Land scarcity is increasing and therefore one of the main challenges for renewable energy in the future is ‘physical space’. While renewables need a lot of space compared to conventional energy sources, and the competition for space is already increasing (food production, housing etc.), leading to higher land costs and opposition. Public resistance to new highly visible windfarms and onshore solar farms is hardening. Large land-based solar farms compete with agricultural use and can have negative effects on the ecosystem, by covering the soil from the solar light. A solution to the stated problems is by placing renewable energy production at sea, among which offshore floating solar energy.
One of the main challenges in offshore floating solar is the continuous wave-induced motion of floaters, and the forces that arise at the most critical points. Dynamically modelling multiple floaters in waves becomes very computationally and time intensive when the number of floaters increases. This research focused on creating and comparing a linear and a non-linear model that calculates motions of multiple floaters under first order wave forces. The aim is to perform these calculations with open-source software only.
The dynamic forces on the floaters are obtained by evaluating the radiation, diffraction and wave excitation potential in the open-source Boundary-Element-Method-Solver NEMOH and showed good resemblance with commercial software packages AQWA and DIFFRAC. The modelling of the motions is performed in time (linear and non-linear RK4-integration-scheme) and frequency domain (linear). The implementation of the dynamic forces in both cases showed to be the most critical for the behavior of the floaters. Especially the determination of the hydrodynamic coefficients: Infinite added mass and retardation function for the time domain simulation are key in getting the same result for the linear time domain and frequency domain. Suppressing non-realistic values of gap resonance between multiple floating bodies increases the efficiency of time-domain simulations. Small differences between the motions of the floaters can cause significant differences in the forcing, therefor it is rather advisable to perform the comparison between the linear and non-linear case in time domain.
A comparison of different wave directions and conditions showed that the results of the linear model are of the same order of magnitude as the non-linear model. For the determination of motions and forcing under first order wave forces this shows that both cases are similar. Nevertheless, other non-linear effects can be added in a follow-up study.

Due to climate change and growing cities, water scarcity is becoming one of the futures biggest problems. On top of that, the population and prosperity of cities around the equator are growing fast. Meaning that the need for electricity, cooling and drinking water will grow fast in the following decades. ROTEC’s vision is that these growing problems require a sustainable approach for the future.
A solution to these challenges can be found in the oceans temperature difference. The top layer of the ocean is heated by the sun, while the deeper layer remains cold. This causes around the equator a temperature difference of more than 20 degrees over the ocean’s depth. This offers a lot of opportunities. It can be used as a vast source for electricity production (OTEC), large scale drinking water production (ROTEC) and for cooling (SWAC). Indonesia is one of the best locations worldwide, due to the easy access of cold deep sea water and the abundant presence of hot surface water. North-Sulawesi has a unique access to these sources. Due to the steep slope of the seabed the cold deep seawater can easily be reached.
Team ROTEC conducted a research in Manado for two months and came up with several solutions that can contribute to a more sustainable and beneficial future of North Sulawesi. There was mainly focussed on performing a need assessment for the capital Manado and the touristic Bunaken Island. This pointed out that Manado can reduce their electricity usage during peak loads by implementing a new way of cooling of malls and hotels along the boulevard. Bunaken needs electricity and drinking water in a way that is more easy to maintain and operate. Data analysis and measurements showed that both Bunaken and Manado have a high theoretical potential, since cold deep seawater is close to shore and found at relative shallow depths.
For Manado a new seawater district cooling system is proposed. This system uses cold deep seawater to cool the large buildings along the boulevard, instead of conventional chiller-cooling-tower units. The solution reduces their electricity usage for cooling by 96% and more electricity is left for the grid of Manado. The yearly costs for the operation of the cooling is 92% cheaper and the investment for the installation is earned back within 6 years after construction. Peak loads in the grid are decreased and emissions reduced; equivalent to 19,000 tons CO2 per year.
For Bunaken an integrated drinking water and electricity solution is found. By just using the temperature difference in the ocean, to produce clean and constant electricity and drinking water from seawater. The proposed installation provides the base load (80kW) for Bunaken for the same price as current solar PV and diesel generators together. Clean drinking water for the villagers is 12 times cheaper than Aqua Danone and 1.4 times cheaper than the not drinkable water from fresh water wells on the island. Such a kind of installation can produce 24/7, is stable and that without the need of fuels.