Caes

=== lowtech ===
compressed air storage lowtechmagazine.com which describes the results of the two papers below. 470watthours per kg is possible(5bar) by using modular small tanks(fire extinguishers). See Wind energy#solution for combining this idea with solar. Stack multiple containers and release the air energy with solenoid valves in sync to drive a turbine. Efficiency is much greater than using a single large container. In short, a higher energy density is obtained by using multiple interconnected storage tanks instead of one large tank. This allows more effective heat transfer to take place, because every air tank acts as an additional heat exchanger. Also, discharging the vessels sequentially allows longer discharge times and solves the problem of pressure drops. These factors are not taken into account in the Wikipedia formula. https://web.archive.org/web/20120214080938/http://www.lightsail.com/tech.html. If a windmill compresses air directly, without converting it to electricity, and the devices are powered directly by compressed air, a small-scale CAES system can become even more energy efficient than the articles states. This strategy, which also works on a larger scale, is explained in the other article: http://www.lowtechmagazine.com/2018/05/history-and-future-of-the-compressed-air-economy.html
https://news.ycombinator.com/item?id=17143163 
https://web.archive.org/web/20160311064449/http://windcompressor.com/html/compressors.html 
https://web.archive.org/web/20100406034214/http://www.gamos.demon.co.uk/just%20gamos%20homepage/henkfnl2.htm 
Energy density (not efficiency, I think) can be ages better if the working fluid is stored in liquid state, but worked in gaseous state. The issue is that you need to either store, exhaust, or immediately use the low pressure gaseous exhaust; and you need a way to source it.
CO2 has a useful working range as a refrigerant, and is somewhat benign. Not sure how easy it is to directly harvest it though, as needed. If you can efficiently separate it from atmosphere, you can store pumping energy as a phase changed liquid. When using that battery, you get usable refrigeration and pressurized gas for mechanical work. The CO2 could be vented afterward (Neutral to atmosphere) or directed to a greenhouse operation.
Lots of hydrocarbons (propane, butane, etc) that have useful working ranges and the expanded gas can be used as fuel. Not sure if there is a practical way to source it though, especially renewably. 
How about using liquified air? It's about a 700:1 volume ratio, can be stored at ambient pressure, only requires a container with some good insulation, can be built in most local machine shops. A US crowd is working on combining this with their rimdrive wind turbines to directly liquify the air (see https://keukaenergy.com/products/) The upside is if having this supply locally it could also be used to provide refrigeration (either by direct air release or during rewarming of the liquid), and possible heating during the compression/liquification phase. I don't know what the smallest feasible size is, but maybe these could be installed in small local clusters of a few houses if individual household size isn't feasible. Pray allow myself to indicate you the remarkable research undertaken by Prof. Alfred Rufer at EPFL regarding this topic of energy storage using compressed air. To my knowledge, he and his staff have put a significant scientific effort to understand the way to achieve the best results for this approach. Certainly very useful to develop simple efficient devices.
https://www.sciencedirect.com/science/article/abs/pii/S0378778820309683 In the October 2020 issue of Energy and Building, someone modeled an analysis of a residential system with High Temperature Thermal Storage, Low Temp Thermal Storage, and Micro CAES (analyzed a system that stores 181 kg of air at 50 bar, seems high pressure!) 
compressed air CAES paper
CAES compressed air buoyancy storage
=== keukaenergy ===
https://keukaenergy.com/products/ or (https://archive.ph/wip/S7ftf) Air can be turned into a liquid by cooling it to around -196oC using standard industrial equipment. Around 700 m3 of ambient air becomes one m3 of liquid air which can then be stored in an insulated vessel. When heat (including ambient or low-grade waste heat) is reintroduced to liquid air, it boils and re-gasifies, expanding 700 times in volume. When this is done in a confined space, it creates high pressure. The high pressure derived from this re-gasification can be used to drive a turbine/generator to produce electricity, thereby making it a viable large scale energy storage solution.
Today’s wind industry is facing three major problems: higher cost, intermittency, and scalability. Recognizing these issues early on gave Keuka Energy the opportunity to work around them. The Rimdrive deals with cost by eliminating the gear box that adds up to approximately 25% of the long-term cost of conventional wind machines. Keuka also uses aluminum blades that are approximately 10% the cost of composite blades. Additionally, the “stayed blade” design offers the ability to build much larger machines capable of operating in lower and higher wind speeds. Energy stored as liquid air addresses intermittency. The conventional wind machine is seemingly simple in design—three blades and a hub mounted to a tower. The eye does not immediately see the complexity and trouble. The gear box is often the Achilles heel of the wind machine due to failures that occur within that system. The Rimdrive eliminates this costly and troublesome component. In addition, the “stayed blade” design will allow wind machines of the future to exceed 100 megawatts on land and larger for offshore wind farms.
=== cryogenic ===
https://cryomatiks.com/ from https://www.youtube.com/watch?v=Fpb7D5vm1vA
https://www.youtube.com/watch?v=tMLu9Dtw9yI liquid air
https://www.cryoenergy.tech/
=== In production ===
* http://zeropollutionmotors.us/
* 
http://www.youtube.com/watch?v=f4w6aJMNXSk SA obtained a production license. The engine has 20 patents to HackPatents via a fronting company in South-Africa.
https://en.wikipedia.org/wiki/Motor_Development_International , http://www.mdi.lu/
http://en.wikipedia.org/wiki/Compressed_air_energy_storage
http://en.wikipedia.org/wiki/Pneumatic_motor
=== air powered electrical generator ===
https://patents.google.com/patent/US20140049047 An air powered electrical generator includes an air compressor, an electrical motor for powering the air compressor and a compressed air storage tank. A fan, a valve and a pipe direct released air onto the fan to thereby rotate the fan and two electrical generators. A battery and a battery charger provide power for the electric motor so that the air powered electric generator can be moved to a pre-selected location to provide electric energy.
=== v8 on air ===
http://www.fuellessusa.com/AIR.html
=== MDI France ===
* http://mdi.lu/
* https://en.wikipedia.org/wiki/Motor_Development_International
* https://www.youtube.com/watch?v=ZHmUmqxXeQ0
* https://www.youtube.com/watch?v=WztEQ_qas1M available from TATA in India
=== engineair ===
* http://www.engineair.com.au/ , https://www.youtube.com/watch?v=ZGiviT-C_oY
* https://www.youtube.com/watch?v=ZGiviT-C_oY 
* https://peswiki.com/directory:engineair
=== youtube DIy ===
https://www.youtube.com/watch?v=MGkKksjPXwc compressed air engine (mrteslonian)
=== compressed air battery ===
http://en.wikipedia.org/wiki/Compressed_air_battery
https://en.wikipedia.org/wiki/Hydraulic_accumulator
http://kair-energy.com/index.php from https://en.wikipedia.org/wiki/Compressed_air_car#cite_note-39 . This is probably a better solution than stirling engines. K°Air’s Pressure Power System: K°Air has developed its multi-patent pending Pressure Power System that is the first and only to harness the Ambient Temperature of the existing surroundings, ranging from only 0°C to 35°C, to produce electricity, as opposed to the 300°C to 650°C required by steam turbines. This Ambient Temperature is sourced from the temperature outdoors; the room temperature of a building, office or home; or the temperature inside a manufacturing or industrial facility, where the temperature is warmer because of the heat generated from operations such as a foundry, manufacturing, mining, pulp & paper, textiles, commercial kitchens & bakeries, or dry cleaning. The K°Air Pressure Power System can achieve an average capacity utilization ranging from 75% to 95% from the Ambient Temperature alone, which can be increased if and when necessary from other sources of green energy including solar, wind or water mills, geothermal, biomass and commercial or industrial heat recovery sources.
The K°Air Pressure Power System represents a major improvement of steam turbine generator systems, which currently produce more than 80% of the world's supply of electricity.
=== links ===
Wind energy
Turbine
Category:Sasecurity
Category:Compressed air storage
Category:Energy