energies Article Thermodynamic Analysis of Three Compressed Air Energy Storage Systems: Conventional, Adiabatic, and Hydrogen-Fueled Hossein Safaei and Michael J. Aziz * Harvard John A. Paulson School of Engineering and Applied Sciences, Pierce Hall, 29 Oxford Street, Cambridge, MA 02138, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-617-495-9884 Received: 10 May 2017; Accepted: 27 June 2017; Published: 18 July 2017 Abstract: We present analyses of three families of compressed air energy storage (CAES) systems: conventional CAES, in which the heat released during air compression is not stored and natural gas is combusted to provide heat during discharge; adiabatic CAES, in which the compression heat is stored; and CAES in which the compression heat is used to assist water electrolysis for hydrogen storage. The latter two methods involve no fossil fuel combustion. We modeled both a low-temperature and a high-temperature electrolysis process for hydrogen production. Adiabatic CAES (A-CAES) with physical storage of heat is the most efficient option with an exergy efficiency of 69.5% for energy storage. The exergy efficiency of the conventional CAES system is estimated to be 54.3%. Both high-temperature and low-temperature electrolysis CAES systems result in similar exergy efficiencies (35.6% and 34.2%), partly due to low efficiency of the electrolyzer cell. CAES with high-temperature electrolysis has the highest energy storage density (7.9 kWh per m3 of air storage volume), followed by A-CAES (5.2 kWh/m3). Conventional CAES and CAES with low-temperature electrolysis have similar energy densities of 3.1 kWh/m3. Keywords: compressed air energy storage (CAES); adiabatic CAES; high temperature electrolysis; hydrogen storage; thermodynamics 1. Introduction Large penetrations of wind and solar energies challenge the reliability of the electricity grid, due to their intermittency and uncertainty. Storage technologies are being developed to tackle this challenge. Compressed air energy storage (CAES) is a relatively mature technology with currently more attractive economics compared to other bulk energy storage systems capable of delivering tens of megawatts over several hours, such as pumped hydroelectric [1–3]. CAES stores electrical energy as the exergy of compressed air. Figure1 is a simplified schematic of a CAES plant. Electricity is supplied by the grid to run the air compressors and charge the storage system. Waste heat is released during the compression phase. Air is stored for later use—often in an underground cavern. During the discharge phase, compressed air is combusted with a fuel, and expanded in a turbine (expander) to regenerate electricity. Currently, there are two commercial CAES plants in operation: Huntorf in Germany (since 1978) and McIntosh in USA (since 1991) [4]. Moreover, there are some smaller projects in operations or in construction and planning phases, most notably General Compression’s 2 MW, 300 MWh project in Texas, USA and SustainX’s 1.5 MW, 1 MWh project in New Hampshire, USA [5]. Energies 2017, 10, 1020; doi:10.3390/en10071020 www.mdpi.com/journal/energies Energies 2017, 10, 1020 2 of 31 Energies 2017, 10, 1020 2 of 31 Figure 1. SchematicSchematic of a generic conventional compressed air energy storage (CAES) system. The prospects forfor the the conventional conventional CAES CAES technology technology are are poor poor in low-carbonin low-carbon grids grids [2,6 –[2,6–8].8]. Fossil Fossil fuel (typicallyfuel (typically natural natural gas) combustiongas) combustion is needed is needed to provide to provide heat to heat prevent to prevent freezing freezing of the moisture of the moisture present presentin the expanding in the expanding air [9]. air Fuel [9]. combustion Fuel combustion also boosts also boosts the work the outputwork ou intput comparison in comparison to solely to solelyharnessing harnessing the energy the energy stored stored in the compressedin the compressed air. air. We develop analytical models to assess the thermodynamicsthermodynamics of two strategies to make CAES greenhouse gasgas (GHG) (GHG) emissions-free. emissions-free. Both Both utilize utilize the temperature the temperature increase fromincrease the airfrom compression the air processcompression to eliminate process the to needeliminate for gas the combustion. need for gas This combustion. heat is generated This heat during is generated the charging during phase. the chargingBecause ofphase. its low Because temperature of its low and temperature correspondingly and lowcorrespondingly exergy, the compression low exergy, heatthe compression is rejected to heatthe ambient is rejected environment to the ambient in the conventionalenvironment CAESin the setup.conventional This heat CAES could, setup. in principle, This heat be could, stored in to principle,heat the expanding be stored airto providedheat the expanding that the temperature air provided of this that stored the temperature heat is high enough.of this stored The primary heat is highmethod enough. to achieve The primary such high method temperatures to achieve is to such increase high thetemperatures operating pressureis to increase of the the compressors operating pressureand to eliminate of the intercoolingcompressors between and to compression eliminate stagesintercooling (i.e., adiabatic between compression). compression This, stages however, (i.e., posesadiabatic practical compression). challenges This, due however, to high operatingposes practical pressures challenges and temperatures due to high operating of the compressors pressures (e.g.,and temperatures metallurgical of limits the compressors on compressor (e.g., blades). metallurgical limits on compressor blades). Physical storage of the compression heat is the core of the Adiabatic CAES (A-CAES) concept—the first first carbon-free CAES system we investigate.investigate. Chemical storage of the compression heat in the form of hydrogen, and combustion of hydrogen instead of natural gas during the discharge phase is is the the second second strategy strategy we we analyzed analyzed.. Hydrogen Hydrogen can can bebe produced produced via via electrolysis electrolysis of steamof steam at athigh high temperatures temperatures (HTE) (HTE) or or water water at at lo loww (ambient) (ambient) temperatures temperatures (LTE). (LTE). The The HTE HTE concept benefitsbenefits fromfrom the the lower lower electricity electricity demand demand of the of electrolysis the electrolysis process process at higher at temperatures.higher temperatures. Utilizing Utilizingthe high-temperature the high-temperature heat of compression heat of compression lowers the lowers electricity the demandelectricity of demand hydrogen of production hydrogen productionin the CAES-HTE in the CAES-HTE system. This system. saving This is achieved saving is at achieved the expense at the of higherexpense electricity of higher demand electricity of demandthe air compressor of the air compressor which, in CAES-HTE,which, in CAES-HTE, operates atop highererates at pressures higher pressures with limited with or limited no cooling. or no Thecooling. CAES-LTE The CAES-LTE concept isconcept comparable is comparable to the conventional to the conventional CAES system CAES (diabatic system compression (diabatic withcompression the use ofwith coolers the use between of coolers compressor between stages).compressor However, stages). hydrogen However, is hydrogen produced is onsite produced with onsitea low-temperature with a low-temperature electrolyzer. electrolyzer. Electrolysis of steam (HTE) instead of water (L (LTE)TE) requires more thermal but less electrical energy. FigureFigure2 illustrates2 illustrates the the theoretical theoretical energy energy requirements requirements as functions as functions of the electrolysis of the electrolysis reaction reactiontemperature temperature (see Appendix (see AppendixA for details). A for In adetails). high-temperature In a high-temperature electrolyzer, steamelectrolyzer, is disassociated steam is disassociatedin the cathode in to the produce cathode hydrogen to produce and Ohydrogen2−, while andO2− Ois oxidized, while O in the is anodeoxidized to producein the anode oxygen. to Theproduce theoretical oxygen. total The theoretical energy demand total energy of the demand electrolysis of the process electrolysis (change proc iness enthalpy, (change inDH enthalpy,) equals ΔtheH) electricity equals the demand electricity (reversible demand work, (reversible i.e., change work, in thei.e., Gibbschange free in energy,the GibbsDG )free plus energy, the heat Δ demandG) plus theof the heat reaction demand (change of the inreaction entropy (change multiplied in entropy by the multiplied reaction temperature, by the reactionDS ×temperature,T) from a source ΔST of) fromat least a source as high of a temperatureat least as high as thea temperature reaction temperature. as the reaction While temperature. the total energy While demand the total (enthalpy energy demandchange) of(enthalpy electrolyzing change) steam of electrolyzing increases at highersteam in temperatures,creases at higher its electricity temperatures, demand its decreases.electricity Thedemand savings decreases. in electricity The savings consumption in electricity of the consumpt electrolyzerion come of the at electrolyzer the expense come of its at higher the expense heat load. of Therefore,its higher heat electrolysis load. Therefore, of steaminstead electrolysis of water of stea couldm instead be particularly of water attractive