Published at : 28 Jul 2023
Volume : IJtech
Vol 14, No 5 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i5.6149
Muhammad Arif Budiyanto | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI, Depok 16424, Indonesia |
Sultan Alif Zidane | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI, Depok 16424, Indonesia |
Gerry Liston Putra | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI, Depok 16424, Indonesia |
Achmad Riadi | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI, Depok 16424, Indonesia |
Riezqa Andika | Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI, Depok 16424, Indonesia |
Gerasimos Theotokatos | Department of Naval Architecture, Ocean and Marine Engineering, University of Strathclyde, G4 0LZ, UK |
The
development of liquefied natural gas (LNG) carrier ships is increasing rapidly
along with the demand for alternative energy sources. This study aims to
analyze the performance of an LNG-fueled propulsion system for a small-scale
LNG carrier ship with a combination gas-electric steam turbine system (COGES).
The performance of the propulsion system is analyzed based on the power output
generation and the environmental effect of the system. The total power output
was evaluated using thermodynamic analysis and the environmental impact was
measured by carbon emissions using the life-cycle assessment approach. The analysis
results show the COGES propulsion system potentially generates total power of
7600 kW at peak load. The desired ship operational speed of 12 knots can be
achieved at 24% load of the total power output. The COGES propulsion system
also indicates a low emission than other systems with a carbon emission is
0.149 kgCO2/kWh.
COGES; Propulsion system; Small-scale LNG carrier
In the transportation
industry, natural gas is a promising alternative fuel source that offers benefits
such as improved combustion efficiency and a decrease in greenhouse gas
emissions
LNG carriers are ships equipped with tanks
designed to transport liquefied natural gas with temperatures below -162°C (Bai and Jin, 2015). LNG carriers vary in
cargo capacity, ranging from small capacities starting at 1000 m3 to
the largest cargo capacity currently reaching 266,000 m3.
A Small-scale LNG carrier
is an LNG carrier that has a carrying capacity of up to 40,000 m3 (Guerrero, 2019). The
small-scale LNG carrier is used for
remote areas with shallow water draught. In its development, the use of small-scale LNG
carriers needs to be reviewed in terms of economic value and transportation
costs (Budiyanto et al., 2020).
In transporting LNG using sea transportation,
it is necessary to pay attention to the use of the propulsion system to obtain
efficiency and economic feasibility considerations. International Maritime
Organization (IMO) has formulated an energy efficiency design index policy for
the ships. According to research based on data from registered ship engines,
tankers and gas carriers are among the ship types that produce the most carbon
emissions (Budiyanto, Adha and Prayoga, 2022). The
small-scale LNG carrier thus is designed to feature a propulsion system that
uses natural gas or LNG as fuel to facilitate more affordable and
environmentally friendly gas transportation
In terms of energy efficiency, numerous
alternative propulsion systems with higher efficiency values have been proposed
including the dual fuel diesel engine and the combined cycle. The ship
propulsion system mainly consists of three main parts, i.e.: the prime mover,
the transmission system, and the ship propulsion device. The design of the
ship's propulsion system will depend on the type of ship, the main size, the
ship’s speed, the stern model, and the hull model (Lin et al., 2020). The propulsion system of the main part of the
ship is closely related to the thermal power generation cycle
Combined gas turbine electricity and steam
(COGES) is a propulsion system that uses a gas turbine to generate electric
power, which is then utilized to drive an electric motor that is connected to a
shaft propeller
The purpose of this study is to analyze the
proposed COGES propulsion system for a small-scale LNG carrier. The
contribution of this study is twofold, which is providing a comparison of the
power output of COGES with conventional systems and understanding the
environmental impact of the proposed system.
2.1.
Case study of LNG carriers
The
case study used in this research is a small-scale LNG carrier with a capacity
of 7500 m3. The size of small LNG carriers has capacities varying
from 1000 - 40,000 m3 (TGE-Marine, 2022; Wärtsilä, 2022; Bai and Jin, 2015). No classification regulates the dimensions of small LNG
carriers. The use of small LNG carriers is attractive for short-distance
shipping inter islands with limited draft depths and small amounts of cargo
Table 1 Ship dimensions of the case study
Figure 1 The required power at the desired speed of the case study
2.2.
Proposed design of COGES propulsion system
The
design of the small-scale LNG carrier propulsion system proposed in this study
is COGES which consists of two power generators each sourced from a gas turbine
and a heat recovery steam generator. The system diagram of the proposed COGES
design is shown in Figure 2. The design of the COGES propulsion system consists
of several main components, namely gas turbines, heat recovery steam
generators, condensers, deaerators, pumps, and generators. The thermodynamic
system analysis of the proposed COGES design was carried out using the
Cycle-Tempo application to obtain the output power generation and total
efficiency of the design system. Cycle-Tempo is commercial software to analyze
and optimize the thermodynamics of the energy system hambient is
environment enthalpy/ambient
s is entropy environment
sambient is environment entropy/ambient
m is mass flow
Tambient is
environment temperature/ambient (K), Q in is the calorie
input of system (kW), W is performance output of the system (kW), and E(x,n)
is exergy rate (kW).
Figure 2 Proposed design of COGES propulsion system
An
analysis of the effect of boiled gas (BOG) produced from the LNG carrier cargo
tank was carried out to investigate the effect of BOG as turbine fuel on the
performance of the system. The power
output generated by the design system will also be influenced by the amount of
fuel obtained from the BOG of the ship's cargo. It is necessary to take into
account the amount of BOG produced by the ship. The ship's BOG is also
influenced by the boil-off rate (BOR) of the number of shiploads, therefore
several variations will be made based on the BOR of cargo starting from the
range 0.1%-0.3% per day. Equation 5 shows the calculation of BOG, where V is
LNG cargo volume (m3), is LNG density (kg/m3), and t is
shipping time (hours).
In this
study, carbon emissions are estimated based on the power output (kWh) for each
propulsion system. The estimation was carried out using the life cycle
assessment software application, namely SimaPro
Performance
analysis of the COGES was conducted by thermodynamic analysis to determine the
amount of power output that can be generated by the designed system. The power
output of the COGES propulsion system is shown in Figure 3. According to the
thermodynamic analysis results, the proposed COGES potentially generate up to
7600 kW of power at full load. To attain the ship's operating speed of 12
knots, which requires 1832 kW, the COGES system only needs to load around 24%
of the entire design load. These results are consistent with the power output
trendline of other similar studies, which in this study also stated that the
load needed to meet the ship's propulsion power requirements was 33% (Nirbito, Budiyanto
and Muliadi, 2020). This shows that the use of the COGES design
system makes the work of the main components lighter so that the use of the
system will be more durable because it does not always work at maximum load
conditions. In actual operation, the power output produced by COGES is used to
power all systems on board the ship in addition to the propulsion system.