Using seebeck effect to increase the intensity of cooling condenser of a small refrigerating machine



The article deals with the problem of using Seebeck effect in power the fan that cools down the condenser in small refrigerating machines. In small refrigerating machines of compression types with relatively small capacity we do not use the Seebeck effect to power the fan and at the same time for blowing and cooling the condenser of the refrigeration unit. The work shows the feasibility of its usage in small refrigerating machines, in particular household refrigerating appliances thermoelectric converters, implementing the Seebeck effect. Using heat on the surface of the compressor and chill the refrigerator compartment produces a voltage of sufficient power to power the fan for cooling the condenser so that the fan does not consume electricity from the network. It is shown that the use of thermoelectric converters and modern fans are designed for cooling personal computers, it is advisable to apply it in domestic refrigerators, as this operation will ensure efficient heat removal from the condenser, without the electricity spending from the grid.

Preliminary calculations revealed heat removal amount from the condenser of the refrigeration machine using modern fan and the thermoelectric Converter is advisable to perform a design of small refrigerating machines with a smaller condenser, but if it is equipped with a thermoelectric Converter to power the fan that cools the heat exchanger.

Key words: small refrigerating machine, household refrigerating device, a cooling condenser, fan, the thermoelectric Converter

Introduction

The object of research is the Seebeck effect in small refrigerating machines. We investigated the possibility of reducing specific energy consumption of a compression refrigerator by the use of thermoelectric converters and fan with high efficiency, which leads to an increase in the intensity of cooling of the refrigerant in the condenser.

The reduction of energy consumption of refrigeration equipment are constantly paid attention, and during its rapid development, and at the present time.

It is known [1] that to intensify the process of condensation of the refrigerant in several ways:

‒ increase surface area of heat exchange;

‒ to increase the coefficient of heat transfer from the condenser surface to the ambient air;

‒ the decrease in the total thermal resistance of heat transfer due to the reduction of its components (specific thermal resistance);

‒ the increase of the temperature difference in heat transfer (mean temperature difference), the use of evaporative cooling».

Along with all above said the classical methods of increasing the heat removal from the condenser to the cold are used, some of them are well-known study and other are «non-traditional» methods.

The heat transfer from the condenser surface can also be increased by increasing the velocity of the air near the surface of the capacitor, for example, using a fan and a thermoelectric transducer, or by using evaporative cooling surface condenser, [4,5], or using movable capacitor [6,7]. Interesting is the option of cooling the condenser and the compressor of the refrigeration unit simultaneously [8].

When you heat a solid surface with a gas medium in natural convective heat transfer, it is known that the heat transfer coefficient is usually not more than 20–80 W/m²∙grad. One of the commonly used methods of cooling the surface of the condenser is forced ventilation.

To increase the intensity of the condensation process of the refrigerant other methods are used, such as presented in the publication [9]. The use of fan for cooling the surface of the condenser is widely used in display coolers, which use compact compressors condensing unit with one or two fans, usually the capacity of these fridges is over 400 HP

Small refrigerators with relatively high refrigeration capacity was traditionally applied as method of cooling the condenser by natural convection, and is considered [1] to be the rational use of fan for cooling the condenser. However, studies [11],add this tradition with a new approach. It is proposed to use a thermoelectric Converter to produce electricity from the running compressor of the refrigeration unit. The use of Seebeck effect will enable the heat generated by compressor refrigerating machines to convert into electricity and the latter to be used for blowing the surface of the capacitor. Thus, the fan is not consumed electricity from the network, and allows to improve the heat removal from the surface of the capacitor. The heat exhausted from the compressor is usually dissipated into the environment, complementing the heat from the condenser. Typically, the capacity of the exhausted heat from the working refrigerating machine is more than power get cold. Great Park for refrigeration compression refrigeration equipment in the aggregate can have very large dissipation to the atmosphere.

Materials and methods

Consider the scheme of cooling of the condenser of the refrigeration machine a small fan which will be powered by a thermoelectric Converter can solve the problem of determining the amount of heat discharged from the surface condenser during its blowing surface of a fan.

Fan capacity refers to the volume of air passing through the fan per unit time. For example, cm³/min, or in units. Suppliers of fans for personal computers measure the performance of the fan in cubic feet per minute (Cubic Feet per minute, CFM). A characteristic feature of the fan is always specified by the manufacturer. (1 foot cubed equals 28 320 cm³ = 0,02832 м³, 1 ft³/min = 28320 cm³/min or 472 cm³/sec.)

The air flow generated by the fan determines how much heat you can take from the condenser per unit of time.

We denote the total capacity of the heat we have load on the condenser .

Theoretically, the magnitude of this heat load can be obtained from calorimetric calculation of refrigeration cycle as a specific design of the refrigerator.

Let us denote the temperature difference on the condenser surface and the ambient air —.

Let the air mass m, which is supplied to the surface condenser and later, is heated by ∆T in time t. Then he transferred quantity of heat will be:

(1)

where is the heat capacity of air at constant pressure. The dimension of the parameters: , , c.

The volume of air supply per unit time characterizes the fan performance , which should ensure the rate of heat dissipation from the condenser which is Q_t per unit of time.

Express the mass of air using its density and volume:

Then, in unit time will be given heat:

,(2)

,

.

Where flow rate of the air flow through the fan for discharging heat output .

Where

.(3)

For dimensions taken in the expression (1):

Or , or

For fans with the specified performance СFМ, it is necessary to consider that 1 m³/min = СFМ of 30.48 (cubic pounds per minute), or:

.(4)

Results

Substituting the results in the formula (3) the heat load on the condenser, temperature difference, density and specific heat of the air, we can calculate the capacity of the fan.

The actual performance of the fan for removal of thermal energy from the surface of the condenser Q_t must be greater than calculated due to partial scattering of the air flow when it is blowing on the surface of the capacitor. This dispersion can be considered as a design parameter as the coefficient of dispersion of blood flow. Depending on the shape of the air flow and the shape of the condenser the value of this coefficient may range from 0.5 to 0.9.

The actual performance of the fan:

(5)

Let's consider an example.

Let the power, the exhaust from the condenser 60 W, the temperature difference between the surface condenser and the surrounding air is 25 ⁰C.

For the approximate calculation accept density of air at t=25 ⁰C and a pressure equal to one atmosphere: C=1100,0 j/kg·deg, then the data given by formulas (3) and (4), assuming Kp =0.6, one will get:

(6)

For relatively accurate calculations the humidity and the pressure in the measurement of fan performance must take into account. For humidity of 60 %, air density is approximately equal to 0.95 kg/m³

Specific heat of air depends on air humidity. For dry air which is equal to .

Full heat capacity of moist air is the sum of the heat capacities of dry air and steam:

Specific heat capacity is usually referred to 1 kg of dry air:

, .

Then

.

where d is the moisture content of the air in kg/kg c. в.

Using the expression (6), perform estimating calculations linking heat load on the condenser and fan performance. The results of the calculation are given in table 1.

To indicate air flow in CFM dimensions, from the expression (6), we get:

.

Here [W] = ft cube /min.

Table 1

Example calculation of fan performance

Q, Wat							60
	20	30	40	20	30	40	20	30	40
W,	957	643	487	1950	1267	975	2925	1950	1462
W, CFM	0,034	0,022	0,017	0,068	0,044	0,034	0,103	0,068	0,052

As can be seen, in order to ensure heat removal from the surface of the condenser by the fan airflow, for given surface temperatures of the condenser and the ambient temperature, it is necessary to create an air flow which is directly proportional to the dissipated thermal power. For example, if the heat load on the condenser has a capacity of 30.0 W, ambient temperature 25 °C, the surface temperature of the condenser 45 °C, the diffusion flow К_р = 0.8, then the necessary airflow fan W must be equal to 861 cm³/min. Or for the cooler (fan for PC cooling) W = 0,030 СFM.

Discussion and conclusion

The airflow seems to have 861 cm³/min whichis not too much, so the airflow is able to provide even a relatively low powered fan. However, you must keep in mind that the airflow generated by the fan and the air flow blown on the surface of the condenser is not the same thing. If the fan is installed in the housing of the refrigerator or fan is included with condensing unit, its performance will differ from that specified in the technical documentation.

It is known that specified in the documentation, the fan capacity is calculated under ideal conditions, without resistance they create to the air flow. In real conditions the path of the air flow generated by the fan, there are always obstacles that reduce the volume of air pumped through the fan per unit time, and increase the difference between the pressure of the air flow generated by the fan, and the pressure in the environment (atmospheric pressure).

In the General case, we can assume that the static pressure of air flow from the fan is a function of the speed of the fan: This function is called the characteristic curve or the flow characteristics of the fan.

Here are some examples of fans that can be used to cool condensers in domestic refrigerators.

Name	Dimensions mm	Power, W	Speed rev/ min	Performance, CFM	Performance cm3 / min	Noise dB	Price rub.
KDE1204PFV2	40x40x10	1	5800	7		27	230
KDE1205PFV2	50x50x10	1,1	4300	11		26	370
KDE1209PTB1	92x92x25	1,8	2800	49		34	320
PMB1275PNB1.AY	75x75x30	3,6	3400	13,6		43,5	560
PMD1209PTB1.A(2)	92x92x25	5,5	4200	77		48	490

Thus, the calculations show that the heat removal from the condenser of the refrigeration machine with modern fan is advisable to perform a design of small refrigerating machines with a smaller condenser, but it must be equipped with a fan for cooling the condenser.

The use of heat from the compressor as additional power source, can solve two problems: to heat sink energy from the compressor and to convert this energy into electricity.

In this case, it is appropriate to use fans to cool the condenser and increase the intensity of cooling providing improved performance of the refrigeration cycle and, ultimately reducing the average energy consumption of the refrigeration machine.

We have also proposed to increase the power of the fan and its rational use to provide a refrigerator battery. This will allow to use low thermal load on the unit, to accumulate energy, and when we have increased load (e.g. when we are loading the refrigerator for Cabinet products) to use more effectively to intensive heat sink, thereby reducing the overall power consumption of the refrigerator.

References:

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