<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>hydrox24</author><text>The short answer is that pumped hydro is mature technology with a pretty wide range of places it can be implemented (at least in Australia[0], which has a mountain range going down the east coast where everyone lives). A-CAES has three advantages, which are in my opinion aren&amp;#x27;t very fundamental:&lt;p&gt;1. It takes up less space on the surface than most PHES. This... is almost always a marginal benefit.&lt;p&gt;2. It doesn&amp;#x27;t require building a reservoir or dam. These are very well regulated in Australia and elsewhere, and the downsides are known so they are quite slow to get approval.&lt;p&gt;3. It&amp;#x27;s a bit quicker at 2.5 - 3.5 years as opposed to 3-7 years.[1] This is a bigger advantage than it looks if you have some tricky renewable energy targets to hit by 2030 (see our 42% emissions reductions target as well as an 82% renewable energy target)&lt;p&gt;I can&amp;#x27;t see this gaining traction outside of a few locations in Australia, at least. I wouldn&amp;#x27;t be surprised if A-CAES is only briefly viable as a result of subsidies and cheap government financing.&lt;p&gt;[0]: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;re100.anu.edu.au&amp;#x2F;#share=g-fa5a20c9c63f6ed6343a7e7573a05fe4&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;re100.anu.edu.au&amp;#x2F;#share=g-fa5a20c9c63f6ed6343a7e7573...&lt;/a&gt;&lt;p&gt;[1]: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;www.csiro.au&amp;#x2F;-&amp;#x2F;media&amp;#x2F;Do-Business&amp;#x2F;Files&amp;#x2F;Futures&amp;#x2F;23-00033_SER-FUT_REPORT_RenewableEnergyStorageRoadmap_WEB_230310.pdf&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.csiro.au&amp;#x2F;-&amp;#x2F;media&amp;#x2F;Do-Business&amp;#x2F;Files&amp;#x2F;Futures&amp;#x2F;23-00...&lt;/a&gt;</text><parent_chain><item><author>greenbit</author><text>I must have missed something. Why not just use the water without the compressed air, i.e., pumped hydro? There must be some advantage, but they didn&amp;#x27;t seem to say. I&amp;#x27;d guess maybe if your lower reservoir were underground, the water-only would require the generator systems to be down there, too, which would mean access for people as well, and being down a mine with a small lake&amp;#x27;s worth of water overhead seems pretty hazardous. Whereas by forcing air down to push water up, that whole below ground aspect can be almost entirely passive. Maybe?</text></item></parent_chain></comment><story><title>A company is building a giant compressed-air battery in the Australian outback</title><url>https://www.wired.com/story/hydrostor-compressed-air-battery-california-australia-energy-climate/</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>foota</author><text>I think you can store energy here not just in the gravitational potential energy of the water, but also in the compression of the air. I _think_ this means that you can get away with a smaller cavern than you could for just pumped hydro.&lt;p&gt;You might think that you&amp;#x27;d need a large amount of water to make the energy release work, but I think it works like this. The force of the water on the air&amp;#x2F;water interface is dependent not on the reservoir volume, but on the weight of the water in the column (which depends only on the height of the shaft).&lt;p&gt;By digging a very deep shaft, you can have a very large force of water on the interface, and moving that interface an equal distance hence releases more energy than it would with a shorter shaft.&lt;p&gt;This way, you can store an arbitrary (up to your ability to compress air to a sufficient pressure, dig a deep shaft, and keep everything from blowing up) amount of energy.&lt;p&gt;I think if you have a 1 square meter cross section of shaft, and the shaft is 1 kilometer deep, then at the bottom the force of the water above is the weight of 1 square meter * 1 kilometer, or 1000 cubic meters of water, or 1 million kg.&lt;p&gt;The force then is 9.8×10^6 newtons.&lt;p&gt;Pressure is force&amp;#x2F;area, or 9.8×10^6 newtons&amp;#x2F;square meter here (since we have a unit area).&lt;p&gt;There&amp;#x27;s a formula for the energy in compressed air, it&amp;#x27;s... involved. I downloaded an excel file from here: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;ehs.berkeley.edu&amp;#x2F;publications&amp;#x2F;calculating-stored-energy-pressurized-gas-vessel&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;ehs.berkeley.edu&amp;#x2F;publications&amp;#x2F;calculating-stored-ene...&lt;/a&gt;&lt;p&gt;It says under this pressure, a 1000 cubic meter tank of air at this pressure stores ~5000 KWh.&lt;p&gt;The one planned in California is supposed to store 4000 MWh, so I guess they have a tank that is ~a million cubic meters.&lt;p&gt;A water tank of the same size would store ~2700 MWh of energy (e.g., pumping that much water up a 1KM shaft requires that much energy), so it does seem to be more efficient.&lt;p&gt;It may also be that hot air turbines are cheaper and easier to maintain than hydropower turbines, but I&amp;#x27;m not certain.</text><parent_chain><item><author>greenbit</author><text>I must have missed something. Why not just use the water without the compressed air, i.e., pumped hydro? There must be some advantage, but they didn&amp;#x27;t seem to say. I&amp;#x27;d guess maybe if your lower reservoir were underground, the water-only would require the generator systems to be down there, too, which would mean access for people as well, and being down a mine with a small lake&amp;#x27;s worth of water overhead seems pretty hazardous. Whereas by forcing air down to push water up, that whole below ground aspect can be almost entirely passive. Maybe?</text></item></parent_chain></comment><story><title>A company is building a giant compressed-air battery in the Australian outback</title><url>https://www.wired.com/story/hydrostor-compressed-air-battery-california-australia-energy-climate/</url></story>