# Energy output calculations for the very big heat exchanger [closed]

It recently emerged the concept of using deep wells as heat exchangers. It can be visualized as a long (2000 m) vertical tube with an inner diameter of 162 mm (the outer pipe diameter is 177.8 mm, made of N-80 grade steel). Inside is another tube with a 73 mm outer diameter and 62 mm inner diameter, made of J-55 grade steel. Theoretically, there shouldn't be any contact between the tubes, and the inner tube is placed more or less in the center. The inner tube is 10 meters shorter than the outer tube and has an open hole at the bottom.

At a depth of 2000 m, we have a temperature of 70 degrees C and a geothermal gradient of 3 degrees C per 100 m. Hence, the temperature decreases by 3 degrees C every 100 m as we go up. We can have two circulation modes:

1. Direct: water goes down through the inner pipe and comes up between the inner and the outer pipes.
2. Reverse - water comes up through the inner pipe.

Question 1: Which flow is optimal from a thermodynamic point of view?

Question 2: What circulation rates (m³/day or l/s) can we have in both cases? What incoming temperature (hotter) can we have at the surface? How much can we take (20-30-etc degrees) in the secondary contour? What energy quantities in Watts can we have here?

Where I should start with my calculations? The biggest problem i can see is that water, which flows down will be cooling the up-flowing water and vice versa.

Thank you all very much in advance!

Dainius

• What about the issue that you are cooling the surrounding formation rock as time progresses, thus lowering the rate of heat transfer to the fluid? Commented Jul 12 at 12:13
• So what you're describing is the idea of geothermal energy. Except that you might go even deeper (depending on geology) and aim for a cavern that is already filled with water so you get a bigger surface area to exchange heat. Also, you're isolating the tubes from each other (there are coaxial systems but with isolation, but in most cases you want two bore holes)... Commented Jul 12 at 12:30
• @kruemi, two boreholes means big energy consumption, because usually lifting fluid from the formation is easier than injecting it back into another wellbore. Using "well as heat exchanger" concept we are making a large vessel from the wellbore (approx. 30.000 liters), isolating it from formation (any water bearing horizons) and circulating this fluid inside the system. More on topic: thinkgeoenergy.com/… Commented Jul 12 at 12:59
• @ChetMiller, very good point - during time we are cooling surrounding rocks to some extent, but the main thermal energy source is from U-Th decay reactions in crystalline basement and water horizon in Cambrian which is ~160 m thick and extends laterally in 20-30 thousand square kilometers, so, I assume that in a scale of human lifetime this effect could be negligible. Commented Jul 12 at 13:02
• @Dainius there is phyically no difference if you're using one hole with a coaxial tube or two holes connected to the same well. We always have the same system of communicating tubes. You want a closed loop system? That's nice but still the same. Just keep in mind that over 2000m in a steel pupe both liquids will have basically the same temperature at the surface. The hot water will have give all its heat energy trough the steel tube to the cold water in the other tube. Commented Jul 12 at 13:04