The convection loop started in reverse again this morning, so I re-plumbed the water side of the system with the hot side going in to the upper element port rather than through the lower element port and the chimney pipe. That didn’t work either. Since I was using shark-bite fittings on the solar loop, I switched the solar hot and cold lines just so see if that would make difference. Nope. I switched them back, cooled down the whole system by refilling it with cool water, and it immediately started pumping in the correct direction and the solar side stayed much cooler than yesterday when the convection loop was flowing in reverse.
We installed 6 sensors on the tank and 4 on the solar loop plumbing around the tank. Tomorrow well install some thermocouples on header and maybe down into some of the evacuated tubes.
It seems that this project is not going to be trivial. We set the 30-tube collector up and started to take temperature readings. It is definitely heating the tank. The thermometer inserted into the shell and against the upper part of the tank shows that the temperature of the tank reached 140F, but the hot side of the heat exchanger never got above 120F. I started looking more closely and burned my ankle when it rubbed against the cold side of the hx. I put a thermometer on it and it showed 175F, just a little below the 180F solar loop.
I was running the solar loop pump at about a gallon per minute at almost no delta-T. I thought that the low delta might be the problem, so I slowed the pump with an in-line valve. When I slowed the pump to about 1/2gpm the header started to boil and steam. That means that a pump rate below 1/2gpm may not be fast enough to transfer heat from the system. So, the geyser pump will have to be faster than that. We don’t have a good measure for how fast a double cricket flows. I will need to look back at the excel program Eldon and Dale wrote to size dragon systems. The current system does have a flow meter on the solar loop. We haven’t calibrated it, but it seems to be working well. I am gong to buy some hall-effect flow sensors to see if we can use them for determining flow rates in sealed systems.
While insulating the solar loop and substituting copper for pex pipe at the head I dropped the pump/flow-meter segment and broke the fittings out of the pump. I ordered a replacement Friday night from Amazon and it arrived by 10AM Sunday! It’s running (in reverse), but it’s heating the tank.
I’m going to pull out the chimney tube and seal it better. I noticed that it was loose in the brass fitting when I inserted it.
Installing the Heat-Pipe Tubes
To get some baseline data on the 30-tube array and the heat-exchanger we installed the collector rack above a fence in my front yard with a 50 gallon tank and a SolarPad heat exchanger. The large can on the end of the header is an expansion tank open to the air. This would be untenable in a normal system, but it will allow the water in the system to expand without breaking anything. Because we installed it directly above the electric pump, and because it was on the low side of the header (that started out to be level) we had to move it to the other end.
We had the system up and running by noon and decided to go get some lunch. When we returned the the system had stagnated and did not seem to be pumping. We decided to install an old flow meter in line with the pump to see for certain. We did the plumbing in a hurry without covering the collectors with a tarp. We refilled the system and started up the pump. It worked for a few minutes, then ground to a halt, but the motor still seemed to be turning with no flow. We swapped out the pump, and the system ran for the rest of the day. We also installed a ball valve above the pump so that we can now control the flow rate.
Next on the list: install the Raspberry Pi with 20 sensors and a pump relay. We’re going to build a simple pyranometer so that we have the ability to compare runs from one day to the next. We tested the Pi last summer and developed a very reliable python data logging program. We are still wrestling with the geyser pumper design for this much larger system.
I just received a shipment of tubes and heat-pipes for my next series of geyser pump /heat pipe experiments. I am going to attempt to geyser pump a 30-tube collector array. I will start with a standard Apricus 30-tube header acting as an evaporator for a heat-pipe. The condenser for the heat-pipe will be the exterior surface of a parallel array of geyser-pumping lifters. Last August, I was able to pump a 10-tube collector with a single lifter, but the pump rates were insufficient for a high efficiency system.
I’ve also purchased a lot of Raspberry Pi and Arduino boards and sensors to instrument the experiments. I will try to be more diligent in my posting of developments this summer.
I have decided to test just the effectiveness of the heat-pipe header’s ability to transfer heat to a condenser pipe inserted through it. I have built several prototypes that use geyser pumping to transfer the working fluid through the heat-pipe condenser, but the results were confusing, so I’m going to decouple the problem by eliminating the geyser pumping in lieu of an electric pump that will pump the tank water through the heat-pipe condenser and back to the tank.
The copper coil heat exchanger was a little too wide for any of my containers, so I built a soft tank from 10′ of mylar coated bubble wrap and a garbage bag.. It’s well insulated and leak tight. I’m going float Styrofoam balls on the surface for added insulation. I got this idea from Tom Gocze almost 30 years ago. He still sells them at the link below. So far I the tank hasn’t exceeded 140F, so I haven’t really tested the high temp limits.
This is the first attempt at using the header and vertical stand pipe as a heat-pipe with a 10-tube collector. It seems to be functioning well, but there is a significant ΔT across the steam pipe header. In full sunlight the dead end of the header runs at about 270F and the exhaust side runs steadily at 230F. The geyser pump exhaust runs at about 160F and is moderated by the tank temperature. The ΔT across the heat-exchanger is about 20F indicating a good pump rate from the geysering.
My next experiment will be to insert some kind of wicking agent into the header pipe to more effectively draw liquid to the dead end. My theory is that the dead end of the heat pipe is over heating because the liquid is evaporating before it reaches that end, thus the high ΔT across the header.
As I was starting to design the straight-through header I mentioned in the previous post, I was preparing a presentation about heat-pipes to an inventors group I meet with weekly. While playing with a naked heat-pipe and handling it while heating the evaporator end in a cup of hot water, I was amazed at how effective it was at transferring heat. I did some googling and found formulas for describing how effective it is, and I was even more amazed. I dawned on me that I could create a heat-pipe that extends upward from the header. I tried this with my little 4-tube show model. I just attached an elbow on one end and plugged the other end of the header. I attached a vertical 3/4″ x 12″ pipe onto the elbow and capped the end with an evacuation port. I added enough water to half fill one of the two header pipes, evacuated it from the top port and set it in the sun. The temperature of the top of the vertical pipe was only a few degrees below the temperature of the header pipe. I insulated it, but left the top 2 inches of the vertical pipe open to the air. The temperature of the naked pipe got up to 280F in partial sunlight.
I realized that I might be able to transfer the heat from the header to a vertical nucleating riser using this idea. I built one the next day and it worked great. The only problem was there was a large temperature difference between the two ends of the header. The capped end was getting much hotter because the liquid dropping back down into the header from the condenser was not flowing all the way to the capped end, so it was overheating. I have read that horizontal heat pipes requires some sort of wicking or capillary agent to move the liquid. One promising idea is to line the header with copper screen. Even with the large delta T the system was pumping heat to the heat-exchanger and storage tank.
I have only a few digital thermometers to determine the temperatures in the systems. I need better temperature data.
The vacuum tube geyser pump worked well for about a week, Then, one day, it locked up. The header pipe approached almost 350F, but the vapor condenser remained relatively cool. The system pressure stayed well below one atmosphere, approximately 12 In. Hg vacuum. My theory is that rather than nucleate boiling, the fluid in the header just evaporated, and the header emptied to its ends where a surface of water maintained an equilibrium evaporating just enough into the header to maintain slightly positive pressure. The rest of the system lost enough heat to maintain a vacuum in the vapor condenser. I was able to start the pumping again by drawing a little bit of vacuum or letting a little air in to the system, and then it would pump for the remainder of the day, but would not start pumping the next day.
The solution to this would be to modify the header so that it has active artificial nucleation sites, but the holy grail I am seeking is to use the headers without modification, but I plan to test the nucleation theory by building a straight through 3/4″ I.D. header with a 3/4″ O.D. nucleator.
To build a straight-through header, I will have to build wrap-around sockets for the heat-pipe condensers. I’ll build it with removable high-temperature insulation and install thermal sensors at several points on the header. I’ll start with a 4-socket header, and if it works, I’ll try to pump a 10-tube slave collector. If that doesn’t work, I’ll try building a 10-tube geyser pumping collector with a straight-through, nucleating header.
Solar domestic water heating is a non-starter. Even though it makes practical sense, it’s not sustainable in a market driven by artificial financial incentives. Systems that should be installed for less than $3,000 are inflated to $10,000 so that participants can maximize incentives. I played in this arena for almost 10 years before I grasped the irony of this vicious cycle.
Instead of pursuing the impossible U.S. SDHW market, I have decided to direct my attention to larger, higher-value systems. I think I have identified one that may optimally exploit the advantages of this system: space heating in northern tier states that have brutally cold ambient temperatures, high cost of electricity, no access to natural gas, and mostly clear daytime skies. My parents live in just such a situation. They burn about 13 cord of wood every winter to heat their home near Grand Marais, Minnesota. They live in an area that will never have access to natural gas, and because coal plants are being retired in Minnesota, the electrical rates are high.
I am designing a solar heating configuration that would use a small evacuated tube/heat pipe collector that is optimized for geyser pumping. In the development of the Copper Cricket we balanced the pumping function with the heat collection/loss function, but in this new system I intend to maximize the pump function in the pumper unit so that it can drive the heat transfer fluid through an array of standard Apricus evac, tube solar collectors. At this point, I envision about 100 tubes being pumped by 6. The tubes will be mounted nearly vertical to capture the maximum winter insolation, while being almost hidden from the summer sun. I may even build a short awning over them to shade them from falling sleet and summer sun. I plan to mount the tubes two feet above the roof so that they will be above most standing snow.
One of the homes is already heated with a hydronic in-floor system, the other has a forced air system with an easy-to-modify plenum that could house a hot water radiator.
I have two applications for this space heating solution within 100 yards of each other. My goal is to design the systems and install them in time for the 2016/2017 winter. Both of these homes are on the shore line of Lake Superior and both have southern exposure.