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XYLENE POWER LTD.
MATCHING GENERATION TO LOAD:
In any electricity system, in order to regulate the system voltage, it is necessary to continuously match total instantaneous electricity supply to total instantaneous electricity demand. This matching process is simplified if the electricity system contains energy storage subsystems that can absorb and release energy at controlled rates.
Energy storage is used to continuously and efficiently match the daily, weekly and seasonal variations in total generator output power to the daily, weekly and seasonal variations in electricity demand. Energy storage is also used to reduce the sizes of required transmission/distribution and primary generation.
APPLICATIONS OF ENERGY STORAGE:
Wind and tidal energy generators at remote locations need behind the meter daily energy storage to level their net generation rates, so as to minimize their transmission capacity requirements. Behind the meter energy storage at a generator can be used to modify the that generator's net power output profile to match the available load profile, subject to local transmission-distribution constraints.
As the dependence on non-fossil fuel generation increases more energy storage behind load customer meters will be required to match the load customer profile to the transmitted energy supply profile. Behind the meter energy storage at a load can be used to modify that load's net power input profile to match the available generation, subject to local transmission-distribution constraints.
There are substantial costs associated with energy storage that were not anticipated in the OPA FIT price formulation. A subtle but important technical issue is that FIT generation and related energy storage should be configured so that the generator can ride through transient grid faults and does not have to rely on other generation to black start.
Electro-chemical energy storage may be used at generation and load sites to provide voltage regulation, reactive power and black start when and where required.
Centrally located pumped hydraulic energy storage may be used for absorbing energy surpluses and for meeting energy deficits. In the absence of sufficient energy storage, a combination of generation constraint and load constraint is required to match generation to load.
OTHER REQUIREMENTS FOR ENERGY STORAGE:
Nuclear generators are generally base loaded to minimize nuclear reactor thermal stress. The electricity system requires other generation types and/or energy storage to achieve load following. Nuclear generation is most economic when there is relatively little seasonal variation in the total load.
Run-of-river generators provide seasonal electricity generation that in Ontario is much higher in the spring than in the autumn. Run-of-river generators can easily be constrained for load following. However, the constrained portion of the generator's potential energy output is usually discarded.
Wind generators in Ontario need on-site electro-chemical storage to average their daily outputs. Failure to purchase electo-chemical energy storage along with wind generation has the practical effect of committing Ontario to extra transmission costs and long term large scale use of other generation for balancing of wind generation. Due to the seasonality of wind generation in Ontario wind generation will be most economic when the total grid load is consistently higher in the winter than in the summer. Seasonal shifting of the provincial load profile requires large scale pumped hydraulic energy storage between Lake Erie and Lake Ontario.
All non-fossil fuel generation types except solar need to shift energy from the daily off-peak period to the daily on-peak period, where the energy has greater value.
Energy storage adjacent to wind generator sites serves four important functions:
1. Shifting excess energy from times of energy surplus to times of energy deficiency;
2. Filtering out random short term variations in generator output;
3. Providing generator black start capability and reactive power;
4. Reducing the cost of switchgear required for network protection;
5. Reducing associated transmission capital costs;
6. Proving local voltage regulation.
Energy storage adjacent to load sites serves three other additional important functions:
1. Filtering out random short term variations in load power input;
2. Providing emergency backup power;
3. Increasing the average efficiency of the transmission/distribution system by reducing the high I^2R losses that occur at times of peak load.
CENTRAL ENERGY STORAGE:
Due to statistical averaging of the outputs of many geographically distributed renewable energy generators the total generator output power variation is less than the sum of the individual generator random output power variations. Similarly the total load input power variation is less than the sum of the individual load random input power variations. This statistical averaging mechanism is known as diversity. If the energy storage is centrally located diversity reduces the amount of energy storage required to match the total generation to the total load. However, this cost saving is offset by an increase in transmission cost.
DISTRIBUTED ENERGY STORAGE:
In terms of reducing the requirement for transmission/distribution the optimum location for daily energy storage is behind generator and load customer meters. By reducing the metered individual generator output power variations and the metered individual load input power variations voltage is regulated, transmission/distribution costs are reduced and the difference between total non-dispatched generation and total non-dispatched load that must be met with dispatched generation and dispatched load is minimized.
SMART GRID:
The overall electricity system performance can be further enhanced by use of a "smart grid" in which each energy storage unit has sufficient intelligence and communication network connectivity to recognize times when there is a net grid energy supply surplus and times when there is a net grid energy supply deficiency. In response to a net grid energy supply surplus the energy storage units should absorb electrical energy. In response to a net grid energy supply deficiency the energy storage units should release electrical energy. This result is achieved by programming the energy storage units to have a negative slope output power versus voltage characteristic. Above nominal voltage the unit charges. Below nominal voltage the unit discharges. The nominal voltage setting can be remotely adjusted to reflect the actual amount of stored energy.
If there is a loss of network communications the energy storage control should default to regulating the net power flow out of each generator and the net power flow into each load customer so as to regulate voltage and optimize use of local distribution and local energy storage.
RATES:
The metering system and electricity rates should reward non-dispatched generators and non-dispatched loads for providing energy storage that reduces local power fluctuations and that enhances overall grid voltage stability.
SEASONALITY:
Renewable generation in Ontario has a seasonal component. The monthly average wind generation during the winter is about two times the monthly average wind generation during the summer. One means of meeting the peak summer electricity load is to use a very large reservoir (Lake Erie-Lake Ontario) hydraulic energy storage system to store surplus energy during the winter for use during the following summer. The alternative to this seasonal energy storage is to build sufficient additional generation with daily energy storage to meet the summer peak load and to also build additional load that can be dispatched off during the summer. Neither of these means of meeting the peak summer load is inexpensive.
ENERGY RATE FOR UNCONSTRAINED WIND GENERATION:
Via a number of 20 year wind power purchases the Ontario Power Authority (OPA) has determined that in 2009 the cost of unconstrained land based wind generation, with no energy storage, is about:
$.145 / kWh.
The OPA currently offers $.135 / kWh. The federal government has an incentive program that in the past has paid $.01 / kWh.
This wind generation typically has a winter capacity factor of 40%, a summer capacity factor of 20% and an annual capacity factor of 30%.
One kW of such generator peak plate capacity produces its owner a gross revenue of about:
1 kW X (8766 h / year) X 0.3 X ($.145 / kWh) = $381.32 / year
The OPA has further found that the capital cost of land based wind generation is about $2800 / peak plate kW, indicating that the maximum simple payback period for wind generator financing is:
($2800 / peak plate kW) / ($381.32 / peak plate kW-year) = 7.34 years
The actual payback period for investors is longer due to the costs of site leasing and wind generator maintenance. However, an investment in unconstrained wind generation may make sense if it shelters other income from tax.
ENERGY RATE FOR WIND GENERATION WITH DAILY ENERGY STORAGE:
Assume that about 30% of the electrical energy generated is lost in the daily energy storage system, so on an annual basis 1 kW of wind generator peak plate capacity with daily energy storage exports to the grid:
1 kW X .30 X 8766 h / year X .7 = 1840.86 kWh / year
The blended cost of financing the daily energy storage system is reasonably estimated to be about $.10 / kWh of filtered energy. Hence the maximum allowable capital cost of the daily energy storage system per kW of wind generator peak plate capacity is:
1840.86 kWh / KW-year X $.10 / kWh X 7.34 years= $1351.19 / kW
Since the output of the daily energy storage unit inverter is only about 0.5 kW per kW of wind generator peak plate capacity, the capital budget for the energy storage unit can be as much as:
$1351.19 / kW X (1 kW / .5 kW) = $2702.38 / kW of inverter output capacity.
The cost of wind generated energy delivered to the grid from daily energy storage becomes:
($.145 / kWh) / .7 = $.2071 / kWh ~ $.21 / kWh.
When the cost of financing the storage system is included, energy from storage costs:
$.21 / kWh + $.10 / kWh = $.31 / kWh
The corresponding average cost of filtered electricity is about:
($.31 / kWh + $.145 / kWh) / 2 = $.2275 / kWh ~ $.23 / kWh
SEASONAL CONSTRAINT:
If due to seasonal generation variation 33% of this generation (half of the average winter generation) has to be constrained off the cost of the remaining generation remains:
$589.08 / year
or
($589.08) / .67(2103.84 kWh) = $.4179 / kWh
and the average output in the summer is:
(.67 X 2103.84 kWh) / 8766 h = 0.174 kW per kW of wind generator peak plate capacity.
Thus, subject to the aforementioned assumptions relating to the efficiency and cost of daily energy storage, with no seasonal energy storage the average cost of wind generated electricity for feeding a constant load is $.4179 / kWh.
The average cost of wind generated electricity can be slightly reduced if the surplus energy available in the winter can be stored for use during the following summer.
SEASONAL WIND ENERGY STORAGE:
If instead of being constrained off half of the average winter generation is stored at an efficiency of 50%, the total available generation per annum per kW of wind generator peak plate capacity is:
(2103.84 kWh / year) X (5 / 6) = 1753.2 kWh
at an average cost not including seasonal storage financing of:
$589.08 / 1753.2 kWh = $.3360 / kwh
and the average generation in the summer is about:
1753.2 kWh / 8766 h = 0.2 kW per kW of wind generator peak plate capacity.
VALUE OF SEASONALLY STORED WIND ENERGY:
The annual energy value savings per kW of wind generator plate capacity achieved by use of seasonal energy storage are:
1753.2 kWh / year X ($.4179 - $.3360) / kWh = $143.59 / year
One fifth of the electricity supplied is drawn from seasonal storage. Hence achieving these cost savings requires recovering from seasonal storage:
(1753.2 kWh /year) / 5 = 350.64 kWh / year
Hence the average value of each recovered seasonally stored kWh is:
$143.59 / 350.64 kWh = $.4095 / kWh
FINANCING SEASONAL ENERGY STORAGE:
Consider 1 kW of generation fed from seasonal energy storage. Assume that this generation operates on average 2000 hours / year. Then this 1 kW generator produces a gross revenue of:
($.4095 / kWh) X (2000 h / year) = $819 / year-kW
If the maximum payback period that can be financed is 7.34 years, the maximum capital cost for seasonal energy storage that can be financed is:
7.34 years X $819 /year-kW = $6011.46 / kW.
It has been shown that the combined inefficiency and capital cost of daily energy storage for leveling wind generator output increases the average cost of wind generated energy from $.145 / kWh to about $.42 / kWh. Addition of central seasonal energy storage improves energy availability in the summer but only slightly reduces the average cost of wind energy.
ENERGY STORAGE AND TRANSMISSION:
In the absence of electrical energy storage located adjacent to a generator, the associated transmission must be sized to match the peak generator output. For unconstrained wind generation without daily energy storage (annual capacity factor = 30%) the cost of transmission / kWh-km is about three times the cost of transmission / kWh-km for base load nuclear generation (annual capacity factor = 90%). Hence, there is an economic incentive for locating some of the energy storage close to wind generation to reduce transmission costs.
IMPACT OF ENERGY STORAGE ON TRANSMISSION COST:
It is generally accepted that the capital cost of rural dual circuit 500 kVAC transmission is about:
$3,000,000 / km-3000 MW peak = $1.00 / km-kW peak
The annual blended cost of interest, capital amortization and maintenance for the transmission of 1 kW of peak generation without energy storage may be as much as:
0.2 / annum X $1.00 / km-kW = $.20 / kW-km-year
When the average transmission distance becomes of the order of 1000 km, the annual cost of peak transmission becomes:
1000 km X $.20 / kW-km-year = $200.00 / kW-year
If the generator capacity factor is only 30%, this cost must be amortized over:
0.3 X 8766 kWh / kW-year = 2629.8 kWh / kW-year
leading to a transmission cost per kWh of:
($200.00 / kW-year) / (2629.8 kWh / kW-year) = $.076 / kWh
If behind the meter daily energy storage improves the net generator capacity factor from 30% to 60% the transmission cost savings are:
0.5 X $.076 / kWh = $.038 / kWh
This cost saving is significant.
UNCONSTRAINED WIND ENERGY COST SUMMARY:
| STORAGE TYPE | CAPACITY FACTOR | ENERGY COST | TRANSMISSION COST |
|---|---|---|---|
| No Storage | 30% | $.145 / kWh | $.076 / kWh-1000 km |
| Daily Storage | 60% | $.28 / kWh | $038 / kWh-1000 km |
| Seasonal Storage | 90% | $.42 / kWh | $.0253 / kWh-1000 km |
ELECTRICITY RATE FAILURE:
At this time electricity rates in Ontario do not financially enable either daily or seasonal energy storage. The present generator compensation rates do not sufficiently encourage storage of renewable energy when it is surplus for later use when renewable energy is in short supply. The load customer electricity rates do not sufficiently encourage off-peak energy storage at the load to relieve on-peak grid congestion. The Green Energy Act Feed-In Tariffs do not adequately reflect either the time value of energy or the value to transmission of high generator capacity factor. All of these rate failures are indirectly caused by the present use of fossil fuel electricity generation for load following.
The net result of this electricity rate failure is that there is less energy storage connected to the Ontario electricity system than in the past. However, as fossil fuel generation is constrained to reduce CO2 emissions there will be an increased requirement for energy storage to match generation to load. This requirement will eventually force a significant change in the electricity rate structure.
Energy storage systems will not be built or operated unless there is sufficient on-going benefit to the energy storage system owner to justify the required capital investment, the ongoing operating and maintenance costs and the loss of revenue that the energy storage system owner would otherwise receive for alternative use of the same land or building space. There must also be long term electricity rate certainty. Until such time as the Ontario Energy Board (OEB) adopts electricity rates that are sufficient to meet the reasonably projected long term electricity system costs, there will be no electricity rate certainty.
It is worth noting that during the 1960s Ontario Hydro offered electricity rates with 10 year guarantees that encouraged many developers of large buildings to incorporate energy storage systems into their buildings. By 1983 there were over 20,000,000 square feet of commercial space in Metro Toronto with energy storage for electrical load control. These energy storage systems were almost all taken out of service during the period 1983 to 1997 when changes to the electricity rates diminished the financial benefits of using energy storage systems. This author, acting on behalf of the Urban Development Institute, advised the OEB in 1981 that the then trend in Ontario Hydro electricity rates would lead to these energy storage systems being taken out of service and that is exactly what transpired.
FINANCIALLY ENABLING ENERGY STORAGE:
Any energy storage system relies on the ratio:
(on-peak electricity cost/kWh) / (off-peak electricity cost/kWh)
being sufficient to fund the storage system energy losses, as well as to fund the storage system's capital amortization, operating, maintenance and overhead costs. In a practical energy storage system this ratio must be in the range of 3.0 to 4.0 and the energy storage system efficiency must be greater than 50%. However, the importance of this ratio is not currently recognized by the present Ontario electricity rate structure. The financial requirements of energy storage systems should be taken into account in any future determination of electricity rates, carbon taxes or generation incentives. Ideally, behind the meter energy storage systems should be used in conjunction with a daily load factor dependent electricity rate, as described in the webpage titled Electricity Congestion Factor.
PRACTICAL EXAMPLES:
Some understanding of the funding issues relating to energy storage systems can be gained by considering two time dependent electricity rate examples:
Marginal Case:
Assume the on-peak electricity rate is $.24 / kWh, the off-peak electricity rate is $.08 / kWh and the energy storage system efficiency is 50%. If the energy storage system charges off-peak and discharges on-peak, the gross income per discharged kWh is:
$.24 / kWh - (($.08 / kWh) / .5) = $.08 / kWh
Favourable Case:
Assume the on-peak electricity rate is $.24 / kWh, the off-peak electricity rate is $.06 / kWh and the energy storage system efficiency is 75%. If the energy storage system charges off-peak and discharges on-peak, the gross income per discharged kWh is:
$.24 / kWh - (($.06 / kWh) / .75) = $.16 / kWh
Note that seemingly minor changes in storage efficiency and electricity rate doubled the amount of money available for funding the energy storage system capital, operating and maintenance costs. Thus,the economic viability of energy storage is sensitive to both the energy storage system efficiency and the ratio of on-peak to off-peak electricity rates.
REQUIRED RATE CHANGE:
In Ontario, in order to financially enable energy storage, the Ontario Energy Board must implement time dependent transmission/distribution rates as well as time dependent energy rates.
On this web site it is proposed that, as an alternative to simple Time-Of-Use (TOU) electricity rates, both daily energy and daily transmission/distribution electricity rates for non-dispatched load customers should decrease exponentially with increasing daily load factor. Similarly the non-dispatched generator compensation rate should decrease with decreasing generator capacity factor. This proposal financially enables energy storage while avoiding grid instability and local distribution problems associated with simple time block Time-Of-Use (TOU) electricity rates.
RIVER FED RESERVOIRS:
River fed hydro-electric reservoirs are a highly efficient and readily controllable means of energy storage. If a river fed reservoir feeds a hydro-electric hydraulic turbine, and if the reservoir has sufficient area and depth to store half of the river flow for 12 hours, the flow through the turbine for 12 hours at night can be set at 0.5 X the daily average river flow and the flow through the turbine for 12 hours during the day can be set at 1.5 X the daily average river flow. This arrangement results in a 3:1 change in the downstream water flow rate and results in an electricity generation rate during the day that is three times the electricity generation rate during the night. This arrangement is a practical way of realizing daily load following generation provided that a daily 3:1 change in downstream water flow rate is acceptable.
In principle, river fed reservoirs can also be used to balance minute by minute variations in wind generation. However, there are occasional weather conditions that cause the total wind generation to drop very quickly. Under those circumstances the downstream water flow rate from wind balancing hydro-electric facilities increases very quickly, causing dangerous conditions for anyone in, on or close to the downstream river. To mitigate these dangerous conditions it is necessary to constrain the rate of increase of downstream river flow and to partially balance a rapid drop in wind generation by shedding dispatched load or by using another type of quick responding energy storage such as distributed electro-chemical energy storage.
An advantage of river fed reservoirs is that the energy storage efficiency is generally over 90%. There is a small decrease in energy storage efficiency as the storage level decreases.
Construction of river fed reservoirs generally requires favourable river valley geography and low cost adjacent land. It is generally economically unfeasible to build such reservoirs in urban areas.
A significant problem with river fed reservoir hydro-electric generation is that it uses fresh water for on-peak electricity generation that might otherwise be used for irrigation of high elevation farm crops. As global warming increases irrigation requirements, the cost of fresh water for on-peak hydro-electric generation will increase.
An example of a large river fed reservoir is the Grand Coulee Dam on the Columbia River in Washington State, USA and its up-river seasonal storage dams in British Columbia. In past years Columbia River hydro-electric power was used to meet US electricity demand peaks as far east as Chicago.
PUMPED HYDRAULIC ENERGY STORAGE:
If the available river fed reservoirs are all fully utilized, then the alternative is pumped hydraulic energy storage. With pumped hydraulic energy storage when there is surplus electricity water is pumped up hill from a low level reservoir to a high level reservoir, storing gravitational potential energy. When there is a deficiency of electricity water runs back downhill from the high level reservoir to the low level reservoir via a hydraulic turbine to generate electricity. A pumped storage system between two nearby lakes can easily store energy for about one day. Pumped hydraulic storage is a good way of efficiently converting nuclear base load power into daily load following power.
Pumped hydraulic storage between two very large lakes such as Lake Erie and Lake Ontario can store sufficient energy for seasonal balancing of wind generation.
Pumped hydraulic energy storage systems are characterized by rapid changes in the water flow rate and direction between the high level reservoir and the low level reservoir. These rapid flow changes are inherently dangerous. To ensure safety the public must be completely excluded from the river or canal system connecting the two reservoirs.
A disadvantage of pumped hydraulic storage is that, due to its rural location, there is usually an associated major transmission cost. There are also changes in water level that have minor ecological consequences. The daily water level change is similar to the tide change at a sea port.
An example of a pumped hydraulic energy storage system is:
Racoon Mountain in Tennessee. View
Racoon Mountain Cross Section
Another example of a pumped hydraulic energy storage system is:
Dinorwig in the UK.
Another example of a pumped hydraulic energy storage system is:
Robert Moses Dam at Niagara Falls in New York.
Another example of a pumped hydraulic energy storage system is:
Bath County Pumped hydraulic power station
ELECTRO-CHEMICAL ENERGY STORAGE:
In an electro-chemical energy storage system surplus AC electricity is converted into DC and then is stored as chemical energy. When there is a deficiency of electricity the chemical energy is converted to DC and then is converted again into three phase AC using a static inverter.
Electro-chemical energy storage has quick response allowing it to be used for balancing rapid changes in wind generation.
An important advantage of electro-chemical energy storage is that it can be located at both distributed generation sites and at urban load sites to minimize transmission/distribution costs.
The chemical systems that are most suitable for stationary electricity energy storage are sodium-sulfur-nickel chloride (Na-S-NiCl2) and vanadium. These chemicals have a high DC electrical efficiency (84% - 91%) allowing 65% to 79% of stored electricity to be recovered as AC. Na-S-NiCl2 energy storage systems are suitable for daily generation or load leveling. Vanadium energy storage systems can be used for daily or weekly generation or load leveling.
The materials used in construction of sodium-sulfur-nickel chloride and vanadium energy storage systems are common, relatively inexpensive and readily available in Canada. We put hundreds of tons of sodium chloride on our streets after every snowfall. We export mountains of sulfur obtained from processing metal-sulfide ores and from refining of petroleum. We have large reserves of the other required materials: iron, nickel, and vanadium.
Sodium-sulfur-nickel chloride electro-chemical energy storage is presently used on a medium scale in Japan, and on a smaller scale in trial installations in the USA, Canada, Ireland, Uk, Switzerland and Italy. The principal suppliers of sodium-sulfur-nickel chloride energy storage systems are NGK Insulators Ltd. in Japan and MES-DEA in Switzerland. The high energy density of sodium-sulfur-nickel chloride energy storage modules allows them to be used for fleet vehicle propulsion. These modules can also be used for daily load leveling in applications as small as single family homes and small commercial establishments. Disadvantages of sodium-sulfur-nickel chloride technology are high temperature operation (~300 degrees C), complex charge control and a limited number of charge-discharge cycles (typically 2000) before the energy storage cells must be recycled. A feature of Na-S-NiCl2 energy storage systems is that when installed adjacent to major buildings the waste heat can be recovered for use in domestic hot water and space heating.
A vanadium based electro-chemical energy storage system is manufactured by Prudent Energy in Richmond, British Columbia. This vanadium based system has the advantages that: it operates at room temperature allowing efficient energy storage over long time periods, its charge control is inherently simple and it offers more charge-discharge cycles than are presently available from sodium-sulfur-nickel chloride energy storage systems. The disadvantages of a vanadiaum based energy storage system are that the energy density is low, making this technology unsuitable for vehicle propulsion, and that the energy storage chemicals are liquid (sulfuric acid) rather than solid at room temperature. In this author's practical experience, widespread use of this energy storage technology in residential or small commercial applications would almost certainly result in significant amounts of dilute sulfuric acid being dumped into city sewers.
THERMAL ENERGY STORAGE:
In thermal energy storage an electrically powered heating or cooling system is run during electricity off-peak periods to store large tanks full of hot water, ice pellets or molten salt. The stored thermal energy is subsequently used for heating or cooling during electricity on-peak periods. Thus the heating or cooling electricity load is effectively shifted from the on-peak period to the off-peak period. Thermal energy storage has the advantage of relatively high overall efficiency as compared to other forms of daily energy storage.
Thermal energy storage systems are both large and heavy and hence either must be engineered into a building during its original design or must be retrofit built on adjacent land. However, between the retrofit energy storage system location and the building there is often a public road that is legally difficult to cross with fluid pipes and electrical cables. It is usually uneconomic for an energy storage system to pay the LDC transmission/distribution rate for a short electricity connection across a road. The facility owner needs to be able to obtain an easement for an underground connection across a public road without involving the LDC.
HYDROGEN PRODUCTION:
Most hydrogen for industrial purposes is presently obtained by reforming natural gas. However, production of hydrogen from natural gas releases large amounts of CO2 to the atmosphere. Also hydrogen obtained by reforming natural gas contains carbon contaminants that shorten the operating life of fuel cells. Purer hydrogen can be obtained via either the CuCl water separation process or via electrolysis of water. Both of these processes require large amounts of input energy. The limitation of these processes is that the produced gaseous hydrogen is difficult to store, so an energy storage system based on hydrogen gas is not suitable for seasonal energy storage.
AMMONIA ENERGY STORAGE:
Liquid ammonia (NH3) is an essential input for making a wide range of chemical products, including nitrogen fertilizers. Presently the required hydrogen is obtained by reforming natural gas. However, if there is a sufficient fossil carbon emissions tax the required hydrogen can be obtained via either the CuCl water separation process or via electrolysis of water. Both of these processes require large amounts of input energy. The produced hydrogen can be combined with nitrogen to form liquid ammonia which can be readily stored and transported. Large swings in available nuclear heat or electricity supply can be balanced by varying the hydrogen and ammonia production rates.
Production of liquid ammonia using off-peak electricity is a process that lends itself to dispatched load shedding for seasonal balancing of wind generation and for grid voltage regulation.
HYDROCARBON ENERGY STORAGE:
In principle biofuels could be used for long term energy storage. Liquid biofuels have the advantages that they are easy to store and transport and hence can be used to conveniently provide energy when and where required with minimal environmental impact. This feature makes liquid biofuels in high demand as fuels for transportation and rural heating. Except in extraordinary circumstances, liquid biofuels are not used for stationary electricity generation.
Production of liquid biofuels using off-peak electricity is a process that lends itself to dispatched load shedding for seasonal balancing of wind generation and for grid voltage regulation. The dispatched electricity would be used for efficient production of ethanol or butanol.
KINETIC ENERGY STORAGE:
In kinetic energy storage surplus electricity is stored as kinetic
energy in a flywheel. When there is a deficiency of electricity on the grid the
kinetic energy in the flywheel is converted back into electricity. Flywheels typically store
energy from seconds to minutes. Flywheel energy storage systems are primarily used to dampen voltage oscillations on electricity transmission/distribution systems. Utilities generally include the costs of kinetic energy storage in their delivery costs. A representative supplier of flywheel energy storage systems is Beacon Power.
ELECTROSTATIC ENERGY STORAGE:
In electrostatic energy storage the surplus energy is stored as an electic field in a device known as a super capacitor. Super capacitors can be realized using carefully controlled alternating thin layers of metal and barium titanate. Super capacitor technology is relatively new and is presently unproven in utility applications. Super capacitor technology development is presently aimed primarily at the electric automotive market. This author has serious safety concerns relating to utility size super capacitors. The energy density and discharge rate makes these devices potentially explosive.
ELECTROMAGNETIC ENERGY STORAGE:
In electromagnetic storage electrical energy is stored in magnetic fields and is released in a
controlled manner a few milliseconds later. The primary uses of electromagnetic energy storage are for AC/AC voltage change, for AC/DC/AC static power inversion and for power factor correction. Energy can be efficiently stored for a long time in a magnetic field realized using superconductoring electomagnets, but the capital cost of such energy storage is generally prohibitive for electric utility applications.
INVERTERS:
AC/DC/AC static inverters are integral to electro-chemical and electrostatic energy storage and are also used with micro-turbines, at certain power system boundaries (eg. Ontario-Quebec border) and in support of underwater cables or very long overhead transmission lines.
This web page last updated June 16, 2010.
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