When planning utility-scale PV plants and Battery Energy Storage Systems (BESS), engineers inevitably face a critical decision: How should the local fire water supply be organized?
The cost of an error in this design phase is high. Choosing the wrong type of reservoir can lead to massive budget overruns (CapEx), weather-related construction delays, and significant challenges during the eventual decommissioning and site restoration after 20 to 30 years.
In today’s European market, three primary solutions are used: underground concrete cisterns, above-ground corrugated steel tanks, and industrial flexible fire water tanks (pillow tanks). Below, we objectively compare these three options regarding construction time, costs, and the complexity of civil engineering and assembly.
1. Concrete Cisterns
The traditional, massive solution. They are mostly installed underground, which saves surface space.
The Hidden Disadvantages: The biggest drawback is the enormous effort required for civil engineering. You need excavators, must transport tons of excavated soil, set up complex formwork, and utilize concrete pumps. The entire process is extremely dependent on precipitation and frost. Furthermore, the demolition and disposal of such a structure after the lease expires often cost almost as much as the initial construction.
2. Corrugated Steel Tanks
A popular alternative to concrete. They consist of bolted, galvanized steel sheets and an internal PVC liner.
The Hidden Disadvantages: While the steel itself is often cheaper than a massive concrete structure, marketing materials frequently omit the cost of site preparation. A steel tank strictly requires the pouring of a precisely leveled concrete ring foundation. Additionally, a mobile crane is needed to assemble the sheets at height, and the actual assembly time usually takes 1 to 2 weeks.
3. Flexible Fire Water Tanks (Pillow Tanks) by Flexsol
An innovative approach that is increasingly becoming the standard for European EPC contractors. The tank consists of high-strength, fabric-reinforced technical PVC (up to 1300 g/m²).
The Main Advantage: Zero civil engineering. The tank is delivered to the site folded on a pallet. It is simply rolled out on a leveled sand bed over a protective non-woven fabric (geotextile). Installation takes only 1 to 2 days—without a single cubic meter of concrete and without heavy lifting equipment.
Comparison Table: Concrete vs. Steel vs. Flexsol
For a better overview, we have summarized the key parameters for establishing a fire water reservoir with a volume of 100 to 300 m³.
Evaluation Criteria |
Underground Concrete |
Corrugated Steel |
Flexsol Pillow Tank |
Foundation & Civil Engineering |
Excavation, formwork, reinforcement, and pouring |
Pouring of a reinforced concrete ring foundation |
None required (leveled ground + sand bed only) |
Special Equipment Required |
Excavator, concrete pump, trucks for soil transport |
Mobile crane, concrete mixer (for foundation) |
None required (manual rollout by crew) |
Construction / Assembly Time |
3 to 5 weeks (incl. concrete curing) |
1 to 2 weeks |
1 to 3 days |
Weather Dependence |
Very high (no pouring during rain/frost) |
Medium (no crane/foundation work in wind/rain) |
None (assembly possible in almost any conditions) |
Capital Expenditure (CapEx) |
High (from €150–300+ /m³ incl. civil works) |
Medium (from €80–150 /m³) |
Low (from €30 /m³) |
Decommissioning & Restoration |
Extremely complex and expensive (heavy demolition gear) |
Complex (disassembly, disposal of ring foundation) |
Very simple (drain, fold, and transport) |
Conclusion: The Decision for Lowering CapEx
If your project is located in an urban area where every square meter of land is extremely expensive, an underground concrete cistern may be justified.
However, for 90% of utility-scale PV plants and remote BESS projects where sufficient surface space is available, Flexsol’s flexible tanks are the clear winner. They eliminate the risk of weather-related construction delays, reduce foundation costs to zero, and completely solve the problem of future site decommissioning.
Request the technical specifications and drawings (CAD/PDF) for Flexsol tanks from our engineers today to integrate them into your current projects.