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Improved efficiency of a rotary active magnetic regenerator
Magnetocaloric cooling (and heating) group DTU
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Organization
Theoretical background • MCM property effects on AMR performance • Regenerator effects on AMR performance Experimental efforts at DTU • Device background and description • Device improvements • Experimental results
Other work at DTU • Heat transfer optimization • ENOVHEAT project
2 11
November 2015
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Studying the Effect of Regenerator Effectiveness on AMR
• Start with a fixed regenerator shape – 13x18 mm, 100 mm long (large prototype) and operating conditions
• Temperature span is 25 K
• Set a range of utilizations
• Scale the heat transfer in order to hold NTU constant
𝑈𝑈 =𝑐𝑐𝑓𝑓 �̇�𝑚 𝜏𝜏𝐶𝐶𝑓𝑓 + 𝐶𝐶𝑠𝑠
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Active Regenerator Performance vs NTU - 25K span
4
0 0.2 0.4 0.6 0.80
1
2
3
4
5
6
7
Utilization
Coe
ffici
ent o
f per
form
ance
NTU = 10NTU = 25NTU = 50
0 0.2 0.4 0.6 0.80
50
100
150
200
250
300
350
Utilization
Coo
ling
pow
er (W
)
NTU = 10NTU = 25NTU = 50
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Active Regenerator Efficiency, Fixed Cooling Power, 25 K span
5
20 25 30 35 40 45 50 55 604.2
4.4
4.6
4.8
5
5.2
5.4
5.6
NTU
CO
P
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Packed Sphere Regenerator Performance
• High heat transfer performance –Small particle sizes available
• Relatively easy construction, including layered regenerators
• High pressure drop
6 11 November 2015
𝐶𝐶𝐶𝐶𝐶𝐶 =�̇�𝑄𝑐𝑐
�̇�𝑊𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 + �̇�𝑊𝑝𝑝𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚
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Packed Sphere AMR Performance
7 11 November 2015
f=2 Hz, ∆T=24 K
400 600 800 1000 1200 14000
50
100
150
200
250
300
350
400
Fluid flow rate (L/hr)
Coo
ling
pow
er (W
)
0.7 mm spheres0.6 mm spheres0.5 mm spheres0.4 mm spheres0.3 mm spheres0.2 mm spheres
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Packed Sphere, no pressure drop
8 11 November, 2015
400 600 800 1000 1200 1400 1600 1800 2000 22000
100
200
300
400
500
600
700
800
Fluid flow rate (L/hr)
Coo
ling
pow
er (W
)
0.7 mm spheres0.6 mm spheres0.5 mm spheres0.4 mm spheres0.3 mm spheres0.2 mm spheres
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Reduction in cooling power due to viscous dissipation
9 11 November, 2015
0.2 0.3 0.4 0.5 0.6 0.70
50
100
150
200
250
300
350
400
450
500
Sphere diameter (mm)
Red
uctio
n in
max
coo
ling
pow
er (W
)
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Packed Sphere Discussion
10 11 November, 2015
• Pressure drop is a huge problem –Cooling power reduction –COP reduction due to increased pump power –Requires higher regenerator housing strength
• Pressure drop can be reduced by increasing cross-sectional
area but this usually makes life difficult for magnet designers
• Room temperature AMRs with packed sphere regenerators will probably never be commercially relevant for widespread applications
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Parallel Plate Regenerators
11 11 November, 2015
• Exhibit attractive pressure drop to heat transfer characteristics
• Relatively low disruption of the fluid flow and low specific
surface area mean that small dimensions are required for high NTU
–Roughly 50 µm plate thickness and 50 µm spacing
• Constructing high performance plate regenerators is difficult
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1st rotary AMR prototype
12
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First large active magnetic regenerator (AMR) at DTU
• Maximum load of 1010 W at zero temperature span • Temperature span of 25.4 K at no-load • Maximum frequency 10 Hz
• Temperature span of 20.5 K at 100 W • Temperature span of 18.9 K at 200 W • Temperature span of 13.8 K at 400 W
23 Jan 2015 13
K. Engelbrecht et al., 2012, Int. J. Ref., 35(6): 1498-1505.
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Main challenges from previous AMR
14 11 November
2015
• Increased motor work
• Heat dissipation subtracts from cooling power
Valve and seal friction • Decreases cooling
power
Heat leakage Only 37% MCM in magnet gap (cross section) • Expensive magnet
wasted • Uneven torque
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Design focus points
Magnet: • 2D AMR model optimization combined with FE magnet optimization • Mechanically simple and efficient rotation relative to regenerator
Regenerator: • 2D AMR model optimization of bed dimensions for magnetic field • Utilize magnetized volume: Minimize regenerator housing • Minimize uneven torque: Minimize bed spacing • Minimize regenerator heat leakage: insulating air gap Flow system: • Control flow profile in beds based on 2D AMR model optimizations • Minimize friction • Eliminate internal leak paths
15 11 November
2015
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Magnet design
16
Φ [deg]
B [T]
Z [mm]
B [T]
Φ [deg]
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Regenerator design
17
0.066 mm
0
0
40oC
0
0.27 GPa
• 11 compartments • Separation walls: 0.5 mm Glass fiber • Housing: 0.5 mm stainless steel
cylinders
von
Mie
ses
stre
ss
Tem
pera
ture
D
ispl
acem
ent
Structural simulations at ∆T = 40 K and P = 10 bar
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18
Gd, Tc = 18 oC Gd97.5Y2.5, Tc = 14 oC
Gd95Y5, Tc = 9.5 oC Gd90Y10, Tc = -1 oC
• Gd/GdY spheres from
SANTOKU for enhanced ∆T
• 1.7 kg MCM • Sphere diameter: 0.3 mm -
0.6 mm • Housing takes up 9.7 % of
regenerator cross section
Ø 104 mm
Regenerator design
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Regenerator with flow system assembly
19
Poppet valve system
Active magnetic regenerator
Cold side check valve system
Cam rings
Rotating magnet
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Final construction…
20 D. Eriksen et al., 2015, Int. J. Refrigeration available online
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The machine in action
21
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An unexpected phenomenon
22
Adjustment of one out of the 22 valves can greatly impact performance
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Flow balancing in multiple regenerator AMRs
23
• Total flow into and out of the regenerator is equal.
• In each individual bed, it is possible to have a slightly different flow rate in each flow direction
• Distributions of regenerators with small variations in their flow resistance were modeled with our 1D numerical model
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Flow imbalance effects on AMR performance
24
D. Eriksen et al., 2015, submitted to Int. J. Ref.
Std. Dev. in flow resistance [%]
0 1 2 3 4 5
Coo
ling
pow
er [W
]
40
60
80
100
120
140
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Flow balancing: main conclusion
25
• Flow resistance in each bed in both directions must be adjusted before experiments can be run (22 valves).
• Best adjustment technique is yet to be determined
• All following results are for normalized flow resistances in each direction at a given fluid flow rate
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Experimental performance
26
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
CO
P
Temperature span [degC]
Series1
Series2
Improved COP demonstrated
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
5.00 10.00 15.00 20.00 25.00 30.00
Coo
ling
pow
er (
W)
Temperature span (K)
1 Hz operation 3 L/min flow rate
New AMR Prev. AMR
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Improving efficiency measurements: shaft power evaluation
27
• Previous results were based on wall plug power measurements
• Motor efficiency is not well known
• Includes other parasitics such as motor controller
• A more reliable method is to report shaft power (Lozano 2015)
• Expected actual efficiency can be calculated using the efficiency of the pump and motor
• Allows accurate assessment of parasitic losses such as bearing and valve drag
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COP measurements for shaft work
28
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Performance highlights to date
• Maximum temperature span 29.2 K @ 1.4 Hz and 3.4 L/min fluid flow
• COP of 3.32 (shaft power) with cooling power of 82 W and 15.3 K temperature span @ 1.0 Hz and 2.5 L/min fluid flow
• Maximum cooling power so far is 160 W at a temperature span of 5.5 K @ 0.47 Hz and 3.8 L/min fluid flow
• Full characterization has been delayed by flow system adjustment, minor component failures and component optimization
29 11 November 2015
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Incorporating thermal storage
30
Cooling cabinet
Storage tank pump
Cold storage tank
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Operating with thermal storage
31
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Examples of high thermal performance in nature
32 11 November, 2015
Average ø36 µm Average
ø84 µm
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Topology optimization of a heat sink
More and bigger fins with increasing Δp Bumps on fins increase with increasing Δp
0.01 Pa 0.1 Pa 0.5 Pa 1.0 Pa
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Concept of a magnetocaloric heat pump • 2 kW is approximately the required heating power for a normal house in
the 2015 Danish building code for energy use. A temperature span of the order of 30 K and a COP of 5 would make a competitive heat pump.
• In a water-to-water heat pump the secondary heat exchangers may not be necessary.
• The heat pump can run on renewable energy sources, such as wind.
34 10 Sept. 2014
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Building a Magnetocaloric device • Everything has to fit together – also the external parts.
35 10 Sept. 2014
Magnet Material
Regenerator
Flow system
Heat exchangers Control strategy
System
integration
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ENOVHEAT goals and highlights (see poster session)
• Studied the effect of variability in Curie temperature for a cascaded regenerator
• Studied effects of having a variable cross-section of the regenerator
• Studying techniques to reduce magnet volume
• Design of an improved efficiency device (used for a heat pump)
• Develop building integration techniques and control strategies
36 10 Sept. 2014
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Conclusions
Compact rotary AMR prototype was designed and built • COP is more than double compared to previous device while maximum
temperature span is increased by ~4 K – Better magnetic force balancing – Low friction / no internal leakage valve system – Improved flow control – Layered regenerator bed
• COP reported for shaft power rather than wall plug power
• Controlling flow imbalance in each regenerator bed is fundamental to
attaining high performance in multi-regenerator AMRs
• Thermal storage concept demonstrated but more experimental/analysis work needs to be done
• Full experimental characterization remains a future task
37
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38
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The AMR as a smart device From Per Lundqvist’s keynote talk at ICR2015 “The role of heat pumps in the smart energy system” • All devices need to be smart • Heat pumps will be a necessary part of the smart electricity grid
and smart thermal grid • Heat pumps are an “enabling technology” for renewable energy
• AMR heat pumps meet the requirements for smart heat pumps – Can communicate with external systems – Variable cooling power/operating conditions – Can be coupled to thermal storage
• Single phase fluid may be more practical for smart applications
39
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Some initial experimental results
40 11 November, 2015
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COP, preliminary results
41
CO
P
∆T [K]
1st DTU prototype
𝐶𝐶𝐶𝐶𝐶𝐶 =�̇�𝑄𝐻𝐻𝐻𝐻𝐻𝐻𝑚𝑚𝐻𝐻𝑚𝑚
�̇�𝑊𝑝𝑝𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 + �̇�𝑉 ∙ ∆𝐶𝐶𝑚𝑚𝐻𝐻𝑟𝑟.