In this lesson, we will discuss topics that lead to answers to all the questions in the scenario above. We will provide PV designers with a basic understanding of batteries to help them choose the best battery to fit their application. Although the majority of PV applications are grid-connected, designers may encounter off-grid PV systems with storage systems if they are involved in that market sector. That said, participating in this lesson will put you in the right direction when selecting the right technology for your application and will help you perform basic assessments for optimal design.

Learning Outcomes

At the successful completion of this lesson, students should be able to:

• Identify types of electrical energy storage technologies and how they relate to (Energy or Power) responses.
• Identify battery parameters and specifications including capacity, efficiency, discharge rate, state of charge and depth of discharge.
• Interpret battery manufacturer datasheets to match PV system requirements.
• Describe factors that influence the lifetime of a battery, such as temperature and charge and discharge control.

Due to the advancement in the distributed generation systems for electricity, there is an increased demand for energy storage at both small and large scale applications. As we learned in lesson 1, PV is one of the fastest growing technologies in the electrical generation market, and in some cases it requires storage systems. In this lesson, we will focus on the complementary energy source that is usually coupled with a PV system, which is the storage system.

Storage is needed in PV systems to overcome the intermittency of the energy generated. These variations could be caused due to daily or monthly solar irradiance fluctuations. Daily fluctuation occurs as a result of the change of solar irradiance within the 24-hour period; while the seasonal (or monthly) fluctuations occur due to the change of solar irradiance across the summer and winter as seen in Figure 3.1. If we observe the monthly energy production of a 1 kW PV system installed in State College, PA, we can see that the generation increases during the summer months while the energy generation drops during the winter months. We can see that the PV system will not generate the same amount of energy each month. This means we need another complementary system to help even out the energy difference throughout the year.

Figure 3.1: Monthly Energy Production for 1kW PV system in State College, PA

Credit: Developed using SAM

In addition to solving the critical intermittency generations issue of solar PV, storage systems provide:

1. A potential resource for shifting energy from one time to another
2. A local source of energy to supply peak demand and potential enhancement for better reliability and resiliency.
3. A method to enable load shifting to improve asset utilization and defer other capital investment

How do we make an optimal choice of a storage system?

Depending on the desired application intended for a storage system, there are many factors that affect the selection decision. In general, each application requires either more power from the storage system or it may require more energy. In Figure 3.2, the Ragone plot illustrates the power/energy density of various storage technologies. It can be seen that some technologies, such as fuel cells, can generate higher energy density (in wh/kg) than technologies with lower energy density such as capacitors. In other words, fuel cells can supply energy for longer time periods than can capacitors. In contrast, capacitors have higher power density (W/kg) than fuel cells, which means capacitors supply higher power for short periods than do fuel cells. For example, a capacitor is a good choice for high power density requirements, but it is not the optimal choice for high energy density, as we can see in Figure 3.2.

Figure 3.2: Ragone plot representing the Energy density vs Power density of different storage systems

Credit: Stan Zurek (raster) [GFDL [5], CC-BY-SA-3.0 [6]], via Wikimedia Commons

For most solar applications, we need a good balance between both high energy density and a high power density.

Batteries are electrochemical devices that convert chemical energy into electrical energy. Batteries are classified as primary and secondary batteries.

Primary batteries

• Convert chemical energy to electrical energy and cannot be recharged.
• Examples include zinc carbon batteries and alkaline batteries.

Secondary batteries

• AKA rechargeable batteries, convert chemical energy to electrical energy and are rechargeable when the chemical reaction is reversed using forced electrical energy.
• Examples include lead-acid batteries and lithium-ion batteries.

There are several kinds of available secondary battery technologies that could be used for different applications, such as lead-acid and lithium-ion batteries. Lead-acid batteries use the oldest and most mature battery technology available, although lithium-ion batteries are being heavily researched and used recently (more work needs to be done regarding the costs! they may still not be competitive in some areas in the world).

So how do we compare batteries for best candidate for PV application?

Let's look at the Ragone plot specific to available batteries. This is slightly different from the Ragone plot shown earlier. Figure 3.3 illustrates the comparison between various battery technologies in terms of gravimetric energy density and volumetric energy density. If we compare battery technologies based on both the energy per volume and energy per weight, we can see that lead-acid batteries have less energy density than Li-Ion batteries. As you move on the "x" axis, the gravimetric energy density increases. In other words, the battery offers higher energy per unit of weight. On the "y" axis, the volumetric energy density increases as we go up. In other words, the amount of energy in higher per unit of volume.

Figure 3.3: Ragone plot showing the Volumetric energy vs Gravimetric energy of different battery types

Credit: Ragone Plot for diff Li batteries [7] / Chem511grpThinLiBat / CC BY-SA 4.0 [8]

Volumetric energy density is the amount of energy stored per unit volume of battery. The typical unit of measurement is Wh/l. We can observe that the higher the volumetric energy density, the smaller the battery size.

Gravimetric energy density is the amount of energy stored per unit mass of the battery. The typical unit of measurement is Wh/kg. We can also observe that the greater the gravimetric energy density, the lighter the battery.

As shown in Figure 3.3, lead-acid shows the lowest volumetric and gravimetric energy densities among the batteries, while Li-ion exhibits the best combination.

That said, let's look a little bit more closely at the lead-acid battery.

Similar to most batteries, the lead-acid battery consists of several individual cells, each of which has a nominal voltage of around 2 V. Lead-acid batteries could have different types of assembly. For example, the common lead-acid battery pack voltage is 12 V, which means 6 cells are connected in series.

When the battery is recharged, the flow of electrons is reversed, as the external circuit doesn't have a load, but a source that has a higher voltage than the battery can enable the reverse reaction. In a PV system, this source is nothing but the PV module or array providing solar power and can charge the battery when the sun is available. As we learned previously in Lesson 1, the use of storage is more common in the stand-alone PV systems, because there is no other source of power to support the PV array when the sun is not available. In other words, the loads are at the mercy of the availability of the sun. In that case, an energy storage option such as batteries can be very useful. As an example, a typical daily solar irradiance profile is shown in Figure 3.4. If we observe the orange curve that represents the daily solar irradiance, we can see that a significant amount of energy is generated during the daytime while no energy is generated during the nighttime. On the other hand, the daily energy demand represented in the blue curve shows that energy is needed all day long, with higher demands at certain time periods. When we put the daily load demand curve (aka daily load profile) on the same figure, we see that a significant energy demand exists when there is no sun.

Figure 3.4: Daily solar irradiance (Orange) and daily load profile (Blue) for State College, PA.

Credit: Developed using SAM

For utility-interactive systems, the excess energy is fed back to the grid while the load demand can be supplied from the grid when the sun is not available.

As for a stand-alone system without storage, even though the sun has more than enough power during the day, the system fails to utilize this excess energy to power the loads when the sun is not available.

With the introduction of battery storage, the excess energy from the sun during the day can be stored in a battery and then used later to meet the load demand when the sun is not available. This is represented in highlighted areas A1 and A2 in Figure 3.5, below, for excess solar power and evening load demand respectively.

The perfect match occurs when area A1 equals to area A2 and that can be accomplished by perfectly sizing the solar PV system to meet the average daily load energy demand. Furthermore, excess solar energy can be stored using Battery systems.

Figure 3.5: Excess solar energy highlighted (green) and daily load highlighted (red) for State College, PA.

Credit: Developed using SAM

In summary, we have seen different types of battery technologies and discussed why lead-acid can be the battery of choice for PV systems globally but Li-ion has become the dominant technology recently. We will talk in detail about battery parameters in the next topic. We will also see how managing battery parameters is a whole new optimization challenge on its own.

Batteries are the final commercial product that are delivered to customers and that require some data provided from the manufacturers to allow customers to evaluate the performance of different battery types in terms of capacity rating, allowable DOD, and temperature operating ranges. Most datasheets come with some curves that a PV designer should be able to perfectly interpret for best design practices.

In this section, we will discuss basic parameters of batteries and main factors that affect the performance of the battery.

The first important parameters are the voltage and capacity ratings of the battery.

Every battery comes with a certain voltage and capacity rating. As briefly discussed earlier, there are cells inside each battery that form the voltage level, and that battery rated voltage is the nominal voltage at which the battery is supposed to operate.

The capacity refers to the amount of charge that the battery can deliver at the rated voltage, which is directly proportional to the amount of electrode material in the battery.

The unit for measuring battery capacity is ampere-hour or amp-hour, denoted as (Ah). The capacity can also be expressed in terms of energy capacity of the battery. The energy capacity is the rated battery voltage in volts multiplied by battery capacity in amp-hours, giving total battery energy capacity in watt-hours (wh). In general, it is the total amount of energy that the device can store.

You must be wondering what is the significance of amp-hours as the unit of battery capacity? The unit itself gives us some important clues about battery properties. A brand new battery with a 100 amp-hour capacity can theoretically deliver a 1 A current for 100 hours at room temperature. In practice, this is not the case due to several factors, as we will see later.

C-rate

Let's move to another important battery parameter, called the C-rate. C-rate is the discharge rate of the battery relative to its capacity. The C-rate "number" is nothing but the discharge current, at which the battery is being discharged, over the nominal battery capacity. It is calculated as the following:

Where

"I_dis" is the discharge current

"C_non" is the nominal battery capacity

The discharge rate is sometimes referred to as C/”number” and that number is the number of hours it takes the battery to be fully discharged. In other words, it is the inverse of the previous notation, and it is calculated as the following:

For example, a C-rate of 1C for 100 Ah capacity battery would correspond to a discharge current of 100 A over 1 hour. Or it can be represented as C/1. On the other hand, a C-rate of 2C for the same battery would correspond to a discharge current of 200 A over half an hour. Or it can be represented as C/0.5. Similarly, a C-rate of 0.05C implies a discharge current of 5 A over 20 hours. Or it can be represented as C/20. Finally, the same battery can be discharged at 1 A over 100 hours, and that corresponds to 0.01C or C/100. In general, C-rate depends on charging and discharging current.

Efficiency

Since there is no energy conversion system that is 100% efficient, the term efficiency represents the system capability to transfer energy from the input of the system to the output. Each battery type comes with different efficiency rating as discussed in EME 812 (9.3. Battery storage - Table 9.1) [4], and usually we talk about efficiencies of both charge and discharge combined.

Battery efficiency is the ratio of total storage system input to the total storage system output. For example, if 10 kWh is pumped into the battery while charging, and you can effectively retrieve only 8 kWh while discharging, then the round trip efficiency of the storage system is 80%.

Let's discuss another important battery parameter, the state of charge or SOC. It is defined as the percentage of the battery capacity available for discharge, so thus, a 100 Ah rated battery that has been drained by 20 Ah had an SOC of 80%. Another parameter that complements the SOC is the depth of discharge or DOD, which is the percentage of the battery capacity that has been discharged. Thus, a 100 Ah battery that has been drained by 20 Ah has a DOD of 20%. In other words, the DOD and SOC are complementary to one another.

Now we come to a very important parameter: the cycle lifetime of the battery. Cycle lifetime is defined as the number of charging and discharging cycles after which the battery capacity drops below 80% of the nominal value. Usually, the cycle life is specified as an absolute number. However, to be more precise, cycle life and other battery parameters are affected by changing ambient condition such (temperature in this case).

So what is the relationship between the battery parameters? The cycle life depends heavily on the depth of discharge. This can be seen in Figure 3.6 for a typical flooded lead-acid battery. If we look at the effective capacity at different depth of discahrge (DOD) rates for a lead-acid battery, we can see that the cycle number diminishes as the DOD increases.

Figure 3.6: The effective capacity (%) vs cycle number at different DOD rates for a flooded lead-acid battery.

Credit: Developed using SAM

Cycle lifetime also depends on the temperature. The battery lasts longer under colder temperatures of operation. Furthermore, we can observe from Figure 3.6 that for a particular temperature, cycle lifetime depends non-linearly on the depth of discharge. The smaller the DOD, the higher the cycle lifetime. However, such a higher cycle life would also mean that those additional cycles you gain can only help you for a smaller depth of discharge. Thus, it could be said that the battery will last longer if the average DOD could be reduced over its normal operation. Also, battery overheating should be strictly controlled. Overheating could occur due to overcharging and subsequent overvoltage of the lead-acid battery. We will learn more about voltage and charge control of the battery in the next section.

While battery life is increased at lower temperatures, there is one more effect that needs to be considered. The temperature affects battery capacity during regular use, too. As seen in Figure 3.7, the lower the temperature, the lower the battery capacity. The Higher the temperature, the higher the battery capacity.

Figure 3.7: the effective capacity (%) vs temperature for a flooded lead-acid battery

Credit: Developed using SAM

Battery Aging Factors

It might not seem scientific, but it is even possible to reach an above rated capacity of the battery at high temperatures. However, such high temperatures are severely detrimental to battery health.

When we say that a battery has a limited cycle life, or that it has completely "run out of juice," what exactly does that mean? Is it related to the aging effect of the lead-acid battery?

There are several factors that contribute to the aging of any battery. Sulphation is one of the major causes of aging. And if the battery is not fully recharged after being heavily discharged, that causes sulphate crystals to grow, which cannot be completely transformed back into lead or lead oxide. As a result, the battery slowly loses the mass of active material and therefore discharge capacity will be lower. Corrosion of lead grid at the electrode is another common aging factor. This leads to increased grid resistance due to high positive potentials.

Moving further, when the battery loses moisture, it causes the electrolyte to dry out, which occurs at high charging voltages, resulting in loss of water. It is referred to as gassing effect and may limit battery lifetime. This should be taken care of with routine maintenance by adding distilled water to the battery.

Researchers have developed maintenance free lead-acid batteries for solar systems that exhibit very high lifetimes. However, these are also high-end products and can be more expensive.

After we covered all basic battery parameters and characteristic curves, a designer should be able to make the best selection for a product depending on the application. But how do designers put these batteries in place? Is there only one size for all batteries, and it is scalable? Or do they make a custom design battery for each project? We will answer these questions in the next section.

Batteries are usually installed in groups for PV applications. In this case, the parallel and series connection of batteries is referred to as the Battery Bank. Lead-acid batteries are usually rated at 12 V, 24 V or 48 V. This voltage is determined by the series and parallel interconnection of several batteries. The voltage needs to meet the load or inverter voltage level requirements.

The series and parallel connection principles are similar to PV modules where we add voltage when connected in series while current is added for parallel connections of batteries. Similar to PV, groups of batteries connected in parallel are called a Battery String. As for the capacity rating of a battery bank, it is similar to the current principle. When connecting batteries in series, the capacity is not added. As for a parallel connection, the capacities add up.

Figure 3.8 illustrates the series and parallel connections of batteries and the corresponding voltage and current. As can be seen, batteries can be connected in series, parallel, or both. In this case, each battery with "V" for voltage and "I" for current is connected either in series or parallel with other similar batteries. The total voltage and current depends on the wiring type. In case of series connection, the total voltage of three batteries will be 3V while the current is similar to the current of a single battery. When three batteries are connected in parallel, the voltage equals the voltage of a single battery, while the total current is the sum of the currents of all batteries for a total of 3I. When two strings of three batteries are connected in parallel, the total voltage will be 3V, while current is summed for the two strings for a total of 2I. 

Figure 3.8: Series and parallel connection of battery bank.

Credit: Mohamed Amer Chaaban

It is recommended to have as few battery strings as possible to avoid voltage differences that may create power loss. In larger PV installations where more battery banks are required, it is recommended to connect more batteries in series rather than parallel strings. An example of a mobile bus that is converted to a solar stand-alone system with batteries is shown in Figure 3.9.

Figure 3.9: Bus turned into a mobile demonstration of sustainable living. Its interior is powered with solar energy and batteries.

Credit: Ecological Bus Project, Solar battery bank below [9] / ArtofCulturalEvolution / CC BY-SA 3.0 [10]

Design considerations

When designing a battery bank for a specific location, a good design will ensure that the battery bank is perfectly:

1. sized so the energy capacity matches the load requirements.
2. sized for maximum and minimum voltage requirements for the desired application.
3. ventilated to meet temperature requirements.
4. maintained to maximize the utilization of batteries.

This week, you will continue working on your Report on Part B.

Let's go back to our scenario from the beginning of this lesson. You came back from the solar trade show, and you collected some datasheets for battery types and technologies from different manufacturers. Now that you are fully knowledgeable about the basic information of battery technology types, efficiency, the capacity rating, and voltage and current ratings, you can work on the report needed by your director. In addition, you can confidently recommend the best battery candidate for the solar firm for which you work.

As a solar designer, you are equipped with the right information to choose battery technology based on the factors that affect performance of batteries when they are installed, such as rate of discharge and the ambient temperature. Finally, you can suggest methods to increase the cycle life of the battery bank.

The next lesson will discuss the heart of any PV system, which is the "Power Conditioning Unit." We will cover a variety of topics starting from basic characteristics to factors that affect performance, and finally, stringing tools considering inverter's parameters that lead to optimal PV system design.