Thermal energy storage for cooling applications: from devices to systems

Dai, Siyuan ORCID: 0000-0001-5499-4184 (2024). Thermal energy storage for cooling applications: from devices to systems. University of Birmingham. Ph.D.

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Abstract

Reducing energy consumption and carbon emissions in the building cooling is crucial for achieving the global target of net-zero emissions by the middle of this century. Latent Heat Storage (LHS) using phase change materials (PCMs), known for their high energy storage density and effective temperature control, provides an effective way to achieve energy savings and emission reduction. This PhD study aims at an in-depth analysis of the performance of PCM-based cooling devices and systems in various application scenarios using both modelling and experimental methods.
First, a scenario of integrated thermal energy storage (TES) with an air-conditioning (AC) system. At the device level, a compact plate-fin heat exchanger (PF-HEX) was studied which encapsulates the PCM and integrates with an air conditioning system for peak shaving of electrical loads. The results showed that under an airflow condition of 1 m/s, the charging and discharging times of the PF-HEX were reduced by 25.53% and 23.70%, respectively, compared to the use of a non-fin heat exchanger (NF-HEX). By increasing the airflow speed to 3 m/s, the time reduction was more pronounced, with charging and discharging times decreasing by 37.50% and 36.84%, respectively. These observations indicate that the heat transfer at the air side is the rate-limiting factor, and the use of the fin structures can significantly enhance the heat transfer between the PCM and the heat transfer fluid (HTF), and the simulation results showed that the heat transfer coefficient could be enhanced by approximately 6.4 times. The experimental results also demonstrated that the TES device based on the PF-HEX design achieved a maximum peak charging rate of 3 kW, a stable charging rate of up to 1.7 kW, with a charging efficiency between 72.07% and 87.67%, and a peak discharging rate of 2.1 kW, with discharging efficiency ranging from 88.06% to 95.54%. Furthermore, the TES device demonstrated stability in the cooling power, maintaining a stable outlet air temperature of 18°C for up to about 4 hours at an inlet airflow speed of 0.56 m/s.
Second, a scenario of TES integrated to an AC for a telecommunication cooling system. TES-AC system with a free cooling system, both experiments and simulations showed a significant improvement in the cooling efficiency of telecommunications base stations (TBSs). Under typical operating conditions, the TES device could be fully charged during a 7-hour low electricity period and provide cooling to the TBS room alone for about 13 hours and 12 minutes, maintaining a stable indoor temperature throughout. The TES device extended the cooling time from the original about 8 minutes to between 79 and 287 minutes, depending on the existing cooling capacity. The TES-AC system with free cooling demonstrated a significant reduction in cooling energy consumption and electricity costs, with a reduction from 31.47% to 80.54%, and from 2% to 80%, respectively. Dynamic simulation of the system over a year using a Nanjing telecommunication base station demonstrated that the TES-AC system improved the average Coefficient of Performance (COP) from 2.60 to 3.83, significantly impacting the TBS cooling needs by providing approximately 17.38% of the cooling demand and contributing to approximately 49.69% of the electricity cost savings. Simulation of multi-city globally showed that, except for under extremely hot or cold regions, positive energy and cost savings were obtained. The extensive use of the TES-AC system in China could result in an annual reduction in electricity consumption of approximately 5.5 TWh, a reduction in carbon emissions of approximately 3.06 Mt, and a saving of approximately $447 million in electricity costs. A sensitivity analysis revealed that the power consumption of telecom equipment, under an average outdoor temperature condition with typical insulation walls, are significant factors affecting the energy consumption of TES-AC systems. In contrast, the differences in electricity prices between peak and off-peak periods, the power consumption of telecom equipment, and the type of wall mainly affect the investment payback period.
Third, a scenario of TES integration with a cold warehouse. Simulation studies revealed a significant annual fluctuation in cooling load, ranging from 20 to 60 kW in a warehouse in Zhengzhou. Sensitivity analyses indicated that the forced convection heat transfer coefficient and the PCM thermal conductivity exert a significant influence on the overall heat exchange coefficient between the PCM cooling plates and the air. The impact of the shell material's thermal conductivity was found to be the least significant.
A comparative study utilising different quantities of PCM revealed that 6 tons of PCM in TES plates demonstrated the most favourable economic performance, with an investment payback period of just 2 years and 6 months. An increase in the proportion of PCM boards in one of the zones in the warehouse, Zone 5, was found to give a significant optimal cooling distribution, resulting in shortening the payback period to 2 years and 4 months.
Fourth, a scenario of TES integration for data centre cooling, particularly for emerging cooling. This study showed that the effective cooling power was the most significantly affected by the inlet air temperature, with higher temperatures resulting in higher effective cooling rates. Experiments demonstrated that the depth of TES discharge was 82.9%, with an average discharging power of 677W, which could extend the emergency cooling by 126 minutes in the event of AC system failure. An economic analysis demonstrated that the TES-AC system could offer significant economic benefits. The annual cost savings could be increased from 8.41% to 28.77% through using different amounts of PCM used in the TES devices. The static and dynamic payback periods were found to lower than 2 years and 8 months, respectively, demonstrating its good cost-effectiveness.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):