Evaluation of TEMS Designs in Yachts

Abstract

The superyacht sector is under increasing pressure to reduce operational emissions and improve onboard energy efficiency. One relevant opportunity is the better use of thermal energy, since yachts require heating and cooling under varying operating conditions while propulsion and power-generation systems can provide recoverable waste heat. Evaluating the effectiveness of a Thermal Energy Management System (TEMS) during the design stage is therefore challenging, because both thermal demand and heat availability depend on operational profile, climate zone, occupancy level, and system configuration.

This thesis develops a method to evaluate yacht TEMS designs across multiple operational scenarios. The method combines Multi-Criteria Decision Analysis (MCDA), adapted Scenario-Based Reasoning (SBR), and analytical calculations. MCDA provides the overall comparison structure, SBR is used to evaluate performance-based Key Performance Indicators (KPIs) across representative scenarios, and analytical calculations are used for design-dependent KPIs. The selected KPIs are Load Coverage, Coverage Durability, Usage Ratio, Combined Heat, Power, and Cooling (CHPC) Efficiency, Volume Ratio, Mass Ratio, and Cost Ratio.

The method is applied to M/Y Obsidian, an 84.2 m Feadship yacht used as reference case. The scenario matrix combines seven climate zones, three operational profiles, three occupancy levels, and four representative weather-parameter cases, resulting in 252 evaluated scenarios. For each scenario, the weather conditions, thermal demand, useful heat and cooling generation, and performance-based KPIs are calculated. The design-dependent KPIs are then determined from the installed volume, mass, and estimated cost of the TEMS components.

Two configurations are evaluated. Design 1 represents the baseline TEMS, based on engine heat recovery, an electrically driven chiller-heat-pump branch, an electric heater, and thermal storage. Design 2 is introduced as a first improvement iteration, in which the electrically driven chiller is downsized and an absorption chiller is added to use engine high-temperature heat for cooling production during anchor and sailing conditions.

The results show that both designs cover the required thermal demand in all evaluated scenarios. In Design 1, the most critical cases occur in high-occupancy Mediterranean scenarios, where the recovery heat exchanger, electric heater, and heat tank become highly loaded. The analysis also shows that a large share of the available engine high-temperature heat remains unused, indicating that the main limitation is not the availability of recoverable heat, but the capacity and configuration of the recovery path.

Design 2 improves the use of recoverable engine heat in anchor and sailing conditions and leads to a small increase in CHPC Efficiency in warm representative scenarios. However, the improvement is profile-dependent and does not affect harbour operation, where no engine heat is assumed to be available. In addition, the absorption chiller increases the installed volume, mass, and cost of the system.

The final ranking combines CHPC Efficiency, Volume Ratio, Mass Ratio, and Cost Ratio using equal weighting factors and ratio-to-best normalization. Under these assumptions, Design 1 is ranked first in all weather-parameter cases. Design 2 therefore shows a technically relevant improvement, but the efficiency gain is not sufficient to compensate for its larger spatial, mass, and economic impact.