1. Geothermal Resources of Uganda
The main geothermal resources of Uganda are located in the west of the country centred on Lake Albert and Lake Edward in the districts of Kasese, Hoima, Bundibugyo and Nebbi. This area is underlain by the Western Branch of the East African Rift System (EARS), a continental rift zone where the African tectonic plate is in the process of splitting into two (Figure 1).
Figure 1. Map of the East African Rift System
Source: After Chorowitcz, 2005, cited in Harðarson, 2014
The EARS is formed of two main branches: the Eastern and Western Rift. The Eastern Rift runs from the Afar Triangle in Eritrea, Djibouti and Northern Ethiopia south into Kenya and Northern Tanzania. The Western Branch runs from Northern Uganda, south through Rwanda and Burundi, into western Tanzania and on into Malawi, Zambia and Mozambique (Figure 1).
In rift zones such as the EARS the driving force for tectonic activity is thought to be the presence of hot plumes of molten rock rising from deep in the Earth’s mantle (mantle plumes). These give rise to hotspots in the asthenosphere, which results in both the melting and thinning of the rocks at the base of the Earth’s crust and the rifting process through extensional tectonics. This creates the opportunity for relatively shallow hydrothermal systems to develop that can be economically tapped and used as a source of geothermal energy.
Controversy exists regarding the nature of the subsurface heat sources in the EARS, including the size, number and nature of mantle plumes (Harðarson, 2014). What is clear is that the geological forces in the EARS region are not acting in a uniform way, hence the existence of the two distinct systems within the EARS: the Western Branch and Eastern Branch. In general, the Eastern branch is characterised by greater tectonic activity than the Western branch. The temperature and nature of the heat source in the mantle across the region may be the driving force behind differing effects in the Eastern and Western Branch. The result of these general trends is that the geothermal resources of the Western Branch may be more complicated than the Eastern Branch, because of the thicker layers of basal rocks through which to heat is to be conducted to the near-surface layers, and possibly lower temperatures in the underlying mantle.
According to Harðarson (2014), “The differences in volcanism and uplift/subsidence in the Eastern and Western branch, respectively, undoubtedly reflect different mantle temperatures with temperatures underneath the Eastern branch probably 100-150°C higher than underneath the Western branch (White et al., 1987).” On this note, he concludes that “Geochemical studies in Western rift...in some areas...indicate that the geothermal activity in the Western branch is dominated by low temperature activity (<150°C reservoir temperature)”. Consequently, there is greater uncertainty about the geothermal energy potential of the Western Branch relative to the Eastern Branch. Notwithstanding this uncertainty, Harðarson (2014) also noted that “in a few places higher temperature [geothermal resources] might be found, of up to about 250°C” and in some instances the use of binary technology for electricity generation may be possible".
Research into Uganda’s geothermal energy resources has been ongoing since 1993 focussed on the four main geothermal areas that have been identified, namely: Katwe (Kasese district), Kibiro (Hoima district), Buranga (Bundibugyo district) and Panyimur (Nebbi district; Figure 2).
Figure 2. Geothermal Resources Map of Uganda
Source: Bahati, 2011
The local geology in these locations is characterised by various surface manifestations indicative of a the presence of a shallow geothermal/hydrothermal system such as hot springs, warm springs, gaseous emissions, travertine, hydrothermally altered rocks, mineral precipitates and thermophyllic grass (Kato, 2013).
The status of exploration activities in different geothermal prospects is shown below (Figure 3). As outlined, a range of surface studies and shallow well measurements have been undertaken (e.g. thermal gradient (T/G) wells to around 300 m; Gislason et al., 2008), but no deep, full-size, test wells have been drilled to date, and no commercial geothermal reservoirs have been fully identified. Consequently, uncertainty persists regarding the nature of the resource in all geothermal prospects, and there is an urgent need to gain a deeper understanding of the resource quality in Uganda in order to unlock its use.
To date only two deep exploration wells have been drilled in the Western Branch of the EARS, at Karisimbi, Rwanda, in 2013. This work was overseen by various Icelandic and Japanese experts and involved the use of a reservoir model prepared by the Institute of Earth Science and Engineering (IESE), New Zealand (Rutagarama, 2015). Problematically, both wells showed a thermal gradient of only 30°C/km and failed to provide any evidence of the existence of a hydrothermal system in the area (Rutagarama, 2014; Rutagarama, 2015). As a result, drilling was halted. The results from the Kirisimbi drilling programme has set back perceptions about the geothermal resource potential of the Western Rift. On the other hand, Onacha (2012) in a summary of the status of geothermal energy in the Western Branch of the EARS highlighted that some good prospects exist, outlining evidence of subsurface temperatures of 200-220°C at Kibiro and lower enthalpy resources at Katwe (140-200°C) and Buranga (120-150°C). Despite the failure of the test drilling in Rwanda, the evidence in Uganda provides a strong indication of the presence of an economically viable geothermal resource, and suggests further investigation is warranted.
 This includes various technical assistance projects with agencies such as the Icelandic International Development Agency (ICEIDA; at Kibiro) and studies by the Japanese International Cooperation Agency (JICA) in the south west of the country (Kato, 2013)
Source: Geothermal Resources Department estimates, 2016
Figure 3. Uganda Geothermal Resources Exploration Status
2. Power Generation Potential
In terms of the total geothermal resource potential of Uganda, initial estimates made in 1982 stood at 450 MW (McNitt, 1982). This has now been upgraded to over 1,500 MW (Uganda Vision, 2040). The upgrade is in line with most other countries included to the 1982 regional assessment (Table 1). For example, McNitt (1982) estimated resource potential for Kenya at 1,700 MW, whereas the latest estimates have revised the potential to 7,000-10,000 MW and similarly in Tanzania the latest resource estimate is >5000 MW, up from 650 MW in 1982. The variability in resource estimates highlights the challenge involved in making regional extrapolations of resource potential, and shows that they tend to be quite conservative. As such, it is fair to speculate that Uganda’s resources could be several times greater than estimated by McNitt in 1982, and more aligned to the long-term estimate of 1,500 MW of installed capacity envisioned under Vision 2040.
Furthermore, new understanding of the geology of the Western Branch of the EARS is emerging based on the analogue of other fault-controlled geothermal systems. Such fault-controlled systems are different to the magmatic systems of the Eastern Branch of the EARS in Kenya and Ethiopia, and more similar to parts of the western USA and Anatolian plate margin in Turkey. These provinces are both active geothermal energy producers. For example Turkey has around seventeen (17) geothermal power plants with an average capacity of 30 MW and a total installed capacity of around 400 MW, and has near-term targets to increase installed capacity to 750 MW by 2018 and more than 1,000 MW by 2020. It is also a significant direct user of geothermal heat, with more than 2,800 MWth of capacity, mainly in balneology (spas) and heating applications (domestic, hotels). The State of Nevada in the USA has eighteen (18) geothermal power plants with an average capacity of 40 MW and a total installed capacity of over 600 MW, and an estimated resource potential of 2,500-3,700 MW. These analogues provide further evidence of the viability of the geothermal potential of Uganda.
 An area known as the Basin and Range Province, which stretches east from the Sierra Nevada mountains through the states of Utah, Nevada, Arizona, New Mexico and into Northern Mexico.
Table 1. East African Geothermal Resource Estimates
3. Direct Use Potential
The potential for direct use is difficult to estimate as it depends on the proximity of the heat source to potential end-users. Salt mining is an industry already utilising geothermal resources at Katwe and Kibiro. Artisanal wellness spas using geothermal waters are also known exist in Uganda (e.g. Rwagimba Hot springs, Kabarole District), and at least one example is known of where warm of water from a geothermal spring is being used for heating and bathing in a hospital (Kisiizi hospital, Rukungiri district). It is clear, however, that other potential applications exist especially in respect of food processing and tourism sectors, both of which are already present in the districts where geothermal resources are located. A summary of the range of potential direct uses by varying temperature is outlined here.
Development of a ‘cascade’ geothermal system is also an area gaining increasing interest. In these systems, the waste heat remaining after successive processes is ‘cascaded’ to different industries depending on their heat needs. An example schematic of such a system is shown here.
A recent example of the growing interest in cascade systems is that of the Geothermal Development Company (GDC) of Kenya. As part of its mandate to “promote alternative uses of geothermal resources other than electricity generation”, the GDC recently launched a cascade pilot at the newly-developed Menengai steamfield, as described further below (Box 1).
The Menengai Geothermal Project, about 180 km northwest of Nairobi, is the most recent large-scale geothermal development activity in Kenya. In developing Menengai, the Kenyan parastatal Geothermal Development Company (GDC) adopted a holistic approach in the development of the project, aiming to make extensive use of the geothermal resource beyond just power generation. In collaboration with USAID (United States Agency for International Development) GDC assessed the potential for a number of direct uses which identified several viable applications. Four direct use schemes have now begun operations, linked together in a cascade system. The schemes, currently at pilot-scale, have been set up on a single well pad of MW-03, which is currently venting waste exhaust heat from a back pressure wellhead plant of around 2-3 MW. The project is supplying water at around 75°C heated via a heat exchanger from the geothermal waters.
The first activity in the cascade is a dairy, in which the hot water is used to pasteurise milk. The milk, located inside a constantly stirred drum, is surrounded by a jacket filled with the heated water; once the milk reaches a temperature of 68°C, the hot water is removed from the jacket and replaced by cold water. Some of the hot water from the dairy is then piped to a laundry and used to wash items in a washing machine (geothermal heat is also used for drying). The remaining water from the dairy is then transferred to an aquaculture project. The water, at a temperature of around 40°C, is mixed with cold water to lower the temperature to an optimised fish growth temperature of 29°C. Finally, once the water has been filtered, it is used to water tomato plants inside a geothermally heated greenhouse (the greenhouse is only heated at night). The filtered ammonia from the aquaculture project is diluted and used to fertilise the plants. The pilot-level projects are expected to be scaled-up in the near future to help local farmers and communities.
Box 1. Cascaded system of direct use at the Menengai Geothermal Project, Kenya
4. Co-production of Minerals by Extraction from Geofluids
The nature of Uganda’s geothermal resources, being largely formed in a sedimentary, fracture dominated system as opposed to a magmatic system, means geothermal fluids can have high mineral content that may be amenable to mineral recovery. The areas around Katwe and Kibiro already utilise geothermal deposits to produce salt. Other recoverable minerals in geothermal brines can include silica, zinc, lithium, manganese and a range of other rare Earth elements. Geothermal fluids could also be used in conjunction with the minerals industry in a fairly simple ore extraction method known as ‘heap leaching’ to extract precious metals such as gold and silver.
There are limited examples of such applications today, however. Silica is generally an issue for geothermal power generation due to the scaling problems that it can cause, but the preferred approach is to manage scaling through temperature management rather than extraction. Commercial silica has been produced at a geothermal power plant at Mammoth Lake in California, USA, where its recovery was a co-benefit from the installation, in the early 2000’s, of a brine-fed evaporative cooling system used to increase plant efficiency during warm weather. A minerals and power coproduction facility was built at Salton Sea in California, USA (Salton Sea V, commissioned 2000) involving recovery of around 30,000 tonnes of zinc per year and using about 40% of the plants 49 MW output, the remaining power being sold to the grid. In the case of the latter, the zinc recovery plant closed down around 2004, however, due to operational difficulties and depressed zinc prices.
5. References and Further Reading
Bahati, G., 2011. Status of geothermal exploration and development in Uganda. Presented at Short Course VI on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Oct. 27 – Nov. 18, 2011.
Chorowicz, J., 2005. The East African rift system. J. African Earth Sciences, 43, 379–410.
Harðarson, B.S., 2014. Structural geology of the Western branch of the East African Rift: tectonics, volcanology and geothermal activity. Paper presented at Short Course IX on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Nov. 2-23, 2014
Kato, V., 2013. Geothermal exploration in Uganda: Status Report. Paper presented at Short Course VIII on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Oct. 31 – Nov. 22, 2013
McNitt. J.R., 1982: The geothermal potential of East Africa. Proceedings of the Regional Seminar on Geothermal Energy in Eastern and Southern Africa, Nairobi, Kenya, June 15-21 1982.
Mnjokava, T.T., 2014. Geothermal Development in Tanzania – Status Report. Presented at Short Course IX on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, Nov. 2-23, 2014.
MOE, 2011. Scaling-up Renewable Energy Program (SREP) Investment Plan for Kenya. Submitted by the Government of Kenya to the Climate Investment Fund (CIF) Board, September 2011.
MWIE, 2015. Highlights of the Ethiopian Geothermal Sector. Presentation to the Global Geothermal Alliance Stakeholder Meeting, Nairobi, by the Ministry of Irrigation and energy, Federal Democratic Republic of Ethiopia. June, 2015
Onacha, S.A., 2012. Challenges and Opportunities of Geothermal Exploration and Development in the Western Branch of the East Africa Rift Valley. Proceedings 4th African Rift Geothermal Conference 2012, Nairobi, Kenya, 21-23 November 2012
Rutagarama, U., 2014. Geothermal exploration and development in Rwanda. Presentation by Geothermal Development Unit Energy, Water and Sanitation Limited (EWSA Ltd) to the Geothermal Donor Collaboration Meeting and Geothermal Workshop for Donors and Decision-makers 26th to 28th May, 2014
Rutagarama, U., 2015. Geothermal Resources Development in Rwanda: A Country Update. Proceedings World Geothermal Congress 2015, Melbourne, Australia, 19-25 April 2015
Uganda Vision 2040. Available at: http://npa.ug/uganda-vision-2040/