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ADSORPTION-DESORPTION SOLAR REFRIGERATOR


by Needful Provision, Inc. (NPI) (This article Copyright 2005 Needful Provision, Inc., USA. This page is a mirror of part of thier page at http://www.needfulprovision.org/main/solaroven.php to ensure that it is not lost. I will remove this mirror as requested.)

The solar refrigerator, developed by NPI, is a simplified and improved version of prior solar adsorption-desorption refrigerators ---refrigerators developed in recent years in Germany, France, and the U.S. All of these solar refrigerators have no moving parts. Water is used as the working fluid. The naturally occurring, highly porous silicon compounds (zeolites) are used as adsorbers. NPI’s unique solar powered refrigerator produces about 5 lbs. of ice daily per cu. ft. of storage space.

NPI’s seeks to help meet food storage needs for Third World families, with locally constructed solar refrigerators having the following parts:

1. An insulated, open-top solar collector box, about 7 ft. x 7 ft. (painted black), with

two-layers of glass (or other glazing) attached to the open-top. This box is used to collect solar heat via exposure to direct sunlight. A single 2 1/16-inch hole is drilled, on one side, to allow the exit of a 2-inch PVC pipe (see drawing).

2. A container, for zeolites, made of 6-inch PVC pipe generally forming a square

that fits inside the above collector box ---with four added cross-sections as shown on the drawing to follow. (Pipe should be black PVC, or paint the outside black.) Locate 2-inch PVC pipe in the center of the above 6-inch pipe, after drilling the 2-inch pipe to provide numerous, scattered 3/16-inch holes. Fill around this smaller pipe with ¼ to ½ -inch zeolite gravel (some 300 cubic inches per lineal foot).

This gravel is to hold smaller pipes in the center of the 6-inch pipes, and to help perform an adsorption-desorption function. All the holes in the 2-inch pipe must be drilled so they fit inside the 6-inch PVC pipe. Cap the opening of the 6-inch pipe joint using a 2 1/16-inch hole to allow the 2-inch pipe to pass through. Use PVC glue to seal the crack between the cap and the 2-inch PVC pipe. (The overall pipe system is of a size to fit within the above solar collector box.)

3. A vacuum hand-pump is used to help maintain a partial vacuum in the above

Described pipe container system sealed with PVC glue to prevent air leaks. (A good hand-operated vacuum pump, with gauge, is available from Triple Helix for US $39.95 --tel. 1-803-360-7872 or see website: http://www.triplehelixspores.com/hand_pump.htm.)

4. An attached heat exchanger, made from copper or brass tubing, also acts as a

condenser to convert hot water vapor into water, or water droplets.

5. An insulated cooler (refrigerator) box of at least 4 cu. ft. may be located in the ground or partly below ground to improve insulation (also use an insulated top door). This size is suggested to meet the food cooling needs of a family of four.

6. A one (1) cu. ft. sealed container to be located inside the above cooler box. At the

various stages of operation, this container holds a combination of water, water vapor, and ice. There is an airtight cap, over a 2-inch hole, that may be opened to add water as needed to keep this container at least 2/3rds full of water and/or ice. A sealed, attached float-gauge indicates the level of the water/ice without opening this container. (Loss of water vapor, and loss of the partial vacuum, may occur if any of the above components develop a leak.)

7. An adapter and appropriate glue or sealing materials are used to connect the 2-inch PVC pipe to the copper or brass tubing used for the condenser (item 4 above). Use solder, or similar means, to fix (and seal) the tube-end to the sealed container (item 6 above). Air leaks will greatly reduce the cooling efficiency of this refrigerator.

Concept of operation is based on the fact that when cool (at night) the zeolite acts somewhat like a sponge soaking up or adsorbing the water vapor ---then when heated during the solar day, this water vapor is desorbed or released. Operating under a partial vacuum, the water vapor moves with high efficiency under low pressures. At about 100F, water vapor begins to desorb from the zeolite. This water vapor is condensed into water droplets as heat is given off by the heat exchanger. The resulting water runs down, using gravity, into a sealed storage tank inside the cooler.

During the night, zeolite is cooled close to ambient temperature and starts adsorbing water vapor. Two sides of the solar collector box are opened to allow night air to help cool the zeolite inside the PVC pipes. Liquid water, in the storage tank (a tank also operating as an evaporator) adsorbs heat from the space to be cooled and is converted into water vapor. Since the system is sealed under very low pressure, any water remaining in the storage tank is then frozen into ice. This ice then melts slowly during the next day providing sustained cooling at reasonably constant temperatures.

The Zeopower Company, and EG Solar, both manufacture commercial versions of solar adsorption cooling (or refrigeration) systems. Other companies, like SunDanzer, manufacture solar coolers (or refrigerators) that use conventional compression-type refrigeration achieved by electrical systems based upon photovoltaics. NPI’s cooler (as above described) was developed to provide food storage for the poorest of the poor, and is not intended to be a commercial system. The NPI cooler has been designed to allow village users to easily obtain their own inexpensive means of local food preservation, with no power other than solar. Every effort has been made to avoid any possible patent infringement related to existing, somewhat similar, commercial solar powered refrigerators.

Zeolite deposits may be found worldwide, and the abundance of zeolite is part of the successful application of solar refrigeration. In many areas, zeolite is sold commercially for use in odor control. An example commercial source is USA Zeolite (with zeolite products shown on the company website: http://www.ZeoliteDepot.com).

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