_{1}

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According to the strategic reurbanization of the Egyptian population from the densely populated areas, numerous new settlements with residential housing areas will be set up in the countryside in the near future. The scheduled areas are mainly situated in hot arid areas. The use of the high temperature air in these areas in electricity generation for the residential houses is quite important, since the success of the reurbanization project depends mainly on providing urban buildings with a suitable level of infrastructure and facilities. Solar chimney power plant (SCPP) is one of the promising power generation facilities that use solar energy for electricity production. It is a solar thermal power plant that utilizes a combination of solar air collector and central updraft tube to generate a solar induced convective flow which drives pressure staged turbines to generate electricity. In this paper, a description of the functional principle of the SCPP components is presented and some results from designing a medium scale unit using simple mathematical model are given.

The growing demand in energy and concerns about depletion of natural resources and global warming has led states worldwide to consider alternatives to the use of fossil fuel for energy production. Several countries especially in Europe have already increased their renewable energy share by about 6% - 10%, and it is expected to increase further to 20% by year 2020. For Egypt excellent resources of solar energy exist. The local demand for energy and electricity is rapidly growing at the same time where the two major energy sources of the country, namely oil and natural gas, are dewendling.

Egypt strategy approved in February 2008, aims to increase the contribution of renewable energies by 20% of the total electricity generation by the year 2020. The country sees renewables as an important potential industry worthy of development, and one which plays well to its comparative advantage of a large, well educated and comparatively low-cost workforce, and to its strong trade links throughout the Middle East and North African regions, as well as Europe and United States. Need for promoting renewables for power generation arises, also, from the social perspective of energy within the context of sustainable development.

Solar chimney power plant (SCPP) is one of the promising power generation facilities that use solar energy for electricity production. It is a solar thermal power plant utilizing a combination of solar air collector and central updraft tube to generate a solar induced convective flow which drives pressure staged turbines to generate electricity.

There are three conditions that make Egypt a very favorable country for this type of solar power plants. First, Egypt location on the world map makes it an advantageous place for benefiting from solar energy. It lies in the North-African Sun Belt with flat desert topography and perennially clear skies favorable to commercial solar technologies [^{2}/day, about 130% higher than Germany; yet per capita use of solar technologies is less than 10 % of Germany’s. Finally, there are large suitably flat areas of land available either in Sinai, east desert, west desert or in Upper Egypt [

Many attempts have been carried out to evaluate the performance of SCPPs. Some researchers such as Mohamed H. [

Egypt is one of the sunbelt countries that enjoys one of the largest potentials of solar energy applications as shown in the Solar Atlas map [^{2} and the sunshine duration ranges between 9 and 11 h/day from North to South with very few cloudy days. The regions of high solar intensity are located south Egypt, where the solar intensity ranges between 6 - 7 kW/m^{2}∙day. The solar intensity decreases in the directions to the north, where it has values lower than 5.4 kW/m^{2}∙day along the north borders of the country.

In Europe, for example in Italy, the solar radiation varies from 3.6 to 4.7 kWh/m^{2}/year, with average sunshine hours of 6 to 7.5 h/day. In Germany, the solar radiation intensity ranges from 2.3 to 3.0 kWh/m^{2}/year, with average sunshine hours of 4.5 to 5.1 h/day.

A schematic diagram of the solar chimney power plant is presented in

1) The height of the collector from the ground was considered large enough to disregard the pressure drop in the collector section. However, this assumption deviates in the district near the chimney inlet.

2) Radiation to the chimney is ignored since the surface area of the collector is much larger than the surface area of the chimney. Therefore, heat transfer equation is only considered for the collector. The temperature change across the chimney is small, hence T_{4} = T_{3}.

3) The process across the turbine is assumed to be isentropic (i.e. reversible and adiabatic).

4) Heat transfer from the system to the surrounding was ignored and only heat transfer from the surrounding to the collector is considered.

5) The analysis is based on steady flow assumption which is an approximation because solar radiation is transient in nature.

6) The flow is incompressible across the chimney since Mach number is below 0.3.

7) Uniform solar heat flux averaged over the day period is used in the calculation.

The mass flow rate across the collector is conserved and pressure drop is neglected. p_{2} = p_{1} as explained in the assumption. The energy equation for the collector section is used to calculate the mass flow rate inside the solar chimney as follows:

where ^{2}), A_{c} is the collector area (m^{2}), and h_{1} and h_{2} are the enthalpy of air at collector inlet and outlet, respectively (J/kg×K).

Across the turbine, the pressure head is related to turbine head as shown in Equation (2). Entropy across the turbine is constant hence the pressure expansion is assumed to be isentropic.

where H_{T} is the turbine head (m), g is the gravitational acceleration (m/s^{2}), _{2}, and s is the specific entropy (J/kg×K).

The density of air across the turbine is assumed to be constant since the wind speed is not high, compared to the speed of sound, and there is no heat added or removed across the turbine. In addition, the pressure drop across the turbine is very small since the study assumes the use of a one stage wind turbine.

The air draft in the chimney is modeled using the following modified Bernoulli equation [

where f is the friction factor, D_{3} is the chimney diameter (m), H_{chimney} is the chimney height (m), K_{in} and K_{out} are loss coefficients at inlet and outlet sections respectively.

From the second assumption introduced in Section 3.1, the temperature change across the chimney is small, which means:

For standard atmosphere and using

where T_{1} is the air temperature at collector inlet (K), β is the laps rate (K/m); p is the pressure (Pa), and R is the air gas constant (J/kg×K).

The expected turbine work is calculated according to the equation:

where h_{2} and h_{3} are the enthalpy at turbine inlet and outlet, respectively.

Without turbine, a maximum flow speed of v_{tower,max} is achieved and the whole pressure difference is used to accelerate the air and is thus converted into kinetic energy. Using the Boussinesq approximation [

where ΔT is the temperature rise between collector inlet and outlet; T_{3} - T_{1}.

The second-law efficiency of the solar chimney power plant is defined as turbine extracted work over the available work as follows:

Finally, the overall plant efficiency is calculated as follows:

A SCPP model with the specifications mentioned in

The above set of equations have the unknowns:_{2}, p_{3}, s_{3}, _{4}. In order to solve theses equations, an iteration technique using a FORTRAN-language program was carried out so that the SCPP performance parameters could be predicted under different geometrical parameters and operating conditions.

Figures 4-9 show the effects of different geometry parameters on the performance of the SCPP. These parameters are: chimney height, chimney diameter, and collector diameter.

Chimney height | 445 m |
---|---|

Chimney diameter | 54 m |

Collector diameter | 1110 m |

Solar intensity | 300 W/m^{2} (for Aswan-Egypt) |

Air temperature | 303 K |

Turbine head | 200 m |

Laps rate | 6.5 K/km |

K_{in} | 0.5 |

K_{out} | 1.0 |

Friction factor | 0.01 |

height of the chimney and that; as chimney height increases, one expects that power output increases. The increased chimney height decreases the maximum velocity in the chimney, which in turn decreases the friction losses. Since the friction losses are proportional to the velocity [

Another important point is that; the higher the chimney means higher driving force which is the buoyancy difference. The rate of increase in this driving force is higher than the friction losses due to chimney height; therefore the second-law efficiency is expected to increase with the chimney height, as indicated in

The effect of collector diameter on solar chimney is shown in

The effect of chimney diameter on the solar chimney is shown in

This section investigates the effect of the monthly incident solar intensity of Egypt on the SCPP performance if a SCPP with the specifications mentioned in

^{2}. The minimum output power of 2.6 MW occurs in December, where the incident solar radiation has a minimum value of 179.0 W/m^{2}. These results show that; an average value of 3.8 MW could be obtained from a single unit has the specifications mentioned in

Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
---|---|---|---|---|---|---|---|---|---|---|---|---|

G | 4.6 | 5.6 | 6.5 | 7.3 | 7.4 | 8.0 | 7.8 | 7.4 | 6.6 | 5.7 | 4.8 | 4.3 |

This paper presents a simple mathematical model to predict the performance of a solar chimney power plant for different geometry and operating conditions. Four performance parameters were considered in the analysis: turbine power, maximum air velocity inside the chimney, second-law efficiency and the overall plant efficiency. The study results show that, the chimney height and diameter, collector diameter and solar intensity are critical parameters for building a solar chimney power plant. The chimney height has a very strong effect on the second-law efficiency and total generated power. The chimney diameter has a small effect except for very small chimney diameter where friction becomes a dominant factor. The collector diameter has a very small effect on second-law efficiency since the friction losses minimized by the wide inlet area and low average velocity inside the collector. The solar radiation intensity values in Egypt allow the generation of 4.8 MW in summer and 2.6 MW in winter if one medium size SCPP with the specifications mentioned in

Mahmoud Bady, (2015) Towards Sustainable Power Generation Using Solar Chimney. Open Access Library Journal,02,1-9. doi: 10.4236/oalib.1101469