Measurement And Application of Performance Characteristics Of A Free Piston Stirling Cooler

Measurements were performed to characterize the performance of a Free Piston Stirling Cooler over a wide range of temperatures and heat lifts. The temperature range investigated was from –120°C to +5°C on the cold head side and from 30 to 60°C at the warm head, while heat lifts from 10 to 100W were evaluated. The publication discusses aspects of the experimental part of the investigation, including a description of a method to quantify thermal losses at the cold end side. Characteristic maps of measured system efficiency (COP) and input power were drawn as a function of the operating parameters. A regression model was applied to the experimental results, allowing calculation of the Stirling Cooler performance at any operating condition. Subsequently, the regression model has been used for a comparison study between Rankine and Stirling based refrigeration systems. By means of a case study on an upright domestic freezer, it is shown that the obtained Stirling performance characteristics are useful to predict the energy consumption of the final product. The case study also includes those aspects, which have to be taken into account in order to make a proper comparison between a Stirling and a vapor compression based refrigeration system, such as cold/warm side heat exchanger efficiencies. It is concluded that competing efficiency levels on products can be obtained with the Stirling Cooler. NOMENCLATURE Q Heat [W] T Temperature [°C] P Power [W] V Voltage [V] COP Coefficient of Performance [-] I Current [A] m& Massflow [kg/s] R Resistance [Ω] h Enthalpy [J/kg] L Coefficient of induction [H] UA Heat transfer coefficient [W/K] INTRODUCTION To properly compare the characteristics of a Stirling based refrigeration system with a Rankine based system (usually applied), first some practical differences between both systems are discussed. Hereafter the measurement system used to characterize the Stirling cooler is explained followed by some of the test results. These test results enable a theoretical comparison of Stirling and Rankine where an example is given for a domestic freezer. Finally the main findings are summarized. COMPARISON BETWEEN RANKINE AND STIRLING In the Rankine system refrigerant is transported by a compressor, which pumps the refrigerant from a low to a relatively high pressure. Heat is absorbed in the evaporator and rejected in the condenser. The expansion device reduces the relatively high pressure from the condenser to the relatively low evaporation pressure. Within the Stirling cooler heat is absorbed at the cold head due to an expansion process and rejected at the warm end due to a compression process. Within an application of this cooling system often additional heat exchangers are necessary to absorb or reject the heat as the surface areas of the heads are limited. For the compressor applied in a Rankine system the efficiency of the compressor is typically expressed by its coefficient of performance, which is calculated according to the following formula: (1) input cool rankine P Q COP = The COP of the compressor corresponds with the performance of the compressor at a specific operating point i.e. a specific condensation and evaporation temperature. Globally, different standards apply, e.g. ASHRAE and CECOMAF. These standards only represent the COP at exactly prescribed conditions. In the practical Rankine cycle the COP can be completely different. Therefore, the COP from the catalogue data cannot directly be related to the practical cooling system. For comparison with the Stirling COP it is recommended to calculate the actual cooling capacity on a Rankine system with the following formula (for stationary conditions): (2) ) ( inlet evaporator exit evaporator cool h h m Q − = & It needs mentioning that the massflow of the compressor can be derived from catalogue data for a certain condition (evaporation, condensation temperature). However, the enthalpy values, representing the inlet and exit conditions of the evaporator), are different for each application. For the Stirling cycle the efficiency of the cooler is also expressed with its COP. This COP is calculated according to the following formula: (3) input Stirling P Q COP = The COP of the Stirling cooler depends on the cold head temperature, warm head temperature and heat lift. In principle, one can compare the COP of the Rankine compressor with the COP of the Stirling cycle applying formula 2 and 3. However, one should take into account that generally Rankine refrigeration systems are controlled with a thermostat. This thermostat switches the compressor on or off, which differs from the free piston Stirling cooler, which operates in a continuously running mode. The on/off behavior in the Rankine system results in extra losses caused by the following effects: • Thermodynamic losses; the average condensation temperature is higher and the average evaporation temperature is lower with respect to a continuously running compressor with adjusted capacity. • Start/Stop losses; at the moment the compressor switches off, vapor from the condenser enters the evaporator. This vapor condenses in the evaporator yielding an extra heat load into the appliance. • At the start of the compressor the current of the compressor is relatively high with respect to a continuously running compressor. It needs mentioning that for variable speed compressors these losses are avoided. However, in some cases also these kind of compressors have to operate in an on/off mode, when low cooling capacities are demanded. For the Stirling cooler the amount of heat from the cold and warm head has to be transported through adequate heat exchangers. This transport yields the following losses: • Temperature losses at the cold and warm heads caused by the extra heat exchangers. In case of secondary fluids temperature losses of the fluids exist, while for a fin construction with a large surface, the fin efficiency will be lower than unity. • Pump losses if secondary fluids are used. It has to be noted that generally the complete construction of a refrigeration system changes, if a Stirling cooler is used instead of a normal Rankine compressor. This makes comparison of both cooling systems not a straightforward task. THE MEASUREMENT SYSTEM With respect to the measurements of the Stirling cooler the following details are mentioned (See figure 1). • The Stirling motor evaluated was of the type M100B, serial number 121. • The maximum voltage supply of the cooler was 12V (AC), at a frequency of 60 Hz. • The Stirling cooler was supplied without a (feed back) control unit (for normal use this unit is implemented). • The warm head of the motor was cooled with water of a controlled temperature, supplied by a pump. • An electrical heating element was placed on the cold head. The heater was operated by means of a DC Voltage supply allowing to set different heat lifts. • In order to minimize the heat flow from the ambient to the cold head, the cold head was insulated with Armaflex material. • The Stirling cooler was put in a climate chamber. Measurements were taken at 25°C ambient temperature. • Data from the tests was taken after a stable operating period of at least 0.5 hours. Motor shell M100B, serial number 121 Water inlet Water exit Water cooling Warm head regenerator Insulation Armaflex Motor supply Heat lift supply Electrical heater Cold head