35,000-kW-hr Later- A User's Experience of Photovoltaic Solar Panels
Authors: Teale G.S.M. 
Published: December 2007

Petroleum Development Oman (PDO) has nearly 3 years' field experience with photovoltaic solar panels powering an extensive chain of microwave radio repeaters along main oil pipelines. Solar generator and load currents are integrated by onsite instrumentation and the results have been recorded monthly. Computer analysis has allowed detailed performance monitoring as well as building up a data bank for the economic design of new stations.

Introduction

PDO owns and operates a 620-mile [1000-km] microwave chain extending through a substantial part of Oman to provide telephone and data links in support of oil operations. Nineteen of the 28 microwave sites are located remote from other oil installations and they are powered by photovoltaic solar panels. While companies in many parts of the world have been experimenting with solar panels for powering microwave radio equipment, PDO is unique in having almost 3 years' operational experience with an extensive chain of solar powered sites. This paper highlights some of the factors that we found to be important and discusses the systematic gathering of results. These results provide a data base to chart the performance of the existing solar panels and serve as an aid in the design of future sites. For terrestrial applications, there is a paucity of life test data and it is important to spot any significant deterioration before it affects the PDO communications system.

Typical Site

The block diagram of the solar power system at a typical site is shown in Fig. 1. Note that although the power supply is divided into two, neither part on its own is sufficient to run the station indefinitely. Integrators measure cumulative charge from the two panel banks (A and B), the load current, and the insolation (peak sun hours), which is the amount of sunshine. The latter is measured directly by the equipment in kW-hr/m2. Results are displayed on nonresettable mechanical counters.

Important Factors

In Oman, dust plays a very significant part in panel output. Monthly cleaning (brushing) of panels is essential at all sites. The scale of the dust problem is such that the charge current can increase by up to 30% after brushing. At one location, Haima, where the solar panels are mounted on the tower, bird droppings have the serious effect of obscuring individual cells in the panel. Not only do these obscured cells not produce electricity, but because all the cells in a panel are connected in series, they substantially reduce the voltage available by becoming reverse-biased. Washing these panels is the only way to maintain acceptable performance. Temperature is also an important factor in panel output. Because the cells are made of silicon they have the normal silicon diode voltage/temperature coefficient of -2 mV/C. This may not seem much, but there are 72 cells in series in each panel (nominal cell output is 0.5 V). For every degree rise in temperature the voltage of the whole panel decreases by 0.144 V. The panels are tested in the factory at 77 degrees F [25 degrees C] by a flash (similar to a photographic flash) so that the temperature remains low. Our panels sit in the sun in the desert where the ambient temperature can reach 122 degrees F [50 degrees C]. Since the silicon cells are under glass (like a greenhouse), they get considerably hotter, regularly reaching about 167 degrees F [75 degrees C]. This is 90 degrees F [32 degrees C] above the test point, so the voltage will be reduced by about 7 V (50 × 0.144). Since the open circuit panel voltage at 77 degrees F [25 degrees C] is only about 40 V this loss is no longer insignificant compared with the battery voltage of 26 to 28 V. The result is that the panels produce substantially less charge current in the summer. Another important factor is that the load is not constant. In common with any equipment using a switching regulator, the ratio consumes a constant amount of power so that as the voltage decreases the current increases. To put it in perspective, the load will consume 15% more current at 23 V than it does at 26.5 V. This can be a serious problem unless it is recognized and accounted for. If a site is in deficit (less current generated than consumed) the needed current will come from the battery and the volta-e will decrease. As voltage decreases, more current is needed to keep the power constant and the site goes further into deficit, lowering the voltage even more. This is an unending circle, and unless something changes, the battery will run flat and the site will go off the air. To prevent this and ensure that a site will recover by itself after a deep discharge, a margin of at least 15 % excess charge is needed over and above the normal load.

JPT

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