# Development and studies of low capacity adsorption refrigeration systems based on silica gel-water and activated carbon-R134a pairs

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## Abstract

Adsorption Refrigeration Systems (ARS) are gaining considerable importance in view of their applications in several areas and in particular to transport industries. The refrigerants used in these systems are also quite acceptable from the points of view of Global warming and Ozone depletion. In our efforts to develop such refrigeration systems for transport vehicles, a sub-atmospheric Silicagel-Water ARS (SWARS) and a positive pressure Activated Carbon / R134a ARS (ACRARS) have been designed, fabricated and experimentally studied to evaluate their performances. A lumped parameter simulation model has been used to describe the dynamic behavior of these systems. The rate of adsorption / desorption of the refrigerants in both cases are assumed to be governed by the Linear Drive Force (LDF) model. The amount of refrigerants in adsorbent at equilibrium conditions are assumed to be described by the equations for modified Freundlich and Dubinin-Astakhov models for SWARS and ACRARS respectively. The simulation models are numerically solved by finite difference method with the simulation programs coded in MATLAB. The simulation results are found to be in good agreement with experimental results in both cases. A two-bed Silicagel-water adsorption refrigeration system has been built and it is well known that the refrigeration is produced at the evaporator. If the refrigeration power is not used to cool an external heat load (for example, by allowing the circulation of water through the heat exchanger coil embedded in the evaporator), the evaporator reaches the lowest temperature. This is the *no-load* condition at which the refrigeration power produced by the system is zero. When the evaporator temperature is higher than this lowest temperature, a finite refrigeration power is produced which increases with increasing evaporator temperature. The above Silicagel-water system reaches the lowest temperature of 5.3°C at no load conditions and produces a refrigeration power of ~ 284 ± 9 W at 18°C, which refers to the average temperature (*T*_{avg}) of the flowing water through the heat exchanger coil within the evaporator. An experimental COP of 0.52 has been measured for this system. On the other hand, the simulation model predicts a refrigeration power of 325 W at 18 °C with a COP of 0.55. Using Activated Carbon - R134a pair, a four bed adsorption refrigeration system has been designed and developed. The refrigeration system is designed such that it can be operated in three different configurations and they are: (A) A single bed (i.e. all four beds arranged in parallel), (B) A twin bed, (i.e. the four beds get grouped in two pairs and undergo the opposite processes of the adsorption cycle and (C) Four independent beds each undergoing the different processes of the adsorption cycle. The measured lowest temperatures under no-load conditions are 14.5 °C, 13.3 °C and 11.9 °C for configurations A, B and C respectively as against the predictions of the simulations which are 13.3°C, 12.5 °C & 11.4°C. The experimentally measured refrigeration powers are 430 ± 13 W, 556 ± 17 W and 657 ± 20 W at the *T*_{avg} temperature of 28.5 °C for configurations A, B and C respectively. On the other hand, the refrigeration powers predicted by simulation are 450 W, 582 W and 690 W for the respective configurations. The measured COP values are 0.5, 0.65 and 0.70 for configurations A, B and C respectively as against those predicted by simulation which are 0.54, 0.67 and 0.73 respectively. It is observed that the experimental results are reasonably in good agreement with those predicted by the simulations. The highlight of the present work is that the Activated Carbon - R134a Adsorption Refrigeration System produces continuous refrigeration power which can be very useful for practical applications. Efforts are now underway to adopt this refrigeration system for cooling of truck cabins.

## Nomenclature

*UA*_{ads}Overall conductance of adsorber bed during adsorption process (W/K)

*UA*_{con}Overall conductance of condenser (W/K)

*UA*_{eva}Overall conductance of evaporator heat exchanger (W/K)

*UA*_{des}Overall conductance of adsorber bed during desorption process (W/K)

*Cp*_{ads}Specific heat of adsorbent (kJ/ kg K)

*Cp*_{hxads}Specific heat of adsorbent bed heat exchanger (kJ/kg K)

*Cp*_{hxcon}Specific heat of condenser heat exchanger (kJ/kg K)

*Cp*_{hxeva}Specific heat of evaporator heat exchanger (kJ/kg K)

*Cp*_{w}Specific heat of refrigerant (kJ/kg K)

*Cp*_{rv}Specific heat of refrigerant vapour (kJ/kg K)

*COP*Coefficient of performance

*SCP*Specific Cooling Power (W/kg of adsorbent)

*D*_{so}Pre-exponential constant in the kinetics equation (m

^{2}/s)*E*_{a}Activation energy of surface diffusion (kJ/kg)

*h*_{fg}Latent heat of vaporization of Refrigerant (kJ/kg)

*M*_{ads}Mass of adsorbent (kg)

*M*_{hxads}Mass of Heat exchanger of adsorbent bed (kg)

*M*_{hxcon}Mass of condenser heat exchanger (kg)

*M*_{hxeva}Mass of evaporator heat exchanger (kg)

*M*_{weva}Mass of refrigerant in evaporator (kg)

*M*_{totalads}Total mass of adsorbent used in the refrigerator (kg)

- \( \dot{m} \)
Mass flow rate (kg/s)

- \( {\dot{m}}_{cads} \)
Mass flow rate of cold water to adsorber bed (kg/s)

- \( {\dot{m}}_{hads} \)
Mass flow rate of hot water to adsorber bed (kg/s)

- \( {\dot{m}}_{ccon} \)
Mass flow rate of cold water to condenser (kg/s)

- \( {\dot{m}}_{ceva} \)
Mass flow rate of chilled water to evaporator (kg/s)

*P*Pressure (Pa)

*P*_{sat}(*T*_{ref})Saturation vapour pressure of refrigerant in evaporator (Pa)

*P*_{sat}(*T*_{ads})Saturation vapour pressure of refrigerant in Adsorbent (Pa)

*Q*_{e}Cycle average cooling power (W)

*Q*_{h}Cycle average heating input (W)

*Q*_{st}Isosteric heat of adsorption (kJ /kg)

*q*Uptake of adsorbent at a given time (kg/kg of adsorbent)

*q*_{0}Coefficient in RHS of D-A equation (kg/kg of adsorbent)

*q*_{ads}Adsorption uptake (kg/kg of adsorbent)

*q*_{des}Desorption quantity (kg/kg of adsorbent)

*q*^{∗}Equilibrium uptake (kg/kg of adsorbent)

- \( {q}_{ac}^{\ast } \)
Equilibrium uptake (kg/kg of activated carbon)

- \( {q}_{sg}^{\ast } \)
Equilibrium uptake (kg/kg of silica gel)

*X*Concentration (kg of adsorbate /kg of adsorbent)

*R*Universal gas constant (kJ/kg K)

*R*_{p}Average radius of Silica gel (m)

*T*Temperature (K)

*T*_{cadsin}Cold water inlet temperature to adsorber bed (°C)

*T*_{cadsout}Cold water outlet temperature from adsorber bed (°C)

*T*_{hadsin}Hot water inlet temperature to adsorber bed (°C)

*T*_{hadsout}Hot water outlet temperature from adsorber bed (°C)

*T*_{con}Temperature of condenser (°C)

*T*_{cconin}Temperature of cold water inlet to condenser (°C)

*T*_{cconout}Temperature of cold water outlet from condenser (°C)

*T*_{eva}Temperature of evaporator (°C)

*T*_{cevain}Temperature of chilled water inlet to evaporator (°C)

*T*_{cevaout}Temperature of chilled water outlet from evaporator (°C)

*T*_{ref}Refrigerant Temperature (°C)

*T*_{ads}Adsorbent Temperature (°C)

*T*_{avg}Average Temperature of flowing water through evaporator (°C)

*T*_{inlet}Inlet Temperature of flowing water through evaporator (°C)

*T*_{outlet}Outlet Temperature of flowing water through evaporator (°C)

*t*_{cycle}Cycle time (s)

*t*Time (s)

*A*(*T*_{ads})Coefficient A in Freundlich Equation

*B*(*T*_{ads})Coefficient B in Freundlich Equation

*D*Coefficient of D-A equation

*n*Power coefficient in RHS of D-A equation

*A*_{0}Constant of expansion of

*A*(*T*_{ads}) as power series*A*_{1}First order coefficient of expansion of

*A*(*T*_{ads}) (*K*^{−1})*A*_{2}Second order coefficient of expansion of

*A*(*T*_{ads}) (*K*^{−2})*A*_{3}Third order coefficient of expansion of

*A*(*T*_{ads}) (*K*^{−3})*B*_{0}Constant of expansion of

*B*(*T*_{ads}) as power series*B*_{1}First order coefficient of expansion of

*B*(*T*_{ads}) (*K*^{−1})*B*_{2}Second order coefficient of expansion of

*B*(*T*_{ads}) (*K*^{−2})*B*_{3}Third order coefficient of expansion of

*B*(*T*_{ads}) (*K*^{−3})*K*_{ads}Overall mass transfer coefficient during adsorption (s

^{-1})*K*_{des}Overall mass transfer coefficient during desorption (s

^{-1})

## Nomenclatures of symbols used in Annexures

*m*Mass of water (kg)

*t*Time (s)

- \( \dot{m} \)
Mass flow rate (kg/s)

*T*Temperature (K)

- Δ
*T* Temperature difference (K)

*W*_{m}Uncertainty in measurement of mass of cooling water (kg)

- \( {W}_{\dot{m}} \)
Uncertainty in measurement of mass flow rate (kg/s)

*W*_{Q}Uncertainty in measurement of cooling power (W)

*W*_{t}Uncertainty in measurement of time (s)

*W*_{Δ T}Uncertainty in measurement of temperature difference (K)

*L*(*T*)Latent heat of vaporization of water at temperature

*T*(kJ/kg)*x*Fraction of water vaporized (kg/kg of original quantity)

*P*Pressure (Pa)

*P*_{1}Vapour pressure corresponding temperature

*T*_{1}*P*_{2}Vapour pressure corresponding temperature

*T*_{2}*T*_{avg}Average Temperature of flowing water through evaporator (°C)

*T*_{inlet}Inlet Temperature of flowing water through evaporator (°C)

*T*_{outlet}Outlet Temperature of flowing water through evaporator (°C)

## Notes

### Acknowledgements

The authors wish to acknowledge the financial support from Ingersoll Rand, Bangalore for carrying out research work in the area of adsorption refrigeration. They are also thankful to CCT staff for their valuable helps in the fabrication of the experimental setup.

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