“As the costs of solar PV modules continue to reduce, and those of batteries follow a similar downward trajectory, solar-plus-storage is in growing demand among utilities and solar developers.
The US is leading the trend, where these types of clean energy power stations are starting to produce electricity competitively with gas peaking plants, especially when other revenue streams from grid services are factored in.
To further push down the levelised cost of energy (LCoE) of solar-plus-storage and maximise the amount of megawatt hours (MWh) of solar-generated electricity that can be fed into the grid, energy suppliers and developers are turning to direct current (DC) coupling these installations.
Compared with alternating current (AC) coupling, DC coupling the PV array and the battery storage system in front-of-meter installations, such as utility-scale plants, is a much newer, less standardised approach. This had led some US utilities to begin piloting these configurations to see how the technology performs. On the supply chain side, balance of plant (BoP) equipment manufacturers are delivering more standardised and simpler to use power electronics equipment for enabling DC-coupled plants.
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According to financial and technical analysis undertaken by Dynapower for DC-coupled solar-storage under the Solar Massachusetts Renewable Target (SMART) programme, an owner of a solar-plus-storage system comprising a 3MW PV array, a 2MW (AC) PV inverter, which is DC coupled to a 1MW/2MWh energy storage system, will be able to capture 265,388kWh of clipped solar energy annually, resulting in US$1.5 million of additional annual revenue, compared with an AC-coupled solar-storage system. The return on investment is estimated to be 5.6 years.
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One often-cited disadvantage of doing DC-coupled projects is that the batteries have to be installed in a distributed way throughout the solar array, compared with an AC coupled installation where one central container of batteries is installed.
While it is technically possible to do a DC-coupled central battery storage system, it would entail a high number of long cable runs, making it more practical to co-locate the batteries with the central inverters distributed throughout the installation.
Gupta says: “If a site is space constrained, it may be better to do AC coupled. With DC coupling, blocks of energy storage are distributed over the site and are connected to blocks of storage.””
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Verbruggen, Sara. Energy Storage News 9 October 2019.
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