Numerical simulations of the solar chimney power plant concept with short diffuser
Sandro Nižetić, PhD, Associate Professor,
Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, R.Boškovića 32, 21000 Split, Croatia,
Željko Penga, Research Assistant, PhD student
Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, R.Boškovića 32, 21000 Split, Croatia,
Muslum Arici, Assistant Professor, PhD,
Kocaeli University, Department of Mechanical Engineering, Umuttepe Yerleşkesi, Kocaeli 41380, Turkey,
Abstract: This paper deals with the numerical simulations of the alternative renewable energy concept. Alternative energy concept is solar power plant with short diffuser which utilizes convective vortices as heat engine, and were finally it is possible to produce mechanical work for the electricity production via turbine. The previously mentioned concept is already developed from the theoretically point of view but before the potential experimental investigations it is crucial to perform numerical simulations. Therefore, this paper presents preliminary results related to the CFD simulations of complex alternative renewable energy system, where certain issues (influential input parameters, parameters that affect vortex genesis and stability, surrounding conditions, etc.) have been addressed in order to provide important base for the experimental approach and to check realistic possibility for realization of considered alternative energy concept.
Keywords: solar chimney, numerical simulation, renewable energy, energy concept.
1. INTRODUCTION
Development of the alternative renewable energy systems is crucial to reach general goals related to the restrain of the energy consumption and reduction of carbon dioxide emission in the whole World. UN climate change conference in the Paris [1] has set goal to limit global warming effect to less than 2°C compared to the pre-industrial levels. Further, at the moment by usage of the existing, market available and mostly used renewable energy technologies, previous goals would be hardly reached. Hence, we seek for new energy solutions, i.e. energy concepts that will be based on renewables and that will ensure stable energy supply.
Solar energy is nowadays widely used for heat or electricity production where more efficient are solar thermal systems in comparison whit the photovoltaic systems, [2,3]. On a large scale there is a large potential in the case of the concentrated solar systems (parabolic trough, heliostats, linear Fresnel reflectors, etc.), where we already have a prototype plants that are successfully running and producing electricity from the Sun. For example, currently in Morocco construction of the large concentrate solar power plant has started and it is plant of 580 GW in capacity (capable to cover population of around 1.1 million), [4]. Obviously we are entering in the era of large scale solar power plants.
This paper deals with the one specific energy concept where convective vortices are assumed to be used as a sources of the mechanical energy (shaft work) for the turbine drive, i.e. electricity production. Currently there are two theoretically developed concepts, first AVE (Atmospheric Vortex Engine) by Michaud, [5] and second concept of a Solar power plant with short diffuser, [6]. I this study we are dealing with the proposed concept, [6].
The objective of this paper is to report preliminary results related to the CFD analysis of the alternative energy concept (the solar power plant with short diffuser)and which is an important base for the further consideration of the demanding experimental approach.
2.OVERIVEW OF THE ALTERNATIVE RENEWABLE ENERGY CONCEPT
Solar power plant with short diffuser (SPPSD) was proposed by Ninić and Nižetić, [2], based by their previous theoretical research findings, [7-10]. Simplified schematic overview of the SPPSD is presented on the Fig. 1 were it is clear that propose alternative energy concept is consisted from three main parts, i.e. from the solar collector S (collector roof), short diffusor, D, deflector F and specific fluid flow of gravitational vortex column (GVC).
Figure 1 Concept of the SPPSD, [7].
GVC is specific kind of the rotating fluid flow which consists from the updraft of the warm air UF and of the downdraft of the cooled air, DF, Fig.1. Air downdraft in the central section of the GVC is extremely important for vortices genesis and maintenance, [11]. Further, nozzles in the system (N, Fig.1.) are crucial for vortices genesis (as well as also an element of control) as it ensures the water spray flow in the zone inside the diffuser section (addition of the water spray has got favorable effect on the enthalpy of the humid air as it increases working availability of the air). The working principle is simple, surrounding air enters at the periphery of the solar collector and passing through collector it has been heated. Passing towards the central section of the SPPSD warm air is forming GVC, which is in essence rotating fluid flow (rotation is ensured by the helical blades in the central section that are causing circular movement of the air). As already mentioned water nozzles are providing air with the water spray and it is assumed to be happening only in the stage of the initial operation. Finally, by previously explained way, a pressure difference has been produced between surrounding air and air inside the SPPSD. Produced pressure potential could be than used for turbine drive, i.e., for the electricity production. An analytical approach for estimation of the produced pressure potential in the case of the convective vortices was elaborated in [12]. Hence, previously elaborated operating principle needs to be proved by experimental approach, however first step is to provide an CFD analysis to explore all possible risks and to provide on quality way possible experiment. Elaboration of the specific experimental small-scale attempts of the similar AVE concept [5] (concept that also deals with the work production from the convective vortex system) has been provided in the [13], which in essence is a prove that it is possible to create and maintain by artificial way convective vortex.
3.NUMERICAL MODELLING
Almost every CFD model in the literature [14-16] deals with investigation of laboratory-scale vortex setups. Although small-scale vortex installation can be useful to determine the effect of different operating parameters on the overall shape and size of the vortex, it does not enable setup of realistic boundary conditions present during the occurrence of natural gravitational vortex columns (i.e. waterspouts, hurricanes, tornadoes). The driving force of the atmospheric vortex is the pressure difference between theair at the ground level, and at a certain geodetic altitude. This pressure difference is a result of a temperature gradient between the warm ground-level air and cooler air in the lower portions of the stratosphere. The temperature gradient causes the uplift of ground level air of higher temperature and lower density, driven by buoyancy, while the cross-winds induce the circumferential velocity of the flow. Air at higher geodetic altitude has lower temperature, and thereby higher density, and the difference between the densities of ground level air and air on certain geodetic altitude results in circular motion of the air, where the warm air is driven upwards, while the cool air is moving towards the ground. Therefore, in this work, a CFD model was developed to enable the study of the described atmospheric conditions on the global vortex operating parameters – size and shape of the vortex, and the mass flow rate of the uplifting air driven by the pressure difference. In order to account the change in temperature and pressure of atmospheric air with geodetic height, polynomial expressions are introduced in terms of user-defined functions. The influence of relative humidity of air was not considered in this study due to a high computational complexity of multiphase-simulations with heat transfer for domains of this size and the lack of experimental data to verify the simulation results. The simulations are carried out in a commercial software ANSYS CFX.
3.1. Geometry
The geometry was based on the works of Ninić and Nižetić [6], with different configurations of the guide cone and the guide vanes. The maximum diameter of the short diffuser is fixed at 30 m. Figure 1 shows the geometry of the CFD model where the total geodetic height of the model was chosen 2000 m. The height of the domain enables full development of the vortex in height and gives insight in vortex height under realistic operating conditions. The shape of the guide vanes shown in Figure 2a., and the shape of the guide cone shown in Figure 2b. are a result of extensive CFD study of different geometry parameters on the shape of the vortex. Since the concept is novel, there are no available experimental installations from which the geometry parameters could be implemented. In order to simplify the guide vane geometry and reduce the number of parameters for the vortex shape study, geometry of the guide vanes is a two dimensional profile extruded in the vertical direction.Different guide cone geometries were studied, the first one studied consisted of a cylinder with guide cone on the top, but it resulted in a very limited range of operating parameters. By omitting the cylinder and leaving only the guide cone of a relatively primitive shape, the operating range was significantly increased.
Figure 2 Numerical domains: a) solar chimney with short diffuser, guide vanes and solarcollector; b) shape of the guide cone; c) domain overview with mesh refinement region
The influence of the solar collector plate size is studied for three different outer diameters. Furthermore, the solar collector plates have a prescribed temperature field dependent on the collector radius, and are used to heat up the incoming air for a nominal pressure difference between the inlet and the outlet of the simulation domain.
3.2. Mesh and boundary conditions
Considering the scale of the model, the mesh used for the simulations was adapted on a global scale by defining element size of the mesh refinement region shown in Figure 2c. Mesh type was hybrid, consisting of tetrahedral elements with prism layers on the walls of the solar collector, guide vanes, guide cone and the chimney walls, while the transition between tetrahedral and prism elements is achieved by wedge elements. Tetrahedral mesh was used due to a complex geometry involved. In order to avoid very high computational requirements, the number of finite volumes was kept in a range of two million cells. Grid dependency study has shown that global parameters changed very little with higher volume count than 2 million cells. Since the experimental data is not available for local parameters, which could be used for calibration of the model, the simulations with relatively coarse mesh are used only to study the global parameters of the vortex. Dimensionless wall parameter y+ was kept below the value of 300 in respect to the upper limit value applicable for the RANS k-e turbulence model.Figure 3 shows boundary conditions applied for the simulations.
Figure 3 Boundary conditions, model not in scale
3.3. Inlet
Pressure inlet was defined on the outter surface of the solar collector. Ideal gas was defined as material, where the inlet temperature of the air was defined to be 30°C. Full buoyancy effects are considered, as well as the change of ideal gas properties with temperature and pressure.
3.4. Opening
Definition of realistic boundary conditions on the opening was achieved by implementing user-defined functions for gauge pressure and temperature dependency on geodetic altitude, in form of polynomial expressions. Gauge pressure was defined as
(1)
where represents the absolute atmospheric pressure of 101.325 Pa. While the temperature dependency on geodetic altitude was defined as following,
(2)
The polynomial expressions are used to calculate the density of air ideal gas.
3.5. Solar collector surfaces
The solar collector consists of two portions – upper and lower, as shown in Figure 3. In order to simulate the effect of heating of the incoming air to the chimney, temperature fields are defined on the collector surfaces. Since the upper portion is considered to be made of a material characterized by high solar transmission coefficient, the lower portion of the collector will have a higher overall temperature profile than the upper portion, due to a green-house effect. The data from literature [17] suggests that a reasonable temperature difference between the collector outer and inner diameter is in order of 25 – 30°C. The lower portion is considered to have a similar temperature distribution with slightly a higher overall temperature profile by 5°C. The ambient air temperature is defined to be 30°C, corresponding to desert-like climate where the installation would be most applicable. Temperature profile over the collector surfaces is approximated with linear expressions, for the upper and lower portion separately by defining the proper user-defined functions inside the software, as shown in Table 1. Parameter R in Table 1 represents diameter of the solar collector.
Table 1 User-defined temperature profile for the solar collector upper and lower portions for different collector diameters
Parameter / Value / UnitSolar collector diameter / 200 / m
Temperature distribution upper collector surface / / K
Temperature distribution lower collector surface / / K
Solar collector diameter / 600 / m
Temperature distribution upper collector surface / / K
Temperature distribution lower collector surface / / K
Solar collector diameter / 1.000 / m
Temperature distribution upper collector surface / / K
Temperature distribution lower collector surface / / K
4.PRELIMINARY RESULTS OF THE CFD ANALYSIS
Results of the CFD analysis, that will be presented in the continuation of the paper are obtained for diameter of the solar collector of 600 m. Mass flow of the air has ranged from approximately 30.700 kg/s up to 46.000 kg/s and maximal achieved height of the vortex was 669 m. Pressure potential, i.e. pressure difference between surrounding air and air in the central zone of the convective vortex has ranged from 50 hPa up to 120 hPa. Absolute velocity profiles with streamlines are presented on the Figure 4 for previously specified general circumstances.
Figure 4Absolute velocity profiles with streamlines
From the Figure 4 it is noticeable that vortex is established in the atmosphere for above specified conditions, however vortex height is significantly lower one than predicted by theoretical approach. Specific pressure profile is presented on the Fig. 5 where it is clearly noticeable pressure potential between the surrounding air, and air in the central zone of the rotating fluid flow. Circular velocities are being presented on the Fig. 6 and it is also noticeable that central part of the fluid flow rotates as the solid body, so there is a linear dependency for the circular velocity with the specific radius of the vortex in considered region. Maximal achieved magnitude of the circular velocity was around 70 m/s which is in order of magnitude that can be found in the case of the natural convective vortices. Figure 7 shows a distribution of the vertical velocities where it is important to detect a one detail. Namely in the central section of the vortex fluid flow we have negative values of the vertical velocities, i.e. air downdraft has occurred in that region. Previously CFD gained result were predicted in theoretical investigations of the proposed concept, [7,8,11]. Hence, it is obvious that important factor for genesis and maintenance of the specific fluid flow in the surrounding atmosphere is necessary existence of the air downdraft (in the central section of the convective vortices system). Regarding general data gained through the CFD analysis it can be concluded that all numerically gained values for pressure difference and velocities are in agreement with the available field data for the natural convective phenomena’s in the nature, [18] (although it is not easy to compare them as they are not same vortex structures). Later means that on one way preliminary CFD analysis gained reasonable results that hardly can be directly validate as in the basis we deal with the artificial fluid structure so we don’t have available experimental results (but we can indirectly validate with the available data from the natural convective structures).
Figure 5Pressure contour
Figure 6Circular velocity distribution for different altitude
Figure 7Vertical velocity distribution at different altitudes
Therefore, in this stage we can provide a wide range of the CFD gained scenarios that needs to be based on the realistic boundary conditions. Moreover, gained results should be carefully validated, as previously mentioned, validation with the available data from the meteorologist scientists that are dealing with the observation of the natural convective phenomena’s.
5.CONCLUSIONS
This paper deals with the numerical modelling, i.e., CFD analysis of the alternative energy concept. The crucial part of the considered alternative energy concept is specific fluid flow, i.e. convective vortex. Convective vortex is source of the pressure potential that can be used for turbine drive, i.e. for the electricity production. Gained preliminary CFD results for pressure difference and velocities showed reasonable results and in general agreement with the order of magnitude when compared to the natural convective phenomena’s (i.e. in agreement whit the available observation data). However, more intense CFD analysis needs to be obtained in order to get more precise and more realistic data for all circumstances that can affect specific convective vortices genesis and maintenance in the surrounding atmosphere. Finally, previously mentioned circumstances are important to shape future possible experimental attempt to realize proposed alternative energy concept and that it will be in the focus of the future research work.
NOMENCLATURE
h –geodetic altitude, m