To build upon the previous post, marine renewables have a fantastic and readily available resource in the waves and the tides, yet while fossil fuels have to be found, extracted, transported and refined prior to use, marine renewables still seem to come out worse off; why is this?

Boat and turbine in water

Energy density Starting with energy density, yes, water is better than the air used by a wind turbine - a similar output tidal turbine is about 1/4 of the diameter of a wind turbine - but this is still much, much less than found in fossil fuels. Fossil fuels pack around 34-39MJ/l whilst a tidal flow of 2.5m/s is an order of magnitude less than that.

The power density of fossil fuels represents a challenge to other emerging technologies such as electric vehicles, where in this case batteries are struggling to compete with the power density of petrol and diesel. Again, the power density in MJ/l is similar to that of a tidal flow, so an order of magnitude less than these "conventional" fuels.

Yield and Reliability In power generation yield and reliability are fundamental, but often conflicting. In order to generate a large yield, which means lots of income, there is a tendency to pursue more advanced, complex or novel designs which may not be as reliable. On the other hand, there are devices that are developed from well-established technologies and are very reliable, but often these are not best optimised for energy extraction and return a lower yield. This engineering balance is a fundamental challenge for marine renewables.

Cost of Marine Operations

Building upon the reliability issue identified in the previous point, if a marine energy device has an issue, it often results in an expensive and/or complex marine operation to access, retrieve or repair the device. This highlights the need for high reliability, which often comes at a cost or to the detriment of yield. The alternative is to have designed for marine operations, which is becoming more popular; i.e. buoyant nacelles, floating devices to move away from heavy lift operations.

Lack of Large Scale Generation

To date marine energy has been showcased in single device demonstrators or small pilot arrays; the lack of large farms generating power has prevented cost reductions through economies of scale and restricted the learning to the few devices deployed. Having more devices in an array not only facilitates the benefits of economies of scale, but also the potential for shared tooling and maintenance equipment, effectively reducing the cost of each asset.

Hopefully this blog has brought to attention the significant challenges facing marine renewables and a better understanding of how hard it really can be. However, whilst there are challenges, it is a fantastic natural, sustainable resource that we should make efforts to exploit. Given that marine renewables get three out of the four power generation OPEX costs for free (cost of fuel, transport of fuel, carbon taxes, O&M), it has great potential to succeed. Many clever minds are tackling the problems we have highlighted in this blog and real progress is being made in the field of marine renewables, so expect them to form a significant part of the renewable energy mix in the future!