Dr Michael McDonagh – OPz Batteries,
The best choice for stationary applications?
The world of stationary batteries is not standing still. What is the best battery choice for this rapidly expanding market?
The world is rapidly changing. More and more industries, organisations, regions and countries need consistent and reliable electric power on demand. National grids are often struggling to meet peak power demands and, in some countries, planned blackouts by city or area are commonplace. In the industrialised countries with mature infrastructures there is definitely a strain on supply and sometimes peak events, damage or accidents can lead to prolonged blackout periods. On the other side of the coin, developing economies can have problems with supplying power in remote areas where is no national grid to supply power. Then there is the modern requirement of storing energy from variable or renewable resources whose power outputs can be intermittent and sometimes unpredictable. Wind and solar generators can be both. These and even more predictable energy sources such as tidal generators can also provide power at inconvenient times, i.e. not at peak demand periods. There are many applications both grid-related (frequency control, peak shaving, arbitrage etc.) and local UPS, standby power, cost saving etc. which are also an essential part of modern commerce.
These are just a few of the applications which need stationary energy storage facilities. Fig. 1 gives a breakdown of the general categories where batteries in fixed locations are used to store energy:
Unfortunately, the data is not current. This is due to the lack of freely available information on this growing and more lucrative market. The fastest growing sectors which are Energy Storage and UPS for data centre applications, are big business and can be investor driven commercial operations. Because of this there is potential profit in providing the relevant information, so companies like Reuters are indeed finding their information to be in demand. For this reason, statistics will not be as coherent or directly applicable as I would like. However, they will be sufficiently on the mark to provide an understanding of the current trends in this high growth market sector.
If we scrutinise the abbreviated list provided in Fig. 1 it can be seen that there are very different requirements for these applications. The largest section of others we can assume is mostly Energy Storage. According to Reuters, the lead acid battery stationary market was valued at 8. 3 billion USD in 2017 and consisted of the following sectors:
Oil & Gas
Railway Backup Systems
For the purposes of this article few can combine some of these markets due to the similarity in their battery requirements. Table 1 shows the demands of these applications on the installed batteries.
|Application||Typical size||Max Discharge||Discharge frequency||Rate of discharge|
|Energy Storage||1 – 50 MWh, Max – 290||80%||Daily||0.2 C10|
|UPS||0.5-500kWh||20%||Infrequent / Weekly||0.05 C10|
|Emergency / Backup||0.5kWh-10MWh||80%||Infrequent / Weekly||0.08 C10|
|Rail / Cable / Security||0.1-5kWh||60%||Daily||0.1 C10|
Table - 1 Stationary Applications and their requirements
Some of these applications can be subdivided further. Starting with Energy Storage, this is perhaps the fastest growing stationary application. In the past 20 years it has been recognised that energy storage, either directly from the grid as in utility scale operations, or as an addition to variable renewable energy generators, has a whole range of benefits. Table 2 below gives a reasonably comprehensive list of the different uses to which grid scale energy storage can be put. What is immediately evident is that some of the applications like energy Arbitrage have significant commercial benefits and an industry based on purchasing reserves of stored energy at low prices then reselling to distributors at peak demand times or when energy generators are struggling. The ability to efficiently harness renewable energy is becoming more important and countries and regions struggle to deliver lower CO2 emissions from increasing industrialisation and consumer demands. Grid scale energy storage along with other markets like remote telecoms towers are burgeoning markets where batteries are ideally placed to enable their growth. Of all the battery storage technologies which are available, lead acid, and particularly the OPzS and OPzV designs, can provide a very cost-effective solution for most of the stationary markets.
Table 2 Description of Energy Storage applications
|Energy Arbitrage||Can store energy and meter its use for purchase and resale at a profit|
|Frequency Regulation||Sudden dips in frequency due to heavy loads can be prevented by use of instantaneous battery power|
|Capacity Firming||Use of storage to provide energy to fill-in when variable generation (i.e. solar and wind) output is below the generator’s rating to provide constant power output|
|Power Quality||A measure of the level of voltage and/or frequency disturbances|
|Black Start||"The reliable restoration of the grid following a blackout. This requires a generating unit to start without an outside electrical supply, or to automatically remain operating at reduced levels when disconnected from the grid"|
|Peak shaving/Demand-side management/Load levelling||Zll activities or programs undertaken by Load-Serving Entity or its customers to influence the amount or timing of electricity they use|
|Backup power (nuclear and fossil fuel generators)||Provides power in the event of a power failure for commercial and industrial facilities. Can also be used by electricity power plants to provide electricity if the generators cannot cope or fail|
|UPS||Uninterruptible Power Supply is a device that provides battery backup when the electrical power fails or drops to an unacceptable voltage level. Can be used for domestic or industrial applications|
One thing that we should not be concerned about is the energy capacity which we have in our national grids. What we lack is the ability to meet our power demand at peak periods, rather than the ability to meet our total energy needs. Many industrialized countries can generate more than the total daily energy requirement, but are at, or near, their generation capacity for peak consumption periods. In the UK, for example, maximum peak demand hovers around 60 GW with a supply capability of about 75 GW but is often significantly less due to frequent breakdowns. This means that on occasions peak demand can outstrip generator supply. This contrasts with India, whose electricity demand touched an all-time record high of 176.724 gigawatts in March this year, despite having an installed capacity of 350.162 GW. However, many states in India do experience planned and unplanned power cuts and peak electricity supply has been low. This has been explained by reference to issues such as the precarious finances of some state-owned electricity distribution companies, which prevent them from being able to procure the required amount of power.
The Indian government claims that in the nine months of FY19, peak demand grew at 7.9% as compared to 2.8% in the corresponding period in FY18. It attributed this increased power demand to the spread of household electrification, increased supply to agricultural consumers, low hydropower generation and extended summers. Almost 50% of India’s power generation capability is derived from hydroelectric plants. This means that other generators account for around 170GW of potential output. With this in mind, the propensity for energy storage is enormous, with definite benefits to be had from holding larger reserves of energy rather than increasing or switching on additional generators when required.
Apart from supply of electricity and avoiding blackouts, energy storage can solve many problems such as keeping the supply frequency at the right level due to its instant response capability. Then there is the peak demand issue and the harnessing of renewable energy which is never produced at a convenient time for our peak requirements. The cost of installing energy storage rather than generation is also favourable, particularly if the most cost-effective battery option is chosen. Yes, as you may have guessed it is the familiar lead acid chemistry which gives the best all round value. This is not only true of capital cost but also of the lifetime cost and the financial return on investment. In Energy Storage, the main Achilles heel of lead acid, its low energy density is not a significant factor in its successful operation. Since there is no movement and plenty of space available within buildings and with batteries stored on concrete floors, weight and volume are not really important issues.
The main requirements for all of the many aspects of Energy Storage are fast response times of several seconds, efficient energy conversion and long calendar and cycle life. The OPzS and OPzV ranges have response times of milliseconds and the best cycle and calendar life of all the different LAB designs. Fig. 2 shows the salient features of the Microtex OpzS. The OPzV is different in two respects: it has an immobilised GEL electrolyte and a pressure relief valve to keep the oxygen and hydrogen produced on charge, inside the cell for recombination. The features shown, in particular the tubular plate with a spine for a grid and a multitube holding in the active material, are the keys to the long cycle and calendar life of this design of cell.
Of the lead acid options OPzS has a better all-round performance in most stationary applications, particularly for cycle life, but has the disadvantage of requiring topping up maintenance along with that associated cost. When calculating the levelised cost of energy storage (LCOES), it is important to include the full costs of the battery installation and maintenance. When determining the best battery option it is important to understand the real costs, particularly when appraising a different battery chemistry. With li-ion for example , it is often the battery pack cost which is quoted, leaving out the cooling, safety and fire arresting equipment. In some cases, it is only the cell costs with not even the battery pack and management system which are included in the calculation. The levelised cost of energy (LCOE) can easily be determined from the following relationship:
LCOE = sum of all costs over life of battery/sum of all outputs over life of battery.
The costs over battery lifetime include the charging of the battery with electricity. In this case the output/input expressed as a percentage is used for the calculation of costs.
The output over the lifetime is critically dependant on the cycle life of the battery, the higher the better. This reduces the cost of providing electricity according to the relationship given above. This again is a source of both confusion and error when making the calculation for the battery purchase. In the case of lead acid, the life of the battery is critically dependant on its depth of discharge (DOD). The lower the DOD the higher the battery cycle life (Fig. 3). Many customers will try to minimise the top line by keeping the capital cost to a minimum and purchasing the smallest capacity battery to do the job. In fact a battery only 50% larger will give a DOD of 50% instead of 80% and will practically double the cycle life. In this situation the system and installation costs are virtually unchanged, it is just the price of the battery cells which has increased. In other words, you get an LCOE almost half of the minimal capital case for the additional 50% higher battery cost. The benefits don’t stop there: the charge efficiency now goes from less than 80% to well over 90%, giving a further reduction in your LCOE.
Figure 3 Depth of Discharge (DoD) vs. cycle life
The benefits of using batteries to the Energy Storage business are quite evident. The question of which battery is less simple. Currently li-ion is the dominant chemistry used globally in this growing application. The reasons for this are not entirely clear unless the marketing tactics are considered. The main reason given by the installers of BESS systems for using li-ion are that it has a better LCOE than PbA due to the higher cycle life and superior charge/discharge efficiency. Going back to the numbers I have just given for an enhanced OPzS battery you can see that with double the cycle life and the improved charge/discharge efficiency, the li-ion batteries still are more expensive but do not give a better life or efficiency. There is a practice developing to use second life li-ion cells which were previously used in EV battery packs. It has been shown that these cells can become unsafe due to internal dendrite growth creating a short circuit. Problems in South Korea and the US where second life li-ion BESS installations have caught fire, and the lack of end of life recycling facilities, all point to the need for further evaluation of the true LCOE for li-ion systems. Perhaps some better marketing by the LAB industry is needed.
The traditional applications of UPS and standby power still account for more than 50% of the global stationary market. As can be seen from Table1, their respective requirements are somewhat different. For the UPS market the batteries have to provide occasional short bursts of power to ensure that equipment is not affected by sudden power drops or cut-outs. Generally, this results in a shallow and infrequent discharge of the battery packs. The battery packs are usually kept in enclosures or cabinets on constant low voltage float charge for most of their calendar life. In this instance it is not DOD or cycle life which is the dominant requirement, it is calendar life. On constant float charge the calendar life is almost exclusively dependant on the corrosion resistance of the alloys used in the battery grid. The other considerations are water loss in flooded systems and the use of VRLA OPzV cells.
In either the no maintenance OPzV or the low maintenance OPzS designs the effective design life can be more than 20 years with the right spine alloy and the right active material ingredients. In this respect the OPzS and OPzV ranges offered by Microtex are class-leading products (Fig. 4). Designed by a respected German battery scientist and manufactured with the latest low gassing and corrosion resistant lead-calcium-tin alloys, they offer an unbeatable package of performance, reliability and life. Microtex achieve these characteristics for their batteries by having optimum active material balance for the positive and negative plates with high-pressure die cast positive spine grids made from the perfectly balanced lead-calcium-tin alloy, which is a critical component of the cell. It not only is a conductor of the electric current generated by the active material of the plate, it also has to ensure that there is a good bond between the AM and the grid to minimise the internal resistance and prevent paste shedding as the battery cycles.
In UPS applications the battery is on constant low voltage float charge which, for the positive means it is constantly being oxidised (corroded). The specific alloy used by Microtex to manufacture the spines is the culmination of decades of R&D and commercial experience. It provides the best possible blend of corrosion resistance and low gassing properties of any available lead- calcium alloy. End of life for UPS applications is generally marked by the complete corroding through of the positive grid. In both OPzS and OPzV designs it is often either positive grid corrosion and/or battery dry out through water loss due to gas evolution which is the cause of eventual battery failure. The use of the specially formulated Microtex positive grid alloy minimises the effects of both of these failure modes to provide the longest lasting UPS lead acid battery on the market. With the OPzS range the clear SAN container is beneficial in being able to see the electrolyte level and state of health of the plates.
The other main stationary applications are standby/emergency power, telecoms, renewables and signalling. I have grouped these applications together as they are mostly deep discharge applications and have similar battery requirements. Again, good cycle life, deep discharge resistance and low maintenance are key parameters in battery choice. Renewables and telecoms have a common operating pattern as they are routinely (daily in most cases) discharged and recharged. The depth of discharge depends on the how long the battery is required to last compared with the capital outlay. The lower the DOD, the higher the initial cost. Operators have to decide which battery to use based on financial as much as technical reasons. The effect on LCOE has been discussed for the BESS situation and holds equally true for the telecom and renewables industries.
The renewables application takes advantage of storing energy which is often produced intermittently and at inconvenient times. This is true in both behind the meter and in front of the meter applications at domestic, local and national level. Battery storage enables the harvesting of energy from, say, wind turbine installations, which can be unpredictable and unusable because they are not required at the time of origin, to be released when required, say at peak demand periods. Again, this is just as true for domestic as it is for grid scale installations. Solar power is another example which, although predictable is often generated when there is little demand. In these examples the energy input/energy output (charge/discharge) cycle is usually a daily occurrence. In this respect it is very similar to the telecoms industry, both totally electric and hybrid diesel systems. In all of these instances, batteries are usually daily discharged and recharged, normally between 60% and 80% DOD.
For most stationary applications the most effective all round energy storage solution in terms of cost, response time, power delivery and energy storage capability is a battery (Fig. 5).
Most of the domestic and commercial scale operations will be located in areas or circumstances where any other form of energy storage such as pumped hydro, compressed air, flywheels etc. are not appropriate. As discussed earlier, the construction of the tubular plate and the alloys used for the positive grid in the Microtex OPzS and OPzV provide the highest possible cycle life with minimal gassing rates (water loss). That makes this design the most appropriate and most cost-effective option in the majority of these applications. The use of tubular plate cells and monobloc batteries for solar power installations for example, is well established. For ultra-long cycle life and deep discharge capability, the choice of the 2V OPzS is the most appropriate. However, it comes with the additional price tag of topping up maintenance. In some cases, particularly in remote applications topping up with water is not an option. In these cases there is the OPzV range from Microtex with all of the features found in the OPzS range but with a gelled electrolyte and sealed VRLA operation. It is very true to say that standby power is a growing and increasingly important market of the future. For this reason, it makes perfect sense to use the most experienced and advance lead acid battery company available. Many companies will make this claim, but very few have the experience and track record of Microtex to truly deliver.
Michael McDonagh – Chief Technical Officer – Microtex