No doubt it is a dynamic environment for electrochemical energy storage. Seeing the various market opportunities, price developments and participants entering and exiting the battery market demonstrate a very dynamic landscape. Besides the established technologies such as lead-acid batteries (with all its different versions) and lithium batteries, some believe the next big opportunity lies in various form of flow batteries.
So why should anybody care about a technology like saltwater batteries? What are these saltwater batteries all about?
What is behind the saltwater battery?
Saltwater batteries have an electrolyte which is a form of saltwater and therefore gives the name to these forms of batteries. They have no flow, no moving or rotating parts and from an architectural perspective they are similar to the common lead batteries. The difference is in the materials used. Saltwater batteries consist of all natural materials, are non-toxic, non-flammable, non-explosive and are considered the safest battery technology on the market. Although it is a fairly young technology it has proven its reliability in the market for years with the longest applications being in the field for more than eight years and over 16.000 batteries deployed worldwide to date.
Although the technology seems simple at first sight, a lead-acid style battery but using only natural and harmless ingredients, a substantial material scientific effort is needed to get the Anode material, Cathode Material and Electrolyte right. And – as everything in the battery industry – it takes a long time to figure out the right material mix and perform the needed tests over multiple cycles. Improvement often comes in small increments, based on repeated testing and practical experience, over a long time.
The strength of this technology is without doubt its robustness. It is one of the few battery technologies which are truly maintenance free. Why so? Because saltwater batteries inherently allow a full discharge, so you can completely empty the battery without harming the battery or making a negative impact on its cycle life. The battery can even be days or weeks without any state of charge. Therefore no battery management system (BMS) is required to actively control the state of charge.
The robustness of this technology manifests itself also in the wide operating temperature window. Starting from -5 degree Centigrade up to 50 degree Centigrade is the specified temperature window. It is interesting to note, that the battery becomes more efficient (in terms of round trip efficiency) in the higher temperature areas. Even if it is getting above 50 degree centigrade the battery can be used without any risk as the technology cannot burn or explode, but with temperature above 50 degree Centigrade potential detrimental effects on the lifetime can be experience as the electrolyte starts to gas and a lower level of electrolyte will impact negatively on the cycle performance of the battery. The gassing however is not harmful and can be even carried out in enclosed rooms; the natural air circulation in an average room is sufficient to ensure a safe environment for humans and animals.
A key metric for any battery is cycle life. How many charge and discharge cycles are available in saltwater batteries? The current generation of saltwater batteries hold 5,000 cycles assuming they are used according to product specification and assuming the average depth of discharge is at 80%. After those 5,000 cycles the product is far from end of life, but the remaining capacity is at least 70% of its original nominal capacity. As the product does not become unstable in any way (explosive, inflammable) the saltwater battery can still be used beyond 5,000 cycles. So it is dependent on the application if 70% storage capacity is still adequate for the particular application.
When it is finally time to dispose of a saltwater battery, its non-toxic materials allow for ease of disposal. There are no materials or chemicals which require special disposal instructions.
The perfect battery which is superior to all others has not been found yet. So, with our saltwater batteries come also some weaknesses. One of them is the C-Rate, which is limited to 0,5 C. In other words, only half of its capacity can be discharged at once. This means for applications which require a lot of energy in a very short time, saltwater batteries might be a wrong fit. Our saltwater batteries like to charge and discharge over a 5-10 hour period. This makes them perfect for solar charging during the day and discharging throughout the night. The reason for the limitation in respect to C-rate lies within the battery design and more specifically relatively high internal resistance. Of course, this an area where we are focusing our research and development with the expectation to increase the C-Rate to at least 0,75 C within the near future.
Another weakness (related to C-rate) is on the energy density front. Saltwater batteries are currently about double the size of comparable lithium products, consequently making them unfit for applications like E-Mobility. Naturally this is the second area where our research and development team are focusing.
Besides all the technical aspects, in many practical projects its costs what matter. So how do saltwater batteries compare in terms of costs in relation to lead and lithium batteries? A benchmark has been done based on European market prices, storage (battery) only, as the peripheral electronics like inverters or Energy Management System (EMS) would distort the cost comparison.
A cost comparison of battery storage – lead, lithium, saltwater: How much is a kilowatt-hour of lithium-ion, lead or salt water batteries? What should be considered for a cost comparison of batteries?
Comparing cost of a battery / storage system is a science in itself. On the one hand, many parameters must be taken into account; on the other hand, many assumptions must be made, such as the ambient temperature. Then there is an additional topic. At first glance every battery seems to show fantastic values on the data sheet. Whether these values from the data sheets are consistent and the assumptions fit, is another matter. This also includes the issue of whether the values given in the data sheet comply with guarantee and warranty provisions? After all, what use are 6,000 cycles on the data sheet if there is no guarantee for them? A cost comparison of battery storage is therefore an extensive matter.
A simple way of Cost comparison of Battery storage: Costs / kWh
Due to many different factors it is hard for customers to make an objective cost comparison. There is nobody to blame for still using the old and simple formula: cost / kWh instead of digging through the mountain of information on datasheets and warranty terms. We from BlueSky Energy did the work and analysed four practical examples in detail. We have carried out an analysis in order to better understand where the saltwater battery technology stands today (August 2018) regarding costs in relation to commonly used lead or lithium products. We looked at two common lead acid batteries and one common lithium product in detail and compared them to our saltwater battery. The analysis takes into account data sheets and warranty conditions in order to tease out the costs per kilowatt hour of each of the four listed products.
Results of simple cost comparison battery storage
At the first quick glance, the lithium product wins with 0.099 Euro / kWh before salt water with 0.11 Euro / kWh and the two lead products with 0.14 and 0.17 Euro / kWh. Anyone who checks the analysis in greater detail recognizes that the price advantage of the lithium product is primarily due to the high number of cycles of 6,000 specified in the data sheet. These cycles are not guaranteed in the warranty terms. So the question arises, what are 6,000 cycles good for if the manufacturer does not guarantee for them? An analysis conducted by cleanergyreviews rates the lithium product at 3,650 cycles.[i] New data sheets state 4,500 cycles for this product. Just to get a sense of the complexity of costing battery storage, the cost of the same lithium battery would be 0.16 EURO / kWh for 3,650 cycles and 0.12 EURO / kWh for 4,500 cycles.
Cost comparison of Battery storage including the number of cycles according to the guarantee terms
Accurate calculations will use for their cost comparison only the number of cycles covered by the warranty provisions. The two lead batteries offered clear and simple warranty conditions. Also the saltwater battery states clear rules of warranty. In contrast, the lithium-ion battery has very special warranty conditions. There are five years guaranteed to be free from defects. That’s – assuming 1 cycle a day – 1,825 cycles. Although there is a 15-year warranty, this only applies to self-discharge degradation. No number of cycles is guaranteed. The replacement of defective products is guaranteed up to 10 years, but considering a declining time scale. In other words, 1,825 cycles are guaranteed (pro-rated) and then another 1,825 cycles with a reduced pro-rated calculation. Although it is not entirely ‘correct’, we will use 4,500 cycles of battery storage for the lithium-ion product for cost comparison purposes. With strict translation of the warranty, we could only assume 1,825 cycles. The following overview shows a cost comparison / kWh of battery storage without inverter but including installation.
Effective cost comparison and important parameters for comparing battery storage
- Usable capacity. How many kWh per cycle can be used by the battery? Meaning how many kWh can be discharged from the storage. The usable capacity of the battery storage degrades (decreases) the more frequently the battery is used. Depending on the number of cycles, degradation is between 2% and 4% per year. The usable capacity differs from the nominal capacity or gross capacity because depth of discharge (DoD) limits the nominal capacity. You can calculate the usable capacity by multiplying the nominal capacity and depth of discharge (in%).
- Number of cycles. How often you can use the battery until reaching a remaining capacity of 70% (see point 1 above)
- Efficiency. How much of the energy charged into the battery can then be taken out as usable energy again. The efficiency is given in %.
Calculation of effective costs of battery storage
In the first step you can calculate the energy throughput with the parameters of usable capacity, number of cycles and efficiency.
Usable capacity x number of cycles x efficiency = energy throughput in kWh
In the second step we set into relation the initial costs or investment costs (for the electricity storage, transport, installation) to the energy throughput.
Initial costs, investment in EUR / energy throughput in kWh = cost of electricity storage / kWh
If the electricity storage requires ongoing support or maintenance, these costs are to be summed over the number of cycles and added to the investment costs.
Now the cost / kWh per electricity storage can be calculated by setting the initial costs in relation to the energy throughput.
Comparison shows that lead products have the lowest initial costs. Due to low number of cycles and low depth of discharge, lead batteries have increased costs per kWh over time. In the test, lithium-ion batteries and saltwater electricity storage have an advantage over lead batteries.
Increased ambient temperatures and consequences for cost comparison of batteries
Temperature is a very important factor in the performance of batteries or electrical energy storage solutions. Narrow temperature windows for lead batteries described in this study of +15 ° to + 25 ° Celsius or + 10 ° to + 30 ° Celsius limit the performance. As the temperatures get cooler, the capacity of the battery decreases. This is well known. But what happens when temperatures are increased? A study by ‘PowerThru’ shows that lead-acid battery life is reduced by 50% when the temperature is 8.3 ° C above the specification level.[ii] There is a serious negative cost impact due to reduced lifetime if the battery is working in high temperature ranges. The costs per kilowatt hour in EUR increase for the two lead products to 0.29 Euro / kWh or 0.23 Euro / kWh. In conclusion, if you expect increased temperatures, then lead batteries will be more expensive compared to lithium or salt water batteries. Although at first glance the initial costs are low, the effective costs are higher than other products.
Generally lithium products are also sensitive in terms of temperature. The analysed lithium product has active cooling and is therefore specified at 0 ° Celsius to 50 ° Celsius. When working with active cooling, the values given in the data sheet are unlikely to be effective in terms of efficiency (94%) because cooling requires further energy.
This again proves how difficult it is to evaluate and compare batteries and above all to calculate the right costs.
Summary Cost Comparison Battery Storage
One thing is clear, the perfect battery for all applications does not exist! In the cost comparison of battery technologies, saltwater has cost advantages over lithium. If we are strictly evaluating the warranty of the lithium product salt water beats lead and lithium.
Is saltwater battery technology the New Big Thing in the Storage Market? No, it depends on the field of application. But if the weaknesses of saltwater technology (space requirements and C-rate) do not play a critical role, the saltwater electricity storage technology offers a cost-effective alternative. Saltwater energy storage systems also score points alongside the core advantages in terms of safety, freedom from maintenance and environmental friendliness.
Applying saltwater batteries – From theory to the practical application
How can saltwater battery be deployed in practical projects in the field? In general, saltwater batteries are available on a 12 volts, 24 volts and 48 volts basis. This means the battery inverter needs to comply to those nominal voltages. Besides the nominal voltage, the battery has a certain charge profile beginning at 37 volts at the lower end (the battery is empty) up to 60 volts (battery is fully charged). The 60 volts is also the cut-off voltage. The inverter needs to be set in accordance with those voltage ranges, similar to lead batteries’ charge profile. As long as the battery inverter can work in those ranges, saltwater batteries can be applied easily and are connected as common lead batteries are.
Naturally you will not do big electrical energy storage project (e.g. in the Megawatt area) on a 48 volt DC basis. In those cases, for bigger storage application the voltage can be increased by serially wiring the batteries. Bigger installations are exclusively executed with a Battery Management System (BMS), which ensures that the multiple batteries are balanced, making bigger systems more robust.
To ease the installation, connection boxes are available which connect the batteries via a central hub, secured with fuses to the battery inverter. The connection box ensures an easy installation for installers and a secured connection, in case of battery failure.
It is common knowledge and highlighted in this paper that temperature is an important factor for battery performance. To enable online monitoring and offline management the saltwater battery has an intelligent Energy Management System (EMS) as an integrated option. Besides temperature control and energy flow management, the EMS has the capability to easily integrate multiple energy sources (Solar/Photovoltaic, Wind turbines, Diesel Generators, Grid etc. ) on one side and enables a smart usage of surplus energy (power-to heat, E-Charging, heat-pumps or other appliances). On the load side the EMS enables peak-shifting to ensure a more balanced load if certain appliances are in use.
Summary – new generation saltwater battery
Although still fairly unknown in the market, saltwater batteries are beginning to position themselves as the third battery technology – besides lithium and lead – in the market. Compared to the multiple new battery technologies, which look promising and are researched on, saltwater electrical energy storage has proved its practical readiness for years in thousands of applications. Saltwater batteries are certainly not a perfect fit for every storage market segment as the C-Rate limitation with 0,5C and the energy density make them unfit for certain markets (e.g. E-mobility). However saltwater batteries prove to be a good alternative, where a robust, simple, safe and reliable electrical energy storage solution is needed.
Dr. Thomas F. Krausse,
BlueSky Energy: www.bluesky-energy.eu
[i] See cleantechnical: https://www.cleanenergyreviews.info/blog/2015/11/19/complete-battery-storage-comparison-and-review
[ii] See study from Powerthru: http://www.power-thru.com/