SOLAR POWERED VACCINE STORAGE REFRIGERATOR
Typically, a usable storage volume of 28 liters (1 cf) or larger will meet the needs of a remote clinic. The PV systems are generally sized assuming that the refrigerator is operating in a 32°C (90°F) environment while making 2.2 kg (4.8 lbs) of ice per day. The average insolation level for the most overcast month is generally used to size the PV array and the battery bank is typically sized for five days of storage. Battery banks usually range in size from 300 to 500 amp hrs. The peak output of the solar array may be between 8 to 20 amps depending on insolation levels and the efficiency of the refrigerator.
Deep cycle batteries are usually not produced in developing countries making funding, purchasing and transporting them to remote sights difficult. In addition, funds may not be available to pay for the batteries when replacement is necessary. Recycling the used batteries is also very difficult and I suspect usually not attempted.
The all copper cooling system of the Sun Frost RFVS does not incorporate any moving part except for the compressors and is rugged both mechanically and electrically. The interiors of the refrigerator and freezer are made of smooth, molded fiberglass with no exposed cooling fins to puncture, making it easy to clean and defrost. It also features convenient front loading design and completely rustproof cabinet and hardware.
ARRAY SIZING & LOAD SHEDDING
To determine the potential effectiveness of load shedding, we carried out a computer simulation of the system. For the simulation, we picked two locations in Africa with poor solar resources for that continent, Akuse, Ghana and Inongo, Zaire. The data was obtained from the “World Radiation Data Center” website (http://wrdc-mgo.nrel.gov). The data gave total daily radiation on a horizontal surface. Since our sites were near the equator, horizontal data could be used directly. We obtained 5 years of data for Akuse, Ghana and 20 years of data for Inongo, Zaire. In the future, satellite data may be available for all of the tropical world from “NASA’s Solar Radiation Data Set” being developed at NASA Langley Research Center (http://eosweb.larc.nasa.gov/).
Knowing the system’s power requirements and the size of the solar array, the number of sunlight hours required to power the system can be directly calculated. The solar data gave us the number of sunlight hours received each day throughout the year. A spreadsheet was used to keep track of the battery’s daily state of charge. With load shedding, if the battery was depleted more than a predetermined amount, the freezer compartment would shut off and would not turn on again until the battery was fully charged.
The Sun Frost RFVS consumed only 32 amp hrs per day in a 32°C (90°F) environment while making 2.2 kg (4.8 lbs) of ice per day. This is the lowest energy consumption we have seen for a vaccine storage unit. The refrigerator (shown in Fig. 1) incorporates two variable speed Danfoss compressors. The speed of each compressor was set at its lowest setting. At this low speed, each compressor consumed only 3 amps in a 12 volt system. Keeping the current draw small is beneficial because it maximizes battery efficiency and capacity. The thermostats are connected so that only one compressor can run at a time. This is accomplished with two commercial grade thermostats, one of which contains a single throw double pole switch.
At night, with a load of fixed size and no battery charging, the system voltage will be a good indicator of the battery’s state of charge. The set point voltage to turn off the freezer section can be fairly accurately predetermined by placing a 3 amp load on a fully charged battery while monitoring the amp hours removed and the battery voltage. When the desired number of amp hours are removed from the battery, record the voltage, this voltage is the set point for the freezers low voltage disconnect. In the simulations carried out, we assumed that the battery capacity is 100 amp hrs and that the freezer compressor is shut off when the battery is discharged more than 25 amp hrs. The set point voltage on the low voltage disconnect for the freezer compressor will then be about 12.2 volts.
The original strategy envisioned to minimize the size of the battery bank was to utilize load shedding. However, after examining several simulations, we found that in addition to load shedding, increasing the array size could also be an effective method of reducing the necessary storage capacity. This strategy would of course not make economic sense when working in the northern part of the United States because the average daily insolation in December is 3 to 7 times less than the average daily insolation in June. Therefore, a uniform daily load in December would require an array 3 to 7 times larger than in June.
In northern latitudes, insolation levels during a cloudy winter day are particularly low because the days are short and the sun angle is low. With a low sun angle, sunlight must travel a greater distance through a cloud layer before it reaches the earth’s surface. In tropical areas, the days are more uniform in length and the sun is higher in the sky, making it easier for sunlight to penetrate a layer of clouds. For Akuse, Ghana, the average insolation level for July (the poorest solar month of the year) was 4.0 equivalent sunlight hours or 4.0 kWh/m2. In 1981, the average insolation for July was below average (3.58 sunlight hours); in April (the sunniest month of the year) the average was 5.68 sunlight hours. The sunniest month then received only 1.6 times as much sun as the poorest solar month. This is in sharp contrast to the northern United States. Similar results were obtained for Inongo, Zaire.
It can be seen by comparing Fig. 2 and Fig. 3 that as the size of the array is increased so that the system can be powered in 3 rather than 4 sunlight hours, the required storage decreases from 3.8 days to .8 days. With only .8 days (28 amp hrs) of storage occasionally needed, a single 100 amp hr battery will provide sufficient storage.
The initial costs of both the 3 sunlight hour & 4 sunlight hour power systems will be about the same. The system designed to operate the refrigerator in 3 sunlight hours will require an 11.7 amp array, while the system designed to power the refrigerator in 4 sunlight hours will require an 8.8 amp array. Comparing the two systems, the system with the smaller array will require a battery bank which will provide 3 days of additional storage. Assuming the marginal cost of a PV array is $120 for each additional amp of generating capacity and that the marginal cost of an additional amp hour of storage for the deep cycle battery bank is $2.88 (assuming a deep cycle battery can be discharged 75%), the PV system with the larger array will only cost an additional $65. However, in the long run, the system with the larger array will work out to be much less expensive since one PV panel will out last a number of battery banks. An additional advantage to using the power system with the larger array is that the batteries will be left in a partially charged state for a much shorter time period (3 days compared to 71 days) which will help to prolong their life. Leaving batteries in a partially charged state increases sulfation and decreases a battery’s effective life.
Load shedding can also decrease required storage capacity and the length of time a battery remains partially charged. In the above example with the array sized to operate the system in 4 sunlight hours, load shedding as shown in Fig. 4 will decrease the number of days of required storage to .88 days and the maximum number of days the battery will operate in a partially charged state will be reduced from 71 days to less than 10 days. Load shedding will require the freezer compartment to be turned off only 13 days per year.
An additional benefit of load shedding is that as the battery ages and looses its capacity, the refrigerator compartment will remain operational until perhaps 80% of the battery’s capacity is lost. As battery capacity is lost, the number of days the freezer compartment is not operational will of course increase. With load shedding, the initial battery provided with the system could be a 100 amp hr deep cycle battery. When this battery needs replacement, it could then be replaced by a locally produced truck battery.
The battery will typically be discharged only 12% each night and be fully charged the next day. With an array sized to pwer the refrigerator in 4 sunlight hours for perhaps 5 days each year, during periods of the most overcast weather, the battery will be discharged about 25 amp hrs or 25%. With this type of duty, a truck battery should give 2 to 5 years of service. An additional benefit of using load shedding is that if the refrigerator is installed where insolation levels are unknown, underestimated or abnormally reduced by forest fires, etc. then the vaccines will still be reliably stored and only the freezer compartment will be effected.
These techniques are also applicable to other PV applications in the tropics. In a village power system for example, instead of having the complications of a backup generator and large battery bank, incorporating an oversized array and load shedding will form a reliable stand alone system with a modestly sized battery bank that is sustainable and better for the environment.
Fig. 1: Sun Frost RFVS vaccine storage refrigerator.
Fig. 2: Days Storage required for an array sized to power the refrigerator
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