have been investigated and thoroughly discussed. 2. Mathematical model 2.1. Physical problem Figure 1(a) illustrates the geometry of the 2D-problem. The nacelle is air-tight, therefore the physical problem splits in two independent parts. (i) Internal sub-problem: Heat transfer within the nacelle by confined natural convection and radiation; (ii) External sub-problem: Heat exchange with the surroundings by radiation and by forced convection of the axial air flow over the nacelle.
Fig. 1. (a) Geometry of the problem; (b) Grid topology.
The internal sub-problem is solved numerically using the CFD method. However, a suitable correlation taken from the literature has been used to approach the external sub-problem. The following practical considerations and simplifying assumptions are adopted: (i) Transient turbulent regime is considered; (ii) Heat transfer by radiation is considered; (iii) The gravity impact (buoyancy force) is considered. Thus, heat transfer by confined natural convection takes place within the nacelle; (iv) Heat generation (resulting mainly from the electrical generator) is idealized as an isothermal condition, represented by a hot plate at temperature TH ; (v) The cooling system is idealized as an isothermal condition, represented by a cold plate (i.e. nacelle cover) at temperature TC , which represents the cooling temperature; (vi) The external air flow is assumed to be horizontal with a constant free stream temperature T and a constant horizontal velocity U , where the tower and blades effect is neglected. 2.2. Governing equations The governing equations, expressed in the Cartesian coordinate system, consist of the unsteady Reynoldsaveraged Navier-Stokes and energy equations, where Boussinesq approximation is considered for the temperaturedensity dependence. For turbulence modelling, the shear-stress transport (SST) k model with four additional
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transport equations has been considered. As it has been shown by Wu et al. [4], this model has the best performance in terms of predicting turbulence quantities in confined natural convection. Air within nacelle is transparent and assumed to be non-absorbing and non-emitting. Therefore it is considered as a non-participating medium. The nacelle cover and the generator wall are assumed to be gray, opaque and diffuse emitters and reflectors of radiation with an emissivity of 0.9 . For such conditions, the surface-to-surface radiation model (S2S) has been selected to model heat transfer by radiation within the nacelle. 3. Numerical method Since the heat exchange of the nacelle with the surrounding environment is modelled through a suitable correlation, only the nacelle inner part constitutes the computational domain. For the temperature field, the isothermal conditions have been prescribed for nacelle cover (i.e. cold plate at TC ) and for the electrical generator wall (i.e. hot plate at TH ). The non-slip conditions are prescribed for velocity field. To carry out simulations, structured rectangular meshes have been used. The grid is refined in the vicinity of the isothermal walls where high temperature and velocity gradients take place. Figure 1(b) illustrates the topology of the grid. ANSYS FLUENT code has been used to solve the resulting governing equations along with the specified boundary conditions. PISO algorithm is selected for the pressure-velocity coupling, with a time-step of 0.001s . 4. Results and discussions 4.1. Problem statement and mesh dependency study A typical 850kW commercial wind turbine has been considered. The geometry of the nacelle has been assimilated to a horizontal circular cylinder of L D 13m 3m . The generator is represented by a coaxial cylinder of l d 3.22m 1.4m placed at a distance of 0.48m from the back side of the nacelle (cf. Fig. 1(a)). In all the study, a safety limit of 100 C is considered for the hot plate temperature TH . It is worth mentioning that for cooling temperature TC varying from 20 C to 30 C and for such dimensions of the nacelle, Rayleigh number based on the longest distance between hot and cold plates is Ra 1012 . Therefore, the air flow inside the nacelle is highly turbulent. To obtain the solution in such conditions, a time-dependent approach is required. For the external subproblem, typical values of 55 C and 5m / s are considered for the temperature and the velocity of the free stream air, respectively. To determine the appropriate grid size that would produce mesh-independent numerical results, simulations have been performed for typical boundary conditions TC 0 C . The resulting inner heat rate qin calculated by Eqt. (1), has been evaluated numerically over the hot plate (i.e., the generator wall G ) by considering different grids. T qin k ds, (1) G n G where k is the air thermal conductivity. n and s are, respectively, the local coordinate normal to the wall and the curvilinear coordinate. Figure 2 shows the mesh dependency study results. As it can be seen, the resulting inner heat rate is convergent and becomes independent of the grid size for number of nodes greater than 10 6 .
Fig. 2. Mesh dependency study.
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4.2. Temperature field and streamline contours The resulting distribution of temperature and streamline contours are shown in Figure 3, where TC 0 C and TH 100 C . As it can be seen, the behavior of these fields seems to be quite reasonable, thus confirming qualitatively the validity of the proposed numerical method. In particular, the thermal stratification phenomenon of the air on the left side of the nacelle driven by the concentration of the hot air in the upper part of the nacelle which is due to the decrease in the density, thus confirming the suitability of the adopted Boussinesq approximation.
Fig. 3. (a) Temperature distribution (
C ) and (b) streamline contours ( 10 3 kg / s ) within the nacelle.
4.3. Nacelle thermal behavior analysis To investigate properly the nacelle thermal behavior, two key parameters are assessed: the resulting average air temperature within the nacelle, Tav , calculated as 1 Tav Td, (2) where denotes the inner surface area of the 2D-nacelle, and the cooling capacity required for the cooling process, qC , that is to be determined through the energy balance equation qC qin q ext , (3) where qin is the heat rate released by the electrical generator and calculated numerically using Eq. (1). qext is the heat rate exchanged between the nacelle and the surrounding environment, that is the sum of the radiation and the air forced convection heat exchange rates, such that qext A(T4 TC4 ) hA(T TC ), (4) with being the Boltzmann constant and A the nacelle cover area. In order to determine the convection heat transfer coefficient h , the heat exchange of the nacelle with the surrounding environment is modeled using the equation (5) proposed by Wiberg et al. [5] to correlate the case of a horizontal circular cylinder placed in an upstream axial flow affected by turbulence-generating grid. (5) Nu 0.155 Re0.674 , where Nu is the Nusselt number averaged over the cylinder surface, defined as Nu hD / k . Re is the Reynolds number based on the cylinder diameter given by Re U D / , where is the kinematic viscosity of the air. This correlation is suitable for the purpose of the present study since the nacelle is long enough to make the reattachment length of the turbulent flow lower than the nacelle length. However, it is to be mentioned that the considered air free stream conditions are such that Reynolds number value exceeds slightly the range of 8.9 10 4 6.17 10 5 indicated by the latter authors. Figure 4(a) shows the variation of the resulting inner and external heat rates along with the required cooling capacity as functions of the cooling temperature. Figure 4(b) shows the variation of the average temperature within the nacelle. As it can be seen, a linear trend characterizes all these evolutions. Indeed, the required cooling capacity (W ) can be written as function of the average temperature within the nacelle ( C ) , as qC 855Tav 69698. (6)
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Fig. 4. (a) Evolution of (a) heat rates and (b) the average temperature within the nacelle as functions of the cooling temperature.
Furthermore, from Figure 4(a), it is clear that the lower is the cooling temperature, the more energy-requiring is the cooling process. This is again a qualitative validation of the numerical method and the suitability of the used correlation for the modeling of the external sub-problem. We observe also that the external heat exchange contribution to the required cooling capacity is more important than the resulting inner part. Moreover, the external part is increasing more significantly as the cooling temperature decreases. This confirms the drastic impact of the Saharan weather conditions on the thermal behavior of the nacelle. However, it is of interest to notice that the contribution of the internal heat exchange by radiation and by natural convection constitutes an important part. Indeed, the heat rate released by the generator, q in , constitutes 22% of the required cooling capacity. Therefore, the thermal loads generated within the nacelle constitute a significant factor not to be neglected during the thermal design of the nacelle cooling system. 5. Conclusion This paper addresses the problem of cooling of a horizontal axis wind turbine nacelle operating in the Algerian Saharan climate, i.e., challenging weather conditions. To investigate the thermal behavior of the nacelle, a typical commercial wind turbine has been considered. A numerical method has been used to assess the heat exchange by radiation and turbulent natural convection within the nacelle. Heat exchange with the surrounding environment is modeled using a suitable correlation. The heat generation and the cooling system idealized as isothermal conditions are represented respectively by hot and cold plates in the computational domain. Temperature and flow field within the nacelle have been obtained and discussed. The thermal behavior of the nacelle has been assessed by considering the required cooling capacity and the average temperature within the nacelle. Comparison of the contributions of inner and external heat exchange rates has shown the prominence of the environment thermal loads. However, it has been confirmed that appropriate values of required cooling loads are also dependent on the accurate predictions of heat exchange within the nacelle by radiation and turbulent natural convection. Acknowledgements The Laboratoire de Génie Mécanique et Développement (LGMD) of Ecole Nationale Polytechnique of Algiers and the Algerian Government are gratefully acknowledged for their resources and support. References [1] Smaili A, Masson C, Taleb SR, Lamarche L, Numerical study of the thermal behaviour of wind turbine nacelle operating in Nordic climate, Numerical Heat Transfer Part B: Fundamentals 2006;50(2):121-141. [2] Tran LD, Masson C, Smaili A, A stable second-order mass-weighted upwind scheme for unstructured meshes, International Journal for Numerical Methods in Fluids 2006;51:749-771. [3] Smaili A, Tahi A, Masson C, Thermal analysis of wind turbine nacelle operating in Algerian Saharan climate, Energy Procedia 2012;18:187196. [4] Wu T, Lei C, On numerical modelling of conjugate turbulent natural convection and radiation in a differentially heated cavity, Int. Jour. of Heat and Mass Transfer 2015;91:454-466. [5] Wiberg R, Lior N, Heat transfer from a cylinder in axial turbulent flows, Int. Jour. of Heat and Mass Transfer 2005;48:1505-1517.
