One of the design objectives of Van de Soleil is to run the refrigeration system on system power, 24V DC in the case of the current prototype. This is necessary in order to avoid conversion loss, which in Man the Van, was measured at 23%.
As 24V DC freezer systems are unavailable, we built our own using a 24V DC compressor, two 24V DC fans (for the evaporator and condenser) and a 24V DC defroster system. All the other components are conventional.
The Condenser Sub-System
The core of the system is a 3/4 horsepower, 24V DC compressor, shown at the bottom left of the picture. In the middle is a 24V DC milspec fan, mounted at the back of the condenser coils. The blue unit is a standard filter, and the upright black unit at bottom right is an accumulator. The assembly is mounted in a sliding rack, which can be jacked down for maintenance, under the bed of the truck. it is connected to the freezer tubing by a pair of chemical-barrier flexible tubes and by 0-gauge power cables. The remainder of the components are standard.
The Evaporator Assembly
The evaporator assembly is a standard unit, purchased without a defroster and with a 120V AC fan. We replaced the fan with another 24V DC milspec 11" fan, wired to the control board. It runs whenever the compressor runs. A standard expansion valve accomplishes the phase transition of the R134A refrigerant to the vapor state, which absorbs heat from the surroundings to accomplish the refrigeration effect. The cold refrigerant vapor absorbs its heat when pushed through the evaporator coil, and the fan pushes the cold air into the freezer compartment.
The Defrost Sub System
The final custom piece is the defrost unit. The evaporator coil frosts up due to moisture in the freezer. It is necessary to periodically turn off the freezer and heat up the coils to thaw the ice buildup. The assembly shown has the system on-off switch at top left, a mechanical 24-hour clock which can be set to activate for 15-minute periods according to how it is set. When activated, the temperature control at top right switches on and turns off again when a preset temperature is reached at a probe in the evaporator coil. When activated, the solenoid in the center bottom energized a circuit in the evaporator via a fuse at bottom right.
The defrost circuit is a 20-foot length of resistance wire threaded through the evaporator coil body in twelve glass tubes. It has a resistance of an ohm and a quarter and heats up to about 150 degrees F to accomplish the defrost cycle.
At the end of its fifteen minutes of activation, the timer 'normally closed' contact energizes a pressure switch, which controls the cycling of the compressor. The system is designed to cycle on about 40% of the time, at a vacuum pressure of about 5 PSI and a freeser temperature in the range of 5 to 15 degrees F.