Solar

Update to solar electric incentive change

On October 19, we announced that we would reduce incentives for residential and commercial solar electric systems, effective November 9. During that three-week period, we received more than 290 incentive applications totaling more than 14.5 megawatts of capacity. Unfortunately, the amount of funding requested exceeded the budget resources available to the solar electric program for 2009 in Pacific Power territory.

Energy Trust solar incentives are subject to availability of funding and offered on a first-come, first-served basis. Complete incentive applications for projects in Pacific Power territory received prior to the point when the Pacific Power budget became fully subscribed will be accepted at the former, higher incentive rates. Applications received after that time will be eligible for the new, lower incentive rates, pending approval of sufficient funding in our 2010 budget on December 18, 2009. Read the full announcement that was emailed to solar electric trade allies on November 20.

Clarification to the active project limit for trade allies

As reported in last month's INSIDER, the active project limit for individual trade allies has been increased to 30 projects total, including no more than 12 non-residential projects and no more than 12 Pacific Power projects. Please note that now the project count per trade ally is based on the number of sites rather than the number of customers in the trade ally's queue. For example, if you are applying for systems on five "Joe's Supermarkets," it will count as five projects in your company's active project list. Additionally, third-party system owners are subject to the same active project limit as the trade allies. Third party-owned projects will count toward both the trade ally's and the third party system owner's active project limits.

The active project limit requires that trade allies complete some projects before beginning others, helping Energy Trust spread its incentives over time and support a broad solar industry. Please contact if you have any questions about your active project queue.

Notes from the field: String sizing

One of the basics of good photovoltaic system design is sizing the string(s) of modules to work well with a given inverter. Many inverter manufacturers provide string-sizing calculators that perform the basic calculations for you. However, these online calculators are not guaranteed and generally should not be used in place of crunching the numbers yourself. Unfortunately, Energy Trust often has to reject applications where the trade ally has relied on an online calculator without looking carefully at the numbers.

We recommend that contractors perform at least two basic string-sizing calculations: 1) find the open circuit voltage at record low temperature, and 2) find the maximum power voltage at record high temperature. As a refresher, these calculations are presented below:

Open circuit voltage at lowest temperature

If a string is too large, then at very low temperatures there is the potential for a voltage excursion that could damage the inverter. To calculate the highest open circuit voltage, gather the following information from the module data sheet and a weather almanac, such as www.weather.com (search on your zip code, and then select Averages).

Variable:	Symbol:	Example:
Open Circuit String Voltage at Standard Test Conditions		36.9 V
Open Circuit Voltage Temperature Coefficient		-0.33%/°C
Record Low Ambient Temperature		8°F = -22.2°C
Lowest Cell Temperature		8°F = -22.2°C

Given this information, the equation below can be used to calculate the highest open-circuit voltage expected from the string, where n is the number of modules in series:

For example, for a string of 12 modules:

If the maximum voltage calculated is above the inverter voltage window, please consider a different string configuration.

Maximum power voltage at highest temperature

If a string is too small for an inverter, it will be prone to dropping out of the voltage window at high temperatures, reducing energy output just when the most solar resource is available. To calculate maximum power voltage at record high temperature, first gather the following information in the table below. Note that cell temperature will be considerably higher than ambient temperature. For the purposes of this equation, Energy Trust assumes that an array mounted on a roof will be 35°C above ambient temperature (30°C for a rack mount system or 25¡C for a pole-mount system).

Variable:	Symbol:	Example:
Maximum Power String Voltage at Standard Test Conditions		29.6 V
Maximum Power Voltage Temperature Coefficient		-0.45%/°C
Record High Ambient Temperature		106°F = 41.1°C
Highest Cell Temperature		169°F = 76.1°C

Given this information, the equation below can be used to calculate the lowest maximum power voltage expected from the string at high temperature, where n is the number of modules in series:

For example, for a string of 9 modules:

If the maximum power voltage is below (or even close to) the minimum inverter voltage, you should consider a different design. It's a good idea to do this same calculation with average high temperature. Remember that system voltage will degrade over time, so you don't want to size a system too close to the bottom of the voltage window.

Our inspectors use Energy Trust's string sizing calculator, developed by Renewable Energy Associates. You're welcome to download and use this Excel-based tool, as well, but it never hurts to double check the numbers with the equations above.

For more information, visit Energy Trust's Renewable Energy—Solar program pages, or call 1.866.368.7878.