Minimum heat consumption with maximum comfort

Minimum heat consumption with maximum comfortHow the energy patchwork "Alter Mühlhof" became an efficient system

A historic power generator in the Haunstetten district of Augsburg was no longer quite in keeping with the spirit of the times: Sustainable, yes, because it was driven by hydroelectric power, but still not productive enough because it frequently shut down due to excessive temperatures. And most of the heat was wasted, even though the house and swimming pool needed it. A Bobing-based crafts company restructured the heat utilisation with the help of the 'rendeMIX' mixing system. The success exceeded all expectations.

Turbine House

Fig. 1: Turbine house and Lochbach inlet to the "Alter Mühlenhof" turbine, Haunstetten. The hydroelectric power plant has been running since 1907 with one and the same Francis turbine. It supplied mechanical energy for many hundreds of years until 1880.

The "Alte Mühlhof" in Haunstetten, Augsburg, is one of the technical monuments in the Fugger city area. The watermill at the Mühlhof probably dates back to the 12th or 13th century. It is still in service today. Of course, it is no longer powered by an overshot or undershot water wheel - overshot: water from above onto the paddles, undershot: water below against the paddles - but by a more modern technology, namely a Francis turbine. It has been tirelessly "grinding" electricity in a generator since 1907. This flows into the public grid. Such ecological electricity generation is remunerated by the state via the Renewable Energies Act at around 10 cents per kWh, depending on the quantity. The exact price of the Augsburg energy supply company later. This is because it contributes to the amortisation of the investment and has a lot to do with the optimised building technology.

A monument to technical culture

Industrial culture in Augsburg

Fig. 2: Professor Wilhelm Ruckdeschel described the probably 700-year history of the Lochbach power station in his book "Industriekultur in Augsburg". Brigitte Settele Verlag, Augsburg, ISBN 3-932939-44-1.

In the past, the kinetic energy of the Lochbach Falls drove, among other things, the circular saw in the courtyard via a transmission, and later an art mill. Since 1880, however - the Francis turbine came 27 years later - the little river has conjured up light in the Guten Stuben. At the turn of the century, it must have seemed like magic to some of the residents.

The efficiency losses of the 76-kW generator and the mechanics make themselves felt in heat. When the generator is working diligently, it turns the mill house into a sauna. In the past, the excess temperatures could be ventilated away by opening windows and doors. Environmental protection today no longer accepts this in two respects. Firstly, it is not in keeping with the spirit of the times to give away valuable heat calories, and secondly, the neighbours might - monument protection or no monument protection - come to court if they had to listen to the squeaking of the mechanism, which is still around 60 dB (A), day in, day out. On the other hand, such a living testimony to the highly developed industrial culture of the late medieval Free Imperial City of Augsburg has new significance today because the still functioning "Wasserkunst", as Augsburg's technical historian Wilhelm Ruckdeschel respectfully calls it, converts renewable energy into electricity.

So the current owner of the "Alter Mühlhof", the Walter Settele family, commissioned various planning offices and installation companies to find a solution for heat recovery. The circuit was to take into account the five different consumers, namely firstly the hot water preparation, secondly the swimming pool water in the main house, thirdly the radiator circuit (high temperature), fourthly and fifthly two underfloor heating circuits (low temperature) in the swimming pool and in the residential building.

How does the heat get into the swimming pool?

Heat pump and buffer tank

Fig. 3: The heat pump (left, Vitocal/Viessmann) and the 300-l buffer cylinder (Vitocell/Viessmann) are responsible for heat recovery in the turbine house.

As the first stage of renovation, the client agreed to the installation of an air/water heat pump (Viessmann) in the turbine house and a quieter new gearbox (Renk AG) to replace the 80-year-old gear rims. However, further proposals, some of which were realised, did not bring the expected success, or the implementation failed due to the cost issue. In some cases, the solution was to cost between 10,000 and 15,000 euros.

However, Walter Settele did not give up because he has a year-round user in the form of the swimming pool. In stages, several companies finally patched together a concept for him that did not deserve the name "concept". It still wasted far too much. Expressed in figures, gas consumption in 2006/2007, before the remodelling, was 150,000 kilowatt hours. A real paradox: even in summer, the Setteles had to heat the pool water with an expensive gas heater because the turbine would stop or shut down when the generator was running at over 35 kilometres per hour. oC. It used to exceed this threshold several times. Only the air/water heat pump in the immediate vicinity of the generator did what was required of it. It cooled the room temperature in the generator station down to a tolerable level. This meant that the building could be used quasi-publicly for demonstrations, birthdays, seminars and other events.

Swimming pool water filter

Fig. 4: Swimming pool water filter. The black cylinder on the left is the heat exchanger for the pool water.

If there is an excess temperature in the station of about 5 °C, the heat pump starts up and stores the yield in a buffer tank. A local heating pipe transports the calories obtained in this way to the two reheating boilers (condensing, 2 x 40 kW, Vaillant) and to the distributor in the residential building. If there is no demand for heat, the generator switches off - see remark overtemperature. And the demand was covered relatively quickly at that time because the switching scheme was not capable of serving the different temperature levels on the consumer side. It was content with this or that consumer.

New process Multi-way mixer

Main house "Alter Mühlhof

Fig. 5: Main house "Alter Mühlhof". Approx. 500 m2 of living space, a swimming pool hall and the pool water are to be heated.

With the consequence of the aforementioned expensive 150,000 kilowatt hours of gas purchased in the mild winter of 2006/2007. Walter Settele therefore ventured a new attempt. He spoke to his house installation company, Albert Kohl Wasser und Wärme GmbH from Bobingen. With its 28 employees, the company is well positioned for the future, with a focus on heating technology, solar technology and photovoltaics. The company promised to help:

Through publications in this magazine, "Sanitär- und Heizungstechnik", the master craftsman's company came into contact years ago with a fitting that is not a product but a system, the Baunach mixer. To anticipate the result straight away: The system builder re-planned the distribution and the control, placed three "rendeMIX" between the heat source and the heat consumer, and the relatively hard winter of 2008/2009 presented the Setteles with the result: a saved 55,000 kilowatt hours. Gas consumption dropped from 150,000 to 95,0000 kilowatt hours. This amount is not climate-adjusted, but the demand in 2008/2009 was probably even much higher than in the warm winter of the reference year 2006/2007. In this respect, the savings speak all the more in favour of the method.

Freely programmable control

Fig. 6: Freely programmable control system from Technische Alternative, Amaliendorf/Austria

The "rendeMIX" mixer stands for a new method of heat distribution as well as charging and discharging buffer storage tanks. It guarantees the lowest return temperatures and thus the highest condensation heat gains by connecting a low-temperature circuit downstream of a high-temperature circuit, thus turning the return of the first circuit into the flow of the second circuit. Exploiting the calorific value with this architecture is one thing, halving the circulation volume and thus halving the auxiliary energy for the circulation pumps is another. Apart from the fact that this series connection gets by with one less circulation pump.

Lowest return temperatures

Sub-distributor heating circuits

Fig. 7: Sub-distributor heating circuits.

The patent of H.G. Baunach GmbH & Co. KG, Hückelhoven, consists primarily of a water supply in the fitting that takes into account the different hydraulic conditions of the radiator heating and the underfloor heating as well as the temperature requirements. Without the "rendeMIX" unit, an installation company would have to make many detours and install many fittings to achieve a similar result - lowest return temperature. Not only condensing boilers work more efficiently with the mixer. Stratified storage tanks, solar systems, heat pumps, transfer stations and distribution networks also benefit from the large Delta T.

Distribution technology rendeMIX

Fig. 8: Mixer group. The three units on the left are the "rendeMIX" multi-way mixers, the two units on the right are fitting boxes for the uncontrolled heat supply of the heat exchanger swimming pool water and the hot water preparation (in the picture Johannes Jacob, Albert Kohl Wasser und Wärme GmbH).

Johannes Jakob from Albert Kohl explains the principle using Haunstetten as an example: "In principle, the multi-way mixer is divided into a hot, a warm and a cold zone. Since we install it downstream of a high-temperature circuit, the hot zone is the 60-grade return flow from the swimming pool heat exchanger or the water heater. These two consumers are controlled directly from the hot district water of the turbine station or from the gas boilers, so to speak, without regulation."

The heat pump in the turbine or generator house generates the up to 80 degree flow to the consumers with a still acceptable annual performance factor, since the intake room air can reach up to 40 degrees Celsius. oC approaches. "We now send the 60 degree return from the swimming pool heat exchanger to the radiator circuit on the upper floors of the apartment building and on to the two underfloor heating systems. Three 'rendeMIX' units take care of the volume flow and temperature control. What happens in rendeMIX 1, which is responsible for the radiators? First of all, they are set to 60/40 oC designed. The 60 oC, but they only need it in the coldest period of the year. In the majority of cases, the mixer is fed either from the return swimming pool, which at the level of the rendeMIX 1 is perhaps still 55 oC, add colder water, or from the return flow radiator circuit, which only has a 40 oC has."

Three times 'rendeMIX

Although the radiator return combines with the swimming pool/hot water return and with the returns of the two underfloor heating systems to form a single line, there is of course a temperature gradient towards the boiler along the length of the line, also due to the return from the individual circuits.

Back to Johannes Jakob. "rendeMIX 2 takes the return flow from the radiator as the supply flow from the floor circuit of the swimming pool hall. The hydraulics work in a similar way as just described, but with a modification. If the return flow of the radiator is not sufficient to heat the swimming pool hall at freezing temperatures, rendeMIX 2 mixes hot water from the supply flow of the local heating pipe. If the weather conditions permit lower heating water temperatures, rendeMIX 2 draws cold water from the return line."

rendeMIX 3 for the underfloor heating "Living" in the upper floors regulates according to the same pattern; Johannes Jakob connected the two underfloor circuits in parallel. He divided the low-temperature heating into two main circuits because the year-round heating of the bathhouse and the six-month heating of the residential building have completely different heating seasons, and the calculated water volumes also differ considerably. The heat supply can be optimised via parallel connection.

Not quite parallel

Whereby "parallel connection" does not correctly reflect reality. If both underfloor heating systems are in operation, the flow temperature of the last rendeMIX, rendeMIX 3, is below the flow temperature of rendeMIX 2. This is because the return flow of the preceding underfloor heating system (rendeMIX 2) has pushed the total return flow back down by a few degrees. This is precisely the goal, to enter the condensation heat exchanger with the coldest water. rendeMIX 3 must therefore usually draw and mix more water from the higher-temperature sections than rendeMIX 2 in order to reach the desired setpoint.

That's about the processes in and the function of the "rendeMIX" stations. Of course, an efficient control technology and a sensible control strategy must be superimposed on the whole system. The company Albert Kohl Wasser und Wärme GmbH relied on the know-how of the planning department of H.G. Baunach GmbH & Co. KG, which supported him with its software in the design and adjustment of the mixer units. The actual networking was configured by the Austrian control specialist Technische Alternative, Amaliendorf. Their product range extends from simple differential controls to multi-circuit controllers for solar and pump management to freely programmable multi-talents for almost all applications. This is exactly what Albert Kohl GmbH was looking for, a control system that is not overloaded with unasked-for functions and thus reacts in a vulnerable manner, but is customised to the application.

Reward from the Augsburg public utility company

Their sensor technology decides, for example, whether the buffer water flows directly into the floor pipe coils and into the radiators or whether the boiler(s) must add heat. The buffer water in the 300-litre storage tank does not constantly have 70 or 80 oC. In winter, the waste heat from the generator is not sufficient to ensure comfort in the house. The boiler might then charge to only 30 or 40 oC. The local heating line therefore generally runs through one of the two boilers, regardless of whether it is to start or not.

Due to the heat recovery, the renewed gearbox, the intelligent switching scheme and an investment that was more than 50 per cent of a new system, Stadtwerke Augsburg corrected the specific feed-in tariff. The "Alte Mühle" now falls under the EEG Renewable Energies Act as well as the EEWG Renewable Energies Heat Act. As a result, the remuneration has climbed from 7.65 to 9.67 ct/kWh. With around 400,000 kilowatt hours of electricity now being generated annually, around EUR 40,000 is now flowing back, EUR 8,000 more than two years ago. The Baunach mixer can take credit for part of this, but in any case for the 55,000 kWh of gas reduction saved.

Walter Settele

Fig. 9: Landlord Walter Settele in front of the 76-kW generator of the Francis turbine.

Johannes Jakob: "What we found was minimal comfort with high consumption. With the new control strategy and the 'rendeMIX' units, we were able to convert the system to the 'minimum heat consumption with maximum comfort' type. You probably can't invest your money better and, above all, more safely."

Bernd Genath

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Minimum heat consumption with maximum comfort


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