sCO2 power cycle design without heat source limitations: Solar thermal particle technology in the CARBOSOLA project

Supercritical CO2 power cycles have been proposed to lower the levelized cost of electricity generated by Concentrating Solar Power (CSP) plants due to their high thermal efficiency and low equipment cost. In this study, a simplified techno-economic model was developed to compare the performance of molten salt and solid particle CSP technologies with various sCO2 cycle layouts and parameters. It was found that systems employing particle technology consistently have a lower levelized cost than molten salt systems, mainly due to the latter’s high storage system cost, caused by a low temperature spread. Furthermore, less complex process layouts without reheat or intercooling and even without recompression render lower levelized cost, which is caused by increasing costs for compressors, motors and recuperators in high-performance layouts. Compared with the reference system based on a steam power block, the best sCO2 processes achieve similar LCOE values but not the oftenproposed significant improvements. These findings are highly dependent on some of the cost models, mainly for the primary heat exchanger and for indirect power block costs, which will be refined in a next step. INTRODUCTION Supercritical CO2 (sCO2) power cycles have the potential to reach considerably higher thermal efficiencies than state of the art steam cycles while minimizing the size and number of components. To reach high thermal efficiencies, the average temperature at which heat is supplied to the cycle has to be very high. Concentrating solar power (CSP) technology allows for this as the heat transfer medium downstream the sCO2-primary heat exchanger is reintroduced into the solar receiver, making CSPsCO2 processes appear like a perfect match. Unfortunately, heat transfer media used in commercial CSP plants have limiting upper and lower temperature constraints. Currently, the maximum temperature reached in these plants is approximately 565 °C for molten salt, which does not allow for using the most efficient high-temperature sCO2 cycles. Contrary to these state-of-the-art heat transfer media, certain ceramic particles have no temperature limitations within the relevant technological range (0 °C. 1000 °C). This leads to the following potential advantages when combined with sCO2 cycles: • Very high temperature sCO2 processes can be employed, leading to high thermal efficiencies. • High approach temperatures to the primary heat exchanger (PHX) can be realized, leading to smaller heat transfer area requirements and, therefore, costs. • Even when a small sCO2 temperature rise is desired in the PHX in order to improve cycle efficiency, the temperature spread between hot and cold particles can still be kept comparatively large due to the high approach temperature. This temperature spread has a direct and significant effect on the cost of the thermal energy storage (TES) system cost [1]. Particle technology therefore allows for employing high temperature, highly recuperated sCO2 power blocks (PBs) reaching thermal efficiencies in excess of 50 %. Although it is tempting to define a system for maximum efficiency, the choice should be based on the techno-economic optimum of the whole plant. The recompression cycle is commonly seen as the most efficient layout for an sCO2 PB and has been proposed numerous times for integration with CSP [1-4]. Although this layout achieves high thermodynamic performance, it requires large and costly internal recuperators for this as well as a small temperature rise in the PHX. This leads to increased costs of the cycle equipment but also of the solar components, mainly the TES system. Due to their much lower cost, simple recuperated cycles, for example, have been found to be competitive on a techno-economic level [3, 5]. Other studies have found that, particularly for molten salt systems, partial cooling layouts can be beneficial as they increase the PHX temperature rise [6]. Besides the choice of the process layout, their main parameters (e.g. turbine inlet temperature, upper and lower cycle pressure, terminal temperature differences in all heat exchangers, .) also have a strong impact on the cycle components’ costs as well as on the solar components (via the PHX). This adds up to a large number of variables with non-obvious system-wide optima. DOI: 10.17185/duepublico/73961 2 Within Work Package 2 of the CARBOSOLA project, funded by the German Ministry for Economic Affairs and Energy, Siemens Energy and DLR are assessing the economic potential of sCO2 cycles for CSP power plants. The present study concerns the initial step of this undertaking: the pre-selection of a small number of CSP technologies and sCO2 processes, which are expected to have the highest techno-economic potential. The main areas described are the definition of investigated systems and boundary conditions, their thermodynamic modeling, economic models and, finally, results. The used models employ numerous simplifying assumptions due to the lack of maturity of, and therefore data for, the technologies. This is particularly true for the cost models of some major sCO2 and particle components. In further works, the identified processes will be designed in more detail leading to improved cost and performance models. BOUNDARY CONDITIONS AND DESCRIPTION OF INVESTIGATED SYSTEMS The main boundary conditions for the modeled power plants are presented in Table 1. The location has been chosen because the Redstone Solar Power Plant is planned to be erected there and reference data exists from previous projects [7]. The location is noteworthy for a high annual direct normal irradiation (DNI) and rather moderate mean ambient temperatures. Two different CSP technologies are investigated and modeled: State of the art molten salt (MS) and next-generation solid Particles (Pa). The maximum operating temperature of MS has been set to 565 °C. In a further variant, salt temperatures of up to 615 °C are assumed to be reachable although this has not yet been proven. This variant is meant to show the potential of sCO2 cycles in combination with evolutions to the MS technology [7]. It was assumed that the MS receiver system operates at the same efficiency for all receiver outlet temperatures, which is obviously optimistic for the 615 °C case. The maximum particle temperature has been set to 900 °C in all variants. All assumptions presented in Table 1 regarding the MS system (except for the receiver efficiency dependency on receiver inlet temperature, which is based on internal studies), were derived from a previous project [7]. The data for the Pa system is mainly based on internal studies at DLR for systems employing the CentRec© particle receiver. More information on the technology can be found in recent publications [1, 8]. The solar multiple, which is a measure for the oversizing of the solar field with respect to the steam generator, of the Pa systems was adjusted to render the identical annual electricity output as the MS systems. TECHNO-ECONOMIC MODEL The simplified annual yield model for the pre-selection process does not include hourly time steps for the performance calculation of the solar field but instead uses average annual values (shown in Table 1) for the subsystems. This allows for the comparison of the performance of tens of thousands of variants of the sCO2 PB. The subsystem costs for each variation of the PB or solar technology is calculated by setting the design point electricity yield (according to Table 1) and calculating the necessary rating of all subsystems via their design point efficiencies. This would result, e.g., in a larger solar field for a less efficient power block (all other efficiencies unchanged). The focus of this study is the sCO2 PB, its predicted performance and cost as well as the influence it has on the overall plant performance and cost. As mentioned above, the simplified model used here only requires a design-point simulation of the PB, meaning that part-load behavior is not modeled. The performance of the cycles and the rating of their components is calculated in the power plant simulation software Ebsilon Professional V. 14.03 by STEAG Energy Services GmbH. Thermodynamic results were validated with data from the literature and very good agreement was found. Some of the major assumptions and ranges of optimization parameters are provided in Table 2. Parameter Value