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U. Muntwyler
12.3.3 Components of Off-Grid DC PV Applications
The electric components of such a system are:
• PV generator
• Battery (so far mainly lead-acid)
• Voltage control of the battery
• DC consumers
• Mounting structures/lightning protection/cables/circuit breakers etc.
The critical technical component of such a system is the battery. The lead-acid battery
should never be completely discharged. If this happens, the battery starts to loose
active lead material and its plates become covered with sulfate. The battery capacity
differs according to the temperature range of operation, and according to the value of
the discharge current. For this reason, it is important to know the value of the battery
discharge current, in order to be able to assess the capacity C x of the battery. The
term C x denotes here the capacity of a battery, which is discharged in x hours. Thus,
C 20 is the capacity of a battery, which is discharged in 20 h. For photovoltaic off-grid
applications, C 20 or C 100 are appropriate. Even regular strong discharges adversely
affect the lifetime of the battery. The battery therefore needs an active protection
against rapid discharge. The reference signals used are: battery voltage and battery
temperature. Some more advanced discharge protections also consider the discharge
current. The most important factor is the battery voltage.
In lead-acid batteries constant control of the voltage is important. If due to overcharging, the voltage rises to a value that is more than 15–20% higher than the nominal value, an explosion can take place. Note that the storage temperature T storage of
lead-acid batteries is also important. If T storage < 0 °C, freezing occurs, leading to
destruction of the battery. If T storage > 20 °C, battery lifetime decreases by half for
every temperature increase of 10 °C in T storage . This is a challenge for photovoltaic
off-grid installations in hot regions (Table 12.1).
The life-time of the whole installation is often limited by the lifetime of the leadacid batteries. The latter varies strongly, depending on the depth of discharge (DOD).
DOD is defined as the percentage of the capacity that is discharged.
At this moment, lead-acid batteries are well established in the market. They have to
be replaced after about 6–10 years depending on the use and the average temperature
of the battery. At the time of writing (2020), the cost of current from lead-acid
batteries is still about 5 times lower than the cost of current from Li-ion batteries.
However, the price of Li-ion batteries is steadily decreasing, so that we can expect
more and more Li-ion batteries in the future. In the case of Li-ion batteries, we have
to control each battery element separately, through a Battery Management System
(BMS).
Further mobile DC—applications are: PV installations for gliders, glider trailers,
boats. In future we will see electric cars with PV on their car body.
U. Muntwyler
12.3.3 Components of Off-Grid DC PV Applications
The electric components of such a system are:
• PV generator
• Battery (so far mainly lead-acid)
• Voltage control of the battery
• DC consumers
• Mounting structures/lightning protection/cables/circuit breakers etc.
The critical technical component of such a system is the battery. The lead-acid battery
should never be completely discharged. If this happens, the battery starts to loose
active lead material and its plates become covered with sulfate. The battery capacity
differs according to the temperature range of operation, and according to the value of
the discharge current. For this reason, it is important to know the value of the battery
discharge current, in order to be able to assess the capacity C x of the battery. The
term C x denotes here the capacity of a battery, which is discharged in x hours. Thus,
C 20 is the capacity of a battery, which is discharged in 20 h. For photovoltaic off-grid
applications, C 20 or C 100 are appropriate. Even regular strong discharges adversely
affect the lifetime of the battery. The battery therefore needs an active protection
against rapid discharge. The reference signals used are: battery voltage and battery
temperature. Some more advanced discharge protections also consider the discharge
current. The most important factor is the battery voltage.
In lead-acid batteries constant control of the voltage is important. If due to overcharging, the voltage rises to a value that is more than 15–20% higher than the nominal value, an explosion can take place. Note that the storage temperature T storage of
lead-acid batteries is also important. If T storage < 0 °C, freezing occurs, leading to
destruction of the battery. If T storage > 20 °C, battery lifetime decreases by half for
every temperature increase of 10 °C in T storage . This is a challenge for photovoltaic
off-grid installations in hot regions (Table 12.1).
The life-time of the whole installation is often limited by the lifetime of the leadacid batteries. The latter varies strongly, depending on the depth of discharge (DOD).
DOD is defined as the percentage of the capacity that is discharged.
At this moment, lead-acid batteries are well established in the market. They have to
be replaced after about 6–10 years depending on the use and the average temperature
of the battery. At the time of writing (2020), the cost of current from lead-acid
batteries is still about 5 times lower than the cost of current from Li-ion batteries.
However, the price of Li-ion batteries is steadily decreasing, so that we can expect
more and more Li-ion batteries in the future. In the case of Li-ion batteries, we have
to control each battery element separately, through a Battery Management System
(BMS).
Further mobile DC—applications are: PV installations for gliders, glider trailers,
boats. In future we will see electric cars with PV on their car body.