Choosing the right heat exchanger for electric or hybrid marine cooling

In just a few years, electric and hybrid propulsion has gone from a niche market enterprise to a serious mainstream solution for marine propulsion systems.

Originally seen as an option for small leisure craft, its continued development is now providing systems that deliver over 100 kW of clean, emission-free power!

As propulsion systems develop, one constant issue is the need to keep the heat generated within operating parameters, to ensure its performance and eliminate system failure due to heat overload.

Areas requiring cooling can include the battery pack and on-board charger, AC-DC convertor, DC-DC convertor, the electric drive motor, transformers, invertors plus on-shore battery charging stations.

Although all systems require different levels of heat dissipation, there are general guidelines that can be applied to establish the typical size of the heat exchanger required.

For example, an electric motor will commonly lose 6% approximately of the rated power in the form of heat. Therefore, a 100 kW motor may need a unit capable of removing 6 kW of thermal energy. The associated components will typically require a further 3% of the motor’s power to be dissipated, whilst additional cooling may be required for the batteries.

Coolant flow rates also affect the performance of a heat exchanger and usually vary from one electrical system to another. So, the 100 kW system example above would require at least 9 kW of heat rejection. The performance graphs below illustrate that if the coolant flow through the system is 40 l/min, a Bowman EC100 would be suitable for the job. However, if the coolant flow rate was only 15 l/min, a larger FC100 heat exchanger would be required, as a low coolant flow rate significantly reduces the cooling capability of any heat exchanger!

EC Series Heat Exchangers

FC Series Heat Exchangers

Figures based upon: Coolant type: 50/50 water/glycol. Sea water inlet temperature: 30 °C at 50 l/min. Coolant inlet temperature: 50 °C.

Bowman heat exchangers are easily integrated into the design of the propulsion system and a single unit is usually all that is required. However, in some installations, a secondary circuit may be required where components (such as the battery) have specific cooling requirements. The illustrations below show three typical scenarios for cooling electric and hybrid systems.

Two heat exchangers, installed in series, control the differing cooling requirements for the battery bank and invertor/drive circuits.

A single heat exchanger is used to cool the dual invertor/drive circuits in this installation.

A single heat exchanger cools both the hybrid motors and invertors, plus two separate coolers installed within this hybrid system.

Electric and hybrid systems are often designed to operate with sea water temperatures of 30 °C plus, so selecting the right unit is critical to performance and reliability. However, by supplying the following information, Bowman can provide a computer-aided product selection, to recommend the most appropriate heat exchanger for an application:

Coolant type and concentration

Heat to be dissipated (in kW)

Required coolant outlet temperature (in °C)

Coolant flow (in l/min)

Sea water temperature (in °C)

For more information, contact the sales team on +44 (0) 121 359 5401 or email [email protected].