Green heat supply

Large-scale heat pumps use industrial waste heat, water


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Large-scale heat pumps use industrial waste heat and waste water to generate new heat and hot water. We take a closer look at one of the largest projects of this kind in Europe, and the global implications for this technology.

Industrial processes and refrigeration activities always generate heat. Today, many private households use geothermal energy or ambient air efficiently to generate heat. For industrial needs, waste heat and waste water serve as ideal energy sources due to their higher basic temperatures.

Large-scale heat pumps can put the energy of this waste heat to good use for heating or for providing hot water, for instance, in local heating consortiums, thus making a significant contribution to climate protection.

“Furthermore, the heat pump branch makes increasing use of natural refrigerants such as ammonia, carbon dioxide or even water,” explains Monika Witt, chairperson of eurammon, the European initiative for natural refrigerants.

“These stand out not only by being environment-friendly – ammonia and water have no global-warming potential at all, and that of carbon dioxide is negligible – but also, above all, through their energy efficiency.”

Energieverbund Schlieren (Schlieren energy consortium) in Zurich, with two large ammonia heat pumps at its core, is one of the largest projects of its kind in Europe.

It was planned, financed and built by ewz, one of Switzerland’s largest energy-service providers. SSP Kälteplaner AG was involved in the project as refrigeration specialist, and was responsible for planning the ammonia equipped heat/refrigeration system.

As a first step in 2006, a central energy installation was fitted in the Mülligen letter-sorting centre, followed by the commissioning of the central energy installation in Rietbach in 2009. The ammonia heat pumps in both systems have a heating capacity of around 5.5 MW each.

By using ammonia as a refrigerant, it is possible to achieve the necessary high-flow temperatures of around 80°C. When finally completed, the energy consortium will produce annual savings of around 48 700 MWh worth of fossil fuels, corresponding to a reduction in carbon emissions of 8 100 t/y.

The ammonia heat pump in the Rietbach central energy installation produces its heating and cooling capacity, on the one hand, by using the energy from the treated wastewater of Werdhölzl sewage plant in Zurich, which was previously discharged into the river Limmat. Furthermore, it also uses the waste heat from a nearby computing centre. Over the whole year, the heat pump thus covers 70% of the heat demand.

Additional oil- or gas-fired heat generators are available to cover peak demand. The generated heat is transported by a pipeline network to the individual properties where transfer stations then convey it to the building’s internal distribution systems.

The cold or heat that is required by the Rietbach central installation is generated by two refrigerating machines that work independently on the refrigerating side, together with coupled high-pressure heat pump compressors.

The two machines are each connected up in series on the cold water side and on the heating side. This is followed by cooling down or heating up in two stages with optimised COP for heating and cooling.

The four low-pressure refrigerating machines for generating cold water are equipped with frequency converters for fully variable operation. Eight compressors are installed to operate the heat pumps, and can all be switched on and off individually.

The two de-superheaters in the low-pressure circuit supply additional heat in cold water operation, which is passed on to the energy consortium via an energy storage device.

Fully variable
In the summer, the treated wastewater is used directly for cooling via an intermediate circuit. If the temperature level of the wastewater is too high for direct cooling, the refrigerating machine is then used.

If the waste heat of the refrigerating machine cannot be used in the summer, this is recooled by the river Limmat.

If the temperature of the river Limmat exceeds 25°C, a dry cooler is provided for emergency cooling. The system permanently monitors the energy efficiency of refrigeration/heat production. All major parameters are measured so that any energy losses or changes in the system are detected and remedied swiftly.

Since early 2009, environment-friendly heat supply has also been installed in the Swiss town of Rheinfelden for more than 1 000 residential dwellings in Augarten and Weiherfeld, together with a nearby commercial estate.

On behalf of AEW Energie AG, and with support from EnergieSchweiz, together with the municipal authorities, Johnson Controls installed a heat pump system that uses treated wastewater from the Abwasserreinigungsanlage Rheinfelden (ARA, or Rheinfelden sewage plant) as a heat source for the existing local heating consortium.

In this manner, the refrigeration experts have made a major contribution to linking the ecologically-valuable wastewater energy with the total energy demand in Rheinfelden.

The heart of the system consists of two heat pumps that were supplied by Johnson Controls, with an output of 1 250 kW each. These were placed at the end of the ARA’s secondary sedimentation tank.

These are two-stage pumps that operate with ammonia, thus achieving greater efficiency than with synthetic refrigerants, another plus point in the eco-balance. Hot water is supplied to the dwellings by the district heat pipes.

To this end, an approximately 500-m-long district heat pipe was installed between the heat pump system in the ARA and the Augarten central energy installation, where it was then connected to the existing local heating consortium. Just about 1 500 m of additional district heat pipes were installed to connect up the new dwellings in Weiherfeld.

Two different operating modes are available, depending on the ultimate demand for the heat output. In the summer, the heat pumps provide the whole network directly with water at a temperature of 67°C as the hot process water supply.

ARA’s heat pump system also has a hot water tank with a volume of 50 m³ to cover peak demand for hot water in the morning. When the demand for heat increases in the winter, three existing natural gas boilers are responsible for controlling and reheating the flow to the heat pumps.

Altogether the new heat pump system produces effective heat of around 14 000 MWh a year, thus covering around two thirds of the annual heat energy demand of the residential estates in Augarten and Weiherfeld, amounting to around 22 000 MWh. This saves 1.25 million m³ natural gas a year, reducing carbon emissions by 2 650 t/y.

There are already plans in place to expand the district heating system, so that the nearby KUBA leisure centre can be both heated and cooled using the two existing heat pump systems.

Consideration is also being given to using the waste heat generated by the refrigeration plant of the nearby ice skating rink. This waste heat, which is currently discharged into the air, could help to cover peak demand, particularly during the winter.

The town of Drammen, with its total of 60 000 residents, is located around 40 km south-west of Oslo. Here, too, the town has opted for the natural refrigerant ammonia for heating in future.

At the moment, the British company Star Refrigeration, with its Norwegian partner Norsk Kulde, is working on the installation of one of the world’s largest municipal ammonia heat pump systems.

The core of the complex system is Neatpump, the heat pump developed by Star Refrigeration.

It operates with the natural refrigerant ammonia, and generates heat from the waste heat of large-scale refrigeration systems and air-con plants, and of industrial processes and from wastewater.

The heat pump generates water at a temperature of up to 90°C, which is then used to heat large buildings.

The scheme was completed in January 2011, with the system anticipated to supply a total heating capacity of up to 15 MW and provide environment-friendly heating for more than 6 000 residents and companies in Drammen.
The heat pump is operated by environment-friendly hydroelectric power. These examples show that manufacturers are increasingly willing to use natural refrigerants.

“Companies are working intensively at implementing sustainable concepts for heat pumps and realising a large number of great role model projects,” says Witt. “This is of increasing significance, particularly in the context of global climate protection targets.”e impact of it is pretty effective.”

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