Performance modeling of a hybrid Diesel
generator-Battery hybrid system
Central University of Technology
Energy Postgraduate Conference 2013
Presentation outline
• Introduction
• System Layout
• Design variables
• Optimal operation control
• Operation strategies
• Conclusion
Introduction
Small rural area electrification challenges:
Reliable electrical power supply lacking, high cost of AC grid extension, rough
topography.
Diesel generator in rural electrification:
• Advantages: Low initial capital costs and generate electricity on demand.
• Disadvantages: Costs (operation fuel, maintenance running, replacement
lifespan),
Environmental impacts (noise and pollutions).
Hybrid Diesel – Battery system:
By optimally designing the hybrid generator-battery system and applying optimal
operation
control
strategies,
the
overall
running
environmental impacts can be sensibly reduced.
costs
and
well
as
the
This paper will propose an approach for optimal operation control of a
hybrid Diesel generator - battery system with the aim of minimizing the
operation cost while meeting the load energy requirements.
The paper will be limited at the problem formulation and the development of
the mathematical model for the performance of all the hybrid system’s
components.
System Layout
The proposed hybrid system is
composed of a diesel generator, a
battery
storage
system,
an
inverter, a charge controller as well
as AC and DC loads.
The hybrid system model is
based on a description of current
flow from the different sources,
taking into account the losses and
the
impact
of
the
operating
decisions along the way up to the
loads.
Design variables
Diesel generator:
The instantaneous output current from the DG depends on variables such as the type or the
size of the DG used (XDG, Size, Type). It can be expressed as:
I DG  I DG,max  X DG %
Battery storage system
The instantaneous output current from the whole battery bank depends on variables such as
the type or the size of the battery used (XBat, Size, Type), the number of battery strings in parallel
XBat. It can be expressed as:
I Bat(t )  I Bat,max, (t )  X Bat%( t )
Inverter
The size of the inverter XInv is expressed in terms of its AC output power.
PInvOut  PInvIn  Inv  X Inv
If XR, DC is the percentage of current from the DC bus (IDC) redirected to the DC load, the
input current to the inverter can be expresses as (1-XR, DC). XR, DC is an operation variable.
I InvOut
VDC ,Bus
 I DC ,Bus  (1  X R ,DC )   Inv 
VAC ,Bus
Transfer switch
Its position (XS) is an operation decision variable. If XS=0, all the AC load is supplied by the
DG and the inverter is off. If XS=1, all the AC load is supplied by the DC bus through the
inverter.
Battery charger
The size of the battery charger XBC in terms of its DC output power.
PBCOut  PBCIn  BC  X BC
The percentage (XR, AC) of current from the DG (IDG) redirected to the battery charger is:
I DG  X R , AC 
I BC Out  VDC , Bus
 BC  VAC , Bus
Optimal operation control
Objective function:
The aim is to minimize the fuel consumption cost from the DG during the
operation time. This can be expressed as:
N
min
2
(
aI
 DG, j  bI DG, j  c)  p
j 1
Where: N is the number of sampling intervals within the operation range or
period of the system.
a ,b, c are coefficients.
p is the price of the fuel.
j is the jth sampling interval.
IDG,j is the output current from the DG at jth sampling interval.
Constraints:
• Load balance
I i ,Load
 0 : Excess

 I i ,Supply  0 : Balance
 0 : Unmet

• Variable limits
Operation limits
Design limits
• Battery limits
0  X DG% ; X R , AC ; X R , DC  1
X S [0,1]
X i ,Size,Type
min
 X i ,Size,Type, j  X i ,Size,Type
max
SOC Bmin  SOC B, j  SOC Bmax
Operation strategies
• Diesel generator favourite: If the DG (CF) is between some preset values
[CFX1%, CFX2%] and the battery state of charge (SOC) is between some preset
values [SOCX1%, SOCX2%], then the DG is preferred.
• Battery system favourite: If the battery state of charge (SOC) is between
some preset values [SOCY1%, SOCY2%] and the DG capacity factor (CF) is
between some preset values [CFY1%, CFY2%], then the battery is preferred.
Conclusion
The purpose of this paper was to propose an approach for optimal operation
control of a hybrid Diesel generator - Battery system with the aim of meeting the
load energy requirement while minimizing the operation costs all through the life
of the system. The sizing optimization variables, operation decisions variables to
be optimized at time t, as well as the constraints for the optimal sizing and
operation control of the hybrid system have been described. The time
independent operation strategies can be chosen by making a search through
potential settings for the hybrid system optimal operation control.
For further studies, the developed model can be integrated to the hybrid
system’s total life cycling cost and then be used by any of the traditional
(gradient-based
methods)
or
modern
(artificial
algorithms to find the best solution for the system.
intelligence)
optimization