Optimization of the Emissions Profile of a Marine Propulsion System Using a Shaft Generator with a MMPT Based Control System and the Use of EEDI and EEOI

Abstract

Nowadays marine propulsion systems based on thermal machines that operate under the diesel cycle have positioned themselves as one of the main options for this type of applications. The main comparative advantages of diesel engines, compared to other propulsion systems based on thermal machines, are the low specific fuel consumption, residuals and their higher thermal efficiency. However, its main disadvantage lies in the emissions produced by combustion, such as carbon dioxide (CO₂), oxide sulphur (SOx) and oxide nitrogen (NOx). These emissions are directly related to the operating conditions of the propulsion system [1].

Over the last decade, the International Maritime Organization (IMO), has adopted a series of regulations to reduce these emissions [2, 3], based on the introduction of an energy efficiency design index (EEDI) and an energy efficiency operational indicator (EEOI). EEDI is mandatory for any new ship and the EEOI is optional to be applied. In this context, adding a shaft generator [4] allows to reduce the nominal design power of the auxiliary generation system and, under nominal operating conditions, the propulsion plant, favouring lower EEDI and EEOI values, which means lower CO₂, SOx and NOx emissions [5, 6].

However, the use of shaft generators can only be justified if the propulsion system operates, most of the time, under 75%–80% of the maximum continuous rating (MCR) design. In addition, the incorrect operation of the shaft generator can result in overloading the main engine, which means an increase of CO₂, NOx and SOx emissions.

The present work proposes a selective control system with a Maximum Power Point Tracking (MPPT) that allows operating the shaft generator in Power Take Off (PTO) or Power Take In (PTI) mode ensuring that the main engine operates, always, at the optimum point to generate minimum CO₂, SOx and NOx emissions.

Appendices

Annex A – EEDI Ship’s Equation

$$ \frac{{\left( {\prod\nolimits_{j = 1}^{M} {f_{j} } } \right)\left( {\sum\nolimits_{i = 1}^{nME} {p_{ME\left( i \right)} \,*\,} C_{FME\left( i \right)} \,*\,SFC_{ME\left( i \right)} } \right) + \left( {P_{AE} \,*\,C_{FAE} \,*\,SFC_{AE*} } \right) + \left( {\left( {\prod\nolimits_{j = 1}^{M} {f_{i} } *\sum\nolimits_{i = 1}^{nPTI} {P_{PTI\left( i \right)} - \sum\nolimits_{i = 1}^{neff} {f_{eff\left( i \right)} \,*\,P_{AEeff\left( i \right)} } } } \right)C_{FAE} \,*\,SFC_{AE} } \right) - \left( {\sum\nolimits_{i = 1}^{neff} {f_{eff\left( i \right)} *P_{eff\left( i \right)} *C_{FME} *SFC_{ME} } } \right)}}{{f_{i} *f_{c} *Capacity*V_{ref} *f_{w} }} $$

• Main engines emissions:

$$ \left( {\mathop \prod \limits_{j = 1}^{M} f_{j} } \right)\left( {\mathop \sum \limits_{i = 1}^{nME} P_{ME\left( i \right)} *C_{FME\left( i \right)} *SFC_{ME\left( i \right)} } \right) $$

Where:

• f_j Correction factor for ship specific design elements.

• P_ME Power of main engines.

• C_FME Main engine conversion factor between fuel consumption and CO₂ emission.

• SFC_ME Main engine specific fuel consumption.

• Auxiliary engines emissions:

$$ \left( {P_{AE} *C_{FAE} *SFC_{AE} } \right) $$

Where:

• P_AE Power of auxiliary engines.

• C_FAE Auxiliary engine conversion factor between fuel consumption and CO₂ emission.

• SFC_AE Auxiliary engine specific fuel consumption.

• Shaft generators/motors emissions:

$$ \left( {\left( {\mathop \prod \limits_{j = 1}^{M} f_{j} *\mathop \sum \limits_{i = 1}^{nPTI} P_{PTI\left( i \right)} - \mathop \sum \limits_{i = 1}^{neff} f_{eff\left( i \right)} *P_{AEeff\left( i \right)} } \right)C_{FAE} *SFC_{AE} } \right) $$

Where:

• f_j Correction factor for ship specific design elements.

• P_PTI Power of shaft motor divided by the efficiency of shaft generator.

• f_eff Availability factor of innovative energy efficiency technology.

• P_AEeff Auxiliary power reduction due to individual technologies for electrical energy efficiency.

• C_FAE Auxiliary engine conversion factor between fuel consumption and CO₂ emission.

• SFC_AE Auxiliary engine specific fuel consumption.

• Efficiency technologies:

$$ \left( {\mathop \sum \limits_{i = 1}^{neff} f_{eff\left( i \right)} *P_{eff\left( i \right)} *C_{FME} *SFC_{ME} } \right) $$

Where:

• f_eff Availability factor of innovative energy efficiency technology.

• P_eff Output of innovative mechanical energy efficient technology.

• C_FME Main engine conversion factor between fuel consumption and CO₂ emission.

• SFC_ME Main engine specific fuel consumption.

• Transport work:

$$ f_{i} *f_{C} *Capacity*V_{ref} *f_{w} $$

Where:

• f_i Capacity factor.

• f_c Cubic capacity correction factor.

• V_ref Ship speed.

• f_w Weather factor

• Capacity or dwt rating for VLCCs.

Annex B – EEOI Ship’s Equation

$$ EEOI = \frac{{\mathop \sum \nolimits_{j} F_{j} *C_{Fj} }}{{m_{Cargo} *D}} $$

Where:

• j Fuel type.

• FC _j Mass of consumed fuel j.

• C_Fj Fuel mass to CO₂ mass conversion factor for fuel j.

• m_cargo Cargo carried (tonnes)

• D Distance in nautical miles corresponding to the cargo carried.