A place in the sun

Small-scale solar photovoltaic plants are proving to be an efficient way of generating electricity locally. E&T visits a 1MW facility on Spain's Costa del Sol to look at a facility in action.

Nestled on a hilltop above Totana on the Spanish Costa del Sol lies a seven-acre solar oasis. Surrounded by groves of olive trees on the bed of an abandoned reservoir is a 1MW photovoltaic facility that provides power to the local community.

In the scheme of things, 1MW is small - the two largest photovoltaic solar parks are in Spain: the Olmedilla PV Park, in Olmedilla de Alarcon, uses 62,000 solar photovoltaic modules to generate 60MW; and the Puertollano PV Park, in Ciudad Real Province, Castilla-La Mancha, generates 50MW.

Like most of these small-scale solar developments, the solar plant is owned by an investment company - on this occasion Global Capital Finance. The requirement was for a high-performance photovoltaic plant that could be up and running within six months. The seemingly undue haste was to meet the deadline for feed-in tariffs in Spain.

The owners turned to ABB, and the net result was a turnkey plant that was up and running well within the timeframe, and that produces 2.2GWh electricity per year, performing at a highly creditable 77 per cent efficiency.

The turnkey solution was completed in a record time of four months, and is based on ABB's modular concept for photovoltaic power plants. It includes solar trackers, electrical balance of plant, instrumentation and controls. The equipment is delivered pre-assembled and factory-tested to reduce on-site work and time.

The project scope also covered installation and commissioning, as well as the associated civil works. The fast-track delivery enabled Global Capital Finance to meet the time criteria and qualify for the maximum feed-in tariff for photovoltaic power plants.

ABB's photovoltaic concept includes optimisation technologies that will help increase the plant performance ratio to around 80 per cent, nearly 5 per cent higher than average industry standards for such applications. This clearly leads to higher energy efficiency and translates into increased power production and higher revenues for the operator.

The first impression on arriving at the facility is the sheer size. Seven hectares is an awful lot of real estate to generate power for fewer than 1,000 homes. There are, however, some environmental legislations in the Murcia area of Spain that make this necessary. No structure is allowed to be higher than the trees that surround the farm, and, as olive trees do not even reach the 2m mark, the height restrictions are severe.

Row upon row of photovoltaic panels mounted on beams stretch as far as the eye can see. Each large panel consists of three photovoltaic panels that are mounted in a frame and positioned on a beam. The site consists of 600 of these large panels arranged on beams along 14 rows.

Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into electricity. The photovoltaic effect refers to photons of light knocking electrons into a higher state of energy to create electricity. The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode.

Solar cells produce direct-current electricity from light, which can be used to power equipment or to recharge a battery. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid-connected power generation. In this case, an inverter is required to convert the dc to ac.

Control

The intricate control systems allow for two axes, so that it can chase the sun throughout the day, ensuring maximum collection of the energy. Solar tracker systems can generate up to 35 per cent more power than fixed PV systems, and ensure maximum productivity even during periods of weak or low sunlight. Totana is a prime example of this highly scalable concept at work.

The panels generate power at 400V dc, which is then sent to the grid at 20kV ac after passing through inverters and transformers. Inverters are essential components in sustainable energy production. They transform the direct current electricity generated by installations like solar panels into alternating current, either for immediate consumption or for transfer into the AC power grid.

The facility is usually unmanned, and the fenced perimeter guarded by cameras monitored from the head office hundreds of miles away in Madrid. Fortunately, the only breaches of security to date have been the local wildlife, primarily rabbits, and the only harm they cause is to chew through cables, necessitating cable troughs rather than the traditional buried cables.

Security is not the only measure that can be controlled from Madrid; the entire operation of the plant is dependent on this remote authority. The small control room located on site, which houses two computer terminals, is merely for show. There are nine inverters controlled by three panels, in order to maximise efficiency. The power from the solar modules can be fed to the appropriate inverter allowing a high degree of flexibility and greater efficiency.

Climate

Another challenge for the facility is the harsh environment of wind and dirt. Winds in excess of 80km/h can cause damage to the modules, so if the system senses high winds the facility will go into a 'safe mode', whereby the panels are set in a horizontal position. To ensure maximum efficiency, the panels are cleaned twice yearly, but the control system monitors each panel's efficiency and if it drops below an acceptable parameter extra cleaning is carried out.

There are clearly limitations to the viability of this type of system. In many areas the cost of land would have been prohibitive, and without the generous feed-in tariffs on offer in the region it is doubtful that the project would ever have got off the ground. But in reality it is just another example that different renewable technologies will flourish in different regions depending on climate and market incentives.

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