Inverter Shoot-Out continues [RE-wrenches]

Antony Tersol tony at appliedsolarenergy.com
Tue Mar 30 09:01:02 PST 2004


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As a wrenching organization, our focus is also overall system value, but from a slightly different perspective than the CEC's.

In order to determine the value of a system, among my concerns are:

1. Properly configuring potential systems.  This requires knowing the characteristics of both the modules and the inverters, and understanding the interplay between those characteristics and the range of conditions expected at the installation site (daily and yearly insolation, temperature, wind, sun position, ...), OR relying on others (distributors, manufacturers, other wrenches) for pre-configured systems that are properly engineered for the locations of my customers.

2. Predicting the real-world performance so that I can estimate the economic value and set realistic expectations for my customers.  In addition to system performance characteristics, this requires knowing the local conditions (weather, shading, ...) and having some method of simulation.  The model can only be verified by comparison with actual real-world performance.

3. The reliability of components, and the customer service of the manufacturers.  Here I must also be concerned with the business model and ethics of the manufacturer.  Will they still be around through the life of the system (or at least the life of the warranty)?  How responsive are they when there is a problem?  What is the MTBF of their products?  Do they readily provide the numbers that characterize their product?  (module makers - where are your temperature coefficients?)

For these reasons, we need both the real world performance data of complete systems AND laboratory testing of equipment to isolate the variables so that the individual components are characterized.  Good testing protocols and reasonable quality control by the manufacturer would lead to meaningful results with the testing of a few units of each model (the Swiss module testing facility randomly chooses from distributor's stock to prevent manufacturer's "cherry-picking" the test subjects - http://leee.supsi.ch/ Look at LEEE-Tiso).

Because of the variability of the real world, real world performance data needs to be available from large numbers of systems.  This would allow us to refine our predictions and simulations, akin to translating auto sticker MPG into our  own driving experience.  The real world data will only be meaningful if we understand the causes of the variability.  Where will this data come from?  Back in January I offered to collect this data from the wrenches.  Only Joel responded.  Is he the only one who has collected data, or is there no interest in real world performance?  

We also need performance data about the manufacturers themselves.  This is where this list contributes, both in allowing us to see firsthand the responsiveness of the companies, and through the sharing of wrenches' experience with customer service.   

Antony

-----Original Message-----
Date: Mon, 29 Mar 2004 08:46:03 -0800
From: "Bill Brooks" <billb at endecon.com>
Subject: RE: Inverter Shoot-Out continues [RE-wrenches]



Ezra,

The focus of our work with the CEC is overall system value and I agree with
you that system performance is more important than any one component's
performance.

In order to determine the value of a system, one must consider many, many
things. Here are just a few:

1. System documentation--does it explain the installation well enough for a
qualified person to install it properly so it will work properly?

2. System hardware--is the system hardware (not just modules and inverters)
designed to last as long as the solar array--materials selection, quality,
etc...? In the case of the inverter, what is the expected life and what is
to be done when and if it fails?

3. System troubleshooting--if a problem occurs, how does the customer find
out that there is a problem and how easy is it for the service person to
troubleshoot the issue?

4. System performance--does the system perform as expected?

This performance issue is the portion we have been discussing. It is only
one of several important issues--but the longterm performance is certainly
the most important of all issues. How do you determine system performance?
One way is to characterize each part that impacts performance for a generic
system (not counting orientation, shading, and soiling which are site
issues)--the PV array and the inverter. Computer modelling of the array and
inverter can help provide some initial sense of matching and performance.
That's why we need the inverter test--to characterize it. Lastly, a one or
two month field test of the product is needed to validate that the model has
accurately predicted actual performance.

Of course this does not necessarily address system degredation and other
important issues, but there is data for accounting for these issues that can
be incorporated into long-term models fairly easily. When you get into
predicting performance in year 17, models are just educated guesses anyway.

Bill.

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