Thermodynamic based resource classification of renewable geothermal energy in Nigeria

Abstract

Geothermal resources are a source of renewable alternative energy, providing heat from below the Earth's surface. This is often regarded as “energy under our feet,” and has the potential to contribute significantly towards energy diversification and energy security. Geothermal heat has two primary applications: heating/cooling and electricity generation. As an oil-producing country with geopressured zones, Nigeria has considerable geothermal resource potential, with the capacity to reduce the overt dependence on fossil fuels. Categorizing and classifying her resource potential is of utmost importance. Temperature and enthalpy are two parameters that have been used previously to classify geothermal resources, but are inadequate for the purpose of estimating geothermal system potential. Two fluids (e.g., saturated water or saturated steam) can have the same temperature in different phases, with resulting differences in their ability to do work. The concept of exergy, based on the second law of thermodynamics, describes the maximum amount of useful work that can be extracted from a system and is widely used in the field for resource accounting. In this study, potential geothermal sites in Nigeria are classified based on their Specific Exergy Index. Data used were collated from published literature, although due to lack of a consistently developed and maintained database on geothermal resources in Nigeria, not all known areas may have been characterized here. Data obtained from the fields/sites (oil wells, natural gas, and geothermal) were classified using the exergy method. The results indicate that Nigeria's geothermal sites/fields are low exergy resources with the capacity to contribute to the nation's total energy use. Furthermore, the absence of a consistent geothermal database and coherent geothermal energy policies indicates that research and policy measures are needed to aid all stakeholders in developing the nation's geothermal resources.