For some time now, a small municipality in Nordfriesland has been making a name for itself in energy technology, as the supplying of heat through the new network from Bosbüll is anything other than standard. Through a power-to-X concept and a triple-sector coupling, the Ü20 wind and solar power installations remain profitable even in post-EEG operation, while heat supply to the community of 250 souls remains secure. The model project shows how a green energy future can be intelligently written.

On September 8, 2021, Bosbüll, a municipality near the Danish border, celebrated the launch of the first power-to-heat (PtH) grid within the German state of Schleswig-Holstein with a “Wärmefest” – although even before the ceremony, the households and the one large agricultural business had been benefitting from connection to the 2,680-meter (8,792-foot) heating grid.

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The starting point was the heat management center that Yados GmbH, responsible for the technical realization of the PtH solution and the control system, had designed, constructed and set up in Bosbüll. The energy center housed in a 60-tonne concrete cell stands directly next to the first customer: a sow farm. In the immediate vicinity of the energy center is also an eFarm base, to which the heating network is physically and sectorally coupled. At a distance of about one kilometer, the newly expanded heating grid reaches the 25 Bosbüll households that were connected in the first construction phase. More private and commercial customers are to follow in a second step.

From post-EEG operation to energy technology blueprint

Two citizen wind and solar parks have been supplying electrical energy to the municipality of Bosbüll for years. This has economic advantages on the one hand for the citizens, who have a financial stake in the parks, and on the other hand for the municipality itself, which can finance numerous new projects through the tax revenue from it. This keeps the created value within the region and contributes to its economic stability. At the end of 2021, the EEG subsidy for two of the wind farms expired, and more will fall out of funding in the coming years. Also the solar park is losing its subsidizing at the end of the decade. But with the power-to-X project that couples the sectors of electricity generation, heat preparation and fuel production, the alternative energy sources remain economically viable and the municipality benefits from an original, ecological and economical supply solution.

BAFA funding – complicated but rewarding

With the aim of applying for funding from the German agency for economy and export monitoring (BAFA), the companies Bosbüll Energie GmbH and Bosbüll Energie GbR were founded. Since, according to Schleswig-Holstein municipal law, the municipality may not be involved in a GbR (Gesellschaft bürgerlichen Rechts) and the energy producer and energy consumer must be the same entity, the municipality itself was forced to withdraw from the planned actual participation. After long, extensive planning and design work, the application for the BAFA support program Wärmenetzsysteme 4.0 was finally created. The rewarding result: Of the 1.9 million euros of planned expenditures, 1.6 million is able to be funded.

Responsible for the conception, planning and implementation of the joint project is GP Joule GmbH. Its cooperation with actors in the community had already started in 2009, as the company from Nordfriesland planned and constructed the solar park. Since then, it has been responsible for managing the technical operation. Furthermore, GP Joule is along with Windpark Bosbüll a partner in the limited liability company Bosbüll Energie GmbH and, as the initiator, responsible for the hydrogen project eFarm.

With power-to-heat into the heating network

The PtX project is based on two pillars: On the one hand, a sophisticated power-to-heat solution using air-to-water heat pumps is supplying heat via the community’s own heating network, and on the other hand, a power-to-gas plant is producing hydrogen for a supraregional H₂ transportation concept.

To meet the annual demand for the connected households of about 500 MWh_therm in total, and for the large agricultural business of about 600 MWh_therm, three flow-controlled air-to-water heat pumps have been installed in Bosbüll. They convert the renewably generated electricity from the citizen energy parks, a total output capacity of 240 kW, into thermal energy for the heating network. An electric heating insert with a power of 750 kW supplements additional thermal energy by heating water in a 14-meter-high (46 feet) 84-cubic-meter storage tank. The energy can be temporarily stored for up to four weeks this way. In the energy center are additionally a Hoval Max-3 boiler for peak load coverage and a hydraulic pump station for distributing heat.

If the hydraulics are in flow, the system is in balance

To reach the maximum efficiency for the entire thermal energy system, the involved producers and consumers must be tuned to each other and be operating as closely as possible to their respective optimal efficiencies. For this, the hydraulics station of the company Yados, based in Hoyerswerda, is responsible. On the one hand, it optimizes the combined working of the energy generators, heat generators, heat storing components and heat distributors. On the other hand, the hydraulics station basically ensures that thermal energy is available at the right place at the planned time in the required quantity – and all this using as little drive energy as possible.

In addition, the heat storage vessel is not only integrated into the system in such a way that only heat loading and unloading volumes flow through it, but in a way that improves the temperature stratification. For demand-driven district heating network operation, low return temperatures are crucial. They not only influence the volume flows, the transmission capacity and the electrical pumping effort, but minimize flow and heat loss at the same time. The flow temperature is 70 to 85 °C, while the return temperature is about 50 to 55 °C.

Smart control of the system

Another important element to fully exhaust the high efficiency potential of a sophisticated sector coupling concept is a well-coordinated measurement, control and regulation system. It takes on the complex task of precisely coordinating the system integration of all components involved in the process. The eastern German heating network specialists have installed the supervisory control and data acquisition (SCADA) system named YADO|LINK for this purpose. To directly coordinate the most important district heating system parameters, this control system regulates and links not only all the systems in the energy center but also the heat transfer stations and their built-in DDC controllers. On a large and user-friendly 21.5-inch display, the responsible persons can call up and view all grid-relevant data and information for real-time monitoring: temperatures, pressures, fault messages, etc. To make the installation and start-up of the control system as simple as possible, the engineers delivered the whole thing as two combinable control cabinets, fully pre-assembled and wired, to Bosbüll.

Safety and efficiency improvement through control technology

The control system generally serves as a coordination tool for all decentralized generation, distribution and transfer processes for thermal energy, electricity and cooling. Its task includes realizing the entire heating system operation according to defined target specifications. For this purpose, an automated real-time monitoring system collects all the relevant pieces of data and then evaluates them. If deviations occur on the consumer or generator side, the regulation function kicks in and adjusts the corresponding operation of the involved component.

Linked sensors, actuators and modular regulation units provide the control system with the information required for this purpose. A large number of complex function queries are processed. If urgently needed, the production, storage or distribution processes can be automatically or manually adjusted. Such urgent cases occur not only in the event of technical faults, in outage situations or in the event of sudden changes in output demand; external conditions such as an unforeseen heat wave or a spontaneous drop in temperature could also demand this.

A strategic optimization of the heating system management is also possible through a continuous evaluation of all system-immanent target and actual values, and by deriving recurring trends or even long-term predictions from the information collected. Control systems of the newest generation are important adjustable factors for further improvement of the energy supplying efficiency.

Furthermore, the control system can contribute to stabilization and to greater comfort in the provision of heat. Primary energy consumption can be decreased by eight to ten percent on average – in particularly use-intensive cases by even up to 30 percent – through this system-based monitoring and regulation of the heat grid operation and controlling of the return temperatures.

Heat substations for stable grid management

In addition to an intelligent high-level system control, the heat transfer components are of central importance for optimal operation of a district heating network. In the small Nordfriesland municipality, smart home-compatible transmission substations connect the building heating installations on the consumer side with the district heating network. As regulating connection elements, they transmit the heat medium, hydraulically separated by a plate heat exchanger, depending on the demand, temperature and pressure. A direct digital control (DDC) installed in the substations calculates the required flow temperatures for this with consideration of all relevant – external and individually defined – parameters such as weather conditions or time and convenience settings for the user. In the planning stage are more adjustments for the heating systems on the consumer side. Additionally, the pipes in the municipality are maximally insulated to keep heat losses in the district heating network as low as possible.

On the road with power-to-gas

In addition to powering the district heating network, energy from the citizen wind and solar farms is also used for the production of green hydrogen. This is vital for decarbonization of the transportation, heating and industrial sectors in the long term. The hydrogen produced in Bosbüll is used for the refueling of hydrogen vehicles.

The power-to-heat system is ideally complemented, through connection to the eFarm project, with a power-to-gas or, alternatively, power-to-fuel concept. This sustainable H₂ mobility project aims to achieve a modular, expandable hydrogen infrastructure in the regional district Nordfriesland. Of the now five polymer electrolyte membrane (PEM) electrolyzers installed there, two are located in Bosbüll. With a combined capacity of 450 kW, the two generate a total of about 200 kg of hydrogen per day from regionally generated solar and wind power.

In the process, the electrolyzer splits, with the help of an electric current, distilled water into oxygen plus free electrons and positively charged H⁺ ions at the anode side of its electrodes. The H⁺ ions diffuse through the proton-conductive membrane to the cathode side, where they join with electrons and become molecular hydrogen.

The efficiency of the electrolyzers in Bosbüll lies at up to 95 percent. The reason for this is, among other things, that the waste heat from H₂ production of around 100 MWh_therm is directed into the heating grid or temporary thermal energy storage. After its production, the green hydrogen is transported to two H₂ refueling stations, in Niebüll and in Husum. A compressor provides the required refueling pressures of 350 bar for buses and other commercial vehicles with 350-bar tanks, and 700 bar for passenger cars and light commercial vehicles.

Two public fuel cell buses that were procured as part of the project use the energy released by the reaction of hydrogen and oxygen as drive power. One full tank of fuel in the buses is enough for 400 kilometers (249 miles), which corresponds to a regular day of service. In addition to the two buses, the project volume is also counting on 30 passenger cars with fuel cell drive systems. These will be gradually handed over to the new owners. Also new from the regional district is a driving school that now, instead of two diesel cars, uses two FC cars for its driving lessons. For the passenger cars, one tank of fuel is enough for up to 600 kilometers and costs around 60 euros.

Targeted use of surplus power

A major problem in the generation of renewable energy has been and remains how to store energy from the sun, wind and water in larger quantities and over longer periods of time. The volatile power peaks and negative residual loads from alternative energy sources raises the need for grid and system safety measures like the power generation shut-offs as feed-in management (Einsman-Schaltungen) regulated in item 14 of the German renewables law (§ 14 EEG 2021). The Ausfallarbeit, the energy that the grid didn’t receive as a result of these curtailments, reached the enormous sum of 6.1 TWh_el in 2021.

To take advantage of this large decarbonization potential, PtX projects are converting the surplus renewable energy into electric heating or stored thermal loads or into another energy carrier, like hydrogen in this case. The energy concept of Bosbüll as well prefers to targetedly use the surplus from the wind and solar parks that would otherwise be curtailed and therefore lost in order to avoid overloading the power grid. They put the excess energy into keeping the inhabitants warm and able to drive around.

Promising energy vision for the future

And it’s going further: The Municipality of Bosbüll is currently in the process of designating a further building area this year to which the new regeneratively powered heating network will be brought. And due to the current world and geopolitical developments, requests for further household connections to the new heating network are piling up at the office of Ingo Böhm, Bürgermeister of Bosbüll. In addition, construction of another solar farm in 2023 has been finalized, according to plan. The energy from the plant will go primarily into hydrogen production. Furthermore, ten more hydrogen-powered public FC buses are to be procured in the near future.

The project in Bosbüll is a model that, especially in these times, when the fastest routes to decarbonization have the highest priority, can serve as a blueprint for the energy concepts of other communities. Through the power-to-heat system alone, the Nordfriesland municipality is able to save 180,000 liters of heating oil per year. Furthermore, local energy procurement solutions always have the advantage of general market-independence – not only in terms of security of supply, but also in terms of protection against price volatility. And not to be forgotten: The added value resulting from the project stays in the community and benefits all of its citizens.

In maintaining the efficiency of such coupled renewable energy systems at a high level, the quality of the overall system and the masterful coordination of the individual components via control technology play an enormous role. The combination of regeneratively produced electrical and thermal energy with smart storage and distribution concepts is certainly one of the draft horses that can contribute to the rapid achievement of the energy turnaround.

This article (German language) was first published in the magazine bbr Leitungsbau|Brunnenbau|Geothermie.

Author: Martin Gentner, YADOS GmbH, Hoyerswerda, Germany
Source: GP JOULE GmbH, Source: YADOS GmbH