Evaluation Of Zn-Br Flow Battery, Li-ion Battery System And Pb-acid Battery For Solar Households

You are working as an engineer for a PV company in Brisbane. The PV company is interested in providing a battery option for their customers and would like you to evaluate what makes the most sense for their customers. The average customer has a 5 kW PV system for a household of 4 – 2 adults and 2 children. They have already narrowed down the options to – Zn-Br flow battery, Li-ion battery system or a standard Pb acid battery.

Design methodology

The reaction in the battery cell is a reduction-oxidation (Redox) where there both oxidation and reduction of chemical reaction occurs at the same time. The anode is electrode where the oxidation occurs, Cathode is the electrode where the reduction occurs, an electrolyte is a liquid where the current flows internally and finally, the external circuit which is basically electrical conductor which permits the flow of electrical currents generating energy in a galvanic cell (Butler, 2011). The Electrical power in the battery is generated from the two combined half-cell equations of zinc and bromine. And the reaction at the anode is referred to as oxidation since the zinc metal is taken into the electrolyte and form two zinc ions.

Zn (s)            Zn²⁺ (aq) + 2e^- ( aq)

While on the cathode side the reaction is referred to as reduction since the Bromine ion is reduced from two anions to zero (bromine liquid) as seen below;

2Br^- + 2e^-            2Br (l)

 The above two half-cell equation are combined make one general equation which then helps to generate electrical energy and the storage of electric power (Mbadi, 2010). 

Zn ( s) + 2 Br^- (aq)             Zn²⁺ (aq) +  2 Br (l)

 For this design the cell reaction is separated by a membrane or a salt bridge represented by the ||, the electrodes always appeared on the outer side while the reactions of the electrolyte are on the inner side (Johnston, 2010).  These two phases are separated with |. The species in the same state is separated with a semicolon (;) and the concentrations are shown in ().  A summarized Notation is illustrated below   

Zn| Zn²⁺ || 2 Br^- |2 Br

In this type of battery, we can use any strong acid as the electrolyte, for example, Sulphuric acid (H₂SO₄).

A battery is like a capacitor having two plates having negative and positive charges (being at the cathode and anode) separated by a distance d as shown in figure two below (Dubey, 2014);

The concept of charging and discharging a capacitor will clearly explain how and that this battery can be recharged. As in capacitors the following expressions are employed (Shaw, 2013);

V =   

 K = ?r/?0

C = ( ?0 A k )/ d

 Q= CV = ( ?0 Ak )/d

 Where;  ?r is relative permittivity of dielectric material ?0 is relative permittivity of vacuum k is a dielectric constant of a material ( dielectric ) and C is the capacitance given in Farads ( Coulomb/ volt).

This battery also employs the concept of the capacitor to help store electric energy (Mubarak, 2010).   As in the capacitor electrical energy (electric potential) which is simply work required to move one unit charge (q)

V = E/q

Electrostatic potential energy = dE/Vdq

And the Capacitance in capacitor, C= Q/ V

Therefore the energy in a single cell E= ½ C (V cell) ²

 During charging of this battery an external source of voltage will pull electrons from the negative electrode of the battery (cathode) via an external circuit (conducting wires)  to the positive  electrode ( anode) and this will make Li-ion  flow from the negative terminal( cathode)  to the positive terminal ( anode ) (Deambi, 2011). This movement occurs within the liquid electrolyte Sulphuric acid (H₂SO₄).  While during the discharge of this battery, the process of charging is reversed (Spagnuolo, 2013).  The Li-ions travel from the anode and cathode via the liquid electrolyte Sulphuric acid (H₂SO₄). And the electrons flow via the external circuit (conducting wire) to the cathode from anode resulting in the generation of electrical power (Sonnenenergie, 2012). 

Working mechanism of this battery

The basic unit of this battery are the cells, so the first design consideration should focus on the cells of the battery. The electrochemical reaction that releases and stores energy occurs in the cells which include the separator, bipolar electrodes, electrolyte storage and the aqueous. 

In this design we have to we have to consider some basic factors of the battery like the battery type, battery bank DC voltage, type of the charge controller, module type among other specifications. For this design, we will use the battery type as a flooded type or wet cell. The battery bank Dc voltage used will be 48 V voltage DC. 48 type is employed since it gives an output power of more than 2kW (Snow, 2013).

The number of battery required for this design is obtained as follows; From the above Net cell reaction of a given cell we can see that we produce 1.85 V between Zn and Br, therefore a single battery of 26 cells is used to produce;

1.85*26 = 48.1 voltage for a single battery. These cells are connected in series to ensure that the overall voltage is the sum of all the voltage produced by each cell.   Each battery is assumed to give about 14 A. This will give a power of 48.1× 14 = 673.4 the banks of the batteries are arranged in series so the total number of batteries needed to be connected in series is given below (Solanki, 2014);

673.4n= 5000 watts

n= 5000/673.4

n= 7.425

This can be rounded off to 8 batteries put in series.

The number of the batteries = 8 (The number of batteries is surrounded off to the next number since there are losses between a battery to another. The diagram below shows how the batteries can be arranged in series;

This battery with the selections of 26 cells per battery with each battery producing 48.1 Voltage DC, will be installed in the household to serve the six individual of 4 adults and 2 children.  For this 26 cell module design, maximum power point tracking controllers (MPPT) will be employed.  This is because these controllers have optimum energy harvest as well as wider input range of voltage which is between 48 voltages to 60 voltage even though they are relatively expensive.  For this design, the amount of electricity consumed per day is of a great importance (DeBlasio, 2012).  

The cathode and the anode are made from a collector of particles of powder that are bonded together into a three-dimension electrode.  During the discharging, the battery begins in the bulk of anode particle, then undergoes diffusion of solid-state particles and at the surface, it dislodges the electrons enters the electrolyte that occupies the pores of the electrode. The ion will then be moved via the electrolyte (diffusion) to the cathode (Sayigh, 2012). As the ions enter the cathode it will undergo solid state diffusion in the cathode.  The electrons will have to go through the collection of the solid-state particle to the collector at the metal current where it is extracted and employed to power the device.

 Zn/Br batteries usually discharge at a relatively low rate of 15-30mA /cm². A discharge – charge profile of 26 cell stack is illustrated in the diagram below. The amount of the charge is put on the zinc loading which is well-defined at 100 %. The amount is always lower than the total Zn²⁺ dissolved in the electrolyte charge duration and efficiency of charging are obtained at the end of the charge.

The design will work best for this small household of size individual with 4 adult and 2 children, but when more electrical power is required then  I would recommend the addition of cells in the battery or designing producing more batteries which are connected in series to the already 7 connected ones.   This will ensure that the amount of power produced will highly increase.  And since these batteries are charged using PV the solar which charges them should be exposed to the sunlight to attain the optimum design specifications of 5000 watts.

 For the safety considerations of this battery is that at high temperature, some battery components and cells will break down and this will result in an exothermic reaction which in turn may cause accidents.  To avoid this accident from occurring I would recommend that if this battery is used in a hot environment then the battery must be put under the shade a place which is cool and dry all the times. The cost of all components required for the design of this PV battery system is relatively cheaper as compared to paying the bill monthly. Averagely the household can use about 65kWh per month which results to   70*7.5 = 525 cents, this will actually make it easier to use this designed battery.

Conclusion

In summary, with the design of this PV battery system will highly improve the storage of energy which may be lost after generation from the solar panel. This is realized by the equations of the capacitor for the energy storage.  This design of a rechargeable battery basically obtains its power from PV solar during charging.  The batteries after the design are arranged in series to ensure that the total power is the sum of respective batteries.  Since in the battery there are cells which operate like a capacitor (two plates of opposite charges), it is possible to charge these batteries by employing the charging of the capacitor and storing the electrical energy in form of charges using the below equations.

Electrostatic potential energy = dE/Vdq

And the Capacitance in capacitor, C= Q/ V

Therefore the energy in a single cell E= ½ C (V cell) ²

References

Butler, P. C. (2011). Zinc/Bromine Batteries. Chicago: IEEE.

Deambi, S. (2011). From Sunlight to Electricity: A Practical Handbook on Solar Photovoltaic Applications. Manchester: The Energy and Resources Institute.

DeBlasio, R. (2012). Performance Criteria for Photovoltaic Energy Systems. Stoke: Solar Energy Research Institute.

Dubey, S. (2014). Fundamentals of Photovoltaic Modules and Their Applications. Washington: Royal Society of Chemistry.

Johnston, W. (2010). Battery Storage Design for. Manchester: Newness.

Knibbe, R. (2012). Electrochemistry . Hull: Newness.

Konarova, M. (2011). Electrochemistry & Corrosion. London: Adventure press.

Luo, B. (2013). Nanomaterials for Energy Storage. Sydney: Australian press.

Mbadi, J. (2010). Battery storage trials in Queensland. Chicago: Newness.

Mubarak, A. (2010). DESIGNING OF A PV/WIND/DIESEL HYBRID ENERGY SYSTEM. Stoke: Lulu.

Sayigh, A. A. (2012). PV battery design for engineering works. Hawaii: Newnes.

Shaw, M. (2013). Introduction to Photovoltaic System Design. Chicago: Jones & Bartlett Publishers.

Snow, M. (2013). Designing with Solar Power: A Source Book for Building Integrated Photovoltaics (BIPV). Chicago: Routledge.

Solanki, C. S. (2014). Battery design system: A Manual for Technicians, Trainers, and Engineers. Tokyo: HI Learning Pvt.

Sonnenenergie, D. G. (2012). Planning and Installing Photovoltaic Systems: A Guide for Installers, Architects, and Engineers. Amsterdam: Routledge.

Spagnuolo, G. (2013). Power Electronics and Control Techniques for Maximum Energy Harvesting in Photovoltaic Systems. Hull: CRC Press.