Fixed solar installations reach their maximum yield at midday but trackers allow modules to make the best of the sun's rays at any time of day, says Alexander Lenfers.
In view of decreasing public subsidies for solar energy in many countries, it is becoming increasingly important for photovoltaic modules to operate more efficiently and is leading to a greater emphasis on keeping the overall costs of a system down. This is why tracking systems have come into focus since they offer advantages such as substantially higher yields as compared to fixed installations. Reduction in land prices due to the increased use of brownfield sites for photovoltaics or dual use such as parking lots and side road lanes have boosted the attractiveness of solar power.
Tracking systems are especially suitable for turning available areas over to power generation but they cost more than fixed installations. The price depends on the actual scale and location of the installation and on the type of ground where they will be installed. The actual cost of the project will fluctuate and the additional price component taken by the tracking system within the total budget cost is difficult to gauge. Usually, the additional cost ranges from 7 to 25 per cent and the yield may increase by 20 to 25 per cent on a single-axis tracking system or 30 to 45 per cent on a dual-axis tracking system, depending on the intensity of the sun at the location. This means that two factors play a role in deciding the quality of the investment – first, the selection of a suitable location, and second, the type of tracking system.
Tracking systems ensure maximum efficiency for the operator, despite low initial outlay and servicing costs. Intelligent solutions such as backtracking, remote-controlled units and very high material stability are good arguments for an investment that will pay off – especially with government subsidies on the decrease in the highly-competitive European market.
Acceptance in Europe, growth in Asia
Although subsidy cuts have made market conditions more difficult for photovoltaics in Europe, 2010 was a record year and Germany has shown a continuously high level of acceptance for solar power. Also the recent incident at the Japanese Fukushima Daiichi nuclear power plants is expected to substantially reinforce interest for solar power. The photovoltaic market in US, China, India and southern and eastern Europe have been showing continuous expansion potential. These countries still have sizeable unused area available. There has been an increase in investments in USA and China is well on its way to becoming one of the world leaders in solar power.
India too has also been contributing towards the worldwide boom in solar energy – the country has set its sights on generating 20 gigawatts of solar power by 2022 in an impressive solar mission. Even now, the government has agreed to projects with a nominal output totalling 1,100 MW and off-grid island solutions generating 200 MW. Doyens in the field have indicated that contracts worth €13 billion in total are positioning India as one of the pioneers in solar energy funding.
Dual-axis tracking systems
Trackers can play a pivotal role in the long-term success of a solar installation. The system determines the sun's position from the location, date and time. This optimises the module's surface alignment to capture the sun's rays. The advantage of this principle is that it works regardless of weather conditions, while avoiding sudden unnecessary and unpredictable movement. The module's position moves like a watch. The tracker moves the module in a way that can be planned right down to the minute. The control system supports networking, connection to wind sensors and remote monitoring. Depending on the location, solar power plants will be able to generate up to 40 per cent more output with the dual-axis tracking systems compared to fixed module designs. The systems are stable against torsion, robust and durable and all the steel parts are hot-dip galvanised. The tracker is modular in construction and the individual elements can be bolted on, making assembly very easy. In addition, the tracking systems save a lot of space and shipping them to worldwide destinations in standard containers is fast and inexpensive. The industrial drives are durable, low-maintenance and particularly stable. Compared to the smaller 60, 70 and 80 models, the 120 model boasts of a significant increase in performance and is suitable for string and central inverter system designs. Equipped with 240 W modules, it yields PV performance levels of 17 kilowatts peak (kWp) per tracker.
Backtracking prevents collectors from shadowing one another in low sun conditions. This feature eliminates shadow from sunrays for the most part and increases module yield. In the morning, evening and in winter depending on the location, the sun is low and the rays are horizontal. This is where backtracking comes into the picture – it automatically reduces the angle of elevation to prevent trackers from casting shadows on one other – which is essential in ensuring maximum yield from the sun's energy. The control system automatically determines when, at what time, where and in which period how much deflection is needed, ensuring that the tracker returns to an ideal position as soon as the sun reaches a suitable position. While fixed solar installations reach their maximum yield only at midday, solar trackers allow modules to make the best of the sun's rays at any time of day and ensure relatively even distribution of solar power throughout the day. This allows operators to predict the amount of power generated and the amount available while giving grid operators a basis to plan on. These benefits are also important in future energy concepts where solar investments are no longer refinanced on feed payments but need to be negotiated with power supply companies in direct agreements if solar power is to make a significant contribution to a reasonably constant power supply.
Dual use – even in adverse conditions
Tracking systems contribute towards making the best of the ground area. Gardens and parks, stormwater basins, waste-water treatment facilities and agricultural areas can still be used for their original purpose in addition to solar power generation. They can be mounted on pylons, allowing areas such as parking lots and agricultural areas to be used for solar power. In addition, they allow a rational use of unused side strips near private properties.
The company has already demonstrated how solar energy can even be harvested on areas that do not seem to be highly suited to efficient solar power in a solar park in Weiterstadt near Frankfurt, Germany. At 22,000 sq metres, the stormwater basin also hosts a solar park with 60 solar trackers in an installation that is unique in Germany. What makes this installation so extraordinary is that the trackers can do without foundations in the earth and the drives, control electronics and cabling are mounted a few metres above grade. The stormwater basin can also be flooded without causing any operational issues.
Realised together with the municipal government, the aim of the project was to boost the municipality's finances by supplying electricity. The trackers ensure significantly higher power output compared to fixed module installations and the photovoltaic area totalling 3,600 square metres is planned to generate around 565,000 kilowatt hours per annum. This represents a yield of €180,500 while reducing carbon dioxide emissions by around 396 tonne and providing power for around 125 families – a showcase in efficient combined area utilisation. Other options for tracking systems for PV modules are showcased at the company's own demonstration solar park based at the headquarters, with 20 trackers on an area that used to belong to a waste-water treatment plant.
Foundations - above or below grade
Solid foundations are essential for smooth operations on adverse ground conditions and classical concrete foundations are usually used, especially in smaller projects. The tracker is mounted onto these foundations that are either below grade or depending on ground conditions, above grade. The tracker pylon is lowered directly into the foundation, or is bolted onto threaded posts.
First, an assessment is needed to clarify ground conditions and design the foundation for the tracker in drilled and piled foundations. A steel member is turned into a drilled foundation and the member is somewhat wider at the top to allow the tracker pylon to be concreted in. In piled foundations, a drilled hole is filled up with concrete and the tracker pylon is lowered into the hole or bolted onto threaded posts. Either option is highly suitable for large-scale solar projects as the foundations are less expensive and time-consuming to lay compared to classical concrete foundations.
Tracking systems mounted onto a central pylon present a wealth of additional benefits – not only in terms of additional yield as compared to fixed systems but also in terms of evenness in power generation at almost full capacity from morning to evening and preventive safety protection in modules and inverters.
Represented by a branch office in New Delhi, Kemper has been investing in innovative technologies since 1977. The company is an internationally active company based in Vreden, Germany, on the border near the Netherlands. Its business activities include welding and machining, automation and solar power. It develops, manufactures and sells dual-axis tracking systems for PV module areas of 60 sq metres and more. Having successfully concluded its first solar projects, it plans to increase its market share in this new business in the medium term. In addition to its registered office in Vreden, it has production locations in Germany, Shanghai and Prague with 8 local offices and many distributors across the globe. It plans to present a new generation of trackers at the upcoming Intersolar trade fair in Munich, Germany, in June 2011.
The author is COO at Kemper, Vreden, Germany. Views are personal.
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