HIGH-POTENTIAL WORKING FLUIDS AND CYCLE CONCEPTS FOR NEXT-GENERATION BINARY ORGANIC RANKINE CYCLE FOR ENHANCED GEOTHERMAL SYSTEMS

A Department of Energy funded study of advanced cycles and high-potential working fluids for Organic Rankine Cycles (ORC) for use in Enhanced Geothermal Systems (EGS) and their impact on cost of electricity has been conducted. The work completed to date, in coordination with AltaRock Energy, Inc., characterized the performance of advanced cycles and high-potential working fluids for EGS resource temperatures. A cash flow model to estimate the levelized cost of electricity (LCOE) is being developed. This model includes high resolution cost models for the EGS reservoir in combination with the power plant. The impact of power cycle, working fluid, and geothermal well specifications on LCOE and optimized plant configurations are evaluated. From an available list of more than 17,000 pure components, 35 working fluids were identified as high-potential. In addition to the numerous fluids that were screened from commonly available sources, additional fluids were screened from vendors that are less common or even not on the market yet. An additional 3 working fluids were included for comparison to the current state-of-the-art. The performance of the working fluids was evaluated in a subcritical ORC, supercritical ORC, and trilateral flash cycle and compared to the performance in baseline subcritical ORCs. The primary advantage of the supercritical cycle or trilateral flash cycle over the subcritical cycle is a better match between resource cooling curve and working fluid heating curve. The lack of constant temperature evaporation allows the heat source to be cooled to a lower temperature despite a similar pinch point as in a comparable subcritical cycle leading to greater utilization of the geothermal resource. The supercritical cycle offers significant improvement in net power output compared to a baseline subcritical cycle. The topics that will be presented include the modeling efforts for the supercritical, subcritical and trilateral flash cycles, the cost & economic modeling of EGS plants and results of the comparison with baseline subcritical cycles resulting in a 30-50% increase in net power output and optimized LCOE calculations. INTRODUCTION Enhanced or Engineered Geothermal Systems (EGS) have the potential to become a significant sustainable power source for the future [1]. Unlike classic geothermal applications, EGS is not limited to specific geologies with hot, high porosity geology and water. Significant research and production plants are under construction or operation to take advantage of this abundant renewable energy opportunity (Figure 1). These efforts vary significantly in size, technology and purpose. Figure 1: Typical EGS Setup EGS is a key green technology to deliver zero emission energy to millions of US households. Improvement in energy conversion technology is highlighted as one of the key requirements for increased acceptance and successful development of EGS [1]. Successful development of advanced fluids and energy conversion technology under this program will aid in enhancement of the U.S. lead in geothermal science and technology. The design capability for optimal energy conversion efficiency at the plant can be combined with drilling cost models to find an optimal design point between drilling depth and energy conversion plant investment. In addition, such models allow for the evaluation of system performance under a range of boundary conditions. If geothermal source temperature variation over time can be predicted, the energy conversion system can be designed to have optimal efficiency over a range of source temperatures. A similar analysis can also be used to improve output performance under heat sink night and day temperature variation. By providing for optimization over the lifetime of the complete EGS system, it is expected that further economic and output performance benefits can be realized. Review of available Concepts/Technology: The focus of this project is cost efficient power generation from Engineered or Enhanced Geothermal System (EGS) resources. EGS, like most geothermal projects, is characterized by a substantial amount of the project cost (50-80%) being bound in the well field. Significant effort is underway to reduce the cost per kW installed by technology development, one of the ideas being improvements in the energy conversion system. Classical higher temperature hydrothermal plants use flash steam systems. In newer, lower temperature plants Organic Rankine Cycles (ORC) are a common energy conversion system. ORC’s use organic liquids as working fluids that relative to water have lower critical pressure and temperature allowing the cycle to run more efficiently at low temperatures and at reasonable pressure levels. A fluid is chosen depending on the application with the most common ones being hydrocarbons like n-Pentane, iso-Butane or refrigerants like R134a or R245fa. Figure 2: Subcritical Rankine Cycle The approach to improve the energy conversion side of EGS systems is focused on evaluating advanced cycles relative to the subcritical Rankine cycle, namely the supercritical and the trilateral flash cycle. An example of all three cycles is shown in Figure 2,3, and 4. Another cycle of interest uses fluid mixtures to take advantage of a gliding boiling point; however, previous GE studies have shown this technology not being cost effective, due to heat exchanger design considerations to avoid de-mixing of the fluid mixture and the lower heat transfer coefficients. Figure 3: Supercritical Rankine Cycle Figure 4: Trilateral Flash Cycle The ideal energy conversion system has: 1. greater than atmospheric condenser pressure at low temperatures, 2. optimal heat source utilization where heat source curve and working fluid boiling curve are parallel with minimal temperature difference (pinch point) and 3. manageable working fluid pressure at maximum source temperature. For a given resource temperature each cycle has a preferable fluid and pressure levels to work with/at. The subcritical and the supercritical cycles are the most mature systems with the supercritical cycle likely operating at higher pressure levels (higher cost and more pump power required) and need for additional engineering on the supercritical heat exchangers and special attention paid to the pressure ratio limits on the expander. The trilateral flash cycle with its flash expansion would require a higher mass flow rate for a similar power output, thereby requiring more pump power compared to a subcritical cycle. In addition a twophase expander is currently commercially not available and R&D effort would be necessary for development. Depending on the specific customer requirements there are several cycle design options that may help the system economics. A recuperator can be employed to use some of the superheat available after the expansion process (e.g. when using R245fa) to preheat the working fluids after the pump before it enters the preheater. Even though this piece of equipment increases the cycle efficiency, it does not necessarily increase the net power output of the plant, depending on the change in heat utilization. From an economics point of view the heat transferred does not have to be rejected in the air cooled condenser and therefore required a smaller air cooled condenser. In case that a lot of solubles are in the brine and a minimum temperature is required to prevent scaling in the heat exchangers, a recuperator can raise the reinjection temperature to prevent precipitation or the use of chemicals to prevent scaling. One key component of the cycle is the expander. The ideal expander provides high conversion efficiency at the design point and at the same time maintains the high efficiency in off-design operation. Unlike industrial waste heat recovery applications with very predictable operating conditions, geothermal applications always carry the risk of the unknown resource. Sufficient information on the resource is available when the actual flow test is performed, relatively late in the overall project. Even then the resource will decline over time and cause a change in the operating conditions. For smaller applications expanders like scroll, screw or piston expanders may be applicable. However, the required flow rate for EGS applications are pushing into the area of radial and axial expanders. Radial expanders provide the advantage of relatively high efficiency under part load conditions, while axial expanders shine with very high design efficiency; they also have a significant efficiency loss for any off-design case. Fluid Selection Process: A review of similar undertakings in the literature has identified several groups of fluids that best fit the needs of the ORC application. Fluids were gathered from literature [2,3,4,5,6,7] commercial software such as Engineering Equation Solver (EES), Aspen HYSYS, Aspen Properties, DIPPR database, NIST RefProp, and from internal GE sources. The full list of available fluids contained more than 17,000 pure components. Fluids considered suitable for the current application were hydrocarbons, which can be categorized into saturated, unsaturated, cyclic/heterocyclic, aromatic, and halogenated8. The halogenated hydrocarbons are also classified as refrigerants. Alcohols, perfluorocarbons, a variety of refrigerants and siloxanes were also considered. It is essential to note that by classifying the fluids into chemically similar groups, the fluids used in cycle analyses will be reduced by assuming chemically similar fluids to exhibit similar cycle behavior [6]. In addition to the numerous fluids that were screened from commonly available sources, additional fluids were screened from vendors that are less common or even not on the market yet. In doing so, fluid vendors have been identified and contacts were made with several vendors. The selection of working fluids for Organic Rankine Cycle was based on fluid properties that affect (i) cycle feasibility, (ii) envi