Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

D. Yogi Goswami, Ph.D.

Committee Member

Elias Stefanakos, Ph.D.

Committee Member

Wenbin Mao, Ph.D.

Committee Member

Sarah (Ying) Zhong, Ph.D.

Committee Member

George Philippidis, Ph.D.

Keywords

High-Temperature, Numerical Model, Phase Change Material, sCO2, Transient

Abstract

Mitigating the effects of climate change will require rapid deployment of a carbon-free electricity system that includes all available forms of zero-carbon energy. Both renewables and nuclear show great promise, but both come with drawbacks. Power production from wind and solar is limited to times when their respective resources are available, while nuclear is limited in its ability to adjust its output according to the ever-varying demand for electricity. Additionally, the systems must be affordable. One way to reduce the cost of thermal power plants (concentrated solar power and nuclear) is to use new power cycles, such as the supercritical carbon dioxide (sCO2) Brayton cycle, that operate at higher temperatures (700–1000 °C) to achieve higher efficiencies (~50%+).

As for the flexibility challenges, they can be overcome using energy storage. For thermal power generators (nuclear and concentrated solar power), thermal energy storage is of particular interest due to its potential for low cost and high-efficiency operation. Thermal energy storage has traditionally been performed in a two-tank direct sensible heat storage configuration, which pumps liquid storage material (typically molten salts) between a hot tank and a cold tank, interfacing with the power cycle through a heat exchanger. However, the commonly used nitrate salts are limited to temperatures below 600 °C, so alternate materials and/or designs are required for coupling with the sCO2 Brayton cycle.

In the literature, research has been performed to reduce the cost and increase the efficiency of thermal energy storage using both sensible and latent heat approaches as well as a hybrid approach that combines the two. These studies are limited in that they typically focus on temperature ranges suitable for current-generation CSP plants rather than the higher temperatures of advanced power cycles. In this study, a sensible/latent hybrid thermal energy storage system is explored through a numerical model with the goal of developing a low-cost, high-efficiency system for use with the sCO2 Brayton cycle.

First, a MATLAB program implementing a numerical model of a packed bed thermal energy storage system was developed based on models from the literature. The model was validated against experimental data from two studies, and its performance was found to be acceptable.

Next, a hybrid sensible/latent thermal energy storage system was evaluated with two heat transfer fluids, air and sCO2, with varying quantities of PCM. When the two fluids were compared with the same mass flow, sCO2 was shown to have a small advantage over air. When they were compared with the same volumetric flow, the higher density of sCO2 allowed it to transfer significantly more heat in the same amount of time, but the stabilization effect provided by the PCM was diminished.

It was then realized that using sCO2 directly in the storage system would involve significant engineering challenges related to the high pressure. A new approach was selected that used recirculated air at atmospheric pressure in the storage system while a heat exchanger allowed it to interact with the sCO¬2 in the power cycle. Two configurations using this setup were evaluated: one included a high-temperature (HT) PCM and rocks, while the second added a low-temperature (LT) PCM. These configurations were studied 1) as single cycles using fixed charging and discharging times and 2) as a series of 5 cycles with charging and discharging times controlled by the temperature of the sCO2 leaving the heat exchanger. Varying volume fractions of PCM (0-100% for HT-PCM, 0-10% for LT-PCM) and varying HT-PCM melting points (657, 670, and 680 °C) were explored. It was shown that 1) the heat exchanger causes a significant reduction in system temperatures; 2) the temperature stabilization effect is strongest when the HT-PCM temperature is relatively low compared to the charging inlet temperature of 700 °C and the volume fraction is around 15%; and 3) adding the LT-PCM leads to significant increases in energy stored/recovered and significantly extends the overall cycle in the multi-cycle approach.

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