Solar Energy Storage Methods: Best Ways to Store Solar Power

The Sun is a great source of energy. Ideal to produce electricity, abundant, easily accessible and free. With the latest technology developments, it has become possible to generate electricity from solar energy with the use of photovoltaic systems (PV), and bring this energy into the residential, commercial and industrial sector.

Among the technology options, the one that has quickly gained popularity and that still has the lowest costs, is a grid-tied photovoltaic configuration. This system consists of a set of solar panels, an inverter (or micro-inverters) and a combiner box that gathers all the wiring of the panels.

A grid-tied system receives power in the form of direct current (DC) from solar panels. The inverter, then, transforms this power into alternating current (AC) and sends the energy into the main panel of your house.

What happens when the grid goes down?

Solar panels generate electricity from the sunlight during the day. This means that if the grid goes down at night, solar panels will not generate electricity and therefore you cannot power your house.

At least, this is what most people think when they install a photovoltaic system. But many people also get surprised when their solar system disconnects when the grid goes off during the day!

How can you get electricity when the grid goes down?

First, it is important to know why the grid-tied solar system disconnects.

There are two main reasons:

• Protection of your appliances

Since the sun radiation is a variable source of energy, there will be occasions when the power produced by your solar panels will not be sufficient to supply the full load.

If your panels are unstably producing electricity during the day and they are connected to the main panel, your electrical appliances could experience flickering, drops in voltages and other issues that could damage them [1].

• Security standards of utility companies

The security reason focuses on the safety of the utility technical workers. In most cases, the grid goes off because of a failure in the distribution system. The utility company, therefore, sends technical experts to fix the problem.

If your solar system would continue generating power during the blackout while it is connected to the grid, utility company employees could be seriously injured by a back-feed in the line (distribution line with electrical current from the energy injected by your solar system).

So the question remains:

Can solar panels provide electricity for your house when the grid goes off?

The answer is: Yes! They can.

How to store solar energy at home?

When you install a grid-tied solar system, the power grid acts as an immense source of energy storage.

On the other hand, there is also a possibility of storing solar energy in batteries. In this scenario, a solar battery bank simply acts as a replacement of the grid. So, if you add a battery bank to your PV system, you will be able to have electricity even when the grid goes off.

Solar battery storage FAQ

How do batteries store solar energy?

In 1793, Alessandro Volta constructed the first model of the battery. Volta discovered how to successfully convert electrical energy into chemical energy (and vice versa).

He found that in order to produce electric current two different metals in the form of electrodes need to be placed into a solution known as the electrolyte – an ionic liquid that conducts electricity.

This reaction happens when electrons from one electrode are transferred to the other electrode through the “electrical circuit”. The electrode that acts as the positive terminal (anode) allows electrons to flow out of it (= oxidation), while the negative terminal (cathode) receives these electrons (= reduction). That is how it is possible to store electricity in the chemical form.

This first battery model was known as a voltaic cell and represents generally a value close to 2 volts. It is possible to achieve higher voltages by combining several cells together and summing up their voltage – that is how a battery pack is created.

Generally, with 3, 6 or 12 cells it is possible to obtain battery packs of 6 volts, 12 volts and 24 volts respectively [2].

Now that you got an idea of the basic principle of how batteries store electricity, you can better understand how they store solar energy.

Once the radiation from the sunlight hits solar panels, photons release electrons. This makes DC current flow through solar cells. These electrons are then collected by the bus bars in the panels and sent through electrical wires into the charge controller.

Charge controller is a device that is programmed to charge batteries in such a way that the exact current flows in and the voltage limits are within the acceptable boundaries.

The process of battery charging involves several phases:

1. bulk charge
2. absorption charge
3. finish charge
4. floating charge

 The cycle begins with a constant charge current between 10 to 20 percent of nominal rated current. Then, when the battery reaches the absorption voltage, the incoming current begins to slowly decrease while the voltage increases until the finish charge phase begins. The current stabilizes again to the minimum value until the battery enters into the floating charge [3].

How to store batteries?

Batteries are an essential component of any backup solar system. Naturally, they need a good storage space in your household.

Solar battery storage space cannot be any place. You need to take some important criteria into consideration. Remember that batteries function based on chemical reaction, and therefore, factors like temperature, humidity, dust, pressure and other can affect the battery lifetime, efficiency and in some cases even its safety.

Presence of any of these elements can lead to leakage currents in the battery, which can lead to self-discharge, short-circuits, moisture and sulfation.

Charge controllers must also be placed in clean and ventilated areas with easy access. For both components, relative humidity values should remain below 90 percent [3].

Temperature is another important consideration. High operating temperatures will shorten battery efficiency. It is recommended that these devices are stored in areas with ambient temperatures close to 25 degrees Celsius (77 degrees Fahrenheit).

Although solar batteries are able to work in a relatively wide range of operating temperatures (depending on the type of solar battery), the average range oscillates between minus 20 to plus 50 degrees Celsius (-4 to 122°F) for VRLA batteries, and between 20 to 45 degrees Celsius (68 to 113°F) for ventilated batteries (more information will be discussed below).

Batteries should not be placed by any means close to heat sources like radiators or heaters, and should not be exposed to cold weather either [3].

The different types of solar batteries

Batteries for any solar system must fulfill one fundamental prerequisite: they must be deep-cycle batteries.

To supply electricity throughout the night implies that solar batteries often get drained to their minimum levels. Batteries that are not conceived as deep-cycle would get damaged if they would be submitted to such operating patterns every day. In some cases, they could even set on fire. That is why the so-called ignition batteries cannot be chosen for solar energy applications.

Batteries contain two types of electrodes which are from different metals – depending on the type of selected metals, the properties and parameters of a battery will change.

Currently, there are multiple battery types on the market.

These are:

• Lithium-ion batteries
• Nickel batteries
• Sodium sulfur batteries
• Flow redox batteries
• Lead acid batteries

The best option: Lead Acid Batteries

But none of these types has better performance and is as cost-effective as the lead acid batteries [4]. Lead acid batteries have an electrode filled with lead oxide (PbO2) and the other electrode is filled with pure lead (Pb). The electrolyte used in this type of battery is the sulfuric acid.

Lead acid batteries can be divided into two main groups:

• Vented Lead Acid Batteries (VLA)

These batteries have small holes that are closed by removable plugs. They allow you to verify the level and specific gravity of the electrolyte as well as refill distilled water lost during chemical reactions.

These batteries generate gases produced by the internal chemical reactions, and release them into the atmosphere, this is why they require periodic maintenance and continuous refilling of distilled water. They must be always placed in the correct angle (upside) to avoid any fluid spills.

Due to these reasons, these batteries are not the most commonly used for solar applications, although some designers use these batteries for their lower costs.

VLA batteries can be divided into three main categories:

• Starting, lighting and ignition batteries – not used for solar purposes;
• Deep cycle batteries – used for solar applications as they are designed to supply relatively low amounts of current over long periods of time, and have higher maximum number of deep discharge cycles;
• Stationary batteries – stationary batteries are used mainly to supply energy in control operations. They have a long lifetime and can endure deep cycle routines, but they must be constantly charged (floating charge) to compensate losses due to self-discharge [6].

• Vented Regulated Lead Acid Batteries (VRLA)

These are the top-rated battery option for solar PV applications. VRLA batteries are regulated and partially sealed to avoid evaporation of the electrolyte. The battery contains a valve which is sensitive to pressure and automatically controls the emission of gases. The valve is normally closed, but if it detects a high level of pressure inside the battery, the valve opens to release the pressure.

A great advantage of these batteries is that they recombine oxygen and hydrogen through the electrochemical process that allows them to recover lost water in the energy conversion. This process is 99 percent efficient, which is why it is generally assumed that there are no water losses, and therefore, no maintenance is needed from your side.

The downside of these batteries is that they are more sensitive to temperature changes and they do not allow to reliably check the State of Charge (SOC).

They can be divided into two main groups:

• Gel batteries – Gel batteries add a particular compound of silicon to the electrolyte, which changes its consistency to a gel. This process allows them to have a longer lifetime (more than VLA) and also allows them to endure more cycles of charge and discharge.
  
  Gel batteries also perform deep cycles with high temperature values and even with vibrations. They offer a stable discharge voltage and do not require maintenance at all. They can be placed in any position (because the electrolyte is gel and does not spills) and are also resistant to low temperatures [3,5].
• Absorbed Glass Mat (AGM) batteries – these batteries contain electrolyte that is absorbed by a fiber glass base that acts like a sponge and immobilizes the sulfuric acid. These batteries also allow the performance of deep cycles, but they can withstand higher voltage charges than gel batteries. This at the end means higher efficiency [3,5].

How long do solar batteries last?

The battery performance and condition over its lifetime determines how long it will last.

To understand this, it is necessary to explain some technical parameters of the battery. One of them is the maximum depth of discharge (DoD).

The DoD refers to the maximum amount of energy that can be extracted from a fully charged battery without damaging the battery.

The other parameter of interest is the maximum number of cycles (charges and discharges) that the battery is designed to endure. This value is intrinsic to the depth of discharge, as the higher the DoD, the less maximum number of cycles [8].

Now that those parameters are clear, the lifetime expectancy of a battery will depend on these two factors, on the environmental and operational conditions, and on the selected type of battery.

VLA Deep-cycle batteries will have 3-5 years of life expectancy, while at the same time have a lifetime expectancy of 1,500 cycles.

Stationary batteries will usually have 15-30 years of lifetime expectancy (the highest value among lead acid batteries) and VRLA will usually have between 5-10 years of life expectancy with a range between 250 – 500 cycles [6].

This means that you will need to replace the batteries at least once over the lifetime of the solar system.

The best batteries for solar power storage

In general, the Lead Acid batteries are the best batteries for solar power storage – particularly, the Vented Regulated Lead Acid Batteries are the best solution for homes or buildings where ventilated areas are limited or where batteries need to be placed inside the house.

The selection of the solar battery type for your solar PV system also depends on:

• the specifications of the inverter
• the nominal voltage of the system
• charge regulation requirements
• autonomy and size
• maximum depth of discharge
• self-discharge
• efficiency
• operating temperatures
• maintenance availability
• costs
• lifetime expectancy [7]

 Besides, you must also keep in mind the capacity and power of the battery. The capacity (measured in Ah) will tell you how much energy the battery can deliver within a specified time.

The other important parameter to take into account is its power rating. Power rating tells you how much power is battery able to deliver instantly – generally measured in kW.

Another parameter to observe is the useful capacity. This parameter reflects how much capacity is available for the use from the battery, while not exceeding the maximum depth of discharge limit. It is generally measured in Ah and is calculated by multiplying the nominal capacity by the maximum depth of discharge.

As you can see, there is not a rule of thumb to decide which battery is the best for your solar system. The key lies in balancing all these factors and making the decision which battery suits the needs and the requirements of your home better.

How many batteries do I need for my PV system?

That is a question that actually depends on several aspects:

• Type of your PV system 

There are two main types of PV systems with a battery backup: a grid-tied system with battery and the off-grid configuration.

If you choose the first option, your solar battery system does not need to be so big because blackouts in the most western countries usually do not last over prolonged periods. Your solar system should be fine even with a small battery bank.

But if you choose the off-grid option, your system needs to be completely self-dependent. Your battery bank will need to cover all your energy demand.

• Energy demand (Wh)

For this parameter, you need to measure the average energy demand of your entire household. To get this number, check the power utility bills to get the closest estimate.

Remember that the consumption you get in a month also includes activities that are more of a luxury to use in the off-grid scenario. Therefore, select only the most essential electronic appliances, such as lighting, electronic outlets, fridge, TV, and try to rule out extra loads like microwaves, irons, dryers, washing machines, heaters or air conditioners.

Once you have calculated the basic load your household has, check the power demand of each device in watts and multiply the value by the number of hours you want to run them without the grid power.

Remember the more hours you choose, the bigger and more expensive your PV system will need to be. Once you get all those values (in Watt-hours), sum them up and you will get the energy demand that your batteries need to cover.

• Autonomy

This parameter represents the number of days you want your system to operate without the power grid. In off-grid systems the minimum value is 3 days, while in the grid-tied systems with battery backup the autonomy is between 12 to 24 hours.

• Capacity (Ah)

The capacity of the battery determines the amount of energy it is able to provide within a specified time. Capacity is usually measured in Ampere-hours (Ah) and is usually accompanied by the C_x acronym, where the “x” establishes the number of hours when the battery can provide a constant electrical current.

As an example, if the battery is 200 Ah C_10, then it can discharge a constant of 20 A for 10 hours.

If you would like to know the amount of energy it can deliver in kWh, you need to multiply this number by the voltage of the battery (typically 12 V) [9].

• Voltage of the system (V)

Voltage of the system depends on the amount of power you want to back up. In general, most home appliances consume between 800 to 1,600 Watts. In this case, the voltage you need is 24 Volts.

When power ranges between 1,600 to 3,000 Watts, then a 48 Volt bank is the right choice.

For higher power demand, increase the voltage accordingly, but keep in mind that current levels should remain below 100 A in the total circuitry [11].

Once all of these parameters are clear, then you can estimate the capacity of the power bank by using this formula:

The next step is to choose a battery model and check its capacity (C_CB) as well as the voltage (V_B). With these two parameters you will be able to determine how many batteries you need to connect in parallel and how many in series.

Storing solar energy without batteries

Batteries are the most used form of solar energy storage, but there are even other options to store electricity of your PV system.

One of them is directing the electricity from your PV to water electrolyzers, which generate hydrogen gas. Hydrogen is then stored and used as feedstock for fuel cells to generate electricity when needed. This is called R&D solution and is more suited for industrial applications.

Another option is to store electricity in super capacitors, which can be later discharged to generate electricity when needed. This method is very expensive.

A brilliant option is to store solar electricity in the form of potential energy of water pumped to higher elevations. When needed, this stored water potential can be converted into kinetic energy and spins turbines, which generate electricity (a combination of hydroelectricity and PV) [12].

There is also an option to store solar energy in the form of heat, which is the main form of storage in concentrated solar power plants, where the heat transfer fluid passes through the receiver (where all the heat is concentrated), absorbs thermal energy and then stores it in hot thermal tanks that are available for usage when the electricity is needed.

Finally, one of the most interesting solutions to store PV electricity comes from E.ON – the German utility company. E.ON customers can freely feed the excess electricity to the “E.ON Solar Cloud,”  which is a virtual electricity account that can be accessed at any time. You can even access the data from your mobile application to check how much stored energy is available in the cloud.

The utility makes sure that the grid is balanced all the time by taking the feed-in and the consumed electricity from this cloud. This highly innovative solution is perfect for solar energy storage, as no capital nor maintenance costs are involved. The proposal was recently announced in January of 2018, so it is just a matter of time until it will be applied in other places as well [13].

Can solar panels store energy for later use?

No, solar panels only generate electricity. They are not able to store energy in any way.

The concept of solar panels is to transform the radiation of the sunlight into DC electricity and send it to the main panel of your house.

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