SunXtender Technical Manual

Document No. 6-0100 Rev. H
Page 25 of 43
6.2 Days of Autonomy
As everybody knows, the sun does not shine with equal intensity every day, nor does it shine at
night and during inclement weather. Cloud cover, rain, snow, etc. diminish the daily insolation
(Insolation is the amount of solar energy delivered to the earth’s surface, measured in W/m
2
or
kWh/m
2
/day). A storage factor must be employed to allow the photovoltaic battery system to
operate reliably throughout these periods.
In addition, it is desired to obtain the best service life of the battery by limiting its average daily
depth of discharge. This storage factor is commonly referred to as “Number of Days of Battery
Autonomy.” The number of days is established by evaluating the peak hours of sun per day for
the lowest insolation month of the year with the solar array oriented for maximum output during
that month.
The minimum number of days that should be considered is 5 days of storage for even the
sunniest locations on earth. In these high sun locations there will be days when the sun is
obscured and the battery’s average depth of discharge should not be more than 20% per day.
The recommended days of autonomous storage are shown in the following table:
Table 6-1. Recommended Days of Autonomy
Insolation Level (kWh/m
2
/day)
Days of Autonomy
4.5+
5
3.5 to 4.5
6
2.7 to 3.5
7
2.0 to 2.7
8
< 2.0
10 or more
6.3 Temperature Considerations
The temperature of the battery is a major factor in sizing a PV system. Battery capacity is
reduced significantly in cold temperatures. For example, a battery that operates continuously at
-18ºC (0º F.) will only provide about 60% of its normal room temperate capacity. Appendix C
provides a chart of capacity versus temperature at various discharge rates.
Battery calendar and cycle life are also affected by temperature. As a rule of thumb, the battery
life decreases by 50% for every 10⁰C rise in temperature. Thus, a battery that lasts 6 years at
25⁰C will last 3 years at 35⁰C, 1.5 years at 45⁰C, and 0.75 years at 55⁰C. Similarly, a battery
that lasts 1000 cycles at 25⁰C will last 500 cycles at 35⁰C, 250 cycles at 45⁰C, and 125 cycles at
55⁰C.
It should be realized that the temperature of the battery itself and ambient temperature can be
vastly different. While ambient temperatures can change very quickly, battery temperature
change is much slower. This is due to the large thermal mass of the battery. It takes time for the
battery to absorb temperature and it takes time for the battery to relinquish temperature.
The earth is a great heat sink which can be used to attenuate temperature fluctuations of the
battery. By locating the battery in an underground chamber, it will be better insulated from
extreme ambient conditions. The battery capacity will improve at cold ambient temperatures, so
a smaller battery may be selected. The cool underground temperature will prolong battery life,
reducing replacement costs. Of course, there is added cost associated with underground
installations. Therefore, a cost/benefit analysis should be performed to determine if locating the
battery underground is economically justified.