A numerical study of vertical solar chimney for enhancing stack ventilation in buildings

Transcription

1 Plea24 - The 21 th Conference on Passive and Low Energy rchitecture. Eindhoven, The Netherlands, September 24 Page 1 of 5 numerical study of vertical solar chimney for enhancing stack ventilation in buildings Wang Liping 1 and Li ngui 2 1 Department of building, National University of Singapore, Singapore 2 School of Environmental & Municipal Engineering, Xi an University of rchitecture & Technology, Xi an, P. R. China BSTRCT: Natural ventilation is an important means to improve thermal comfort conditionings and to reduce the energy consumption for residential buildings. Solar chimney system can be used to enhance the natural ventilation effectiveness, which can make full use of solar energy to produce upward momentum to a mass of air, thereby converting the thermal energy into kinetic energy. In this paper, a numerical study has been carried out to predict mass flow rate, temperature field and velocity field for vertical roof solar chimney under steady-state conditions. Suggestions for optimum construction design of solar chimney have been put forward based on large numbers of simulation results. Results showed that for vertical roof solar chimney with both ends open, the optimum gap-toheight ratio 1/2 was found in the range of chimney gaps investigated, compared with Trombe wall optimum ratio 1/1. The predicted ventilation rate increases with solar radiation and solar chimney height. Meanwhile, reverse flow can be observed at the outlet when the chimney gap increased to a certain value. These findings are helpful in designing a natural ventilation solar chimney for residential buildings. Conference Topic: 7 -energy built environments Keywords: Solar chimney; Natural ventilation; Numerical simulation 1. INTRODUCTION Saving energy and sustainable development are two themes in building constructions after the international energy crisis in Energy required for heating and cooling of buildings in approximately 3% of the total world energy consumption [1]. Natural ventilation and renewable energy utilization are widely used to improve the indoor air environment for buildings and reduce the energy consumption of air conditioning. The comfort indoor environment in summer is normally obtained by air conditioning or ventilation including mechanical ventilation and natural ventilation. Natural ventilation not only can save energy and life cycle costs, but also can alleviate the environmental burden from the by-products of energy consumption. The purpose of natural ventilation is to replace (or partly replace) the air conditioning systems in certain regions, climates and seasons or periods. The annual summary report of International Energy gency (IE) [2] shows that for the wellinsulated office buildings, a well-controlled and energy-efficient natural ventilation system can reduce more than 5% of energy requirement. In building construction, solar energy is widely used in two modes for indoor thermal comfort. One is positive solar house, which uses the photovoltaic materials converting solar energy to electricity. The other mode, passive solar house, is increasingly developing in the world. How to make the building healthy, comfortable with minimum costs is common concern of architects and engineers. Proper building orientation and layout for surroundings, smart integration of indoor space and outdoor figures, and skilful selection of building materials, frame work and construction not only reduce heating energy for building in winter by collecting, maintaining storing and distributing solar energy, but also decrease the indoor temperature in summer by preventing solar radiation from indoor space. Warm winter and cool summer could be achieved by passive solar house with low energy consumption. The building envelopes (roofs and side walls) will never be treated as passive thermal resistances, but as solar energy collections. Passive solar heating in sunny district could reduce conventional heating cost by 7%, even in cloudy district 3% energy consumption can be saved in United States and Europe [3]. Since energy crisis in 1973, 2, residential buildings and 15, commercial buildings have been built up in merica [3]. Solar chimney as one of the effective means to enhance natural ventilation has been utilized since it integrates solar energy and natural ventilation organically. Solar chimney system is a natural draft device, which uses solar radiation to provide upward momentum to a mass of air, thereby converting the

2 Plea24 - The 21 th Conference on Passive and Low Energy rchitecture. Eindhoven, The Netherlands, September 24 Page 2 of 5 thermal energy into kinetic energy [4]. Typical solar chimney can be applied into three main fields: power generation, drying and natural ventilation. In this paper, solar chimney for enhancing natural ventilation has been investigated. There are a few outstanding advantages of solar chimney for enhancing natural ventilation. Under this kind of dynamic condition, the design temperature for thermal comfort can be increased up to 28 with natural ventilation, which can prolong the period for natural ventilation and reduce the times for running air conditioning. It can improve the indoor air quality by supplying outdoor fresh air and avoid the sick building syndrome due to exposure to air conditioning for a long time. Personal psychological needs to communicate with nature could be satisfied with natural ventilation to a large extent. It makes use of available solar energy to promote natural ventilation, cooling and heating. ll this can promote the development of healthy buildings. With the increasing needs of ventilation systems in building which are efficient and in harmony with nature, solar chimney is increasingly used in passive solar houses or hybrid ventilation systems. Replace night air conditioning system, remove the heat stored during the daytime in building and reduce the energy consumption of air conditioning for the next day. In addition, the effects of indoor ventilation by solar chimney partly depend on atmospheric conditions. In spring and autumn, outdoor air condition could satisfy thermal comfort for indoor environment, which will drastically reduce energy consumption. The outdoor temperature during the night is much lower than that in the daytime, so that night air conditioning can be replaced by natural ventilation, which may reduce the heat storage in the daytime and reduce the energy consumption further for the next day. Therefore, there is a need to carry out investigation to optimize solar chimney design to promote natural ventilation. The objective of this paper is to provide guidelines for optimum design of vertical solar chimney to obtain the maximum ventilation rate. Therefore, the effects of natural ventilation by stack effect are not only related to the temperature difference, but also related to the geometric parameters (height, air gap and width) and the opening location of solar chimneys. In this paper, the effects of air gap width, height and solar radiation on mass flow rate have been investigated for solar chimney with vertical solar collector (Fig.1). There are four commonly used methodologies for naturally ventilation: theoretical method, region meshwork method, computational fluid dynamics (CFD) and experiment (wind tunnel, tracer gas). In this paper, solar chimney models have been simulated with CFD software, which has been used for different models, heights, opening widths, air gaps and solar intensities of solar chimney to carry out the simulations. Experiment studies have been used to validate the CFD simulation results. During the process of simulation, the density of grids has been increased until there is almost no variation between the two results (the error should be less than 1%). Thus, the grid independent result has been obtained. Temperature field and velocity field in solar chimney have been analyzed. 2.1 turbulence model For roof solar chimney with vertical solar energy collector, three-dimension turbulence model has been selected since the width of the model is comparative with the length of the model. Standard k ε model with standard wall function calculation method has been selected. hot air heat storage material insulation material 2. METHODOLOGY The utilization of passive solar energy does not completely involve the exploration of new technologies, but is mostly the problem of architect design. Natural ventilation in solar chimney is caused by stack effect. This kind of convective heat transfer belongs to heat transfer in a limited space. That is to say, heated air can not expand freely and increase the thermal layer thickness. Therefore, the temperature increase in the limited space is higher than that of free space heat transfer, and the buoyancy force is much greater. nd thus, air flow will speed up. However, with the increase of buoyancy force, the air velocity and airflow resistance also increase. Generally, conduction is the main heat transfer means for small temperature difference and very small air gap. However, heat transfer is very near to free space heat transfer for big wall gap. roof W indoor air Figure 1: roof solar chimney with vertical solar collector 2.2 Parameters and calculation conditions The calculation ranges of three dimension model W(width) H(height) L (length) are as following. W=.1m ~ 2.m, H=1.m ~ 3.m, L=1.m. Heat fluxes which are the part of solar radiation transmitted through the windows in heat storage wall are

3 Plea24 - The 21 th Conference on Passive and Low Energy rchitecture. Eindhoven, The Netherlands, September 24 Page 3 of 5 separately 1W/m 2, 2W/m 2, 3W/m 2, 4W/m Simulation method number of different configurations of roof solar chimney with vertical solar collector have been simulated. To find the rules for the variation of flow rate of solar chimney, the effects of heat fluxes S 1, S 2,chimney height H, air gap width W, width of inlet and outlet out on ventilation flow rate have been investigated. (1)Keep the constant of W,T T a,s 1,S 2, out, investigate the effects of chimney height H on mass flow rate m ; (2)Keep the constant of H, T T a, S 1, S 2, investigate the effects of air gap W, out on mass flow rate m ; (3) For each condition above, the effects of heat fluxes S1, S2 on mass flow rate m have been investigated. ctually, each variable in real project varies in a relative small range. However, it is difficult to obtain the variation trend of mass flow rate in the certain range. In this simulation study, the ranges of variables have been extended in the simulation. Some extreme conditions have been calculated. Therefore, the variation rules for mass flow rate of solar chimney could be recognized more clearly and guidelines for solar chimney could be provided. 3. VLIDTION To test the reliability and accuracy of the turbulence model for the calculation of roof solar chimney with vertical solar energy collector, the simulation results using the standard k ε model with standard wall function were compared with the experiment results [5].The solar chimney height for both experiment and simulation was 1.5m,the length was.62m,and air gap width was ranged from.1m to.6m. The heat storage wall was heated by uniform heat flux varied from 2W/m 2 to 6W/m 2. The heat storage wall consisted of.12mm thick stainless steel plate and 1mm thick fiberglass for insulation. 3mm thick plexiglass was opposite to the heat storage wall. The two sidewalls consisted of 6mm thick plxiglass and 5mm thick polystyrene. Fig.2 describes the comparison results of mass flow rate between simulation and experiment. When the uniform heat flux is 4W/m 2 and the air gap width varies from.1m to.6m. The maximum relative error between experiment results and simulation results is 18.5%. The average relative error between experiment results and simulation results is about 9.5% simulation experiment ir gap width(m) Figure 2: mass flow rate comparison between simulation and experiment(uniform heat flux=4w/m 2 ) From the comparison results, it can be seen that simulation results are well consistent with the experiment results. It could be concluded that using the mathematical model for the simulation of roof solar chimney with vertical solar collector is reliable. However, there are still minor errors between the simulation and experiment results. There are two main reasons for the deviation. The first one is the difference of thicknesses and characteristics for materials between simulation and experiment. The second reason is radiation heat transfer has not been considered in simulation. 4. RESULTS ND DISCUSSION 4.1 Temperature field s shown in Fig.3, it can be seen that air temperature inside the vertical solar chimney increases along the chimney height and is nonuniform along air gap width. Temperature is quite high near heat storage wall. With the increase of solar radiation intensity and solar chimney height, air temperature inside solar chimney increases. Figure 3: the temperature profile for vertical solar chimney along the air gap width 4.2 Velocity field From the velocity field Fig.4, it can be seen that the distribution of air velocity along the air gap width for vertical solar chimney is non-uniform. ir velocity near heat storage wall is much higher than that in the middle. ir velocity inside the solar chimney will decrease with the increase of air gap width and

4 Plea24 - The 21 th Conference on Passive and Low Energy rchitecture. Eindhoven, The Netherlands, September 24 Page 4 of 5 increase with the increase of solar radiation intensity. From the flow display of simulation results, it could be clearly recognized that reverse flow near the outlet of vertical solar chimney at a big air gap width as shown in Fig.5. Figure 4: the velocity profile for vertical solar chimney along the air gap width (H= 3m) Figure 5: velocity profile for vertical solar chimney (W=.6m, H=3m) 4.3 Mass flow rate The main characteristic of solar chimney with vertical collector model is that the air gap width of roof solar chimney with vertical solar collector is equal to the width of inlet and outlet. Therefore, there is much difference between roof solar chimney and Trombe wall solar chimney in the aspect of optimum air gap width The effects of solar radiation The mass flow rate obviously increases with the increase of solar radiation. s the main power for solar chimney, solar radiation is transmitted through the clear glass and then is mainly absorbed by the vertical heat storage wall to increase air temperature inside. The hot air goes up by the stack effect, which promotes the indoor natural ventilation. With the increase of solar radiation intensity, the heat gain of heat storage wall was increased. s a result, the temperature of air inside the chimney was also increased. Fig.6 shows the increase of mass flow rate with solar radiation intensity for roof solar chimney with vertical solar collector W=.1m W=.5m W=1.m W=1.5m Uniform heat flux(w/m 2 ) Figure 6: the variation of mass flow rate with solar radiation (H=3m) The effects of air gap width and inlet width There are two factors responsible for the increase of mass flow rate. One is the increase of stack effect. The other one is the reduction of pressure losses. The increase of air gap width and inlet width decreases pressure losses of the system. On the other hand, the intensity of reverse flow near the outlet has been increased with the increase of air gap width. The occurrence of reverse flow reduces mass flow rate. Therefore, there must be an optimum air gap width for specific height of vertical solar chimney to obtain the maximum ventilation rates. Fig.7, Fig.8 and Fig.9 show the variation of mass flow rate with inlet width for the height of roof solar chimney 1m, 2m and 3m respectively. From the result it can be seen that the mass flow rate firstly increases with the increase of air gap width and slightly decreases after a certain value of air gap width. The maximum ventilation rate could be obtained when the air gap width is.8m for 1m high solar chimney. The optimum air gap width is 1.m for 2m high solar chimney. The optimum air gap width is about 1.5m for 3m high vertical solar chimney. It could be concluded that there is an optimum ratio of air gap width to height. The optimum ratio for roof solar chimney with vertical solar collector about 1/2 is much bigger than 1/1 for Trombe wall solar chimney [6]. Comparing with Trombe wall solar chimney, the inlet width for vertical solar chimney has been increased with the increase of air gap width. Thus, the pressure losses for air flow have been reduced due to the increase of openings, which lead to obtaining the maximum ventilation rate at big air gap width.

5 Plea24 - The 21 th Conference on Passive and Low Energy rchitecture. Eindhoven, The Netherlands, September 24 Page 5 of S1=1 S1=2 S1=3 S1= ir gap width and inlet width(m) Figure 7: the variation of mass flow rate with air gap width and inlet width (H=1m) S1=1 S1=2 S1=3 S1= ir gap width and inlet width(m) Figure 8: the variation of mass flow rate with air gap width and inlet width (H=2m) S1=1 S1=2 S1=3 S1= ir gap width and inlet width(m) Figure 9: the variation of mass flow rate with air gap width and inlet width (H=3m) The effects of roof solar chimney height Fig.1 shows the variation of mass flow rate with the increase of roof solar chimney height. From the results, it can be seen that with the increase of roof solar chimney height, the mass flow rate increases drastically. This is mainly due to the rapid increase of stack effect. There are two main reasons for the increase of stack effect. Firstly, the surface area of heat storage wall increases with the increase of chimney height. Therefore, more solar radiation has been obtained and air temperature inside the solar chimney has been increased. s a factor of stack effect, density difference between outside and inside increases. Secondly, chimney height is one factor for stack effect. Therefore, it can be said that the increase of solar chimney height brings the obvious increase of mass flow rate S1=1.5 S1=2 S1=3 S1= Solar chimney height(m) Figure 1: the variation of mass flow rate with solar chimney height (W=.6m) 6. CONCLUSION The simulation results achieved agree well with the experimental results, which indicates the reliability of CFD with k ε turbulence model and the method of wall function. It can be observed in airflow profiles that there is reverse flow near the outlet of solar chimney when air gap width has been increased to a certain value. For roof solar chimney with vertical solar collector, it is commended that chimney height is better than 1m. The optimum ratio of air gap width to chimney height is nearly 1/2. It is suggested that the ratio should be no more than1/2 for a solar chimney. Mass flow rate of solar chimney obviously increases with the increase of solar radiation intensity and chimney height. Nomenclature S 1 Uniform wall heat flux (W/m 2 ) S 2 Uniform heat flux for glazing(w/m 2 ) H Solar chimney height(m) W ir gap width(m) ----Inlet width(m) in out ---Outlet width(m) m Mass flow rate(kg/s) T in ----inlet air temperature ( ) T a ----outdoor air temperature ( ) REFERENCES [1] [2] Hybrid Ventilation nnex35. (2).IE annual reports. Sydney. [3] Edward S, Cassedy. (2) Prospects for sustainable energy: a critical assessment. Cambridge University Press. [4] Padki, M.M., and Sherif, S.. (1999). On a simple analytical model for solar chimneys. Int. J. Energy Res., 23, [5] Chen Z. D., Bandopadhayay P., Halldrosson J., Byralsen C., Heiselberg P., and Li Y. (23). n experimental investigation of a solar chimney model with uniform wall heat flux. Building and Environment, 38, [6] Gan, G. (1998). parametric study of Trombe walls for passive cooling of buildings. Energy and buildings, 27,