When we set out to build a water heater that could accept heat from any source — solar collectors, a heat pump, or both at once — the hard part was never the tank itself, but what happens inside it. In August 2024, Solimpeks completed a TÜBİTAK 1707 R&D Order project (no. 3225050), carried out in close collaboration with Innorma R&D, Solimpeks' R&D start-up, to design and manufacture plug-and-play boilers that integrate multiple thermal energy sources for higher efficiency and simpler installation.
What did the TÜBİTAK 1707 project deliver?
The project produced prototypes in four capacities, each engineered for a distinct role:
| Capacity | Key design feature | Primary role | | --- | --- | --- | | 300 L | Compact, user-friendly format | Household applications, favoured in the European market | | 500 L | Perforated diffuser guiding cold inflows downward | Buffer tank duty, stabilising temperature fluctuations | | 800 L | Nested coils — DN40 outer for heat input, DN16 inner for water flow | Higher heat-transfer efficiency without a larger footprint | | 1000 L | Spiral lower coil plus a chimney-like baffle | Larger-scale demand with faster hot-water delivery |
All four boilers draw on diverse heat inputs — solar energy through one coil, a heat pump via another — while delivering consistent hot water. The plug-and-play format keeps setup requirements minimal, making the units ideal for retrofits in homes and small facilities. It also serves broader objectives, including the United Nations Sustainable Development Goals: energy savings, reduced emissions, and export potential across Europe, the Middle East, and Africa.
How did we engineer and test the prototypes?
The work ran over 24 months in structured work packages. We began with the 300 L and 1000 L units, preparing CAD drawings to TS EN 12897 and ASHRAE 118.2. Two innovations stood out: the spiral lower coil, which minimises mixing of thermal layers, and the chimney-like baffle, which accelerates natural convection of heated water and shortens the time to usable hot water. Coils were fabricated from flexible 316L stainless steel and passed welding checks, immersion leak tests, and hydrostatic pressure tests at 8 bar. Expansion tanks, circulation pumps, air vents, and a Resol DeltaSol controller were integrated for intelligent operation.
Optimisation combined TRNSYS modelling with NSGA-II algorithms to refine coil lengths, raising outlet temperatures by 2–3°C and cutting internal temperature gradients by more than 11°C. By mid-2023, prototyping extended to the 500 L and 800 L models; CFD simulations evaluated charging to 50–60°C and discharges at 16–24 L/min, and validation against preliminary tests showed strong correlations, with slight variances in actual fluid dynamics.
In the final phase, concluding by mid-2024, units were tested under standardised conditions — ambient temperatures of 18–20°C, inlet water at 13–15°C — charging stored water to 50°C or 60°C before discharging at varying flows, with and without a mixing valve holding a 40°C output. Temperature data recorded by programmable logic controllers closely matched the CFD predictions.
What challenges did we face, and what did the project achieve?
Refined coil geometries forced revisions to support structures and welding processes for the flexible tubing; early batches showed minor leaks in immersion tests until crimp parameters were adjusted. At set-points above 55°C the test rig's heat pump reached its operating limit, so the electric backup element took over (the fallback path these boilers are designed to provide), which added test cycles to the timeline. Advanced insulation options were outside the project's scope; they would have added only marginal efficiency gains.
The results justified the effort. The 800 L nested coils achieved 20–30% superior energy transfer, yielding 300–500 litres of 40°C water per cycle. The 500 L diffuser extended discharge periods, strengthening its buffering role. Overall yields reached 400–700 litres at elevated flows with the mixing valve, often surpassing simulation expectations in sustained performance. A pivotal moment came during deliberations on coil pitches, which culminated in an open-crimp hybrid that balanced surface area and longevity after iterative prototypes.
Who are these water heaters for?
Anywhere with steady thermal demand and no appetite for complex installs: homes, hospitals, hotels, dormitories, and factories. Contractors and energy companies are key users too, drawn by the rising call for systems that outperform older equipment while being kinder to users and the planet. With capacities from 300 to 1000 litres, the range spans small-apartment retrofits to larger commercial loads.
What comes next?
This project is one of 8 TÜBİTAK-supported projects Solimpeks has carried out since 2009, and it reinforces our position in innovative thermal energy storage. Building on earlier work — EU Horizon 2020 building-retrofit initiatives and TÜBİTAK-funded developments such as hybrid PVT systems — we are exploring further integrations, such as automated source switching for optimised operation. We welcome inquiries about partnerships or a detailed discussion of the technology.
