Energy performance and optimization of a transcritical R744 heat pump for electric ferries
This study demonstrates that an optimized transcritical R744 heat pump system with a parallel gas-cooler layout and decentralized PI control offers a reliable, environmentally friendly, and energy-efficient thermal management solution for zero-emission electric ferries, outperforming standard R410A systems across a wide range of operating temperatures.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The journey toward zero-emission travel is reshaping how we move people across water. Electric ferries, powered by batteries instead of diesel engines, offer a cleaner, quieter future for coastal communities. Yet, these vessels face a unique challenge that their diesel predecessors never had: they generate almost no waste heat. Traditional ships burn fuel to move, and that burning process creates a massive amount of excess warmth that naturally heats the passenger cabins. Electric ferries, however, rely entirely on their batteries for both propulsion and comfort. In cold climates, keeping a ferry warm can drain a significant portion of the battery's energy, sometimes consuming nearly half of the total power available. This creates a difficult trade-off where the need for heat shortens the distance the ship can travel. To solve this, engineers are turning to heat pumps, devices that move heat rather than create it, but they must find a way to make these systems work efficiently in the harsh, variable conditions of the sea without adding too much weight or complexity.
Researchers at the University of Southern Denmark and its partners set out to find the best way to manage this thermal balance for electric ferries. They focused on a specific type of heat pump that uses carbon dioxide as its refrigerant, a natural substance that is safe for the environment and non-flammable. Unlike older systems that use synthetic chemicals, carbon dioxide operates differently when the outside air is warm, requiring a specialized design to function correctly. The team built a detailed computer simulation of a ferry's heating and cooling system to test how well a carbon dioxide setup would perform compared to the standard systems currently used on ships. Their goal was to see if this natural refrigerant could keep passengers comfortable while preserving the battery's energy for the journey, all without making the ship too heavy to be practical.
The study began by comparing a standard system using a common synthetic refrigerant against a new design using carbon dioxide with a flooded evaporator, a component that allows the refrigerant to fully cover the heat exchange surface for better efficiency. The simulations revealed that the performance of the carbon dioxide system depends heavily on the outside temperature. When the air was very cold, dropping to minus five degrees Celsius, the standard system was slightly more efficient. However, as the temperature rose, the carbon dioxide system began to pull ahead. At four degrees Celsius, it was nearly a quarter more efficient than the standard system, and even in hot summer weather at thirty-five degrees, it maintained a significant advantage. This suggests that for most of the year, the natural refrigerant is the superior choice, provided the system is designed to handle the specific demands of a ferry's heating loop.
To improve the system's performance during the coldest winter days, the researchers tested several different configurations. They explored adding extra components, such as additional heat exchangers, to help the carbon dioxide release its heat more effectively. One specific design, which added a second air-cooled unit working in parallel with the main system, proved to be the most effective. This setup allowed the system to beat the standard baseline even at minus five degrees Celsius, delivering a heating efficiency that was nearly six percent better. However, this improvement came with a cost: the added hardware increased the total weight of the system by twenty-five percent. The team had to weigh this extra mass against the energy savings, concluding that the trade-off was acceptable for electric ferries where maximizing battery range is critical.
Managing these systems requires a sophisticated control strategy because the behavior of carbon dioxide changes drastically as temperatures shift. The researchers developed a control system that uses four separate loops to monitor and adjust the compressor speed, valve openings, and fan speeds in real time. This decentralized approach allowed the system to react quickly to changes in the weather without needing complex, heavy computing power. In their simulations, the system responded smoothly to sudden temperature shifts, maintaining stable operation without dangerous spikes in pressure or inefficient cycling. The control logic successfully kept the system running along its most efficient path, proving that a relatively simple control method could handle the complex physics of a carbon dioxide heat pump.
The final analysis brought together the energy gains and the physical weight of the different designs. While the most efficient layout added significant weight, the researchers determined that the energy saved over the life of the ferry would likely offset the initial penalty. The study confirmed that a well-designed carbon dioxide heat pump is a viable and environmentally friendly solution for electric ferries, capable of handling both winter heating and summer cooling. By using a natural refrigerant and optimizing the system layout, these vessels can maintain passenger comfort without sacrificing their electric range. The work highlights that while no single solution is perfect for every temperature, a flexible, optimized system can provide a reliable path forward for the future of clean maritime transport.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.