Performance test and numerical study of salt hydrate-based thermochemical heat storage materials at middle-low temperature

doi:10.11949/0438-1157.20201484

Abstract

Abstract:

Using hydrated salt K₂CO₃·1.5H₂O and expanded graphite (EG) as chemical heat storage materials and porous matrix respectively, a composite heat storage adsorbent K₂CO₃@EG was developed. The desorption, adsorption performance and cycle stability of the composite sorbent and pure salt without EG-doping were compared and analyzed. The results show that the desorption temperature of the composite adsorbent is reduced, and the adsorption kinetics of adsorbate is obviously improved, which can effectively avoid deliquescence. After fifteen consecutive desorption-hydration cycle experiments, the heat storage density of pure salt and composite adsorbent decreased by 27.6% and 10.9%, respectively. In addition, the numerical results of the thermal storage unit preliminarily verify the feasibility of the thermal storage system.

Key words: salt-hydrate, thermochemical heat storage, composite sorbent, kinetics, heat transfer, energy storage density, cycle stability

摘要：

以水合盐K₂CO₃·1.5H₂O和膨胀石墨（EG）分别作为化学蓄热材料和多孔基质，研制了复合储热吸附剂K₂CO₃@EG。对该复合吸附剂和未掺杂膨胀石墨的纯水合盐就脱附储热、吸附性能、循环稳定性等方面进行了对比分析。结果表明，复合吸附剂所需的脱附温度降低，对吸附质的吸附动力学性能也有明显提升且可有效避免潮解现象。经过连续15次的脱附-水合循环实验后，纯盐和复合吸附剂的储热密度分别下降27.6%和10.9%。此外，对储热单元的数值研究结果初步验证了该蓄热体系的可行性。

关键词: 水合盐, 热化学储热, 复合吸附剂, 动力学, 传热, 储热密度, 循环稳定性

CLC Number:

TK 02

LI Wei, WANG Qiuwang, ZENG Min. Performance test and numerical study of salt hydrate-based thermochemical heat storage materials at middle-low temperature[J]. CIESC Journal, 2021, 72(5): 2763-2772.

李威, 王秋旺, 曾敏. 水合盐基中低温热化学储热材料性能测试及数值研究[J]. 化工学报, 2021, 72(5): 2763-2772.

Figures/Tables 11

Fig.1 Schematic diagram of hydrated salt-based chemical heat storage system

Fig.2 SEM images of expanded graphite (a), pure salt (b), and composite sorbent of K2CO3@EG (c)

Fig.3 Adsorption of pure salt and composite sorbent at 30℃ with (a) 29% RH and (b) various vapor pressures

Table 1 Equilibrium adsorption capacities and corresponding times of sorbents

工况	纯K₂CO₃		CS15
工况	平衡吸附量/(g/g)	所需时间/min	平衡吸附量/(g/g)	所需时间/min
30℃，29% RH	0.1906	360	0.168	250
30℃，40% RH	0.1924	350	0.169	230
30℃，55% RH	0.24	400	0.2031	360

Fig.4 TG-DSC results of the pure salt hydrate and the composite sorbent

Fig.5 Water uptakes and energy storage densities of pure K2CO3 and CS15 sorbent in cyclability testing

Fig.6 Schematic of the integral heat storage pipe (a) and a half of the symmetrical structure and boundary conditions (b), grid independence verification (c)

Table 2 Governing equations and descriptions

控制方程		描述
反应动力学	$? α ? t = A f e x p - E a R T (1 - α) 2 / 3 p v p e q$	α为转化率；p_v, p_eq分别为蒸汽动态压力和平衡压力；A_f为指前因子；E_a为反应活化能；R为通用气体常数
克劳修斯-克拉贝隆方程	$l n p e q p r e f = - Δ H r R T e q + Δ S r R$	p_ref为参考水平压力；ΔH_r为反应焓；ΔS_r 为熵
质量守恒及质量输运	$(1 - ε) ? ρ s ? t = ρ s ? ? α ? t$ (solid) $ε ? ρ v ? t = S w - ? (ρ v u) + D g Δ ρ v$ (gas)	ε为吸附剂反应床孔隙率；D_g为蒸汽在多孔吸附剂内扩散系数
质量源项S_w	$S w = ρ s ? α ? t M v M s$	ρ_s 为储热吸附剂密度；M_v/M_s 为蒸汽摩尔质量和储热吸附剂摩尔质量比值
湿空气混合物	$? ? t ε ρ m + ? ? ρ m u = S w$	ρ_m 为湿空气密度；u 为气体速度
多孔储热材料内流体流动	$? ? t ρ m u + u ε ? ? ρ m u = ? ? [- ε p v I + μ m ? u + ? u T - 23 μ m ? ? u I] + S w u ε - ε μ v k u$	k为反应床渗透率；μ_m为水蒸气黏度
能量守恒	$(ρ C p) e f f ? ? T ? t = ? (λ e f f ? T) ? - C p v ? ρ v ? u ? ? T + q ˙$	热源 $q ˙ = ± ρ s M s ? α ? t Δ H r$ , 正负号取决于是吸附或脱附

Table 2 Governing equations and descriptions

控制方程		描述
反应动力学	$? α ? t = A f e x p - E a R T (1 - α) 2 / 3 p v p e q$	α为转化率；p_v, p_eq分别为蒸汽动态压力和平衡压力；A_f为指前因子；E_a为反应活化能；R为通用气体常数
克劳修斯-克拉贝隆方程	$l n p e q p r e f = - Δ H r R T e q + Δ S r R$	p_ref为参考水平压力；ΔH_r为反应焓；ΔS_r 为熵
质量守恒及质量输运	$(1 - ε) ? ρ s ? t = ρ s ? ? α ? t$ (solid) $ε ? ρ v ? t = S w - ? (ρ v u) + D g Δ ρ v$ (gas)	ε为吸附剂反应床孔隙率；D_g为蒸汽在多孔吸附剂内扩散系数
质量源项S_w	$S w = ρ s ? α ? t M v M s$	ρ_s 为储热吸附剂密度；M_v/M_s 为蒸汽摩尔质量和储热吸附剂摩尔质量比值
湿空气混合物	$? ? t ε ρ m + ? ? ρ m u = S w$	ρ_m 为湿空气密度；u 为气体速度
多孔储热材料内流体流动	$? ? t ρ m u + u ε ? ? ρ m u = ? ? [- ε p v I + μ m ? u + ? u T - 23 μ m ? ? u I] + S w u ε - ε μ v k u$	k为反应床渗透率；μ_m为水蒸气黏度
能量守恒	$(ρ C p) e f f ? ? T ? t = ? (λ e f f ? T) ? - C p v ? ρ v ? u ? ? T + q ˙$	热源 $q ˙ = ± ρ s M s ? α ? t Δ H r$ , 正负号取决于是吸附或脱附

Fig.7 Temperature evolutions of reaction bed and outlet air and conversion degree during discharging

Fig.8 Distributions of conversion degrees of sorption bed at different times

Fig.9 Effects of different relative humidity on adsorption bed temperature

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