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In the evaporator, chilled liquid water is produced by pulling a vacuum on the liquid water returned from the room-air contactor. At this low pressure, a portion of the liquid water evaporates causing the remaining liquid water to chill to a low temperature. This chilled liquid water is pumped out of the evaporator and is sent to the room-air contactor where it again cools the indoor air.

The water vapor produced in the evaporator is compressed and sent to the condenser. In the condenser, the compressed water vapor condenses onto cool liquid water coming from a cooling tower. The heat of condensation warms the liquid water, which is then pumped out of the condenser and sent back to the cooling tower, where it cools.

Because this is an open system, liquid water directly contacts air. A small amount of air dissolves in the liquid water, and is removed from the liquid water at the vacuum conditions. This air collects in the condenser and is purged; for simplicity, the details are not described here. Also, salts will accumulate in the liquid water, which must be purged as well.

The water-based air conditioner described above has been known for many years, but it has not been commercialized because a suitable compressor has not been available until the development of StarRotor technology. The compressor must be able to compress large volumes of water vapor - a typical 3-ton air conditioner must compress about 1200 cubic feet per minute of low-pressure water vapor. The StarRotor compressor can accomplish this both efficiently and economically.

The water-based air conditioner provides indoor comfort comparable to traditional vapor-compression air conditioners that use Freon refrigerants. Scientific studies have proven that Freon refrigerants damage the ozone layer. The Montreal Protocol has banned CFCs (chlorofluorocarbons) and has temporarily allowed HCFCs (hydro chlorofluorocarbons), which are less damaging to the ozone layer. However, HCFCs are scheduled to be banned because they are potent greenhouse gasses. Of course, water is environmentally benign.

In addition to being environmentally benign, the water-based air conditioner is more energy efficient than traditional Freon-based vapor-compression air conditioners. The primary reason for the increased efficiency is that heat is rejected at the wet-bulb temperature, not the dry-bulb temperature. Note in the schematic, even though the outdoor dry-bulb temperature was 90°F, the condenser was only 85°F. In a traditional Freon-based vapor-compression air conditioner, the condenser temperature would be about 110°F. In this example, the water-based air conditioner rejects heat at a 25°F cooler temperature than a traditional air conditioner.

If the relative humidity of the outdoor air were lower - such as occurs in a desert - then the water-based air conditioner performs even better. For example, if the outdoor air were still 90°F but with a relative humidity of 20%, then the condenser temperature would be only 75°F, or 35°F cooler than a traditional air conditioner.

Air conditioner efficiency is rated according to the SEER (Seasonal Energy Efficiency Rating). The SEER rating is based upon the following assumptions: room air = 80°F, 50% RH; air exiting evaporator coils = 67°F, 100% RH; ambient dry bulb = 82°F. SEER describes how many Btu's of cooling per W·h of electricity input. The electrical input includes not only the compressor, but also the indoor and outdoor fans that move air. Table 1 shows the expected SEER of the water-based air conditioner at a variety of relative humidities. Table 1 also shows the SEER of traditional Freon-based air conditioners and the expected energy savings the water-based air conditioner will give.

Table 1. Expected SEER of Water-Based Air Conditioner
and Its Energy Savings (in %) Compared to Conventional Air Conditioners.