historic and recent progress in solar chimney power plant enhancing technologies

Renewable and Sustainable Energy Reviews 58 (2016) 1269–1292

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Historic and recent progress in solar chimney power plantenhancing technologies

Hussain H. Al-Kayiem a,n, Ogboo Chikere Aja b,1

a Mechanical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysiab Mechanical Engineering Department, Curtin University Sarawak, CDT 250, 98009 Miri, Sarawak, Malaysia

a r t i c l e i n f o

Article history:Received 11 October 2014Received in revised form1 August 2015Accepted 28 December 2015

Keywords:Energy conversionIntegrated solar systemSolar energySolar chimney power plantUpdraft solar heating

x.doi.org/10.1016/j.rser.2015.12.33121/& 2016 Elsevier Ltd. All rights reserved.

esponding author. Tel.: þ60 53687008.ail addresses: [email protected].: þ60 169495674.

a b s t r a c t

Upon the basic idea of the updraft solar heating, the solar chimney was proposed and implemented as amodel and a prototype by many research and industrial bodies. Although the system efficiency is below2%, but it is a promising technology to harness and convert the solar energy to electric power throughthree basic components; namely: solar collector, tower or chimney, and wind turbine-generator unit. Thelow efficiency, the bulk size and the high dependency on the solar irradiation are the major issuesexperienced in the solar chimney power plants. Since the implementation of the first prototype inManzenares, numerous attempts have been reported to enhance the performance of the system. Thepresent paper is compiling most of the reported attempts to enhance the performance of the solarchimney power plant. The conclusion drawn is that the system performance can be enhanced con-siderably via integration with another source of thermal energy, or by using efficient solar thermalenergy storages. This paper provides a platform to the researchers in the field to understand and getdetailed literature on the enhancing technologies of the solar chimney power plant current updates.

& 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12701.1. Energy scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12701.2. Energy and the environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12701.3. Solar energy systems as alternative for power generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1272

2. Fundamentals of solar chimney power plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12722.1. SCPP timeline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12732.2. Investors on SCPP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1275

3. SCPP components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12763.1. Open solar-air collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1276

3.1.1. Open-solar-air collector models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1276
3.2. The chimney . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1277
3.2.1. Chimney principles and processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1277
3.3. Power conversion unit (PCU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1278
3.3.1. SCPP turbo-generators (turbine(s)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12783.3.2. Air flow passage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1278

4. Performance of SCPP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1280

my (H.H. Al-Kayiem), [email protected] (O.C. Aja).

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4.1. Enhancement models of SCPP performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12815. Cost modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285

5.1. Components cost analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12855.2. Electrical power cost model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1287

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1288Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1288References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1289

1. Introduction

Solar updraft power plants (SUPP) are known as low tem-perature solar power plants, which utilise the solar radiation towarm up the atmosphere air, as working fluid. As proved techni-que, the solar chimney power plant (SCPP) is representing a simpleconfiguration of the SUPP, but it is not efficient, where it comprisesfour conversions of energy. Solar radiation is converted to thermalenergy in the absorbing medium, the thermal energy is convertedto kinetic energy in the collector passage, the kinetic energy isconverted to mechanical energy in the wind rotor, and finally, themechanical energy is converted to electrical power through thegenerator. This series of conversions caused the system efficiencyto be as low as less than 0.1%. Numerous research works have beenreported attempting to enhance the system performance throughimprovements of all the conversion processes starting from thesolar absorbing mechanism up to the generation of electricalpower. Review papers on the SUPP have been reported by Mustafaet al. [1], but it is limited to some suggested ideas to enhance theSUPP by integration with another resources of energy. Anotherreview paper on the SCPP is reported by Zhou et al. [2]. Theauthors presented large number of related works, but with littleon the enhancing methods of the SCPP. The third review paper onthe SCPP is reported by Dhahri and Omri [3]. The paper presentssome historical background of the SCPP and some CFD simulationand experimental investigations. Aja et al. [4] reported a reviewpaper with suggestion for modification on the SCPP. They havesummarised. Few enhancing attempts of the SCPP by otherresearchers.

The present review paper on the SCPP totally differs from theabove mentioned four review papers in terms of comprehensivelyand presentation manner of the practical problems and thereported solutions. The paper includes discussion on the energyroles on the civilisation development and the roles of the solarenergy, in particular. Then, the paper summarises the conversiontechnologies of the solar energy to useful energy that can be uti-lised by the human being for the daily domestic, industrial andpower generation activities. As the SCPP is the material of thepaper, the authors paid good effort to present a comprehensivehistorical background of the SCPP technology. Then, the paperpresents all the attempts reported, so far, aiming to improve thesystem performance. The authors extended the usefulness of thepaper by including cost modelling of the SCPP, which make thepaper useful for the technical researchers as well as to theindustrial and business people.

1.1. Energy scenario

Energy is the ultimate capacity for a body to do work whichsupports human activities, technological advancement, economicgrowth, maintenance of different individual life styles, securityand many more needs [5]. All types of activities, living or non-living, depend on energy. In other words the quality of human lifedepends to a large extent on energy availability. Thus, energy is

the basis of every social development and economic advancementand it is the convertible currency of technology [6].

Energy requirements vary and depend primarily on the pros-perity of a nation (industrial capacity and income per person),climatic conditions (average winter and summer temperatures),human populations, efficient use of energy and energy conversiontechnologies. In general term, the global energy demand has beenaggravated by the world population growth which is rated at 1.4%per year for the years 2000–2012 [7] coupled with the growingtrend of economic advancements and the choice of upper classlifestyle.

The growing energy demand poses some pressure on naturalresources which leads to deforestation, greenhouse gas (GHG)emissions, global warming and other environmental problems [8].The energy employed by any nation to meet her energy demand isdependent on the most easily accessible primary energy sourcesupported by the nation's policy. It may also connect to relation-ship one nation has with other nations which has cheaper primaryenergy source when the nation which need the energy can affordthe mature technology for such energy conversion to useful andend-user products.

Fossil fuels are non-renewable energy sources which depletewith time. Fossil fuels have been the major primary energyresources that have contained over 86% of the continued growingglobal energy demand [9]. The conventional energy supplies, suchas fossil fuel and nuclear energy have environmental burdens thatcome with their usage. Similarly, fossil fuels are being exhausted ata fast rate, and utilisation of fossil fuels together with net defor-estation contributes to environmental degradation, increase GHGemission and global warming [10–12].

A report on the world's primary energy consumption by fuelsfor the years 2004–2014 in million tonnes of oil equivalent asreported by BP energy outlook [13–23] is analysed as shown inTable 1 with consideration of the percentage contribution of thedifferent fuels (fossil, nuclear, hydroelectricity and renewableenergy). The World primary energy consumption in the year 2012and 2014, as shown in Table 1, infers that 86.94% and 86.31% of theglobal energy need was supplied from fossil fuels (oil, gas andcoal) representing 10847.70 and 11158.40 million tonnes of oilequivalent (MTOE) in the respective years. Similarly, in 2012, about4.49% of global energy supply came from nuclear, 6.66% fromhydroelectricity, while only about 1.90% was supplied fromrenewable energy (RE) sources (Geothermal, Solar, Wind, Woodand Waste) out of the 12476.6 MTOE consumed in 2012. In 2014,nuclear energy contributed 574.0 MTOE (4.44%), hydroelectricitycontributed 879.0 MTOE (6.8%) and renewable energy resourcecontributed 316.9 MTOE (2.45%) of the 12928.3 MTOE consumedin 2014 [23]. It was observed that the global energy demand isgrowing at an annual rate of 2.1%.

1.2. Energy and the environment

The use of conventional energy sources, such as fossil fuel andnuclear energy has some side effects on the environment andindirectly on human health. Non-renewable energy resources like

Table 1World primary energy mix usage for year 2004–2014 in million tonnes oil equivalent (MTOE) and percentage (%) [13–23].

Year Fossil fuel Nuclear Hydropower Renewable energy Total

Oil Natural gas Coal Total (MTOE) (%) (MTOE) (%) (MTOE) (%) (MTOE)

(MTOE) (MTOE) (MTOE) (MTOE) (%)

2004 3798.6 2425.2 2798.9 9022.7 87.68 625.1 6.07 643.2 6.25 N/A N/A 10291.02005 3836.8 2474.7 2929.8 9241.3 87.70 627.2 5.95 668.7 6.35 N/A N/A 10537.22006 3910.9 2558.3 3041.7 9510.9 87.71 634.9 5.86 697.2 6.43 N/A N/A 10843.02007 3952.8 2637.7 3177.5 9768.0 88.01 622.0 5.60 709.2 6.39 N/A N/A 11099.22008 3959.9 2717.3 3286.4 9963.6 88.06 620.2 5.48 731.4 6.46 N/A N/A 11315.22009 3882.1 2653.1 3278.3 9813.5 87.90 610.5 5.47 740.3 6.63 N/A N/A 11164.32010 4031.9 2843.1 3532.0 10407.0 86.89 626.3 5.23 778.9 6.50 165.50 1.38 11977.72011 4081.4 2914.2 3628.8 10624.4 86.91 600.4 4.91 794.7 6.50 205.60 1.68 12225.12012 4130.5 2987.1 3730.1 10847.7 86.94 560.4 4.49 831.1 6.66 237.40 1.90 12476.62013 4179.1 3052.8 3867.0 11098.9 86.66 563.7 4.40 861.6 6.73 283.00 2.21 12807.22014 4211.1 3065.5 3881.8 11158.4 86.31 574.0 4.44 879.0 6.80 316.90 2.45 12928.3

N/A stands for “Not Available”.

Transportation 13.5%

Industry 10.4%

Other fuelConsumption 9%

Fugitive emission 3.9%

Land use change 18.2%

Electricity and Heat 24.6%

Agriculture 13.5%

Waste 3.5%

Carbon dioxide CO2 77%

Methane CH4 14%

Nitrous oxide N2O 8%

HFCs, PFCs, SFs 1%

Fig. 1. World GHG emissions flow chart [30].

H.H. Al-Kayiem, O.C. Aja / Renewable and Sustainable Energy Reviews 58 (2016) 1269–1292 1271

the fossil fuels deplete gradually with time [10,11], while the useof the fossil fuels and the growing rate of deforestation as a resultof development contribute to a great extent to environmentalpollution; increase GHG in the atmosphere and consequentlyglobal warming [8–12].

The option of using nuclear energy as alternative to fossil fuelhas its challenges in relation to its waste disposal coupled with thethreat of radiation exposure in the case of plant accident/failure.Some examples of nuclear power plant failures include the ThreeMile Island unit 2 (TMI2) nuclear power plant, Pennsylvania, USAwhich failed on March 28, 1979, the Chernobyl Nuclear PowerPlant (Chernobyl 4) in Ukraine which failed on April 26, 1986 andthe Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan inci-dent which occurred on 11th March, 2011. The report on the abovementioned nuclear power plant failure has devastating effects onboth human (health and psychological effect) and the environ-ment in general as the release of radionuclides contaminates the

air within the environment, affects the soil, thereby hinderingsome agricultural and social activities [24–28].

The need for solutions to energy demand challenges cannot beover emphasised as the global energy demand is expected todouble by 2050 while the means to meet the demand are limited[29]. To combat the above mentioned challenges associated withconventional energy supply and usage, there is a need for analternative/renewable energy to supply a good percentage of theglobal energy demand and strive to be the major energy source forpower generation. Secondly, some of the non-renewable energysources are also raw material for other industrial activities which ifother alternative/renewable energy supplies mix can be employed;the usage life span of the non-renewable resources can beextended. Considering the expected doubling of the global energydemand and the impending energy shortage [29], one solution tosolving the problem is to tap more into the renewable energysources.

Fig. 2. Schematic view of SCPP.


The global energy demand has continued to increase as shownin Table 1 but the means of meeting this demand has been mainlythe use of non-renewable energy resources. The use of non-renewable energy sources contributes to emission of GHG to theatmosphere and global warming while the flaring exhaust hot fluegas to the atmosphere contributes to thermal discomfort at thelocation where the process occurs.

Globally, coal has been recorded as the highest carbon fueldominant in electric power generation which contribute greatly tothe emission of GHG to the atmosphere leading to climate change.Similarly oil has contributed in no little quota to emission in thepower generation industries and the transport sector where it hasnear monopoly status as the major source of energy [30,31].

The power generation sector is the highest contributor to glo-bal GHG emission in 2005 which accounts for about 25 percent ofglobal GHG emissions as shown in Fig. 1 [30]. In the power gen-eration sector, electricity generation accounted for 68% of theemission which is about 17% of the total global GHG emissions.Heat (including combined heat and power) was rated at about 5%of worldwide emissions, and other energy industries account forroughly 3 percent [30,31].

The use of conventional energy supply sources such as fossilfuel burdens the environment with greenhouse gas emission andglobal warming while the flared hot flue gas causes thermal dis-comfort in the environment where the process occurs. As a meansto reducing the above mentioned side effects of using convectionalenergy, alternative and renewable energy need to be harnessedthe more.

1.3. Solar energy systems as alternative for power generation

Ultimately, all energy supplies on earth are derived directly orindirectly from solar energy. The Sun is an ubiquitous form ofenergy, but not as yet an economic one [32]. Solar energy provides acontinuous stream of energy which warms us, causes crops to growvia photosynthesis, and heats the land and sea differentially tocauses winds and consequently waves and rain. Tidal rise and fall isthe result of gravitational pull of moon and sun and geothermalheat the result of radioactive decay deep in the earth [6]. The beautyof solar energy is its free supply, abundant in nature (ubiquitousform of energy) and characterised of environment-friendliness, butis not yet economical as compared to the well-established tech-nologies of conventional energy power plants [32].

Solar energy can be converted into other forms as useful energyemploying different technologies such as photovoltaic cells (PV)for direct solar energy conversion to electricity; flat plate solarcollectors (FPSC) and concentrated solar collectors (CSC) for eitherheating or indirect electrical power generation.

The indirect electrical power generation system which arecharacterised of high temperatures are classified as CSC technol-ogies. The CSC technologies include parabolic trough collectortechnology, linear Fresnel collector technology, concentrating solartower technology, and solar Stirling dish technology. On the otherhand, FPSCs are low temperature solar-thermal systems whichfind wide application in domestic household hot-water heatingand space heating. FPSC are used in industrial systems either tosupply low-temperature demands or to preheat the heat transferfluid before entering a field of higher-temperature concentrating,collectors. SCPP is also classified as one of the low temperaturesolar energy conversion system which converts the energy fromthe sun indirectly to electricity. It utilises the earth-ground as theabsorber plate which is covered with transparent cover raisedsome height above the absorber to cause fluid flow that can beharnessed for energy generation. For the purpose of power gen-eration, if the performance efficiency of SCPP is improves and the

cost of installation reduces, SCPP can be employed for commercialelectrical power generation [33].

The SCPP is a solar-thermal power generation system whichutilises a combination of three technologies to harness and con-vert solar energy to electrical energy. The principle involves theabsorption of solar energy in an open-solar-air collector (green-house) which heats up the air in the enclosure. The heated air rises(buoyancy effect) due to density drop and exit the greenhousethrough the chimney (chimney effect). At the base of the chimney(exit of the open-solar-air collector and or the inlet of the chim-ney), wind turbine(s) are installed to harness the kinetic energy inthe buoyancy driven hot air and convert it to electrical energy withthe help of generator. In other words SCPP is a solar thermal powerplant utilising a combination of a solar air collector and a centralupdraft tube to generate convective air flow which drives sta-tioned wind turbine(s) to generate electricity [34,35]. The open-solar-air collector of the SCPP is a form of flat plat collector whichabsorbs direct and diffused solar radiation.

In line with other solar thermal energy generation systems, theSCPP has low efficiency but is favoured with well-developedtechnologies that make of the system. The materials for thedevelopment of the system are easily sourced locally. Thus, theplant has a promising future as large-scale solar-electric powerplant. The SCPP is characterised with long life span, little main-tenance, no combustible fuel, no cooling water and it is free ofGHG emissions. The SCPP technologies are simple, reliable and candeveloped in technologically less developed countries, which aresunny and often have limited raw material resources for otheradvanced technologies [33,34].

2. Fundamentals of solar chimney power plant

The SCPP is a solar thermal energy conversion system that utilisesa combination of three technologies (greenhouse, chimney and tur-bine) to harness and convert solar energy to electrical energy. Theprinciple involves the absorption of solar energy using open-solar-aircollector (greenhouse) to heat up air (working fluid) in the domain.The heated air rises due to density variation as a result of temperaturechanges and leaves the domain through the chimney by buoyancy(chimney/stack effect), as shown in Fig. 2. The chimney acts similar toa penstock in the hydropower system to guide the buoyant airthrough the turbine and exit to the atmosphere. At the base of thechimney (exit of the open-solar-air collector and or the inlet of thechimney), wind turbine(s) coupled with generator(s) is/are staged toharness the kinetic energy in the moving air and convert it to elec-trical energy. In conclusion, SCPP is a solar-thermal power plant uti-lising a combination of an open-solar-air collector and a central

Fig 3. Leonardo da Vinci's chicken barbecue with a windmill and chimney [50].

Fig. 4. Solar engine project proposed by Col. Isidoro Cabanyes [51,53].Fig. 5. Prof Bernard Dubos's proposed solar aero-electric power plant [53,54].


updraft tube/chimney to generate a convective air flow which driveswind turbine(s) to generate electricity.

The SCPP is characterised with long life span, little main-tenance, no combustible fuel, no cooling water and is free ofGHG emissions [33,34]. Because the plant technology is simple,reliable and can be constructed with potential available mate-rials, SCPP technologies can be developed in technologicallyless developed countries which are sunny and often have lim-ited raw material resources.

One major problem of SCPP is the low conversion efficiencyas determined by the thermal performance of the system[4,36–44]. However, the conversion efficiency of SCPPincreases with the solar chimney height, open-solar-collectorarea and solar radiation intensity [34]. The power output of aSCPP is a function of the mass flow rate of the working fluidwhich is determined by the collector area and the chimneyheight. For SCPP to be economically viable for commercialpower production a high gigantic solar chimney and largeopen-solar-air collector area are required to generate highmass flow rate/driving force to rotate the turbine(s) [32–35,45–48]. Higher conversion efficiency for large-scale SCPPwill lead to reduction in the produced energy cost.

2.1. SCPP timeline

The concept of updraft has been in practice for centuries. Thefamous Italian genius Leonardo da Vinci (1452–1519) created theearliest system, which uses hot rising air in a chimney to drive awindmill which rotates his roasting spit connected to the windmillabove a fireplace as shown in Fig. 3 [40,49,50].

After centuries, in 1903, a Spanish artillery colonel, IsidoroCabanyes first described the SCPP. His public proposition “Proyectode motor solar” (solar engine project) was described using anapparatus consisting of an air heater attached to a chimneydesigned house and a wind propeller place in the house to extractthe energy in the buoyant air for electricity generation as shown inFig. 4 [51–53].

In 1926 Prof Engineer Bernard Dubos proposed to the FrenchAcademy of Sciences the construction of a SCPP to be located inNorth Africa with its solar chimney on the slope of a sufficientlyhigh mountain as shown in Fig. 5 [53,,54]. Dubos's SCPP wasdescribed and published in 1931 by Hanns Günther [53,,2,,55].

Günther's analysis of Dubos proposal inferred that an ascend-ing air speed of 50 m/s can be reached in the chimney, where theenergy can be extracted using wind turbines. An experimentaldescription of the proposed system by Dubos is shown in Fig. 6

Fig. 6. Solar chimney proposal of Prof Dubos presented by Günther, 1931 [55].

Fig. 7. Improvements in or relating to apparatus for generating power from solarheat [56].

Fig. 8. Schematic Diagram of Nazare’s SCPP patent [57,58].


where the plate and the spirit lamp represent the Sahara desertand the solar heat, respectively; the small wind wheel at the top ofthe chimney represents the wind turbines. The description indi-cated that if the spirit lamp is positioned under the plate, warm airflows concentrically through the plate reaching the tube. Conse-quently, the ascendant flow impels the wind wheel [55].

In 1956, a patent was granted to Ridley [56]. His patent wasdesigned to incorporate two chimneys, where the first chimneywall is heated by hot air generated from the greenhouse such thatcold air is drawn to the turbine at the base of the first chimneythrough a tunnel connecting to the second chimney. Thus thesecond chimney supplies cold air by downdraft principle whilefirst chimney creates updraft due to temperature difference at thechimney walls and creates the channel for the air to exit, see Fig. 7.

Another record of an early patent application on the SCPP was aFrench patent granted in 1964 to Nazare [57,58]. The model of theinvention is proposed for a tower of 100–300 m height with itsshape approximately that of a diffuser as shown in Fig. 8.

Up to the 1970s, there was scarce information about the SCPPsystems, and this is possibly due to the oil boom and the devel-opment of conventional power generation systems that shifted theenergy supply source to the fossil fuel energy. Since after the oilcrisis in the 1970s the need for energy saving and alternativeenergy ignited an energy revolution. Renewable energy systemshave become the topic of study for many researchers with differ-ent theoretical and experimental studies being published.

Some of the related developments in SCPP since after the oilcrisis in the 1970s include the patents granted to Lucier [59–63]between 1975 and 1981 as shown in Fig. 9.

SCPP resurfaced to limelight when Prof Schlaich in 1978 pre-sented the SCPP technology in a congress [64]. Between 1980 and1982, Prof Schlaich with his colleagues designed and constructedthe first SCPP prototype, Fig. 10, on a site provided at Manzanares,Spain by the Spanish Utility Union Electrica Fenosa, while theproject was funded by the German Ministry of Research andTechnology [33–35,65,66].

Fig. 9. Diagram of Lucier's patents [59,63].

Fig. 10. Manzanares SCPP [33].


The Manzanares SCPP prototype is composed of a 195 m highchimney of 10 m diameter; an open-solar-air collection area(greenhouse) of 45,000 m2 (244 m diameter) which generated amaximum power output of about 50 kW [34,45,67]. The trans-parent cover of the open-solar-air collector was mainly plasticmembranes which covered about 40,000 m2 of the total collectorarea while the rest area close to the chimney connection wascovered with glass. The mean height between the ground and thetransparent cover was about 1.85 m which allows ambient air toflow into the greenhouse. The temperature rise in the greenhousewas about 17 K above the ambient while the air velocity reachedan average of 12 m/s during turbine operation. The chimneyhoused a 4-bladed single wind turbine which is situated at thechimney base to harness the energy in the buoyant air [33].

The plant was operated from 1982 to 1989 until it was finallydecommissioned [53]. A brief on the design criteria, energy bal-ance and cost of the Manzanares SCPP and the fundamentalinvestigation report of the plant was presented by Haaf el al. [65].To buttress the success of the SCPP, test results of the Manzanarespilot plant was reported showing feasibility of the technology [67].

SCPP requires large area for the collector which might beexpensive in city-centres. Study by some researchers has shownthat the most suitable construction sites of large-scale SCPPs arevast desert regions where land may be cheap or free because ofthe large land area requirement [68]. The applicability of usingSCPP in solving the energy need in rural areas has been investi-gated and was found suitable for power generation in remote

areas which will help solve the energy problem in less developedcountries [69–71].

2.2. Investors on SCPP

Since the successful proving of the SCPP technology using theManzanares prototype, SCPP underwent a revolution with manycountries planning for the installation of the plant. In 2001, theAustralia government voted in support of the construction of a200 MW SCPP in Buronga, north of Mildura, Australia. The exclu-sive Australian license for Solar Tower Technology was awarded toEnviroMission to champion the establishment of SCPP in thecountry [72]. The Plant was proposed to have 1000 m high solarchimney with 120 m chimney diameter and 7000 m collectordiameter [2,72–75]. To enhance the night time power production,it was planned to include additional thermal storage mediumwhich would cover 25% of the total collector ground area withexpected 24 h operation. The 200 MW plant was planned toincorporate 32 pressure-staged horizontal-axis turbines of6.25 MW each, symmetrically distributed on the ground close tothe stack inlet. The evaluated air temperature difference betweenthe working fluid and the ambient air is expected to be about 35 Kand the updraft average velocity would be around 15 m/s. Theplant has been downsized from 200 MW to 50 MW. The re-engineered tower will now be 78 m diameter, 480 m tall chim-ney structure, surrounded by a 3300 m diameter glazed solarcollector. At the perimeter, the roof will be separated from theground to 2.4 m high and the open-solar-air collector will gradu-ally slope up to a height of about 15 m at the chimney base[73,74,76].

Another large-scale SCPP is planned for South Africa [77–88]with about 6900 m diameter collector, 1500 m high chimney and160 m chimney diameter. The choice of the 1500 m high chimneyis based on the scale economy of chimney height effect. Accordingto Schlaich [34], the energy output of SCPP with 1500 m highchimney yields about three times the power output of a 750 mhigh chimney of equal collector area [78–81,87].

A SCPP of 200 kW in Jinshawan, Mongolia, China startedoperation in 2010. The 1.38 billion RMB (USD 208 million) projectwas started in May 2009 and it was aimed to build a facilitycovering 277 hectares and producing 27.5 MW by 2013. Thegreenhouses will also improve the climate by covering movingsand, restraining sandstorms [2,89,90]. Another development inChina is the proposal for building 1000 m high solar chimney for

Fig. 12. Irregular solar collector configuration [110].


power generation and tourism development in Shanghai, Chinawas presented and its simulation was performed by HuazhongUniversity of Science & Technology team [2].

A SCPP is planned to be sited at Ciudad Real, Spain, entitledCiudad Real Torre-Solar which would stand 750 m tall, covering anarea of 350 hectares (3,500,000 m2) with expected power outputof 40 MW [2,89].

In Brazil, the MCTI News of the Ministry of Science and Tech-nology in 2004 announced its plan of building a SCPP in the stateof Maranhão (Brazilian Northeastern region) [91] but has notcommenced construction [92].

Botswana's Ministry of Science and Technology designed andbuilt a small-scale solar chimney system for research. The planthas 2 m chimney internal diameter of 22 m height and wasmanufactured from glass-reinforced polyester material while col-lector area was approximately 160 m2. The roof was made of a5 mm thick clear glass that was supported by steel framework[89].

The Namibian government in 2008 approved a proposal for theconstruction of a 400 MW solar chimney called the “Green Tower”.The tower is planned to be 1500 m tall and 280 m in diameter, andthe base will consist of a 37 km2 greenhouse in which cash cropscan be grown [2,89,93,94].

Other planned projects include Sri-Lanka's 200 MW project,three 200 MW projects in the USA, the 200 MW plant in Rajas-than, India and 200 MW plant in China [95]. Many SCPP experi-mental setups focused on plant performance enhancement withmodification have been proposed and investigated by differentresearchers across the globe [4,38,39,54,96–109].

3. SCPP components

SCPP is made up of three main components namely: thegreenhouse (open solar air collector), the solar chimney (solartower) and the turbine. This section will discuss some of the workscarried out on the different components.

3.1. Open solar-air collector

The open solar-air collector (greenhouse) of a SCPP is the heatexchanger of the power plant where the solar radiation is absor-bed and converted into thermal energy. The thermal energy isconverted to kinetic energy when the absorbed heat in the col-lector is transferred to the air in the greenhouse. The open solar-air collector consists of support matrix, column structure, trans-parent roof and the ground. Fig. 11 shows the structural design ofManzanares SCPP open solar-air collector.

In order to reduce the turbulent friction losses that may resultin the open-solar-air collector, Bonnelle [110] proposed a new

Fig. 11. Solar collector structure of the Manzanares SCPP [33].

collector concept with ribs containing their branching as shown inFig. 12. This proposed new design of the open solar-air collectorhas air flow guides that direct the generated buoyant air towardsthe chimney thereby reducing heat losses.

The ground which is a part of the open solar-air collector actsas natural thermal storage medium as natural soil has certainthermal storage capacity. But the thermal storage capacity of thesoil depends on the soil type and may not meet the needed energyfor the night or cloudy day operation of a SCPP. Thus the nightmode of the SCPP is always faced with little or no power genera-tion, hence to combat this challenge and improve the performanceof SCPP during night time, some measures were proposed.

To improve the thermal absorption and heat storage capacity ofthe ground, Pretorius [42] introduced intermediate secondary roofbetween the transparent canopy and the ground. Investigation onthe improvement of the open solar air collector by utilising dif-ferent varieties of glass quality and various types of soil wasconducted Pretorius and Kröger [84]. The use of water-filled tighttubes embedded to the ground surface was introduced by Kreetz[111,,33]. The water filled tube enhancement was also furtherinvestigated by Bernardes [112].

Other thermal storage media include the use of solar pond.Davey [113] proposed the use solar ponds as thermal storagemedium under in the open solar-air collector to enhance heatstorage and combat the poor power generation associated withthe night operation of SCPP. Other enhancement proposals andmodels are discussed in the performance enhancement modelssection of this literature review.

3.1.1. Open-solar-air collector modelsSeveral analytical and numerical models have been developed

to study the fluid flow, thermal field process and performance ofopen solar-air collector. Schlaich [34] presented an analyticalmodel of the SCPP collector. Different numerical models have beendeveloped for the collector by Kröger and Buys [114], Gannon andVon Backstrom [115], Hedderwick [116], Kröger and Burger [117],Bernardes [112], and Pretorius and Kroger [84]. It was shown thatthe process involved heat transfer from the ground to the adjacentair in the greenhouse and thus was considered as control volume[41,42,83–85]. Without external force like wind, the air gainsenergy by convection while the velocity increases as the flow getscloser to the chimney. Thus, the changes of the air dynamics alongthe radius of the collector from the collector periphery to thecentral chimney was considered to be slow and steady state wasassumed [41,85,116]. Considering the state of the flow, the con-tinuity, momentum and energy equations can be summarised asfollows:

Continuity,

∂∂r

ρairvcoll�exitrcollhg� c� �¼ 0 ð1Þ


Momentum

ρairvcoll�exithg� c∂vcoll� exit

∂r¼ �hg� c

∂psys�air

∂r�τsys�air ð2Þ

where ρair is the density of air at any point within the collectorwith reference to the collector radius

rcoll is the collector radius,vcoll-exit is the velocity of air at the collector exit,hg–c is the air flow gap height (the distance between the ground

and the cover)τair is the shear stress in buoyant air at the open-solar-air col-

lector due to friction between the air and the collector walls(ground and transparent cover) and the drag resulting from thestruts that support the transparent cover above the ground. Pre-torius and Kroger [42,85] presented the expression for τ as part ofthe collector losses.

psys–air is the pressure difference between the air in the systemand the air outside the system (ambient air)

The energy transfers to the collector are the energy gain by theground and the cover from the sun, Sg and Sc, respectively; heattransfer from the ground by conduction to the underlying earthcrust, qg-cond; convective heat transfer to the air from the ground,qg-conv and radiation heat transfer to the transparent cover fromthe ground, qg-rad; the convective heat transfer from the air to thecover , qair-conv; the convective heat transfer from the cover to theambient caused by wind effect, qc-wind; the radiation heat transferfrom the cover to the sky, qc-rad and the useful energy, qu. Theenergy transfer can be presented with respect to the collectorcomponents (ground and the transparent cover) and the air.

Energy balance at the ground

Sg ¼ qg� radþqg� conv

þqg� condqg� cond ¼ �kg∂Tg

∂h

��z ¼ 0

ð3Þ

Energy balance at the cover

Scþqg� radþqair� conv ¼ qc�windþqc� rad

Scþqg� rad ¼ qc�windþqc� radþqair� conv ð4Þ

In Eq. (4), the convective heat transfer is considered in twosituations: the first case represents a situation where heat istransferred from air to the cover (air temperature is higher thanthe cover temperature) and the second case is heat transfer fromthe cover to the air when the cover temperature is higher than theair temperature, thus the convective heat transfer equation is asshown in Eq. (5).

Energy balance in the flow

qg� conv ¼ qair� convþqu

qg� convþqair� conv ¼ qu ð5Þ

Several models have been developed based on these equationsat the collector section of SCPP. Most of the models assumed theabsorbed solar radiation in at the cover to be negligible [118–126].

3.2. The chimney

The chimney of a traditional SCPP is situated at the centre ofthe collector. It is the thermal engine of the plant, which createssuction that draws ambient air into the collector and enhancesbuoyancy in the hot air in the system. It is a pressure tube withlow friction losses due to the chimney surface to volume ratio. Ithas been reported that the mass flow of the updraft air isapproximately proportional to the collector air temperature riseand to the solar chimney height [2,33].

The chimney height of SCPP contributes greatly to the effi-ciency of the chimney and the total efficiency of the system

[33–35,45,127]. Literature infers that for a commercially viableSCPP, the chimney height should be reasonably tall to about1000 m high but with reduced height, an increase in the collectorarea can stand as a compensation for the chimney height[33,34,127]. Based on the characteristic height, the chimneystructure faces threat from external forces that might result fromwind storm or other environmental/natural forces and as such, thedesign must be well planned and structured to overcome suchthreats [128–131].

Many designs of the chimney structure have been proposed tocombat the effects of the natural forces with some considerationson construction cost without compromising the structural integ-rity and strength. The chimney can be constructed using guyedtubes as used for Manzanares [33,34,45,47,65,127], reinforcedconcrete, corrugated metal sheets, and cable-net with cladding ormembranes where the best method was suggested to be free-standing reinforced concrete as it is characterised with long lifespan of about 100 year in dry climate and availability of con-struction material even in less developed countries [34,127,132].

The effect of chimney height and diameter was investigated bySchindelin [133] and found that the height of the chimney was amajor determinant for chimney efficiency while the increase inchimney diameter reduces frictional losses.

3.2.1. Chimney principles and processesThe SCPP is dependent on the pressure differential created by

the relatively buoyant warm air in the chimney and the corre-sponding column of denser ambient air. Schlaich [34], presentedthe pressure difference created by the chimney as Eq. (6)

Δptot ¼Z Hch

0ρamb�air�ρair� in� �

dh ð6Þ

For dry air, the perfect gas relation gives the density, ρ¼ p=RT;considering the air at inlet of the collector with temperature Tair-inand air at chimney base at temperature Tair-base while assumingdry adiabatic lapse rate for the air inside and outside the solarchimney and the chimney base, the pressure potential wasexpressed as Eq. (7) [134,135]

Δptot ¼ pair� in 1� 1�0:00975Hch=Tair� in� �1�0:00975Hch=Tair�base� �

( )3:50@

1A ð7Þ

Noting that air inside the SCPP undergoes some thermo-dynamic processes, the conservation equations for continuity,momentum, and air energy in the chimney were presented as Eqs.(8)–(10) respectively [41,85].

Continuity

∂∂h

2πρair� chvair� chrchhch� �¼ ∂

∂hρair� ch

_Vair� ch

� �¼ 0 ð8Þ

Momentum

ρair� ch_Vair� ch

∂ _Vair� ch

∂h¼ �∂pair� ch

∂h�2τair� ch

rch�ρair� chg ð9Þ

where ρair-ch is the density of air at any height along the chimneyrair-ch is the chimney radius,vair-ch is the velocity of air along the chimney height,hch is the air any height along the chimney till exitτair-ch is the shear stress in buoyant air as the air flows up the

chimney_Vair� ch is the volume flow rate of air in the chimneyAir flow energy in the chimney

Cpair� chTair� ch∂∂h

ρair� ch_V air� ch

� �þρair� ch

_V air� ch∂∂h

Cpair� chTair� ch� �

Fig. 13. Schematic view of SCPP turbine configurations (a) single vertical-axisturbine configuration (b) multiple vertical-axis turbine configuration and(c) multiple horizontal-axis turbine configurations [34,154].


þ ∂∂h

ρair� chg _V air� ch ¼ 0 ð10Þ

where Cpair-ch is the specific heat of air at a specific height of thechimney under consideration.

Tair-ch is the temperature of air at the chimney heightconsidered.

The air flow in the chimney has been modelled by differentresearchers as either compressible [136–138] or observing theBoussinesq principle of density variation with change in tem-perature [139–143]. A comparison of compressible flow model ofair flow in the chimney and Boussinesq model of air flow in thechimney using computational fluid dynamics (CFD) software wasconducted by Zhou et al. [144] for 1500 m high solar chimneyproposed by von Backström and Gannon [137].

3.3. Power conversion unit (PCU)

The power conversion unit (PCU) of large-scale SCPP consists ofone or several turbo-generators (turbine coupled with generator,where the size is dependent of the design air mass flow rate), flowpassage from collector exit to solar chimney inlet, and grid con-nection. It may include a diffuser designed to the chimney wallbehind the turbine for single vertical-axis turbine configuration.

3.3.1. SCPP turbo-generators (turbine(s))The turbo-generator is the energy conversion equipment of the

SCPP which converts the kinetic energy in the moving air intomechanical/electrical energy. It is characterized by the number ofrotor blades, specific speed of blades, axial orientation (vertical/horizontal axis) and turbine diameter. The performance of SCPPturbo-generator is measured between the performance of windturbine and gas turbine [40,145–152]. Turbine installation loca-tions, configurations and layouts have been proposed by differentresearchers for either the exit of the chimney [64] or the con-ventional location at the chimney inlet/base [33–35,45,47,153].

Bonnelle [110] compared the static pressure fields of airflow indifferent solar chimneys where the turbines were respectivelyplaced at the base and the top. The comparison showed thatrelative negative pressure appeared in the chimney when theturbine was placed at the chimney base, while relative positivepressure appeared when the turbine was placed at the chimneytop which can be related to the fact that static pressure must droplargely from the upstream to downstream of a turbine. In moststudies of SCPP, the turbines were proposed to be staged at thechimney base mainly due to the inconvenience arising frominstallation and maintenance of turbine generators at the chimneytop for large-scale SCPP. Considering the chimney base for theinstallation of the turbine(s), many configurations/arrangementshave been studied for single and multiple turbines installation.

The single vertical axis turbine was the first employed SCPPturbine which was designed by Schwarz and Knauss [154] andused in the experimental investigation of the Manzanares proto-type. Single vertical-axis turbine was presented by Schlaich [34]for harnessing the energy of the buoyant air in the SCPP and wasdesigned such that the blades arms extends from the turbinepropeller at the centre of the chimney base to walls of the chimneybase with small clearance while at the exit of the turbine, thechimney is designed for proper air diffusion for pressure dropcontrol Fig. 13. Single vertical-axis turbine for a large-scale SCPPhas the advantages of simplicity of the flow passage and smallnumbers of parts while the disadvantages are the huge sizes ofturbine which is a challenge in the manufacturing, handling/transportation, the huge torque that can be generated whichwould require huge generator and lack of redundancy [145–148].The multiple turbines of SCPP are arranged in either vertical or

horizontal configuration [34]. The vertical configuration is forvertical shaft turbines which are integrated into the chimney(Fig. 13b) while the horizontal shaft configuration locates theturbines around the chimney circumference with their axes per-pendicular to the chimney axis (Fig. 13c) [34].

A vertical shaft configuration reduces cyclical stress on thecomponents due to gravity but requires a thrust bearing to carrythe weight of the whole rotor but for a horizontal shaft config-uration, the pressure after the turbine section is sub-atmosphericwhich makes sealing of the horizontal to vertical flow transitionsection necessary [40].

Considering the different arrangements, a comparison of effi-ciency, energy yield and impact of the various losses on the overallperformance of the three proposed configurations by Schlaich [34]conducted by Fluri and von Backström [150] shows that the singlevertical axis turbine configuration has a slight advantage abovemultiple vertical-axis and multiple horizontal-axis turbine con-figurations with regards to efficiency and energy yield, but its peakoutput torque is tremendous, which requires huge generator andmaking its drive train costly and its feasibility questionable forhuge turbine. Similar conclusion was reported by Kolb andHelmrich [155] on CFD study and comparison of a single vertical-axis and the multiple horizontal axis turbine configurations for a200 MW plant.

3.3.2. Air flow passageThe air flow passage is the path through which generated

buoyant air flows from the collector to the chimney base andturbine. The air flow passage configuration induces differentmagnitude of pressure losses [40,124,125,155–157]. To reducethese pressure losses, Kolb and Helmrich [155] propose the useintake geometry with converging sections and a transition from

Straight junction Curved junction

Conical chimney Slanted canopy

Curved junction withconical deflector

Fig. 14. Configurations for air flow passage for Bernardes et al [157] study.

Table 2Mass flow rate of different geometric configurations [158].

Case Hc1 (m) Rt1 (m) Rin (m) Rex (m) m ̇ (kg/s)

Straight junction 0.02 0.05 – – 8.31�10�4

Curved junction 0.02 0.05 0.1 – 1.08�10�3

Slanted junction 0.07 0.05 0.1 – 1.29�10�3

Conic solar chimney 0.02 0.1 0.1 – 1.92�10�3

Curved junction withguiding cone

0.02 0.05 0.1 0.12 1.1�10�3

Fig. 15. Schematic layout of SCPP of different configurations: (a) reference plant; (b) sloping collector with a cylindrical chimney; (c) constant absorber-cover gap heightsystem with convergent chimney; (d) constant absorber-cover gap height system with divergent chimney; (e) sloping collector with convergent chimney; (f) a slopingcollector with divergent chimney [159].


rectangular to circular and analyse it with CFD. Müller [156],conducted CFD study on the proposed shape by Kolb and Helmrich[155] for multiple vertical shaft turbines and found that such acentrepiece intake geometry can reduce the inlet losses by 43%. Aninvestigation on the different design for the air flow passage wasconducted by Bernardes et al [157] using five geometric

configurations (straight junction, curved junction, slanted junc-tion, conic solar chimney, and curved junction with guiding cone),as shown in Fig. 14.

The geometric configurations, parameters and the mass flowrate for the study is presented in Table 2. The analysis showed thatstraight junction case gave smaller flows, due to the occurrence of

Fig. 16. Schematic diagram of turbine layouts (single rotor and counter rotatingturbines – with or without IGVs) [40].


junction flow recirculation. The recirculation reduces with thesmoothening of the junction. The curved junction allowed higherflows, the use of conic solar chimney offers highest mass flow ratewhich validates the claim of Yan et al. [158]. They also inferred thatthe introduction of a deflector such as guiding cone at the centredid not bring major thermal or hydrodynamic improvements.Suggestion for more investigation was recommended by theauthors.

Koonsrisuk and Chitsomboon [159] presented a computationalsimulation study on the influence of the flow area parameters ofSCPP on the behaviour of the air flow. They considered the effect ofdifferent collector inlet areas and chimney exit area, while theareas of the collector exit and the chimney entrance constant. Thesix configurations considered are as shown in Fig. 15.

Their results inferred that a divergent chimney increases thestatic pressure, mass flow rate and power compared to thecylindrical chimney. The convergent chimney was characterizedwith equal power output as the convergent chimney. The slopingcollector was found to enhance the static pressure across the roofand the power at the roof entrance. Finally they inferred that SCPPwith sloping collector and divergent chimney of exit to inletchimney area ratio of 16 can produce 400 times the power of thestraight chimney.

The guiding of the generated hot air at the collector area to theturbine (horizontal to vertical transition) is important in the SCPPin order to reduce energy losses which can arise from recirculationof the hot air at the collector exit. To achieve the proper guide ofthe hot air to the turbines, inlet guide vanes (IGVs) are necessary.For large-scale SCPP, the IGVs can be variable, and can serve tocontrol the plant output and to close off the turbine flow passage(s) for emergency/maintenance [40]. Some investigations on IGVsof SCPP were presented in [40,145–148,152]. Gannon [145]designed a single vertical shaft turbine with inclusion of inlet

guide vanes. Kirstein and Von Backström [125] present a CFDinvestigation of the effect of IGVs on air flow and the performanceof SCPP with single vertical axis turbine. Fluri [40] investigated theeffect of IGVs on horizontal-axis turbines of four configurations(single rotor turbine without IGVs, single rotor turbine with IGVs,counter rotating turbine without IGVs, and counter rotating tur-bine with IGVs) as shown in Fig. 16. He found that the single rotorlayout without IGVs is the simplest and cheapest layout but itstotal-to-static efficiency is low, because the swirl at the turbineexit cannot be recovered. For the three other layouts the max-imum total-to-static efficiency is much better and lies in a narrowband, with the counter rotating turbines performing slightly betterat low speeds, which leads to a higher torque for the same poweroutput. Cai [160] investigated axial counter rotating turbine lay-outs with and without guide vanes and considered the rotors to beat different speeds making the exit swirl component to be equal tothe swirl component at the inlet. It concluded that for a counterrotating turbine, the load capacity per unit engine length is muchhigher than that of normal turbine. Counter rotating turbines withand without IGVs was also investigated by Denantes and Bilgen[161]. They studied the counter-rotating turbines application inSCPP and determined the preliminary design parameters and theiroperating conditions. They modified the analytical model of VonBackstrom and Gannon [148] to accommodate layouts withcounter rotating rotors and to compare the performance of thesingle rotor with counter rotating rotors with IGVs. They foundthat the single rotor layout has a higher efficiency at the designpoint but a lower efficiency at off-design conditions.

4. Performance of SCPP

Several mathematical models have been developed since theinception of the Manzanares SCPP prototype in order to predictSCPP performance. Haaf et al. [65] present the model, which theyused for the design of the Manzanares pilot plant. Mullet [37]presented an analysis to derive the overall efficiency and sig-nificant performance data for SCPP and it was claimed thatnumerical values were consistent with the information fromManzanares pilot plant. Pasumarthi and Sherif [64] show a moredetailed model, which was verified against their experimentalresults [162] and results of the Manzanares pilot plant. Gannonand Von Backström [115] adapt standard gas cycle in defining astandard solar chimney cycle and compare the results from theirsimple model to experimental results of the Manzanares plant.Pastohr et al. [163] used FLUENT – a commercial computationalfluid dynamics (CFD) software package – to model the Manzanaresplant and compared the CFD result with result from their analy-tical model which was developed for the collector.

Hedderwick [116] modelled the SCPP and the total performancemathematically using energy and draught equations with respectto boundary conditions, which consisted of the environmentalconditions that were applicable at the reference location.

To enhance the performance of the SCPP, Kreetz [111] intro-duced the use of water-filled tight tubes or bags as heat storagedevice at the open solar air collector. Pretorius [41] and Pretoriuset al. [83] presented SCPP performance model and find that thetotal performance deteriorates with the presence of ambientwinds but chimney shadow does not significantly affect totalperformance. A more comprehensive SCPP performance modelwas presented by Pretorius and Kröger [85] which was an upgradeof the model of Hedderwick [116].

Bernardes [112] developed a comprehensive model of SCPPperformance and investigated the possibility and impact of usingwater-filled bags on the collector floor as heat storage device andfinds that its implementation smoothes out the daily fluctuation in


power output and also increases the power output after sunset butreduces the peak power output during the day.

Pretorius [42] presented a model of SCPP to include the impactsof ambient wind, various temperature lapse rates, nocturnaltemperature inversions and the use of the collector as a green-house or agriculture on the performance of the plant. As the totalperformance of the SCPP is a product of the performance of thecollector, turbine and the chimney, von Backstrom and Fluri [149]used simple analytical models to indicate the importance offinding the turbine pressure drop for maximum power output.Various types of soil for the collector ground and a variety of glassqualities for the collector roof were simulated by Pretorius andKröger [84] for their effect on plant performance.

Fig. 17. The use of water-filled tight tube as heat storage in SCPP [33,111].

Fig. 18. SCPP model of Pasumarthi and Sherif integr

4.1. Enhancement models of SCPP performance

One of the first proposed enhancements of the SCPP was theintroduction of water-filled tight tubes in the open-solar-air col-lector for heat storage. The water-filled tight tubes (Fig. 17) asthermal storage medium was introduced by Kreetz [33,,111]. Theidea was based on the principle that the specific heat capacity ofwater is much higher than that of soil and from natural convectionpoint of view, the heat transfer between water in the tube and thetube wall is much more efficient than that between ground surfaceand the soil layers underneath. The tubes or bags are filled withwater and tightened to avoid evaporation. The tubes are paintedblack to enhance solar radiation absorption and transfer of thegained thermal energy to the water during the hours of sunshine(Fig. 17A). During the night time when the air in the collector startsto cool down, the water releases the thermal energy it storedduring day time. The volume of water in the tank corresponding towater layer thickness is selected according to the desired char-acteristics of power output profiles during the day and night. Theanalysis as in Fig. 17B shows that with the water filled tight tubes,power can be generated in the night but with drop in the powergenerated during the hours of sunshine.

Pasumarthi and Sherif [64,162,164] developed SCPP modelwhich incorporates black canvas and absorber plate to enhanceenergy conversion at the greenhouse as shown in Fig. 18.

Bilgen and Rheault [165] proposed a SCPP with sloped solarcollector along a hillside and short chimney for high latitudelocation (Fig. 19) to contain the challenges associated with tallchimney. They claimed that the system had higher performancesfor locations at high latitudes than the use of traditional SCPP.

In order to reduce the associated cost of constructing the rigid solarchimney of SCPP, Papageorgiou [54,166–174] proposed a replacementof the rigid concrete solar chimneys with lighter than air inflated

ated with black canvas and plate [64,162,164].

Fig. 19. Schematic of the SCPP with sloped surface at high latitudes [165].

Fig. 20. Floating solar chimney model of Papageorgiou [54,166,174].

Fig. 21. Sketch of the proposed SCPP design for mountainous regions [176].

Fig. 22. Schematic diagram of SCPP with mountain hollow solar chimney: (a) topview and (b) vertical view [100].


fabric structures called Floating Solar Chimneys (FSC). The proposedFSC technology can be constructed for 1500–3000 m high chimneyusing polyester fabric as presented in Fig. 20. The lifting force is fromtoroidal tubes filled with less-dense than air gases, like He/NH3.Several patents have been granted to this invention in USA, Australia,EU, China, India and South Africa [170–174].

In a recent publication about the FSC, Papageorgious et al. [175]stated that using 5% of the existing desert or semi desert unusedland fields in all continents, the FSC can generate 50% of theelectricity demand in the areas. Serag-eldin [176] proposed the useof steep mountains or cliffs with expanded valleys as chimney andcollector ground respectively such that the chimney stack wouldbe replaced by a duct running up the mountain/cliff while thevalley serves as the collector area. His proposal is similar to that ofDubos as was presented by Günther [53]. The chimney is proposedto be inclined at 45° to a horizontal valley as shown in Fig. 21 withthe turbine(s) horizontally staged at the at the bottle-neck sectionof the chimney before the exit air is expanded through thedivergent chimney to the atmosphere. The work claims that theproposed design would cut cost on the construction of the chim-ney and also improve cycle efficiency.

Another novel design concept of the SCPP was introduced byZhou et al. [100] which consists a giant solar collector surroundinga hollow space excavated in a mountain. The giant hollow space isconnected to the exit of the solar collector as shown in Fig. 22where the solar radiation is absorbed and stored as heat in the soilof the mountain keeps the hollow warm thereby creating moresuction and buoyancy as the heat is released even in the night.Their analysis claimed that such concept provides safety andreduces cost of construction materials as compared to the con-ventional chimney structure.

Cao et al. [177,178] numerically modelled and studied theperformance of sloped SCPP which uses the mountain as the col-lector and also as part of the chimney height. The system as shownin Fig. 23 operates on same principle as the traditional SCPP.

Zhou and Yang [179] extended investigation on floating SCPPby stiffening the floating chimney to the side of mountain. Asshown in Fig. 24, the collector is the base of the mountain whichimplies that the selection of the collector location must be con-sidered based on the location. For their study it is a location in thenorthern hemispheres meaning the mountain foot for the collectorshould face south.

Alrobaei [96] proposed hybrid geothermal/PV/SCPP (Fig. 25) forprospective SCPP in the south region of Libya to enhance

Fig. 23. Schematic diagram of sloped SCPP [177,178].

Fig. 24. Novel solar thermal power plant with a floating chimney stiffened on amountainside, segment by segment [179].

Fig. 25. Hybrid geothermal/pv/SCPP [96].

Fig. 26. Intermediate secondary roof to enhance heat storage in the soil of opensolar air collector [42].

Fig. 27. Hybrid solar pond and SCPP by Davey [113].


performance and combat the night low energy production asso-ciated with SCPP. Pumps were employed to pump geothermal hotwater from underground and circulate it through pipes embeddedon the soil surface of the open solar-air collector thus heating upthe adjacent air to generate artificial wind for power generationwhile the PV generates DC electricity from the absorbed sunlightwhich is converted using inverters to AC power.

Pretorius [42] introduced an intermediate secondary roofunder the first roof of the SCPP collector (Fig. 26), which dividedthe collector into top and bottom parts. At the bottom section, heintroduced an airflow regulating mechanism to control the airmass flow between the intermediate collector and the ground. Airflow through the bottom section is effectively controlled by

increasing or decreasing the pressure drop across the regulatingmechanism. By regulating the air that flows over the ground sur-face, using the regulating mechanism, the plant can store andrelease energy from the ground to regulate the power output ofthe plant. When less power is required, the bottom section isclosed and energy is stored in the ground and when powerdemand is high, the bottom section is opened in a controlledmanner, producing air flow under the secondary roof and releas-ing the energy stored in the ground.

A conceptual application of hybrid SCPP and solar pond toproduce electrical power and distilled water at a site adjacent tothe sea was proposed by Wang et al. [180,181]. Another conceptualdesign that uses solar ponds as a supplementary thermal energyand storage of heat for SCPP was patented by Davey in 2008 [113]as shown in Fig. 27. This concept was adopted for the proposedlarge-scale EnviroMission's SCPP where saltwater ponds areplanned to sit outside the solar collector and trap heat in layers ofsaltwater during the day and at night the heat is released togenerate hot air and power the turbines. Solar radiation is cap-tured in the ponds and stored in the bottom level of the ponds

Fig. 28. Hybrid solar pond and SCPP by Akbarzadeh et al. [182].


such that the hot brine can be continuously extracted using heatexchanger at about 35–50 °C above ambient temperature tomaintain air velocity that can sufficiently generate power day andnight, throughout the year [32,74].

Similarly, investigation on hybrid SCPP with salinity gradientsolar pond was conducted by Akbarzadeh et al. [182] where twodifferent configurations, Fig. 28, were studied. Both configurationsused pumps to circulate the hot brine in the collector area andchimney base through a heat exchanger thus creating thermaldifference between the ambient air and the air in the SCPPgreenhouse.

More researches have been conducted on the hybrid SCPP withseawater desalination by Zuo et al. [183–190]. They developed anintegrated SCPP with seawater desalination. The integrated systemconsists of five major components; chimney, collector, turbine,energy storage layer and basin solar still as shown in Fig. 29.

Another investigation on enhancing thermal performance ofmodified solar chimney was proposed which employs flue gaswaste heat as a supplementary thermal energy for the system. Theproposed design used metal sheet as the absorber plate/collectorinstead of the soil. The absorber plate stands as a heat exchangerserving the purpose of extracting heat from hot flue gas that flowsunderneath the bottom part of the collector and in the top/frontpart of the absorber plate, which was housed by a transparentcover, it absorbs the solar energy to generate air in motion thatrotates the turbine at the chimney base to generate electricity. Thefirst laboratory model was designed and investigated experimen-tally by Al-Kayiem et al. [109] and simulated numerically by Al-Kayiem et al. [191], as in Fig. 30. The system had one absorberplate inclined at 45° and the flue gas was supplied by experimentalGas Turbine unit, as shown in Fig. 30a.

The system has been modified and extended for outdoorexperimentation, as shown in Fig. 31. The absorber plate and theflue channels were constructed to comprise two parts. The firstpart was inclined by 15°, while the second part was inclined by 45°and connected to chimney via conical diffuser at its upper outlet.The details of the design and the experimental results werereported by Aja [39] and patent number MyIPO: PI 2011001224[192].

Azeemuddin et al. [193] and Azeemuddin et al. [194] suggestedan enhancement technique using waste heat energy in the fluegases passing through conduits in the solar collector (Fig. 32). The

process of the heat and fluid flow was simulated using ANSYSsoftware and validated through comparison with Manzanaresprototype results. The simulated model showed good enhance-ment for the performance as well as it contributes to the reductionof global warming by reducing the flue gases temperature beforeexhausting to the atmosphere. The proposed hybrid techniquebefits to generate electricity 24 h.

Ninic and Nižetić [44] invented, and patent, a SCPP with shortdiffuser and low-temperature solar collector. Their inventionclaims to generate warm air at the low-temperature collectorwhich flows to the chimney base to forms a complex vortexstream and exit through a short diffuser (Fig. 33), in which thephenomenon is similar to a tornado funnel which they referred toas the gravitational vortex column (GVC). At the chimney base, theupdraft air rotates the turbine and exits the turbine in a circularupward swirl hence forming a sort of vertical fluid. They claim thatdue to the rotation, the pressure in the vertical swirl is lower thanambient air at the same altitude with the radial pressure differ-ence greater at the bottom of the vertical fluid swirl such that nearthe ground level, just above the turbines it displays a significantlylower pressure than the ambient. They patent also claimed thatthe generated vertical vortex stream field acts like an extension forthe solar chimney, which creates operating conditions favourableto the turbines.where S – solar collector, D – short diffuser, E –

spiral canals, F – diffuser deflector, GVC – gravitational vortexcolumn, 1 – glass cover, 2 – absorbing ground, 3 – air guides, 4 –

diffuser neck, 5 – diffuser body, 6 – diffuser exit opening, 7 –

deflector surface, 8 – circulation pumps, 9 – nozzles, 10 – axial airturbines, 11 – transmission mechanism, 12 – electricity generators,13 – canal with water moisturising the warm air, 14 – rainwatercollecting canal, 15 – warm air stream, 16 – cold air stream [44].

Kashiwa and Kashiwa [195] presented a conceptual utilisationof solar cyclone for harvesting fresh water from atmosphere whichalso was inferred in the conclusion that it can be utilised for powergeneration. The system is characterised by expansion cycloneseparator placed at the base of the chimney, along with a turbinegenerator, the greenhouse and the chimney as shown in Fig. 34.The expansion cyclone separator condenses and removes atmo-spheric water while the turbine harnesses the energy from themoving air.

On the quest for increased power output from renewablepower generation system, Yabuz [196] proposed and investigated a

Fig. 29. Schematic diagram of the integrated SCPP with water desalination [183–190].

Fig. 30. Modified inclined solar chimney integrated with flue gas source.(a) Experimental setup, (b) velocity vector as predicted by CFD simulation.[109,191].


hybrid utilisation of the SCPP coupled with a solar receivermounted at the chimney top and heliostat mirror situated somedistance away from the collector inlet base as shown in Fig. 35.

The solar receiver and the heliostat generate steam for steampower plant while the SCPP generates its energy from its princi-ples. The process was to utilise the chimney for dual purpose aspressure tube to the SCPP and tower for a solar receiver.

5. Cost modelling

The cost of construction and operation of any power plant isimportant in understanding the economic feasibility of the plant.For the SCPP, many researchers have presented some cost models.Schlaich [34] and Schlaich et al. [35] presented estimated costs forall components of SCPP for various plant sizes (5 MW, 30 MW and100 MW) and evaluated the levelised electricity cost (LEC) and alsoperformed sensitivity analysis of LEC to the interest rate and thelength of the depreciation period. In their model, the land wasconsidered to be free while the cost of the collector was con-sidered only for the glazing materials and the supporting struc-ture. The materials and the labour costs are dependent on thelocation and size of the plant. Bernardes [112] modelled the cost of

a SCPP with consideration to LEC and derived a parametric costmodel for the collector, solar chimney and the power conversionunit of a 100 MW capacity plant. The configuration of the plantswhich the researchers analysed the cost is presented in Table 3.

In Table 3, the calculated electrical output for the various plantsconfiguration was presented. The electrical power generated perannum varied from 281 GWh/a to 320 GWh/a for same plantcapacity. To cost of each plant is analysed in Table 4. The costanalysis and the associated LEC per kWh as presented by Fluriet al. [197] showed that there is great difference in the cost of theplants modelled by Schlaich [34] and Schlaich et al. [35] from themodel presented by Bernardes [112]. Based on the variation in theplants costs and the cost of electricity generated, Fluri et al.[197,198] also presented a cost model for the SCPP with inclusionof the impact of carbon credits on LEC which they compared theirmodel results with other SCPP cost model. Fluri et al. [197,198] puttogether the fundamental assumptions on the various plantspresent a well detailed cost analysis for the SCPP.

5.1. Components cost analysis

Fluri et al. [198] detailed the cost of the various components ofSCPP following cost assumptions shown in Table 5. The costassumptions may vary from location to location based on avail-ability of the materials and nearness to procurement. The modelpresented insight on materials and the estimate cost followingreports from previous models and simulation results of the per-formance of SCPP.

Fluri et al. [197,198] also did a comparative life cycle costanalysis for SCPP versus coal-fired power plant as presented in

Fig. 31. Hybrid solar flue gas chimney power plant [39,192].

Ground

Canopy

Chimney exhaust

Flue gas panels

Ambient air inlet Flue gas inlet

Flue gas re-injected in the chimney above the turbine outletto the collector

Fig. 32. Simulation model of a solar chimney integrated with flue gas panels[193,194].

Fig. 33. Schematic view of SCPP with short diffuser invented by Ninic and Nižetić[44].

Fig. 34. Schematic view of Solar Cyclone and the expansion cyclone separator[195].


Fluri et al. [198]. The report of Fluri et al. [198] showed anapproximately 2.5 times higher initial cost for the collector and thechimney than Bernardes [112]. However Fluri [40] showed that thematerial assumed in Fluri et al. [198] was too high. Fluri et al. [197]improved the model of Fluri et al. [198] and they compared theirresults to two selected reference SCPPs of 100 MW respectivelyproposed by Schlaich et al. [35] and Bernardes [112]. Fluri et al.[197] also evaluated the power output of the two selected refer-ence SCPPs using Pretorius [42] thermodynamic model and foundthat the peak power output for Schlaich et al. [35] and Bernardes[112] 100 MW reference SCPP were 66 MW and 62 MW respec-tively. With this peak power output, they evaluated the LEC forSchlaich et al. [35] 100 MW SCPP with the same economic para-meters (i.e., interest rate¼6%, inflation rate¼3.5%, and deprecia-tion period¼30 years) and got €0.270/kWh while originallySchlaich et al. [35] estimated the LEC per kilowatt-hour at €0.1. ForBernardes [112] 100MW SCPP, with economic parameters (interestrate¼8%, inflation rate¼3.25%, depreciation period¼30 years, andconstruction period¼2 years), using Bernardes's model, theyevaluated the LEC per kilowatt-hour as €0.43 which was originallyestimated by Bernardes [112] as €0.125/kWh. Pretorius [42] inchapter four of his PhD thesis also presents a simple cost model forthe SCPP using different chimney heights (500–1500 m) withvarying diameter and different collector diameters at varying inletheights.

Fig. 35. Hybrid Heliostat and SCPP for more energy production [196].

Table 3Configurations of 100 MW SCPP developed by Schlaich [1], Schlaich et al. [2] andBernardes [3] and the electrical power output per annum.

Configuration Schlaich [1] Schlaich et al.[2]

Bernardes [3]

Capacity, MW 100 100 100Chimney height (m) 950 1000 850Chimney diameter (m) 115 110 110Collector diameter (m) 3600 4300 4950Glass roof height at inlet (m) 6.5 3 3.5Electricity output/Annual poweroutput (APO) (GWh/a)

305.2 320 281

Table 4Cost comparison of 100 MW SCPP from Schlaich [1], Schlaich et al. [2] andBernardes [3] models.

Configuration/process Schlaich [1] Schlaich et al.[2]

Bernardes [3]

Euro Euro Euro

Collector cost 134.8 131.0 190.0Chimney cost 68.2 156.2 64.4Power conversion unit cost 79.8 75.0 76.7Roads, buildings, workshops 2.0 n/a 2.71Infrastructure 4.3 n/a 6.5Planning, site management 2.9 n/a n/aRounding 8.1 n/a n/aEngineering, tests, misc. n/a 40 n/aElectrical installations n/a n/a 10.5Insurance n/a n/a 1.58Overall investment cost 300.0 402.0 352.4Operation and Maintenance cost(1st year), (Million Euro)

1.0 1.9 1.0

Depreciation period (y) 20 30 30Construction period (y) n/a n/a 2Real interest rate,% n/a 6 8Nominal interest rate (%) 8 n/a n/aInflation rate (%) 3.5 n/a n/aLEC (Euro/kWh) 0.1045 0.1000 0.0370

Table 5Cost assumptions guide to the analysis of SCPP components [4].

Components cost Price in Euro

Collector Glass cost/m2 € 11.40Collector canopy structural support costSteel columns (IPEAA120)/m € 8.66Cable guys/m € 1.84Angle beam/m € 12.75Concrete cost/m3 € 100.00Reinforcement steel cost/ton € 750.00Reinforcement quantity/m3 € 7.50Cover cost – horizontal/m2 € 21.88Cover cost – vertical/m2 € 15.63Chimney partHigh performance concrete cost/m3 € 125.00Normal concrete cost/m3 € 100.00Reinforcement cost/ton for 120 kg/m3 € 750.00Construction, concrete cover and labour cost/m3 € 312.50Materials hoisting cost/ton € 250.00Ring stiffener cost/ton. € 1250.00Construction and labour on the ring stiffeners andhoisting of ring stiffeners/link

€ 250.00

The transport cost/ton over an average distance of300 km

€ 21.00

Material and construction costs of chimney supportcolumn with 12 shear walls of 50 m�8 m�0.5 m/column (foundation)

€ 18.45 million

Chimney shell for 110 m diameter chimney 4� cost of founda-tion structure

Power conversion unit for 66 MWTurbine cost € 2.82 millionCentral structure € 0.25 millionDucts € 2.42 millionSupports € 0.10 millionGenerators € 3.95 millionPower electronics € 1.95 millionControl € 0.18 millionBalance of station € 15.23 millionCapital cost of CPU installation (total) € 26.87 million


5.2. Electrical power cost model

The modelling of electrical power cost of SCPP takes intoaccount the plant operation and maintenance cost. The operationand maintenance cost of the plant is dependent on the size of theplant. Following the report of the cost comparison of 100 MWSCPP as shown in Table 4, the operation and maintenance cost forthe various plants varied in the range of 1.0 M€ to 1.9 M€. Con-sidering interest rate, i, inflation, f, and design depreciation life ofthe plant, N, the present equivalent value of the operation andmaintenance cost of the plant (POM) over the life time can be

evaluated using Eq. (11) [199].

POM ¼ A1

f �11þ f1þ i

� �N

�1

" #ð11Þ

where A1 is the cash flow at the end of the first yearThe understanding of the present equivalent value of the

operation and maintenance cost of the plant over its life time will


aid the understanding of the present total value of the plant. Thusthe equivalent annual cost over the plant life time, A, can bedetermined using Eq. (12) considering the present value of all costrelating to the plant [199]

A¼ Pi 1þ ið ÞN1þ ið ÞN�1

" #ð12Þ

In the analysis presented by Bernardes [112], a consideration ofthe construction year was a factor in the determination of theinvestment cost of the plant with the assumption that the loan forthe construction of the plant was taken before the start of theconstruction thus, the capital cost will include the interest ratepaid till the end of the construction. Including the interest rateincurred through the construction period, the future equivalentvalue of the plant, F, on completion can be evaluated using Eq. (13)

F ¼ P 1þ1ð ÞN ð13ÞThe levelised electricity cost (LEC) of the plant can be deter-

mined by dividing the equivalent annual cost of the plant by theannual power output (APO) as presented in Eq. (14)

LEC ¼ AAPO

ð14Þ

The SCPP being an environmental friendly energy system canbe considered beneficial by evaluating the LEC with considerationof the carbon credit impact of the plant [197]. Following theimpact of coal-fired power plant which emits 0.95 kg of CO2/kWh[200] in analysing the LEC of SCPP electrical power output, the CO2that can be reduced by using SCPP as power system will bedetermined by multiplying the annual power output (APO) of SCPPby the amount of CO2 emitted per kWh using coal-fired powerplant. Thus the CO2 emission reduced by using SCPP is as shown byEq. (15)

_CO2� reduced ¼ APO 0:95� 106kg=GWh� �

ð15Þ

Considering Eq. (15), the benefit earned from the carbon creditwhich a reduction in one ton of CO2 is valued at about €27.00[201]. The LEC evaluation with the benefits earned from carboncredit can be evaluated as Eq. (16)

LEC ¼ A

APOþ APO� 27� 0:95� 106� � ð16Þ

Other researchers that presented cost modelling of SCPPinclude Gholamalizadeh and Mansouri [98] who presented anapproximate cost model to illustrate the thermo-economic opti-mal configurations of the pilot SCPP in Kerman, Iran. Nizetic et al.[202] presented an approximate costs analysis of a SCPP of 550 mchimney height, 82 m chimney diameter and 1250 m collector roofdiameter at Mediterranean region, which included a total invest-ment estimate and the levelized electricity cost. The parametersconsidered in the cost analysis include inflation rate of 6.0% p.a.,maintenance and repair costs of 5.5% p.a., interest rate of 6.0–10%p.a., under 20–40 years amortisation period. The cost structureanalysis showed that the chimney bears approximately 30–50% oftotal costs, the collector roof constitutes about 20–40% of theexpenditures but if the collector roof is made of special plasticfilm, the investment is reduced by about 30% compared to thecosts associated with the traditional glass covering. Depending onthe orientation of the collector roof, the price of single glass roofamounts to 6.0–9.0 €/m2, for a reinforced concreted chimney, thecost amounts to 250–500 €/m2 and for the turbine which dependon the nominal power of turbine, it was estimated for 200 MWturbine, amount to 700 €/kW, while, 5 MW cost about 1600 €/kW.They concluded that the average price of electrical energy

produced by a SCPP was 0.24 and 0.78 €/kWh which was con-siderably higher compared to the other power sources.

Cervone et al. [203] conducted a basic economic analysis offloating SCPP of collector diameter 2700 m, chimney height2500 m, and chimney internal diameter 100 m for a location in theMediterranean region focus on Sicily, Italy. They concluded thatthe theoretical energy production was 302 GWh/year, but maydrop by 13.6% under wind influence. The construction cost wasestimated at 222-351 million euro and the energy production costat 18.4–29.0 euro cent.

In a recent publication about the floating solar chimney,Papageorgious et al. [175] discussed a scale analysis of the systemusing a simplified cost model and inferred that the system cangenerate energy at lower cost of approximately 45 USD/MWhusing a moderate height FSC structures of 650 m.

Cao et al. [204] performed an economic analysis and comparedthe suitability of conventional SCPP with sloped SCPP in Lanzhou,Northwest China. In their study, they considered the influence ofinvestment cost, expected payback period, inflation rate, andlevelised electricity cost (LEC). The results reveal that SCPPinvestment is influenced by the plant configuration and the priceof the construction materials. The sloped SCPP was more cost-effective than the conventional SCPP of same size and poweroutput during the system life span. This is due to the reducedvolume of material when associated with the slope chimneystructure as the chimney is designed to lean on high-mountain.They also inferred that large scale SCPP holds good competitive-ness with conventional fossil fuel power plants.

Aja et al. [38] presented analysis of the economic benefit ofincline SCPP integrated with flue gas waste heat. The resultsshowed that the plant can be established in cities while the landcan still be used for other purposes. It also showed that with theinclined collector, the chimney height will be reduced but analysisdid not present financial evidence to the claim.

6. Conclusions

The total efficiency of SCPP is low (less than 2%) which is aproduct of the collector efficiency, chimney efficiency and theturbine efficiency. The collector efficiency varies with respect tothe collector material (soil type/absorber plate material andtransparent cover material), location, available solar radiation andthe slope of the collector. The open solar-air collector accounts forabout 50% of the investment cost of a SCPP and about 30% of theoverall system losses [112]. Improving its performance offers thepotential to make the SCPP cost competitive. The chimney effi-ciency has been found to be mostly dependent on the chimneyheight. Most research has focused on structural design of thechimney while a few numerical investigations have been con-ducted on varying shapes of the chimney. Conventionally thestraight chimney with uniform cross sectional area has beenmostly promoted which is based on the gigantic height associatedwith the commercial SCPP. Chimney performance enhancement israrely reported in literature. The turbine performance is estimatedto be in the range of 60–80%. This has only been supported withone experimental investigation [147] while others are analyticaland numerical investigations.

Acknowledgements

The authors would like to acknowledge Universiti TeknologiPETRONAS (UTP) (STIRF 24/ 07-08) for the logistic and financialsupport to produce and publish the paper. The second author


highly appreciates UTP support to carry out his PhD study underthe Graduate Assistance (GA) scheme.

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