Double harvest from the fields

Tue, 03/01/2017 - 10:28


Ground-mounted photovoltaics and agricultural use do not have to be mutually exclusive. Scientists from the renowned Fraunhofer Institute for Solar Energy Systems (ISE) now want to run a research project to develop a market-ready PV system which enables the harvesting of both energy and food from the same area.

When Professor Adolf Goetzberger, founder of the Fraunhofer Institute for Solar Energy Systems ISE, first formulated the idea of agro-photovoltaics, mass-market PV was still a long way off. In 1981 there was no demand for high-mounted PV systems, under which agriculture could be carried out. Thus, his first applications for research projects were turned down.

In 2011 things turned around. In the boom phase of the PV market the food or fuel debate caused feelings to run high. Solutions for the land use competition between growing food and growing energy crops were sought. The Fraunhofer ISE revived Goetzberger’s idea and was able to receive funding. On 18th September 2016 came the big day; on a field in the village of Herdwangen-Schönach in southern Germany near Lake Constance, an agro-PV system with 194 kW of capacity went into operation.

The first task for project leader Stephan Schindele had been to find project partners. The total budget of the consortium is € 3.2 million, of which € 2.8 million has come from the Federal Ministry of Education and Research.

Irradiation simulation for optimum design

The Fraunhofer ISE started off by developing a solar irradiation simulation. This told the researchers how the system had to be built to provide optimum growing conditions for the plants beneath the modules. The results showed that for optimal spreading of the solar irradiation, it is better to align the system towards the south-west or the south-east. Additionally, there is an advantage to having a larger gap between the rows of modules. As a consequence of this, the module rows are approx. 40 % further apart than in conventional ground-mounted systems.

Based on this data the institute put the project development and construction out to tender, and now the system has been built. The construction is 8 m high, with a drive-through height of 5 m. The modules are fixed statically at an angle of 20 %, with a gap of 18 m between the supports. This is many times the width of conventional agricultural machines and 95 % of the area beneath the agro-PV system can be used agriculturally, with just 5 % being lost to the supports.

Details were also paid attention to, such as the question of how rain will fall on the area. There would be an effect on the plants if the rain were to come down very unevenly. There is thus a small gap between each of the two module rows installed one above the other. The water falling on the upper row of modules should therefore not flow across onto the lower row, but drop down in the area between them.

Bifacial modules for higher yields

The German manufacturer SolarWorld makes bifacial modules which can also use the solar irradiation falling on the underside of the modules. Although they are more expensive than conventional glass to foil modules, they have higher yields. A further advantage is that they fulfil the national requirements for overhead glazing. The glass on the front and rear sides is safety glass, as is also used in shopping centres and other uses for overhead glass. This protects the people working beneath them.

The contract for the subconstruction was won by the Austrian manufacturer Hilber Solar. It developed a mounting system with spider anchors for the ground fixing. The lack of a concrete foundation makes it possible to have an agricultural use right up to the supports and a full dismantling at the end of the system lifetime.

The spider anchors could also be described as having a root structure. For each foundation point, six to twelve long and thin iron stakes like tent pegs are screwed into the ground at different angles through an iron plate. Originally, a depth of 4 to 6 m was planned. The spider anchors must withstand a pull of 15 tonnes and not even move by half a centimetre. “The ground was loamier and wetter than we thought, however,” says Tabea Obergfell, who heads the technology department at the Fraunhofer ISE. As a result, longer anchors with a length of up to 8 m were screwed into the ground.

For the converters, Obergfell says that the Chinese manufacturer Huawei was the only one able to meet the technical specifications. Due to having the powerful bifacial modules, the planned system configuration will see peak voltages of over 1,000 V being achieved. Additionally, data transfer via Bluetooth must be possible, as the installation height makes access impossible without taking extra steps. Five inverters of 36 kW each were installed, which also control the system as a whole.

Reference area for comparison

Construction began at the beginning of August 2016. Four weeks later, the modules, mountings and inverters had been installed. The test area covers 2.5 ha in total, of which the PV system takes up approx. one third of a hectare. Below the PV modules four crops will be grown simultaneously over the project period – wheat, clover-grass, potatoes and celery. On the rest of the test field the project team has set up a reference area of the same size and with the same crops, but without any PV modules. By making a direct comparison the scientists will determine which types of vegetable or field crops are suitable for the agro-PV system and can ensure a double use of the land area which is as efficient as possible.

The solar electricity will primarily be used by the farm community which owns the field. Excess electricity will be fed into the public electricity grid, but bought up by the green electricity supplier EWS Schönau at a price of 5 ct/kWh. “The energy supplier could buy at 3.5 ct on the electricity market, so they are paying an extra 1.5 ct per kilowatt hour,” says Schindele, but this amount is the energy supplier’s contribution to the consortium.

The Fraunhofer ISE has calculated that the generation price for electricity from this agro-PV system is 11.5 ct/kWh. This is more than for electricity from a conventional ground-mounted system, which can generate electricity under the same conditions starting at 7 ct/kWh. It is especially the substructure which has led to the extra costs. No aluminium was used, for example, but galvanised steel. A high-mounted system does have advantages, stresses Schindele: “The farmer can farm beneath it.  Additionally, there are no weed control costs with an agro-PV system compared to a conventional ground-mounted system and the costs for leasing the land are also cheaper.”   

The aim of the research project is to develop a market-ready system. The project team now has until 30th June 2019 for this work. The system itself is planned to be in operation for a total of 20 years.

Making agriculture more attractive

Thomas Schmid, founding member of the farm community, is supporting the project in order to be able to steer farmers’ interests towards sustainable pathways, as he puts it: “Generating energy now brings in more money than growing food. With suitable regulations, agro-photovoltaics could enable the production of both from the same field: food production and energy generation.” He is convinced that agro-PV could make growing food interesting again.

Stephan Schindele also sees many reasons for having this type of system. “We must preserve our fertile land and use it well,” he appeals. It will be a challenge in the future to feed the growing world population. The problems with our climate also make a stronger use of solar technology necessary.

He sees the markets for agro-PV systems as being both at home and abroad. He names Qatar as an example, which can currently only meet two percent of its food consumption itself through salt and fish, but which wishes to make itself less dependent on imports.

10-fields programme

Initially, further projects in Germany are planned. Meanwhile, however, the frameworks have changed strongly once again. PV expansion has shrunk considerably and under the current EEG, agro-PV systems would have no chance of winning a tendering process. “There is thus a need for further research projects to demonstrate the applicability and to reduce agro-PV system costs through learning processes,” says Schindele. The Fraunhofer ISE is thus pushing for a 10-fields programme, in which further initiatives can be financed. There are already ideas for these next projects.

In one of them, Schindele and his team want to combine green electricity and organic eggs. “Organic hens require more space. You would only have to raise a conventional ground-mounted PV system by one metre, so that mobile chicken coops could be placed between the rows of modules, whilst simultaneously generating electricity,” explains the project manager.  

A second project is to be linked to fruit growing. In the Lake Constance region, approx. 30 % of orchard areas are covered with hail protection netting. Instead of the nets, modules could be installed above the fruit trees, protecting them from hail and preventing the fruit from getting sunburn.

Agro-photovoltaics would also be suitable for hops, which are plants requiring lots of shade.  Wine-growing could also be considered. “Many wine-growers complain of having too much sunshine and they have thought about techniques of providing shading,” says Schindele. Agro-photovoltaics could be one such technique. Farmers could use the electricity there and then in electric tractors.

Project partners

  • Fraunhofer Institute for Solar Energy Systems ISE
  • BayWa r.e.
  • Elektrizitätswerke Schönau (EWS)
  • Farm Community Heggelbach,
  • The Institute for Technology Assessment and Systems Analysis (ITAS) at The Karlsruhe Institute of Technology (KIT)
  • Hohenheim University
  • Regional Association Bodensee-Oberschwaben (Lake Constance-Upper Swabia)

Further information

Project APV-Resola
agrophotovoltaik.de

Fraunhofer ISE
ise.fraunhofer.de

Farm Community Heggelbach
hofgemeinschaft-heggelbach.de