It is, in fact, a VRLA battery of tubular battery plate construction, but which recombines hydrogen and oxygen using an immobilised electrolyte. In this case, the electrolyte is immobilised using fumed silica to turn the liquid electrolyte into a solid gel.
This is in contrast to the other lead acid VRLA battery range which uses a glass mat of very fine fibres to absorb the acid-like blotting paper and immobilise it in this way. This range of VRLA batteries is known as AGM (Absorbed or Absorptive, Glass Mat). This glass mat technology depends upon having a uniform pressure across the face of the mat, otherwise, the gas recombination process will not work.
For this reason, it is unsuitable for a tubular positive plate construction and is only used for batteries with flat positive plate designs.
The two important features of OPzV battery cells are the tubular plate construction and the immobilised (GEL) electrolyte. The tubular positive plate gives the advantage of extra acid contact for the PAM via its rounded, rather than flat shape as shown in Fig. 1 From this, it can be seen that the additional contact area is approximately 15% compared with its flat plate counterpart.
Fig-2-Typical stationary
OPzV battery bank in steel rack.jpg
Figure 1 Additional acid area in contact with tubular plate surface.jpgThis better utilisation results in a higher energy density, whilst the gauntlet holds active material firmly against the conductor to minimise the battery resistance and prevent loss of PAM from shedding during deep cyclic operations.
The immobilisation of the electrolyte in the OPzV battery has the dual benefits of allowing operation of the cells in different orientations without spillage and also it enables the gases produced by electrolysis of water on charge to recombine and prevent the water being lost. Fig. 2 is a typical installation in a stationary application. The ability to store cells on their sides enables a space-efficient racking system and allows easy access to battery terminals for maintenance checks.
The recombination aspect is critical to many, particularly remote stationary installations. It means that battery maintenance can be carried out at much-increased intervals since no water topping up is required. It also removes the need for expensive ventilation equipment which is designed to remove potentially explosive gases produced when the battery is being charged.
The problem of gas evolution with flooded cells derives from the electrochemistry of the lead-acid battery. The production of hydrogen and oxygen can occur at very low cell voltages. Fig. 3 shows the relationship between the rate of gas evolution and lead-acid cell voltage.
Fig 3 Oxygen and hydrogen evolution as a function of cell potentials
Fig 4 Oxygen recombination with hydrogen in a VRLA cellIn this diagram, both the positive and negative plates are shown as single potentials and the difference is the overall cell voltage. As can be seen, even at 2.0 volts per cell there are measurable quantities of gas evolved from a flooded system, and at 2.4 VPC on a charge, the water loss and gas generation are considerable. For this reason, a recombinant design of the cell is the best way to ensure a safe installation with minimal or no water loss during normal cycle duties.
To understand how a gel battery is able to facilitate a recombination reaction, we need to look at the structure of the gelled electrolyte when it is in service. First, however, a knowledge of the reactions causing water electrolysis followed by hydrogen and oxygen evolution (gassing) would be useful.
The breakdown of water due to electrolysis is fairly straightforward:
Overall 2H2O → 2H2(g) + O2(g)
Positive 2H2O → O2(g) + 4H+ + 4e– (oxidation)
Negative 2H+ +2e– → H2 (Reduction)
In both cases for cathode and anode there is a release of gas due to the electrochemical action of either adding electrons (negative electrode) or removing electrons (positive electrode). The method by which the gases, or ions can recombine to form water is not completely understood and there is more than one explanation. The most widely accepted is:
O2 + 2Pb → 2PbO
2PbO + 2H2SO4 → 2PbSO4 + 2H2O
2PbSO4 + 4H+ + 4e– → 2Pb + 2H2SO4
In this model, it is necessary to persuade the gaseous oxygen produced on the positive, to travel to the negative plate. This would not happen in a flooded lead acid cell with a liquid electrolyte.
When oxygen and hydrogen are produced in a liquid electrolyte, they form bubbles which rise to the surface, then into the headspace of the cell and are ultimately released into the atmosphere. The gases are not then available for recombination. However, in a gelled electrolyte, a recombinant action is created by the drying out of the GEL which forms small cracks and fissures in the structure. In this case, the oxygen formed from water electrolysis is able to migrate from the positive to the negative electrode, due to the pressure created by the gas evolution.
Small cracks and fissures are able to store the gases which then migrate by diffusion through the gel to other voids in the matrix until the distance between the electrodes is filled with gas (Fig. 4). The recombination reaction, however, is relatively slow compared with the evolution rate, which means the internal pressure of the cell increases during charging. The gases are prevented from venting out by the pressure relief valve, keeping them available for recombination after the charging process has ended.
The two main features which characterize this range are, firstly, it recombines the hydrogen and oxygen produced on charge, back to water within the electrolyte making it essentially maintenance-free and safe in enclosed spaces.
Secondly, it has a tubular positive plate which imparts greater active material retention under deep discharge conditions to provide a longer cycle life. The OPzV battery range is essentially a deep discharge, high cycle life, maintenance-free lead-acid battery. Because of its immobilised electrolyte, it also has the benefit of being able to store it on its side whilst in operation, without acid leaking from the vent. In essence, this orientation makes the battery a front terminal design, providing similar operational benefits in addition to its other advantages.
However, there are downsides to these two advantages: the high deep cycle life does come at the expense of high rate discharge, or cold-cranking ability, both of which are significantly lower when compared with its AGM flat plate counterpart. The gas recombination is considerably slower than the rate of gas generation. For this reason, the charging process takes longer than a flooded cell, typically up to 15 hours.
Bearing in mind the above discussion, it is fairly clear that this design of the OPzV battery is most suited for those applications where there is difficulty in maintaining the battery and it is required to have frequent, perhaps regular deep discharges combined with a long calendar and cycle life. Because of its relatively low CCA performance, the discharge profile would typically be current draws of 0.2C amps or less over a period of several hours. Although it is fair to say that OPzV battery and cells can provide intermittent, reasonably high discharge currents of up to 2C amps during a normal duty cycle.
The recharge time, which is typically 12 to 15 hours to recharge a battery, limits the amount of gas that can be produced on charge. This is achieved by charging with a voltage limit, typically 2.23 to 2.45 volts per cell. Fig. 5 shows a typical charging profile for an OPzV battery. This reduces the current going into the battery and consequently extends the charging time. This is also an important factor when considering different battery markets and their operational profiles. With these considerations in mind, the most suitable application for the OPzV battery is predominantly heavy duty and industrial.
OPzV batteries offer sealed maintenance free tubular gel battery performance. While the OPzS battery in SAN containers require very minimal maintenance throughout its designed 20 year life on float applications.
An OPzS battery is housed in a transparent SAN (Styrene Acylonitrile) container. The OPzV battery is housed in an ABS (acrylonitrile butadiene styrene) container. Which is not transparent, yet very sturdy and will not bulge. The transparent SAN container is necessary in mission critical applications. OPzV batteries are usually installed in remote locations where periodic annual topping-up poses a challenge.
Fig 5 Recharging OPzV at 2.4 VPC
Fig 6 Stationary markets overviewLooking at the broad categories in both market sectors, we have:
• Stationary
– Solar power: diesel hybrid, off-grid generation and storage, domestic storage
– BESS
– Standby Power
– UPS
• Rail (Rolling Stock applications)
– Emergency lighting
– Diesel locomotive starter
– Signalling
Motive Power
• Traction
– Warehousing: Forklift trucks, electric hand trucks, AGV
– EV: Golf cart, Rickshaws
• Leisure:
– Marine
– Caravan
– Camping
Of the above-listed applications, it is those which require frequent deep battery discharges, with time to fully recharge, for which the OPzV battery is best suited. In a stationary battery application, it would be solar power, BESS and standby power which ticks all the boxes.
For railway applications, the train lighting and air conditioning battery and railway signalling battery are the best applications for OPzV battery. The railways need a deep cycle battery which is capable of deep discharge cycles in times of power outages. This is best provided by a tubular battery plate and not a flat plate battery. Considering the huge network of operations of the railways, a maintenance-free battery like the OPzV battery would be a boon to the railways.
The OPzV battery range is not suited for Traction applications such as golf cart batteries & forklift battery. There are practical considerations such as the use of breakable ABS containers instead of the polypropylene cases used in forklift battery for example. Non-flexible ABS cell jars would easily break if it were to be tightly packed into the steel battery trays of forklift trucks. The Gel OPzV battery design calls for more volumes of active materials which will increase the standard dimensions of a forklift battery.
The leisure market generally opts for lighter weight and higher energy density monoblocs, particularly for caravan and camping applications. The same is generally true of the marine battery applications, which apart from electric boats, uses marine batteries for broadly similar uses of refrigeration, navigation and lighting, and also as with camping, there is limited space for battery storage.
The major use for OPzV battery is the stationary battery market. The common thread throughout all of the subdivisions in this sector is that the location of the batteries is fixed. Fig. 6 gives a breakdown of the industrial battery market with the main stationary applications of telecoms, UPS, standby power and battery energy storage systems (BESS), having about a 90% of the share of a 15 billion USD global market. Unlike the traction, leisure and rail applications (excepting signalling) the stationary battery stay fixed in a single location and are generally hard wired into a power supply system. However, the similarity ends there.
Some applications such as UPS in telecoms and load levelling/frequency control in BESS will require brief or short discharges of high power at random intervals, spending a high proportion of their life on a charge, whilst others such as solar and standby power will be deeply discharged at regular intervals.
For this reason, OPzV battery is most suited for those sectors of the stationary market that are deeply discharged, regularly or randomly, but certainly frequently. In this category, we can include all solar power installations with larger-scale diesel/solar hybrid installations being the ideal candidates for the longer-lasting more robust construction of the OPzV battery.
The maintenance-free aspect of the OPzV battery is important here, particularly in remote areas where topping up of batteries would be extremely expensive and add to the cost, thereby reducing the ROI to the provider. Similarly, domestic installations benefit from the lack of expertise required in maintaining battery electrolyte levels. Overtopping, topping up at the wrong State of Charge (SoC) of the battery and even neglect are common features in domestic battery usage.
Of all the stationary categories, it is perhaps the burgeoning ESS market, which some consider will reach 546 billion USD by 2035, which offers the most opportunities for exploitation of the OPzS design. Table 1 lists the diverse outlets of batteries within the category of BESS whilst Fig. 7 gives a chart of the global storage capacity by primary use. Of these, demand response and energy sales are the most likely uses where regular deep discharges would be required. In all of these cases, it is likely that the installations are around 1 MWh or greater, located near power stations or distribution substations and operated either automatically or remotely.
Table 1 Commercial use of BESS at utility and behind the meter scales
| Value Stream | Reason for dispatch | Value | Who? |
|---|---|---|---|
| Demand charge reduction | Reduce load - peak shaving | Lower bill by reducing demand charges | Customer |
| Time of use/Energy arbitrage | Battery dispatch during peak periods when energy costs are high | Lower retail electricity bill | Utility or customer |
| Capacity/demand response | Dispatch power to grid in response to events signaled by utility or ISO | Payment for capacity service | Utility,customer, DR agregator |
| Frequency regulation | Battery injects or absorbs power to follow a regulation signal | Payment for regulation service | Utility, ISO, Third party |
| Energy sales | Dispatch during times when locational marginal prices (LMP) are high | LMP price for energy | Customer, third party |
| Resiliency | Battery dispatch to provide power to critical facilities during outage | Avoided interruption costs | Utility, ISO, third party |
| Capital deferment | Support voltage or reduce load locally | Prevents costly infrastructure upgrades | Utility, ISO |
Fig 7 Global battery storage capacity by primary case use
Fig 8 India’s cumulative installed power capacity mixAnother, as yet limited application is that of EV charging stations. There are many advantages to having a BESS alongside the grid supply.
For all of these reasons, a maintenance-free, deep discharge OPzV battery with a high cycle life is the best option. Added to this is lead acid’s low cost/kWh, making this design of an OPzV battery and chemistry an ideal option to achieve a good ROI and low capital cost option for BESS stations and substations.
Renewables
A major part of the BESS market is that of renewables. Naturally occurring sources, predominantly solar and wind power are making fast progress in becoming major contributors to many countries’ total energy production. Fig. 8. Shows India’s current proportion of installed energy generation with renewables at over 35% of the total power supply. Of all the renewable energy sectors, the fastest growing technology is probably solar energy. .
The solar energy capacity increased by around 24 per cent in 2018 with Asia dominating the global growth with a 64 GW increase (about 70% of the global expansion in 2018). Both wind and solar are ideal candidates for energy storage as they cannot be switched on and off to order. The International Renewable Energy Association (ARENA) predicts that PV will reach 8519 GW by 2050, becoming the second-largest global source of power Fig. 9. The trend is considered to be true for both on and off-grid applications with domestic installations growing at around the same rate as industrial and grid-scale enterprises.
Yes. Gel batteries are good for solar applications. This is because of the following characteristics
They are sealed maintenance free batteries
Wide operating temperatures of -20°C to 55°C
Does not get affected by acid stratification
Grid corrosion is minimal
Premature capacity loss (PCL) is lower compared to AGM VRLA
Fig 9 IRENA projection to 2050 for PV installed capacity in total Renewable Sources
Fig 10 Site power requirements for Telecom installations for 2G 2 – 4G and 5G according to HuaweiThe most variable is obviously wind energy, and the ability to store energy when it is generated and release it when required is a major advantage. Use of stored energy allows peak demand periods to be satisfied even if the wind is not blowing nor the sun shining. It can mean drastic reductions in capital investment for energy generation. Most countries have a peak power demand of around 3 to 5 times the background usage for just a few hours a day. In the UK, for example, the peak demand in the morning and evening is around 69GW for approximately 2 hours.
This contrasts with a steady underlying demand of 20 to 25 GW for the other 20 hours of the day. Instead of having energy generators lying idle for long periods due to overcapacity, it makes sense to have fewer wind turbine generators operating at full capacity, all day, storing their energy in batteries, for use at peak demand times.
Telecommunications and Standby power.
Currently, telecommunication towers account for around 1% of global energy use. With off-grid towers being constructed at a rate of 16% per year, there are challenges for providing safe, consistent power whilst reducing CO2 emissions. For this reason, off-grid power solutions combining diesel generators, batteries and solar panels are increasing. Rising fuel costs also contribute to high operating expenses. If we add to these the increasingly restrictive governmental and environmental regulations, then a global situation arises where the use of diesel will be restricted, paving the way for use of renewable energy and therefore battery storage.
Typical remote telecommunication towers will be powered by hybrid energy systems of diesel and solar power where the use of batteries to store solar energy will reduce diesel fuel consumption. Depending on the size of the station, 100% solar power can be used with battery storage to enable nighttime use. However, not only are more towers being constructed but also energy demands per station are also increasing particularly with the introduction of 5G networks Fig. 10. Maintenance-free OPzV battery offers significant benefits in terms of cost per cycle and also provide the highest level of reliability and performance in remote telecom installations. Typically, these stations will require frequent, long periods of battery discharge without maintenance or regular checks.
Leisure
The remaining categories of leisure and rail have some unique aspects. Both of these have vehicles that carry the battery which is used as a source of power for lighting and other support systems. In most cases, the battery is not the source of power to move the vehicle, but it is still regularly deeply discharged. In the case of marine use, it may be for the navigation system or refrigerator on board a boat and is recharged from a diesel engine or solar panels depending on the boat design.
However, for electric canal boats, for example, it would be a traction application with identical usage patterns to an FLT or EV. In all cases the deep discharge and long cycle of the OPzV battery combined with the lack of maintenance are the properties which are required for these applications.
Railway energy requirements are difficult to categorise under most standard headings. However, within that group, there is the category of stationary signalling. This effectively has the same battery requirements as that of solar power. The category of train lighting battery & air conditioning battery, although on a moving platform, has a similar deep discharge requirement but is irregular and unpredictable, and therefore has similar requirements to standby power applications.
For this reason, deep discharge OPzV battery is the most suitable choice for train lighting battery & air conditioning battery, particularly as they do not need expensive maintenance and will avoid the possibility of damage resulting from poor maintenance. The other railway category of diesel starting is closer to an SLI rather than an industrial requirement and OPzV batteries are not ideal for this use. In diesel-electric locomotives, there is a separate diesel locomotive starter battery.
The battery applications so far discussed are based on current market requirements. There are, however, emerging applications for electrochemical energy storage which are yet to be commercially introduced. One new requirement is that of EV charging stations. There are several reasons why battery energy storage would be of benefit in this application. Firstly, there will be high output surges, probably greater than the incoming supply, due to fast and multiple charging of EVs. In this case, the use of stored battery energy would reduce the demand on the grid supply meaning a smaller electricity sub-station requirement and a lower capital cost.
Secondly, peak demand charges could be avoided due to using stored battery energy for the demand peaks which would result in a constant, low power draw from the grid. Thirdly, battery storage would also enable the use of variable renewable power sources, by storing energy when it is generated from PV arrays or wind turbines and using this energy to supplement the grid supply. All of which considerably reduces both capital outlay and operating costs.
Another possible OPzV battery application derives from the opportunity to use power generation from telecom towers by building excess renewable capacity into them and selling power to surrounding communities via mini-grids. This would not only help to mitigate the cost of building and operating telecom towers by having an additional revenue stream for the provider, but also enable countries with an underdeveloped grid network to provide much needed electrical power to remote communities.
In all the OPzV Gel battery applications discussed, it is the structure, chemistry and design of the OPzV battery which provides the key to satisfying the market requirements. The use of lead-acid chemistry, with the high cycle life, low capital and running costs and virtually zero maintenance characteristics of this technology, make the OPzV battery range a logical if not unbeatable choice for most stationary applications. In tandem with this, the materials, design and quality of construction are of equal importance. All have to be of premium quality in order to ensure that the plate can withstand the daily expansion and contraction of the Positive Active Material (PAM) when the OPzV battery is discharged and charged each day.
