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## RENEWABLE ENERGY POWERED SINGLE-STAGE GREEN COOLING SYSTEM

Over the past three decades, solid adsorption cooling has attracted a lot of attention being one of the eco-friendly cooling techniques. These systems can be activated by low-grade thermal energy using eco-friendly adsorbent-refrigerant pairs with minimal global warming potential. Additionally, the price of fossil fuels is sharply rising and people are becoming more aware of environmental issues, which opens up a wide range of thermally powered adsorption cooling systems. Therefore, this paper focuses on the performance assessment of a renewable energy–powered two-bed single-stage green cooling system using biomass-derived activated carbon–ethanol as the adsorbent–adsorbate pair. Moreover, a parametric investigation is also carried out in order to study the impacts of various factors, such as evaporator temperature ($$T_{\text{evap}}$$), ambient temperature ($$T_{\text{amb}}$$), and regeneration temperature ($$T_{\text{reg}}$$). It is observed that the cooling capacity (CC) and coefficient of performance (COP) increase with increasing the regeneration temperature. A new performance index, defined as the ratio of the mass of adsorbent to the mass of fuel ($$m_{ad}/m_{cf}$$) which interlinks the renewable energy system with the chiller system, has also been introduced and found to be decreasing with an increase in regeneration temperature. The lowest $$m_{ad}/m_{cf}$$ and maximum COP are estimated to be 7.45 kg/kg and 0.52, respectively at $$T_{\text{evap}}$$ of 7°C, $$T_{\text{amb}}$$ of 32°C, and $$T_{\text{reg}}$$ of 90°C.

Key words: adsorption cooling, renewable energy, Dühring diagram, eco-friendly pair, COP

1. INTRODUCTION

The economic growth of a nation is highly associated with energy consumption that leads to energy scarcity due to the high standard of living and human comfort (Chauhan et al., 2022a). Worldwide, heating ventilation, and air conditioning (HVAC) systems consume a huge share of high-grade energy for cooling comfort resulting in the depletion of fossil fuel resources and the production of greenhouse gases. Therefore, the thermal energy-driven cooling system is one of the best alternatives and can work on low-temperature, low-grade energy sources (Pan et al., 2022) using eco-friendly working pairs (Chauhan et al., 2019). Despite these advantages, the traditional adsorption-based cooling system performs poorly due to its intermittent and complex operation, poor adsorption capacity of adsorbent material, as well as the design of the sorption reactor. In order to enhance its performance, various strategies such as heat recovery (Xu et al., 2018; Yagnamurthy et al., 2022), mass recovery (Duong et al., 2018), novel and composite adsorbents (Pal et al., 2019), extended surface heat exchanger (Zhang and Wang, 2020), and micro-scaled tubes (Pahinkar et al., 2015; Chauhan and Kumar, 2018) have been covered in the available literature.

Recently, several research efforts (Jribi et al., 2017; Sur et al., 2020; Pal et al., 2020) have been made toward the usage of activated carbon as an adsorbent for adsorption heat transformation applications. According to the available literature, biomass-derived highly porous activated carbon sorbents have been synthesized and found to be significantly effective in terms of adsorption capabilities and surface area (Pal et al., 2019; Chauhan et al., 2022b). Therefore, the present work aims to study the effects of various operational parameters on performance indices of renewable energy-powered, single-stage green cooling systems using biomass-derived activated carbon-ethanol pairs. The detailed working principle of the proposed system has been discussed in a subsequent section along with its schematic and thermodynamic cycles.

2. THERMALLY-DRIVEN COOLING CYCLE

The thermally driven cooling system works on the principle of adsorption–desorption of refrigerant vapor on the porous surface of adsorbent material. A proposed schematic of a renewable energy-powered, two-bed, single-stage green cooling system and its actual thermodynamic cycle is shown in Fig. 1. It comprises a cooling fluid unit, chilled fluid unit, and hot fluid unit. The sorption reactor or thermal compressor is used to lift the vapor of refrigerant from the evaporator level (low-pressure level) to the condenser level (high-pressure level) through the adsorption–desorption phenomenon. This high-pressure vapor of refrigerant in the condenser releases the heat to the cooling fluid flowing from the cooling tower, where the ambient air carries the heat from the cooling fluid via evaporative cooling. The work of the metering device (expansion valve) is to drop the pressure from the condenser level to the evaporator level; however, the chilled fluid flows in the evaporator to release the heat to the refrigerant liquid. Afterward, it gets converted into the vapor phase and goes to the sorption reactor. In the sorption reactor, the refrigerant vapor adsorbs in liquid form inside the pores of the adsorbent while circulating the cooling fluid from the cooling tower. On the other hand, the regeneration of the adsorbent bed takes place by providing hot fluid from the heating unit. In this study, a heating unit consisting of a thermal energy storage tank along with a renewable energy–based highly efficient clean and green combustion device [developed and patented recently by Tyagi (2022)] is utilized for regeneration of the adsorbent bed.

Figure 1. Proposed schematic and thermodynamic cycle of a thermally driven cooling system (Reprinted from Chauhan et al. with permission from Elsevier, Copyright 2022a)

3. MATHEMATICAL MODELING

The time-independent theoretical modeling for a two-bed, single-stage renewable energy–powered cooling system is presented in this section. The highly porous biomass-derived adsorbent material along with ethanol is taken as a working pair for cooling performance assessment. The fitting parameters ($$W_{o}$$, $$E$$, and $$n$$) and thermophysical properties of the studied adsorbent are taken from the literature (Pal et al., 2017). The theoretical modeling for adsorption uptake, energy balance, and performance indices are summarized in Table 1. The efficiency of the renewable energy combustion device $$\eta_{b,\text{device}}$$ and the calorific value of combustion fuel (CV$$_{cf}$$) are also given in the literature (Himanshu et al., 2022).

TABLE 1: Mathematical modeling for renewable energy–powered green cooling system

Models Formulations Nomenclature
Adsorption Modeling $$T$$ – temperature (K)
$$T_{\text{ads}}$$ – adsorption temperature (K)
$$T_{\text{reg}}$$ – regeneration temperature (K)
$$P_{s}$$ – saturation pressure (kPa)
A, B, C – Antoine constants for ethanol (Khanam et al., 2018)
$$m_{\text{hex}}$$ – mass of heat exchanger metallic body (kg)
$$m_{ad}$$ – mass of adsorbent (kg)
$$c_{p,\text{hex},m}$$ – specific heat of heat exchanger material (kJ/kg·K)
$$c_{p,ad}$$ – specific heat of adsorbent (kJ/kg·K)
$$m_{ad,\text{ref}}$$ – mass of adsorbed refrigerant (kg)
$$t_{\text{ads}}$$ – adsorption time (s)
Change in adsorption uptake $$\Delta W=W_{\max}-W_{\min}$$
Adsorption uptake $$W=W_0 \exp\left[-{\left(\frac{ RT}{E}\ln\left(\frac{P_{s}}{P}\right)\right)}^{\!n}\,\right]$$
$$P_{s}={10}^{\left[{\rm A}-\frac{{\rm B}}{{\rm C}\,+\, T}\right]}$$
Isosteric heat of adsorption $$-Q_{st} =RT^{2}\dfrac{\partial}{\partial T}(P_{s})+E\,\left\{-\ln\left(\frac{W}{W_{o}}\right)\!\right\}^{1/n}$$
Energy Balance
Heat exchanger metallic body ($$Q_{\text{hex}}$$) $$Q_{{\text{hex}}}=m_{{\text{hex}}}\, c_{{p,{\text{hex}},m}} \left(T_{\text{reg}}-T_{{\text{ads}}}\right)$$
Refrigerant or adsorbate ($$Q_{\text{ref}}$$) $$Q_{\text{ref}}=m_{ad,\text{ref}}\, c_{p,\text{ref}} \left(T_{\text{reg}}-T_{\text{ads}}\right)$$
Adsorbent ($$Q_{ad})$$ $$Q_{ad}=m_{ad}\, c_{p,ad} \left(T_{\text{reg}}-T_{\text{ads}}\right)$$
Total heat input $$(Q_{h,\text{in}})$$ $$Q_{h,\text{in}}=Q_{{\text{hex}}}+Q_{\text{ref}}+Q_{ad}+Q_{st}$$
Performance Indices
Cooling capacity (CC) CC$$\; =\dot{m}_{\text{ref}} \,{\Delta h}_{\text{evap}}$$
$$\dot{m}_{\text{ref}}=\dfrac{m_{ad,\text{ref}}}{t_{\text{ads}}}=\dfrac{m_{ad}\, \Delta W}{2 t_{\text{ads}}}$$
Coefficient of performance (COP) COP$$\; =\dfrac{\text{CC}\cdot t_{\text{ads}}}{Q_{h,\text{in}}}$$
Mass of adsorbent to combustion fuel ($$m_{ad}/m_{cf}$$) $$\dfrac{m_{ad}}{m_{cf}}=\dfrac{Q_{\text{use}}}{Q_{h,\text{in}}}$$
$$Q_{\text{use}}=\eta_{b,\text{device}}\, m_{cf}\,{\rm CV}_{cf}$$

4. DISCUSSION OF RESULTS

This paper carries out a theoretical investigation on renewable energy–powered green cooling systems considering $$T_{\text{reg}}$$ of 90°C, $$T_{\text{cond}}$$ of 30°C, $$T_{\text{evap}}$$ of 7°C, $$t_{\text{ads}}$$ of 100 s, and $$T_{\text{ads}}$$ of 30°C. A parametric investigation of the impact of working temperatures on performance indices is discussed in Sections 4.1–4.5.

4.1 Effect of Treg on ΔW

The change in uptake shows the definite numeric value for the difference between the amount of the vapor adsorbed and desorbed at low- and high-pressure conditions. The variation in change in uptake corresponding to the regeneration is shown in Fig. 2. Herein, it can be seen that the change in uptake increases with an increase in hot fluid temperature. This is because of the decrease in the minimum value of uptake while the maximum value of uptake remains constant irrespective of regeneration temperature. On the other hand, this difference is noted as greater toward the low ambient temperature.

Figure 2. Variations in the change in uptake with respect to regeneration temperature

4.2 Effect of Adsorption Uptake on Qst

The isosteric heat of adsorption describes the suitability of the adsorbent–adsorbate pair for cooling or heat pump applications. The high value of adsorption heat indicates the applicability of the adsorbent material for cooling applications; however, its low value represents the heat-pumping application. Figure 3 describes the variations in isosteric heat of adsorption with respect to the adsorption uptake. It is found that the high value of adsorption heat is needed at a lower value of uptake and decreases with an increase in uptake value. At a regeneration temperature of 50°C, it is estimated as maximum throughout the uptake values. It shows that lesser adsorption heat is required for a high regeneration temperature. Therefore, this working pair is more suitable for an adsorption heat pump.

Figure 3. Variations in isosteric heat of adsorption with respect to uptake value

4.3 Effect of Tevap on Dühring Diagram

The Clausius Clapeyron diagram or Dühring diagram shows the refrigerant and compression cycles on the pressure-temperature–concentration plane. It has mainly four processes, such as preheating, desorption, precooling, and adsorption. The shape and width of the diagram depend on the evaporator temperature. From Fig. 4, it can be seen that the width of the diagram is noted more at the high value of the evaporator; or in other words, it becomes narrower toward the chilling temperature, wherein this width mainly shows the change in uptake value that was discussed in Section 4.1.

Figure 4. Effect of evaporator temperature on P-T-w plane

4.4 Evaluation of Cooling Capacity and Coefficient of Performance

In order to evaluate the performance in terms of cooling capacity (CC) and coefficient of performance (COP), the effect of ambient temperature is studied by keeping the regenerator temperature of 90°C and chilling temperature of 7°C. It is observed that the CC is higher at higher ambient temperature; whereas, COP is found to be more at low ambient temperature, as shown in Fig. 5. This is because of the larger difference between the maximum and minimum uptake values, as previously discussed. The CC at 38°C of ambient conditions is calculated as approximately two times higher than that of the ambient temperature of 32°C; whereas, COP is found to be ∼ 11% higher at a temperature of 38°C in comparison to 32°C ambient conditions.

Figure 5. Effect of ambient temperature on CC and COP at $$T_{\text{reg}} =$$ 90°C and $$T_{\text{evap}} =$$ 7°C

A new parameter, viz, the ratio of the mass of adsorbent material to the mass of combustion fuel ($$m_{ad}/m_{cf}$$), is introduced in our previous works, i.e., bioenergy-powered adsorption cooling system. The lowest value of this parameter shows the better suitability of the working pair at different ambient conditions. As described in Fig. 6, the chiller system associated with low ambient temperature requires a lesser amount of adsorbent material per kilogram of combustion fuel used in the combustion device. At an ambient temperature of 32°C, the mass of adsorbent per kilogram of combustion fuel is found to be less (∼ 10.5%) than that of the higher temperature value.

Figure 6. Effect of ambient temperature on $$m_{ad}/m_{cf}$$ at $$T_{\text{reg}} =$$ 90°C and $$T_{\text{evap}} =$$ 7°C

5. CONCLUDING REMARKS

The steady-state analysis of a two-bed, single-stage green cooling system driven by a pellet-based clean combustion device is carried out for various ranges of working temperatures. A processed biomass waste material is utilized as the combustion fuel to produce the thermal energy for the activation of the adsorbent bed. However, the waste biomass-derived activated carbon has been utilized as an adsorbent with ethanol refrigerant for cooling purposes. Therefore, the complete cooling system along with the heating unit is named a green cooling system in this paper. A new performance index parameter, which plays the role of bridge between the chiller unit and heating unit, is studied under different ambient conditions. It is concluded that cooling capacity (CC) at 38°C of ambient temperature is found to be approximately double compared to CC at 32°C of ambient temperature; however, coefficient of performance (COP) at 38°C of ambient temperature is found to be 11% higher than that of COP at 32°C of ambient temperature. Furthermore, the regeneration of the adsorbent bed at 32°C of ambient temperature requires ∼ 10.5% less mass of adsorbent per kilogram of combustion fuel than that of 38°C. It can be concluded that a clean and green sorption cooling is possible and has a very high potential; however, more in-depth analysis is to be carried out and the R&D group at the Indian Institute of Technology Delhi is working in this direction.

ACKNOWLEDGMENTS

The first author (P.R.C.) thankfully acknowledges the financial assistance in the form of fellowship due to the Department of Energy Science and Engineering, Indian Institute of Technology Delhi. Both authors are also thankful to the knowledgeable reviewers and editor for providing fruitful suggestions to modify the paper into the present form.

#### REFERENCES

Chauhan, P.R., Kaushik, S.C., and Tyagi, S.K. (2022a) A Review on Thermal Performance Enhancement of Green Cooling System Using Different Adsorbent/Refrigerant Pairs, Energy Convers. Manag., 14: 100225.

Chauhan, P.R., Kaushik, S.C., and Tyagi, S.K. (2022b) Current Status and Technological Advancements in Adsorption Refrigeration Systems: A Review, Renew. Sustain. Energy Rev., 154: 111808.

Chauhan, P.R. and Kumar, R. (2018) A Comprehensive Review on Heat Transfer Enhancement and Pressure Drop Characteristics of Nanofluid Flow through Micro-Channels, Ann. Chim. Sci. Matér., 42: 363–385.

Chauhan, P.R., Verma, A., Bhatti, S.S., and Tyagi, S.K. (2019) An Overview on Mathematical Models of Adsorption Refrigeration System, J. Mater. Sci. Mech. Eng., 6: 275–278.

Duong, X.Q., Cao, N.V., Hong, S.W., Ahn, S.H., and Chung J.D. (2018) Numerical Study on the Combined Heat and Mass Recovery Adsorption Cooling Cycle, Energy Technol., 6: 296–305.

Himanshu, Kurmi, O.P., Jain, S., and Tyagi, S.K. (2022) Performance Assessment of an Improved Gasifier Stove Using Biomass Pellets: An Experimental and Numerical Investigation, Sustain. Energy Technol. Assess., 53: 102432.

Jribi, S., Miyazaki, T., Saha, B.B., Pal, A., Younes, M.M., Koyama, S., and Maalej, A. (2017) Equilibrium and Kinetics of CO2 Adsorption onto Activated Carbon, Int. J. Heat Mass Transf., 108: 1941–1946.

Khanam, M., Jribi, S., Miyazaki, T., Saha, B.B., and Koyama, S. (2018) Numerical Investigation of Small-Scale Adsorption Cooling System Performance Employing Activated Carbon-Ethanol Pair, Energies, 11: 1499.

Pahinkar, D.G., Garimella, S., and Robbins, T.R. (2015) Feasibility of Using Adsorbent-Coated Microchannels for Pressure Swing Adsorption: Parametric Studies on Depressurization, Ind. Eng. Chem. Res., 54: 10103–10114.

Pal, A., Thu, K., Mitra, S., El-Sharkawy, I.I., Saha, B.B., Kil, S.H., Yoon, S.H., and Miyawaki, J. (2017) Study on Biomass Derived Activated Carbons for Adsorptive Heat Pump Application, Int. J. Heat Mass Transf., 110: 7–19.

Pal, A., Uddin, K., Saha, B.B., Thu, K., Kil, H.S., Yoon, S.H., and Miyawaki, J. (2020) A Benchmark for CO2 Uptake onto Newly Synthesized Biomass-Derived Activated Carbons, Appl. Energy, 264: 114720.

Pal, A., Uddin, K., Thu, K., and Saha, B.B. (2019) Activated Carbon and Graphene Nanoplatelets Based Novel Composite for Performance Enhancement of Adsorption Cooling Cycle, Energy Convers. Manag., 180: 134–148.

Pan, Q.W., Xu, J., Ge, T.S., and Wang, R.Z. (2022) Multi-Mode Integrated System of Adsorption Refrigeration Using Desiccant Coated Heat Exchangers for Ultra-Low Grade Heat Utilization, Energy, 238: 121813.

Sur, A., Sah, R.P., and Pandya, S. (2020) Milk Storage System for Remote Areas Using Solar Thermal Energy and Adsorption Cooling, Mater. Today: Proc., 28: 1764–1770.

Tyagi, S.K. (2022) Biomass Pellet Based Combustion Devices, Patent No. 397919 (Application No. 201811019556).

Xu, S.Z., Wang, R.Z., and Wang, L.W. (2018) Reply to “Letter to the Editor on ‘Temperature–Heat Diagram Analysis Method for Heat Recovery Physical Adsorption Refrigeration Cycle–Taking Multi-Stage Cycle as an Example’” by A. Bejan, Int. J. Refrig., 90: 280–286.

Yagnamurthy, S., Chauhan, P.R., Saha, B.B., and Tyagi, S.K. (2022) Entropy Generation Minimization of an Advanced Two-Bed Adsorption Refrigeration System, Int. Commun. Heat Mass Transf., 139: 106461.

Zhang, Y. and Wang, R. (2020) Sorption Thermal Energy Storage: Concept, Process, Applications and Perspectives, Energy Stor. Mater., 27: 352–369.

#### 参考文献

1. Chauhan, P.R., Kaushik, S.C., and Tyagi, S.K. (2022a) A Review on Thermal Performance Enhancement of Green Cooling System Using Different Adsorbent/Refrigerant Pairs, Energy Convers. Manag., 14: 100225.
2. Chauhan, P.R., Kaushik, S.C., and Tyagi, S.K. (2022b) Current Status and Technological Advancements in Adsorption Refrigeration Systems: A Review, Renew. Sustain. Energy Rev., 154: 111808.
3. Chauhan, P.R. and Kumar, R. (2018) A Comprehensive Review on Heat Transfer Enhancement and Pressure Drop Characteristics of Nanofluid Flow through Micro-Channels, Ann. Chim. Sci. Matér., 42: 363–385.
4. Chauhan, P.R., Verma, A., Bhatti, S.S., and Tyagi, S.K. (2019) An Overview on Mathematical Models of Adsorption Refrigeration System, J. Mater. Sci. Mech. Eng., 6: 275–278.
5. Duong, X.Q., Cao, N.V., Hong, S.W., Ahn, S.H., and Chung J.D. (2018) Numerical Study on the Combined Heat and Mass Recovery Adsorption Cooling Cycle, Energy Technol., 6: 296–305.
6. Himanshu, Kurmi, O.P., Jain, S., and Tyagi, S.K. (2022) Performance Assessment of an Improved Gasifier Stove Using Biomass Pellets: An Experimental and Numerical Investigation, Sustain. Energy Technol. Assess., 53: 102432.
7. Jribi, S., Miyazaki, T., Saha, B.B., Pal, A., Younes, M.M., Koyama, S., and Maalej, A. (2017) Equilibrium and Kinetics of CO2 Adsorption onto Activated Carbon, Int. J. Heat Mass Transf., 108: 1941–1946.
8. Khanam, M., Jribi, S., Miyazaki, T., Saha, B.B., and Koyama, S. (2018) Numerical Investigation of Small-Scale Adsorption Cooling System Performance Employing Activated Carbon-Ethanol Pair, Energies, 11: 1499.
9. Pahinkar, D.G., Garimella, S., and Robbins, T.R. (2015) Feasibility of Using Adsorbent-Coated Microchannels for Pressure Swing Adsorption: Parametric Studies on Depressurization, Ind. Eng. Chem. Res., 54: 10103–10114.
10. Pal, A., Thu, K., Mitra, S., El-Sharkawy, I.I., Saha, B.B., Kil, S.H., Yoon, S.H., and Miyawaki, J. (2017) Study on Biomass Derived Activated Carbons for Adsorptive Heat Pump Application, Int. J. Heat Mass Transf., 110: 7–19.
11. Pal, A., Uddin, K., Saha, B.B., Thu, K., Kil, H.S., Yoon, S.H., and Miyawaki, J. (2020) A Benchmark for CO2 Uptake onto Newly Synthesized Biomass-Derived Activated Carbons, Appl. Energy, 264: 114720.
12. Pal, A., Uddin, K., Thu, K., and Saha, B.B. (2019) Activated Carbon and Graphene Nanoplatelets Based Novel Composite for Performance Enhancement of Adsorption Cooling Cycle, Energy Convers. Manag., 180: 134–148.
13. Pan, Q.W., Xu, J., Ge, T.S., and Wang, R.Z. (2022) Multi-Mode Integrated System of Adsorption Refrigeration Using Desiccant Coated Heat Exchangers for Ultra-Low Grade Heat Utilization, Energy, 238: 121813.
14. Sur, A., Sah, R.P., and Pandya, S. (2020) Milk Storage System for Remote Areas Using Solar Thermal Energy and Adsorption Cooling, Mater. Today: Proc., 28: 1764–1770.
15. Tyagi, S.K. (2022) Biomass Pellet Based Combustion Devices, Patent No. 397919 (Application No. 201811019556).
16. Xu, S.Z., Wang, R.Z., and Wang, L.W. (2018) Reply to “Letter to the Editor on ‘Temperature–Heat Diagram Analysis Method for Heat Recovery Physical Adsorption Refrigeration Cycle–Taking Multi-Stage Cycle as an Example’” by A. Bejan, Int. J. Refrig., 90: 280–286.
17. Yagnamurthy, S., Chauhan, P.R., Saha, B.B., and Tyagi, S.K. (2022) Entropy Generation Minimization of an Advanced Two-Bed Adsorption Refrigeration System, Int. Commun. Heat Mass Transf., 139: 106461.
18. Zhang, Y. and Wang, R. (2020) Sorption Thermal Energy Storage: Concept, Process, Applications and Perspectives, Energy Stor. Mater., 27: 352–369.