Desiccant-Based Solar Air Conditioner: Lab-Scale Performance Test and Modifications

Tech Ingredients

Introduction to Desiccant-Based AC [0:00]

• Previous Benchtop Model & Efficiency [0:39]
  • A previous benchtop model demonstrated an electrical efficiency more than twice as high as a typical window air conditioning system.
  • This efficiency was achieved using a desiccant-based system that does not rely on a compressor or freon gas.
  • The desiccant used is a concentrated solution of calcium chloride in water.
• Goal of Current Project: Lab Cooling [0:55]
  • Based on the successful benchtop test and viewer feedback, the system was upgraded and substantially enlarged.
  • The objective of this video is to attempt to cool the Tech Ingredients lab with this larger air conditioning system.

System Components and Operation [1:41]

• Air Flow Path [1:41]
  • Room Air Intake and Centrifugal Fan [1:41]
    • Room air is drawn through a lower duct from the lab's window.
    • This is facilitated by a DC brushless centrifugal fan, typically used for hydroponics or forced-air heating systems.
  • PVC Column and Recirculation [1:59]
    • The fan forces air down into a 150mm (6-inch) diameter PVC WYE fitting.
    • Air then travels up a straight column to the top.
    • From the top, the air is rerouted back into the room, creating a closed loop of air.
• Desiccant Flow and Dehumidification [2:33]
  • Calcium Chloride Solution as Desiccant [2:33]
    • A concentrated (42%) cold calcium chloride desiccant showers down the straight column in a counter-current direction to the airflow.
  • Humidity Absorption and Collection [2:49]
    • Millions of small desiccant droplets provide a large surface area for interaction with the air.
    • The desiccant has a strong affinity for water, pulling humidity out of the air.
    • The now-diluted desiccant accumulates at the bottom of the column.
    • The air returning to the room has been dehumidified and refrigerated.
• Desiccant Regeneration Process [3:11]
  • Pumping to Solar Water Heater [3:11]
    • An aquarium pump moves the diluted desiccant from the reservoir to a solar water heater.
  • Heating for Water Release [3:39]
    • The solar water heater raises the desiccant temperature to between 40-60°C.
    • At this higher temperature, the desiccant has less affinity for water vapor.
  • Outdoor Air for Dehumidification [3:58]
    • The heated desiccant then flows to the top of a second column, where it's sprayed down by a showerhead.
    • Another duct booster (centrifugal fan) forces outdoor air upward through this column.
    • The warm desiccant releases its absorbed humidity into the outdoor air, regenerating its concentration.
• Desiccant Cooling and Re-circulation [4:38]
  • Heat Exchanger (Radiator) for Initial Cooling [4:38]
    • The warm, concentrated desiccant collects at the bottom of the second column.
    • A second aquarium pump moves this desiccant into a heat exchanger (fan-mounted radiator) where outdoor air cools it.
  • Evaporative Cooling Tower for Supercooling [5:00]
    • The cooler desiccant then travels to a second radiator, which acts as an evaporative column.
    • Cold, humid air passes through this radiator, pulling off additional heat through evaporation of plain water.
  • Coaxial Wort Chiller for Final Cooling [5:28]
    • The cold water from the evaporative cooler is then pumped through the outer coil of a coaxial stainless steel wort chiller.
    • This further reduces the temperature of the desiccant flowing through the inner tube with more heat transfer.
    • The supercooled, concentrated desiccant then travels back to the original tower, completing its loop.

Modifications and Advantages over Previous Design [7:43]

• Larger Diameter Tubing [7:49]
  • The current system uses 150mm (6-inch) diameter PVC tubing, a substantial increase from the 100mm (4-inch) tubing in the benchtop model, allowing for greater airflow.
• Showerheads instead of Bio-balls [8:17]
  • Initially, bio-balls were used, but they blocked airflow.
  • Switching to common showerheads created millions of small droplets, providing sufficient surface area for interaction without restricting airflow, and is cheaper.
• Outdoor Placement of Evaporative Cooler [9:15]
  • The evaporative cooling column is now outside, preventing the reintroduction of humidity (from water evaporation) into the cooled room.
  • This also contains any potential bacterial or algal growth (like Legionella) outside the living space.
• Bactericidal Desiccant, Non-Consumable [9:42]
  • The 42% calcium chloride desiccant solution is bactericidal, killing bacteria and dust mites, and acting as a mechanical filter for debris.
  • The desiccant is theoretically never consumed (unless leaked or excessively dirty), making it a non-consumable component.
  • The only consumable is the plain water used for evaporative supercooling.

Solar Water Heater Details [10:23]

• Construction and Cost [11:00]
  • The solar water heater is extremely inexpensive, costing approximately $45 to build.
  • It consists of about 50 meters of high-density polyethylene (HDPE) tubing (1.5cm / 0.5-inch ID) formed into a large coil.
  • The coil is painted with flat camouflage black paint to absorb sunlight.
  • The unit is capped with an e-glass window.
• E-Glass for Heat Absorption [11:26]
  • E-glass blocks thermal infrared radiation, trapping heat inside the collector.
• High Efficiency and Waste Heat Utilization [12:35]
  • Unlike photovoltaic cells (20-22% efficient), this solar water heater can achieve absorption efficiencies of 80-85%.
  • It can absorb about 1400 watts at noon at the equator.
  • The system doesn't require very high temperatures, allowing it to leverage inexpensive waste heat sources (industrial waste heat, hot air from conventional AC exhausts).
  • Mounting it on a roof can also block solar heat that would otherwise warm the building.
• Design Credit to "Desert Sun 02" [13:09]
  • The design for the solar water heater was presented on the "Desert Sun 02" YouTube channel.

Performance Testing and Results [13:41]

• Power Consumption [14:31]
  • The entire system, including pumps and fans, draws approximately 165 watts of electrical power. [15:17]
• Outdoor Weather Conditions [15:25]
  • Outdoor temperature: 17.3-17.4°C (around 63°F). [15:38]
  • Outdoor relative humidity: ~54%. [15:38]
• Indoor Temperature and Airflow Measurements [16:09]
  • The return air duct has a diameter of 175mm (7 inches). [16:30]
  • Air velocity out of the duct is measured at approximately 5.0 meters per second. [16:55]
  • Air density is 1.22 kg/m³. [17:31]
  • The system moves 0.145 kg/sec or 145 grams/sec of air. [17:41]
  • Initial indoor room temperature: ~18.8°C (65.8°F), dropped to ~12.6°C (54.8°F) after 1.5 hours. [18:28, 23:31]
• Cooling Power Calculation [23:44]
  • The system removed approximately 580 watts (580 Joules per second) of heat from the room air, based on the temperature drop and airflow. [23:55]
• Dehumidification Performance [24:28]
  • Comparing initial and final relative humidity values using a water vapor graph, the system showed negligible dehumidification. [26:15]
  • This is attributed to the lab's wooden structure absorbing and releasing humidity, as well as significant solar gain through large windows.
• Coefficient of Performance (COP) [26:54]
  • Considering only the electrical input (165 watts) and the cooling power (580 watts for temperature reduction), the COP is approximately 3.5. [26:54]
  • A COP of 3.5 is comparable to a SEER (Seasonal Energy Efficiency Ratio) of 14 for a typical household air conditioning system.
  • The solar energy used for desiccant regeneration is "free" and not included in the electrical COP calculation.
• Challenges and Future Optimization [27:06]
  • The main challenge is dealing with the large thermal mass of the wooden lab structure and direct sunlight.
  • Optimization is needed for fans and managing dynamic temperature changes.
  • The system has the advantage of being DIY, not using refrigerants, and being quieter than compressor-based systems.
  • Future projects will involve further optimization and testing of seasonal applications.