Exploration into the potential for a low-enthalpy geothermal power plant in the Cape Fold Belt

Martin, Jonathan William (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: South Africa has long been dependent on coal and other fossil fuels for cheap electricity generation. While there has been an increase in utilising renewable energy over the last two decades, the main focus has been on solar and wind, while geothermal energy is not considered by the main power producer Eskom or private energy producers and only limited research has been done by academics. With advances in technology that harness geothermal energy from a wider range of temperatures, geothermal resources as low as 85oC have been reported attainable when using low-enthalpy technologies as such binary systems. This makes geothermal energy a reality for regions in South Africa where moderately high geothermal gradients exist; making sure the geothermal gradient is high enough to obtain necessary heat energy from 3-4km depth. The initial high level assessment of the geothermal potential of the Cape Fold Belt region was done through accessing seven hot springs found to have the highest temperature from previous studies. Temperature measurements, amongst other parameters, were taken as close to the source as possible as well as collection of water samples for ICP-AES analysis for major cations. The cation concentrations from the ICP-AES analysis allowed for geothermometry calculations to be conducted which gave the minimum temperature estimates of the reservoirs of each hot spring. Both the surface temperature measurements and the estimates of the reservoir temperature resulted in two locations that were in the top three for both measurements. These two locations were Calitzdorp and Caledon, having surface measurements of 47oC and 45oC and estimates of the reservoir temperatures of 117oC ±13oC and 108oC ±21oC respectively. The analysis of the Oudtshoorn region, where the Calitzdorp hot spring is located, was conducted using published geophysical data in the form of a magneto-telluric (MT) survey that was carried out in 2005 by the Agulhas-Karoo Geoscience Transect project. The MT data was presented in a paper by Weckmann et al. (2012) as a cross sectional profile from Mossel Bay to Prince Albert to a depth of 30km, where a large region of low resistivity was found below the Oudtshoorn basin. The Calitzdorp hot spring is positioned at the surface above this region. The geological cross sections and regional interpretation presented in this study infers that a major syncline of the Cape Supergroup exists below the basin, potentially as deep as 10km, and covers the low resistivity area from the MT profile. This led to the inference that the large region of low resistivity is most probably due to a large water reservoir. This potential reservoir is about 40km in length with a depth of 2.5km to 7km at its thickest, tapering out towards the edges. The depth to the top of the potential reservoir and the estimated reservoir temperature from the geothermometry results in a geothermal gradient of 39oC/km ±4.3oC/km. Thus Calitzdorp was identified as a promising location for further exploration, ideally deep boreholes or more geophysical surveys, to validate the existence of a reservoir and take down-hole temperature measurements. The depth and size of this potential reservoir would make it a favourable candidate for a pilot low-enthalpy geothermal power plant within the Cape Fold Belt and South Africa.

AFRIKAANSE OPSOMMING: Suid-Afrika is al lank afhanklik van steenkool en ander fossielbrandstowwe vir goedkoop kragopwekking. Hoewel die benutting van hernubare energiebronne vir kragopwekking toegeneem het in die afgelope twintig jaar, is die klem deurentyd op son- en windkrag, wat nie-deurlopend is, sonder enige oorweging van geortermiese energie. Geotermiese bronne by 85oC is al as haalbaar gerapporteer wanneer lae-energie strategieë soos binêre sisteme aangewend word. Gevolglik kan geotermiese energie potensiaal inhou vir Suid-Afrika met matige geotermiese gradiënte wat voorkom in sekere dele van die land met die voorbehoud dat dit hoog genoeg is om geo-vloeistof vanaf ‘n diepte van 3-4km te verkry. Die aanvanklike hoëvlak evaluering van die geotermiese potensiaal vir die ‘Cape Fold Belt’ streek is gedoen deur sewe warmwaterbronne, wat in vorige studies as dié met die hoogste temperatuur uitgewys is, te assesseer. Lesings vir temperatuur en ander parameters, sowel as watermonsters vir ‘ICP-AES’ analise van die belangrikste katione, is so naby as moontlik aan die bron geneem. Die katione konsentrasies vanaf die ‘ICP-AES’ analise is gebruik om geotermometriese berekeninge te doen, sodat die minimum temperatuur beramings vir die reservoirs van elke warmwaterbron verkry kon word. Ná die verkryging van die oppervlak temperatuurlesings en die beramings van die reservoir temperatuur is twee liggings verkry wat in die top drie vir beide metings was. Hierdie twee liggings, nl. Calitzdorp en Caledon, het oppervlak temperatuurlesings van 47oC en 45oC en beramings van reservoir temperature van 117oC ±13oC en 108oC ±21oC, onderskeidelik, gehad. Die analise van die Oudsthoorn streek, waar the Calitzdorp warmwaterbron is geleë is, is uitgevoer met behulp van gepubliseerde geofisiese data in die vorm van ‘n ‘magneto-telluric’ (MT) opname. Hierdie opname was uitgevoer in 2005 deur die Agulhas-Karoo Geoscience Transect projek. Die MT data is gepubliseer in ‘n artikel deur Weckmann et al. (2012) as ‘n deursnitprofiel vanaf Mosselbaai tot Prins Albert tot ‘n diepte van nagenoeg 30km, waar ‘n groot area van lae resistiwiteit gevind was onder die Oudtshoorn vallei. Die Calitzdorp warmwaterbron is geleë by die oppervlak bokant hierdie area. Die geologiese deursnitte en streekswye interpretasie aangebied in hierdie studie het tot die interpretasie gekom dat ‘n groot sinklien van die Kaapse Supergroep onder hierdie vallei bestaan, wat moontlik tot ‘n diepte van 10km strek en die area van lae resistiwiteit, getoon in die MT profiel, behels. Gevolglik was die afleiding gemaak dat die groot area van lae resistiwiteit as gevolg van ‘n groot water reservoir is. Hierdie reservoir is omtrent 40km lank met ‘n diepte van 2.5km tot 7km in die middel en wat sywaarts afneem. Die diepte tot die bokant van die reservoir en die beramings van reservoir temperature het ‘n geotermiese gradient van 39oC/km ±4.3oC/km tot gevolg. Calitzdorp is dus as ‘n belowende ligging bestempel vir verdere ondersoek, byvoorbeeld addisionele geofisiese opnames of diep boorgate, om die bestaan van ‘n reservoir onder die Oudtshoorn vallei te bevestig en om lesings vir temperatuur met diepte te versamel. As gevolg van die diepte en grootte van hierdie potensiële reservoir kan dit as ‘n ideale kandidaat dien vir ‘n loodsprojek in die ‘Cape Fold Belt’ en Suid-Afrika, in die vorm van ‘n lae-entalpie geotermiese kragsentrale.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107815
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