Elk as Differential Controller for Solar HW

[politics123]

Hi Experts --

So, I have a solar H/W heater on order, and as I was buying all the parts, I got to thinking: "Why should I pay $150 for a differential controller, if, my Elk M1 can probably do the same work." I was hoping someone could double check my work/RP rules or point out any gotcha-s.

A Solar H/W differential controller measures two temperatures: at the solar panel, and at the storage tank. They are simple devices that turn on an electrical outlet (eg: for a water pump) when the temperature at the flat-plate collector ("panel") is ~10 degrees hotter than the water in the tank. It turns off the electrical outlet when the temperature at the panel is only 5 degrees hotter than the tank. They usually have two additional rules:
1) don't turn on (or turn off) when the panel is below a set limit (eg: below 90 degrees)
2) don't turn on (or turn off) when the tank is already super-hot (eg: above 140 degrees)

The way I thought to solve this:

Get the temperatures

• Whenever every 2 minutes, get the temperatures from the panel and tank. Store the temperatures into counters and increment two counters to reflect +5 and +10 degree variables.
  c_waterheater=temp_waterheater
  c_waterheater_10 = temp_waterheater
  c_waterheater_5 = temp_waterheater
  c_panel=temp_panel
  c_waterheater_10 += 10
  c_waterheater_5 += 5
  Toggle Output 208

Rule to turn on

• Whenever Output 208 is on
  and temp_waterheater < 140 degrees
  and temp_panel > c_waterheater_10
  and temp_panel > 90 degrees
  THEN turn on solar pump (output 207)

Rules to turn off

• Whenever Output 208 is turned off
  and output 207 is turned on
  and temp_panel < 90 degrees
  then turn output 207 off
• Whenever Output 208 is turned off
  and output 207 is turned on
  and temp_waterheater > 140 degrees
  then turn output 207 off
• Whenever Output 208 is turned off
  and output 207 is turned on
  and temp_panel< c_waterheater_5
  then turn output 207 off

Since I have a booster pump, I actually have to turn on two outputs. The second output only needs to be on for a minute, then turned off.

• Whenever Output 207 is turned on
  then turn output 206 on for 1 minute
• Whenever Output 207 is turned off
  then turn output 206 off

NOTE: The rule to turn off output 206 may not be necessary, given that output 206 is on for only 1 minutes, and the system could only cycle on/off every four minutes.


I think: Every 2 minutes, I'm going to check the temperatures. Every four minutes, I'll either turn the solar HW system on or off, depending on the conditions.

Cost savings ($190 for differential controller + probes) + power to drive
Costs: (unknown) for relays and Elk temperature probes.
Advantages: I can turn the system off/on remotely, I can also purchase a 30amp relay for the H/W heater's heating element and have the two interact (eg: don't turn on water heater burner if the solar portion is operational)
Disadvantages: I can't think of any

Thoughts?

Issues:
I need a couple inexpensive relays and temperature probes.
-- I know Elk sells a 9100, but I don't need/want X-10 control. Anyone recommend a couple 120VAC (5-10amp) relays that can be driven from an Elk Output?
-- Any news on when the Elk 1-wire interface will be released? Other options? (the sensors have to measure the temperature of water in a pipe, and as long as they can measure the temperature of a pipe, I'll be okay)

 

[shenandoah75]

You might check out:

http://www.cocoontech.com/index.php?showto...hl=shenandoah75

I don't have temp sensors where i can play with the rules, but i think i had trouble coming up with the section you're trying to deduce in your first block of rules... for one, the greater/less than comparison seem to only allow you to operate on custom setting in the latter argument, not counters... trying to remeber the other issues i saw (i know one specific to me was that elk's proprietary temp sensors wouldn't work due to location of the M1G)

But if you get this to work - i'd love to know...

-brad

 

[Spanky]

The upper temperature limit on the temp sensor from Elk would be a problem. I had a solar hot water collector and it could get to the 200 degree F range. Too hot for the ELK temp sensor and it work.

 

[shenandoah75]

Spanky still seems to be hush-hush on 1-wire must be working on setting ViziaRF on fire...

 

[politics123]

Spanky still seems to be hush-hush on 1-wire must be working on setting ViziaRF on fire...

He gets us all excited, but then we have to wait. <sigh> However, I'm sure it'll be worth it.

 

[murphy]

Hi Politics123,

KIS (keep it simple).

And I suppose, the waterheater has additional electric heating. Just in case there is no sunny day or night. And I think the thermostat for the electric-heater is set to a value, a little below your lower water temp limit.

I would write only 3 rules:

WHENEVER EVERY 120 sec
AND t-waterheater < t-panel
AND t-waterheater < 140°F
THEN solar-pump ON
Remark: this will work even below 90°F, but even then you get help from the sun, if there is any

WHENEVER EVERY 3 sec
AND t-waterheater => t-panel
THEN solar-pump OFF
Remark: If there is no more hot water on the roof, avoid pumping hot water on top ! Make the max time for this case so short, that the hot water from the heater cannot travel the complete way to the roof. This will limit losses. However: if the triggertime is longer, you are not loosing a significant value of the efficiency. Because at first the temp-differences are small.

WHENEVER EVERY 30 sec
AND t-waterheater > 140°F
THEN solar-pump OFF
Remark: just to prevent overheating. But IMHO I would set the limit higher, to 190°F. This way I can grab as much sun-energy as possible. And there are mixing-valves available to mix the temperature for your warmwater-pipes down with additional cold water. Cooper-pipes can handle more than boiling water, some plastics not more than 140°F. I'm not a plumbing-expert.

So far, at a first glance. This should be improved by rules which are counting the events and the times, as

WHENEVER every 60 sec
AND solar-pump is ON
THEN ADD 1 to counter#a(solar-pump-minutes)

WHENEVER solar-pump becomes ON
THEN ADD 1 to counter#b(solar-pump-starts)

WHENEVER every 60 sec
AND t-panel > 140°F
THEN ADD 1 to counter#c(overheat-minutes)

etc...

Add rules for displaying and weekly/monthly/yearly resetting the counters as you like an overview.

Let me add:
with the ELK-M1G -only the "gold" model- you can set an input to operating-mode 34.
Then the input is converting the voltage to a digital number.
I don't know the resolution and accuraty but it sure is sufficient, even if it is "only" an 8-bit AD converter.
In this mode the input can be a voltage from 0 to 12 Volt.
So you can use nearly all kinds of temp-sensors with very little other hardware.

 

[znelbok]

There should be no reason why you cant do this.

I am going to do something similar. You should consider using the temp sensor that come with the unit. Might be a little easier to install than trying to fit something else.

I am keeping my controller (comes with the tank and panels), and letting the elk monitor the temp for my internal display. I will have to remove te pu;; resistor for this as the controller has its own voltage supply and I just need to measure the voltage and not the resistance.

As for the rules, keep them simple and to the point.

Mick

 

[shenandoah75]

Murphy,

The logic is simple, but how are you going to put that into elk rules... The temp comparison only allows comparing a temp input to either:
fixed value / customer setting / counter

and you can't change the custom settings via rules unless i'm missing something... Looks like you can set the counter a temp probe, but i'd be worried about sequencing. i.e. every thirty seconds, etc - you don't know which rule will fire first, so the counter may not get updated until after the rule fires... Ok, so maybe that's not a big deal after all, you lose X seconds of heating... Can you verify this is what you were thinking?



I'm also really interested in some temp probe ideas that would withstand the temps on a black solar panel in florida. As per spanky, i'm sure that could reach close to 200F up there...

-brad