Renewable resources: energy storage in offshore applications

Renewable resources: energy storage in offshore applications

Renewable resources are becoming increasingly popular, onshore as well as in large offshore systems. However, there is one considerable problem that is currently restricting the growth of the market: all energy that is being produced, be it by harnessing the power of the sea,  the sun or of the wind, has to be deployed immediately. Any surplus which cannot be used instantly is irrevocably wasted. Moreover, renewable sources tend to be unstable in that natural conditions may change suddenly, which directly affects the output power. The solution to this issue is obvious: to invent a way of storing energy for later usage. 

Dual chamber technology allows independent energy storage

With their project FLASC, engineers from the Faculty of Engineering at the University of Malta have found a way to do so. They have developed a procedure for offshore systems that allows surplus energy to be effectively stored. Compressed air is used for energy storage. Similar solutions that are already in use rely on hydrostatic pressure, which in turn is dependent on water depth. In contrast, the FLASC dual chamber technology allows for an independent pressure range, no matter the depth of water. That way, surplus energy can be securely stored and released at specified intervals that can be set individually. This ensures that changes in the natural environment do no longer directly affect the output power.

Exact measurement with STS ATM/N/T sensors

The whole technology relies on stable air pressure which has to be guaranteed at all times. For this, FLASC uses high quality STS ATM/N/T sensors. The sensitive sensors measure air pressure and temperature at three different spots in the system. With housing material made of resistant titanium, the sensors are perfectly equipped for permanent usage in salt water. Thanks to the integrated temperature sensing element PT100, they are able to cover a temperature measuring range from 5 to 80°C.  All collected data is transferred to the SCADA system, where it can be monitored in real-time.

The force of water: Renewable energy from the seas

The force of water: Renewable energy from the seas

The idea of harnessing the force of the seas for energy generation is not a new one. The main challenge lies in developing efficient energy conversion systems that keep costs low whilst barely impacting the environment. A highly promising project termed REWEC3 has emerged in this regard in Italy.

The Resonant Wave Energy Converter (REWEC3) is an advanced technology that produces electrical power from the energy of the sea’s waves. The first instance of this type has been successfully constructed at the port of Civitavecchia. Its functional principle is based on Oscillating Water Column (OWC) systems.

OWCs exhibit great potential as a renewable energy source of low environmental impact. When water levels around and within an OWC rise, air is displaced inside a collecting chamber by this water motion and then driven back and forth through a Power-Take-Off (PTO) system. The PTO system in turn converts this air movement into energy. Amongst the models that convert air motion into electricity, the PTO system takes the form of a bidirectional turbine. This ensures that, regardless of airflow orientation, the turbine always rotates in the same direction, thus providing for continuous energy.

The REWEC3 system in Civitavecchia arose from a research project at the Mediterranea University of Reggio Calabria and is operated today by the enterprise. The installation essentially consists of a reinforced caisson made of concrete. This caisson has a vertical shaft on its wave-facing side (1), which, through an opening (2) to the sea, on the one side, as well as by a deeper-sitting opening (4), is connected to an inner chamber (3) on the other side. This inner chamber contains water in its lower section (3a) and an air pocket within its upper reaches (3b). An air duct (5) connects this air pocket to the ambient air through a self-rectifying turbine (6). Wave movements create pressure changes at the entrance to the vertical shaft (2). The water inside the shaft thus rises and falls within the shaft interior (1). In this way, the air pocket in the shaft’s upper section compresses or expands. Airflows within the air duct (5) then drive the self-rectifying turbine (6).

The principle of REWEC3 installations thus exploits wave movements in the sea for power generation. The air within the air chamber is alternately compressed (by wave peaks) and decompressed (by wave troughs) so that an alternating airflow is created inside a duct which in turn drives a self-rectifying turbine. The electrical energy is subsequently produced by a coaxial generator.

The advantages of REWEC3 installations in power generation speak for themselves:

  • They do not impinge visually upon the landscape, since they are barely detectable from the outside.
  • They absorb the effects of waves and moderate the impact of storms on the coastline.
  • Marine fauna are not endangered due to the elevated position of the turbines.
  • An installation of one kilometer in length can produce 8,000 MWh annually.

A system such as the REWEC3 obviously requires a reliable and rapid monitoring of pressure differences arising from impacting waves. Following extensive tests, the researchers at the Mediterranea University opted for the highly precise  ATM.1ST/N level sensors from STS. Crucial to this decision in favor of ATM.1ST/N pressure transmitters were the very short response times of < 1ms / 10 … 90% FS, as well as their very good long-term stability across a wide temperature range. Additionally, the fact that measuring instruments from STS, thanks to their modular construction, can be easily adapted to various requirements also spoke loudly. The ATM.1ST/N level sensors deployed can even be readily configured for use with the data loggers from National Instruments.

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