Tidal Stream vs. Wind Energy: The value of cyclic power when combined with short-term storage in hybrid systems by Dr Danny Coles
Tidal stream energy is at the pre-commercialisation stage, with 29 MW of capacity installed globally to date. The UK has led the way in the development of tidal stream energy, with a total of 17 MW installed so far, and an additional 2 MW scheduled for installation in early 2021 . UK projects with a total installed capacity of around 124 MW are at an advanced pre-installation stage, and are ready to bid into the Governments energy system auction round at the end of 2021. The latest national scale resource estimate concludes that tidal stream energy could provide approximately 11% of the UK’s current annual electricity demand .
The economic viability of tidal stream energy, along with other technologies, is typically assessed based on Levelised Cost of Energy (LCoE). LCoE quantifies the average net present cost of electricity generation. A shortfall of LCoE is that it does not consider whole-system costs, which include costs incurred on the grid side of the connection with the grid. For example, introducing variable renewable power will result in a greater need for reserve power capacity and/or energy storage. This will help ensure demand is met during periods of low renewable power generation. As the penetration of renewable power capacity increases, power variability will also increase, leading to significant demand matching challenges. This is reflected in the HM Government’s recent Offshore Wind Sector Deal Industrial Strategy report , with plans for a new System Management and Optimisation Task Group to establish innovative system integration solutions.
Figure 1. Horizontal axis tidal stream turbine being installed at the MeyGen project, in the Inner Sound between the Island of Stroma, and mainland Scotland. Image courtesy of SIMEC Atlantic Energy.
The part European funded Tidal stream Industry enerGisER project (TIGER), led by the Offshore Renewable Energy (ORE) Catapult, is working with researchers and modellers to investigate how the predictable nature of tidal stream could help manage this power variability in the future.
Recent research, “Tidal Stream vs. Wind Energy: The value of cyclic power when combined with short-term storage in hybrid systems” , has begun to quantify the whole-system value tidal stream energy can bring to hybrid energy systems. The research, published in the Tidal Turbines special issue of the Energies journal, investigates ways to displace oil generated electricity using a hypothetical scenario based on the island of Alderney, located in the Channel Islands.
Alderney uses oil generators to provide the majority of its electricity, but has a significant wind and tidal stream energy resource. Two hybrid systems are considered, the first uses a tidal stream turbine, a lithium-ion battery, and a back-up oil generator, as shown in Figure 2. The second uses the same system architecture, but the tidal stream turbine is replaced with a wind turbine. Both the tidal stream turbine and the wind turbine have a power rating of 1.5 MW and produce approximately the same energy per year (6.3 GWh and 6.8 GWh respectively). The battery has a storage capacity of 3 MWh. The turbines are the primary source of power used to meet the island’s demand. If the turbines are generating more power than required, it is stored in the battery. If the battery is already full during these periods, the excess turbine power is curtailed. When the demand cannot be met by the turbine and/or the battery, demand is met by the back-up oil generators.
Figure 2. Hybrid systems schematic
Results demonstrate that tidal stream power is well suited to integration with short term storage in order to displace oil. Tidal power is cyclic, providing four power periods per day, separated by slack tide, which last for around 2 hours. The cyclic nature of tidal power enables the battery to charge during high tidal resource periods, and discharge during slack tide, thereby absorbing the tidal energy within the hybrid system to minimise curtailment and reliance on the oil generator.
In contrast, the wind hybrid system curtails high levels of energy due to the variable nature of the wind resource. The wind resource is generally more persistent than the tidal resource, meaning periods of high/low resource have a longer duration. This is demonstrated in Figure 3, which shows a typical power generation power time series from the tidal and wind turbines, along with Alderney’s electrical power demand. During high wind periods, excess turbine power charges the battery, but the battery has no way of discharging once full since the wind turbine can meet the demand on its own. This causes high levels of excess turbine power curtailment once the battery becomes fully charged.
Figure 3. Temporal variation in tidal and wind power, as well as Alderney’s electrical power demand, over a 2 week period in January 2013.
During low wind periods, when the wind turbine cannot meet the demand, the battery discharges but then remains empty in the absence of excess wind power. In these periods the system must rely on oil generated power alone. Comparisons between the tidal and wind hybrid systems are provided in Table 1. Relative to the wind hybrid system, the tidal hybrid system emits 34% less carbon by displacing a greater level of oil. The tidal hybrid system also saves £6.4 m on oil expenditure over the 25 year life of the project. This assumes a flat cost for electricity generation by the back-up oil generator of £0.31/kWh over the 25 year operating period of the systems.
It is shown that increasing the battery storage duration has very little impact on the performance of the wind hybrid system. It is also found that an additional two wind turbines must be installed in order to reduce carbon emissions to the levels achieved by the tidal hybrid system.
Table 1. Summary of the hybrid systems performance
To view outputs from the models that demonstrate the performance of the tidal and wind hybrid systems, see below:
After the publication of this research, a similar study was conducted by Blackfish Engineering . This work came to the same conclusion that the tidal hybrid system was more effective at displacing oil and reducing carbon emissions than an equivalent wind hybrid system.
The next stage of the research in 2021 will contextualise the cost savings with respect to the capital and operational expenditure of tidal stream and wind turbines. Through quantification of these costs, the hybrid system design can be optimised to establish the mix of technologies required to displace oil most cost effectively. Cost effectiveness will be quantifies based on LCoE, as well as whole system costs that also incorporate incurred grid costs.
To view the full research paper on ‘tidal stream vs wind energy the value of cyclic power when combined with short-term storage in hybrid systems’ – click here.
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