You want to build at the economic minimum and optimised for energy. You are looking for energy supply concepts that go beyond the heat pump, the photovoltaic array or the conventional battery storage unit. You are open to any technology, and the highest attainable degree of energy autonomy in the building is your goal?
We advise you on the current state of the art and on today's technical options for energy supply — in residential as well as in non-residential construction.
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The starting point is not the product but the building: its demand across the year, its storage capacity and its interplay with the surroundings. Only from that does the sensible technology follow — technology-neutral and without committing to a type of plant in advance.
Consulting and calculation Energy concepts, including sorption and latent heat storage Building simulation for demand coverage (yield of heat pumps and solar thermal systems) Yield forecasts for wind turbines (hourly simulation, horizontal and vertical axis) Talk to us while your project is still in the concept phase — that is where it is decided what remains economically feasible later on.
The right battery is not decided by the price per kilowatt-hour alone, but by storage duration, number of cycles, power demand and installation site. Lithium-ion batteries are strong at short, powerful discharges and have the higher efficiency. Redox flow batteries come into their own where discharge is steady over many hours and cycling continues for decades with little capacity loss.
How a redox flow battery works The energy is not held in solid electrodes but in two liquid, water-based electrolytes in separate tanks. Pumps drive them through a cell stack in which a membrane separates the two circuits; charging and discharging happen there. Because the tanks determine the energy and the stack determines the power, both can be sized independently: twice the storage duration only needs twice the tank volume and electrolyte. In vanadium systems both sides contain the same element in different oxidation states, so crossover through the membrane costs efficiency and, temporarily, capacity, but does not consume the electrolyte; rebalancing both sides recovers the capacity. The water-based electrolyte is non-flammable, but contains sulphuric acid.
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Structure of a vanadium redox flow battery
Two tanks of liquid electrolyte, one pump each, a cell stack with membrane in the middle and an inverter to the grid. Energy depends on tank volume, power on the cell stack.
Inverter
Grid / building
Charge ⇄ discharge
− +
Cell stack with membrane
determines power (kW)
Negolyte
V²⁺ / V³⁺
Posolyte
V⁴⁺ / V⁵⁺
Tank volume determines
energy (kWh)
Tank volume determines
energy (kWh)
Pump
Pump
Structure of a vanadium redox flow battery. The tanks determine the energy, the cell stack the power — both can be sized independently. Power capability For the three systems compared here, maximum discharge power is about 0.23 to 0.34 C relative to rated capacity (for CellCube and Endurium Enterprise, the maximum usable energy stated in the datasheet). Depending on design they can only sustain close to full power for two to four hours: CellCube states 467 kW for 2 hours for the FB 333-4, Invinity 75 kW for 2.25 hours for the VS3 and 150 kW for 4 hours for the Endurium Enterprise. At two and 2.25 hours (CellCube, VS3) the batteries deliver only 61 and 73% of their rated capacity respectively, the Endurium Enterprise 94% at four hours; their full capacity is only available at eight to ten hours. This is mainly because cell voltage and thus available power fall as the state of charge drops, and high currents increase internal losses. Manufacturers therefore give pairs of power and duration instead of a fixed C-rate. Variants designed for longer durations have a lower C-rate: the CellCube FB 333-8 has twice the capacity at the same power, i.e. 0.17 C, and, based on the datasheet figures (230 kW for 12 hours), delivers only about 90% even at 12 hours; the datasheet gives no reason for this. The datasheets give no figure for short-term overload beyond maximum power; CellCube merely lists an ‘Overrating functionality’. For the Invinity Endurium Enterprise, the datasheet states that maximum power of the base version is available continuously, without rest periods, for four hours. Lithium batteries, by contrast, can also be designed for one hour or less.
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Usable energy depending on discharge duration
Calculated from the manufacturers’ datasheets: power times discharge duration relative to rated capacity. CellCube FB 333-4: 61 percent at 2 hours, 82 percent at 4 hours, 90 percent at 6 hours, 100 percent at 10 hours. Invinity VS3: 73 percent at 2.25 hours, 90 percent at 4 hours, 101 percent at 8 hours.
0%
25%
50%
75%
100%
Rated capacity
0
2
4
6
8
10
12
Discharge duration at constant power [h]
usable energy
CellCube FB 333-4 (1,525 kWh): 467 kW for 2 h = 61% of rated capacity CellCube FB 333-4 (1,525 kWh): 311 kW for 4 h = 82% of rated capacity CellCube FB 333-4 (1,525 kWh): 230 kW for 6 h = 90% of rated capacity CellCube FB 333-4 (1,525 kWh): 152 kW for 10 h = 100% of rated capacity Invinity VS3 (230 kWh): 75 kW for 2.25 h = 73% of rated capacity Invinity VS3 (230 kWh): 52 kW for 4 h = 90% of rated capacity Invinity VS3 (230 kWh): 29 kW for 8 h = 101% of rated capacity CellCube FB 333-4 · 61%
Invinity VS3 · 73%
CellCube FB 333-4 · 1,525 kWh Invinity VS3-022 · 230 kWh Usable energy depending on discharge duration, calculated as power × duration relative to rated capacity from the manufacturers’ datasheets. At two and 2.25 hours respectively, the two systems shown deliver only 61 and 73% of rated capacity. Manufacturers state response times of less than 5 to less than 200 milliseconds, depending on the system; from standstill Invinity states under one minute for the Endurium Enterprise, presumably because the pumps first have to circulate the electrolyte. The datasheets permit the full depth of discharge from 0 to 100%, and cycle life is very high to practically unlimited. Manufacturers state annual capacity loss of below 0.2% (Invinity Endurium Enterprise) to below 0.5% (Invinity VS3, CellCube); CellCube specifies at most 10% over 30 years. The trade-off is a DC round-trip efficiency of at most 73 to 84%, depending on the system.
Key data of selected vanadium flow systems (datasheet values; C-rate and floor load calculated) System Capacity Max. discharge power C-rate DC efficiency max. Mass Floor load CellCube FB 333-4 1,525 515 0.34 84 135 ≈ 22 Invinity VS3-022 230 78 0.34 75 24.6 ≈ 16 Invinity Endurium Enterprise (string of 2 modules, base version) 640 150 0.23 73 55 (2 × 27.5) ≈ 18 (per module)
Sources: CellCube datasheet release 4.2 ; Invinity VS3-022 datasheet ; Invinity Endurium Enterprise datasheet (09/2025) . Floor load calculated as evenly distributed from the mass and footprint of one unit (CellCube: complete system, 7.5 m high; Invinity: one module). The Endurium Enterprise is also available in a larger version with 188 kW / 700 kWh.
Prices Hardly any manufacturer publishes list prices; offers are made per project. The price per kilowatt-hour falls with storage duration because only tanks and electrolyte grow with it, while the cost of the cell stack is spread over more kilowatt-hours: with only two hours of storage a vanadium system can reach US$700 to 900 per kWh according to the cost estimate cited below and is then not competitive with lithium; for Western turnkey systems at typical durations of two to eight hours the same source gives US$450 to 750 per kWh, with short durations at the upper end and above; at ten hours the same design can drop towards US$350 to 450 per kWh; Chinese gigawatt-hour-scale projects with leased electrolyte have brought reported costs down towards US$300 to 450 per kWh, according to the same source. The electrolyte accounts for 40 to 60% of system cost, rising with duration. Leasing the electrolyte instead of buying it cuts the initial investment by 35 to 40%, but the lease payments add up again over 25 years.
Vanadium flow system prices, with lithium iron phosphate for comparison Case Duration Price ≈ €/kWh China, award 09/2025 (Rongke, 2.5 MW / 10 MWh) 4 h 1.96 CNY/Wh ≈ 255 Estimate, turnkey, Western suppliers, typical durations 2–8 h 450–750 US$/kWh (up to 900 at 2 h) ≈ 395–660 (up to 790) Estimate, same design 10 h down towards 350–450 US$/kWh ≈ 305–395 Lithium iron phosphate, China, average bid 08/2026 4 h 0.53 CNY/Wh ≈ 69
Sources: Sina Finance, 26 Sep 2025 (Rongke Power award) ; How to Store Electricity, VRFB cost estimate 2026 (blog) ; Tencent News, 8 Sep 2026 (lithium prices August 2026) . Converted at the rate of 23 Sep 2026: €1 = CNY 7.67 = US$1.14 (finanzen.net EUR/CNY , EUR/USD ). The Chinese prices generally apply to delivery of the storage system in China; construction, grid connection, transport to Europe and duties are not included. The vanadium figure is an award price, the lithium figure an average bid; for 2-hour lithium systems the average bid in August 2026 was 0.69 CNY/Wh.
Use in buildings For single-family homes there is practically nothing on offer: the smallest unit from the manufacturers listed here, the Invinity VS3, has around 230 kWh but is only offered in arrays from 2 MWh; VFlowTech starts at 500 kWh (FlowBox), and the other manufacturers mostly supply systems in the megawatt-hour range. VoltStorage had offered a 6.8 kWh home unit, discontinued it in October 2021 , turned to commercial-scale storage and ceased operations in 2025 ; ESS Tech, Inc. acquired its IP and assets in 2026. The technology plays to its strengths – cycle life and longevity – with long storage durations and many cycles per year: in districts, commercial sites and on the grid, for example to shift solar power into the night or to run a heat pump for hours. Most of the large plants in operation are designed for three to five hours, Oxford for 2.5 hours, the system at Fraunhofer ICT for ten hours. For load peaks, inrush currents and fast charging the power is too limited. One option is to combine the flow battery with a lithium battery that handles the peaks while the flow battery carries the base load and the many cycles; at Energy Superhub Oxford both technologies work together. The electrolyte is heavy: evenly distributed over the footprint this gives a calculated 16 to 22 kN/m² for the systems in the table and around 27 to 29 kN/m² for the 3 MWh CellCube variants, locally more under feet and skids. That can rarely be accommodated on a suspended floor slab; these systems belong on their own ground slab or strip foundation. Because the electrolyte contains sulphuric acid, a spill bund and secondary containment at the site also need to be checked. Manufacturers Rongke Power (Dalian, China) — one of the largest manufacturers, more than 3.9 GWh installed, production capacity 1 GW / 4.5 GWh per yearSumitomo Electric (Japan) — active in redox flow battery research since 1982, grid storage in Japan and CaliforniaInvinity Energy Systems (UK, Canada) — VS3 for systems of 2 to 40 MWh, Endurium Enterprise for commercial and industrial sites with 4 to 80 MWh, Endurium for large plants from 12 MWh CellCube (Austria) — systems of 1.5 to 3 MWh, according to the datasheet more than 30,000 cycles and a 30-year lifeVRB Energy (Canada, manufacturing in China) — MW class, 1 MW power modulesSCHMID Group (Germany) — vanadium flow systems; Everflow joint venture with Nusaned (SABIC) for manufacturing in Saudi ArabiaVFlowTech (Singapore) — FlowBox (500 kWh) and FlowMax (2 to 3 MWh) container systems, custom FlowFlex plants from 7.5 MWhESS Tech, Inc. (USA) — Horizon iron flow battery for 20 hours and more; acquired VoltStorage's IP and assets in 2026An overview of just over 30 manufacturers, including some no longer active, is kept by Vanitec, the vanadium industry association .
Systems in operation 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Large redox flow batteries in operation, storage capacity in MWh
Jimusar, Xinjiang (CN): 200 MW / 1,000 MWh; Wushi, Xinjiang (CN): 175 MW / 700 MWh; Dalian (CN): 100 MW / 400 MWh; Minami-Hayakita, Hokkaido (JP): 17 MW / 51 MWh; Pfinztal (DE): 2 MW / 20 MWh; Oxford (UK): 2 MW / 5 MWh.
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Storage capacity [MWh]
Jimusar, Xinjiang (CN) China Three Gorges, Rongke · since 2025
Jimusar, Xinjiang (CN): 200 MW / 1,000 MWh (5 h) 1,000 MWh 200 MW
Wushi, Xinjiang (CN) Rongke Power · since 2024
Wushi, Xinjiang (CN): 175 MW / 700 MWh (4 h) 700 MWh 175 MW
Dalian (CN) Rongke Power · since 2022
Dalian (CN): 100 MW / 400 MWh (4 h) 400 MWh 100 MW
Minami-Hayakita, Hokkaido (JP) Sumitomo Electric · since 2022
Minami-Hayakita, Hokkaido (JP): 17 MW / 51 MWh (3 h) 51 MWh · 17 MW
Pfinztal (DE) Fraunhofer ICT · since 2017, full build-out 2025
Pfinztal (DE): 2 MW / 20 MWh (10 h) 20 MWh · 2 MW
Oxford (UK) Invinity · since 2021
Oxford (UK): 2 MW / 5 MWh (2.5 h) 5 MWh · 2 MW
Large redox flow batteries in operation, storage capacity in MWh. Links to the systems are in the list below (Dalian under the Wushi entry). Jimusar, Xinjiang (China) — 200 MW / 1,000 MWh, China Three Gorges with Rongke Power as supplier, reported fully operational on 31 Dec 2025; the first gigawatt-hour-scale flow battery, next to a 1 GW solar parkWushi, Xinjiang (China) — 175 MW / 700 MWh, Rongke Power, completed December 2024, grid-forming; succeeded the 100 MW / 400 MWh Dalian plant commissioned in 2022 as the largestMinami-Hayakita, Hokkaido (Japan) — 17 MW / 51 MWh, Sumitomo Electric, since April 2022; balances 162 MW of wind power from 15 wind farmsPfinztal, Baden-Württemberg (Germany) — 2 MW / 20 MWh at Fraunhofer ICT, coupled directly to a 2 MW wind turbine via the DC link; a pilot plant since 2017 , in research operation at full build-out since 2025 and, according to a press release of June 2025 , Europe's largest vanadium flow batteryEnergy Superhub Oxford (UK) — 2 MW / 5 MWh from 27 VS3 modules, Invinity, in operation since late 2021, officially opened 2022; combined with a 50 MW / 50 MWh lithium battery (Wärtsilä) ; according to Invinity, the flow battery is intended to act as first responder for a large share of system use, sparing the lithium batteryRetrieved 23 Sep 2026.
For a specific project it pays to compare with thermal storage: where heat is ultimately needed, a heat store – from a hot-water buffer tank to a latent heat or sorption store – can be the cheaper solution.
Building energy-efficiently is not in itself building in a climate-friendly or ecologically sustainable way. A building interacts with its immediate environment and with the climatic conditions of the present and the future.
The points below are those that should be considered at an early design stage. They do not list every requirement exhaustively; they give a first overview.
Windows and glazed areas A low U-value minimises heat loss across the envelope. Setting the windows deep into the reveal exposes them to little direct solar gain in summer, which lowers the cooling load. With the low sun of winter the gain is welcome instead. In the 2226 project in Lustenau, the 76 cm wall produces reveal depths of around 70 cm that exploit precisely this effect. Roof lights and roof windows should be avoided in such a concept.
Covering heat and cooling demand To minimise heating demand, the waste heat that occurs anyway — from processes, people, equipment and lighting — should be used. Helpful measures are:
Components with high internal thermal storage capacity — effective to a depth of roughly 10 cm depending on the material Open, connected spaces to distribute internal heat gains Generous room heights: a large air volume lowers the CO₂ concentration and ventilation heat losses Controlled removal of lighting and process waste heat, returned through a heat exchanger Short cross-ventilation intervals through controlled vents instead of permanent mechanical ventilation A well-designed building form can reduce the cooling load to zero. Where technical cooling is nevertheless required, the heating technology should be chosen so that it also serves for cooling in summer. Heating and cooling ceilings are more suitable than wall or floor surfaces because they remain exposed. Thermally activated building systems respond slowly and are unsuitable in residential use.
Energy supply The aim is to draw as little energy as possible from suppliers. It must be examined which local sources can be used — wind, water, sun, geothermal energy, air — and what quantity can be generated, transported and stored on site for flexibility.
Photovoltaic arrays on roofs and façades Small wind turbines (horizontal or vertical axis) Small turbines in flowing water Storage with as many charge cycles, as large a capacity and as good a recyclability as possible — on an industrial scale, vanadium redox flow batteries for instance Heat generation by combustion is defensible only where the fuel releases no more CO₂ than is bound during its regeneration.
Microclimate and rainwater Green roofs and façades, including the areas beneath the photovoltaic array, counteract solar heating of the components Shading must not cost daylight — compensating with artificial light raises energy consumption. In front of large glazed areas on lower floors, trees at sufficient distance are suitable A planting scheme for the outdoor areas counteracts the heating of sealed surfaces; the sealed proportion should be minimised Traffic areas with low thermal conductivity and storage capacity, built to be permeable The building in the climate of 2050 Mean annual precipitation is rising — drainage must be sized for the increase Avoid hollows and high degrees of sealing outdoors; green roofs delay run-off Build access areas to be permeable The number of hot days above 30 °C rises markedly — measures against local heat stress are therefore obligatory at concept stage Basis: internal criteria paper “Climate-Neutral Construction — Climate and Ecology Criteria”, as at 30 August 2026.
Five factors govern the procurement and use of building materials in climate-neutral construction.
Low production energy demand, using CO₂-neutral primary energy Low transport energy demand — the case for local resources Recyclability and compostability CO₂ storage capacity, with a view to a negative balance Renewable raw materials Examples are hemp-lime blocks in a timber or concrete frame for the external wall, or a solid concrete structure with hemp or wood fibre insulation. Flat roof areas can be built as glulam or timber joist decks; above-rafter and between-rafter insulation in wood fibre boards or as blown natural fibre. Renewable raw materials bind CO₂ permanently — set against this is the energy needed for manufacture and transport.
Ecological building materials are not universally compostable, but they are recyclable without environmental pollutants provided no harmful additives were used in production. That deserves attention — as does the separability of the component layers: what cannot be dismantled by material type ends up as waste rather than raw material.
Building physics data Insulation materials from renewable raw materials — building physics data Material λ ρ μ c DIN 4102-1 DIN EN 13501-1 Flax mats 0,039 30–40 1–2 1.550–2.300 B2 E Hemp (loose wool) 0,045 50–60 1–2 2.200 B2–B1 E, C-s2, d0 Hemp jute 0,043 35–40 1–2 2.300 B2 E Hemp mats 0,043 30–110 1–2 1.600–2.300 B2 E Wood fibre (loose) 0,040 30–45 1–2 2.100 B2 E Wood fibre mats 0,038 40–55 1–3 2.100 B2 E Wood fibre boards 0,040 110–270 2–5 2.100 B2 E Wood shavings 0,045 90–360 2 n/a B2 E Wood wool boards 0,090 330–500 2–5 2.100 B1 B, s1, d0 Jute mats 0,039 30–40 1–2 2.350 B2 E Cork board (expanded) 0,040 120 5–10 1.800 B2 E Cork-clay board 0,080 200–300 10 1.254 B2–B1 E Sheep wool 0,036 20–90 1–2 1.300–1.730 B2 E Reed boards 0,065 150 3–6,5 1.200 B2 E Seagrass 0,045 65–75 1–2 2.502 B2 E Straw bales 0,052 85–115 2 2.000 B2 E Blown straw insulation 0,043 105 2,8 2.100 B2 E Cellulose fibre (loose) 0,039 28–65 1–2 2.100–2.544 B2 E to B-s2, d0 Cellulose boards 0,042 70–145 2–3 2.000 B2 E Polystyrene (expanded) — comparison 0,035 11–30 20–100 1.400 B2–B1 E Stone wool boards — comparison 0,035 15–130 1–2 830–1.000 A1 A1
λ as design value. Source: market survey “Dämmstoffe aus nachwachsenden Rohstoffen”, Fachagentur Nachwachsende Rohstoffe e. V. (FNR), order no. 317, 10th edition 2019, p. 13.
Dynamic stiffness Values for impact sound insulation Material s′ in MN/m³ Cellulose 20 Hemp fibre felt 22 Sheep wool 50 Flax mats 57 Cork granulate 120 Wood fibre boards 30–150 Reed 208 Wood wool boards 300 Cork boards 500
Sorted ascending — the lower s′, the better the impact sound reduction. Source: FNR, order no. 317, 10th edition 2019, p. 11.
For sustainability certification it is recommended not to select XPS, PUR or EPS insulation unless it is genuinely required.
Principal Tino Josef Ritter Degree M.Sc. in Engineering (Energy Systems) Practice Planning and consulting practice for building physics and energy efficiency Established 1 January 2016 Network
Thermal performance decides energy demand, comfort and freedom from damage — and it is won or lost in the details.
Verification and detailing Thermal performance certificates for residential and non-residential buildings Component build-ups and U-value calculation Detailed thermal bridge calculation instead of blanket surcharges Assessment of junction details in solid and timber construction Simulation Building simulation (VDI 6007) Thermal simulation (DIN EN ISO 13791) The calculated thermal bridge assessment regularly pays for itself where the blanket surcharge costs an efficiency level — we tell you in advance whether that is the case for your building.
All planning and calculation services required to meet sound insulation requirements — in solid and in timber construction.
Verification to DIN 4109-1:2018 and DIN 4109-5:2020 Residential buildings, non-residential buildings and heritage-listed properties Airborne and impact sound insulation between occupancy units Component and junction details in timber construction Assessment of external noise and façade requirements In multi-storey timber construction in particular, early coordination pays off: the acoustically viable solution often determines the floor build-up.
Moisture is the most common cause of building damage. Whether a component stays dry is decided by the interplay of build-up, use and air change.
Moisture control Verification to DIN EN ISO 13788 Verification to DIN 4108-3:2024 Hygrothermal simulation for critical build-ups Ventilation Ventilation concepts for new build and retrofit Sizing and assessment of ventilation measures After a window replacement in an existing building in particular, the concept is the most effective protection against moisture damage.
We provide all services and calculations for planning and funding under the programmes of KfW and BAFA.
Accreditations: KfW, BAFA, WTA Residential KfW Efficiency House (BzA and BnD) Non-residential KfW Efficiency House (gBzA and gBnD) Individual measures, residential and non-residential, KfW and BAFA Heritage-listed residential (BzA and BnD) Heritage-listed non-residential (gBzA and gBnD) Individual measures on heritage-listed residential and non-residential property, KfW and BAFA BzA — confirmation for the application · BnD — confirmation after completion · gBzA / gBnD — the corresponding confirmations for non-residential buildings. These are the designations used by the German funding programmes and are given here in their original form.
How it runs Initial discussion and classification of the project Calculation and comparison of the attainable efficiency levels Construction supervision during the works Funding rates, programme numbers and application deadlines change regularly. We establish the position applicable to your project in the initial discussion.
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Provider Tino Josef Ritter, M.Sc. in Engineering (Energy Systems) — Planning and consulting practice for building physics and energy efficiency Address Feuerwehrstrasse 44, 77933 Lahr, Germany Mobile +49 (0) 176 8232 9156 Landline +49 (0) 7821 274 501 4 Email ritter@bauphysik.direct Established 1 January 2016 Dispute resolution We are neither willing nor obliged to take part in dispute resolution proceedings before a consumer arbitration board.
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§ 9 Consumers' right of withdrawal Consumers have a statutory right of withdrawal in the case of contracts concluded away from business premises and in distance contracts. The details are set out in the withdrawal instruction provided separately.
To be completed: the full withdrawal instruction together with the model withdrawal form pursuant to Article 246a EGBGB. Without it, the withdrawal period extends to twelve months and fourteen days.
§ 10 Governing law and jurisdiction The law of the Federal Republic of Germany applies, to the exclusion of the UN Convention on Contracts for the International Sale of Goods. Where the Client is a consumer habitually resident in another state, the mandatory consumer protection provisions of that state remain unaffected (Article 6 Rome I Regulation). Where the Client is a merchant, a legal person under public law or a special fund under public law, the exclusive place of jurisdiction for all disputes arising from the contractual relationship is Lahr, Germany. The Contractor remains entitled to sue at the Client's general place of jurisdiction. In relation to consumers the statutory places of jurisdiction apply. For consumers domiciled in the European Union, Articles 17 to 19 of Regulation (EU) No 1215/2012 apply. For contracts with clients domiciled in the Republic of Türkiye: disputes shall be finally settled under the arbitration rules of […]. The seat of arbitration is […]; the language of the proceedings is German or English. § 11 Final provisions Amendments and additions to the contract require text form. This also applies to any waiver of this clause. The Contractor is neither willing nor obliged to take part in dispute resolution proceedings before a consumer arbitration board. Should any provision be or become invalid, the validity of the remaining provisions is unaffected. The statutory provisions take the place of the invalid provision. As at 30 August 2026. Convenience translation — the German version prevails.