Solar power for a piglet farm

Tue, 10/01/2017 - 17:01


Detlef Koenemann

SUN & WIND ENERGY
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A large number of farms depend on power supplied by diesel generators. An example from Spain now shows that these fossil fuel-powered systems can be almost completely replaced with PV systems.

Most regions of Spain are sparsely populated, and the power supply is not sufficient in some areas of the country. Farming, however, is almost everywhere, and since farms need electricity, there are thousands of diesel generators in use to cover the needs of farms in remote areas. These fossil fuel-powered off-grid systems are not environmentally friendly and do not save resources. On top of that, the large amounts of fuel they consume make them expensive. Given the high levels of solar irradiation in Spain and the constantly falling cost of PV systems, it makes sense to supply farms with electricity using solar energy.

This is why the agricultural company Cereals Torremorell S.A. (CETOSA) opted for photovoltaics. CETOSA built a piglet farm near its corporate headquarters in Algerri, approximately 130 kilometres west of Barcelona, which has been powered by a PV system since April this year. Three Fronius Symo 20.0 inverters convert solar power into AC power that is either consumed directly by the farm or stored in two battery stacks. Six Victron Quattro inverter chargers, each of which have 10 kW rated power, are connected to the inverters and pass the electricity through four Victron BlueSolar charge controllers into the batteries that cover the electricity demand during the night.

The Victron devices form the core of the off-grid system because they are also connected to the backup diesel generator, which is fired up whenever the batteries are exhausted, for example after several days of bad weather. The inverter chargers are only disabled when the batteries are fully charged and the generated solar power is completely used up by the farm.

But this is rarely the case. If the batteries are fully charged during the day, then the PV system usually supplies more power than is needed. "This makes automatic power reduction necessary", said Francisco Heredia, who is responsible for technical support at Fronius Spain. "Our inverters have a special setup with a range of functions that ensure the stable operation of micro-grids", he added.

Power always under control

One of the most important of these functions is the frequency droop. The inverter and inverter charger communicate with each other to regulate the power and to prevent an excessive increase of the frequency in the micro-grid.

When the batteries are fully charged and the system is generating more solar energy than is being consumed, the inverter chargers slowly increase the frequency. At 51 Hz, the inverters react by reducing the PV power until the frequency does not increase any further. In extreme cases, it can reach 52.8 Hertz. At that point, PV power is suppressed completely. It is reduced to zero. The batteries can then be discharged and can completely take over supplying power. Voltage-dependent power reduction is also possible, and this guarantees the stability of the grid at all times.

A common monitoring system was implemented using the Victron Color Control Display CCGX to combine the installation into a single system. This is possible because the CCGX and Fronius Datamanager are on the same grid. The plant operator can always access the off-grid system's current data using the Victron Monitoring Portal. The online portal Fronius Solar.web (www.solarweb.com) provides comprehensive presentation and analysis of the PV system data.

The two battery stacks consist of a number of lead-acid batteries. Each has a capacity of 75 kWh and weighs 2.1 tonnes. Lithium batteries weigh significantly less, but they were not taken into consideration for this project because lead-acid batteries are less expensive. The purchase cost per kilowatt hour capacity is significantly lower. Weight does not matter for stationary applications. This means that lead-acid batteries are still competitive, even if you take into account that the rated capacity cannot be fully utilised because deep discharge would considerably shorten the life of lead-acid batteries.

The operating results from the first seven months are now available. The average power consumption of the piglet farm during this period was between 4 and 6 kW. The daily requirement is between 100 and 150 kWh. Approximately half of this amount of power came directly from the solar generator and the other half from the battery. The contribution made by the diesel generator was negligible during the past summer. There were only two days in October when the diesel generator had to be fired up and generate electricity for a few hours.

The diesel generator may need to be started more often during the winter. Next spring, the operators will know how much fossil fuel is needed to ensure the power supply of a farm this size. The piglet farm in Algerri could become a model for similar businesses and contribute to replacing fossil fuels almost completely in the power supply of off-grid systems.