A very offbeat alternative solution to glycol water mix problems

[julianm1234]

I thought there might be some interest in my somewhat offbeat solar thermal design as it eliminates issues with Glycol. I am also looking for some feedback on differential temp controllers as my design has caused me a problem.

I will provide the short version now, and if anyone is interested in my design, and the journey to where I am now, I can expand on it.

I obtained some flat panels on the cheap, and was detailing my design. Having researched solar thermal systems I identified one significant issue (for me at least) as being the choice of thermal fluid, with freezing/boiling and decomposition being interrelated issues. As a retired engineer from the petrochemical industry I decided there must be a better solution, and after investigation decided to try kerosine as my transfer fluid. Foreseen problems were leaks, and seal material incompatibilities which I dealt with. An unforseen problem was that due to it being capable of withstanding the panel static temperature of potentially up to 165C (i.e. if my tank hit the upper set point I could stop the pump with no concerns about boiling in the panels) the temperature sensors were able to see higher temperatures than they normally would.

End result is that I have a system that works well (or at least did when the controller was working correctly), can withstand freezing conditions, does not boil/overpressure even in static sunny conditions, should never have thermal decomposition issues, is non corrosive, and which once the control glitches are ironed out should thus be able to run without any maintenance.

My hope is that someone can point me in the direction of a reasonably cheap solution to my failed differential temperature controller, or, if not a complete kit (which is a cheap off the shelf chinese one), then at least equivalents to the failed sensors which are 3 wire DS18B20, where it turns out the sensor is only rated to 125C and the cable to 85C. I suspect at the panel end it might see up to 140C in reality (I don't forsee actually reaching 165C) and there the sensor is in a very short thermowell so it is just the sensor that sees the extreme temperature, and in the storage tank I can’t see me ever setting the protection thermostat, and thus the temperature that sensor will see, above 95C.

Currently I am running the pump on a timer rather than on temperature control. Not as efficient but still gaining me energy.

[J.P.M.]

See my post under the other thread you also posted this same information to.

J.P.M.

[LucMan]

I thought there might be some interest in my somewhat offbeat solar thermal design as it eliminates issues with Glycol. I am also looking for some feedback on differential temp controllers as my design has caused me a problem.

I will provide the short version now, and if anyone is interested in my design, and the journey to where I am now, I can expand on it.

I obtained some flat panels on the cheap, and was detailing my design. Having researched solar thermal systems I identified one significant issue (for me at least) as being the choice of thermal fluid, with freezing/boiling and decomposition being interrelated issues. As a retired engineer from the petrochemical industry I decided there must be a better solution, and after investigation decided to try kerosine as my transfer fluid. Foreseen problems were leaks, and seal material incompatibilities which I dealt with. An unforseen problem was that due to it being capable of withstanding the panel static temperature of potentially up to 165C (i.e. if my tank hit the upper set point I could stop the pump with no concerns about boiling in the panels) the temperature sensors were able to see higher temperatures than they normally would.

End result is that I have a system that works well (or at least did when the controller was working correctly), can withstand freezing conditions, does not boil/overpressure even in static sunny conditions, should never have thermal decomposition issues, is non corrosive, and which once the control glitches are ironed out should thus be able to run without any maintenance.

My hope is that someone can point me in the direction of a reasonably cheap solution to my failed differential temperature controller, or, if not a complete kit (which is a cheap off the shelf chinese one), then at least equivalents to the failed sensors which are 3 wire DS18B20, where it turns out the sensor is only rated to 125C and the cable to 85C. I suspect at the panel end it might see up to 140C in reality (I don't forsee actually reaching 165C) and there the sensor is in a very short thermowell so it is just the sensor that sees the extreme temperature, and in the storage tank I can’t see me ever setting the protection thermostat, and thus the temperature that sensor will see, above 95C.

Currently I am running the pump on a timer rather than on temperature control. Not as efficient but still gaining me energy.

Only 1 problem using kerosene for heat transfer fluid is that it is considered toxic to humans. Using it for domestic hot water would be a problem if the heat exchanger were to develop a leak and entered the domestic loop. A double wall HX would be required at minimum.
A simple drainback system would work with just tap water, no worries of freezing, overtemp, or toxicity. Keep it simple.

IMC instruments or Goldline are the controllers that I use and recommend.

[julianm1234]

Only 1 problem using kerosene for heat transfer fluid is that it is considered toxic to humans. Using it for domestic hot water would be a problem if the heat exchanger were to develop a leak and entered the domestic loop. A double wall HX would be required at minimum.
A simple drainback system would work with just tap water, no worries of freezing, overtemp, or toxicity. Keep it simple.

IMC instruments or Goldline are the controllers that I use and recommend.

A valid point, and one which I should have covered in my design points.

My thermal stores are the unpressurised type with a solar coil at the bottom, and a water coil at the top. The thermal store itself is filled with the treated heating circuit fluid. So I have the double hx as you suggest should be the case. Any leakage would be into the heating circuit fluid, and any concurrent leaks from the water coil would soon show up as continuous overflow liquid.

As or the drawback system, yes it does provide a solution but requires you to have a system that is built without any chances of airlocks, and demands a greater pumping load. It would also require to be controlled from the overtemp limit switch on the thermal store as well as whatever you determine as the start finish times. I wanted to come up with an alternative that once the initial design issues are sorted, would be simpler to run and maintain. It remains to be seen i that is the case but I believe it is promising.

[julianm1234]

An update in case anyone is interested.

As could be expected from so radical a design there have been some teething issues, which now all seem to be in the past. Now it appears to be able to cope with any extreme it sees. Winter weather well below 0C no problem, full sunshine with the tank at it's upper temp limit and the pump shut off and 130C+ in the panels, no problem, however it has taken some trial and error to get here, which I shall briefly describe.

Pump selection. I went through 3 pumps to get the right one. First pump (a Wilo ZRS with 110C rated windings) worked fine initially but for some unknown reason the impeller broke. It's replacement only had windings rated to 90C and failed due to overheat. I then discovered that the Grundfos Alpha 2L had 110C rated windings, an electronic control head that could be removed and thus mounted elsewhere, and a rotor can that was split allowing access inside it. This was then modified by replacing main gasket and vent O’rings with Viton, completely removing the rubber dirt seal inside the rotor, and creating cabling to allow the control head to be mounted away from the hot pump. It now runs 90C kerosine with no difficulty.

The differential temp controller. This also had problems coping with the extremes of temp. First cheap one from china used 3 wire sensors which failed on the first sunny day. The second controller used NTC chips which should have been good for up to 110C+. However the cabling was not, and the one in the tank thermowell which only reached 85C melted it’s insulation, and the panel one also failed even though it only had the stainless sensor in a very short thermowell. So I researched the NTC’s and found similar ones that were rated for 185C, bought a number and made tails for them using silicone insulated cable. These then burnt out at around 70C which made me look again at the specs. It turned out at higher temps the power rating of them was very very low and the design of the controller was applying 12V to them. After much calculation I determined that if I inserted a 3k6 resistor in series I could maintain the power seen by the thermistors to below their rated power (variable, 0.01W @120C) under all conditions. Fortunately this has not significantly impacted the control function and the system no operates even on the hottest days. I have added a high temp thermostat on the pump inlet to make sure it won’t exceed the winding rating (and to protect the plastic pipe on that part of the circuit), but since the pump is positioned after the tank coil it has never seen the cut off temp anyway. I have seen temperatures of 135C or thereabouts coming from the panels, but after the coil this is under 90C.

Thermal expansion is taken up by an expansion vessel. This was another area I had problems. I had, previous to the kerosine idea, bought an expansion vessel suited for water. The diaphragm was thus EPDM or some such. I decided just to use it anyway as I felt that if the diaphragm failed it didn’t really matter as it was there only as an air pocket. In reality the diaphragm failing did cause me short term problems. Initially I had it at a slight pressure (<0.5bar) and on failure it injected material into the system which kept clogging the pump impeller. However after repeatedly cleaning the pump impeller out over the period of a week then it ran clean. The failure also meant the internal pressure dropped to atmospheric, and that is what I now run the system at. The vessel merely acts as an expansion vessel, and does not provide any pressurisation function.

End result is that the system runs, and can sit stalled, with no issues. The kerosine should have none of the gelling issues that water/glycol mixes have at high temperatures, it won’t freeze, and runs with a very low pump wattage as all the pump has to do is circulate and not lift from a tank which a self draining system has to do.

[bcroe]

Thanks for the great report. You need an EE type to design a controller
that is compatable with your required sensors. The one I made for my
hot water circulator is an extremely simple on/off affair, with known
hysteresis and a temp readout, but perhaps your function is more
complex?

Unfortunately lack of sun, and temps down to -34C make such equipment
only useful part of the year here, so I rely on PV solar panels to collect
the energy for heating. Bruce Roe

[julianm1234]

Thanks for the great report. You need an EE type to design a controller
that is compatable with your required sensors. The one I made for my
hot water circulator is an extremely simple on/off affair, with known
hysteresis and a temp readout, but perhaps your function is more
complex?

Unfortunately lack of sun, and temps down to -34C make such equipment
only useful part of the year here, so I rely on PV solar panels to collect
the energy for heating. Bruce Roe

My latest incarnation of the diff temp controller as described works fine, new sensors and the series resistors sorted it. It is much the same as the one you describe in function, but is now capable of potentially reading right up to the panel stagnation temp of 165C.

You might be interested to know that kerosine is liquid above -40 (F or C it happens to be the same). You could potentially run it where you are with no winter issues, apart from the pump using more energy due to the higher viscosity. Mind you it wold only give you a little preheat in the colder months, even here in the winter it is only preheating my hot water to reduce the energy input. Realistically if you did at least you could leave it filled over winter, just sitting waiting for better weather but coming to no harm, even if it did freeze I don't think it expands like water, so it would not damage the pipes.