Photovoltaic Systems

Due to their lower costs, PV systems are now common features of buildings since an increasing number of national and local codes are requiring net-zero energy and even net-positive energy designs for new and deep retrofit constructions. In Section 9.3.3, first different PV system configurations are discussed. Then, a simplified analysis procedure is presented to evaluate the annual energy generated and the cost-effectiveness of PV systems. Finally, specific components and their design guidelines for a building-integrated PV system are described.

PV System Configurations

Several configurations for PV systems can be specified for residential and commercial buildings. The most common design categories include stand-alone systems for buildings typically located in remote sites and grid-connected PV systems for buildings served by electrical utilities. A brief outline is provided of the two main PV system design categories and their associated components.

9.3.3.1.1 Stand-alone PV Systems

Generally, for stand-alone applications, the PV system is the only generation source of electrical power needed to serve all the building loads. However, in order to reduce the size and the installation cost of the PV systems, fuel-fired generators

Typical layout of a stand-alone PV system

FIGURE 9.6 Typical layout of a stand-alone PV system.

such as those outlined in Section 9.2 can be added as part of stand-alone systems. Several configurations can be considered for PV stand-alone systems depending on the building type, site, and needs. The simplest form of stand-alone PV systems is powered exclusively from the PV array without the need of any storage and inverting capabilities to serve DC electrical loads. However, even when the building has only DC electrical loads, batteries are included nearly universally to ensure continuous power supply. When some or all the building loads require AC power, an inverter is needed to convert DC to AC power. Figure 9.6 illustrates a layout for a PV stand-alone system that serves both DC and AC loads. In addition to the PV array, a battery, and an inverter, the PV stand-alone system shown in Figure 9.6 has a charge controller to regulate the battery operation as well as a combiner box that houses the various disconnects and protection devices for the PV array. The load centers are generally power panels that include protection devices for the branch circuits that serve various building electrical loads. The basic features and design considerations for all the components of PV systems are described in more detail in Section 9.3.3.3.

9.3.3.1.2 Grid-Connected PV Systems

Grid-connected PV systems are suitable for most buildings located within communities served by the utility grid. Several configurations can be considered to design grid-connected PV systems based mostly on cost and resilient requirements. In particular, several systems are designed without storage capabilities (i.e., batteries) since the grid can provide power when the PV array does not generate any electricity. However, these systems depend significantly on the grid both when the PV system generates excess power and during nights and cloudy days. Figure 9.7 shows a grid- connected PV system with a battery bank connected to an inverter to meet typically critical AC loads.

Typical layout of a grid-connected PV system with a battery bank

FIGURE 9.7 Typical layout of a grid-connected PV system with a battery bank.

Analysis of PV System Performance

The performance of the PV system depends mainly on the power generated by its PV arrays made up of connected modules that include several cells. The efficiency of PV cells and PV arrays is typically by their I-V curve as presented in Figure 9.8, measured under standard test conditions (STC) with an ambient temperature of 20°C, irradiance of 1000 W/m2, and wind speed of 1 m/s. Specifically, Figure 9.8 indicates five key parameters that characterize the PV system, and they are typically included as part of the nameplate ratings provided by the manufacturers. These parameters include open circuit voltage (Voc), short circuit current (Isc), maximum power voltage (VMP), maximum power current (/w>), and maximum power (PMAx)- A maximum power point tracking (MPPT) is commonly used to ensure that the PV arrays operate to generate the maximum power, PMAX, under various loading and ambient conditions.

Typical current-voltage or I-V curve for a PV array

FIGURE 9.8 Typical current-voltage or I-V curve for a PV array.

The conversion efficiency of commercially available PV modules ranges from 13% to 23% under rated conditions even though higher efficiencies are reported mostly for laboratory prototypes. While most of the PV panels used in rooftop systems are made of crystalline silicon cells with efficiency around 15%, thin-film PV modules are being used for building integrated PV applications (such as shingle-PV systems) and their efficiency is improving and reaches currently 20%. The efficiency of PV cell is affected by various operating conditions, including the cell temperature as indicated by Eq. (9.4):

where

  • • qR is the cell efficiency at TR.
  • T(. is the cell temperature.
  • • (3 is the temperature coefficient of efficiency, which should be obtained from the manufacturer’s specifications.

In particular, Eq. (9.4) indicates that the efficiency of the PV cell and the entire PV system is reduced when the cell temperature increases due to higher ambient air temperatures.

A simple way that can be used to assess the overall PV system performance is to utilize the efficiencies of all of its components. In particular, for a typical PV system with a PV panel, an inverter, and an MPP tracker, the overall system efficiency can be estimated as follows:

where

  • • 1bv,Modui< is PV panel efficiency.
  • rmv is inverter efficiency (typically over 95%).
  • • Пmppt is maximum power point tracker efficiency (typically over 95%).
  • • is voltage regulator efficiency (typically over 95%).

The electrical energy generated. Egen, by the PV system can be estimated on hourly basis:

where

  • • APV is the area of the PV panel
  • G„e, is the net incident radiation per unit area hitting the PV panel and depends on the tilt angle, orientation, and tracking system used for the PV system.

Several tools are available to estimate the electrical energy generated by PV systems, such as PVWatts, TRNSYS, and PV-Chart (Krarti, 2017). In order to maximize the power output of PV panels, tracking and/or concentrating systems are utilized. Tracking systems allow solar panels or reflectors to be oriented toward the sun to ensure optimal collection of solar radiation. The main goal of a tracking system is to minimize the incidence angle of solar rays hitting a PV panel. Two types of tracking systems are commonly used for PV panels: single-axis trackers and dual-axis trackers.

  • Single-Axis Trackers. These systems allow the PV panels to rotate around one axis: either vertical or horizontal. The vertical axis trackers are recommended for high-latitude locations where the sun position is low and the summer days are long. The horizontal axis trackers are more suitable for tropical regions.
  • Dual-Axis Trackers. These systems can allow the PV panels to rotate around both vertical and horizontal axes. Therefore, the dual-axis trackers can follow the sun’s position independent of the PV system location.

Figure 9.9 compares the monthly electrical energy generated from a 500-kW PV array with (a) fixed tilt, (b) 1-axis tracking system, and (c) 2-axis tracking system located in three sites: Boulder, Berlin, and Riyadh. As noted in Figure 9.9, the performance of the PV system depends significantly on the location as well as the mounting and tracking type.

In order to assess the cost-effectiveness of adding rooftop PV systems as part a retrofit measure for existing buildings, cost analysis methods outlined in Chapter 3 can be used. In particular, the LCC or levelized cost of energy (LCOE) can be considered for PV systems to determine their cost-effectiveness compared to the baseline case of simply purchasing all the electricity needs from the grid. For renewable energy systems, the LCOE can be estimated as indicated in Chapter 3 [refer to Eq.(3.31)], using the installation cost, IC, and the operation and maintenance cost rate, r0&M:

Instead of Eq. (9.6), which requires hourly analysis, the annual generated electricity, Egen, can be readily estimated as follows:

where

  • CAP is the capacity of the system.
  • CF is the capacity factor of the system.
  • downtime is the fraction in percent when the system is not operating during one year.
Monthly electricity generated from a 500-kW PV systems with (a) fixed tilt

FIGURE 9.9 Monthly electricity generated from a 500-kW PV systems with (a) fixed tilt,

(b) 1-axis tracking system, and (c) 2-axis tracking system for three locations: Boulder. Berlin,

and Riyadh.

The cost-effectiveness of the PV system depends highly on the installation costs and the available incentives available to promote building integrated renewable energy technologies. Example 9.3 illustrates the LCOE and LCC-based analyses to assess the cost-effectiveness of a rooftop PV system relative to the grid.

Example 9.3

Consider a 5-kW rooftop PV system that can be installed at a cost of $3000/kW and maintained at a cost of $0.10/kWh. The building is connected to the grid, which provides electricity at $0.10/kWh. Assuming the PV system has a capacity factor of 20%, an annual downtime of 2%, and that the grid does sellback electricity, determine the following based on a life cycle of 20 years and a discount rate of 5%:

a. The LCOE of the PV system.

b. The LCC of the PV system and the baseline (i.e., grid only) system when the average daily electrical load for the building is 75 kWh/day and the power demand is never below 5 kW during daytime throughout the year.

SOLUTION

a. LCOE Estimation

Using Eqs. (9.7) and Eq. (9.8) with CAP = 1 kW for the PV system, Egon and LCOE values based on the parameters provided for this problem, including USPW = 15.37 years, CF = 0.20, 1C = $3000, rOAM = $0.01/ kWh, and downtime = 2%:

b. LCC Analysis

Using the LCC analysis outlined in Chapter 3, the LCC can be estimated based on the installation costs (1C = 5 kW x $3000/kW = $15,000) and EC the annual energy cost, EC:

The annual output for the PV system with a capacity of 5 kW can be estimated as indicated by Equation (9.8):

Without the PV system, the grid has to supply the annual building electrical load of Ebldg = 75 kWh/day x 365 days/year = 27,375 kWh/year.

For the grid-connected PV system, the grid has to provide the following annual load to meet the 75-kWh/day load during 365 days/year:

Using the LCC analysis, the LCC value for the baseline (i.e., grid only) and the grid- connected PV system can be estimated as:

And

Combined with the results obtained using the LCOE analysis, the grid-connected PV system is not cost-effective compared to the grid-only option.

 
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