Solar water heater + controller

[Simen]

Just got a solar water heater for free: it had some leaks, but was easily fixed with some small hoses.:


The heater are 1.2m X 6m; 7.2m², supposedly around 5kW.

Ofcourse i had to test it, so i connected a small 12V, 1A pump @15liters/min, and dragged out a bath tub i was just going to throw away anyway... I filled the tub up with around 230 liters of water @ around 14deg. C. One hour later, the temp was 32deg. C., so what choice had i but to slide into the tub with a glass of wine...   An hour later, i was slightly tipsy, and extremely hot! The watertemp was 42+ deg. C. The ambient temp was 19, the tub was in the shadow and it's made of emanelled metal.

Now, this was fun, so i had to make a controller for the pump to turn it on/off when it was sunny/cloudy, and here is the result:


I used a PicAxe 08M and a flat-topped diode as a solar sensor, since it produces a low voltage that are even and stable enough at approx. 160deg (sun)angle, and a MosFet to switch on/off the pump. I also added a Thermistor as a temp sensor, but thermistors are too much trouble calibrating, so i've ordered some digital temp sensors. The picaxe sends the solar value, temp and whether the output fet are on or off via serial port/usb to a pc...

Here are the basic code i used in the PicAxe:

Code: [Select]

Start:
SYMBOL Sol = w0
SYMBOL Temp = w1
SYMBOL Counter = b8
SYMBOL OutOn =b9
Counter = 0
OutOn = 0

Main:
ReadADC10 1,Sol
ReadADC10 2,Temp
SerTXD("Sol:", #Sol, " Temp:", #Temp, " Output is:", #OutOn)
SerTXD(10,13)
If Sol <= 255 AND OutOn = 1 Then
Inc Counter
EndIf
If Sol >= 260 AND OutOn = 0 Then
Inc Counter
EndIf
If Sol >= 260 AND OutOn = 1 AND Counter > 0 Then
Counter = 0
EndIf
Pause 2000
If Counter = 5 AND OutOn = 1 Then Cloud
If Counter = 5 AND OutOn = 0 Then Sun
Goto Main

Cloud:
Low 4
OutOn = 0
Counter = 0
Goto Main

Sun:
High 4
OutOn = 1
Counter = 0
Goto Main
The on/off values i found by trial and error, monitoring the values in sun/clouds and the sensor at different angles, and i give a cloud 10-12 seconds to pass before turning the pump off/on. I'm probably going to add an override on/off button, for those hazy days...

And to top it all; the pump gets it power from my solarpanels, making it a 100% free hot bath. (Well those i paid for, but nevermind that... )

[DamonHD]

You're a very brave man to hazard testing your own equipment.  More wine next time, just for medicinal purposes I say.

Nominally insolation is ~1kW/m^2 at noon so that could just about be a 'marketing' 5kW I suppose.

Anyhow, very good.  I should think about one of those for us in winter since a 5kWh/day (average London mid-winter) contribution to heating might be well worth having.  Where did it come from?

Rgds

Damon

[ghurd]

I see them on ebay, 4x20', so I bet they come from China!

Interesting post.
A friend is toying with the idea of using one for domestic hot water.  Somehow.
G-

[SparWeb]

Wow that looks like a lot of fun.  The good life - powered by the sun.

Gee I've got lots of those bits and pieces around my place, too, especially the bottles of creativity...

[willib]

Simen how are the connections made between the horizontal tubes and the headers?

[Simen]

I do not know what brand it is; got it from a friend that had 4 of them which he used for his swimmingpool, which it is originally designed for.

I'm toying with the thought of insulating it, so i can use it for pre-heating domestic hot water year round, but that would be some construction work... I'll probably put it on the roof on the annex seen in the background, for the summer at least.

The horizontal tubes - which are approx. 1/6" ID - are fastened together just like a computer ribbon cable. Looking inside the 1.5" headers, one see 1/6" holes for each third horizontal tube; i think 3 tubes come together just before the headers, inside the welding. (The horizontal tubes and the headers are heat-welded together.)
Halfway inside the 1.5" in/out header, there's a restriction wall with a 1/2" hole through. When using a powerful pump, like a 1.5hp swimmingpool pump, part of the water goes through that hole, bypassing the thinner tubes; protecting the thinner tubes against too high pressure. Currently i have plugged that hole...

[TomW]

simen;

I would not connect that to a pressurized water system. Period.

It might work in a static pressure exchanger system to heat a vat of unpressurized water like garygary has on his builditsolar website?

Just off the top thoughts.

Tom

[sk windpirate]

Hii folks

Our Daughter's pool came with one of them heaters. I made a 2x4 frame, plywood back and 6mill poly over it. If the pool gets to hot we shut it off. They work very good.

Might try swinning pool places.

Ron

[Simen]

jeez...
I got to throw away that tub... Spent 4 hours in there today...
I loaded my trailer with junk this morning to take to the junkyard, and before i left at 09:30, i took a bath; Temp: 34-35deg. C. When i came back from the said yard, i was looking forward to a bath but - it was 46 deg. C. in the tub...
(Had to add some cold water...)

Regarding pressure; i think the collector are rated for around 30-40psi, but with my 'repairs' (slice free a single hose with hole in it, and cut out the section with hole and replace it with some flexible hose), i think i'll stay with the pressure the pump manages to make, which isn't much at 15l/min. (4gal./min.) This mean that i'm probably limited to use it as a pool/Large wooden bath tub(spa - kinda) usage...

I'm going to make 2 changes to the controller: add an Override on/off button, and make the pump stop/start at a given temperature; maybe with an adjustable potmeter...

[Simen]

FYI:
I've added an override button that turns off output regardless of the state of the output. Are the output on, it turns it off; are the output off, the controller confirms 'override' mode by flashing the output twice.
(I was planning on using the Override button to turn the output on, but after several scolding baths, i found it more important to turn it off... )

I also replaced the led with an ldr; the led started to give varying voltage depending on the ambient temperature. I haven't calibrated the ldr yet, so it remains to see if the ldr are more stable to temperature variations...

For those interested; here's the new code for the picaxe, with comments... I might draw up the schematic if there's an interest.

Code: [Select]

Start:
SYMBOL Sun = w0 'ADC10 value for sunlight.
SYMBOL Temp = w1 'ADC10 value for temp.
SYMBOL Counter = b8 'A counter used to delay start/stop when clouds pass by.
SYMBOL OutOn = b9 'Whether Output is on or not.
SYMBOL Override = b10 'Whether Override Off are active or not.
Counter = 0
OutOn = 0
Override = 0

Main:
Pause 2000
ReadADC10 1,Sun 'Read in Sunlight value.
ReadADC10 2,Temp 'Read in Temp value.
SerTXD("Sun:", #Sun, " Temp:", #Temp, " Output is:", #OutOn, " Override is:", #Override)
SerTXD(10,13) 'And send some data out to the serial port...
If Pin3 = 1 AND OutOn = 1 Then 'Check if Override are pressed when output are on.
Override = 1 'Activate Override Mode
Low 4 'Turn off Output.
OutOn = 0
Goto Main 'And skip the rest of the code; return to top.
ElseIf Pin3 = 1 And OutOn = 0 AND Override = 0 Then 'Check if Override are pressed when output are off
Override = 1 'and Override Mode are off; set Override on.
High 4 'Confirm by 'flashing' the output twice during 700mS.
Pause 300
Low 4
Pause 200
High 4
Pause 300
Low 4
Goto Main
ElseIf pin3 = 1 AND Override = 1 Then 'If Override button are pressed when in override mode
Override = 0 'Turn off override mode
High 4 'and confirm by turning the output on.
OutOn = 1
EndIf
If Override = 1 Then 'If Override Mode are on and no button pressed
Goto Main 'do nothing and return to top.
EndIf
If Sun <= 258 AND OutOn = 1 Then 'If a cloud passes by and output are on
Inc Counter 'Increment counter.
EndIf
If Sun >= 260 AND OutOn = 0 Then 'If the sky are blue and output are off
Inc Counter 'Increment counter
EndIf
If Sun >= 260 AND OutOn = 1 AND Counter > 0 Then 'If the sky are blue, output still on and a cloud has passed
Counter = 0 'reset counter.
EndIf
If Counter = 5 AND OutOn = 1 Then Cloudy 'If there's still cloudy after 10sec and output are on; turn it off.
If Counter = 5 AND OutOn = 0 Then Sunny 'If the sky still are blue after 10 sec and output are off, turn it on.
Goto Main

Cloudy: 'Cloudy...
Low 4 'Turn off output and clear the counter.
OutOn = 0
Counter = 0
Goto Main

Sunny: 'Sunny...
High 4 'Turn on the output.
OutOn = 1
Counter = 0
Goto Main
Sorry about the formatting in the 'Code' box; it ignores tab it seems... (Comments starts with an ' ...)

[willib]

I loaded my trailer with junk this morning to take to the junkyard, and before i left at 09:30, i took a bath; Temp: 34-35deg. C. When i came back from the said yard, i was looking forward to a bath but - it was 46 deg. C. in the tub...

I usually always make a withdrawal at the junkyard , not a deposit 
It rarely happens that i don't eventually need whatever it was that i was getting rid of.