GreenMountain Engineering

PV in the 1960s

For some quick entertainment, I thought I'd share a few scientific papers about PV from "back in the day".  From well before the 1977 founding of the Solar Energy Research Institute (now NREL), and back when oil was less than $20 a barrel (in 2009 dollars).

In 1963, the year the Beatles released their first album and Iron Man debuted as a comic book character, Joseph Loferski published "Recent Research on Photovoltaic Solar Energy Converters". This paper described silicon's lead as a material for high-efficiency (15% efficient, that is) solar cells, but mentioned some of the other materials under exploration, such as GaAs (13%), CdTe (6%), and CdS (6%). Note that the record 1-sun cell efficiencies of these materials are currently 25% (Si PERL cell, 1999), 26.1% (GaAs thin-film, from 2008 I believe), and 16.7% (CdTe, 2001).

In 1964, the year the Shinkansen high-speed rail system was inaugurated in Japan and Dr. Strangelove was released (and nominated for four Academy Awards), R. J. Tallent and E. J. Zapel published "Structrual and Electrical Performance of a Concentrating Solar Cell Panel". This paper described the design of a CPV module that used reflective aluminum to reach a 1.9x concentration ratio, as shown in the module and system test images below. I'm a fan of the Boeing Solar Research Laboratory bus.

And in 1965, the year of the civil rights march from Selma to Montgomery, DEC's unveiling of the PDP-8 microcomputer, and the first wide area network connection (between Massachusetts and California, the states that house the two GreenMountain Engineering offices), E. L. Ralph published "Use Of Concentrated Sunlight With Solar Cells For Terrestrial Applications", another early CPV paper describing a simple conical optic, the need for tracking, and an increase in efficiency with low concentration (via an increase in Voc), which must be balanced against losses due to series resistance and increased cell temperature.

Cheers and Happy Holidays!

This blog will likely go on hiatus until 2010, as we focus on wrapping up various end-of-year project work.

New Tracker Accuracy Paper

I just finished up a busy week with Chris at the EUPVSEC 24 in Hamburg. I plan to post something more about the conference in the coming week, but several people have asked me for a copy of the poster we presented about solar tracker accuracy (co-authored with assistance from ISFOC)-- so the paper is now available for download on our papers page.

It includes a dozen sets of measured, real-world accuracy data for a variety of different solar trackers by different manufacturers (mostly trackers designed for concentrating photovoltaic applications with more strict accuracy requirements). If you have any feedback or questions, I'd be interested to hear them-- you can email me at mdavis (at) greenmountainengineering.com

IEEE PVSC 34 Paper Available

My sources tell me that Brandon and Max had a great time mingling with the "solarati" at the IEEE Photovoltaic Specialists Conference last week, and the presentation was a success!  The full paper (abstract below) is available on our Papers page.

Solar Tracker Accuracy: Field Experience, Analysis,
 and Correlation With Meteorological Conditions

B. Stafford1, M. Davis1, J. Chambers1, M. Martínez2, D. Sanchez2
1GreenMountain Engineering, LLC, San Francisco, CA, and Somerville, MA, USA
2Instituto de Sistemas Fotovoltaicos de Concentración S.A., Puertollano, SPAIN

Abstract: Tracker performance is a significant factor in the energy production of PV systems, especially concentrating photovoltaic (CPV) systems. The nonlinear relationship between CPV optic pointing accuracy and energy production means that simple metrics such as mean pointing accuracy are not sufficient for predicting performance of these systems.

Additionally, trackers are currently a significant component of system cost. Understanding the real causes of tracking errors in the field (as well as which types of errors have a less significant impact on energy production) is an important step towards the development of lower-cost tracking systems.

In this paper we present a collection of real-world data, including tracking error and corresponding meteorological data, gathered over a period of months from commercial solar trackers installed at Instituto de Sistemas Fotovoltaicos de Concentración S.A. (ISFOC) in Puertollano, Spain. We present several relevant mathematical tools for analyzing this data, and draw conclusions about the implications of tracking errors for system energy production.

Full Paper

Solar Tracker Characterization

The question of tracking was one of the many CPV-related topics discussed at the recent ICSC-5 conference and the adjacent meeting of the IEC Technical Committee 82 / Working Group 7.

I gave a talk about tracking accuracy covering several possible metrics and summarizing real-world field data from a variety of CPV trackers collected with the Trac-Stat SL1. If you’re interested, you can view the presentation here. It wasn’t designed to be a stand-alone presentation, so some of it may not be clear without the accompanying discussion. At the very least, the background on tracking and slides of data may be of interest.

Overall, there was agreement that when describing tracking accuracy, a “direct” method must be used - that is to say actual outdoor measurement of the angle between the sun in the sky and the pointing axis of the tracker using some optical instrument - not an indirect method based on a tracker controller’s internally-reported position or the calculated precision of software algorithms or encoders. 

There are so many mechanical sources of error in a tracker (manufacturing and assembly tolerances of the tracker, foundation, and modules, structural deformation, thermal expansion, and so on) that measurement is essential. 

Make plans based on calculations, make decisions based on data.

See you at ICSC-5!

Three engineers in our San Francisco office have been inspiring plenty of jealousy because they're heading down to Palm Desert next week for the 5th International Conference on Solar Concentrators (ICSC-5).

In the Wednesday morning session, Max Davis will give a presentation titled Understanding Tracker Accuracy and its Effects on CPV, co-authored by Tyler Williams of GreenMountain with María Martínez and Daniel Sanchez of ISFOC.

We'll post the presentation after the conference, but drop us a line if you'll be there.