Off-Grid Solar Power

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Off-Grid Solar Power

Battery ventilation is key to safe operation.

 

Off-grid Photovoltaic (PV) systems are most common in remote areas not served by utility power.  These systems may be of any size, but typically serve a single residence, which may be some distance from available utility power.

solar module

Typical PV module array

PV modules consist of an array of cells which create electrical energy during daylight hours. The output of the PV array is direct current (DC).  In order to have power available during nighttime hours, the distinguishing feature of off-grid installations is the presence of a battery bank of sufficient size to power the anticipated loads during overnight hours.

The requirements for a safe and reliable installation are defined in the articles of NFPA 70, the National Electric Code (NEC).  Among the applicable items, electrical requirements for bonding and grounding (article 250.4), rating for outdoor use (article 110.3), as well as those requirements specific to Photovoltaic systems (article 690) apply to off-grid PV installations.  Off-grid systems are referred to as Stand-Alone Systems in the NEC.

The block diagram depicts the architecture of an off-grid PV system.  The presence of a generator is optional, in which case nighttime power would only be available from the battery bank.  The conversion of DC into AC power for household loads is provided by the Inverter, which draws from the battery bank during non-daylight hours.

offgrid diag

Typical block diagram of an off-grid system.

Deep-cycle traditional lead-acid batteries still dominate the off-grid PV market as they offer the most capacity for the money.  However, care must be exercised in their placement.  All lead acid batteries, particularly flooded (or wet cell) types, will produce hydrogen and oxygen gas under both normal and abnormal operating conditions.  Flooded lead acid batteries will outgas at varying rates under almost all conditions, even in storage where minor amounts of gas will be produced due to the normal evaporation of water and the tendency to self-discharge. In normal operation (float voltage), flooded lead acid batteries are kept in a state of maximum voltage potential in order to maintain maximum power reserve.

battphoto

Battery damaged from explosive ignition of hydrogen

Most codes and regulations require commercial-scale flooded battery systems to be installed in dedicated battery rooms, which are physically separated from other areas in the facility. Also, it is not a good practice to co-locate other electrical or electronic systems in this same battery room area.  However, these requirement are not always adhered to in residential off-grid system installations, and this has set the stage for losses from battery explosions resulting from ignition of hydrogen.  Hydrogen-air mixtures can ignite with very low energy input, 1/10 that required igniting a gasoline-air mixture. With an energy input of as little as 19μJ, the hydrogen / oxygen gas mixture near the battery can be explosively ignited. For reference, an invisible spark or a static spark from a person can cause ignition.  Resulting losses can range from loss of the battery bank, escalating with acid spillage, as well as additional property damage and injury to persons nearby.  The obvious solution to this hazard is prevention, achieved by providing sufficient ventilation to prevent buildup of hydrogen in the vicinity of lead-acid batteries.

IEEE Standard 484 addresses the ventilation requirements for safe operation of a battery bank.  There are a number of regulatory agencies and bodies that have established well defined standards and guidelines on proper ventilation design and facility requirements/calculations for battery rooms and power systems, including OSHA, ANSI/ASHRAE, IEEE, NFPA, and others here in the U.S. These requirements and recommendations must be considered in design of off-grid PV systems to ensure safe operation throughout the intended service life.