Elsevier

Energy

Volume 173, 15 April 2019, Pages 691-705
Energy

A novel cryogenic energy storage system with LNG direct expansion regasification: Design, energy optimization, and exergy analysis

https://doi.org/10.1016/j.energy.2019.02.047Get rights and content

Highlights

  • A feasible design of cryogenic energy storage process with LNG regasification.

  • A novel design for efficient air sub-cooling via two-stage LNG pressurization.

  • A thermodynamic based optimization to maximize net power output.

Abstract

Recovering the remaining cold energy from the regasification process is one of the key challenges of the overall LNG value chain. This paper aims to develop a cryogenic energy storage system (CES) integrated with LNG direct expansion regasification (LNG–CES) that can recover cold energy and store it as cryogenic energy using air as the working fluid. Cold energy of LNG is available in two forms: thermal energy by heat exchange and shaft work by expansion, while the cryogenic storage process requires compression and cooling. The supply and demand of LNG direct expansion and cryogenic energy storage processes are well balanced. Therefore, a combined LNG–CES process to store energy will prove efficient. This study proposes an industrial-feasible design for the LNG–CES process and energy optimization to maximize net power output from the process. Moreover, a novel process design is proposed to recover cold energy lost during LNG regasification more efficiently. Energy optimization results of the proposed design demonstrated an 11.04% increase in the net power generation from the feasible configuration of the base design. Additionally, the cause of this improvement was studied using thermodynamic analyses.

Introduction

The 2018 Outlook for Energy (ExxonMobil, 2018) [1] forecasts liquefied natural gas (LNG) trade to meet one-third of the natural gas demand in the next three decades. LNG is preferred for long-distance transportation because its volume is approximately 600 times less than its same mass in gaseous phase [2]. Natural gas liquefaction requires cryogenic conditions (around −160 °C), which results in a large amount of energy being consumed during the liquefaction process [3]. Therefore, LNG is necessary to be regasified before it is transported to end users [4]. However, regasification without energy recovery results in most of the cold energy of LNG being wasted [4]. In general, the LNG cold energy is wasted to seawater during regasification, and it can be recovered by applying cold energy recovery processes [5]. To address this issue, different approaches towards LNG cold energy recovery have been reported [6].

Kanbur et al. [7] reviewed various processes that are used to recover LNG cold energy. They found that since its introduction, LNG cold energy has been utilized mainly for power generation. Thus, the major concern of LNG cold recovery is electricity production. Choi et al. [8] attempted to recover LNG cold energy for power generation by adopting five process configurations: direct expander, Rankine cycle, direct expander with Rankine cycle, two-stage cascade Rankine cycle, and three-stage cascade Rankine cycles. The net power, and thermal and exergy efficiencies of each configuration's performance were evaluated. They concluded that the three-stage cascade Rankine cycle using propane as the working fluid exhibited the best performance. Sun et al. [9] suggested an LNG regasification power plant with a Rankine cycle using a mixed composition working fluid. They reported that a decrease in exergy loss in the heat exchanger afforded the highest efficiency with a relatively simple process configuration. Gomez et al. [10] proposed an LNG power plant by applying closed Brayton and Rankine cycles. They performed case studies by applying different working fluids—helium or nitrogen for the closed Brayton cycle and carbon dioxide, ammonia, ethanol, or water for the Rankine cycle. They concluded that the best working fluid for the closed Brayton cycle was helium while carbon dioxide worked best for the Rankine cycle. Additionally, in their subsequent work, Gomez et al. [11] performed a thermodynamic analysis of a combined system comprising a closed Brayton cycle with helium, Rankine cycle with carbon dioxide, and fuel combustion. They analyzed the effect of different variables, including LNG pressure, compressor and turbine inlet temperatures, and compression ratio. Garcia et al. [12] proposed Rankine cycles in series as a heat sink to utilize the LNG cold energy. The proposed power plant used the following sequence: Rankine cycle with argon, Rankine cycle with methane, direct LNG expansion, and Rankine cycle with methane or R14. This group [13] also proposed a power generation process design based on two cascaded Rankine cycles with a direct LNG expander. They performed a sensitivity analysis of the pinch-point temperature and natural gas outlet pressure. Fazlollahi et al. [14] suggested a cryogenic carbon-capture system with energy storage. They proposed a design that stored energy using natural gas as the working fluid at off-peak times and captured carbon dioxide during on-peak times. Sun et al. [15] performed exergy efficiency optimization to elucidate the most efficient configuration of the power generation process by LNG cold recovery. Bao et al. [16] proposed a two-stage condensation Rankine cycle system using propane as the working fluid. Ghaebi et al. [17] suggested an ammonia–water cooling and power generation system that used LNG as the heat sink. They performed energy, exergy, and economic analyses, and sensitivity analysis of key variables. Ferreira et al. [18] studied the optimal working fluids for Rankine cycles with LNG cold recovery using a multi-objective optimization approach via genetic algorithm. Lee and Mistos [19] suggested using an optimization methodology for the selection of multi-component working fluids. The objective of optimization was to minimize the area between the hot and cold composite curves. Lee et al. [20] proposed a process design methodology of an organic Rankine cycle based on the LNG cold recovery process, using superstructure optimization approach. Le et al. [21] suggested an LNG cold utilization process, including pressure and thermal energy recovery. Dutta et al. [22] analyzed the economic feasibility of power generation processes coupled with LNG regasification. They proposed a genetic algorithm based on the economic optimization of various power generation processes via a combination of LNG direct expansion and organic Rankine cycle.

Some studies have also focused on the air separation unit and oxy–fuel combustion power plant. Aspelund and Gundersen [[23], [24], [25], [26]] proposed a liquefied energy chain that utilized LNG cold energy for the oxy–fuel power plant and carbon capture. In the onshore sector, LNG cold energy is utilized to separate air, and liquefy nitrogen and carbon dioxide. Here, natural gas is used as the fuel in oxy–fuel combustion, while nitrogen and carbon dioxide are produced during power generation. In the offshore sector, the cold energy of liquid carbon dioxide and nitrogen is used to liquefy natural gas. Mehrpooya et al. [27] introduced two-stage Rankine cycles integrated with LNG and solar energy. In their work, the LNG cold energy reduced the condensate pressure of the system and produced extra power. Mehrpooya and Sharifzadeh [28] proposed an oxy–fuel power generation cycle using LNG as the heat sink and a solar cycle as the heat source. In a later study, Mehrpooya et al. [29] suggested a coal-gasification process combined with an air separation unit. In this system, LNG was used for the heat sink, air separation unit, and cryogenic carbon capture unit. Mehrpooya and Zonouz [30] performed exergy and sensitivity analyses for an LNG cold utilization power plant coupled with an air separation unit, oxy–fuel combustion, and carbon capture.

At the same time, several studies have performed on energy storage systems on energy storage systems over the last decade [31]. Among these systems, the cryogenic energy storage (CES) is the most suitable for large-scale energy storage because of its unique characteristics, including low internal energy but high exergy. The interest in CES systems has been renewed because of amid growing environmental concerns surrounding power plants. Li et al. [32] assessed a process that used a cryogen as an energy carrier and performed economic analysis in a subsequent work [33]. Li et al. [34] also proposed a CES system coupled with nuclear power plants to balance the demand and supply during on- and off-peak hours. Abdo et al. [35] compared various CES systems that apply Rankine cycles, Linde–Hampson processes, and Claude processes with the basic CES system. Grossmann and co-workers [36] proposed a combined process that used an air separation unit and CES system. They proposed strategies through mathematical optimization with two product options: electricity generation from liquid air and sale of the liquid product. Sciacovelli and Ding [37] suggested a CES system with packed-bed cold thermal storage using air as the working fluid. To develop an efficient energy storage system, Park et al. [38] applied the CES system to a two-stage Rankine cycle with an LNG regasification process. In their process, the air recovers the cold energy of LNGoff peak, while liquid air generates power on peak. They stated that their system was an economical way to recover LNG cold energy into the CES system. However, because of the low capacity of the system, the utilize LNG cold energy recovery as an auxiliary energy source in the energy grid can be a better way than as the main energy source.

To address this issue, we developed a flexible energy storage and release process as an auxiliary to the energy grid from our previous work [39]. In the proposed system of the previous work, the direct expansion process is applied to the LNG regasification process, while air is continuously liquefied and stored using cold and work from the direct expansion LNG regasification process. As a successive work, the current study proposes an industry-feasible design that considers the following key industrial constraints: (1) maximum pressure of the cryogenic liquid air storage tank; (2) air humidity removal; and, (3) compressing seawater to use it as a heat source. In addition, a novel LNG–CES process design that can store a larger amount of air per unit mass of LNG by recovering a greater amount of cold energy from the LNG is proposed. To find the optimal design and operating conditions, a thermodynamic-based mathematical model has been developed. The proposed LNG–CES processes are optimized within a mathematical optimization model to maximize net power output. The novelty of this work is summarized as:

  • An industry-feasible configuration of cryogenic energy storage process combined with LNG direct expansion regasification

  • A novel design of air sub-cooling via two-stage pump configuration for LNG

  • A mathematical optimization based on thermodynamics to maximize total net power generation

Section snippets

Industrial feasible design

The base design of the LNG–CES process in literature [39] assumed a pressure of 210 bar for cryogenic liquid-air storage. However, the permissible pressure limit of a cryogenic liquid-air storage tank is only 250 psig (approximately 18 bar) [40]. Therefore, to find an industry-feasible design, the configuration of the cryogenic storage section of a LNG–CES base design should be modified. Fig. 1 illustrates liquid-air storage process configurations for the base design (assuming high-pressure

Mathematical model

A mathematical optimization model is developed, based on the thermodynamics, to optimize and analyze the energetic performance of the process. The simulation optimization model for the LNG–CES process is developed using an equation-oriented optimization software, gPROMS. The advanced Peng–Robinson equation of state is applied to calculate the thermodynamic properties of the process [44]. Each equipment unit is modeled individually based on heat and material balance, and connected following the

Results and discussions

The process simulation of the base case was performed using the developed mathematical model. The total net power generation of the base case was found to be −17.279 kW per 1 kg/s LNG flow rate. The negative total net power is mainly due to the partial vaporization of LNG after the expansion at the valve. The vapor phase fraction at the valve outlet is 0.514 and only 48.6% of feed air was liquefied and stored (885.61 kg/h out of 1822.51 kg/h). It revealed a large amount of exergy loss because

Conclusions

This study focuses on the industrially feasible design and optimization of CES systems with LNG direct expansion regasification process. The main objective of the LNG–CES system is LNG cold energy recovery via heat and power integration. This system has the potential to achieve high efficiency energy storage. However, several industrial constraints must be considered first before commercializing the developed process. This study adopts additional configurations for the conceptual design to help

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Education (Grant Number: 2018R1 A6A3A03011666).

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