Solar Freezer - 06/27/05             

International Solar Energy Society World-wide Information System for Renewable Energy

Our family hurricane preparedness plans include an off-grid 24 cubic foot freezer run by solar power. According to Kill-A-Watt(tm), it runs on less than 200 watts and draws less than 2 amperes. The duty cycle is about 60%. A minute after startup, the readings were 159w, 170va .93pf, 1.4a, 121.9v.

24 cu ft freezer w Aims 1250 watt modified sine inverter and 6 plus 8-volt golf cart batteries.

The modified sine did not work well with the freezer and sometimes failed to start it up. Under load, it drew 2x as much from the batteries than the pure sine replacement. I also upsized it for good measure.

24 cu ft freezer w Aims 3000 watt pure sine inverter and 6 plus 8-volt golf cart batteries.

When I first tried the modified sine with a 12-volt trolling battery, it ran down to 10 volts in 10 minutes and shut down. Paralleling 12-volt batteries (as suggested in the instructions) seems to be a bad idea because of unlimited equalizing current flows that might occur when not under load. The Trojan(tm) T105 and T875 golf cart batteries are much better and the extra 2 volts gives more headroom. Some electrical profile data was collected from a microMeter(tm) mMII processor installed on the house mains panel.

24-hour freezer demand from microMeter(tm) (540 readings in ten-watt units)

24-hour refrigerator demand from microMeter(tm) (540 readings in ten-watt units) Tiny spikes are probably the door light

Orig mM s/w display, see #4 and #9, others were not saved

The green jumper between batteries was calibrated to give ampere data from millivolt readings. Although mM can read dc mv, there was a need for ground isolation, so manual readings were taken. Amperes=1.4508 * mv. With 10.5 mv we had about 15.23 amperes flowing in the battery circuit under load. The refrigerator data is included for comparison but with its higher demand and usage, the freezer is a better choice for solar operation.

Closeup of calibrated green jumper

Closeup of Aims 3000 watt pure sine inverter

Also shown is a switchbox to flip the freezer from mains to inverter and a Kill-A-Watt(tm) measurement device. The switchbox also has a tiny trickle charger (< 1 ampere) built in. Another charger simulates the not-yet-received Kyocera(tm) panels. They will provide 14 amps when the sun shines.

2nd Closeup of Aims 3000 watt pure sine inverter

Funding has been allocated for renewable energy research. Adaptation of hardware is to be made to track DC current flows and energy storage using Hall devices. The bracket for the battery post has been fabricated and will use 2 50-amp chips, one for outflow and one for inflow.

Battery bracket w/o chips.

Panels just arrived!


mMI processor hookup for ac and dc measurements

A mMI processor was set to apparent power mode to measure the dc mv. The chips produce .5 to 4.5 volts to represent -50 to +50 amperes. Shown with the processor is an offset circuit (stuffed into the 3/4" port) to shift the 2.5-4.5 volt range to 0-2 which then feeds two resistive voltage dividers (10k+100) to scale to output to mv. Calibration is done in the software file (below). For #15, 20.4mv represents +50 amps so the CT size is set to 50*23/20.4 = 56.

CT.DAT file

  1 - outlet 1           "0020,1.00,120,000,999                       .
  2 - outlet 2           "0020,1.00,120,000,999                       .
  3 - outlet 3           "0020,1.00,120,000,999                       .
  4 - outlet 4           "0020,1.00,120,000,999                       .
  5 -                    "0000,1.00,120,000,999                       .
  6 -                    "0000,1.00,120,000,999                       .
  7 -                    "0000,1.00,120,000,999                       .
  8 -                    "0000,1.00,120,000,999                       .
  9 -                    "0000,1.00,120,000,999                       .
 10 -                    "0000,1.00,120,000,999                       .
 11 -                    "0000,1.00,120,000,999                       .
 12 -                    "0000,1.00,120,000,999                       .
 13 -                    "0000,1.00,120,000,999                       .
 14 -                    "0000,1.00,120,000,999                       .
 15 - batt out  20.4mv   "0056,1.00,014,000,999                       .
 16 - batt in y 19.1mv   "0060,1.00,014,000,999                       .

1st sun 5pm 4 amps!

cheap home depot connections!

I tested the chips to be sure about current flow before mounting. There are two ways people think about it. They both seemed a little high (about 2.6v w no current flow). After torching them onto my board and praying I didn't overcook them, they zeroed in nicely to 2.5v. The designer must have planned ahead!

Hall chips added to battery bracket.

battery bracket installed and measuring amps in and amps out. Yellow extension cord runs out to panels.

mMI records it all. A 5v p/s feeds the chips. A standard charger simulates panels at night.

All I need to do is keep channel 15 and channel 16 balanced.

16 catches up to 15 after compressor shuts down. Charger (since it's nighttime) supplies 16. Note that it is powered from outlet #3 on ac channel #3.

compressor only ran a few hours and full charge was restored at end of day.

Also, I am resurrecting an earlier Peltier-based project. In spite of the abysmal efficiency of pV * Peltier, I marvel at the fact that this could chill a gallon of beer. The only thing consumed besides sunlight is the 2 gpm cooling water flow.

More sun-frost without cooling water.

9/2/2005 - Actual solar generation falls well short of the needs of the freezer operating under conventional thermostat regulation. It is believed that modification of the duty cycle and acceptance of higher sub-freezing temperatures can reduce the energy requirement by a factor of four. The following spreadsheet shows that the two KC125 panels with 6 hours of average daily sunshine can support a 10-hour off period followed by a 2-hour continuous run (at higher currents).