POTENTIAL FOR DIRECT UTILISATION OF GEOTHERMAL ENERGY AT THE LAKE BOGORIA GEOTHERMAL RESOURCE

Transcription

1 PROCEEDINGS, Kenya Geothermal Conference 2011 Kenyatta International Conference Center, Nairobi, November 21-22, 2001 POTENTIAL FOR DIRECT UTILISATION OF GEOTHERMAL ENERGY AT THE LAKE BOGORIA GEOTHERMAL RESOURCE Jack Kiruja Geothermal Development Company Ltd P.O. Box Nairobi ABSTRACT Kenya s geothermal resources are located mainly along the Kenya Rift Valley. Lake Bogoria geothermal prospect is one of the prospects, which the government of Kenya has mandated the Geothermal Development Company (GDC) to explore, develop and exploit. Lake Bogoria National park within which the prospect is located is a protected area due to its heritage of wildlife and birds, which support its tourism industry. The lake in particular is the center of the diverse ecosystem of Bogoria. Therefore, protection of the lake from human activity has been given a lot of emphasis. Lake Bogoria geothermal prospect has moderate temperatures which are not ideal for power production using the conventional power plants. However, the resource can be utilized for electricity generation using binary technology plant technology or for direct applications such as swimming, bathing and balneology, industrial and commercial applications which will complement tourism. This paper highlights the potential for direct use applications at Lake Bogoria geothermal prospect and the technology required. Key words: Lake Bogoria, Direct Use, Honey processing, Hides treatment, Therapeutic uses. INTRODUCTION Geothermal is a clean and renewable energy source found in many parts of the world especially along the tectonically active areas. Unlike the convectional energy sources such as fossil fuels which are known to emit large quantities of CO 2 gas into the atmosphere, geothermal energy is associated with very little emissions as shown in figure 1. Furthermore, geothermal energy is utilized close to its indigenous location and therefore transport over long distances is not necessary like is the case with oil and coal. Geothermal resources are classified as low, medium or high enthalpy depending on the thermodynamic conditions of the reservoir. High enthalpy resources are used for electricity generation while low and medium enthalpy resources are mainly for direct use applications. Overview of Lake Bogoria geothermal resource FIGURE 1: CO 2 emission from electricity generation from different energy sources, Rybach (2010). Lake Bogoria geothermal prospect is located in Baringo county North of Nakuru. Preliminary studies indicate that the heat source of the prospect is due to intrusives in the subsurface since there is no observable central volcano. The geothermal system in Lake Bogoria is fracture controlled. The major geothermal manifestations associated with the prospect are geysers and hot springs which can be observed along the shores of the lake with temperature of 85 C-98 C and about 40 respectively (Lagat, 2008). Convective heat loss from all outflow zones with temperatures above 35 C is 437 MWt and

2 the flow is 760 kg/s (Mwawongo, 2000). Geothermometry studies indicate that the reservoir fluids are in the low to medium enthalpy range (Karingithi, 2005) which means that the resource is best suited for direct use applications or for electricity generation using binary technology. Geothermal Energy utilization Geothermal energy can either be utilized for electricity generation or for direct heat utilization. Electricity generation is the most common mode of utilization of high enthalpy fluids. However, with the use of binary power plants it is possible to produce electricity from geothermal fluids at lower enthalpies. Direct heat applications utilize geothermal fluids from the low and medium enthalpy resources. There are several applications for utilizing geothermal energy directly and they depend to a large extent on the energy content of the fluid as described by Lindal (Figure 2). These are classified as agricultural applications, aquaculture, space conditioning, industrial applications and bathing and swimming. FIGURE 2: Geothermal Utilization at different temperatures (Lindal, 1973) Kenya s geothermal resources are mainly high enthalpy and are therefore better suited for electricity generation. However, direct use is also possible either as a standalone project or as an operation that utilizes waste fluid from electricity generation. The latter case is done to achieve high utilization efficiency of geothermal energy in the fluid. Utilization challenges such as silica scaling impose a limit on the amount of energy that can be extracted from the geothermal fluid before disposal. This therefore calls for prior detailed chemical analysis of the fluid before utilisation projects are undertaken. Silica in the geothermal reservoir is in equilibrium with other dissolved solutes but a change in thermodynamic state of the reservoir fluid may result in deposition. The factors that determine the onset and the rate of silica deposition are its concentration in the fluid and the saturation conditions. POTENTIAL FOR DIRECT USE AT LAKE BOGORIA PROSPECT Geothermal energy is usually utilized close to the location where it is found. This is because transporting the fluids over long distances results in temperature and pressure losses and high capital costs. However, with proper designs and cost analysis, the fluid can be transported for long distances. This has been successfully done in Iceland where geothermal fluid has been transported to more than 60 km (Ragnarsson, Hrólfsson, 1998). Therefore, the feasibility of direct utilization applications depends to a large extent on whether there is a market for them in the locality where the resource is situated. The area around Lake Bogoria has a semi-arid climate and the major economic activities are livestock rearing and bee keeping. It is also a popular tourist destination because of the wildlife and geothermal manifestations. These sectors can benefit immensely from the geothermal energy through direct utilization. The possible applications include honey processing, processing of animal products such as beef, skins and hides and in recreational facilities such as warm pools, saunas and steam baths. Therapeutic uses of geothermal fluid Geothermal water mainly from the low temperature resources has been used for therapeutic and recreational reasons for thousands of years. The Japanese, Turks and the ancient Romans were the pioneers of this practice but other societies have adopted it. Swimming pools Geothermally heated swimming pools and hot baths have been constructed in several places in the world where geothermal resources are located. These are mainly for recreational and social purposes. The use of geothermal water for treatment and preventive therapy of ailments is also a common practice. The important factors to consider in balneology are the temperature of the water and it mineral content. Swimming pools and hot baths are supplied with warm water directly from a low temperature geothermal system if the chemistry of the water is

3 conducive. Otherwise, a heat exchanger is used to transfer the heat from the geothermal water to the fresh water which is used in the pool as shown in figure 3. Hot Geothermal Water Flashing Unit (Steam Generated) Steam Bath FIGURE 4: Operating principle of a steam bath FIGURE 3: Heating of a swimming pool using geothermal water Steam bath (Saunas) A steam bath has moist air with a relative humidity close to 100%. The temperature inside the steam bath is usually between 43 C and 47 C since higher temperatures would lead to scalding (wise geek, 2003). The use of steam baths is associated with several health benefits such as: improving blood circulation cleaning and rejuvenating the skin easing muscle tension promoting feelings of relaxation and wellbeing enhancing detoxification processes The steam in the steam bath is produced using a steam generator which heats the water until it boils. However, with the availability of hot water from a geothermal resource, heating of the water is not necessary. Instead, a flashing process at atmospheric pressure is applied to obtain steam from the already hot water. Furthermore, geothermal water contains dissolved minerals which have a therapeutic effect on the human skin. The generation of steam for a steam bath is illustrated in figure 4. Animal product processing Rearing of cattle and goats is a common practice among many communities in Kenya. The area around Lake Bogoria is one of the areas in the country where livestock were traditionally kept as a measure of wealth and the residents of this area continue with the practice to date. However, due to modernization and drought occasioned by the unreliable rain patterns, the communities are now selling their cattle for slaughter in exchange for money and foodstuffs. The products that come from the slaughter houses are skins or hides, meat, horns and hooves. Skins and hides treatment Immediate processing and preservation of hides and skins is important to preserve their quality and value. This is because they start to degrade immediately after removal from the carcass due to autolysis and bacterial action. The processing, treatment and preservations can be enhanced using the thermal energy through direct utilisation. The leather making process is divided into three main steps as described below (Co-products compendium, 2009): a) Removal of extraneous tissues and structure from the skin or hide b) Stabilisation of collagen (tanning). This is done by chemical cross-linking of the protein chains in the hide or skin to provide resistance to bacterial degradation and increase the resistance of collagen to heat. c) Conversion into leather. This is a series of processes which alter the colour, texture and appearance of the tanned product so that it can be used for a particular purpose. The flow chart below shows a summary of the treatment of hides and skins (figure 5).

4 Fleshing Soak Unhairing (Sulphide/Lime) Meat processing Cattle are slaughtered mainly for beef which is a common source of food in Kenya. Beef is highly perishable and therefore it is important to process and preserve it so that it remains fresh for as long as possible to make it fit for human consumption. Figure6 below shows the common beef processing operations. Delime (Ammonium salts/co 2 Bate (Enzyme) Pickle (Salt/Acid) Chrome tan [Cr(111)] Tawing Dry Finish (Polymers) FIGURE 5: Leather making process Chromium tanning process is done in a bath at a temperature of about 40 C in order to achieve a good chemical reaction between collagen in the skin and the chromium salt. Tawing, the treatment of the tanned material with aluminium salts in order to obtain a leather product with pliability, stretchability, softness and quality, is yet another process that requires the input of heat in the leather making process. It is done at a temperature of about 30 C (Co-products compendium, 2009). Tanning and tawing processes require a substantial amount of thermal energy. This is because the processes take up to two days at elevated temperatures to achieve the desired results. A cheap and convenient source of energy would be the geothermal fluid from the nearby Lake Bogoria geothermal prospect. FIGURE 6: Beef processing operations Canning, chilling and cold storage are the major beef preservation methods. These are performed in order to increase the shelf life of beef. Hot water or steam is required to achieve the objectives of these operations. The geothermal fluids from Lake Bogoria geothermal prospect can be utilized to meet the entire or most of the energy needs of these operations as discussed below. Cold Storage After slaughter of the cattle, clean carcasses are conveyed to a cold storage area for rapid chilling. A thorough chilling is essential within the first 24 hours of slaughtering otherwise the carcasses may sour. Air chilling is the most common method for cooling beef. The most desirable temperature for chilling beef is 0 C. Since warm carcasses will raise the temperature of a cold room, it is good practice to lower the temperature of the room by 5 C below freezing

5 before the carcasses are brought in for storage. Beef undergoes maturation and should be held for at least a week at 0 C before being sent to the butchery for retail (P2Rx, 2008). Cold storage of beef can be achieved by the use of absorption chillers which utilise hot water as the source of energy. The most appropriate absorption chilling machine for this operation uses a mixture of water and ammonia as the working fluid. The water temperature requirements and the evaporation temperature for this chiller are shown in figure 7(colibri-bv, 2004). bees get the nectar from flowers of plants which were traditionally used for medicinal purposes. It is a very popular product and the locals sell the pre-processed honey in bottles by the roadside. The processing of honey entails both mechanical and thermal processes. The main mechanical processes are honey extraction and filtration while the thermal processes are mainly heating and cooling as shown in figure 8. Uncapping Extraction Liquification Pre-heat and Straining FIGURE 7: Hot water and evaporation temperature for different temperatures of cooling water (colibribv, 2004). Canning Canning is a method of preserving food in which the food contents are processed and sealed in an airtight container. Canned beef has a typical shelf life ranging from one to five years. Canning is performed by two methods: precooking and raw pack. In precooking operations, the beef is boiled or steamed before canning while in raw pack, the beef is put into cans and then cooked. In both cases above, the cans are finally sealed and then sterilised using hot water or steam (P2Rx, 2008). As indicated earlier in the Lindal diagram, the appropriate temperature for food canning is about 140 C which is within the temperature range of the fluids of Lake Bogoria geothermal prospect. Honey processing. Beekeeping is a major economic activity around Lake Bogoria. The major products of beekeeping are honey and wax. Honey is normally consumed in it natural unprocessed state as liquid, crystallized or in the combs. Honey is used for its medicinal and nutritive value as well as an additive to food recipes. The honey from Lake Bogoria area is believed to be of high quality and high medicinal value because the Micro-filtration FIGURE 8: Honey processing Processing (Enzyme inactivation) Vacuum evaporation Cooling Heating of extracted honey at 45 C causes any wax present to melt. The molten wax, together with any other impurities, float on top of the honey forming a layer of scum which is then scooped out. The second heating of the honey up to 60 C prevents the crystallisation and fermentation of the sugar in the honey, which forms about 80% of the mass of honey (profile on honey processing, 2009). Geothermal energy is a cheap source of energy which can be used in the thermal processing of honey. Geothermal water at about 100 o C occurs naturally at the Lake Bogoria geothermal prospect. The hot water offers an abundant, adequate, cheap and readily

6 available source of thermal energy at the vicinity of the beekeeping area. CONCLUSION In Kenya, especially along the Rift Valley, geothermal energy can be used to improve the livelihood of the local communities. There are many potential direct utilization of geothermal energy which have not been tapped mainly due to ignorance and lack of focus and coordination. With the creation of GDC, whose one of its mandate is to promote direct uses of geothermal energy, research is going on, on how to use the geothermal energy and its byproducts for economic and social gains of the people living in these areas. Most of these applications are related to agricultural activities like farming, livestock rearing, fishing, beekeeping. Other areas of utilisation are in balneology and tourism. At the Lake Bogoria prospect, the readily available geothermal energy can be used in processing of animal products such as beef, hides and skins, milk and honey and for therapeutic use. Use of geothermal energy in such processes will replace need for use of the expensive and environmentally unfriendly fossil fuels. Such applications will be sources of employment and therefore assist in poverty eradication. A few challenges may emerge in the utilisation of the Lake Bogoria geothermal resource. The location of REFERENCES Advanced leather solutions (2000):Leather care for unfinished leather. Advanced leather solutions, Inc. website: Colibri-bv (2004): Heat source for the NH 3 Absorption Refrigeration Systems. Colibri-bv. Website: Co-products compendium (2009):Hides and skins. MLA Monthly report, 14 pp. Karingithi, C. and Wambugu, J. (2004): The geochemistry of Arus and Bogoria geothermal prospects. Proceedings of the world geothermal congress, 6 pp. Lagat, J. (2008): Geology and geothermal resource utilisation options at Arus-Lake Bogoria prospect, northern Kenya rift. KENGEN, 24pp. the resource in a protected area which may lead to challenges in obtaining the necessary licenses. Disposal of the spent geothermal water may also need to be addressed for environmental reasons. The high initial cost of the project and in particular the cost of transmission and distribution pipelines could also hinder the commencement of the project. Convective heat loss from all outflow zones with temperatures between 35 C and 90 C is 437 MWt and the flow is 760 kg/s which are indicators of the thermal energy in the geothermal water. This energy can be utilized in hides and skins treatment, honey processing, therapeutic and recreational facilities and cleaning of the abattoirs while higher temperatures are required for beef canning and absorption refrigeration. RECOMMENDATIONS The following should be looked at before the proposed direct uses in Lake Bogoria geothermal prospect are implemented. 1. Carry out a thorough survey to identify the feasible projects to be implemented. 2. Engaging the local communities and the relevant environmental agencies to ensure acceptability of the projects. 3. Assessment of the chemistry and energy content of the geothermal fluids from the prospect. Lund, J.W. (1996): Balneological use of thermal waters. Geo-Heat Center Klamath Falls, OR, 10 pp. Mwawongo, G.M., 2005: Heat loss assessment at Arus and L Bogoria geothermal prospects. Kenya Electricity Generating Company Ltd., internal report, 7 pp. Pollution Prevention Resource Exchange (P2Rx) (2008):Meat Processing Operations.Environmental Sustainability Resource Center. Website: wrrc.p2pays.org/p2rx/index.cfm Ragnarsson, A. and Hrólfsson, I., (1998): Akranes and Borgarfjordur District Heating System. GHC bulletin, 4 pp. Wise Geek (2003): What is a steam bath? Conjecture corporation. Website: