Performance evaluation of solar chimney power plants in Iran

The solar chimney power plant is a simple solar thermal power plant that is capable of converting solar energy into thermal energy in the solar collector. In the second stage, the generated thermal energy is converted into kinetic energy in the chimney and ultimately into electric energy using a combination of a wind turbine and a generator. The purpose of this study is to evaluate the performance of solar chimney power plants in some parts of Iran theoretically and to estimate the quantity of the produced electric energy. A mathematical model based on the energy balance was developed to estimate the power output of solar chimneys as well as to examine the effect of various ambient conditions and structural dimensions on the power generation. The solar chimney power plant with 350 m chimney height and 1000 m collector diameter is capable of producing monthly average 1-2 MW electric power over a year.     

Performance analysis of conventional and sloped solar chimney power plants in China

The solar chimney power plant (SCPP) has been accepted as one of the most promising approaches for future large-scale solar energy applications. This paper reports on a heat transfer model that is used to compare the performance of a conventional solar chimney power plant (CSCPP) and two sloped solar chimney power plants (SSCPPs) with the collector oriented at 30° and 60°, respectively. The power generation from SCPPs at different latitudes in China is also analyzed. Results indicate that the larger solar collector angle leads to improved performance in winter but results in lower performance in summer. It is found that the optimal collector angle to achieve the maximum power in Lanzhou, China, is around 60°. Main factors that influence the performance of SCPPs also include the system height and the air thermophysical characteristics. The ground energy loss, reflected solar radiation, and kinetic loss at the chimney outlet are the main energy losses in SCPPs. The studies also show SSCPPs are more suitable for high latitude regions in Northwest China, but CSCPPs are suggested to be built in southeastern and eastern parts of China with the combination to the local agriculture.     

Simulation of a pilot solar chimney thermal power generating equipment

A pilot experimental solar chimney thermal power generating equipment was set up in China. A simulation study was carried out to investigate the performance of the power generating system based on a developed mathematical model. The simulated power outputs in steady state were obtained for different global solar radiation intensity, collector area and chimney height. By intercomparison, it is found that the simulated power outputs are basically in agreement with the results calculated with the measurements, which validates the mathematical model of the solar chimney thermal power generating system. Furthermore, based on the simulation and the specific construction costs at a specific site, the optimum combination of chimney and collector dimensions can be selected for a required electric power output.     

Analysis of some available heat transfer coefficients applicable to solar chimney power plant collectors

A solar chimney power plant consists of a translucent collector which heats the air near the ground and guides it into the base of a chimney at its centre. The buoyant air rises in the chimney and electricity is generated through one or more turbines in or near the base of the chimney. Various studies about solar chimney power plant performance have been published. Different calculation approaches with a variety of considerations have been applied to calculate chimney power plant performance. In particular, two comprehensive studies are relevant, namely those of (Bernardes, M.A.d. S., Voß, A., Weinrebe, G., 2003. Thermal and technical analyses of solar chimneys. Solar Energy 75, 511-524; Pretorius, J.P., Kröger, D.G., 2006b. Solar chimney power plant performance. Transactions of the ASME 128, 302-311). The paper compares the methods used to calculate the heat fluxes in the collector, and their effects on solar chimney performance. Reasons for the discrepancies between the predictions of the two models are given. In general the Pretorius model produces higher heat transfer coefficients and higher heat rate fluxes for both the roof and for the ground surfaces. The two approaches lead to very similar air temperature rises in the collector and thus, similar produced power.     

Analysis and feasibility of implementing solar chimney power plants in the Mediterranean region

This paper analyzes the feasibility of solar chimney power plants as an environmentally acceptable energy source for small settlements and islands of countries in the Mediterranean region. For the purpose of these analyses, two characteristic geographic locations (Split and Dubrovnik) in Croatia were chosen and simplified model for calculation of produced electric power output is also developed. These locations possess typical characteristics of the Mediterranean climate. The solar characteristics of the chosen geographic locations are shown along with characteristic meteorological data. A solar chimney (SC) power plant with a chimney height of 550 m and a collector roof diameter of 1250 m would produce 2.8–6.2 MW of power. The average annual electric power production of this SC power plant would range between 4.9 and 8.9 GWh/year, but in reality from 5.0 to 6.0 GWh/year in average. An approximate costs analysis, which included a total investment estimate, was performed. The levelized electricity cost was also calculated. It is found that the price of produced electric energy by solar chimney power plant in Mediterranean region is considerably higher compared to the other power sources.     

Available energy of the air in solar chimneys and the possibility of its ground-level concentration

Solar chimneys are defined as low temperature solar thermal power plants, which use the atmospheric air as a working fluid, where only one part of the thermodynamic cycle within the plant is utilized. The available work potential that atmospheric air acquires while passing through the collector has been determined and analyzed. The dependence of the work potential on the air flowing into the air collector from the heat gained inside the collector, air humidity and atmospheric pressure as a function of elevation are determined. Various collector types using dry and humid air have been analyzed. The influence of various chimney heights on the air work potential are established. The possibly higher utilization factors of the available hot air work potential without the use of high solid chimneys are discussed. It has been shown that the vortex motion flowing downstream of the turbine can be maintained under pressure and can possibly take over the role of the solid structure chimney. Thus, a part of the available energy potential acquired in the collector would be used to maintain the vortex flow in the air column above the ground-level turbine. Basic conditions for the maintenance of such a vortex flow are described and compared to the tornado phenomenon.     

Vertical evacuated tubular-collectors utilizing solar radiation from all directions

A prototype collector with parallel-connected evacuated double glass tubes is investigated theoretically and experimentally. The collector has a tubular absorber and can utilize solar radiation coming from all directions. The collector performance is measured in an outdoor test facility. Further, a theoretical model for calculating the thermal performance is developed. In the model, flat-plate collector's performance equations are integrated over the whole absorber circumference and the model determines the shading on the tubes as a function of the solar azimuth. Results from calculations with the model are compared with measured results and there is a good degree of similarity between the measured and calculated results. The model is used for theoretical investigations on vertically-placed pipes at a location in Denmark (Copenhagen, lat. 56°N) and at a location in Greenland (Uummannaq, lat. 71°N). For both locations, the results show that to achieve the highest thermal performance, the tube centre distance must be about 0.2 m and the collector azimuth must be about 45–60° towards the west. Further, the thermal performance of the evacuated solar-collector is compared to the thermal performance of the Arcon HT flat-plate solar-collector with an optimum tilt and orientation. The Arcon collector is the best performing collector under Copenhagen conditions, whereas the performance of the evacuated tubular collector is highest under the Uummannaq conditions. The reason is that the tubular collector is not optimally tilted in Copenhagen but also that there is much more solar radiation “from all directions” in Uummannaq and this radiation can be utilized with the tubular collector. It is concluded that the collector design is very promising—especially for high latitudes.     

Modeling and numerical simulation of solar chimney power plants

The solar chimney power plant is a simple solar thermal power plant that is capable of converting solar energy into thermal energy in the solar collector. In the second stage, the generated thermal energy is converted into kinetic energy in the chimney and ultimately into electric energy using a combination of a wind turbine and a generator. The purpose of this study is to conduct a more detailed numerical analysis of a solar chimney power plant. A mathematical model based on the Navier-Stokes, continuity and energy equations was developed to describe the solar chimney power plant mechanism in detail. Two different numerical simulations were performed for the geometry of the prototype in Manzanares, Spain. First, the governing equations were solved numerically using an iterative technique. Then, the numerical simulation was performed using the CFD software FLUENT that can simulate a two-dimensional axisymmetric model of a solar chimney power plant with the standard k-epsilon turbulence model. Both the predictions were compared with the available experimental data to assess the validity of the model. The temperature, velocity and pressure distributions in the solar collector are illustrated for three different solar radiations. Reasonably good quantitative agreement was obtained between the experimental data of the Manzanares prototype and both the numerical results.     

Solar multiple optimization for a solar-only thermal power plant, using oil as heat transfer fluid in the parabolic trough collectors

Usual size of parabolic trough solar thermal plants being built at present is approximately 50 MW_e. Most of these plants do not have a thermal storage system for maintaining the power block performance at nominal conditions during long non-insolation periods. Because of that, a proper solar field size, with respect to the electric nominal power, is a fundamental choice. A too large field will be partially useless under high solar irradiance values whereas a small field will mainly make the power block to work at part-load conditions.This paper presents an economic optimization of the solar multiple for a solar-only parabolic trough plant, using neither hybridization nor thermal storage. Five parabolic trough plants have been considered, with the same parameters in the power block but different solar field sizes. Thermal performance for each solar power plant has been featured, both at nominal and part-load conditions. This characterization has been applied to perform a simulation in order to calculate the annual electricity produced by each of these plants. Once annual electric energy generation is known, levelized cost of energy (LCOE) for each plant is calculated, yielding a minimum LCOE value for a certain solar multiple value within the range considered.     

Accuracy of theoretical models in the prediction of solar chimney performance

A solar chimney is a solar power plant which generates mechanical energy (usually in terms of turbine shaft work) from a rising hot air that is heated by solar energy. The present paper compares the predictions of performances of solar chimney plants by using five simple theoretical models that have been proposed in the literature. The parameters used in the study were various plant geometrical parameters and the insolation. Computational Fluid Dynamics (CFD) simulation was also conducted and its results compared with the theoretical predictions. The power output and the efficiency of the solar chimney plants as functions of the studied parameters were used to compare relative merits of the five theoretical models. Models that performed better than the rest are finally recommended.     

Case study of solar chimney power plants in Northwestern regions of China

A solar chimney power plant, which is expected to provide electric power for remote villages in northwestern China, has been analyzed in this paper. Three counties in Ning Xia Hui Autonomous region, namely, Yinchuan, Pingluo, and Helan, where solar radiation is better than other regions of China, were selected as pilot locations to construct solar power plant. The solar power plant chimney, in which the height and diameter of the chimney are 200 m and 10 m, respectively, and the diameter of the solar collector cover is 500 m, is able to produce 110˜190 kW electric power on a monthly average all year. Some parameters, such as chimney height, diameter of the solar collector, ambient temperature, solar irradiance and the efficiency of wind turbine, etc. which influence the performance of power generation, are also analyzed.     

太阳能烟囱发电新技术

介绍了一种太阳能烟囱发电新技术。太阳光辐射透过太阳能集热棚，加热集热棚下面的地面，被加热的地面与集热棚内的空气进行热量交换，使其温度上升，被加热的空气上升并进入与集热棚中部相连的烟囱，在烟囱内上升气流推动涡轮发电机旋转、发电。整个太阳能烟囱发电技术的能量转化以及效率可以从三个部分来分析：通过集热棚太阳能转化为空气热能，通过烟囱将热能转化为动能，通过涡轮发电机将动能转化为电能。另外，总结了太阳能烟囱发电技术的优缺点，指出它是一种适合于我国西部地区的一种能源开发新途径。     

Critical evaluation of solar chimney power plant performance

This paper evaluates the influence of a recently developed convective heat transfer equation, more accurate turbine inlet loss coefficient, quality collector roof glass and various types of soil on the performance of a large scale solar chimney power plant. Results indicate that the new heat transfer equation reduces plant power output considerably. The effect of a more accurate turbine inlet loss coefficient is insignificant, while utilizing better quality glass enhances plant power production. Models employing Limestone and Sandstone soil produce virtually similar results to a Granite-based model. The plant collector height is found to differ from previously obtained optimal values.     

Maximum power-conversion efficiency for the utilization of solar energy

The overall efficiency of solar thermal power plants is investigated for estimating the upper limit of their practical performances. This study consists of the theoretical optimization of the heat engine and the optimization of the overall system efficiency, which is the product of the efficiency of the solar collector and the efficiency of the heat engine. In order to obtain a more realistic performance of the solar thermal power plant, the solar collector concentration ratio, the diffused solar radiation and the convective and radiative heat losses of the solar collector are taken into account. Instead of the classical Carnot efficiency, the efficiency at maximum power is used as the optimal conversion efficiency of a heat engine. By means of simple calculations, the optimal overall system efficiency and the corresponding operating conditions of the solar collector are obtained. The results of the present work provide an accurate guide to the performance estimation and the design of solar thermal power plants.     

CFD analysis on flow and performance parameters estimation of solar updraft tower (SUT) plant varying its geometrical configurations

A 3D numerical model is developed to estimate and analyze the flow and performance parameters of solar updraft tower (SUT) plant. The effects of geometrical parameters, such as chimney height and collector roof angle are studied. A turbulent, renormalization group (RNG) k-ε model and discrete ordinates (DO) model is used to solve the governing equations. It is concluded that with an increase in collector roof angle, air velocity increases but air temperature decreases. There is 31% velocity enhancement when the chimney height is increased from 3 to 8 m. The overall, chimney and collector efficiencies and power output are estimated to be 0.00354%, 0.0465%, 81.4% and 0.255 W, respectively.     

Solar gas turbine systems: Design, cost and perspectives

The combination of high solar shares with high conversion efficiencies is one of the major advantages of solar gas turbine systems compared to other solar-fossil hybrid power plants. Pressurized air receivers are used in solar tower plants to heat the compressed air in the gas turbine to temperatures up to 1000 °C. Therefore solar shares in the design case of 40% up to 90% can be realized and annual solar shares up to 30% can be achieved in base load. Using modern gas turbine systems in recuperation or combined cycle mode leads to conversion efficiencies of the solar heat from around 40% up to more than 50%. This is an important step towards cost reduction of solar thermal power. Together with the advantages of hybrid power plants—variable solar share, fully dispatchable power, 24 h operation without storage—solar gas turbine systems are expected to have a high potential for market introduction in the mid term view.In this paper the design and performance assessment of several prototype plants in the power levels of 1 MW, 5 MW and 15 MW are presented. Advanced software tools are used for design optimization and performance prediction of the solar tower gas turbine power plants. Detailed cost assumptions for the solarized gas turbine, the solar tower plant and further equipment as well as for operation and maintenance are presented. Intensive performance and economic analysis of the prototype plants for different locations and capacity factors are shown. The cost reduction potential through automation and remote operation is revealed.     

Cost analysis of solar chimney power plants

Several cost models for large-scale solar chimney power plants are available in the literature. However, the results presented vary significantly, even in cases where the input parameters and the used models are supposedly very similar. The main objective of this paper is to clarify this matter by comparing previous cost models to a newly developed alternative model. Further, the impact of carbon credits on the levelised electricity cost is also investigated.A reference plant is introduced, with dimensions and financial parameters chosen specifically for the purpose of making the results of this analysis comparable to those of previous publications. Cost models are presented for the main components of a solar chimney power plant, i.e. the collector, the chimney and the power conversion unit. Results show that previous models may have underestimated the initial cost and levelised electricity cost of a large-scale solar chimney power plant. It is also shown that carbon credits significantly reduce the levelised electricity cost for such a plant.     

Numerical simulation of the solar chimney power plant systems coupled with turbine

Numerical simulations have been carried out on the solar chimney power plant systems coupled with turbine. The whole system has been divided into three regions: the collector, the chimney and the turbine, and the mathematical models of heat transfer and flow have been set up for these regions. Using the Spanish prototype as a practical example, numerical simulation results for the prototype with a 3-blade turbine show that the maximum power output of the system is a little higher than 50 kW. Furthermore, the effect of the turbine rotational speed on the chimney outlet parameters has been analyzed which shows the validity of the numerical method advanced by the author. Thereafter, design and simulation of a MW-graded solar chimney power plant system with a 5-blade turbine have been presented, and the numerical simulation results show that the power output and turbine efficiency are 10 MW and 50%, respectively, which presents a reference to the design of large-scale solar chimney power plant systems.     

Examining potential benefits of combining a chimney with a salinity gradient solar pond for production of power in salt affected areas

The concept of combining a salinity gradient solar pond with a chimney to produce power in salt affected areas is examined. Firstly the causes of salinity in salt affected areas of northern Victoria, Australia are discussed. Existing salinity mitigation schemes are introduced and the integration of solar ponds with those schemes is discussed. Later it is shown how a solar pond can be combined with a chimney incorporating an air turbine for the production of power. Following the introduction of this concept the preliminary design is presented for a demonstration power plant incorporating a solar pond of area 6 hectares and depth 3 m with a 200 m tall chimney of 10 m diameter. The performance, including output power and efficiency of the proposed plant operating in northern Victoria is analysed and the results are discussed. The paper also discusses the overall advantages of using a solar pond with a chimney for production of power including the use of the large thermal mass of a solar pond as a practical and efficient method of storing collected solar energy.     

Experimental investigation of solar tower with chimney effect installed in CRTEn,Tunisia

This paper studies the performance of a solar tower power plant (STPP) with chimney effect based on renewable energy proposed for electricity production. That's way, a solar tower prototype was constructed and tested in the Research and Technology Centre of Energy (CRTEn), Borj Cédria, northern Tunisia.The design involves heating air using solar energy and the chimney effect to raise the hot air up the chimney stack. The hot air velocity increases by the use of a convergent nozzle to reach a suitable velocity which can run the wind turbine. The kinetic energy of the hot air is then converted to electricity by the wind turbine.During this study, the influence of the climatic conditions of Borj Cédria site (insulation, ambient temperature) as well as the chimney height and the collector diameter on the amount of electricity production were investigated.The distribution and the evolution of the temperature at different positions of the prototype as well as the electrical energy produced were determined.The results reveal that when the temperatures reach 45 °C, the electric power reaches an average value of about 0.3 W/m² for a solar tower prototype with 8 m of diameter and 2 m of height chimney.