Ship Electrical Load Analysis and Power Generation Optimisation To Reduce Operational Costs
Ship Electrical Load Analysis and Power Generation Optimisation To Reduce Operational Costs
Ship Electrical Load Analysis and Power Generation Optimisation To Reduce Operational Costs
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Paola Gualeni
Università degli Studi di Genova
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Abstract—Nowadays the amount of electrical power generated term performance and economic balance between initial
on-board ships is drastically increased, especially for the All investments, running costs and revenues. From what concern
Electric Ships (AES) where all the energy needed is supplied by new design approaches, the Electrical Power Load Analysis
the electrical power system. In this context, the traditional (EPLA) is one of the phases where the need of innovation seems
methods to calculate the power demand and select the size of the to be stronger. The EPLA consists in the evaluation of the
generation system have become inadequate, since they are based electrical power demand in different ship operational scenarios
on very dated assumptions. Aim of this paper is to present an (e.g. cruising, manoeuvring in port, operation and emergency
optimum problem in order to correctly and efficiently size the scenarios are typical). The power required to generators is
generation system. The power demanded will be calculated using
traditionally evaluated using specific coefficients (e.g. load
the traditional approach based on Load Factors. The optimum
problem will be solved using the Genetic Algorithms, and provide
factors, diversity factors and utilization factors), which might
the optimal size, load factors and unit commitment for each have different values for each typology of load installed on
generator in each ship operative scenario. board and for each scenario under analysis. The sum of the
previous results provides the value of the maximum power
Keywords—Electrical power load analysis, power generation, demand. After this phase, it is possible to proceed with the
energy efficency, marine power system, optimisation, genetic generators sizing, the study of possible energy storage
algorithms. solutions, perform fuel consumption evaluation, and perform an
economic assessments on the system cost, both in the short and
I. INTRODUCTION long term perspective. Considering the generators design and
sizing, it is usual to select them with the same rated power in
In recent years, an increasing interest on energy efficiency order to reduce the complexity of the process (e.g. reduce both
and environmental issues has been revealed. These are two very the installation costs and the complexity of the generators
closely related topics. The International Maritime Organization managements) and decrease the need of spare parts. As a result,
(IMO) has recently adopted measures to reduce ships emissions the initial investment cost of generators is minimized by the
of Greenhouse Gases (GHG) i.e. the Energy Efficiency Design shipyards. On the other hand, this approach does not
Index (EEDI) and the Ship Energy Efficiency Management specifically consider as design objective the generators
Plan (SEEMP). The EEDI has become mandatory for new ships efficiency as well as their management costs reduction. In case
and the SEEMP for both new and existing ones [1]. Aim of the of systems with very low energy efficiency characteristics,
EEDI, in addition to the reduction of GHG, is to stimulate some improvements have been proposed in literature, such as
technical developments and innovation in design as well as in in [3]-[7]. In this work, the demand of electrical power will be
ships management. Significant improvements in ship energy evaluated using the traditional factors approach. The aim of this
efficiency depend on a mix of design and operational factors. work is to propose a possible approach to perform an EPLA and
This include the implementation of new technologies (e.g. more select the optimal size of the power generation system with the
efficient engines, improved hull shapes, highly performing focus on the management costs, rather than the investment cost.
propellers, low resistance coatings or air cushions, means for This methodology will be tested on a case study in order to
heat recovery [2]), combined with daily operational practices highlight the economic benefits in adopting this kind of
(e.g. weather routing and other means to measure and enhance approach instead of the traditional one. A traditional bulk
energy efficiency), both on board and ashore. Even, the most carrier with mechanical propulsion system has been selected as
traditional type of ship, the bulk carrier ships, can be designed case study.
as innovative eco-friendly ships, when detailed consideration is The rest of the paper is organized as follow: Section II
given to all the factors that may contribute to energy savings. reports a description of the model selected in order to perform
This perspective should be the driven objective of the whole an EPLA, Section III describes the problems statement for the
ship life-cycle; from design, through construction and optimisation, Section IV reports the case study with the main
operation, to recycling. From the economic point of view, new results and Section V draws some conclusions.
technologies require large investments that cannot be made if
there is not a high degree of certainty concerning their long‐
II. ELECTRICAL POWER LOAD ANALYSIS AND An alternative approach is to consider together two different
GENERATORS SIZING factors. These, as it was for the LF, are defined for each load in
The main task of the Electrical Power Load Analysis is to every ship operative scenarios. In normal condition, the power
calculate the electrical power required by the users installed on- absorbed by a load is less than that indicates by its nominal
board the ship. As a direct result of this analysis, it is possible power rating. The Utilization Factor (ku) can account for the
to evaluate the power that the generators have to supply in the time in which the load in switched on rated with the total time
principal ship operative scenarios. When the power required to that it could be in use as reported in (3). The Demand Factor
the generation system is known, it is possible to select the (DF), which is the inverse of the most known Diversity Factor
generators size, number and typology in order to minimize the (ks), is defined as the ratio between the maximum demand of a
installation and management costs. system (or maximum demand of a user) and the total connected
load on the system (4).
A. Traditional Factors Approach to Perform the EPLA
In order to perform an EPLA some information about the tonij
kuij = (3)
ship are required. In this context, the main document containing Tj
the input information is a complete list of the electrical users
installed on-board the ship. Further, when available, some data
PMAXij
from similar ship or sea trials may be useful in order to increase DFij = (4)
the accuracy of this analysis. The traditional method to perform PNOMij
an EPLA is based on factors, which can partially account of the
behaviour of each user in the different ship operative scenarios. Where:
The Load Factor (LF) is the most used in naval field [8]. - ku is the utilization factor of the i-th load in the j-th
The LF is defined as reported in equation (1). scenario
- is the time the i-th load is switched on in the j-th
1 T scenario
LFij = pij τ)dτ (1) - DF is the demand factor of the i-th load in the j-th
Tj ·PMAXij 0 scenario
Where: Consequently, from the previous (1), (3) and (4) it is easy to
- LF is the load factor of the i-th load in the j-th scenario understand that the Load Factor is equivalent to the ratio
- T is the time period of reference for the j-th scenario in between the Utilization Factor and the Demand Factor (5).
hours [h]
- is the maximum power of the i-th load in the j-th kuij
scenario in [kW] LFij = (5)
DFij
- p τ) is the instantaneous value of power absorbed by
the i-th user in the j-th ship operative scenario. Once the total load absorbed by the users in each ship
operative scenario has been calculated, it is possible to evaluate
As it is defined in equation (1), the LF is the average value the generation power PGEN . The generators are sized
of power absorbed by each user in each ship operative scenario
considering the worst combination of the former conditions (i.e.
in reference of the maximum load during a given period (i.e. in worst ship operative scenario).
the considered scenario). Load Factors can be calculated from
actual data for a single day, for a month or for years. Its value
is always less or equal than one, because the maximum demand B. Generators Sizing
is always more than average demand. Furthermore, it could be First result of the Electrical Power Load Analysis is the
used for determining the overall cost per unit generated. In fact, possibility to select the generators (e.g. size and number) as
higher is the LF lesser will be the cost per unit. When the shown in Fig. 1.
designers decide to adopt the LF in order to perform an EPLA
it is not necessary to introduce other factors. It is possible,
indeed, to calculate the total power absorbed by the users in
each ship operational scenario as reported in equation (2).
Where:
- PABS is the total power absorbed by the users in the j-th
scenario
- PNOM is the nominal power value of the i-th load in the Fig. 1 - Inputs and Outputs of EPLA
j-th scenario
- N is the total number of electrical load considered.
Considering the EPLA it would be possible to select the number of cylinders for the same type of generator. As a result
generators considering installation, management, service costs of this consideration, shipyards and ship owners might select
and weight. In order to perform this analysis, an optimum generators of the same brand but with different characteristics,
problem has been formulated and implemented. improving efficiency, performances and reducing space and
weight.
III. POWER GENERATORS OPTIMAL SIZING, MODEL
FORMULATION B. Operational Costs, Objective Function Formulation
In literature, several optimal problems have been presented Due to the aim of this work, the objective function (8)
in order to correctly size generators, energy storage and defines the cost represented by a generators option, taking into
photovoltaics [7]-[12]. The model presented in this work is account the operational costs and, in lower part, the installation
focused on the optimal sizing for generators considering the costs (i.e. it is possible to select generators with homogeneous
management costs and partially for the investment costs. One sizes).
of the innovations here presented is the possibility to join an
optimisation problem focused on the generators size (i.e. it is G S
possible to select generators with different or homogeneous Pij
F= · Tij ·SFOCij ·FC (8)
sizes) with a traditional EPLA, which identifies the demand of ηij
i=1 j=1
electrical power.
Where:
A. Operational Costs, Problem Statement - F is the cost of the generators option [€]
In order to calculate the operational costs of a certain - G is the number of generators
solution (i.e. every generators configuration under exam) the - S is the number of scenarios
fuel oil consumption (FOC) in a given condition has to be - Pij is the power rated by the i-th generator in the j-th
defined. For a specific operating condition, the FOC is strongly scenario [kW]
dependent on the load factor (GLF) of the generators. - ηij is the operating efficiency
Moreover, it depends on the efficiency of the generators (η), the - Tij is the operating time of the i-th generator in the j-th
power absorbed by the users (PABS ), the line losses (Pl ), the scenario [h]
number and the operation time (t) of each generators, the rated - SFOCij is the specific fuel oil consumption of the i-th
power (PG ) and on the specific fuel oil consumption (SFOC) generator in the j-th scenario [g/kWh].
that is characteristic of each generator, as defined in (6). - FC is the unit cost of the marine diesel oil [€/t]
The generation costs are all mainly dependent on the power
FOC= f(GLF,η,PABS ,Pl , t, PG , SFOC) (6) rated by generators (10).