dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
1034
share rss forum feed


54067323

join:2012-09-25
Tuscaloosa, AL

1 recommendation

A Simple Solar DHW Differential Controller Project.

»Free Hot Water From The Sun. And It Works Too. ;-)

A Simple Solar DHW Differential Controller Project.

I needed a basic and inexpensive method for controlling a circulation pump connected to a solar DHW heater.

The pump is used to circulate cooler water from a ground level storage tank up to the solar collector and then pushed back down the storage tank, now the most common method for doing this is to buy 15 or so watt solar panel and connect it to a semi-reliable 12 volt pump. I have two problems with this design, the first one being the pumps only last a year or so and then either lock up or spin slowly until they do lock up. The second problem is the solar panel must be sized large enough to overcome the start-up resistance of a cool pump with a shaft-seal and also have the power to overcome the startup current needed. Overcoming this problem results in utilizing a panel that will over-run the pump as the sun wanes into the west wasting stored heat.

My solution which is what most legit solar installers utilize and that is a 120 volt shaft-less magnetic drive pump, which has little startup resistance (which no longer matters), but more importantly, has no shaft seal to leak in the middle of the night or while I am away for a week or two.

However to run a “real” pump even a small Little Giant 105 watt pump, requires some type of control and for that I decided to go the KISS route and cobble together a differential controller utilizing two LM34 precision temperature sensors and a couple offset voltage comparators.




This is a block diagram of how it works.




To prep the LM34’s for potting I connected to them to pre-cut lengths of two conductor shielded cable, utilizing red for power, black for LM34 return and shield drain for ground, once the connections where completed they where insulated with heat shrink.







I built four of these pickups two for the diff controller and two for panel mounted volt meters as temperature readouts.

That completed, I placed al of them at the same time into a bath of mineral oil heated to 80 degrees and sorted them by indicated temperature, the reason being I wanted the ones that read higher to be in the storage tank and the ones that read lower up in the solar panel, as doing so would provide a good, but not excessively wide temperature offset for the controller to work with.

The probes to contain the LM34’s where constructed from one half inch copper tubing with a cap soldered to one end with the other end of the tube pushed through a nice snug hole drilled into a three quarter inch brass pipe cap, once in place another touch of solder ensured a good tight leak proof seal.













A mix of 3 to 1 epoxy was poured into the half inch tube to a depth of four inches and then two of the rinsed off and sorted LM34 where lowered into the epoxy to the bottom of the tube and then pulled back about an half inch to knock off any trapped bubbles of air.

Once the epoxy fully kicked the remainder of the tube was filled with a polyurethane resin given a good 2% kick with MEKP.




The probes where then installed, wired back to the controller and they say the rest is history.












SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
Reviews:
·Verizon FiOS
Nice, thanks for posting.

I keep reading waiting for a question

I have one, that has nothing to do with the heater. Did you check the LM34's against each other? I working on a project using the LM35 and I can see the difference in parts. There is also some error due to self heating.
--
"Facts do not cease to exist because they are ignored." - Aldous Huxley

"Omne ignotum pro magnifico."


54067323

join:2012-09-25
Tuscaloosa, AL
said by SparkChaser:

Nice, thanks for posting.

I keep reading waiting for a question

I have one, that has nothing to do with the heater. Did you check the LM34's against each other? I working on a project using the LM35 and I can see the difference in parts. There is also some error due to self heating.

I just happened to do that and they do vary a bit.

said by 54067323:

That completed, I placed al of them at the same time into a bath of mineral oil heated to 80 degrees and sorted them by indicated temperature, the reason being I wanted the ones that read higher to be in the storage tank and the ones that read lower up in the solar panel, as doing so would provide a good, but not excessively wide temperature offset for the controller to work with.



SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to SparkChaser
I have made several temp sensors using LM34's and they were all pretty close if you added a heat sink to them. The self heating seemed to add the most difference.

I also used audio wire for the connection, the only difference was that I painted on plasti dip for the insulation. That way it got a liquid tight seal. If you go that route you have to make sure the plasti dip only get on the bottom of the LM34 or else you will see some serious self heating.

My most common use for the sensors I make is to attach them to the condenser return line, and compare to an other one used as an ambient temperature probe, to view the differential temp. When it starts to get high I know it's time to go clean the condenser.

Dirty condenser's is my number 1 cause for loss of cooling at transmitter sites.


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
Reviews:
·Verizon FiOS
said by SmokChsr:

I have made several temp sensors using LM34's and they were all pretty close if you added a heat sink to them. The self heating seemed to add the most difference.

The amount of self heating took me a little by surprise, although if I had read the entire spec sheet, it wouldn't have. It doesn't take to much of a sink to set them right.

I made 20 last week and you're right there was some difference but only significant, to my project, on a couple.

SmokChsr See Profile How are you fastening them to the condenser?
--
"Facts do not cease to exist because they are ignored." - Aldous Huxley

"Omne ignotum pro magnifico."


54067323

join:2012-09-25
Tuscaloosa, AL
said by SparkChaser:

[I made 20 last week and you're right there was some difference but only significant, to my project, on a couple.

The four I used came out as +.2, +.1, -.2, -1.1, degrees off of the oil bath temperature.

Now once encapsulated with epoxy into the probes they are all reading a tad bit higher due to self heating, but the amount of deviation from the actual temperature did not change.


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to SparkChaser
said by SparkChaser:

How are you fastening them to the condenser?

Actually I attach them to the output line from the condenser since that would seem to be the best location to determine the condensers effectiveness. I'll put a couple wraps of silicon tape around the line to provide for cushioning of the leads then a dab of heat sink compound, just off the tape and place the LM34 there and secure it with a few more laps of silicon tape. (that's the fancy way) It also works if you just simply tape it to the out line with electrical tape, but you'll have to re tape every so often as the heat will deteriorate the electrical tape in a few years.


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
Thanks SmokChsr See Profile clever application.


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to 54067323
said by 54067323:

The four I used came out as +.2, +.1, -.2, -1.1, degrees off of the oil bath temperature.

I'll admit that I did not test to this level, my application was not that critical. I just put them on the bench and compared to ambient. I imagine if I was doing something really critical I would want to compare them in 2 different solutions to also verify tracking.

I found the easy way to test self heating is to be all set up, and then apply power to the device and watch the rise of the temperature reading. If they were somewhat isolated it was not uncommon to see a 3-4 deg F rise. Small slip on heat sinks greatly reduce the self heating.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to SparkChaser
Click for full size
I just wanted to point out that converting to current right at the source avoids many problems with remote temperature sensing.

That can be done with LM34 (as shown above) but the AD590 does it in a single milspec (-55C to +150C) chip which can be supplied with upto 30V for long cables.

Of course precision digital temperature sensors avoid the analog issues altogether and could be easier to interface when there is a microcontroller in the project anyway.


54067323

join:2012-09-25
Tuscaloosa, AL

1 edit
reply to SmokChsr
said by SmokChsr:

My most common use for the sensors I make is to attach them to the condenser return line, and compare to an other one used as an ambient temperature probe, to view the differential temp. When it starts to get high I know it's time to go clean the condenser.

Dirty condenser's is my number 1 cause for loss of cooling at transmitter sites.

If I might ask what is the differential you consider to be the trip point? I have a site where we had two roof top package units (out of site out of mind) where the only indication a problem had occurred was when the BO back at the studio received a building over-temp alarm, by then one unit had already brunt out a compressor and the other compressor was really pissed off and cycling it's overtemp switch, the problem was caused when the surrounding farms harvested their fields which they do each and every year, the chafe collects on the condenser coils and pushes the head pressure through the roof.

My solution was to install pressure switches on the high side charging ports, but that caused another problem of false alarms during summer months as the startup pressure would close the switch momentarily shooting an alarm back to the studio, causing the BO to panic. I rigged a workaround using a time delay, but I think monitoring the condenser return temp combined with the pressure would be a better solution.

Of course the correct solution would be regular inspections, but try selling that to the GM.

So we build bridges…


54067323

join:2012-09-25
Tuscaloosa, AL
reply to SmokChsr
said by SmokChsr:

I'll admit that I did not test to this level, my application was not that critical. I just put them on the bench and compared to ambient. I imagine if I was doing something really critical I would want to compare them in 2 different solutions to also verify tracking.

My primary concern was not accuracy, but to ensure I didn’t have an LM34 that read high up in the panel, versus one that read lower in the storage tank, as that would cause the pump to circulate heated water into the panel, when in fact the panel was at a temperature lower than storage.

I actually pulled the 1 meg resistors out of the design as they caused too much hysteresis, the “semi-matched” potted LM34’s worked out to have a 5 degrees offset which is perfect for this application.

It’s crude and simple but will last forever.


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to 54067323
said by 54067323:

If I might ask what is the differential you consider to be the trip point?

That seems to vary somewhat, depending on the cooling system and the site. The typical is that 50deg above ambient indicates a problem. Still you may have to watch the normal clean behavior of your unit to know the best setting. I have a couple units that have the condenser facing West fully exposed to the afternoon sun, those may go a bit more on a summer afternoon.

In fact I'm looking at one of my unit's via remote control right now that has a 40deg at 2AM, that tells me that one needs cleaning very soon, it hasn't tripped an alarm yet, but it will.

In Florida other than dirt, we have a serious problem with algae clogging it up. Most of the AC units (window & wall hung) arrive set up with the drain dumping into the condenser pan to increase efficiency. The first thing I do is move that to draining out side the unit. Other wise I can lose those to algae in a few weeks.

Where I'm lucky enough to have a dual system, I like the Bard dual unit controller set to alternate primary unit duties once a week. If the primary fails, the temp goes up just enough to trip (separate) the room temp alarm, usually by the time I get the alarm and call in the room is already cooling off using the secondary unit.