A New Solar Hybrid Refrigerator/Freezer
Sun Frost developed a hybrid refrigerator/ice pack freezer, the FRH-3. The Sun Frost FRH-3 has a SDD freezer and a battery powered refrigerator. The SDD freezer is the same size as our F-1 (http://www.sunfrost.com/batteryless_direct_drive_freezer.html) and incorporates the same technology which allows the freezer to operate at low levels of insolation. The F-1 has been successfully tested in Colombia this past year. Testing was supported by the BMG Foundation. The refrigerator has a net storage capacity of 32 liters and is battery powered. The refrigerator compartment is extremely efficient at 32 deg C, it consumes only 96 watt hours/day or 8 amp hours/day at 12 volts. The FRH-3 can be powered by two 140 watt module’s at any location in the tropics. The cooling systems are independent and one module will be connected to each cooling system.
A 140 watt module will provide enough energy to run the refrigerator section with only 1 KWH/m^2/day. At poor solar locations in the tropics levels of insolation at or below 1 KWH/m^2/day may occur once or twice a year. This is based on the examination of many years of data on the WRDC data base (http://wrdc-mgo.nrel.gov/). We primarily looked at locations with low levels of insolation. At this level of energy consumption for 363 days per year the battery will have to supply only 4 amp hours/day to keep the refrigerator operating at night. For one or two days per year the load will be 8 amp hours.
In terms of the Autonomy Tool the array oversize factor will be at least 3.5; an array oversize factor of 1.25 is all that has been required by WHO. Using a larger oversize factor greatly reduces the necessary storage capacity of the battery and also reduces the amount the battery is discharged.
In a conventional battery powered combination unit the freezer is also battery powered. The freezer is a major part of the load, and this load could vary considerably depending on the quantity of ice manufactured. Eliminating the freezer load significantly reduces the amp hour draw on the batteries. In addition the efficiency of the refrigerator was increased by incorporating a newly developed evaporator.
A battery as small as 40 amp hours would typically be cycled only 10% each day; each night 4 amp hours will be used to keep the refrigerator operating. The expected life of a battery is highly dependent on the percent discharged. Data for MK Battery DEKA gel cell shows that the cycle life of a battery increases from 600 cycles to 6000 cycles as the percent discharged decreases from 80% to 10%. We expect the life of a high quality battery to be at least 10 years. If the battery is replaced by a lower quality locally available battery in this shallow cycle application we expect the life of the battery to be at least 3 years. A 40 amp hour battery is about the size of a small car battery. Loss of battery capacity is typical mode of failure for a battery, however, even if a 40 amp hour battery loses 80% of its capacity it will continue to operate the refrigerator.
Advantages of a Hybrid System:
- System is sized to operate at any location in the tropics. Sizing is typically a problem with SDD refrigerators. From the PQS Sheets “For solar direct drive units, the correct sizing of the solar panel array for a specific site is complex and critical. It must be agreed with both the appliance manufacturer and with the Qualified Supplier of the solar energy system at the time of ordering.”
- The system will be considerably less expensive than an SDD refrigerator/freezer. In a poor solar location the savings could be as much as 40%.
- The refrigerator and power system will be smaller and easier to transport than a SDD refrigerator/freezer.
- Connection will be plug and play.
- Since the panel powering the refrigerator is significantly oversized the battery will typically be full early in the day. Excess power could than be used to power a second battery which could be used for lights or cell phone charging, etc. Typically 24 amp hours or 288 watt hours will be available for other applications. This could light eight 5 watt LEDs (40 watt incandescent equivalent) for 7 hours each day.
- In emergency situations where a spare battery is needed the auxiliary battery could be used to run the refrigerator. A spare battery will then always be available.