Many vessels have been dismantled based on energy efficiency considerations, becoming an environmental problem and contributing to the non-sustainability of materials. Revamping of existing ships in view of improving their operational costs may be seen as a sustainable approach.

The article focuses on the use of a specific procedure to determine and evaluate the ship energy improving actions and their technical and economic evaluation, therefore allowing the support to the revamping or scrapping decision. This article uses a number of existing ships as examples.

Energy efficiency is intimately connected with fuel costs and these with the ship competitiveness in a market as well as with its environment. In 2007 shipping was responsible for approximately 3.3 per cent of global carbon dioxide emissions (1 billion tonnes). If shipping were a nation that amount would make it the sixth largest emitter of carbon dioxide in the world, surpassed only by China, the United States, Russia, India and Japan.

International shipping, excluding domestic shipping and fishing vessels, emitted 2.7 per cent of global emissions (870 million tonnes) that same year. The majority comes from cargo vessels, which account for 89 per cent of total gross tonnage of the global fleet.

Ship emissions are not only limited to carbon dioxide and other pollutants including SOx, NOx, PM, VOCs and CO are also emitted and responsible for a costly impact on the societies such as cardiorespiratory and oncologic diseases.

On average global shipping has grown by 3 per cent annually over the last three decades and emissions are projected to grow by more than 20 per cent by 2020 and 50 per cent by 2030, above 2007 levels. In the absence of emission reduction policies, emission scenarios predict a doubling to tripling of 2007 emission levels by 2050.

However, a sizable portion of the current fleet is relatively young and will potentially be in service for many years to come. Approximately half of the world fleet is 20 years old or younger. However, by gross tonnage about half the fleet is 10 years old or younger.

Figure 1 – Energy segregation of a small cargo vessel head sea, Beaufort 6.

Since the average life of a ship is 30 to 40 years, these ships will foreseeably be on the water for decades to come. Consequently, reductions in emissions that result from phasing in of more efficient vessels will be very slow. Having said that what can the industry do to overcome this problem?

The answer is not straightforward as there is a number of limiting factors that may be contrary to directions not allowing for an easy solution.

Despite the nowadays technical advances on ships systems, most of the ship-owners are still ordering their vessels with a minimum of energy saving systems, they are usually looking at the cheaper and not necessarily the most efficient. In fact, the cost of not investing in ship energy systems can cost many times their investment through the ship life.

On the other hand, market freight rates are not helping shipping to order energy efficient ships, by the contrary, they push for cheap vessels and while constraining the left money to a minimum, so that no money for investment is available.

Finally, despite all the technological advances in the main and auxiliary machinery, the thermal efficiency of a slow speed two-stroke engine is just about 50%, which means 50% of the fuel is wasted. The overall thermal efficiency of a typical small feeder vessel of 6000 TDW is only about 33% efficient, which means that 67% of the energy is wasted.

Summarising it can be said, that the solution does not fall over the ship-owners, but also over the market player’s freighters and authorities that can implement a CO2 trading scheme, similar to the one existing for the shore industry.

During the recent years, ships have been scrapped due to their alleged impossibility of not being sufficiently efficient in terms of energy, and have been replaced by vessels that unfortunately have only slightly higher energy efficiency. In fact, since about one and a half century ship energy efficiency have been just around 25 to 35%, this means that in the best, 65% of the energy is wasted in its heat form. Only a fraction of the fuel energy used by the ship’s main engines actually ends up generating propulsion thrust.

This is illustrated in Figure 1, which represents a small well-maintained cargo ship moving at about at 15 knots in Beaufort 6 head weather condition. The bottom bar in this diagram represents the energy input to the main engine from the fuel to which one needs to have the energy used on auxiliary boilers and generators. In this case, 43% of the fuel energy is converted into shaft power while the remaining energy is lost in the exhaust or cooling systems and radiation.

Due to further losses in the propeller and transmission system, only 28% of the energy from the fuel that is fed to the main engine generates propulsion thrust in this example. The rest of the energy ends up as heat, as exhaust, and as transmission and propeller losses. The majority of this remaining 28% is spent overcoming hull friction, while the remaining energy is spent in overcoming weather resistance and air resistance, as residual losses and for generating waves. Additional to this is the fuel energy for operation of auxiliary engines.