2nd WSEAS/IASME International Conference on SOURCES (RES'08) Corfu, , October 26-28, 2008

FLOATING SOLAR CHIMNEY TECHNOLOGY FOR

Christos D. Papageorgiou Associate Prof National Technical University of Athens Nymfon 1b, 14563 Athens Greece

Abstract: - Solar chimney technology is a very French president Nicolas Sarkozy political initiative promising solar thermal electricity generating for a closer cooperation of MENA countries and EU. technology. Solar chimney power plants have three The solar chimney power plants are usually referred major parts. A large circular greenhouse, a tall as solar updraft power plants cylinder in the center of the greenhouse named solar http://en.wikipedia.org/wiki/Solar_updraft_tower chimney and a set of air turbines, around or in the and their proposed solar chimneys are reinforced solar chimney, geared to appropriate electric concrete structures. A low cost alternative of the generators. concrete solar chimney is the The technology is appropriate for desert or semi Chimney (FSC) www.floatingsolarchimney.gr. The desert areas with high solar irradiation and limited solar chimney power plants, due to their similarity to strong winds or sand storms. The technology is cost hydroelectric power plants, were named by the competitive to any other solar technology (PVs or author Solar Aero Electric Power Plants (SAEPPs). CSPs) and does not demand any water for its In the previously mentioned sites there are a lot of operation. Due to the ground thermal storage, the references related to the solar chimney technology. technology it is generating a continuous electric The solar chimney technology was experimentally power output 24x365, thus can enter to the electric tested in Manzanares of Spain, where a small grid at least up to 50%. prototype of 50 KW was built in 1982 and All these benefits are making the FSC technology successfully tested for 6 years, by the team of Prof the most appropriate technology for the desertec J. Schlaich. Part of the results by the operation of project. The desertec project (www.DESERTEC.org this small demo is appearing in the book [1] and in ) is proposing the construction of a HVDC electric the reference [2] . grid, connecting Europe with MENA area. Solar A thermodynamic cycle analysis of the solar electricity could be generating in MENA area and chimney power plant operation was given by Prof transmitted to Europe though the HVDC grid. The Backstrom and his associates in a series of papers desertec project is supported by French president [3,4,and 5]. Nicolas Sarkozy political initiative for a closer Floating solar chimney technology was presented by cooperation of MENA countries and EU. the author in a series of papers [6,7,8,and 9]. Most recently the author presented a paper [10] for the Key-Words: - Floating Solar Chimney Desertec application of the FSC technology in desert areas of project China with adequate horizontal irradiation. Similar desert areas, even with higher solar irradiation, exist 1. Introduction in USA, South America, Australia, south and North Africa and India.

It is estimating that with FSC technology, operating The purpose of this paper is to present the Floating on 1% efficiency and using a 2-3% of the existing solar chimney (FSC) technology and to give the unused desert or semi desert areas, we can generate important benefits of this technology that make it the at least 50% of the world electricity demand. proper candidate for the Desertec project. According In European countries there are places of adequate to desertec the huge desert or semi desert areas of solar horizontal irradiation but the land is very the Middle East and North Africa (MENA) countries expensive because there are no desert or semi desert can be used for solar electricity generation that could areas. However in the nearby to Europe MENA be transmitted to European countries though a countries there are huge unused desert or semi desert HVDC grid. The desertec project is supported by

ISSN: 1790-5095 216 ISBN: 978-960-474-015-4 2nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 26-28, 2008

areas with appropriate solar characteristics that can generators that transform to electricity a part of the be used for solar electricity generation. This solar thermodynamic energy of the moving air mass. electricity can be transmitted to the European The floating in the air, lighter than air, “Floating electric grid through an appropriate HVDC or Solar Chimney” (FSC) is a low cost alternative of UHVDC electric grid. the reinforced concrete solar chimney structure. The Floating Solar Chimney technology is the FSCs can easily be constructed to heights up to 1 appropriate technology for the desertec project, the Km. The figure (1) is representing the FSC power main reasons are: plant and its operation. a. The technology is cost competitive to any other solar technology. This means that FSC technology can generate electricity in much lower direct cost per produced KW in comparison to any other solar electricity generating technology. b. The FSC technology is operating continuously (24x365) thus can replace fuel consuming base load power plants in the destination countries. c. The technology demands no water for its operation, as for example demands the concentrating plants (CSPs) for cleaning and cooling of their mirrors. Clean water is very valuable in desert or semi-desert areas of MENA.

Figure1.Floating Solar Chimney Power Plant in 2. Floating Solar Chimney (FSC) operation technology presentation Due to its patented [11] construction the FSC as a A Floating Solar Chimney (FSC) power plant is free standing lighter than air structure is bending made of three basic parts: when external winds appear as shown in the figure(2). • A large solar collector with a transparent

roof supported a few meters above the ground, open at its perimeter (the greenhouse). • A tall lighter than air cylinder in the center of the solar collector (the Floating Solar Chimney ) Direction of Wind • A set of air turbines geared to appropriate electric generators placed in a circular path Main around the FSC (the turbo-generators) Chimney The solar irradiation warms the ground below the made of roof of the greenhouse and consequently the air parts inside it. The warm air becomes lighter than the Heavy ambient air and tends to escape though the solar Mobile Base Chimney chimney, up drafting to the upper atmospheric Seat Folding Lower layers. New ambient air is entering in the Part Greenhouse through its open periphery that, as is moving towards the FSC, becomes warm by the Fig 2. Schematic diagram of the FSC solar irradiation and is also up drafting through the FSC etc. Thus the first two parts of the FSC power A small part of this cylinder is shown in the next plant form a huge passive thermodynamic machine figure(3). circulating the air from the ground to the upper As shown in the figure the FSC is made by a series layers of the atmosphere. In the path of the airflow of of the warm air are placed appropriate air turbines, successive tubular balloon rings made of fabric. with inlet guiding vanes, geared to electric

ISSN: 1790-5095 217 ISBN: 978-960-474-015-4 2nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 26-28, 2008

to electricity through their air turbines geared to their appropriate electric generators. Furthermore both power plants efficiencies are proportional to their Rin g heights (falling water height or up drafting air height). In fig. (4) The annual efficiency of a typical SAEPP is shown as function of its FSC height.

SAEPP of 4sqKm solar collector in a place of annual solar irradiation 1750KW/sqm Balloon 2.5 with gas

2 Compre fab ssed air ric 1.5

Fig 3. A small part of the fabric cylinder of the FSC % efficiency 1

The polyester fabric of the tubular rings and the rest 0.5 parts of the FSC, is similar to the polyester fabric already used for the construction of air balloons or 0 airships. An extensive presentation of “light” 400 600 800 1000 1200 1400 1600 1800 2000 variable height of Floating Solar Chimney in m of internal diameter 60m structures is given by Prof Beukers in [11]. These Figure 4. Annual efficiency of a typical SAEPP as tubular balloon rings can become lighter than air function of its FSC height containing special balloons filled with lighter than air gas (He or NH3). In order to keep the rigidity of The annual efficiency is defined as the ratio of the the structure the balloon tubular rings should be over produced electricity in KWh to the annual solar pressed with ambient air. Thus the whole fabric irradiation arriving on the greenhouse roof. For cylinder can not be deformed by external winds or example if in the place of a installation of a SAEPP by the operational sub pressure and can be a free the annual horizontal irradiation is 2000 KWh/m2 standing lighter than air structure. Through this free and the greenhouse of the SAEPP has a roof of 4 standing cylinder the warm air of the greenhouse is Km2 (4 million m2), 8000 GWh/year irradiation solar up drafting. When external winds appear the energy is arriving on its roof. If its FSC height is structure is bending due to its inclining special 900m than approximately by the diagram its patented heavy base. Of course its up drafting efficiency is 1.0 % thus the annual electricity operation is not interrupted by the inclining position production is 80 GWh/year. of the structure, however the operating height of the The annual efficiency, see J. Schlaigh in [1] and C. solar chimney it becomes smaller. The external Papageorgiou in [8], can be estimating, as a product winds, for a properly dimensioned FSC, have a of three efficiencies, the efficiency of the greenhouse marginal effect on its average annual operating estimated to 55%, the efficiency of the Turbo height see ref [7]. generators estimated to 80% and the efficiency of the FSC estimated to 2.6% per Km height of the 3. Solar Aero Electric Power Plants FSC. That is why the overall SAEPP efficiency for a (SAEPPs) main characteristics Km FSC is about 1.15 %. However by theoretical analysis, not yet published The FSC power plants named by the author as Solar by the author, the greenhouse efficiency is achieved Aero Electric power plants (SAEPPs) are similar to only if there is a double glazing roof. The inner hydroelectric power plants. In hydroelectric power glazing could be made of a thin crystal clear plastic plants the dynamic energy of the falling water, due sheet, hanged below the outer strong glazing of the to gravity, is partly transformed to electricity roof. For single glazing roof, as calculated by the through water turbines geared to appropriate electric analysis of Bernades et al [12] the greenhouse generators. In the SAEPPs the dynamic energy of efficiency is not more than 40%. the warm air, due to buoyancy, is partly transformed

ISSN: 1790-5095 218 ISBN: 978-960-474-015-4 2nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 26-28, 2008

Due to the ground thermal storage Bernades [12] and MENA area a cost optimal prototype SAEPP should Pretorius [13] have shown that the SAEPP can have the following dimensions: operate all year round 24 hours per day. Typical • A square solar collector of 2 Km side and daily operation curves for an average day of the year surface area of 4 Km2 is shown in the fig.(5), with and without artificial • A Floating Solar Chimney of ~900m height thermal storage and of 64 m internal diameter (and ~70 m external diameter) SAEPP of 4MW ,DD=1000m,H=700m,d=34m,Wy=1750KW/m2 • A set of several air turbines, geared to 180 appropriate electric generators of 20 MW 160 overall rating power output

140 ground only This SAEPP will generate more than 80 GWh of plus tubes electricity yearly. The construction cost of this 120 SAEPP will not be higher than 40-48 million EURO. 100 Thus the construction cost per yearly produced KWh

80 is approximately 0.5-0.6 EURO. The onshore wind turbines they have a similar figure 60 (0.5-0.6 EURO) for their investment cost per yearly 40 produced KWh. Assuming that SAEPPs and onshore produced power % and solar irradiation % 20 wind farms should have almost equal operation and maintenance costs, they should have an almost equal 0 0 5 10 15 20 direct production cost per produced KWh. However solar time in hours the SAEPPs are prevailing of wind turbines because they are generating a continuous electric power Figure 5. Typical daily production curves of the profile while the wind turbines intermittent. SAEPP ACCIONA Energy will build two concentrating solar power plants (CSPs) of 50 MW capacity each, Care should be taken for the correct evaluation of in Palma del Río (Córdoba, southern Spain) The the inner diameter of the FSC in order the SAEPP to facilities represent an investment close to 500 operate properly. For a rough estimation of the million EURO and their entry into service is planned proper FSC diameter an air speed inside the FSC of for 2010. The CSP plants will produce 244 million 10 m/sec should be assumed for the summer KWh per annum. By the figures, the investment cost operation of the SAEPP. of the CSP plants is estimated to 2.0 EURO per In ref. [6] the author has proposed an algorithm yearly produced KWh. This means that CSPs will through which the produced average electric power have a much higher direct KWh production cost in by the SAEPP can be calculated, as function of its comparison to SAEPPs KWh. Furthermore the mass flow, given the dimensions of the SAEPP and SAEPPs does not demand any water as the CSPs and the annual solar irradiation of its place of are generating a continuous electric power profile installation. A short presentation of this analysis is while the CSPs intermittent. Thus the superiority of appeared in Appendix I . the SAEPPs in comparison to CSPs is obvious. An optimal operation of the SAEPP, can be If the FSC heigh of the model SAEPP will be achieved by the proper control of the inlet guiding limited to 250 m, the rating power of the model vanes blade pitch of its air turbines see ref. [5]. SAEPP will become ~5 MW, its construction cost ~36 million EURO and its annual electricity 4. An optimized FSC technology power generation ~18 GWh (leading to a construction cost plant prototype for the desertec of ~2.0 EURO per yearly produced KWh). Even in this case where the construction costs of the SAEPPs and CSPs are the same, the rest benefits of the By the previous description it is evident that the FSC SAEPPs (no water demand and continuous electric technology power plants (SAEPPs) demands 2 power profile) make SAEPPs superior to the CSPs horizontal square lands of several Km surface areas for desertec and any other similar application. and floating solar chimneys of high height and In an area of 20 Km x 20 Km a farm of 100 similar proper internal diameters. Following the model SAEPPs can be built generating yearly 8 approximate analysis of appendix I and assuming an average annual solar irradiation of 2000 KWh/m2 in

ISSN: 1790-5095 219 ISBN: 978-960-474-015-4 2nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 26-28, 2008

TWh of electricity with a rating power of 2 GW, that • An air turbine of ~22 m diameter with a will be available in summer noon. rotor of 12÷16 blades and inlet guiding For Greece, for example, a set of four such farms in vanes (stator) appropriate areas, in one or two adjacent MENA • A four pole induction generator of 1 MW countries, could generate and provide to the Greek • A gear box to adjust the rotating frequency electric grid, through a set of UHVDC lines, yearly of the air turbine to the grid frequency of up to 32 TWh (50% of its annual electricity demand) estimated transmission ratio ~60 RPM/1500 with a maximum power of 8 GW supplied in RPM summer noon of high electricity demand. • An electric transformer of ~1 MW to adjust However in order to prove the viability and the cost the output voltage of the generator to the effectiveness of the FSC technology a demonstration grid voltage SAEPP of 1 MW that could be installed in a south The demo SAEPP will operate 24 hours per day 365 European country is necessary. days per year. If necessary its continuous operation could be secured by a set of tubes filled with water 5. Initial dimensioning of a placed on the ground of the inner part of the demonstration pilot SAEPP of 1MW greenhouse (artificial thermal storage). The model SAEPP output average daily electric For a d emo project SAEPP in a south European power will be proportional to the daily horizontal country the following objectives should be fulfilled: solar irradiation in the area, while its daily power • Its power rating should be at least one MW profile will have a minimum near the sun rise and a producing several million KWh per year in maximum after the noon. order to prove its importance as a renewable alternative technology. 6. Conclusion • Its solar collector surface area should be ~500000 m2 (50 hectares), in order to prove In the paper a short presentation of the FSC its low construction cost and its ability to technology and its respective power plants withstand any possible external adverse (SAEPPs) was given. conditions (strong winds, rain, snow etc). The SAEPPs demand no water for their operation • Its Floating Solar Chimney should be 400m and produce a continuous electric power profile ÷ 500 m high, in order to prove its ability to 24x365. withstand any external conditions (winds, Their construction cost per yearly produced KWh is rain, possibly snow, thunderstorms etc) and approximately 0.5-0.6 EURO. its easy handling and maintenance. In comparison the concentrating solar power plants Taking all these in consideration a pilot SAEPP (CSPs) they have a construction cost of ~ 2.0 EURO should h ave the following dimensions: per yearly produced KWh (four times more 2 expensive than SAEPPs). The CSPs demand water • Solar collector area 0.5 Km • Solar collector roof height minimum 2 m for cleaning and cooling their mirrors and produce a • FSC height ~450 m intermittent electric power profiles. The desertec project is proposing the construction of • FSC internal diameter ~24 m (using lifting a HVDC electric grid, connecting Europe with tubular balloons of 3 m diameter, its MENA area. Solar electricity could be generating in external diameter will be ~30 m) MENA area and transmitted to Europe though the Assuming that in the area of installation of the demo 2 HVDC grid. The desertec project is supported by SAEPP an annual solar irradiation of 1700 KWh/m , 2 French president Nicolas Sarkozy political initiative. an average irradiance G =200 W/m is used for the av By the comparison it is evident that the proper solar calculating procedure of the appendix I. technology for desertec project is the FSC The o u tput data of the calculating procedure of technology. appendix I with the previous data are: A model SAEPP of 20 MW for desertec and a pilot • Annual production by the SAEPP ~ 4.0 SAEPP of 1 MW are dimensioned. GWh The demo SAEPP has enough power output that is • Rating power of the SAEPP ~ 1 MW necessary in order to prove the cost effectiveness The electricity production unit could be a set and the advantages of the FSC technology for its composed of: large-scale application. Taking into consideration

ISSN: 1790-5095 220 ISBN: 978-960-474-015-4 2nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 26-28, 2008

that there is an urgent demand for solar renewable T4 is the appropriate root of the polynomial energy in Europe in order to meet the demands of equation: policies for greenhouse gases elimination, I hope 4 3 2 wTwTwTwTw =++++ 0, where w1, w2, that the proposal demo SAEPP project will be 41 42 43 544 supported by the market and the states in the area. w3, w4, w5 are given by the relations: 2 21 −= kCw )1( Appendix I = 2 − − TCnkCw ΄ 22 ( T 42 )

= − + − 211 TCnkCCw ΄ An approximate procedure for the derivation of the 323 ( ) T 42 equation describing the operation of the SAEPP i.e. = − TnCw ΄ 1+ CC −= ΄CTnw 34 T 4 ( 21 ) , 5 T 14 , the electric Power Output P as function of the where: moving air mass flow m has been derived by the = ⋅ / CHgC , ′ −⋅= /1 TCTT author in ref [6]. 1 p 034 ()01 2 A short presentation of the results of this analysis is 2  ch 4 ⋅⋅⋅⋅= CpAmRaC p )2/())/(( given below. = −1 + CnTC , π ⋅= dA 2 4/ The derived equation is the following: 33 ( To ) 1 ch 2 5.3 p  ( ⋅−−−⋅⋅= TCTCTmCP 424103 ) 4 o −= TCpp 01 )/1( and: 0 0 2 Where T03 (in K) is the entrance stagnation air R=287 J/Kg C, g=9.81 m/sec 0 temperature in the air turbines and m the warm air and Cp=1005 J/Kg C. mass flow in Kg/sec. T03 is also the exit air temperature by the solar collector thus can be - p0 is the ambient atmospheric pressure defined exclusively by the solar collector thermal - ηT is the overall efficiency of the air turbines and analysis given the mass flow m . generators - k is the FSC’s friction loss coefficient and An approximate procedure for T03 calculation is - α kinetic energy correction coefficient. given by Shlaigh in ref [1]. An approximate equation 0 Average values for T0 and p0 are T0=296 K and relating the exit solar collector air temperature T03 to p0=101300 Pa. A usual value for α is 1.1058. its input air temperature T02 valid for the circular Solar Collector is given by: An average value for ηT is 0.8, for a well-designed air turbine operating around its optimum point of ta·G·Ac= m ·Cp·( T03 - T02) + β·Ac· (T03-T02) - β is the approximate thermal power losses operation. coefficient of the Solar Collector (to the An average value for k is given by the formula: environment and ground) per m2 and 0C of the k=0.25+0.012·H /d. The necessary data for the calculation of the average temperature difference (T03-T02) . An average value for β is ~3.8÷4 W/m2 /0C (for double glazing solar Electric Power Output Pav as function of mass flow collectors). m are the following figures: The average annual solar horizontal irradiation Wy - G is the horizontal irradiance on the surface of the 2 solar collector. The average solar horizontal (in KWh/m ) And the dimensions: irradiance Gav is given by: Gav=annual horizontal irradiation in the place of installation of the SAEPP, H= FSC’s height in m (in KWh/m2) divided by 8760 hours d= FSC’s internal diameter in m - ta is the average of the product: {roof Dc= Solar Collector’s outer diameter in m. transmission coefficient for solar radiation X soil The diameter of theAir Turbines is calculated absorption coefficient for }. An average approximately by the formula:

value for the coefficient ta for a double glazing roof rt = / Ndd rt where Nrt is the number of the air is ~ 0.70 . turbines around the bottom of the FSC. - Ac is the Solar Collector’s surface area. The entrance height in the outer diameter of the solar Thus an approximation for the function T03 ( m ), is: collector is ~2 m and the inner height of it is not less   T03 ( m )= [ ta·G / (β +m ·Cp/Ac) ] –T02 than drt. In an inner wall higher than drt the Nrt air Where T02 is, approximately, equal to the ambient turbines are placed with horizontal axis, geared to 0 temperatureT0 (in K), plus 0.5 degrees (due to their respective induction generators, around the ground thermal storage around the Solar Collector). bottom of the Floating Solar Chimney. This inner

ISSN: 1790-5095 221 ISBN: 978-960-474-015-4 2nd WSEAS/IASME International Conference on RENEWABLE ENERGY SOURCES (RES'08) Corfu, Greece, October 26-28, 2008 wall around the FSC should have a diameter bigger Solar Energy Engineering, August 2006, Vol 128 than (drt•Nrt)/π. pp.302-311.

References: [1] Schlaich J. 1995, “The Solar Chimney: Electricity from the sun” Axel Mengers Edition, Stutgart. [2] Schlaich J. e.al 2005, “Design of commercial Solar Updraft Tower Systems-Utilization of Solar Induced Convective Flows for Power Generation” Journal of Solar Energy Engineering Feb. 2005 vol 127, pp. 117-124 [3] Gannon A. , Von Backstrom T 2000, “Solar Chimney Cycle Analysis with System loss and solar Collector Performance”, Journal of Solar Energy Engineering, August Vol 122/pp.133-137. [4] Von Backstrom T, Cannon A. 2000, “Compressible Flow Through Solar Power Plant Chimneys”. August vol 122/ pp.138-145. [5] Gannon A. , Von Backstrom T 2003, “Solar Chimney Turbine Performance”, Journal of Solar Energy Engineering, February Vol 125/pp.101-106. [6] Papageorgiou C. 2004 “Solar Turbine Power Stations with Floating Solar Chimneys”. IASTED proceedings of Power and Energy Systems, EuroPES 2004. Rhodes Greece, july 2004 pp,151- 158 [7] Papageorgiou C. 2004, “External Wind Effects on Floating Solar Chimney” IASTED Proceedings of Power and Energy Systems, EuroPES 2004, Conference, Rhodes Greece ,July 2004 2004 pp.159-163 [8] Papageorgiou C. 2004, “Efficiency of solar air turbine power stations with floating solar chimneys” IASTED Proceedings of Power and Energy Systems Conference Florida, November 2004, pp. 127-134. [9] Papageorgiou C. 2005 “Turbines and Generators for Floating Solar Chimney Power Stations”. IASTED Proceedings of Power and Energy Systems, EuroPES conference Benalmadena Spain June 2005 [10] Papageorgiou C. 2007 “floating solar chimney technology- a solar proposal for china” Proceedings of ISES, Solar World Congress 2007, Beijing, China, 18-21 September 2007, Volume I, pp.172- 176 [11] Beukers A., Hinte E van. 1998 “Lightness: The inevitable renaissance of minimum energy structures” Amsterdam: 010 Publishers. [12] Bernades M.A. dos S., Vob A., Weinrebe G., 2003 “Thermal and technical analyses of solar chimneys” Solar Energy 75 ELSEVIER, pp. 511-52. [13] Pretorius J.P., Kroger D.G. 2006,“Solar Chimney Power Plant Performance“, Journal of

ISSN: 1790-5095 222 ISBN: 978-960-474-015-4