Portable Window Freezer + AC (abbreviated F-AC) is a portable fridge-freezer with all or part of the freezer used during summer for cooling indoor air. During summer, the top external heat exchanger is connected to the freezer and an air fan is placed behind the heat exchanger (or it can be a fan coil unit) for using a section (or all) of the freezer for cooling indoor air. Like a conventional window or portable AC, the back of the F-AC has access to outdoor air to expel the heat generated by its condenser outdoor during night when its cooler than day, thereby saving electricity. The water inside the freezer is frozen during night when it's colder than day time. During hot days, the ice, frozen at night melts to cool indoor air. It uses the phase change materials (water/ice) latent heat of fusion and heat of solidification to store and release thermal energy. Instead of water-ice, other, more efficient phase change material (PCM) may be used. To save space, the insulated water-ice reservoir and pump may be positioned on top (outside) of the freezer-fridge compartments (not shown).

True Portable Spot Cooling. A regular/conventional portable window AC must remain attached to window or have access to outdoor air while cooling indoor air - to transfer the room's heat to outdoor. Therefore it cannot be moved away from window while cooling indoor air. Since F-AC is attached to window during night to freeze water in its reservoir, during day when it's being used, it can be detached and moved away from window because the ice that was frozen during night is used for cooling, not the evaporator.

Not having an internal air duct and air fan, but instead using a regular larger external air fan not only reduces the size and manufacturing cost of the device but also increases its efficiency. In addition it reduces indoor air pollution caused by inaccessible (hard to clean) dirty internal fan coil of an older AC.

"According to the U.S. Department of Energy, if you're running a large fan at the same time as your air conditioner, you could potentially raise the thermostat as much as 4 degrees Fahrenheit. That might not seem like much, but it's energy savings that puts money in your pocket."

F-AC freezes water at night. The ice is used for cooling indoor air during day when it's warmer than at night. It saves more electricity in areas where there is a high temperature difference between day and night (dry climate). Although in hot humid climate it saves less electricity, it compensates for it by dehumidifying indoor air, thereby increasing comfort and satisfaction. Assuming that the temperature difference between day and night is 20 F, the energy savings will be 20% or more. In addition F-AC recycles indoor air, which further adds to its energy efficiency.

Even the perfect system decreases in efficiency with increased outside temperature, dropping about 2 percent per degree Celsius (about 1 percent per degree Fahrenheit). In the real world, the drop in efficiency is even more dramatic; so it's not an illusion when your air conditioner seems to be working harder as the outside temperature increases.

The opposite of that occurs with F-AC - its efficiency rises (cools faster) with rising temperature. The higher the temperature difference between the ice and indoor air (the hotter the room), the faster heat transfers from indoor air to water-ice-reservoir, the faster the ice melts in the reservoir.

Variations

There are several variations. One is that internal portion of F-AC (the part that is inside the refrigerator/freezer) has its own compartment with its own evaporator for freezing the PCM (Phase Change Material, water/ice) reservoir during night. To reduce manufacturing cost and improve performance, it's best to have the PCM reservoir on top of F-AC where it's close to the heat exchanger on top of device and the freezer preferably under it. Another variation would have the internal portion of F-AC inside the freezer compartment. Another variation has the PCM reservoir inside the refrigerator (uses a PCM that has a higher freezing/melting temperature than water).

To reduce the volume of PCM reservoir (save space), other PCMs (other than water/ice) may be used that have lower (or higher) PCM temperature than water/ice. Since the ideal temperature inside the freezer is 0F (-18C), a PCM with freezing temperature slightly higher would be for example, Eutectic PCM E-15. It has a freezing/melting temperature of 5F (-15C) slightly higher than the ideal temperature inside the freezer compartment and its latent and specific heat is only slightly lower than water/ice. Lowering the freezing temperature of the PCM enables F-AC to save more energy at night. All that saved thermal energy can then be used for day time cooling. Saving more thermal energy (at lower temperature) allows for having a smaller volume of PCM reservoir. An additional benefit would be that the anti-freeze circulating through the frozen PCM, will get much colder and therefore it can cool the indoor air faster when the colder anti-freeze circulates inside the heat exchanger on top of the device.

Typically, during winter, spring and autumn the AC is not used. During this time, the external section of the pipe is disconnected from its internal section. Then the pipes, the top air fan, the top heat exchanger (and optionally) the water-ice reservoir are removed, and it can no longer cool indoor air. Only during summer when AC is needed, is a section of used to house the water-ice reservoir for cooling indoor air. During summer, at night, the portable is either moved outdoor or its condenser has access to outdoor to exchange heat with outdoor air (through a window, an opening in the wall or an air duct). During the hot days of summer when AC is needed, the evaporator freezing the water-ice reservoir is turned off to allow the ice to melt, thereby saving electricity during peak hours of usage. The ice that was frozen the night before, gradually melts as the pipe circulates anti-freeze inside the reservoir, exchanging room's heat with the ice.

Since does the job of two devices (Refrigerator/Freezer and Air Conditioner), ideally it should have a larger insulated internal body to accommodate the additional water-ice reservoir, and a more powerful motor and compressor to freeze more water in less time to enable it to not only cool food but also cool indoor air.

Calculations

By definition, at 32F (0C), a one ton (12000BTU/hr) Air Conditioner (abbreviated as "AC") can freeze 2000LBs of ice in 24 hours . A half a ton AC (equivalent to 6000BTU/hr) therefore freezes 1000LBs of ice in 24 hours. A half a ton AC or 6000BTU/Hr AC cools a room of 150-250 SQFT.

A 6000 BTU/hr AC freezes 1000/24 = 41.67 LBs per hour (rounded to 42LBs) of water.

To make it more realistic, I have assumed that the AC only runs 5 hours/Day (for example 1-6 PM).

How many cubic feet is 42 LBs of water ? The weight of one cubic foot of water is 62.42718356 pounds. Therefore (42LBs of water x 5Hrs/Day) / 62.43LBs = 3.36Cubic Feet.

The volume of ice is about 10% greater than water. Therefore the volume of the water/ice reservoir should be 3.36 x 1.1 = 3.7 Cubic feet (~ 4 Cubic Feet) assuming the 6000BTU/Hr AC runs 5Hrs/Day. For each additional hour that the AC runs it requires a 0.75cubic Ft. larger water/ice reservoir. [(42LBs / 62.43Lbs) x 1.1 (ice has more volume than water) = 0.75 Cubic Ft.]

For example: Whirlpool Model # WRT311FZDB 4.2484 $808 source

20.5 cu. ft. Top Freezer Refrigerator. The freezer compartment is 6.08 cu. Ft. and the ideal temperature inside the freezer is about 0F (-18 C). Therefore this model has more than enough room in its freezer compartment to accommodate a 4 cubic feet of water/ice reservoir.

Second Example: Frigidaire 7.0 Cu. Ft. Chest Freezer White $180. Source

In this example, the size of PCM (Phase Change Material, water/ice) reservoir can be greater. During hot summer days the portable chest freezer can be converted to and used for cooling indoor air instead of (or in addition to) freezing food.

As shown previously, a 6000 BTU/hr equivalent AC requires 4 cubic feet of PCM reservoir. Therefore a 6.5 Cu. Ft. PCM reservoir (slightly smaller than the size of the above chest freezer) would do the job of a X BTU/hr AC. Calculating for X = 6.5x(6000/4) = 9750BTU/hr AC (can cool ~ 425 Sq. Ft. http://northcool.com/air_conditioner_size.html ) . The advantage is that not only is it more economical than buying a 9750 BTU/hr AC, but unlike the AC the chest freezer can be used as a chest freezer during cooler months of the year (spring, winter, and autumn) or whenever AC is not needed for cooling air. Unlike an AC that is only useful during hot summer months. To improve its cooling ability a more power water pump may be used. Also, an antifreeze with lower viscosity is easier for the pump to circulate especially through narrow internal pipes of the heat exchanger. That would also improve its cooling effect.