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 Melting Latent heat (kJ/kg) Price ($/kg) temperature (°C) ZnCl 2 - NaCl - KCl 203 NaOH - Na 2 CO 3 210 222 ~0.62 KNO 3 (54wt%) - NaNO 3 (46wt%) 161 NaNO 3 - NaNO 2 226-233 Ca(NO 3 ) 2 (45wt%) - NaNO 3 (55wt%) 230 ~110 ~0.33 Ca(NO 3 ) 2 - NaNO 2 200-223 Ca(NO 3 ) 2 - LiNO 3 235 200 ~1.84 LiNO 3 (12wt%) - NaNO 3 (18wt%) - KNO 3 (70wt%) LiNO 3 (57wt%) - NaNO 3 (43wt%) 193 248 ~5.8 LiNO 3 (49wt%) - NaNO 3 (51wt%) 194 265 ~5.1 LiNO 3 (87wt%) - NaCl(13wt%) 208 360 ~8.7 LiNO 3 (45wt%) - NaNO 3 (47wt%) - Sr(NO 3 ) 2 (8wt%) 200 199 ~4.9 Material investigations contain two parts: 1. Research stage Table 2 Market prices of some salts Salt Price (per Metric Ton) 1) Binary system with lithium nitrate Ca(NO 3 ) 2 $250 ~ $280 2) Ternary system of LiNO 3 (12wt%) - NaNO 3 (18wt%) - NaNO 3 $300 ~ $500 KNO 3 (70wt%) KNO 3 $700 ~ $900 2. Industrial application stage LiNO 3 Around $10,000 $400 ~ $500 NaNO 2 1) KNO 3 (54wt%) - NaNO 3 (46wt%) NaCl $50 ~ $100 2) Ca(NO 3 ) 2 (45wt%) - NaNO 3 (55wt%) $950 ~ $1000 ZnCl 2 3) Ternary system of LiNO 3 (12wt%) - NaNO 3 (18wt%) - KCl $500 ~ $900 Na 2 CO 3 $180 ~ $250 KNO 3 (70wt%) Sr(NO 3 ) 2 Around $3,000 4) Other new ternary or quaternary systems NaOH $350 ~ $450
Binary system with lithium nitrate (Research stage) • LiNO 3 (87wt%)- NaCl(13wt%) • LiNO 3 (57wt%)-NaNO 3 (43wt%) • Ca(NO 3 ) 2 -LiNO 3 (To be test….)
Heat storage system 0.2 m 0.4 m 0.14 m 1.5 d o 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 D o : 50 mm Inside pipe diameter D in : 20 mm Pipe length: 1 m
Heat storage system • High temperature heat exchanger: Exergy tube – in-tube • High temperature oil pump heat exchanger ½’’ NPT male (Turbine pump) inner tube connections and 1’’ • M pumps CM MAG-M series NPT female outer tube boss. magnetically coupled centrifugal pump. Differential head: 6m; capacity: 2-15 L/min; working temperature: up to 300 ° C . • Highly dynamic temperature control system: Julabo • IC-LPM industrial paddle wheel series flow meter: • 2-20L/min • Operating temperature: 350 ° C
U-value calculation 1. Mass of the PCM 𝐸 𝑃 𝐸 𝑗𝑜 2 ) 2 = 1.96 × 10 −3 𝑛 2 ; Pipe 2: 𝐵 2 = 𝜌( 2 ) 2 = 3.14 × 10 −4 𝑛 2 (1) Cross area: (A) Pipe 1: 𝐵 1 = 𝜌( (2) Volume of the PCM (VP) 𝑾 𝑸 = 𝐵 1 − 𝐵 2 𝑴 = 1.646 × 10 −3 𝑛 3 (3) Mass of the PCM (MP) 𝑵 𝑸 = 𝝇 𝑸 ∙ 𝑾 𝑸 = 𝟑𝟒𝟔𝟔 × 1.646 × 10 −3 𝒍𝒉 = 3.88𝒍𝒉 300𝒍𝑲 (4) Suppose the latent heat of the PCM 𝑰 𝑸 = 𝒍𝒉 ; total latent heat: 𝑴 𝒊𝒇𝒃𝒖 = 𝑵 𝑸 ∙ 𝑰 𝑸 = 3.88 × 300 𝒍𝑲 = 1162.9 𝒍𝑲 2. Suppose the flow rate inside the heat transfer tube is 𝒘 𝒈 = 𝟏. 𝟔𝒏/𝒕 0.𝟔𝒏 𝟒 𝒈 = 𝑩 𝟑 ∙ 𝒘 𝒈 = 3.14 × 10 −4 × = 𝟐. 𝟔𝟖 × 𝟐𝟏 −𝟓 𝒏 𝟒 /𝒕 (1) The volume flow rate ( 𝒘 𝒈 ) 𝒘 𝒕 𝒈 ∙ 𝝇 𝒈 = 𝟐. 𝟔𝟖 × 𝟐𝟏 −𝟓 × 𝟖𝟓𝟕𝒍𝒉 (2) The mass flow rate ( 𝒏 𝒈 ) 𝒏 𝒈 = 𝒘 = 𝟏. 𝟐𝟐𝟖𝒍𝒉/𝒕 𝒕 𝒘 𝒈 ∙𝒆 𝒋 0.5×0.02 (3) Reynolds number at T=250 ° C 𝑆𝑓 = 𝜃 𝑔 = 1.2×𝟐𝟏 −𝟕 = 8333.333 So the flow is turbulent flow 𝜃 𝑔 ∙𝐷 𝑄𝑔 ∙𝜍 𝑔 1.2×𝟐𝟏 −𝟕 ×2.72×713 (4) The Prantle number (Pr) 𝑄𝑠 = = = 19.722 0.118×𝟐𝟏 −𝟒 𝑙 𝑔 (5) The Nusselt number 𝑂𝑣 = 0.023 × 𝑆𝑓 0.8 × 𝑄𝑠 0.4 = 0.023 × 8333.333 0.8 × 19.722 0.4 = 103.84 𝑂𝑣 𝑔 ×𝑙 𝑔 103.84×0.118 = 612.656 𝑋/𝑛 2 𝐿 (6) The heat transfer inside the tube ( ℎ 𝑗 ) ℎ 𝑗 = = D in 0.02 (7) The minimum overall effective heat transfer coefficient estimation 1 1 = 43.3𝑋/𝑛 2 𝐿 𝑉 = = ℎ 𝑗𝑑 + 𝜇 𝑡 1 𝑙 𝑡 + 𝜇 𝑄 612.656 + 0.001 1 + 0.015 𝑙 𝑄 19 0.7 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
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