Ongoing Project: Additive Manufacturing of Composite High-Pressure Storage Tanks
High-pressure gas storage tanks are important component in compressed air energy storage systems. They allow building and operating modular systems since air storage in tanks represents the potential energy storage where air is stored till demand increases. This potential energy depends on two components: (i) the tank volume and (ii) the tank storage pressure. Remember that the polytropic equation that caverns the energy storage takes the form:
Energy = constant = ±è±¹â¿
Where n is the specific heat ratio.
As for the volume, it can be either one large, cavernous space where air is compressed over a long period of time until operational pressures are reached, or by using smaller interconnected tanks to allow partial discharge according to a predetermined plan. The former works well for seasonal storage, where the caverns capacity is built gradually and the letter is best suited for applications that require dynamic control over the charge/discharge cycles.
The other important component is the operating pressure. According to equation (1), the higher the operational storage pressure, the higher the energy available for the pneumatic equipment. Indeed, it is intuitive to notice that the higher the operational pressure the higher the stored potential energy, but the energy relationship is nonlinear (raised to exponent, n).
The main practical challenge in installing the storage tanks is the maximum operating pressure the tank material can handle before failure. In thin-walled pressure vessels, the relationship between the internal pressure and the generated stresses in the tank material. The two stress types are circumferential (hoop) stress and longitudinal (axial) stress.
These two stresses are correlated with the storage pressure as follows:
σh = pr/t
Where p is the internal pressure, r is the tank radius and t is the material thickness. This is also shown in the figure below:
The axial (longitudinal) stress is defined as half of the hoop stress:
σa = ½ pr/t
The storage tanks can thus either be a subterranean cavern in the form of a depleted oil/water well or salt mine as these geological caverns are naturally and inherently impermeable and amenable to mass air/gas storage. If caverns are unavailable, then tanks have to be purchased or manufactured based on the expected operating pressures. For compressed air energy storage the expected pressures are between 200-300bar (or 3000-4000psi). These tanks are readily available at various storage capacities, but the situation is different for storing lighter gases such as hydrogen, as vessels made of metal will suffer from either embrittlement if hydrogen atoms manage to embed themselves in the metallic intertitles or hydrogen would escape from between the metal molecules and leak into the atmosphere.
For such reasons, R&D in manufacturing custom gas storage tanks to be technically and economically feasible has to be investigated via additive manufacturing technologies.
Current work has examined building tanks from layers of polymeric materials as follows:
| Materials | Structure | Layer Amount |
| Kevlar | Plain 175g | 18 layers |
| Carbon | Plain 200g | 8 layers |
| Kevlar | Plain 175g | 2 Reinforcements |
| Epoxies | AMPREG 31 |
1.5 kg |
The resulting tank was subjected to a burst test and failed at 40bar, which was although disappointing, but promising.