article 7 imece2011-65819 solar
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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.
Ali K. Abdel-Rahman
Mechanical Engineering Department, Faculty of Engineering,
Assiut University, Assiut (71516), Egypt. [email protected]
Mahmoud Ahmed
Mechanical Engineering Department, Faculty of Engineering,
Assiut University, Assiut (71516), Egypt. [email protected]
Wael M. Khairaldien
Mechanical Engineering Department, Faculty of Engineering,
Assiut University, Assiut (71516), Egypt. [email protected]
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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els, many theorility of buildinn that SC heig
ney power planmponents suchter of the collincludes one ofor the SCPP r problem of mined by the the conversion
overview of thprincip
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der the first rooosed water-fillr roof [3], anSCPP [4]. ge-scale SCPP
power electroal transition sector exit to SCf any SCPP. Thhe SC turbinese SC turbines ly because ofintenance of tu
SCPP. ance has beeners [7-12] us
main objectives a theoretical wbe scaled up. retical studies ng a large scalght plays the m
nt (SCPP), as sh as a solar lector, and poor several turbcomponents SCPP is its thermal perfoefficiency of
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of support mhe ground undeabove it flowinen by buoyan
mance of SCPpreviously propare including aof [2], to add led tanks on tnd a use of s
P consists of onics, a grid ection (HTVTSC inlet. Turbinehe typical SC t are usually pwere proposed
f inconveniencurbine generato
n experimentasing small to
of these studiwork and to get
Utilizing thes[12-22] have
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shown in Fig. 2collector, a Swer conversiobine generatorare simple anlow conversioormance of thSCPP increase
ey power plant
matrix, columer the roof heatng radially from
ncy. In order tPP during nighposed by manan intermediatthermal storagthe natural sosolar ponds fo
one or severainterface and S), i.e., the flowe generators arturbine is of thlaced at the S
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ney will enhaased. n the present aat the top of thof chimney to vo large chimney
SCPP. Largeor research word in the collecto, which drives
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
osses of PCU ector and in the
ure–entropy dialysis with systeric air at SC outletllector outlet air,
n the solar chimlower pressure himney when eported by NAses by 6.5oC aospheric pressulevel till 2750
y showing speata show that ould deliver ay creating theance its perfo
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
e SC except tu
agram of air staem losses [23]t, 02: collector i 04: SC outlet air
mney is mainly and higher wicompared to
ASA [24] that tat 1000m heighure decreases 0m. Furthermoeed of 60m/ssolar chimney
air flows with ese driving fo
ormance as if
concept of adnd a solar heatate the temperaoposed.
2011 by ASME
500 m) are no
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of temperaturapproximately
e and to the SChown in Fig. ir entering fromntropy increasecollector outle
creases slightlyturbine flow
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conce10m cchimnmodewith eleme10cm
Urook
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
m. Under the collebed that absor
OF THE NEWeasing the heiperature and pddressed in chimney exit
ation solar drnot addressed
on refrigeratiod for general comney exit cout over the tu
milar to that usfor the new demney and heat
atic overview ochimney top an
base
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
himney, Fig. 5, eter, 3m collecom 50m to 100ymmetric modh to get the trolled to be le
ector a thick rb solar radiati
W DESIGN ight of a solapressure at its the current
t could be redriven system. d before for son solar driveooling systems
uld be reducedurbine section,sed in [26]. esign of addingting unit at the
of a solar chimnnd passive heate
minary study In the first p
e frame for thspeed. Then
he temperature chimney base combined. Th
n the chimney .
reliminary examwith 200m col
ctor largest heig00m is assumeddel is built usin
most accurateess than 20cm
layer of higheon during suns
ar chimney bexit is a noveproposal. Th
duced using a This positiv
solar chimneyen systems ars [25]. Also th
d by heating th, using passivFig. 4 shows g a cooling unchimney base.
ney aided withting at chimney
is done in twphase numericahe effect of th, in the seconat the chimneon the air flow
hen the study ireal height an
mination of thllector diameteght, and varioud for the curren
ng ANSYS-CFDe results. Thand higher tha
er heat capacitshine hours an
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e the stored eng non-sunshineg day time). Tollector is assuure at the collecure. The colleiabatic. The bled based on t
ndent pressure spheric pressur101325Pa andmpressible and
Schematic dra
- The regulaThe preliminaryl have shown tchimney perf
ity significant
C
Chimn
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nergy to the fle periods (nigThus, the temp
umed to be 60o
ctor entrance isector and the cboundary condthe chimney hand temperatu
re and temperad 300oK respecd turbulent ther
awing of a solaCFD mo
ar chimney ry results for the great effectformance, Figtly increases
Chimney
Collector
ney
al ne
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
the first phast of the chimneg. 6. The a
with the in
Location oTur
LocationTu
C
H
2011 by ASME
et to the turbincloudy intervae ground undel the time. Ththe atmospheriare assumed t
chimney exit iaverage heigh
ASA [24]. Thvel are assumemodel is solvelysis.
th its simulation
se of the CFDey height on thverage airflow
ncrease of th
of Horizontal rbines
n of Vertical urbine
Collector
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ney height, Fiel, for simplicigh altitudes is ner decrease thet. This effect
hts. Furthermoure drop afte
parative analysi
6. The airflowturbine re
7. The effect ovelocity enter
b- Cooling thIn the second pney height by hney exit is conng the chimneney base is ex
g. 7. It is woty, the airflownot counted ine pressure at thwill increase
ore, the currener the horizois.
w pattern in m/sgions for chim
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
s at the horizonmney height of 2
height on the al and horizonta
it cept of virtuallyimney base andlatter is examinn the concept out cooling the
at in the currenatmospheric a
model which wiit due to ventuat high chimnenot consider th
since it is
ntal and vertica200 m
airflow averageal turbines
y increasing thd by cooling thned first withou
of heating the chimney exi
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nt air ill
uri ey he a
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Finallyperforbound25oCheightvelociby 1%Both chimn
c-T
movesof prefor a cexit. the croof turbimpaccoolin
Fig
y both effectsrmance under tdary conditions
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
E
y e o y w s
e. e
e ct d y h
ns er it
5 Copyright © 2011 by ASME
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
6 Copyright © 2011 by ASME
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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