A telescopic divergent chimney for power generation based on forced air movement: Principle and theoretical formulation

doi:10.1016/j.apenergy.2014.04.086

Applied Energy

Volume 136, 31 December 2014, Pages 873-880

https://doi.org/10.1016/j.apenergy.2014.04.086 Get rights and content

Highlights

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A new power generation concept based on forced updraft air in a divergent chimney.
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A theoretical model was developed to investigate the forced updraft system.
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A self-sustained updraft is reached when air velocity reaches a threshold value.
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No external mechanical work is needed when the forced updraft reaches steady state.
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The extractable energy has an exponential relationship with the chimney heights.

Abstract

We present a new renewable energy technology concept for electricity generation based on an upward momentum created by a balanced forced air movement, which is initiated by an air blower, inside a telescopic divergent chimney constructed with adiabatic materials and equipped with a gas turbine. A theoretical model based on the principles of mass and energy conservation, and continuity was developed to investigate the forced air flow inside the system using a criterion defined as the difference between ambient pressure and static pressure at the outlet aloft of the chimney. The model results indicated that, as the air flow increasing to reach a threshold, there exists a steady state when the pressure difference equals to zero. At this state, a constant air flow velocity is attended such that the system becomes stable and the action of the blower is no longer required. In this condition, there is a non-linear relationship between the threshold velocity and the pressure difference with the geometry of the chimney, the installed load and ambient conditions are important dependent variables. With the estimated threshold velocity for chimneys from 15 to 150 m, the steady-state distributions of velocity and energies (internal, potential, kinetic, enthalpy and extractable mechanical energy) at five cross-sections of the chimney were evaluated. The maximum velocity is found at the section with the minimum area, where maximum mechanical energy can be extracted. The maximum velocity can reach ∼45 m/s in a 100-m high chimney. The extractable energy is found to have an exponential relationship with the chimney heights. The novelty of the concept is that when the air is forced to reach a threshold velocity in the system, the air can maintain a steady state without further supply of driving power.

Introduction

This aim of this paper is to introduce a new device that produces energy through a telescopic divergent chimney that transforms internal energy of air into mechanical energy by forced air movement and subsequently electricity. This chimney system is structurally quite similar to the solar chimney power plant (SCPP), which is a device for transferring heat to mechanical energy then to electricity but with a straight chimney [1], [2], [3]. The first prototype of SCPP was operated in Manzanarès of Spain from 1982 to 1989 and it showed that it is possible to produce electricity power using natural thermal convection [1], [2]. The convective air flow of the SCPP is created by the temperature difference of warm air heated by solar radiation at ground surface and the ambient temperature of the air aloft of the chimney [4], [5], [6], [7]. It is structured with a straight chimney, a huge canopy on the ground level, a turbine and a generator [8], [9]. Research investigations revealed that the output efficiency of the solar tower is highly dependent on the height of chimney and the acreage of the heat-collecting canopy [10], [11].

Instead of the straight chimney design which is commonly used in the SCPP, the new device adopts a divergent structure [12] constructed with adiabatic materials. Different from the natural thermal convection in the straight chimney, which will not be able to reach an efficiency of 1% of the incident solar radiation [13], the divergent chimney operates with an air movement artificially initiated by air blowers or other devices. This air flow is called forced air movement in this paper. When it starts to operate, the air blowers pull the air into the chimney with a certain speed. As the flow velocity increases to a threshold value, the whole system reaches a steady state in which the air flow stays almost at a constant speed without the action of the blowers any more. The chimney using forced air movement does not require the heat-collecting canopy or solar radiation for heating the air. Thus, it is a new technology for electricity generation and this technology is less dependent on ambient conditions, especially sunlight than the solar tower.

Section snippets

Conceptual design and operation

For easy understanding, the telescopic divergent chimney is expressed in a 2-D structure as shown in Fig. 1. The device is structured with a ground-level circular canopy for collecting air from the ambient environment, which is directed by a blower through cambered ducts to the inlet of a telescopic chimney situated at the center of the canopy. The chimney is constructed by adiabatic materials with the smallest diameter at the bottom where a turbine and a generator are located, from which the

Assumptions

Given the 2-D structure and operating process, we build up a theoretical model based on the assumptions below:

(I)
In air flow with low Mach number (<0.3) (i.e. with low flow velocity), the air is usually considered as an incompressible gas [14]. Our computations through a CFD model indicated that the Mach number of air flow in chimneys lower than 100 m is much less than 0.3. As such, we assumed that the gas in our system in incompressible for convenient physical analysis and numerical computations.
(II)

Threshold velocity for self-sustained forced air movement

The performances of four different scales (15, 50, 100 and 150 m) divergent chimney were estimated by the formula (16) and the Eq. (17). Table 1 below summarizes the geometry of the four chimneys in terms of chimney height, areas of inlets dip angel of chimney wall and their performances in terms of the threshold velocity, maximum velocity, mass flow rate and maximum extractable mechanical power. The readers are reminded that the geometric design of the chimneys may not be optimal, which will be

Conclusion

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While some studies analysed the effect of Hch and diameter on flow and performance parameters (Bouabidi et al., 2018; Hu et al., 2017; Lal et al., 2016; Patel et al., 2014), very few studies (Chan et al., 2014; Hu et al., 2017; Mehla et al., 2019) have dealt with CC/DC - SUT plant. Most of the studies of diverged chimney are based on either theoretical analysis (Chan et al., 2014; Hu et al., 2017) or 2D numerical analysis (Bouabidi et al., 2018). It is identified that the chimney is one of the most influential components and highly sensitive towards flow phenomena and performance properties of an SUT.
A 3D numerical model was made to analyse the effect of divergence angle of chimney (Φ_ch), ambient temperature (T_a), solar flux (I) and turbine efficiency (η_tur) on wind flow and performance parameters of a solar updraft tower (SUT). A parametric study was carried out by varying Φ_ch from 1 to 5° for different I (600–900 W/m²) and the results were compared with results for existing cylindrical chimney SUT (CC-SUT) plants. The range of T_a selected was 293–318 K and η_tur was 0.54–0.9%. Five different configurations of divergent chimney SUT (DC-SUT) plants were modelled. It was noticed that at Φ_ch of 2°, the average velocity of air at chimney base was enhanced by 58.9% compared to CC-SUT plant. The average absorber plate temperature increased from 316.4 to 332.85 K with an increase in I from 600 to 900 W/m² for Φ_ch of 2°. The maximum overall efficiency, theoretical and actual power output of the DC-SUT plant were 0.0307%, 3.99 W and 2.67 W, respectively. The exergy of DC-SUT was 58% higher when Φ_ch was increased from 1 to 2° at I = 900 W/m². Correlations were generated to calculate performance parameters as functions of geometrical parameters.
Computational study on the effect of collector cover inclination angle, absorber plate diameter and chimney height on flow and performance parameters of solar updraft tower (SUT) plant
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Ming et al. [22] briefly reviewed developments in SUT powerplant and numerically performed a case study based on the impact of reverse wind flow on large SUT powerplants having structural change in collector cover. From the literature, there are several studies concentrated on estimating and analysing flow and performance parameters of SUT through numerically as well as experimentally [3,5,6,13–19]. Most of the numerical studies used RNG k-ɛ model for turbulence prediction and discrete ordinate (DO) model to solve RTE (radiative transfer equation) [1,3,5,7].
A computational model is developed to investigate the influence of geometrical configurations such as chimney height (H_ch), absorber plate diameter (D_ap) and collector plate angle (θ_cr) on flow and performance characteristics of solar updraft tower (SUT) plant. The diameter of the chimney (D_ch) and air entrance gap (e) selected for numerical simulations are 0.6 m and 0.1 m, respectively. Parametric study is carried out by varying H_ch of 3–8 m, θ_cr of 20°-35° with an increment of 5° and D_ap of 3.5–12 m. A turbulent model (RNG k-ɛ) is incorporated for considering turbulence effect and Discrete ordinates (DO) model incorporated for estimation of radiation heat transfer inside the setup. It is found that a slight decrease of temperature and increase of air velocity when θ_cr is increased. It is noticed that air velocity is enhanced (up to 44%) while the H_ch is increased from 3 to 8 m. Power output, overall, chimney and exergy efficiencies are estimated. Correlations are developed to estimate the performance parameters in terms of geometrical configurations. From the parametric study, it is concluded that for the D_ch and e of 0.6 m and 0.1 m, respectively, better performance can be achieved at a H_ch of 7 m, θ_cr of 30° and D_ap of 5 m.
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Few experimental data showed that divergent chimney helped increase the mass flow rate of air in the solar chimney power plant (Koonsrisuk, 2009; Okada et al., 2015; Ohya et al., 2016). Chan et al. (2014) developed a theoretical model of a telescopic divergent chimney for power generation without heat input in the chimney inlet (for example, due to the solar collector) but based on forced air movement, in which the air follows incompressible gas assumptions. Zhou et al. (2017b) developed a theoretical model of solar chimney power plants with variable chimney inlet-to-outlet area ratio based on compressible ideal gas assumptions, and studied the performance of the plants.
Simulations of divergent-chimney solar power plants (DSPPs) are conducted, and the DSPP performance studied by changing chimney outlet-to-inlet area ratios (COAR, representing the degree of divergence) over a wide range of values. Study method involves the use of total pressure potential (TPP) which consists of buoyancy and static pressure recovery in which an effective pressure potential recovery coefficient (EPPRC) is employed. Results show when the COAR is large enough, the boundary layer separation (BLS), flow stall and backflow will occur, vortex be formed, and a part of flow area be blocked. Due to the backflow from the ambient cool air, the temperature greatly decreases above the BLS point, possibly causing the reduction of buoyancy of the divergent chimney. With COAR increasing, the TPP initially increases and reaches a maximum for COAR = 8.7 then decreases. The mass flow rate and the power output have the same variational trend, while the collector temperature rise has the inverse variational trend. A maximum power of 231.7 kW is attained at COAR = 8.7, which is 11.9 times as high as that for COAR = 1. Before the occurrence of flow stall, the EPPRC decreases slowly due to the gently thickening of boundary layer and is higher than 0.91. While after the occurrence of flow stall, mainly due to the vortex and backflow the EPPRC greatly decreases and is found to decrease gradually with COAR. The effective ground heat flux increases the power output, but has little influence on the characteristics of the flow stall in the chimney, specifically the EPPRC.
Computational and experimental studies on solar chimney power plants for power generation in Pacific Island countries
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In their work, they also reported the variation of temperature and velocity along the collector. Chan et al. [33] tested a new concept of using a blower to force the air through a divergent chimney until the air flow reaches a steady state and the flow sustains itself thereafter. Li and Liu [34] carried out an experimental study of an SCPP under controlled laboratory conditions at three different heat fluxes of 500, 600 and 700 W/m2 and used a phase change material for latent heat absorption and release.
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View full text

A telescopic divergent chimney for power generation based on forced air movement: Principle and theoretical formulation

Highlights

Abstract

Introduction

Section snippets

Conceptual design and operation

Assumptions

Threshold velocity for self-sustained forced air movement

Conclusion

Sol Energy

Energy

Appl Energy

Appl Energy

Renew Sustain Energy Rev

Energy

Sol Energy

Solar chimneys: part II: preliminary test results from the manzanares pilot plant

Int J Sustain Energy

Solar chimneys part I: principle and construction of the pilot plant in Manzanares

Int J Solar Energy

The solar chimney: electricity from the sun