• Energy Research

Abstract The basic absorption thermal energy storage cycle suffers from low energy storage efficiency and density, while the conventional H2O/salt working fluids risk crystallization problems. To achieve crystallization-free thermal batteries with improved energy storage performance, a hybrid compression-assisted absorption thermal energy storage cycle using H2O/1,3-dimethylimidazolium dimethylphosphate is proposed. The fluid property is modeled using non-random two liquid equations and the hybrid cycle is modeled using heat/mass transfer/conservation equations; both models are validated against measurement data with good accuracies. The effect of pressure ratio on the transient behaviors and energy storage density/efficiency is investigated to characterize the performance enhancement. Besides, different hybrid cycles are compared with the basic cycle to identify the best-performing thermal battery. Results show that, with both generation and absorption enhancement, the concentration difference is significantly enlarged from 0.230 to 0.480 by the hybrid cycle with a pressure ratio of 2.0. Meanwhile, with energy storage efficiencies maintained at 0.715–0.794, the energy storage density is effectively increased from 110.3 to 199.2 kWh/m3. Through valve switching, three hybrid cycles are realized, i.e., hybrid cycle with charging/discharging compression, hybrid cycle with charging compression, and hybrid cycle with discharging compression. Comparisons indicate that the hybrid cycle with discharging compression is the most energy-efficient, with the highest energy storage efficiency of 0.816, compared to 0.794 of the hybrid cycle with charging/discharging compression and 0.790 of the hybrid cycle with charging compression. In terms of energy storage density, the hybrid cycle with discharging compression performs quite close to the hybrid cycle with charging/discharging compression, only slightly lower by 1.6–4.3%. However, compared to the hybrid cycle with charging compression, the energy storage density of the hybrid cycle with discharging compression is significantly enhanced by 11.4–52.0%. In summary, the hybrid cycle with discharging compression is the best cycle comprehensively considering the energy storage efficiency and density. This study aims to provide theoretical supports and suggestions for the development of advanced thermal battery cycles.

Abstract Desiccant dehumidification is an alternative solution for independently processing air humidity, which can simultaneously accommodate the sensible and latent heat loads even in high-humidity areas (e.g., Hong Kong). However, there is still a lack of comparative investigation in practical applications between the traditional condensation dehumidification and desiccant dehumidification, which is of great significance for revealing the effectiveness of desiccant dehumidifiers. Therefore, this study conducted on-site measurements in four different types of buildings. Moreover, the annual performance of different dehumidification solutions based on the set values of real buildings were simulated to further compare the dehumidification capacity, energy consumption, and cost performance. Results showed that the indoor humidity control of the measured buildings was substandard when using condensation dehumidification, and it was significantly improved after installing desiccant dehumidifiers. Compared with overcooling & reheating condensation dehumidification that achieved a similar humidity level, the annual energy savings of the desiccant dehumidification were respectively up to 4.9% and 7.7% for an office and a hotel based on the simulation results. The economic analysis indicated that desiccant dehumidification was more cost-effective than condensation dehumidification with overcooling & reheating. Finally, comprehensive evaluations were presented to facilitate the selection of dehumidification solutions.

Thermal battery plays an important role in renewable energy utilization towards carbon neutrality. The novel absorption thermal battery (ATB) has excellent performance but suffers from serious capacity attenuation. To address this problem, two capacity regulation methods, i.e., variable solution flow and variable cooling water flow, are proposed to achieve a demanded discharging rate. The effects of the two regulation strategies on the dynamic discharging characteristics and overall storage performance are comparatively investigated. To demonstrate the adjustability of the output capacity, several stable discharging rates are successfully maintained by the proposed methods. To maintain a higher discharging rate, the stable discharging time has to be sacrificed. As the demanded output increased from 0.5 kW to 6.0 kW, the stable discharging time decreased from 781.8 min to 27.9 min under variable solution flow and from 769.9 min to 30.7 min under variable cooling water flow. With the increase of solution or water flow rate, the energy storage density is improved, while the energy storage efficiency is slightly increased first and decreased later. The regulation method of variable water flow shows relatively lower energy storage efficiency due to the larger pump power. This study could facilitate reasonable development and application of ATB cycles.