Making Liquid Oxygen from Scratch Using Only Electricity
0:00:21 The goal is set: to produce liquid oxygen using only electricity as the input, requiring intermediate physics and chemistry steps.
0:01:11 Electrolysis of water (using potassium hydroxide electrolyte) is demonstrated to produce hydrogen and oxygen gas but is deemed unsuitable due to requiring water replenishment and producing water vapor, which is problematic for liquefaction.
0:02:41 Pressure Swing Absorption (PSA) is introduced as the preferred method to extract oxygen directly from ambient air (21% O2) using only electricity for the process.
0:03:32 The PSA process is explained: Air is filtered, compressed, cooled, and passed through zeolite molecular sieves which preferentially absorb nitrogen under pressure. Two zeolite tanks are used alternately, one absorbing nitrogen while the other releases its captured nitrogen, allowing for continuous oxygen enrichment (targeting >90%).
0:07:40 An inexpensive oxygen concentrator unit is shown and its basic operation (flow control, questionable O2 concentration display) is demonstrated.
0:08:39 A teardown of the oxygen concentrator reveals its internal components: filters, oil-free compressor, heat exchanger fan, motorized switching valve, two zeolite tanks, and control PCB. It confirms the absence of a real oxygen sensor, making the front panel display inaccurate.
0:13:08 An ultrasonic oxygen analyzer is introduced to accurately measure O2 concentration. Its teardown reveals it works by measuring the time-of-flight and Doppler shift of ultrasonic pulses through the gas in a tube, utilizing the density difference between oxygen and nitrogen and compensating for temperature.
0:15:17 Measurements show the concentrator produces ~77% oxygen at 1 L/min, improving only to ~83% when flow is restricted below 0.5 L/min, falling short of the expected 90%+ purity likely due to the small size of the zeolite tanks in the cheap unit.
0:16:35 The phase diagram of oxygen is discussed, highlighting that it cannot exist as a liquid at room temperature regardless of pressure, requiring cryogenic temperatures (below 90 K / -183°C) for liquefaction.
0:19:54 A Sterling cycle cryocooler (salvaged from a superconducting filter unit) is chosen as the method to achieve the necessary cryogenic temperatures using only electricity, avoiding the need for consumables like liquid nitrogen.
0:20:50 The operation of the Sterling cryocooler is explained: An electrically driven piston oscillates, compressing and expanding helium gas. Heat is rejected at the warm end, and extreme cold is generated at the cold tip, aided significantly by a high-surface-area regenerator material. Vibration is managed by a counter-oscillating mass.
0:27:25 A custom setup is built to run the Sterling engine, featuring extensive fan cooling for the heat rejection area, vibration damping mounts, and an insulated chamber for the cold tip.
0:33:17 A standard silicon diode is used as a cryogenic temperature sensor attached to the cold tip, leveraging the relatively linear relationship between its forward voltage and temperature at a constant low current (100µA). A potentiometer is added to slightly offset the reading, aiding control loop stability in the custom setup.
0:37:14 The complete experimental setup is shown: Oxygen concentrator output is dried with silica gel, flow-regulated, and fed into the cryocooler chamber. The cryocooler is controlled via PC software, monitoring temperatures and power.
0:41:01 The cryocooler runs, reaching temperatures below -100°C in minutes and approaching the target temperature (around 77K reported, ~67K actual due to offset) after about 35 minutes, consuming ~126W at the engine.
0:42:58 After 1.5 hours of operation, the insulated collection cup is removed, revealing successful liquefaction of oxygen.
0:43:18 The collected liquid oxygen displays its characteristic pale blue color and demonstrates paramagnetism by being visibly attracted to a strong neodymium magnet. Its potent oxidizing nature is shown through rapid combustion of a wooden stick.
0:45:25 The total power consumption for the process (oxygen concentration + cryocooling) is measured at approximately 310 Watts during the 1.5-hour run.