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  • Author Author: JWx
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Using (obsolete) home appliances to optimize energy consumption - blog 3, instrumentation

JWx

Intro

In the previous blog post we have researched the method of controlling household refrigerator operations, but doing it blindly/open-loop can lead to sub-optimal results. Thus, some measurement method has to be prepared. To not make big modifications in the fridge construction (which can lead to decrease of thermal insulation, water vapor condensation and other problems), I have decided to use wireless temperature sensor. 

Sensor

Use of an outdoor weather station sensor was inspired by the story I have read sometimes in the past, when somebody was troubleshooting his fridge by measuring temperature inside using such a sensor. That choice is not an intuitive one - after all, what is the most recognizable characteristic of the fridge? That people can stick magnets to it's walls. Which means that they are made of ferromagnetic metal. That probably shields RF signal. But that's theory - let's see if correct.

To not create controversies with other fridge users, my outdoor sensor was put into food-certified plastic bag and then placed in the freezer (first) before being moved to the uppermost shelf, near the termostat. Below is the photo of it - it is a second-hand Oregon RTGN318 working on 433 MHz frequency band, using proprietary (but well-researched) communication protocol. 

Oregon outdoor sensor

Data gathering station

To obtain data from the sensor, small SBC (OrangePI Zero) with RTL-SDR dongle was used

OrangePi

It has some issues - mainly: limited memory (256MB) and a CPU with the tendency to running hot (especially inside a case and with daughterboard installed), but as it already serves as a print server, it is most convenient hardware for quick tests.

On this SBC, typical software stack was configured:

  • rtl_433 to control SDR module and decode messages,
  • rtl to influxdb relay to forward messages to the database,
  • influxdb as a database (in fact, this time influxdb had to be started on different machine due to memory shortage caused by growing dataset)
  • grafana as visualization tool

Rtl_433 influxdb relay was copied from the example (/usr/share/doc/rtl-433/examples/rtl_433_influxdb_relay.py) and then modified to include correct database connection parameters.

Then, rtl_433 and data relay were executed in different screen sessions (it is a temporary installation after all) using following commands:

python3 ./rtl_433_influxdb_relay.py

while `/bin/true` ; do date; rtl_433  -R 12 -F syslog:127.0.0.1:1433 ; sleep 10; done

As we see, rtl_433 is loading only one protocol parser (-R 12) and executes in the loop - version from official system repository was crashing every few days.

By default, relay script is loading data into the "rtl433" database, each sensor type into different measurement (table equivalent).

Data can then be selected using sql-like query language:

select * from "Oregon-RTGN318" where time > now() - 1d;

[...]

2024-01-31T16:48:31.532522Z 1 1 50 181 -4.1
2024-01-31T16:49:24.322516Z 1 1 50 181 -4.1
2024-01-31T16:49:24.424488Z 1 1 50 181 -4.1
2024-01-31T16:51:10.22942Z 1 1 50 181 -3.9
2024-01-31T16:51:10.292678Z 1 1 50 181 -3.9
2024-01-31T16:52:03.184808Z 1 1 50 181 -3.7
2024-01-31T16:52:03.313972Z 1 1 50 181 -3.7

or displayed in visualization tool like Grafana

image

As we can see, temperature curve is quite interesting - our mechanical thermostat seems to have hysteresis window of about 10°C

Parents

  • I've just been timing my fridge/freezer here and its compressor runs for about 9mins and then is stopped for about 25mins before running again.

  • in reply to beacon_dave

    Interesting - it seems that cooling cycle is much shorter... I wonder how long could the compressor start be delayed if overfreezed (set lower temperature, let it reach it, return to the previous setting and measure time)?  Is it type of fridge with mechanical thermostat or something with electronic controller?

  • in reply to JWx

    Looking at a service manual, it appears that the rotary dial in the fridge compartment is controlling a damper in the air duct that draws cool air from the freezer section through the fridge. Which means potentially there is scope for control of the damper setting and fan speed independent of the freezer temperature setting.

  • in reply to beacon_dave

    I wonder if there is a easy way of changing settings of electronic controller or override it, in similar way to overriding mechanical thermostat by the relay connected in parallel... I think that also important would be to assess the quality of thermal insulation - by setting the temperature low and check how much time it takes to return to the typical temperature - mine has poor insulation and heats pretty fast...

  • in reply to JWx

    Looks like you can parallel across the output of the relay if you needed to. 

    image

    Is your freezer compartment normally kept full ? 

  • in reply to beacon_dave

    yes - I have even put some cooling inserts to increase heat capacity. I think that changing (lowering) temperature in the freezer is rather harmless, the problem may arise when cooler part goes below freezing point...

    image

  • in reply to JWx

    Longer term, a standalone freezer may be the easier the option in that case. However perhaps need to keep in mind increased duty cycle for running the compressor for longer durations during the summer months. Not only will it be running longer due to warmer ambient temperatures but also due to the peak solar generation months. May also want one with a good automatic defrost cycle to prevent damage from increased icing up of the internal plumbing.

    Alternatively, the style of fridge-freezer where the fridge compartment is chilled by cold air drawn from the freezer compartment may work as the fan shouldn't run when the fridge temperature is below its set point threshold, and even if it does you can always interrupt it with the external controller.

Comment
  • in reply to JWx

    Longer term, a standalone freezer may be the easier the option in that case. However perhaps need to keep in mind increased duty cycle for running the compressor for longer durations during the summer months. Not only will it be running longer due to warmer ambient temperatures but also due to the peak solar generation months. May also want one with a good automatic defrost cycle to prevent damage from increased icing up of the internal plumbing.

    Alternatively, the style of fridge-freezer where the fridge compartment is chilled by cold air drawn from the freezer compartment may work as the fan shouldn't run when the fridge temperature is below its set point threshold, and even if it does you can always interrupt it with the external controller.

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