From a magnet to a heat pump

The magnetocaloric effect (MCE) is the thermal response of a magnetic material to an applied magnetic field. Magnetic cooling is a promising alternative to conventional vapor compression technology in near room temperature applications and has experienced significant developments over the last five years. Although further improvements are necessary before the technology can be commercialized. Researchers were mainly focused on the development of materials and optimization of a flow system in order to increase the efficiency of magnetic heat pumps. The project, presented in this paper, is devoted to the improvement of heat pump and cooling technologies through simple tests of prospective regenerator designs. A brief literature review and expected results are presented in the paper. It is mainly focused on MCE technologies and provides a brief introduction to the magnetic cooling as an alternative for conventional vapor compression technology. has experienced significant developments over the last five years. Although further improvements are necessary before the technology can be commercialized. Researchers were mainly focused on the development of materials and optimization of a flow system in order to increase the efficiency of magnetic heat pumps. The project, presented in this paper, is devoted to the improvement of heat pump and cooling technologies through simple tests of prospective regenerator designs. A brief literature review and expected results are presented in the paper. It is mainly focused on MCE technologies and provides a brief introduction to the magnetic cooling as an alternative for conventional vapor compression technology. magnetic refrigeration, magnetocaloric effect, test device modelling. by and Weiss and Piccard (Smith et al. 2012). the mid-1920s and Giauque proposed a method of adiabatic demagnetization to reach very low temperatures (Smith et al. 2012). The discovery of magnetocaloric materials (MCM) with Curie temperatures ( T C ) temperature zero vapor a ΔT span of 15.5 K and efficiency of 18% was developed at DTU Energy, it is described in Eriksen et al. (2016). A brief introduction to the MCE as an alternative to the conventional vapor compression device has been given in the present paper. The existing test machine, built at DTU Energy, has been described. Additionally, the interested reader might find more information about the rotary prototype of the magnetic cooling technology in Eriksen (2016). published


Introduction
Kristina Navickaitė*, Henrique Neves Bez, Kurt Engelbrecht, Christian R. H. Bahl Technical University of Denmark, Department of Energy Conversion and Storage Frederiksborgvej 399, 4000 Roskilde, Denmark The magnetocaloric effect (MCE) is the thermal response of a magnetic material to an applied magnetic field. Magnetic cooling is a promising alternative to conventional vapor compression technology in near room temperature applications and has experienced significant developments over the last five years. Although further improvements are necessary before the technology can be commercialized. Researchers were mainly focused on the development of materials and optimization of a flow system in order to increase the efficiency of magnetic heat pumps. The project, presented in this paper, is devoted to the improvement of heat pump and cooling technologies through simple tests of prospective regenerator designs. A brief literature review and expected results are presented in the paper. It is mainly focused on MCE technologies and provides a brief introduction to the magnetic cooling as an alternative for conventional vapor compression technology.
The magnetocaloric effect (MCE) was discovered by French and Swiss physicists Weiss and Piccard (Smith et al. 2012). In the mid-1920s Debye and Giauque proposed a method of adiabatic demagnetization to reach very low temperatures (Smith et al. 2012). The discovery of magnetocaloric materials (MCM) with Curie temperatures (T C ) near room temperature (RT) has opened an opportunity for magnetic refrigeration (MR) to become an alternative to conventional vapor compression devices. Furthermore, MR technology is environmentally friendly since it has zero vapor pressure, no Ozone Depletion Potential (ODP), no direct Global Warming Potential (GWP), it has potential to be more compact layout since the working material is solid, and has the potential to work more silently than conventional compressors (Bahl et al. 2008, Barbosa et al. 2014, Engelbrecht et al. 2011, Eriksen et al. 2015, Smith et al. 2012.
The MC cooling technique is based on the thermal response of a magnetic material when magnetized/demagnetized, such as a temperature increases when the field is increased and vice versa. The isothermal entropy change is negative upon magnetization and positive upon demagnetization, see Fig. 1. This scenario is valid when an ordinary MCE takes place and is analogous to the negative change in entropy associated with the isothermal compression of a gas.
MR near RT is a desirable, but at the same time technically challenging idea. Challenges are related to device engineering. Barclay et al. (1981) patented the active magnetocaloric regenerator (AMR) with its own 'distributed refrigeration' cycle which can provide refrigeration over a temperature span that is much larger than the adiabatic temperature change of the material. Nevertheless, an efficient AMR device requires a large magnetic field. Rowe et al. (2006) concluded that An MR material with randomly oriented spins is magnetizedthe temperature of a material increases adiabatically (a) and then entropy decreases under isothermal conditions (b) (RT) has opened an opportunity for magnetic refrigeration (MR) to become an alternative to conventional vapor compression devices. Furthermore, MR technology is environmentally friendly since it has zero vapor pressure, no Ozone Depletion Potential (ODP), no direct Global Warming Potential (GWP), it has potential to be more compact layout since the working material is solid, and has the potential to work more silently than conventional compressors (Bahl et al. 2008, Barbosa et al. 2014, Engelbrecht et al. 2011, Eriksen et al. 2015, Smith et al. 2012. The MC cooling technique is based on the thermal response of a magnetic material when magnetized/demagnetized, such as a temperature increases when the field is increased and vice versa. The isothermal entropy change is negative upon magnetization and positive upon demagnetization, see Fig. 1. This scenario is valid when an ordinary MCE takes place and temperature of a material increases adiabatically (a) and then entropy decreases under isothermal conditions (b).
MR near RT is a desirable, but at the same time technically challenging idea. Challenges are related to device engineering. Barclay et al. (1981) patented the active magnetocaloric regenerator (AMR) with its own 'distributed refrigeration' cycle which can provide refrigeration over a temperature span that is much larger than the adiabatic temperature change of the material. Nevertheless, an efficient AMR device requires a large magnetic field. Rowe et al. (2006) concluded that the lower magnetic field might be compensated by increasing the operating frequency. However, Nielsen and Engelbrecht (2012) showed that in case of a long AMR (200 mm) the importance of the thermal conductivity increases when a device is operated at a high frequency -the higher the operating frequency, the higher the thermal conductivity is necessary.
Seeking to optimize the configuration of machines, attention is mainly paid to the performance of it. There are several ways to increase the efficiency of an MR such as optimizing the flow system, optimizing the magnet assembly, improving a b Discovered compositions are available in a variety of shapes and packing of new materials should be tested before applying them in larger and more complex prototypes. The properties of a series of such MCMs are tested in a vertically oriented versatile device -the test machine, see Fig. 2. It is designed in a way to allow the variation and control of many experimental parameters. A brief introduction to the main parts and operation of the test machine are given in this paper, more detailed description of the device and operational conditions are given in Bahl et al. (2008) and Engelbrecht et al. (2012). The device is built in a temperature controlled cabinet. The hot reservoir (1) is placed above the regenerator (2) and is linked to the forced convection heat exchanger (HEX) (3) and the cold reservoir (4) (below the regenerator) is covered by thermal insulation (not shown). A Halbach cylinder permanent magnet is used as a magnetic field source, with an average flux density in the bore of 1.03 T. The temperature of the hot end (T hot ) is controlled via the ambient (air inside the cabinet) temperature.
During operation a regenerator is moved vertically by a stepper motor while the magnet is kept in a stationary position (see Fig. 3). Magnetization and demagnetization of the MCM is achieved in this way and a temperature span across the thermal reservoirs is built up. The heat transfer fluid is moved through the regenerator by mean of a displacer in the cold end. The entire device is in thermal contact with the same ambient air i.e. hot end is thermally linked to the ambient via a forced convection heat exchanger while the cold end is thermally insulated using foam tubing. The test machine can be operated in different combinations of various parameters, such as a different piston (displacer) stroke (amplitude) and velocity. The heat transfer fluid flow is provided by pushing the piston backwards and forwards. In this manner the fluid velocity and volume of the flow can be controlled. The velocity of the fluid directly affects utilization, which represents the ratio of the thermal capacity of the fluid that moves into regenerator to the thermal capacity of the solid regenerator material. The expression to define utilization (φ) is given below:

Fig. 2
A photo of the test machine installed in the temperature controlled cabinet (a) and the schematic drawing (b)

Fig. 3
The test machine with the magnetic regenerator a) outside and b) inside the magnet applying them in larger and more complex prototypes. The properties of a series of such MCMs are tested in a vertically oriented versatile device -the test machine, see Fig. 2. It is designed in a way to allow the variation and control of many experimental parameters. A brief introduction to the main parts and operation of the test machine are given in this paper, more detailed description of the device and operational conditions are given in Bahl et al. (2008) and Engelbrecht et al. (2012). The device is built in a temperature controlled cabinet. The hot reservoir (1) is placed above the regenerator (2) and is linked to the forced convection heat exchanger (HEX) (3) and the cold reservoir (4) (below the regenerator) is covered by thermal insulation (not shown). A Halbach cylinder permanent magnet is used as a magnetic field source, with an average flux density in the bore of 1.03 T. The temperature of the hot end (Thot) is controlled via the ambient (air inside the cabinet) temperature. During operation a regenerator is moved vertically by a stepper motor while the magnet is kept in a stationary position (see Fig. 3). Magnetization and demagnetization of the MCM is achieved in this way and a temperature span across the thermal reservoirs is built up. The heat transfer fluid is moved through the regenerator by mean of a displacer in the cold end. The entire device is in thermal contact with the same ambient air i 1 2 4 (2) The test machine can be operated in different combinations of various parameters, such stroke (amplitude) and velocity. The heat transfer fluid flow is provided by pushing the pisto this manner the fluid velocity and volume of the flow can be controlled. The velocity of the fl which represents the ratio of the thermal capacity of the fluid that moves into regenerator to th regenerator material. The expression to define utilization () is given below: ( 1 ) where: ṁ f -mass of the fluid pushed through the regenerator in one direction; c f and c s the solid, respectively; m s -mass of the solid regenerator (Neves Bez et al. 2016).
To find the optimal working point of the MCM, T hot is set slightly above the T C of the tests are run under the following conditions: 1. At different piston amplitudes, when the utilization (Eq. 1) of a regenerator i at the same temperature (slightly above the T C ) and piston velocity conditions. The point whe highest is selected to be the optimal piston amplitude for the remaining tests of the same regen 2. At different piston velocities, when the optimal piston amplitude is known. T temperature (same as at the first step) and piston amplitude conditions. The point where the tem selected to be the optimal piston velocity for the remaining tests of the same regenerator. 3.
At the different temperature range, when the piston amplitude and velocity is is known, a test against temperature change is made in order to find the peak of the temperatur The test machine can be operated in different combinations of various parameters, such as a stroke (amplitude) and velocity. The heat transfer fluid flow is provided by pushing the piston b this manner the fluid velocity and volume of the flow can be controlled. The velocity of the fluid which represents the ratio of the thermal capacity of the fluid that moves into regenerator to the t regenerator material. The expression to define utilization () is given below: ( 1 ) where: ṁ f -mass of the fluid pushed through the regenerator in one direction; c f and c s -s the solid, respectively; m s -mass of the solid regenerator (Neves Bez et al. 2016).
To find the optimal working point of the MCM, T hot is set slightly above the T C of the wo tests are run under the following conditions: 1. At different piston amplitudes, when the utilization (Eq. 1) of a regenerator is k at the same temperature (slightly above the T C ) and piston velocity conditions. The point where highest is selected to be the optimal piston amplitude for the remaining tests of the same regenera 2. At different piston velocities, when the optimal piston amplitude is known. The temperature (same as at the first step) and piston amplitude conditions. The point where the tempe selected to be the optimal piston velocity for the remaining tests of the same regenerator. 3.
At the different temperature range, when the piston amplitude and velocity is co is known, a test against temperature change is made in order to find the peak of the temperature s To find the optimal working point of the MCM, T hot is set slightly above the T C of the working material and series of tests are run under the following conditions: 1 At different piston amplitudes, when the utilization (Eq. 1) of a regenerator is known. The sample is tested at the same temperature (slightly above the T C ) and piston velocity conditions. The point where the temperature span is the highest is selected to be the optimal piston amplitude for the remaining tests of the same regenerator.
2 At different piston velocities, when the optimal piston amplitude is known. The sample is tested at the same temperature (same as at the first step) and piston amplitude conditions. The point where the temperature span is at the peak is selected to be the optimal piston velocity for the remaining tests of the same regenerator.
3 At the different temperature range, when the piston amplitude and velocity is constant.
When working point is known, a test against temperature change is made in order to find the peak of the temperature span of the tested sample.
Two types of regenerators have been tested in the test machine. The parallel plate regenerator is shown in the Fig. 4. a). The irregular particle regenerator is shown in Fig. 4 b) and c). The MCM and their characteristics are not discussed further in this paper. More information about the both types of the regenerators might be found in  and Neves Bez et al. (2016), respectively.
The results for three regenerators, in the form of irregular particles (see Fig. 4 b and c) are presented in this paper. All of the presented regenerators were bonded with a small amount of epoxy which is meant to maintain the mechanical integrity of the regenerator. A water based solution (2 wt%) of anti-corrosion inhibitor ENTEK FNE was used as a heat transfer liquid. All the samples have the same mass (95 g), and average specific heat (500 J/kgK) (the difference in the specific heat of the samples might be neglected) (Neves Bez et al. 2016). The bonding epoxy has poorer thermal properties than pure MCM, nevertheless it is necessary for structural integrity of the regenerator. Thus it is important to measure the MCE characteristics of the regenerators before applying the material on a larger prototypes or magnetic heat pumps. Particularly this material is will be used in the ENOVHEAT (Efficient Novel Magnetocaloric Heat Pumps) prototype, which is under design at DTU Energy. More information can be found at the project's home page http:// www.enovheat.dk/.

Fig. 4
The MCM regenerator a) stacked plate regenerator b) top view and c) side view. Geometry of the housing of the regenerators ø~38mm (outer) and h=40mm.
at the same temperature (slightly above the T C ) and piston velocity conditions. The point where the temperature span is the highest is selected to be the optimal piston amplitude for the remaining tests of the same regenerator. 2. At different piston velocities, when the optimal piston amplitude is known. The sample is tested at the same temperature (same as at the first step) and piston amplitude conditions. The point where the temperature span is at the peak is er in this paper. More information about the both types of the regenerators might be found in  and Neves Bez et al. (2016), respectively.
The results for three regenerators, in the form of irregular particles (see Fig. 4 b and c) are presented in this paper. All of the presented regenerators were bonded with a small amount of epoxy which is meant to maintain the mechanical integrity of the regenerator. A water based solution (2 wt%) of anti-corrosion inhibitor ENTEK FNE was used as a heat transfer liquid.
at the same temperature (slightly above the T C ) and piston velocity conditions. The point where the temperature span is the highest is selected to be the optimal piston amplitude for the remaining tests of the same regenerator.
2. At different piston velocities, when the optimal piston amplitude is known. The sample is tested at the same temperature (same as at the first step) and piston amplitude conditions. The point where the temperature span is at the peak is er in this paper. More information about the both types of the regenerators might be found in  and Neves Bez et al. (2016), respectively.
The results for three regenerators, in the form of irregular particles (see Fig. 4 b and c) are presented in this paper. All of the presented regenerators were bonded with a small amount of epoxy which is meant to maintain the mechanical integrity of the regenerator. A water based solution (2 wt%) of anti-corrosion inhibitor ENTEK FNE was used as a heat transfer liquid.
at the same temperature (slightly above the T C ) and piston velocity conditions. The point where the temperature span is the highest is selected to be the optimal piston amplitude for the remaining tests of the same regenerator.
2. At different piston velocities, when the optimal piston amplitude is known. The sample is tested at the same temperature (same as at the first step) and piston amplitude conditions. The point where the temperature span is at the peak is er in this paper. More information about the both types of the regenerators might be found in  and Neves Bez et al. (2016), respectively.
The results for three regenerators, in the form of irregular particles (see Fig. 4 b and c) are presented in this paper. All of the presented regenerators were bonded with a small amount of epoxy which is meant to maintain the mechanical integrity of the regenerator. A water based solution (2 wt%) of anti-corrosion inhibitor ENTEK FNE was used as a heat transfer liquid.
In order to measure and log the temperature of the ambient, cold and hot end, the type E thermocouples and Pico TC-08 thermocouple data logger (Pico logger) were used.
A Pico logger has 8 channels for thermocouples and the cold junction compensation (CJC) measuring ambient temperature is used in this work. The features of the device are as follow: _ measureable temperature range is -270 +1820 °C; _ temperature accuracy sum of +/-0.2% of reading and +/-0.5 °C; _ voltage accuracy sum of +/-0.2% of reading and +/-10mV; _ conversion time 100 ms per thermocouple channel; _ sampling rate is up to 10 samples per second; _ resolution 20 bits.

Set-up for measurements
The optimal working point of the tested regenerators was at a piston amplitude of 20 mm and piston velocity 15 mm/s (φ = 0.45), see Fig. 5 and Fig. 6. The working point was tested only for the sample of 2 wt.% epoxy. All of the presented samples kept their mechanical integrity during the tests, indicating that 2 wt % epoxy is adequate from a mechanical standpoint. The volume of the epoxy bonding the MCM particles was varied in order to find the minimum necessary amount to maintain the mechanical integrity of the regenerators.

Results and discussion
All the samples have the same mass (95 g), and average specific heat (500 J/kgK) (the difference in the specific heat of the samples might be neglected) (Neves Bez et al. 2016). The bonding epoxy has poorer thermal properties than pure MCM, nevertheless it is necessary for structural integrity of the regenerator. Thus it is important to measure the MCE characteristics of the regenerators before applying the material on a larger prototypes or magnetic heat pumps. Particularly this material is will be used in the ENOVHEAT (Efficient Novel Magnetocaloric Heat Pumps) prototype, which is under design at DTU Energy. More information can be found at the project's home page http://www.enovheat.dk/.

Set-up for measurements
In order to measure and log the temperature of the ambient, cold and hot end, the type E thermocouples and Pico TC-08 thermocouple data logger (Pico logger) were used.
A Pico logger has 8 channels for thermocouples and the cold junction compensation (CJC) measuring ambient temperature is used in this work. The features of the device are as follow:  measureable temperature range is -270 +1820 °C;  temperature accuracy sum of +/-0.2% of reading and +/-0.5 °C;  voltage accuracy sum of +/-0.2% of reading and +/-10mV;  conversion time 100 ms per thermocouple channel;  sampling rate is up to 10 samples per second;  resolution 20 bits.

Results and discussion
The optimal working point of the tested regenerators was at a piston amplitude of 20 mm and piston velocity 15 mm/s ( = 0.45), see Fig. 5 and Fig. 6. The working point was tested only for the sample of 2 wt.% epoxy. All of the presented samples kept their mechanical integrity during the tests, indicating that 2 wt % epoxy is adequate from a mechanical standpoint. The volume of the epoxy bonding the MCM particles was varied in order to find the minimum necessary amount to maintain the mechanical integrity of the regenerators.  No-load temperature span as a function of utilization for a two layers regenerator.

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Journal of Sustainable Architecture and Civil Engineering 2016/1/14 The regenerators with 3 wt% and 4 wt% of epoxy were tested at the same working point. As it was expected, the regenerator with 2 wt% of epoxy showed the best performance: ∆T span = 13.6 K at T hot = 301 K (28 ºC). The peaks of the samples with 3 wt% and 4 wt% were ∆T span = 12.8 K (at T hot = 301 K (28 ºC)) and ∆T span = 12.2 K (at T hot = 300 K (27 ºC)), respectively.
The obtained results agree with the previously published knowledge and suggest that increasing the number of layers in the regenerator in the form of powders may lead to a broader temperature span (Eriksen 2016, Neves Bez et. al 2016. However, a more powerful and more efficient machine is necessary to allow us to perform a full test of multilayered regenerators. Thus, it is planned to build an advanced testing device with higher operating frequency and an adjustable magnetic field. The main purpose of the machine would be kept the same -to test and optimize active magnetic regenerators on a small scale before implementing them in larger, more complex prototypes, such as the rotary prototypes, built

Fig. 6
No-load temperature span as a function of fluid velocity for a two layers regenerator

Fig. 7
No-load temperature span as a function of the temperature of the hot end for a two layers regenerator.

Fig. 6.
No-load temperature span as a function of fluid velocity for a two layers regenerator.

Fig. 7.
No-load temperature span as a function of the temperature of the hot end for a two layers regene The obtained results agree with the previously published knowledge and suggest that increasing in the regenerator in the form of powders may lead to a broader temperature span (Eriksen 2016, Ne However, a more powerful and more efficient machine is necessary to allow us to perform a full regenerators. Thus, it is planned to build an advanced testing device with higher operating frequen magnetic field. The main purpose of the machine would be kept the same -to test and optimize active m on a small scale before implementing them in larger, more complex prototypes, such as the rotary pro Energy (Bahl2013, Eriksen 2016). The design and arrangement of the machine is not well discussed y that the new device should be designed to be very flexible with regards to materials, regenerator ge conditions. Also, the available literature suggests that the temperature of a heat-sink must be decided a is going to be designed.
Finally, the efficiency and coefficient of performance (COP) of magnetocaloric prototypes h increased over the past three years -Okuamura and Hirano (2013) presented a device with a COP of 2. their previous prototype, later Jacobs et al. (2014) published results about a prototype, operating at a CO of 10 K. The latest published prototype with a COP of 3.6 at a ΔTspan of 15.5 K and efficiency of 18% w Energy, it is described in Eriksen et al. (2016).

Conclusions
A brief introduction to the MCE as an alternative to the conventional vapor compression device present paper. The existing test machine, built at DTU Energy, has been described. Additionally, the in find more information about the rotary prototype of the magnetic cooling technology in Eriksen (2016) Fig. 6. No-load temperature span as a function of fluid velocity for a two layers regenerator.
The regenerators with 3 wt% and 4 wt% of epoxy were tested at the same working point. As regenerator with 2 wt% of epoxy showed the best performance: ∆Tspan = 13.6 K at Thot = 301 K (28 º samples with 3 wt% and 4 wt% were ∆Tspan = 12.8 K (at Thot = 301 K (28 ºC)) and ∆Tspan = 12.2 K (at respectively. The obtained results agree with the previously published knowledge and suggest that increasing in the regenerator in the form of powders may lead to a broader temperature span (Eriksen 2016, Ne However, a more powerful and more efficient machine is necessary to allow us to perform a full regenerators. Thus, it is planned to build an advanced testing device with higher operating frequen magnetic field. The main purpose of the machine would be kept the same -to test and optimize active m on a small scale before implementing them in larger, more complex prototypes, such as the rotary pro Energy (Bahl2013, Eriksen 2016). The design and arrangement of the machine is not well discussed y that the new device should be designed to be very flexible with regards to materials, regenerator geo conditions. Also, the available literature suggests that the temperature of a heat-sink must be decided a is going to be designed.
Finally, the efficiency and coefficient of performance (COP) of magnetocaloric prototypes h increased over the past three years -Okuamura and Hirano (2013) presented a device with a COP of 2. their previous prototype, later Jacobs et al. (2014) published results about a prototype, operating at a CO of 10 K. The latest published prototype with a COP of 3.6 at a ΔTspan of 15.5 K and efficiency of 18% w Energy, it is described in Eriksen et al. (2016).

Conclusions
A brief introduction to the MCE as an alternative to the conventional vapor compression device present paper. The existing test machine, built at DTU Energy, has been described. Additionally, the in find more information about the rotary prototype of the magnetic cooling technology in Eriksen (2016) at DTU Energy (Bahl2013, Eriksen 2016). The design and arrangement of the machine is not well discussed yet, though. It is clear that the new device should be designed to be very flexible with regards to materials, regenerator geometry and operating conditions. Also, the available literature suggests that the temperature of a heat-sink must be decided a priori if a new device is going to be designed.
Finally, the efficiency and coefficient of performance (COP) of magnetocaloric prototypes has been significantly increased over the past three years -Okuamura and Hirano (2013) presented a device with a COP of 2.5 at a ΔT span of 5 K of their previous prototype, later Jacobs et al. (2014) published results about a prototype, operating at a COP above 2 at a ΔT span of 10 K. The latest published prototype with a COP of 3.6 at a ΔT span of 15.5 K and efficiency of 18% was developed at DTU Energy, it is described in Eriksen et al. (2016).

Conclusions
A brief introduction to the MCE as an alternative to the conventional vapor compression device has been given in the present paper. The existing test machine, built at DTU Energy, has been described. Additionally, the interested reader might find more information about the rotary prototype of the magnetic cooling technology in Eriksen (2016).
No-load temperature spans have been presented for the regenerators with varying amounts of bonding epoxy. It is seen that the smaller amount of epoxy in the volume of the regenerator, the better is the performance. The minimal amount of epoxy should be incorporated in order to sustain the mechanical integrity of the regenerator, though. As it is presented in the results section, the utilisation for all of the tested regenerators was φ = 0.45, which corresponded to a