Sunday, 28 August 2022

"Dispatchables" with OpenHAB and PowerPal

I read a while back about the concept of "dispatchable" energy sources - namely, ones that can be brought on- or off-stream at virtually no notice, at a desired output level. As an enthusiastic solar-power owner/operator, the idea of tuning my energy consumption to also be dispatchable, suited to the output of my rooftop solar cells, makes a lot of sense.

My first tiny exploration into this field will use OpenHAB to automate "dispatch" of a non-time-critical task: recharging some batteries, to a time that makes best use of the "free" solar energy coming from my roof.

Just to be clear, I'm referring to charging domestic AA and AAA batteries here; I'm not trying to run a PowerWall!


To get the necessary insight into whether my house is running "in surplus" power, I'm using my PowerPal PRO which offers a simple RESTful API. If you send off a GET with suitable credentials to{{SERIAL_NUMBER}}
you get something like:

    "serial_number": "000abcde",
    "total_meter_reading_count": 443693,
    "pruned_meter_reading_count": 0,
    "total_watt_hours": 4246285,
    "total_cost": 1380.9539,
    "first_reading_timestamp": 1627948800,
    "last_reading_timestamp": 1659495300,
    "last_reading_watt_hours": 0,
    "last_reading_cost": 0.00062791666,
    "available_days": 364,
    "first_archived_date": "2021-04-13",
    "last_archived_date": "2022-08-02"

It's pretty straightforward to translate that into an openHAB Thing definition using the HTTP Binding that will get us the current watt-hours reading every 60 seconds (which is how often the device phones home)

Thing http:url:powerpal "PowerPal" [
  refresh=60] {
      Type number : powerUsage "Newest Power Usage" 
      [ stateExtension="/api/v1/device/000abcde", 
      mode="READONLY" ]
You can get MyPowerPalAPIKey as used above, by opening the PowerPal mobile app and going to Guidance -> Generate an API Key.

That's it for the "physical" (Thing) layer. Lets move up the stack and define an Item that we can work with in a Rule.

Number:Power currentPowerUsage "Current Power Usage [%d W]" 

... and if you're me, nothing will happen, and you will curse openHAB and its constant changes. Make sure you've actually got the HTTP Binding installed or it'll all just silently fail. I wasn't able to see the list of Official Bindings because of some weird internal issue. So I had to do a full sudo apt-get update && sudo apt-get upgrade openhab before I could get it.

Then, fun times ensued because the PowerPal API uses a slightly-strange way of providing authentication, which didn't fit very well with how the HTTP binding wants to do it. I had to go spelunking through the binding's source code to figure out how to specify the Authorization header myself.

Now we can finally get to the "home automation bus" bit of openHAB ... we define a rule that's watching for power usage changes, and triggers my Broadlink SP2 smart power switch on or off depending on whether we're net-zero.

rule "Charge batteries if in power surplus"
  Item housePowerUsage changed 
  logInfo("dispatchable", "Power: " + housePowerUsage.state);

  if (SP2_Power.state === ON && housePowerUsage.state > 0|W) {
    logInfo("dispatchable", "Charger -> OFF");
  if (SP2_Power.state === OFF && housePowerUsage.state == 0|W) {
    logInfo("dispatchable", "Charger -> ON");

And we're all done!

What's that weird |W stuff? that's an inline conversion to a Number:Power object, so that comparisons can be performed - a necessary, if slightly awkward aspect of openHAB's relatively-new "Units Of Measurement" feature.

What does it look like? Here's the logs from just after 9am:

09:06:37 [dispatchable] - Power: 3 W
09:07:37 [dispatchable] - Power: 2 W
09:08:37 [dispatchable] - Power: 3 W
09:09:37 [dispatchable] - Power: 2 W
09:12:37 [dispatchable] - Power: 3 W
09:13:37 [dispatchable] - Power: 2 W
09:16:37 [dispatchable] - Power: 1 W
09:18:37 [dispatchable] - Power: 0 W
09:18:37 [dispatchable] - Charger -> ON

So the query to PowerPal is obviously running on the 37th second of each minute. There are "missing" entries because we're only logging anything when the power figure has changed. You can see the panels gradually creating more power as the sun's incident angle/power improves, until finally at 9:18, we hit power neutrality and the charger is turned on. Not bad.

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