Geothermal Series – Part 6

Introduction

Building a tank is a difficult and expensive task. However, it is just as difficult and just as expensive to find a tank that is appropriate, or have one custom built. In this post, we will go over the design and construction of the geothermal tank.

If you are not up to date on the previous posts in this series, please take a minute to read them.

Part 1: DIY Geothermal System Design
Part 2: Geothermal Trenches – Digging for Energy
Part 3: Ground Loops
Part 4: Geothermal Mud
Part 5: Sensor Network

Design

Purpose

The purpose of the tank is to provide a means to submerge the air conditioner condenser in the geothermal fluid so that the heat can be exchanged between the fluid and the condenser. A method for moving the fluid in and out of the tank needs to be established, as well as a path for the refrigerant to flow in and out of the tank.

Condenser

The condenser has a 90 degree bend in it as it sits in the outdoor unit of the mini-split. I briefly considered making a tank big enough to fit the condenser in it as is, but that would require too much geothermal fluid to fill the tank.

If I could modify the condenser to be straight, I could make the tank much smaller and use less geothermal fluid. With a straight condenser, I could make the tank 48 inches wide, 36 inches tall and 8 inches deep.

Penetrations

The next consideration for the tank are the penetrations. While it would be nice to make a tank without any holes, we have to get water and refrigerant in and out of the tank. In order to accomplish this, I found bulkhead connectors that I could put through the holes. These connectors had a pipe thread on the inside that fit could take in a PEX connector. For the refrigerant, I piped those up through two holes near the top of the tank, above the water line, where the risks for leaks was minimal.

Flow Considerations

The tank needs to flow water over the coils of the condenser. The condenser is wide and tall, but thin. The flow enters through the large face and exit through the other large face of the condenser. This maximizes the surface area of the coils to the flow. I decided to try and duplicate this setup in the tank design.

It is easier to do one entry point and one exit point for the water inlets, but that limits the flow and allows for dead spots where the fluid can stagnate. Instead, I decided to have four inlets and four outlets for the main circulation pump. The inlets and outlets are arranged in a 2×2 grid with the rows being 1/3 the distance from the top and bottom with respect to the designed water level. The columns are 1/4 distance from each side. The inlets and outlets each connect to their own manifold to combine the flow into a single pipe.

The is another inlet and outlet pair connected to a separate pump, that as of now is set to circulate the fluid. My future plan is to use that pump to cycle fluid through a solar heater during the winter months to help improve overall efficiency. However, that is much later.

Tank Construction – Take 1

After I decided to build my own tank, I needed to figure out what materials I wanted to use and how to assemble it. I decided on using 3/4 inch plywood and facing it with fiberglass with vinyl ester resin. I’ve seen this on videos of people who build boats, and I thought “How hard could it be?”.

After I built the tank, I immediately realized the problem that I had. While the tank had large flat surfaces to fiberglass, it was completely closed on five sides and the opening was 48 inches wide and only 8 inches tall. With an opening that narrow, I could not get the fiberglass material down inside and roll it into the corners. That doesn’t mean that I didn’t try!

After going back and forth on whether or not I could fix the problems, I decided to abandon that design altogether. And given the lessons I learned on the next iteration of the tank, I’m glad I did.

Tank Construction – Take 2

For the second attempt, I decided to still use the same materials, but change how I assembled the tank. I would first make the bottom of the tank and fiberglass the inside. Then I would make a box to extend the tank up to the desired height. That part would also be fiber-glassed ahead of time. Then I would use a lap joint to put he two parts of the tank together. I would then put two layers of fiberglass along the seam in the inside of the tank.

I got the box completed and I felt good about it. I drilled out the holes for the bulkhead connectors for the fluid ports and sealed those up with epoxy and glass bead mixture. Everything looked good.

Testing

Then I tested the tank by filling it with water. At first, everything looked good, but as soon as the water got above the joint connecting the two parts of the tank, water began to leak. It took a while to realize that the water was coming from the joint, because it ran down the inside of the fiberglass covering the outside of the tank and dripped from the bottom.

After I drained the water and let everything dry out completely, I started to inject resin into the seam from the outside using a syringe. I was amazed at how much resin the seam took, considering I pre-sealed the surfaces with resin ahead of time. I also applied several coats of resin to the inside surface to help seal any places where there may have been holes. Even after all that work, the tank still leaked.

Last Effort

By this point, I had put a lot of effort into this tank and wasn’t ready to give up on it yet. I decided to try Flex Seal to see if that would help. I bought two cans of the roll-on type and went to work coating the inside of the tank. Three layers were applied on the inside, giving each layer a full day to cure. Then I tested the tank again, and while it did better, it still leaked!

Tank Construction – Third Time’s a Charm!

After the previous two attempts, I was frustrated and disappointed. I decided to switch things up. I’m admittedly not much of a woodworker. Nor do I have any skills doing fiberglass. But I am pretty good with metal. So I decided that the next tank would be made of metal.

Type of Metal

Since this was a water tank, there were only two types of metals that I would consider. Stainless steel and aluminum. After checking on the price and availability of both materials, I was starting to reconsider this option. Then I remembered that my neighbor had a large sheet of 1/4″ aluminum plate that he said I could have. I found that I could make most of the tank with this plate and just have to buy a few feet of plate to make the sides and the lid.

With aluminum as my medium, I then had to figure out how to put it together. The problem was that I didn’t own a TiG welder at the time. So I ended up getting a spool gun for my MiG welder and that would allow me to MiG weld aluminum. I only had a few small pieces to practice and get my settings dialed in, but I felt that they were close.

Construction

I was able to use my plasma cutter to cut the large plate into the desired sizes. Then I 3D printed a template to use the plasma cutter to cut the holes for the bulkhead connectors. I welded some pieces of aluminum angle to the insides of the panels to help hold them in the proper place while I tacked everything together.

From there, it was a matter of welding. I welded the sides of the tank first. That way, I could weld one corner, then do the opposite corner to keep the heat down. I found that welding aluminum is very tricky when it comes to heat soak. You have to let it cool down between welds. A lesson I would soon learn.

More Problems

When I got to the bottom plate, I had to weld four sides of one plate. Two 48 inch seams and two 8 inch seams. By this point I was a little too confident and I welded the two 8 inch seams first as they were the furthest apart. But then, without thinking, I immediately started welding one of the two long seams. It started out fine, but then the heat caught up and started to create some problems. The weld still looked okay, but not great. While I waited for a while to start the second long seam, it wasn’t long enough. And to further compound my problems, halfway through the weld, I ran out of shielding gas.

After a refill of the gas, I was able to finish the weld, but the two long seams looked terrible. I touched up what I though needed touching up and then went to testing.

Testing

I filled the tank up and immediately had leaks from those two long seams. The good thing about it was that it was easy to identify where the leaks were happening. I circled them with a marker and then drained the tank and did some spot welding to fill the little holes and tried again. I went about this for days! It seemed that every time I fixed a spot, a new spot opened up next to it.

I finally had to grind out the welds on the bottom and start completely over. That ended up fixing the problem. However, I wasn’t going to feel confident in this tank over the long term, so I poured in enough vinyl ester resin into the bottom of the tank to fill it up about an eighth of an inch. That would help make sure any little pin holes were completely sealed. Testing of this setup went well, but highlighted another problem.

Thermodynamics

I noticed during my testing that condensation was quick to form on the outside of the tank. That meant that the temperature gradient between the tank and the outside air was large enough to cause the moisture to collect on the surface. This created a problem in identifying leaks, also showed one of aluminum’s flaws in this scenario.

In thermodynamics, heat moves from areas of higher temperatures to areas of lower temperatures. The rate at which it does this is based on the thermal conductivity of the material. For example, the insulation in your house has a low thermal conductivity. It slows the rate at which your house changes temperature, which allows your A/C or heat to keep up.

Unfortunately, Aluminum has a very high thermal conductivity. Which means that the temperature between the inside of the tank and the outside of the tank will reach equilibrium very quickly. This is the opposite of what is desired in this situation. During the summer, we want the inside of the tank to be cool, and during the winter, we want the inside of the tank to be warm. That is hard to maintain with aluminum.

In order to combat this problem, I had to insulate the tank. I went with two layers of protections. The first was 1 inch thick insulation panels that were cut to size and then sealed with duct tape (the real stuff!). Then I built a wood frame around the tank (which I already planned to do) and filled the spaces between with fiberglass insulation.

Thank You

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