UTILITY GRID-TIE PV SYSTEM DESIGN

Utility Grid-Tie PV System Design consist of PV modules, output cables, a module mounting structure, AC and DC disconnect switches, inverter(s), grounding equipment, and metering system, as shown in the diagram below. The Grid-Tie System Worksheet is designed to help contractors size a PV array to offset all of their client's electrical usage with the largest system that would be cost-effective to install. A smaller system can reduce part of the electric bill, and in locations with tiered or progressive rates, it may have a faster financial payback. Compare the worksheet result with the amount of space available to mount the PV array in order to get a rough idea of the maximum PV array size.

Above is a diagram of a typical battery-less grid-tie system (utility inter-tie). Many grid-tie inverters have built-in DC disconnect switches, while some have both a DC and an AC disconnect. Some models also contain a PV array string combiner so a separate one may not be necessary. Separate over-current protection of each series string of modules in a PV array (typically provided in the array combiner box) is required only if there are three or more series strings of modules connected to a single inverter input. Inverters with multiple MPPT input channels can have one or two series strings per channel without individual string fusing.

Worksheet: Grid-Tie PV System Design

Determine PV array size for a grid-connected system.

________ Step 1: Determine the daily average electricity usage from the electric bills.

This will be in kilowatt hours (kWh). Due to air conditioning, heating, and other seasonal usage, it is a good idea to add up all the kWh for the year and then divide by 365 to find the average daily usage.

________ Step 2: Find the location's average peak sun-hours per day. Calculate 3 - 5 sun hours.

________ Step 3: Calculate the system size (AC watts) needed to offset the average usage.

Divide the daily average electricity use by average sun-hours per day. For example, if the daily average electricity use if 30 kWh and the site is in Central California, system size would be: 30 kWh / 5 h = 6 kW AC. Multiply kW by 1000 to get AC watts.

________ Step 4: Calculate the number of PV modules required for this system.

Divide the system AC watts in Step 4 by the watt rating of the modules to be used, and then divide by the inverter efficiency, usually 0.94 to 0.98, to get the total number of modules required. Round this number up to the next whole number of modules. For best results, use the module's PTC watt rating that is found on the California Energy Commission's Approved Equipment List (www.gosolarcalifornia.org/equipment) rather than the manufacture's peak rating.

Select inverter(s) and determine string configuration

________ Step 5: Select the inverter/module combination from the table on the below that is 

closest to the desired system size.

The table on below shows inverter and module combinations for our most popular modules and grid-tie inverters with a 600 VDC maximum voltage limit. For a given inverter and module combination the table displays the recommended number of series strings of modules and the number of modules per string for temperatures between 14 °F and 104 °F. Where the inverter will support more than one string of modules, the table shows the number of modules that can be used with multiple strings.

Sizing is accurate in locations where the maximum temperature is lower than 104 °F and the minimum temperature is higher than 14 °F. In locations where the minimum temperature is lower than 14 °F, the maximum number of modules per string may be lower.

The line labeled "PTC" is the expected output of the modules at normal operating temperature in full sun. the approximate power output of a system in full sun will be the number of modules multiplied by the watt rating of the modules and then multiplied by the inverter efficiency from the second column in the table. Other factors, such as high or low temperatures, shading, array orientation, roof pitch, and dirt on the modules, will affect the system's actual output.

To learn more about the products you will need for a residential and/or commercial application visit: Net-Energy.us Your place for renewable energy products.

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