Overview

This lesson will overview energy storage options for large-scale solar facilities. Clearly, energy demand rarely coincides with energy generation. Being bound to daily solar activity cycle at a certain locale, solar energy conversion systems are intermittent by nature, therefore, using energy at nighttime requires technologies to store energy on site. Photovoltaic systems, which convert natural solar resource into electric power, require means for electrical energy storage, while CSP systems may be better off storing thermal energy. The thermal storage principles and technology were discussed in the previous lesson, and Lesson 9 is primarily concerned with the technologies used to store electric power. For storage, electrical energy is often converted to other kinds of energy; for instance, potential mechanical, kinetic mechanical or chemical energy, which would be stored as fuel. Energy storage research has been accelerated over the recent years to address the need for compact and economically efficient storage technologies and is currently define the rate implementation of the commercial renewable energy systems. This lesson readings provide an overview and resources for learning the storage technology principles, with understanding that some of those options are much more advanced and rapidly evolving than others.  

Learning Outcomes

By the end of this lesson, you should be able to:

1. explain the technical principles of energy storage used in utility-scale solar plants;
2. understand the performance metrics for storage systems;
3. articulate technological, environmental, and economic cons and pros of energy storage implementation.

9.1. Options for energy storage

Because solar energy supply is variable in time, energy storage is an important issue. Energy storage is used to collect the energy generated by the solar conversion systems (thermal or photovoltaic) in order to release it later on demand. This can be a situation when sufficient power is produced during the day, and stored energy is used during the night. Also, when insolation conditions are ideal, the solar system may produce enough power for the target application, but on dull days, direct energy supply from collectors is diminished, and the energy from the storage is used to compensate the deficit. Energy storage devices help to smooth out differences and minor fluctuations in energy supply caused by shading, passing clouds, etc. Development of efficient and cost-effective energy storage is considered the main bottleneck of the universal development of solar systems.

Video: How solar energy got so cheap, and why it's not everywhere (yet) (7:53)

There are quite a few different technology options for energy storage, which are briefly outlined below:

1. Grid. For grid connected solar systems, the most natural and cost-effective way would be to store energy in the grid. The main idea here is that the DC power from a solar facility (array or farm) is converted to AC power and is fed to the grid and further on is used for on-site or off-site applications. This way, the grid acts as the medium that collects energy from different power-making facilities (renewable or non-renewable) and redistributes it as necessary. Since a grid does not really represent a separate system that is part of a solar plant, it will not be discussed further in this lesson.
2. Fluid. Fluid-based storage is typically used with solar thermal systems. Unlike grid, which stores electrical energy, fluids store thermal energy. Fluids, such as water, oil, molten salt or others act as a medium for absorbing heat. The main idea is that the solar radiation heats the heat-transfer fluid which is accumulated in the tank. The tank is insulated, so the hot fluid keeps its temperature for a substantial period of time. When needed, the heated fluid is used in a heat-exchanger to produce steam for the electric generator. This type of thermal energy storage was discussed in more detail in Lesson 8.
3. Battery. A battery is an electrochemical device that stores chemical energy in internal components and releases energy as electricity, which is generated through electrochemical reactions. Batteries are reversible, i.e., can be charged and discharged, and the parameters of these processes are regulated to avoid damage by overcharging or over-discharging. Battery life is expressed in number of charge-discharge cycles. There are many different types of batteries, some of which will be discussed further in this lesson.
4. Hydrogen. The idea behind hydrogen storage is that electricity generated by solar PV systems can be used to electrolyze water - to split it to hydrogen and oxygen. Further, hydrogen gas is collected and can be used as a fuel. One of the highly efficient devices "converting" hydrogen back to electricity is H₂/O₂ fuel cell, which has zero carbon footprint during operation.
5. Compressed air. In this case, the electrical energy produced by a PV solar system is used to run compressors to compress massive amounts of air and store it in underground, above-ground, or underwater containers. Later on, when energy is needed, the air is decompressed and is supplied to a turbine to generate electricity. Compressed air energy systems (CAES) promise high efficiencies, although this technology is not yet widely implemented.
6. Pumped storage hydropower. The available energy can be used to pump water into an elevated reservoir for storage. When power is needed, the water can be discharged under gravity to run a turbine, which is connected to a generator to produce electricity. The same as compressed air systems, the pumped storage technology has high energy return on investment, although it may require special topographical conditions and water availability in order to be used.

All of the above options for energy storage should be employed with understanding the facility needs and capacity. What energy storage is efficient for small residential systems may be insufficient or too costly when scaled up to the utility-size systems. Determining capacity of energy storage for a particular solar project is an important technical and economic issue. For example, if the capacity of the storage is too large compared to the energy produced by the solar conversion facility, the total system cost will be unnecessarily increased. On the contrary, if the capacity of the storage is too small, that leads to energy dumping and overall unsatisfactory plant performance.

In the following sections, we will discuss different energy storage options that can be possibly applied to utility scale solar systems.

9.2. Battery storage

Batteries are commonly used to store electric energy generated by off-grid renewable energy systems, and also to mitigate the sharp fluctuations of power for on-grid systems. While there are many different types of battery technologies, some are more applicable to utility scale energy storage than others. Applicability to large systems depends on such factors as cost of materials, ability to scale up with no ill effects or performance loss, and design and operation mode.

Some well-known examples of battery types used as stationary storage system for PV solar are listed in Table 9.1

Note: The costs in the table are based on standard assumptions for the applications and technologies considered, and on expert opinion. They are meant to be used for comparative purposes. The actual costs of any storage system depend on many factors and the assumptions and the means of calculating some of the values are subjective and continue to be debated, even among experts in the field (Sandia National Laboratories).

For quite a while, lead-acid batteries have been the first choice for off-grid PV applications. This lead-acid battery technology has been around since the 19th century and, historically, service providers have more knowledge and tools to deal with those systems. But, despite their long existence and widespread use, lead-acid batteries remain one of the lowest energy-to-weight and energy-to-volume battery designs, which means they are too big and heavy for the amount of energy they provide. This technology is inexpensive and reliable, and it may be a while before it is replaced by more advanced types on a wide scale.

The following reading provides more information on the battery storage types and lead-based batteries, specifically.

Li-ion battery technology

Li-ion battery is one of the rapidly advancing technologies preferred for employment in conjunction with solar systems due to high storage capacity, high charging rates, light weight, and relatively long service life. However, the technology cost is still high and can be a limitation on the utility scale. Some of the very attractive features of Li-ion batteries are high power output and high charge-discharge efficiency. They can also withstand more charge-discharge cycles than lead-acid batteries.

The principle of operation of the Li-ion battery is discussed below.

A schematic representation of a generic Li-ion battery is given in Figure 9.1. Roughly, Li-ion cell consists of three layers: electrode 1 (cathode) plate (usually lithium cobalt oxide), electrode 2 (anode) plate (usually carbon), and a separator. The electrodes inside the battery are submerged in an electrolyte, which provides for Li+ ion transfer between the anode and cathode. The electrolyte is typically a lithium salt in an organic solvent.

Figure 9.1. Li-ion battery system and charge transfer processes

Credit: Wikimedia Commons / Mark Fedkin.

During the charging process, a DC current is used to withdraw Li⁺ ions from the cathode and to partially oxidize the cathode compound:

LiCoO₂ → Li_1-xCoO₂ + xLi⁺ + xe^–

The released Li⁺ ions migrate through electrolyte towards the anode, where they become absorbed in the porous carbon structure:

xLi⁺ + xe^– + C₆ → Li_xC₆

At the same time, electrons travel through the external circuit (electrolyte is not electron conductive).

During the battery discharge, the reverse process takes place. Li⁺ ions spontaneously return to the cathode, where electrochemical reduction occurs.

Limitations of the Li-ion batteries are rooted in the material properties.

For example, the LiCoO₂ ⇔ Li_1-xCoO₂ $$conversion is only reversible with x<0.5, which limits the depth of the charge-discharge cycle. But with a wider variety of materials available, research is underway to develop new generations of Li-ion batteries.

For example, take a look at the Sigma Aldrich [7] website, which lists multiple alternatives for cathode, anode, electrolyte, and solvents.

Flow Batteries

Flow batteries, unlike solid-state batteries, have their chemical components dissolved in liquid solutions, which can be pumped through the electrodes in a flow. If you are familiar with the concept of fuel cell, it is something similar in principle of operation, although it is still a closed loop system. A flow battery cell itself can be small, while the solutions can be contained in external storages. One of the advantages of the flow batteries is almost instant replacement of the electrolyte liquid, thus eliminating any gradient or concentration fluctuations at the electrodes. The main difference between the conventional batteries and flow batteries is that the energy is typically stored in the liquid phase in flow batteries. So, increasing the size of the storage tanks for the liquids allows easy scale-up of the battery to match a specific application.

Zinc-bromine flow battery storage

Zinc-bromine battery is a type of hybrid flow battery. It uses zinc bromine as the working solution, which is stored in two compartments, separated by a porous membrane. One compartment has a negative zinc electrode and the other compartment has a positive bromide electrode. During charge, supplied electricity (e.g., from a solar conversion system) is used to electroplate metallic zinc (Zn) on the negative electrode, while bromine (Br₂) is generated on the positive electrode. During discharge, the opposite process occurs: Zn is dissolved to form Zn²⁺ ions in solutions, and bromine is converted back to bromide ions (Br^-).

Here are the electrochemical reactions involved in this process:

Zn²⁺ + 2e^- → Zn(s)   - Reduction of zinc during battery charging

2Br^- → Br₂(aq) + 2e^-    - Oxidation of bromine during battery charging

The overall reaction is therefore:

Zn²⁺ + 2Br^-⇔ Zn(s) + Br₂(aq)

This reaction proceeds to the right on charging and to the left on discharging. The standard electrode potential for the overall reaction is 1.85 V, which is the maximum theoretical voltage that can be expected from a single cell. The battery cells are stacked to increase the overall storage capacity of the system.

The battery compartments are made of inert plastic. Unlike common batteries, which store electrolyte within the reaction chamber, zinc-bromine batteries have solution storage in the external tanks, from where it is circulated through the electrodes (flow battery type). The external bromide solution storage also helps maintaining required concentration of bromide throughout the reaction cycle.

This technology has been commercialized by ZBB EnerStore company, which engineered zinc-bromine batteries into 50 kWh modules, scalable up to bigger storage systems. Each module is a stand-alone system that includes all necessary software and hardware. Some advantages of this technology include high-energy density (75-85 Wh/kg), stability, i.e., good resistance to performance degradation, ability to operate at full output within a wide temperature range. Unlike most batteries, ZBB EnerStore batteries use non-reacting electrodes (i.e., electrodes are not reactants, but simply are substrates for reactions to take place), which helps minimize loss of performance from repeated cycling.

Watch this video (5:19 minute) for a demo of ZBB EnerStore solution for zinc bromide battery technology:

Video: ZBB EnerStore (5:19)

Vanadium Redox Flow Batteries

This type of battery utilizes the multiple redox states of vanadium (V) in its charge-discharge cycles. Vanadium is present in the dissolved form in the sulfuric acid medium, and because it is all-vanadium system, this type of battery is not susceptible to performance loss due to cross contamination.

During charging, the following half-reactions occur in two separate compartments of the battery:

V³⁺ + e– → V²⁺

VO²⁺ + H₂O → VO₂⁺ +2H⁺ + e^–

Electrons are supplied from the solar energy conversion system as DC current onto non-reacting electrode dipped in the V³⁺ solution. As a result, V³⁺ is reduced to V²⁺. At the same time in the other compartment, vanadium (IV) species VO²⁺ is oxidized to vanadium (V) species VO₂⁺, releasing the electron. On discharging, these reactions are reversed.

The summary process is expressed through the following reaction:

VO²⁺ + V³⁺ + H₂O ⇔ V²⁺ + VO₂⁺ + 2H⁺

The total voltage generated by a single vanadium redox flow battery is around 1.25 V in ideal case.

The main benefits of the vanadium redox flow batteries ability to go through "unlimited" number of cycles; they have a long lifespan (>20 years), quick charging, and high efficiency of the charge-discharge cycle (~80%). They are also more environmentally friendly in terms of component toxicity than many other types of batteries.

The vanadium redox flow battery technology is potentially suitable for extra-large utility scale applications. For example, the 200 MW VRB battery facility in Dalian, China, is expected to significantly increase the stability of the electric grid by supplying power during peak hours and emergency black-starts. Development of such a mega facility was enabled by its co-location with the VFB cell manufacturing factory, which is tapping into local vanadium resources. The Dalian battery is expected to become operational in 2020. Nearby wind power facilities have been forced to curtail electricity production – this battery facility hopes to reduce curtailing significantly.

Check out the story here [11]. 

Additional resources:

Review paper: Blanc, C. and Rufer, A., Understanding the Vanadium Redox Flow Batteries, in Paths to Sustainable Energy, Dr. Artie Ng (Ed.), ISBN: 978-953-307-401-6. pp. 333-337(Access the article in Canvas).

This paper is quite technical as it describes different models used to analyze the performance of the vanadium redox flow batteries. Read sections 1 and 2, which describe the electrochemical principles behind the battery operation. Reading further sections may be useful if you have a special interest in this topic, but is not required.

9.3. Compressed Air and Pumped Hydro

Compressed Air Storage

Compressed air storage technology may become an efficient solution of storing energy generated by large solar plants. The concept is as follows.

Air is used as the energy transfer medium. During the daytime, solar power is used to heat and compress air in an airtight chamber. When energy is needed, that compressed air can be expanded through a turbine or another expansion device to drive a generator to create electricity. Compressed Air Energy Systems (CAES) have been in use in some conventional power plants, and they are making a come-back as energy storage systems for renewable energy plants.

Traditionally, CAES technology used underground geological formations, such as salt caverns, as reservoirs for compressed air. While this approach was effective at some locations, it was not universal, as geology in some areas may be just unsuitable. A newer approach with CAES is to use human-made chambers - large pipes, such as those used for natural gas pipelines. While it involves more construction and installation, this type of artificial storage can be employed virtually anywhere and scaled up to the required capacity by simply using longer pipes. 

Based on this reading, answer the following self-check questions:

Pumped Hydro Energy Storage

Pumped-storage hydropower (PSH) is the type of storage technology that is based on storing energy in the form of potential energy of water. It consists of two water reservoirs at placed at different elevations connected by discharge channel. The available energy can be used to pump water to the upper reservoir (recharge phase), and energy is released when water moves back down to the lower reservoir through the turbine (discharge).

Pumped reservoir, Grand Coulee Dam, WA.

Image credit: Robert Ashworth via Flickr [14]

Closed loop PSH storage does not need to be connected to an outside natural body of water, and all the water is re-circulated.

This storage technology is not new. The first commercial systems employed for storage were implemented in the 1970s, and the design changed very little since then. According to U.S. DOE, pumped-storage currently accounts for 95% of all utility-scale energy storage in the United States. However, additional investments are considered in innovative pumped storage technologies to explore its potential for storing non-dispatchable renewable power generated from utility scale wind and solar farms and improving grid resiliency and reliability.

Listen to the recent podcast that discusses the potential of various energy storage options for utility scale renewables: 

What to do you think? Which method of those mentioned in the discussion has a better chance to become the main player for storing mega- and giga-amount of power?  

9.4. Hydrogen storage

In this section, we will discuss how solar energy can be stored in the form of hydrogen gas. Hydrogen (H2) is a common industrially used chemical and fuel, which can be obtained from water by electrolysis or by reforming of natural gas. Electrolysis is of special interest in the energy storage context, since it converts electric energy into something storable. The process of electrolysis involves passing electric current through water or another aqueous solution, which initiates the electrochemical reaction:

H₂O ⇔ H₂ + 1/2O₂

The basic idea is that the electricity generated by solar PV systems during daytime can be used to run electrolyzers to split water into hydrogen and oxygen gases. Hydrogen is collected and stored in one or another form. When energy is needed, hydrogen can be used for combustion or for electrochemical conversion (in a fuel cell) to recover energy as heat or electricity. Hydrogen provides a new form of energy economy, which complies with the present-day environmental requirements. For instance, hydrogen combustion does not result in any carbon emissions, and water and heat are the only products. Electrochemical utilization of hydrogen in fuel cells is thermodynamically efficient and environmentally benign. Fuel cells can be used for both stationary power generation and transportation. Unlike other forms of energy storage, hydrogen can be transported and used at a different location.

There are a few advantages of the hydrogen energy storage in solar plants:

1. Hydrogen generation by electrolysis is a well-established technology. Hydrogen is used in multiple branches of industry, so the procedures for its handling are well developed.
2. Water electrolysis uses low-voltage DC current, which is compatible with the output from the PV cells.
3. Hydrogen can be stored with minimal losses.
4. Hydrogen can have multiple uses - electricity generation, heat and power generation.
5. Hydrogen is an environmentally benign substance, its combustion does not produce harmful emissions, it is volatile and is easily dissipated.
6. There are multiple ways of hydrogen transportation (pipelines, tanks, hydride compounds).
7. Existing infrastructure for natural gas can be adapted to hydrogen use.

Electrolysis

There are different types of electrolysis that can be used for hydrogen generation. All of those methods use the available electricity to drive otherwise non-spontaneous electrochemical reaction. Electrochemical cell is required to realize electrolysis process. A typical electrochemical cell for electrolysis consists of two electrodes connected to an electric circuit submerged in the working solution. The electrode compartments are separated from each other with a gas-impermeable membrane, which does not allow hydrogen and oxygen mix, while allowing conduction of ions.

Use the following reading material to learn the scientific background and engineering principles for different electrolyzers.

Note on hydrogen safety

Due to high reactivity, hydrogen storage brings up some safety issues. At certain conditions, reaction between hydrogen and oxygen can lead to explosion (Figure 9.4), so storages for those gases should be separated, and leak detection is critical.

Figure 9.4. The character of the reaction of hydrogen with oxygen.

Credit: Shriver and Atkins, 1999

Solar hydrogen is attractive due to its carbon-free footprint, in contrast to any hydrocarbon fuels. Hydrogen has a very high energy density (142 MJ/kg), which is much higher than that of gasoline (~47 MJ/kg). This is due to its very light atomic mass. On the other side of the medal, hydrogen as a volatile gas must be contained in a compressed form, and the traditional gas cylinders used for storage and transportation of hydrogen have been a concern. Such cylinders, usually about 35 kg in weight contain only 700 g of hydrogen (at 200 bar of pressure), which is equivalent to only 2.85 liters of gasoline (Komoto et al., 2009).

The production of hydrogen via water electrolysis is most relevant to utility-scale PV systems. The efficiency of the electrolyzer is above 75% at optimized conditions. Even higher efficiencies are promised at elevated pressure, although in this case extra energy needs to be spent for isothermal compression. Much more than production, hydrogen "packaging", i.e., converting it to a convenient form for storage, can be much more costly and energy-expensive. These forms include (1) compressed hydrogen, (2) liquefied hydrogen, and (3) metal hydrides.

In option (1), compressed hydrogen is much more usable compared to that at atmospheric pressure due to much higher energy density (2.54 MJ/liter at 200 bar versus 0.0127 MJ/liter at 1 atm) (Komoto et al., 2009). Further compression to 800 bar brings its energy density to 10.1 MJ/liter - close to that of liquid hydrogen. However, multistage compression is estimated to take up to 8-12% of the total energy that hydrogen contains as fuel (referring to higher heating value), and mechanical and electrical losses can increase this value even further. Option (2), hydrogen liquefaction requires even more energy - 30 to 50 MJ/kg hydrogen converted. Option (3), metal hydrides are special compounds that can bind hydrogen by forming chemical bonds under certain temperature and pressure conditions and release hydrogen when temperature and pressure change. Simple physical pressurizing hydrogen into hydride form is possible, but is not very attractive due to the large amount of metal hydride needed (50 kg per 1 kg H₂ stored) (Bossel, 2006). Chemical binding of hydrogen in metal hydrides is much more promising.

Some examples of hydride compounds are CaH₂, MgH₂, LiH, NaAlH₄, LiAlH₄. Because metal-hydrogen bonds are quite strong, energy input is needed to release hydrogen from hydride. So, temperatures for hydrogen generation need to be raised to 120-200 ^oC. Nevertheless, the hydride storage is theoretically more efficient. For example, if we consider the reaction

$Ca{H}_2+2H_{2}O=>Ca(OH{)}_2+2H_2-$ reaction of hydride with water,

we can see that by molar ratios, 42 g of CaH₂ release 4 g of H₂ (gas), so it is roughly 4:1 ratio; at the same time, compressed storage in the cylinder provides storage of only 1 kg of hydrogen per 50 kg of storage tank, i.e., 50:1 ratio. Because of the obvious advantage of the metal hydride, substantial research effort is underway to improve this technology.

Summary and Final Tasks

In this lesson, we looked at very different technologies used to store solar energy. Those technologies differ in physicochemical principles, scale, and impact, and require quite different scientific and engineering background for detailed analysis. In that sense, this material is challenging. At the same time, it is important for solar specialists to be aware of various energy storage options, as well as recent innovations, and to be able to apply those options to specific conditions. Hopefully, this lesson has been a good step in that direction and will motivate you to learn more and to elevate your expertise down the road.

Please complete the following activities to complete this lesson.

References for Lesson 9