The human race is moving slowly but steadily towards renewables. The sheer force of the wind or the sun can run turbines and power entire cities. Imagine, then, watching TV on a particularly sunny Sunday afternoon, when your favourite show is rudely interrupted by a wayward cloud. A small price to pay? The issue, however, is much larger than minor discomforts to our daily lives.
One of the primary obstacles for making renewable energy mainstream is intermittency — a point that’s often brought up in debate. Technologies like wind or solar can generate power only when the wind is blowing or the sun is shining. Apart from making it difficult to meet second-to-second demands of modern living, intermittency could also threaten the stability of a power grid. The logical next step is to develop effective energy storage solutions that can kick in when the sun or wind bail out. A number of establishments are looking at viable options to produce the battery of the future. Tesla’s lithium grid battery seems like a promising (albeit expensive) option. Some innovators are even considering simple 8th grade physics to convert potential energy to kinetic energy — pumping water uphill or loading boxcars with rocks and running them up a mountain
In this mix of energy storage solutions, compressed air is a strong contender. It can be stored in pressurized chambers and released at will to generate power. Sounds simple enough? Here’s where things start to get complicated. Compressed air is stored at a pressure much higher than atmospheric pressure. To counter this pressure difference, the walls of the chambers must be thick. This greatly increases the cost of manufacturing, making this methodology commercially impractical.
However, under the sea, the pressure difference between the air inside the container and water can be equalized. The result is that compressed air can be easily stored in a container as thin as a balloon! Hyrdrostor, a startup in Canada capitalized on this simple principle of physics.
Excess electrical energy is used to compress air using an advanced compression system. The compressed air is stored in rigid-walled accumulators or energy bags that are anchored underwater (at least 25 meters deep and ideally 100 meters or more). Here, the air is held until there’s demand for electricity. On demand, the system reverses the flow of air, allowing the weight of the water to force the air back to surface under pressure.
A key challenge for implementation of this technology is the heat generated during the process. Over 90% of the energy used in compressing air gets converted into heat. Temperature of the air rises upto a whopping 1202 °F! This heat can be completely lost to the water, undermining the efficiency of the system. Hydrostor got around the problem with an off-the-shelf heat exchanger. This technology captures the heat during compression and adds it back to the airstream on exit, optimising the efficiency of the system.
When it comes to clean energy, a number of questions lie before us today. An underwater compressed air storage system could boost us into a utopian future. Sometimes, to find the right solutions, one has to delve deeper (quite literally).