Analysis of an integrated packed bed thermal energy storage system for heat recovery in compressed air energy storage technology
Introduction
Currently, the worldwide installed capacity for electrical energy storage (EES) is dominated by pumped hydroelectric energy storage (PHES). In 2015, with 145 GW installed, PHES represented about 97% of the global EES capacity [1]. The power ratings of the existing PHES plants are in the range of 1 MW up to 3 GW with a cycle efficiency of 70–85% [2]. Despite PHES is a well-known, mature and efficient solution it has also some major limitations such as: applicability limited to suitable locations and relatively low energy density, which translates into a considerable environmental impact.
In the field of large-scale EES, a valid alternative to PHES is represented by compressed-air energy storage (CAES). CAES plants operate on a “decoupled” Brayton cycle. During electric energy storage, the air compression operation occurs and electricity is absorbed from the grid to activate a motor-compressor train (see Fig. 1). The thermal energy produced during compression is removed by means of intercoolers and after-coolers and the high-pressure low-temperature air is then stored in a large air reservoir, usually a cavern. When electric energy is requested from the grid, the compressed air is extracted from the reservoir, is flown and heated in a combustion chamber and, at high enthalpy, it is expanded in a gas turbine that drives itself an electric generator. As of today, two industrial-scale CAES plants are successfully in operation: the 321 MW Huntorf plant in Germany, and the 110 MW McIntosh plant located in Alabama, (U.S.). Commissioned at the end of 1978, the Huntorf plant is the world’s first CAES plant, whereas the McIntosh CAES plant, commissioned in 1991, can be considered as a second-generation CAES in which a recuperator is exploited to pre-heat the compressed air before entering the combustion chamber. With this enhancement the electricity-to-electricity cycle efficiencies of McIntosh reached 54% vs. 42% of the Huntorf plant [3].
Since several CAES concepts have been proposed/developed, a general classification can be based upon how thermal energy is managed during air compression/expansion stages [3]. If thermal energy is wasted during compression and provided prior to expansion by burning natural gas in a combustion chamber, the CAES concept is known as diabatic (D-CAES); Huntorf and McIntosh are D-CAES plants. Conversely, if thermal energy produced during compression is stored into a thermal energy storage (TES) system, from which is recovered before expansion, the associated CAES concept is known as adiabatic (A-CAES) [4]. The development of the A-CAES concept was the subject of the research project “ADELE” (2010–2013) [5] with the construction of the world’s first 260 MW prototype expected as outcome of the “ADELE-ING” project (2013-present) [6]. Since in the A-CAES concept there is no need of burning fuel for the air heating process before expansion, the expected round-trip efficiency is in the order of 70% [5]. As a consequence, the TES system becomes a key component for a successful commercial implementation of the A-CAES technology.
TES systems can store thermal energy in the form of sensible heat, latent heat [7] or by thermochemical reactions [8]. The large majority of the high-temperature TES systems nowadays in operation in concentrating solar power (CSP) applications [9] or industrial process heat recovery [10], store sensible heat with a two-tank storage configuration [11]. However, and by considering its high-efficiency, affordability and simplicity, the single-tank or thermocline storage technology represents a valuable alternative [12], [13], [14].
This work reports a detailed performance analysis of a packed bed TES system exploited to store the thermal energy produced during the compression stage of an existing CAES plant, i.e. charging operation, to be then reused to increase the enthalpy of high-pressure air prior to be expanded in the CAES plant turbine during electric energy production phase.
For this purpose, initially a single-tank solution was studied for continuous and complete charge/discharge cycles. Hereinafter, assuming this first simulation as a reference case, different TES enhancement alternatives were analysed. Among them, the impact of the tank volume size, the number of TES tanks used, and the overall heat management strategy were studied. The results of this part of the study were used to define the multiple benefits of the proposed storage technology. A pre-charge stage was analysed to possibly enhance the TES cyclic performance. Finally, the performance of the packed bed TES under partial charge/discharge operations, due to a realistic CAES daily operational scheme, was also evaluated for a month of continuous operation.
Section snippets
CAES plant operational parameters
As a real plant of reference for this study, the Huntorf CAES plant was chosen. It is the world's first and longest-operating CAES facility and is located near Huntorf in Northern Germany. This plant is designed for a peak power generation of 2 h at full load with an air mass flow rate of 417 kg/s and a power of 290 MW. The full charge of the two caverns, acting as high pressure air reservoirs, occurs in 8 h with an air mass flow rate of 108 kg/s. Plant components are designed to operate with a
Momentum equation
The interstitial fluid movement through the packed bed was modelled using the commercial computational fluid dynamics (CFD) software ANSYS-Fluent v.16.2. In particular, the porous media formulation was applied. The porous media model considers the domain as a continuum adding a source term () to the momentum equation for describing the interference of the packed bed on the fluid flow:where is the fluid density, the fluid velocity, the static
Materials properties
The HTF considered in this study is air with thermo-physical properties assumed as a function of temperature and pressure (see Fig. 2). The air density was evaluated with the Peng-Robinson equation of state [27]. Viscosity and thermal conductivity values show a negligible dependence on pressure. Consequently, these properties were considered as only dependent on temperature (see Fig. 2b and d). The air specific heat at different pressures (46 bars, 56 bars and 66 bars) shows a noticeable spread
Reference case: single-tank packed bed TES
In this section the thermal performance of a single-tank TES, subjected to a total of 20 consecutive charge/discharge cycles, is evaluated. In Fig. 6 the resulting HTF outflow temperature, during charging (l.h.s) and discharging (r.h.s.), is presented. Fig. 6a clearly shows that the HTF outflow temperature remains constant at 20 °C throughout the first 8 h of charging. However, from the second cycle this temperature starts increasing gradually reaching a stable thermal behaviour after 14–15
Summary and conclusions
The performance of a packed bed TES system integrated in a CAES plant was analysed in detail. This proposed solution is known as adiabatic-CAES (A-CAES). In an A-CAES plant, the TES system allows to store the thermal energy recovered from the air compression stage and to, use it to increase the enthalpy of the high-pressure air prior of its expansion in the power block. With this enhancement, the electricity-to-electricity cycle efficiency of an A-CAES plant is higher than that of a CAES plant.
Acknowledgements
The financial support given by the Swiss Federal Office of Energy (SFOE – OFEN – BFE), under the framework of SolAir-3 Project (“SI/500926”), the Swiss Commission for Technology and Innovation through the Swiss Competence Centre for Energy Research for Heat and Electricity Storage (SCCER-HaE), and the ELKARTEK 2016 (KK-2016/00037 – CICE2016) funding program of the Department of Economic Development and Infrastructures of the Basque Country Government – Spain is gratefully acknowledged.
Iñigo
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