Hydrogen as Energy Backup for the Hexicon (Mechanical Project)

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The island of Malta is highly reliant on fossil fuels for its power (99%), and due to climate mitigation policies implemented by EU the Maltese government is required to have 10% of its power generation from renewables by 2020.

To achieve these energy goals, the Maltese government has expressed interest in investing on a Hexicon platform to produce 9% of the Maltese energy demand. The Hexicon platform is a floating structure capable of carrying a wide range of renewable energy generators. The Hexicon platform proposed for Malta is meant to have a rated capacity of 54MW distributed by vertical and horizontal wind energy converters.

Nevertheless, due to the irregular nature of wind the Hexicon platform would still use diesel generators on-board as backup power; this inherently defeats the purpose of the Maltese investment, and therefore a Hydrogen backup system was proposed and investigated for its technical and economic viability.

A literature study was carried out on renewable hydrogen system in order to familiarise with the type of markets and the best way to apply the technology to the scenario at hand. Four markets were established, small-scale, transportation, stand-alone power systems, and large buffering systems; the large buffering system is the most appropriate for the study, and taking this type of system into account, the most appropriate hydrogen generation and utilisation system were then identified. It was established that the system is composed of three parts, electrolyser, storage tanks and fuel cells stacks.

However, an additional water purification system is necessary; this is due to the fact that the Hexicon platform will be located offshore, and salt water is not appropriate for the electrolyser. A literature study was then performed to identify the most appropriate equipment for each stage of the process; it was established that a Reverse Osmosis (RO) system will be used to purify the water, an alkaline electrolyser will be used to generate the Hydrogen, the Hydrogen will then be stored in pressure vessels (at 30bar), thus also requiring compressors, and the recovery of energy will be performed by a proton exchange membrane (PEM) fuel cell (FC) stack.

A study was carried out to establish the models to use for each equipment, and based on the hourly demand for Malta, as well as the hourly winds, a first estimate of the size of each equipment was established. The system model was developed in the HOMER software, which unfortunately did not model the desalination plant.

The Hexicon (in the design considered in this study) is not able to provide Malta with 9% of the energy demand; this was mainly due to the low wind conditions. In addition to this, it was understood from the literature study that a hydrogen system backup system, i.e. a buffering system, would not be applicable to the scenario initially proposed in this thesis due to the low renewable energy penetration, and also due to the fact that the Hexicon would be connected to the grid, rendering such a system defunct. A micro-grid scenario was assumed and developed.

This scenario tried to assess how low the demand would need to be in order to make a hydrogen project feasible. Different percentages were tried and the only one that met the constraints was one with 1.1% of the Maltese demand.

The system would consist of a 3MW Fuel Cell, a 4.5MW electrolyser, and hydrogen storage for 10.5tonnes. The NPC of this system would be approx. 130 Million €, with an initial investment of approx. 71 Million €, LCOE of 0.257€.kWh-1, and a Hydrogen cost of approx. 20€.kg-1. While other economic indicators show viability, for example, a short payback time of 3.5 years based on the revenue from the excess electricity, the cost of hydrogen suggests that it is too expensive.
Source: KTH
Author: Rebello de Andrade, Filipe

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