Water heating. PV panels or their water filled cousins for heating tanks in the loft?

Help me with my ignorance if you would please. What do the letters "SA" stand for ?

Since he referred to "that SA system" I presumed the clue was in a previous post. It turns out that it was a reference he made to a South African system.

More than likely. Both JPM and ampster have been sparing for a while on the forum.

HI how do i post????

Hi you just did, go to the front page of the forum them click on the sub forum you want to post in then press new post or new topic and your good to go, cheers

These are exactly the issues I decided to eliminate in my own design of system. As a retired engineer from the petrochemical industry I decided there must be a better solution than glycol water mix which is the root of all the above issues, and after investigation decided to try kerosine as my transfer fluid. Foreseen problems with it were leaks, and seal material incompatibilities which I dealt with. However an unforseen problem was that due to it being capable of withstanding the panel static temperature of potentially up to 165C 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), it will withstand freezing conditions, will not boil/overpressure even in static sunny conditions, should never have thermal decomposition issues, does not cause corrosion, and which once the control glitches are ironed out should thus be able to run without any maintenance. I anyone wants more detail I can provide it.

I do have one hope, which 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 but in the tank I can’t see me ever setting the protection thermostat above 95C. Currently I m running it on a timer rather than on temperature control.

What type of HX does the system have ?

What pump are you using ?

What do you figure the collector tube wall to kerosene film coefficient to be as f(transport properties, fluid velocity)? As I seem to recall, those film coefficients were about 20% of so of water or a water/glycol mixture which will have a negative effect on the thermal performance of both a liquid cooled solar collector as well as the secondary HX in a rather significant way. That will lower the collector heat exchanger factor and so impair performance. See Duffie & Beckman: "Solar Engineering of Thermal Processes", Sec.10.2, for details.

Does the AHJ know what the heat transfer fluid is ?

As for controllers, any controller that uses 10 Kohm sensors ought to work well provided the sensors a re correctly placed and insulated.. Two controllers I've used are a delta T model from Heliotrope General (no longer made but maybe a used one might be found - very robust) and a Goldline model GL-30-LCO which I believe is still being built.
I've stagnated liquid cooled flat plate collectors up to ~ 175 C. and the controllers + sensors were fine afterwards.

You might have found a working method to avoid some of the problems associated liquid cooled plate solar collectors, but it looks to me like you created as many or more, some of which you may not have considered or evaluated.

Thanks for the response, I will look into the availability of those controllers you mention.

So, in response you your questions, points and concerns, my key design points are below.

Panels are sputtered finish flat panels that the fluid runs directly through in small bore copper with a 165 C stagnation temp. The tank HX is a grooved/finned solar coil in a thermal store.

The pump is located in the cool section, after the tank solar coil so that it will only ever see temps less than 100C.

Pump is currently set to stop when the upper tank temp reaches 80C, this can readily be altered if it causes issues.

Pump seals have been replaced with Nitrile and Viton. All valves use ptfe cup seals (I accidentally used some which contained o’rings, these had to be replaced), I later discovered that full bore ball valves typically use ptfe, whereas reduced bore use rubber o’rings.

The pump is otherwise a std wilo canned rotor heating pump with a plastic impeller.

Sealed expansion vessel pressure is set to atmospheric to prevent air injection to system if/when the bladder fails. Bladder failure is not considered to be an issue. The vessel can continue to operate with a direct air to fluid interface in the vessel.

The vessel is sized for 6 times the thermal expansion of the loop content at stagnation temp.

The vessel is located so that the inlet is at the bottom in order to prevent air bleeding out due to gravity.

The vessel inlet is also on the cold section of the loop.

The vessel is slightly pressurised during fill by the pump static pressure so that it starts with some liquid in it.

The closed system thus operates just slightly above atmospheric pressure under all conditions. No high pressures to spray hot kerosine if a leak develops.

System was pressure and leak tested with air, and soapy water on the joints, to 5bar after completion and before filling.

In terms of heat transfer film coefficients, the over-riding factor in fluid heat transfer calculations is kinematic viscosity. The viscosity of kerosine is less than pure glycol at 25C but slightly greater than a water glycol mix (when new), which in itself is significantly greater than water. So yes, the performance at 25C is likely to be slightly worse than a (fresh) water glycol mix. However what viscosity the water glycol mix goes to when ageing is anyone's guess, more or less solid in the worst case. I can't say it has caused me any problem. I am also unable to find comparable temp vs viscosity charts to compare them across the full operating range so it is quite possible that kerosine is better at higher temps than a glycol water mix, even when new. It works for me so I am satisfied.

What is AHJ? Ok, done some research, looks like you are referring to the US authority, I am in the UK, there isn't such a formalised process. Here it is down to the designer to be competent and have taken on board the associated risks, and I believe I have ticked the boxes.

The only likely future problem I forsee relates to the pump impeller, I would have been happier with a metal one.

Yes I have a very similar post elsewhere, I initially responded to the post on this one, then thought it might be useful as a starting post and added some additional info.

I run my attic located HPWH once a day in winter, and once every 3 days in summer. This is done by setting the water temp high during hottest times and solar gen (between 1 to 4:30 depending on time of year, with 3 hours at the high setting), rest of day is it OFF and lowest temp setting as added measure.
Also I do not run the elements, just HP.
Reducing operating cycles prolongs life span. Also a flame or element have higher heat stress to components then the HP.

Another item is a filter with scale-reducing material. This will reduce material buildup inside tank. Or a whole house micro particle water filter.

My old Bradford-White gas water heater was looked after and lasted almost 21 years before replaced, I fully expect my new Rheem HPWH to last longer.

I decided to update on my experience over the past year or so of running my system, as it was just in it's infancy when I last posted.

As JPM predicted there were a few issues I had created, all are now solved.

I thought I had found and modified a suitable pump, but it did not last long, for some reason the impeller broke. Finding a better alternative took a couple of trial and errors, the first did not have winding temperature ratings high enough (90C instead of 110), the second I was not able to replace all the rubber components. Finally I came across the Grundfos Alpha 2 which turned out to be ideal.As well as replacing all the seals that I had on the initial Wilo, I was able to get inside the can and remove a rubber seal that was there to exclude corrosion products, which in my system there will be none. Also the electronics were in a removable plug in head, which meant they could be located away from the hot pump for longevity. The pump has performed flawlessly for a year now. There is a temp limit switch on it's inlet that stops it running if the inlet temp goes above 90C, pump windings are rated at 110C so giving a factor of safety.

The problem of the controller took some work also. I bought a simple differential controller off eBay that used NTC type sensors. As sold it was rated for 120C, and although it worked for a while it did pack in come the warmer weather. So I asked the vendor what spec the sensor was and researched alternatives that might work. I found some that were capable of over 180C but they had to have a very limited current flow, which required a series resistor to limit the current at high temperatures. This impacted on the design differential temperature trigger, but it has not in reality been a problem.

I also discovered that the section of plastic pipe on the cold side of the loop was unsuitable, and was being dissolved by the still relatively hot kerosene, seemingly it was only suitable for kerosene at normal ambient temps, not anything above around 50C, so that was all replaced with copper.

Following these changes the system has run though the summer and winter with no issues. If it gets too hot in summer, it just stalls, and starts again when the tank temperature drops. In practice the tank temperature has been the only limiting temperature, as I have it set at 80C and the pump inlet temperature thus never gets to 90C. Sitting static on a hot day causes no difficulties, and similarly a winters day of -10C. In summer I have seen tank inlet temperatures of 125C. In the winter on a sunny day latitude 56 the tank can manage 50C, but on very cloudy days in the winter I get nothing. Typically in December it has kept the tank at between 12 and 20C since we have not had very much sun, but still a useful preheat. 7-8 months of the year it provides all the hot water demand.