Electricity and Heat Storage Options (JR104)

Electricity and Heat Storage Options (JR104)

Introduction

Storage of energy is receiving much attention, since the renewable energy contributions, particularly, from wind and solar sources are variable in nature. Wind and solar vary: on an average day by time of day; on monthly basis;  due to freak , unpredictable occurrences that remove the VRE from the grid  ; and cyclic occurrences that result in lower than normal output of wind and solar. To circumvent this variability of RE the grid needs to: have base load capacity that is usually run on coal or gas ; strengthening of the grid by means of addition of inductive and capacitive loads to counteract over and under voltages ; sensing devices to detect abnormal conditions l; and fast acting devices that mitigate these abnormal occurrences . Where de-carbonization is an objective fossil fuels are not a considerations and therefore storage technologies and systems are needed to store power in glut periods to supply the grid in lean supply periods. Batteries are the obvious answer, batteries are expensive. Battery development is ongoing and can be viewed at https://javedrashid.blogspot.com/2018/10/developments-in-battery-technology.html.

Storage of energy technologies other than battery is discussed in this article.  Energy storage is also an important factor in space heating applications.  Energy storage systems provide a wide array of technological approaches to managing our power supply in order to create a more resilient energy infrastructure and bring cost savings to utilities and consumers. These options can be divided them into six main categories:

·    Solid State Batteries- a range of electrochemical storage solutions, including advanced chemistry batteries and capacitors

·    Flow Batteries- batteries where the energy is stored directly in the electrolyte solution for longer cycle life, and quick response times

·    Flywheels - mechanical devices that harness rotational energy to deliver instantaneous electricity

·    Compressed Air Energy Storage - utilizing compressed air to create a potent energy reserve

·    Thermal - capturing heat and cold to create energy on demand

·    Pumped Hydro-Power - creating large-scale reservoirs of energy with water

The main drawback of solar power is that we’re yet to develop reliable, dense, and long-term storage for the energy that it generates. Our only realistic option at this time is batteries, but they’re quite expensive, use on rare or polluting materials, and have a limited capacity. The current research, however, might provide exactly the breakthrough that the industry needs — the new compound, a specialized fluid called solar thermal fuel, can store and release solar heat for up to 18 years.

Energy storage in the region can help grids maintain quality and reliability as well as optimising off-grid PV power self-consumption. Much of today’s focus is on batteries, although energy storage includes a wide range of technology families, from pumped hydro and flywheels to molten salts with concentrated solar power (CSP). Interest in batteries is mainly driven by cost reductions that have come about as a result of manufacturing volumes arising from vehicle electrification. The cost of batteries 10 years ago was a multiple of today, Even batteries can be broken down into a range of chemistries, including lead-acid, redox flow, sodium-sulphur and lithium-ion. In liberalised markets, batteries have been very successful in providing ancillary grid services

Thermal Energy Storage

Thermal energy storage (TES) is achieved with widely differing technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, or months later, at scales ranging from individual process, building, multiuser-building, district, town, or region. Usage examples are the balancing of energy demand between daytime and nighttime, storing summer heat for winter heating, or winter cold for summer air conditioning (Seasonal thermal energy storage). Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials
Other sources of thermal energy for storage include heat or cold produced with heat pumps from off-peak, lower cost electric power, a practice called peak shaving; heat from combined heat and power (CHP) power plants; heat produced by renewable electrical energy that exceeds grid demand and waste heat from industrial processes. Heat storage, both seasonal and short term, is considered an important means for cheaply balancing high shares of variable renewable electricity production and integration of electricity and heating sectors in energy systems almost or completely fed by renewable energy

Solar energy storage

Most practical active solar heating systems provide storage from a few hours to a day's worth of energy collected. However, there are a growing number of facilities that use seasonal thermal energy storage (STES), enabling solar energy to be stored in summer for space heating use during winter. The in Alberta, Canada, has now achieved a year-round 97% solar heating fraction, a world record made possible only by incorporating STES.

The use of both latent heat and sensible heat are also possible with high temperature solar thermal input. Various eutectic mixtures of metals, such as Aluminum and Silicon (AlSi12) offer a high melting point suited to efficient steam generation, while high alumina cement-based materials offer good thermal storage capabilities

Molten-salt technology

Sensible heat of molten salt is also used for storing solar energy at a high temperature. Molten salts can be employed as a thermal energy storage method to retain thermal energy. Presently, this is a commercially used technology to store the heat collected by concentrated solar power (e.g., from a solar tower or solar trough). The heat can later be converted into superheated steam to power conventional steam turbines and generate electricity in bad weather or at night. It was demonstrated in the Solar Two project from 1995-1999. Estimates in 2006 predicted an annual efficiency of 99%, a reference to the energy retained by storing heat before turning it into electricity, versus converting heat directly into electricity. Various eutectic mixtures of different salts are used (e.g., sodium nitrate, potassium nitrate and calcium nitrate). Experience with such systems exists in non-solar applications in the chemical and metals industries as a heat-transport fluid.

The salt melts at 131°C (268 F). It is kept liquid at 288°C (550°F) in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566°C (1,051°F). It is then sent to a hot storage tank. With proper insulation of the tank the thermal energy can be usefully stored for up to a week.  When electricity is needed, the hot molten-salt is pumped to a conventional steam-generator to produce superheated steam for driving a conventional turbine/generator set as used in any coal or oil or nuclear power plant. A 100-megawatt turbine would need a tank of about 9.1 metres (30 ft) tall and 24 metres (79 ft) in diameter to drive it for four hours by this design.

Single tank with divider plate to hold both cold and hot molten salt, is under development  It is more economical by achieving 100% more heat storage per unit volume over the dual tanks system as the molten-salt storage tank is costly due to its complicated construction. Phase Change Material (PCMs) are also used in molten-salt energy storage

Several parabolic trough power plants in Spain ^] and solar power tower developer SolarReserve use this thermal energy storage concept. The Solana Generating Station in the U.S. can store 6 hours worth of generating capacity in molten salt. During the summer of 2013 the Gemasolar Thermosolar solar power-tower/molten-salt plant in Spain achieved a first by continuously producing electricity 24 hours per day for 36 days.

In terms of installed capacity, TES is arguably a more mature technology than batteries, with more than 22 GWh deployed worldwide compared to a total battery storage volume of just 6.6 GWh .The technology is also falling rapidly in cost, with current pricing at between €50 and €60 per MWh Furthermore, molten salt allows for a much higher volume of storage than batteries, with megawatt-level capacities and duration of up to 20 hours allowing for competitive round-the-clock operations with CSP. In contrast,  , lithium-ion battery systems rarely surpass four hours of storage today.

Heat storage in tanks or rock caverns 

A steam accumulator consists of an insulated steel pressure tank containing hot water and steam under pressure. As a heat storage device, it is used to mediate heat production by a variable or steady source from a variable demand for heat. Steam accumulators may take on a significance for energy storage in solar thermal energy projects.  

Large stores are widely used in Scandinavia to store heat for several days, to decouple heat and power production and to help meet peak demands. Inter seasonal storage in caverns has been investigated and appears to be economical.^[

Heat storage in hot rocks, concrete, pebbles etc

Water has one of the highest thermal capacities Heat capacity - 4.2 J/(cm³·K) whereas concrete has about one third of that. On the other hand, concrete can be heated to much higher temperatures – 1200°C by e.g. electrical heating and therefore has a much higher overall volumetric capacity. Thus in the example below, an insulated cube of about 2.8 m would appear to provide sufficient storage for a single house to meet 50% of heating demand. This could, in principle, be used to store surplus wind or PV heat due to the ability of electrical heating to reach high temperatures. At the neighborhood level, the Wiggenhausen-Süd solar development at Friedrichshafen has received international attention. This features a 12,000 m³ (420,000 cu ft) reinforced concrete thermal store linked to 4,300 m² (46,000 sq ft) of solar collectors, which will supply the 570 houses with around 50% of their heating and hot water. Siemens builds a 36 MWh thermal storage near Hamburg with 600°C basalt and 1.5 MW electric output. A similar system is scheduled for Sorø, Denmark, with 41-58% of the stored 18 MWh heat returned for the town's district heating, and 30-41% returned as electricity

Miscibility gap alloy (MGA) technology

Miscibility gap alloys   rely on the phase change of a metallic material   to store thermal energy Rather than pumping the liquid metal between tanks as in a molten-salt system, the metal is encapsulated in another metallic material that it cannot alloy with (immiscible). Depending on the two materials selected (the phase changing material and the encapsulating material) storage densities can be between 0.2 and 2 MJ/L.
A working fluid, typically water or steam, is used to transfer the heat into and out of the MGA. Thermal conductivity of MGAs is often higher (up to 400 W/m K) than competing technologies  which means quicker "charge" and "discharge" of the thermal storage is possible. The technology has not yet been implemented on a large scale.

Electric thermal storage heaters  

Storage heaters are commonplace in European homes with time-of-use metering (traditionally using cheaper electricity at night time). They consist of high-density ceramic bricks or feolite blocks heated to a high temperature with electricity, and may or may not have good insulation and controls to release heat over a number of hours.

Ice-based technology

Several applications are being developed where ice is produced during off-peak periods and used for cooling at later time For example, air conditioning can be provided more economically by using low-cost electricity at night to freeze water into ice, then using the cooling capacity of ice in the afternoon to reduce the electricity needed to handle air conditioning demands. Thermal energy storage using ice makes use of the large heat of fusion of water. Historically, ice was transported from mountains to cities for use as a coolant. One metric ton of water (= one cubic meter) can store 334 million joules (MJ) or 317,000 BTUs (93kWh). A relatively small storage facility can hold enough ice to cool a large building for a day or a week.
In addition to using ice in direct cooling applications, it is also being used in heat pump based heating systems. In these applications the phase change energy provides a very significant layer of thermal capacity that is near the bottom range of temperature that water source heat pumps can operate in. This allows the system to ride out the heaviest heating load conditions and extends the time frame by which the source energy elements can contribute heat back into the system

Cryogenic energy storage

This uses liquefaction of air or nitrogen as an energy store.   Pilot cryogenic energy system that uses liquid air as the energy store, and low-grade waste heat to drive the thermal re-expansion of the air, has been operating at a power station in Slough, UK since 2010.

Hot silicon technology

Solid or molten silicon offers much higher storage temperatures than salts with consequent greater capacity and efficiency. It is being researched as a possible more energy efficient storage technology. Silicon is able to store more than 1MWh of energy per cubic metre at 1400 °C

Pumped-heat electricity storage

In pumped-heat electricity storage (PHES), a reversible heat-pump system is used to store energy as a temperature difference between two heat stores.

Isentropic

One system which was being developed by the now bankrupt UK Company Isentropic operates as follows. It comprises two insulated containers filled with crushed rock or gravel; a hot vessel storing thermal energy at high temperature and high pressure, and a cold vessel storing thermal energy at low temperature and low pressure. The vessels are connected at top and bottom by pipes and the whole system is filled with the inert gas argon.
During the charging cycle the system uses off-peak electricity to work as a heat pump. Argon at ambient temperature and pressure from the top of the cold store is compressed adiabatically to a pressure of 12 bar, heating it to around 500°C (900 °F). The compressed gas is transferred to the top of the hot vessel where it percolates down through the gravel, transferring its heat to the rock and cooling to ambient temperature. The cooled, but still pressurized, gas emerging at the bottom of the vessel is then expanded (again adiabatically) back down to 1 bar, which lowers its temperature to -150°C. The cold gas is then passed up through the cold vessel where it cools the rock while being warmed back to its initial condition.
The energy is recovered as electricity by reversing the cycle. The hot gas from the hot vessel is expanded to drive a generator and then supplied to the cold store. The cooled gas retrieved from the bottom of the cold store is compressed which heats the gas to ambient temperature. The gas is then transferred to the bottom of the hot vessel to be reheated.
The compression and expansion processes are provided by a specially designed reciprocating machine using sliding valves. Surplus heat generated by inefficiencies in the process is shed to the environment through heat exchangers during the discharging cycle  
The developer claims that a round trip efficiency of 72-80% is achievable.  This compares to >80% achievable with pumped hydro energy storage.  
Another proposed system uses turbomachinery and is capable of operating at much higher power levels. Use of Phase Change Material (PCMs) as heat storage material would enhance the performance further.

Endothermic/exothermic chemical reactions

One example of an experimental storage system based on chemical reaction energy is the salt hydrate technology. The system uses the reaction energy created when salts are hydrated or dehydrated. It works by storing heat in a container containing 50% sodium hydroxide (NaOH) solution. Heat (e.g. from using a solar collector) is stored by evaporating the water in an endothermic reaction. When water is added again, heat is released in an exothermic reaction at 50 °C (120 °F). Current systems operate at 60% efficiency. The system is especially advantageous for seasonal thermal energy storage, because the dried salt can be stored at room temperature for prolonged times, without energy loss. The containers with the dehydrated salt can even be transported to a different location. The system has a higher energy density than heat stored in water and the capacity of the system can be designed to store energy from a few months to years.
In 2013 the Dutch technology developer TNO presented the results of the MERITS project to store heat in a salt container. The heat, which can be derived from a solar collector on a rooftop, expels the water contained in the salt. When the water is added again, the heat is released, with almost no energy losses. A container with a few cubic meters of salt could store enough of this thermo chemical energy to heat a house throughout the winter. In a temperate climate like that of the Netherlands, an average low-energy household requires about 6.7 GJ/winter. To store this energy in water (at a temperature difference of 70 °C), 23 m³ insulated water storage would be needed, exceeding the storage abilities of most households. Using salt hydrate technology with a storage density of about 1 GJ/m³, 4–8 m³ could be sufficient
As of 2016, researchers in several countries are conducting experiments to determine the best type of salt or salt mixture. Low pressure within the container seems favorable for the energy transport especially promising is organic salts, so called ionic liquids. Compared to lithium halide based sorbents they are less problematic in terms of limited global resources, and compared to most other halides and sodium hydroxide (NaOH) they are less corrosive and not negatively affected by CO₂ contaminations

Chemical storage

Swedish researchers have developed a new liquid that can store solar heat for almost two decades. “The energy in this isomer can now be stored for up to 18 years And when we come to extract the energy and use it, we get a warmth increase which is greater than we dared hope for.”

Solar fuels work similarly to a rechargeable battery that substitute’s sunlight and heat in lieu of electricity. The team’s compound is a molecule (norbornadiene) in a liquid form that researchers at the Chalmers University of Technology, Sweden have been developing for over a year. It’s composed mainly of carbon, with some hydrogen and nitrogen atoms thrown in. So, up to now, it’s a pretty standard organic compound.

What makes this fluid stand out is its interaction with sunlight. When exposed to sunlight, the bonds between the molecule’s atoms get rearranged and stabilize in an energized form — an isomer (called quadricyclane). This transforms heat energy from the sun into chemical energy that can be stored and released. The isomer itself is stable enough to last unaltered for up to 18 years (which is a lot), even at room temperatures.

When the energy is needed, the ‘charged’ fluid can be drawn through a catalyst that unpacks the molecule to its original form. The excess chemical energy is given off as heat.

A prototype rig using this new fuel is already undergoing tests on one of the university’s buildings, the team adds. The system is based on a circuit that pumps the fluid through transparent tubes under a concave reflector (this focuses sunlight on the fuel). The charged fuel is then pumped into storage. The whole installation acts much like a sunflower, tracking the Sun as it moves across the sky.

When the energy is needed, the fluid is filtered through the catalyst, warming it by 63 degrees Celsius (113 degrees Fahrenheit). The team hopes that the heat can be used in various roles around the house — heating systems, dishwashers, anything and everything, really — before being pumped back to the roof once again.

“We have made many crucial advances recently, and today we have an emissions-free energy system which works all year around,”  

So far, the researchers have tested their fuel through 125 such cycles without observing any significant damage to the molecule. Furthermore, they report that one kilogram of the fuel can store 250 watt-hours of energy — which is double what a Tesla Powerwall can boast. However, they’re confident that there are still areas where the fuel can be improved. They hope to have the system generate at least 110 degrees Celsius (230 degrees Fahrenheit) more with further tweaks.

“There is a lot left to do. We have just got the system to work. Now we need to ensure everything is optimally designed,” says Moth-Poulsen. Moth-Poulsen thinks the technology could be available for commercial use within 10 years.

Pumped Hydro 

Pumped-storage   is a type of hydroelectric energy storage used by electric power systems for load balancing. The method stores energy in the form of gravitational potential energy of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost surplus off-peak electric power is typically used to run the pumps. During periods of high electrical demand, the stored water is released through turbines to produce electric power. Although the losses of the pumping process makes the plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of peak demand, when electricity prices are highest. The principles on which a pumped hydroelectric plant works can be explained as follows: It is an arrangement which stores electricity as gravitational potential energy. At times when electricity is plentiful and cheap — for instance, on a sunny day for solar energy — a pumped hydro plant uses it to move water up to higher elevation. When electricity is scarce — after dark, in the solar example — it drops the water down, passing it through a hydroelectric turbine to harvest the energy. 

Where possible pumped storage can also store energy, a low and high level reservoir works to store energy. During low price periods energy is used to pump water to the high level storage and during high price periods the water is released to provide power to the grid .Pumped hydro solutions is an mature technology widely used where ever the conditions allow this.

Bricks

The principle used in the pumped storage hydro can be replicated with material other than water. The material that is raised to a higher elevation doesn't have to be water. Companies are currently creating gravitational systems that move gravel up the side of a hill and use the same underpinning principle - when energy is needed, the gravel is released and the weight drives a mechanical system that drives a turbine and generates electricity.

Renewable energy could reliably power the grid at peak times using an eco-friendly and cost-effective storage solution designed by Swiss start-up Energy Vault. The technology, which works by moving concrete bricks around, has more longevity than batteries and more versatility than pumped hydroelectricity — two major current methods for storing energy.

It offers a possible solution to one of the key problems of renewable energy sources like wind and solar, which need to be stored for later use, because they are produced intermittently.Energy Vault says the system delivers base load power cheaper than fossil fuels.

"The world needs rapidly scalable and sustainable energy storage solutions to meet one of the most urgent challenges — the need to decarbonise our energy generation," said Energy Vault co-founder and CEO Robert Piconi.

"In addition to the vital environmental benefits that it provides, the system's radical reduction in cost per kilowatt-hour and overall levelised cost of storage enables our customers to provide dispatchable and base load power cheaper than fossil fuels for the very first time."

Energy Vault announced it was making its technology commercially available in November, at the same time as it announced a partnership with Indian company Tata Power to build its first plant, a 35 megawatt-hour system, in 2019.

Energy Vault's autonomous system combines proprietary software and a six-arm crane to move blocks of concrete in response to changes in energy production and demand.

It works on the same principle as a pumped hydroelectric plant, which stores electricity as gravitational potential energy. At times when electricity is plentiful and cheap — for instance, on a sunny day for solar energy — a pumped hydro plant uses it to move water up to a higher elevation. When electricity is scarce — after dark, in the solar example — it drops the water down, passing it through a hydroelectric turbine to harvest the energy.

Energy Vault's system replicates that effect with 35-tonne bricks, stacking them into a tower when electricity is abundant and releasing them to generate energy.The system has advantages over hydro, because it doesn't require a specific topography and doesn't have the negative environmental impacts. Energy Vault says its system is also more efficient and cost-effective, offering savings of 50 per cent or more over existing solutions. The blocks are made with waste concrete, and the lifetime of one plant is upwards of 30 years.

It also avoids the pitfalls of chemical storage solutions like Tesla's Power wall, which degrade over time, requiring new batteries to be manufactured.

Pumped Compressed Air: Rocks off the UK’s coast could be used as long-term storage for renewable energy. The energy storage method involves pumping compressed air into local, porous sandstone formations, which can later be released to generate large quantities of electricity. Using such a technique on a large scale could store enough compressed air to cover the UK’s winter energy needs — when demand is highest such systems are is already in use in certain sites in Germany and the US. CAES involves using an electric motor to compress air and pump it at high pressure into porous geological layers when energy is plentiful. When supply can’t keep up with demand, this air is released to power turbines that generate electricity and feed it into the grid. In essence, CAES-type systems act as a compressed-air electrical battery.

Small pumped storage hydroelectric power plants:

The purpose of PWS up to now has been network storage, i.e. bulk storage of “surplus” energy on the grid to balance demands during peaks. The growth of renewable energy and the move to distributed generation, micro-grids and survival of sections of the grid have created a demand for smaller storage units, either to balance output or ensure security of supply.

Head (m)	Volume requirement (m3)	Approximate dimensions of reservoir
1000	366	100 m2 x 3,66 m
500	732	200 m2 x 3,66 m
250	1464	400 m2 x 3,66 m
100	3660	1000 m2 x 3,66 m
50	7320	1000 m2 x 7,32 m
20	18 300	2500 m2 x 7,32 m
Water volume requirements for various pressure head.

Small PWS in the range of tens of MW or even several hundred MW can be constructed above ground and using existing infrastructure. Items such as abandoned mines, quarries or even high rise buildings have been investigated as means to provide the necessary storage, and several projects are underway worldwide. Small PWS have been successfully combined with wind farms in island applications, but mainland sites are now being considered as well  

In addition to network storage of hundreds of MW used to balance the entire wind production for an area, small PWS systems are being considered as supplementary storage to wind and solar farms ranging in size from 15 to 300 MW. The amount of storage required varies with the operating regime of the plant, but typically small PWS plants range from 10 to 300 MW in size with storage ranging from one to four hours at full capacity.

It should be fairly obvious that hydropower, especially PWS, requires both a large amount of water and a high head. To generate 1 MWh of energy with the above system would require 366 m3 of water, which is roughly equivalent to a reservoir with 100 m² of surface area and a depth of 3,66 m.

Small PWS will not generally be associated with high heads of water, and it is clear that the volume of water required could be a limiting factor for small PWS systems. Where artificial upper reservoirs are anticipated, this could be a limiting factor. For a river or dam based hydropower system the flow rate will be determined by the flow rate available from the feeding river. For a PWS system the flow rate will be determined by the required power generation capacity and the turbine type. Pipe sizes will be determined by the required flow rate. The flow rate will be limited by the head and the pipe size. This severely limits the use of some existing infrastructure, such a mine shafts, as the ground level dam would be considerable in size, and underground storage would consist of tunnels which may require sealing.

Small PWS technology requirements are considerably simpler from the large reversible pump turbines used in large schemes, and in many cases, pump technology run in reverse can be used. One configuration uses two pumps in a dual configuration  Such a configuration has the advantage that pumping rates and generation rates could differ, a factor which is not possible with fixed speed or even variable speed pump/turbines. The pumping rate could be adjusted to match the amount of spare energy available. 

  Abandoned mines is an option which deserves consideration. Some have depths in excess of 500 m, and almost all have water accumulated in the underground workings. The use of abandoned mine shafts as PWS is suggested regularly   but no detailed study has been done to my knowledge. The biggest problem would be the mine head reservoir, as to store say 20 MWh with a mine depth of 500 m would require maybe 20 000 m³of water storage taking efficiency into account. In some cases the mine dump or slurry dam could be considered as an existing infrastructure to base the reservoir on but no detailed studies have been done. There are numerous studies and proposals in the literature on the use of abandoned mines for PWS systems and there are a few firm projects planning to use abandoned mines. The Kidston project in Australia is a typical example of an open cast mine planned for re-use as PWS . As envisaged, the plan was to develop a 330 MW pumped hydro project on the old Kidston gold mine, involving water transfer between two pits at different altitudes.  There is a 20 Gℓ dam, which was built to service the mine, 18 km away. The Kidston project owns piping between the dam and the mine and it has water rights of 4,6 Gℓ annually should there be a drought. The basic plan was to connect two dams which are located at approximately 190 m vertically from each other. The initial plan involved three 110 MW reversible pumps, which will pump water from below to the top dam when there is excess power, or generate electricity via water from the top dam entering the lower dam. There is already a 132 kV transmission line providing connection to the North Queensland grid and this will be enhanced with a further 275 kV line. There is also a planned 50 MW solar PV farm, and this will be the highest quality solar resource in Australia, with strong community support. A binding agreement with Ergon Energy has been announced for power uptake. The planning for the solar PV farm in the old tailings area, can now proceed with the certainty that the power produced is sold into the grid and there is the possibility of extending this solar PV farm to 150MW. The project is expected to cost 20 to 25% of a traditional pumped storage system.

There are similar plans to use abandoned open pit mines for combined solar/ PWS in other countries.

There are several proposals in different countries to use underground mines as the basis for PWS. Among the more ingenious use the existing mine cavities for both the upper and lower reservoirs, thus obviating the need for new reservoirs above ground level. This makes it possible for the pumped storage project to be completely contained within the existing mine cavities. An example is the Prosper-Haniel coal mine in Germany  The PWS is designed to generate about 200 MW with a storage capacity of 4 h. The upper reservoir is planned to hold 600 000 m³ of water, and the mine is about 600 m deep. The mining complex comprises 26 km of horizontal shafts, which would be used as the lower reservoir. The mine design has an added benefit from the air shafts, which will discharge hot air as the lower reservoir fills up, and heat will be recovered using heat exchangers.

Small PWS are quite common alongside major rivers, but are limited in capacity, firstly by the head available and land available for the upper reservoir.A concept, developed by the Australian national university and based on small scale PWS is pairs of reservoirs, typically 10 ha each, are separated by an altitude difference of between 300 and 700 m, in hilly terrain or ex-mines and away from rivers, and joined by a pipe with a pump/turbine. Water circulates between the upper and lower reservoirs in a closed loop to store and generate power.

Very little water is apparently required relative to conventional fossil fuel power stations. Estimated stations could be in size from 50 to 500 W and with a storage time of 4 to 20 h. Problems with initial filling and compensation for evaporation and leakage. Such a network of small scale PWS is claimed to be able to provide sufficient storage capacity to allow operation from 100% renewable energy sources.  The concept of combining wind power with pumped hydropower has been taken to the extreme by GE in the Gaildorf project, located in Germany’s Swabian-Franconian forest on the Limburger Berge uplands  The project will comprise four of GE’s new 3,4 MW 137 turbines and a 16 MW pumped storage hydro-electric power plant supplied under a separate agreement Germany are storing water for hydroelectricity inside wind turbine towers allowing the towers act like massive batteries once the wind stops blowing. It’s the first major example of the two technologies being physically integrated to supply reliable renewable energy.

The four-turbine project stores energy by pumping water about 35 m up inside the turbine structure itself. Basins around each base will store another 45 mℓ. When the wind stops, water flows downhill to generate hydroelectric power. A man-made lake in the valley below collects water until turbines pump the water back up again. The reservoirs are connected by a polyethylene penstock. The pumped storage powerhouse is a standardised, modular design. The base of each wind turbine is to be used as a water reservoir, increasing tower height by 40 m and extending tip height to a record-breaking 246,5 m. The lower reservoir for the pumped storage plant lies in a nearby valley, 200 m below the wind turbines. The combined wind and hydro power plant will be able to provide balancing power for fast-response stabilisation of the grid, according to GE. This would be an additional source of earnings on top of normal participation in the wholesale electricity market.

During times of peak demand and high electricity prices, the hydro plant will be in production mode. When demand and prices are low, the hydro plant will be in pump mode, pumping and storing water in the upper reservoir for later use. The net effect will be to use the stored hydro capacity to balance the intermittent nature of wind power through the optimal use of energy at different times of the day.

The wind farm, will feature the tallest turbines in the world at 246,5 m. The wind turbine generators will sit at a hub height of 178 m, while the lower 40 m of each tower and its surrounding area will be utilised as active water reservoirs to store energy. At its full capacity, it would produce 13, 6 MW, along with another 16 MW from the hydroelectric plant.   

Off grid wind and solar systems rely mainly on battery storage to provide a usable reliable supply of electricity over the full day’s usage. The use of micro PWS systems for regulating domestic power for small off grid applications has been proposed in a paper by using a PWS with head of 15 m and separate pump and turbines, coupled with solar and wind generators, the system is capable of providing both long term and short term energy storage   The study showed that a net financial benefit resulted from altering the solar and wind power system into a solar-wind pump-storage power system  

Battery Storage Paves Way for a Renewable-powered Future

March 26, 2020 reve

Battery storage systems are emerging as one of the key solutions to effectively integrate high shares of solar and wind renewables in power systems worldwide. A recent analysis from the International Renewable Energy Agency (IRENA) illustrates how electricity storage technologies can be used for a variety of applications in the power sector, from e-mobility and behind-the-meter applications to utility-scale use cases.

Utility-scale batteries, for example, can enable a greater feed-in of renewables into the grid by storing excess generation and by firming renewable energy output. Furthermore, particularly when paired with renewable generators, batteries help provide reliable and cheaper electricity in isolated grids and to off-grid communities, which otherwise rely on expensive imported diesel fuel for electricity generation.

At present, utility-scale battery storage systems are mostly being deployed in Australia, Germany, Japan, United Kingdom, the United States and other European countries. One of the larger systems in terms of capacity is the Tesla 100 MW?/ 129 MWh Li-ion battery storage project at Hornsdale Wind Farm in Australia. In the US-State of New York, a high-level demonstration project using a 4 MW?/?40 MWh battery storage system showed that the operator could reduce almost 400 hours of congestion in the power grid and save up to USD 2.03 million in fuel costs.

In addition, several island and off-grid communities have invested in large-scale battery storage to balance the grid and store excess renewable energy. In a mini-grid battery project in Martinique, the output of a solar PV farm is supported by a 2 MWh energy storage unit, ensuring that electricity is injected into the grid at a constant rate, avoiding the need for back-up generation. In Hawaii, almost 130 MWh of battery storage systems have been implemented to provide smoothening services for solar PV and wind energy.

Globally, energy storage deployment in emerging markets is expected to increase by over 40% each year until 2025.

Figure 1. Stationary battery storage’s energy capacity growth, 2017-2030

Currently, utility-scale stationary batteries dominate global energy storage. But by 2030, small-scale battery storage is expected to significantly increase, complementing utility-scale applications. 

The behind-the-meter (BTM) batteries are connected behind the utility meter of commercial, industrial or residential customers, primarily aiming at electricity bill savings. Installations of BTM batteries globally is on the rise. This increase has been driven by the falling costs of battery storage technology, due to the growing consumer market and the development of electric vehicles (EVs) and plug-in hybrid EVs (PHEVs), along with the deployment of distributed renewable energy generation and the development of smart grids. In Germany, for example, 40% of recent rooftop solar PV applications have been installed with BTM batteries. Australia aims to reach one million BTM batteries installations by  2025, with 21 000 systems installed in the country in 2017. 

Figure 2. Services provided by BTM battery storage systems

Overall, total battery capacity in stationary applications could increase from a current estimate of 11 GWh to between 180 to 420 GWh, an increase of 17- to 38-fold.

Read IRENA’s full Innovation landscape briefs on Utility-scale batteries and Behind-the-Meter batteries. 

Find more information about enabling technologies in IRENA’s Innovation Landscape briefs: Enabling Technologies https://www.evwind.es/2020/03/26/battery-storage-paves-way-for-a-renewable-powered-future/74182

 

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