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Thermal Energy Storage for Medium
Temperature Industrial Process Heating
Dan Zhou
CREST
Loughborough University
------Progress
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Progress
• Materials Update
• System Update
• System Performance
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Materials
Salt Price (per Metric Ton)
Ca(NO3)2 $250 ~ $280
NaNO3 $300 ~ $500
KNO3 $700 ~ $900
LiNO3 Around $10,000
NaNO2 $400 ~ $500
NaCl $50 ~ $100
ZnCl2 $950 ~ $1000
KCl $500 ~ $900
Na2CO3 $180 ~ $250
Sr(NO3)2 Around $3,000
NaOH $350 ~ $450
PCMs
Melting
temperature
(°C)
Latent heat (kJ/kg)
Price ($/kg)
ZnCl2 - NaCl - KCl 203
NaOH - Na2CO3 210
KNO3 (54wt%) - NaNO3 (46wt%) 222 161 ~0.62
NaNO3 - NaNO2 226-233
Ca(NO3)2 (45wt%) - NaNO3 (55wt%) 230 ~110 ~0.33
Ca(NO3)2 - NaNO2 200-223
Ca(NO3)2 - LiNO3 235
LiNO3(12wt%) - NaNO3(18wt%) - KNO3(70wt%) 200 ~1.84
LiNO3(57wt%) - NaNO3(43wt%) 193 248 ~5.8
LiNO3(49wt%) - NaNO3(51wt%) 194 265 ~5.1
LiNO3(87wt%) - NaCl(13wt%) 208 360 ~8.7
LiNO3(45wt%) - NaNO3(47wt%) - Sr(NO3)2(8wt%) 200 199 ~4.9
Table 1 Potential molten salt mixture as medium temperature heat
storage media
Table 2 Market prices of some salts
Material investigations contain two parts:
1. Research stage
1) Binary system with lithium nitrate
2) Ternary system of LiNO3(12wt%) - NaNO3(18wt%) -
KNO3(70wt%)
2. Industrial application stage
1) KNO3 (54wt%) - NaNO3 (46wt%)
2) Ca(NO3)2 (45wt%) - NaNO3 (55wt%)
3) Ternary system of LiNO3(12wt%) - NaNO3(18wt%) -
KNO3(70wt%)
4) Other new ternary or quaternary systems
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Binary system with lithium nitrate (Research stage)
• LiNO3(87wt%)- NaCl(13wt%)
• LiNO3(57wt%)-NaNO3(43wt%)
• Ca(NO3)2-LiNO3 (To be test….)
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Heat storage system
Double pipes heat exchanger: heat transfer pipe can
be smoothed pipe or enhanced pipes
Outside pipe diameter Do: 50 mm
Inside pipe diameter Din: 20 mm
Pipe length: 1 m
0.4 m0.2 m
0.14 m
O.D 0.015 m
1.5 do
Helical coiled tube
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Heat storage system
• High temperature oil pump
(Turbine pump)
• M pumps CM MAG-M series
magnetically coupled centrifugal
pump. Differential head: 6m;
capacity: 2-15 L/min; working
temperature: up to 300°C .
• IC-LPM industrial paddle wheel
series flow meter:
• 2-20L/min
• Operating temperature: 350 °C
• High temperature heat
exchanger: Exergy tube –in-tube
heat exchanger ½’’ NPT male
inner tube connections and 1’’
NPT female outer tube boss.
• Highly dynamic
temperature control
system: Julabo
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U-value calculation
1. Mass of the PCM
(1) Cross area: (A) Pipe 1: 𝐴1 = 𝜋(𝐷𝑂
2)2= 1.96 × 10−3𝑚2 ; Pipe 2: 𝐴2 = 𝜋(
𝐷𝑖𝑛
2)2= 3.14 × 10−4𝑚2
(2) Volume of the PCM (VP) 𝑽𝑷 = 𝐴1 − 𝐴2 𝑳 = 1.646 × 10−3𝑚3
(3) Mass of the PCM (MP) 𝑴𝑷 = 𝝆𝑷 ∙ 𝑽𝑷 = 𝟐𝟑𝟓𝟓 × 1.646 × 10−3𝒌𝒈 =
3.88𝒌𝒈
(4) Suppose the latent heat of the PCM 𝑯𝑷=300𝒌𝑱
𝒌𝒈; total latent heat: 𝑳𝒉𝒆𝒂𝒕 = 𝑴𝑷 ∙ 𝑯𝑷= 3.88 × 300 𝒌𝑱 = 1162.9
𝒌𝑱
2. Suppose the flow rate inside the heat transfer tube is 𝒗𝒇 =
𝟎. 𝟓𝒎/𝒔
(1) The volume flow rate (𝒗 𝒇) 𝒗 𝒇 = 𝑨𝟐 ∙ 𝒗𝒇 = 3.14 × 10−4 ×
0.𝟓𝒎𝟑
𝒔= 𝟏. 𝟓𝟕 × 𝟏𝟎−𝟒𝒎𝟑/𝒔
(2) The mass flow rate (𝒎 𝒇) 𝒎 𝒇 = 𝒗 𝒇 ∙ 𝝆𝒇 = 𝟏. 𝟓𝟕 × 𝟏𝟎−𝟒 ×
𝟕𝟒𝟔𝒌𝒈
𝒔= 𝟎. 𝟏𝟏𝟕𝒌𝒈/𝒔
(3) Reynolds number at T=250°C 𝑅𝑒 =𝒗𝒇∙𝒅𝒊
𝜂𝑓=
0.5×0.02
1.2×𝟏𝟎−𝟔= 8333.333 So the flow is turbulent flow
(4) The Prantle number (Pr) 𝑃𝑟 =𝜂𝑓∙𝐶𝑃𝑓∙𝜌𝑓
𝑘𝑓=
1.2×𝟏𝟎−𝟔×2.72×713
0.118×𝟏𝟎−𝟑= 19.722
(5) The Nusselt number 𝑁𝑢 = 0.023 × 𝑅𝑒0.8 × 𝑃𝑟0.4 = 0.023 ×
8333.3330.8 × 19.7220.4 = 103.84
(6) The heat transfer inside the tube (ℎ𝑖) ℎ𝑖 =𝑁𝑢𝑓×𝑘𝑓
Din=
103.84×0.118
0.02= 612.656 𝑊/𝑚2𝐾
(7) The minimum overall effective heat transfer coefficient
estimation
𝑈 =1
1ℎ𝑖𝑐
+𝜆𝑠𝑘𝑠+𝜆𝑃𝑘𝑃
=1
1612.656
+0.00119 +
0.0150.7
= 43.3𝑊/𝑚2𝐾
Suppose the inlet temperature of the heat transfer oil is 300
°C
The charging time is around 1 hour. The charging time can be
shorten by enhanced pipes, such as finned pipe.
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Performance calculation
Figure 1 Heat storage system performance
Figure 2 Influence of inlet temperature on the performance
Figure 4 Influence of effective heat transfer rate on the
performance
Figure 3 Influence of heat transfer fluid velocity on the
performance
