Ideal Adsorption Isotherm Behavior for Cooling Applications

Abstract

Purely heat-driven refrigeration has the potential for high primary-energy efficiency, especially when powered by waste heat or solar thermal sources. This paper presents a novel expression for the ideal adsorption step location as a function of operating conditions. This methodology is then applied to a hypothetical stepwise material to evaluate its intrinsic efficiency. This analysis technique is then extended to allow facile efficiency comparisons for any adsorbent-refrigerant pair using an adsorbent's isotherm and heat of adsorption properties. This work focuses on limitations to efficiency due to the equilibrium thermodynamics. It is found that a stepwise adsorbent can have a single-effect intrinsic efficiency of as high as about 85% of Carnot, assuming typical adsorbent specific heats and uptake capacity. Using these tools, we analyze the maximum ratio of cooling to heat input (coefficient of performance) for two adsorption pairs, zeolite 13X-water and UiO-66-water, which are found to have maximum coefficients of performance of 0.52 and 0.88 for a cold-side temperature of 10 °C and an ambient temperature of 30 °C, respectively. Meanwhile, the maximum fraction of Carnot cooling is 37% for zeolite 13X-water and 67% for UiO-66-water. Moreover, these peak fractions of Carnot occur at much higher regeneration temperatures for 13X (196 °C) than for UiO-66 (60 °C). These two materials could be coupled in a two-stage cascading triple-effect adsorption cycle that operates with a combined coefficient of performance of 1.50 at a regeneration temperature of 196 °C, a cold-side temperature of 10 °C, and an ambient temperature of 30 °C.

Toward Optimal Metal–Organic Frameworks for Adsorption Chillers: Insights from the Scale‐Up of MIL‐101(Cr) and NH2‐MIL‐125

“…As discussed in Section 3.1, the position of the characteristic curve has a high influence on the actual change in loading in the application. [68] For this reason, Erdős et al [21] screened several MOFs regarding their adsorption capacity for a fixed application. Also, Boman et al [69] screened several adsorbent materials by considering the position of the characteristic curve.…”

Alcohol-based adsorption heat pumps using hydrophobic metal–organic frameworks

“…In addition, a few theoretical approaches have been proposed to study the performance of AHP devices based on hypothetical and idealized adsorbents. Bagheri and Schiffres 64 proposed a method for finding the ideal step location as a function of the operating conditions and applied it to test systems based on 13X-water and UIO-66-water working pairs. Jiang et al 65 proposed a framework based on the ideal adsorption isotherm behavior to obtain the thermodynamic limits of an AHP system.…”

Monolithic Zirconium‐Based Metal–Organic Frameworks for Energy‐Efficient Water Adsorption Applications

“…[17] To date, MOFs have displayed benchmark performance for a wide variety of water adsorption applications, including atmospheric water harvesting, adsorbents-based chillers, and adsorbent-based heat pumps due to their characteristic Type V, S-shaped isotherms (Figure S1, Supporting Information). [18][19][20][21][22][23][24][25][26] Studies on MOF-based adsorbent refrigeration have seen significant improvements in COP (e.g., MIP-200 = 0.81) [23] when compared with zeolites [27] and silicates. [11] Similarly, the use of MOFs has seen dramatic reductions in the heat requirements for adsorbent regeneration (≤100 °C) compared to zeolites.…”