For when the sun doesn’t shine…
A battery stores electrical energy in a reversible chemical reaction. The renewable energy (RE) source (PV, wind, or hydro) produces the energy, and the battery stores it for times of low or no RE production. Most batteries employed in renewable energy systems use the same electro-chemical reactions as the lead-acid battery in your car.
But, unlike your car battery, they are specifically designed for deep cycling. Most renewable energy systems have batteries that store between hundreds of times more energy than a car battery. However, this doesn’t guarantee you will have a consistent performance with batteries. One should consider backup power in case your batteries become discharged due to a lack of renewable energy in the RE system or over-consumption of energy.
There are many brands and types of batteries available for RE systems. It is important to find the right battery for your situation and wallet. The two most common batteries are the L-16 and golf cart sizes. With proper care, RE system batteries have a lifetime of five to ten years, but there are more expensive batteries that are warranted to last ten to twenty years.
Battery capacity is rated in amp-hours. 1 amp-hour is the equivalent of drawing 1 amp steadily for one hour, or 2 amps steadily for half an hour. A typical 12 volt system may have 800 amp-hours of battery capacity. This battery can draw 100 Amps for 8 hours if fully discharged and starting from a fully charged state. This is the equivalent of 1,200 watts for eight hours (watts = amps x volts), or about the same power consumed as running a small hairdryer for eight hours.
However, completely discharging your battery decreases its longevity, and can ruin it in short order. Most home power users will only tap into a portion of available capacity to keep their batteries alive longer. Opinion varies as to the appropriate depth of discharge, but most agree that 50% (and many say 30%) is the maximum a battery should be routinely discharged.
Never go below 80% depth of discharge. 50% means that the above 1200 watt hairdryer would only be used for 4 hours instead of the 8 indicated by the maximum capacity of the batteries.
Batteries typically are encased in plastic and need to be wired together in series and parallel strings by the installer. Some larger batteries are pre-wired and encased in steel containers.
Using DC Power
Using the power directly from the source…
Low voltage DC appliances (mostly 12 VDC) can be operated directly from batteries or photovoltaic modules. For many years good inverters to power the standard 120 VAC appliances common to most modern homes did not exist. Many DC appliances were developed to accommodate these systems, including DC incandescent and fluorescent lighting, televisions, stereos, refrigerators, and even vacuum cleaners and washing machines. These are mostly 12 volt, though some appliances are available in 24 volt models.
Inverters are greatly improved now, making 120 VAC appliances the standard, but many off-the-grid homes still use low voltage DC appliances. Using DC loads is a more efficient use of energy because inverters have a 50 to 95 percent efficiency, depending on the amount of power being consumed through the inverter and the make/model of the inverter.
Due to declining demand, fewer DC appliances are being manufactured. Most of these are being used in third-world applications where inverters and other sophisticated electronics are still beyond the financial means of the users.
Back Up Power
When the sun doesn’t shine and the wind doesn’t blow…
Backup power is needed for those times when your system’s batteries are discharged, usually when you’ve consumed too much energy or when there has not been enough renewable energy coming into your system. This happens to photovoltaic (PV) systems during long periods of cloudiness, especially during the shorter days of winter. It can happen to hydro systems during the dry season, or to wind systems during calm weather. If you live outside of town in a remote area, a generator is vital in case you run out of power.
There are basically two kinds of generators used in RE systems. The most common is the standard 120/240 VAC generator you can buy at a hardware or department store. Since they produce alternating current, they require a battery charger to change it to DC for your batteries. This is not the most efficient way to do things. First, most battery chargers cannot take advantage of the full power available from an AC generator, causing them to run at less than their full efficiency. Second, the charger itself has built-in inefficiencies, especially if it has a large transformer.
The other kind of generator is low voltage DC. These generators are capable of putting nearly their full power into a battery bank since no additional charger is required. The disadvantage of these is that they cannot be used independently of an inverter-based system as a household 120 VAC backup.
If you have access to any combination of solar, hydrogen, and wind, you have the advantage of access to complementary systems. Often when one isn’t able to produce, the other is at its prime. Hydrogen can be used as a source of energy and has hardly any detrimental effects on the environment. Research a variety of renewable energy sources you can utilize to ensure minimum consequences including biomass and geothermal power.
Backup power can come either from a generator or from the utility grid. Most renewable energy users have the goal of not using backup power. In the case of a generator, it’s a noisy, pollution-belching machine. Connecting to the utility grid may not be desired.
Essentially, if you need back up power, your best option is a solar powered generator as they are more environmentally friendly. Any invention, whether it be a vehicle or a household appliance, that can benefit the environment should be the more appealing choice.
Solid State Batteries
High energy density, long cycle life, durability, and safety are among the chief concerns of battery manufacturers today. While conventional lithium-ion liquid electrolyte batteries have enjoyed market domination across industries ranging from portable electronics to electric cars, issues with safety, expensive sealing agents, and catastrophic failure modes caused by the liquid electrolyte have shown that the technology has plenty of room for improvement.
Replacing the liquid electrolyte with a solid could be the solution the battery industry has been looking for. Here’s everything you need to know about solid state batteries.
What are Solid State Batteries?
Solid state batteries are batteries that forgo the conventional liquid electrolyte for a solid one that is to say a battery made up entirely of solid components. As with conventional batteries, they are made up of a cathode, an anode and an electrolyte.
The primary difference lies in the mechanism through which ions travel from one electrode to another through a solid electrolyte membrane.
How Solid State Batteries Work
Regardless of chemistry, solid state batteries use redox reactions to store and deliver energy. Oxidation occurs at the anode, reduction occurs at the cathode and the battery is able to use this phenomenon to store energy (charge) and release it (discharge) as necessary.
During discharge, ions travel through an ion-conductive solid matrix instead of the ionic salt saturated solvent state of typical liquid electrolytes.
Solid State Electrolyte
Solid state electrolytes are fast ion conductors solids that allow ions to move freely throughout the solid’s crystalline matrix. Fast ion conductors are best thought of as a material that lies between crystalline solids that possess a regular structure with fixed ions and structure-less liquid electrolytes with freely flowing ions.
Solid electrolytes often come in the form of gels, glasses, and crystals with novel internal structures. In solid state batteries, solid electrolytes must meet a combination of high ionic conductivity, low internal resistance, and high electronic resistance.
The higher the ionic conductivity is the better the power density and the lower the internal resistance of the battery. The better insulating the solid electrolyte is to electrons, the lower the self-discharge rate and the higher the charge retention. The choice of solid electrolyte depends on the chemistry of the battery, and the ions available for conduction. In the case of lithium ion solid state batteries, a solid electrolyte like LiI/Al2O3 is an excellent Li+ conductor.
Advantages of Solid State Batteries
The best way to understand why solid state batteries are so exciting is to look at the problems caused by liquid electrolyte in lithium ion batteries on the market today. Much of the bulk found in lithium ion batteries is due to separator systems and safety precautions required to deal with the catastrophic failure modes of lithium batteries. Let’s take a look at some of the more pressing problems scientists hope this technology will be able to solve.
No Electrolyte Leakage
The most obvious advantage of solid state batteries is the avoidance of electrolyte leakage. If you’ve ever had to deal with the messy aftermath of some old AA batteries left behind in an old toy, you’re already somewhat familiar with the problem. In order to function, the battery needs a medium through which ions can be transferred during discharge and charge. If a cell dries out, due to exposure, rupture the battery will no longer be able to function.
In higher rate applications electrolyte leakage can be devastating, creating a fire hazard, providing paths for electrical shorts and other problems. Using a solid electrolyte inherently avoids this failure mode. Solid state batteries can help manufacturers by removing the need for advanced sealants, pressurizing electrolytes, and including flame retardant fail safes.
No Thermal Runaway
In batteries, a thermal runaway reaction is a series of cascading exothermic reactions that are accelerated by an increase in temperature that occurs when a cell rapidly discharges its stored energy.
The consequences of this reaction are rising internal cell temperature, rising pressure, venting of flammable gases in the liquid electrolyte, and the risk of explosion and shrapnel. The liquid electrolyte in lithium ion cells is highly flammable, and leakage due to rupture can lead to disastrous consequences, especially in scenarios like an automobile crash. Replacing the flammable liquid with a solid electrolyte can prevent thermal runaway from occurring.
No Dendrite Formation
Cycle life or the total number of charge/discharge cycles a battery can perform is the main metric used by the industry to judge the operating life of a battery. A key limiter on conventional liquid electrolyte batteries is the tendency for metal deposits to form within the battery during charge. These deposits can form dendrites that penetrate through separator material and potentially cause a short.
On a fundamental level, liquid electrolytes are also attacking the electrodes within the battery themselves. The metals will slowly dissociate into the surrounding liquid medium over time, with the ebb and flow of electrolyte during cycling. The more cycles a cell experiences, the more deposits will inevitably form within the cell leading to a short. A solid electrolyte avoids this problem entirely allowing the cells to survive hundreds of thousands of cycles.
Commercial Applications
While solid state batteries have been around for a long time, it is only in recent years that the technology has started to make some sizable steps towards commercial applications.
Advances in material science, computer modeling techniques, electrochemistry and manufacturing have opened up new possibilities to the battery industry. More recently, the British electronics giant Dyson invested $15 million USD into the Michigan based solid state battery company Sakti3, following the ranks of General Motors, Khosla Ventures, Itochu and Khosla Ventures among others for a total of $50 million as of March 2015.
Sakti3 has remained tight-lipped on the exact materials used in their solid state lithium ion battery technology, but their use of large advanced computer modeling algorithms coupled with their focus on manufacturability and process technology has earned them intense interest from major investors in the industry. The future is bright for solid state batteries.
Battery Storage Safety
Batteries do not belong inside your living space. They have dangerous chemicals in them, so they must be contained to avoid spills. They also put out hydrogen and oxygen gas while being charged, so they should be vented to the outdoors. Their tops and connections must be periodically cleaned to avoid energy losses.
Batteries must also be routinely topped off with distilled water. Finally, they need to be “equalized” with an occasional controlled overcharge to keep the individual cells at equal states of charge.
