Numerical and Analytical Modeling of Solar for Chimney Combined Ventilation and Power in Buildings

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Covenant Journal of Engineering Technology (CJET) Vol. 2, No. 1, June. 2018

An Open Access Journal Available Online

Layeni A. T. 1, Nwaokocha C. N.*1, Giwa S. O. 1, Olamide O. O.1

1Department of Mechanical Engineering, Olabisi Onabanjo University, Ago Iwoye, Nigeria * [email protected], [email protected]

Abstract- Analytical and numerical studies were carried out to investigate the performance of the combined solar chimney for power generating system and ventilation based on a developed mathematical model. Numerical solution of the problem was based on the continuity, momentum and energy equations for turbulent, steady-flow using k-ε model and the finite volume method using ANSYS Fluent CFD package. The analysis domain was a 2-D room of 4 x 4 m2 with solar chimney of various dimension attached. The results obtained revealed that Chimney Height (CH), Collector Width (CW), Solar Heat Flux (SHF) and ambient wind speed were found to be the most important factors in the design of the SC. Results showed that the room mass flow rate increased from 1 kg/s with no wind effect to about 30 kg/s with induced wind of 1 m/s. The mass flow rate increased from about 6 to 9 kg/s at CH of 5 and 8 m respectively for no wind condition and SHF of 400 W/m2. Power outputs were obtained for the average velocity of the chimney, collector area and chimney height. It was observed from results obtained from both the numerical and analytical analysis that power outputs of the power generating systems increases with increase in heat energy in the collector space area which is a function of the global solar radiation intensity, the collector area, and the chimney height. The power outputs results showed that with SHF of 400 W/m2 for CH of 5 and 9 m were 33 and 85 W/m2 respectively. The respective power output for SHF of 200 and 1,000 W/m2 were 25 and 47 W/m2. Furthermore, the optimum values of CH, CW and SHF were 5 m, 1 m and 417 W/m2 respectively under no wind condition with room temperature of 300 K and chimney velocity 0.12 m/s. It was also observed that with the increase in the mass flow rate in the chimney, the ventilation requirements were adequately met. Keywords: Solar collector; Chimney; Turbine generator; Power output, Ventilation. 36

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I. Introduction at the end of 2011. However, since At the present stage of technology 2004, photovoltaic passed wind as the development fossil fuels are the main fastest growing energy source, and source of energy in the world. since 2007 has more than doubled Current energy production from coal every two years. At the end of 2011 and oil is damaging to the the photovoltaic (PV) capacity environment and non-renewable, worldwide was 67,000 MW, and PV which necessitated the research into power stations are popular in other sources of energy which are Germany and Italy (Alex Morales, renewable. It is noted that inadequate 2012; Renewable Energy, 2012). energy supplies can lead to high Moreover, while many renewable energy costs which invariably leads energy projects are large-scale, to low standard of living as well as to renewable technologies are also poverty. There is therefore need for a suited to rural and remote areas, switch to renewable sources of where energy is often crucial in energy. human development. Climate change Renewable energy, according to Odai concerns, coupled with high oil et al (2016) Renewable energy, is prices, peak oil, and increasing energy which comes from natural government support, are driving resources such as sunlight, wind, increasing renewable energy rain, tides, waves and geothermal legislation, incentives and heat, which are renewable. It is commercialization in many further reported according to the developed and developing countries. Renewables 2011 Global Status New government spending, Report that about 16% of global final regulation and policies helped the energy consumption comes from industry weather the global financial renewables, with 10% coming from crisis better than many other sectors. traditional biomass, which is mainly According to a 2011 projection by the used for heating, and 3.4% from International Energy Agency, solar hydroelectricity. New renewables power generators may produce most (small hydro, modern biomass, wind, of the world’s electricity within 50 solar, geothermal, and biofuels) years, dramatically reducing the accounted for another 3% and are emissions of greenhouse gases that growing very rapidly. The share of harm the environment (Ben Sills, renewables in electricity generation is 2011; UNEP, 2007; Zhou, et al., around 19%, with 16% of global 2007a). electricity coming from Schlaich and Wolfgang (2000) in the hydroelectricity and 3% from new Encyclopedia of Physical Science and renewables (Renewables 2011 Global Technology, Third Edition, stated Status Report). that “Sensible technology for the use According to current research, the use of solar power must be simple and of wind power, widely used in reliable, accessible to the Europe, Asia, and the United States, technologically less developed is increasing at an annual rate of countries that are sunny and often 20%, with a worldwide installed have limited raw materials resources, capacity of 238,000 megawatts (MW) should not need cooling water or

Layeni A. T, et al CJET (2018) 2(1) 36-58

produce waste heat and should be otherwise consumed for mechanical based on environmentally sound ventilation and/or cooling which can production from renewable be achieved successfully with the use materials.” This is where other of a solar chimney ventilation system technology of harnessing the solar (Gontikaki, et al., 2010). energy is sought after, that will Background produce the required energy at a cost Solar Chimney Ventilation System lower that of the PV solar energy Chimneys or channels are used in technology. The solar chimney meets various applications such as heating these conditions and makes it and ventilating of buildings, drying of possible to take the crucial step agricultural products, and various toward a global solar energy other passive systems such as for economy. Schlaich and Wolfgang cooling electronic components (Bar- (2000) reported that Economic Cohen and Krauss, 1988; Bilgen and appraisals based on experience and Michel, 1979; Ekechukwa and knowledge gathered so far shown that Norton, 1999). even solar chimneys rated at 100 and A Solar Chimney (SC) ventilation 200 MW are capable of generating system differs from a conventional energy at costs comparable to those chimney in that at least one wall is of conventional power plants. This is made transparent; solar radiation a major reason to develop this form enters the chimney through the glazed of solar energy utilization further. In part and heats up the walls. The a future energy economy, solar temperature of the air inside the SC chimneys could thus help assure channel rises due to heat transfer economic and environmentally from the walls and the resulting appropriate utilization of energy in buoyancy drives the airflow through sunny regions. the channel. The SC pulls air from

Another application of the solar the interior of the building, which is chimney is in the area of ventilation replaced by fresh air through systems for buildings. Natural openings or other paths, and natural ventilation of buildings can be ventilation is accomplished. achieved with solar-driven, Performance of the SC is primarily buoyancy-induced airflow through described by the induced ventilation the solar chimney channel. Gontikaki, flow rates; in case heat harvesting is et al (2010) noted that the also of interest, air temperature in the exploitation of sustainable energy channel is the other important sources in relations to the functional performance indicator (Gontikaki, et demands of buildings (for heating, al., 2010; Hirunlabh, et al., 1999; ventilation, cooling, etc.) can Nouanégué and Bilgen, 2009; contribute significantly to energy Schlaich and Wolfgang, 2000; savings in buildings, and thus to Sudapom and Bundit, 2006). alleviating of the present Burek and Habeb (2007), in their environmental, economic and social report on an experimental problems related to conventional investigation into heat transfer and energy practices. Passive (natural) mass flow in thermosyphoning air ventilation of buildings is a heaters, such as solar chimneys and successful means to save energy Trombe Walls. Their principal results

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showed that the mass flow rate And that the thermal efficiency of the through the channel was a function of system (as a solar collector) was a both the heat input and the channel function of the heat input, and not depth given by dependent on the channel depth given m/Q0:572 and by 0:712 0.298 m/s ; h/Qi

Figure 1: Cross-Section and Top View of Solar Chimney

Nouanégué and Bilgen (2009), in Their results showed that the their Numerical study by conjugate proposed combined ventilation heat transfer carried out on solar system improved the ventilation. chimney systems for heating and When the mean solar radiation during ventilation of dwellings, produced a day from 8:00 to 18:00 h is 550 results showing that the surface Wm-2, the mean airflow rate in the radiation modifies the flow and combined ventilation system reaches temperature fields, affects the Nusselt 0-32 m/s, which is 18.5% higher than number and the volume flow rate, that (0.27 m/s) in the individual both in a positive way, and improves chimney ventilation system under the the ventilation performance of the same conditions. Furthermore, results chimneys. showed that the effect of night

Zhou et al (2007a,b) studies a ventilation on the combined solar combined solar house with a solar house is better, although less energy chimney and a solar water collector. is drawn out from the room by Theoretical analyses were carried out comparing chimney airflow to investigate natural ventilation in temperatures and rates when the the solar house, compared with that in dampers are open after 18:00 hour, the individual chimney ventilation with those when the dampers are system under the same conditions. open day and night.

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Gontikaki et al (2010) in their report Habeb (2007) derived that the mass 0.572 stated their parametric findings of the flow rate is proportional to Qi 2 SC. In parametric analysis, where Qi (W/m ) the uniform heat parameters are varied to determine flux supplied to the back wall. the sensitivity of the system’s According to Gontikaki et al (2010) performance against each other. They Wind is the second most influential stated that the most influential climatic parameter, as it can create parameters fall under the category of positive or negative pressures at the geometrical (e.g. wall height, gap outlet of the SC and thus obstruct or width), construction (e.g. type of enhance the airflow. In the outdoor glass, insulation) and climatic (e.g. experiments by Arce, et al. (2009) at solar radiation, wind) parameters. a full-scale SC, the highest airflow The effect of wall height and cavity rates coincided with the highest width has in some cases been studied recorded wind velocity. and expressed separately, while in Maerefat and Haghighi (2010a), others inextricably, in terms of the examined the heat and mass transfer height-to-width aspect ratio (also characteristics of a Solar Chimney referred to as height-to-gap ratio). Air (SC) and an Evaporative Cooling flow increases with height, since the Cavity using low-energy- back wall’s heat gains increase: in a consumption technique to improve parametric study on Trombe walls, it passive cooling and natural is reported that an increase in wall ventilation in a solar house. The height by a quarter is equivalent to an results obtained show that the system increase in heat gains by three is capable of providing good indoor quarters. In another study air flow air condition during daytime even rates increased in when the wall with low solar intensity of 200 W/m2. height increased from 3.5 to 9.5m (a Further results also show that relative cavity width of 0.3m was humidity is a factor and at RH lower considered). With respect to the than 50%, the system performs cavity width, it was found that the effectively even at 40o C. Another induced flow rate increases with analysis carried out by Maerefat and increasing width. Haghighi (2010b), to investigate The intensity of solar heat flux is the cooling and ventilation in a solar motive force for the operation of the house using solar chimney (SC) SC and is thus the most determinant together with earth to air heat factor for its performance. In the exchanger (EAHE) for passive experimental study of Chen et al. cooling as a low-energy consuming (2003) varying values were technique for removing heat from a considered for the uniform heat flux building in the hot seasons. The on the back wall and the airflow rate results show that the solar chimney was found to rise by 38% for a can be used to power the threefold increase of heat flux (from underground cooling system during 200W/m2 to 600W/m2). Mathur et al. the daytime, without any need of (2006) found that airflow rate electricity. increases linearly with solar radiation. DeBlois et al. (2013), investigated the Based on experimental investigations performance and energy level of the on a small-scale SC, Burek and roof type solar chimney, and noted it

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being a low cost energy reduction and turbines. In the collector, solar method for passive ventilation. The radiation is used to heat an absorber results obtained indicate that the (ordinarily soil or water bags) on the ventilated roof provided cost-free ground, and then a large body of air, cooling and passive ventilation in all heated by the absorber, rises up the climates and seasons. The results also chimney, due to the density observed that stack effect rather than difference of air between the chimney natural convection was the driving base and the surroundings. The rising force. Further results show that the air drives large turbines installed at system reduced the cooling load by the chimney base to generate about 80%, while cross ventilation electricity. The concept of solar alone by 50% percent. chimney power technology was first Koronaki (2013), investigated conceived many years ago first ventilation performance to assess the described in a publication written by effectiveness of three high thermal a German author, Gunther in 1931 inertia solar thermosyphonic (Zhou, et al, 2010), again presented configurations, used as night natural in 1978 and proven with the ventilation systems. The results operation of a pilot 50 kW power obtained show that a thermal chimney plant in Manzanares, Spain in the with duct attached behind the early 1980s (Pretorius & Kröger, absorber facing West was the most 2006; Liu, et al., 2005; Schlaich and effective design with predicted flow Wolfgang, 2000; Zhou, et al., 2010). rate 98% higher than the a similar Pretorius and Kröger (2006) size conventional SC. examined the effect of a convective Solar Chimney Power Plant heat transfer equation, more accurate Man learned to make active use of turbine inlet loss coefficient, quality solar energy at a very early stage: collector roof glass and various types greenhouses helped to grow food, of soil on the performance of a large chimney suction ventilated and scale solar chimney power plant. The cooled buildings and windmills Results show that the new heat ground corn and pumped water. transfer equation reduces plant power Schlaich and Wolfgang (2000) stated output considerably. Further results the solar chimney's three essential show that the effect of a more elements - glass roof collector, accurate turbine inlet loss coefficient chimney, and wind turbines - have is trivial, and using better quality thus been familiar from time glass enhances plant power immemorial. A solar-thermal production. It was also discovered chimney power plant simply that using Limestone and Sandstone combines them in a new. The soil produced similar results to technology combines three Granite. components: a collector, a chimney

Layeni A. T, et al CJET (2018) 2(1) 36-58

Figure 2: Principle of the solar chimney: glass roof collector, chimney tube, wind turbines

Fluri and von Backstrom (2008a) area ratio but improves only slightly carried out performance evaluation with reducing the diffuser area ratio using analytical models and below unity. optimisation techniques on several Zhou et al (2009) in their report turbo-generator layouts of SCPP. found that the results based on the Results show that slight changes in Manzanares prototype show that as modelling approach have significant standard lapse rate of atmospheric impact on the performance estimate. temperature is used, the maximum Further results conclude that the power output of 102.2 kW is obtained single rotor layout without guide for the optimal chimney height of 615 vanes performs very poorly. The m, which is lower than the maximum counter rotating layouts provide the chimney height with a power output highest peak efficiencies, however at of 92.3 kW. Sensitivity analyses are relatively low speeds, which leads to also performed to examine the undesirable higher torque for the influence of various lapse rates of same power output. Further study by atmospheric temperatures and Fluri and Von Backstrom (2008b) collector radii on maximum height of investigated the performance of the chimney. The results show that Power Conversion Unit of a SCPP maximum height gradually increases and its effect on the plant. The results with the lapse rate increasing and go obtained show that single vertical to infinity at a value of around 0.0098 axis turbine has a little better K m –1, and that the maximum height efficiency and energy yield due to for convection and optimal height for some loss mechanisms are not maximum power output increase with present, however, the output torque larger collector radius. value makes its feasibility less Pretorius and Krögerb (2009) in their desirable. Further results show that report presented results indicating redesigning the flow passage reduces that windy conditions impair plant aerodynamic losses and improve performance considerably, while performance. Results show that the nocturnal temperature inversions PCU efficiency depreciates cause significant reductions in night- significantly with increasing diffuser time output. Petela (2009), carried out

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a thermodynamic analysis of energy respectively, due to solar energy and and exergy balances on a simplified the greenhouse effect produced in the model of solar chimney power plant solar collector. Air at the ambient (SCPP). The results obtained show temperature T∞ enters into the that an energy input of 36.81 MW of collector and is heated to the mean solar radiation to SCPP, corresponds temperature Tf in the solar collector. to 32.41 MW input of radiation The energy balance equations for the exergy. different components of the system are given as follows: Proposed combined ventilation and power solar chimney Collector and Chimney Glazing: Zhou et al (2009) describes solar chimney power technology, designed (1) to produce electric power on a large- where, S1 denotes solar radiation scale, as utilizing solar energy to absorbed by glazing cover; produce ventilation that drives wind hrap is the radiation heat transfer turbines to produce electric power. coefficient between absorber and The generation of power in the glazing cover; hp is the convective process of passive ventilation is the heat transfer coefficient between main objective of the study; the glazing cover and airflow in the solar development of a combined collector; ventilation and power solar chimney. Ut denotes the overall heat loss II. Mathematical Model coefficient from glazing cover to Zhou et al (2007b) in their study ambient, including convection by developed a mathematical model wind, radiation heat transfer from based on energy balance to predict glazing cover to sky and conduction the performance of the solar chimney through glazing. thermal power generating equipment Absorber: for different conditions. The following assumptions are made: 1. Air follows the ideal gas law. (2) 2. The mathematical model is considered to be in steady state. where, S2 denotes solar radiation absorbed by absorber; 3. There is no friction or leakage considered in the system. ha is the convective heat transfer coefficient between absorber bed and 4. Only the buoyancy force is airflow in the solar collector; considered, and wind-induced natural ventilation in the ambient Ub denotes the overall heat transfer coefficient from the absorber to the is not included. 5. The temperature of the natural earth. ground under the heat insulator Collector airflow: bed is equal to that of the ambient. Figure 2 shows the physical model for the solar chimney thermal power (3) generating system. The absorber and where, Acoll is the collector area; the glazing can be heated to temperatures of Ta and Tp,

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Q denotes the useful heat energy Boussinesq Model treats density as transferred to the moving airflow by constant value in all the equations convection. except in the buoyancy term of the

The mean temperature of hot air, Tf , momentum equation, given as can be given by, follows (4) � = �0 1 − �Δ� → �Δ� ≪ 1 (9) Δ�� = ��Δ� where, To is the collector outlet airflow temperature; (10) ε denotes the constant in mean Chimney temperature approximation, which is Assuming a uniform velocity and recommended as 0.25 by Hirunlabh temperature inside the chimney the et al. (1999), theoretical flow rate will be (Ortega, 2011):  Energy balance in the chimney (11) (5) (12) where, cp is the specific heat of air; m denotes the mass flow rate of hot  Approximation velocity profile airflow in the solar collector, which is constant equal to the mass flow rate of hot (13) airflow passing through the solar  Drag coefficient Vs constant pressure collector outlet and can be calculated loss by the following equation, (6) (14) where ρ is the air density at the solar  Bernoulli equation collector outlet; Ach is the cross-sectional area of solar chimney; V denotes hot air velocity at the solar collector outlet. According to Schlaich J. (1995), V (15) can be expressed as (7) The value of Cd is calculated as a where g is the acceleration due to function of the sum of coefficient of gravity and pressure loss of the inlet, the nozzle, Hch is the height of the the outlet and the friction of the walls chimney. System’s Power: Power output Pout can be found as (Schlaich, 1995), (16) In theory, Cd is a function of the flow (8) rate, because the factor f depends of where ήw is the turbine generator the Reynolds’s number Re, but � ∈ efficiency, usually between 50% and 0.04, 0.015 which gives a �� ∈ 0.75, 90%. 0.788, which is small difference Ortega (2011) stated in the analyses (Ortega, 2011). Therefore Cd can be of solar chimney design, that the

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considered only as the function of the analysis are the Navier-Stokes height: equations, the Continuity and (17) momentum equations and the energy equation for the heat transfer analysis of the system. Additional transport (18) equations are modelled for turbulent Natural ventilation takes place when flow. In Reynolds averaging, the buoyancy pressure overcomes the solution variables in the sum of all flow pressure losses. A instantaneous (exact) Navier-Stokes mathematical model based on equations are decomposed into the Bernoulli’s equation is used to mean (ensemble-averaged or time- evaluate the system flow rate averaged) and fluctuating (Maerefat & Haghighi, 2010a). The components. For the velocity chimney net draft is given by the components: following formula (21) where and are the mean and fluctuating velocity components (j = 1,2,3). Likewise, for pressure and (19) other scalar quantities: where, c is the pressure loss j (22) coefficients, ζ is the chimney friction where ᶲ denotes a scalar quantity. ch Substituting expressions of this form factor, V is the chimney air for the flow variables into the ch instantaneous continuity and velocity, momentum equations and taking a ρchout is the air density at chimney outlet, time average yields the ensemble- averaged momentum equations, ρa is the ambient air density. The kinetic energy efficiency is one written in Cartesian tensor form as: of the solar chimney properties that (23) allow the comparison of the solar chimney with other technologies. This efficiency can be calculated as the coefficient between the kinetic energy and the power generated in the solar collector: (24) Equation (23) and Equation (24) are the Reynolds-averaged Navier-Stokes (RANS) equations and have the same general form as the instantaneous (20) Navier-Stokes equations, with the Computational Fluid Dynamics velocities and other solution variables (CFD) Analysis now representing ensemble-averaged The CFD analysis of this study is (or time-averaged) values. Additional performed using ANSYS FLUENT terms now appear that represent the software (ANSYS, 2011). The effects of turbulence. Reynolds mathematical model for the CFD stresses ( ) are modeled in

Layeni A. T, et al CJET (2018) 2(1) 36-58

order to close Equation 24. The In the equations above, Gk represents Boussinesq hypothesis is related to the generation of turbulence kinetic the Reynolds stresses to the mean energy due to the mean velocity velocity gradients: gradients, and Gb is the generation of turbulence kinetic energy due to buoyancy. ANSYS FLUENT solves the energy equation in the following form: (25)The Boussinesq hypothesis is used in the k – Ԑ models employed in the study. The standard k – Ԑ model is based on model transport equations for the turbulence kinetic energy (k) (28) and its dissipation rate (Ԑ). Where is the effective conductivity (k + kt, where kt is the turbulent thermal conductivity,

defined according to the turbulence model being used), and is the diffusion flux of species . The first (26) three terms on the right-hand side of Equation (8) represent energy transfer due to conduction, species diffusion, and viscous dissipation, respectively.

Computational Domain

(27)

Figure 3: 2D Computational Domain

III. Results and Discussion outputs of the power generating The analytical model was simulated systems increases with increase in the using an Excel Spread Sheet, while Heat Energy in the collector space the numerical simulation was carried area which is a function of the global out using ANSYS Fluent package. solar radiation intensity and collector The power outputs simulated by the area, and chimney height. program are obtained for different conditions, as shown in Figures 15 Analytical Results: and 16. The results show that power

Figure 4: Effect of Chimney Height on Chimney Air Velocity (Analytical)

Figure 5: Effect of Chimney Height on Power Generated

47 Layeni A. T, et al CJET (2018) 2(1) 36-58

Figure 6: Effect of Chimney Air Velocity Power Output Ra = 1010

Figure 7: Effect of Chimney Width on Air Mass Flow Rate

Figure 8: Effect of Chimney X-Sectional Area on Air Mass Flow Rate

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Figure 9: Effect of Chimney Height on Power Generated

Figure 10: Effect of Chimney Air Velocity Power Output Ra = 1010

Figure 11: Comparison of Heat Energy (Power Generated) in the Collector with Chimney Kinetic Efficiency

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Figure 12: Solar Collector Efficiency with Increase in Solar Heat Flux

Figure 13: Effect of Collector Area on Heat Energy in the Collector Space

Figure 14: Effect of Chimney Width on Air Mass Flow Rate

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Numerical Results:

Figure 15: Effect of Chimney Height on Mass Flow at Outlet at no Induced Wind, CH of 5 m and SHF of 400 W/m2 Condition for Ra = 1012 – 1014

Figure 16: Effect of Chimney Gap on Mass Flow at Outlet at no Induced Wind, CH of 5 m and SHF of 400 W/m2 Condition for Ra = 1012 – 1014

Figure 17: Effect of Chimney Height on Power Output for Ra = 1012 - 1014

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Figure 18: Effect of Flow Velocity on Power Output Ra = 1014

Figure 19: Effect of Solar Heat Flux on Power Output for Ra = 1012 - 1014

Effect of Solar Heat Flux, Chimney al, 2005; Pretorius and Kröger, 2006; Height and Chimney width on Zhou et al, 2009). Outlet Mass Flow Rate and Effect of Solar Heat Flux and Chimney Velocity Chimney Height on Power Output The outlet mass flow rate, which is The power output per unit area was also a function of the chimney found to increase from 39 to 61 W/m2 velocity was observed to increase for solar radiation of 200 and 1,000 from 0.5 to 0.8 kg/s at Ra = 1014 for W/m2 at Ra = 1014, with a solar SHF of 200 to 1 000 W/m2 with a radiation to flow power conversion of chimney height of 5 m and width of 1 19.5 % and 6.1 % respectively. The m. The velocity increased from about effect of the chimney height on the 0.6 to about 2 m/s for chimney height power output of the SC was 154.6 % of 1 m to 10 m. This confirms increase from power output of 44 to observations from literature that the 112 W/m2 at chimney height of 5 to 9 2 chimney height is a major factor in m at solar radiation of 400 W/m and 51 14 determining the power output of the Ra = 10 . The conversion effect SC (Schlaich and Schiel, 2000; Liu et (ratio of power to chimney height)

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are 8.8 and 12.4 for heights of 5 and 9 power output instead of the m respectively. rectangular channel used. Effect of Flow Velocity on Power Optimised Model Analysis Output The optimisation criteria have been The flow velocity is another major set with model constraints that will factors of the power output of the SC. produce optimum effect for both Results show that an increase in room ventilation and power channel velocity from 0.05 to 1.5 m/s generation. Two (2) optimal solutions caused the power output to increase were found that placed the chimney from 3.1 to 92.1 W/m2, while an height and the solar collector breadth increase to 5 and 10 m/s resulted in a at values of 5 m and 1 m respectively. power output of 307.1 and 614.1 This solution minimised the chimney W/m2 respectively. It was noted that height and the solar collector breadth, the channel velocity was affected by which will reduce the size of the factors, which include the ambient system and consequently reduce the wind velocity, the temperature cost of the system. difference between the ambient The minimisation of the solar heat temperature and the channel flux was between 200 and 600 W/m2, temperature (which was a function of as a result of the average solar the solar heat flux) and the geometry insolation experienced in the of the SC. However, the factor within Southern Nigeria Region. The human control was the geometry of optimised solutions found for the the system. At constant ambient solar heat flux for the two optimum conditions, it is suggested that a (2) solutions are 407 and 417 W/m2 convergent divergent channel may which is close to the average range of help in increasing the velocity at Solar Heat Flux values for Nigeria required points for an increase in (Osueke et al., 2013).

Figure 20: The temperature contour of the SC system at heat flux of 200 – 1,000 W/m2 The results from the optimised model value of 300 K (Figure 20). This showed that the room temperature increase in velocity at the cavity as was conserved and maintained at the the room air enters the chimney is

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due to the boundary layer effect at major contributor to the effectiveness the cavity and the suction pressure of a SC system: In that respect, thus differential; coupled with the stack would improve the results from the effect within the chimney, the optimised model. velocity increase was observed within Effect of Solar Radiation on the chimney from 0.05 m/s to 0.12 Ventilation flow pattern and m/s.. The chimney temperature was Temperature within the Room found to have increased from 300 K The outlet mass flow rate, which is to 314.7 K and 315.1 K for both also a function of the chimney solutions. velocity was observed to increase The velocity at the junction between from 0.5 to 0.8 kg/s at Ra = 1014 for the chimney and the horizontal solar SHF of 200 to 1 000 W/m2 with a collector also showed an increase in chimney height of 5 m and width of 1 velocity from 0.05 m/s to 0.07 m/s for m. The flow within the room showed both optimised solutions. However, it that with cross ventilation the flow was observed that for power across increases in velocity as the generation it will be more desirable to solar radiation increases, creating two put the turbine at a position close to pressure zones above and one below the top of the Chimney, since it is this the cross flow section (Figure 21). part that correspond to maximum This is due to the pressure differential velocity. created by the cross flow and the Moreover, Gontikaki et al. (2010) temperature vis-à-vis density of air at noted that wind is a highly influential the two zones. climatic parameter and its effect is a

Figure 21: The flow pattern of the SC system at heat flux of 200 – 1,000 W/m2

IV. Conclusion being used as a means of ventilation In conclusion, the Solar Chimney was and electric power generation in analysed for potentials for providing buildings in Nigeria. The study aimed

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at the development of a solar was found to increase with both chimney model for the dual purpose increase in solar radiation and of passive ventilation and renewable chimney height. It is therefore power generation in buildings. In this concluded from the performance study analysis were carried out using analysis that the combined solar basic equations of thermos-fluids to chimney is a feasible a means of carry out sensitivity analysis of the improving energy efficiency of model. The mass flow rate increased buildings. Furthermore, it is noted as chimney height increases. It was that combined solar chimney had also observed that the mass flow is a better results at locations with higher direct function of the solar radiation solar radiation and wind conditions input. The power output of the system for both ventilation and power.

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