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Dr. Cool answers to the frequently asked questions about the FPSCs (Stirling Coolers) and Portable Refrigerators.

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Answers to the questions about FPSC

Q-01. How does the FPSC cool?

A-01. FSome people may be familiar with Stirling Engines but the Stirling Cooler is a less well known variant that also utilizes the Stirling thermodynamic cycle. Both Stirling Engines and Stirling Coolers fall under the classification of ‘Heat Engines'. When you add heat into the system, the Heat Engine produces power (either mechanical or electrical). Some better known types of heat engines are Internal Combustion (Otto Cycle), Diesel (Diesel Cycle) and Gas Turbine engines (Brayton Cycle). A Stirling Cooler is essentially a Stirling Engine, running backwards. Instead of forcing heat into the system and extracting power, as you do with a Stirling Engine, you are providing power to the engine (Stirling Cooler) and the engine (Stirling Cooler) is extracting heat from its environment. Because of this heat extraction effect, the heat acceptor of the Stirling gets colder and colder as more and more power is provided to the engine (Stirling Cooler) to pump the heat that is absorbed from the environment, away from the system. A FPSC is a Free-Piston Stirling Cooler. This type of Stirling is distinguished, from traditional kinematic Stirling coolers, by having no mechanical timing mechanism to define the phase relationship between the internal components. Instead, a FPSC relies on gas pressure and resonate tuning for control. In the simplest terms, the FPSC alternately compresses and expands a fixed amount of gas with a piston while controlling where the compression and expansion takes place. Heat exchanger assemblies separating the compression and expansion spaces provide for the creation of a thermal gradient which allow the FPSC to extract heat from the region surrounding the expansion space and reject heat to the region around the compression space. This process is repeated many times per second and can ultimately produce temperature differences, between the expansion and compression spaces, of several hundred degrees Celsius. By attaching external heat exchangers to the expansion (heat absorbing) side of the machine, heat can be moved from a remote location, by a pumped liquid, forced convection or a refrigerant, to the FPSC and then rejected to the environment.

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Q-02. What is the difference between a compressor type (evaporation-condensation type) and a FPSC?

A-02. A compressor based cooling systems consists of four main components: compressor, condenser, expansion valve and evaporator. The compressor acts to compress a gaseous refrigerant, which is then forced through the condenser where the heat picked up during the last pass through the evaporator and the heat generated by compressing the refrigerant is dumped to the ambient, cooling the refrigerant until it liquefies. The liquefied refrigerant then flows through the expansion valve and is allowed to expand back into a gas in the evaporator. As the refrigerant expands back into a gas its temperature drops and it can now absorb heat from the cooled space through the evaporator and return to the compressor to repeat the cycle. This process is temperature limited by the physical properties of the refrigerant. In order to achieve temperature differences greater than about 70°C requires the use of multiple refrigerants and often multiple compressors in what are called cascade or auto-cascade Rankine systems. These systems are notoriously unreliable, inefficient, expensive and noisy. The FPSC does not use what would be commonly called a refrigerant. In fact the FPSC can operate with virtually any compressible fluid (gas). We use Helium because it is inert, has good heat transfer characteristics and will not liquefy until about -269°C.
Because the FPSC does not rely on the eutectic properties of its working fluid to pump heat but instead relies on simple ideal gas laws, the temperature differential that can be created is theoretically unlimited. Additionally, without the necessity of pumping a refrigerant around a loop, a FPSC cooling system can be completely modular, greatly simplifying service and repair. And finally because the heat pumping capacity of the FPSC is a function of the compression ratio, by simply modulating the stroke of the piston the system capacity can be adjusted to exactly match the heat leak into the system. This means that the FPSC will only use the amount of energy absolutely required to maintain a set point temperature and the set point temperature can be maintained with much greater accuracy than a thermostatically controlled system.

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Q-03. Why is the FPSC environmentally-friendly?

A-03. As mentioned earlier, the working fluid in a FPSC is Helium and as a noble gas is, under realistic conditions, completely inert, with no GWP (Global Warming Potential) or ODP (Ozone Depletion Potential) at all. Additionally, the FPSC, like all Stirling machines has the theoretical potential to realize the greatest thermal efficiency possible.

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Answers to the questions about SC-C925 Portable Freezer Cooler

Q-01. What are the inner dimensions and capacity?

A-01. Length 335mm x Width 225mm x Height 340 mm. Inner capacity: approx. 25 liters.

* Click to see the drawings

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Q-02. How many PET bottles or cans can it hold?

A-02. The following quantity can be contained.

> PET bottles (1.5 liters/12 liters) ... 6
> PET bottles (500 ml) ... 25
> Cans (500 ml) ... 30
> Cans (350 ml) ... 37

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Q-03. How does the FPSC control the temperature?

A-03. The FPSC control electronics use the feedback signal from a thermistor (semi-conductor based, variable resistance, temperature sensor) to determine the temperature of the cooled space and dynamically adjusts the stroke of the FPSC piston, to modulate the thermodynamic capacity (how much heat it can move) of the FPSC, to match the heat leaking into the space, in order to maintain a constant set-point temperature. This control scheme varies greatly from that used by compressor based systems, which rely on a thermostat and compressor duty-cycle to maintain a set point temperature. This process subjects the system to high current, repetitive, start-stop loads and large temperature swings in the cooled space. Capacity modulation is not practical on most compressor based systems because they do not work well with high turn-down ratios (reduced flow rates) as the efficiency of the system falls dramatically as the flow rates drop much below the system optimum.

* Click to see the user's manual

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Q-04. How many hours does it take to cool down the drinks contained in the cooler?

A-04. It takes about 12 hours to cool 3 PET bottles of water (each 2 liters, in total 6 liters) to 6°C.
It is recommended to cool the drinks before putting them in the freezer/cooler when it is used in a vehicle.
When the inside of the freezer cooler is empty, the inner temperature will cool down from 30°C to 7°C within 30 minutes from supplying power.

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Q-05. How many hours does it take to make ice?

A-05. It takes about 6 hours to make ice with an ice cube tray (plastic) filled with 1 liter of water at 30°C. (FREEZE-2 mode, ambient temperature 20°C)
Please note that the time for making ice will differ depending on the amount of water.

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Q-06. How many hours does it keep the contained foods/drinks cool after the power is turned off?

A-06. It takes about 1 hour for the inner temperature to rise from -18°C to 0°C with the ambient temperature at 30°C and the inner space of the freezer/cooler empty. Please note that the time for the inner temperature to rise differs depending on the load contained in the freezer/cooler.

*In case of cooling
It is recommended to additionally use ice if the power is turned off for a long period of time.

*Please do not turn off the power during freezing.

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Q-07. Can I keep an ice cream?

A-07. Ice cream should be kept at or under -18°C. This freezer/cooler can be used for keeping ice cream by setting the inner temperature to -18°C (FREEZE 2 mode).

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Q-08. How far should the Cooler be kept from other objects?

A-08. We recommend keeping the air exhaust vent and the air intake screen of the freezer/cooler at least 15cm from all objects.
If the air exhaust vent and/or the air intake screen are hindered from ventilating, the freezer/cooler will not operate to its full cooling capacity.

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Q-09. Can it be installed on an uneven floor?

A-09. Yes, but the cooling capacity will be temporarily reduced if it is inclined. If it is reinstalled horizontally, operation will return to normal, full capacity. The temporary reduction in capacity is a result of reduced refrigerant flow in the CO2 thermosiphon, used to pull heat from the inner chamber.

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Q-10. Is its cooling performance higher than that of peltier type (thermoelectric cooling) refrigerator?

A-10. The FPSC (Free-Piston Stirling Cooler) integrated in this product has 5 times better cooling capacity than a peltier-type refrigerator.
As such, our product offers better cooling performance, higher reliability and significantly reduced energy consumption when compared to thermoelectric refrigerators.

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