Techno-economic Assessment of Latent Heat Thermal Energy Storage Integrated and Solar Assisted Heat Pump for Domestic Hot Water Systems


Student thesis: Doctoral Thesis

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Awarding Institution
Award date12 May 2022


The rapid escalation of building energy consumption has raised concerns worldwide. More alarming, this increment will likely aggravate the current energy crisis and environmental problems. Aiming to address the problem of high non-renewable energy consumption related to buildings, multi-renewable energy systems integrated with the storage technique have been constantly considered to simultaneously satisfy the requirement of load profiles whilst enhancing the energy flexibility.

In this study, a suitable latent heat thermal energy storage (LHTES) medium with a desirable phase-change behaviour for the air source heat pump (HP) system was prepared to enhance the flexibility potential of the heating device. The phase-change temperature of the salt hydrate was adjusted using melting point modifying agents. The supercooling and phase separation issues were alleviated using the nucleator and thickener, respectively. Each agent content was optimised to afford improved thermal energy storage performance without compromising the phase-change properties of the samples. Then, the thermophysical properties and crystalline structures of the final phase-change material (PCM) were characterised. The resultant PCM yielded a desirable melting point and a high latent heat.

The study further provides insight into the integration of the LHTES technique with the air source HP system. Firstly, the compatibility of the PCM to the heating device and the dynamic thermal behaviour of the entire heating system were experimentally evaluated. The LHTES medium effectively achieved the phase-change stage at the inflection point of approximately 47 °C. This finding indicates that the phase-change area (PCA) of the PCM can be adapted to the working condition of the air source HP system. In addition, comparative experiments were performed to investigate the effects of the operating conditions on the charging and discharging performance of the heating system, and then the optimal operating parameters were explored. The coefficient of performance (COP) of the HP system increased with the rise in the heat transfer fluid (HTF) flow rate and decrease in working fluid source temperature. However, the power drawn by the HP system presented an inverse relation to those variables.

A reasonable heating system should be developed to take full advantage of the storage medium. In this study, an experimentally validated numerical model representing the dynamic performance of the LHTES integrated HP heating system was established. The overall performance with respect to the incorporation of the LHTES technique into the air source HP system was assessed in view of meeting the residential hot water demand of subtropical dwellings. A numerical study was performed to identify the desirable storage medium that could match the heating system and determine the optimal design parameters. The thermal buffering amount and critical design factors were specified to be able to achieve peak load shifting without compromising thermal comfort whilst also ensuring low power or expense. Then, the impacts of the environmental variations and operation strategies on the energy and economic performance of the heating system were assessed. Results demonstrated that the tariff-based operation strategy outperformed the other operation options in terms of economic performance.

Solar energy was introduced to the heating system to further promote energy conservation and cost-saving potential. A test rig was constructed to evaluate the thermal performance of the LHTES integrated and solar-assisted HP heating system. The test results revealed the potential to implement the developed heating system for hot water production. Then, the impacts of the charging modes and operating conditions on the thermal behaviour of the heating system were explored. Compared with the single heating mode, the entire heating system exhibited considerably enhanced overall efficiency under the combined heating mode owing to the introduction of solar energy. The combined heating mode substantially accelerated the charging process of the storage medium. Nonetheless, the heat collection efficiency of the solar assisted HP heating system slightly varied with the flow rate of the working fluid because of the difference in power consumption.

A simulation platform describing the energy transmission of the heating system was also built to systematically investigate the system performance in relation to the energy and economic aspects. The practical application performance of the LHTES integrated and solar assisted HP heating system in real scenarios concerning dynamic meteorological conditions and various demands of end-users was evaluated. The optimal component size for promoting renewable energy penetration and decreasing electricity cost was determined. A reasonable operation framework was developed to enhance the economic and energy-saving benefits of the developed heating system. Results illustrated that a certain increment in the solar collection area could substantially benefit system performance, resulting in a high seasonal performance factor and a low operation cost. The oversized HP reflects a slight improvement in the overall efficiency of the system; however, it would require a high payback period (PBP). Compared with other system configurations and operation strategies, the hybrid heat source that considered the demand-side management was a more profitable solution for achieving low carbon emission and a short PBP. Overall, the solar assisted HP combined with the LHTES technique allows building heating systems to achieve energy saving and environmental sustainability. In addition, the occupied space of the system can be effectively decreased when the PCM with a large volume storage density is used.

    Research areas

  • Air source heat pump, Latent heat thermal energy storage, Phase-change material, Solar assisted heat pump, Energy utilization efficiency, Economic viability, System sizing, Operation strategy