Fundamentals and Applications of Supercritical Carbon Dioxide (SCO2) Based Power Cycles

Klaus Brun, Peter Friedman, Richard Dennis

• Publisher: Woodhead Publishing
• Year: 2017
• ISBN:
  9780081008058
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

Fundamentals and Applications of Supercritical Carbon Dioxide (SCO2) Based Power Cycles aims to provide engineers and researchers with an authoritative overview of research and technology in this area. Part One introduces the technology and reviews the properties of SCO2 relevant to power cycles.

Other sections of the book address components for SCO2 power cycles, such as turbomachinery expanders, compressors, recuperators, and design challenges, such as the need for high-temperature materials. Chapters on key applications, including waste heat, nuclear power, fossil energy, geothermal and concentrated solar power are also included. The final section addresses major international research programs.

Readers will learn about the attractive features of SC02 power cycles, which include a lower capital cost potential than the traditional cycle, and the compounding performance benefits from a more efficient thermodynamic cycle on balance of plant requirements, fuel use, and emissions.

• Represents the first book to focus exclusively on SC02 power cycles
• Contains detailed coverage of cycle fundamentals, key components, and design challenges
• Addresses the wide range of applications of SC02 power cycles, from more efficient electricity generation, to ship propulsion

1

Introduction and background

G. Musgrove¹, and S. Wright^{2 1}Southwest Research Institute, San Antonio, TX, United States ²SuperCritical Technologies, Inc., Bremerton, WA, United States

Abstract

By taking advantage of fluid properties near the critical region, power cycles operating with a supercritical fluid can obtain high thermal efficiencies near 50%. The specific selection of CO₂ as the supercritical fluid allows the power cycle to be paired with a range of heat sources that include conventional fuel, nuclear, and renewable energies. Also a benefit to the power cycle is the fact that the high density of the supercritical fluid allows small-scale machinery compared to conventional cycles utilizing air or steam. Although there are benefits to a supercritical cycle and the use of CO₂, there are a number of technical challenges and considerations to be made in the design of supercritical CO₂ power cycles. In this introductory chapter, an overview is given of the primary considerations and challenges that are discussed in more detail in later chapters.

Keywords

Auxillary equipment; Cycle controls; Heat exchanger; Power cycle; sCO₂ power cycle applications; Turbomachinery

Overview

By taking advantage of fluid properties near the critical region, power cycles operating with a supercritical fluid can obtain high thermal efficiencies near 50%. The specific selection of CO₂ as the supercritical fluid allows the power cycle to be paired with a range of heat sources that include conventional fuel, nuclear, and renewable energies. Also a benefit to the power cycle is the fact that the high density of the supercritical fluid allows small-scale machinery compared to conventional cycles utilizing air or steam. Although there are benefits to a supercritical cycle and the use of CO₂, there are a number of technical challenges and considerations to be made in the design of supercritical CO₂ power cycles. In this introductory chapter, an overview is given of the primary considerations and challenges that are discussed in more detail in later chapters.

Key Terms

Auxillary equipment, Cycle controls, Heat exchanger, Power cycle, sCO₂ power cycle applications, Turbomachinery.

1.1. Introduction

A power cycle is a collection of processes and machinery used to generate useful energy from heat or momentum sources. For example, wind is considered a momentum source and fuel is considered a heat source. In keeping with the purpose of this book, power cycles using a momentum source are neglected and power cycles using a heat source are the main focus. While there are a number of different power cycles, the most commonly used for large-scale power generation are the Brayton cycle and the Rankine cycle (Fig. 1.1). A simple observation reveals that the Brayton cycle lies completely within the single-phase gas region, while the Rankine cycle spans the vapor and liquid phases separately. In either cycle, heat is added or removed at constant pressure, a pump or compressor increases the pressure before heat addition, and an expander (turbine) reduces the pressure and extracts work from the cycle. The simple cycle configurations shown in Fig. 1.1 can be modified to increase the cycle efficiency by using reheating, intercooling, and recompression among others. Cycle modifications for supercritical CO₂ (sCO₂) will be discussed in later chapters.

Figure 1.1 Brayton and Rankine cycles.

In considering the process of converting heat energy, the differential temperature between the hot and cold reservoirs of the cycle drives the cycle efficiency. As discussed in more detail in Chapter 3 Section 3.2.2, the concept of an upper limit to the cycle efficiency is termed the Carnot efficiency and can be approximated from the hot and cold reservoir temperatures of the cycle (Eq. 1.1) and varies linearly with the ratio of hot and cold temperatures, as shown in Fig. 1.2. For a given cold reservoir temperature, however, the Carnot efficiency is nonlinearly dependent on the hot temperature. Therefore, the slope of the cycle efficiency curve decreases as the hot temperature is increased. While the Carnot efficiency is considered the upper limit to thermal efficiency of the cycle, the details of the power cycle, such as the actual process and the machinery, affect the efficiency of the energy conversion. In most cases, it is difficult to achieve 50–60% of the Carnot efficiency.

ηCarnot=1−TCTH

(1.1)

In 1968, publications by Angelino (1968) and Feher (1968), presented concepts for a supercritical power cycle. In one formulation, Feher describes the supercritical power cycle as an alternative to the Rankine and Brayton cycles that operate either in the liquid or supercritical region, never crossing into the gas–liquid two-phase region, as shown in Fig. 1.3. In this concept, the fluid is pumped as a liquid, heat is added at constant pressure, the fluid is expanded in the supercritical state, and heat is rejected at constant pressure. The heat rejection is noted to be performed in a recuperator to offset the heat requirements during heating. Feher also mentions another formulation of the cycle that crosses into the two-phase region of the fluid. This formulation was mentioned by Feher to unknowingly have also been published by Dekhtiarev (1962) and was also the topic of Angelino's work (1968).

Figure 1.2 Carnot efficiency limit.

Figure 1.3 Supercritical cycle concept (Feher, 1968).

1.2. Overview of supercritical CO₂ power cycle fundamentals

The choice of sCO₂ for a power cycle comes from two distinct reasons. The first is that a power cycle using a supercritical fluid can increase the cycle efficiency by taking advantage of fluid properties near the critical region. The second reason is that CO₂ has a nearly ambient critical temperature (31°C), which allows the sCO₂ power cycles to be paired with a wide range of heat sources and also allows variations of the cycle to be operated with heat rejection to near-ambient sinks. For example, the near-ambient critical temperature can allow an sCO₂ Brayton cycle that rejects heat to air or water at ambient conditions.

Following from the descriptions of Feher and Angelino, the cycle layout for most implementations of the sCO₂ cycle have not changed. They include an expander, heat exchangers to add heat and remove heat to the cycle, a recuperator heat exchanger, and either a pump or a compressor or both to compress the fluid. The thermodynamic analysis by Angelino compares the performance of different cycle variations to show that those with recompression and reheat provide the highest efficiency, as shown in Fig. 1.4. The loop configurations that have been tested or designed typically using the reheat or recompression cycle layout, which may or may not include a pump to be a condensing cycle. An example of a recompression cycle layout is shown in Fig. 1.5.

1.2.1. Cycle machinery and balance of plant

The major components that are generally discussed in sCO₂ power systems are the compressors (or pumps), expanders (turbines), and the heat exchangers including recuperators, the primary heat exchanger, and the waste heat rejection heat exchanger. Compared to steam power systems or gas turbines, the design and operation of these components are unique because of the physical properties such as high power density, high pressure, low viscosity, and rapid changes that occur when the component must operate near the critical point or as a dense-phase supercritical fluid. This leads to component designs that have features that often challenge the standard designs for the major components in most power plants, such as power density, seal leak rate, and small size.

A number of other c...

Table of contents

• Cover image
• Title page
• Table of Contents
• Woodhead Titles
• Copyright
• List of contributors
• The Editors
• Foreword
• Overview
• 1. Introduction and background
• 2. Physical properties
• 3. Thermodynamics
• 4. High-temperature materials
• 5. Modeling and cycle optimization
• 6. Economics
• 7. Turbomachinery
• 8. Heat exchangers
• 9. Auxiliary equipment
• 10. Waste heat recovery
• 11. Concentrating solar power
• 12. Fossil energy
• 13. Nuclear power
• 14. Test facilities
• 15. Research and development: Essentials, efforts, and future trends
• Index