Stadtwerke Gießen is building a state-subsidised iKWK plant by June 2026. At its heart are three large heat pumps that extract heat from the River Lahn.
Decarbonising heat production across Germany is considered by experts to be a key component of the national energy transition. Stadtwerke Gießen (SWG) has been pursuing this goal for much longer than politicians. SWG has been working for decades to gradually reduce theCO2 emissions associated with its heat generation. A current major project fits seamlessly into this strategy: the iKWK PowerLahn plant. iKWK stands for innovative combined heat and power generation and the name PowerLahn says it all. At the heart of the plant are three large heat pumps that utilise the water of the River Lahn as a heat source.
The term iKWK, introduced by politicians, refers to the combination of three basically well-known and proven technologies that can be subsidised. Such an overall system must consist of a highly efficient CHP plant, a power-to-heat element (P2H) and an innovative heat generator. Solar thermal energy, geothermal energy or heat pumps can be considered as i-components. "We decided to combine two combined heat and power (CHP) plants, an electric boiler and three water-to-water heat pumps," says Matthias Funk, Chief Technical Officer at SWG, explaining the concept.
Doing the maths
In view of the investment required, most iKWK systems can only be operated economically with state subsidies. However, obtaining these usually proves to be a real challenge. Simply having the required components is not enough. Other strict criteria must also be met. For example, the P2H module must provide at least 30 per cent of the thermal output of the CHP. In addition, the heat pumps must be designed and operated in such a way that they supply at least 35 per cent of the heat generated by the CHP units each year. "Our engineers had to make very precise calculations here. Both in terms of the specifications themselves and the subsequent operation," recalls Matthias Funk. Specifically, they came up with the following result: SWG is installing two CHP units, each with an electrical output of 4.5 megawatts and a thermal output of 4.7 megawatts. The P2H module contributes 2.7 megawatts of thermal output. And the three heat pumps supply 1.774 megawatts each, but will only run from April to September - i.e. when they work particularly efficiently due to higher water temperatures.
These explanations alone illustrate the complexity associated with an iKWK project. "The numerous variables, especially at the beginning of the work, are obviously still deterring many companies from taking a closer look at the matter," surmises Matthias Funk. This hypothesis can be substantiated by two details relating to the awarding of grants. The tendering process for subsidies is basically a reverse auction. Bidders apply for the funding they need to operate economically. The most favourable bids are awarded the contract until the pot is empty. "We won the bid with the almost maximum subsidy of 12 cents per kilowatt hour of electricity," says Jens Schmidt, Commercial Director of SWG. In addition, SWG was able to win 40 per cent of the budget distributed nationwide in this tendering period. In fact, there were not enough applicants to distribute all the available funds.
Sufficient financial resources and technical expertise are necessary, but not sufficient conditions for the success of a project like PowerLahn. "A project like this cannot be realised without excellent project management," Jens Schmidt is convinced. An ambitious schedule must be adhered to - under all circumstances. "Because we are using subsidised funding, we have to put the plants into operation on a fixed date," adds Jens Schmidt. Because every delay costs real money.
One factor is particularly critical for adhering to the tight schedule. As with other construction projects, SWG has to obtain the relevant licences for all PowerLahn components. The crux of the matter is that iKWK is not only breaking new ground for SWG, but also for the relevant authorities. The fact that the Lahn is also a federal waterway complicates matters further. "We have focussed on maximum transparency right from the start, are in very close contact with everyone involved and exchange information as quickly as possible," says Jens Schmidt, describing the process.
Active climate protection
There is a simple reason why SWG is going to this enormous effort. "We have been pursuing the goal of continuously reducing the use of fossil fuels in the production of our district heating for many years. If it hadn't been for PowerLahn, we would have come up with something else that was similarly climate-friendly," explains Matthias Funk. In fact, SWG is due to modernise parts of its plant fleet over the next few years - specifically, two gas turbines and two steam boilers are to be replaced. "In this respect, the funding for iKWK systems came at just the right time for us," recalls Jens Schmidt.
PowerLahn is also contributing to a project of the city of Giessen, the owner of SWG. The municipality has set itself even more ambitious climate targets than the federal government and is aiming to be climate-neutral by 2035. "As a traditional municipal utility, we naturally have a responsibility here. With PowerLahn, we are making an important contribution," summarises Jens Schmidt. The reduction in carbon dioxide emissions to be realised thanks to the project underlines this assessment. In future, the three heat pumps will contribute around ten per cent of Giessen's district heating - completely emission-free. Compared to conventional gas boilers, they will save around 7,767 tonnes ofCO2 per year.
Two locations
An important criterion for the subsidy, which many would probably find difficult to fulfil, was not an issue for SWG from the outset: all components must feed into the same heating network. Because SWG has already developed large parts of Giessen with the corresponding pipelines, the engineers were able to plan with two locations. "That saves money and a lot of authorisation work," explains Matthias Funk. In fact, SWG only has to construct one new building for the heat pumps. It is being built not far from the SWG headquarters on the other side of the Lahn. A good two kilometres to the south-west as the crow flies - in the existing Leihgesterner Weg CHP plant - SWG will have the two highly efficient BHWKs and the P2H module installed. "Existing buildings already offer sufficient space here. This simplifies many things, saves money and also makes the project a little more sustainable," says Matthias Funk happily. Because every cubic metre of concrete not required and every tonne of reinforcing steel saved savesCO2.
The environment benefits twice over
The expected reduction inCO2 emissions thanks to PowerLahn is not the only positive effect for the environment. As already mentioned, the three heat pumps will primarily operate in the warmer months for reasons of efficiency. This is precisely what improves the living conditions for many animal species native to the Lahn. This is because climate change is causing the water to heat up more frequently and more strongly, especially in summer. This is not exactly conducive to the well-being of fish, snails and other inhabitants. The higher the temperature, the less oxygen the water can bind. Heat pumps counteract this harmful development. After all, they extract heat energy from the river and thus lower the water temperature again. This makes life easier for the animals in the river.
However, SWG is not content to simply build and operate a highly efficient, climate-friendly CHP plant. "Our aim is also to get people excited about our project, to get them on board," Jens Schmidt summarises. For precisely this reason, SWG is not erecting a purely functional, square concrete building on the banks of the Lahn. Instead, a functional building will be created that is also aesthetically pleasing and will have large, round windows in the façade facing the carriageway. Matthias Funk puts it like this: "We are providing a view of the technology and thus ensuring transparency in the truest sense of the word. Our aim with PowerLahn is to make the energy transition tangible here on site." This also includes SWG using various channels to communicate the topic. A specially produced film will soon explain exactly what will be created, how the plant works and how it fits into the existing district heating network. The film is part of a dedicated website that SWG is using to accompany the project - it can be found at www.stadtwerke-giessen.de/powerlahn
Conclusion: With the PowerLahn project, SWG is taking the heat supply in Giessen to a completely new level and bringing it significantly closer toCO2 neutrality. In fact, the plant is setting new standards in terms of sustainable energy supply and may also serve as an example for other companies in other municipalities.