Adsorbent screening for thermal energy storage application

Adsorbent screening for thermal energy storage application

Solar Energy Materials and Solar Cells 196 (2019) 119–123 Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal h...

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Solar Energy Materials and Solar Cells 196 (2019) 119–123

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat

Adsorbent screening for thermal energy storage application ⁎

T

Ye Hua, Burcu Ugur, F. Handan Tezel

Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis-Pasteur, Ottawa, Ontario, Canada K1N 6N5

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermal energy storage Water vapor adsorption Energy density Breakthrough tests Adsorbent screening

In this study, alkaline salt was added into the activated alumina/zeolite 13X hybrid adsorbents to improve the energy density of the adsorbents and lower the regeneration temperature. A series of activated alumina/zeolite 13X hybrid adsorbents with/without 10% alkaline salt added were tested using an experimental system built and designed in our lab. The effects of the regeneration temperature and relative humidity (RH) on the materials’ behavior and their experimental energy density at different operating conditions were discussed. An energy density of 215 kWh/m3 was obtained with the regeneration temperature of 250 °C at 90% relative humidity.

1. Introduction Due to the cold climate in Canada, up to 63% of residential energy consumption and about 56% of commercial and institutional energy consumption are accounted for space heating [1]. One way to lower the high utility bill in winter months and save energy is by storing the excess heat produced in factories, thermal power plants and renewable energy sources, and releasing the stored heat for building heating when it is needed. Thermal energy storage (TES) provides a solution to store heat generated from different types of energy sources (traditional or renewable) and correct for the mismatch between the energy supply and demand. Thermal energy can be stored as sensible heat, latent heat, physical adsorption heat or chemical heat [2]. TES system using phase change materials (PCMs) is one of the popular heat storage systems for house heating due to the large thermal energy storage capacity of PCMs [3–5]. For example, the (Na2HPO4·12H2O) and ((NH4)Al(SO4)·6H2O) eutectic mixtures is a promising PCM for heat storage, which demonstrated a latent heat of fusion of 288.13 kJ/kg at melting temperature between 41.98 and 60.42 °C [4]. However, the phase change process (liquid to solid phase and vice versa) of a specific type of PCM happens at a relatively narrow temperature range, which may limit the operating climate and the applications. This study focused on the adsorption TES system that utilizes reversible adsorption-desorption phenomena (e.g. water vapor adsorption on solid adsorbents) to store and release energy. When water vapor and the dry adsorbent material are put into contact, heat releases from the exothermic adsorption process referred to as discharge. The adsorbent material eventually becomes saturated and can be regenerated with warm dry air to desorb moisture. Under the influence of a heat supply,



water molecules are desorbed from the adsorbent material, which is an endothermic process referred to as charging. The adsorption TES technology has three major advantages compared to other energy storage methods: (1) Low environmental impacts: no toxic compounds or chemicals are used in the process, which minimize the risk of chemical leaking and pollution; (2) The adsorption process is fully reversible, which results in the high energy efficiency of the adsorption TES system; (3) The energy can be stored inside the material forever: the thermal energy is stored in the system in the form of chemical potential (heat of adsorption). Unlike latent or sensible heat storage, there is no heat loss due to the temperature differences between the system and the surrounding environment during the storage period. Therefore, minimum thermal insulation is required for the heat storage of the adsorption TES system. Comparing to the PCMs TES system, the adsorbents in the adsorption TES system can be regenerated at a much wider range of regeneration temperature, and the system is more tolerable for different operating conditions. Many materials can be used as adsorbents in TES systems, and zeolites are the most economic and efficient so far. Zeolites are crystalline aluminosilicates built of SiO4 and AlO4- tetrahedra units. The AlO4- tetrahedra units introduce the negative charges to the framework, which are usually neutralized with alkaline and alkaline earth cations. The biggest disadvantage of using zeolite for adsorption TES system is the high regeneration temperature. Regeneration temperature of above 150 °C or even 200 °C are required to dehydrate zeolite to low enough water content to achieve an energy density of about 200 kWh/m3 [6–8]. The high regeneration temperature required for zeolite materials narrowed the heat sources available for the adsorption TES system, therefore, limited the application and the flexibility of the TES system. In order to take the advantage of the adsorption TES system and achieve

Corresponding author. E-mail address: [email protected] (F. Handan Tezel).

https://doi.org/10.1016/j.solmat.2019.01.052 Received 18 September 2018; Received in revised form 11 December 2018; Accepted 21 January 2019 Available online 03 April 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.

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higher energy density at lower regeneration temperature, studies on impregnating salt into a zeolite matrix have been conducted. Researches showed that by impregnating zeolite 13X with 10 wt% CaCl2 salt solution could increase its water adsorption capacity by 5.7% with the water uptake of 0.37 g/g [9]. And some salts that have been used in chemical TES systems, such as MgSO4 powder, the theoretical energy density can be as high as 476 kWh/m3 for a total hydration reaction [2]. In our previous research, an activated alumina/zeolite 13X hybrid adsorbent showed high energy density for TES application [10]. In this paper, a series of activated alumina/zeolite 13X hybrid adsorbents with/without 10% alkaline salt added were tested for TES applications. A bench scale adsorption TES experimental set-up was designed and build in our lab for the TES material testing. Various experimental conditions with different regeneration temperature and relative humidity were used to simulate the real-life operating conditions. The amount of energy released during the adsorption process and the practical/experimental energy densities at each condition were calculated based on the experimental data.

Fig. 1. Schematic illustration of the thermal energy storage system. Table 2 Adsorbent size and adsorption column properties. Properties

Value

Adsorbent size Column length Column inner diameter Column outer diameter Column volume

8–12 mesh 6.9 cm 3.4 cm 3.8 cm 62.8 cm3

2. Material and methods warm air regenerated and dried the adsorbents packed inside the column until the RH of the air at the column outlet was close to zero (less than 2% RH). (2) When the column and the adsorption material were cooled down to room temperature after regeneration (the dry air continuously passed through the column during the cooling down period), the three-way valve was switched to direct the air flow to the by-pass pathway. By adjusting the flow rate and the mass flow ratio of the dry and wet air streams, a moist air mixture with desired flow rate and RH (as high as 96%) can be obtained. Once a stable moist air flow was achieved, the three-way valve would close the by-pass pathway and direct the moist air mixture to the packed column pathway to start the adsorption process. (3) During the adsorption step, the adsorbents in the packed bed column adsorbed the moisture in the moist air mixture and the stored energy was released due to heat of adsorption, which heated up the air carrying the moisture. The adsorption process was completed when the inlet and outlet column temperatures were equal, and the outlet RH reached the level before the adsorption step started, indicating that the adsorbent was saturated.

Four commercially available adsorption materials were provided by Axens SA (Brockville, ON, Canada) for testing. A zeolite 13X sample were purchased from Delta Adsorbent Ltd. (Roselle, IL, USA) for the comparison of the experimental results. The sample number, names and manufacturers are listed in Table 1. A thermal energy storage experimental system using packed bed adsorption column was designed and built in our lab (see Fig. 1 for the schematic illustration of the thermal energy storage system). An oil filter was installed in the compressed air supply line to remove any impurities before connecting to the dry and wet streams. A mass flow controllers (MFC) was installed at the upstream of the dry and wet air streams, respectively, to control the flow rate of air through each stream. A drying column filled with desiccants was installed in the dry air stream to remove moisture in the air, therefore, the relative humidity (RH) of the dry air stream was controlled less than 2%. An ultrasonic humidifier was installed in the wet air stream to generate moist air with RH up to 96%. By adjusting the flow rates of the dry and wet air streams, a moist air mixture with desired flow rate and RH can be obtained. A total flow rate of 24 standard liter per minute (SLPM) were used in all breakthrough experiments based on our previous research [11]. Candidate adsorption materials were packed in an adsorption column. The adsorbent size and properties of the adsorption column are presented in Table 2. The moist air mixture passes through an electrical heater and the adsorption column before exhausting out of the system. A by-pass was built to avoid any undesired air flow through the heater and the adsorption column during the experiments. Thermocouples and hygrometers were installed along the system to monitor the temperature and relative humidity of the air stream at selected points. The experimental data were recorded by a system control program designed using the LabVIEW system design platform. Each cycle of the experiment consists of three steps: the regeneration; the RH set-up and the adsorption. (1) During the regeneration step, dry air was heated up by passing through the heater to the selected regeneration temperature before entering the packed column. The dry

3. Results and discussion 3.1. Adsorbent screening Regeneration temperature of 120 °C was used for adsorbent screening process, which is compatible with solar collectors [2] and 50% inlet RH with a total flow rate of 24 standard liter per minute (SLPM) were used in the breakthrough experiments. The breakthrough RH and temperature curves are shown in Figs. 2 and 3, respectively. Due to the high flow rate (24 SLPM) and relatively short column length (6.9 cm), the water vapor broke through immediately (less than 2 min) after the adsorption started. The complete adsorption equilibrium inside the packed columns were reached after about 3 h (200 min). A discrepancy between the inlet and outlet RH was observed for all the samples (see Fig. 2) due to the friction and pressure drop inside the system between the two hygrometer measuring points. The

Table 1 List of sample number, names and manufacturers.

1 2 3 4 5

Sample

Manufacturer

Zeolite 13X Activated Alumina Activated Alumina with 10% Alkaline added 50/50 Mixture of Activated Alumina and Zeolite 70/30 Mixture of Activated Alumina and Zeolite with 10% Alkaline added

Delta Adsorbent Ltd. (Roselle, IL, USA) Axens SA (formerly Rio Tinto Alcan Inc.) (Brockville, ON, Canada)

120

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actual RH inside the adsorption column should be between the inlet and outlet RH values. The average of the inlet and outlet RH was used for data analysis of the adsorption process. The stored thermal energy was released simultaneously during the adsorption process. As demonstrated in the temperature figures (see Fig. 3), the differences between the column outlet and inlet temperatures vary between 8 and 12 °C depending on the adsorbent used. Samples 1 and 5 showed the highest temperature increase at the column outlet during the adsorption, while sample 3 and 5 showed the longest time span for energy releasing. Sample 3 released about 12.78 kJ of energy during the adsorption process, which led to the highest experimental energy density at the given condition among the five adsorbent candidates. The energy density for each experiment can be calculated from the concentration and temperature breakthrough curves using Eq. (1):

Energy Density =

Fig. 2. Water vapor RH breakthrough curves for the five adsorbents with 120 °C regeneration temperature and 50% inlet RH.

t ∫0 (Um × Cp×∆T ) dt

V

(1)

where Um is the mass flow rate (kg/s) of the moist air passed through the column, which can be calculated from the wet and dry air volumetric flow rate measured by MFCs using ideal gas law and the

Fig. 3. Temperature curves of the five adsorbents during adsorption with 120 °C regeneration temperature and 50% inlet RH. 121

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Table 3 Experimental results comparison of selected adsorbents at 120 °C regeneration temperature and 50%RH.

1 2 3 4 5

Sample

Energy released (kJ)

Energy density (kWh/m3)

Zeolite 13X Activated Alumina Activated Alumina with 10% Alkaline added 50/50 Mixture of Activated Alumina and Zeolite 70/30 Mixture of Activated Alumina and Zeolite with 10% Alkaline added

5.83 8.66 12.78 8.62 11.18

25.79 38.30 56.52 38.14 49.47

concentration; Cp is the heat capacity of the moist air (kJ/kg K) inside the packed bed column; ΔT is the temperature difference (K) between the column inlet and outlet obtained from the temperature data; and V is the volume of the packed bed column used in the experiments. The amount of energy released during the adsorption and the calculated energy densities are presented in Table 3. Adding activated alumina to the zeolite 13X sample increased its energy densities as high as the pure activated alumina sample. Adding alkaline can further increase the energy density. It seems that adding alkaline have the most impact on the energy density of the activated alumina, as sample 3 (activated alumina with 10% alkaline added) showed the highest energy density among the five adsorbent samples. 3.2. The effect of inlet RH The five adsorption materials were also tested using different RHs between 10% and 90% to examine the effect of inlet RH on the amount of energy released. The experimental energy density at selected RH are presented in Table 3 and in Fig. 5. As shown in Table 4 and Fig. 4, using the same regeneration temperature (120 °C), higher energy density was obtained at higher RH for all the samples. Sample 3 (activated alumina with 10% alkaline added) showed the highest energy density, while sample 1 (zeolite 13X) showed the lowest energy density among the adsorbents studied. Changing the inlet air RH has the most effect on the activated alumina material (sample 2), especially at the 50–90% RH range.

Fig. 4. Comparison of experimental energy density of the five adsorbents at different RH with 120 °C regeneration temperature.

3.3. The effect of regeneration temperature Sample 5 (70/30 mixture of activated alumina and zeolite with 10% alkaline added) was tested to represent the behavior of alkaline impregnated activated alumina/zeolite hybrid samples with different regeneration temperatures at 90% inlet RH. As demonstrated in Fig. 5, the experimental energy density increases with increasing regeneration temperature. A linear relationship between the experimental energy density and the regeneration temperature with coefficient of determination (R2) of 97.7% was observed at the 120–250 °C regeneration temperature range. An energy density of up to 215 kWh/m3 has been

Fig. 5. Comparison of experimental energy density of Sample 5 (70/30 Mixture of Activated Alumina and Zeolite with 10% Alkaline added) at 90% RH using different regeneration temperatures.

Table 4 Experimental energy density of selected samples at different RH with 120 °C regeneration temperature. Sample name

1 2 3 4

5

Zeolite 13X Activated Alumina Activated Alumina with 10% Alkaline added 50/50 Mixture of Activated Alumina and Zeolite 70/30 Mixture of Activated Alumina and Zeolite with 10% Alkaline added

observed for the regeneration temperature of 250 °C. Although higher regeneration temperature has positive impact on the energy density, the actual regeneration temperature that can be used with the thermal energy storage system is limited by the renewable energy generation resource. For the most commonly used solar thermal collector – flat plate collector (FPC) – temperature up to 100 °C can be obtained with good efficiencies [12].

Energy density (kWh/m3) with inlet air of RH = 10

RH = 30

RH = 50

RH = 90

8.08 5.29 9.35

19.61 20.70 27.37

25.79 38.30 56.52

50.66 124.13 133.40

7.81

24.76

38.14

86.22

6.96

27.24

49.47

108.28

4. Conclusion A series of activated alumina/zeolite 13X hybrid adsorbents with/ without 10% alkaline salt added were tested to examine their performance of water vapor adsorption for thermal energy storage application. The activated alumina with 10% alkaline added (sample 3) showed a larger temperature increase and the longest breakthrough time, which resulted in the highest energy density at the experimental 122

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condition (120 °C regeneration temperature and 50% inlet RH) among the adsorbents studied. Increasing the regeneration temperature and the RH of the inlet moist air increases the experimental energy density of the samples. With 120 °C regeneration temperature and 50% inlet air RH, an experimental energy density of 56.52 and 49.47 kWh/m3 can be reached by sample 3 (activated alumina with 10% alkaline added) and sample 5 (70/30 mixture of activated alumina and zeolite with 10% Alkaline added), respectively, which is twice as much the energy density received as using the standard zeolite 13X sample (sample 1). An energy density of 215 kWh/m3 was obtained with the regeneration temperature of 250 °C at 90% relative humidity for sample 5.

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Acknowledgement Special thanks to Axens SA (formerly Rio Tinto Alcan Inc.) for providing the adsorbent samples. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the NSERC Energy Storage Technology (NEST) Network, Canada. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2019.01.052. References [1] Statistics Canada, Households and the Environment: Energy Use, Gov. Canada. (n.d. ). 〈http://www.statcan.gc.ca/pub/11-526-s/2013002/part-partie1-eng.htm〉. (Accessed 15 March 2018).

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