A review on adsorption heat pump: Problems and solutions

Read from local Zotero full-text PDF (group 5183627 / Joule Heist).

Key findings

• Basic cycle = 4 steps on the adsorbent isoster: isosteric heating (a-b), isobaric desorption (b-c), isosteric cooling (c-d), isobaric adsorption (d-a); cooling effect occurs during isobaric adsorption (d-a). COPref = Qe/(Qab+Qbc); SCP = Qe/(m*tau_cyc).
• Three problem areas for AHP: (1) intermittent/discontinuous cooling, (2) operation under high vacuum (water/zeolite systems), (3) poor heat & mass transfer in the low-conductivity adsorbent bed. Research is organized around advanced cycles, new/improved pairs, and bed design.
• Advanced cycles: uniform-temperature adsorber heat recovery between two beds can raise COP by up to ~50%. Dous & Meunier cascade (zeolite-water high stage + active carbon-methanol low stage) achieved experimental cooling COP of 1.06. Thermal-wave two-stage systems (Saha, Hamamoto) utilize low-grade solar/waste heat at 40-95 C.
• Working-pair comparison (Table 3, max adsorbate capacity / heat of adsorption / energy density / temp range): water-zeolite 13X 0.30 kg/kg, 4400 kJ/kg, 1290 kJ/kg, 30-350 C; water-zeolite 4A 0.22 / 4400 / 1250; water-silica gel 0.20-0.37, ~2500 kJ/kg, 600-1000 kJ/kg, 20-130 C; water-charcoal 0.40 / 2320 / 1200; methanol-activated carbon 0.32 / 1400 / 590, 20-140 C; water-mordenite/clinoptilolite lower capacity (0.11-0.12).
• Pair selection criteria: high latent heat, non-corrosive/non-toxic, chemical & thermal stability, high adsorption capacity and conductivity, low cost; interaction must be physical adsorption (reversible). Water/zeolite needs high vacuum; NH3/zeolite runs at 4-11x ambient pressure; Wang & Zhu binary NH3+H2O fluid can run near ambient pressure, mitigating the vacuum problem.
• Bed designs split into uncoated (pellet/granule/fiber, fins to boost heat transfer, voids for mass transfer; activated carbon fibers have higher pore volume/area than silica gel and no hysteresis) and coated (adsorbent coated on pipe/fin/metal foam for fast heat & mass transfer).
• Restuccia et al. coated bed of finned tubes with SWS-1L (CaCl2 in mesoporous silica gel) gave optimal cycle time 20-40 min and cooling COP 0.17-0.48; copper-foam and hydrothermally grown zeolite supports also demonstrated.
• Practical advantages of AHP vs mechanical heat pumps: driven by waste/solar/geothermal heat, no moving parts (low maintenance, long intervals), no corrosive/toxic refrigerants, built-in thermal storage; main drawback is lower COP and salt-corrosion-limited life in advanced (salt) systems.