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September 17, 2007

More on energy storage – Flywheel Batteries

Category: Flywheels – Dan 8:03 pm

Energy storage is the least talked about, and most challenging problem facing green energy today. There’s plenty of potential power from either wind or solar, but these sources are intermittent. What’s needed is reliable storage.

There are four broad general ways to store energy[1]:

  • Chemical, with batteries being the most popular form. But in truth all fossil fuels are, in effect, chemical energy stores, and storing hydrogen is another effective chemical means of storing energy.
  • Mechanical, which might be water pumped uphill, compressed gas, or kinetic energy in a flywheel.
  • Electrical, either capacitors or inductors.
  • Thermal, which is not particularly efficient but if your source is thermal (as with CSP), this might be a cost effective approach.

There’s clearly a lot of research going on around batteries, especially since they are suitable for any mobile application. Electric or hybrid cars rely on batteries, and alternatives (such as flywheels or compressed hydrogen) so far do not look nearly as promising. There’s also considerable research going into storing hydrogen.

One option that gets far less press are flywheels, which store energy in a disk spinning at high speeds. These are commonly called “Flywheel Batteries”. Currently traditional lead-acid batteries today have an advantage in terms of cost, but flywheels have far longer service lives (>20 years vs. 3 or 4 for lead-acid batteries), a wider range of operating temperatures, and high power density.[1]

25 kWh Flywheel from Beacon Power

Imagine for a moment the potential of combining a solar PV system with a modest flywheel battery with a 25 KWHr capacity. This combination installed in a small energy-efficient home would have the potential of providing nearly all of the power needed over a 24 hour period. Properly sized, the PV panels could provide sufficient power during the day to both power lights and electric appliances, and spin up the flywheel in the basement. Peak AC requirements would closely coincide with peak PV generation. When the sun goes down, load could be pulled from the flywheel battery. A grid connection covers any under or over generation during the course of the day.

What’s the advantage of such a system? There are several:

  • Near continuous power – The flywheel battery effectively extends the power generating time of the PV system, providing a nearly continuous power source.
  • Long service life – Both the PV system and the flywheel battery are virtually maintenance free and have >20 year service life given today’s technology.
  • Modular design – Both the PV panels and the flywheel batteries are produced as manufactured components that can easily be shipped to a site and installed. Need more capacity — add more units.
  • Local source – There are no transmission losses, because most of the power generated on site is used on site.
  • Grid Stability – From the grid connection’s perspective, the system is relatively stable. The combination of flywheel battery and PV panels means that the house can handle much of it’s own power regulation. Could these systems work off of the grid? Certainly, but having them on the grid provides advantages with local power regulation.

Flywheel batteries are manufactured today, but are primarily sold as short-term high current uninteruptable power supply sources to even out fluctuations in the grid. Some manufacturers include Tribology Systems, Powercorp, and Beacon Power.

What’s not to like? Cost is the primary objection, but as with PV systems, the prices are falling as quantity ramps up. 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June 19, 2007

NREL “Wind to Hydrogen” Facility

Category: Hydrogen,Wind power – Dan 6:31 am

NREL (the National Renewable Energy Lab) recently published information on their new experimental “Wind to Hydrogen” facility. This is an idea that has been promoted for some time by the Leighty Foundation, and it’s a clear example of “Smart Green Energy”.

The challenge: Wind power is intermittent. The solution: Use unneeded power to generate hydrogen which is stored on-site. This hydrogen is then converted back to electricity in a fuel cell when the wind isn’t blowing and power is needed.

“By marrying wind turbines to hydrogen production, we create a synergy that systematically reduces the drawbacks of each,” Richard Kelly, Xcel Energy chairman, president and CEO

While mobile hydrogen storage is a problem (see “Mythbusters – Hydrogen will fuel our cars?“), there’s no problem with industrial-scale hydrogen storage, especially where wind power is generated (which by it’s nature is out in the wide open spaces).

NREL’s Wind2H2 project is designed to analyze the tradeoffs using different types of wind generators, different approaches to convert the electricity to hydrogen, and issues related to the integration of these technologies as well as the operation of electrolyzers with different gas output pressures.

The NREL site also has a very cool animation to show the different configurations being tested.NREL “Wind to Hydrogen” Animation

May 7, 2007

Energy Storage 101

Category: Transmission/Storage – Dan 8:29 am


One of the biggest challenges with new alternative energy solutions such as wind and solar is that these sources are em>intermitent; power is only generated when the wind blows or when the sun shines. The good news is that there is vast potential, either wind or solar could in theory provide the entire US with our power needs. But the bad news is there is no obvious solution to provide storage of the energy from these intermittent sources. Fossil fuels are a form of high-density stored chemical energy — that’s why they are so popular. The stored energy can be transported to where it’s needed, and released (by burning) when it’s needed.

There are several ways to store energy on an industrial scale; let’s look at a few:

  • Compressed air – In compressed air energy storage, electricity is used to pump air into a sealed underground cavern to a high pressure. The pressurised air is then kept until needed, at which point it drives turbines as it is released. Note that the compressed air can be mixed with natural gas and they are burnt together, as in a conventional turbine plant, which improves efficiency as the compressed air will lose less energy (one problem with these systems is that the air heats when compressed, and this heat must be eliminated — then when it expands, it cools and this reduces efficiency in the turbine). [1]
  • Pumped water storage – If you have a hill (over 100 meters) nearby with a suitable reservoir, and a supply of water, you can create what amounts to a reversable hydroelectric facility. Excess power isused to pump water up to the reservoir, and when energy is needed the water runs back down through turbines. Hydropower is 80% efficient, but this affects you in both directions so the result is about 64% conversion efficiency. [2]
  • Hot fluid – Generally, heat isn’t an efficient means to store energy. The exception is when heat is what you started with, which is clearly the case with concentrating solar power (CSP). Trough-based CSP systems use long parabolic mirrors to heat a working fluid. This hot fluid is then used to generate steam to drive a generator. Some CSP plants have a reservoir of fluid to store heat generated during the day for use at night
  • Batteries – Batteries store electricity as chemical energy. There are battery configurations of several megawatts, making them suitable for various utility applications, and of course smaller configurations are common and widespread.
  • Hydrogen storage – Water can be electrolized into hydrogen and oxygen, and the hydrogen stored for later conversion to electricity when needed using a fuel cell. Note this solution can be combined with a pipeline so that hydrogen becomes the energy tramismission medium, rather than using electrical lines.

There are other options as well, such as flywheels, capacitors, and superconducting magnets, but the above four technologies are the likely best near-term energy storage alternatives for industrial scale energy storage.

Storage pros and cons

None of these storage solutions are perfect. Because energy is being converted to a different form, all storage solutions waste some of the original energy. But all of the above storage solutions have different pros and cons, and these lead to some logical conclusions. Before looking at some smart solutions, let’s consider the pros and cons of each technology:

Compressed air

The investment in a compressed air storage system is in compressors and turbines. These components affect the rate of energy storage/generation. The storage medium – the cavern within which air is kept – is site dependent but is usually vast (in terms of potential mWh storage) compared to the rate (in terms of mW) of the generators. This makes compressed air ideal for stranded wind generators which themselves are limited by the maximum generation rate, but might experience generation fluctuations of days, weeks, or even seasons.

Pumped storage

Pumped storage is limited by lack of suitable sites; you need appropriate geography, plus plenty of water. Pumped storage facilities are also a less desirable option environmentally, because the upper reservoir isn’t much use for anything else — it fills and empties like a bathtub, often on a daily basis. Beyond these limitations, pumped storage has many of the characteristics of compressed air — the volume of storage is quite large compared with the rate of storage or recovery. Pumped storage is also ideal for stranded energy generation.

Hot fluid

As previously stated, storing energy as a hot fluid makes sense only when hot fluids is what you start with. Within the context of CSP facilities, investing in storage makes sense in the context of shifting peak power demand. In other words, peak energy generation at CSP will be during the middle of the day when the sun is highest; peak energy demand is likely to be later in the day and into the early evening. The quantity of fluid stored need only be enough to carry the load over into the evening.


Batteries have characteristics almost opposite those of compressed air or pumped storage. The cost of expanding storage volume is linear; double the number of batteries and you’ll double the storage. Batteries are also relatively expensive (driven in part because the storage medium is a manufactured product vs. a cavern or a reservoir). The result of these characteristics is that batteries make the most sense to even out intermittent power sources near the point of consumption, where the value of the energy is highest. An ideal use for batteries, for example, is to extend the time that energy is available from residential photovoltaic installations.

Hydrogen storage

Hydrogen is relatively easy to generate given a supply of water which can be separated using electrolysis. And while hydrogen is bulky to store, there are no major technical obstacles. Hydrogen falls somewhere between batteries and compressed air storage in characteristics; there’s a higher investment required in the equipment to electrolyze water and with fuel cells to convert hydrogen to electricity. Hydrogen can be stored in suitable underground caverns as with compressed air. In addition, there are several proposals to build pressurized hydrogen pipelines from stranded energy sources. The pipeline would not only serve as the energy transmission technology, but would contain a significant quantity of hydrogen and thus be a storage medium as well. I’ve included more information on this in the early article “What About Hydrogen?” Arguably, this pipeline approach is the best use of hydrogen storage, because it is then well integrated into the overall source / transmission / storage infrastructure.

April 3, 2007

Mythbusters – Hydrogen will fuel our cars?

Category: Hydrogen,Mythbusters – Dan 4:02 pm

I guess my blog entry on hydrogen for transmission and storage created a bit of confusion regarding the most common “talk”, that is hydrogen to power cars. To wit:

Hydrogen Can Provide Long-Term Energy Security Through Use Of Diverse Domestic Resources. The President’s Hydrogen Fuel Initiative and the FreedomCAR partnership will reduce America’s need for imported oil and help clean the air by aiding the development of hydrogen fuel cells and affordable hydrogen-powered cars. Together, these two initiatives constitute a commitment of $1.7 billion over five years.[1]

This was directly off the website. I suppose the good news here is that, in more recent white house documents[2] the President isn’t talking so much about hydrogen and cars.

So let me go on record as saying that, in terms of an automotive fuel, hydrogen is a dumb idea. This is a myth that needs to be dispelled. There are better alternatives, both for the near and long term.

Hydrogen – The Perfect Fuel?

But wait? Isn’t hydrogen the perfect fuel? Consider:

  • Abundant – Hydrogen is the most abundant element in the universe
  • Clean – Hydrogen burns clean – no carbon dioxide will come out of your tailpipe
  • Easy electricity – Feed hydrogen into a fuel-cell stack and you get clean electricity
  • Powerful – Pound for pound, hydrogen has three times the energy of gasoline

What’s not to like?

Major technical hurdles for automobiles

Technology review just did an article on the BMW “Hydrogen 7”, a prototype hydrogen vehicle. Their conclusion:

In the context of the overall energy economy, a car like the Hydrogen 7 would proba­bly produce far more carbon dioxide emissions than gasoline-powered cars available today. And changing this calculation would take multiple breakthroughs–which study after study has predicted will take decades, if they arrive at all. In fact, the Hydrogen 7 and its hydrogen-fuel-cell cousins are, in many ways, simply flashy distractions produced by automakers who should be taking stronger immediate action to reduce the greenhouse-gas emissions of their cars.[3]

BMW's Hydrogen 7 Sedan
“BMW’s Hydrogen 7 Sedan”

Why such a negative quote? Let’s consider the current technical realities:

The “hydrogen tank”

We take filling up the tank with gas for granted. Don’t smoke, leave your phone in the car, try not to spill any, and don’t top off. Otherwise, you end up with 10 to 20 gallons of fuel that, at normal temperatures, can sit in your tank for months, propel your car for 300 miles or so, and is relatively unlikely to leak or blow up.

What’s the hydrogen tank alternative. The April’07 issue of Scientific American just did a comprehensive article on “Gassing Up with Hydrogen”[4] that looked at five different technologies for on-board hydrogen storage. The problem is that, although hydrogen, pound for pound has three times the energy content than gasoline, at normal temperatures and pressures hydrogen has only 1/3000 the energy density of gas: a 20 gallon tank of hydrogen would only propel your car about 500 feet!

The solution will fall into one of three categories:

  • Compressed hydrogen – Simple. If hydrogen has 1/3000 the energy density of gas, then compressing it 3000 times should solve the problem. Except that would require pressuring the tank to over 40,000 psi — 8 to 10 times the pressure of a good scuba tank. While theoretically possible, this requires both new advances in tank technology, special protection so the tank isn’t ruptured in an accident, and a means of providing ultra-high pressure hydrogen at the filling station.
  • Liquified hydrogen – To maximize density, you can liquify hydrogen. In liquid form, hydrogen has about 30% of the energy density of gasoline, so you’d need a tank that was roughly 3 times as big for the same fuel range. Hydrogen liquifies at -253°C (about -420°F). The challenge; you need lots of insulation, it takes lots of energy to cool hydrogen to this temperature, and you’ll constantly loose fuel as it slowly boils off. And there are additional challenges at the filling station. Spill gas on your hands and it smells bad. Spill liquid hydrogen on your hand, and you can snap off your fingers.
  • Chemical compaction – The third means of storing hydrogen is to leverage the fact that hydrogen, when bound to the right materials, packs even closer than with liquid hydrogen. There is intensive research in this area. One promising area, tmetal hydride’s, have attained hydrogen capacity of 2% of the total material weight. Unfortunately this still means that you’d need a 1,000 lb fuel storage system for a 300 mile driving range.

There is another option to have on-board hydrogen — you can generate it on board from natural gas or gasoline. While practical in many ways, this produces just as much CO2 as you would with a traditional natural gas or gasoline powered car.

The bottom line here: this is a big problem, and will remain so without some major technical breakthroughs.

Running your car on hydrogen

Once you’ve stored your fuel, you can consume it and drive around. There are two basic approaches here: burning the hydrogen in something resembling a conventional internal combustion engine, or converting the hydrogen to electricity in a fuel cell to drive electric motors.

The BMW Hydrogen 7 burns hydrogen in a fairly standard engine. Indeed Ford is now producing hydrogen internal combustion engines[5]. This technology is relatively feasible today. This approach can create better potential acceleration and a more thrilling ride, but it’s way less efficient than using fuel cells and electric motors.

The alternative is to create an electric car and include an on-board fuel-cell stack to convert the hydrogen to electricity. While elegant in concept, creating small fuel cells that work at normal temperatures without major compromises is a big challenge[6].

Hydrogen infrastructure

Hydrogen cars need to “fill up” at hydrogen fueling stations, which means we need infrastructure to generate hydrogen and distribute it on a widescale basis. Currently, the most inexpensive way to generate hydrogen on an industrial scale is to strip it from fossil fuels[3]. From the standpoint of global warming and carbon reduction, this of course makes no sense, as what remains from this process is CO2. The alternative is to split hydrogen from water, which requires electricity. At the point where we have extensive renewable wind or CSV generation capacity, this might be cost effective, but it’s certainly not today.

Distribution is another challenge. One proposal to solve this problem is to generate hydrogen from natural gas, which already has a distribution network. This however means that you’re creating CO2; if you’re going to do this, it would make more sense to run cars directly off the natural gas.

As with the challenge of storing hydrogen on board, the hydrogen infrastructure is a big problem.

Smarter alternatives

Are there better alternatives to hydrogen powered vehicles? Absolutely. Let’s look at this from a rational engineering perspective:

First, what makes sense to maximize efficiency? While internal combustion engines have seen major improvements over the decades, they are still an inefficient power suource. The most efficient gasoline powered car ever made was perhaps the 1003 Honda Civic VX, EPA rated at 51 mpg, yielding an efficiency of 0.52km/MJ (MJ = megajoule, a quantity of energy)[6]. Compare this with the estimate for the soon-to-be-released Tesla Roadster (an all-electric car), which has an estimated efficiency of 2.18 km/MJ – over four times as efficient as the Honda. The point is that electric motors are far more efficient for powering vehicles than internal combustion engines.

Second, what’s the best way store energy on the vehicle? Note that “Best” is a subjective term, and it’s tied to infrastructure issues (it’s easy to buy a gallon of gas down the street; the same cannot be said for H2), the design question of consider how far you want to go before refuelling, and the engineering question of what form do you want the energy in? (Gas may be a great fuel, but you can’t run an efficient electric motor on gas). Gasoline and diesel are best in terms of energy density (12.2 KWh/kg and 13.7 KWh/kg)[7]. Ethanol is an acceptable liquid alternative at 7.8 KWh/kg. Batteries have much lower energy density, at best a bit over 0.1 KWh/kg.

Third, what’s the best source of the energy? This also has the subjective “best” issue. I’ll argue that best in this case includes flexible, and for that reason, electricity is the best energy source, because we can create it in a variety of ways, and there’s an infrastructure already in place to deliver it.

OK, too many words. What does this mean in terms of the best approach for automobiles. Given these three points, and wearing my engineering hat. I’ll argue that the best approach is:

  • Electric motors – They are the most efficient
  • A plug-in hybrid architecture – It’s allows the use of electricity for modest trips without the range and fast fueling limitations of an electric-only car
  • Ethanol as the on-board liquid fuel – Ethanol (especially over time) can be generated from non-fossil sources, it has a reasonably high fuel density, and the current fuel delivery infrastructure can evolve fairly smoothly to support this


This was a mythbuster. This post has lots of data, and it may have some controversial points, but my bottom line is that hydrogen as an automotive fuel is a non-starter. Does anyone disagree?

March 30, 2007

A deeper look at energy transmission

Category: Transmission/Storage – Dan 2:06 pm

In my recent post “The Energy Big Picture” I argued that there are four components to the energy big picture: Sources, Transmission, Storage, and Use. If you consider the big picture, you must consider all of these together. In this post, I’ll take a closer look at the transmission part of the picture, and show why it’s closely tied to the other three components.

Energy Distribution in the US Today

There is a vast infrastructure today providing energy transmission. Too often we think only of the electricity grid, but transmission today includes:

  • Natural gas transmission – There are 300,000 miles of natural gas transmission lines[1] in the United States. Note this doesn’t count the local distribution network, which adds another million miles or so of pipes.
  • Oil transmission – There are about 200,000 miles of liquid oil pipelines [1] in the United States. However, unlike natural gas, most local distribution (except airports) is handled by tanker trucks.
  • AC High Voltage Electric Grid – There are about 154,000 miles of high voltage (>230kV) electric transmission lines[2]. The electric grid is divided into three major segments: the Eastern Interconnection, the Western Interconnection, and the Texas Interconnection. As with natural gas, this doesn’t count local electric distribution systems.
  • DC High Voltage Electric Transmission – This is a relatively small component of the electrical distribution system, comprising only about 3,300 miles today[2]. With DC it is possible to transmit higher voltages over long distances. The best known line in the US is the Pacific Intertie, which transmits 3,100 MW of power from the Dalles Dam on the Colombia River to Southern California.

Source and transmission relationships

If we look down the list of new energy technologies, they have to be taken in context of source and transmission. Here are some examples:

  • Wind power – There’s enough wind power available in the plains states to power the entire country. But to tap it, we would need to extend the electric power grid throughout the region.
  • Concentrating Solar – There’s enough space in the southwest to generate at least 25% of US demand. But while the markets in southern California and Arizona are nearby, moving the rest of this power across the country requires major grid investments.
  • Hydrogen – Hydrogen is an energy carrier. Whether we use it to power cars (I’ll reserve my opinion here for another post), as a means to store energy generated by wind or CSP, or as a major energy carrier, we’ll have to create a new pipe distribution infrastructure equivalent to the natural gas system.


What this means, is that we have to think about energy transmission at the same time we think about new sources. Wind, CSP, geothermal, and biomass advocates have to take into account how the power gets to the markets where it will be used.

March 28, 2007

What about Hydrogen?

Category: Hydrogen – Dan 10:00 am

Hydrogen is often quoted as a new alternative energy source, particularly with respect to what we’ll put in our gas tank. I will argue that hydrogen is a likely part of the energy future. But it is not a new energy source, and using it as an automotive fuel is not a smart use for hydrogen.

Hydrogen compared with Natural Gas

The closest equivalent to hydrogen today is natural gas, with respect to it’s potential uses and behavior. The obvious difference is that natural gas is a fossil fuel that can be extracted from the earth, whereas elemental hydrogen does not naturally exist in commercially available quantities and so has to be generated. Here’s a simple chart to show the similarities and differences:

Natural Gas Hydrogen
Normal state Gas Gas
Highly compressable Yes Yes
Energy Content 1040 BTU/cubic foot 320 BTU/cubic foot
Practical to transport in pipes Yes Yes
Practical to store underground Yes Yes
Sources Underground extraction Electrolize water or reform natural gas

We all think of natural gas as an energy source. But beyond being a source, natural gas is also an energy carrier, allowing practical energy distribution from source to consumer, and it serves as an energy storage mechanism. Indeed, to balance seasonal demand, natural gas suppliers regularly store natural gas in salt caverns during slack periods to have energy in reserve during peak times. It’s no wonder natural gas is in favor.

Hydrogen can serve as a natural gas alternative for two of these three crucial characteristics: transmission and storage.

Pairing hydrogen with alternative energy sources

Alternative energy sources like wind, concentrating solar power (CSP), or geothermal energy, all share the same good-news/bad-news characteristics:

  • The good news – There’s a huge power potential. We could supply the entire US energy needs by wind power from the midwest or CSP in the desert southwest.
  • The bad news – These sources are intermittent, and they are hundreds or thousands of miles from the demand. In the industry, these are known as “stranded” sources.

Here’s where hydrogen can, and perhaps should, play a staring role. Given a source of energy and a supply of water, it’s fairly straightforward to generate hydrogen. If it’s practical to pipe hydrogen long distances, and store it until needed, one could almost redefine a large wind farm as a hydrogen generation facility.

This topic has considerable depth. I first came to understand some of these ideas after a conversaton over dinner at the PowerGen conference with Bill Leighty of the Leighty Foundation, which has been funding research into this area for some time. I’ll provide more on these ideas, with more information sources in future posts!

March 22, 2007

The energy big picture

Category: General,Transmission/Storage – Dan 7:44 am

I want to return to a theme I touched on a week ago in the “What’s the answer? Yes” entry. Too often we look at a potential solution like wind or concentrating solar power (CSP), and say “won’t solve the problem — it’s intermittent”, or “it only works in the desert”.

This issue touches directly on energy policy. We need to think of the energy big picture. And I’m going to argue that there are four components to that big energy picture: Sources, Transmission, Storage, and Use.

  • Sources – Wind, CSP, Photovoltaic — these are all sources. There’s a lot of data that shows that there’s plenty of environmentally benign energy to be had. For example, the TREC project (a Club of Rome initiative) shows that a CSP facility covering a fraction of the Sahara desert could provide power for the entire world. The challenge here is that these great CSP sites are far from the major population centers. This leads us to…
  • Transmission– Our current major power sources, coal and oil, are cost effective today in part because of elaborate transmission and distribution systems. The US is crossed by a network of pipelines, an electric power grid, and coal fired plants either are colocated with the mine or at the other end of a major rail line connecting mine to plant. Indeed one enormous energy use in the US is energy transmission. No energy strategy is complete without considering the investment required to get that energy to market.
  • Storage – As important as transmission is the ability to store energy for use when it’s needed. Why is gasoline a great transportation fuel? Because a relatively small and lightweight quantity can be stored easily in a gas tank until it’s needed. Indeed, you can think of fossil fuels as a giant tank of stored solar energy that, for the past century, we’ve been drawing down. The piles of coal outside a coal-fired utility, and the tanks of gasoline at your local filling station are all examples of functional and cost effective energy storage. The big challenge with many alternative energy sources is storage; it’s easy and cost effective to convert wind to electricity, but it’s not so obvious how to store it until it’s needed. As our energy strategy evolves, we have to address this storage challenge.
  • Use – Finally, the nature of delivered energy is important. Electricity is a wonderful and flexible energy medium, but without better batteries (a.k.a. chemical storage), it can’t power automobiles. There are industrial processes that require specific fuel sources. I cook, and believe me a gas stovetop is superior.

Much of the challenge to move to a carbon-free energy economy is how to evolve the current entrenched system to adapt to new cleaner sources, while addressing the new problems that arise with transmission, storage, and use. This will be a focus on ongoing posts.