PV System Components

As noted for both stand-alone and grid-connected configurations, several components have to be specified to ensure that the PV system operates safely under various conditions. In this section, specific guidelines are outlined to select the adequate specifications and characteristics for the main components of PV systems. Figure 9.10 illustrates the electrical design specifications for a grid-connected system without batteries (Krarti, 2017).

PV Array

The PV size both in terms of peak power output rating (i.e., watts) and roof surface area requirement (i.e., ft2 or m2) depends on several factors, including, but not limited to, the building location, building architectural features, roof available area, PV array efficiency, and shading effects. The PV size in watts is generally determined using a detailed analysis accounting for the building site, the load profile.

Electrical layout for a grid-connected PV system

FIGURE 9.10 Electrical layout for a grid-connected PV system.

and especially the cost-effectiveness to estimate the optimal PV power rating. The roof area required for the PV array is a function of the layout of the roofs, local applicable codes such as life safety and accessibility, type of PV module and its efficiency, and mounting type. For common applications, the power density of commercially available PV modules ranges from 6 W/ft2 (64.6 W/m2) for high- efficiency panel systems to 15 W/ft2 (161.5 W/m2) for low-efficiency panel systems. Manufacturer’s data can be used to estimate the PV power density and, ultimately the roof area needed to install the PV system. For instance, a 175-W crystalline silicon PV module has a surface area of 14.4 ft2 (1.38 m2), resulting in a pow'er density of 12.2 W/ft2 (131.3 W/m2).

Rooftop PV arrays should be located in areas and positions that avoid any extended shading impacts. Typically, the optimal orientation for the PV array when mounted in a set of fixed racks is south for sites located in northern hemisphere and north for sites in the southern hemisphere. For the same fixed-rack PV systems, the optimal tilt angle is slightly less than the site latitude angle even though detailed analysis is recommended to account for the special architectural features and local weather patterns.

Batteries

The size of batteries and any other electrical storage systems depends specifically on the magnitude and the duration of the electrical loads to be met by these storage systems (i.e.. hours or days when the primary electricity sources are not available, including the PV systems and/or the utility grid). In particular, the average daily electricity use determines the capacity of the required batteries in terms of the maximum and average depth-of-discharge rates. The main characteristics required to specify a battery suitable for a stand-alone or a grid-connected PV system include its capacity, rate of charge and discharge, and the state-of-charge as briefly outlined. [1]

For PV installations with batteries, a storage or charge controller is highly recommended to limit the discharge rate and ensure that the batteries do not over discharge and maintain a minimum level of depth-of-discharge. The charge controller can also provide protection against overcharging the batteries from the source of electricity (i.e., PV panels). For safety, the voltage rating for the batteries should not exceed 50 V for residential buildings. Typically, 48 V battery banks can be obtained by connecting 24 of 2 V individual batteries in series. When batteries over 50 V ratings are used, they should be located in protected enclosures accessible only by authorized personnel.

The capacity of a battery bank required for a building-integrated PV system can be determined based on the autonomy goal, that is, the desired number of days for a fully charged bank can meet safely the desired building electrical loads. For instance, a battery of 1,000-Ah can safely meet 250 Ah daily load for three consecutive days. Indeed, the remaining storage capacity is 250 Ah at the end of the third day, which is higher than 200 Ah (i.e., 20% of 1,000 Ah), the desired minimum allowable depth-of-discharge. For critical applications of stand-alone PV systems, such as vaccine refrigeration and telecommunication structures, autonomy periods of at least three days are recommended to estimate the battery capacity. When hybrid systems are specified to combine PV systems and CHP or other generation systems, lower autonomy periods can be used. In particular, an 8-hour period of autonomy is sufficient for the size of the battery for grid-connected PV systems.

The charge controllers for the batteries have maximum input voltage and current ratings that the PV system should not exceed when generating electricity. Specifically, the maximum current rating of the charge controller should exceed 125% of the short-circuit current, Isc, of the PV array based on NEC requirements (NEC, 2017). Moreover, the maximum voltage of the charge controller should be larger than the open-circuit voltage, Voc, of the PV array. The operation of the charge controllers are controlled by various set points to regulate the charging and discharging rates of the batteries using disconnects linked to the PV array to limit overcharging as well as to the load to avoid overdischarging. The common set points used by charge controllers include the following:

  • • Voltage regulation (VR) set point defines the maximum voltage level the battery is allowed to reach before disconnecting it from the PV array in order to avoid overcharging. A set point for the array reconnect voltage (ARV) can also be set for interrupting-type controllers to allow the battery to receive current again from the PV system and thus to be charged.
  • • Low-voltage disconnect (LVD) set point is the voltage threshold the battery is allowed to attain before it is disconnected from the loads in order to limit the overdischarging and exceeds the desired depth-of-discharge level. When the battery is sufficiently recharged and reaches the load reconnect voltage (LRV) set point, the load is again connected.

The charge controllers can have other functions to optimize the charging and discharging levels of the batteries. In addition, the charge controllers can include MPPT capabilities to ensure that the PV array operates to generate the maximum power, as noted in Figure 9.5, during various operating conditions. MPPT charge controllers can be effective in maximizing the generating power of the PV systems, especially during cold and sunny days.

  • [1] Size of a battery expresses the total energy storage capacity in terms ofAmpere-hours (Ah) or kilowatt-hours (kWh). For instance, a 12-V batterywith a capacity of 480-Ah can store 5.76 kWh. In addition to its age, thedischarging/charging rates and the operating temperatures can affect theactual capacity of the battery. Specifically, high discharge rates and lowoperation temperatures reduce the battery capacity. • Rate of discharge or charge (expressed in Amps) for a battery providesthe ratio of the battery capacity (expressed in Ah) and the time duration(expressed in hours) of the discharging and charging period. For example, a480-Ah battery has a discharge rate of 48 A over a 10-hour period. • State-of-charge expresses in percentage the available storage capacity compared to the rating when the battery is fully charged. The depth-of-dis-charge is the percent of the storage capacity depleted from a fully chargedbattery. Thus, the state-of-change and the depth-of-discharge add to 100%.For most batteries, the maximum allowable levels of depth-of-discharge inany cycle is 80% so that the state-of-change is kept at least 20% in order toprotect the battery, especially during extreme cold conditions.
 
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