Energy Efficiency Improvements in Household Refrigeration Cooling Systems

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Energy Efficiency Improvements in Household Refrigeration Cooling Systems

Doctoral Thesis By

Erik Björk

Division of Applied Thermodynamics and Refrigeration Department of Energy Technology

Royal Institute of Technology

Stockholm, Sweden 2012

Trita REFR Report 12/02 ISSN 1102‐0245

ISRN KTH/REFR/12/02‐SE ISBN 978‐91‐7501‐306‐0

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Abstract

This thesis is based on eight articles all related to the characteristics of the cooling system and plate evaporator of a household refrigerator. Through these articles, knowledge is provided that can be used to increase the operational efficiency in household refrigeration. Papers A, B and C focus on heat transfer and pressure drop in a commonly used free convection evaporator – the plate

evaporator. Applicable correlations are suggested on how to estimate the air side heat transfer, the refrigerant side pressure drop and the refrigerant side heat transfer. Papers D, E and F hold a unique experimental study of the refrigerant charge distribution in the cooling system at transient and steady state conditions. From this cyclic losses are identified and estimated and ways to overcome them are suggested. In paper G the topic “charging and throttling” is investigated in an unparalleled experimental study based on more than 600 data points at different quantities of charge and expansions device capacities. It results in recommendations on how to optimize the capillary tube length and the quantity of refrigerant charge. Finally, Paper H holds a thermographic study of the overall cooling system operating at transient conditions. Overall, a potential to lower the energy use by as much as 25 % was identified in the refrigerator studied. About 10 % was found on the

evaporator’s air side. 1‐2 % was identified as losses related to the edge effect of the evaporator plate. About 8 % was estimated to be cyclic losses. About 5 % was found in cycle length optimization.

It is believed that most of these findings are of general interest for the whole field of household refrigeration even though the results come from one type of refrigerator. Suggestions of simple means to reduce the losses without increasing the unit price are provided within the thesis.

Sammanfattning

Avhandlingen innehåller åtta artiklar som behandlar karaktäristiken hos kylsystemet och förångaren i ett hushållskylskåp. I artiklarna finns kunskap som syftar till att minska elenergiförbrukningen i kyl och frysskåp. Artiklarna A, B och C fokuserar på värmeöverföring och tryckfall i en vanligt

förekommande kylskåpsförångare. Användbara beräkningssamband föreslås vilket är viktiga redskap för den som vill utforma förångare med hög effektivitet. Artiklarna D, E och F fokuserar på

kylsystemets köldmediefördelning från vilka de cykliska förlusterna identifieras och uppskattas. I artikel G undersöks hur systemets energiförbrukning beror av systemets fyllnadsmängd och strypning. Resultatet är rekommendationer om hur dessa parametrar ska optimeras. Artikel H innehåller en termografisk studie av kylsystemet under cyklisk drift. Totalt identifierades en potential att minska energiförbrukningen med motsvarande ca 25 % i det undersökta kylskåpet. Omkring 10 % återfanns på förångarens luftsida. 1‐2 % var kantförluster på kylskåpsförångaren. Omkring 8 % var cykliska förluster. Ca 5 % lägre energiförbrukning erhölls med kortare cykellängd. Resultaten har ett allmänt intresse för hela kyl‐ och frysskåpsbranschen även om resultaten kommer från studier av ett kylskåp. Avhandlingen innehåller förslag på enkla åtgärder som syftar till att minska de identifierade förlusterna utan att öka kylskåpets pris.

Keywords: Household refrigerator, Domestic refrigerator, Free convection, Plate evaporator, Heat transfer, Flow boiling, Isobutane, R600a, Pressure drop, Two‐phase, Cooling system, Charge inventory, Cyclic losses, Thermography.

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Acknowledgement

This research project has been financed by the Swedish National Energy Agency (Energimyndigheten) and Electrolux AB through the research programs Klimat 21 (1998‐2001) and EffSys (2001‐2004), both focusing on energy systems based on heat pumping technologies.

Special thanks are directed to my supervisor at KTH, Professor Dr Björn Palm, who patiently have been awaiting this thesis to be finalized and to Professor emeritus Eric Granryd for thorough manuscript review and warm support.

Special thanks are also directed to Benny Andersson for experimental help and scientific discussions.

Benny was employed for a long time at the Electrolux R&D household refrigeration department and had very valuable hands‐on experience within the field.

From Electrolux AB, Dr Per Wennerström appeared as supervisor. Klas Andersson and Leif Strindberg played important roles at the project start‐up.

Many persons have provided experimental help, interesting scientific discussions and other support during this project. Among them are Joachim Claesson, Peter Hill, Benny Sjöberg, Karl‐Åke Lundin, Per Lundqvist, Jan‐Erik Nowacki, Richard Furberg, Jaime Arias, Hans Havtun, Rahmatollah

Khodabandeh, Åke Melinder, Inga Du Rietz, Nabil Kassem, Primal Fernando, Sanheeva Witharana, Wilmosiri Pridasawas, Yang Chen, Monika Ignatowicz, Raul Anton, Claudi Calizzo, Jörgen Wallin, Getachew Bekele, Ehsan Haghigi, Cecilia Fransson, Anders Johansson, Hatef Madani, Mumayun Maqbool, Gunne Eriksson, Fredrik Lagergren, Martin Forsén, Paul Westin, Samer Sawalha, Oxana Samoteeva, Wahib Owhaib, Rashid Ali, Aleh Kliatsko, Marino Grozdek, Arrie Setiawan, Arturo Carrera, Johan Nordenberg, Anders Herolf, Anders Nilsson, Alexandre Rücker, Aumnad Phdungsilp, Elin Isgren, Shota Nozadze, Dimitra Sakellari and perhaps someone else that I have forgotten.

Finally, to my wife Ewa and my children Klara and Åke; You are the best!

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Thesis outline

This doctoral thesis is outlined as a compilation thesis (in Swedish sammanläggningsavhandling) which means that a number of published scientific articles are attached to an introduction part (in Swedish kappa). The purpose of the introduction part is to present the research problems, goals, methods and results and to show the relevance of the attached articles to the completeness of the thesis.

The introduction part is subdivided into 5 chapters. The first chapter introduces the reader to the subject of household refrigeration and the race for energy efficiency. It also holds the aim and scope of the project. In chapter 2 the case study refrigerator, which is used for most of the experiments, is described. Chapter 3 is a summary of the thesis’s eight scientific articles. This chapter is further subdivided into part I, which deals with heat transfer and pressure drop of the evaporator studied, and part II, which deals with the overall cooling system behaviour. Chapter 4 summarizes other work carried out within the research project relevant to the thesis. In chapter 5, the overall thesis results are summarized together with some conclusions that can be used to make tomorrow’s refrigerators and freezers less energy consuming. Finally, in the appendix the articles on which this thesis is based are collected.

Thesis articles

The eight articles listed on the next page can be found in full text in the thesis appendix.

Papers A to C are devoted to the characteristics of a household refrigerator evaporator. More specifically: Paper A concerns the air side heat transfer. Paper B concerns the refrigerant side heat transfer. Paper C concerns the refrigerant side pressure drop. The results are given in the form of recommended correlations to calculate heat transfer and pressure drop.

Papers D to H focus on the cooling system characteristics. Paper D describes an experimental method on how to measure the refrigerant charge in different parts of the cooling system. This method was developed to obtain the experimental results used in papers E and F. In paper E the refrigerant charge distribution was measured at transient conditions. In paper F it was measured at steady state conditions. Paper G is a parametric study of the cooling system at varied throttling and charging conditions. Finally, Paper H gives an overall view of the cooling system characteristics as seen through an infra‐red camera.

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A. Björk E., Setiawan T., S., Palm B., 2003, Air side heat transfer of a domestic refrigerator plate‐type evaporator. Presented at The Eurotherm Seminar No 72, Valencia, Spain B. Björk E., Palm B., 2008, Flow boiling heat transfer at low flux conditions in a domestic

refrigerator evaporator. International Journal of Refrigeration 31, pp. 1021‐1032

C. Björk E., 2002, Pressure drop in a plate evaporator for refrigerators. Proc. 1st International conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT), Kruger National Park, South Africa

D. Björk E., 2005, A simple technique for refrigerant mass measurement, Applied Thermal Engineering 25, pp. 1115–1125.

E. Björk E., Palm B., 2006a, Refrigerant mass charge distribution in a domestic refrigerator. Part I. Transient conditions, Applied Thermal Engineering 26, pp. 829‐837

F. Björk E., Palm B., 2006b, Refrigerant mass charge distribution in a domestic refrigerator. Part II. Steady state conditions, Applied Thermal Engineering 26, pp. 866‐871

G. Björk E., Palm B., 2006c, Performance of a domestic refrigerator under influence of varied expansion device capacity, refrigerant charge and ambient temperature. International Journal of Refrigeration 29, pp. 789‐798

H. Björk E., Palm B., 2010, A thermographic study of the on‐off behaviour of an all‐refrigerator, Applied Thermal Engineering 30, pp 1974‐1984.

The major work in all of these papers were made by the first author except in paper H which is based on a master thesis work by Mr Johan Nordenberg (Nordenberg, 2001). In paper A experimental help was provided by Mr Arrie Setiawan. In paper G experimental help was provided by Mr Gunne Eriksson. Dr Björn Palm appears as co‐author in papers A, B, E, F, G and H following the tradition of acknowledging the role of the supervisor. His contribution to the papers was mainly manuscript review.

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1. Introduction

Background

Household refrigerators and freezers are found in almost every home in the industrialized parts of the world and in increasingly larger number elsewhere. It is estimated that the global annual

production is more than 90 million units (Harrington, 2009). In Asia the annual production is about 45 million units with China alone accounting for more than 30 million units. Production in Europe is around 25 million units and North and South America is about 20 million units a year.

With an expected lifetime of 10 to 20 years the stock of household refrigerators and freezers operating in this moment is more than one billion units. Evidently this gives a significant impact on the global energy consumption. In Sweden household refrigeration is the second largest consumer of electricity in the average household (Energiläget, 2009). The department of energy in the U.S.A.

estimated that household refrigeration is responsible for 7.2 % of the average household energy consumption (Bansal et al., 2011). Melo and Silva estimated that about 6 % of the produced electrical energy is used by household refrigerators and freezers worldwide (Negrao and Hermes, 2011). Thus, it is not surprising that household refrigeration is a target for energy consumption controls in the EU and in many countries around the world.

Internationally, there are about 60 countries world wide that have some sort of program to regulate the energy efficiency of refrigerators and freezers, mostly in the form of mandatory comparative energy labelling and Minimum Energy Performance Standards (MEPS). These programs have proven to be an effective tool to reduce the energy consumption (Mahlia and Saidur, 2010). Within Europe, the European commission directive of 1994 (94/2/EC) made it compulsory to energy label household refrigerators and freezers. The objective with this was to encourage consumers to favour appliances and equipment with high electrical efficiency, thus encouraging the producers to improve the efficiency of their appliances. Furthermore, in the directive of 1996 (96/57/EC) on energy efficiency requirements, the most energy consuming units were banned. For the producers the message was clear: energy efficiency is important!

Two technologies are often discussed as tools to significantly increase energy efficiency in the next generation household refrigeration units; vacuum insulation panel and variable speed compressors.

However, among other technical barriers such as uncertainties about performance and reliability over a typical life expectancy of approximately 20 years (Bansal et. al., 2011), these techniques are expensive which have so far prevented a wide introduction on the market. Even though energy consumption is an important factor for the consumer, the first cost, as seen on the price tag is often the most important.

Therefore it is interesting to explore other low cost solutions to lower the energy consumption. Of course, if such solutions can increase the operational efficiency without increasing the unit price it is even better. Moreover, such solutions can in most cases be combined with variable speed

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compressor or vacuum insulation panels, which might be more common in the future, to achieve an even higher efficiency.

From the second law of thermodynamics it is known that the efficiency of a heat pump system, which is the key technology used in household refrigeration, depends on the temperature levels at the cold side evaporator and the warm side condenser. A higher evaporation temperature or a lower condensation temperature gives higher system efficiency. In other words, a smaller temperature lift from the cold to the warm side increases the system efficiency. This also means that the efficiency of the heat exchangers, in terms of operating with small temperature differences, is important for the overall efficiency. The governing equation for the ideal Carnot cycle operating as a cooling machine is:

2 1

2

T T COP_Carnot T

= − (1)

Where T is temperatures in Kelvin and 1 and 2 denotes the temperatures at the high and low temperature side.

COP stands for Coefficient of Performance which, for a cooling machine, is defined as the ratio between the useful cooling energy and the needed work:

work energy cooling

COP = ₍₂₎

Obviously a high COP is desirable. With a typical refrigerator storing temperature of 5°C and an ambient temperature of 25°C the COP ideally becomes 13.9 (Eq. 1). This would however require that the heat exchangers (evaporator and condenser) are at the same temperatures as their surroundings, which only could be achieved with heat exchangers having infinite surface areas. In practice, cost and size limitations give heat exchangers that must operate with a temperature difference to their surroundings, and thus the system must operate with a larger temperature lift compared to the ideal case.

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Figure 1 The refrigeration cycle plotted in a h-log(p)-diagram. The heat exchanger pressure drops are illustrated through the inclined evaporator and condenser temperatures (somewhat exaggerated in the figure). Temperatures are at typical levels for a household

refrigerator.

With an evaporation temperature of ‐15°C (delta T 20 K to the surrounding air on the evaporator side!) and a condensation temperature of 40°C (delta T 15 K on the condenser side!), which can be taken as normal values, the COP ideally becomes 4.7¹ (Eq. 1). It is seen that the temperatures play a large role for the system efficiency and thus that the heat exchanger efficiency, in terms of having small temperature differences to their surroundings, is important. For instance, from equation 1 it can be calculated that for a household refrigerator the efficiency increases about 2.2 % for each degree higher evaporation temperature². Using typical freezer temperatures (40°C/‐25°C) the increase of the COP for an ideal refrigerant cycle is about 2.1 % per degree increased evaporation temperature. It is clear that any improvement to the evaporator that will decrease the temperature lift will give higher system efficiency.

It is also known that capacity control by intermittent run (on‐off cycling) lowers the overall efficiency.

These so called cyclic losses can partly, but not fully, be explained with the temperature losses that was discussed in the previous paragraph. For example, at start‐up refrigerant is redistributed over the heat exchangers during which the capacity and efficiency is lowered. This effect, caused by

(‐15°C) (5°C) (25°C) (40°C)

Surrounding temperature

Ideal temperature lift

Actual temperature lift made by compressor Pressure (temperatures

in two phase region)

Enthalpy Cabinet air temperature

Temp.diff. from heat resistance at air side

Temp.diff. from heat resistance at refrigerant side and pressure drop

1 It is interesting to note that one relation (Granryd et al., 1999) indicates that the real COP is about 40 % of the ideal “Carnot”

COP for a small 1 kW cooling system. If this number is used in the example given the COP for the real refrigerator should be 40 % of 4.7, or about 1.9. This can be compared to normal values of the COP in household refrigerators of about 1-3.

2 In fact, inspection of compressor data indicates an even higher increase of 3 %/K in some cases. Therefore, in the following, 2- 3 %/K will be used as the estimated variation in efficiency with varied evaporation temperature

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improperly charged heat exchangers, operating with superheat and subcooling, means increased temperature lift and thus lower system efficiency. An example of another kind of loss, not related to temperature difference, is after the compressor shuts down, when the system pressure equalizes and refrigerant flows from the condenser to the evaporator. This adds latent heat to the refrigerated space.

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Aim and scope

The purpose of this work is to provide knowledge that can be used to increase the operational efficiency in household refrigerators and freezers.

The focus has been the characteristics of a free convection plate evaporator and of the cooling system of a household refrigerator. For the evaporator, the concern was to find applicable correlations to predict heat transfer and pressure drop; internally in the refrigerant tube and externally on the air side. For the cooling system a better understanding in general was sought with some key questions in mind.

Research questions

The following research questions can be formulated:

What relationships should be used to predict

‐ The air side heat transfer of a typical free convection plate evaporator?

‐ The refrigerant side heat transfer?

‐ The refrigerant side pressure drop?

These relations are important when designing highly efficient cooling systems. As was already mentioned in the first chapter the thermodynamic laws state that the

temperature lift from the cold side evaporator to the warm side condenser should be as small as possible. It has been found that the efficiency increases by 2‐3 % when the evaporation temperature increases 1°C. It follows that the temperature difference between the storage volume in the household refrigerator and the evaporation temperature should be as small as possible.

How is the refrigerant charge distributed in the cooling system at transient and steady state conditions?

The knowledge from these questions is essential to estimate and understand the cyclic losses and to find means to reduce them. It is also expected that the results would contribute to a deeper understanding of the cooling system behaviour.

How does the efficiency depend on the quantity of charge and the expansion device capacity?

This is a classical topic within household refrigeration. It is a common belief that an optimum exists, resulting in the lowest energy consumption for a certain combination of charge and capillary tube capacity. The result may tell if there exist a large potential for energy saving just by finding another combination between quantity of charge and capillary tube length.

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Research methodology

The results of this thesis are mainly based on experimental work. Pure simulations using state of the art tools (FEMLAB) were also used, but to a lesser extent.

At an early point in the research project it was decided to use one type of household refrigerator as a case study. Most experiments were therefore conducted on this refrigerator, more or less modified for the experiments. The Electrolux refrigerator ER8893C can be described as a free convection, cycle defrost, on‐off controlled, single‐door, upright, all‐refrigerator. In short this means that no fans are used at the heat exchangers, that defrost occurs when the compressor is at rest without additional heating and that temperature is controlled by the compressor simply switching on and off. The reason for selecting this refrigerator as the case study test object was that it was a common product on the Swedish and European market. Obviously such decision is always open for criticism. One benefit of having the same type of test object for different experiments is that comparison between tests is possible. It should also be mentioned that the difference between the cooling systems in this refrigerator and the cooling system in other refrigerators or freezers (or combinations) in reality is small. Typically a freezer has lower evaporation temperature and higher capacity than a pure refrigerator. Different types of heat exchangers are also used (forced convection and free

convection). Therefore, in many cases it is possible to draw general conclusions from the case study results that are applicable for the whole field of household refrigeration.

Different experimental studies were conducted; flow visualization of the refrigerant flow through an observation glass evaporator, thermo‐graphic study of the overall cooling system including the heat exchangers, heat transfer and pressure drop measurements in the evaporator, heat transfer measurements at the evaporator air side, charge inventory at different parts of the cooling system, parametric study of varied refrigerant charge and expansion device capacity. In one case a separate experimental setup with an isolated test section was used. This was to measure the refrigerant side heat transfer which called for a more controlled environment.

The most important results of the various experiments were published in reviewed journal articles and conferences relevant to the research field.

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The need for refrigerated space

It is estimated that one third of all perishable food is lost in one way or another (Lorentzen, 1978).

Although other food preservation technologies exist, such as ionising radiation, modified

atmosphere, chemical preservatives, freeze drying, high hydrostatic pressure etc. (Zhou et al., 2010) refrigeration and freezing is probably the technique causing the least change from the fresh state in terms of nutrition and taste.

Moreover, a fast growing population, urbanisation, a need to even out seasonal variations in production and sales, and a food industry that needs to produce in locations distant from the consumers are all factors that create a large demand for refrigerated storage space. Hence, refrigeration as a food preservation technology has become an important industry over the last century.

It is a well known fact that almost all processes in nature run more slowly at lower temperatures.

This knowledge is widely used within food preservation.

Figure 2 shows the result of a test panel who has judged the acceptable storing time for different kinds of food at different temperatures. It is seen that lower temperatures give longer storing times.

Time (days)

Temperature (° C)

1: Chicken a) good packing b) bad packing c) cut and fried

2: Fat fish 3: Lean fish 4: Beef 5: Orange

6: Apple a) normal storage b) storage in CO2 atmosphere 7: Egg 8: Banana 9: Peas 10: Raspberry 11: Strawberry

Figure 2 The time of storage for different kind of food until the first sign of deterioration is detected in taste or quality. Adopted from Granryd et al. (1999).

As a rule of thumb, the time for a certain chemical process doubles if the temperature is decreased by 10 °C. Looking in the diagram, it is seen that this rule of thumb holds as a rough estimation of the acceptable storing time for all the listed foods except for Raspberry and Strawberry.

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In a more detailed view the processes responsible for the gradual deterioration can be grouped in four different processes, which are all affected by the temperature (Lorentzen, 1978).

1. Metabolic processes that slow down with lower temperature and stop completely when the tissue is frozen.

2. Physical processes which follow precise laws. Surface drying is proportional to the vapour pressure difference between the surface, whose pressure follows the saturation pressure curve, and the vapour pressure in the atmosphere above the surface, whose pressure also depends on the relative humidity in the air. With lower temperatures, this difference is bound to be smaller and thus less surface drying occurs. Freezing of tissues can in some cases be detrimental since the cell walls in the frozen tissue can be damaged by the ice crystals that are formed in the freezing process. It has been found that the ice crystals are smaller in a more rapid freezing process compared to a slower one. Therefore one should strive for a rapid freezing process. Another effect upon freezing is that concentration of salts may appear in the remaining, unfrozen, liquid, which lowers the freezing point at these locations.

3. The chemical processes approximately follow exponential temperature dependence.

However, factors such as availability of oxygen and enzymes intervene.

4. The microbiological processes are diverse and depend besides temperature of the sanitary conditions and the humidity. Each of the thousand strains of bacteria has its own growth curve, having its own activity maximum and a lower temperature threshold where activity is strongly reduced. The human pathogens have their activity maximum at about 310K (37 °C) and their lower threshold at 273K. At typical freezing temperature (255K) they are

completely inactivated, but will activate as soon as the temperature rises above the lower threshold.

In living products such as fruit, vegetable, cheese and egg the biological process (1) dominates. For non‐living but unfrozen products (such as chilled beef, fish or chicken), the microbiological processes (4) dominate. In the case of frozen products (living or dead) at lower temperatures than about 263K the deterioration is purely caused by physical (2) and chemical (3) processes.

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Historical remarks

Prior to the invention of the household refrigerator snow and ice were used to refrigerate and preserve food. Historically, ancient cultures (Chinese, Hebrews, Greeks, Romans, and Persians) stored snow and ice throughout the year in different types of ice houses or covered wells. The ice was harvested from winter lakes or was brought down from the mountains. Later, ice boxes were used in homes in which the ice were stored to keep the internal box cold. This highly work intensive transport of thermal energy existed at least into the 1950s when it was outclassed by the household refrigerator. In 1958, 94 % of the U.S households owned refrigerators (Radermacher and Kim, 1996).

Today almost all modern homes have household refrigerators, typically powered by electricity and operating by the vapour compression cycle.

Figure 3 Ice harvesting in Spy Pond in the 19th century (from Wikipedia public domain)

In 1834, Jacob Perkin developed the first vapour compression operated refrigeration machine, using ether as refrigerant. However, a number of developments were still needed to make the refrigerator a standard appliance in the normal household. The wiring of homes with electricity, the development of smaller electrical motors, the hermetically sealed unit that eliminated the belt, the halogenated hydrocarbon used as refrigerants that made it possible to use a simple capillary tube as expansion device and eliminated the risk of explosions, poisoning and unpleasant smells, are some of the important steps taken (Radermacher and Kim, 1996).

In 1974 it was found (Molina and Rowland, 1974) that the chlorinated refrigerant used up to then, R12 (also named Freon 12), accumulated in the stratosphere where it damaged the ozone layer that shields the earth from cancer causing ultra violet solar radiation. Another halogenated refrigerant HFC‐R134a was therefore introduced, having similar properties as R12. This refrigerant is the preferred choice today in the U.S, and some other countries of the world as it is a non‐flammable

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refrigerant. However, it has also been found that this refrigerant contributes to the global warming and therefore another refrigerant is the preferred choice in Europe and in many other countries. This refrigerant, Isobutane (HC‐R600a), is a non‐halogenated hydrocarbon refrigerant. To handle the flammability of this refrigerant, safety regulations stipulate how the refrigerator should be designed to avoid fire, including leak protected cooling systems and spark free electronics (Gigiel, 2004).

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Configurations in Household Refrigeration

Household refrigerators & freezers are thermally insulated compartments in which food can be stored at reduced temperatures hereby extending the shelf life. The refrigerator (or fridge) has a storing temperature above 0 °C (typically 0 to 10 °C) making it suitable for fresh food and vegetables.

The freezer has a temperature below 0 °C (typically ‐6 to ‐18 °C) making it suitable for frozen food and longer storing times. Normally, there also exist special‐purpose compartments within the refrigeration unit to provide a more suitable environment for storage of specific food. For example, a warmer compartment for maintaining butter is often found in the refrigerator door. A high‐humidity compartment for vegetables and fresh food are also common in a refrigerator.

Refrigerators and freezers are available in several styles. All‐freezers can be found as upright freezer or as chest freezer. Combinations of refrigerators and freezers can be found as top‐freezers, bottom‐

freezers, side‐by‐side or as a separate freezer compartment located within the larger refrigerator compartment. All‐refrigerators, which the case study in this work is an example of, are typically upright³.

The configurations vary considerably by region, but at a global level, top freezers are the most common (nearly 40 %), bottom freezers are next at about 33 % and side‐by‐side combinations are about 13 %. The remaining types are mostly all‐refrigerators or other configurations including separate freezer compartments (Harrington, 2009).

Qualities that are desirable in a good cabinet are, according to the ASHRAE handbook (2010):

1. Maximum food‐storage volume for the floor area occupied by the cabinet 2. The best in utility, performance, convenience, and reliability

3. Minimum heat gain

4. Minimum cost to the consumer

Other ways to classify the household refrigeration units is how heat is transferred at the heat exchangers. The difference between natural convection and forced convection is that no fan is used in the first case. Sometimes natural convection heat exchangers are referred to as “static” or

“passive”. The opposite, forced convection heat exchangers, are sometimes referred to as “dynamic”

or “active”.

The way to defrost is either automatic or manual. In automatic defrosting one can separate cycle defrost (where defrosting occurs in the off‐cycle) from the heater defrost (where a heater is

activated during defrost). The cycle defrost is only possible in an on‐off cycling refrigerator where the cabinet air temperature is higher than 0 °C. In manual defrosting the defrosting must actively be started, for instance by switching on an electrical heater or by turning off the cooling system. The latter is typically used for natural convection freezers.

3 Even though chest freezers converted into all-refrigerators exist having very low energy consumption.

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One can also separate the way to control the capacity. In on‐off cycling the compressor is switched on and off with the relative on‐cycle being longer with increasing capacities. This is the dominating technique to control capacity in household refrigeration. In variable speed capacity control the compressor is varying its capacity through speed‐modulation. However, normally in combination with on‐off cycling since it is difficult to achieve a sufficient reduction of the compressor speed to perfectly match the heat load.

The basic refrigerator

The cabinet and the cooling system are the main components of a household refrigerator/freezer.

Today, almost universally, polyurethane foam is used as insulation material to minimize the thermal leakage. In Europe, cyclopentane is the favoured blowing agent⁴ which gives a typical thermal conductivity of about 0.02 W/m K. The foam is either expanded directly into the insulation space between the plastic inner liner and the steel outer shell, which gives a rigid sandwich construction, or is used to build slabs that are mounted together to form a cabinet.

The cooling system typically operates by the vapour compression principle. In Figure 4 it is seen why.

The Coefficient of Performance (COP) is higher than other available techniques. It should be noted, however, that the absorption technique can also be directly operated by heat instead of electricity which in certain applications is a benefit for this technique.

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Vapour

compression

COP ≈ 1‐3

Absorption

COP ≈ 0.2‐0.3

Thermoelectric

COP ≈ 0.1‐0.2

Figure 4 Different cooling principles and their typical efficiencies (ASHRAE handbook, 2010 and Granryd et al., 1999). COP (Coefficient of Performance) is a quality number defined as

the ratio between the useful cooling energy and the supplied work.

In vapour compression the fixed relation between saturated vapour pressure and temperature (the vapour pressure curve) is used to create two thermal conditions; one at a high temperature where heat is rejected and one at a low temperature where heat is absorbed. By combining a compressor with an expansion device to separate a high pressure side condenser (from which heat is rejected while refrigerant is condensing) and a low pressure side evaporator (into which heat is absorbed while refrigerant is evaporating) a heat pump is formed. At the price of mechanical work, thermal energy is transferred from a lower to a higher temperature. The cooling system, used in household refrigeration, is basically a heat pump that absorbs thermal energy from the cabinet’s inside and rejects it to the outside to maintain a climate at reduced temperature.

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compressor

evaporator cap. tube

condenser

filter drier

compressor

evaporator cap. tube

condenser

filter drier

+ +

-

Figure 5 Main components (left) and a schematic view (right) of a typical household refrigerator cooling system. The arrows show the path of the working media (refrigerant)

between the different components.

Figure 5 shows the basic components used in a vapour compression refrigerator in a “component‐

depicted” and schematic view.

• The hermetic compressor is normally reciprocating with an induction split‐phase motor. It is capsulated in a robust steel shell to stand high pressure and to minimize noise. It is mounted on the bottom of the cabinet on rubber feet to further lower the noise level. The typical capacity ranges between 50 and 250 W.

• The steel pipe condenser is designed to stand high pressures. Into this the compressed refrigerant gas is pumped and brought to condensation. While doing so heat is rejected. The condenser seen in Figure 5 is a wire on tube condenser. Other common condenser types are plate on tube and various forced convection fin packages.

• After the condenser the filter drier follows. This device prolongs the system’s lifetime by filtering out particles and by absorbing water that may remain in the cooling system after manufacturing. The drier is filled with small Zeolite pellets, each with a porous, molecular sieve surface.

• Next, the capillary tube follows. This is basically a narrow pipe section through which the throttling or expansion occurs. The thin copper tube (d = 0.33‐1.5 mm, L = 2‐5 m) restricts

Suction line heat exchanger

-

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sweating (water condensation on the suction line surface close to the compressor) and to increase the overall efficiency.

• Finally, the evaporator follows. This is made of aluminium to prevent corrosion. In this, refrigerant is vaporized while heat is absorbed from the internal cabinet. An accumulator is located at the evaporator outlet, either as an integrated part of the evaporator or as an external device. To complete the cycle, the refrigerant vapour is being sucked back to the compressor where it is compressed. As was mentioned in the previous point, this suction line runs in parallel with the capillary tube to form a suction line heat exchanger. Other types of evaporators are wire on tube (freezers) and various fin packages (forced convection).

• Not shown in the Figures is the refrigerant. Today, following the phase out of Ozone depleting refrigerants, the refrigerants used are R134a and Isobutane (R600a). A typical quantity of charge is 20‐200 g.

Overall, one can summarise that the cooling systems in household refrigeration are characterized by low cooling capacities (50‐250 W), low quantities of refrigerant charge (20‐200 g), a refrigerant accumulator located at the evaporator outlet, a hermetically sealed cooling system, a capillary tube expansion device which is in heat exchange with the suction line, and a cooling capacity typically controlled by intermittent run (on‐off cycling).

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Historical remarks about energy use reduction

When refrigerators and freezers became common in the average household the need to lower their energy consumption was brought on the agenda. Different factors have contributed to the decreased energy consumption that is seen in household refrigeration since the 1970’s. The 1974 energy crisis highlighted the energy conservation topic in general. The following increase in energy prices

motivated the consumer to buy low energy products. Moreover, legislation has obliged producers to declare the energy consumption of their products measured at standardized conditions. This made it much easier for the end user to compare energy consumption before buying a product. Additionally, different energy programs have pushed the most energy consuming units out of the market.

Examples of energy declarations are seen in Figure 6.

Figure 6 Examples of energy declaration in Europe and USA

Rosenfeld (1999) showed that the average energy use in U.S. refrigerators increased from less than 400 to 1800 kWh/year from 1947 to 1974. In the same period the internal volume grew from 8 to 18 cubic feet (226 to 510 l). After 1974 the average volume has only slightly increased to 20 cubic feet

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Figure 7 shows the energy consumption of household refrigerators, freezers and combinations (refrigerator‐freezer) in Sweden 1980‐2000. As can be seen, the energy reduction is significant over the period. For the refrigerators the reduction is more than 30 %. For the freezers and combinations the reduction is about 50 %. After the year 2000, the trend has continued. In 2012 the average energy consumption of a 250 l product was about 0.45 kWh/24h (refrigerators), 0.81 kWh/24h (combinations) and 0.80 kWh/24h (freezers).

Figure 7 Average energy consumption on the Swedish market for a 250 l product. Data from the Swedish Consumer Agency.

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Figure 8 shows some of the most important modifications that have been used to lower energy consumption during the last decades. The modifications are primarily taken from Electrolux AB products and do not fully represent a worldwide view of the subject.

The most significant reduction was achieved by better and thicker insulation material. During the 1960´s, the insulation material was changed from mineral wool and cork to polyurethane foam. This change lowered the heat leakage by around 50 %. In addition, the general wall thickness has

increased from around 25 mm to 40 mm thus lowering the heat leakage even more.

+

-

More material in windings and stator, run capacitor

Larger heat- exchangers

Milk

Plastic muffler

el.motor

Semi-direct intake

Thicker and more effective insulation

Figure 8 Some of the modifications that have increased the energy efficiency during the last decades.

The physical size of the heat‐exchangers has typically increased over the years. By doing this, the temperature lift is decreased thus increasing the system efficiency. However, many refrigerators sold today contain foamed‐in evaporators, which from an energy point of view is a poor solution.

The compressor efficiency has been increased by different means. The electrical efficiency (indicating how good the electrical energy is transformed into mechanical work) has been increased by more material in the windings (copper) and in the stator (steel). The cylinder inlet gas temperature has been reduced by semi‐direct intake and a plastic muffler. This increases the compressor isentropic efficiency (indicating how ideal the compression is). A run capacitor is often added to permit the compressor start winding to give a helping torque even at running conditions.

As was already mentioned in the first part of the introduction, techniques exist to further reduce the energy consumption, but the increased cost has so far prevented a wider break through. A shift from polyurethane foam into vacuum panels as wall insulation could reduce the heat leakage to half and

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2. The refrigerator used in the case study

ER8893C is a single compartment upright household all‐refrigerator. The declared energy consumption is 0.68 kWh/24h (energy class B). It has the following typical data (Small variations occurred with different specimens used in the experiments. Consult the various papers for a more detailed description):

 Cabinet: (External dimensions: 1.75^×0.6^×0.6 m and 0.04 m wall thickness), 350 l internal volume, UA value 2.3 W/K.

 Evaporator: free convection, (0.66^×0.49^×0.0014 m), aluminium, plate type, back wall located (20‐25 mm distance to back wall), integrated downstream located accumulator.

Refrigerant line length (including accumulator) 6.02 m. Internal hydraulic diameter 3.2 mm.

Total internal volume 114 ml whereof accumulator volume 46 ml in which approximately half the volume can store liquid at steady state condition. UA value about 3.7 W/K.

 Condenser: free convection, (1.33^× 0.51^× 0.008 m), steel, wire on tube (53 vertical wires on each side of the tubing, each of diameter 1.5 mm) positioned with 25 mm distance to the cabinet back wall. The refrigerant flow is horizontally downward (see Figure 9). Internal volume 135 ml. Internal/external tube diameter 3.5/5.0 mm. UA value about 7.7 W/K (condenser in original location and cabinet located against a wall)

 Capillary tube expansion device (2.54 m length and 0.60 mm internal diameter) with coaxial type suction line heat exchanger of 2 m length. The capillary tube adiabatic inlet and outlet sections are 0.5 and 0.04 m.

 Filter drier: molecular sieve with internal free volume 11.3 ml.

 Piston compressor (HL60AH, ZEM HQY70AA and HQY75AA) with low pressure oil sump and ca 265 ml mineral oil charge. Cooling capacity (HQY70AA) 118 W and COP 1.49 at ASHRAE conditions (55/‐23.3 °C).

 Refrigerant: nominal refrigerant charge 33 to 36 g of Isobutane (R600a)

 Capacity control by intermittent run (on‐off cycling) with self‐defrosting in every off‐cycle

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Evaporator

Condenser

Compressor

Figure 9 Household refrigerator ER8893C front and backside.

In Figure 9 (left picture) the plate type, free convection, semi hidden evaporator is located at the upper part of the back wall in the cabinet. The picture to the right shows the condenser and the compressor at the refrigerator back side. Both pictures include some experimental equipment (thermocouples and pressure transducers).

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660,00 30,00 25,00

30,00

evaporator

30,00

defrost water chute duct behind evaporator

600,00

496,00 240,00

520,00

25,00

duct behind evaporator

evaporator (wall position) condenser slot

handle

Figure 10 Household refrigerator ER8893C in a top and side view

In Figure 10 the household refrigerator cabinet is depicted in a top and side view including the dimensions of the evaporator. Note that the evaporator is positioned with a small distance to the back wall (ca 25 mm) as to provide for air to circulate behind the evaporator.

Figure 11 The left picture shows the plate evaporator back side with its integrated refrigerant channels. At the left side in this picture the capillary tube suction line heat exchanger (that connects the evaporator to the cooling system) is visible. The right picture depicts a close-

up of the refrigerant tube cross section.

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Figure 11 depicts the back side of the plate evaporator (left) and a close‐up of the refrigerant tube cross section (right). In the left picture one can see the refrigerant tubing with a number of U‐bends.

The upper part has an area with parallel tubes connected by a number of vertical short tubes. This is the integrated accumulator located at the evaporator outlet. In this, refrigerant is accumulated to compensate for different running conditions. For aesthetic reasons the evaporator front side is flat while the back side contains the refrigerant tubing. This plate evaporator is manufactured from two plates of aluminium with a bonding zinc‐layer in between. The plates are pressed together in a tool which has the desired refrigerant tube layout milled on one side. While heated, the refrigerant tube is inflated whereas the zinc‐layer bonds the plates to each other. The result is an evaporator, flat on one side and with a non‐circular internal cross section. This type of plate evaporator is sometimes referred to as a roll‐bond evaporator or simply a bond evaporator. A more detailed description of the heat exchanger is given in papers B and C.

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3. Summary of appended papers

This chapter holds a summary of the appended papers. It is split into two parts to follow the thesis main topics; the evaporator and the cooling system. Therefore, part 1 concerns heat transfer and pressure drop of the case study refrigerator plate evaporator (Papers A, B and C). In part 2 the focus is shifted to the cooling system (Papers D, E, F, G and H). Paper D presents a new technique to accurately measure refrigerant quantities in different parts of the cooling system. Papers E and F holds results from experiments conducted with this technique. Paper G is devoted to the classical topic of optimal charging (quantity of refrigerant charge) and throttling (capillary tube diameter or length). Finally, in Paper H a thermo‐graphic study of the overall cooling system operating at on‐off cycling conditions is presented.

Energy Efficiency Improvements in Household Refrigeration Cooling Systems