Wattlab, located in Rotterdam, builds solar hatches for inland vessels in collaboration with the Belgian ship hatch manufacturer Blommaert. Wattlab covers both new and existing hatches with extremely thin solar panels for this purpose.

Hans Heynen
Hans Heynen.

This article appeared in Dutch in SWZ|Maritime’s April 2023 inland navigation special and was written by Hans Heynen (pictured on the right), maritime journalist, hans.heynen@casema.nl.

On sunny days, the solar hatches produce more than enough power to replace the generator. In combination with a large set of traction batteries or a set of lithium-ion batteries, it is possible to store enough power to get through the night without a generator or shore power.

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TU Delft

Wattlab was founded by former TU Delft students Bo Salet, Siebe Roefs and David Kester. During their studies, they were active in TU Delft’s Nuon Solar Team, which became world champion several times with the solar car Nuna.

‘We were already using these wafer-thin solar panels there and we wondered whether there were more applications,’ says director Bo Salet. ‘That brought us to dry cargo shipping. There is a lot of space for solar panels on hatches, but they must be very thin to be able to stack the hatches without damaging the solar panels. They must also be light enough for these hatches. Traditional solar panels are too thick and heavy.’

Durability tests

Wattlab extensively researched whether the wafer-thin panels were strong and durable enough for use in shipping.

‘A number of circumstances are different on a ship than on land,’ states Salet. ‘For example, we simulated the influence of low-frequency vibrations occurring on board over a period of fifty years. That resulted in no extra relegation. We also tested the effect of extended cycles with significant deflections, the effect of overtopping waves and the effect of dangerous dry cargo on the solar cells and the other components, including hydrochloric acid solutions used in marine cleaning. That was no problem at all. When someone starts working with a grinder, you must cover the solar panels. But then you would also cover the normal hatches.’

To avoid stepping on the solar panels, walkways have been kept free on the solar hatches in the longitudinal direction of the ship between the solar panels.

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Three-phase electrical power

The direct current generated by the solar panels, of around 40 Volt per panel, is converted per solar panel directly into 240 V alternating current, via micro-inverters concealed under the hatch, and then per hatch into three-phase 380 V alternating current. It flows via a 7-wire plug connection per hatch and a central cable running along the coaming to the engine room, where it supplies the on-board power supply via a smart box and, when supply exceeds demand, charges the batteries.

Salet: ‘Using microinverters per panel provides a major safety benefit and makes the system more efficient. When one panel does little or nothing due to covering or a shadow, the rest simply continues to produce.’

The crew can see the yield of the solar cells on a smartphone or laptop via an App that communicates with the smartbox.

So far, ten inland vessels with lengths of 80 to 135 metres have been equipped with two or more solar hatches. ‘At least ten more will be added in the coming months,’ says Salet.

All the ships that have been sailing with solar hatches for more than six months have recently ordered one or more additional solar hatches. ‘The electricity cables are already there and often the electrician had already put some extra plugs in the coaming during construction,’ according to Salet. ‘The plugs are always on the side where you are standing when you move the hatches with the hatch crane. Disconnecting or reconnecting the plugs then proceeds in one effort.’

Practical experience

The inland ship Mededinger (80 x 8.21 metres, 1205 tonnes), owned by the Van Wijngaarden family, was one of the first to buy a set of solar hatches last year.

‘Our ship was extended from 73 to 80 metres a year and a half ago,’ says skipper Wim Wijngaarden. ‘So, a new hatch had to be added. We eventually requested two quotes from Blommaert, one with and one without solar panels, to compare the costs. We decided to order two narrow solar hatches instead of one wide hatch, to make moving easier. Each with nine solar panels. They were installed in March 2022 and performed well that summer. Until the days became shorter, then the yield was no longer sufficient. At the end of last year, we therefore decided to equip an existing wide hatch with another eighteen panels. That led to some discussion, but we thought it would be a shame to throw away a good hatch. With 36 panels, our system now has a peak power of 13,000 watts. This allows us to run a pump, the air conditioning, to cook and do the laundry at the same time.’

To take full advantage of the solar power, even in the dark, the Van Wijngaarden family installed three heavy inverters with battery chargers from Victron, each with a capacity of 8 kW and the heaviest available set of traction batteries (1550 Ah at 24 Volt.). The big advantage of all this, according to Van Wijngaarden, is that he is no longer dependent on shore power or a generator when the ship is moored.

‘On sunny days we have a luxury problem now. The batteries are quickly charged when you don’t have large electricity users running and then you see the solar cells scaling down on the App. We recently installed a switch in the wheelhouse that allows us to turn the water heater on and off. When a lot of power is produced, we switch on the boiler. For example, we last switched it on last Friday and on Monday the water was still warm, thanks to the good insulation of the boiler.’

Since the solar cells are on the ship’s hatches, they hardly bother Van Wijngaarden. ‘We often make long journeys, so the hatches don’t have to be opened and closed all the time. With grit and gravel, I make sure that the crane does not swing over the solar hatches during loading or unloading. Then I ask them to wait until I have moved them. Dusty cargo does not harm the panels. You can just spray it off after loading or unloading.’

Dusty cargo does not damage the solar panels and can be rinsed with water.
Dusty cargo does not damage the solar panels and can be rinsed with water.

When Van Wijngaarden is home over the weekend, he monitors the charge level with the App. ‘When I see that we are not going to make it with the sun alone, I remotely switch on the generator for a while, as there is always something using electricity on a ship.’

Generator-free holiday

With the first eighteen solar panels installed in March 2022, the number of generator hours fell over the whole of 2022, compared to 2021, from 2500 to 1500 hours.

‘In the summer of last year, we went on holiday for three weeks without having to start the generator once,’ says Van Wijngaarden. ‘The sun has to shine regularly to achieve this, of course, because the three heavy inverters have a fairly high stand-by consumption. I turned them off once, with shore power on, and that saved 15 amps, 5 amps per battery charger/inverter. It is being investigated whether this can be done more economically.’

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Summer heat

On land, when solar panels become very hot on a windless day in the bright sun, they will produce less electricity.

‘On a ship you then have the advantage of passive cooling through the sailing wind,’ says Salet. ‘The solar panels do not get as hot as bare aluminium hatches, because part of the solar energy is dissipated in the form of electricity. The solar panels are stuck to the hatches with industrial double-sided tape, leaving about 1.5 mm of space between the hatch and the solar panel. So, they are not directly on it. But on a hot day, it’s a good idea to spray some water over the panels and hatches now and then. Then the yield will immediately increase.’


As with other solar panels, the yield decreases over the years. ‘The degradation of Wattlab’s solar panels is no different from the standard degradation,’ says Salet. With an average measured degradation of 0.5 per cent per year, this means that after 25 years, the yield of the solar hatches is still about eighty per cent of the original yield.

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When sailing with ADN (dangerous) cargo, the panels must be temporarily de-energised and therefore do not supply any energy for a while. ‘The solar shutters power group marked red by the electrician will then be switched off,’ says Salet. ‘When sailing with closed hatches and loose ADN cargo, the solar panels must be covered with an opaque tarpaulin. When sailing with open hatches and containers with ADN cargo, the solar hatches must be placed as far as possible (> 2 metres) from the ADN containers.’

Subsidy options

There are now also requests from France and Germany for Wattlab solar hatches. ‘Those countries offer attractive subsidies for making inland vessels more sustainable,’ says Salet. ‘In France you get thirty per cent back from the national government and up to twenty per cent can be added per region. An interest-free loan can be taken out for the remaining amount. In Germany, you can get a sixty per cent and sometimes even eighty per cent subsidy for sustainability with solar panels. We are now working on agreements with a German shipping company to fit all the hatches of a diesel-electric inland vessel with solar cells.’

In the Netherlands, only Energy Investment Allowance (EIA) and a Small-Scale Investment Allowance (KIA) are possible for the solar panels. ‘We are currently discussing better subsidy options with the Minister of Climate and Energy, Rob Jetten, and the Minister of Economic Affairs and Climate, Micky Adriaansens,’ states Salet. ‘Solar panels reduce the emissions of CO2, NOx and particulate matter. Based on data from a skipper who has an electric drive, it appears that on an annual basis, about ten per cent of the total fuel consumption can be reduced by solar hatches.’

Panels for tankers

Wattlab has taken the first steps towards developing solar panels for inland waterway tankers, which must meet the strict ATEX (explosion safety) requirements. ‘We are doing this together with the Rotterdam VT-Group,’ explains Salet. ‘Then you should think of solar panels on canopies above the trunk deck. We are also looking at inland shipping in North and South America. There, gigantic push convoys with hatches on all barges are used. When you fill those hatches up with solar panels, the electricity yield can make a substantial contribution to the propulsion.’

Picture (top): The inland ship Mededinger (80 x 8.21 meters, 1205 tons) of the Van Wijngaarden family with two smooth solar hatches with nine solar panels each. An existing hatch was recently transformed into a wider third solar hatch with eighteen panels. On the Mededinger, the solar panels lay three-wide on the hatches, with 30 cm walkways in between (photos Wattlab).