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.
Van de Soleil has several assumptions and design parameters which need to be explicit.
- Currently, about 60% the space under the solar panels can be frozen. This was due to an estimate we did in the design phase. This limitation can be lived with in several applications, and we expect that eventually this limitation will go away.
- Current solar panels are about 20% efficient. It is widely believed that the theoretical limit for current designs is about 32%, which should be reached in 20 years. New technologies could increase this limitation.
- Van de Soleil has a 14 inch gap between rows of panels. This is a 15% energy penalty which could be eliminated either by fitting smaller panels in the gap, or having custom panels built in a size which can use all the roof space. Using 8 4'X4' panels would increase current 300W panels from 1800 watts to 2170 watts with current panel yields.
- There's space for one additional panel on the cab roof, which would increase yield by a further 300W to 2470W. This is a 45% increase on Van de Soleil's yield from two year old panels, which would remove the freezer size limitation.
- Future designs would reduce panel weight by incorporating thin-film panels as part of the freezer box roof structure.
- The system is freezer only - not refrigerated or air-conditioned temperatures. This is due to a technical feature which actually makes freezing more efficient than refrigeration. There are several applications where this limitation is acceptable, and there is also a work-around for higher temperature loads.
- Counter-intuitively due to the Ideal Gas Law, it requires about twice as much energy to cool a load to 38 degrees than to freeze it to zeeo degrees. This is because P = VT, where P is pressure, V is volume and T is temperature. In a refrigeration system, volume is constant. Therefore P is proportional to T. As P is maintained by work done in the compressor, the system works less hard at lower temperature.
- The work-around, if a refrigerated load is required, is to use the same technique as a regular refrigerator. The freezer is maintained at low temperatures, and cold air is vented into the refrigerated portion of the box. Refrigerated temperatures can be obtained in a separate enclosure.
- Food distributors already used this mixed load configurations. A freezer box has an additional door at the front passenger side. An insulated divider is placed behind the frozen part, and air is circulated behind the partition to create a refrigerated section. This is common practice in the food distribution industry.
- The system is designed to allow for 24 hour operation.
- Although the system is designed for 24 hour operation, in practice, for one-man operation, the creature comforts of the driver (air conditioning, hot showers, creature comforts) are available at places where shore power is also available, and also greatly exceed the energy requirements of the freezer, so this point is moot except for multi-day "custom critical" applications, where two or more drivers take it in turns to drive and sleep in a sleeper cab. We recommend an additional engine-driven alternator for 100% reliable operation in this role.
- The system is designed to accept loads which are already at freezer temperatures. This has not been a limiting factor with Man the Van, as it has always been possible to have suppliers freeze the load prior to pick-up. A high temperature load causes the refrigeration system to run 100%, without cycling, until the load temperature is adjusted, which exceeds the energy budget. In practice, Van de Soleil has been able to adjust to higher-temperature loads, but there may be issues.