Comment on My customisable solar hot water system controller (project in progress)
dgriffith@aussie.zone 11 months agoAt my latitude, never below 8 or 9 degrees C in winter. Maximum temperature is about 35 or 36 degrees C in summer.
Older systems here were simple thermosyphon designs on the roof with the holding tank closely coupled directly above the collectors and they were quite effective.
My system has a 330 litre mains pressure hot water tank, with the usual cold and hot fittings bottom and top. There is a third inlet about 1/3rd of the way up the tank, and a little bit above that is a heater element and a controlling thermostat. The lower third of the tank is circulated out via the cold inlet and up through the collector by the pump, which can do about 300 litres an hour. Hot water returns from the collector at the 1/3rd location, and rises to the top of the tank via stratification. The copper pipes to and from the collectors are 1/2" and are insulated, but their surface area to volume ratio is quite large and heat is easily lost over the 15 or so metres of pipework.
On sunny days I can see the inlet temperature at the bottom of the tank slowly rise from approx 20-25 degrees C to 30-35 degrees C as the bottom third circulates, the hotter water rises to the top to give the small temperature rise I can see at the top of the tank. Essentially I want a higher temperature returned to the tank which I suspect can rather paradoxically be done by increasing the circulation rate when running the pump, peaking the collector temperature higher then dumping that back to the tank rapidly, rather than slowly circulating it and losing the heat in the lines.
I have the service manual for the controller and it mentions frost protection, where it will circulate warmer water back to the collectors in low temperatures, but this would be insufficient for your colder climate.
At some stage I would also like to control the heater element, it runs on an off peak circuit which is switched by the electricity company when it suits them, so it is cheaper to run. It also means that it regularly “tops up” the top 2/3rds of the tank temperature, which negates the solar contribution a fair bit. I would like to be able to disable that top up if the system can sense adequate heating from the collectors, but that requires switching control to a 3kW element, and something I will leave alone for now.
roterabe@discuss.tchncs.de 11 months ago
Hmm, the heater element being controller disregarding your will is a bit of a bummer.
Thanks for clarifying, so you are indeed circulating the water itself, it’s not a separate line used as a heater. In this case you really are at the mercy of either your pump, pipe length, ambient temperature, insulation and/or sun intensity.
Have you monitored the temperature at the panel on the roof itself? Also, what type of insulation are you running on those pipes?
dgriffith@aussie.zone 11 months ago
It’s generic foam pipe insulation, approximately 3/4" thick.Originally there was also a 3-4 meter section of pipework from the collector output to inside the roof that was completely uninsulated, I’ve gotten that sorted.
I bought the place a couple of years ago and after replacing the seized circulating pump early last year I was pretty disappointed with the system’s performance.
At that point I put onewire sensors on the tank and lines on the ground level and measured it for a few months and didn’t see much heating at all from the collector.
I couldn’t get to the collectors for a long time, not having a three storey ladder, but I got suspicious after sunny + windy days resulted in very little heating. Finally got up there about three months ago to discover that bare section of pipe and insulated that.
Collector temperature I haven’t measured using the original analog sensor yet, I just let the original controller do its thing to get a baseline. But currently in a “warm” Australian spring the maximum outlet temperature at ground level is a brief peak of about 65 degrees C at around 1pm. Going off the circulation rate of the pump it’s probably 40 litres of water at that temp before it starts dropping back down in the afternoon.
Insulating that bare section of pipe improved performance but it’s still not fantastic, so now it’s time to drive the pump with my own algorithm and see if I can improve things.
roterabe@discuss.tchncs.de 11 months ago
Yeah, that seems like a good way to go. 65 degrees at ground level is what I’d also expect. My assumption up top would be close to 90/95 degrees. Driving the pump more often would be interesting.