Optimization of Energy Management of A Microgrid Based On Solar-Diesel-Battery Hybrid System
Optimization of Energy Management of A Microgrid Based On Solar-Diesel-Battery Hybrid System
Optimization of Energy Management of A Microgrid Based On Solar-Diesel-Battery Hybrid System
https://doi.org/10.1051/matecconf/201817101006
E3PE 2017
Abstract— Hybrid systems, which are composed of loads, electricity storage, and the conventional source (diesel
combinations of diesel generators, battery energy storage system genset) [5]. In addition, integrating a battery energy storage
and renewable energy resources such as photovoltaic, are system with the hybrid plant provides significant dynamic
outlined as a recommended approach for off grid power supply operation benefits such as higher stability and reliability of
options for remote areas applications. Since these systems are not
power supply.
connected to an infinite source of energy, they must be well
designed and controlled to satisfy the demand load. This study More recently, hybrid renewable energy systems (HRESs)
presents an efficient battery management strategy for the are gaining popularity for their environmental friendliness and
charging and discharging of the batteries in a hybrid renewable potential economic benefit. However, HRESs have not yet
energy systems by controlling the energy flow between different dominated in the market due mainly to their high price. One of
components of the system. In order to simulate the developed the major factors leading for this high price is the high battery
battery management strategy, the components of the hybrid cost. Thus, manufacturers have been focusing on the reduction
system are studied and modeled using mathematical models. of battery cost to increase their market share. An effective
During modeling of the battery storage system, the effects of battery management strategy (BMS) can reduce the required
aging and capacity degradation were taken into consideration.
number of battery cells and extend the replacement period,
The simulation of the strategy was based on a case study, where
it is validated to be functional. Finally, the optimization of the hence reducing the battery cost. As part of our objective of
strategy has been done by working on its critical parameters. designing such a BMS, we will in this study focus on
extending batteries’ life with respect to the whole investment
Keywords— Hybrid renewable energy systems; energy life which has significant impact on the battery cost.
storage; battery; PV; energy management strategy; optimization. Battery life is defined as the duration (measured in the
number of cycles or elapsed time) of a rechargeable battery
I. INTRODUCTION until it degrades irreversibly and cannot hold a useful capacity
Public interconnected power grids are composed of of the underlying applications [5]. That is, the battery capacity
complex combinations of generation plants, substations, monotonically decreases with time, and will never be
transformers and transmission lines, which supply electricity recovered. Among the various reasons for battery capacity
to cities, businesses and industry. In addition, there are smaller degradation, the discharge/charge rate is reported to be the
independent power grids that provide power to islands or most critical. For example, a continuous exposure to high
remote areas, which have limited or no access to public discharge current leads to fast capacity degradation thus
interconnected grids. Traditionally, small stand-alone grids are shortens the battery’s life, which is unavoidable due to sudden
electrified by diesel generators. However, the renewable changes in the power requirement, such as motor start-up. To
energy resources are attractive sources of power, since they extend the battery life, researchers focused on management
can provide sustainable and clean power. Hybrid plants can be strategies that use more than one charging/discharging modes.
an integration of diesel generators with renewable energy In this study we will elaborate an advanced battery
resources such as photovoltaic. Because of its intermittent and management strategy that best saves battery life-time
irregular nature, PV generation makes hybrid system respecting some constrains and system conditions. We will
management harder. Consequently, for some authors [1-2], PV take a case study and size the hybrid system, so we can take it
production into the grid is considered to be limited [3]. One of as a reference to simulate our strategy.
the major challenges for PV systems remains in the matching II. TOPOLOGY OF THE HYBRID SYSTEM
of the intermittent energy production with the dynamic power
demand. A solution is to add a storage element to these Before focusing on each component in the system, the
nonconventional and intermittent power sources [4-5]. In this overall topology must be defined in order to specify these
case, the hybrid system is composed of a PV generator, local components and define their role. A scheme of the studied
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution
License 4.0 (http://creativecommons.org/licenses/by/4.0/).
MATEC Web of Conferences 171, 01006 (2018) https://doi.org/10.1051/matecconf/201817101006
E3PE 2017
C. Battery
III. ENERGY CONVERSION MODELING OF HYBRID SYSTEM
a) Modeling of the battery
A. PV system The model chosen to be used in this study is a model that
The method selected to model the PV array, in this study, takes into account the state of charge of the battery. It is a
involves simply using mathematical equations to calculate the dynamic model that adopts the simple model where the battery
current and voltage generated at the specified temperature and is represented by an open circuit ideal voltage source and a
irradiance from the power generated at Standard Temperature fixed resistance (Figure 3), but with variable voltage source
Conditions (STC) by analyzing the factors concerned and that depends on the SOC and constant internal resistance.
determining how they would affect the power generated. STC
is defined as 1000W/m2 irradiance, temperature of 25C and
air mass of 1.5 (corresponds to 48.2 incidence angle). If the
charge controller includes a MPPT system, the PV panels
always work at the maximum power point of the I = f (V)
curve, so the output current of the PV generator is as follows
Figure 3: Simple Circuit Model [8]
[6]:
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MATEC Web of Conferences 171, 01006 (2018) https://doi.org/10.1051/matecconf/201817101006
E3PE 2017
The relation between the state of charge (SOC) and the (9)
open circuit voltage (Voc) is supposed to be linear as in the
following equation [8]: Where Idisch is the absolute value of the discharge current. CN is
the nominal battery full capacity. The end of life of the battery
(5)
is reached when the degradation of the battery capacity
Where Umax is open circuit voltage at full battery and U min is reaches 80% of its nominal capacity. This capacity loss by
the open circuit voltage at empty battery, these values are degradation can be calculated from the Schiffer equation [8]:
taken from the battery manufacturer datasheet.
(10)
The charging current of the battery can be controlled by
the charging voltage on the battery terminals. This current can Where ZIEC is the total number of cycles estimated by the
be calculated as in the following equation [9]: manufacturer according the IEC standards. Therefore, the
(6) remaining battery capacity is difference between the initial
Where a is the coefficient that allows the adjustment of the nominal capacity and the capacity loss by degradation:
equation, it can be calculated by substituting experimental (11)
values of current and state of charge.
The state of charge must also be modeled so our battery
IV. BATTERY MANAGEMENT SYSTEM
model is complete. The state of charge is changed in two
cases, charging and discharging [10]. In order to elaborate, simulate and validate the
management strategy, a data base is necessary. For this reason,
During charging:
we recovered the data of solar irradiance, ambient
(7) temperature, and of electrical load of the diesel engine on the
Where ηbat is the round-trip efficiency of the battery, it reflects experimental site situated at Canada (Latitude: +48.83,
the charging efficiency of the battery, since not all energy Longitude: -64.48) [10-11]. These data will be used to sizing
injected in the battery can be stored, certainly there will be the hybrid renewable energy system in order put a strategy
losses. This factor has values between 0.7 and 0.85 during compatible with it.
charging and is equal to 1 during discharging process. t is the
charging time and Cb is the full battery capacity [10].
During discharging:
(8)
b) Cycling and Capacity Degradation
Modeling lifetime of batteries is a very important aspect of
hybrid power system simulation because the uncertainty
associated with the expected lifetime of the batteries makes
the estimates of cost of energy of the projects very uncertain.
Since the life cycle cost of the batteries is one of the
significant power system expenses it is a major source of
uncertainty for potential power system investors.
Many factors affect the life of the batteries, including the
depth of the charge – discharge cycles, the current, the cell
voltage, the performance of the charge controller (e.g., voltage
and state of charge limits and regulation), the length of time
that the batteries are in a low state of charge, the time since the
last full charge, the temperature, etc.
Many studies have been published about the simulation
and optimization of renewable stand-alone systems including
batteries. However, the battery lifetime has always been
estimated in fixed values based on the experience of many
researchers or it has been estimated by calculating the number
of equivalent full cycles. In the best cases, it may be estimated
using the cycle counting method (CCM).
Figure 4: Energy management strategy flow chart
The CCM assumes that a complete cycle is reached the
Ah-throughput is equal to the nominal battery capacity. The Taking into consideration the previously discussed
estimation of the lifetime consists of adding the charge (Ah constraints and priorities, and starting from the studied models
throughput) cycled by the battery and calculating the number of the system components, and taking the data of the case
of full cycles (ZN) as follows [8]: study as a reference, we elaborated the control and
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MATEC Web of Conferences 171, 01006 (2018) https://doi.org/10.1051/matecconf/201817101006
E3PE 2017
management strategy of the PV-Diesel-Battery hybrid system. As we see in the Figures 5 and 6, load varies too much
The flow chart in Figure 4 shows the strategy as an infinite with time. The strategy must handle this variation well and
loop that reads inputs from an external source each period of satisfy the load. The following is the PV power produced; it
time and gives the outputs during this period. After the system seems to be very similar to the solar irradiation. This
parameters are initialized, the inputs must be read, which are convergence is due to the absence of the effect of temperature,
the solar irradiation G, the demanded power by load P load and since this effect is very slight at temperature near 20°C.
the ambient temperature Ta. This data is taken from the case
study profile table that gives a sample every 10 minutes.
According to the mathematical model of the PV system, the
temperature of the panel and the PV system output current and
voltage are calculated. Losses in the converters are also taken
into consideration. Based on these information, the power
flows are correctly dispatched between the different sources
(PV, diesel), loads (principal and dump loads) and battery
energy storage in order to ensure the minimum cost per
kilowatt-hour produced from the hybrid system and a long
cycle-life of the batteries.
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MATEC Web of Conferences 171, 01006 (2018) https://doi.org/10.1051/matecconf/201817101006
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true character of batteries and agrees with the charging hour of operation and maintenance of the system. The
characteristic given by the manufacturer [12]. operating costs include the price of fuel consumed, the costs
Figure 8 shows the variation of the state of charge of the of generator maintenance and cost battery storage system over
batteries. During the charging times, the SOC increases and the total life time of batteries [13-19]. In order to fair compare
during the discharging times it decreases. Also it is clear that strategies we will calculate the hourly operating cost of the
there is a limitation of the SOC decrease to 0.3 or 30% of system as an average of total life time.
battery capacity. The cost of diesel fuel in Canada is 1.3 USD per liter [13].
Figure 9 shows the evolution of the consumed battery The generator oil must be changed every 250 operating hours
cycles. As the battery model previously described, the cycle [13]. The oil capacity of the generator is 10.6 liters; the
counting is done during the discharge of the batteries, and this average price of the liter is 7 US dollars [13], so the oil cost
is clear in the graph. As we see also in Figure 18, the capacity will be 10.6*7/250 = 0.29 USD per operating hours. The other
of the battery is slightly decreasing, and this is a result of maintenance works can be estimated to be 0.13 USD/hr. Thus,
degradation due to cycling. Figure 10 and Figure 11 show the the oil changing and maintenance cost can be totally
generator output power and the fuel consumed. They seem to calculated as 0.29 + 0.13 = 0.42 USD per operating hours.
be proportional, and this result was expected from the The batteries have limited life time and must be replaced
characteristic given by the manufacturer [7]. As it is clear the periodically, so we must include the price of batteries divided
generator only turns on at the time where the state of charge on the working hours to calculate the hourly operational costs.
reached 0.3 (between 1500 and 2000 minute), which is when The cost of a 200Ah Yuasa lead acid battery is 300 USD [13].
the batteries are almost empty and there is no solar irradiation. The system contains 56 batteries; their total cost is 16’800
USD.
The following formula lets us evaluate the operating costs
of the system under the management strategy:
(12)
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MATEC Web of Conferences 171, 01006 (2018) https://doi.org/10.1051/matecconf/201817101006
E3PE 2017
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MATEC Web of Conferences 171, 01006 (2018) https://doi.org/10.1051/matecconf/201817101006
E3PE 2017