Power grid disturbances pose a great risk to the safety and operation of essential energy-consuming infrastructures such as data centers and hospitals. This DMT Superproject aims to address these risks by designing a system that provides a fast response backup power supply, protecting the user against the loss of critical information and catastrophic failure.
The solution developed here is a Compressed Air Energy Storage (CAES) System. When the grid is operational, a compressor injects air at high pressure into storage vessel. In the event of power outage, the pressure vessel discharges its stored air to power a turbine coupled to a generator, converting the air's energy back into electricity.
Advantages of the system include low environmental impact and high reliability. The brief of the project is to design an integrated CAES system where three sub-groups are responsible for the three sub-assemblies.
The conceptual design includes two-stage compression and expansion processes, requiring interstage cooling and reheating respectively. A scaled-down proof-of-concept prototype is being manufactured and tested. The prototype acts as an important first step in the development of commercial CAES systems.
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DMT07A Compressor
As the first sub-assembly of the larger CAES system project, DMT 7A is tasked with the development of an air compressor to pump air inside the pressure vessel (DMT 7B) to a higher pressure. For the full-scale system, the compressor is required to pressurise the air inside the vessel to 16 bars . A 2-stage reciprocating compressor with an intercooling stage was designed that satisfies this requirement but was deemed unfeasible due to its prohibitive cost.
A proof-of-concept prototype featured scaled-down technical specifications is instead designed and manufactured, consisting of a single-stage reciprocating compressor delivering compressed air at 4 bars, with a designed flowrate of 250L/min. Particular attention was paid to the interfacing between the compressor and the vessel, ensuring proper integration within the larger super-project. An intercooling system was also designed and tested to serve as the proof-of-concept prototype that would be attached to a theoretical second stage . Our prototype seeks to provide critical performance parameters via testing that will verify our thermodynamic model and its feasibility as a full-scale design.
As the manufactured product is designed with the proposed full-scale design in mind, there is scope for an extension to this project, where the second stage can be developed to meet the original 16 bar requirement.
For the use cases detailed in the full project brief, such as data centres, it is a project requirement for the energy supply to have a rapid response. These use cases are commonly covered by batteries in uninterruptible power supply (UPS) systems, but batteries are expensive, environmentally damaging, and degrade. This project aims to provide an alternative to this system which improves upon the shortcomings of current solutions. The mechanical storage of energy as pressure may offer a more cost-effective and sustainable solution.
Group 7B is responsible for the ‘air receiver' components of the system, alongside the compressor (7A) and turbogenerator (7C) groups. This sub-project comprises the storage and discharge of air to meet the customer product, and turbine operation requirements. The objectives were: design a pressure vessel to meet real full system requirements; design a pressure vessel outlet system to meet the real turbogenerator inlet flow conditions; manufacture a prototype pressure vessel and outlet system for proof-of-concept tests; and ensure the prototype and real system meet industrial safety regulations and standards.
A 0.5 m3, 20 bar vessel with 40 mm of Rockwool insulation was designed with the calculated parameters. The manufacture of this bespoke vessel proved however to be cost prohibitive, thus an off the shelf vessel was selected for the prototype. This prototype, with an experimental outlet assembly, will allow for the verification of subsequently proposed models that focus on the flow of air to the turbine. The outcome is a prototype system designed for manufacture and testing, and a provisional real design to be developed further with prototype findings.
In order to design a compressed air energy backup system, it is essential for the overall system performance to integrate a high-efficiency expander which runs during transient discharging operation. The mechanical work will be consumed by a generator to produce 10kW, 230V electrical power output at 50Hz. After extensive literature research, a radial turbine was identified as the optimal expander for the project application. In order to assess the performance of the expander, an up-scaled turbine was designed, made and tested with the aid of an existing turbocharger.
The rotor design process involved creating the 1-D geometry to identify the optimal turbine blade angles, generating the 3D geometry and performing CFD simulations. Once a consolidated design was established, FEA was performed to ensure the structural integrity of the rotor. Secondary flows and tip clearance losses were evaluated to ensure the turbine met efficiency requirements.
The volute was designed to deliver the required mass-flow rate and angle to the rotor, while maintain a linearly decreasing A/R (cross-sectional area over radius ratio). The inlet connection was based on the existing flange of the testing rig and the outlet shroud profile was designed to maintain a 0.5 mm tip clearance.
The final configuration reported a 82.4% total-to-static isentropic efficiency at a pressure ratio of 3, an operating temperature of 77°C, 0.59 degree of reaction. At a rotational speed of 60 krpm and mass-flow rate of 0.2 kg/s, the turbine is capable of delivering 13.8 kW.
The aim of the testing is to map the performance characteristics of the turbine at various operating parameters. In the context of the overall system, these results will allow for the identification of the optimal operating point in a discharging pressure vessel.
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