By Amanda Trabant, Jorge Melero

Before speaking about the photovoltaic sector, please allow us to briefly explain some aspects about Spain, which over many centuries has had many major achievements, but also great fiascos. As is well known around the world, the Spaniards of the 15th century sailed across the Atlantic and not only put to rest the debate over the earth being round or flat, but also to discover new continents. At the same time, this discovery led to the brutal exploitation of the new riches that were present in the New World. It was famously said that the sun never sets in the new empire that the Spaniards constructed following the famous voyage of Christopher Columbus. This same sun is now a new treasure that the Spaniards have begun exploiting with the same fervor that was once used in the new world, but with a new goal of leading the renewable energy revolution.

Widest Variety of Installations

In either case, things have never been easy for Spain. The country had its golden age which ended in the mid 17th century and it spent many centuries falling behind in the race to enter the modern age. From a combination of the geographical diversity found on the Iberian Peninsula to the proud Spanish heritage of being independent, the nation has a history of leading by doing, which sometimes leads to many individuals each leading in their own direction. As can be seen from the spectacular, albeit somewhat disconcerting, rise of Spain to the top of the league of photovoltaic installations in 2008, here in Spain it is hard to deter people when there is a favorable economic opportunity and there is freedom to invent personalized solutions. It can easily be said that Spain now has the widest variety of installations than any other markets, including all the different flavors of modules, inverters, mounting structures and environmental conditions that exist; however, it remains to be seen if this solar success story marks the beginning of a new golden age or will be relegated to history as a bubble market.

At Alatec we have seen over 500 MW of what is installed within Spain. With all the diversity present in Spain, an excellent testing ground has been created. As a result and please pardon our modesty; Alatec has had the pleasure to be an expert witness to all of the possibilities of solar technology. While it may be too ambitious a task for the space at hand to pretend to share some of the knowledge that we have absorbed by viewing the solar fields of Spain, we hope that some of our insights will prove to be valuable.

Maximizing the Efficiency

The first idea that should be highlighted is to remind everyone that a solar installation, regardless of its overall size, is essentially a power station. While modularly interconnecting string after string of modules may come to seem like child’s play, an installations purpose is none other than to produce electricity. Obviously, in order to maximize the economic benefits, the more energy the better. As strange as it sounds, this concept has not always been followed. The fundamental question that should always be asked is: how can the efficiency or the amount of energy produced be maximized? All of the important principles for maximizing the efficiency of an installation are well known and easily found in books, magazines and on the internet. The following will briefly summarize some of these concepts.

The inclination and orientation of the modules should correspond to the optimum depending upon the location of the site and the panels should be installed at a sufficient distance so that they do not project excessive shading onto each other. There are software programs that can simulate the shading over the installation throughout the whole year. A modest investment in these types of tools is recommendable. Finally, a common sense approach always prevails. For example, please do not augment the security system by placing light poles throughout a perfectly designed solar installation, so that the poles project shadows on some of the modules during every hour of everyday of the year.

Mismatch Matters

It is also important to take into account mismatch. Panels that are in the same string should have Imp values that are similar. This means that, at the very least, modules with the same model number and construction characteristics should be interconnected and depending upon the manufacturer’s tolerance it may be advisable to further sort the modules using their test data supplied by the manufacturer. Please do not mix technologies or differently configured modules within the same string or on the same input channel of the inverter. The end results will be disappointing. Additionally, when utilizing the test data supplied by the manufacturer a simple verification procedure can be implemented. The product of the Imp and Vmp should equal the power of the module. While this may seem basic, programing a spreadsheet to confirm that this is the case can easily identify errors in the manufacturer’s control systems.

Mounting Structure

The orientation of the panels on the mounting structure should be considered. This is typically called either portrait or landscape mounting. For example, in the case that the modules are to be installed in a ground mounted, fixed angle installation with various rows of modules, the shading will tend to affect the lower portion of the bottom row of modules to varying degrees over the course of the year. This means that the portrait or landscape orientation could mean that all of the bypass diodes are activated when there is shade present or that a reduced number are activated. The magnitude of the effect will change over the course of the year, but will be repeated each day. Taking into account the shading patterns, if any, is also important for designing the individual series of the modules. Forming the series connections of modules in an S-type, vertically repeating fashion is typically not recommendable. Series strings formed by modules from the same row can optimize the energy output from the installation; however, as in all cases the specifics of each installation should be considered. Finally, the modules will produce more energy at lower temperatures. As such, the mounting structures should be open and the distribution of the modules should allow the wind to circulate as freely as possible.

Inverters, One of the Key Installation Elements

With regard to the inverters, it should come as a surprise to no one that they are one of the key elements of an installation and perhaps one of the most delicate. Again please pardon a statement about the obvious, but the modules should be interconnected to the inverter in agreement with the specifications of the manufacturer. In the majority of cases the service department of the company selling the inverter can be consulted in order to confirm that the proposed design is optimum. Particularly true in the case of rooftop installations, interconnecting modules that are mounted at distinct orientations to the same input channel of the inverter is not recommendable and will have a negative impact upon the efficiency of the inverter. This effect will vary over the course of the year and in function of the degree of difference between the orientations. Please note that when planning to install modules on each side of the roof and following the inclination, that the modules on each side of the roof will have a distinct orientation. Also with regard to designing the number of modules in each string, the minimum turn-on voltage and the maximum power point tracking range of the inverter should be considered. This means not only modeling the operational parameters at STC, but also modeling the electrical characteristics of the strings over a wide range of temperatures, sunlight levels and over time. Delaying the wakeup time of the inverter every morning can have an important impact on the efficiency of the installation.

Sensitive to Temperature

Inverters are electronic components and as such are sensitive to temperature. Based on our experience, many of the problems in photovoltaic installations are directly related to the temperature of the inverters. In this sense and especially for installations that are located in zones with high temperatures, a properly designed refrigeration system is important. Similarly, if the inverters are to be installed within a building, it is important to ensure that there is sufficient space between the inverters themselves and the walls, so that there is sufficient air flow available to the inverters. Additionally, it is important to take into consideration the environment where the inverters are located. For example, in the case of an environment with high levels of dust, not only the electronic circuits of the inverter can be affected, but also the dust can interfere with the fans and refrigeration systems, which could adversely affect the operating temperature of the inverters. Thus, it is important to properly plan the maintenance for the refrigeration systems of the inverters and their immediate environment. In some cases, additional air filters on the air inlets of the building may be recommendable.

Wide Variety of Mounting Structures

No discussion about the Spanish market would be complete without touching on the wide variety of fixed, single and two axis mounting structures that have been installed. This tends to lead to a lively debate, but to simplify, the basic decision is simply an economic one. The additional energy yield that either single axis or double axis trackers offer must be superior to their additional cost in terms of both the initial investment and ongoing maintenance costs. We have seen installations with trackers, and in particular two axis trackers that have comfortably outperformed their original production forecasts. While on the other hand, there have been installations that have had significant start-up delays and still others that have had serious structural problems with the cement foundations. These issues are frequently highlighted as a result of extreme bad weather that, while infrequent, is predictable over a 25 year timeframe. We can attest to the fact that trackers can fly; even though, that is not something that is usually included in their technical specifications. At any rate, there is now a significant amount of experience both in the design, construction and operation of installations with single and double axis trackers that hopefully will serve as a guide in the future.

Design of the Installation

There is no doubt that the design of the installation is important with regard to the resulting efficiency, but the maintenance that is scheduled and performed is also critical. Obviously, the number of module cleanings that are planned as part of the O&M contract each year and the control mechanisms are important. For example and depending upon the characteristics of the site where the modules are installed, four cleanings per year should not necessarily translate into cleanings every 3 months. A careful study of the weather patterns, seasonal activities around the site and the optimal production times should be taken into consideration when scheduling the cleaning of the solar field. Also, it should be noted that dirt build up will occur to a greater extent on modules that are installed at low, non-optimum inclination angles, which also will increase the losses from the dirt buildup due to having more incident sunlight at oblique angles.

Maintenance, Diagnosis and Monitoring

The concepts that have been mentioned earlier will help optimize the efficiency of the installation, but logically the efficiency is not the whole picture. Of equal importance is the fact that the plant continues to operate at an appropriate level during its expected life of at least 25 years. Appropriate tools for diagnosing the performance of the plant are important. Periodic monitoring of the solar modules and electrical connections with an infrared camera can identify problems that are not visible to the eye and also identify potential future failures.

Additionally, in order to be able to measure and evaluate the efficiency of the plant, adequate monitoring, communication and control systems are also essential. All critical parameters of the plant should be monitored in real time and depending upon the configuration of the plant it may be recommendable to monitor the DC performance of each of the strings. In the particular case of having two axis trackers installed, the proper monitoring of their position is important. Finally, a well-designed meteorological system that is comprised of high quality instruments will ensure that an accurate evaluation of the performance level of the plant is achieved.

Beyond these tips and considerations, one should never forget to apply common sense, which of course applies not only to photovoltaic plants. We hope that our experience will prove to be of service to others and that the future development of solar energy continues to be one of a triumph of sanity.

Amanda Trabant is Industrial Engineer MBA. Trabant has more than 20 years of experience within the solar energy sector including product development, project engineering, manufacturing and marketing. She also has ample experience in international markets such as China, Africa, Europe and the United States.

Jorge Melero is Technical Director of the Renewable Energy and Environment Department at Alatec (http://www.alatec.es/). Melero has more than 9 years of experience working within the renewable energy sector.

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