Graduate Studies, UNL

 

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–

First Advisor

David Yuill

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Moe Alahmad, Seunghee Kim, Xiaoqi Liu

Department

Architectural Engineering

Date of this Version

2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Architectural Engineering

Under the supervision of Professor

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, the author. Used by permission

Abstract

Energy storage (ES) has become an essential element of modern energy systems, particularly with the rapid growth of renewable energy sources. While these sources are central to global decarbonization efforts, their intermittent nature creates significant challenges for both grid operators and end users. To address these challenges, a wide range of storage technologies have been developed ranging from large utility-scale to small behind-the-meter units. Among these, Compressed Air Energy Storage (CAES) stands out for its long lifespan, low storage cost per unit of energy, and ability to provide both electricity and thermal energy. However, small- and micro-scale CAES systems, unlike large-scale units that employ caverns, rely on storage tanks which significantly increases the investment required for such systems. Therefore, integrating CAES systems into existing infrastructures has been explored to reduce capital costs. One such example is the use of building foundation piles, where storage volume and maximum allowable pressure are dictated by structural constraints rather than energy optimization. These limitations reinforce the importance of carefully designing the expander and discharge strategy to maximize useful work output.

This study addresses a critical and previously unexplored question: How does a reciprocating piston expander (RPE) perform when operated under free-flow discharge, without a pressure regulating valve (PRV), and connected to a finite-size storage tank that mimics building foundation piles? To answer this question, RPE performance under free-flow discharge is modeled based on thermodynamic fundamentals, with correlations developed to calculate overall work output under both the free-flow and throttled flow discharge. Notably, the analysis explicitly incorporates expansion-ratio mismatch and residual-mass losses, two loss mechanisms that remain understudied in RPEs. By directly comparing the two discharge scenarios, this dissertation addresses whether the inclusion of a PRV is justified in small-scale systems.

Through modelling the free flow performance and the development of theoretical correlations for estimating overall useful work, it was determined that the free-flow strategy has a consistent superior performance over using a PRV in the range of parameters studied. Moreover, the developed correlations show that overall useful work is a function of four distinct parameters: 1. Storage tank volume, which is shown to act as a linear scaling factor, 2. Storage pressure, which has a linear effect under throttled flow and is nearly linear in free flow, 3. Stopping pressure, which is the PRV setpoint pressure in the throttled valve and final pressure in the free-flow scenario, and is shown to have a quadratic correlation with overall work output, and 4. The expander geometry, which by itself is divided into the expander cylinder’s maximum volume, the crank-to-rod ratio, and the clearance volume ratio. Geometric ratios have modest influence: a higher crank-to-rod ratio and a smaller clearance ratio both improve performance, with clearance having a larger effect. Furthermore, it is proven that the cylinder’s maximum volume does not impact the performance of the expander. These insights suggest that system designers should prioritize pressure optimization, and, where stability and control requirements allow, adopt free-flow discharge to maximize efficiency in small-scale CAES.

Advisor: David Yuill

Download

Share

COinS