this post was submitted on 01 Aug 2023
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Do It Yourself
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Make it, Fix it, Renovate it, Rehabilitate it - as long as you’ve done some part of it yourself, share!
Especially for gardening related or specific do-it-yourself projects, see also the Nature and Gardening community. For more creative-minded projects, see also the Creative community.
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I'll preface with my qualifications, so if a more qualified person comes along you can disregard me. I'm an engineer who has taken a few thermodynamics courses and has worked as an engineer for a hvac manufacturing plant. I've never done anything strictly related to geothermal, but I've read a decent bit about it (and watched Technology Connections' video on the subject, it's a good entry point)
You may want to call up a company who does geothermal cooling and see what options you have, they've gotten pretty creative on how to bury the cooling lines. (See the video mentioned before)
Going the route of just sticking a large water tank underground probably won't do a ton. I expect that you will have a poor surface to volume ratio, which means poor heat transfer, which means you'll saturate your thermal mass fairly quickly. What this may allow you to do is run your HVAC system during the night/morning when it's much more efficient, and 'charge' your thermal mass for the hottest part of the day.
Assuming you use 300kg of water in a day, and you can get a 10°C delta, my very rough back of the napkin math says you're only going to have about 3 kWh of cooling from just the cold water, which is a decent bit, but it's not a ton. Best case scenario you cut your cooling needs by around 10-20%.
I'm too lazy to do the math of the heat exhange with the ground, but my bet would be you're better off spending any money you have set aside for this on better insulation techniques and/or a proper geothermal cooling system.
I do like your creative idea though
As regards your point on the budget, i don't think that two car radiators, some insulated tube and a small water pump will cost much, more so if I can run the pump off a small solar panel in the day time.
All said and done, if there is nothing in theory that advises me against trying this, i would like to give it a shot and document it so that someone else might learn from it.
Thanks again!
Engineers describe heat transfer with a "heat transfer coefficient", and the rate of heat transfer is this coefficient multiplied by the temperature difference. So you can calculate what the heat transfer coefficient must be by measuring room air temperature initially, water temperature initially, and then running your system for a little while and measuring the room temperature again. The smaller room area you can cool the more accurate this will be. You will need to look up heat capacity and density of air (easy to find), and the temperature change of the air with the volume of the room and the temperature change will together give you an amount of heat you removed from the air to the water. Simple!
While I have a background in science, this is not the specialization i chose and so almost all of it is almost Spanish (won't say Greek, since at least I've heard of the words) to me.
I thank you for summarising the science of it though. It gives me enough keywords to start learning.