“I’ve been following your website for years and I like it very much. I’ve been reading your article from 2018 about off-grid compressed air energy storage and I’ve been thinking about a possible solution regarding the varying pressure problem which you mention about small scale, low pressure vessels.

What about using a constant pressure reservoir? My idea is to use a long airtight bag stored in a trench and covered with sand, that would inflate and deflate pushing the sand up and down. The height of the sand column would determine the inside pressure of the bag, which would remain constant across it’s whole inflation range. The sand would also act as a thermal mass, taking and restoring heat to the air.

I’ve read that similar storage ideas exist [1] but involve putting the bags deep underwater, which is not practical for off-grid purposes. Unlike a deep-water reservoir, a bag put under sand would not try to float, so there would be no need for complex anchors at the bottom.”

[1] http://euanmearns.com/a-review-of-underwater-compressed-air-storage/

Update. Reader Erich Wälde comments: 

> Unlike a deep-water reservoir, a bag put under sand would not try to float, so there would be no need for complex anchors at the bottom.

This is not entirely correct. The bag will work it’s way up! Around the edges some sand will always travel from “above” the bag to “below” the bag — unless the bag has considerable “flaps” around it, which will extend well into the neighboring area, i.e. complex anchoring, I’m afraid.

Other example: I bought a house several years back. In the garden there is an underground tank, which was used to store oil (heating). Heating was changed to use natural gas rather than oil, so the tank became unused. In order to fully abandon its operation, we had to get it cleaned out AND we had to show that the tank is not in the parking lot. If it were there, then it would have to be filled up with concrete or sand. Otherwise it would work it’s way up, very slowly, due to its buoyancy force. This is in Germany and local administration knows about each underground tank. Its well regulated.

So: You can’t trick physics 🙂

GIWHs are currently the least expensive form of energy storage available. Utilities can use fleets of grid-enabled water heaters for load shifting, demand response, arbitrage, ancillary services, or to respond to unexpected grid-stabilization events. Traditional dissemination of new water heater technology has been a painstakingly slow process, but water heater rental programs may greatly accelerate this process.

Read more: Battery Killers: How Water Heaters Have Evolved into Grid-Scale Energy-Storage Devices, David Podorson.

Related: How sustainable is stored sunlight?

“Graining cereal crops is a basic, century old business and it will continue to be as important as ever before for centuries to come. Before the age of oil grain milling was entirely based on renewable energy. It was either done by wind energy, hydropower, animals or manpower. For the last century the traditional grain milling has been mainly replaced by electricity and fuel driven milling.”

“The Solar PV Grain Mill works to the same principle like any conventional, electrically driven mill. The mill has a very efficient 3-phase AC motor which is directly coupled to the graining system. The main invention of the system is, and that makes it unique among PV systems, that it is a “direct drive system” without the need of batteries. The Solar PV generator converts solar radiation into electricity, and the generated electricity is directly feeding the motor drive. There are no additional conversion losses, such as energy storage losses in batteries, battery maintenance or replacement costs, which are a common problem in conventional Solar PV off-grid systems.”

Read More: Solar Milling. Via Engineering for Change.

I would like to add that the direct drive system also eliminates the high energy use caused by the production of the batteries, which can make solar PV off-grid systems everything but sustainable. Therefore, storing work instead of energy — the solar mill only operates when the sun shines — is a very interesting strategy in sunny regions.

Related:

• The Bright Future of Solar Thermal Powered Factories
• Back to Basics: Direct Hydropower
• Wind Powered Factories: History (and Future) of Industrial Windmills
• Pedal Powered Farms and Factories: The Forgotten Future of the Stationary Bicycle

The energetic implications of curtailing versus storing solar- and wind-generated electricity, Charles J. Barnhart et al, Energy & Environmental Science, Issue 10, 2013. Open access. Via Eurekalert and Yale Environment 360.

“Rail cars are pushed to the top of a grade using excess power from renewable energy plants or when electricity demand is low. Then, when the wind drops, the sun stops shining, or electricity demand rises, the rail cars are released back down the hill, generating electricity through regenerative braking. The technology integrates recent advantages in motor/generator traction drives and power control technologies with proven rail technology to produce a reliable and highly capable system that approaches an 80% charge/discharge efficiency.”

The company, founded in 2010, is operating a pilot project in the United States. More information can be found at the ARES website and in this Gizmag article, which has more pictures. Videos of the system can be found here, here, and here. ARES is a great low-tech project that deserves praise for its sustainable thinking. One problem might be that climbing slopes is not what trains are good at, while it is the vertical distance travelled that matters. The consequence is that the method requires a lot of space. A 50 MW rail energy storage system needs an eight kilometre track on an eight percent grade with 32 vehicles, each weighing 300 tonnes.

There exists a low-tech method that needs less space. Energy Cache, an American firm founded in 2009, has been testing a method of gravity energy storage which is based on aerial ropeways. When it comes to transportation of passengers and goods in hills and mountains, aerial ropeways are more efficient than railways in terms of energy and space, and they are much cheaper to build. Ropeways might have the same benefits for energy storage.

KDD (edited by Alice Yaxley)

Read more: The most important man in energy storage? Try Archimedes.

Quoted from: The Manufacturer and Builder, Volume 0016 Issue 2 (February 1884).

The GravityLight uses a sack of sand or stones to gradually pull a piece of rope through a dynamo mechanism which generates electricity to power an LED. It is a cheaper and more sustainable option than a solar powered light, which requires not only a solar panel but also a battery. The product is aimed at the developing world and its makers raised 400,000 dollars at indiegogo.

The technology could be further improved by including pulley mechanisms that were used to operate human powered cranes and lifting devices in pre-industrial times. This would allow a person to lift heavier weights and thus power more powerful lights.

To be precise, the light is not powered by gravity. It is muscle-powered, while gravity stores the energy and fulfills the role of a battery. Hat tip to Bernd Vleugels.