Power from Dirt

Part of the year, I live in Vermont, where there is a lot of interest in renewable energy sources. They want to use wind or solar or wood or biofuels but almost all the tree-huggers skip the part about all those renewable energy sources combined would not meet the demand and we would still need a coal, gas or nuclear power plant to make up the difference. I decided to try to make up something that really could give enough energy for a household but would also work year round and be independent of weather, temperature and would use a fuel that is cheap and renewable. That is a big set of requirements and it took me several months to work out how to do it. It turns out that it can be done with dirt and some rocks and a little electronics.

As I have said many times, I worked for NRL and then DARPA while I was active duty in the Navy and then for other labs and in my own R&D company when I got out of the military. While I was at DARPA, they worked on an idea of using piezoelectric devices in the shoes of soldiers to provide electricity to low powered electronics. It turned out to be impractical but it showed me the power of piezoelectric generators.

I also work at NRL when they were looking into thermal-electric generators to be used on subs and aircraft. Both subs and planes travel where the outside is really cold and the insides are really hot and that temperature differential can be used to create electricity. I had a small involvement in both these projects and learned a lot about harvesting energy from micro-watt power sources. I also learned why they did not work well or could not be used for most situations back then but that was 22 years ago and a lot has changed since then. I found that I could update some of these old projects and get some usable power out of them.

I’ll tell you about the general setup and then describe the details. The basic energy source starts out with geothermal. I use convective static fluid dynamics to move heat from the earth up to the cold (winter) above ground level – giving me one warm surface (about 50 degrees year round) and a cold surface – whatever the ambient air temperature is, in the winter.

I then used a combination of electro and thermal-mechanical vibrators attached to a bank of piezoelectric crystal cylinders feeding into a capture circuit to charge a bank of batteries and a few super capacitors. This, in turn, powers an inverter that provides power for my house. The end result is a system that works in my area for about 10 months of the year, uses no fuel that I have to buy at all, has virtually no moving parts, and works 24×7 and in all weather, day and night. It gives me about 5,000 watts continuous and about 9,000 watts surge which covers almost all the electrical needs in my house – including the pump on the heater and the compressor on the freezer and refrigerator. I’ll have to admit that I did get rid of my electric stove in order to be able to get “off the grid” entirely. I use propane now but I am working on an alternative for that also. So, if you are interested, here’s how I did it.

The gist of this is that I used geothermal temperature differentials to create a little electricity. That was used to stimulate some vibrators that flexed some piezo-electric material to create a lot more electricity. That power was banked in batteries and capacitors to feed some inverters that in turned powered the house. I also have a small array of photovoltaic (PV) solar panels and a small homemade wind mill generator. And I have a very small hydro-electric generator that runs off a stream in my back yard. I use a combination of deep cycle RV, and AGM and Lithium and NiMH batteries in various packs and banks to collect and save this generated power. I total, on a good day, I get about 9,500 watts out. On a warm, cloudy, windless and dry day, I might get 4,000 watts but because I get power and charge the system 24/7 but use it mostly only a few hours per day, it all meets my needs with power to spare.

Today it was 18F degrees outside. Last night it was 8F degrees. From now until mid-April, it will be much colder than 50 degrees above ground. Then we have a month or less in which the air temp is between 40 and 60 followed by about 3 months in which the temps are above 70. Then another month of 40-60 before it gets cold again. That gives me from 20 to more than 40 degrees of temperature differential for 10 months of the year.

Using these two differential temperatures, I hooked up a bank of store-bought, off-the-shelf solid-state thermal electric devices (TEDs). These use “Peltier” elements (first discovered in 1834) to convert electricity to heat on one plate and cold on another. You can also reverse the process and apply heat and cold to the two plates and it will produce electricity. That is called the “Seebeck effect”, named after a guy that discovered it in 1821. It does not produce a lot of electricity but because I had an unlimited supply of a temperature differential, I could hook up a lot of these TEDs and bank them to get about 160 volts at about 0.5 amps on an average day with a temperature differential of 20 degrees between the plates. That’s about 80 watts. With some minor losses, I can convert that to 12 volts at about 6 amps (72 watts) to power lots of 12 volt devices or I can get 5 volts at about 15 amps (75 watts) to power a host of electronics stuff.

Then, I dug a hole in the ground – actually, you have to drill a hole in the ground. Mine is 40 feet but the deeper the better. It has to be about 10-12 inches in diameter. If you have a lot of money and can customize the parts and then you can use a smaller diameter hole. I salvaged the cooling coils off the back of several commercial grade freezers to get the copper pipes that have those thin metal heat sinks attached to them. I cut and reshaped these into a tightly packed cylinder that was 10″ in diameter and nearly four feet long, containing nearly 40 feet of copper pipes in a wad of spiral and overlapping tubes – so it would fit in my 40’ deep by 12″ inch diameter hole. Down that deep, the hole filled with water but the water was still about 50 degrees. I wrapped the heat sinks in several layers of wire fence material. This was aluminum screens with about ¼” holes. I used two long copper tubes of 1 inch diameter to connect the two ends of the coil to the surface as I sank them to the bottom. All the joints were soldered and then pressure tested to make sure they did not leak.

Just before and after it was sunk into the hole, I pushed some marble sized pea rocks into the hole. This assured that there would be a free-flow of water around the heat sink lines without it becoming packed with clay. I bought a 100 foot commercial grade water hose to slip over the two pipes to cover them from the surface down to the sunken coils. This hose has a thick hard rubber outside and soft rubber on the inside and had a 1.75 inch inside diameter. It was designed to use with heavy duty pumps to pump out basements or ponds. It served as a good sleeve to protect the copper tubes and to insulate the pipes. To insulate it further, I bought a can of spray expanding foam. The kind that you use to fill cracks and it hardens into a stiff Styrofoam. I cut the can open and caught the stuff coming out in a bucket. I then diluted it with acetone and poured it down between the hose and the copper pipe. It took about 18 days to dry and harden but it formed a really good insulating layer so that the pipes would not lose much heat or cold while the fluid moved up and down in the copper pipes. The two copper pipes sticking out were labeled “UP” and “Down” and I attached the down pipe to the bottom of a metal tank container.

The next part is another bit of home brew. I needed a large thin metal sandwich into which to run the “hot” fluid. To make it would cost a fortune but I found what I needed at a discount store. It is a very thin cookie sheet for making cookies in the oven. Its gimmick is that it is actually two thin layers separated by about a quarter inch of air space. This keeps the cookies from getting too hot on the bottom and burning. I bought 16 of these sheets and carefully cut and fused them into one big interconnected sheet that allowed the fluid to enter at one end, circulate between the layers of all the sheets and then exit the other end. Because these sheets were aluminum, I had to use a heliarc (also known as TIG or GTAW welding and I actually used argon, not helium) but I was rained by some of the best Navy welders that work on airframes of aircraft. The end product was almost 6 x 6 feet with several hose attachment points into and out of the inner layer.

I then made a wood box with extra insulation all around that would accommodate the metal sandwich sheet. The sheet was then hooked up to the UP hose at one end and to the top of the tank/container that was connected to the Down hose. Actually, each was connected to splitters and to several inlet nad outlet ports to allow the flow to pass thru the inner sandwich along several paths. This made a complete closed loop from the sunken coils at the bottom of the hole up the UP tube to the 6 x 6 sheet then thru the tank to the DOWN tube and back to the coils.

Now I placed my bank of Peltier solid-state thermal-electric modules (SSTEMs) across the 6×6 sheet. Attaching one side of the SSTEMs to the 6×6 sheet and the other side to a piece of aluminum that made up the lid of the box that the sandwich sheet was in. This gave me one side heated (or cooled) by the sandwich sheet with fluid from the sunken coils and the other side of the SSTEMs was cooled (or heated) by the ambient air. The top of the flat aluminum lid also had a second sheet of corrugated aluminum welded to it to help it dissipate the heat.

So, if you are following this, starting from the top, there is a sheet of corrugated aluminum that is spot welded to a flat sheet that forms the top of the box lid. Between these two sheets that are outside the box and exposed to the air, there are air gaps where the sine-wave shaped sheet of corrugated aluminum meets the flat sheet. This gives a maximum amount of surface area exposed to the air. In winter, the plates are the same temperature as the ambient air. In the Summer, the plates have the added heat of the air and the sun.

The underside of this flat aluminum sheet (that makes up the box lid) is attached to 324 Peltier SSTEMs wired in a combination of series and parallel to boost both voltage and current. The lower side of these SSTEMs is connected to the upper layer of the thin aluminum of the cookie-sheet sandwich. This cookie-sheet has a sealed cavity that will be later filled with a fluid. The lower side of this cookie sheet is pressed against the metal side of a stack of three inch thick sheets of Tyvek house insulation. The sides and edges of all of these layers is also surrounded by the Tyvek insulation.

I then poured 100% pure car antifreeze into the tank on the copper up/down tubes. I had to use a pump to force the antifreeze down to the coils and back up thru the cookie sheet back to the tank. I ran the pump for about 6 hours to make sure that there was no trapped air anywhere in the system. The tank acted like an expansion tank to keep the entire pipe free of any trapped air. The antifreeze was the thick kind – almost like syrup – that would not freeze at any temperature and carried more heat than water would.

It actually began to work very fast. The top of the large flat hollow sheet had filled with fluid and it got cold from the ambient air. This cooled the antifreeze and the cold fluid wants to sink down the DOWN pipe to the sunken coils at the bottom of the hole. The coils meanwhile were heating the fluid down there to 54 degrees and that wanted to rise up the UP pipe. As soon as the heated fluid got up to the top, it cooled in the hollow sheet and sank down the DOWN tube again. This is called dynamic convective thermal fluid circulation or some just call it thermal siphoning.

The transfer of heat up to the surface creates a continuous temperature differential across the plates of the Peltier SSTEMs and then they create about 160 volts of DC electricity at about 0.5 amps or about 80 watts of electricity. I needed to use a solar panel controller to manage the power back to a usable 12 to 14 volts to charge a bank of batteries. But I am not done yet.

I added a second flat aluminum sheet on top of the corrugated aluminum- like a sandwich. This added to the surface area to help with heat dissipation but it also was to allow me attach 100 piezoelectric vibrators. These small thin 1.5″ diameter disks give off a strong vibration when as little as 0.5 volts are applied to them but they can take voltages up to 200 volts. They were 79 cents each from a surplus electronic online store and I bought 100 of them and spaced them in rows on the aluminum lid. Along each row, I placed a small tube of homemade piezoelectric crystals. I’m still experimenting with these crystals but I found that a combination of Rochelle salt and sucrose work pretty well but more importantly, I can make these myself. I’d rather use quartz or topaz but that would cost way too much.

The crystal cylinders have embedded wires running along their length and are aligned along the rows of piezoelectric vibrators. They are held in place and pressured onto the vibrators by a second corrugated aluminum sheet. This gives a multi-layer sandwich that will collectively create electricity.

One batch of the SSTEMs is wired to the 100 piezoelectric vibrators while the rest of the SSTEMs feed the solar controller to charge the batteries. I had to fiddle with how many SSTEMs it took to power the vibrators since they will work with a very small amount but they do a better job if they are powered at a higher level.

The vibrators cause a rapid oscillation in the cylinders of Rochelle salt and sucrose which in turn give off very high frequency, high voltage electricity. Because the bank of cylinders is wired in both series and parallel, I get about 1,500 volts at just over 200 milliamps, or about 300 watts of usable electricity.

It takes an agile-tuned filter circuit to take that down to a charging current for the batteries. I tried to make such a device but found a military surplus voltage regulator from an old prop-driven aircraft did the job. This surplus device gave me an initial total at a continuous 13.5 volts DC of about 22 amps charging power fed into a bank of deep cycle AGM batteries.

I found that the piezo vibrators had a secondary and very unexpected positive benefit. Since the vibration was also felt in the circulating antifreeze and the SSTEMs, it seems to have made them function more efficiently. There is more heat transfer in the dynamic convective thermal fluid circulation than the normal formulas and specs would dictate but I think it is because the vibration of the fluid makes the thermal transfer in the cookie sheet panel more efficient. The SSTEMs are boosted in output by several watts of power. So when everything is running, I am getting about 340 watts of charging power on a continuous basis. Of course this fluctuates as the temperature differential changes but I rarely get less than 250 watts and sometimes as high as 400 watts.

A local recreational vehicle (RVs, trailers, campers, boats) dealer removes the very large deep cycle AGM batteries from their high-end RVs’ solar systems even when they have been used very little. He lets me test and pick the ones I want and sells them for $20. I have 24 of them now that are banked to charge off the thermal and piezoelectric devices and then feed into several inverters that give me power for lights, heaters, fans, freezers, TV’s etc. The inverters I use now give me up to 5,000 watts continuous and up to 12,000 watts of surge (for up to 2 hours) but I have set the limit of surge to 9,000 watts so I do not damage the batteries. The 24 deep cycle batteries could give me 5,000 watts continuously for up to several days without any further charging but so far, I have found that I am using only about 25% to 35% of the system capacity for about 80% of the time and about 80% of the capacity for 20% of the time. The high usage comes from when the freezer or refrigerator compressors kick on and when the heater boiler pumps kick on. As soon as these pumps start and get up to speed, the load drops back to a much lower value. The rest of the time, I am just using CFL and LED lights and my computer.

I finished this setup in September of 2011 and it worked better and better as the winter temperatures dropped in November and December. I had to make one adjustment. The piezo vibrators made too much noise and the aluminum plates made them sound even louder. I have since added some noise dampening and now I can’t hear it unless I am outside and standing near it. The dampening I used was two 4’x8’ sheets of thick heavy-duty foam used to put in horse and cattle stalls to keep the animals from standing on freezing cold ground. These were $30 each and have isolated the sheets from the wood and metal frame but still allows the vibrators to do their thing on the piezo tubes and the cookie sheet SSTEMs.

I have lots of meters and gauges on the system to monitor temperatures and power outputs and levels and so far nothing seems to be fading or slowing. There are slight changes in the charge levels of the batteries due to changes in the ambient air temperature but that has been less than +/- 10% so far. I was concerned that the cold antifreeze would freeze the water around the sunken coils but so far that has not happened. I think it is because there is a fairly rapid turnover of water at that depth and the coils just don’t have a chance to get that cold.

I’m also going to experiment with rewiring the whole thing to give me perhaps 60 volts output into the bank of batteries that are wired in series to make a 60 volt bank. This is the way that electric cars are wired and I have obtained a controller out of a Nissan Leaf that uses a bank of batteries in a 60 volt configuration. It should be more efficient.

The whole system cost me about $950 in batteries, fittings, hoses and chemicals and a lot of salvage of used and discarded parts. I already had the inverters and solar controller. I also had a friend that drilled the hole for me – that would have cost about $400. The rest I got out of salvage yards or stripped off old appliances or cars. It took about 3 weeks to build, working part time and weekends. I estimate that if you had to pay for all of the necessary parts and services to build the system; it would cost about $3,000. By the end of next year, I will have saved about half that much in electricity. As it is, I will have a full payback by about March of 2013.

I still have a hookup to the city power lines but since installing this system, I have used only about 10 kilowatts. I think that was mostly when I used my arc welder and did not want to suck too much out of the batteries. A side benefit has also been that since September, when I first started using it, there have been 4 power outages – one lasted for two days….for my neighbors. I never lost power.

I have not done it yet but I can also setup an electric meter that allows me to sell electricity back to the power company. When I integrate this whole system with my solar PV array, I might do that but for now, I can store unused power in the batteries for later use and since I won’t run out of fuel, I don’t need to recover any extra ongoing costs.

Since this system has no expendable fuel, no moving parts, no maintenance and no costs, I expect it will be functional for the next 15 to 20 years – maybe longer. Actually, I can’t think of why it will ever stop working.

Sept 2012 Update:

This system has been running for a year now. My power company monthly bill is averaging about $28/mo. But it goes lower almost every month. I am still running several AC units during the summer and have used ARC welding and electric heaters in the winter.

My estimate of costs and maintenance was a little optimistic as I discovered my 79 cent piezo vibrators were worth every penny – they lasted about 6 months. I have since replaced them with a bunch of salvaged parts used in claxon alarms on board Navy ships. These normally run on 28 volts but I did not need them to be loud so I found that if I fed them with 4 volts, I got the vibration I needed without the noise and they are so under-powered that they will likely last for years.

During the Spring and Fall, the system was not too productive because the temperature differential was usually less than 10 degrees but in the hottest part of the summer, I was back up to over 300 watts of total output with differentials of 20+ degrees between the 50 degree ground and the 70 to 85 degree summer heat.

I was not hit by the floods of last Spring but my place in the woods experienced torrential rains and the water table rose to nearly the surface. In all that my system continued to work – in fact, I noticed a slight improvement in performance since the temperature exchange rate improved with the heavy flow of underground water.

I still have not hooked up to a reversing electric meter but I did calculate that I would have made a net $290 over the past year instead of paying out $28/mo average. If I added in my solar PV system, my small $400 vertical wind generator and the 60 to 100 watts I get from a tiny hydroelectric setup I have on a small steam that runs thru my property, I would have had a net gain of over $1,000. Not bad for a bunch of junk and surplus parts and a little help from the dirt under my lawn.

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