The Design of a Small-Scale Pumped Heat Energy Storage System for the Demonstration of Controls and Operability

Abstract

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.

Transient Analysis and Control of a Brayton Heat Pump During Start-Up

“…Further work is then required to assess the magnitude of these issues and to investigate whether possible solutions may negatively affect the start-up time of the plant, therefore nullifying the economic effort of using more sophisticated heat exchangers. Finally, it is worth mentioning that the considered Brayton heat pump showed a start-up time consistent with the ones reported for a similar cycle specifically designed for energy-storage purposes [17], where start-up times in the 1.38−2 h range have been reported depending on the opening the compressor recycle valve opening. Here, differences in the results may be due to the different designs of the components, operating modes, temperature levels, and heat exchangers' size or technology, as the system are meant for different purposes.…”

“…Despite not optimizing the gas turbine speed ramp for quick start-ups, a start-up time of around 30 min was reported. Smith et al [17] presented the design of a laboratory scale pumped thermal electricity storage demonstration plant in order to test the control strategies and demonstrate the system operability and criticalities. Start-up times of around 1.38 h and 2 h were reported depending on the compressor recycle valve opening.…”

Impact of the Regulation Strategy on the Transient Behavior of a Brayton Heat Pump

“…Furthermore, the heat flow rate transferred through the heat exchanger was determined based on the ε-NTU method [18], summarized in Equation (11):…”

“…Zühlsdorf et al [10] suggested that technically and economically feasible heat pumps capable of supplying heat up to 280 • C could be developed by leveraging equipment already available in the oil and gas industry. Under this approach, various concepts aiming to supply heat at temperatures higher than 150 • C have been put forward in the literature for industrial and energy storage applications [5,11]. However, these systems need to be developed, realized, and integrated into relevant industries.…”

Dynamic Modelling and Part-Load Behavior of a Brayton Heat Pump

“…Several concepts capable of delivering heat above 150 °C have been proposed in the literature for industrial purposes [5]. Further prototypes have also been proposed for energy storage applications [10]. Nevertheless, these heat pumps must be developed, built, and integrated into the relevant industries.…”