Minimum PV source size

The size of the PV source, which is determined based upon the load evaluation and weather and solar resource evaluation will determine the necessary size of the charge controller. The charge controller must be selected at the same time as the PV source as the charge controller type - PWM or MPPT - will also determine the possible configurations of PV modules.

In this phase of the design process, more than in any other phase, it is necessary to explore different designs using PV module, series and parallel wiring configurations, and charge controllers in order to achieve the highest performance at the lowest cost possible. This phase may have to be performed several times.

An off-grid PV system that depends upon the PV as its single charging source requires an array that is sufficiently sized to be able to generate sufficient energy to both meet the energy needs of the users and to recharge the energy storage system under less than ideal conditions. Any sizing decisions should therefore lean towards an oversized PV source.

Step 1: Deteremine PV source loss parameters

The power rating of a PV module is generated under ideal testing conditions in a laboratory (See Standard test conditions), thus to build a PV system that is going to generate the necessary amount of energy under less than ideal conditions it is necessary that the estimated production that a PV module will generate is reduced to function under the anticipated conditions of use. There are many different parameters that need to be accounted for and without performing extensive testing each of these parameters will largely be an estimate on the part of the designer. It is important to use conservative numbers as an over-sized system is always preferrable to an under-sized system.

• Module degradation parameter: The PV module degradation parameter accounts for the gradual reduction in the performance of PV modules over time. This is a factor that should be taken into account as a PV system should be designed to function for more than a decade. Module degradation not only affects performance, but can lead to a reduction in voltage over time that impedes the proper charging of the energy storage system. The amount that the performance and voltage decreases per year largely depends upon the quality of the module:

• High-quality modules: approximately .33% per year leading to a value over 10 years of .967 (96.7% of original power rating).
• Lower-quality modules or generic brands: approximately .5% per year leading to a value over 10 years of .95 (95% of oiginal power rating).

• Shading loss parameter: Parameter that accounts for any production losses that the system may experience due to shading from nearby obstacles - mountains, trees, other buildings - during the course of the year. The only way to get a accurate value for this parameter is to evaluate the shading at the site using a took like a the Solar Pathfinder. The value for this parameter is completely site dependent. If it is anticipated that there will be no shading losses, then the value for this parameter should be 1. If there is a 5% loss, then a value of .95 should be used.
• Soiling loss parameter: Parameter that accounts for the build-up of dirt on the surface of a PV module, which reduces production as it doesn't allow light to reach the cells of the module. The appropriate value for this parameter depends primarily upon the climate and location in which the system is installed, but also the tilt and cleaning frequency of the PV modules. Frequent rainfall helps PV modules to self-clean. If a system is installed in a location with minimal windblown dust that receives regular rainfall or in which modules are cleaned weekly, then losses may be as low as 1%. If a system is installed in a location with significant windblown dust that receives very infrequent rainfall or in which modules are never cleaned, then losses may be as high as 50%. With PV module cleaning every two months a value of .97 (3% loss) is recommended for locations with minimal windblown dust and a value of .93 (7% loss) is recommended for locations with significant windwblown dust.
• Wiring loss parameter: Parameter that accounts for the loss of voltage due resistance in DC system wiring. The actual value depends upon the system design. Total DC wiring losses - roundtrip from PV source to energy storage system - should be kept below 4%. A value of .96 (4% loss) is recommended for this parameter.
• Module mismatch parameter: Parameter that accounts for the loss of production that results from PV modules having different characteristics that result from manufacturing inconsistencies. This only applies for systems that have more than 1 PV module. The proper value varies depending upon the module used, but a value of .98 (2% loss) is recommended for this parameter.
• PV source temperature loss parameter: Parameter that accounts for losses that result from PV module cell temperatures in excesss of the standard test condition value of 25°C, which are common except in the coolest locations on earth. Cell temperatures are generally 10C-30ºC above ambient temperatures in full sunlight, therefore ambient temperatures must be around 0ºC for cell temperatures to be at 25ºC. The temperature of a PV module depends upon the climate, insolation, and how the module is mounted. PV module manufacturers provide a loss coefficient - typically called Temperature coefficient of max power and measured in %/°C - on the specifications sheet of each module that can be used to estimate losses based upon the conditions at the site. The value from the specifictions sheet of a module can be used in these calculatons if a module has been chosen, but a standard average value of (-.48%/°C) will work for both poly and monocrystalline modules.

The mounting system will also affect the ability of the PV source to cool itself. A mounting system temperature adder should be added to the maximum temperature that is used to calculate losses due to temperature:

• 20°C for pole mount
• 25°C for ground mount
• 30°C for roof mount

PV source temperature loss parameter	= 1 + (maximum ambient temperature + mounting system temperature adder - 25°C) x temperature coefficient of max power %/°C ÷ 100)

Total PV source loss parameter	= module degradation parameter × shading loss parameter × soiling loss parameter × wiring loss parameter × module mismatch parameter × PV source temperature loss parameter

Step 2: Determine charge controller type

There are two different charge controller types - pulse width modulation (PWM) and maximum power point tracking (MPPT) - each of which has advantages and disadvantages that are are detailed in Charge controller types. The charge controller will be sized further on, but a charge controller type must be selected at this point to proceed with the design.

PWM:

The PV source must be configured to operate at the charging voltage of the energy storage system and below the maximum PV source current rating of the charge controller.

• Nominal system voltage: 12V, 24V, 48V.
• Maximum PV source current: 6A-60A

MPPT:

The PV source must be configured to operate below the maximum PV source voltage rating of the charge controller, above the minimum PV source voltage based upon the maximum charging , and below the maximum PV source current rating of the charge controller.

• Nominal system voltage: 12V, 24V, 48V
• Maximum PV source voltage: varies up to 600V
• Minimum PV source voltage: depends upon nominal voltage and charge controller type
• Maximum PV source current: up to 100A+

Step 3: Charge controller efficiency parameter

All charge controllers lose a certain percentage of all energy that is produced as heat as it is transferred to the energy storage system and loads. PWM charge controllers are more efficient than an MPPT charge controller, nonetheless the design of a MPPT charge controller will lead to it outproducing a PWM charge controller under most conditions - see Charge controller for more info. Specifications sheets for MPPT charge controllers will often give a maximum efficiency rating that is typically an overestimate - it is better to use a more conservative value. Two seperate designs may be performed with each type of charge controller to determine the best systerm design. The specification sheet for a particular MPPT charge controller can be consulted for a maximum effiency number, but this number will typically be an over-estimation of efficiency.

Charge controller efficiency parameter values for different charge controller types:

• Pulse width modulation(PWM) charge controller: .99 (99% efficient)
• Maximum Power Point Tracking (MPPT) charge controller: .95 (95% efficient)

Step 4: Energy storage efficiency parameter

All energy storage systems lose a percentage of all energy produced to heat as it is stored and released to power loads.

Energy storage efficiency parameter values for lead acid battery types:

• Flooded lead acid (FLA) batteries : .75 (75% efficient)
• Valve-regulated lead acid (VRLA) batteries: .85 (85% efficient)

Step 5: Deteremine minimum size of the PV source

Finding the minimum size of the PV source will inform the rest of the design.

Minimum PV source size	= average daily Watt-hours required ÷ design insolation ÷ Total PV source loss parameter (from step 1) ÷ Charge controller efficiency parameter (from step 3) ÷ Energy storage efficiency parameter (from step 4)

Notes/references

National Renewable Energy Laboratory - Understanding Light-Induced Degradation of c-Si Solar Cells