A Solar Ammonia Absorption Icemaker

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

Key findings

• Working pair is calcium chloride salt (adsorbent) + pure anhydrous ammonia (refrigerant) in an intermittent (day/night) cycle; salt is preferred over water because it does not evaporate during heating, avoiding the dilution problem of water-ammonia. Charge forms the compound CaCl2·8NH3.
• Generator is a 3-inch non-galvanized (black iron) steel pipe at the focus of a parabolic trough collector with 7 x 20 ft (140 ft²) collecting area, oriented east-west so only seasonal (not daily) tracking is needed.
• During the day cycle, ammonia boils out of the generator as hot gas at ~200 psi; at 250 °F, six of the eight ammonia molecules bound per salt molecule are available (working ratio 6:1 of the 8:1 total). Roughly 3/4 of the ammonia collects in the storage tank by day’s end.
• At night the cooled CaCl2 reabsorbs ammonia, pulling it back through the condenser coil as it evaporates from the evaporator tank, freezing water in bags around the tank — producing about 10 pounds of ice per cycle.
• Ammonia’s heat of vaporization is ~3/5 that of water, among the highest of practical refrigerants, which drives the strong cooling effect.
• Condenser is a 21-ft coil of 1/4-inch non-galvanized pipe rated at 2000 psi, immersed in a water bath (half of a 55-gallon drum); evaporator sits in an insulated box (scrap chest freezer).
• Under good sun a generation cycle completes in about 3 hours, far less than a day, so the unit works on hazy/partly cloudy days; covering the collector then induces the night cooling cycle.
• Total materials cost ~$510, including 15 lb ammonia ($15) and 10 lb calcium chloride ($10). Materials in ammonia contact must resist corrosion — non-galvanized steel and stainless valves used; system runs over 200 psi. Future design proposes staggered multiple generator pipes plus an ammonia reservoir for continuous (rather than intermittent) refrigeration.