DC-coupled energy storage systems ideal for real-time trading

By Andrew Tang, Vice President, Energy Storage and Optimization at Wärtsilä Energy

With more opportunities to sell energy to new energy markets, energy developers are thinking creatively about wasted energy and harnessing greater efficiency. There are two major evolutions in energy storage technology that solar energy producers choose when purchasing new systems: Solar energy projects are leveraging the efficiency of DC-coupled design in energy systems, precisely with the emergence of bids in the market as a new industry standard. As more developers link solar systems to energy storage, the convergence of these two trends serves as an anchor design for future utility-scale solar and storage projects to ensure that these systems are as optimized and impactful as possible.

A representation of Wärtsilä’s 40-MW / 80-MWh DC-coupled hybrid solar and storage system.

Wärtsilä recently announced a solar PV and storage project that both captures new evolutions and illustrates the most efficient designs and revenue-generating systems available today. The Wärtsilä system, a 40 MW / 80 MWh energy storage system located in Mitchell County, Georgia, will enable a subsidiary of RWE Renewables (Hickory Park Solar) to sell nearly 200 MW of solar power generation to Georgia Power Company.

RWE Renewables received the project through an advanced DC-coupled solar + storage design that can increase energy delivery during peak times and is designed to facilitate the integration of locally produced energy into Georgia’s power grid.

DC systems are back

The first electrified cities connected energy assets and charged via direct current; however, soon after, most of the electricity system became dependent on alternating current electricity because of its better ability to transmit energy over long distances; However, to bring energy for relatively short distances, AC conversion requires additional steps, equipment converters, and energy to convert and transport the electricity. Today, most co-located solar and energy storage systems use AC power design to connect the two assets. Despite all operating on DC power, the two disparate systems convert their energy into AC power before connecting.

The DC-coupled system distributes modular batteries over different points in the solar panel, so that the energy does not have to travel as far. This means that the energy from the solar panel does not have to be converted to alternating current to reach the batteries. DC systems offer improved system efficiency, lower plant-or-plant costs and the ability to recover excess solar energy when the photovoltaic cells reach their peak and would otherwise be cut off. These benefits are multiplied in larger projects. The RWE project is Wärtsilä’s first DC-coupled system. It will also be the largest application of Wärtsilä’s GridSolv Quantum solution, a fully integrated modular energy storage system. GridSolv Quantum’s compact design is highly optimized for DC-coupled systems, as the battery can be connected to the inverters distributed across the entire PV field relatively inexpensively, compared to standard 40-ft containers.

The design basis of optimized utility-scale solar projects is shifting to more distributed on-site batteries to capture DC efficiency. Greater system efficiency translates into greater energy yield. As new energy markets emerge, those returns can become new sources of income. However, the future of solar + storage is more than just hardware; software and advanced computing are a critical next step in ensuring that utility-scale storage systems in conjunction with renewables are optimized and not lost.

The emergence of market bidding as the norm

The ability to interact with the wholesale energy markets without having to wait for real-time markets is becoming ubiquitous. At a basic level, energy storage and advanced analytics today generally function as a central computing hub for any power generation resource, tracking and responding to market parameters to maximize revenues for the owner and / or purchaser of the assets. It can predict how much power an adjacent plant will produce and benefit from and balance for price variations, among other insights.

However, the RWE project goes a step further in project optimization. While Wärtsilä is sophisticated Gemstones Digital Energy Platform will control the entire hybrid plant, this project is the first application of Wärtsilä’s new cloud-based IntelliBidder software. IntelliBidder uses machine learning and optimization algorithms based on automated and predicted data and real-time trading for value-based asset management and portfolio optimization.

The RWE project serves two primary market-based value stacking applications: solar reinforcement and DC solar PV clipping recovery, which will improve the predictability of the intermittent generation.

To further optimize renewable energy, the future of solar + storage will encompass two converging trends of energy storage: a shift to more modular and distributed, DC-coupled energy storage systems for solar energy projects and market bidding as the standard. It supports the energy transition by financially optimizing renewable energy sources.

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