Intro
Initial measurements were conducted to asses the feasibility of obtaining working energy storage using refrigerator under test. The objectives were set as following:
- over-freezing during energy oversupply should allow for significant reduction of energy consumed in the rest of the day,
- refrigerator should be able to provide it's default role (it is the only way we can utilize stored energy),
- energy consumption during storage phase is of secondary concern - we assume local PV production (with secondary objective of creating enough of local consumption to prevent an inverter shutdown caused by overvoltage),
Normal operation analysis
First, refrigerator behavior during normal operation was observed. Thermostat was set to "4" on the symbolic scale in the range "0" - "7" on the regulation knob.
Sensor was installed in the freezer chamber at first, then moved to the top shelf of cooler section, near a thermostat.
Inside a freezer chamber
Measurements of temperature in the freezer during steady operation was as below:
With mean temperature of about -14°C, temperature changed from -12.5°C to -16.5°C with period of about 3h. During this time, temperature was rising for about 1:10h and lowering for about 1:40h.
Then, some thermostat lag was observed. On the screen below, first vertical line indicates control knob switch from "4" to "6"
As can be seen, thermostat has switched "off" and undergone a full cycle despite being reconfigured for lower temperature. Then, temperature has lowered to about -23°C.
Moving to the upper shelf
As following data analysis has created suspicion that thermostat is not measuring temperature in the freezer, the sensor was then moved to the upper shelf of the fridge, near thermostat's regulation knob
In this location, following measurements were obtained
On the graph, vertical dotted lines mark thermostat setting change points - first by decreasing temperature setting ("4" -> "6"), then raising it above initial set ("6" -> "3") and at the end returning it to the initial setting of "4". Curve shapes are - as expected - exponential, with lowest obtainable temperature of -8°C.
First observation is that signal shape is similar to the data gathered previously - with full cycle of about 3h. Second one is that temperatures inside a fridge that are displayed frequently on food conservation posters (like below) are different than measured - they seem to be not only averages but averages using very long averaging window.
Another observation is that "ON" temperature seems to be largely the same when regulation is changed, only "OFF" temperature is changing.
What is most worrying is the fact that despite we have lowered the temperature by about 4°C, the unpowered curve still takes about 2h (which can be more clearly seen on following screen: temperature raises from -8°C to about 8°C between 19:30 and 21:34 and from -4°C to 8°C between 1:07 and 3:00), which probably means that thermal resistance of insulation and heat capacity are too low.
To verify that theory, upper shelf of the refrigerator was filled with cooling inserts from the camping cooler, like this:
And the resulting graphs were as below:
As previous, to simulate energy storage system operation, regulation knob was moved to "6" during "storage" phase and then moved to "4" when returning to the normal operation.
Installation of cooling inserts have increased unpowered phase of the cycle from about 2h (for example: 19:20 - 21:30) to about 4-5h (19:50 - 00:40). In the same time, cooling phase increased from ~5:30h (13:50 - 19:20) to about 10h (for example 10:20 - 19:50)
Summary
After conducting series of measurements, it seems that tested refrigerator is not very suitable for energy storage role - cooling performance is limited (resulting in very long cooling times) and thermal insulation not very good (resulting in long times of cooling to obtain preset temperature and step heating curve in lower temperatures range).
Next part will concentrate on temperature distribution issue - it seems that it is very uneven inside a cooling chamber