Storing solar energy: How solar batteries work
Solar panel systems have become one of the fastest-growing sources of energy in the United States. According to the Solar Energy Industries Association, the solar market is expected to double in size by 2023, becoming a $4.5 billion market by that time.
The popularity of solar power has led to the rise of another renewable technology: solar batteries that can store extra solar power for later use. Companies like Tesla are developing batteries that can be installed with solar panels to create “solar-plus-storage” systems for your home. Read on to learn more about residential solar batteries, and find out if you should consider installing a solar-plus-storage system for your home.
The cost of solar is dropping globally.
Solar plus storage: Solar batteries for home explained
To appreciate why you might choose to install a solar-plus-storage system for your home, you first need to understand how a standard home solar PV system functions.
The typical solar energy system includes solar panels, an inverter, equipment to mount the panels on your roof, and a performance monitoring system that tracks electricity production. The solar panels collect energy from the sun and turn it into electricity, which is passed through the inverter and converted into a form that you can use to power your home.
The vast majority of residential solar energy systems are connected to the electricity grid (or “grid-tied”). When your panels are producing more electricity than your home needs, the excess is fed back into the power grid. Conversely, when your home needs more electricity than your solar panels are producing, you can draw power from the electric grid.
In most cases, you receive a credit on your utility bill for the electricity you send back to the grid. Later, when you are using more electricity than your solar panels have generated, you can use those credits instead of having to pay more to your utility. This process is known as net metering.
Classification of Batteries used for Solar Energy Storage
How is solar energy stored in batteries?
Solar batteries work by storing energy produced by your solar panels for later use. In some cases, solar batteries have their own inverter and offer integrated energy conversion. The higher your battery’s capacity, the more solar energy it can store.
When you install a solar battery as part of your solar panel system, you are able to store excess solar electricity at your home instead of sending it back to the grid. If your solar panels are producing more electricity than you need, the excess energy goes towards charging the battery. Later, when your solar panels aren’t producing electricity, you can draw down the energy you stored earlier in your battery for night use. You’ll only send electricity back to the grid when your battery is fully charged, and you’ll only draw electricity from the grid when your battery is depleted.
What this means in practical terms is that homes with solar-plus-storage can store excess solar power onsite for use later when the sun isn’t shining. As a bonus, since solar batteries store energy at your home, they also offer short-term backup power in the event that there’s a power outage in your area.
Batteries used for Solar Energy Storage
Lead Acid Batteries
Invented by the French physician Gaston Planté in 1859, lead-acid was the first rechargeable battery for commercial use. 150 years later, we still have no cost-effective alternatives for cars, wheelchairs, scooters, golf carts and UPS systems. The lead-acid battery has retained a market share in applications where newer battery chemistries would either be too expensive.
Lead-acid does not lend itself to fast charging. Typical charge time is 8 to 16 hours. A periodic fully saturated charge is essential to prevent sulfation and the battery must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation and a recharge may not be possible.
Finding the ideal charge voltage limit is critical. A high voltage (above 2.40V/cell) produces good battery performance but shortens the service life due to grid corrosion on the positive plate. A low voltage limit is subject to sulfation on the negative plate. Leaving the battery on float charge for a prolonged time does not cause damage.
Lead-acid does not like deep cycling. A full discharge causes extra strain and each cycle robs the battery of some service life. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger battery is recommended. Lead-acid is inexpensive but the operational costs can be higher than a nickel-based system if repetitive full cycles are required.
Depending on the depth of discharge and operating temperature, the sealed lead-acid provides 200 to 300 discharge/charge cycles. The primary reason for its relatively short cycle life is grid corrosion of the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Cycling does not prevent or reverse the trend.
The lead-acid battery has one of the lowest energy densities, making it unsuitable for portable devices. In addition, the performance at low temperatures is marginal. The self-discharge is about 40% per year, one of the best on rechargeable batteries. In comparison, nickel-cadmium self-discharges this amount in three months. The high lead content makes the lead-acid environmentally unfriendly.
The service life of a lead-acid battery can, in part, be measured by the thickness of the positive plates. The thicker the plates, the longer the life will be. During charging and discharging, the lead on the plates gets gradually eaten away and the sediment falls to the bottom. The weight of a battery is a good indication of the lead content and the life expectancy.
The plates of automotive starter batteries are about 0.040″ (1mm) thick, while the typical golf cart battery will have plates that are between 0.07-0.11″ (1.8- 2.8mm) thick. Forklift batteries may have plates that exceed 0.250″ (6mm). Most industrial flooded deep-cycle batteries use lead-antimony plates. This improves the plate life but increases gassing and water loss.
During the mid 1970s, researchers developed a maintenance-free lead-acid battery that can operate in any position. The liquid electrolyte is gelled into moistened separators and the enclosure is sealed. Safety valves allow venting during charge, discharge and atmospheric pressure changes.
Driven by different market needs, two lead-acid systems emerged: The small sealed lead-acid (SLA), also known under the brand name of Gelcell, and the larger Valve-regulated-lead-acid (VRLA). Both batteries are similar. Engineers may argue that the word ‘sealed lead-acid’ is a misnomer because no rechargeable battery can be totally sealed.
Unlike the flooded lead-acid battery, both SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge because excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential. To reduce dry-out, sealed lead-acid batteries use lead-calcium instead of the lead-antimony.
The optimum operating temperature for the lead-acid battery is 25*C (77*F). Elevated temperature reduces longevity. As a guideline, every 8°C (15°F) rise in temperature cuts the battery life in half. A VRLA, which would last for 10 years at 25°C (77°F), would only be good for 5 years if operated at 33°C (92°F). The same battery would desist after 2½ years if kept at a constant desert temperature of 41°C (106°F).
The sealed lead-acid battery is rated at a 5-hour (0.2) and 20-hour (0.05C) discharge. Longer discharge times produce higher capacity readings because of lower losses. The lead-acid performs well on high load currents.
Absorbed Glass Mat Batteries (AGM)
The AGM is a newer type sealed lead-acid that uses absorbed glass mats between the plates. It is sealed, maintenance-free and the plates are rigidly mounted to withstand extensive shock and vibration. Nearly all AGM batteries are recombinant, meaning they can recombine 99% of the oxygen and hydrogen. There is almost no water is loss.
The charging voltages are the same as for other lead-acid batteries. Even under severe overcharge conditions, hydrogen emission is below the 4% specified for aircraft and enclosed spaces. The low self-discharge of 1-3% per month allows long storage before recharging. The AGM costs twice that of the flooded version of the same capacity. Because of durability, German high performance cars use AGM batteries in favor of the flooded type.
- Inexpensive and simple to manufacture.
- Mature, reliable and well-understood technology – when used correctly, lead-acid is durable and provides dependable service.
- The self-discharge is among the lowest of rechargeable battery systems.
- Capable of high discharge rates.
- Low energy density – poor weight-to-energy ratio limits use to stationary and wheeled applications.
- Cannot be stored in a discharged condition – the cell voltage should never drop below 2.10V.
- Allows only a limited number of full discharge cycles – well suited for standby applications that require only occasional deep discharges.
- lead content and electrolyte make the battery environmentally unfriendly.
- Transportation restrictions on flooded lead acid – there are environmental concerns regarding spillage.
- Thermal runaway can occur if improperly charged.
Batteries that have lithium as their anode are called lithium batteries. The charge moves from anode to cathode during discharge and cathode to anode during charging. Lithium batteries were introduced way back in 1980-1990s. These batteries have completely revolutionized the portable electronics market such as cellular telephones and laptop computers. Today lithium batteries are finding increased applications in electronics, electric cars, and solar markets. Given its lightweight, high energy density and efficiency, lithium batteries are used in a wide range of portable consumer electronic devices, medical equipment, power backups, solar storage and in electric vehicles.
Lead-acid batteries use plates of lead and lead oxide in a sulfuric acid solution. Commonly used in cars and trucks these batteries are also rechargeable. Lead-acid batteries are less expensive, but they have a shorter lifespan and require regular maintenance. Lithium batteries are much more expensive up front, but they are maintenance-free and have a longer lifespan. The leading popularity of lead-acid batteries is due to the fact that these are not only dependable but are also very cheap. However, this technology is old and technicians prefer lithium batteries to better suit the purpose, given its higher efficiencies and densities. Lithium batteries are more efficient and charge faster when compared to lead-acid batteries. Lithium is a premium battery technology with a longer lifespan and higher efficiency.
What is the need for Lithium Batteries?
Solar energy plus storage is already gained traction since economics is driving the faster adoption of solar systems paired with lithium batteries. These batteries are very versatile and besides storing energy for the house, they can also be used in wholesale energy markets for providing capacity and frequency regulation. Solar plus storage option with lithium batteries are also being considered as a serious option for islands as they find it cheaper to use solar plus storage instead of shipping expensive and dirty oil/ diesel to provide power to its residents. Remote area where electricity is not available, such as rural areas, hill stations, small shops, etc. will greatly benefit from these lithium batteries to be used as a storage option with the environment friendly solar systems.
Advantage of Lithium Battery over Existing Technologies
- Logistics- The main benefits of Lithium Battery is portability, that means we can carry anywhere whereas we need. We never think about overflow of acid from battery. The weight of Lithium battery is 1/4th of Lead acid batteries.
- Fast Charging – Charging within 2 hours- Lithium batteries charge and recharge much faster than lead-acid batteries. As solar expert’s experience is that Lithium Battery can charge within 2 hours where as Lead Acid battery takes almost 10 hours to charge full battery. We know that incoming mobile battery charges in small time and gives full day backup.
- Maintenance Free- Lithium batteries do not require any maintenance to ensure their performance. Few other types of batteries like nickel-cadmium cells require discharge to prevent memory effect. Lithium batteries, on the other hand, have no memory effect which means that they do not have to be completely discharged before recharging.
- High Efficiency- Lithium batteries offer reliable, stable, long-lasting power. Their energy density is higher, which means they have high power capacity. This high energy density enables its usage in devices which have high power requirements like laptops and mobile phones.
- Cycle Life – More than 2000 times- These batteries have a lifespan of over ten years on an average. They can also handle more than 2000 times of charge-discharge cycles.
Lithium batteries are generally smaller and lighter in weight. This is a big advantage which helps its application in various portable consumer electronic devices.
Disadvantages of Lithium Batteries
- Expensive- Though the cost of lithium battery is constantly falling, it is still higher than that of Nickel-cadmium cells. However, given its large number of applications, progress is being made towards improving the technology and reducing cost.
- Inverter Compatibility- At this time, Lithium Battery is using in Automobile and DC Applications.
Liquid Metal Battery
The liquid-metal battery is composed of two liquid metal electrodes which are being separated by a molten salt electrolyte—being self-segregate into three layers based upon density and immiscibility. This battery is considered an innovative approach to solve problems regarding grid-scale electricity storage. Its capabilities enable an improved integration of renewable resources into the electric grid. Furthermore, the battery is expected to improve the overall reliability of an ageing power grid and it can offset the need to build additional generation, transmission, and distribution assets.
With all these potentials, the majority of start-ups and companies are targeting utility-scale energy storage with innovative systems that can help in utilizing iron flow batteries, compressed air, saltwater batteries, and other electrochemical processes. These liquid-metal batteries are made for grid energy storage to balance out intermittent renewable power sources such as solar panels and wind turbines. Aside from that, it can also be used for electric vehicles.
The liquid metal battery is based on research conducted by co-founder Donald Sadoway at the Massachusetts Institute of Technology. Unlike the other storage options on the market, this battery system is quite different as it is the only battery where all three active components are in liquid form when the battery operates. Two liquid electrodes—magnesium and antimony, are being separated by a molten salt electrolyte, while the liquid layers float on top of each other based on density differences and immiscibility.
The system is also being operated at an elevated temperature maintained by self-heating during its charging and discharging, which is the reason why it is in a low-cost and long-lasting storage system.
Ambri as one of the startups manufacturing the liquid metal battery is still continuing to improve the performance and longevity of its batteries—some of its test cells have been running and being offered for almost four years without showing any signs of degradation. The company is also experimenting with other elemental combinations, including lithium, calcium and lead. Because of the simple design and easy-to-source materials, the manufacturing process of the battery will cost far less than other storage technologies for an equivalent amount of storage.
History of Liquid Metal Batteries
Professor Donald Sadoway at the Massachusetts Institute of Technology has conducted research about the liquid-metal rechargeable batteries. Magnesium–antimony and lead-antimony were both used in the experiments at MIT. Upon experiments, both the electrode and electrolyte layers were heated until they became liquid and self-segregate due to density and immiscibility. These batteries may have a longer lifespan as compared to conventional batteries, due to a cycle of creation and destruction where the electrodes undergo during the charge-discharge cycle. Thus, making them immune to degradation affecting conventional battery electrodes.
The technology was initially proposed in 2009 which is based on magnesium and antimony separated by molten salt. Magnesium was selected to be the negative electrode due to its low cost and low solubility in the molten-salt electrolyte. Whereas, Antimony was chosen to be the positive electrode for it is low cost and has a higher anticipated discharge voltage.
In 2011, the researchers managed to demonstrate the cell with a lithium anode and a lead-antimony cathode, which have higher ionic conductivity and lower melting points between 350 to 430 °C. The downside of Li chemistry is it has a higher cost. A-Li/LiF plus LiCl and LiI/Pb-Sb cell with about 0.9 V open-circuit potential operating at 450 °C had electroactive material that costs about US$100 per kilowatt-hour and US$100 per kilowatt with 25 years projected lifespan. Its discharge power at 1.1 A/cm2 is only 44 percent while 88 percent at 0.14 A/cm2.
The experimental data indicated that about 69 percent of storage efficiency, has a good storage capacity of over 1000 mAh/cm2, has low leakage of about < 1 mA/cm2 and a high maximal discharge capacity of over 200 mA/cm2. Furthermore, by October 2014, the MIT team managed to achieve operational efficiency of approximately 70 percent at high charge and discharge rates of 275 mA/cm2, which is similar to the pumped-storage of hydroelectricity systems and higher efficiencies at lower currents. Tests showed that the system would retain about 85 percent of its initial capacity after a 10-year regular use.
Whereas, by September 2014, a study illustrated a disposition using a molten alloy of lead and antimony as the positive electrode, liquid lithium as the negative electrode; and a molten mixture of lithium salts as the electrolyte.
Prior to that, the Liquid Metal Battery Corporation (LMBC) was established to commercialize and market the liquid-metal battery technology in 2010. In 2012, LMBC was renamed as Ambri. The name “Ambri” is derived from “cAMBRIdge” Massachusetts, where the LMBC company is headquartered and where MIT is located at the same time.
In 2012 and 2014, Ambri received $40 million funds from Bill Gates, GVB, Khosla Ventures and Total S.A.
Moreover, Ambri made an announcement in September 2015 about the layoff and pushing back of commercial sales. But they also announced a return to the battery business along with a redesigned battery in the year 2016.
A recent innovation is the PbBi alloy which allows a very low melting point lithium-based battery. This battery uses a molten salt electrolyte based on LiCl-LiI and operates at 410 °C.
Voltage Ineﬃciencies of Liquid Metal Battery
Liquidity provides liquid metal batteries with superior transport properties and kinetics. The operating voltage of an electrochemical cell or Ecell differs from the equilibrium cell potential, Ecell, eq, depending on the current density, dependent losses or voltage ineﬃciencies.
Typical voltage ineﬃciencies include:
Charge transfer losses – which result from the sluggish electrode kinetics.
Ohmic losses – that happened to arise from the electrical resistivity of the cell electrolyte electrodes, and current collectors.
Mass transport – in which the losses are caused by slow diﬀusion of reactants to and products away from the electrode−electrolyte interface.
Advantages of Liquid Metal Battery
Liquid metal batteries can boast an ultrafast electrode charge which transfers kinetics because of the liquid to liquid electrode to electrolyte interfaces, high rate capability, as well as low ohmic losses that are being enabled by highly conductive molten salt electrolytes reaching up to 3 S cm−1. This is also followed by the active mass transport of reactants and products to and from the interface of the electrode−electrolyte by liquid-state diﬀusion. Entirely, these properties enable liquid metal batteries to operate with relatively high voltage eﬃciencies at high current densities.
Liquid metal batteries have a very low cost, too because most of the electrode materials being used are earth-abundant and cheaper than other materials. Moreover, the natural self-segregation of the active liquid components enables simpler and lower-cost cell fabrication as compared with other conventional batteries.
Whereas, the most considered feature of these liquid metal batteries is the continuous creation and annihilation of the liquid metal electrodes during the charge−discharge cycles. This feature allows liquid metal batteries the potential for modern life cycle by rendering them to be immune to microstructural electrode degradation mechanisms that can limit the life cycle of a conventional battery. Aside from being modular in design that can be customized in order to meet some specific needs of customers, the liquid material batteries can also respond to grid signals in milliseconds, store energy up to 12 hours and can discharge slowly over time. Plus, it is indeed easy to install without moving parts in the operation.
All in all, the low cost of materials, simple and easy assemble, and the potential for the long lifetimes’ position of liquid metal batteries are all good features of liquid metal batteries to compete in the grid-storage market.
Disadvantages of Liquid Metal Battery
Despite all the advantages, liquid metal batteries also have some disadvantages, which make them inappropriate to use in portable applications and works. These include elevated operating temperatures which are generally less than 200 °C, it also has low theoretical speciﬁc energy density which is typically less than 200 Wh kg−1, and has comparatively low equilibrium cell voltages which are usually less than 1.0 volts. Aside from that, these batteries have highly corrosive active cell components and high self-discharge rates for some chemistries because of the metallic solubility of the electrode species in the molten salt electrolyte. Additionally, there are three liquid layers that make the operation of battery more sensitive to motion and potentially dangerous when the liquid electrodes are touch, thus leading to a short-circuited cell and fleeting heat generation.
Nickle- Cadmium Battery
Nickel cadmium or NiCd batteries have been around since the early 1900s. Though they may not have the energy density (the power) of other technologies, they provide long life and reliability without complex management systems.
Cost: Nickel cadmium is relatively inexpensive compared with other technologies.
Replacement/maintenance: NiCd batteries are vented to allow gases to dissipate. They traditionally require some watering, but new designs allow the gases to recombine to form water which makes the battery nearly maintenance free. This, along with the ability to tolerate extreme temperatures, makes these batteries ideal for off-grid applications in harsh environments. They have been used for storage in megawatt-sized projects. .
Cycling: NiCd batteries are rugged batteries with a high cycle life. Some companies promise a service life of up to 20 years.
Disposal: Cadmium is a hazardous material. In fact Europe limits the applications NiCd batteries can be used in. Toxic materials must be removed before the battery is disposed of. NiCd batteries can be recycled, however. The cadmium can be extracted and reused in new batteries. The nickel can be recovered and used to make stainless steel.
The chemistry behind flow batteries has long been proven in the power industry and most analysts agree they are ideal for long-duration energy output with very low degradation of components within larger, utility-scale deployments.
With life spans reaching up to 30 years, depending on the electrolyte chemistry, flow batteries may provide unrivaled cost certainty versus other emerging storage technologies on the market. Though flow batteries currently represent a higher upfront capital investment than a similar-sized lithium-ion configuration, they become more competitive when evaluated on a total cost of ownership over a 20- to 30-year lifecycle. Moreover, costs are dropping for flow batteries as technology advances and manufacturing efficiencies are implemented.
In the utility space, flow batteries are best suited for longer discharge durations (six hours or more) in megawatt-scale power increments. Certain use cases favor flow batteries over other storage types. For applications where multiple charge/discharge cycles are required each day, flow batteries are available within milliseconds as loads dictate and they can quickly recharge from a variety of available power sources. In fact, depending on tank configurations, flow batteries can discharge and recharge simultaneously, providing power capacity or voltage support almost indefinitely. Attributes of flow batteries include:
- ■ Demonstrated 10,000-plus battery cycles with little or no loss of storage capacity.
- ■ Ramp rates ranging from milliseconds for discharge if pumps are running, to a few seconds if pumps are not.
- ■ Recharge rates for flow batteries also are reasonably fast.
- ■ Wide temperature ranges for operation and standby modes compared to lithium-ion options.
- ■ Little or no fire hazard.
- ■ Chemistries that pose limited human health risk due to exposures.
- ■ Easy scale-up of capacity by adding electrolyte volume (although that may involve more tanks and piping).
How Flow Systems Work
Though there are dozens of different types of flow batteries, only about 10 to 12 specific chemistries appear ready for commercial applications. All operate on the same basic principle of incorporating liquid electrolyte to function as a source of direct current (DC) electricity that runs through an inverter for conversion to alternating current (AC) power.
In a redox flow battery, catholyte and anolyte are stored in separate tanks, and pumps are used to circulate the fluids into a stack with electrodes separated by a thin membrane. This membrane permits ion exchange between the anolyte and catholyte to produce electricity. The power produced is dependent on the surface area of the electrodes, while the storage duration is a function of the electrolyte volume. For some technologies, the power and energy can be scaled independently, allowing for an easily customizable battery.
In a hybrid flow battery, electroactive material is deposited on the surface of the electrode during the charge cycle and then dissolved back into the electrolyte solution during discharge. For hybrid technologies, the storage duration is a function of both the electrolyte volume and the electrode surface area. While most hybrid technologies can achieve durations of six to 12 hours, power and energy are not fully decoupled.
Flow batteries can be configured as both a single tank, usually for smaller applications, or as a dual tank, usually on a larger footprint. The single-tank systems typically feature zinc or other metal batteries, while dual-tank systems require electrolyte comprised of saltwater, iron, vanadium, or other minerals.
Flow battery system designs change depending on the application and project size. Behind-the-meter commercial systems are commonly kilowatt-scale packaged units that can fit into a typical utility room. For distribution applications in the 1-MW to 5-MW range, containerized and/or modular solutions exist with varying levels of scalability depending on the storage duration requirements. Utility-scale designs in development may have millions of gallons of electrolyte storage, so the industry is trending toward large quantities of stack modules headered together and piped to large, field-erected tanks.
Power stacks and balance-of-system components, such as piping, pumps, seals, cooling systems, and control instrumentation, require more routine maintenance than lithium-Ion configurations. However, if routine maintenance guidelines are followed, flow battery performance should not degrade within the project lifetime. When the operations and maintenance (O&M) costs are compared to lithium-ion capacity augmentation costs required to offset performance degradation, flow battery annual costs are less expensive.
Choosing the right battery
Use a sizing calculator
Battery sizing is essential but often overlooked by users and installers. Batteries in PV systems are routinely undersized due to cost or because the system loads were underestimated. It’s important to know the customer’s power needs and correctly plan. Many online calculators provided by battery manufacturers and other software simplifies determining battery capacity for load requirements.
Consider cost of ownership
There are several factors that should be taken into account when determining the total cost of ownership over the life of the battery.
• Price: A battery with a low price is always attractive, but if low price comes at the expense of quality and battery life, the need for frequent battery replacements could boost the cost over time. That’s why it’s important to consider issues other than price when making the decision.
• Capacity: Battery capacity is important because it’s a measure of the amount of energy stored in the battery.
• Voltage: The battery bank voltage must be considered to ensure it matches the system requirements. The battery bank voltage is often determined by the inverter specifications if installing a DC-to-AC system or by the voltage of the loads in a DC system.
• Cycle Life: The most critical consideration is cycle life, which provides the number of discharge/charge cycles the battery can provide before capacity drops to a specified percentage of rated capacity. Batteries from different manufacturers may have the same capacity and energy content and be similar in weight. But design, materials, process and quality influence how long the battery will cycle.