Multidisciplinary approach for the sustainable utilization of medium-low temperature geothermal resources

A growing interest in the applications of the medium-low temperature geothermal resources can be observed, but often a reference frame in this field does not exist. Different backgrounds are involved in the design and optimization of geothermal projects: Earth Sciences, Engineering, Economics, Environmental Impact. In the practice one aspect often tends to take priority on the others. In this work firstly the elements for a methodological interdisciplinary framework for geothermal projects analysis are given. A clear frame of the state of the art and possible technical-scientific developments are illustrated. The necessity of an "integrated" interdisciplinary approach is underlined, with reference to several case studies. The geothermal potential evaluation tasks and methods are illustrated, and some outlines for an assessment oriented to the resource utilization are described. The sustainability of geothermal projects is analysed under different perspectives and criteria. The main technological issues of geothermal binary cycle power plants are discussed (mainly for small power size), together with technological solutions. The concept of upper limit to the extraction rate is introduced, with reference to an equilibrium point between power production and resource depletion. Direct heat uses are also briefly described and the common issues related to environmental impact and resource durability are then discussed, with a particular focus on the scaling phenomena and the reinjection strategy. An innovative solution for geothermal power production (SBES) is described: the application of the heat pipe principle, in the CLTPT concept, for power production purposes is proposed. The numerical simulation of geothermal reservoirs is an important instrument for the synthesis between the different backgrounds involved in the geothermal energy study. General aspects and its potentialities (historical data matching and forecast of future utilization scenarios) are illustrated. Several numerical models from scientific literature are reviewed (about 21 geothermal fields and related 24 numerical models). The reservoir models of Momotombo (Nicaragua) and Sabalan (Iran) have been realised from literature data and widely discussed. One original model, Monterotondo Marittimo - Torrente Milia (Italy) has been realised in a very multidisciplinary framework and it is also presented. Different utilization scenarios for the case studies are analysed and their sustainability level is discussed. A purely economic approach is considered to be counter-productive for geothermal utilizations, so the thermoeconomic analysis is here applied to geothermal power plants. Momotombo case study and other small size power plants are analysed in order to estimate their thermoeconomic sustainability (exergy balances and cost items are evaluated). The current Italian geothermal energy market situation is briefly described, in relation to the ORC technology diffusion. The small size plants technological and environmental issues treated in this work are then linked to the current way of diffusion in the Italian market. The outlines coming from the global work of analysis are organised in order to give the main features of the proposed methodological approach. The multidisciplinary perspective is reviewed and extended in order to optimize the sustainability of the projects (environmental impact reduction, resource durability).