Lerwick Power Station on Shetland is an ageing power station using a number of large diesel generators. Eventually it will be replaced with more modern technology and the purpose of the energy storage system is to reduce the peak demand on the power station and level out the loads. This should allow downsizing of the new power station, potentially using more efficient and more modern technologies and integrating renewable energies into the grid.
This 1 MWh battery is a pilot project to better understand the potential and impact of grid connected energy storage devices, to determine suitable control mechanisms and to gather long term experience with battery technologies for such applications. Originally a Sodium Sulpher (NAS) battery technology was chosen to be installed in a purposely designed building next to the Power Station. However after the innovative NAS battery had been installed it was decided not to operate it due to the news of a fire in another large NAS battery installation in Japan. It was then decided to replace the NAS battery with a more established valve regulated lead acid technology.
The installed battery is a 6.3MWh energy storage (usable 3MWh), weighing 200 tonnes, operating at approx 20deg C and capable of producing 1.3MW electric power. The battery supplier YUASA claims a life time of 5 years or 1500 cycles for their 1000Ah cells. Lead acid batteries offer very good recyclability and this particular cell type used is suitable for deep cycle operation, however even though lead acid batteries are well established this is still a very new application and achieving claimed life requires sophisticated control protection and management of that battery.
REAP has developed and supplied a custom battery monitoring and management system and assisted YUASA with the commissioning and installation. The battery consists of 12 parallel battery strings and the REAP management system monitors all string currents and voltages, calculates state of charge, communicates with the inverter to limit charge and discharge limits, manages essential equalisation for the lead acid battery, logs data and allows YUASA to remotely access information about usage and battery behaviour. The later point is important for YUASA in order to provide the claimed life time and detect any problems early, understand the usage and the behaviour on this application which in turn will allow them to continuously improve the battery technology and the settings in the BMS.
To specify and implement algorithms that would satisfy both minimal restrictions on operation (max performance) and on the other hand maximise the battery life. Maximising life of such a large lead acid battery is a very challenging task and required us to understand lead acid battery technology and its key operational requirements, whilst appreciating that the end user may wish to use the battery without any restrictions. Other challenges were high accuracy current measurement in order to calculate state of charge and managing a number of parallel strings. Also system testing and validation is challenging because such a large system cannot be rebuild the laboratory for testing. Testing and commissioning on site requires a number of stakeholders to be involved.