Abstract
Viable tidal stream energy resources are often situated around remote locations that may have limited and frequently disrupted grid connections. The slow speed of grid upgrades in these regions is a major barrier to the growth of the tidal stream sector. One proposed solution to this is the introduction of local offtake industrial users that can take advantage of the local generation of green electricity at advantageous pricing as part of a behind the meter power purchase agreement. For local businesses this replaces reliance on expensive retail electricity and heavily polluting diesel generators. This arrangement may also have the potential to attract new industries to these areas. The predictability of tidal stream energy can provide a clear advantage over other intermittent renewable generation technologies.
Although tidal generation is predictable, it is also cyclical, with daily cycles that create four periods of slack water every day, together with monthly and yearly cycles that mean there are large, predictable variations in the power generated. Battery storage has been proposed as a method to smooth this generation, thus providing continuous power to an offtake, but little information is available on how this would work in practice. Tidal power can also be subject to large instantaneous power variations.
The Fall of Warness tidal generation site off the island of Eday, Orkney, which is operated by the European Marine Energy Centre (EMEC), will become subject to constrained grid export as the number of tidal energy converters (TECs) on the site increases. In anticipation of this, EMEC has introduced a hydrogen production plant on its Caldale onshore site on Eday and has installed a vanadium flow battery (VFB) to replicate this challenge. A local wind turbine that can divert its generation during curtailment has also been introduced to the system to investigate whether the offtake could use this intermittent energy beneficially. At time of writing, this plant is not yet fully operational, but site acceptance data is available. This report uses this data, together with tidal stream current speeds at the TEC locations, to model how these could be used to optimise hydrogen production by:
- Maximising the operational time of the hydrogen electrolyser (HE) at the most efficient setpoint; and
- Minimising operations and maintenance costs both by reducing the quantity of startup cycles and using tidal cycles to schedule regular maintenance cycles.
This report shows how the predictability of tidal stream and curtailed wind energy can be combined with energy storage systems to maintain optimum hydrogen production rates for most of the time, with turndown to minimum at neap tides. The periods of lowest generation are approximately six months apart and offer an ideal opportunity for planned maintenance of both the electrolyser equipment and the TECs. The availability of a grid connection, even if limited, enables excess generation to be exported during spring tides and the import of grid electricity for the plant during times of low generation or TECs being unavailable. Full economic modelling can be used to establish the most beneficial operating profile for grid import, increased battery capacity or stopping production during the remaining periods of low generation, however this was beyond the scope of this work. The addition of the wind turbine was shown to have minimal benefit for the EMEC site as curtailed wind energy was limited, however, with careful system design, this benefit could be increased.
Tidal energy converters, combined with energy storage, can deliver stable, near continuous behind the meter electricity for industrial offtake industries. Early detailed modelling, based on local tidal resource and equipment specifications, is important during the design feasibility stage to enable careful sizing and specification to establish the optimal economic operating profile for each individual system.