Technical Issues of Sizing Lead-Acid Batteries for Application in Residential Renewable Energy Systems Mohammad Jafari*, Member, IEEE, Glenn Platt**, Zahra Malekjamshidi*, Jian Guo Zhu*, senior member IEEE *Faculty of Engineering and Information Technology, University of Technology Sydney **Commonwealth Scientific and Industrial Research Organisation (CSIRO) PO Box 123, Broadway, NSW 2007, Australia
[email protected] Abstract— Lead acid batteries are one of the most commonly used storage devices in residential renewable energy systems. This paper explains the procedure of calculating required capacity of lead acid battery for a residential based renewable energy system. A renewable energy system (RES) is introduced based on a topology of multi-port converter. The basic required stored energy is defined using averaged load energy demand and renewable energy generation profiles. The required capacity of lead-acid battery is calculated on the basis of required stored energy considering technical factors such as battery efficiency, operation temperature and number of autonomy days. The technical issues of using Lead Acid batteries are discussed and presented based on the factory provided manuals.
+proposed. At first the total stored energy demand is calculated as basic part. Then some technical issues as percentage of available energy for off-grid operation, operating temperature and battery efficiency are considered to modify the required capacity. The reminder of this paper is organized as follow. In section II the basic structure of RES including MPC is introduced briefly. The method of estimating basic stored energy demand is discussed in Section III. Characteristics of lead-acid batteries, their positive and negative points and technical issues are discussed in section IV. A conclusion is derived at section V.
Keywords— lead acid, battery, residential, renewable energy, multiport, converter
II.STRUCTURE OF RENEWABLE ENERGY SYSTEM
I. INTRODUION
The proposed RES is designed to be used in a residential house as a part of smart grid. Fig.1 shows the electrical schematic of RES in detail. As can be seen RES includes four main blocks including multi-port dc-dc converter, three phase inverter/rectifier, central controller and a bidirectional meter/load management unit. The output from MPC is connected to the three phase inverter and further to the residential load and grid. The MPC included three H-bridge ports connected to the PV panel, fuel cell stack and battery and a multi-winding high frequency transformer at the middle. It can operate in various modes depending on the direction of power flow, availability of energy sources and energy management scenarios. The residential load can be supplied by renewable sources or battery via MPC and the three phase inverter. It also can be connected to the grid via a bidirectional meter. Each house has such RES as a member of smart grid and is connected to the smart grid control center via a wireless communication network and transceiver unit. The RES includes three local controllers which operate in device level to control MPC, three phase inverter and bidirectional meter and load management sections. The control part of RES includes local controllers at device level and a global controller at system level. The global controller selects the operation modes on the basis of existing renewable sources, predicted load demand, solar energy and the objectives of energy management. It also is designed to communicate with smart grid control center and receives the predicted daily load demand and solar energy profiles and sends the required command signals to local controllers. Energy management of residential house especially in islanded
I
n the last decades, there has been a great interest in renewable sources of energy as feasible solution to mitigate environmental issues and reduce the dependence on traditional sources of energy for electrical generation. The need of technology for integrating these non-traditional types of energy into the existence grid has motivated the development of new smart grid concept. The Smart Grid is recognized as feasible solution to the new challenges of existence grid network such as increasing energy demand and penetration of renewable energy sources at the consumer end [1]-[3]. On the other hand residential consumers as an important part of future networks should be able to integrate their renewable sources such as wind turbine and PV into the grid. Multi-port converter (MPC) is able to integrate multiple energy sources and storage devices into a single power processing unit [4]-[7]. A topology of MPC is proposed in this paper combining a common dc and magnetic buses including four ports connecting to the solar PV panel, fuel cell stack, battery and load. A group of lead-acid batteries connected in series and parallel are selected as energy storage in this system. It is required to calculate the appropriate capacity of battery bank for RES to store the surplus renewable energy and manage the energy distribution efficiently. Lead-acid batteries are have been used in renewable energy systems as a mature technology [8]-[10]. The main characteristics of lead-acid batteries and their application in RES are studied in the literature and charging methods are discussed by authors [11]-[14]. In this paper a simple method of defining appropriate battery capacity is
978-1-4673-9130-6/15/$31.00 ©2015 IEEE
mode, bidirectional measurement of energy flow and break down the system in overload and emergency conditions are other responsibilities of global control unit. As can be seen the lead-acid battery is connected to the port two of MPC via a bidirectional buck-boost converter. Based on this structure the battery is charged or discharged into the dc bus of port two to other sources and load. The bus is connected directly to fuel cell stack and via H-bridge port two. Following part explains the simple way of estimating basic stored energy demand using predicted load and PV profiles. III. DEFINITION OF BASIC STORED ENERGY DEMAND A 24-h ahead forcasted PV power and load demand is provided by smart grid control center based on weather forcasts, historic data base and meteorologocal information as shown in Fig.2. Assuming an energy management secnario that compares the predicted load demand with predicted solar energy for a 24 hours time period. The load demand should be covered by solar energy as preferred energy source. The difference depending on the amount of energy should be covered by battery, fuell cell or grid.
(c) Fig.2. 24-h ahead Forcasted (a)-PV power and (b)-load deamnd (c)-the difference power
The predicted load demand and solar power for the 24-h ahead received from smart grid control center can be used to define the PV and load forcasted energies for each ½-h time priodes accoreding to (1)-(2).
~ EPV _ 1 / 2 h (n) =
t0 +( n+1) Te
~ E LD _ 1 / 2 h (n) =
t0 + ( n +1)Te
~
∫P
PV _ 24 h
(t )dt
(1)
(t ) dt
(2)
t0 + nTe
~
∫P
LD _ 24 h
t0 + nTe
T e = 30 min
n ∈ {0 ,1, 2 ,..., 47 }
Using (1)-(2) the difference between predicted load demand and solar power can be calculated for each half an hour time period as ~ ~ ~ (3) ΔE1 / 2 h ( n ) = E LD −1 / 2 h ( n ) − E PV −1 / 2 h ( n ) The total value of estimated PV generated energy, load demand and the difference for 24-hour can be calculated using (4)-(6).
(a)
(b) Port 3
Port 1
Port 2
Bidirectional Meter / Brea ker Load Management
Three
Rfc
Vfc
Q1
D1
Vbt
Q2
Q9
C
D2
Phase Inverter Rectifier
Q4
Q6
Q10
Q8
Central
C Q12
Q14
Lowpass Filter
Re sidential Load
Q13
Q11
VPV
Q7
C
L
Rbt
Q5
Q3
Controller
Port 4 Fig.1.Schematic of proposed multi-port converter
To Microgrid
47 ~ ~ E LD − 24 h = ∑ E LD −1 / 2 h ( n)
(4)
47 ~ ~ E PV − 24 h = ∑ E PV −1 / 2 h (n)
(5)
n=0
n=0 47
~ ~ ~ ΔE 24 h = ∑ [ E LD −1 / 2 h ( n ) − E PV −1 / 2 h ( n )] (6) n=0
The value of stored energy demand depends on the scenario that is considered for RES operation. In our case we assumed that the energy differences less than PREF should be covered only by battery. When the energy difference is more than PREF and less than PFC (max) the difference should be covered only by fuel cell. In case of difference more than fuel cell maximum power, grid covers the difference automatically. This can be mathematically presented as: ~ ⎧ PBT−1/ 2h (n) ⎪ ~ ~ PFC−1/ 2h (n) ΔP1/ 2h (n) = ⎨ ~ ⎪P (Max) + P GD−1/ 2h ⎩ FC
~ ΔP1/ 2h (n) < PREF ~ PREF < ΔP1/ 2h (n) < PFC (Max) ~ PFC (Max) < ΔP1/ 2h (n)
(7)
Based on this assumption the value of energy should be supplied by fuel cell and battery can be defined using (7)-(10). m ~ ~ E FC − 24 h = ∑ T0 PFC −1 / 2 h ( n )
(8)
~ ~ E BT − 24 h = ∑ T0 PBT −1 / 2 h ( n )
(9)
n =1 K
liquid. The chemical reaction at negative (Pb) Electrode can be presented as
Pb + SO4−2 → PbSO 4 + 2e The charged sulfate ion approaches uncharged lead electrode surface, dipole attraction kicks in on close approach and lead atom becomes ionized and forms ionic bond with sulfate ion. The result of this reaction is release of two conducting electrons which give a net negative charge to lead electrode. On the other hand the reaction at positive (Pb) Electrode can be concluded as
PbO2 + SO4−2 + 4 H + + 2e − → PbSO4 + 2 H 2O At this electrode the charged sulfate and hydrogen ions approach lead-dioxide molecule (net uncharged) on surface of electrode and lead atom changes ionization and forms ionic bond with sulfate ion. As a result two water molecules are released into solution. As a result of above mentioned reactions two electrodes are coated with lead sulfate (a poor conductor) and this reduces the concentration of the acid electrolyte. The resulted charges on both positive and negative leads produces a voltage difference across the cell terminals. The resulted open circuit voltage under standard conditions (T = 298°K and 1 molar acid electrolyte) is equal to 2.04V as can be seen in Fig.4.
n =1
~ ~ ~ ~ EGD − 24 h = Δ E 24 h − E BT − 24 h − E FC − 24 h
(10)
where m is the number of half hour time periods that the difference power is higher than PREF and k is the number for less than PREF . The calculated value of battery energy resulted from (9) is called the basic stored energy demand and is used to calculate the battery capacity in the next stage. IV.TECHNICAL ISSUES OF USING LEAD-ACID BATTERIES Lead-acid batteries have been used as energy storage in many electrical industries during the last decades. They are used in a wide range of renewable energy systems to overcome the intermittent nature of renewable resources. The electrical energy stored in a lead-acid is resulted from a chemical reaction that can be briefly as below. The structure of one lead-acid battery cell is presented in Fig.3.
Fig.3. Structure of a lead-acid battery cell
As can be seen the cell is contains of two positive and negative electrodes floating in sulfuric acid as electrolyte
Fig.4. Chemical reaction at lead-acid battery cell and resulted voltage
Several cells are connected in series to form a battery pack. The main advantages of lead-acid batteries compared with other type of batteries are illustrated in Table. I. As can be seen they are relatively mature technology and have been used in electrical industries since 1970. Compared with other batteries they have lower price and need less maintenance which makes them a good choice for low cost renewable energy systems. Looking at the table shows that their selfdischarge is low and they are tolerant against overcharge. They have higher charge and discharge efficiency thanks to their low internal resistance. They also have some disadvantages compared with other batteries such as low energy and power density and life cycle compared with other batteries. Their charge time is relatively long compared with other types of batteries. Table .I shows the main characteristics of five types of commonly used batteries including lead-acid. The positive and negative points of lead-acid batteries are compared with other types. The positive points are enclosed by dashed line and the negative points by solid line.
V. TECHNICAL ISSUES OF LEAD-ACID BATTERIES This section provides a review on main factors that are effective in estimation of required battery capacity. The factors should be considered along with the basic stored energy calculated at previous section IV to guaranty an efficient energy management of RES. TABLE.I CHARACTERISTICS OF VARIOUS TYPES OF BATTERIES
Gravimetric Energy
Ni-Cd
Ni-MH
Lead Acid
Li-ion
Reusable Alkaline
45-80
60-120
30-50
110-160
80 initial
100200 6V Pack -40 to 60Co 1.25 V
200-300 6V Pack
5C 0.2C
>2C
