Use large areas of the basins for solar folding roof

Solar plants are installed on buildings, barns in agriculture, on noise barriers on highways, on dam walls, and even in the middle of reservoirs. Andreas Hügli and Gian Andri Diem thought that the large areas of the water basins at the sewage treatment plants would also be suitable, especially since they are usually built somewhat out of the way and are not very sensitive to the landscape. They did not let themselves be stopped when the sewage treatment plant operators declared that access to the basins must continue to be guaranteed during renovations or operational work. Hügli and Diem founded the company dhp technology AG in 2015 and began to further develop a retractable folding roof for solar energy use that was suitable for these demanding requirements. After two years, the first solar folding roof was realized on the wastewater treatment plant in Chur, and in the meantime eleven systems have been delivered, 30 systems are in the planning stage, five of them in Germany. But that's not all, parking areas are also very well suited for solar production. A folding solar roof was installed on the parking lot of the Kronberg mountain railroad; it supplies electric mobility with renewable electricity and also provides shade for drivers on hot summer days.

Solar folding roof, a Swiss innovation

The plant components of the solar folding roof are assembled by the aforementioned company in its own production and put together at the treatment plants. The company's specialists also take care of the operation of the solar folding roof and the permanent check via digital monitoring and can thus guarantee safe operation and constantly optimize the electricity production. Hügli and Diem are convinced of their product; this is also reflected in the fact that they give a 25-year warranty on the solar modules.

And what do the wastewater treatment plant operators say?

 "The system works without any problems, the solar folding roof retracts and extends when it should, and it covers 20% of our electricity consumption as planned," Curdin Hedinger from ARA Chur was already able to report on the first pilot system. This is also confirmed by operations manager Markus Wendler from ARA Davos, because more solar power can be produced in the mountains than in the lowlands, especially in winter when the demand for electricity is high: "Even when it snows, the folding roof remains protected thanks to the weather-controlled retraction mechanism and can then supply power again with the first rays of sunshine." The wastewater treatment plant operators particularly appreciate that on hot days, which are becoming more frequent, the personnel are protected from heat under the folding solar roof and that wastewater treatment improves because the shade reduces algae growth in the secondary clarifiers.

Concrete example ARA Bassersdorf

The responsible persons of the ARA Bassersdorf had the possibilities of solar energy clarified by the specialist Simone Bützer of the engineering office Hunziker Betatech AG in 2019. The investigations showed that with the solar folding roof over the basins of the biology, the preclarification and the secondary clarification 261'000 kWh/a electricity can be produced, on the roofs of the various, partly small buildings 61'000 kWh/a as well as on the fence 23'000 kWh/a. Using all possible areas, this results in a total electricity production of 345'000 kWh/a, which corresponds to 33% of the total electricity demand of the ARA, including the considerable, additional electricity consumption for the elimination of micropollution (EMC). In a second step, a pre-selection was made based on an economic analysis. For a photovoltaic (PV) system on the largest roof of the various buildings, the operations building, the prime costs (assumption: 25-30 years, 0-2% interest, including operating costs and taking into account the current subsidy) were calculated at 13 to 15 Rp./kWh and for the folded solar roof (without the primary clarifier) at 15 to 17 Rp./kWh. Thus, about three quarters of the total solar potential can be used, 250'000 kWh/a renewable electricity can be generated annually and 23% of the total electricity demand of the ARA can be covered (cf. chart 1 below).

What was also interesting about the clarifications was that the prime costs have already fallen in the past four years by around 15% for the PV and even by around 40% for the folding solar roof. Because in the first study in 2015 with a massive steel construction over the basins, the amortization costs still amounted to 26 to 30 Rp./kWh. It may be assumed that this trend of falling costs will continue - albeit at a slower rate. In addition, a west-east design can be used to balance solar power production and weather forecasting tools can be used to increase the usable portion for the wastewater treatment plant's own power consumption.

Costs and efficiency

The economic efficiency of solar plants depends on the one hand on the investment and operating costs. In the case of the solar folding roof, the size of the system and the subsoil for safe statics of the comparatively light steel construction play an important role. On the other hand, the financial savings from electricity production depend very much on the usable portion for one's own electricity needs, the prices for electricity purchases or the feed-back tariffs, which are often still low today. It is not enough to look only at today's electricity prices.

It must be taken into account that electricity prices are likely to rise over the next 25 to 30 years. And let's not forget the various subsidies available (including from the respective canton), www.pronovo.ch). In addition to the financial aspects, it is important to consider the overall benefits, because the public sector has an exemplary function, the staff benefits from shade as well as weather protection, and the sewage treatment plant benefits from reduced algae growth.

Potentials in Switzerland

Including the demand for the elimination of micropollutants, wastewater treatment plants in Switzerland consume around 500 million kWh of electricity annually and are among the largest municipal electricity consumers. Around 20% of this is covered by the plant's own electricity production by means of combined heat and power (CHP) plants using sewage gas. By operational increase of the sewage gas quantity, improved efficiencies of the BHKW this portion can be still increased and on the other hand by electricity saving measures the external electricity purchase, which must be paid, can be still further lowered. A first rough projection by InfraWatt has shown that with the solar folding roof over the basins and PV on the roofs of the buildings, the wastewater treatment plants could generate another 110 million kWh/a of electricity (see Figure 2). The implementation of the solar potential alone would be equivalent to the electricity consumption of all municipal administration buildings in Switzerland and, at future electricity prices of 20 Rp./kWh, would generate savings of 550 million Swiss francs for the wastewater treatment plants over the next 25 years. This would also be worth investing in energy measures.

Conclusion

For wastewater treatment plants with a population equivalent of more than 10,000 to 20,000, it is worth considering the folding solar roof option. A photovoltaic system on or at buildings pays off even for smaller wastewater treatment plants. The technologies have proven themselves in practice and can be economically interesting. A recently published study of the canton of Schaffhausen comes to the conclusion (www.sh.ch) that the infrastructure facilities could be increasingly used for solar energy, whereby concrete projects with the solar folding roof can be realized quickly and cost-effectively, especially in parking lots and sewage treatment plants.

Author

Ernst A. Müller, InfraWatt, Association for the Utilization of Energy from Wastewater, Waste, Waste Heat and Drinking Water.