Solar Geothermal - Part II

The day after Christmas, the geothermal installer called me (waking me up) and told me the well drillers were on the way. I began to protest, but then reconsidered since I had been waiting for months to get them on site. Actually I had feared that with the onset of mud season, our window of opportunity might have passed. The road was in terrible shape, but if the drillers were rarin' and ready to give it a try, so was I... I thought the road might just be in good enough shape to push and pull a well drilling rig to the top of the hill. I was ready for some fun.

Miraculously, the top heavy 60,000lb drilling rig teetered slowly up the dirt road to the top of the hill on its own power without going wheels up, but then it got mired in the mud at the top. I used my backhoe to pull him free, but the last 20 yards proved to be even tougher. The clay on top of our hill is slicker than fish snot, and it was starting to rain. The backhoe was not up to the task, so I got the bulldozer and shoved him up the bank into the front yard. I'll have to say that the driver of the well rig (the owner's nephew - 25 years old?) had a fine sense of adventure. After he was in position, he called it a day and rode home with his helper in their Ford truck.

A few days later, they showed back up ready to drill holes. I loaned them some timber cut-offs to stack under their front outrigger because the hydraulic outrigger didn't have enough extension to level the drilling rig on the hill side. It all looked a bit precarious, but the young drill operator was un-fazed. In the picture to the right, you can see one of the carousels holding the rusty drill extensions. I believe they had twelve of these 25 foot drill extensions, which enabled them to drill to 300 feet if need be. Our geothermal contractor usually bores to 150 feet, but he suggested we go another 50 feet in each well on our job to compensate for the height of the hill. His goal was to strike water, because if your geothermal wells are wet, the heat transfer to the surrounding soil is more effective.

Our whole family watched with great anticipation from a second floor window as they drilled the first hole. They began with a 6" bit for drilling dirt, and as soon as they hit the first layer of rock, they extracted that bit and replaced it with a 4" rock bit. The kids and I documented the color of the cuttings that were blown from the hole, as the drill progressed downward. Every 25 feet, the men operating the rig had to add another extension to the drill in the ground, so we could estimate the depth of the drill at any time. Red, brown, yellow, gray, blue-gray... and then from 50 feet and beyond, our notes recorded only varying shades of gray. We saw dark gray and wondered hopefully if that meant water.

After the drill reached 200 feet, the operators began extracting the 25 foot segments one at a time. The first three segments to emerge from the hole were no longer rusty - but instead shiny gray steel. The last five segments to come out had mud stuck to them - the operator confirmed that we had hit water at around 100 feet! After removing the drill from the hole, the operators inserted a 200 foot long loop of 3/4" black PE pipe all the way to the bottom. The loop was actually two pipes with a u-fitting fused to the bottom, so the total length of pipe in each hole is 400 feet.

The next three holes went exactly as the first had. The kids lost interest after the first hole, but I was captivated until the end. When completed, this geothermal system (aka ground source heat pump) will effectively multiply the energy of our solar panels by a factor of at least four - how could I not be excited about that?!

Rather than backfill with grout, the well drillers filled the holes back up with the cuttings from the boring process. I have my doubts about the thermal transfer efficiency of the cuttings (only about 30% of them went back into the hole!), but I speculate that the cuttings never went all the way down the hole and that the bottom of the hole is immersed in water. I think it will work just fine. I tried pouring some water into the holes after the drillers left, and the fine powdery cuttings in the top 5 feet of the hole turned to clay and would let no more water flow in. With the tops of the holes sealed so well, I doubt there is any chance of contaminating ground water with surface runoff - which I believe is the main reason some installers use grout.

A few weeks earlier, I had begun upgrading our inverter system, so it would have the wattage necessary to start the heat pumps. Unfortunately, one of the two new Outback inverters was dead - right out of the box. Not usually a problem since these have a two year warranty, but I had bought these inverters over two years ago and had left them sitting in the box since I didn't yet need the extra capacity. Slightly concerned about how they would treat the warranty situation, I called Outback, and they agreed to send me new boards for the bad inverter - free of charge. This is a class A company. Due to the aborted upgrade, our inverter system looked like the picture on the left while I waited for the boards to show up. Mind you - it was still functioning the whole time!

The boards showed up and I installed them in the inverter. The directions for installing the boards, written for professional installers not home owners (I consider myself somewhere in between), were nonetheless some of the most amusing technical writings I have ever read. One passage read... "Before getting into the unit you need to make sure you do not have a static charge built up on you or your clothes. The safest way to accomplish this is to stand naked in a mud puddle while disassembling the inverter. Another method would be to...(goes on to describe more conventional ways to avoid static discharge)" . I like a company that takes their customers seriously, while maintaining a sense of humor! With the help of the well written instructions, I successfully completed the inverter surgery without disrobing. Here's a picture of the upgraded system with four functioning inverters - capable of providing 14.4 Kilowatts of continuous power!

Incidentally, the heat pumps will collectively consume less than 4 Kw while running at full power (less than 30% of the inverter system's capacity). I upgraded our system to ensure that the lock rotor amps (startup current) of the heat pump compressors will not exceed the peak capability of the inverters. As an added safety margin, the geothermal contractor tells me that Waterfurnace (the brand of geothermal heat pumps we will be using) are supportive of this unique installation and will provide some special soft-start electronics for the compressor motors.

With all of these preparations, I am confident that these inverters will be able to start and run the two 2 ton units. Some day, running geothermal systems with off grid solar power may be common place. In the mean time, I'm happy to be the guinea pig.