Background article

Large heat pumps, huge potential

The natural refrigerant ammonia impresses in new applications

The use of heat pumps with the natural refrigerant ammonia is well established in industrial applications – especially for high outputs. Now, ammonia-based heat pump systems are beginning to assert themselves in new applications, such as in municipalities, public institutions or the enterprise. They use this environmentally friendly and energy-efficient technology to meet internal demands for heat or even to sustainably heat entire districts. The reasons are of both an economic and ecological nature: in addition to a global warming and ozone depletion potential of zero, NH3 has excellent thermodynamic properties. The results: ammonia is a very efficient refrigerant capable of achieving extremely high efficiency in heat pumps. Three eurammon member companies show what is possible with large heat pumps today, based on current projects.

Norway: district heating from the fjord

The fjords of Norway hold a huge energy potential; the town of Drammen recognised this in 2008 when it tendered an ambitious project. Drammen, which has a population of 65,000, was looking for a sustainable district heating system that used heat pumps to utilise the heat in the fjord water to supply almost an entire district of the town with inexpensive and environmentally friendly heat. The contract was awarded to eurammon member Star Refrigeration, which implemented the project. Star are so convinced of the opportunity of big ammonia heat pumps they have since formed a subsidiary Star Renewable Energy. One decisive criterion for the award was the environmental performance of the proposed solution with ammonia‘. Drammen is the world’s largest heat pump to use a natural refrigerant at 90°C. Managing to boost the heat from a river to a useful temperature level in such an efficient way is a remarkable achievement’, said Dave Pearson, Director of Star Renewable Energy. This is only practically possible with ammonia.’

A prime example with replication potential

The average water temperature of the fjord in winter is 8°C; this drops to 6°C in summer due to melting glacier water. Because of its constant temperature, the water is extracted at a distance of around one kilometre from the shore at a depth of 35 m, and then filtered and transported to the heat pump system. The thermal energy of the water is transferred to a closed circuit here, and successively heated to 90°C by three two-stage heat pumps connected in series and designed for 65 bar. Each heat pump only needs a fill of 1,000 kg NH3. Thanks to the intelligent system design, the ammonia heat pumps achieve a COP of 3.05. The nearly boiling water then passes through a 22 km pipeline system to Drammen and supplies the connected buildings with hot water and heating via a heat exchanger. And at an unbeatable price: one megawatt hour costs just 11 euros. With its capacity of 43 MW, the plant covers about 70% of the district’s total energy needs. In 2013, ‘Drammen district heating’ supplied a total of 90 GWh – and the heat pumps generated nearly three-quarters of this (67 GWh), in a sustainable and cost-efficient manner. An impressive pioneering example, which could be followed in the future by many towns and cities with a similar waterfront location, such as Stockholm or locations on the Danube. The flow rate of the Danube would allow around 2 GW of heat to be extracted.

Germany – silver mine as an energy treasure trove

The district hospital in Freiberg near Dresden is idyllically situated in the Ore Mountains above an old silver mine – and has benefited from this location in a very special way since the spring of 2014. At a depth of 200 metres, in the tunnels below the hospital, mine water flows at a year-round constant temperature of about 14°C. With the aim of using this source of heat, Johnson Controls designed a two-stage ammonia heat pump that heats the hospital. ‘For us it was important to consider all local features, such as access to mine water, when choosing the heat pump system,’ explains D.Eng. Jürgen Brückner from Johnson Controls. The decision was made to use both a heat pump and a combined heat and power plant (CHP), which produces a total heating output of 1.16 MW at peak times.

Well drilled – 200 metres through the gneiss in Freiberg

The mine water at a temperature of 14°C at a depth 200 meters flows from the tunnel at a volume flow rate of 300 litres per second into the River Triebisch. First, some of the water is pumped into a specially constructed machine room on the third shaft bottom. The heat energy of around 600 kW stored in the water is transferred to the NH3 circuit by a plate heat exchanger. A challenge: to route the pipes out of the tunnel to the heating centre on the surface, a 200-metre hole with a diameter of 50 centimetres had to be drilled through the rock. At the heating centre, the temperature of the water is raised by a heat pump cycle. From the supplied compressor capacity of 215 kW results a heat output of 815 kW. A gas-fired combined heat and power plant generates the electrical energy for operating the heat pump, which achieves a heat output of 815 kW. The CHP also focuses on energy efficiency: in order to increase the overall efficiency, waste heat from the CHP is fed into the overall system – this boosts the final temperature of the heating circuit up to 76°C. The entire system thus generates a heat output of about 2 standard cubic metres of gas from an energy equivalent of one standard cubic metre of gas, which translates to a COP value of the heat pump of up to 4.05. Thus, the heat generation costs are now only 57 €/MWh. Thanks to this heat energy, which is produced in a sustainable and cost-effective manner, the hospital can cover some 80% of its heat requirement internally, thus saving heating costs of about 350,000 euros per year and safeguarding its own long-term competitiveness.

Sweden: practising what you preach

Alfa Laval is one of the world’s biggest manufacturers of heat exchangers. At its site in the Gunnesbo area of the southern Swedish city of Lund, the eurammon member is now also an internal role model: since 2013, process heat generated in component production is used for an innovative ammonia-based heat pump system. Alfa Laval thus covers almost the entire heating requirements of the factory and corporate headquarters for heating and hot water using only the existing heat. This is not just highly sustainable, as it avoids exposing the environment to some 140 tonnes of CO2 per year, but the investment also makes great business sense: prior to this, the company purchased about 3,700 MWh of district heating – an enormous cost factor that it will be able to reduce by an estimated 80 per cent with the new heat pump. This means that the investment will pay for itself in just three years.

Double efficiency. Easily implemented.

The commitment with which Jesper Olsen, market manager of Industrial Refrigeration, pushed the project forward internally is noteworthy. For Olsen, the original heat pump design had enormous scope for improvement. ‘I was certain that a heat pump system with twice the efficiency level was possible, using a natural and future-proof refrigerant,’ said Olsen, explaining his dedication. The heart of the newly designed system is a flooded evaporator, which Olsen combined with a U-turn separator. It absorbs the thermal energy of the oil cooling system, which is heated to around 28°C in the factory’s plate production pressing operations. After compression, the high-pressure system conducts the heat in the condenser to a closed heating-water circuit, thus boosting the water temperature to 65°C. The system uses a high-pressure circuit and thus only needs 40 kg of ammonia. All told, the heat pump has a capacity of 827 kW. Really practical: Alfa Laval is using the successful project as an ‘in-house’ reference and showroom for customers and employees.

Three projects. One refrigerant. Many opportunities.

Despite all their differences, the three projects demonstrate a variety of common features – all of the solutions rely on the natural refrigerant ammonia in designing an environmentally friendly, energy- and cost-efficient heat pump solution. In addition, the three projects are not only excellent case studies – they have a model character and show new opportunities in a growing market segment, namely ammonia-based heat pumps in the very-high-performance range – even outside of industrial applications.