Impact of Off-design operation on the effectiveness of a low- temperature compressed air energy storage system Ahmad Arabkoohsar a, * , Hamid Reza Rahrabi b , Ali Sulaiman Alsagri c , Abdulrahman A. Alrobaian c a Department of Energy Technology, Aalborg University, Denmark b Department of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran c Department of Mechanical Engineering, Qassim University, Saudi Arabia article info Article history: Received 5 August 2019 Received in revised form 8 February 2020 Accepted 15 February 2020 Available online 19 February 2020 Keywords: Low-temperature compressed air energy storage Off-design operation Power ramps Performance destruction index Performance maps abstract There is no doubt that the determination of a smart charging-discharging pattern can be very effective in increasing the cost-effectiveness and overall energy efficiency of an energy storage system. For finding the optimal operation strategy of the energy storage unit of a renewable power plant, the electricity spot price, the forecast data of energy availability, and the regulations of the local power market should all be taken into account. In addition to these economic considerations, the effect of deviation from the nominal load (partial-load operation) on the performance of the energy storage system is a critical parameter that directly affects the optimal operation pattern of the system in real-life energy markets. In this study, the effects of partial-load work of a low-temperature compressed air energy storage system on its overall performance are investigated thermodynamically employing real performance maps of all the components of the system. The results of the study indicate that the energy storage system needs to operate around nominal design conditions if it is expected to perform efficiently. The round-trip effi- ciency of the unit approaches 68% at a nominal load while it offers the low efficiencies of 52% and 28% if working at 50% and 10% loads, respectively. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction There will be a serious need for energy smoothers in the future when perfectly renewable-based energy systems with high pene- tration of fluctuating solar and wind energies come into service [1]. The use of energy storage systems can be a smart measure for addressing this challenge by storing the surplus energy of the po- wer plants during off-peak times and giving it back whenever needed or for peak shaving [2]. Energy storage systems are classi- fied into thermal and electricity storage technologies. In spite of thermal storage that has mature states-of-the-art and -practice, where efficient yet cheap heat and cold storage solutions are already in the market [3], electricity storage still has serious un- solved challenges [4]. Battery technologies as the most efficient electricity storage solutions are costly and present low energy density, and alternative solutions (i.e. mainly mechanical energy storage systems) either have not been well developed yet or suffer from specific deficiencies (e.g. special topology, etc.) [5]. Mechani- cal electricity storage technologies include pumped hydroelectric storage [6e8], flywheel [9], pumped thermal electricity storage [10], the new concept of high-temperature heat and power storage in different configurations [11e13], gravity energy storage [14,15], and compressed air energy storage (CAES) [16]. CAES technology is more appropriate for large-scale applica- tions ranging from 50 to 300 MW [17]. A CAES may come into various configurations and depending on its design, it may offer an energy efficiency of up to 80% [18]. The main disadvantages of this technology are the need for special geological sites and immature state-of-the-art [19]. The most advanced design of CAES is called advanced adiabatic CAES (or isothermal adiabatic CAES) [20], while diabatic CAES [21], single-stage adiabatic CAES [22], subcooled CAES [23], and low-temperature adiabatic CAES (LTA-CAES) are its other possible designs [24]. LTA-CAES is one of the latest configu- rations proposed for the CAES technology in which the main objective is to minimize the need for any auxiliary heating pro- cesses. Thus, the temperature of the compressed air flow before the * Corresponding author. E-mail addresses: ahm@et.aau.dk (A. Arabkoohsar), a.alsagri@qu.edu.sa (A.S. Alsagri). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy https://doi.org/10.1016/j.energy.2020.117176 0360-5442/© 2020 Elsevier Ltd. All rights reserved. Energy 197 (2020) 117176