The greatest sustainability challenge facing humanity today is the greenhouse gas emissions and the global climate change with fossil fuels led by coal, natural gas and oil contributing 61.3% of global electricity generation in the year 2020. The cumulative effect of the Stockholm, Rio, and Johannesburg conferences identified sustainable energy development (SED) as a very important factor in the sustainable global development. This study reviews energy transition strategies and proposes a roadmap for sustainable energy transition for sustainable electricity generation and supply in line with commitments of the Paris Agreement aimed at reducing greenhouse gas emissions and limiting the rise in global average temperature to 1.5°C above the preindustrial level. The sustainable transition strategies typically consist of three major technological changes namely, energy savings on the demand side, generation efficiency at production level and fossil fuel substitution by various renewable energy sources and low carbon nuclear. For the transition remain technically and economically feasible and beneficial, policy initiatives are necessary to steer the global electricity transition towards a sustainable energy and electricity system. Large-scale renewable energy adoption should include measures to improve efficiency of existing nonrenewable sources which still have an important cost reduction and stabilization role. A resilient grid with advanced energy storage for storage and absorption of variable renewables should also be part of the transition strategies. From this study, it was noted that whereas sustainable development has social, economic, and environmental pillars, energy sustainability is best analysed by five-dimensional approach consisting of environmental, economic, social, technical, and institutional/political sustainability to determine resource sustainability. The energy transition requires new technology for maximum use of the abundant but intermittent renewable sources a sustainable mix with limited nonrenewable sources optimized to minimize cost and environmental impact but maintained quality, stability, and flexibility of an electricity supply system. Technologies needed for the transition are those that use conventional mitigation, negative emissions technologies which capture and sequester carbon emissions and finally technologies which alter the global atmospheric radiative energy budget to stabilize and reduce global average temperature. A sustainable electricity system needs facilitating technology, policy, strategies and infrastructure like smart grids, and models with an appropriate mix of both renewable and low carbon energy sources.

Geothermal energy has a potential for several applications including geo-exchange, direct thermal application and power generation. Whereas the untapped capacity is over 100 GW globally, its growth realizes only 3-4% growth per year while the global share of electricity generation is less than 1%. Limitations of geothermal energy include scarcity of exploitable sites, remote locations often far from load centers and undesirable gaseous emissions. Project development faces challenges of poor funding, technology and long gestation periods of between 5 and 10 years for conventional power plants. Smooth implementation of geothermal project requires, social acceptance, through minimization of environmental effect, avoidance of adverse effects on the people and giving direct benefits to local communities. It is through both subsurface and power plant technologies that environmental challenges of geothermal power plants are addressed. Research and development into new, efficient and costeffective technologies will enhance safety and environmental integrity with respect to geothermal energy and electricity development. A geothermal power plant project generally goes through exploration or prefeasibility stage, drilling or feasibility stage and development stage which involves development of production wells, reinjection wells, steam gathering system and power plant construction and commissioning. The last phase is power plant operation and maintenance before the plant is finally retired upon end of the generation contract or license. These phases of the cycle before generation all combined take relatively long and there is need to improve. The use of Wellhead power plants currently provide a quick access to geothermal electricity ahead of full development of a conventional power plant hence enabling quicker access to geothermal electricity. Drilling which is a critical process in geothermal resource exploration and development is expensive, and therefore development of geophysical methods to resolve this problem is highly desirable as it could significantly reduce the cost of geothermal energy development hence advanced technology in drilling and upfront activities will reduce costs and risks in development. Better technology in upfront activities will significantly reduce the high risks and costs while power plant conversion technologies and better reservoir management and engineering will increase output, resource availability and efficiency of resource exploitation. Geothermal energy resources are significant and if the technical, financial, environmental, social and logistical challenges are addressed, it can be a major player in global energy and electricity market as renewable, safe and cheap resources. Research and development of technology and solutions to current challenges should be enhanced. The study concluded that technical barriers, high financial costs and long gestation periods prevent faster development of geothermal electricity.

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Strategic planning and management is the way to go for organizations to prepare themselves to sustain and overcome competion in market places. It is important for all organizations in private sector, public sector and nonprofit organizations. It is a process that begins with self-assessment and realization and then reorganization to compete in a business environment. Business strategies form the basis of survival in a competitive environment and should therefore be well developed by the right people and the right organizational levels. Strategy formulation and implementation should be linked by an evaluation strategy to realize strategy success otherwise the strategies remain useless paperwork. Strategy implementation challenges include political interference, limited resources and global economic situations that may be beyond the organizations' control and so organizations should monitor the internal and external environment and make changes or adjustments to prevent strategy failure. This can be realized by effectively evaluating strategy implementation process. A good strategic plan will give significant benefits to organizations like increased profitability and better corporate governance. Strategic management however is expensive, requires significant resources and investment in market research and other resources yet does not always guarantee success. To be successful in strategic planning and implementation organizations, should invest in market research and forecasting, adequate budgeting, as well as recruitment, training and motivation of qualified personnel and having a holistic approach in strategy formulation and implementation by bringing everybody in the organization to contribute and participate. Strategies should be creative and innovative while the execution should be both effective and efficient. Quick and effective feedback will enhance monitoring and evaluation and facilitate successful strategy implementation. Organizations should be customer oriented to deal with competition and hence proper strategy formulation and implementation. It should however be noted that having a strategic plan does not always guarantee success. However, a well-crafted, innovative and creative plan that is well executed will guarantee success. An effective strategy should start with a SWOT analysis which will enable the organization to build on its strengths and utilize opportunities while controlling or managing threats and weaknesses.

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Biogas is competitive, viable, and generally a sustainable energy resource due to abundant supply of cheap feedstocks and availability of a wide range of biogas applications in heating, power generation, fuel, and raw materials for further processing and production of sustainable chemicals including hydrogen, and carbon dioxide and biofuels. The capacity of biogas based power has been growing rapidly for the past decade with global biogas based electricity generation capacity increasing from 65 GW in 2010 to 120 GW in 2019 representing a 90% growth. This study presents the pathways for use of biogas in the energy transition by application in power generation and production of fuels. Diesel engines, petrol or gasoline engines, turbines, microturbines, and Stirling engines offer feasible options for biogas to electricity production as prme movers. Biogas fuel can be used in both spark ignition (petrol) and compression ignition engines (diesel) with varying degrees of modifications on conventional internal combustion engines. In internal combustion engines, the dual-fuel mode can be used with little or no modification compared to full engine conversion to gas engines which may require major modifications. Biogas can also be used in fuel cells for direct conversion to electricity and raw material for hydrogen and transport fuel production which is a significant pathway to sustainable energy development. Enriched biogas or biomethane can be containerized or injected to gas supply mains for use as renewable natural gas. Biogas can be used directly for cooking and lighting as well as for power generation and for production of Fischer-Tropsch (FT) fuels. Upgraded biogas/biomethane which can also be used to process methanol fuel. Compressed biogas (CBG) and liquid biogas (LBG) can be reversibly made from biomethane for various direct and indirect applications as fuels for transport and power generation. Biogas can be used in processes like combined heat and power generation from biogas (CHP), trigeneration, and compression to Bio-CNG and bio-LPG for cleaned biogas/biomethane. Fuels are manufactured from biogas by cleaning, and purification before reforming to syngas, and partial oxidation to produce methanol which can be used to make gasoline. Syngas is used in production of alcohols, jet fuels, diesel, and gasoline through the Fischer-Tropsch process.

The long gestation period, high upfront costs and the risks in the development of central geothermal power plants are the main reasons for the slow rate of geothermal electricity growth and its contribution to the global electricity mix. The overall objective of this study was to make a comparison between central geothermal power plants and wellhead power plants in the delivery of geothermal electricity projects. The study showed that wellhead power plants are generally less efficient compared to central power plants because of higher specific steam consumption, but are financially attractive because of the quicker return on investment, early electricity generation and the lower financial risks. The study showed that permanent wellhead power plants are a better option for geothermal wells with too low or too high steam pressure compared to others in the steam field. Temporary use of wellhead power plants as opposed to their permanent use is preferred when only limited time is available between the commissioning of a wellhead plant and the commissioning of a central power plant in the same steam field. Technical, operational and environmental challenges, including higher specific steam consumption and lower efficiency than central power plants as well as absence of geothermal fluid reinjection system make wellhead plants less economical and less sustainable in resource use. It can thus be concluded that wellhead power plants can reduce the long wait to generate geothermal electricity and make an early return on investment for investors. Both central and wellhead power plants have relatively higher capacity factor than many other power plants and so can be used to supply base load electricity for the grid or off-grid power supply. This study is a review of the central and wellhead power plants and additionally provides policy guidelines in the execution of geothermal electricity projects either as central or wellhead power plants for grid electricity generation.

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