One Pump, Two Pumps, Three Pumps…

How to Reduce Energy Consumption

Pumps use approximately 40 % of a ship’s electrical output. Let’s see how we can reduce their energy consumption.

In the maritime sector, cooling pumps are oversized primarily due to their design criteria. These systems are required to ensure the cooling of a ship’s mechanical components under the worst conditions. A pump’s capacity is calculated using as reference points the seawater’s temperature at 32 °C and an electrical load equal to 110 % of its maximum value (in accordance with ISO standards).

In practice, ships operate at roughly 80 % power and rarely do any of them navigate in 32-degree waters. At that temperature, heat exchangers (plate coolers or other types) are less efficient because their thermal exchange capacity is determined by the difference in temperature between a warm fluid and a cool fluid. Therefore, a cooler with a 7 000 kW capacity at 32 °C will see its performance increase to over 10 000 kW in colder water (20 °C).

Given the above, how do we go about applying one of the first principles of energy efficiency, namely to generate and use only as much energy as needed?

When designing a ship, one of the first traps to avoid is installing two pumps that are each capable of producing 110 % of the ship’s cooling needs (the purpose of the second pump being to ensure redundancy as required by classification societies). It would be far better to install three pumps with each having a 55 % capacity. This way, redundancy is maintained but we have the added benefit of being able to control the pumps’ flow rate by switching one of them off or by installing variable frequency drives (VFDs).

It is easy to understand that, if coolers perform better in cold water, their cooling water flow rate will be reduced.

For the shipowner, these two factors make adding VFDs an inevitable and cost-effective solution. Given that seawater is colder, the coolers’ capacity is increased and the ship’s power output is less than the amount anticipated in the design criteria. Consequently, we can safely decrease the cooling water flow rate as well as reduce the electrical power consumed by the pumps, and thereby achieve energy savings.

To illustrate how much energy can be saved, let’s look at an example of an actual case involving the assessment of a ship:

During its design phase, it was determined that the required cooling water flow rate was 500 m3/h at 3 bars of pressure.

At this point, the engineers were faced with two choices:

A large pump delivering 500 m3/h at 3 bars of pressure;

Or

Three small pumps each having a capacity of 250 m3/h at 3 bars.

At first glance, using one large pump instead of two small ones, as is customary, would have resulted in reduced energy consumption since large pumps are usually more efficient. However, its modulation capacity would have been limited. Even with the installation of a VFD, given the targeted flow rate of 200 m3/h, the pressure would have dropped too significantly and would not have been able to counteract the system’s load losses. Generally, in a closed circuit, a pump’s flow rate cannot be reduced by more than 50 %.

The decision was made to install three pumps, which is a good choice.

However, an energy audit performed while the ship was in operation revealed that it was able to meet its cooling needs with only 200 m3/h at approximately 1 bar of pressure (classic case).

It was shown that just one of the three pumps was sufficient to satisfy the ship’s cooling requirements. As a result, the shipowner estimated that it was not necessary to install VFDs on the system. Instead, it was deemed that ship’s operators will be able switch off one of the pumps as they see fit.

Let’s examine the table below to determine the cost-effectiveness of various scenarios:

The above calculations show that installing VFDs on two of the three pumps would generate savings of nearly $36,000 over and above what could be achieved with the occasional shutting of either of the two pumps. It would also reduce annual GHG emissions by over 230 tons per year. As an added bonus, the pumps would wear down less quickly due to their substantially lower rotation speeds. The actual savings in the latter scenario are difficult to quantify and have therefore not been provided in the above table.

Conclusion:

From a technical point of view, it often occurs that decision-makers must decide between “impressions” formulated by members of their technical department and/or their chief engineers, and actual “facts that are dictated by the laws of physics” and are therefore unchangeable. In addition, hydrodynamics and more specifically the law of affinity are now firmly established without the need for further debate. Adding variable frequency drives makes it possible to automatically adjust pump speeds as needed and, in doing so, they increase savings during manoeuvers as well as at port while at the same time lowering the risk of human error.

If you are going to invest in energy analyses of your ships, the best way to profit from your outlay is to implement the recommendations submitted to you. They were issued by certified professionals. There will always be resistance to change as well as sorcerers’ apprentices who will speak out against recommendations. Unfortunately, it is one of the peculiar features of man. That being said, one cannot cast doubt on the laws of physics, the cost of energy or the resulting GHG emissions.