Dynamic modeling of compressed gas energy storage to complement renewable wind power intermittency

Highlights

• Dynamic models of compressed air and hydrogen energy storage systems are developed.

• The compressed air storage model performance compares well to literature data.

• Performance characteristics of air storage are compared to hydrogen storage.

• Hydrogen storage can better complement wind intermittency for the same resource.

• Hydrogen storage achieves seasonal load shifting and produces transportation fuel.

Abstract

To evaluate the impacts and capabilities of large-scale compressed gas energy storage for mitigating wind intermittency, dynamic system models for compressed air energy storage and compressed hydrogen energy storage inside salt caverns have been developed. With the experimental data from air storage in a salt cavern in Huntorf, Germany, the cavern model has been verified. Both daily and seasonal simulation results suggest that with the same size wind farm and salt cavern, a compressed hydrogen energy storage system could better complement the wind intermittency and could also achieve load shifting on a daily and seasonal time scale. Moreover, the hydrogen produced in the compressed hydrogen energy storage system could also be dispatched as a fuel to accommodate zero emission transportation for up to 14,000 fuel cell vehicles per day while achieving seasonal load shifting.

Introduction

With increasing energy demand and growing concerns for environmental impacts, renewable energy sources are receiving increased attention for their inherent low pollutant and greenhouse gas emissions. Over the past few years, renewable wind power has become one of the fastest growing sectors of the U.S. renewable energy portfolio [1], [2]. In 2010, wind power installation in the U.S. was roughly 5 GW and comprised 25% of U.S. electric generating capacity additions [2]. In addition, 12,100 MW of new wind capacity is expected to be added by the year 2013, with forecasts of meeting 20% of the nation's electricity demand from wind energy by the year 2030 [2]. However, the intermittent and uncontrollable nature of wind power sources introduces new technical challenges for integration into electric power systems, especially as the market share of wind power becomes large. Wind power is intermittent directly due to spatial and temporal wind speed variations [3]. Spatial variability of wind speed is introduced by various climate regions, geographical location, local vegetation and topography [3]. Temporal intermittency and variability of wind power includes typical annual and seasonal variations, synoptic, diurnal and turbulence variations. Due to these uncontrollable intermittencies that act in different time scales, wind power sources could have significant negative impacts on grid operation in different time scales, including those that affect regulation, load following, and scheduling [3], [4], [5]. In the time scale of regulation (seconds to minutes), the impact of the wind intermittency may require significantly more regulation reserves and frequency control [5], [6]. In the load following time scale (minutes to hours), wind intermittency may require a significant increase in the amount of operating reserves [6]. In the scheduling time scale (hours to days), the wind intermittency may result in significant economic costs due to disturbing the generation mix versus time across the whole generation portfolio [3], [6]. At the current trends, higher renewable wind power penetration will occur and the large-scale integration needs to be implemented to accommodate the increased wind power penetration. To deal with the increased variability introduced by large-scale wind power generation on the power systems, several methods are proposed to complement the wind power intermittency and to improve the ability to integrate the increasing wind capacity. Methods focused on demand side management such as demand response technologies are being considered to reduce and manage the wind intermittency [7], [8]. Improvements in the operational flexibility of conventional power plants will also complement the intermittency of wind power [3], [4]. But with high market adoption of wind power it is expected that these measures alone will not be able to fully complement wind power, nor can these measures store wind power that would otherwise be curtailed (i.e., when production exceeds demand). Introducing energy storage technologies is a promising option to manage and complement wind intermittency [6], [7], [9], [10].

There are many different energy storage technologies currently available, each with its own advantages and constraints. Hydrogen energy storage, pumped hydro, compressed air energy storage, various types of battery systems, flywheels, super capacitors, and thermal energy storage are all either being used or investigated for integrating intermittent renewable energy. These energy storage systems can provide frequency regulation, alleviate transmission congestion, defer costs of new construction, provide load shifting, and/or reduce “time of use” and demand charges [11], [12], [13], [14]. Over the past decade, many studies have been focused on modeling and analyzing the technical and economic implications of using energy storage technologies to integrate intermittent wind energy. Many of these studies were focused on small-scale applications such as stand-alone wind and energy storage systems [10], [15], [16], [17]. For large scale integration and large capacity energy storage, the potential applications have been mainly focused upon pumped hydro energy storage, compressed air energy storage, and hydrogen energy storage [6], [11], [18], [19], [20], [21], that could enable a larger storage capacity to accommodate massive energy storage for not only daily load shifting but also seasonal load shifting. Pumped hydro energy storage is the oldest and most widely used method of such massive energy storage, accounting for over 99% of energy storage worldwide. A location with a suitable elevation gradient to provide the gravitational potential energy, and a large amount of storage media (water) are required to achieve large scale pumped hydro energy storage [9], [21]. Compared to pumped hydro energy storage, compressed gas (air/hydrogen) energy storage systems will require much less water and do not require a large elevation gradient. Compressed gas energy storage systems typically use existing underground sites (e.g., a salt cavern), and will have the potential advantage of higher energy storage capacity and much lower cost than batteries and ultra-capacitors, since the amount of stored energy is decoupled from the energy conversion device size [6]. A 2.25 GW rated integrated compressed air renewable energy system has been examined by Garvey et al. [22]. Performance measures of the system simulation indicated that effective turnaround efficiency is over 85% and the wholesale value of the output power may be increased by a factor of 1.3 [22]. Worldwide there are currently two compressed air energy storage facilities in operation, one in Huntorf, Germany, and one in McIntosh, Alabama [23]. To release the stored energy the compressed air is routed to a gas turbine that is also fueled by natural gas to produce electricity. One of the challenges that have plagued recent compressed air energy storage systems is associated with the operation and emissions from high pressure combustion [23]. A model for compressed air energy storage inside caverns has been developed by Raju and Khaitan that simulates the mass and energy balance inside the storage cavern and has been verified with the Huntorf facility, but the dynamics associated with the system were not simulated or discussed [24].

A compressed hydrogen energy storage system typically has an electrolyzer that produces hydrogen from water by using wind energy, and a hydrogen fuel cell that utilizes the hydrogen to provide power to the grid. Compressed hydrogen energy storage has been considered less favorable due to its low round trip efficiency and relatively high cost [25]. However, for integration with large-scale wind energy, large energy capacity and low self-discharge become more important than round trip efficiency [6], therefore compressed hydrogen energy systems with electrolyzers and fuel cells become more attractive as the amount of energy storage required increases. Compressed hydrogen energy storage has been demonstrated by NREL and Xcel Energy with the Wind-to-Hydrogen demonstration project in Boulder, Colorado. In the most recent demonstration, low temperature electrolysis is applied using a proton exchange membrane (PEM) electrolyzer to split water into hydrogen. The PEM electrolyzer achieved a system efficiency of 57% [26]. Moreover, hydrogen energy storage has the additional, synergistic benefit that the hydrogen produced using wind power (that would have otherwise been curtailed) could also be used as a renewable domestic fuel to enable completely zero emission transportation. Gonzalez et al. showed that hydrogen energy storage could drastically increase wind energy penetration since hydrogen produced from excess wind could be used for purposes other than electricity [27].

To further understand the dynamics of compressed air and hydrogen energy storage technologies integrated with large-scale wind power and their impacts on the grid, dynamic system models have been developed and verified for compressed air and hydrogen energy storage systems in this study. A hypothetical wind farm has also been modeled and integrated with measured demand profiles and the dynamic compressed air and compressed hydrogen energy storage models. In this overall system model, the implemented energy storage transforms the intermittent renewable resource into a power plant that can produce dispatchable power. The compressed gas energy storage acts to buffer the intermittent nature of wind power. Dynamics associated with wind power/demand fluctuations, timescale variations, hydrogen and compressed air conversion technologies, and storage size variations are simulated and discussed.