The Future of a Smart Battery Energy Storage

Joseph Cilia (jcilia@um.edu.mt, joseph.cilia@abertax.com) , Klaus Dieter Merz (kd.merz@abertax.com), Malcolm Tabone (malcolm.tabone@abertax.com), Neville Azzopardi (neville.azzopardi@abertax.com), Björn Mentzer (bjoern.mentzer@abertax.com)

 

Abstract

This paper is based on research carried out on the sunny island of Malta. With oil prices being high and likely to further increase and PV installations now much more affordable, PV generated energy already reached grid parity. A smart system of batteries combined with PV, self-consumption and the option to trade energy with the grid promises to become profitable indeed.

This paper calculates cost and return on investment of such a system. It also discusses the technologies required to take ful l advantage of smart batteries controlling PV energy, its self-consumption, storage, selling to- and supplementing from the grid.

 

 

 

Introduction

The recent drop in PV component prices, particularly panels and the continuous increase in oil prices will create a new market for battery energy storage. During the last year, the price of PV panels has dropped to an interesting value. This was a shock to some and reactions are still ongoing, with European PV manufacturers even going to court claiming that China is creating unfair competition by subsidising the PV market. However, when one looks at the financial side of things, one just needs to take the right decisions and moves, to turn the situation favourable again. Let’s take a simple example : assuming that a year ago the PV panel cost was 300 € and now it is 150 €. The reaction of some European countries was to immediately

 

drastically drop the feed in tariff from 0.40 € to 0.20 € or less – i.e. removing all the support to promote renewable energy that they set to meet their own renewable targets. The obvious move was to shift the support from the renewable energy generated to the European PV production. But this is called subsidy in Europe and therefore cannot be done, even if this means subsidising the PV products from China as was actually the case till a year ago with the extra 0.20 € for the renewable energy generation!

 

                 Figure 1: Where does the battery stand when PV reaches grid parity?

 

There is only one explanation to the above; i.e. the price of PVs has reached grid parity and some will say Bingo! While others will go Bang. And where is the battery in all this? The battery, as always in the last century, is there to support the oil or nuclear fired engines with all their pros and cons. Starting with the car we use and ending up with the electricity or heat we need, even if this means killing the air we breathe!

The authors see a bright future to a battery which is there to support renewable energy [1], [2] and use oil or nuclear fired engine as an emergency backup. I.e. turn the battery role around. Some questions crop up, such as; what is required for such a change in role?

 

The authors carried out a number of studies on the island of Malta and are actively working on components that will help the battery industry to adapt to this change.

 

 

 

Energy Generation on small islands

From a review of 65 small island power systems, it is clear that heavy fuel oil and light fuel oil are the only fuels being used. This makes small islands’ economies very vulnerable to variation in oil prices. Apart from this, islands within the European Union such as Malta and Cyprus have to meet their respective targets for energy generated from renewable sources.

In order to evaluate the existing situation of the power generation for small island systems, the Nesis data book [3] was analyzed and the following conclusions were deduced: 

• All islands run on HFO/LFO In most cases the prime mover is an ICE run from diesel  
• The medium sized islands have a steam turbine and / or a gas turbine 
• Some islands have a small percentage of hydro power 
• In rare cases there is a very small percentage of energy from wind 

 

It is very clear that for both islands the electricity cost depends on the price of oil which has been fluctuating over the past years creating at times high fluctuations in the energy tariffs thereby making photovoltaics and battery storage an attractive option.

 

 

As can be seen in figure 2a, there is a continuous increase in the price of oil. If we consider the price over the last 5 years one can see that the price of crude oil is increasing by 8% if the 2008 peak is ignored and by 10% if this is considered. One should also point out that the price of oil hit the absolute highest value of $140 in 2008 and at this time it was being predicted that it might also reach the $200 value. On the other hand the price of PVs over the last 5 years has been dropping at a rate of 17% (fig 2b). In fact on small islands like Malta, with a high degree of solar energy, PV energy has already reached the ‘true’ cost of

electricity.

 

 Figure 2a: Price of crude oil over                        Figure 2b: Price of PV over the last 5 years.

                   the last 5 years

 

Photovoltaic energy generation in Malta -Industrial and domestic case studies

The authors have designed and built a photovoltaic plant for research in collaboration with IBC Solar on the premises of Abertax Technologies (www.abertax.com) which has now been running for the past four years. The 18kWp PV Plant generates approximately 3% of Abertax energy needs. The Plant consists of 9 different setups which were designed for analysing different technologies and layouts as shown in figure 3. One of the systems is of a hybrid type with a VRLA battery storage system.

 

 

 

 

 Figure 3: Photovoltaic research        Figure 4: Monthly output energy from the Abertax research plant

                 plant at Abertax

 

 Figure 4 shows the monthly energy generated since its commissioning. The output of each one of the 9 setups can be monitored separately, daily and the authors have compiled a number of interesting results by analysing and comparing the different setups [4]. Currently the authors are extending the plant size to 220kW which will generate 28% of the company’s consumption.

 

 Abertax also runs a fleet of 19 electric vehicles which are also used to cover most of their daily errands and to field test all the battery ancillaries they develop. Both the hybrid system and the electric car fleet batteries are being continuously monitored through Abertax Online Battery Management [5].

 

 

Abertax has installed more than 200 domestic PV installations and these arebeing monitored. Figure 6 shows the installation that will be referred to in this paper.  

 

 Figure 6: One of the monitored                       Figure 7: Percentage of PV generated relative to the

                 domestic installation                                       energyconsumption – Monthly average (brown) 

                                                                                      and running average (black)

 

 This installation consists of 12 PV panels each 235W (total of 2820kWp) mounted on a wooden structure and supplying the grid through a grid connected inverter. The house is a 2 storey semi detached villa with a floor space of 290 m . A family of 6 people (2 adults and 4 children) live in this house and the energy consumption and generation have been continuously monitored since December 2011. The system was designed to generate 100% of the yearly family’s energy needs. Figure 7 shows the percentage of energy generated by the PV system relative to the energy consumed by the family. From the running average (black curve) till the end of August, it looks that the calculated target will be reached. It is interesting to note that the monthly average peaks even to 185% in spring and the excess of energy is presently exported to the grid but can also be stored. There is more space available for PV and one can also think of utilising better the existing wooden terrace space. Presently the system is at its optimum angle of 30deg facing south. However now, that the price of PV has gone down; it is financially feasible to reduce the angle to 20deg and fix them in line with the terrace. This means 21deg off south (towards west) which will eventually create space for another 4 panels bringing the peak power up to 3760kWp (i.e. by another 25% taking into account the fact that the angle is not the optimum). The house is equipped with all appliances and presently uses electric heating in winter and air-conditioning for cooling in summer. This family uses three cars, one of which is fully electric with 9kWhr of lead acid batteries that Abertax has been monitoring through its online battery management. The authors estimated that if all the 3 cars were electric a further 6 panels would be required to cover the transport energy needs as well. One has to keep in mind here that on a small island like Malta, the average daily distances travelled are less than 15Km. The latter was concluded in a Master plan for electric transport [6], Abertax research and development department was commissioned to do, for the Maltese Government.

   

The effect of PVs on the electrical energy generated and the Peak power

 The authors believe that due to the amount of energy that a PV system can yield in Malta together with the availability of roof space, 65MWp of PV is possible. The authors used the results of the Abertax industrial PV research plant to extrapolate and estimate the output power and energy if Malta installs 65MWp of PVs.

  

Over the last few years the maximum energy demand has shifted from winter to Summer time due to the increase in energy demand mainly caused by the air conditioning load. In fact if we look at figure 8 which shows the peak load demand of the last 27 years, one can clearly see that the peak power shifted from winter to summer. While there was a continuous increase in the peak winter power up to 2005 this then changed to a steady decrease. The use of heat pumps for heating and cooling has shifted the peak from winter to summer. The peak power peaked in 2007 and then due to a sharp increase in the electricity tariffs, energy efficient measures and the use of renewable sources of energy started to become attractive. The

 

implementation of such measures has controlled even the summer peak as can be clearly seen. It is also interesting to observe that even the total energy demand has been controlled by such measures although some of the sharp decrease in the energy used in 2009 is surely attributed due to reduction of industrial activity caused by the recession.

 

 Figure 8: Winter and summer peak load           Figure 9: Maximum instantaneous peak power demand on

               during the past 27 years.                                   a hot summer day and the effect of a 65MWp PV

                                                                                       (black) plant (August 2011)

  

 

The sharp increase in the peak power summer demand which eventually exceeded the winter peak has made photovoltaic renewable energy become very attractive. This is because PVs will surely help to reduce this peak and its related capital investment apart from the savings of the running fuel costs. In order to study the extent of this effect of PVs during the summer peak, the authors carried out a study of the effect of a 65MW PV plant on the instantaneous power. For this reason the electricity power curve when the demand was the highest was selected for the three past consecutive years. The blue curve in figures 9 shows the hourly power demand on the Malta Power Station during the particular day where the absolute peak power was reached. A peak power of 413MW was reached during the year 2011. The green curves show the instantaneous power that would be generated from 65MWp of PVs and the red curve shows the net power demand from the power station in the presence of such PV power. One can clearly see that 65MWp of PVs would shave the peak almost completely thus reducing the power station capital investment required to source the peak, that is expected to increase in due course. Apart from this, open cycle gas turbines are usually used to cope with this peak and hence the running cost is also very expensive making photovoltaics and battery energy storage profitable during this time as will be shown later in this paper.

  

Effect of high concentration of PV

 Figure 10 shows the peak instantaneous power demand on the power station and as can be seen PV penetration (green part of the columns) is on the increase. This is expected to increase in the coming years especially now that PV reached grid parity in Malta. From a study carried out [7] there are already cases of high voltage in areas of high PV concentration. Figure 11 shows a particular Grid connected PV installation which is experiencing repetitive shutdowns due to the excessive high voltage. This will obviously extend the payback period of the system as it is limiting the export. Battery storage would be the right solution in this case as the PV energy can be saved and then either used later to supply the local needs or exported to

grid.

 

 

 Figure 10: Maximum peak power demand and generation from the Power station and PVs during the

                 last 5 years

 

 

 

 

 

     Figure 11: Loss of PV energy due to high on the grid

 

  

Malta Electricity Tariffs and Renewable Targets

 

Both PV energy and energy storage viability depend on the energy tariffs. The present electricity tariffs are listed in table 1. As can be seen some of the tariffs are very high and surely make renewable energy sources and battery storage very attractive.

 

Like all other European countries, Malta has also its own renewable energy target of 10% which the thorities aim to achieve with the available technologies by 2020. The use of Photovoltaic panels on our flat concrete roofs has the added advantage that they will reduce the airconditioning energy required to cool the place due to the shading that the panels provide. In another paper [4] the authors show that in some cases, the shade of the panels can block an amount of heat energy radiated through the concrete roof equivalent to the rating of the peak power of the panel.

 

 

 Present cost of a domestic grid connected PV system in Malta

Malta is blessed with solar energy and currently the PV energy reached grid parity. At the moment locals are investing in a standard grid connected system as show in figure 12. The most popular system presently used consists of 6 panels of 235Wp and a 1.2kW inverter. i.e. 1410 kWp. Such a system in Malta will generate 2200 kwhrs yearly. Presently such a system installed has an average retail price of 3000€ and occupies around 10m of roof space. This means that the payback period of such a system is 6.5years at 0.21€ and less than 2 years at 0.70€!

 

 

 

Present cost of using lead acid battery storage

The authors calculated the difference between the feed in and exported tariff to make domestic battery energy storage systems viable.

 

 

Let’s assume we have a 48V, 5kWhr battery energy storage.To achieve this we need a 48V 130Ahr battery i.e. 48 x 130 x 0.8 = 4.99kWhr for 80% discharge Let’s also assume that the battery is 80% efficient We will therefore end up with 4.99 x 0.8 = 3.99kWhr Say we get 1200 cycles during the battery life time. This means that: [ 1200 x ((3.99 x discharge tariff) – (4.99 x charging tariff)) ] = Cost of battery + profit

 

 

 Say battery cost is €800 and expected profit of 25%. If we take the normal tariff to be 20 cents then we get; Peak tariff = (83.30 + (4.99 x 20.00)) / 3.99 = 45.89 cents

  

Table 2 shows the normal and peak electricity price in case of an hourly tariff for battery energy storage systems to be profitable. At the moment the most attractive use of battery energy storage in Malta is when used together with a photovoltaic system by heavy domestic consumers which fall in band 5 of the domestic tariff system (refer to table 3). If such a consumer has a photovoltaic system it would be profitable for him to ensure that no PV energy is exported to the grid since this is at the moment reimbursed at a flat rate of 25 euro cents in Malta.

 

The present domestic tariff in Malta depends on the energy consumed however this is expected to change as Malta is presently investing and installing a Smart metering system. Hourly tariff is then expected to be implemented and this will surely make domestic battery energy storage attractive, even without a renewable source, especially due to the peak demand cost.

  

The potential Energy Battery Storage and grid connected PV systems

 Over the past three years there have been a number of grants issued by the Maltese government, which encouraged the use of grid connected photovoltaic systems. These systems are connected in such a way that they feed directly the grid and the tariff for the energy generated is only 0.25 € for the first 8 years and then drops to 0.11 € thereafter. One has to point out here, that in most cases the government gave a grant equivalent to 50 % of the total investment which made a PV system very attractive with payback periods of less than 6 years. However, when you have such an investment, there might be other attractive ways of using this energy especially for those individuals who have to pay a tariff of 0.70 € (as in table 1). During power cuts caused by disasters like that experienced in Japan, one might also face the frustrating situation that with such an investment on the roof he/she cannot make use of it even to run at least the essential loads, such as a fridge. This situation has caused a sharp increase in the use of standby batteries for backup in Japan. There is also now the drive, in certain countries, to encourage self consumption. I.e. limiting the renewable energy generated to your use and this is also increasing the potential of battery energy storage.

 

PV and energy storage System Design requirements

Grid connection of any installation is usually very simple and straight forward as one, just needs to take a tapping from a passing low voltage line outside the building and this requires only minor management and control in cases where day and night tariffs apply.

  

This is not the case with a grid or off grid system with energy storage. While in off grid systems energy storage is a must , in grid systems this is an option which is becoming more and more interesting, especially in cases for example of customers who have to pay 0.70 € for the energy imported and will get only 0.25 € (or eventually 0.11 €) for the renewable energy exported. Such cases will increase in the near future and other interesting opportunities for energy storage will come in, either due to other opportunities such as the self consumption tariff introduced in Germany or due to consumers desperate drive to rely less and less on just the grid ! As pointed already this is the case with a lot of consumers in Japan but there are others such as India that have a very weak grid and get regular grid disturbances.

  

Figure 13 shows an off grid system with energy storage. In such a setup the energy storage is basically charged from any of the available sources including possibly a standby generator. The system management is very important as it has to make sure that the energy generated and the energy consumed has to be balanced with the battery acting as the buffer.

 

 

Presently the current buzz word in the energy sector is the ‘Smart grid’ however very few are aware what being smart is! This drive to become smart is presently coming mainly from energy suppliers and authorities who want to make the domestic users smart by controlling the way they use their energy which is basically load management. But this is already done in off grid systems where you have to limit the instantaneous power due to the limiting component/s in the system. In the case of grid suppliers they would like to make their customers smart to control the peak power and the stability of their distribution network. One can argue that, there is very little difference between an off grid and a smart grid especially if energy storage is used. In fact we will be smarter than the energy providers if as described above we reverse the role of the battery and grid. I.e. have the grid as a backup especially if we have a renewable source of energy. This offers an interesting future to the battery industry as it appears that consumers like to be smart and quite a number are already there with their smart car and smart phone! Germany is leading in this regards as the self consumption tariff it has recently introduced, is basically encouraging the use of the grid as standby. Well done! This was not initiated because they want to increase the sales of energy storage systems but to control the problem that exists in some areas with excessive renewable energy on the grid resulting in grid instability. Figure 14 shows a block diagram of a future smart grid system. The system management in this case has to take into consideration the information coming from the smart meter and react according to the conditions that will be imposed by the energy provider. If one compares figure 13 and figure 14, one can observe that, there are hardly any differences between the two systems. Actually the so called “Smart” system might limit the consumer to follow procedures and rules that suit the energy provider and not the user. The authors’ concept in this paper is to create the Smartness for the user rather than the energy provider.

 

 

It is very clear to the authors that in all cases where energy storage is required, being off grid or smart grid, we need to have a battery with its own management and control. This is the only way to ensure reliable storage and a quality battery service to the end user. It is also a way to allow the customer the possibility to buy a storage system without the trouble of having to worry about its compatibility. If we agree we can join the Smart Club and refer to this as a Smart Battery. The authors therefore see a future for a standard which defines the requirements of a “smart storage system”. I.e. we give the customer the opportunity to buy say “a 48V DC 10kWh smart battery storage” or “a 240V AC 10kWhr smart battery storage”. It should have one power input and output and a standard communication connection. The system should be designed like all of today’s digital equipment i.e. plug and play when connected to a smart system. So if the user connects his smartphone to the battery in the morning, he should get a screen saying for example;

  

“Good morning, I am your Lead Acid Horse.

 

Currently on standby waiting to discharge the 7kWhrs of stored energy or to fill up the 3kWhrs empty capacity If you need further details click on my ‘A La Carte menu’ below”

  

When one speaks of batteries, a lot of users might become concerned of the costs and maintenance required, however the authors show that with the use of their latest state of the art development in battery monitoring, this concern is something of the past. For this paper, the authors used a smart battery setup involving their intelligent battery concept (developed and patented by Abertax Technologies) where the battery has an embedded electronic circuit which can be accessed via the internet at any time.

 

 The authors have done a number of experiments showing how the smart battery setup shown in figure 15 can be used with standard grid connected systems and at the same time, be smart enough to work in a number of possible scenarios including off grid. This means that, in the case of grid failure the energy from the PV system can be used to power the essential loads and charge the batteries for the evening. Excess PV energy during the sunny hours can be used to heat up a water reservoir in the house, in order to ensure that all the power generated is used at all times. In order to kick off the grid connected inverter which has to be isolated from the grid during a power cut, a small inverter powered from the battery bank is used to simulate the grid. Abertax online [5] battery management unit was used to monitor and control the complete setup. This monitoring unit also provides the energy flow to and from the battery.

 

 

 Figure 15: A smart battery energy storage

  

The authors’ smart battery concept was used to prove that it can satisfy the most common scenarios described below. This requires the control and communication with every piece of equipment. In the future domestic equipment with such features will surely be available for compatibility with the Smart grid expectations.

 

Scenario 1 - The smart battery as a stand alone for rural electrification

 This setup is commonly used in rural electrification and there exists specific setups for this use. In such off the shelf systems, the control, monitoring and inverter are usually integrated into one box. The authors however, used a setup as shown in figure 16 where each individual component, including the battery, can communicate its status so that the system management is able to control the energy at all times. In the case of the photovoltaic inverter a standard grid connected inverter was used, while the smart battery inverter was set to emulate the grid. The water heater was utilised as a smart dummy load to ensure that power stability was maintained at all times. This is necessary to ensure that any surplus PV energy that could not be used to either charge the battery or feed the loads is dumped into the hot water reservoir. To emulate the smart part of this experiment, standard laboratory equipment was used. The smart management and control was obtained using a standard remote monitoring system which was programmed accordingly.

 

 

 Figure 16: A standalone system using the smart battery storage

  

Scenario 2 - The Smart battery with Grid connected system with / without a renewable source of energy

This setup was relatively easy to achieve as the presence of the grid always helps to ensure stability of the voltage and frequency even if some loads demand a high current. In order to discharge the energy from the battery, a standard PV grid connected inverter was used and set to discharge the batteries at constant voltage till the battery management sends a signal indicating the 80% DOD threshold. A smart meter was used to get the information of the tariff and the instantaneous power flow in both directions. This enabled us to switch off the non critical loads during the peak tariff and use the battery to support the energy required when the PV was not enough to power the critical loads. With the PV disconnected the battery was used to power up the critical loads. The battery was charged from the PV or from the grid depending on the better value of return, calculated on the tariff at the time.

 

 

 

 

 Figure 17: A grid connected system using the smart battery storage

 

 Scenario 3 - The Smart battery as in scenario 2 but during a power cut

 The last setup is the most interesting as it covers all possibilities. In the case of grid failure, this setup will allow the use of the other energy sources installed. Without the smart battery and management grid connected equipment cannot be used. For example most of the domestic CHPs run from an asynchronous machine which cannot deliver any power unless it is excited from the grid. In this setup the authors made use of the CHP at the University laboratories to show that a smart battery can be used to excite the asynchronous machine through its inverter. In the case of a standard PV grid connected inverter the smart management and control has to ensure, the use of the instantaneous PV power, at all times. However, this can be changed if the inverter software is programmed to allow smart control and reduce the power if necessary.

 

 

 Figure 18: A system using the smart battery storage for operation with and without the grid

 

Conclusion

Small islands like Malta that generate their electricity from fossil fuels are expected to pay dearly for their electricity energy bills. The fact that European countries have agreed to reach specific renewable energy targets forces governments to give incentives to increase the use of renewable sources. The authors have shown that investment in photovoltaic energy has the added benefit, on islands such Malta, of reducing the capital investment required by the fossil fuel generation plant. These scenarios already create an interesting and profitable niche market for both photovoltaic and battery energy storage. It is expected that if the electricity tariff is charged based on the time of the day rather than the consumed energy, the use of photovoltaic and battery energy storage will become much more attractive for both the energy supplier and the consumer. A further advantage of having battery energy storage is the possibility of running your essential loads in the case of a power failure. The authors have shown that their online monitoring and control management system that they developed can be embedded in the Smart energy storage system of the Future. The authors welcome any battery and/or converter (charger /inverter) producer to partner up to develop the components of the complete smart battery concept which will be required in the future.

  

Acknowledgements

The authors would like to thank Abertax Technologies for their financial support in this ongoing interesting research work. Particular thanks go to the founder and chairman of the Abertax Group, Mr. Werner Schmidt and vice –chairman Dr. Martin Florin who have been the motivators of this research work with their aim to ensure a better environment for future generations.

  

References

[1] Joseph Cilia, Neville Azzopardi, Mark Scicluna, Klaus Dieter Merz, “The potential of battery storage and Photovoltaics for small island states” 5th International Renewable Energy Storage Conference 22-24th November 2010

 

[2] Joseph Cilia, Mark Scicluna, Neville Azzopardi, Klaus Dieter Merz, Björn Mentzer, ”The potential of battery energy storage for grid connected domestic renewable sources of energy” 6th International Renewable Energy Storage Conference 22-24th November 2011

 

[3] “Nesis Data Book – Network of Expert for Small Island Systems” published by the Union of the Electricity Industry

 

[4] Neville Azzopardi, Joseph Cilia, Mark Scicluna, ”The performance of the PV research plant at Abertax Research and Development facilities” August 08 – September 11.

 

[5] Björn Mentzer, Joseph Cilia, Klaus Dieter Merz, Malcolm Tabone, “Charge for Energy – Not Batteries” 12 ELBC conference Istanbul 21-24th September 2010

 

[6] Joseph Cilia, Klaus Dieter Merz, Edward Mallia, Martin Florin, ”A Master Plan for the use of electric vehicles in Malta and Gozo” July 2007

 

[7] Cyril Spiteri Staines, ”Technical issues in local PV systems” conference on the potential of renewable in Malta held by MIEMA, March 2012.

 

 

 

 

 

 

 

 

Superior valve design for VRLA batteries

A small part but with huge impact on the battery performance

Klaus Dieter Merz (kd.merz@abertax.com), Joseph Pule (joseph.pule@abertax.com), George Schembri (george.schembri@abertax.com)

Introduction

A lot of Research and development work has been put in the design of an extremely reliable and high quality valve for VRLA batteries. The authors will present interesting research work showing that a high quality valve is essential to ensure quality battery performance. An alarming factor is that most of the valves on the market tend to have a wide range of opening and closing pressures. Apart from that, they tend to degrade in performance rapidly affecting the cells concerned, resulting in unacceptable short cycle life of VRLA batteries. Four years of intensive research and development work on this valve resulted in very interesting findings and finally their work brought about a superior valve, suitable for all VR lead acid batteries. This

superior valve has been in use for the past three years and has been proven by various battery anufacturers. A high quality valve is a must for any application but the effect is even more evident when the battery is used in harsh environments such as the AGM battery for all future stop and go applications.

 

Valve Construction

The standard valve basically consists of three main parts: the valve body, the diaphragm and the valve lid. The precision of the plastic and rubber parts is of utmost importance for the proper functionality of the valve during its operation. A bottom cap is used at the lower end to protect the valve from the direct exposure of the diaphragm to the acid vapour. The o-ring, which should be of the appropriate dimensions depending on the type of battery lid, ensures that no gas leaks occur between the battery lid and the valve body.

                                            Figure 1: Exploded view of the Abertax valve

The diaphragm and the length of the calibrating pin are the most important parts in the valve which determine the pressure settings of the valve. This patented technology uses a disk shape diaphragm design rather than the typical U-shape design as used in most of the safety valves for VRLA batteries. The membrane material and design is the result of intensive research by Abertax R&D and has proved to stand the most harsh environments that some of the VRLA batteries might be exposed to. This diaphragm in combination with the novel lid and body design of the valve outperforms any other valve on the market through a number of advantages namely by addressing the critical features as shown in figure 2.

 Figure 2: Comparison of Critical features between the Abertax valve and a standard valve   

Main valve features to ensure the expected cycle life of a VRLA battery

During their research work, the authors found out that the valve can have a huge impact on the battery performance. They identified that the following characteristics are a must for a high quality valve:

1. Low tolerances in opening – and closing pressure

2. Calibration of the opening and closing pressure

3. A good flow rate at excessive pressure to ensure safety

4. Capability of a long cycle life without degradation in performance

5. Acid resistance

6. Protection against any form of dust particles

7. Self cleaning surface

8. Has to fulfill the ignition test

9. Has to fulfill IATA test requirements

The most important feature to guarantee the expected performance of a VRLA battery is the opening and closing pressure tolerance, the long cycle life and the appropriate pressure setting. Abertax patented valve technology is the only technology which allows the calibration of the opening and closing pressure to specific requirements which depend mainly on the design, construction and material of the battery box.

In particular, the closing pressure and the difference in the operating pressure of each individual valve are very important to guarantee the expected cycle life of a VRLA battery. These specifications are not so critical for a low cost AGM block batteries. The latter are used in standby operations such as UPS applications where the standby voltage of the battery is always below the voltage that causes the battery to gas, i.e. 2.35V. However it is extremely important for a good GEL or AGM battery used in cyclic applications, where the battery is exposed to discharge and charging profiles that will cause gassing regularly. The authors delved into this problem and present interesting results below showing the huge effect that a valve can have on the performance of the battery.

Opening and closing pressure

The Patented Abertax GRS was designed to ensure that the opening and closing tolerances are at their lowest possible values. This and the very low pressure differences of each individual valve are considered to be the most important specifications to guarantee a quality VRLA battery. A number of valves which are available on the market were selected and tested for their opening and closing performance and the best of these was then selected and compared with the Abertax GRS which was calibrated to open at 175mbar. Figure 3a shows the performance of the Abertax GRS while figure 3b shows the performance of the other valves selected. It can be clearly observed that the Abertax GRS outperforms the others. The opening and closing pressure range of the Abertax valves is within a range of +/- 25 mbar and can be calibrated to the

required pressure.

                        Figure 3a: Opening /closing pressure of 100 Abertax valves

                       Figure 3b: Opening /closing pressure of 100 valves of a selected example

Cycle life

During the recharging process gas is generated in the battery cell. The amount of gas generated during the charging cycle depends mainly on battery design, charging regime and temperature. The valve opens during this process between 50 and 250 times. The cycle life of a good quality battery is of around 1,000 cycles, implying that the “valve cycles” could go up to more than 250,000 cycles. The Abertax valve easily reaches over 1 million cycles keeping within the calibrated pressure specifications.

                         
                          Figure 4: The Abertax GRS performance during the cycle test

Effect of valve on battery performance

The impact of a high quality valve on the life time and performance of the battery has been demonstrated with the numerous tests conducted. The vast experience and research work of the authors clearly identifies that the end of life of a good designed VRLA battery is usually a lack of electrolyte. This is mostly attributed due to the gassing during the charging phase. Gassing results from the decomposition of water and this generated gas is released through the safety valve when the internal cell pressure exceeds the opening pressure of the valve. Therefore the battery will be losing the gas which would otherwise be available again for recombination into water. Hence this results in a water loss of the very limited amount that exists in all VRLA batteries. The consistency of the opening and closing pressure between the valves on the same battery is also extremely important as otherwise the cell with a higher pressure valve tends to press on the cell with the lower pressure valve causing it to release even a higher amount of gas with every cycle.

 

Water loss test on GEL block batteries

Several 12 Volt Gel block batteries were fitted with three 30 mbar valves and three 170 mbar valves respectively. These batteries were then continuously cycled and the water loss was calculated after 100 and 200 cycles. This could be done according to an experimental setup which the authors designed to measure the volume of gas released by each cell. As expected, the cells fitted with the low pressure valves (30 mbar opening pressure) had a much higher water loss in comparison to the cells fitted with the 170 mbar valves.

                                                       Figure 5: Water loss test

This high water loss has a significant impact on the acid concentration inside the cells resulting in much higher corrosion and higher voltage level during recharge. Such “unbalanced cells” lead to premature f ailure of the battery.

Test results on OPzV cells

This section demonstrates the performance of a high quality valve in reducing the gassing and eventually water loss in products like OPzV cells which are expected to give a long life. The block and lid material is usually made of very strong SAN plastic material and would therefore allow the use of a valve with high opening and closing pressure.

Standard valves currently used in VR cells have an opening pressure of around 80 mbar. Such OPzV cells fitted with 80 mbar valves have been taken in a C3 to C10 cycles test with continuous internal cell pressure measurement.

Figure 6: Internal cell pressure of an OPzV cell while being cycled with a valve opening at 80mbar


The above graph shows very clearly that the cell pressure of 80 mbar inside the cell is always exceeded. This means a loss of hydrogen and oxygen out of the cells which as explained earlier eventually results in water loss during each and every cycle!

As can be seen in figure 6, the cell pressure does not exceed the 80mbar because the valve is releasing internal generated gas. The “gassing phase” lasts about 60 % of the whole recharging time and therefore results in a critical amount of water loss.

Figure 7: Internal cell pressure of an OPzV cell while being cycled with a valve opening at 200mbar

Interesting to note is the cell internal pressure curve characteristics during the discharging and recharging process. In order to investigate the effect of the valve on this characteristic the authors carried out test with valves set to open at different pressure.

Figure 7 shows this characteristic for a cell equipped with an Abertax GRS valve calibrated to open at 200mbar. It was concluded that while the shape of the characteristics does not vary with pressure, the pressure operating levels inside the cell vary drastically. In fact while the pressure inside the cell fitted with a 200 mbar valve is always positive, the cell wi th the 80mbar valve goes into negative pressure while discharging.

     

                    
Figure 8: Comparison of pressure inside cell during recharge using valves of different pressures

It is very important to note that in the case of the cell with a 200mbar valve there is only a very short period of gassing at the end of the charging phase. Figure 8 shows one cycle and one can observe that in the case of the 200mbar valve, the duration of gassing was only 1 hour out of 14 hours, while in the case of the 80mbar, the gassing was 6 ½ hours. This means that with a high pressure setting of the valve, the gassing time is drastically reduced and hence the water loss is very low.

 

Conclusion

Laboratory tests and field tests have proved that the quality of the safety valve for VRLA batteries is of very high importance for good reliability and long cycle life of VRLA batteries. The patented design of the Abertax valve guarantees the lowest tolerances and longest cycle life of all valves on the market and is one of the important parts in a high quality GEL or AGM lead acid battery.

Acknowledgements

The authors would like to thank Abertax Technologies for its financial support in this ongoing interesting research work. Particular thanks go to the founder and chairman of the Abertax Group, Mr. Werner Schmidt and vice–chairman Dr. Martin Florin who have been the motivators of this research work with their aim to ensure a better environment for future generations.