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[en] Highlights: • Developed model-based efficiency performance metrics for industrial pneumatic systems. • Quantified system efficiency increases due to the Pneumatic Strain Energy Accumulator. • Experimentally validated model efficiency increases ranging from 32% to over 78% - Abstract: A number of national organizations have recently expressed interest in research to develop materials and devices that achieve greater energy storage capacity, power density and increased energy efficiency on the heels of a report finding that the pneumatic sector of the fluid power industry averages only 15% efficiency. One way of improving efficiency is the use of compressed air storage and recycling devices. The pneumatic Strain Energy Accumulator is a recently developed device that recycles exhaust gas from one pneumatic component, stores it in a highly efficient process, and reuses the stored exhaust gas at a constant pressure to power another pneumatic component. This work analyzes system efficiency increases directly attributable to the implementation of a pneumatic strain energy accumulator by applying an analytical methodology for system level efficiency improvement calculations, experimental validation, and compressed air savings projections. Experimentally determined efficiency increases ranged between 32% and 78%, demonstrating that the pneumatic strain energy accumulator can be a viable part of the solution to the fluid power efficiency challenge.
[en] Highlights: • Performance of novel oil-free co-rotating scroll expander presented. • Water injection allows reaching quasi-isothermal expansion process. • Comparison between experimental data, semi-empirical and deterministic models. • Flank leakage, water injection and rotor speed effects have been analyzed experimentally. • Design guidelines for co-rotating scroll improvement proposed. - Abstract: Efficient compressed air energy storage requires reversible isothermal compression and expansion devices. The isothermal compression and expansion processes can either be approached by several stages with intercooling or by the more convenient injection of a liquid, often water. While volumetric machines are readily available for dry processes the compression and expansion of a gas with the presence of liquid is still problematic. The concept of a co-rotating scroll has been identified as a promising technology to cope with the presence of liquid. The current paper discusses the first experimental results of an oil-free co-rotating scroll prototype tested in expansion mode on a wide range of rotational speeds, varying water injection flow rates and with different nominal flank clearances. A maximal overall isothermal efficiency of 34% and a maximum output power of 1.74 kW_e_l were measured with this first prototype, providing the proof of the technical feasibility of the oil-free co-rotating scroll expander concept. The experimental data indicate a positive effect of water injection suggesting good heat transfer behaviour between the water and the air in the individual chambers, which is a result of the relatively long residence time compared to other volumetric concepts. The experimental sensitivity analysis yields a strong dependency of the machine performance on both the nominal flank clearance and on the injected water rate. The analysis through a semi-empirical model suggests the inversion of a classical trend, i.e. the increase in total leakage area with rotor speed. This is resulting from the centrifugal loads acting on the flanks and deforming them to produce increased radial and flank clearances. The injection of water is suggested to significantly decrease the leakage. A deterministic reduced order model of the co-rotating scroll expander was developed in order to better understand the governing phenomena within the machine and to provide design guidelines for further prototypes. A novel leakage model takes into account for the structural deformation of the flanks and the scroll involutes as a result from the rotor speed. By means of this comprehensive thermodynamic model, mechanical power, mass flow rate and exhaust temperature were predicted within a range of ±12% and ±4 K respectively compared to experimental data. The calibrated model suggests an achievable isothermal efficiency of 87% for an improved co-rotating scroll concept, thus offering promising perspectives not only for compressed air storage, but also for wet expansion in Absorption Power Cycles, trilateral flash cycle and Organic Rankine Cycles.
[en] Highlights: • We adapted a turbocharger to work as a micro-CAES system. • We provide an experimental evaluation of this micro-CAES performance. • Overall experimental efficiency was up to 45%, and turbine efficiency 76%. - Abstract: This work presents an experimental evaluation of a micro-CAES system performance, based on the 1st and 2nd laws of Thermodynamics. To build this micro-CAES system we adapted an automotive turbocharger, and also included a generator, an electrical circuit, and a lubricating system. As part of the experimental procedure, we measured the turbine inlet and outlet thermodynamic properties, also the generator power output and the mass flow rate. We imposed the turbine inlet pressure and the electrical load, obtaining as response the generator power output and the turbine outlet temperature. The proposed solution can operate reliably and safely up to 100,000 RPM without load, and 70,000 RPM with 3.5 kW of electrical power output. Overall efficiency was up to 45%, when operating with air at ambient temperatures without external sources of heat.
[en] Highlights: • Thermodynamic analysis is presented for an improved A-CAES combined with PBTES system. • A mathematic model is developed, validated and used to simulate system performances. • PBTES heights have significant influence on the PBTES’s thermal behaviors and the system efficiencies. • The maximum cycle efficiency of the improved A-CAES system is 56.74%. - Abstract: Energy storage technology is a cutting-edge research in the field of new and renewable energy application. In this paper we introduce the concept of an energy storage based on adiabatic compressed air energy storage (A-CAES) combined with packed bed thermal energy storage (PBTES) system. First, the system thermodynamic performance of a typical single cycle is discussed and the effect of PBTES heights is analyzed. The results show that an overall efficiency in excess of 49% is achievable and the PBTES heights have significant influence on the thermal behavior of PBTES, as well as the overall efficiencies. Because there is still heat energy remaining in the packed bed until the discharge process is terminated, an improved A-CAES system with a heat recuperator is further proposed. It is found that this improved system shows a promotion of ∼5% compared with the first present A-CAES system. The cycle efficiency of the improved system increases with the increase of continuous cycles, and then reaches a stable value of 56.74% after around 25 cycles. The main conclusions drawn from this work will be helpful for future development of a high-efficiency A-CAES system combined with PBTES.
[en] Highlights: • A wellbore-reservoir coupled model is developed based on Huntorf CAES plant. • Performance of CAESA can be similar to or even better than CAESC. • The temperature of CAESA shows a smooth variation due to large grain specific heat. • The impact of initial gas bubble volume on the storage efficiency is not significant. • Boundary permeability of the reservoir can significantly affect total storage efficiency. - Abstract: CAESA (compressed air energy storage in aquifers) attracts more and more attention as the increase need of large scale energy storage. The compassion of CAESA and CAESC (compressed air energy storage in caverns) can help on understanding the performance of CAESA, since there is no on running CAESA project. In order to investigate the detail thermodynamic process, integrated wellbore-reservoir (cavern or aquifer) simulations of CAES (compressed air energy storage) are carried out based on parameters of the Huntorf CAES plant. Reasonable matches between monitored data and simulated results are obtained for the Huntorf cavern systems in the wellbore and cavern regions. In this study, the hydrodynamic and thermodynamic behaviors of CAES in cavern and aquifer systems are investigated, such as pressure and temperature distribution and variation in both the wellbore and cavern regions of the CAES systems. Performances of CAESA are investigated with numerical models and compared with the performances of CAESC. The comparisons of CAESC and CAESA indicate that the pressure variation in CAESA shows a wider variation range than that in CAESC, while the temperature shows a smooth variation due to the large grain specific heat of the grains in the porous media. The simulation results confirm that the CAES can be achieved in aquifers, and further that the performance of energy storage in aquifers can be similar to or better than CAESC, if the aquifers have appropriate reservoir properties, which means the gas bubble can be well developed in an aquifer with such properties and the aquifer should have closed or semi-closed boundaries. The impacts of gas-bubble volume, formation permeability, and aquifer boundary permeability on storage efficiency are investigated and the simulation results indicate that the increase of gas bubble volume and permeability can improve the efficiency, but the effect is not significant. The gas bubble boundary permeability has a small effect on the energy efficiency of the sustainable daily cycle but can significantly affect total sustainable cycle times. The analysis of thermodynamic behaviors in CAESA suggests that more attention should be paid to the heat storage, reservoir properties and two-phase flow processes.
[en] Highlights: •Investigated CAES + HPT system concept for offshore wind energy; •Validated cost model for offshore wind farm including CAPEX and OPEX items; •Quantified cost-of-rated-power savings associated with CAES + HPT concept; •Estimated savings of 21.6% with CAES + HPT for a sample $2.92 billion project. -- Abstract: The size and number of off-shore wind turbines over the next decade is expected to rapidly increase due to the high wind energy potential and the ability of such farms to provide utility-scale energy. In this future, inexpensive and efficient on-site wind energy storage can be critical to address short-time (hourly) mismatches between wind supply and energy demand. This study investigates a compressed air energy storage (CAES) and hydraulic power transmission (HPT) system concept. To assess cost impact, the NREL Cost and Scaling Model was modified to improve accuracy and robustness for offshore wind farms with large turbines. Special attention was paid to the support structure, installation, electrical interface and connections, land leasing, and operations and maintenance cost items as well as specific increased/reduced costs reductions associated with CAES + HPT systems. This cost model was validated and applied to a sample $2.92 billion project Virginia Offshore case It was found that adaption of CAES + HPT can lead to a substantial savings of 21.6% of this 20-year lifetime cost by dramatically reducing capital and operating cost of the generator and power transmission components. However, there are several additional variables that can impact the off-shore energy policy and planning for this new CAES + HPT concept. Furthermore, these cost-savings are only first-order estimates based on linear mass-cost relationships, and thus detailed engineering and economic analysis are recommended.
[en] Highlights: •A concise analytical solution for SC-CAES system was presented for the first time. •The analytical solution is universal for SC-CAES and other similar CAES systems. •A method of sectional treatment and Taylor expansion was carried out. •Exergy analysis for SC-CAES system with its analytical model was conducted. -- Abstract: An analytical solution for a novel Compressed Air Energy Storage (CAES) system, Supercritical Compressed Air Energy Storage (SC-CAES) system, was conducted in this paper. The analytical solution can explore the evolution and its reason of roundtrip efficiency varying with system key parameters in depth, while it can also reveal the coupling mechanism of different sections of the system. On that basis, the model of exergy destruction for each part was obtained, and the exergy destruction can be easily calculated. Furthermore, the analytical solution has the character of universality due to the deduced method of sectional treatment, hence it can be extended to other similar CAES systems. Lastly, a sensitivity analysis and an exergy analysis were conducted for SC-CAES system. It is found and proved that the system efficiency varies linearly with isentropic efficiencies of compressor and expander, temperature difference of intercooler and reheater, pressure loss of intercooler and reheater. Meanwhile, the main factors of the varying tendency of total exergy destruction with different parameters are revealed.
[en] Highlights: • A new pneumatic hybrid concept is introduced. • A proof-of-concept prototype system is built and tested. • The experimental system has a round-trip efficiency of just under 10%. • A thermodynamics model is used to predict the performance of modified designs. • An efficiency of nearly 50% is possible with reasonable design changes. - Abstract: This paper introduces a new low-cost hybrid drivetrain concept based on compressed air energy storage. In contrast to most contemporary approaches to pneumatic hybridization, which require modification to the primary power plant, this concept is based on a stand-alone pneumatic system that could be readily integrated with existing vehicles. The pneumatic system consists of an air tank and a compressor–expander that is coupled to the rest of the drivetrain via an infinitely variable transmission. Rather than incorporating more expensive technologies such as variable valve timing or a variable compression ratio compressor, a fixed valve system consisting of a rotary valve and passive check valves is optimized to operate efficiently over a range of tank pressures. The feasibility of this approach is established by thermodynamic modeling and the construction of a proof-of-concept prototype, which is also used to fine tune model parameters. While the proof-of-concept system shows a round trip efficiency of just under 10%, modeling shows that a round trip efficiency of 26% is possible with a revised design. If waste heat from the engine is used to maintain an elevated tank temperature, efficiencies of nearly 50% may be possible, indicating that the concept could be effective for practical hybridization of passenger vehicles
[en] Highlights: • A novel, high-efficiency, scalable, near-isothermal, energy storage system is introduced. • A comprehensive analytical physics-based model for the system is presented. • Efficiency improvement is achieved via heat transfer enhancement and use of waste heat. • Energy storage roundtrip efficiency (RTE) of 82% and energy density of 3.59 MJ/m"3 is shown. - Abstract: Due to the increasing generation capacity of intermittent renewable electricity sources and an electrical grid ill-equipped to handle the mismatch between electricity generation and use, the need for advanced energy storage technologies will continue to grow. Currently, pumped-storage hydroelectricity and compressed air energy storage are used for grid-scale energy storage, and batteries are used at smaller scales. However, prospects for expansion of these technologies suffer from geographic limitations (pumped-storage hydroelectricity and compressed air energy storage), low roundtrip efficiency (compressed air energy storage), and high cost (batteries). Furthermore, pumped-storage hydroelectricity and compressed air energy storage are challenging to scale-down, while batteries are challenging to scale-up. In 2015, a novel compressed gas energy storage prototype system was developed at Oak Ridge National Laboratory. In this paper, a near-isothermal modification to the system is proposed. In common with compressed air energy storage, the novel storage technology described in this paper is based on air compression/expansion. However, several novel features lead to near-isothermal processes, higher efficiency, greater system scalability, and the ability to site a system anywhere. The enabling features are utilization of hydraulic machines for expansion/compression, above-ground pressure vessels as the storage medium, spray cooling/heating, and waste-heat utilization. The base configuration of the novel storage system was introduced in a previous paper. This paper describes the results obtained from a transient, analytical, physics-based thermodynamic system model used for the system design and evaluation of three design configurations (including base configuration). The system model captures real gas effects and all loss mechanisms. The model demonstrates an energy storage roundtrip efficiency of 82% and energy density of 3.59 MJ/m"3. Experimental evaluation of system performance and detailed cost-analysis will be presented in future publications.
[en] Highlights: •A new configuration of compressed air energy storage system is proposed and analyzed. •This system, so-called subcooled-CAES, offers cogeneration of electricity, heat and cooling. •A pseudo-dynamic energy, exergy and economic analysis of the system for an entire year is presented. •The annual power, cooling and heat efficiencies of the system are around 31%, 32% and 92%. •The overall energy and exergy performance coefficients of the system are 1.55 and 0.48, respectively. -- Abstract: Various configurations of compressed air energy storage technology have received attention over the last years due to the advantages that this technology offers relative to other power storage technologies. This work proposes a new configuration of this technology aiming at cogeneration of electricity, heat and cooling. The new system may be very advantageous for locations with high penetration of renewable energy in the electricity grid as well as high heating and cooling demands. The latter would typically be locations with district heating and cooling networks. A thorough design, sizing and thermodynamic analysis of the system for a typical wind farm with 300 MW capacity in Denmark is presented. The results show a great potential of the system to support the local district heating and cooling networks and reserve services in electricity market. The values of power-to-power, power-to-cooling and power-to-heat efficiencies of this system are 30.6%, 32.3% and 92.4%, respectively. The exergy efficiency values are 30.6%, 2.5% and 14.4% for power, cooling and heat productions. A techno-economic comparison of this system with two of the most efficient previous designs of compressed air energy storage system proves the firm superiority of the new concept.