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Forum Index : Solar : Solar hot water build
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| Georgen Guru  Joined: 13/09/2011 Location: AustraliaPosts: 462 |
it almost sounds like there is need for some grease to improve interpretation of rules. George |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Finally got around to fitting the pump and controller to the solar panels. I've been running it as a convection only system for the last 9 months, and while it works, there isn't much reserve in the system. To get the best results, I had a gate valve that I turned off every evening and on every morning. Ahh, nothing like taking responsibility for your next hot shower. Ordered a 12v pump off ebay, about $25 with a bit of searching. Matted to a one way valve. The pump is very quiet, and draws about 1 amp. 
And mounted the controller to the side of the tank. Its all very crude at the moment, once the controller proves itself I'll tidy it up. 
I added another temp sensor to the circuit, so it now has 3 sensors. One at the top of the solar panel, to measure panel temp, one poked into the tank so it sits between the tank and insulation, to measure the tank temperature near the top, and one at the bottom outlet, where the cold water goes out to feed into the solar panel. The software in the little 08M PicAxe works like this. It runs in a loop, testing the 3 temp sensors every 10 seconds. If the panel top temperature is about 5 degrees hotter than the tank temperature, turn on the pump for 20 seconds. I found 20 seconds is enough time to pump all the hot water from the panel into the tank. After 20 seconds, the panel top temperature will drop, as its now full of cold water. Wait 20 seconds, for the panel top sensor to catch up, and then go back into a loop. This routine will pump water from the panel into the tank if its hotter than the tank water, logical. Also, if the tank bottom temp is close to 60 degrees, we have a tank full of very hot water, no point in running the pump, so go to sleep for a few minutes. After 1 day, its worked perfectly, and I have a full tank of hot water, which is a first 
Now my concern is freezing. Last week the water hose from the ram pump, which trickles water onto the yard 24/7, froze... 
So to protect the panels, I was thinking of adding a bit of code that sensed when the panel temperature had dropped to freezing. I tested the sensor with a glass of ice water, and at 0 degrees, I have a reading of 35 at the PicAxe. So if the temperature drops to 35 on the sensor, I was thinking of running the pump for a second every couple of minutes, just to stir up the water in the panel. Not sure if this is the best way to go. In the mean time, I've drilled a 2mm hole in the one way valve, to let a slow trickle of warm water through the panel at night. I'll post the code at a later date. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I done a very similar system a few weeks back with a 18m2 and a lcd panel to show temps, it also saves the Max and Min temps to eeprom so they can be viewed later. Like you i added a defrost mode that monitors the panel sensor temp for close to zero temps and runs the pump for 10 seconds every few minutes if the panel is below 4C. The main program works on the panel temp needs to be above 40C and/or 10C above the inlet temp before the pump will run, then the panel temp needs to fall back by 5C from the temp that triggered the pump start before the pump switches off. My third sensor monitors outlet (HW) temps and records the max to eeprom for later viewing, the sensor was initially for monitoring air temp, but found the panel sensor done this better than a ground level sensor, so its now a outlet temp recorder. Pete. Sometimes it just works |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah I'm tempted to use a more advanced chip at a later date, logging would be nice. This is the code. I have a debug program I used to check the probe temperatures... symbol BottomTemp = B0 symbol TopTemp = B1 symbol PanelTemp = B3 Main: readadc 4, B4 PanelTemp=256-B4 readadc 1, B4 TopTemp=256-B4 readadc 2, B4 BottomTemp=256-B4 debug Sleep 2 Goto Main Typical readings are 35 = freezing 220 = boiling 186 = very hot 69 = cool And this is the main program. I've changed a few settings, pump time is now about 13 seconds. symbol BottomTemp = B0 symbol TopTemp = B1 symbol PanelTemp = B3 Start: Low 0 Main: readadc 4, B4 PanelTemp=256-B4 readadc 1, B4 TopTemp=256-B4 readadc 2, B4 BottomTemp=256-B4 B5=TopTemp+5 if BottomTemp>185 then TankHot if PanelTemp>B5 then PumpOn Sleep 4 Goto Main PumpOn: High 0 Sleep 5 Low 0 Sleep 10 Goto Main TankHot: Low 0 Sleep 50 readadc 2, B4 BottomTemp=256-B4 if BottomTemp<175 then Main goto TankHot My sensors are 10k at 25degreesC, and I'm using them in a series divider with a 4.7k resistor. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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