It is possible to generate energy using waste heat and waste water and supplying it through large-scale heat pumps running on natural refrigerants. Here is an insight into how the environmentally friendly process has worked in a few major district heating and cooling projects.

THE BACKGROUND

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, Chairwoman 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.”

AMMONIA HEAT PUMPS DEMONSTRABLE SUPERIORITY

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 specialists, and was responsible for planning the ammonia-equipped heat/refrigeration system.

As a first step in 2006, a central energy installation was fitted in 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 the heating capacity of around 5.5 megawatts each.

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

The ammonia heat pump in Rietbach central energy installation produces its heating and cooling capacity, on the one hand using the energy from the treated wastewater of Werdhölzl sewage plant in Zurich, which was previously discharged into the river Limmat. On the other hand, it also uses the waste heat from a nearby computing centre. Over the entire 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 needed in Rietbach central installation is generated by two refrigerating machines that work independently on the refrigerating side, coupled with high-pressure heat pump compressors. The two machines are each connected on the cold water side and 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 desuperheaters 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.

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, it is re-cooled by the river Limmat. If the temperature of the river 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 swiftly detected and remedied.

WASTEWATER – A RENEWABLE SOURCE OF ENERGY

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, along with a nearby commercial estate.

On behalf of AEW Energie AG, and with support from EnergieSchweiz, together with the canton and municipal authorities, Johnson Controls installed a heat pump system that uses treated wastewater from the Abwasserreinigungsanlage Rheinfelden (ARA – Rheinfelden sewage plant) as heat source for the existing local heating consortium. In this way, the refrigeration experts have made a major contribution to linking the ecologically valuable wastewater energy with the energy demand in Rheinfelden.

The heart of the system consists of two heat pumps supplied by Johnson Controls with an output of 1,250 kilowatt each; they 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-metre-long district heat pipe was installed between the heat pump system in the ARA and Augarten central energy installation, where it was then connected to the existing local heating consortium. Just about 1,500 metres of additional district heat pipes were installed to connect up the new dwellings in Weiherfeld.

Two different operating modes are available, depending on the demand for 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 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 megawatt hours per 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 megawatt hours. This saves 1.25 million m³ natural gas per year, reducing carbon emissions by 2,650 tonnes. There are already plans 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 ice skating rink in the vicinity. This waste heat, which is currently discharged into the air, could help cover, in particular, the peak demand during winter.

WORLD’S LARGEST MUNICIPAL AMMONIA HEATING SYSTEM

The town of Drammen with its 60,000 residents is located around 40 kilometres southwest 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 ammonia, a natural refrigerant, and generates heat from the waste heat of large-scale refrigeration systems, air conditioning plants and 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. On completion in January 2011, the system will supply heating capacity with a total of up to 15 megawatt 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.

NATURAL REFRIGERANTS GAIN PROMINENCE

The examples cited show that manufacturers are increasingly willing to use natural refrigerants. “Today, already, 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 the global climate protection targets.”