One major goal of sustainable development strategy is to address the severe impact of global climate change by reducing the overdependence and consumption of fossil fuels. To this end, the proportion of renewable energy sources, especially solar and wind energy, in the global energy mix has significantly increased. Although renewable energy sources are abundant, they are intermittent. With the increasing demand for renewable energy, developing flexible, competitive and economical energy storage technologies to balance the production and consumption of the power system is highly desirable.
Among the energy storage technologies, pumped thermal energy storage (PTES) is a promising technology owing to its numerous advantages, including a long-life cycle, no fossil fuel and cheap storage fluid. A typical PTES consists of thermal energy storage systems, heat pump cycles and power cycles. To improve its applicability in different industrial scenarios, different types of PTES systems have been designed. Based on various power cycles, PTES has two main branches: Brayton and Rankine. While Brayton PTES has been extensively studied and developed, PTES based on Rankine cycles remains underexplored despite its potential benefits.
PTES systems can be based on different types of power cycles including organic flash cycle (OFC) and organic Rankine cycle (ORC). ORC is widely used to convert low-temperature waste heat into electrical energy, while OFC is a potential candidate for thermal power conversion due to its higher potential performance. Despite the remarkable attempts to evaluate, analyze and compare different types of OFC, the integration of OFC with PTES system and comprehensive evaluation of their economic and thermodynamic performance is still lacking.
Herein, Dr. Huan Xi and graduate student Wenbiao Tian from Xi’an Jiaotong University conducted a comparative analysis and optimization of PTES systems based on different power cycles. To address the deficiencies of the current research on PTES systems, five PTES systems (one based on basic ORC and four based on OFC as power cycles) were established. The economic and thermodynamic performance of these five systems were explored and compared to establish the best system. Taking levelized cost of storage (LCOS) as the optimization objective, the variation in different indicators like heat pump cycle and thermal efficiency were determined. Their work is currently published in the research journal, Energy Conversion and Management.
The researchers observed that the compressor and the turbine occupied the largest share of the total investment cost, while the throttle valve had the largest exergy destruction. For the five PTES systems, the optimization results showed a descending LCOS with an increase in the storage temperature of the subsystem. This suggested that an increase in the storage temperature within a desirable range could improve the economic performance of the whole system. As a result, the PTES based on ORC (BORC-BVCHP) exhibited the best economic performance with a LCOS value as low as 0.4413 $/kWh at the storage temperature of 403.15 K.
The BORC-BVCHP system also exhibited a maximum exergy efficiency and optimal round-trip efficiency of 23.40% and 31.15%, respectively, with the increased subsystem storage temperature. However, the round-trip efficiency of the other four systems did not significantly change with the increase in the storage temperature. Thus, considering only these four systems would require compromising either round-trip efficiency or the LCOS to select the most appropriate storage temperature. Furthermore, the exergy efficiency and the round-trip efficiency parameters were closely related.
In summary, the economic and thermodynamic modeling of five different PTES systems was reported. The ORC-based PTES system outperformed that based on OFC from both economic and thermodynamic perspectives and is thus more suitable for practical application in PTES systems. In a statement to Advances in Engineering, the authors noted that their findings would enable parameter optimization and appropriate selection of PTES systems under different storage conditions.
Tian, W., & Xi, H. (2022). Comparative analysis and optimization of pumped thermal energy storage systems based on different power cycles. Energy Conversion and Management, 259, 115581.