Guest essay by Eric Worrall
Hydrostor has created an interesting innovation in energy storage. The energy is stored as compressed air, in giant underwater balloons.
Hydrostor’s system works in several steps. Electricity is run through a compressor and converted into compressed air. This compressed air is then sent underwater.
“There, we have a whole series of what are effectively balloons, that fill… like lungs under a lake,” he added.
“They fill with air, and when they’re full you stop charging the system and it can sit there indefinitely. When you want power back, again, a valve opens … air comes rushing out, we run through a low pressure turbine called a turbo expander, and that reproduces power back to the grid.”
Energy storage is becoming increasingly important. In the United States, the Office of Electricity Delivery & Energy Reliability states that the development of technology to store electricity so it’s available on whenever it’s needed would be a “major breakthrough in electricity distribution.”
Read more: http://www.cnbc.com/2015/12/09/underwater-balloons-clean-energy-savior.html
This isn’t the first time I’ve heard of storing energy with compressed air. Hydrostor’s innovation is to reduce the cost, by using water pressure to maintain the storage compression, without the need to create high strength pressure containers.
Using water to supply the pressure to collapse the balloons might also improve the efficiency of energy retrieval. By storing the air underwater, and using the water pressure to collapse the balloon, the air would be retrieved at a near constant pressure, until the balloon was empty. Hydrostor claim an efficiency of 60 – 80%.
Hydrostor seem to be aiming at grid stabilisation rather than storing multiple days worth of grid supply, so it seems unlikely this proposal, as it stands, will fully solve the renewable intermittency issue.
Could compressed air storage be scaled up enough to solve intermittency? Even if you had enough renewable capacity to cover 100% of grid requirements, on average, if you wanted to go further than Hydrostor’s current plans, to completely eliminate the need for backup gas turbines for renewables, you would need to store enough energy to maintain full grid supply for at least a day, more likely several days.
My concern is energy stored as compressed air could, in principle at least, be released all at once, in the event of a storage system failure.
Imagine you wanted to store one day worth of energy to supply a major city, in case the wind didn’t blow that day – enough energy to maintain a 1 gigawatt supply to the grid.
To maintain a 1Gw supply of energy for one day, your storage solution would need to store;
1Gw x 1 day
= 1000,000,000 W x 86400 seconds
= 86,400,000,000,000 Joules of energy
= 86.4 TJ.
This amount of energy is the same magnitude as the energy released by Little Boy, the nuclear bomb which destroyed Hiroshima (63 TJ). An abrupt release of 84TJ of energy next to a populated area would cause serious damage.
How would such a release of energy manifest? During the Lake Nyos disaster, when 100,000 tons of volcanic CO2 at the bottom of Lake Nyos was abruptly released, the rising gas created a 25m Tsunami which did extensive damage to the thankfully mostly uninhabited shoreline.
In the case of Lake Nyos, the Tsunami damage was secondary compared to the lethal effect of normal air being displaced over a large geographic area, by an asphyxiating cloud of concentrated CO2. But a large Tsunami smashing into a densely populated coastal city, adjacent to the storage facility, could still cause serious loss of life, even if the gas bubble which created the Tsunami was breathable air.
Risk concerns aside, in my opinion Hydrostor’s solution is still a very interesting innovation. Storing energy underwater, utilising the natural pressure of the water, should substantially reduce the cost of creating storage “balloons”, compared to other schemes for using air pressure for energy storage, by dramatically reducing the required strength and potentially the cost of the materials used to construct the pressure containers.
To me the idea seems to be a complicated way of using conventional forms of energy to pump water from a lower gravitational potential to a higher gravitational potential, and then when needed recovering some of that energy by placing a dynamo in a pipe and letting the water drive the dynamo as it flows through the pipe on its return to a lower gravitational potential. Unless the plastic of the balloons can store massive amounts of energy by being stretched, isn’t the energy stored by such a system stored in the form of an increased water level? Nature does this all time time–think hydroelectric power from dams. Maybe the proposed method is more efficient than simply raising the water level using a pump–I don’t know; but it sounds like a lot of folderol to me. If used in local reservoirs the proposed “battery” does have the advantage, namely in drought stricken areas it will give the appearance of increased water supplies.
Few places are adequate for pumped water energy storage, you need two pretty large reservoirs.
Air-lifted water energy storage has the advantage of not needing an additional big water reservoir.
steverichards1984 “To match pumped storage in terms of size and output, imagine the upper water catchment area, that’s what is needed in underwater in bags.”
That’s only the volume, you also need depth to give pressure, ½psi per foot of depth, So if you you use a low pressure turbine say 100psi & 50psi to overcome pressure drops we are looking at water depths of 150-200ft. so can’t be inland. Most offshore wind turbines are in water <100ft.
QED a crap idea !!
The necessary size of the reservoir would depend on the elevation of the 2nd reservoir, no?, like voltage in a D.C. circuit dictates how much power you will get with each amp of current.
“The necessary size of the reservoir would depend on the elevation of the 2nd reservoir, no?”
Yes. You use natural geography and you can’t choose heights of mountains.
Or maybe you can if you are “green” and can justify building up an artificial mountain with concrete.
The buoyancy factor of large volumes of air under high pressure makes this crazy expensive.
The balloons have to be sealed at the bottom because the air has to be held at higher pressure than the water pressure if the air is going to do any work.
This is such an obviously unworkable concept as to raise questions about why anyone is willing to spend serous money to study it.
Why is compressing air underwater any better than compressing it into a tank on land if you intend to use it to run a turbine what am I missing?
“Why is compressing air underwater any better than compressing it into a tank”
The pressure in the balloon depends almost only on the depth of the ballon (and a little bit on the shape and tensile force of the rubber).
The pressure in a tank is proportional to the amount of air and also to its temperature, which means the pressure would drop as the air cools down during standby.
What is the lifetime of steel?
What is the lifetime of ballons in water?
Hunter/Philip: Why underwater? Sealed/unsealed?
Underwater the bags have the same pressure inside and out so can be weakly constructed, shore side pressure vessels are strong and costly.
Unsealed – open at the bottom – can be weak and contain air at 1 Bar if the bag was 10m high.
So tethered at 100m underwater the pressure of the air within a 10m bag would vary from approximately 9 Bar to 10 Bar, but you can only use the 1 Bar range.
You would need hundreds of thousands of these devices to do anything near useful.
A logistical nightmare.
To match pumped storage in terms of size and output, imagine the upper water catchment area, that’s what is needed in underwater in bags.
Time to put this idea to bed.
To match pumped storage in terms of size and output, imagine the upper water catchment area, that’s what is needed in underwater in bags.”
That’s only the volume, you also need depth to give pressure, ½psi per foot of depth, So if you you use a low pressure turbine say 100psi & 50psi to overcome pressure drops we are looking at water depths of 150-200ft. so can’t be inland. Most offshore wind turbines are in water <100ft.
QED a crap idea !!
The deeper you go, the more energy you store.
… and the more problems you get for maintenance, the more uneconomical this stuff becomes.
“Unsealed – open at the bottom – can be weak and contain air at 1 Bar if the bag was 10m high.”
I don’t understand.
“So tethered at 100m underwater the pressure of the air within a 10m bag would vary from approximately 9 Bar to 10 Bar, but you can only use the 1 Bar range.”
Why?
When you compress air, it gets hot. As you store the air, that heat escapes to the environment. This will happen even faster underwater.
The claims for 60 to 80% efficiency sound really far fetched to me.
I agree w/others — compressed-air storage is a non-starter due to glaring inefficiencies (where are the engineers in this scheme?). Particularly with the equipment on a sea-bottom — maintenance would be a nightmare. Even worse than constructing offshore pinwheels.
MarkW December 14, 2015 at 9:52 am
Sorry, Mark, but as an erstwhile refrigeration technician I can assure you that your description is not correct. Air conditioning also includes a working fluid that undergoes a phase change from liquid to gas and back again …
w.