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Ahmad HemamiSome time ago, when thinking about the deep sea and transportation of electricity to the shore, it came to my mind that we could possibly use a (huge) battery system which could be charged in a wind farm and then deliver its charge to the onshore grid. Compared with transmission cables, the way that it is right now for all existing wind farms, this is not an ideal solution. Therefore, I set the idea to one side. But with the recent agreement between Canada and Germany, the idea was revived in my mind. If the wind farm is floating on the top of 500 metres or more of water and is far offshore, then the concept is worth being regarded as an alternative. After all, technically and financially, the numbers must be correct and suitable.
By Ahmad Hemami, McGill University, Montreal, Canada
The aforementioned agreement involves generating energy by (offshore) wind turbines in Atlantic Canada, using the harvested power to make hydrogen, and shipping the hydrogen to Germany. The last part may involve changing hydrogen to a hydride (say ammonia) to make the transportation more manageable (safer and less expensive). Hydrogen can be used as a fuel for transportation and heating, as well as for electric power generation, which also includes heating and transportation. If this makes sense, then the idea of transporting electricity in the form of a cargo (like the oil transported in tankers) should not be dismissed.
Many issues are involved in such a scenario (transporting electricity by batteries instead of cables). Basically, we are talking about a huge operation of using rechargeable batteries. At least, it is not something new that needs to be started from scratch. We have seen the developments in the case of electric vehicles at a very small scale and battery energy storage systems (BESS) at a larger scale. These two uses, of course, are not the same from the point of view of frequency and duration of the charge/discharge cycle, which can affect the useful life of a unit.
There has been tremendous progress in developing new batteries, including lessons for their safe use (prevention from catching fire) when in large numbers. For rechargeable batteries, after safety the capacity and the rate of charge and discharge are the main issues. The energy density, which determines the required space and weight, the efficiency, the lifespan and the price become the secondary concerns. The most common and more frequently utilised battery is the Li-ion, used in many electric vehicles and BESS systems. A good source of information about batteries and their potential use for BESSs is the ‘Handbook on Battery Energy Storage System’.
According to the available data, the largest BESS, at present, is 400MW/1,600MWh. This system (in California) consists of 4,500 (Li-ion) battery packs installed in an area of around 15,000 square metres. Like most of the applications of the battery storage systems so far, it is used for peak hours energy release in an electric grid. It can deliver 400MW for 4 hours, or more hours for less output power (e.g. 6 h 24 min if its load is 2.5MW).
Battery packs come from different sources/manufacturers with different sizes and configurations. Just to have an idea for comparison/reference, the specification of a giant battery pack (Tesla’s Megapack), including the electronic converters, cooling system and operation management system, is as follows:
Nearest dimensions 9 × 2.75 × 1.63 metres
Capacity 970kW / 3,880kWh / 4 h
Voltage 380V / 3-phase AC
Weight 38,180kg
For the sake of discussion, consider a 100MW wind farm, the daily production of which is around 800MWh. A daily cycle of 24 hours allows disconnection (of a charged cargo from a farm) and connection (of a ready-to-charge unit) to take place during the few hours of no or little wind. The electrical switching between the two load carriers can be automated in the docking area of a wind farm.
For this wind farm, nominally 1,004 units of the Megapack are required for handling the power requirement if the charge rate is the same as the discharge rate provided that the operating voltage is the same (380V). It takes, however, more time (> 4 hours) for the batteries to charge, since the generating rate is according to the wind turbines’ power curves. From the capacity requirement, the units must hold 800MWh, which is twice as much as the capacity of 1,004 packs. Therefore, 2,008 are needed. The weight of 2,008 units is 76,666 tonnes.
If plausible, a fleet of vessels similar to oil tankers could carry electricity on a continuous basis between a wind farm and the delivery port. The size and allowable weight of the cargo need to satisfy the requirement for the amount of electricity on board. The stacks of battery packs can be on multiple floors inside a carrier ship. A Panamax is a smaller size class of oil tanker around 292.4 metres long, with a beam (width) of 32 metres and a 12-metre draft (distance from the ship’s bottom to the water surface), capable of carrying 50,000 to 80,000 deadweight tonnes (total capacity). For containers, a Panamax vessel can carry up to 4,500 20ft equivalent units. A 20ft container measures 6 × 2.44 × 2.59 metres. Thus, its volume is less than a Megapack.
Although in terms of volume, more battery packs can be put in the vessel, in terms of weight possibly this size vessel reaches about its maximum capacity.
It looks like the scenario described here is more costly than cable transmission because of the continuous need for the fleet of electricity carrier vessels, their crew, their management, and all the associated expenditure. Nonetheless, if this is the way to go, then that is it, for now. The calculations made here are very preliminary, and more detailed calculations are needed, taking into account the many other factors involved. For example, the mega size electric storage units cannot be dealt with like ordinary cargos – keeping them dry and cool, with other safety precautions, dictates special purpose carriers that are designed and allocated only for this purpose.
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