This post looks at fifteen kinds of utility or grid scale energy storage solutions that are either in wide use or have significant potential to supply the energy storage capacity that will help make the grid both more efficient and more robust. These range from pumped hydro, which is by far the most prevalent form of energy storage at this scale to compressed air, thermal storage, advanced batteries, fuel cells and purely electric storage systems.
Electricity is a use it or lose it product with no shelf life to speak of. Oil, gas, coal, wood, water in a reservoir, uranium etc. all can be held back and used as needed without losing much if any of their original potential. Try that trick with an electric current. Capacitors, of course, as their name implies, store electric charge (hence capacity), but what we generally are thinking of when we think of electricity is electric current itself.
Because of this, grid operators must continually dispatch generating capacity to keep the network’s supply and demand in balance at all times and to maintain the grid voltage and (AC) line frequency within very tight operational tolerances. It really is a colossal balancing act, and one that has extremely tight time constraints placed on it. Maintaining a very close balance between supply and demand is important not just because electric current needs to be consumed – for practical purposes pretty much right away, but also in order to maintain network power quality and reliability, both in terms of voltage and AC frequency.
The Balancing Act between Supply and Demand
There are several ways to deal with this supply & demand balancing act. These will act together in a virtuous manner so that the sum will be greater than the parts. Some of the ways that are being pursued to help balance the instantaneous (or very close to it) supply and demand include:
• Better interconnection and long distance transmission capacity in order to be able to better shuttle current around from areas of temporary surplus to regions of deficit. This approach would include UHV transmission and grid interconnects.
• Demand management, enabled by the parallel smart information and communications network now being built out i.e. the smart grid; by the evolution of a near real-time spot market, and by smart appliances that can ramp up or ramp down their energy use in near real time. In moments of transient under-supply signals are sent out to connected energy appliances that can damp down their demand in near real time thus restoring balance, or be signaled to ramp up consumption to soak up over-supply.
• Improved weather prediction resources, both in terms of accuracy; the forward extent of the prediction; and the level of geographical granularity that predictions are given for. Improved weather will make energy planning much smoother than it is without these tools; enabling planned dispatching of power and lowering the costs of doing so.
• Electric energy storage capacity that can soak up over-supply and return current to the grid when supply is short. Electricity can be stored in many very different ways and this is the focus of this post.
To read more about how better long distance electric transmission can help usher in an era of renewable energy generation see our related post: “Ultra High Voltage (UHV) Transmission is the Renewable Energy Interstate“.
Energy Storage the Electricity Sponge
Electric energy can be converted into chemical, potential, kinetic, or electromagnetic energy, in addition to other more exotic storage states. Each of these very different energy stores holds promise and is uniquely suited for important and specific tasks. For example supercapacitors may have a relatively low energy density, but because their power density is so high – they can deliver a lot of power in a very short period of time — they have an important role in transient smoothing of short duration variability and maintaining power quality on the grid.
Storage also is often limited in time. Some kinds of energy storage lose capacity over time; this varies widely from type to type. This may be acceptable however if storage times are short; other factors may carry more weight, such as power density for example.
Energy storage systems have three basic markets, which are: stationary power, transportation power, and portable power… that is: the big utility energy box (home or grid scale); the moving vehicle (car, locomotive, bus, motor bike etc.) and the portable device (laptop, cell phone etc.). Each of these markets has its own particular needs. This post is focusing on the stationary power segment.
Stationary power units have different design and performance considerations than either the portable or the vehicle energy storage market sectors. For starters the energy density of storage system is not as important; it can be quite low if it is also very cheap and environmentally benign. There is also most often a considerable scale difference, which can be immensely huge when one looks at energy storage systems like Gigawatt or more capacity pumped hydro.
Traditional Pumped Hydro the Big Daddy of Them All
Currently traditional pumped hydro accounts for almost all (99%) of the large utility scale energy storage capacity in the world. Japan, with a suitable topography close to large population centers leads with pumped storage facilities accounting for around 10% of its generating capacity. In North America and Europe the figure is around 2.5% and 5% respectively. In 2009, for example, the United States had 21.5 GW of pumped storage generating capacity spread over around 40 pumped hydro facilities, all built at least two decades ago. The EU block (in 2007), by comparison, had 38.3 GW net capacity of pumped storage.
Traditional pumped hydro is limited in how much it can expand, because it requires a geographically suitable site that can link a high reservoir with a low reservoir and move water between the two. It also has a large land footprint – e.g. the two required reservoirs. Pumped hydro projects must go through a long vetting process and often run into concerted opposition from people who they will impact. Projects can be delayed for years and this adds to their cost structure. It is also hampered because despite being cheaper on a per-megawatt basis than other forms of energy storage, pumped hydro projects are usually quite large-scale projects. Typically thee facilities are built to provide around 1,000 MW of storage at a cost of $1 billion to $2 billion, and raising this kind of up front capital is hard to do.
Basically pumped hydro works like this: Off peak electricity is used to power reversible generator/pumps moving water from the low reservoir to the high one and storing its potential energy, which is then run back through generating current during periods of peak demand. Reversible turbine/generator assemblies act as both pump and turbine and the overall efficiencies are pretty good. Approximately 70% to 85% of the electrical energy used to pump the water into the elevated reservoir is recovered.
Because pumped storage plants, can respond to grid load changes within seconds they can help control electrical network frequency (of the alternating current e.g. 50 Hz in Europe and 60 Hz in North America & South America). Power conditioning maintaining both frequency and voltage levels within tight tolerance levels is very important for grid operation
An interesting proposed 1.3 GW pumped hydro project, the Eagle Mountain Pumped Storage Project about 65 miles west of Palm Springs will use a closed loop system of water pumped between two existing depleted mine pits located about 14,000 feet apart, with an elevation difference between the pits of approximately 1,500 feet. It will use groundwater for the initial fillup and to replace water lost by evaporation. This big new energy storage facility will pair very well with the wind and solar resources that are abundant there.
One thing that impressed me favorably about this pumped hydro project is that both the high and the low reservoirs are brownfield open pit mine sites; no pristine valley to get flooded just two pits… literally. Other than the impact of the removal of the groundwater for the initial filling of the facility and for on-going replacement of water lost to evaporation and seepage this plant is transforming an existing brownfield site of little environmental value anymore into a valuable piece of California’s energy infrastructure.
A Cutting Edge Variation of Pumped Hydro
Gravity Power, LLC, a privately-held company, based in Southern California (in Goleta, CA just north of Santa Barbara) is developing a novel grid-scale energy storage system for global commercialization called the Gravity Power Module (GPM). Like pumped hydro the working energy carrier is water that is pumped between a high pressure and a low pressure reservoir running a reversible generator/pump assembly to either produce power by drawing down the high pressure reservoir or store it up by pumping water from the low pressure store back into the high pressure store. In this sense it operates on the very same principles – and thus can also benefit from existing capital equipment, such as the reversible hydro generator/pump assemblies for example – as traditional pumped hydro.
Gravity Powers technology circumvents traditional pumped hydro difficulties related to siting, negative environmental impact, huge land demands, permitting, long-lead times and the very large investment required, by burying it all underground…. literally.
The GPM system uses a very large and very dense high mass piston that is suspended in a deep, water-filled shaft. The piston is equipped with sliding seals to prevent leakage around the piston/shaft interface and its immense mass pressurizes the supporting water column beneath it. A high pressure pipe from the bottom of this shaft enables water to be run or pumped through a generator/pump assembly of the same types now used in pumped hydro systems. The low pressure low energy potential water is returned above the piston adding somewhat to its weight and further pressuring the remaining high energy potential water column.
The massive piston moves up and down the shaft, storing and releasing power in a closed sealed cycle. It is compact with a small land footprint and the units can be clustered together into larger groups. It also is environmentally benign, no toxic chemicals or explosive dangers.
I like the scalable nature of this store that makes it suited to incremental growth of capacity. I also like how this energy storage system could be placed very near the big urban areas of greatest need for this kind of electric capacity. The fact that this energy storage system can take advantage of a lot of already existing infrastructure and technical knowhow from the existing pumped hydro sector is a definite advantage.
I would like to see more details on the costs of the boring of the immense vertical shafts; the long term performance metrics of the shaft seals (that would be an expensive repair job I would think. All in all I think this or something like it is a strong contender in the energy storage sector.
Compressed Air Electricity Storage (CAES)
Compressed air storage, while still far less than pumped hydro has a bigger existing installed capacity than the other energy storage ideas looked at. Most of this capacity is accounted for by two existing sites; one in Germany the other in Alabama. Installed capacity is at 200MW.
Compressed air storage compresses air and stores this potential in suitable underground formations, such as salt domes, depleted gas fields and so forth. The stored energy can then be recovered as required by running the high pressure compressed air through a turbine. It is not quite as simple as that because of how heat and pressure are intertwined. Compress a gas it heats up; expand it and it cools down. The two big existing systems are both diabatic. As they compress the air they shed heat through intercoolers so that it does not get too hot. The air must be re-heated prior to being run through the generator. The overall efficiency is a little over 50%.
This is less than the 70% or so efficiencies it is believed adiabatic CAES systems could reach. These would retain the heat produced by compression and return it to the air when the air is expanded to generate power, utilizing some thermal store such as molten salt, or simple mass such as gravel. However diabatic compressed air storage is easier to do.
There is a spate of new projects in development including: a 270 MW project in Iowa, a 300 MW project in California, a 150MW project in New York, and a new 200MW project in Germany. Together these projects will total almost one Gigawatt of compressed air storage capacity.
Compressed air has the advantage of using mature technology and in not relying on any new exotic technologies or materials. The gas turbines, air compressors, recuperators, injection and extraction wells, and other components used in CAES are all mature technologies that enjoy economies of scale and long design evolution built into them. The knowhow and the people with it are either ready right now or can be ramped quickly. All of this kind of stuff is not the sexiest most glittering story one can tell, but it is all stuff that really matters.
Many of the more intellectually more exciting and eye catching energy storage technologies depend on advanced materials at the edge of human technology and often these beautiful dreams never scale beyond the lab. Compressed air storage may seem a little, shall we say Victorian, but it works and in the end that’s what really matters.
Pumped Heat Electricity Storage (PHES)
Isentropic , a UK based company has developed an elegant idea to store electricity in thermal form. The system comprises of two massive insulated sinks one hot the other cold. These would basically be big thermally well insulated barrels of gravel. The two sinks would be connected through a Isentropic heat pump. As electric energy is stored in the system the heat pump would compress a gas to very high pressures and hence temperatures this high pressure gas would dump its heat into the hot sink slowly raising its temperature and then be suddenly decompressed in the cold pump transferring its “cold” potential into the cold sink. To generate power the system is run in reverse with the thermal gradient potential between the two sinks driving the Isentropic heat pump.
They claim a round trip efficiency of over 72% – 80%. Because gravel is such a cheap and readily available material, the cost per kWh can be kept very low – $55/kWh – and $10/kWh at scale And Isentropic claims that their system offers the following benefits: A low cost compared to some other ideas; a high efficiency; it is not constrained by geology (like pumped hydro is); it is environmentally benign; it is modular and scalable; and it can respond rapidly to load variations.
I like its elegance, use of non exotic materials (unless the Isentropic heat pump itself requires them); its scalability and low environmental impact and risk. It certainly seems like it could be a real contender.
I would like to learn more on the performance metrics over time for the heat pump/generator itself. Are their maintenance issues? What about wear of parts? I would also like to know how far it can be scaled down; is it strictly a utility grid scale solution or can it also be scaled down to service smaller systems?
Molten Salt Thermal Storage
Molten salt batteries (using saltpeter) are a class of thermal storage that seems very well adapted for utility scale concentrated solar power (CSP) plants; being used both as the thermal energy transfer fluid that is heated by the sun and then boils water for example and as a store of collected solar energy directly as thermal potential. No energy transformation step required until electricity is needed. The Sandia National Laboratory National Solar Thermal Test Facility uses molten salt in solar power tower systems because it is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with today’s high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic.
When used with CSP solar towers for example it decouples the collection of the energy from the use. Because of this the solar power facility can become more of a load following generating source, which makes the electricity it produces more valuable. It will significantly raise the capacity value of the resulting electric power over what a similar CSP facility without the thermal storage capacity would have.
Santa Monica-based SolarReserve has secured $140 million in venture capital towards its goal of building a California approved and permitted 110MW capacity CSP with molten salt storage. The plant will be able to operate from afternoon through about eight hours after sunset; closely following peak demand.
Our related post: “Does Concentrated Solar Power Have the Answer to Intermittency Concerns?“, delves further into the subject of molten salt thermal storage and how it is being paired with concentrated solar power.
Ice Storage Systems
Ice storage systems are an interesting class of energy storage that is being used in some HVAC systems to time shift demand for cooling so that it utilizes off peak power to essentially freeze water ice (or a coolant) and then use the thermal potential of the cooled mass to absorb heat during the day without taxing the grid during peak demand periods. If more of these systems are put in place in areas that require cooling they could time shift considerable load from peak periods of demand to much easier to supply off peak periods. They would also work well with variable power sources such as the wind, being able to soak up power when it was abundant for later use when it was needed. One market segment that I personally think ice thermal batteries make good sense for is in data center cooling; decoupling the data centers cooling power from its immediate electric demand profile.
The Grid Scale Batteries
There are several types of battery storage being talked about for grid scale operations. Several more talked about types will be looked at in turn, because they have unique properties and pose different challenges. Batteries are ubiquitous and one of the oldest electric energy storage mediums in existence. Batteries also often present significant environmental risks, employing toxic metals and chemicals. In the world of batteries, it’s usually messy wet chemistry at work. Batteries also are expensive and often have live cycle time below 1000 charge/discharge cycles.
To read on how electric vehicles may one day begin to act like a gigantic virtual battery, helping to smooth out demand and soak up excess electricity, see our related post: “Rising Hopes that Electric Cars Can Play a Key Role on the Grid“.
Sodium Sulfur Batteries
Sodium sulfur battery is a type of liquid metal battery that has a high energy density, charge/discharge efficiency and long cycle life. It has a high operating temperature in the range of 300 to 350 degrees Celsius and employs corrosive materials. However these constraints can be engineered around and its other attributes make it desirable for big stationary power applications such as grid scale energy storage. Attributes that include: a high energy density, high efficiency (up to 90%) and high power density. These batteries are suited for combined power quality and peak demand shaving duties.
Currently this is the form of grid scale battery that has achieved the largest installed capacity with a few hundred megawatts of installed sodium sulfur (NaS) battery energy storage capacity, mostly in Japan. TEPCO – a company whose name is very much in the news these days because it is the utility that owned and operated the Fukushima Daiichi nuclear complex – has pioneered usage of this battery type. In the US, the town of Presidio in Texas has a 32MW capacity (1MW discharge rate) sodium sulfur battery.
These battery types are beginning to pop up paired with wind farms, solar power facilities, and in sub-stations. Newer lower temperature variants that promise reduced costs are also on the horizon. There seems to be enough momentum, especially in Japan, behind this battery type to ensure that it will continue to be improved and will continue to see its cost structures driven down.
Lithium Ion Batteries
Lithium ion batteries, of the kind that power everything from laptops to cell phones, and an ever increasing array of portable devices are also increasingly powering electric and plugin hybrid cars as well. In the US this battery type is also being used in grid scale stationary power applications.
AES Energy Storage, has deployed lithium ion grid scale batteries to five locations in the US, with more in the pipeline. These include an already installed 8MW capacity unit in Johnson City, New York, with a further 12MW due online this year. These batteries will improve the grid responsiveness as well as helping to support the wind development expected in that region. In the Laurel Mountain energy storage project the batteries are being used not only to store energy, but to maintain power quality helping to manage the rapid fluctuations in power output that can characterize wind energy. This is a good example of how batteries can pair with renewable sources to help smooth their output.
Flow Batteries, like fuel cells with which they share many common attributes, allow storage of the active materials external to the battery. These reactants are circulated through the cell stack (reactor) as required. This allows them to scale with reactants stored in external tanks. Flow batteries differ from fuel cells because the chemical reaction involved is often reversible, so they can be recharged without a new supply of new active material.
High power batteries modules are constructed using a multiple stack of cells. Thus power is scaled independent of capacity, which depends on the availability of the externally stored reactants.
Various types of flow batteries exist including: redox (reduction-oxidation) flow battery and the hybrid flow battery. Most interest is focused on the redox flow battery type, which can scale because all of its electroactive components are dissolved in the electrolyte… meaning potentially large amounts can be stored in external tanks. The power of the battery depends on the size or scale of the reactor itself. Hybrid flow batteries cannot scale in this manner, because a part of the active material is incorporated as a solid in the battery cell itself.
Flow batteries require pumps to circulate the “flow” of the active materials through the reactor and are thus more complex than other battery types in this regard.
Several types of flow batteries, utilizing different chemistry are being explored including: Vanadium redox fow battery, which is a rechargeable type that takes advantage of the various oxidation states of Vanadium in order to store/retrieve energy and Zinc/Bromide types.
Zinc/bromide flow battery variants look quite promising. Premium Power, is offering utility grade flow batteries that have an very low cost of just $250 to $350 per kilowatt of capacity. Their zinc/bromide flow batteries are being offered in containerized configuration as large as 2.8MW capacity with a power rating of 500kw. These long lasting low maintenance and environmentally friendly batteries have the potential to begin achieving significant market share in the energy storage sector.
Like batteries fuel cells convert chemical energy directly to electrical energy. However unlike most batteries fuel cells consume reactant from an external source, generating electricity from the reaction between some fuel supply and an oxidizing agent. For example, a hydrogen fuel cell uses a hydrogen reactant and oxygen from the air producing electricity, with water and heat as its by-product. Because the reactor is separated from the reactant, which can be stored separately fuel cells can scale; like a car they keep running as long as they get filled up. Fuel cells do not burn fuel, making the process quiet, pollution free (for hydrogen types) and two to three times more efficient than combustion.
There are several important classes of fuel cells including the following: proton exchange membrane fuel cells (PEMFC), Solid Oxide Fuel Cell (SOFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), and direct methanol fuel cells (DMFC).
Proton Exchange Membrane Fuel Cells (PEMFC)
When most people think of fuel cells they are thinking of hydrogen fuel cells of the proton exchange membrane fuel cell (PEMFC) type. In a hydrogen (PEMFC) fuel cell hydrogen atoms enter at the anode where an anode catalyst reaction strips them of their electrons. The hydrogen atoms are now “ionized,” and carry a positive electrical charge. The protons are conducted through the proton-conducting polymer membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating.
Hydrogen fuel cell use has been principally limited so far by its need for rare and expensive platinum catalyst. However in spite of this the sector has grown and in 2010 more than 80MW of fuel cell capacity was shipped, a drop in the bucket compared to say pumped hydro of course, but the sector is on a rapid growth path and incremental improvements are driving down its costs towards the levels that it needs to achieve in order to become a viable energy storage solution. In fact, according to the DOE report Overview of Hydrogen & Fuel Cell Activities costs have been reduced by 80% since 2002, with 30% cost reduction since 2008 and the sector is on track to meet its $30/kW 2015 price target (with current volume unit price slightly over $50/kw of capacity).
One interesting technological development that bears watching is the recent discovery that a molybdenum sulfide catalyst can replace the expensive platinum catalyst materials that are now required; however this is still some years away from commercialization if it can be scaled.
The unique high energy density hydrogen storage and distribution issues that face the transportation sector are less important for the utility scale stationary power segment. Utility scale hydrogen fuel cells could produce their own hydrogen with electrolysis, store it on site for use in the fuel cell to generate current.
High Temperature Solid Oxide Fuel Cell (SOFC)
These types of fuel cells operate at very high temperatures of between 700 and 1,000 degrees Celsius, and are suited for the role of large-scale stationary energy storage. The very high operating temperature poses some significant materials and design issues; however it does provide opportunities for co-generation of heat. This class of fuel cells promise also includes: high efficiency (around 60%), long-term stability, fuel flexibility, low emissions, and relatively low cost. Some startups, such as Versa Power Systems are producing (or prototyping) SOFC stationary fuel cells in the 2- 10KW capacity range. This is approaching utility scale – imagine these cabinet sized units grouped into larger series.
Flywheel Energy Storage (FES)
Advanced flywheels made from carbon fibers and operating in a vacuum supported by magnetic bearings can achieve rotor RPMs ranging from 20,000 to over 50,000. This is important because the amount of energy potential that a device can store is geometrically proportional to how fast it can spin. Advanced flywheels are able to come up to top speed – and hence energy storage in a matter of minutes and can discharge as quickly. Advanced flywheels with magnetic bearings and operating in a high vacuum are also very efficient and can maintain 97% mechanical efficiency.
One problem is that these very rapidly spinning devices can explode with immense kinetic energy if their rotors fail and these units need to be encased in shielding because of this potential failure mode.
Although more work needs to be done to bring cots down flywheels have several advantages over batteries including: their high power density, the unlimited number of charge and discharge cycles, and the number of charge/discharge cycles that a flywheel can cycle through per unit time.
For these reasons flywheels are beginning to find a niche in the grid frequency regulation market where Beacon Power, for example offers its Smart Energy 25 flywheel. Its advanced flywheels are also being demonstrated as an energy storage system on a wind farm in Tehachapi, California in a project that will incorporate “intelligent agent” controls and flywheel energy storage technology to demonstrate how to enable as much wind-generated electricity to be delivered as possible without exceeding the limits of the locally constrained transmission system.
Supercapacitors or Ultracapacitors
Supercapacitors are well suited to be paired with underlying batteries in hybrid battery/supercapacitors energy storage systems that pair the higher energy density of the underlying battery with the very high power density of the supercapacitors enabling these hybrid devices to accept electric power surges and/or deliver surges of electric power in burst mode, while also being able to store large quantities of energy in the underlying battery system with which they are being paired. Basically the supercapacitor is used as a buffer between the underlying battery and the device, improving the overall performance of the paired energy storage device and also in many cases extending the life of a battery.
In grid scale applications supercapacitors will fill roles related to power quality and grid instability applications, and their use is likely to be driven significantly forward by the impressive gains that Smart Grid supercapacitors have been able to achieve as well as their very long useful operating life. Because supercapacitors are purely electric devices and do not rely on any chemistry they are rated to last through up to 500,000 discharge cycles or more outlasting the other components in a storage system.
Superconducting Magnetic Energy Storage (SMES)
Superconducting magnetic energy storage stores energy in a cryogenically cooled super conducting coil. As long as the superconducting coil is maintained below the critical temperatures required for the material to be super-conducting a DC current will not decay and the magnetic energy can be stored indefinitely. It is highly efficient; however the cost of refrigerating the coolant as well as the inverter reduce efficiency in practice to well below the theoretical levels. This is still frontier stuff, it does however have a potential niche role to play as a short term energy store for power conditioning and reliability purposes.
The Chemical Synthesis of Hydrocarbon Fuels From Electric (Wind or Solar) Power
Although this is beginning to stretch the idea of what people commonly think about when they think about energy storage using transient surpluses of electric power from renewable sources such as the wind to chemically synthesize simple hydrocarbons such as methane from CO2 and water is one thing that is being considered. In a sense one can think of this as an energy storage system. Electricity is stored in the chemical energy potential of the methane, which can be burned or used as a reactant in a fuel cell, when it is needed to generate power.
The utility or grid scale energy storage market is heating up and becoming more important than it has been in the past as renewable energy generation complicates the energy topography of the grid. Energy storage offers a path forward that increases the efficiency and the reliability of the electric grid. It opens up valuable opportunities for peak load shaving thorough time shifting the peak load supply to off peak periods and temporarily storing this energy for use during periods of peak demand. Energy storage promises to help smooth out variability introduced by renewable energy generation and has an important role in helping to maintain frequency (and also voltage) within a tight quality tolerance.
The potential size of this market segment is also very large in the many hundreds of billions of dollars. The electric energy sector will increasingly begin to add on energy storage to complement its generating capacity with the addition of needed storage capacity and when one looks at the size of the potential global market it is easy to understand why so many venture capitalists and energy storage startups are crowding into this space hoping to get even a small piece of what promises to be a very large pie indeed.
So much of this depends on getting the energy market to more accurately price energy in order to facilitate the business models of energy storage, which need to be able to buy cheap and sell when prices rise. Our related post: “The Smart Grid: Why Getting Dynamic Pricing Right Is More Important Now Than Ever” covers this subject.
There are so many ways energy can be stored… it can be overwhelming. This long post has covered in some detail fifteen of these energy storage solutions ranging from pumped hydro to supercapacitors. Though I tried to be comprehensive I am sure that I have overlooked interesting energy storage technologies; as I said there are so many of them. So if any readers know of energy storage systems that I have failed to include in this listing please feel free to mention them in the comment section and bring them to attention.
© 2011, Chris de Morsella. All rights reserved. Do not republish.