In this work, a parametric two-dimensional computational fluid dynamics (CFD) analysis of a solar chimney power plant (a prototype located in Manzanares, Spain) is presented to illustrate the effects of the solar radiation mode in the collector on the plant performances. The simulations rely on a mathematical model that includes solar radiation within the collector; energy storage; air flow and heat transfer, and a turbine. It is based on the Navier-Stokes equation for turbulent flow formulated according to the standard k-ε model. Moreover, the Boussinesq approach is used to account for the fluid density variations. Different solar radiation modes in the collector are compared and discussed. The obtained results are also compared with available experimental results. It is shown that the radiation model is essential to avoid overestimation of the energy absorbed by the plant and that results based on a two-dimensional model can resemble closely those produced by three-dimensional models.

Environmental pollution and declining oil reserves make renewable energies a viable option to provide clean and cost-competitive energy in the future. In fact, solar power plants have become one of the most promising candidates for supplying most of the world’s clean energy needs [

The solar chimney power plant (SCPP) is one of the world’s most ambitious projects meant to produce alternative energy. It is a clean and safe renewable energy plant that could provide significant electrical power. It is capable of operating continuously through the use of the sun during the day and of stored thermal energy in the storage layer at night. The solar chimney power plant combines three well-known principles: the greenhouse effect, the chimney, and wind turbines in an innovative way to convert solar radiation (direct and diffuse) into electricity. The sun generates hot air in the collector. This is drawn upwards by a chimney in the middle of the collector. The air flow drives a turbine installed at the chimney’s base that generates electricity. A schematic of the SCPP is illustrated in

The solar chimney concept was originally proposed by Professor Schlaich of Stuttgart in the late of 1970s [

The complete simulation of the solar chimney power plant consists of 4 sub-models: the solar radiation, the energy storage, the air flow and heat transfer, and the turbine models [

In the literature, various approaches that account for solar radiation were used. For example, Guo et al. [

Algeria is a key player in global energy markets as the largest producer and exporter of natural gas and liquefied natural gas. Algeria’s energy mix depends almost exclusively on fossil fuels, especially natural gas and oil. However, the country has immense potential for renewable energy, especially solar power. Recently, the government has tried to harness this by creating an ambitious renewable energy and energy efficiency program [

This program aims to produce 22,000 MW of power from renewable sources from 2011 to 2030. Of the 22,000 MW, 12,000 MW will be destined for domestic consumption and the remainder for export [

To predict the mode of action of SCPP at sites in Algeria, numerical simulations of the plant heat transfer process are helpful. On the one hand, the prevailing thermal boundary conditions have to be taken into account, but also the reflection properties leading to a reduction of the efficiency. In this numerical case study, different model assumptions are discussed in order to establish realistic predictions for the operation of such plants.

The Spanish SCPP of Manzanares was selected as the physical model for the CFD calculation.

Because the geometry has rotational symmetry, it is sufficient to consider the process in only one intersection. The computational finite volume grids used in this study with and without a heat storage layer are shown in

To simulate the fluid flow and heat transfer through the system, the steady-state conservation equations of mass, momentum, and energy were solved using the commercial CFD package ANSYS Fluent 19.2.

In an SCPP, the Rayleigh number is used to determine the strength of the buoyancy-induced flow. The Rayleigh number in a SCPP as the Spanish prototype is higher than 10^{10}. Thus, convective heat transport clearly predominates. Due to the low viscosity of the air, the Reynolds number is also high, so that the flow inside the system is turbulent. We use the standard k-ε model in the numerical simulation.

To account for the temperature dependence of air density, the Boussinesq approach is used. This model treats density as a constant value in all solved equations, except for the buoyancy term in the momentum equation. In our model, the reverse fan interior boundary condition was given to account for the pressure drop across the turbine at the chimney base. The thickness of the ground in the computational domain was set to 5 m, and the bottom temperature was set as the ambient temperature.

Place | Type | Value |
---|---|---|

Collector inlet | Pressure inlet | P_{gage} = 0 Pa, T_{a} = 293 K |

Canopy | Wall | h = 10 W/m² K, T_{a} = 293 K, Volumetric heat sources due to |

Top surface of the ground | Wall | Volumetric heat sources due to |

Ground | Wall | T = 300 K |

Surface of chimney | Wall | Adiabatic |

Chimney outlet | Pressure outlet | P_{gage} = 0 Pa |

Turbine | Reverse fan | ∆P |

To take into account solar radiation in the simulation, three distinct modes are studied:

The first and second modes do not take into account the greenhouse effect of the collector.

As shown in

where

Type | ||
---|---|---|

Mode 1 | 0 | 800 |

Mode 2 | 32 | 588.80 |

Mode 3 | 37.94 | 593.55 |

To verify the CFD model, the fluid temperature increase through the collector and the upward flow velocity in the case of an unloaded turbine are first compared with experimental results of the prototype in Manzanares, Spain, and with numerical results of Rabehi et al. [^{2}, the air velocity at the chimney inlet is 15 m/s, and the temperature difference in the collector with unloaded turbine attains 20°C as illustrated in

Variables | Spanish prototype | Rabehi work | Present work |
---|---|---|---|

V(m/s) | 15 | 15.95 | 16 |

∆T(°C) | 20 | 24.4 | 24.5 |

In comparison, the CFD results show an average air velocity of 16 m/s and a temperature increase of 24.5°C. Nevertheless, the results of the numerical simulation can be seen as reasonable.

Now we compare the operation without and with turbine. The temperature profile across the collector in the case of unloaded and during turbine operation is illustrated in

where

The collector efficiency

where

Mode 1 | Mode 2 | Mode 3 | |
---|---|---|---|

Collector efficiency | 90.11% | 69% | 68% |

A closer look at the temperature distribution across the ground surface affirms the incorrect assessment of efficiency in Mode 1. As illustrated in

Additionally, as shown in

The significant variation in the temperature profile of the ground surface indicates that previous approaches were unable to accurately describe the system’s heat transfer. The major causes behind this are as follows: 1) because solar radiation is regarded as a heat source in previous models, it differs from the real energy transfer mechanism. 2) the greenhouse effect in solar collectors has not been taken into account.

A 2D steady-state numerical analysis for the SCPP, which consists of the collector, chimney, turbine and energy storage layer, was performed. Three different modes taking solar radiation into consideration are used and their results are compared. The turbine is regarded as pressure-based and the energy storage layer is considered as solid media, aiming at analyzing the pressure, velocity, and temperature distributions of the system. Meanwhile, the influence of the turbine pressure drop on the flow and heat transfer characteristics and the output power of the SCPP.

The numerical results were initially verified using experimental data from Manzanares SCPP and other research. The numerical results were then compared under no-load and turbine operating conditions to determine the effect of turbine operation on SCPP parameters. In addition, the effect of solar irradiation on flow and heat transfer parameters was examined.

The analysis of the performed numerical results can be concluded as follows:

The pressure differences during the turbine operation must be considered in modelling of SCPP, because the temperature inside the collector increases.

Furthermore, variations in solar radiation have a noticeable influence on upwind speed, temperature increase, and chimney outlet parameters under no-load and loaded turbine conditions. These dependencies must be taken into account when designing SCPP plants.

The main result is that disregarding the greenhouse effect in the collector will affect the accuracy of the soil surface temperature estimation, collector efficiency, and power output of the SCPP. Even with simple approaches to this effect (Mode 2), realistic results can be produced.

As a future work, the effect of other factors as the incidence angle of the solar radiation on the flow and temperature fields and the chimney efficiency will be studied. The independence on the temperature of the thermal source term in Mode 3 will be considered. The results obtained from this study will also be applied to a case study in Algeria.

specific heat capacity (

energy (

convective heat transfer coefficient (

solar radiation (

power (

_{gage}

gage pressure (Pa)

volumetric heat source (

_{v}

volume flow rate (

heat flux (

Rayleigh number (−)

solar chimney power plant

absorptivity (−)

emissivity

transmittance (−)

reflectivity (−)

dynamic fluid viscosity (^{−2})

The authors would like to acknowledge the support provided by the German Academic Exchange Service