Article 7 IMECE2011-65819 Solar

1 Copyright © 2011 by ASME

Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011

November 11-17, 2011, Denver, Colorado, USA

IMECE2011-65819

VIRTUAL HEIGHT AIDED SOLAR CHIMNEY: A NEW DESIGN

Khaled I.E. Ahmed Mechanical & Industrial Engineering Department,

Faculty of Engineering, Qatar University, Doha (2713), Qatar.

[email protected]

Ali K. Abdel-Rahman

Mechanical Engineering Department, Faculty of Engineering,

Assiut University, Assiut (71516), Egypt. [email protected]

Mahmoud Ahmed



Wael M. Khairaldien



ABSTRACT

Renewable energy source deployment is growing rapidly as it reduces CO2 emissions and increases diversity and security of supply. Solar chimney (SC) is a promising large-scale power technology, which absorbs solar radiation and converts parts of solar energy into electric power free of CO2 emissions. A major problem of Solar Chimney Power Plant (SCPP) is its low conversion efficiency as determined by the thermal performance of the system. However, the conversion efficiency of SCPP significantly increases with the SC height increase. The current paper proposes a new design of a virtual height aided solar chimney. In this new system the solar chimney is aided with a passive cooling system at the top of the chimney and a passive solar heater at its base to virtually mimic larger heights of the chimney. The new design has been simulated numerically for development and optimization. The numerical study is done in two stages to examine this concept. In the first stage, numerical results are obtained for the effect of the chimney height on the inside air flow speed. Then, in the second stage, the effect of decreasing the temperature at the chimney exit and the effect of increasing the temperature at the chimney base on the air flow speed are examined separately for small chimney heights. Then the combined effect of the two actions is investigated at a wide range of chimney heights. The numerical results have shown that the localized base heating and exit cooling have significantly enhanced the chimney performance for chimney heights up to 500m. A chimney with height of 300m gains an increase in the air velocity more than 25% due to the heating and cooling actions. Virtual height aided Chimney with original height of 300m acts similarly to a conventional chimney with height of 500m due to the effect of

base heating and exit cooling actions. This air flow velocity increase reflects 100% increase in the expected generated electric power. Further detailed results are presented and discussed. INTRODUCTION

In Qatar and despite of low average wind speed for on-shore energy harvesting, off-shore can be a good alternative. The wind energy becomes visible if the solar power chimney concept is employed in Qatar. Qatar is blessed with more than 2200 kWh/m2 annual solar radiation as shown in Fig. 1. Solar chimney (SC) power technology is a promising large-scale power technology [1], which absorbs direct and diffused solar radiation and then converts parts of these solar energies into electric power free of emissions.

Fig 1. Annual global solar radiation [5]

1700 < SR < 2200

SR > 2200

Tcombsituatunit (The treliabefficisystemwith t

Fig

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structup anthe pimprotime, researseconsystemundertherm

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shown in Fig. 2collector, a Swer conversiobine generatorare simple anlow conversioormance of thSCPP increase

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mbining with ass flow of ucollector air tele with losses process denoteswhen its tempeenotes the air e when the temdenotes the aihen the temperavitational forir flow are releaower system a

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n the solar chimlower pressure himney when eported by NAses by 6.5oC aospheric pressulevel till 2750

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article a novel he chimney anvirtually simulay heights is pro

Copyright ©

e heights (> 5rk. or center is thethe air to the the effect o

updraft air is emperature risefor SCPP is shs the heated aierature and enflow at the c

mperature decir behind the erature decrearce. Finally, thased into the at

are also includecomponents a

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agram of air staem losses [23]t, 02: collector i 04: SC outlet air

mney is mainly and higher wicompared to

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air flows with ese driving fo

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concept of adnd a solar heatate the temperaoposed.

2011 by ASME

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ELOPMENT Virtually increeasing the tempept to be aerature at the rption refrigerng concept is ever, absorptiorally addressedure at the chimney base, jusng system simmatic diagram e top of the chim

Fig 4. Schemaive cooling at c

In the current es to examine ts are obtainedney height one, the effect ofand the effect od is examined sluded with relaal height at the

a- The CFD A CFD model iept. A solar chchimney diameney heights froel. A 2-D axisy

mapped meshent size is cont

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OF THE NEWeasing the heiperature and pddressed in chimney exit

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on refrigeratiod for general comney exit cout over the tu

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paper a prelimthis concept.

d as referencen the air flow f decreasing thof heating the separately and ations betweenairflow speed.

Model is built for a pr

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W DESIGN ight of a solapressure at its the current

t could be redriven system. d before for son solar driveooling systems

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reliminary examwith 200m col

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Copyright ©

lowing air inleght time and cperature of theoC constant alls bounded by thimney walls dition at the c

height and the ure given by NAature at sea levctively. The mrmal fluid anal

ar chimney witodel

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2011 by ASME

et to the turbincloudy intervae ground undel the time. Ththe atmospheriare assumed t

chimney exit iaverage heigh

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ore, the currener the horizois.

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of the chimney ring to vertical

he chimney exphase, the concheating the chi

nsidered. The ley base. Thenxamined witho

orth noting thaw speed of the n the current mhe chimney exisignificantly a

nt study does nontal turbines

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to simulate dts of 50m andity entering the

% for each 5K chimney heig

ney exit cooling

- Heating thThe second tecs the air up tow

essure drop at tchimney heighFig. 9 shows oss sections ofbines. Fig. 9, act on increasinng.

8. The effect oentering t

s are combinethese effects. Ts have been reddifferent coold 100m. As e vertical and reduction in th

ghts experiencg.

he chimney bachnique which wards the chimthe chimney exht of 50m with/

the profile of f the collector also, shows tha

ng the airflow

of cooling the to vertical and

Copyright ©

ed to comparThe chimney educed with raning conditionshown in Fighorizontal turhe chimney exce the same

se is heating the

mney exit increxit. This conce/without coolin

f the airflow vand chimney at heating the bvelocity than

chimney exit ohorizontal turb

2011 by ASME

e the chimneyexit temperaturnge from 5oC ts for chimney. 8 the airflowrbines increasexit temperatureeffect for th

e chimney baseasing the effecept is examineng the chimneyelocity throughat the locationbase has highethat of the exi

on the airflow bines

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d- Combined cooling and heating About a 25% increase in the airflow velocity is realized

with the combined actions of heating the chimney base and cooling the chimney exit. Such increase in the air velocity results in a 95% increase in the harvested energy from such arrangement due to the cubic relation between the harvested energy and airflow velocity.

The effects of the combined cooling and heating are examined for chimney heights ranging from 50 – 1000m. It is

worth noting that the cooling and heating is applied at the chimney cross sectional area within height less than 20cm. These localized heating and cooling have shown significant performance enhancement for chimney heights up to 500m, Fig. 10. Chimneys with heights up to 400m gain an increase in the air velocity more than 20% due to the heating and cooling actions, Fig. 11. Chimney with height 300m with heating and cooling actions acts similarly to a chimney with height 500m without these modifications, Fig. 10.

Fig 9. Airflow velocity profile through the cross sections of the collector and chimney at the turbines locations

Fig 10. Airflow average velocities Vx entering horizontal turbines and Vy entering vertical turbine before and after applying heating

chimney base and cooling chimney exit

b) Entering to horizontal turbines a) Entering to vertical turbine


Fig 11. Percentage increase in the air flow velocity and in virtual height due to the action of heating the chimney base and cooling the

chimney exit.

CONCLUSIONS Solar chimneys are significant source of clean electrical

energy. The performance of the solar chimney power plant depends mainly on the height of its chimney. The chimney height has to be at least 500m to gain cost effective power. In the current work a new design concept of heating the chimney base and cooling the chimney exit has been presented. The new proposed design has worked as a chimney with virtual added height. This virtually added height has shown double the generated electrical power for a chimney with original height up to 400m.

The current preliminary results of the current CFD model require few modifications to include all boundary and environmental conditions to confidently relay on. Moreover, comparisons with experimental results are required for the model to gain confidence in its achieved results and in the claimed enhancements.

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