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HYBRID HEAT PUMP FOR WASTE HEAT RECOVERY IN
NORWEGIAN FOOD INDUSTRY
Stein Rune Nordtvedt, Institute for Energy Technology, Instituttveien 18, N-2027 Kjeller
Bjarne R. Horntvedt, Hybrid Energy AS, Ole Deviks vei 4, N-0666 Oslo
Jan Eikefjord, John Johansen, Nortura AS, Rudshøgda, Norway
Stein.Nordtved[email protected]
This paper was published in the proceedings of the 10th International Heat Pump Conference 2011
(www.heatpumpcentre.org/en/hppactivities/ieaheatpumpconference).
Abstract: Traditional food companies usually have facilities for the production of refrigeration
and steam. Typically, these facilities run independently of each other even if they are a part
of the energy system in the companies. There are only a handful of companies that need
steam and in most cases hot water up to 80-100°C is sufficient. Institute for Energy
Technology has done research on energy recovery in an industrial context, and especially
within the food industry since 1995. This has resulted in a hybrid heat pump (compression /
absorption heat pump) using a mixture of ammonia and water as working fluid. The hybrid
heat pump has the capability to deliver hot water up to 100°C, recovering waste heat at
approx 50°C. This paper presents operational experiences with a specific hybrid heat pumps
installation at a Norwegian slaughterhouse. The hybrid heat pump recovers heat from the
refrigeration system and produce hot water at 90°C.
Key Words: industrial heat pumps, waste heat recovery, ammonia-water mixture
1 INTRODUCTION
Traditional food companies usually have facilities for the production of refrigeration and
steam. Typically, these facilities run independently of each other even if they are a part of the
energy system in the companies. There are only a handful of companies that need steam
and in most cases hot water up to 80-100°C is sufficient.
Institute for Energy Technology has done research on energy recovery in an industrial
context, and especially within the food industry since 1995. This has resulted in a hybrid heat
pump (compression/absorption heat pump) using a mixture of ammonia and water as
working fluid (Nordtvedt 2005). The hybrid heat pump has the capability to deliver hot water
up to 100°C, recovering waste heat at approx 50°C. A pilot plant of the hybrid heat pump was
installed in a Norwegian dairy in 2000 and after successful operation over several years the
company Hybrid Energy AS was founded in 2004. The hybrid heat pump installation that will
be described here is the first commercial hybrid heat pump installation in Norway. This heat
pump was installed in a slaughterhouse providing hot water for cleaning purposes and has
been in operation since 2007.
2 THE HYBRID HEAT PUMP TECHNOLOGY
The combination of an absorption process and a vapour compression process with
ammonia-water as working fluid gives a flexible heat pump system. A hybrid heat pump with
a 50% mixture of ammonia and water, can heat water to over 100°C with the same pressure
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rating/equipment. Then this plant can cover quite different temperature ranges than
conventional heat pumps and is particularly interesting in industrial processes.
The hybrid heat pumps (compression/absorption) exhibit some interesting possibilities when
compared to conventional vapour compression heat pumps. High heat sink temperatures can
be achieved at the design pressures of standard refrigerating components due to the boiling
point elevation reached when mixing ammonia with water. The temperature glide of the
ammonia-water mixture can be adjusted to the external heat sink/source fluids by adjusting
the composition. Changing the average composition of the solution circulating between the
absorber and the desorber allows for a change in the saturation pressures in the system, and
a capacity control as the compressor is supplied with lower or higher density vapour.
3 SYSTEM DESCRIPTION
Figure 1 shows a sketch of the hybrid heat pump system. The working cycle consists of a
solution pump pumping solution rich in water from low pressure to higher pressure, and a
compressor which is pressurizing ammonia vapour from low pressure to high pressure. The
solution rich in water is mixed with the ammonia vapour in the absorber, and heat is released
to the heat sink during the ammonia absorption process. The resulting solution leaving the
absorber is rich in ammonia, and is expanded to the low pressure side through the expansion
valve before it enters the desorber. There ammonia is evaporated out of the solution by heat
extraction from the heat source, and the process repeats itself. A solution heat exchanger is
installed for internal heat recovery and increases the cycle efficiency. Figure 2 shows the
hybrid heat pump cycle components in a pressure, temperature, concentration (PTX)
diagram.
Absorber
Desorber
Expansion
valve Solution pump
Compressor
Vapor/liquid
separator
Heat source
Heat sink
45- 50 °C
45-50 °C10-20 °C
Solution
heat exchanger
85- 90 C
Figure 1: Hybrid heat pump sketch
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As the working fluid is a mixture of ammonia and water the evaporation and condensation
processes in the desorber and the absorber respectively, will occur at gliding temperatures.
The ammonia concentration in the solution will increase in the absorber and the mixture
temperature will decrease through the heat exchanger as the saturation temperature of
ammonia is considerably lower than the saturation temperature of water. The process is
reversed in the desorber where ammonia is evaporated out of the ammonia/water solution,
and the saturation temperature in the solution will increase from the inlet to the outlet of the
heat exchanger.
Absorber
Desorber
Solution VVX
Compressor
Solution pump
Ekspention valve
Figure 2: PTX diagram of ammonia-water mixture with hybrid heat pump components
The gliding temperatures of the working fluid mixture during heat extraction in the desorber
and during heat release in the absorber can be an advantage in many processes. If the heat
source is water that has to be cooled from 40 to 10°C, and the heat sink fluid is water that
shall be heated from 50 to 90°C, the temperature glide of the working fluid mixture can be
adapted to the temperature glide of the heat sink and heat source fluid. The compressor in
the hybrid heat pump will then work against a lower temperature lift than the compressor in a
conventional vapour compression heat pump using a pure fluid which experiences a constant
temperature during evaporation and condensation.
4 THE HYBRID HEAT PUMP AT NORTURA RUDSHØGDA SLAUGHTERHOUSE
Nortura SA is one of the main suppliers of meat and eggs in Norway, and has 6,100
employees in different industries in 34 municipalities all over the country. The group is
organized as a cooperative owned by 25,000 Norwegian farmers (Nortura 2011).
Nortura SA has a slaughterhouse at Rudshøgda in Norway. Nortura Rudshøgda slaughter
cattle, pigs and sheep, and do cutting of cattle and pigs. The slaughterhouse produces
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steaks/piece of meat, meat cakes, dough and salted/smoked and marinated products. The
slaughterhouse need large amounts of hot process water at minimum temperature of 83°C.
In 2007 a hybrid heat pump with a capacity of 650 kW on the hot side was installed for this
purpose. The hybrid heat pump is built using a standard reciprocating ammonia compressor
and plate heat exchangers as heat exchange components. Figure 3 shows a system sketch
of the heat pump installation.
Absorber
Desorber
Expansion
valve Solution pump
Compressor
Vapor/liquid
separator
Heat source
circuit
Heat sink circuit 87°C
45- 50 °C
45-50 °C35-40 °C
Heat exchanger
Pump
Accumulator tank
83 °C
From energy recovery system
Solution
heat exchanger
To hot water
consumers
Figure 3: The hybrid heat pump installation at Nortura Rudshøgda
The energy recovery system at Nortura Rudshøgda recovers heat from the refrigeration
system on site. Process water is preheated to approximately 50°C with a low-pressure
ammonia heat pump system, and then further heated to approximately 83°C with the hybrid
heat pump. The hybrid heat pump delivers heat at 87°C to at secondary circuit to avoid
contamination of the water for consumption with ammonia in case of a leakage in a heat
exchanger.
The heat source of the hybrid heat pump is surplus heat at a temperature of 45-50°C from
the low-temperature ammonia compression heat pump plant and waste heat from the
cooling/freezing plant.
As a part of the hybrid heat pump installation, a hot water accumulator tank with a capacity of
200 m3 was installed. The accumulator tank gives the Nortura flexibility in the production, as
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the hot water supply system manages large water withdrawals over periods without large
peaks in the heating system. By heating tap water up to 83°C with recovered heat, the water
can be stored without any danger of legionella problems.
The hybrid heat pump installation has showed an average efficiency of 4.5 over three years.
The energy recovery system saves in total approximately 3.4 GWh annually.
5 REFERENCES
Nortura 2011. www.nortura.no
Nordtvedt, S.R. 2005. “Experimental and theoretical study of a compression/absorption heat
pump with ammonia-water as working fluid,” Doctoral Thesis, Norwegian University of
Science and Technology, 2005:82.
