Modular Matters for Bulgarian BESS: Sigenergy Powers Ultra-Fast Deployment

This article was originally published by ClimateCopy.

The black asphalt shimmers with heat as the road winds through the dappled green of the ancient forests of Strandzha Nature Park, the largest protected area in Bulgaria. Nestled in the Strandzha Mountains near the Turkish border, the landscape here is both rugged and remote.

After a steep descent, the dense trees give way to rolling, rocky terrain leading to a small checkpoint, where Bulgarian officials inspect vehicles entering the southeastern border of the European Union. From here, a sharp veer onto a narrow dirt track suddenly brings the 11 MW Malko Tarnovo solar park into view.

Built in 2021, the project is one of two developed by the family-owned company Trakia MT. Immediately adjacent to the neatly arranged rows of solar modules stand gleaming white columns of individual battery packs – they’ve just been freshly installed, and in record time.

Bulgaria’s solar market has expanded rapidly in recent years. From an installed capacity of 1 GW at the beginning of the decade, it has climbed to 5 GW today. Yet, as is increasingly true elsewhere, Bulgaria’s solar sector is now grappling with the challenges of its own success.

Solar’s share of Bulgaria’s annual electricity generation increased from just 4% in 2022 to 13.3% in 2024. This surge has led to lower daytime electricity prices in the day-ahead market, with zero and even negative price events becoming more common. As a result, a strong business case for battery energy storage systems (BESS) has quickly emerged.

At the same time, lignite coal generation in the country has plummeted from 54% to 26.5%, according to the Transmission System Operator, creating a greater need for voltage and frequency regulation from non-thermal sources – particularly utility-scale BESS.

“Battery installations are underway almost everywhere in the country,” says Dimitar Zarchev, Director of the National Dispatch Center from the Bulgarian Transmission System Operator. “I expect somewhere between 600 and 800 MW to be in place by the end of the year.”

Time-shifting solar generation to meet evening demand peaks will be one of the key functions of the current wave of battery installations, Zarchev adds. There is also growing anticipation that batteries will contribute to voltage control, frequency regulation, and potentially, synthetic inertia in the future. “These functions are currently being tested with producers,” he says.

EU Funding Urgency

Another major driver behind the rapid deployment of BESS in Bulgaria is the availability of public funding. In April 2025, the Bulgarian government announced the selection of 82 battery storage projects, totalling nearly 10 GWh of capacity. These projects are set to receive funding through the National Recovery and Resilience Plan (NRRP), part of the European Union’s broader COVID-19 recovery stimulus package. Altogether, €587 million has been allocated, with grants covering up to 40% of a project’s capital expenditure.

Nikola Gazdov a veteran solar advocate and Chair of the APSTE - Association for Production, Storage and Trading of Electricty, notes that with the current mix of market volatility and public subsidies, first movers could see payback periods as short as four to five years. But developers will have to move fast.

“To receive the NRRP subsidy, battery projects will have to be completed and grid connected by March, 2026 – and all payments should be confirmed, audited, and wired by August,” explains Gazdov.

20 MWh in 10 Days

While a key feature of solar and BESS projects can be their relative speed to deploy, they can (and often do) face challenges that delay commissioning. But with strict deadlines tied to the EU grant funding, lagging execution or grid connection issues could prove to be very costly.

Project delays are often influenced by the remoteness of the location, challenging terrain, and unforeseen technical issues during commissioning. In Bulgaria, these risks are further exacerbated by regulations that restrict utility-scale PV development on highly productive arable land, which includes much of the country’s flat and easily accessible areas.

As a result, most utility-scale solar projects, both completed and in development, are sited on challenging landscapes with difficult ground conditions, such as hard rocky soils. These factors significantly complicate construction and increase risk for developers – making the rapid execution of the Malko Tarnovo project notable.

Set against rolling terrain, Malko Tarnovo is visually striking, with solar modules stretching across undulating green fields. But while visually appealing, the project has faced a major challenge: consistently low, and increasingly negative, daytime electricity prices, a trend becoming more common across many European markets. This pricing shift has eroded revenue potential from the project.

“In Bulgaria, prices can be negative from 10 a.m. to 6 p.m., which means that during peak PV production hours, we’re essentially earning no revenue,” explains Peycheva-Miteva. “So, it was a no-brainer to look for a solution that will allow us to arbitrage, provide grid services, and get the most out of our asset, while stabilizing returns over the long term.”

While subsidies may be enabling much of Bulgaria’s BESS rollout, for Trakia MT, the business case was so strong that it proceeded without public funds. Peycheva-Miteva notes that the investment decision, “further serves to underscore the faith of our company in the quality and durability of Sigenergy’s storage solution.”

Technical Specs

The battery system deployed at the Malko Tarnovo site stands out in several key ways. Unlike the common shipping container format, the system, supplied by Shanghai-based Sigenergy uses a modular design. While primarily targeted at C&I applications, the new SigenStack batteries can be installed in parallel, making them applicable in utility-scale projects as well.

The Malko Tarnovo project consists of 90 Sigenergy 110 kW inverters, paired with a 240 kWh SigenStack batteries. The SigenStacks are designed to IP66 standards, eliminating the need for additional housing while still ensuring ingress prevention against dust and water.

Fire safety is addressed through Sigenergy’s proprietary six-layer safety system (see Figure 1.2 below), which has been independently validated by quality-assurance provider Intertek for compliance with UL 9540A and IEC 62619 standards. As demonstrated in testing, the modular system design prevents fire propagation across a battery array – containing incidents to an individual unit. And after a fire event, a damaged unit can be swapped out, without impacting the function of neighbouring batteries.

Each SigenStack includes integrated EMS (Energy Management System) and BMS (Battery Management System), removing the necessity for external data loggers. The modular configuration is also highly scalable – up to 100 units can be connected in parallel. And the system’s fully networked communication architecture ensures response times of under one second, even across as many as 2,000 devices, according to Sigenergy.

The modular design of the SigenStack platform streamlines both logistics and installation. Traditional containerized battery systems are heavy and cumbersome, often requiring specialized haulage and cranes. In Europe, truck size and weight restrictions can further complicate the transport of containerized BESS, especially to rural or remote sites where narrow roads, weight-limited bridges, or tunnels present additional barriers.

By contrast, SigenStack units can be delivered on smaller trucks and positioned manually, without use of heavy machinery. At the remote Malko Tarnovo site, accessible only via winding rural roads, this modular solution proved especially advantageous.

Speed Meets Scale

Time is often a critical factor in BESS projects – both in how the system responds to market signals and in terms of speed of project execution. The combination of modular hardware, advanced software, and integrated communication architecture delivers clear advantages in terms of both rapid response times and construction.

At Malko Tarnovo, the battery system was installed in just 10 days, followed by a further two for commissioning.

“Previously, we interviewed many customers and EPCs, and most expected a 10 MWh project to take two to three months,” says Roy Zhang, Head of Global Sales and Solution Management at Sigenergy. “With this modular design, featuring plug-and-play connectors and floating terminals – five people were able to complete the installation within 10 days.”

The project marks a major milestone for Sigenergy. Not only is it a major step in Sigenergy’s expansion into utility-scale energy storage, but it also highlights the value that modular energy storage systems can deliver.

“With this project, we’re demonstrating that the SigenStack can be used not only in the large commercial segment, but also in small utility projects,” Zhang adds. “This is just the beginning for SigenStack, and we believe it will prove its capabilities and convince more customers to use SigenStack worldwide.”