Modelling of Electricity Generation Using Medium-temperature Geothermal Resources in Greece

Abstract

In Northern Greece and in some islands in the Aegean Sea there are numerous low enthalpy geothermal fields with water temperatures reaching more than 90 ºC. These fields are located at very shallow depths (100-500 m) in the post-orogenic basins of the area (e.g. basins of Nestos River and Evros River) and in the islands of Samothrace, Chios and Lesvos. The geological and tectonic conditions are favourable for the presence of medium enthalpy geothermal fields (T=90-120 ºC) at greater depths. In the area of Eratino (Nestos River Delta, Eastern Macedonia), two reservoirs have been identified. The first one is the main geothermal reservoir (high-enthalpy) and it is estimated to be lying at a depth of 1500 m. The Greek Public Petroleum Authority has measured 122 ºC at a depth of 1377 m. The second one lies at a depth of 650-700 m. Heat is transferred from the main reservoir to the second one, where the geothermal fluid has a temperature of 70-80 °C. Binary cycle energy conversion systems are successfully used to exploit low-temperature geothermal resources. They are usually constructed in small modular units and can be used efficiently for power generation for both off- and on-grid systems. This paper aims at investigating and assessing the potential of electricity generation using medium temperature geothermal resources in Greece. In order to perform such an investigation, models for small power plant have been developed simulating a KALINA (KCS34) or an Organic Rankine Cycle (ORC). The modeling for this work was performed using ASPEN Plus software. The KCS34 model has been successfully validated with experimental data from the Husavik plant using the same input parameters. The results of the simulation are very close to the actual site data. The ORC model was initially set to produce the same power at identical conditions. After the validation step, the models are used to parametrically study the plant performance for the range of climatic and geothermal conditions in the area of Eratino. The geothermal fluid inlet temperatures considered to be in the range 90–130 ºC, while the return temperature of the brine is assumed to be between 70 and 90 ºC. Energy (and exergy) analysis of the plants is performed in order to define power production limits. Sensitivity analysis of the plants was also performed in order to identify the impact of main parameters such as ammonia mass fraction (KRS34) or the working fluid used (ORC), high and low cycle pressures and working media mass flow on power, efficiency and size of the units. Eventually a methodology is suggested for optimization of plants performance in terms of measures based on both exergetic efficiency and size of heat exchangers.