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An accumulator is a type of electrical battery which can be charged, discharged into a load, and recharged many times. It is composed of one or more electrochemical cells. The term “accumulator” is used as it accumulates and stores energy through a reversible electrochemical reaction.


Der Begriff Arbitrage bezeichnet vereinfacht gesagt das Ausnutzen von Preisunterschieden des gleichen Produkts an unterschiedlichen Handelsplätzen, mit der Absicht schnell und risikoarm Gewinne zu erzielen. In der Praxis bedeutet das, dass ein Händler an der einen Börse ein Produkt preisgünstig einkauft und zeitgleich an der anderen Börse mit höherem Preis veräußert. Solche Geschäfte funktionieren meist nur kurz, da sich durch die Arbitrage die Märkte schnell aneinander anpassen – die Preisdifferenz verschwindet.


In the technical sense of the term, work is the delivery and thus consumption of electrical energy.


The zinc-air battery is a cell that generates voltage through a zinc-oxygen reaction. It is still non-rechargeable at the current state of research. Originally it was used as a coin cell in hearing aids. Today there is some initial research into applications in stationary storage facilities. However, one problem is that this battery type discharges very quickly upon contact with oxygen.


A ZEBRA battery is a rechargeable battery of sodium and nickel chloride cells. It is a high-temperature battery and is used primarily in electric cars and in the defense industry. The abbreviation ZEBRA stands for “Zero Emission Battery Research Activities”.


Electrical voltage is an expression of how much energy is transported or released per unit of electrical charge. Voltage is measured in volt (V).


In the technical sense of the term, work is the delivery and thus consumption of electrical energy.


Watt (W) is the unit of power, which is energy per unit of time.


In Germany, electricity is transported via grids at different voltage levels from very high to low voltage. To enable stable grid operation and prevent damage to connected devices, the voltage on each level has to remain within a defined range. It is possible to compensate for fluctuations by providing reactive power, e.g. from batteries.


Volt – abbreviated with the unit symbol V – is the unit for electric current. One Volt means that one Joule of energy is carried per one Coulomb of electrical charge: 1 V = 1 J / C.


Currently the infeed of energy to the grid by multiple small power generation facilities like wind turbines or solar power plants, biogas or combined heat and power stations is still uncoordinated. Better communication and a centralized administration would enable better control of the power output of these many small power stations and their adaptation to the actual energy demand. Such a control center would function like a single large, virtual power station. This would exploit the advantages of the individual stations while compensating for their weaknesses.


Vanadium redox flow batteries have great potential in stationary storage. The have almost no self-drain. This makes them ideally suited for seasonal storage. Because the energy carrier does not age or wear, they have an almost indefinite service life with low maintenance requirements. Depending on need, power and energy can be scaled separately and flexibly.


The transmission grid is the synchronous grid of very high voltage lines for the interregional transmission of electricity from power stations to transformer stations. Its power flow is determined primarily by the power stations. German transmission grids usually operate at 220 kV and 380 kV.


Electricity and gas suppliers pay system usage charges to the grid operators. These charges are passed on to the end user as part of the working price or the base price. Large-scale users – e.g. in industry – pay system usage charges directly to the grid operators. This money is used to pay for frequency regulation, for the compensation of energy loss, the provision and management of reactive power, the maintenance of black start capability, any redispatch measures in the event of bottlenecks, as well as the cost of power lines.


Beyond the transmission and distribution of electricity, every grid operator is obligated to perform system services, also referred to as ancillary services. In particular, these include frequency and voltage control, restoration of supply, and system control.


System control means the monitoring, direction and control of the transmission grid as well as the coordination of commercial power transport between the balance circuits and traders within a regulation zone.


SMES devices store electrical energy magnetically in coils of supraconducting materials. This allows the current to flow without resistance, so that the charge is maintained without any energy loss. SMES have a relatively low energy density, but can have a high power density. However, currently there are almost no practical applications for this technology.


Storage capacity indicates the maximum amount of energy that can be stored and released. Storage capacity is indicated in watt-hours.


Sodium sulfur batteries have a very long service life and are the most advanced stationary electrical storage technology. Batteries of this type are available industrially in the megawatt range and have been used in Japan for 15 years at various sites. Due to their high storage capacity, these high-temperature batteries are a particularly cost-effective means of balancing daily fluctuations in wind and solar power. In the case of rising demand, production could be further industrialized very quickly. The raw materials are not only non-toxic but also globally available.


The secondary reserve is used to compensate for extended frequency deviations within the affected regualtion zone. It takes over from the primary reserve and can be completely activated or deactivated within five minutes. Similar to the primary reserve, the secondary reserve power is requested automatically by the transmission grid operators and supplied from variable output power stations. There are secondary reserves for regulating up and down.


The basic idea behind the seasonal reserve is to have a sufficient supply of solar energy even in winter. The goal is to store large amounts of electricity from renewable sources over long periods of time. It is to be assumed that we will not need such storage facilities before renewable energies reach a share of about 80% of the total production.


Reactive energy is the share of electrical energy that is consumed without being directly usable, e.g. to power a device. It occurs when a device draws reactive power temporarily in order to build up electromagnetic and electrical fields. Reactive energy oscillates back and forth between the supply and the consumer, so to speak. It is indicated in kVArh and occurs only for AC but not for DC.


When a large-scale power failure occurs (blackout), the power supply must be restored starting from the very high voltage level. Smaller regions are reconnected to the grid first and then linked up step by step. The coordinated startup of producers and consumers is a substantial challenge in the restoration of supply. This requires the use of what are known as black start capable generating units.


In Germany, power generation from wind and solar energy cannot always meet the energy demand completely. The difference between demand and the actual supply of renewable energies is called residual load. In the event of an increase in demand that cannot be met by renewable energies, the remainder is provided by power stations with a variable output.


Reserve power describes the power that a power utility provides in addition to the forecast demand of the consumers. In the event of a power supply failure, it guarantees that important electrical devices remain in operation. Of particular importance is, for instance, the emergency power in hospitals, where life support machinery needs to keep running reliably even in the event of a general power failure.


A redispatch is the operators’ response to an impending overload of the power grid by adapting the output of their power stations to the load. In practice this means that power stations are ramped up or down as needed.


As opposed to effective current, reactive power or reactive current does not generate usable energy. While effective power is used for instance to drive an electrical motor or to generate heat, the reactive power flowing along with it does not contribute to these applications. This is why it cannot be converted into usable energy for the consumer, but it is needed to generate electromagnetic and electrical fields to enable the effective power to do its job.


PV and wind turbines sometimes supply large amounts of energy very suddenly. Storage facilities can be used to smooth these steep generation gradients, also called “ramps”.


A pumped reserve power station uses the surplus energy of other power stations to pump water to a higher level and store it there. When the water is drained again, it drives a generator via a turbine. This enables a quick provision of power in times of peak load. These storage power stations are able to reliably compensate for fluctuations in the power supply from wind and solar energy.


The primary reserve is used for frequency regulation in the European synchronous grid. It is activated automatically whenever the frequency is outside the tolerance range of 49.99 to 50.01 Hz. Activation must be completed within 30 seconds. The four German transmission grid operators buy reserves from various vendors in a weekly, strictly regulated auction. Vendors must be able to supply additional energy at any time in the event of a frequency drop (regulation up) or to remove energy from the grid in the event of over-frequency (regulation down). The payment is exclusively for the continuous provision of the capacity during the relevant week; the market is therefore a capacity market.


Pre-qualification is a qualification test that is performed before entering into market competition. It is for example required for participation in the primary reserve market. Suppliers have to prove, e.g., that the entire agreed primary reserve can be activated smoothly within 30 seconds, with a tolerance for quasi-stationary frequency deviation of no more than ± 200 mHz, and maintained for 15 minutes. The batteries in the Younicos Technology Center have successfully passed this process and have been in the primary reserve market since 2012.


The acronym AC/DC is for alternating current/direct current. Alternating current periodically changes its direction and polarity and is used primarily to supply energy. It is carried in high-voltage lines and supplied to European homes with a frequency of about 50 Hz. With direct current, the amplitude and direction do not change. Batteries are typical sources of direct current.


Ampere – abbreviated with the unit symbol A – is the unit for electric current.


Ampere hour – abbreviated with the unit symbol Ah – is a unit for electrical charge. It describes the amount of electrical charge passing through a conductor in one hour at a constant current and is calculated by multiplying the current with time. If for example a current of 3 A flows for one hour, then the amount of charge is 3 Ah (3 A x 1 h = 3 Ah ). The available charge in a battery is often indicated in ampere hours.


Assured power is the amount of generated power that producers are able to supply with 99% certainty. Assured power can only be maintained for a system of several power stations because any individual station’s availability is below 99%. The availability of batteries can be higher than 99% because batteries are modular assemblies of multiple, mutually redundant units..


One way to think of the principle of a battery or rechargeable battery is to picture two water containers connected by a pipe. The containers are placed at different heights, so that water flows from the upper container to the lower, driving a wheel. The electrodes in a battery act like these containers. The flow consists of electrons instead of water, driving not a waterwheel but any electrical device. The higher the difference in height between the containers, the faster the wheel spins.
In the case of a battery, the height is called “voltage,” the flow is called “electrical power,” and the size of the container is called “capacity.” The higher the voltage, the greater the amount of work done by the battery. The more electrons stored in the electrodes, the longer the duration of the electricity supply. Batteries can be divided into two categories: primary batteries and secondary batteries. Secondary batteries are rechargeable.


A power station is black start capable if it is able to start independent of the power grid. This capability is important in the event of a large-scale power failure and generally utilizes energy from batteries or generators. After a collapse of the electrical supply, black start capable power stations help to restart the grid.


In winter 2009, a surplus energy supply caused the prices of electricity to fall below zero for the first time ever. Such an excess supply occurs when, despite low demand, large amounts of renewable energy are fed into the grid while conventional power stations are still running. Due to their lack of flexibility, conventional stations need to keep running at minimum power even when they are not needed as suppliers in the grid. This is because they take a long time to restart after a stop. Frequent starts and shutdowns also increase wear. In addition, power stations often remain on the grid in order to deliver system services.


A nickel cadmium (NiCd) battery is rechargeable and is highly resistant to overload and complete discharge. However, the use of this type of battery has been severely restricted by law because cadmium is a highly toxic heavy metal. Today these batteries are only used in emergency and alarm systems and cordless power tools.


In a nickel metal hydride (NiMH) battery, an environmentally friendly metal alloy is used as a substitute for cadmium. These batteries are used wherever there is high energy demand but high battery cost must be avoided, for example in home electronics. However, NiMH batteries are sensitive to overcharging, overheating and deep discharge (draining the battery until almost completely exhausted).


Peak shaving is meant to prevent supply bottlenecks and relieve the grid in times when demand is very high. This is done either by having consumers cover their additional power needs by activating generators or storage devices like industrial batteries, or by matching their needs to the supply.


In the energy business, power is the amount of energy generated or consumed within a certain time. Power is given in watt, kilowatt, megawatt, or gigawatt.


Power to gas is a chemical process in which energy from renewable sources is used to generate hydrogen or synthetic natural gas. The first step is to generate hydrogen by electrolysis, followed by methanization, i.e. conversion to methane. This process enables the long-term storage of large amounts of renewable energy for consumption when needed.


Capacitors can store electrical energy without requiring conversion to other forms of energy. They consist of two conducting surfaces – the electrodes – which are set slightly apart, with an insulation layer sandwiched between them. Electrical current causes one of the electrodes to acquire a positive charge, the other a negative. The capacitor stores this charge and is able to release it later. Energy losses are minimal. However, capacitors are only able to store low amounts of energy.


Carbon Capture and Storage or CCS is the separation and underground storage of carbon dioxide (CO2) generated during the combustion of fossil fuels. The goal is to reduce CO2 emissions into the atmosphere. With the development of renewable energies, CCS may become unnecessary in the future.


Electricity is generated either centrally in a large power station, such as a nuclear, coal or gas power station, or decentrally by multiple small stations that separately feed their energy into the grid.


A compresed air energy storage (CAES) facility utilizes the energy of compressed air. When there is low energy demand and a surplus of energy from renewable sources, electrically driven pumps compress ambient air to up to 100 bar. If the compression heat is used as well, the storage is described as “adiabatic”.
The compressed air is then stored in caverns. In times of high energy demand and low supply from renewable sources, the compressed air can be used to generate power by means of a turbine. Currently there are two CAES plants in the world, one of them in Huntorf, Germany.


For a stable power grid, load and generation need to be balanced. However, errors in load forecasts, failures of power stations, loads, or lines, or volatility in the supply of renewable energies may cause temporary imbalances in the grid. The four German transmission grid operators are required to compensate for such fluctuations by providing short-term control power and stabilize grid frequency at 50.00 Hz. This is done through negative control work in the case of over-frequency (> 50 Hz), or positive control work in the case of under-frequency (< 50 Hz). There are three types of control reserves: primary, secondary, and minute reserves (tertiary reserves).


The amount of electrical charge passing through a conductor per unit of time is called current. Current has the unit ampere (A).


A cycle is a complete charge and discharge of the battery. The full cycle is achieved when an amount of energy equal to the storage capacity of the battery has been transferred. A battery’s cycle stability refers to how often the battery can be charged and discharged before its capacity falls below a certain value.


At present, the electrical supply is determined by demand. The idea behind demand site management is to flexibly control both energy demand and energy supply. Wind and solar energy would be used according to availability, significantly reducing the need for conventional power stations run on fossil fuels. Consumers would operate their equipment based on energy availability. However, consumers would need to be continuously informed about the actual supply situation and receive incentives through pricing.


One kilowatt hour – abbreviated as kWh – is the amount of energy consumed in one hour when the power is one kilowatt (1 kW). Prices for electricity and heating are calculated in kilowatt hours.


Consumers are familiar with the kilowatt-hour rate from their electricity bill. It is listed in the bill and indicates the price of one kilowatt hour (kWh) of electricity. The kilowatt-hour rate includes all costs of consumption and covers generation, system usage, concession levies, and all taxes and duties.


This term describes the costs that must be covered in order for a power station to operate without losses. If earnings are below marginal cost, losses result. Marginal cost depends on the operating cost of the power station and includes fuel and wear costs, as well as expenses for emissions certificates. This plays an important role in energy trading, because the energy price determined at the exchange always follows the marginal cost of the most expensive station, known as the marginal power station.


The merit order is the activation order of power stations. It is determined by marginal cost, the minimum cost at which a power station is able to produce electricity. Power stations are activated in ascending order of their marginal cost until the demand is met. The last, and thus, most expensive power station determines the current market price of electricity, the market clearing price (MCP). The merit order effect has been observed from the time renewable energies entered the stage. Since no fuels are needed and solar power is basically always available during the daytime, the use of renewable energies means that fewer conventional power stations are needed. This means the price of electricity decreases because it is determined by more cost-effective power stations.


After 15 minutes, the secondary reserve is replaced by the third regulation level, known as the minute reserve or tertiary frequency regulation. It is activated manually upon the request of the grid operator. A distinction is made between buffer energy for regulation up or down. Power stations which can serve as minute reserve include, for instance, black coal, natural gas, combined heat and power, and pumped storage plants.


Conventional power stations are not infinitely flexible. A certain minimum threshold needs to be exceeded in order to even start the required combustion process. This threshold is about 60% of the rated power for brown coal power stations and 50% for black coal power stations. Because power stations must be able to reduce their output, they usually have to run at a capacity higher than this minimum threshold. This so-called must-run energy from fossil fuels must be fed into the grid. This limits the use of all other forms of energy to the difference between the must-run power and the current demand.


Lead batteries are a very common form of energy storage, used mainly as starter batteries in cars, but also domestically. The cell voltage of lead batteries is 2 V. Usually several cells are combined to achieve voltages such as 6 V, 12 V or 24 V. Energy density and service life are much lower than for lithium ion batteries, but the price is much lower as well. These batteries contain highly caustic sulfuric acid.


Lithium ion batteries are particularly suitable for short-term storage with a time window of minutes or hours. Lithium ion batteries are available with ratings from a few kilowatt to several megawatt and can thus be used both in very small and in very large systems. Their especially high energy density allows a compact design, making them suitable for installation in homes as well as in utility grids. They are also ideal as sources of buffer energy to balance temporary fluctuations.


In the energy industry, load refers to the power used by the consumer.


Base load is the continuous consumption in a grid. The base load is covered by continuously operating power stations (base load power stations) that currently still run on nuclear or brown coal fuel. Mean load or peak load occurs when the base load is exceeded due to temporary increases in demand. Black coal power stations are frequently used as mean load power stations to cover additional demand. In times of very high demand, peak load power stations are also brought into the mix. These include pumped storage plants and natural gas power stations. The increasing use of renewable energies in power generation reduces the need for base load power stations. The idea: Wind and solar are used flexibly as needed to cover the energy demand.


Load shedding means that consumers are disconnected from the grid, e.g. during a blackout. The shutdown of individual facilities is also called load shedding. The shutdown is a precaution and occurs automatically when there is a risk of overload and subsequent breakdown of the energy grid.


The discharge time is the length of time over which a storage system releases energy. The various storage systems have different capacities and activation speeds. Short-term storage systems may provide energy within seconds or minutes, but have a low volume. Daily storage systems, on the other hand, need between one and ten hours until they are ready, but have a higher capacity.


The power from the transmission grid is transformed down to the voltage level of regional distribution grids in transformer stations. Distribution grids carry this energy to the individual consumers. The transport in distribution grids is the responsibility of the distribution grid operators, who are also in charge of maintenance. Germany has around 900 distribution grid operators in total. The expansion of decentralized renewables has made the tasks of the distribution grid operators more complex. They have to accept more power from wind and solar and forward it to transmission grid operators for superregional distribution.


Electric mobility is the use of vehicles powered not with fossil fuels but with electricity. The vehicles have a built-in energy storage (rechargeable batteries) that are charged from the electrical grid. In Germany, several programs exist for the subsidy of electrically powered vehicles, since they play a role in the context of renewable energies: If electric vehicles are powered from renewable energy sources or with hydrogen, then they reduce overall CO2 emissions. At the same time, they can serve as “mobile energy storage units” to balance current fluctuations and stabilize the power grid.


Gas can be stored in a variety of ways. A porous storage system is a natural gas reservoir that can be refilled. Cavern storage uses an artificial storage space. Other forms include above-ground storage tanks or storage pipes close to the ground. Germany has 44 natural gas reservoirs with a total capacity of 19.1 billion cubic meters.


A heat storage system absorbs energy as heat, stores it, and releases it on demand. A distinction is made between sensitive, latent, and thermochemical heat storage systems. Sensitive heat storage systems change temperature when charging and discharging, while latent heat storage systems change not their perceptible temperature but their aggregate phase. Thermochemical heat storage systems store heat by means of endothermal chemical reactions and release it by means of exothermal reactions.


The inertia of rotating masses in a running synchronous generator stores kinetic energy. This stored energy is immediately available to stabilize the system in the event of unforeseen frequency fluctuations.


Hydrogen plays an important role in the renewable energy industry: It can be generated through electrolysis and then stored. Afterwards it is possible to convert hydrogen into electricity.


The power grid consists of power lines, switching and transformer stations, and power stations. The purpose of the grid is the production and distribution of electrical energy. In Europe, most consumers are connected to the synchronous grid of Continental Europe (EONTSO-E) that was formed by connecting many smaller grids. It is much easier to compensate for fluctuations in power generation in large grids than in smaller, insular grids.


Current and voltage in the grid take the form of sine waves with a certain frequency. The regulation of this grid frequency is the responsibility of the transmission grid operators. In the Continental European grid the frequency is 50 Hz.


Electrochemical storage converts chemical energy into electrical energy. A substance that donates electrons reacts with a substance that receives electrons. An electrical cell is an electrochemical storage unit. A battery consists of several connected electrical cells. They enable storage and retrieval of energy independent from the grid.


Flywheel storage is a short-term storage method that is characterized by high power but limited energy density. A flywheel is accelerated by an electrical motor to store energy. When it is discharged, usually the same generator that has powered the flywheel is used to generate power from the kinetic energy.


Generators producing electricity in conventional power stations must rotate at precisely 50 revolutions per second. This is the only way to ensure that alternating current has exactly 50 full cycles per second. If consumption increases unexpectedly and exceeds the supply from the power stations, the generators in the power stations decelerate slightly, causing the grid frequency to drop. The same happens when supply suddenly decreases, e.g. due to the unexpected failure of a power station. Vice versa, the grid frequency rises when the supply is higher or the load is lower than expected. Keeping the grid frequency stable requires active corrections. If the grid frequency is too low, additional supply energy or a reduction of the load is required. These measures are known as regulation up. When the grid frequency is too high, the supply must be reduced or the load increased. This is referred to as regulation down.


Energy is power multiplied by time. Power is the amount of current at any given time. Energy describes how much power is provided over how long a time. For illustration, electricity can be compared with water: The size of the water tank defines the amount of energy that can be stored. The diameter of the inlet and outlet defines the maximum power.
Energy is measured in watt-hours, kilowatt hours, or mega or gigawatt hours (Wh, kWh, MWh, GWh). The units are separated by factors of 1,000: 1 GW = 1,000 MW = 1,000.000 kW. Amounts of electrical energy are measured in watt-hours. If a 1 GW coal power station operates at full power for a day, then it produces 24 GWh of energy. Germany uses about 1,600 GWh of energy per day. The average power is therefore about 67 GW, with peak loads at around 80 GW.
Energy is power multiplied by time. Power is the amount of current at any given time. Energy describes how much power is provided over how long a time. For illustration, electricity can be compared with water: The size of the water tank defines the amount of energy that can be stored. The diameter of the inlet and outlet defines the maximum power.
Energy is measured in watt-hours, kilowatt hours, or mega or gigawatt hours (Wh, kWh, MWh, GWh). The units are separated by factors of 1,000: 1 GW = 1,000 MW = 1,000.000 kW. Amounts of electrical energy are measured in watt-hours. If a 1 GW coal power station operates at full power for a day, then it produces 24 GWh of energy. Germany uses about 1,600 GWh of energy per day. The average power is therefore about 67 GW, with peak loads at around 80 GW.


Energy storage describes the absorption and storage of energy for later retrieval as needed. There are different storage options for different types of energy – such as mechanical, electrical, thermal, or chemical. Energy is usually retrieved in its original form. It may however be stored as a different type of energy. For example, electrical energy may be stored as mechanical energy and released as electrical energy. This is called energy conversion.