Advanced Absorption Thermal Batteries for High-efficiency and High-density Energy Storage

Abstract

Increasing the proportion of renewable/waste energy on the energy supply side is critical to alleviating the energy crisis and environmental problems and achieving a carbon-neutral world. However, wider applications of renewable/waste energy have been mainly hindered by the following challenges: (1) renewable/waste energy is often of low grade (<100 ºC), e.g., solar energy at 45-80 ºC and industrial waste heat at 50-85 ºC, which are difficult to be directly used effectively or economically; (2) renewable/waste energy is available intermittently, e.g., solar energy is restricted by weather and day-night alternation, industrial waste heat is only generated during the production process, thus they are difficult to match the energy demand of end-users. Thermal battery, also known as thermal energy storage (TES) technology, can bridge the energy supply and demand to make up for the mismatch, attracting increasing attention in recent years.

Absorption thermal battery (ATB) stands out due to its excellent energy storage performance and operational flexibility (i.e., cooling, heating, and dehumidification). A series of studies have been conducted to investigate the performance of basic ATB through dynamic simulation and experiment. The charging and discharging characteristics and the storage performance of the basic ATB under various external thermal conditions have been presented and examined. A demonstration project with a solar-driven ATB prototype that has a storage capacity of 5 kWh is built to promote the application of ATB technology.

However, there is still a lot of room for performance improvement for the basic ATB. For example, the energy storage density (ESD) needs to be improved under low charging temperatures, and the energy storage efficiency (ESE) is expected to be enhanced with high charging temperatures. Therefore, this thesis proposed various advanced ATB cycles and investigated them through simulations and experiments, aiming to improve the ESD and ESE and lower the charging temperature. Besides, type II ATB, also known as energy storage heat transformer (ESHT), is proposed to combine energy storage and temperature upgrading for deep utilization of low-grade renewable energy and facilitate the high-grade user demand. Then, some regulation strategies are established to achieve stable and adjustable discharging rates to better match the variable user loads. Finally, comparative investigations among various ATB cycles have been conducted based on a comprehensive evaluation system.

The hybrid compression-assisted ATB (CATB) is proposed to improve the ESD under low charging temperatures (e.g., below 80 °C). With the auxiliary compression, both the generation and absorption processes are strengthened, the concentration glide is enlarged, especially under low charging temperatures, e.g., for a charging temperature of 80 °C, the ESE is increased from 0.58 (the basic cycle) to 0.62 (charging compression), 0.70 (discharging compression), and 0.67 (charging/discharging compression), respectively, with the corresponding ESD improved from 104.8 kWh/m³ to 143.1 kWh/m³, 245.7 kWh/m³, and 282.8 kWh/m³ as well.

The two-stage ATB with absorption-enhanced generation is also proposed for the low-charging-temperature conditions. A prototype with a cold storage capacity of 5 kWh is designed and manufactured. An extremely low charging temperature of 50 ºC is achieved by the two-stage ATB. Under a charging temperature of 70 ºC, the ESD of the two-stage ATB is greatly enhanced from 62.7 kWh/m³ (163.1 kJ/kg) to 100.0 kWh/m³ (260.1 kJ/kg) compared to the basic ATB.

To improve the ESE under high charging temperatures (e.g., over 160 °C), the double-effect ATB is proposed for cascade utilization of the heat sources. The ESE is remarkably enhanced by 57.1-61.6% for cooling and 58.2-61.8% for heating compared to the basic ATB cycle. Moreover, the double-effect CATB is investigated to make up the charging temperature gap (i.e., 100-160 °C) between the basic and double-effect cycles. By combining the advantages of the double-effect and compression-assisted cycles, the double-effect CATB achieves high ESE and ESD simultaneously, with a maximum ESE above 1.3 and ESD over 300 kWh/m³.

Discharging for heating under low ambient temperatures seriously degrades the storage performance of ATB, which is still a major challenge. Therefore, this thesis proposed a phase-change-material (PCM)-assisted ATB to recover and store the condensation heat in the charging process. Then, the recovered heat is used to heat the evaporator to strengthen the evaporation-absorption process. The PCM-assisted ATB can significantly enhance the ESD and slightly improve the ESE under low ambient temperatures.

To facilitate the deep utilization of low-grade renewable/waste energy, the type II ATB, also known as energy storage heat transformer (ESHT) is proposed with the ability of temperature upgrading. The compression-assisted cycle and two-stage cycle are also applied to the type II ATB to achieve a lower charging temperature, higher storage performance, and larger temperature lift.

In spite of the good storage performance, the attenuation problem in output capacity caused by solution dilution hinders the applications of ATB technology. Therefore, this thesis proposed some regulation strategies to achieve stable and adjustable discharging rates to better match the variable user loads, including the flowrate-based capacity regulation and multi-level ATB. Also, a long-term dynamic supply-demand matching case study is provided with detailed and comprehensive simulations.

After that, this thesis conducts comparative investigations among various ATB cycles (i.e., the basic ATB, the CATB, the double-stage ATB, the double-effect ATB, and the double-effect CATB) to reveal rational design principles to accommodate different energy storage scenarios. The systematic, comprehensive, and comparative performance evaluation is carried out from a multi-criteria perspective, i.e., ESD, ESE, EXE, charging temperature, thermal stability, and cost. The effects of charging/discharging/cooling temperatures on energy storage performance are analyzed in three scenarios: short-term cold storage, short-term heat storage, and long-term heat storage. Moreover, recommendations have been made to guide the rational selection and development of ATB cycles under different external thermal conditions.

Finally, perspectives on the development and application prospects of ATB technology are given, which provide suggestions and references for future research.

Date of Award	20 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wei WU (Supervisor)