Going from coal pits to floating solar plants
Commissioned by the renewable energy developer BayWa r.e., the Fraunhofer Institute of Solar Energy Systems ISE investigated the technical potential of floating photovoltaics (FPV) on pit lakes in former lignite mines in Germany. Such FPV plants that basically float on man-made lakes can help to alleviate the competition for land use and help expand PV capacity in Germany. Floating PV technology has several advantages over ground-mounted systems; for instance, greater power production due to the cooling effect of water or higher efficiency of land use.
“Floating PV power plants are a relatively new concept, which holds a large potential for electricity generation worldwide, not least because it allows a land-neutral expansion of photovoltaic capacity,” Andreas Bett, director of Fraunhofer ISE, said. Depending on the scenario, estimates show that up to 500 gigawatts photovoltaic generation capacity are required for a successful energy transformation. Due to the limited amount of arable land, solutions must be developed that are less dependent on land availability. A large advantage of FPV is its higher land-use efficiency (1.33 megawatt per hectare). The water on which the PV array floats also provides cooling which results in somewhat higher energy yields. The PV modules and, in most cases, the inverters are installed on floating devices that are anchored either on the shore or the lake bottom. Former lignite open cast mines are also good locations for FPV since a connection to the grid already exists as a rule. If anchoring on land is not possible, then a conventional ship anchor can be used. However, this can lead to additional costs, which depend on the lake depth. The electricity generation costs for floating PV systems are about 10 to 15 per cent higher than the costs for conventional ground-mounted PV power plants.
Assessing the potential for electricity generation
About 500 pit lakes formed from lignite mining exist in Germany, covering a total area of 47,251 hectares. Most of the lakes are located in the German states of Brandenburg (29.8 %), Saxony-Anhalt (28.2%) and Saxony (15.7%). In order to assess the potential for electricity generation on these lakes, researchers interviewed public authorities, stakeholders and experts in authorisation, planning, installation and water protection. A theoretically useful potential of 56 gigawatts was originally estimated. From this value, the projected area for recreational activities, tourism, nature and land conservation was then subtracted. Pit lakes smaller than one hectare, or with large fluctuations in depth or lacking possibilities for anchorage on shore were rejected for reasons of costs. Conclusively the realistic economic potential was determined to be about 4.9 per cent of the theoretical lake area, equivalent to an installed power of 2.74 gigawatts. The largest potential is located in the Lausitz region and the coal district in Saxony and Saxony-Anhalt. Other types of artificial bodies of water or also natural lakes were not considered in this study, so that the total potential in Germany is probably much larger. In all, Germany has 4,474 man-made lakes. Most of these were formed by surface mining for building materials. This includes 725 quarry ponds and 354 gravel lakes. The share of pit lakes from former opencast lignite mines is only 12.9 per cent.
In the Netherlands, the remuneration for electricity generation is sufficient to economically set up and operate floating PV systems, and BayWa r.e. has already installed systems there. Edgar Gimbel, Head of Power Plant Engineering at BayWa r.e. Solar Project GmbH emphasises: “We have already realised three projects with a total of 25 megawatts in the Netherlands. We just began construction on Europe’s largest floating solar PV plant with 27.4 megawatts. Fraunhofer ISE’s study impressively shows that there is enormous potential for floating PV in Germany. Now is the time to set up a suitable legal framework and simplify the authorisation procedures in order to make use of this potential.”
Creating incentives for floating PV
Germany’s first floating PV power plant began operation in 2019. Due to conditions laid down in the Renewable Energy Act, the nominal power was limited to 750 kilowatts.
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Because the investment costs for floating PV are somewhat higher than ground-mounted plants, they have trouble competing in local tenders. “It would therefore make sense to have innovation tenders specifically for floating PV and other innovative PV plants that still need a market boost. In order to avoid time-consuming changes to the land use plan, the land-neutral floating PV technology should be given a privileged position, as is already the case today for the use of land for wind and nuclear power plants,” says Harry Wirth, division director of Photovoltaic Modules and Power Plants at Fraunhofer ISE. The researchers also recommend that pit lakes from former lignite open cast mines should be recognised as conversion areas under the German Renewable Energy Act. In this way, floating PV projects could participate in the call for tenders from the German Federal Network Agency. To promote the installation of FPV in pit lakes, it could be meaningful to state this use explicitly in the redevelopment framework plans for former lignite mines. To address the aspects of environmental friendliness and public acceptance, investigations carried out on a prototype FPV plant in Germany are recommended. The project team also finds that analysing citizen participation models, specifically addressing implementation of FPV plants, would make sense. (mfo)