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Solar DC-Direct Energy Storage in a Thermal Water Battery for Overnight Cooling

Access to affordable energy is the greatest historical differentiator in development between societies. Thomas Edison strongly argued that DC was a better solution inside the home which was absolutely correct. AC is better for distribution to the home. Many modern devices and appliances already run on AC converted to DC internally from LED bulbs to induction cooktops, inverter refrigerators and dryers.

Focus is not energy savings but FREEDOM from the electrical umbilical shore power  to enjoy the best places comfortably for Marine Vessels, Recreational Vehicles (RVs), a remote Cabin, or village in the developing world

All Heat Exchangers Fall into 3 Reversible Groups 

  • Air-to-Air - commonly referred to as an "Air conditioner" 
  • Water-to-Water - commonly referred to as a "Chiller"
  • Water-to-Air - commonly referred to as an "Air handler" or "Geothermal" 

Open-loop Geothermal is to 2 wells or a pond or a canal. Closed-loop Geothermal is to pipes buried underground. Marine chiller or geothermal systems are the same thing simply one is buried, the other not. Many marine chiller and geothermal systems are two-stage combining one or more water-to-water with one or more water-to-air.

Energy can be Stored Chemically (Battery), Thermally, Kinetically, or Atomically

If we want cooling air conditioning at night powered by solar energy captured during the day we can

  1. Store Solar DC in Big expensive DC batteries, then use a (hopefully DC) air-to-air heat exchanger powered by those batteries all night.
  2. A Thermal Water Battery. Use a DC water-to-water chiller powered directly by DC solar panels during the day to cool the water in a large insulated tank buried in the ground, ideally under the center of our home. Then use a small battery to move cool water through, and spin a fan in, a water-to-air handler. This moves the heat from our bedroom into the tank of cold water throughout the night. 
One BTU is the amount of energy required to raise the temperature of one pound of water by 1 degree Fahrenheit. So a 1 ton 12,000 BTU air handler running for 18 hours would require 216,000 BTUs or for a 40° temperature change from 20° F to 60° F would require 5400 lb of water or rather glycol so it does not freeze at 20 degrees Fahrenheit. Since a gallon of water weighs 8.34 lb we will need 650 gallon tank per ton of cooling. 

A 12 ft by 15 ft bedroom with two people sleeping might require between 5,000 and 8,000 BTUs of cooling throughout an 80° night depending on insulation. So a 2000 gallon tank would provide a buffer of three nights to cool two bedrooms in a typical home.  A 2000 gallon non-pressurized insulated buffer tank is commonly available for $3,500 and a non-insulated tank for $1400 in a heavily insulated space.

We want to cool 2000 gallons 40°F.  That would be 667,200 BTUs. One 24,000 BTU 56v DC water-to-air chiller will cool the water 144,000 BTUs with 6 Summer Peak Sun Hours or the full tank in 4.63 days. Two 24,000 BTU Chillers would be better to reach maximum thermal capacity in 2.3 days, but one 24K BTU chiller would be sufficient for a home where only one bedroom is being cooled. We can balance our chilled water reserve against other energy needs.

Our 24,000 BTU Water to Air heat exchanger would be running at 100% utilization and thus requiring ? watts or X 400 W Solar Panels.

Remember this is only the requirement for cooling at night. During the day we can use an already in place air to air heat exchanger. Or simply calculate our BTU loads by day in common living areas.

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