Converting Low-Head Hydropower into Air Pressure/Electricity

Hydro Siphon

by Emil Morhardt

Low-head hydro, in this case 2 meters, is not normally viewed as a good source of energy for electricity production, but a clever paper from the engineers at Lancaster University, in the UK, suggests using a shore-based siphon to generate air pressure (their field test rig is shown above). Although they didn’t try converting the air pressure to electricity, they certainly could if the economics were right, and it looks like they might be. As they point out, there are other alternatives to water turbines in current use, including Archimedes screws, hydro-venturis, and water wheels, all of which minimize harm to fish, but this approach might be even cheaper. The idea is to siphon water from above a small dam (weir) to below it, and entrap air into the water stream through a tube at the top of the siphon. The air gets compressed in the process and can be bled off for whatever use is handy; generating electricity with it might not be the best use since it will involve energy losses that might be avoided if the air were used directly, say to run some kind of pneumatic machine. Continue reading

Beefing up a Wind Turbine with Compressed Air

by Emil Morhardt

If your wind turbine isn’t going fast enough to meet the demands of the grid, blow on it a little harder: that’s the general idea suggested by Sun et al. (2014) [Shouldn’t somebody named Sun be studying solar rather than wind power?] The concept is a little like a hybrid electric vehicle; if the internal combustion engine isn’t going fast enough, give it a little boost from the electric motor connected to it. Except in this case, it’s that if the wind turbine isn’t going fast enough, goose it with a little compressed air. You might be envisioning a compressed air nozzle pointed at the turbine blades, but there’s a better way: use a motor driven by compressed air to speed up the turbine. One novel aspect to this study is that the device envisioned as a compressed-air motor is something called a scroll expander, or scroll-type air motor, a new type of pneumatic drive, but that doesn’t seem to be central to the idea—any suitable air-driven motor should work. The main point is to have it integrated with the wind turbine so that when needed, it can help out in the short term. Continue reading

Compressed Air Hybrid Vehicles?

by Emil Morhardt (An early compressed air vehicle is shown in the photo above. Not what this paper is talking about though.)

The usual candidate power supplies for the non-fossil-fuel part of hybrid vehicles are chemical batteries, supercapacitors, and flywheels, all powered up using electricity, and generating electricity when their power can usefully replace or supplant the main power source, the internal combustion engine. But these types of electrical storage and the motor/generators they utilize are complex, sophisticated, and expensive, and have barely appeared at all in the developing parts of the world where fossil fuel use is growing fastest. Maybe there is a simpler, cheaper option. One possibility is compressed air energy storage. All you need is a tank (cheap), a reversible compressor (fairly cheap), and a way to link it to the engine. That last part is tricky because the general run of such systems work optimally at a specific pressure, but their performance falls off dramatically as pressures in the tank exceed or fall below optimum as would be expected the tank is being pressurized or depressurized. The simple solution, according to Brown et al. (2014), is to use inexpensive check valves on the tank to prevent over-compression and over-expansion, and an infinitely variable transmission between the compressor and the engine that can operate efficiently at a range of tank pressures. The transmission adjusts by changing the number of thermodynamic cycles of the compressor executed per driveshaft rotation. Continue reading