Elsevier

Energy Conversion and Management

Volume 138, 15 April 2017, Pages 461-473
Energy Conversion and Management

Techno-economic optimization for the design of solar chimney power plants

https://doi.org/10.1016/j.enconman.2017.02.023Get rights and content

Highlights

  • Chimney height and collector area of different designs were optimized.

  • Simple actual and minimum payback periods were developed.

  • Comparative assessment was conducted for different designs configuration.

  • Effects of uncertain parameters on the payback period were studied.

Abstract

This paper aims to propose a methodology for optimization of solar chimney power plants taking into account the techno-economic parameters. The indicator used for optimization is the comparison between the actual achieved simple payback period for the design and the minimum possible (optimum) simple payback period as a reference. An optimization model was executed for different twelve designs in the range 5–200 MW to cover reinforced concrete chimney, sloped collector, and floating chimney. The height of the chimney was optimized and the associated collector area was calculated accordingly. Relationships between payback periods, electricity price, and the peak power capacity of each power plant were developed. The resulted payback periods for the floating chimney power plants were the shortest compared to the other studied designs. For a solar chimney power plant with 100 MW at electricity price 0.10 USD/kWh, the simple payback period for the reference case was 4.29 years for floating chimney design compared to 23.47 and 16.88 years for reinforced concrete chimney and sloped collector design, respectively. After design optimization for 100 MW power plant of each of reinforced concrete, sloped collector, and floating chimney, a save of 19.63, 2.22, and 2.24 million USD, respectively from the initial cost of the reference case is achieved. Sensitivity analysis was conducted in this study to evaluate the impacts of varied running cost, solar radiation, and electricity price on the payback periods of solar chimney power plant. Floating chimney design is still performing after applying the highest ratio of annual running cost to the annual revenue. The sensitivity analysis showed that at the same solar radiation and electricity price, the simple payback period for 200 MW with sloped collector design would almost have the double simple payback period for 5 MW with floating chimney design.

Introduction

The energy demand in the world is in continuous increase [1] due to the population growth and higher rate of consumption per capita to face the improvement of the living standards. Utilization of sustainable and non-depleted sources of energy can face part of this increased demand for energy. The solar energy as a clean and renewable source of energy can play a major role in addressing these challenges. Intensive research efforts to improve the reliability and feasibility of the renewable energy systems can support the dissemination of these technologies.

To some extent, the fundamental, theoretical, and technical issues of solar energy were covered, but still, there are constraints in the practical application of the renewable energy systems. Pretorius and Kroger [2] evaluated technically the effect of design parameters and materials quality on the performance of solar chimney power plant (SCPP) and Tingzhen et al. [3] conducted heat transfer analysis of SCPP after divided the complete system into three regions of the collector, chimney, and turbine. Intermittency of solar and wind energy and high capital cost of renewable energy technologies are some of the challenges facing the dissemination of these clean technologies [4]. Solar photovoltaic (PV) technologies are obstacle by high cost, low financial ability, and limited application of the product [5]. Latent heat energy storage systems were proposed to face the intermittency in solar radiation. Aydin et al. [6] evaluated the impact of latent heat storage systems on the solar energy used for space heating in Istanbul, Turkey. Phase change materials used for latent heat storage systems are found suitable for domestic solar water heaters due to the high storage capacity and stable heat transfer temperature [7]. Heat generated by thermal conversion of solar energy is proposed to be injected into the ground to save energy [8].

Solar chimney used in natural ventilation of buildings by makes use of the difference in air density due to the temperatures difference, which is known as stack effect or buoyancy. Stackeffect increases the air flow rate to cool the building in warm humid climates [9]. Experimental investigations showed that airflow across a small chimney increases with the increase in each of solar radiation and the gap between absorber and glass cover [10]. A chimney is used for lifting air through the solar dryer to dry agricultural products [11]. Maia et al. [12] conducted an experimental study for the airflow inside a solar chimney used for drying of agricultural products and recommended to include the amount of water evaporated from the product into the performance evaluation of the dryer. A similar design consists of black tubes as a collector surrounding the chimney is proposed for nine cities in China with a different application to generate freshwater from the air [13].

Solar chimney power plant (SCPP) is proposed to be used for electricity generation and consists mainly of the solar collector, chimney, and turbine. Zou and He [14] evaluated a hybrid system integrating a solar chimney with a dry cooling tower and found that the output power can be increased up to 20 times compared to the conventional SCPP. An SCPP model of 2.5 m chimney height and 2 m collector diameter was constructed and tested with the application of corona wind to improve the airflow speed to 72% and the output power to about 5 times [15]. An SCPP was designed theoretically based on Jordanian weather conditions with 210 m chimney height and 40 m collector diameter to result in a maximum output power of 85 kW [16]. Electricity generation based on solar chimney technology is being proposed for Australia to generate 200 MW from a power plant with 1000 m chimney height and 5000 m collector diameter [17]. Optimization of the design parameters and geometry of the SCPP would contribute significantly into the feasibility of electricity generation based on this clean technology. The rotational speed of the turbine corresponding to the maximum turbine efficiency has to be slightly increased in order to achieve the maximum output power from an SCPP [18]. The solar collector as well known in other solar thermal applications is made in flat surface from transparent material either of glass panels or plastic film to harness the solar radiation through greenhouse effect and heat the air flowing through the chimney. The heated air flows through the solar chimney due to its density difference between inlet and outlet of the chimney. This heated air is forced the turbine to rotate and to generate electricity [19].

Compared to the electricity generation from conventional sources of energy, SCPPs are still constrained by huge initial cost and need long construction period [20], require large area of land for the collector, and the total system efficiency is very low [21]. The chimney associated with the SCPP is very tall compared to the conventional buildings which add a significant uncertainty with the construction and materials to be used. One of the challenges facing SCCP is the continuity of power generation during the night-time [22].

As SCPP is one of the renewable energy technologies, has the advantage of low environmental impact on air and water [23], unlike the conventional power plants based on fossil fuels. The solar energy source is freely available and storage systems can be added to the SCPPs to recover for the input energy when the solar radiation is unavailable. Black tubes filled with water located under the collector is one of the systems used for thermal energy storage and the amount of filled water depends mainly on the SCPP output power [24]. Al-Kayiem and Aja [25] concluded that SCPP performance can be improved considerably by adding another thermal energy source or by retrofitting a proper storage system. Flat mirrors to intensify the solar radiation on the solar chimney zone are also proposed to improve the SCPP performance [26].

The 50 kW SCPP prototype built and operated in Manzanares, Spain for seven years since 1982 [27] proved the reliability and assured the potential of the technology. The main objectives of Manzanares prototype were to verify the theoretical calculations of the design and to test the SCPP output power and the system efficiency under different weather conditions. The prototype was designed with 194.6 m chimney height, 10 m chimney diameter, and 122 m mean collector radius [28]. Manzanares prototype was operated for a total period of 8611 h at an average of 8.9 h per day and required supervision from one person [29]. Manzanares power plant was uneconomical due to the small size of the operated prototype and the limited power capacity [30]. The turbine for Manzanares prototype was designed to rotate around a single vertical axis, with a single rotor, and without inlet guide vanes [31]. Estimation of SCPP output power based on the input solar radiation was developed by Fathi et al. [32] and the numerical results were compared to the Manzanares prototype as an actual reference case.

Different designs for SCPP were proposed to improve the performance and feasibility of the system. Schlaich [33] recommended the reinforced concrete chimney from the viewpoint of lifespan and cost. Bilgen and Rheault [34] proposed a 5 MW SCPP with sloped collector design for high latitudes in Canada. The sloped collector is designed to be built in a suitable mountain hills to shorten the chimney height. Cao et al. [35] conducted a simulation for a design of sloped SCPP to be located in Lanzhou, Northwest China and found valuable although of the low total efficiency of the system. The sloped design was found more cost-effective compared to the reinforced concrete design [36]. Floating solar chimney is a structure with balloons inflated by a gas lighter than air and found to be 5–6 times lesser in cost than the corresponding reinforced concrete SCPP with a power capacity 40 MW [37]. Zhou et al. [31] reviewed SCPP technologies and expected that with further research in materials, this technology can play a major role in the field of power generation. The improvement in the design and the selected materials for SCPP would be more sustainable if accompanied with techno-economic analysis of the system.

The earlier techno-economic studies for SCPP were carried out focusing on the cost and cash flow with uncertain assumptions for the future values such as interest rate, inflation rate, carbon credits, electricity price, depreciation period, and the lifespan [38]. Fluri et al. [39] developed a new model for SCPP cost and found some underestimations on the previous models regarding the initial cost and the calculated levelized electricity cost. Certain values of the expected interest rates (6% and 8%) were considered to analyze the SCPP feasibility and the calculated levelized electricity costs were found higher compared to the other electricity generation sources [40]. Akhtar and Rao [41] investigated the feasibility of 200 MW SCPP in Rajasthan, India and studied the effects of variation in the interest rate, inflation rate, and operation period on the levelized electricity cost. A cost-benefit analysis [42] showed that the most sensitive factors for the net present value of SCPP are the electricity price and inflation rate while carbon credits, income tax rate, and interest rate are the least sensitive. Some earlier studies targeted optimization of SCPP design from the technical point of views without integration with the economic analysis. Patel et al. [43] optimized the SCPP geometry through computational fluid dynamics (CFD) without consideration of the economic parameters. An optimization methodology for the design parameters of the SCPP was introduced by Gholamalizadeh and Kim [44] based on a triple objective function covering expenditure, total efficiency, and output power. The minimum dimensions of an SCPP sufficient to supply electricity to about fifty rural households were determined by Onyango and Ochieng [45] without taking into account components cost and the associated financial parameters. A comprehensive review of SCPP studies was conducted by Kasaeian et al. [46] and recommended to determine optimized dimensions for chimney and collector, constructing large scale SCPP, and establishing decision-making support procedures for new SCPP. A feasibility study for SCPP is introduced by Okoye et al. [47] to integrate the optimum power plant dimensions and economic aspects for a certain design of reinforced concrete, with a specific location in Nigeria (Potiskum), and with an annual rate of return to calculate the discounted net present value. Guo et al. [48] highlighted the importance of collector and chimney dimensions on the output power of SCPP and optimized specific design with a 100 MW power capacity for Hami region in China.

The novelty of the current study is the integration of design and economical parameters to optimize the performance and feasibility of SCPP. Optimization is indicated through simple payback period to avoid uncertain future economic parameters and investigated different SCPP designs with a wide range of power capacity to represent a generic analysis not specific to a certain geographical location.

The main objective of this paper is to propose a methodology to optimize the SCPP design from technical and economical viewpoints. The key objectives of this paper are:

  • To determine the optimum configuration of the SCPP including the chimney height and the collector area.

  • To develop the minimum (optimum) simple payback period for each SCPP as a reference for optimization.

  • To take the difference between actual and minimum payback period as the indicator for the optimization potential.

  • To conduct a comparative techno-economic assessment for different designs of SCPP.

  • To develop the relationship between payback periods (actual and minimum), electricity price, and SCPP power capacity for each of power plant with reinforced concrete chimney (design-C), sloped collector (design-S), and floating chimney (design-F).

  • To study the impact of the running cost on the payback period for SCPP.

  • To study the effect of solar radiation and electricity price on the payback period for SCPP.

Section snippets

Methods

Three design configurations of SCPP were studied in this paper as shown in Fig. 1 to cover reinforced concrete chimney (design-C), sloped collector (design-S), and floating chimney (design-F). The working principle in the three studied systems is the same and air as the working fluid is heated in the collector after utilizing the incoming solar radiation. The kinetic energy of the working fluid is used to rotate an air turbine located at the bottom of the chimney. This kinetic energy is a

Model description

Payback period is the period of time to get back the investment and it has two forms of simple and very rarely used discounted [49]. The discounted payback period considers the time value of money and hence it is longer than simple payback period.

In this paper, the minimum (optimum) simple payback period is considered as a reference to measure to what extent the actual design is far from the optimum conditions. Using simple or discounted payback period would not affect significantly the

Input data and assumptions for the reference case

An excel based model is prepared to evaluate the actual (SPback) and minimum (SPmin) payback periods with a range of electricity price (ELp) 0.01–2.0 USD/kWh and a representative value of 0.10 USD/kWh was selected based on literature [52], [53], [54] for comparative assessment. The global solar radiation (G) is taken constantly for all designs at 2300 kWh/m2/year [22], [23], [24]. The collector inlet temperature is taken constantly at 302 K (29 °C) throughout the execution of the model as a design

Reference case results

Fig. 2, Fig. 3, Fig. 4, Fig. 5 show the simple payback periods (SPmin and SPback) based on the electricity price (ELp) for the reference case with power capacity 5, 30, 100, and 200 MW, respectively. Design-F is always at the bottom of the graph (see Fig. 2, Fig. 3, Fig. 4, Fig. 5) showing shorter payback periods compared to design-S and design-C. These shorter payback periods are mainly due to the lower initial costs for all power capacities of design-F compared to the other designs (see Table 1

Design optimization

The relationship between simple payback period (SPback) and the chimney height (H) was developed for each model in the reference case of this study. The minimum payback period (SPmin) for each relationship was specified and the corresponding chimney height was identified to represent the optimum (HOPT). The corresponding optimum collector area is estimated after using the obtained HOPT and maintaining the power capacity fixed at the same value of the reference case. Design optimization of each

Sensitivity analysis

The portion of annual revenue assigned to the annual running cost (CR), global solar radiation (G), and electricity price (ELP) are three of the most sensitive factors for SCPP payback periods (see Eq. (10)). CR would determine the level of running cost which is an essential portion in the total SCPP cost. G would determine the level of input energy to SCPP and represents the maximum limit for the output power which affects directly the revenue. ELP is the essential factor for the SCPP revenue.

Conclusions

SCPP design parameters were optimized in this study after related to the economic factors. Chimney height was firstly optimized and the associated collector area was estimated to generate the same output power of the reference case. Simple payback periods and initial cost were used as indicators for SCPP feasibility. SCPP with floating chimney (design-F) has shown shorter payback periods and lower initial costs compared to the other two studied designs (design-C and design-S). The low material

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