Long-duration, grid-scale energy storage technologies provide a potential pathway to enable the full penetration of renewables into the electricity grid while still maintaining grid reliability and security. Among these technologies, pumped heat energy storage (PHES) stores energy thermally using a heat pump and discharges that stored thermal energy with a heat engine when needed. With high potential system performance, implementation versatility, and no geological and geographical constraints, PHES is a promising technology. While PHES leverages many existing component technologies, no air-based PHES system has been built and demonstrated and the path to commercialization requires intermediate demonstration. This paper summarizes the design of a PHES system with the purpose of demonstrating operation and verifying control strategies of a closed air Brayton PHES at lab scale. With lower temperatures and smaller machinery, this facility will provide better understanding of operational limitations, discover first implementation challenges, and reduce risk for the full-scale system. This paper presents the challenges associated with cycle design using off-the-shelf hardware, and how that lead to the development of custom machinery including the turbine aerodynamic flowpath and nearly all mechanical design of the turbocompressors. Finally, cycle analysis updates with as-built hardware and transient modelling are presented. The subject design effort resulted in a simple recuperated lab-scale PHES with three heat exchangers shared between charge and discharge mode and two turbocompressor drivetrains, one for each operation mode. At design point, the system is predicted to have 10% round trip efficiency, generate 5 kW with 1 hr of storage.
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Existing research has demonstrated the viability of supercritical carbon dioxide as an efficient working fluid with numerous advantages over steam in power cycle applications. Selecting the appropriate power cycle configuration for a given application depends on expected operating conditions and performance goals. This paper presents a comparison for three indirect fired sCO2 cycles: recompression closed Brayton cycle, dual loop cascaded cycle, and partial condensation cycle. Each cycle was modeled in NPSS with an air side heater, given the same baseline assumptions and optimized over a range of conditions. Additionally, limitations on the heater system are discussed.
Grid-scale energy storage technologies that provide greater than ten hours of electricity at full power are a critical enabler for full penetration of renewables on electricity grids worldwide. These technologies minimize renewable curtailment, while maximizing grid security and reliability. Other energy storage solutions such as pumped hydro and compressed air energy have geological and geographical constraints that widespread adoption. Pumped Heat Electricity Storage (PHES) systems can store energy thermally in any location at the low costs necessary for displacing fossil fuels, reducing foreign fuel imports, and reducing emissions to maintain the energy security of the world. PHES systems operate in two modes, a charge mode in which the system operates as a heat pump to convert electricity into a thermal potential which is stored in a suitable storage media, and a discharge mode which uses a heat engine to convert the stored thermal energy to electricity. These systems offer a potential Round-Trip Efficiency (RTE) greater than 60%. This is a key parameter of interest for energy storage application systems, defined as the ratio between energy into the system to the energy retrieved. Efficient PHES system design represents a compromise between thermal storage temperatures, system configuration, machinery selection, and minimization of secondary losses. This paper examines design trades associated with turbomachinery, system pressure ratio, and storage temperatures and the impact on RTE for a recuperated full scale PHES system progressing through conceptual design, and wraps up with turbomachinery evaluation and selection and resulting cycle for a small-scale system demonstrator used for demonstrating PHES system startup and operational control.
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