## 1 Introduction  
As a renewable and sustainable resource, the geothermal energy can be well utilised for human activity and industry. Though existed worldwide, the high-temperature geothermal energy that is needed to drive electric generation stations is found in relatively few places [1; 2]. In regions like Aluto Langano of Ethiopia, a great amount of geothermal energy has been detected with diverse temperatures, yet most of them are under-utilized [3].  
As the most common power plant, the single-flash plant is always installed at a newly developed liquid-dominated geothermal. However, the irreversibilities associated with the flashing processes are inevitable [4; 5]. Better energy utilization would be achieved if the energy-waste flashing process is reformed to be an energy-utilization one. Based on this consideration, the total-flow expansion concept, in which the wellhead geofluid is utilized in a two-phase expander/turbine for direct power generation [6; 7; 8] can be integrated to replace the flashing process in the single-flash plant. Extensive experimental and analytical studies were carried out on the total-flow expander/turbine [9; 10] and most focus has been placed on screw expanders for its steady intermittent processes [11; 12; 13]. To date, no commercially successful total flow system is available to replace the traditional entire single flash system in geothermal application. However, the idea of replacing only the energy-waste flashing process with a total-flow expander shows promising potentials [14].  
On the other hand, initiatives to address the concurrent water and power energy issues are largely being pursued separately in geothermal field. Traditionally, most of the high-temperature geothermal energy is exploited through power plant, while low-temperature geothermal energy can be optionally used for desalination and freshwater generation [15]. However, besides of generating power, the high-temperature geothermal flow can be further utilized for freshwater generation by condensing the steam after expansion. Based on this idea, a novel combined power and freshwater generation scheme is proposed. As to the freshwater generation in the geothermal energy field, extra desalination devices and processes are normally needed, which requires mechanical power or electricity consumption [16; 17; 18]. The proposed design in this work aims at eliminating these extra consumptions. The design was previously investigated by [PERSON] et al. [19; 20] on a combined power generation and desalination system using saline water as heat resource. They started by proposing a system for simultaneous desalination and power generation based on trilateral flash cycle (TFC). Comparing to traditional power cycles, trilateral flash cycle can more effectively utilize most of the energy available in low/medium grade heat sources. Steam after expansion is condensed into freshwater and stored instead of being abandoned. By upgrading the expansion device from two-arm rotor to reaction turbines, the performance of the system has been improved significantly, demonstrating a good potential of the combined TFC and water generation concept [21].  
Based on these ideas, an optimized energy system for geothermal energy utilization is proposed and studied in this paper. A total-flow process is used to replace the flashing process to get more power output. Based on the novel designed system, the production of both power and water are studied under variable wellhead conditions, including variable wellhead pressures, temperatures, mass flowrates and vapor qualities. The temperature of the separating point at which the total-flow expansion ends and the steam expansion starts is studied for optimal output. In addition, the efficiency effects of the total-flow turbine on performance of the combined system is also investigated.