Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The meteorological conditions at Eskom’s Majuba Power Station are measured,
evaluated and trended in this dissertation. The results are used to evaluate the current
natural draught wet-cooling tower (NDWCT) design- and performance test
specifications and to compare these to the original design- and performance test
specifications. The evaluation reveals that the design parameters for the NDWCTs at
Majuba Power Station, a cooling system that was originally designed optimally, could
have been determined differently and arguably more accurately by using the wet-bulb
temperature (Tawb) as the main design variable instead of the dry-bulb temperature (Ta).
A new technique to determine optimal NDWCT design and performance test conditions
is consequently proposed. In order to satisfy the atmospheric conditions required for a
successful NDWCT performance test, it is also proposed that the tests be undertaken
between 12:00 and 14:00 during Summer. It is found that the NDWCT inlet Tawb,
measured at specific heights, does not compare well to the far-field Tawb measured at the
same heights when a Tawb accuracy of 0.1 K is required. It is proposed that a more
representative far-field Tawb measuring height of 10 m should be used in future NDWCT
designs as the NDWCT design temperature reference height. The industry-standard
reference height should, however, still be used during temperature profile calculations.
A parametric study of the water-steam cycle and wet-cooling system at Majuba
indicates that during full load conditions, the generated output (Pst) is primarily
dependent on the condenser saturation pressure (pc). The latter is reliant on Tawb, the
temperature lapse rate (LRT) that is represented by the temperature profile exponent (bT),
the main cooling water flow rate (mcw), atmospheric pressure (pa), and wind speed (VW).
Using historical plant data relatively simple methods, enabling the quick and effective
determination of these relationships, are proposed. The plant-specific and atmospheric
parameters required for these analyses are also tabulated.
Two NDWCT effectiveness models, one mathematical (Kröger, 1998) and one
statistical artificial neural network (ANN) model are presented and evaluated. ANNs,
which are not often used to evaluate NDWCT effectiveness, provide accurate NDWCT
temperature approach results within 0.5 K of measured values for varying dependent
variables. This motivates that an ANN, if set up and used correctly, can be an effective
condition-monitoring tool and can be used to improve the accuracy of more empirical
NDWCT performance models. The one-dimensional mathematical effectiveness model
provides accurate results under NDWCT design conditions.
The dependency of Majuba’s NDWCT to the rain zone mean drop diameter (dd) is
evaluated by means of the one-dimensional mathematical model. A reduction in dd from
0.0052 m to 0.0029 m can reduce the NDWCT re-cooled water temperature (Tcwo) so
that the rated pc is reduced by 0.15 kPa, which relates to a combined financial saving
during peak and off-peak periods of R1.576M in 2013 and R1.851M in 2016.
Similar improvements can result in higher savings at other wet-cooled stations in the
Eskom fleet due to less optimally-designed wet-cooling systems. The proposed
techniques should be considered in future economic evaluations of wet-cooling system
improvements at different power stations. / AFRIKAANSE OPSOMMING: Die meteorologiese toestande by Eskom se Majuba-kragstasie is deur die navorser
gemeet en -evalueer. Die resultate word gebruik om die Natuurlike-trek, Nat koeltoring
(NTNKT) se ontwerp- en werkverrigting toetsspesifikasies te evalueer en vergelyk met
die oorspronklike toetsspesifikasies. Die resultate dui daarop dat die
ontwerpsparameters vir die NTNKTs by Majuba-kragstasie, ‘n verkoelings-sisteem wat
aanvanklik optimaal ontwerp is, op ‘n ander, selfs meer akkurate manier bepaal kon
word deur die natbol-temperatuur (Tawb) te gebruik as die hoof-ontwerpsparameter
inplaas van die droëbol temperatuur (Ta).’n Nuwe tegniek wat gebruik kan word om
akkurate NTNKT ontwerp- en werkverrigting toetsspesifikasies te bepaal word
voorgestel. Die tydperk vir die mees optimale atmosferiese toestande, wanneer
NTNKT-toetse uitgevoer moet word, word vasgestel as tussen 12:00 en 14:00 tydens
Somermaande. Dit word bewys, vir ’n Tawb akkuraatheid van 0.1 K, dat die NTNKT
inlaat-Tawb, gemeet by verskillende hoogtes, nie vergelykbaar is met Tawb wat ver van
die NTNKT af op dieselfde hoogtes gemeet word nie. ’n Meer aanvaarbare hoogte van
10 m word voorgestel as die NTNKT ontwerpstemperatuur verwysingshoogte vir
toekomstige NTNKT ontwerpe wanneer die Tawb ver van die NTNKT af meet word. Die
industrie-standaard temperatuur verwysingshoogte moet wel steeds gebruik word tydens
temperatuur-profielberekeninge.
’n Parametriese studie van die turbine se water-stoom siklus en die nat-verkoelingstelsel
by Majuba dui daarop dat die generator se uitset (Pst) hoofsaaklik afhanklik is van die
kondensator se druk (pc) gedurende vol-vrag toestande. Druk (pc) is weer afhanklik van
Tawb, die temperatuur vervaltempo (LRT) wat voorgestel word deur die temperatuur
profiel eksponent (bT), die verkoelingswater-vloeitempo (mcw), atmosferiese druk (pa) en
windspoed (VW). Deur die gebruik van historiese data word redelike eenvoudige
metodes voorgestel om dié verhoudings doeltreffend te bepaal. Die atmosferiese- en
stasie-spesifieke parameters wat benodig word vir dié ontleding is ook getabuleer.
Twee modelle vir NTNKT-effektiweit, ’n wiskundige (gebaseer op Kröger, 1998) en
statistiese kunsmatige neurale-netwerk (KNN) model, word aangebied en geëvalueer.
KNNe, wat nie gereeld gebruik word om NTNKTe se effektiwiteit te evalueer nie,
lewer akkurate NTNKT temperatuur-benadering resultate binne 0.5 K van die gemete
resultate vir wisselende afhanklike parameters. Dié resultate motiveer dat ’n KNN wat
korrek opgestel is doeltreffend gebruik kan word om die toestand van NTNKTs te
bepaal en om die akkuraatheid van ander NTNKT-modelle te verbeter. Die eendimensionele,
wiskundige model lewer akkurate resultate onder NTNKT
ontwerpspesifikasies.
’n Wiskundige NTNKT-model word gebruik om die afhanklikheid van Majubakragstasie
se NTNKTe tot die reënsone druppelgrootte (dd) te bereken. 'n Vermindering
in dd van 0,0052 tot 0,0029 m kan die NTNKT se afgekoelde watertemperatuur (Tcwo),
van só 'n aard verlaag dat pc verminder met 0,15 kPa. Só kan ’n gesamentlike vol- en
gedeeltelike vrag finansiële besparing van R1.576M in 2013 en R1.851M in 2016
behaal word.
Soortgelyke verbeterings aan verkoelingstelsels sal lei tot meer en hoër besparings by
ander Eskom nat-verkoelde stasies. Dié tegnieke moet in ag geneem word tydens
toekomstige ekonomiese evaluasies van verbeterings tot nat-verkoelingstelsels by ander
kragstasies.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/17904 |
| Date | 12 1900 |
| Creators | Ehlers, Frederik Coenrad |
| Contributors | Reuter, H. C. R., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various pagings) : ill. |
| Rights | Stellenbosch University |
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