Ease of installation and better availability to drive shift to AC block solutions

That’s according to Dax Kepshire, who until April this year spent nearly a combined decade at energy storage system integrator Fluence and in energy storage at its parent company AES Corporation, from which Fluence was spun out in 2018. Most recently he was Fluence’s chief operating officer (COO) for the Americas.

A BESS with a four times higher energy density than another requires four times less rigging, anchor bolts, auxiliary connections, DC connections and fire systems, Kepshire said.

Shift to AC blocks

Kepshire said the next big trend we will see is a shift towards BESS units that integrate power conversion technology (PCS) inside the unit, otherwise known as an ‘AC block’ (AC meaning ‘alternating current’ – DC power needs to be converted to AC power to be fed into the grid).

“The move to 5MWh+ blocks has been the main trend of the last year or two. The next one is the move to AC blocks,” Kepshire told Energy-Storage.news.

“Tesla and Sungrow dominate the space as the only truly vertically integrated ones, while Wärtsilä has a product coming and AESI (American Energy Storage Innovations) has a hybrid system which goes one step further than the AC block. All players will get into AC blocks, and the main driver of this is the ease and speed of installation that an AC block can provide.”

Deploying a project with AC blocks means no need for separate installation of large inverters and PCS equipment, instead using string inverters, allowing a direct connection to the grid via medium voltage transformers. AC blocks should also provide higher availability than DC blocks, by limiting the impact of inverter failures, and more decentralised and granular control.

DC blocks may be advantageous for projects which are expected to undergo augmentation, allowing for shuffling of DC capacity behind a central inverter, while the use of separate centralised inverters also provides more flexibility during installation. DC block projects may also require lower capex than an AC block one.

Rimac Energy, part of the electric vehicle (EV) supercar technology firm Rimac, also has an AC block design for its BESS, while lithium-ion OEM Envision is also currently completing certifications for an AC block solution.

Other aspects of the DC vs AC block debate

The topic has provoked a lot of debate and discussion on networking site LinkedIn.

String inverters have more technology risk making diligence more important because of their less widespread US deployment than centralised inverters, posted Michael Carpol, energy storage director for independent power producer (IPP) D.E. Shaw and formerly executive at system integrators Tesla and Wärtsilä.

Carpol added that commoditisation within the DC block segment (a characterisation of the market which is debated) was a sign of a maturing industry, and made it easier to plan engineering, procurement and construction (EPC) services with swappable components.

Shameer Majeed, energy storage senior expert for UAE-based renewables investor Masdar, meanwhile characterised the shift as a ‘race’ between DC block manufacturers and system integrators. DC ‘innovators’ are surging ahead with higher energy densities and better degradation profiles with integrators, who cannot match those, responding with AC block solutions, he said.

Kepshire added in a comment that the shift to AC blocks would be particularly felt in the US market.

Will the convergence to 20-foot, 5MWh plus products continue or reverse?

The current situation is a change from a few years ago when many Western-based system integrators had their own proprietary, modular building block BESS products which typically had lower energy densities.

There is a question over whether this convergence will continue or whether the future could see a reversal of the trend. Many say that we have reached a limit of energy densities packable into a 20-foot space because of the logistical challenges such a weighty product could present.

Another Fluence alumni, Marek Kubik, speculated recently on LinkedIn that the huge jump in battery cell form factor sizes from some manufacturers could mean supplier-specific BESS container designs. Kepshire and Kubik were both at Fluence from its founding.

“My (contrarian?) prediction: based on what I see, we might be heading back in the next few years back to the future – to divergent, supplier specific ESS container designs optimised around their specific cell form factors/partnerships and to stay comfortably within weight and logistics limits for shipping, which are near their limits,” Kubik said.

In our conversation, Kepshire similarly pointed out that the limit of transportable weights means there will be some reconfigurations of what’s inside, but he expected the actual containers to look the same.

“Longer term, I think enclosures will cap at the transportation weight limit, so I think the outside specifications will look similar, but you will see people get more creative in terms of the inside configuration, and cell form factors will go much higher,” Kepshire said.

One to watch in particular is a move from 305Ah-320Ah cells which dominate the market today, to over 500Ah and, in the future, over 1000Ah size cells, which Kepshire said could lead to a ‘reinvention of the DC block’.

Another change Kepshire sees in the BESS industry is a move to higher voltages as the battery cells get bigger, and that will also drive down costs.