The simplest version basically only requires TWO components, both used automobile components. One is a car radiator and the other is an electric radiator fan. They may cost you nothing up to maybe $20 total.
A used car radiator 12-volt fan blower.
A used car radiator, filled with cold 51°F underground city water to cool room air.
This version is based on the fact that ALL municipal water systems bury their city water lines 3 feet deep, primarily to avoid winter freezing of their water pipes. It turns out that the Earth is really massive, and deep soil always remains at very constant temperature. In the Chicago area, all municipal water lines are therefore ALWAYS at 51°F.
When you turn on your COLD water (anywhere near Chicago) after a few seconds the water always comes out at 51°F. Pretty much, FOREVER!
For this version you then just need to use any type of insulated pipe or hose to connect the house cold water supply to this vehicle radiator, and another piece of pipe or hose to carry the used water from the radiator to any house drain.
The radiator is stood NEAR a wall, with the electric fan behind it, to blow hot ( 90°F ) room air THROUGH the radiator and be cooled by the cold 51°F water inside the radiator, and then the cold air is sent out into the room.
A conventional air conditioner is designed to send air out into the room at 62°F temperature, which is wonderfully cool for the occupants. THIS version of the free air conditioner is even better, sending even cooler air out into the room, usually at about 55°F. When the room gets too cool, you actually have three different ways to reduce the cooling! You can partially reduce the cold water flow. You can slow down the speed of the radiator fan, by reducing the voltage supplied to the 12-volt DC motor. Depending on how warm the water was that left the radiator, some or all of that water could be recirculated through the radiator a second time.
This whole system usually costs less than $20 to install, and it can be installed in less than half an hour!
One result is that your municipal water bill will be slightly increased.
In my 11-room house near Chicago, I put four used car radiators in my Master Bedroom, my Office, my Living Room, and a Guest Bedroom. I also made a smaller one for my Doghouse which uses a car heater core, but my dogs do not know how to turn the cooling on and off! For all the air conditioning for the month of August 2021, my four room coolers water bill and electric bill for the whole house has been around 80 cents for that month.
This version is virtually the same as the simpler version described above, with a few differences. NO municipal water is needed or used, so there is no increase in municipal water bills. The water is recirculated forever! You need to buy and use an electric recirculating water pump to raise the water up the central PVC pipe, so there is a slight increase in the home electric bill.
For either of these two versions, any such energy system must be analyzed regarding every single step of its operation, and the WORST performing step is the one that limits the performance of the system. We can list the separate steps here: (1) heat must flow THROUGH the earth so that undesired heat can flow AWAY from any buried component, which in this case may be a discarded (buried hot water heater tank (which does not leak!). (2) next heat from such a warmed tank must be able to conduct into the adjacent cool soil which is surrounding it. (3) Next is the warm water inside the tank must send its heat into the metal of the tank. (4) Next, some (possibly insulated) piping is required to move the cool water from the tank up into a house and then the warmed water back down into the tank. The pipes must have sufficient diameter to allow adequate water flow and also a small pump must recirculate the water around this circuit. (5) Next, the cool water which came up from the tank must pass through a heat exchanger, which in this case will be a standard car radiator (NOT a car heater coil, which is too small!). (6) And finally, a fan needs to blow room air through that radiator to cool the room air due to the cooler water inside the radiator. We find it simplest and cheapest to use a standard 12 volt modern electric car radiator fan, but only running on SIX volts instead of twelve volts, which makes the fan nearly silent (while car radiator fans are generally pretty noisy due to their very high spin rate). A standard 110-volt room cooling fan will work. but usually it does not blow the air hard enough to blow it through a car radiator.
Buying the circulating water pump increases the cost of this version up to about $40, and the labor of using the auger to make the borehole can cause this version to take a day to install.
Where the two simpler versions described above are COLD WATER based systems, this much larger version is a deep soil COLD AIR based system, which involves a LOT of trenching (500 feet, all over the yard) so installation often takes several days. It also involves buying 500 feet of 4" PVC (buried) air tubes to collect the coolness (which can cost $500). But there are no visible car radiators inside the home!
Complete instructions are provided in Academia pages on the internet.
Deep soil temperature is different in various locations on Earth. In Western Canada, the soil is even cooler, so these systems work even better. Near the EQUATOR, deep soil is always very hot, so these systems tend to appear to work minimally well.
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