Home Energy Nuclear Electricity Climate Change Lighting Control Contacts Links



By C. Rhodes, P.Eng., Ph.D.

An energy storage system absorbs and stores energy at times when the uncontrolled demand for electricity is low (and hence there is surplus non-fossil energy) and discharges energy at times when the uncontrolled demand for electricity is high (and hence there is a shortage of non-fossil electricity).

Seasonal energy storage is essential to add value to intermittent renewable generation. Daily energy energy storage behind the meter reduces the cost of nuclear generated electricity. However, neither seasonal nor daily energy storage will be built unless the electricity rate structure allows the owners of the energy storage to make a profit.

An energy storage system is only financially viable if the electricity rate adequately recognizes the benefits of energy storage to the electricity system. Typically for behind the meter energy storage to be financially viable the retail electricity rate must be primarily based on a customer's monthly peak kW or peak kVA so that the customer's cost of a marginal kWh is small enough to allow for energy losses in storage and for cost competition with fossil fuels. This is an issue that electricity rate setting bodies everywhere have failed to address. Daily behind the meter energy storage is required to mitigate the cost of nuclear electricity generation.

There is little merit in building more intermittent renewable generation if there is insufficient seasonal energy storage to support use of the renewable generation. In North America interstate and interprovincial electrical energy exchanges presently occur at the instantaneous value of that electricity. However, that instantaneous value system is not consistent with energy storage because the more storage that is added the lower the difference in value between an on-peak and an off-peak kWh. Transmission connected storage requires a consistently large difference between the on-peak and off-peak price per kWh in order to be financially viable.

The fix for this problem is to value interprovincial and interstate electricity exchangs based on mimimum sustained transfer capacity over a prolonged period such as a month. Then intermittent generators feeding the electricity market would be financially incented to have sufficient dedicated seasonal energy storage to contribute to the base load.

In the absence of sufficient energy storage, constrained surplus generation capacity and dispatchable loads are required to continuously match generation to load. Constrained surplus generation increases the average cost of reliable electricity. Interruptible electricity earns much less revenue per kWh than does reliable electricity.

In a typical electricity system operating without energy storage the total connected generation is about twice the average uncontrolled electricity load. This total connected generation to average uncontrolled electricity load ratio exists because there are normal daily and seasonal variations in the uncontrolled electricity load and because to achieve reliability there is a regulatory requirement for 15% reserve generation at peak load times. The purpose of the reserve generation is to prevent a small unplanned generation and/or transmission equipment failure triggering a cascade grid failure.

In the past the motivation for energy storage was to reduce the average cost of generation by reducing the peak demand on the electricity grid. More recently, as the generation mix has involved more intermitent generation the motivation for energy storage is earning additional revenue from unused generation capacity. Thus demand response programs that focus on peak electricity demand reduction instead of on increasing electricity demand during off-peak times do not adequately meet the electricity system financial requirements.

Every energy storage system has a characteristic energy storage capacity and characteristic energy charge/discharge rates at which it is most cost effective. The viable energy storage system types for multi-day or longer energy storage are gravitational hydraulic and electro-chemical. The viable energy storage system types for daily energy storage are gravitational hydraulic, thermal, electrochemical and compressed air. The viable energy storage system types for short term energy storage for power system stabilization and power factor correction are flywheel kinetic, electrostatic and electromagnetic.

Note that electro-chemical and thermal energy storage systems are most efficient when their outputs are respectively chemical and heat rather than electricity. Usually these energy storage system types are not used to inject electricity back into the grid. Instead these energy storage system types are used to shift energy consumption from times of high uncontrolled electricity demand to times of low uncontrolled electricity demand.

The efficiency of an energy storage system is a function of the number of times the stored energy changes in form. For example, consider a battery bank that is used for energy storage and energy recovery. During battery charging:
1) High voltage AC is converted to low voltage DC
2) Low voltage DC is converted to chemical energy.

During battery discharge:
3) Chemical energy is converted to low voltage DC
4) Low voltage DC is converted to high voltage AC.

Assume that each of the above four energy form conversions is 90% efficient. Then the efficiency of recovery of stored energy is given by:
0.9 X 0.9 X 0.9 X 0.9 = 0.656

A similar efficiency story applies to pumped hydraulic energy storage. During system charging:
1) Electrical energy is converted into pump shaft mechanical energy;
2) Pump shaft mechanical energy is converted into gravitational potential energy;

During system discharge:
3) Gravitational potential energy is converted into generator shaft mechanical energy:
4) Generator shaft energy is converted into electrical energy.

Assume that each of the above four energy form conversions is 90% efficient. Then the efficiency of recovery of stored energy is given by:
0.9 X 0.9 X 0.9 X 0.9 = 0.656

Clearly the efficiency of an energy storage system can be substantially improved if the stored energy can be directly utilized without a form change.

Another way of improving the efficiency of energy storage is to capture the waste heat and use that heat to displace purchased heat. This methodology is particularly applicable to battery storage and thermal energy storage systems.

In any electricity system, in order to regulate the system voltage, it is necessary to continuously match total instantaneous power supplied to the grid to the total instantaneous power drawn from the grid. Available non-fossil fuel renewable energy is usually most efficiently used if the electricity system contains energy storage subsystems that can absorb renewable energy when it is surplus and release the stored energy at a later time at a controllable rate. A common example of such energy storage is a hydroelectric dam holding back a reservoir that accumulates rain water and then releases that stored water through a turbine generating electricity on demand.

In terms of minimizing the requirement for transmission/distribution the optimum locations for energy storage are behind uncontrolled generation and uncontrolled load customer meters. By controlling energy flows in and out of storage the net generator output power variations and the net load input power variations are minimized, the voltage regulation is improved, the transmission requirements and transmission losses are minimized, the generation requirements are minimized and the difference between total uncontrolled generation and total uncontrolled load that must be met with dispatched generation and dispatched load is minimized. Hence energy storage adds value to the electricity system which value is not recognized by the present Ontario electricity rate structure.

Wind, solar and tidal energy generators at remote locations, if motivated by suitable compensation rates, will use behind the meter energy storage to reduce the amplitude of variations in their net power outputs. Reducing the amplitude of these net output power variations reduces transmission/distribution capacity requirements, reduces voltage regulation problems and reduces the need for both balancing and reserve generation to meet temporary shortfalls in total primary generation.

Energy storage behind load customer meters allows the load customers to have short term major daily power variations without imposing these power variations on the electricity grid and hence saves the load customer money by reducing electricity distributor imposed peak kW and peak kVA charges. Another potential benefit of behind the load customer meter energy storage is protection of the load customer from short term failures in the external electricity system.

Transmission connected dispatched energy storage can be used to match variations in total generator output power to variations in total load customer electricity demand. Due to statistical averaging of the outputs of many geographically distributed renewable generators the net generator output power variation is less than the sum of the individual generator quasi-random output power variations. Similarly the net load variation is less than the sum of the individual quasi-random load variations. These statistical averaging mechanisms are known as generation diversity and load 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, use of central energy storage in place of behind the meter energy storage substantially increases the required amount and cost of transmission/distribution.

Water cooled nuclear reactors are usually operated at a nearly constant thermal power to minimize reactor thermal stress and to avoid problems related to reactor poison fission products. Fast neutron reactors have the advantage that they can be modulated to follow the net load on the electricity grid. However, nuclear generation is most economical when operated constantly at nearly full rated power.

In the past use of fossil fuels in thermal generation stations inherently provided the electricity system with energy storage for load following. As fossil fueled generation is taken out of service to minimize CO2 induced climate change, more non-fossil energy storage will be required to efficiently match the total load to the total available generation. In the absence of sufficient energy storage, a combination of constrained generation and constrained load is required to match generation to load. Use of generation constraint increases the average cost of generation. Use of load constraint decreases electricity revenue and decreases return on investment for the participating industrial customers.

An important technical issue is that energy storage systems should be designed to ride through transient grid faults and should not rely on other generators for black start or for providing voltage regulation.

All non-fossil fuel generation types need to efficiently shift energy from a time of surplus to a time of deficiency when energy has a greater monetary value. Thus energy storage systems require an electricity rate that is primarily peak demand dependent so that the cost of marginal energy is relatively small.

Run-of-river generators provide electricity generation that tends to be much higher in the spring than in the late summer. Run-of-river generators can easily be constrained for load following. However, the constrained portion of the generator's potential energy output is wasted.

Wind generators need nearby energy storage to shape their daily net outputs. Failure to purchase energy storage along with wind generation has the practical effect of reducing the market value of wind generated electricity. 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. Successful seasonal shifting of the provincial load profile requires:
1. Shift in space heating from fossil fuels to electricity.
2. Large scale winter production of nitrogen fertilizers;
3. Large scale winter production of synthetic liquid fuels;
4. Pumped hydraulic energy storage between Lake Erie and Lake Ontario;

Energy storage at wind generator sites serves several 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 at load sites serves several other additional important functions:
1. Filtering out random short term variations in load power input;
2. Providing emergency backup power;
3. Reducing the transmission/distribution (I^2 R) thermal losses that occur at times of peak load.

The failure by the Province of Ontario to require that wind generators have matching seasonal energy storage has devalued wind generation in 2016 to about $0.016 / kWh whereas the cost of the wind generation is about $0.117 / kWh. Hence wind generation makes no economic sense in Ontario.

In theory 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 local energy supply surplus and times when there is a local energy supply deficiency. In response to a local energy supply surplus the energy storage units should absorb electrical energy. In response to a local 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 (absorbs energy). Below nominal voltage the unit discharges (liberates energy). The nominal voltage setting can be adjusted to meet system control requirements and energy storage constraints.

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 local voltage and optimize use of local distribution and local energy storage.

This power control methodology is extremely robust and is highly resistant to computer hacking because the distributed generator and distributed load power setpoints are independently controlled by the equipment owners, not the public utility.

However, there will be not financial motivation for smart grid implementation until the methodology for electricity valuation is completely changed to recognize the values of high generation capacity factor and high load factor.

There is also a protective switching problem. In many major buildings the main isolation switch was sized to clear a short circuit fault in which the current is limited by the grid source impedance at the time the building was built. Addition of distributed generation lowers the local grid source impedance which compromises the short circuit fault clearance capability. This issue has tremendous cost and liability implications which have made the "smart grid" concept economically unviable. For safety each building must be controlled so that it does not net export power.

The metering system and electricity rates should adequately reward uncontrolled generators and uncontrolled load customers for providing energy storage that reduces local power fluctuations and that enhances overall grid voltage stability.

At this time electricity rates in Ontario do not financially enable energy storage. The present generator compensation rates do not sufficiently encourage storage of non-fossil energy when it is surplus for later use when non-fossil 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 OPA Feed-In Tariffs do not adequately reflect the value to transmission/distribution of high net generator capacity factor.

Most of the aforementioned rate failures are caused at least in part by the present use of fossil fuel electricity generation for load following. As long as generators bear no cost related to their CO2 emissions and natural gas remains relatively cheap, natural gas will contine to be used for fueling load following electricity generation. However, sooner or later an increase in the cost of natural gas, possibly caused in part by a fossil carbon tax, will 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.

A load customer's electricity bill is primarily made up of peak demand and energy components. Hence the cost of on-peak electricity includes both peak demand and energy charges whereas use of off-peak electricity causes only energy charges.

Any energy storage system relies on the Rate Ratio:
Rate Ratio = (Cost of marginal on-peak electricity / kWh) / (Cost of marginal off-peak electricity / kWh)
being sufficient to fund the storage system energy losses as well as the storage system's capital amortization, operating and maintenance costs. To reasonably financially enable a practical energy storage system this Rate Ratio should be in the range:
(Rate Ratio) > 3.0
and the energy storage system efficiency as defined by:
Efficiency = (Useful Energy Output) / (Energy Input)
must be greater than 50%.
However, the importance of the Rate Ratio is not presently recognized by the Ontario Energy Board and is not reflected in 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, global adjustments, carbon taxes or generation incentives. Ideally, behind the meter energy storage systems should be used in conjunction with a daily peak demand dependent electricity rate. The energy charges should also be time dependent. The global adjustment should vary with time in direct proportion to the energy charge.

Some understanding of the funding issues relating to behind the load customer meter energy storage systems can be gained by considering the folllowing two simple time dependent electricity rate examples:
Unfavorable Case (Rate Ratio = 2.5, Efficiency = 50%):
Assume the on-peak electricity rate is $.24 / kWh, the off-peak electricity rate is:
($.24 / kWh) / 2.5 = $.096 / 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:
$.24 / kWh - (($.096 / kWh) / .5) = $.048 / output kWh
Favorable Case (Rate Ratio = 3.5, Efficiency = 75%):
Assume the on-peak electricity rate is $.24 / kWh, the off-peak electricity rate is:
($.24 / kWh) / 3.5 = $.06857 / 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:
$.24 / kWh - (($.06857 / kWh) / .75) = $.14857 / output kWh

Note that seemingly minor changes in energy storage system efficiency and off-peak electricity rate tripled the amount of money available for funding the energy storage system capital, operating and maintenance costs. Thus,the economic viability of energy storage is very sensitive to both the energy storage system efficiency and the ratio of the marginal on-peak to marginal off-peak electricity rates. Note that in these examples the marginal on-peak enerrgy rate is a composite of (the peak demand charge divided by the number of on-peak hours) plus the on-peak energy charge.

The net result of the present electricity rate failure is that there is much less customer owned energy storage connected to the Ontario electricity system than in the past. During the 1960s Ontario Hydro offered an electricity rate of:
($0.01 / KWh + $6.00 / KW-month)
with a 10 year guarantee that encouraged many developers of large buildings to incorporate energy storage systems into their new buildings. By 1983 there were over 20,000,000 square feet of commercial space in Metro Toronto with behind the meter customer owned energy storage for electrical load control. Toronto Hydro had a further 14,000 single family residential customers with electric hot water heaters under central load control. These energy storage systems were almost all taken out of service during the period 1983 to 1997 when changes in the electricity rates induced by availability of load following fossil fuel generation diminished the financial benefits to end users of 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. Both Ontario Hydro and the municipal electricity distributors were forced to raise their rates simply due to loss of energy market share.

In Ontario, in order to financially enable energy storage, the Ontario Energy Board must increase the Rate Ratio by implementing a new electricity rate primarily based on peak demand.

The delivered electricity cost per kWh for all standard uncontrolled load customers should decrease with increasing daily load factor. Similarly the uncontrolled generator compensation rate should decrease with decreasing generator capacity factor. This rate proposal financially enables daily behind-the-meter energy storage while avoiding instability problems inherent in simple block Time-Of-Use (TOU) electricity rates. Under this proposal during each day a customer cannot obtain electricity at below average cost of production unless that customer first purchases some electricity at above the average cost of production.

It is of fundamental importance that costs associated with load following be embedded in the on-peak rather than the off-peak portion of end users electricity bills. Examples of these charges are global adjustment constraint payments such as amounts paid to owners of natural gas fired reserve generatorion when this generation is not actually producing electricity.

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 such as synthetic liquid fuel plants and fertizizer plants that can be dispatched off during the summer. Neither of these means of meeting the peak summer load is inexpensive. When the total costs of meeting the peak summer load with renewable generation are taken into account, nuclear generation is a bargain.

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 the 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 hydro-electric facilities used to balance wind generation 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 low cost land and favourable river valley geography and topography. It is usually economically unfeasible to build such reservoirs in urban areas, so there are usually substantial related transmission costs.

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 and as far south as San Francisco.

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 water bodies with different elevations 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.

Centrally located pumped hydraulic energy storage may be used for absorbing energy surpluses and for meeting energy deficits.

Pumped hydraulic storage between two very large lakes such as Lake Erie and Lake Ontario can potentially store sufficient energy for seasonal balancing of wind generation in all of Ontario and New York State.

Pumped hydraulic energy storage systems are characterized by frequent rapid changes in the water flow rate and between the high level reservoir and the low level reservoir. These rapid flow rate changes are inherently dangerous. To ensure safety the public must be completely excluded from the river or canal system interconnecting the two reservoirs. At the present the public is largely excluded from the Niagara River, so this issue is not a significant problem in the contemplated Lake Erie-Lake Ontario energy storage system.

A disadvantage of pumped hydraulic storage is that, due to its rural location, there is often an associated major transmission cost. There are also changes in water level that have minor ecological and marine consequences. The daily water level change is usually comparable 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 in Virginia.

Electrochemical processes tend to lend themselves to load shifting. 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 electrolysis of water. Electrolysis of water requires large amounts of electrical energy. The produced gaseous hydrogen is difficult to store, so an energy storage system based on electrolysis of water to produce hydrogen generally has to include either a methanol (CH3OH) or an ammonia (NH3) production facility to efficiently absorb the hydrogen as it is produced.

The details of electrolytic hydrogen production are set out at:

Other industrial chemical production processes that use large amounts of electicity are production of sodium, lithium, chlorine, fluorine and aluminum. Large variations in available electric generation and customer load can be balanced by varying the appropriate chemical production rates under dispatch control.

The hydrogen produced by electrolysis of water can be combined with biomass to form methanol (CH3OH). Methanol, also known as methyl hydrate, is a room temperature liquid that is easy to store and transport and that can conveniently provide energy when and where required. Methanol has long been used as a relatively safe fuel for both racing cars and model airplanes. In a refinery methanol can be dehydrated to form gasoline. At times of exceptionally high electricity demand methanol can also be directly used as a fuel for electricity generation.The details of methanol production are set out at:

The hydrogen produced by electrolysis of water can also be combined with nitrogen to form ammonia (NH3), which as a liquid can readily be stored under pressure. Ammonia is an essential ingredient for making a wide range of essential chemical products including nitrogen fertilizers. Nitrogen fertilizers are generally safe for normal handling although in the wrong hands ammonium nitrate fertilizer can be used for making explosives. Liquid ammonia (NH3) can be shipped by rail tank car, but the consequences of a derailment in an urban area are very serious. One breath of ammonia gas will permanently destroy human lung function. For safety whenever possible ammonia should be shipped embedded in a solid chemical compound such as ammonium nitrate (NH4NO3) fertilizer or ammonium phosphate ((NH4)3PO4) fertilizer. This author points out that in November 2011 the lungs of former Canadian and world ladies figure skating champion Karen Magnussen were seriously damaged by ammonia gas that leaked from the cooling system of an ice rink in North Vancouver, British Columbia.

Chlorine gas is produced by electrolysis of the common salt sodium chloride (NaCl). Chlorine is a precursor to production of a wide range of chemicals. Under pressure chlorine gas can be liquified, stored and in principle can be shipped by rail tank car. However, the consequences of a chlorine tank car derailment in an urban area are extremely serious. One breath of chlorine gas will permanently destroy human lung function. A 1979 derailment of a rail tank car containing 90 tons of liquid chlorine caused 250,000 people to be evacuated from the town of Mississauga, Ontario. For safety whenever possible chlorine should be stored and/or shipped embedded in a chemical compound such as sodium hypochlorite (NaClO) liquid bleach or calcium hypochlorite (Ca(ClO)2) powder bleach.

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 stored chemical energy is converted to DC and then is converted again back into three phase AC using a static inverter.

Electro-chemical energy storage based on nickel-metal-hydride batteries has been extensively developed for improving the fuel use eefficiency of automobiles. The typical cost of this form of energy storage is about $1000 / kWh. The mass of this storage is typically about 40 Wh / kg.

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.

Electro-chemical energy storage has very quick response allowing it to be used for balancing rapid changes in wind generation.

An advantage of electro-chemical energy storage is that it is transportable and can be located at both distributed generation sites and at urban load sites to minimize transmission/distribution costs. A disadvantage of electrochemical energy storage is that it is too expensive for weekly or seasonal energy storage. Under the present Ontario end user electricity rate structure it is almost impossible to financially justify use of electro-chemical energy storage for reducing peak electricity demand.

Lithium ion batteries, as developed for automotive applications, are believed to be too expensive for utility scale energy storage.

The chemical systems that seem to be most suitable for stationary electricity energy storage are sodium-sulfur-nickel chloride (Na-S-NiCl2), sodium-aluminum chloride 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 and vanadium energy storage systems are suitable for daily capacity factor or load factor improvement.

The materials used in construction of sodium-sulfur-nickel chloride and vanadium energy storage systems are common, relatively inexpensive and readily available in Canada. Toronto puts hundreds of tons of sodium chloride on its streets after every snowfall. Canada exports mountains of sulfur obtained from processing metal-sulfide ores and from refining of petroleum. Canada has 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 Rolls Royce in Europe. 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 factor improvement in applications as small as single family homes and small commercial establishments. Disadvantages of sodium-sulfur-nickel chloride technology are high temperature operation (~270 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.

Electro-chemical energy storage systems usually involve sliding tap transformers and 12 pulse inverters to match the three phase line voltage to the charge dependant electro-chemical DC bus voltage while controlling the direction, rate and phase angle of energy exchange. A dedicated controller monitors the amount of chemical energy contained in each cell string and compensates for single cell shorts. A control loop with maximum and minimum power limits sets the direction and rate of energy exchange in proportion to the difference between the RMS line voltage and the software controlled RMS line voltage setpoint. Phase control of the static inverter permits reactive power compensation.

In thermal energy storage an electrically powered heating or cooling system is run during electricity off-peak periods to generate and store large thermally stratified tanks full of molten salt, super heated water, hot water or ice pellets. If possible the heat of fusion related to the phase transition of the storage medium from the solid state to the liquid state or the latent heat of vaporization from a liquid state to a gaseous state is used to maximize the amount of heat stored per unit mass of storage medium. A ground source heat pump based heating and cooling system uses the dry ground below the frost line for thermal energy storage.

Typically thermal storage consists of multiple thermally stratified storage tanks connected series-parallel so some tanks can be taken out of service for maintenance without compromising the entire storage system.

The stored thermal energy is used for space or water heating or cooling during subsequent electricity on-peak periods. Thus the heating or cooling electricity load is effectively shifted from the electricity on-peak period to the electricity off-peak period. Thermal energy storage has the advantage of relatively high overall efficiency and relatively low cost as compared to other forms of daily energy storage.

If the [(on-peak) / (off-peak)] marginal electricity price ratio is sufficiently high and if the cost of cooling water is sufficiently low it may be practical to mate a molten salt thermal energy storage system with an organic turbine to generate electricity during the on-peak period from thermal energy stored using off-peak or interruptible electricity.

Thermal energy storage systems are large and heavy and hence are usually either built into the basement of a building during its original construction or are retrofit built on adjacent property. However, between the retrofit energy storage system location and the existing building there is often a public road that requires a difficult to obtain easement to cross the road allowance underground with fluid pipes and electrical cables. Municipal public utilities frequently oppose such easements as infringements on their monopoly rights. Such easements are often only granted to municipal district heating/cooling systems.

Mortgage funders also often oppose any arrangement whereby the essential services for one property are dependent upon privately owned equipment on another property that has differnt ownership or different mortgage arrangements. Such opposition makes many potential retrofit thermal energy storage opportunities financially non-viable.

In spite of the aforementioned impediments, behind the meter thermal energy storage is usually the most practical method of storing energy for flattening the electricity load profile of major buildings. As compared to a battery based energy storage system a large thermal energy storage system features a thermal energy storage efficiency that is much higher, a working life that is much longer, fire protection requirements that are much less onerous and a cost per unit of recoverable energy that is much lower.

Thermal energy storage systems were incorporated into many buildings in Ontario during the period 1965 to 1981, while the electricity rate for such major buildings was [($6.00 / peak kW - month) + ($0.01 / kWh)]. Almost all of these thermal energy storage systems were taken out of service in subsequent years due to rising off-peak electricity rates that exceeded the cost of heat from natural gas. The resulting loss of market share contributed to the insolvency of Ontario Hydro during the 1990s.

The misleading concept of equating the off-peak electricity rate to the marginal cost of fossil fuel required for off-peak electricity generation was a political and economic disaster from which the Ontario electricity system has yet to recover. The issue is that the marginal cost of non-fossil electricity generation is zero. Uncompetitive pricing of off-peak electricity led to system wide loss of thermal energy storage, loss of market share, lower grid load factor, higher per capita fossil fuel consumption and higher total energy costs for all Ontario consumers. This situation occurred because the Province of Ontario ran coal fired generation in the off-peak period instead of building a new power transmission line from the Bruce Nuclear Power complex to Milton in the Greater Toronto Area. The problem was fixed many years later by construction of the Bruce to Milton transmission line which allowed coal fired generation to be taken out of service.

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 such kinetic energy storage in their transmission/distribution rates. A representative supplier of flywheel energy storage systems is Beacon Power.

In electrostatic energy storage the surplus energy is stored as an electic field in a device known as a ultra capacitor. Ultra capacitors can be realized using carefully controlled alternating thin layers of metal and barium titanate. Ultra capacitor technology is relatively new. Ultra capacitor use in utility applications is presently aimed primarily at the wind turbine market. This author has safety concerns relating to utility size ultra capacitors. The energy stored per unit volume in these devices makes them potentially explosive.

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.

This web page last updated March 3, 2017.

Home Energy Nuclear Electricity Climate Change Lighting Control Contacts Links