Namibia study comparing thermosiphon and PV-to-heat systems sparks interest at ISEC 2026

Photo: Bärbel Epp

ISEC 2026 in Graz, Austria, became a major international meeting point for the energy transition. Around 550 experts from 42 countries gathered to discuss technological developments, global research cooperation, regulatory frameworks and renewable energy projects. The conference was organized for the fourth time by the Austrian research institute AEE INTEC. Two experts from IEA SHC Task 69 on Solar Hot Water for 2030 presented results from the international collaboration: Joseph Shigwedha from the Namibia University of Science and Technology and Prof. Samson Mhlanga from the National University of Science and Technology in Zimbabwe. The photo shows the panel discussion with Mhlanga speaking on the right and Wolfgang Gruber-Glatzl, Head of Industrial Systems at AEE INTEC, moderating on the left. The audience showed strong interest in the first results from a side-by-side comparison of thermosiphon and PV-to-heat systems

Over the past 3.5 years, around 50 experts from around the world have worked within Task 69 to analyse the market status, technical challenges, as well as training and standardization needs for two reliable and cost-effective solar water heating technologies: thermosiphon solar water heaters and PV-to-heat systems.

Mhlanga explained how Southern Africa has benefited from the international cooperation within Task 69: “The Task has not yet directly transformed the Southern African market, but it has created an important foundation with better data, improved skills and stronger standards.” He added that the work helped validate technologies already well suited to local conditions and supported realistic adoption pathways instead of importing unsuitable solutions.

Task 69 has also strengthened the confidence of researchers in Southern Africa. “Task 69 helped us realize that we can contribute to global research not only as system recipients, but also through data analysis and experimental work on the ground,” Mhlanga emphasized during his presentation.

Innovative test configuration for solar water heating systems

One example is a comparison study presented by Shigwedha. The Namibia Energy Institute, supported by Soltrain, installed and monitored three different solar water heating systems. Soltrain is a regional initiative promoting training and demonstration projects for solar and renewable thermal systems in the SADC region.

The test setup included two thermosiphon systems – one with a 2.52 m² flat plate collector and one with a 2.42 m² evacuated tube collector – as well as a PV-to-heat system with 5.2 m² of PV panels. All three systems used 200-litre storage tanks. The generated heat is transferred via heat exchangers into a storage tank located inside the container.

This represents an innovative test configuration because all three systems operate under identical boundary conditions and within one harmonized methodological framework. As a result, the study provides a more robust basis for technology comparison than separate case studies.

Monitoring of three solar water heater systems installed on top of a container (left), combined with a small-scale hot water demand system inside (right).
Photo: Namibia Energy Institute

Comparing thermosiphon and PV-to-heat systems

Since December 2025, heat meters have measured the energy output of all three systems, while temperature sensors collect data at the collector outlet and at the top of the storage tanks. The first four months of monitoring show that the flat plate collector system achieved the highest efficiency. It produced 515 kWh of solar heat with an efficiency of 40 %. By comparison, the PV-to-heat system produced 386 kWh with an efficiency of 15 % (see table below).

Shigwedha explained that flat plate collectors perform especially well during periods of high solar irradiation, but their output drops significantly under cloudy conditions. For example, a daily irradiation drop of 70 % reduced energy production by 80 % in the flat plate collector system, compared to 65 % in the PV-to-heat system.

Online monitoring of the three solar water heating systems with the three heat meters in the middle and the different temperature sensors in the storage tanks and the collector exits.
Graphic: Namibia Energy Institute

Monitored solar yield from December 2025 to March 2026
Source: Namibia Energy Institute

“We found that flat plate collectors achieved the highest energy production, while PV-to-heat systems were less affected by irradiation fluctuations,” Shigwedha summarized. He also noted that the overall system costs are relatively similar. At the same time, PV-to-heat systems offer an advantage because they can be integrated into mini grids, which are becoming increasingly common in rural areas. Excess electricity can then be fed into the local grid.

According to Shigwedha, most consumers in Namibia still prefer flat plate collectors because they perform well under local conditions and are easier to maintain. Evacuated tube collectors, by contrast, are more fragile and require more careful handling. He also pointed out that it is difficult to track the market share of different solar water heating systems in Namibia because all components are imported and their final application is not officially monitored.

The discussions at ISEC 2026 demonstrated that both thermosiphon and PV-to-heat systems can play an important role in the energy transition in Southern Africa. While flat plate collectors currently deliver the highest energy output under Namibian conditions, PV-to-heat systems offer greater flexibility and easier integration into emerging mini grids. The work carried out within Task 69 has not only provided valuable technical data, but has also strengthened regional research capacity and international collaboration in the field of solar water heating.

Websites of organisations mentioned in this news article:

• ISEC 2026: https://isec-conference.at/frontend/index.php
• AEE INTEC: https://www.aee-intec.at/
• National University of Science and Technology in Zimbabwe: https://www.nust.ac.zw/
• Namibia University of Science and Technology: https://www.nust.na/
• IEA SHC Programme: https://www.iea-shc.org/
• IEA SHC Task 69: https://task69.iea-shc.org/
• Soltrain: https://www.soltrain.org/
• Namibia Energy Institute: https://nei.nust.na/