Thermal Energy Storage for Medium
Temperature Industrial Process Heating
------Progress
Dan Zhou
CREST
Loughborough University
Progress
• Materials Update
• System Update
• System Performance
Materials
Table 1 Potential molten salt mixture as medium temperature heat storage media
PCMs
Latent heat (kJ/kg)
Price ($/kg)
ZnCl2 - NaCl - KCl
NaOH - Na2CO3
KNO3 (54wt%) - NaNO3 (46wt%)
NaNO3 - NaNO2
Ca(NO3)2 (45wt%) - NaNO3 (55wt%)
Ca(NO3)2 - NaNO2
Ca(NO3)2 - LiNO3
LiNO3(12wt%) - NaNO3(18wt%) - KNO3(70wt%)
Melting
temperature
(°C)
203
210
222
226-233
230
200-223
235
200
161
~0.62
~110
~0.33
LiNO3(57wt%) - NaNO3(43wt%)
LiNO3(49wt%) - NaNO3(51wt%)
193
194
248
265
~5.8
~5.1
LiNO3(87wt%) - NaCl(13wt%)
LiNO3(45wt%) - NaNO3(47wt%) - Sr(NO3)2(8wt%)
208
200
360
199
~8.7
~4.9
Table 2 Market prices of some salts
Salt
Ca(NO3)2
NaNO3
KNO3
LiNO3
NaNO2
NaCl
ZnCl2
KCl
Na2CO3
Sr(NO3)2
NaOH
Price (per Metric Ton)
$250 ~ $280
$300 ~ $500
$700 ~ $900
Around $10,000
$400 ~ $500
$50 ~ $100
$950 ~ $1000
$500 ~ $900
$180 ~ $250
Around $3,000
$350 ~ $450
~1.84
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
Binary system with lithium nitrate (Research stage)
•
LiNO3(87wt%)- NaCl(13wt%)
•
LiNO3(57wt%)-NaNO3(43wt%)
•
Ca(NO3)2-LiNO3 (To be test….)
Heat storage system
0.2 m
0.4 m
0.14 m
1.5 do
O.D 0.015 m
Helical coiled tube
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
Heat storage system
•
•
•
High temperature heat
exchanger: Exergy tube –in-tube
heat exchanger ½’’ NPT male
inner tube connections and 1’’
NPT female outer tube boss.
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
Highly dynamic
temperature control
system: Julabo
U-value calculation
1. Mass of the PCM
(1) Cross area: (A)
(2) Volume of the PCM (VP)
(3) Mass of the PCM (MP)
Pipe 1: 𝐴1 = 𝜋(
𝐷𝑂 2
) =
2
1.96 × 10−3 𝑚2 ;
Pipe 2: 𝐴2 = 𝜋(
𝐷𝑖𝑛 2
) =
2
3.14 × 10−4 𝑚2
𝑽𝑷 = 𝐴1 − 𝐴2 𝑳 = 1.646 × 10−3 𝑚3
𝑴𝑷 = 𝝆𝑷 ∙ 𝑽𝑷 = 𝟐𝟑𝟓𝟓 × 1.646 × 10−3 𝒌𝒈 = 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
0.𝟓𝒎𝟑
= 𝟏. 𝟓𝟕 × 𝟏𝟎−𝟒 𝒎𝟑 /𝒔
𝒔
𝟕𝟒𝟔𝒌𝒈
𝟏𝟎−𝟒 × 𝒔 = 𝟎. 𝟏𝟏𝟕𝒌𝒈/𝒔
(1) The volume flow rate (𝒗𝒇 )
𝒗𝒇 = 𝑨𝟐 ∙ 𝒗𝒇 = 3.14 × 10−4 ×
(2) The mass flow rate (𝒎𝒇 )
𝒎𝒇 = 𝒗𝒇 ∙ 𝝆𝒇 = 𝟏. 𝟓𝟕 ×
(3) Reynolds number at T=250°C
𝑅𝑒 =
(4) The Prantle number (Pr)
𝑃𝑟 =
(5) The Nusselt number
𝒗𝒇 ∙𝒅𝒊
𝜂𝑓
0.5×0.02
= 1.2×𝟏𝟎−𝟔 = 8333.333
𝜂𝑓 ∙𝐶𝑃 𝑓 ∙𝜌𝑓
𝑘𝑓
=
1.2×𝟏𝟎−𝟔 ×2.72×713
0.118×𝟏𝟎−𝟑
So the flow is turbulent flow
= 19.722
𝑁𝑢 = 0.023 × 𝑅𝑒 0.8 × 𝑃𝑟 0.4 = 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/𝑚2 𝐾
(7) The minimum overall effective heat transfer coefficient estimation
1
1
𝑈=
=
= 43.3𝑊/𝑚2 𝐾
1
𝜆𝑠 𝜆𝑃
1
0.001 0.015
+ +
+ 19 + 0.7
ℎ𝑖𝑐 𝑘𝑠 𝑘𝑃 612.656
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.
Performance calculation
Figure 2 Influence of inlet temperature on the performance
Figure 1 Heat storage system performance
Figure 4 Influence of effective heat transfer rate on the
performance
Figure 3 Influence of heat transfer fluid velocity on the performance