Week 6 - Chapter 10 Flashcards
primary battery
A primary battery (or primary cell) is a device constructed of anodes, cathodes, and electrolytes in such a way to allow them to emit a charge.
The devices are constructed to allow the controlled chemical reaction of the components. They operate until the available internal transformations are exhausted, after which the battery is no longer useful. The chemical reactions cannot be easily reversed, and therefore the battery cannot practically be recharged.
Secondary battery
In 1859, Gaston Planté developed the first rechargeable battery (also called a secondary battery) using lead-acid chemistry. This type of battery allowed the chemical reaction to be reversed by the application of current into the battery, which recharged the chemistry and, therefore, the electrical energy available from it.
Mechanical storage
Today’s methods of electricity storage fall into two classes of energy that can be called upon when needed/
The first represents all of the different ways that potential energy can be stored in chemical bonds. Devices that use this electrochemical control method have come to be known collectively as batteries. Depending on the chemistry involved in the structure of the device that is used to manage this chemical conversion, batteries can be subdivided into solid battery or flow battery types.
The second method uses mechanical storage of energy, which can draw on a number of potential energy sources, such as gravitational or elastic energy or even controlled kinetic energy.
- flow batteries
A variation on the secondary battery being explored today separates the components of a typical battery. Flow batteries still use chemical energy to store and deliver electricity through an electrochemical reaction, but they store the electrolytes in separate liquid storage tanks outside the device and feed them in as necessary to accept or deliver charge, as shown in Figure 10.3.
Adding charge reduces the metal, and accepting charge oxidizes it. This redox (reduction + oxidation) process can be repeated many times before the material degrades. Flow batteries come in a number of different chemistries, including vanadium redox, iron-chromium, and zinc-bromide, all of which are still in the stage of research, evaluation, and early initial adoption.
- pumped hydropower
Pumped hydropower (gravitational potential energy)—Pumped hydropower storage is the method of pumping (using electricity) either fresh or salt water to a higher elevation, and storing it in some reservoir for later use (see Figure 10.4). When the energy is required, the water can be run, using gravity, through a turbine to generate electricity. Where the geology allows this, substantial storage can be created cheaply, and the overall efficiency of the process is quite high.
- compressed air energy storage (CAES)
■ Compressed air (elastic potential energy)—Compressed air energy storage (or CAES) uses an electric-powered compressor to force air into a closed container, and the electricity can be recaptured later by the release of that pressure driving an air engine or pneumatic motor. While this technology can be used for vehicles and other portable applications, it is currently used primarily for grid-connected energy applications, which utilize large underground storage (underground CAES) in the form of geological formations or potentially depleted natural gas reservoirs.
- flywheels
■ Flywheels (rotational kinetic energy)—Flywheels are spinning shafts or discs that are accelerated using electricity. The momentum of the flywheel can be converted easily into electricity using a dynamo. Because of the physical momentum of the devices, the amount of energy that can be safely stored in flywheels is limited, but flywheels are increasingly recognized as a technical option of high-power grid-connected storage for ancillary services, with potentially very long device lifetimes and overall high efficiency when constructed properly.
- capacitors
Capacitors are small electronic devices that can store electric charge using electric plates and a separating (dielectric) insulator. Capacitors are used throughout the electricity system to manage the flow of electrons in everything from computer chips to large generating stations. Capacitors do not store large amounts of energy very well, but the ability to charge or discharge their energy quickly gives them an extremely high power rating, or power density.
Supercapacitors (also called ultracapacitors) are capacitors with technical features that allow them to be scaled up to a larger size. This larger size allows them to store meaningfully large amounts of energy and also discharge and recapture at very high power ratings. This makes them suitable for applications such as regenerative braking, as well as other electric vehicle applications, though they are primarily used for power management and not for the energy required to propel vehicles over long distances.
- superconducting magnetic energy storage (SMES)
Superconducting magnetic energy storage (SMES) allows energy storage in a magnetic field. By using supercooled conductors, which have nearly no resistance, energy can be cycled through conduit loops for a long time with very little loss.3 Another advantage of the system is the nearly instantaneous response time to operational signals.
- inverter
- specific energy
■ Specific energy—Specific energy is the amount of energy that can be stored in the device or system per unit of mass (technically, gravimetric energy density to differentiate it from the volumetric kind below). Clearly, being able to store energy in less mass is preferable, particularly when the cost of the device is heavily driven by the cost of the materials used in it.
- specific power
Specific power and power density—Both of the concepts above can be measured as functions of power rather than energy if the application is intended to provide primarily power outputs.
Specific power—Specific power is the amount of power that can be stored in the device or system per unit of mass. Clearly, being able to store power in less mass is preferable, particularly when the cost of the device is heavily driven by the cost of the materials used in it.
- energy density
■ Energy density—Often confused with specific energy, energy density is more about the ability to store energy per unit of volume. Also called volumetric energy density, this becomes vital when space constraints are binding on the application or technology choice.
- power density
Specific power and power density—Both of the concepts above can be measured as functions of power rather than energy if the application is intended to provide primarily power outputs.
power density is more about the ability to store power per unit of volume. Also called volumetric energy density, this becomes vital when space constraints are binding on the application or technology choice.
- round-trip efficiency
■ Round-trip efficiency—This refers to the efficiency with which energy can be stored and then converted back into electricity in the device or system. High round-trip efficiency means that a lot of the input energy that comes back is useful output, minimizing the losses in both the physical and the economic senses.
- parasitic losses
■ Parasitic losses—Batteries that are charged but not in use all have a parasitic loss that reduces the amount of energy stored over time. This can occur through heat dissipation or electric discharge in chemistry batteries as well as evaporation or friction in physical storage methods.
- frequency regulation
Using electric storage for short-term applications has economic advantages compared with using it for longer-term applications. Storage typically has a large upfront capital cost, which needs to be recouped through as much value creation or use as possible.
Frequency regulation is the short-term management of the supply and demand balance in the grid that keeps the system operating within acceptable parameters of voltage and current. It helps avoid tripping or curtailing assets, which can lead to expensive cascading failures. In restructured electricity markets, frequency regulation services are potentially needed at any given moment for standby power and execution in both up (adding) and down (reducing) markets, depending on in which direction the system is imbalanced. Storage solutions are particularly effective at meeting this need due to their requirement to both take in and deliver electricity in balance over a given period of time. They can participate in up markets at some times and in down markets at others, often in rapid succession. These are sometimes referred to as balancing markets in Europe.
Electricity storage, particularly from batteries or electronically controlled devices, has additional advantages in functioning as capacity for system regulation—it has a very fast response time (sometimes reacting in less than one second from receiving an instruction) compared with other types of storage and nearly all types of generators, which need much longer to ramp up their capacity. This makes energy storage a particularly effective spinning reserve (a reserve that can be called on and deliver electricity ultrafast), though adequate compensation from utility operators for this faster response time is not universally provided in regulation or through PUCs.
- demand charge management
From the customer side, storage can be used for demand charge management. This is possible when the customer has a fluctuating power need (often due to equipment that can cycle on and off in unpredictable ways). Such circumstances can result in large demand charges from the utility, which has to be ready to provide the peak power demanded at any time. Storage on the customer side of the meter that can clip that peak power requirement can result in substantially lower demand charges and therefore provide meaningful cost savings to the customer.
- peak shaving
One common goal for electricity storage is to meet some of the same challenges that demand response targets discussed in Chapter 9—reducing the peak energy requirements of the grid, particularly at its most constrained times of the day or year. Very small amounts of energy shifting, or peak shaving, at these moments can have substantial impacts on the system’s power requirements and overall stability. The value of reducing these peak power needs can be significant, since under current Dutch auction market structures, the reduction in the wholesale power price benefits all customers. Also, with storage, a grid operator may be able to deploy fewer generation (and potentially transmission and distribution) assets to prepare for these rare but inevitable peaks. For these reasons, there is value to this peak shaving with the potential for compensation.
- system regulation
The overall flexibility of being able to dispatch electricity from energy storage, like for other forms of generation, helps improve the operator’s system regulation. It provides capacity to the system, which should generally be compensated under whichever method and degree of regulation the utility operator is subject to.
- firming renewable energy
Finally, an emerging power application involves using storage in firming renewable energy. Many emerging renewable energy options, such as wind and solar, rely on kinetic energy resources that are intermittent and not perfectly predictable. While the average amount can be reasonably well known, relying on this energy as being dispatchable is not possible unless it can be stored for minutes, hours, or even days, depending on the nature of the resource. Electric storage is ideal for these applications because of the matching ins and outs and high power needs of these applications. This kind of storage can be located anywhere—from the generator site to the grid—and can provide value for both smoothing and grid regulation at the same time.
- time shifting
Energy applications for storage move chunks of energy from the time they are generated to another time when they are more valuable. This time shifting can be for hours or days, depending on the design of the system and the amount of energy available.Solutions of this type can take a couple of different forms. The first is load shifting, which allows an energy input from a few hours earlier to be delivered at the peak and is compensated by the differential value for the electricity between when it is stored and when it is withdrawn. This particular configuration is also very useful when generation may be closely coincident with the peak but not exactly, such as for solar generation technologies in places with a late afternoon or early evening load peak. Moving the supply of the generated electricity a few hours later greatly improves the economics and functioning of the overall system.
- day-night arbitrage
If energy value drivers only occur once per day, then the ideal proposition is to buy power at the cheapest part of the day (often at night) and sell it back at the peak hours of the next day (i.e., in the middle of the day or late afternoon/early evening). This is termed day-night arbitrage, and Figure 10.10 shows how it works in theory. Because this process also reduces the top and fills in the bottoms of the load curve, it is sometimes referred to as load leveling.
- levelized cost of storage (LCOS)
Storage devices (just like generators) that are used with different capacity factors, incur different O&M costs, or consume different fuels may change the economics of operating these technologies in practice.
Aggregating asset performance characteristics and capital costs into a unit for broader comparison is precisely what the LCOE calculation derived in Chapter 5 was designed to solve for generators. These concepts can be converted for use in electric storage applications with a very similar method, called levelized cost of storage (LCOS).
Includes Overnight, Fixed O&M, Fuel cost
Because the power rating of a storage device is much easier to measure and more persistent over time, standardizing the cost per watt of the device is also easier to calculate. However, for most of the time-shifting and energy applications for storage, evaluating their economics on a per kilowatt-hour ($/kWh) basis is a more appropriate and relevant standardizing cost metric.
Combining these data allows a calculation of LCOS for different technologies in a specific application. A correctly constructed LCOS calculation allows not only comparison of the economics across different storage technologies but also the comparison of storage to other generation technologies that might meet the same market need.
One major caveat in LCOS calculations, however, is that storage often provides many value streams simultaneously. Since storage is a basket of benefits that may include energy, power, risk mitigation, and time shifting—all of which derive from the same capital investment in the storage application—when assessing the economic competitiveness of a storage application, it may not be appropriate to attribute the entire cost of the storage device to just one of the value streams, such as energy.
- economies of scope
economies of scope refer to the cost advantage of a producer who produces at least two different goods simultaneously. The producer has an economy of scope when the combined cost of producing the two goods together is less than the cost of producing both goods separately.
- coproducts
You can allocate some proportional cost to each of the value streams the storage creates. For example, some portion of the costs would pay for capacity value and some portion would be attributed to risk mitigation. Everything left over can then be allocated to the energy application, and then that value can be compared to other storage or generation options that meet the same energy needs. This assumes that these other values can be monetized, which may not always be the case.
This cost allocation problem is not unique to electricity storage, as for nearly all solutions that result in multiple value streams, or coproducts, it is difficult to assess the competitiveness of a single value stream among many. Notably, associated oil and gas (in Chapters 14 and 18) and biofuels (in Chapter 15) share this dilemma.
- grid hardening
Microgrids—Grid hardening (protection against natural or human-made loss of asset use) initiatives, islanding capabilities (i.e., being able to operate when disconnected from the grid) for campus or other isolated grids, and military applications are all creating microgrid benefits and will use increasing amounts of storage to support their deployment.
- fuel cell system
Storing electricity using chemical batteries represents a fairly efficient, if expensive, way to transform electricity through time and space. Chemical batteries are naturally better suited to provide fast response and round-trip efficiency advantages that lend themselves better to power applications. However, scaling chemical batteries up to meet the needs of larger energy capacity storage is limited by the economics of adding more and more material-intensive solid chemical batteries.
In contrast, fuel cells present an alternative to simultaneously provide both short-term power capacity and energy storage for hours, days, or longer through the ability to bidirectionally convert power to fuel and fuel to power. Importantly, their operation allows the disassociation of the power and energy storage dimensions of the device, making their design and use much more flexible across different applications. Initial markets for fuel cells are targeting applications in which the value of their conversion of fuel into electricity is enhanced by their quiet and low-emission operation, as well as by their low weight for some portable applications. However, the potential for bidirectionally linking the electricity system to other gaseous and liquid energy carriers, if realized, would be truly transformative to the global energy system.
Providing a complete fuel cell offering requires understanding not just how the device works but the characteristics of its inputs and outputs as well. Many different versions of available technologies subtly alter the requirements for fuel type, which ranges from pure hydrogen to natural gas and propane. The conversion technology also drives the proportions and exact characteristics of the electricity, heat, and water that fuel cells produce, altering their suitability for various markets. Typically, fuel cells consume hydrogen and oxygen by combining them with a catalyst, or material that enables or accelerates the chemical conversion.
The assembly of catalysts is called the fuel cell stack, and the stack must be surrounded by other components to safely and efficiently manage the inputs and outputs, or balance of plant. Collectively, these components are the whole fuel cell system.
- centralized reforming
Obtaining hydrogen –
Centralized reforming (gas and coal)—The main method of hydrogen production used today is the steam methane reformation of natural gas, and this hydrogen is predominantly used in the production of ammonia as well as in oil refining. A major issue with this method is that the energy content in the resulting hydrogen is substantially less than that of the natural gas used in its production (i.e., losses happen), which means that natural gas is more cost-effective in any fuel application in which both hydrogen and natural gas are viable competitors.
- distributed reforming
Obtaining hydrogen –
Distributed reforming (gas)—Distributed reforming usually uses steam reforming of natural gas in smaller equipment much closer to where the hydrogen will be used. Higher losses occur due to the smaller scale and lower efficiency of the reforming equipment, but this process reduces the need for hydrogen transportation and storage infrastructure. High-temperature fuel cells such as MCFCs and SOFCs can perform onboard reforming, which further reduces hydrogen delivery and storage requirements.
- electrolysis
■ Electrolysis (electricity)—Using electricity to separate water into hydrogen and oxygen is technically simple, but it is often costly and inefficient. Larger centralized electrolyzers tend to be more efficient and cost effective, but they also require either large local users or a distribution and storage infrastructure.
- direct solar to hydrogen
■ Direct solar to hydrogen (solar)—Instead of converting solar energy into electricity and then creating hydrogen using electrolysis, other methods of direct conversion of solar energy to hydrogen formation are theoretically available and have been tested in the laboratory. These include photoelectrochemical water splitting (using dissolved metal complexes), photobiological methods (using algae), or thermochemical (using solar heat to split water molecules). Today these methods are very inefficient (<1%), but research continues to improve them.
- power-to-gas
Another emerging variation on the hydrogen infrastructure generates hydrogen from intermittent and surplus sources to support the existing natural gas infrastructure, which comprises technologies collectively referred to as power-to-gas. Increasing penetration of renewables, lack of responsiveness of nuclear baseload, and limited demand flexibility create conditions of occasional excess electricity supply driving down market prices. Using the hydrogen generation techniques described above, or even fuel cells running in regenerative mode, this excess electricity can be converted into hydrogen.
