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 into new energy markets, energy developers are thinking creatively about wasted energy and harnessing greater efficiency. There are two important evolutions in energy storage technology that solar power producers are opting for when purchasing new systems: solar projects are leveraging the efficiencies of DC-coupled design in energy systems just at the emergence of market bidding as a new industry standard. As more developers pair solar systems with energy storage, the convergence of these two trends serve as an anchor design for utility-scale solar and storage projects going forward to ensure these systems are as optimized and impactful as possible.

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

Wärtsilä recently announced a solar PV and storage project that captures both of these new evolutions and illustrates the most efficient designs and revenue-generating systems out there right now. 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 generation from the solar PV panels to Georgia Power Company.

RWE Renewables was awarded the project through a sophisticated DC-coupled solar + storage design, which can increase energy delivery during peak demand times and is designed to facilitate integration of locally produced energy to the Georgia Power grid.

DC systems are back

The first electrified cities connected energy assets and load through DC energy; however, shortly after, most of the electricity system turned to rely on AC-based electricity due to its better ability to send energy long distances; however, to bring energy relatively short distances, AC conversion requires additional steps, equipment inverters and energy to convert and transport the electricity. Today, most co-located solar and energy storage systems leverage AC design to connect the two assets. Therefore, despite each running on DC energy, the two disparate systems convert their energy into AC before connecting.

The DC-coupled system distributes modular batteries across different points in the solar array, so the energy does not have to travel as far. As a result, the energy from the solar array does not need to be converted to AC to reach the batteries. DC systems offer improved system efficiency, lower “balance-of-plant” costs and the ability to recapture excess solar when the photovoltaics reach their peak and would otherwise be clipped. With bigger projects these benefits get multiplied. 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 since the battery can be connected to the inverters that are distributed across the entire PV field rather inexpensively, compared to standard 40-ft containers.

The design basis of optimized utility-scale solar projects is moving toward more distributed batteries on-site to capture the efficiencies of DC. Greater system efficiencies translate into greater energy yield. As new energy markets emerge, those yields can become new sources of revenue. However, the future of solar + storage is more than just hardware; software and advanced computing is a critical next step to ensure utility-scale storage systems coupled with renewables are optimized and do not go to waste.

The emergence of market bidding as the standard

The ability to interact with wholesale energy markets today, without having to wait for real-time markets, is becoming ubiquitous. At a basic level, energy storage and advanced analytics today widely function as a central computing hub for any power generation asset, tracking market parameters and reacting to maximize revenue for the asset owner and/or off taker. It can forecast how much power an adjoining plant will produce and take advantage of and balance for price variations, among other insights.

The RWE project, however, takes project optimization a step further. While Wärtsilä’s sophisticated GEMS Digital Energy Platform will control the entire hybrid plant, this project is the first application of Wärtsilä’s new cloud-based IntelliBidder software. IntelliBidder leverages machine learning and optimization algorithms based on automated and forecasted data and real-time trading for elevated value-based asset management and portfolio optimization.

The RWE project serves two primary market-based value-stack applications: solar firming 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 include two converging trends from energy storage — a shift toward more modular and distributed, DC-coupled energy storage systems across solar projects and market bidding as the standard. It is supporting the energy transition by financially optimizing renewables.