Floating power platforms for offshore cold-ironing

Floating Power Platforms for Offshore Cold-ironing

Dr. Shantha Jayasinghe¹, Mr. Gamini Lokuketagoda¹, Dr. Hossein Enshaei¹, Professor Dev

Ranmuthugala¹, Dr. Daniel Liang²

1. Australian Maritime College, University of Tasmania, Launceston, Tasmania, 7250 Australia

2. DNV GL - Energy, Singapore

Email: shanthaj@utas.edu.au

Abstract The colloquial term ‘cold-ironing’ refers to connecting a ship to shore power when it is at

berth, as its main and auxiliary engines, made of steel/iron, are shut down, literally becoming cold. In

the current environment where strict emission regulations govern virtually every ship operation, shore

power has become an essential service sought by ships at berth in emission controlled ports. At present,

these ships in port can receive shore power only when they are at berth, usually during the transfer of

cargo or passengers. However, there are many more ships anchored in and around the ports awaiting

access to berths. Efficient supply of shore power to these anchored ships is a challenge and thus they

rely on their on-board power generation systems to supply essential loads. As the waiting time can vary

from a few hours to weeks, emissions from these ships are significant, especially as they are clustered

together in close proximity to land. Therefore, if an efficient and effective way of powering anchored

ships with clean or low emission power is available, it can significantly contribute towards reducing

emission around busy ports as well as the operating cost of anchored ships.

In an attempt to address this problem, authors propose a floating power platform for ‘offshore cold-

ironing’. The proposed system consists of a fuel cell, a battery bank, and a small LNG engine driven

generator set installed on a floating platform such as a barge and moored in close proximity to the

anchored ships to supply them with their required electrical power. Nowadays, with the advancements

of the fuel cell technology, installation of a suitable fuel cell stack on a floating platform such as a barge

has become feasible. Nevertheless, fuel cells response slowly and thus fast dynamic loads such as

dynamic positioning load in adverse sea conditions can easily push the fuel cell beyond its safe operating

range, possibly creating power system instabilities that can result in blackouts. In addition, the floating

platform itself needs dynamic positioning or some form of control to maintain its position relative to

supplied ships, further influencing the dynamic load.

The battery bank can support the fuel cell to cope with such loads. However, in extreme situations, even

the battery bank will discharge rapidly and the fuel cell may struggle to charge it. Therefore, when the

state of charge of the battery bank drops below a defined lower threshold, the LNG engine-driven

generator set starts and provides boost power to charge the batteries. As the generator is used only when

the battery charge is low, and the emission from the LNG is relatively low in comparison to other fossil

fuels used on-board, the proposed system can be considered as a low emission technology solution. The

feasibility of the proposed concept of floating power platform, from the power system control

perspective, is investigated in this paper through modelling and simulation, with the results clearly

showing the efficacy of the proposed hybrid power system to supply the dynamic loads encountered on

anchored ships.

Keywords: Battery, cold ironing, fuel cell, LNG engine, power management, shore power.

1. Introduction

Shipping is considered essential for the growth of the global economy as it accounts for more than 90%

of the goods transported locally and internationally [1]. Therefore, current economic and population

growth across the world requires a complementary growth in the shipping industry, with greater demand

for the number and size of ships plying the major economic routes. Unfortunately, this results in a greater

use of fossil fuels, in turn increasing the global share of greenhouse gas (GHG) emissions from ships

and thus contributing significantly to climate change and other environmental issues [2, 3]. In an attempt

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to reduce these emissions, many countries and regions have imposed strict emission regulations and

defined emission control areas (ECAs) around their coasts [4, 5]. Ships that visit ports in these areas or

sail through these regions are required to take measures to comply with emission regulations [6]. Cold-

ironing is one such measure where ships at berth shut down their relatively high emission engines and

receive power from shore connections [7, 8]. Even though the generation of shore power may use similar

fossil fuel, (e.g. diesel fuel), it is possible to do so within a more controlled environment and thus can

yield to a net reduction in emissions [9]. Thus, the use of shore power is increasing in popularity in

comparison to running on-board engines.

At present, ships can receive shore power only when they are at berth for cargo operations or passenger

transfer. Ships at anchor, waiting for their turn, are unable to access shore power. Currently, there is no

efficient and effective way to supply them with shore power and thus on-board main and/or auxiliary

engines have to supply essential loads. As the waiting time can vary from a few hours to weeks,

emissions from these ships could be significant. Therefore, an innovative solution is required for the

efficient and effective supply of clean or low emission power for anchored ships, especially around busy

ports such as in Singapore.

As proposed in [10] an offshore floating renewable power station is a promising solution for supplying

anchored ships with clean power. The ‘offshore cold-ironing’ platforms presented in this paper is a

continuation of this idea. An overview of the proposed system consisting of a fuel cell stack, a battery

bank, and a small LNG engine-driven generator is shown in Figure 1. A schematic diagram of the

corresponding power conversion system is given in Figure 2. The coupled LNG engine and generator

set does not run continuously. It is used only to give a power boost when the battery is discharged below

a certain level, and will shut down once fully charged. This ensures low emission due to the sparing use

of the engine and the relatively low emissions of the LNG in comparison to other commonly used marine

fuels. Therefore, the proposed system can be categorised as a low emission technology solution. The

combined system can be installed in a platform such as a barge and moored closer to the anchored ships

to supply the required electrical power. As reported in [11], installing a fuel cell power system in a

floating platform, e.g. barge, is feasible.

A typical ship power system provides service and propulsion loads [12]. Although service loads may

experience fluctuations, their magnitudes and rate of change usually fall within operational envelop of

fuel cells. However, difficulties arise with propulsion loads that are linked to dynamic positioning,

especially at rough sea conditions. Similarly, the moored platform may not hold to its position during a

rough sea and therefore a suitable control mechanism is required for position fixing. Therefore, load

dynamics caused by propulsion motors and thrusters can easily push the fuel cell away from the stable

operating region causing blackouts. In such situations, the battery bank can support the fuel cell to keep

it within its operating envelop. When the combined fuel cell and battery system is unable to supply the

load, the LNG engine is started automatically to supply a power boost. These combined operations bring

complexities into the proposed floating power system and thus its performance heavily depends on the

effectiveness of the control and power management technologies used within the system.

Various control and power management schemes are proposed in relevant literature for similar hybrid

power systems. These techniques can be broadly classified as traditional PID based controls [13-16],

model reference based controls [17-19], and learning based controls [20, 21]. Out of these three

categories, the traditional PID based controls are the simplest and thus the most widely used.

Nevertheless, model reference based control schemes render fast transient response while learning based

controls are generally immune to parameter changes. As the focus of this paper is to investigate the

feasibility of the proposed system, simple PI controllers and a straightforward power management

strategy which is based on the battery state of charge measurement were adopted. In this study, the

proposed system, its control scheme and the power management were modelled in the

MATLAB/Simulink environment, with simulations carried out to test their performance under dynamic

loading conditions. Simulation results show that the proposed floating hybrid power system is capable

of supplying dynamic loads without triggering blackouts.

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Figure 1 An overview of the proposed system

Figure 2 Schematic diagram of the proposed ‘offshore cold-ironing’ system

404

2. System Modeling

2.1 Fuel Cell Model

A schematic diagram of the fuel cell model used in this study is shown in Figure 3. In this model, the

open circuit voltage (E_oc), exchange current (i₀) and the Tafel slope (A) are calculated using equations

(1), (2) and (3) respectively [22, 23].

i

 _fc



1





NA ln



i₀



ST /3 1



d

Figure 3 Fuel cell model

E_oc K_cE_n

(1)

(2)

(3)

 G

^zFk^P_H^{ P}

O₂

2

RT

i_o

e

Rh

RT

A 

zF

where N is the number of cells in the fuel cell stack, R is the universal gas constant (8.3145 J/mol K), F

is Faraday's constant (96485 C/mol), z is the number of moving electrons, E_nis the nernst voltage which

is the thermodynamics voltage of the cells that depends on the temperatures and partial pressures of

reactants and products inside the stack (V), α is the charge transfer coefficient which depends on the

type of electrodes and catalysts used, P_H2is the partial pressure of hydrogen inside the stack (atm), P_O2

is the partial pressure of oxygen inside the stack (atm), k is the Boltzmann's constant (1.38 × 10^–23J/K),

h is the Planck's constant (6.626 × 10^–34Js), ΔG is the size of the activation barrier which depends on

the type of electrode and catalyst used, T is the temperature of operation (K), K_cis the voltage constant

at nominal condition of operation, R_fcis the internal resistance of the fuel cell stack, S is the Laplace

variable and T_dis the response time (s).

2.2 Engine and Generator Models

The engine is represented as a first order delay with the time constant _enas shown in Figure 4. The

governor of the engine controls the fuel supply to the engine and thus controls the engine torque, T_ENG

,

to regulate the engine speed under varying load conditions. The corresponding closed loop speed

controller of the small LNG engine and the generator set is represented as in Figure 4, where T_eis the

electrical load and J_genis the equivalent inertia of the rotating parts.

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1

J _gen

K_i_iS 1

_enS 1

S

Figure 4 Engine model and closed loop speed controller of the small LNG engine and generator

set

3. Control and Power Management

3.1 Fuel Cell Power Controller

The interfacing dc-dc converter, shown in Figure 2, is used to control the fuel cell power by controlling

the current passes through it. The corresponding controller block diagram is shown in Figures 5, where

P_fc^*represents the power reference for the fuel cell stack and V_FCrepresents the fuel cell voltage. The

power reference can be adjusted to suite system requirements. Nevertheless, due to the slow dynamics

of the fuel cell it cannot respond to fast changes. Therefore, generally, the fuel cell power reference is

changed slowly. Hence, in this study, it is set to a constant value which in turn ensures smooth operation

of the fuel cell stack. This value is then divided by the fuel cell voltage to generate the current reference,

I_FC^*. As shown in Figure 5, the actual current passing through the dc-dc converter, I_FC, is then compared

with the reference and the error is passed through a PI controller to generate the duty cycle, D_fc. The

duty cycle, which varies between 0 and 1, compared with a triangular carrier signal in the pulse width

modulation (PWM) unit to generate gate pulse for the transistor Q7.

*

Figure 5 Fuel cell power controller (P_fc- power reference, V_FC- fuel cell voltage, I_FC^*- current

reference for the interfacing dc-dc converter of the fuel cell stack, I_FC- output current of the dc-dc

converter, D_fc- duty cycle of the dc-dc converter)

3.2 Battery Power Controller

Due to the slow dynamics of the fuel cell, sudden load changes cause deviations in the dc-link voltage.

The battery power controller, shown in Figure 6, senses these deviations by comparing the actual dc-

*

link voltage, V_dc, against the set value, V_dc. The error is passed to a PI controller to generate the duty

cycle, D_Bat, for the corresponding dc-dc converter. As described above, the PWM unit generate gate

pulses for Transistors Q₉and Q₁₀based on the duty cycle. This controller attempts to restore the dc-link

voltage. As a result of this voltage restoration effort, battery power varies. This controller is simple and

easy to implement as it automatically takes care of both charging and discharging of the battery bank.

*

Figure 6 Battery power controller (V_dc- dc-link voltage reference, V_dc- measured dc-link voltage,

D_fc- duty cycle of the interfacing dc-dc converter of the battery bank)

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3.3 Generator Power Controller and Power Management Strategy

A battery state of charge based simple power management strategy is used in this study where the LNG

engine automatically starts when the state of charge of the battery bank falls below a certain threshold

to provides boost power. For the sake of simulation, the generator power reference P_gen^*, was set to a

constant value in this study. Whenever the sum of the generator power and fuel cell power exceeds the

load demand, the surplus gets stored in the battery bank and thus it gets charged. As shown by the

hysteresis block in Figure 7 the generator runs until the state of charge of the battery reaches the upper

threshold before cutting out. The corresponding controller is shown in Figure 7.

Figure 7 Engine power controller (SoC - state of charge of the battery bank, P_gen^*- power reference,

*

V_dc- measured dc-link voltage, I_g- current reference for the interfacing dc-dc converter of the

generator, I_g- output current of the interfacing dc-dc converter, D_gen- duty cycle of the dc-dc

converter)

4. Simulation Results

The proposed hybrid power system was modelled and simulated using the MATLAB/Simulink software

to test its performance at dynamic loading conditions. System parameters of the simulation setup are

given in Table I. Load power variations used in this study are shown in Figure 8(a) by the trace marked

‘Load power’[24]. As mentioned above, fuel cell power reference was set to a constant value. As evident

in Figure 8(a) by the traced marked ‘Fuel cell power’ the controller is able to maintain the fuel cell

power at the set value. The battery bank absorbs the fluctuations present in the load power as shown in

Figure 8(b) by the trace marked ‘Battery power’. The negative battery power shown during the 0-30s

period indicates that the fuel cell power is larger than the load demand and thus the battery gets charged.

The plot of corresponding state of charge variation in Figure 8(b) shows an increase during this period.

After that, the load power exceeds the fuel cell power and thus the battery bank is discharged causing

its state of charge to drop.

In order to show the performance of the engine power controller, the lower and upper thresholds of the

battery state of charge was set to 64% and 65% respectively. Practical values of theses thresholds can

be significantly different to these values, depending on the application environment. The points at which

the battery state of charge meets these thresholds are marked by vertical dashed lines in Figure 8. When

the Battery state of charge drops below 64%, the generator starts and delivers power to the common dc-

bus. As mentioned above, the engine power is set to a constant value in this study and thus the

corresponding generator power remains constant as shown in Figure 8(a) by the trace marked ‘Generator

power’.

Whenever the sum of the generator and fuel cell power exceeds the load power, the battery bank is

charged. This is evident in the variations of the battery state of charge shown in Figure 8(b). When the

battery state of charge reaches 65%, the engine cuts out, thus the generator power drops to zero as shown

in Figure 8(a). These results verify the ability of the proposed hybrid power system to supply dynamic

loads while ensuring smooth operation of the fuel cell stack through appropriate control and power

management.

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Table 1 System parameters of the simulation setup

Rated power of the fuel cell stack

No load voltage of the fuel cell stack

DC-Link voltage reference

350kW

900V

600V

Nominal voltage of the battery bank

Maximum capacity of the battery bank

Rated power of the generator

480V

400Ah

100kW

440V

Output line voltage (V_LL-rms

)

Output frequency

60Hz

Load power

Fuel cell power

Generator power

Battery power

Time (s)

(a)

Time (s)

(b)

Figure 8 (a) Load power, fuel cell power, battery power and generator power, (b) State of charge (SoC) of

the battery bank.

5. Conclusion

This paper proposes a hybrid power system for offshore cold-ironing of anchored ships, which consist

of a fuel cell stack, battery bank and a small LNG engine-driven generator set. In this study, a simple PI

controller based control system and a battery state of charge based power management strategy were

used to verify the operation of the proposed hybrid power system under dynamic loading conditions.

Simulation results show that the fuel cell and battery combination is able to supply dynamic power

demands, while the small LNG engine-driven generator provides boost power to charge the battery bank

when its charge falls below a set threshold. Based on the results, it can be concluded that the proposed

hybrid power system is capable of supplying dynamic loads that can occur in a ship’s power system

while at anchor, ensuring smooth operation of the fuel cell stack. It is thus a viable option to provide

efficient, effective and low emission power to a large fleet of vessels anchored in and around congested

ports across the world. Assessing the performance of the proposed system against actual power demands

recorded from anchored ships would be a possible step to take the proposed concept to the next level.

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