The topic of this dissertation concerns solar refrigerators. An extensive literature review of this subject will be performed. Solar refrigerators can be divided into three primary categories which include PV refrigerators, solar absorption refrigerators, and solar mechanical refrigerators (i.e., those which drive a compressor through the use of a fluid). Several characteristics of these solar refrigerators will be discussed including the basic principles of operations of each of these three types of solar refrigerators, their efficiencies, their economics, and some of their applications. A literature review of solar mechanical refrigerators will demonstrate that this type of solar refrigerator is impractical for any application. PV solar refrigerators are the most pragmatic form of solar refrigerators. A subtype of PV solar refrigerator, known as the PV direct form of solar refrigerator, will be discussed within the aforementioned contexts. Of particular importance is the fact that some PV direct solar refrigerators make use of a thermal storage device as opposed to a battery.
The use of solar energy has been gaining interest for quite sometime. This interest has been further momentum as more focus has been placed upon environmental concerns (such as climate change as well as the possible health effects of carbon-based gases being produced by a wide variety of human activities), concerns about the number of energy sources by which humans can meet the demands of their activities, and political tensions which have resulted from nations attempting to locate the aforementioned energy sources which they require to meet their nation's needs. Much of the past sources of energy has been fossil fuel based such as petroleum/gasoline, diesel fuel and coal. Nuclear fission has also played a role in finding alternative sources of energy. However, nuclear fission has also had its share of issues including how to safely dispose of the spent fuel. As a result, there has been placed a large emphasis upon the search for clean alternative energy sources. Three of these clean, namely wind, geothermal, and hydroelectricity. Nonetheless, there are issues associated with the three aforementioned forms of clean energy. For example, geothermal and hydroelectricity both require the appropriate location near a river or geyser. The energy generated from the wind may vary upon the local climate and may, therefore, also be subject to large regional scale variations.
It is largely the result of the reasons stated within the previous paragraph which has led to the interest and popular.ity of the use of solar energy. The use of solar energy does not have many of the drawbacks mentioned toward the end of the previous paragraph. The use of solar energy is not restricted to any one region of the world. PV (i.e., photovoltaic) panels can be placed at any location with the goal of collecting sunlight to power human activities. While this collection of sunlight, by PV panels, may also be subject to weather variations (just as wind is also subject to such variations), it is likely that some sunlight will reach any given location over the course of any given period of time. That is, while the quantity of sunlight may be much less as the result of weather factors (e.g., clouds and overcast), it is likely that some amount of the radiation from the Sun. It may be questionable as to whether or not the quantity of sunlight will be sufficient to provide power for any given application, but there will be some quantity of solar power available as long as the Sun is above the horizon. Therefore, only the Arctic Circle and the Antarctic Circle will contain locations at which there will be no sunlight striking the Earth (and, therefore, will not be able to make use of solar energy) for a substantial amount of time.
The question becomes what is the best way of collecting the sunlight to convert it into useful energy to power the appliances, and other devices, used by humanity. There are two possibilities. The first possibility consists of the use of extensive collection devices. The philosophy of this first possibility is to produce solar energy on a large scale. There are several ways of producing solar power on such a large scale. Examples of such solar farms include a proposed solar farm located at Palm Springs, California and a proposed solar farm which will be located near Pflugerville, Texas [Desert Sunlight Solar Farm n.d.; Price]. Such solar farms often make use of a large number of PV cells. However, the large scale production of solar energy may also use a different approach. This second approach creates heat by focusing a large number of parabolic mirrors on to a substance such as water or hydrogen [Wade]. For example, a solar farm which makes use of a set of mirrors, to focus upon a Stirling engine, is located in Southern California [Wade].
The second possibility is that the collection of solar energy is performed at the individual level. This can be accomplished at the level of the home or one or more set of appliances. This set of appliances can be anything ranging from one or more lights to a television. One such important appliance is that of a refrigerator. Of course, as with any other technology, refrigerators come within several different types of varieties. For example, some types of refrigerators make use of the transference of thermal energy. Such refrigerators make use of a physical principle known as absorption to create the cooling effect needed for refrigeration. Other types of refrigerators make use of a compressor to create this cooling effect.
This dissertation will examine those solar refrigerators which make use of the two categories of refrigeration processes which were mentioned within the previous paragraph. Those types of refrigerators which make use of a compressor often require AC power and, therefore, make use of the power supplied by a utility company. However, solar panels typically generate DC power. Therefore, there is a third type of solar refrigerator which goes by the category PV refrigerators. This type of refrigerator drives a DC powered refrigerator. This is not to say that solar energy cannot be used to drive a compressor which is based upon the Rankine cycle. It can. That is, solar energy can be used to either drive a DC powered refrigerator or a refrigerator which contains a compressor, which is based upon the theory of the Rankine cycle, as well as an absorption refrigerator. Later in this dissertation, all three of these different categories of solar refrigerators will be discussed. Also, a particular subset of the PV refrigerators will also be presented. This particular subset of PV refrigerators is known as PV direct refrigerators.
The philosophical approach to this dissertation will be to perform a review of the literature by which to come to the set of conclusions which it will make. This set of conclusions will consist of a set of recommendations concerning 1) whether or not any of these types of solar refrigerators provided advantages of other types of refrigerators (which are not powered by means of solar energy) and 2) (assuming that it is found that solar refrigerators do have some advantages over those refrigerators which are powered by other means) which of these types of solar refrigerators are best suited for which applications. Indeed, the bulk of the first portion of this dissertation will consist of the investigation of the set of criteria which will be used to make the aforementioned set of recommendations. These criteria will consists of considerations such as the method of operations of each of the types of solar refrigerators, their efficiencies, and their economics. Of course, this set of criteria will be framed within the context of some of the applications for solar refrigerators. These applications will be presented within a chapter by that same name. However, these applications will be further discussed within the chapter in which the final set of recommendations is performed. This final set of recommendations will be made within the final chapter of this dissertation.
History of Solar Refrigerators
The history of solar refrigerators should be more properly thought of as a conflation of the history of solar energy and the history of refrigerators. As a result, this chapter of this dissertation will be divided into three portions. The first portion will be devoted to the history of solar energy. The second portion will discuss the history of refrigerators. The third and final portion of this chapter will combine these two topics to discuss the history of solar refrigerators.
The use of the Sun's energy to power human activities was being done as early as several hundred years B.C.. To be more specific, the concept of the magnifying glass was developed during the seventh century B.C. and sunlight was used to start religious torches in Greece and Rome within the third century B.C. [Energy Efficiency n.d.]. The Chinese used sunlight for this same purpose [Energy Efficiency n.d.].
The use of solar energy was extremely important to the Romans. The Romans used sunlight to warm the interior of their bathhouses. The Romans developed what is referred to as the Justinian by the early sixth century A.D. [Justinian Code n.d.]. Solar energy was so important to this culture that this first Justinian Code included a set of "Sunrights" to ensure that Sunrooms were placed within houses as well as public buildings [Energy Efficiency n.d.]. However, this use of sunlight to heat the places of human habitation was not limited to the ancient Greeks, Romans, and early Chinese cultures. It was actually used by several early cultures located around the globe. For example, it was used by the Anasazi who resided within North America around 1200 A.D. The Anasazi would take great care to choose southward facing cliffs on which to construct their dwellings [Energy Efficiency n.d.]. This would help to ensure that their dwellings would capture the maximum amount of sunlight to heat these same dwellings [Energy Efficiency n.d.].
A more technological approach to the use of solar energy has also been around for quite some time. The world's first solar collector was developed in the year. In 1816, Robert Stirling created what is now referred to as the dish/Stirling system [Energy Efficiency n.d.]. This system can be used to generate electrical power through the use of solar energy. In fact, this system was briefly mentioned within the introduction within the context of solar farms.
The first step in developing photovoltaic (PV) cells was taken in 1839 with the actual initial discovery of the photovoltaic effect. The discovery of the photovoltaic effect gained additional importance, from a modern perspective, with the discovery of selenium in 1873 by Willoughby Smith. The first solar cell was made by Charles Fritts in the year 1883 through the use of selenium. Another major breakthrough, which would have significant implications of the later manufacturing of PV panels, was the ability to create single crystal silicon. This ability was developed by Jan Czochralski in the year 1918. This ability is closely connected with the later development of the world's first silicon photovoltaic cell which was created in 1954 by Daryl Chapin, Calvin Fuller, and Gerald Pearson working at Bell Labs.
The above advances were complemented by several scientific advances near the beginning of the 20th century. Einstein documented and discussed the photoelectric effect in 1905. Einstein received the Nobel Prize in physics in 1921 for the discovery of this phenomenon. Robert Millikan experimentally verified this phenomenon 11 years later.
These advances were, of course, followed the actual application of photovoltaic technology. However, photovoltaic technology is not the only ingredient contained within solar refrigerators. There is the refrigerator itself. It is the history of refrigeration to which this chapter will now turn its attention to.
The first steps toward artificial refrigeration were taken by Robert Boyle who simply demonstrated the temperature at which water boils could be lowered by placing the water under increased pressure. The first known attempt to create artificial refrigeration occurred within the middle of the 18th century when William Cullen observed that the temperature of the water within a container could be lowered by the container's evacuation. The first true advance, within the science of refrigeration, could be said to have occurred as the result of the work of the English confectioner Thomas Masters toward the middle of the 19th century. Masters had invented a type of ice cream maker which created its own ice [Quinizio]. Frederick Tudor described the details of the way in which ice should be cut and properly stored within the first half of the 19th century [Jha]. Prior to this humans had to make use of whatever cooling agents (i.e., naturally occurring ice and snow) which nature provided.
Indeed, as might be suspected as the result of the previous paragraph, the roots of the modern refrigerator are to be found within the 19th century. In 1834, Jacob Perkins became the first individual to develop a system of refrigeration that could produce ice [Diamond and the National Geographic]. The device which Perkins designed contained many of the components of a modern refrigerator such as compressor, condenser and an evaporator. While this first refrigerator did not attract much attention, its function caught the captured the interest of a medical physician by the name of John Gorrie who believed that cool temperatures provided certain medical benefits. As a result, Gorrie was inspired to improve Perkin's design to develop a refrigeration device which would be more effective in the way it produced ice as well as a device to cool air. This occurred in the year 1848, 14 years after Perkin's original invention. The first true refrigerator was invented by Karl von Linde in 1871. In 1873 an individual by the name of James Harrison wanted to ship meat to Europe. Of course, at that time, the only way to accomplish this task was to find a way of preserving the meet as it was transported to Europe by a ship. This is what Harrison attempted. However, the meat spoiled as the result of the fact that no one aboard the ship knew how to operate the device which was to keep the meat preserved through the process of refrigeration. It was only after the second world war that refrigerators became a common appliance within the home.
There are issues surrounding this important appliance. Many of the household refrigerators which were made within the years following their introduction contained gases which could potentially harm the environment upon the discard of these same appliances. Other considerations also have in one way or another become important. For example, as will be discussed within the chapter titled Applications of Solar Refrigerators, their use has become important within the transport of vaccines, blood, and other biological products in rural areas. This particular application has the potential to save countless lives.
This last statement brings us to the history of solar refrigerators. However, this history is very brief because of the recent invention of the solar refrigerator. Indeed, even in the possible applications of solar refrigeration are relatively recent. For example, it was not until 1992 that the use of solar refrigerators to transport vaccines was investigated.
Types of Solar Refrigerators
There are several types of solar refrigerators. One of the primary goals of this study is to draw conclusions is to make recommendations as to which kind of solar refrigerator should be used within the context of a set of criteria which will be enumerated and discussed elsewhere within this thesis. This section will present the types of solar refrigerators which are now, or have been, available on the market. There are several types of solar refrigerators that use different methods to cool their interior. These include using the absorption cycle to cool their interior, using a mechanical approach which incorporates a solar powered Rankine engine, and the use of photovoltaic cells to create electricity which drives a compressor, and nocturnal passive cooling [Prakash and Gang]. The example of PV direct-drive will be discussed within the context of solar refrigerators which make use of photovoltaic cells to operate a compressor.
The first type of solar refrigerator, which was listed within the previous paragraph, makes use of the physics of the absorption cycle to cool in the interior of the solar refrigerator. This type of solar refrigerator makes use of a refrigerant material to create the cooling. To be more specific, solar absorption refrigerators make use of the transfer of heat to create refrigeration. As a result, this category of solar refrigerators can be divided further into two types of refrigerators depending upon the nature of the state of the refrigerant material. The first category makes use of a liquid such as LiBr – H2O, H2O-NH3, LiCl-H2O, NH3-LiNO3, R22-DMF, and NH3-NaSCN [Prakash and Gang]. The second category of solar refrigerators, which make use of the absorption cycle to cool their interiors, make use of a solid refrigerant material. Examples of solid refrigerant materials that can be used include Silicagel – H2O and Zeolites.
The physics of the absorption cycle is the same whether the solar refrigerator makes use of a solid refrigerant or a liquid refrigerant. To be specific, the basic idea is to sublimate (in the case of solid refrigerants or evaporate (in the case of liquid refrigerants) the material used by the refrigeration process [West]. The principle of the conservation of energy (i.e., the first law of thermodynamics) implies that the energy to sublimate or evaporate the refrigerant material must be acquired from the interior of the solar refrigerator and, thereby, cooling its interior.
What is important, within the context of this type of solar refrigerator, is that the cooling process is driven by a thermal process as opposed to a mechanical process which makes use of a compressor. However, the role of the compressor within the mechanical driven system is now played by an evaporator. The evaporator allows the refrigerant material to be converted into a corresponding gaseous substance. This is what produces the refrigeration effect.
The fact that the same cooling process is used by both solar absorption refrigerators and their corresponding traditional counterparts implies that both types of refrigerators make use of the same internal structure to produce their refrigeration. To be specific, both types of absorption refrigerators (i.e., normal and solar) contain a condenser, separator, generator, evaporator, and absorber [Darbyshire]. Absorption refrigerators also contain both a refrigerant and a second substance (usually a fluid) referred to as an absorbent. In all forms of absorption refrigerators, the purpose of the refrigerant is to remove heat. This removal of heat is accomplished by allowing the refrigerant to be vaporized within the evaporator [Sadik, Smimov, Avelino]. The purpose of the absorbent is to transport that the refrigerant back to the beginning of the absorption cycle.
After passing through the evaporator, the refrigerant material may either be recycled by the solar refrigerator or expelled to the atmosphere. If the refrigerant material is recycled, then it is referred to as a closed absorption cycle. In contrast, if the refrigerant material is expelled to the atmosphere, exterior to the solar refrigerator, then the process is referred to as an open absorption cycle. In what follows, this section will only consider closed absorption cycles.
As was just mentioned, a closed absorption cycle recycles the refrigerant material. This is accomplished by first passing the refrigerant material through a device referred to as the absorber. The absorber places the refrigerant material into the appropriate solution. For example, if the refrigerant material is ammonia, then the evaporator transforms liquid into gaseous ammonia which is next put into a solution of water and ammonia by the absorber [Klein and Reindl]. At this point, it should be noted that many refrigerant materials release heat upon placing them into solution. That is, this process is exothermic. This last statement is true, for example, within the case in which the refrigerant material is liquid ammonia. As a result, the solution containing the refrigerant will need to be cooled prior to completing the process which will recycle the refrigerant material. In any event, the solution containing the refrigerant material then passes through a mechanical pump and a heat exch. The presence of the mechanical pump does imply that some power must be used to run the pump. However, the efficiency implications of this will be discussed within the chapter, of this thesis, titled Solar Refrigerator Efficiencies.
Next, the solution enters into a device which is referred to as the generator. The purpose of the generator is to dehydrate the solution which contained the refrigerant. That is, it is the function of the generator to remove the refrigerant from the absorber so that the absorption cycle can repeat itself [Sadik et al.]. If this dehydration is not thoroughly performed and some of the water remains with the refrigerant, then there exists the potential risk that the minimum temperature of refrigeration will be raised.
The previous few paragraphs dealt with solar refrigerators which make use of the absorption cycle to cool their interiors. As was implicitly mentioned within the above discussion, as well as within the introductory paragraph to this chapter, another way of cooling their interior is to make use of a mechanical approach to operate a compressor which, in turn, is used to drive the compressor associated with refrigeration. There are two ways in which this compressor can be driven with solar energy. The first is through the use of a set of PV panels which produce a DC current and voltage. Such a solar refrigerator is referred to as a PV solar refrigerator and will be discussed later within this chapter. The second method is to drive a Rankine engine. This second type of solar refrigerator drives this Rankine engine by heating some fluid through the use of solar collectors. It is this second mechanism which this present chapter will concern itself with over the course of the next few paragraphs.
The first step in discussing this type of solar refrigerator is to discuss the fundamentals of the Rankine cycle. The Rankine cycle is likely to be familiar to many engineers and physicists because it is essentially a reversible version of the steam engine [Reichl]. The term reversible and the other thermodynamic terms used within this thesis are defined within Appendix A. In any event, the Rankine consists of four sequential steps. These steps can be graphically represented by the diagram displayed within Appendix B. These four steps are:
1. Water at a reduced temperature is adiabatically transferred from a lower temperature and a lower pressure (Ta,Pa) to a higher temperature and higher pressure (Tb,Pb).
2. The water is isobarically heated from the temperature and pressure given by (Tb,Pb) to the temperature and pressure (Tb',Pb). This step then converts this water into steam. The temperature and pressure becomes equal to (Tb',Pb) at the intermediate stage of this second step. Finally, this steam is further heated to a temperature and pressure of (Tc,Pc).
3. The steam at a temperature and pressure (Tc,Pc) is then adiabatically expanded to the temperature and pressure (Td,Pd) where the pressure Pd is equal to the original pressure Pa.
4. The steam at a temperature and pressure of (Td,Pd) is then isobarically condensed back to the original temperature and pressure of the Rankine cycle (Ta,Pa).
There are a couple of comments which require immediate mention within the context of the above described Rankine cycle used within a realistic solar refrigerator. First, the realized version of such a Rankine cycle makes use of either a piston or turbine. Second, while the above discussion made use of water to describe the Rankine cycle, an actual solar refrigerator can make use of other types of liquids besides water.
An important consideration of these types of solar refrigerators, which make use of a Rankine cycle to derive a compressor, concerns the efficiency of the Rankine cycle relative to the efficiency solar collector being used to heat by a second fluid which, in turn, is used by the Rankine cycle. To be specific, while the efficiency of the Rankine monotonically increases with temperature, the efficiency of the aforementioned solar collectors decreases with temperature. To overcome this difficulty, the movement of the Sun over the course of the day can be tracked. Therefore, the solar collectors can be attached to a device which performs this tracking. One problem with this solution is that such tracking devices can add to the cost and weight of the solar refrigerator. In any event, the temperatures of the fluid which is heated by the solar collector can be as much as between 100°C and 200°C (of course, it should be kept in mind that this fluid, heated by the solar collectors, is used to heat a second fluid which is directly used by the Rankine cycle). More on the efficiency of this type of solar refrigerator, as well as the efficiencies of other types of solar refrigerators, will be discussed within the chapter titled Solar Refrigerator Efficiencies.
Another type of solar refrigerator makes use of photovoltaic (PV) cell to create a DC current to operate the compressor producing the refrigeration. To be specific, this type of solar refrigerator typically makes use of the DC current produced by a set of solar panels to drive a DC motor. In turn, this DC motor is used to operate the DC compressor of the solar refrigerator.
One issue associated with this type of solar refrigerator concerns the ability of the solar panels to supply a DC current. On one hand, the solar panels will have some maximum specified amount of DC electrical current and DC electrical voltage which they can produce. On the other hand, these solar panels can produce DC electrical power which is much less than the DC electrical power which is needed by the DC motor which is used to operate the compressor of the solar refrigerator. Furthermore, considerations such as whether and/or climate may adversely impact the amount of solar radiation striking the solar panels. In general, the efficiency (which will be further discussed within the chapter of this thesis titled Solar Refrigerator Efficiencies) of this type of solar refrigerator will depend upon both the amount of solar radiation which is collected by the panels as well as the temperature of the panel.
There are two classes of PV solar refrigerators. The first makes use of batteries to store electrical energy during those times during which the radiation from the sun is not available (e.g., during cloudy days. The characteristics of the battery associated with this type of solar refrigerator will be discussed over the course of the next few paragraphs. Following that will be a discussion of PV solar refrigerators which are referred to as PV direct solar refrigerators. This type of PV solar refrigerators makes use of a couple of innovations including the use of a thermal storage device.
In any event, the possibility of certain types of solar refrigerators requiring the need to store electrical energy, as opposed to the storage of thermal energy, requires the use of a battery. Indeed, as was mentioned above, the use of a battery to store the DC electrical energy produced by a PV panel, can be used to maintain power to the solar refrigerator during cloudy weather or at night.
There are several issues with regards to the use of batteries to store the electrical energy which is produced by the solar panels associated with a PV solar refrigerator. They can add cost as well as weight to the solar refrigerator. Another issue associated with the use of batteries, to store the DC electrical energy produced by the PV panels, concerns the lifetime of the battery. Hysteresis will inevitably shorten the lifetime of a battery. Several PV solar refrigerators make use of lead- acid batteries [Ghassemi]. As implied by its name, this type of battery makes use of lead plates which are suspended within a electrolytic solution of sulfuric acid. Many companies and individuals suggest that the battery hold a sufficient amount of charge to last for five days. If the battery contains an amount of charge which is smaller than that needed to power the nuclear best that refrigerator for less than five days, then the battery may require a sufficient amount of charging that the lifetime of the battery may be significantly compromised [Solar Direct n.d.]. Finally, it should be noted that it is recommended that the batteries, associated with PV solar refrigerators, be located within a place which is extremely well ventilated. This is to reduce the possibility to reduce the possibility of the hydrogen gas that may be emitted by the battery from creating an explosion as well as reducing the possibility of the electrolyte, contained within the battery, from producing a toxic discharge.
The need for a battery can be eliminated within a PV solar refrigerator by making use of a thermal storage device. This type of solar refrigerator is termed a PV direct-drive or a PV direct solar refrigerator. The name refers to the fact that a direct connection is made between the panels of the solar refrigerator and the DC compressor of the solar refrigerator. It is important that the solar refrigerator is insulated as much as possible to ensure that the thermal storage is effective. Furthermore, a microprocessor-based system is used to operate the aforementioned DC compressor whose speed is able to vary. It should be noted that the use of a microprocessor can be used within the context of either a solar refrigerator which stores energy through the use of batteries or a solar refrigerator which stores energy thermally. This chapter has decided to place the mention of this microprocessor here because of its ability to help conserve thermal energy by managing the speed, and hence the power, of the DC compressor.
The thermal energy, of a PV direct solar refrigerator, is stored using a phase change material. This phase change material should possess a high latent heat of fusion (this is in addition to the insulation requirements mentioned within the last paragraph. As the name implies, a phase change material is one which changes from a solid phase to a liquid phase or vice versa. Of course, the latent heat of fusion of this material corresponds to the quantity of heat required to melt 1 kg of a solid substance into the same volume of the same liquid substance. Typically, many substances can store more thermal energy by changing their phase from a solid to a liquid than by simply making use of their inherent specific heat (i.e., by simply storing thermal energy by remaining within the same phrase. Usually, this phase change material is comprised of an inexpensive nontoxic water based solution, with the appropriate set of reasoning properties, is brought to bear. Examples of substances which are appropriate for use of phase change materials include paraffin, eutectic salts, and a type of salt hydrate known as Glauber salt. A calculation of the mass of the required phase change material is performed using the thermal properties of the substance as well as the rate at which heat escapes the solar refrigerator. This calculation is performed with the goal of assuming that a sufficient amount of the phase change material (i.e., the water based solution described earlier within this paragraph) to provide enough cooling to maintain the refrigeration, of the interior of the solar refrigerator, for a period of seven days at an ambient temperature of 29.5°C.
This chapter of this thesis has discussed several types of solar refrigerators. It needs to be noted that each of these types of solar refrigeration can be combined with a more conventional approach to refrigeration. This can be accomplished by combining a solar refrigerator with some other mechanism to power the refrigerator. This secondary source of power may come from a wide variety of power sources such as natural gas or the electricity supplied by a local power company. Whatever the secondary source of power used to operate this type of solar refrigerator, it may be referred to as a hybrid solar refrigerator.
Solar Refrigerator Efficiencies
The chapter of this thesis titled Types of Solar Refrigerators looked at types of solar refrigerators which can be categorized according to three primary different types (i.e., mechanically operated solar refrigerators which make use of physical considerations such as the Rankine cycle, absorption solar refrigerators, and photovoltaic (PV) refrigerators. This chapter of this thesis will now examine the efficiencies of each of these three of solar refrigerators.
There are actually three perspectives of efficiency which can be discussed within the context of solar refrigerators. The first perspective relates to the efficiency of the solar panels of the refrigerator. That is, this perspective is relevant to the amount of solar radiation collected by the panels of the refrigerator. The second perspective relates to the physical mechanism producing the effect of refrigeration. Of course, these two perspectives are not completely independent of one another. As a result, the third perspective concerns the "total efficiency" of the solar refrigerator. That is, the total efficiency of a solar refrigerator is the product of the efficiencies just mentioned within the context of the first two perspectives. To be even more specific, this third perspective takes into account both the efficiency of the solar panel made use of by the refrigerator as well as the efficiency of the physical mechanism made use of to produce the effect of refrigeration. The first perspective (i.e., that of the efficiency of the solar panels made use of by the refrigerator) will be briefly discuss within the paragraph following the present paragraph. Furthermore, this discussion will not make reference to the three types of solar refrigerators which were discussed within the appropriate chapter. The second and third perspectives will be discussed throughout the remainder of this chapter and will explicitly involve the three primary difference types of solar refrigerators which were discussed within the chapter of the corresponding title.
As was mentioned within the introductory paragraph of this chapter, the first perspective of efficiency that will be discussed is that of the efficiencies of the solar panels collecting the solar radiation. As was also mentioned within the introductory paragraph, this perspective of efficiency is independent of the fact of the physical mechanism used to produce the effect of refrigeration. That is, this first perspective of efficiency does not depend on whether the solar refrigerator makes use of the thermodynamics produced by the physics of absorption, by the physical mechanism associated with those solar refrigerators which operate a compressor through mechanical means (e.g., the Rankine cycle), or whether the solar refrigerator makes use of a photovoltaic cell to operate a DC motor to drive a compressor.
The process by which solar radiation is converted into DC electrical current and DC electrical voltage is largely based upon the physics of classical electrodynamics and quantum mechanics. In particular, this process does not make use of the principles of thermodynamics. As a result, the concept of the efficiency of a solar panel does not equate with the concept of thermodynamic efficiency, but is more congruent to the percentage of the solar radiation reaching the solar panel that is able to be converted into electrical power by the panel. That is, the larger the amount of the spectrum of the solar radiation which is able to be captured by the solar panel the larger the defined efficiency of the solar panel. A theoretical solar panel which was able to convert all of the solar radiation reaching it into electrical power would have an efficiency of 100%. Therefore, in general, the efficiency of a PV module can be defined as the percentage of the incident solar radiation that is absorbed by the PV module [Schaeffer and Pratt].
The Sun delivers 85000 * 1012 Watts on a yearly basis. This translates into a solar radiation intensity of 1000 W per square meter occurring at a location at which the solar radiation strikes the PV module at a perpendicular angle (e.g., at a point of the Earth which is directly facing the Sun on either the fall equinox or the spring equinox). In any event, a solar radiation flux of 1000 W per square meter, striking a PV module perpendicularly, is referred to as "full Sun." Some slightly older sources state that the efficiency of commercially produced solar panels is roughly between 10% and 12%. In contrast, slightly newer sources state that while mass produced PV modules have an efficiency of approximately 15%, there are solar panel technologies (e.g., crystalline silicon cells) which possess efficiencies up somewhere between 20% and 25%. This concludes the portion of this chapter which discusses the efficiency of the solar panels used by the solar refrigerator.
The remainder of this chapter will discuss the efficiency of solar refrigerators from the perspective of the mechanisms which they make use of to produce the effect of refrigeration as well as the way in which this efficiency conflates with the efficiency of the solar panels that were discussed within the preceding paragraphs. Of course, as was mentioned within the introduction, these two items will be intermixed within the remainder of this chapter.
The efficiency of the first type of solar refrigerator which will be discussed is that of solar absorption refrigerators. The efficiency of such a solar refrigerator is equal to the product of the efficiency of the PV modules and the lower coefficient of performance of the absorption mechanism used to cool the interior of the refrigerator.
The coefficient of performance, or COP, is equal to the rate at which heat is removed from the contents of the refrigerator divided by the sum of the work required to operate the pump connecting the absorber with the generator and the rate at which heat is being supplied to the generator by the PV module or (Qf = the rate at which heat is being removed from the contents of the refrigerator, Qg = the rate at which heat is being supplied to the generator by the PV module, P = the power required to operate the mechanical pump connecting the absorber to the generator within the solar absorber refrigerator, and COP = coefficient of performance)
COP = Qf / Qg + P
There exists mechanisms by which no mechanical pump is required between the absorber and generator. For example, some solar refrigerators may make use of gravity or thermosiphoning in place of a mechanical pump to drive the fluid from the absorber to the generator. In such a case, the variable P can be set equal to zero because of the absence of a mechanical pump. As a result, the above equation can be written as:
COP = Qf / Qg
In any event, it can be shown that the coefficient of performance, of an absorption refrigerator, not containing a pump, is equal to (Te = temperature at which the refrigerant evaporates, Th = temperature at which heat is supplied to the generator, and Ta = ambient temperature).
COP = Th – Ta / Ta - Te
Prior to finishing our discussion of the efficiencies of solar absorption refrigerators, there are two more comments which require mentioning. First, when the above coefficient of performance is combined with the efficiency of the PV module associated with the solar absorption refrigerator, then efficiencies do not typically exceed 10% or 0.10. Second, this efficiency can be further enhanced by engulfing the circuit of the solar absorbent refrigerator with the same refrigerant which is used to cool the solar refrigerator.
The next type of solar refrigerator which this chapter will discuss is the solar mechanical refrigerator. As was mentioned within the chapter titled Types of Solar Refrigerators, this form of solar refrigerators use solar energy to drive a mechanical compressor which, in turn, creates the cooling effect for the interior of the solar refrigerator. As was described within that chapter, this mechanical compressor often corresponds to a Rankine cycle. The reader is referred to that chapter for a discussion of the Rankine cycle.
In the chapter titled Types of Solar Refrigerators, it was mentioned that while the efficiency of the compressor increases with the temperature of the vapor (water or otherwise), the efficiency of the solar panels decreases with rising temperatures. As a result of this consideration, the overall efficiency of a solar mechanical refrigerator may not be equal to its optimal efficiency if its solar panels are not effectively aligned with the incident solar radiation. Furthermore, the discussion of that chapter mentioned that one way to overcome this issue was to make use of a tracking device which maintained the alignment of the solar panels with the incident solar radiation. Finally, it should be noted that it was also mentioned, within that chapter, that the addition of such a tracking device is its additional cost and weight.
However, even with such a tracking device, there are still remaining issues concerning the overall efficiency of solar mechanical refrigerators as the result of the considerations presented within the previous paragraph. For example, even if the tracking device allows the solar panels of the refrigerator to maintain their alignment with the incident radiation, there can be other environmental factors which lower the temperature of the vapor of the fluid that is being used by the Rankine cycle. The quantity of the solar radiation reaching the panel may be subject to variations over the course of a day as the result of changes within the cloud cover. The ambient temperature may cause the interior of the solar refrigerator, containing the vapor, to cool and thereby reducing the efficiency of the Rankine cycle. Of course, this last possibility can be eliminated by simply insulating the interior of the solar refrigerator from its exterior.
Finally, it should be mentioned that (if needed) the above described tracking system can be replaced by other technologies. That is, there is an alternative solution to the use of tracking devices used to enhance the temperature of the fluid which is heated to drive the Rankine cycle of the solar mechanical refrigerator. This alternative consists of making use of a compound parabolic mirror or the use of multi-cover flat plate collectors [Klein and Reindl]. Just as with the use of tracking devices, the use of either compound parabolic mirrors or multi-cover flat plate collectors can raise the temperature, of the fluid which is being used to drive the Rankine cycle, to between 100° Celsius and 200° Celsius. Notice that this range of temperatures is above the boiling point of water.
Next, this chapter will discuss PV (photovoltaic) solar refrigerators. Recall that, from the discussion within the chapter titled Types of Solar Refrigerators can be categorized as PV solar refrigerators in general or as PV direct solar refrigerators. The next paragraph will discuss the efficiency of PV solar refrigerators in general while the paragraphs following this next paragraph will discuss the efficiency of PV direct solar refrigerators in particular. With regards to PV direct solar refrigerators, the emphasis will be placed upon the fact that this specific type of solar refrigerator makes use of a thermal storage device.
As was just mentioned within the preceding paragraph, this chapter will now examine the general considerations which are involved in considering PV solar refrigerators in general. Earlier within this chapter it was mentioned that the efficiency of a solar panel can be defined as the percentage of the available solar radiation which is collected. However, a second definition which is perhaps more relevant to PV solar refrigerators is that the efficiency is equal to the amount of DC electrical power created by the PV panels divided by the incident flux of the solar radiation. This efficiency, in general, will depend upon the magnitude of the incident flux of the solar radiation as well as the ambient temperature of the PV. This is because of the fact that the voltage which is created by the PV panel will be a function of the incident flux of the incident solar radiation and the ambient temperature of the PV panel. That is, there will exist a specific voltage, for a given value of the incident flux of solar radiation and ambient temperature of the PV panel, which will maximize the voltage produced (and hence the amount of DC power delivered) by the PV panel. Furthermore, if the PV solar refrigerator is to operate at its maximum possible performance, the actual voltage produced by the PV panel must be approximately equal to the aforementioned maximum voltage. In any event, this definition of the efficiency of a PV solar refrigerator yields a typical value of between 8% and 10%.
Now this chapter will turn its attention to the discussion of the efficiency of PV direct solar refrigerators. Once again, the reader is referred to the chapter titled Types of Solar Refrigerators for a discussion of the mechanisms used by this form of solar refrigerator. As mentioned within that chapter, this type of solar refrigerator makes use of a variable speed DC compressor. One important reason for the use of a veritable speed DC compressor relates to considerations of efficiency. To be more specific, a fixed speed compressor would require a sufficient amount of sunlight in which to operate. Therefore, the compressor would not begin operation until the sun had rose to a sufficient altitude within the morning sky and, similarly, would quit working once the sun had dipped below the corresponding altitude within the late afternoon or early evening sky. In turn, this implies that the thermal storage device may use of by PV direct solar refrigerators (please see the comments within the chapter titled Types of Solar Refrigerators) is likely to be required for a longer time span during the day and may also waste power at those times during the day at which sunlight provides a greater amount of photovoltaic power than is required by this form of solar refrigerator. Indeed, it is estimated that a fixed speed DC compressor would make use of only 50% of the available power which can be delivered via solar radiation. In contrast, the use of a variable speed compressor is able to make use of 75% of the available sunlight. Paramount to accomplishing this significant increase in the available amount of sunlight is the use of the microprocessor which is mentioned within the chapter titled Types of Solar Refrigerators. This microprocessor helps to enhance the efficiency of the PV direct solar refrigerator by ensuring that the solar collection device is operating at its peak during that time interval during which the compressor is operating.
Applications of Solar Refrigerators
In order to make well-informed recommendations, concerning solar refrigerators, it is important to consider their potential applications. Of course, one of these applications concerns the preservation of both food and beverages. This particular application will be discussed within a residential context as well as a recreational context. Next, this chapter will discuss the refrigeration of food and beverages within the context of less developed countries. This last discussion will include the use of solar cooling mechanisms to produce air conditioning within these undeveloped countries. The final application which will be discussed within this chapter concerns the possible solar refrigerators to preserve blood, vaccines, and/or other biological products within less developed nations.
The first application which this chapter on the applications of solar refrigerators will discuss is their use within the context of preserving food and beverages within both the context of a residential setting as well as recreational setting. The residential setting for solar refrigerators will be discussed over this and the next few chapters while the recreational applications, in preserving food and beverages, will be discussed following the discussion of solar refrigerators occurring within the context of residential settings.
There are several possibilities for making use of solar refrigerators to preserve food and beverages within a residential setting. One type of residential refrigerator is that of a SunDanzer [SunDanzer n.d.]. As was mentioned within the chapter providing the literary review, the SunDanzer was originally developed by NASA [SunDanzer n.d.]. Since that time it has become the present brand name.. Many of this brand name's products make use of PV solar refrigerator technology [SunDanzer Products, n.d.]. Some of this brand name's products even make use of the battery free (thermal storage) technology of PV direct solar refrigerators [SunDanzer Products n.d.]. More about the SunDanzer brand-name of solar refrigerators will be briefly mentioned later within this chapter within the discussion of solar refrigerators for recreational purposes.
Another important consideration of solar refrigerators, for residential applications, concerns the technology known as grid technology. Grid technology not only relates to the use of solar refrigerators, but also to the use of solar applications within the context of a residential environment. To be more specific, grid PV technologies relates to any solar application for which the PV system is connected with a home's electrical utility [Build It Solar n.d.] Such solar grid technologies can help to generate power for some of the appliances within their home and can return some of the power generated by the PV panels to the electrical utility connecting the home via the grid [US Department of Energy n.d.].
This type of technology (connecting a PV application to an electrical grid poses its own problems. For example, some of these systems do make use of batteries. Such grid PV systems require specialized charging and maintenance characteristics. Of course, if the consumer of such a grid PV system wishes to sell the utility company the excess power which is generated by the PV system, then some device will need to be installed which allows DC power to be converted to AC power. Of course, such systems are readily available, but they add additional cost.
Finally, it should be mentioned that occasionally the manufacturers of solar refrigerators suggest the use of solar refrigerators which made use of a dual compression for regions which have a warmer climate. Of course, it can be anticipated that if such a dual compressor is required, then it will place a greater strain on the energy storage device (whether it be a battery or thermal storage unit) during its nocturnal operation or its use during those cloudy during which there is insufficient sunlight to power the refrigeration.
The third application which will be explored, for the use of solar refrigerators, concerns their possible use as a means of preserving food and drinks for recreational purposes within developed countries. A brief search of the Internet illustrates that there are several companies which can either provide solar refrigerators and/or make recommendations on the acquisition of such solar refrigerators.
For example, the online issue of Backwards Home Magazine [Yago n.d.] has an entire webpage devoted to the use of solar refrigerators within a recreation context. This particular article confirms what was said earlier, within this chapter, that there exists several companies which are able to provide the size of solar refrigerators which may be desirable to recreationists. Such companies include Polar Power Inc. (it should be noted that this company also mentions the fact that solar refrigerators can be used to preserve vaccines within rural areas), SunDanzer (which sells both sells both PV solar refrigerators which require a battery as well as PV direct solar refrigerators which make use of a thermal storage device which eliminates the need for a battery), and Solatron (which sells solar refrigerators which are appropriate to both recreational use as well as for residential use).
Another application of solar refrigerators concerns the production of milk. This particular application has been described by Tuszyriski et al. within the book titled Solar Energy in Small-Scale Milk Collection and Processing, Issue 39. Indeed, the process of milk production requires, at certain steps, the need to chill the milk to a given temperature through the use of absorption cooling. Historically, cold water was used to accomplish this task. However, it was realized that the principle used by solar absorption refrigerators could also accomplish this same cooling task. Solar absorption refrigerators make use of a thermal mechanism to produce the refrigeration effect. More details of the principles used by solar absorption refrigerators are discussed within the chapter titled Types of Solar Refrigerators. In any event, the only limitation of the ability of the principles used by solar absorption refrigerators to produce the magnitude of the cooling necessary for this step, within the production and collection of milk, is for sufficient sunlight to be collected by the PV panels to drive the required absorption process. Of course, the amount of sunlight collected by the PV panels can be enhanced through the use of tracking devices. The use of such tracking devices was described elsewhere, within this dissertation, with regards to their use by PV refrigerators. Finally, it should be mentioned that there are eight volumes for which the use of the principles of solar absorption refrigerators should be taken into account. These eight sizes are (the following are within units of kilograms) 32, 43, 65, 108, 161, 247, 387, and 624. These masses of ice correspond to the amount which must be evaporated using solar energy to produce the required cooling effect.
Furthermore, the article by Yago [n.d.] provides suggestions to readers as to considerations they may wish to contemplate in choosing a solar refrigerator which best fits their recreational needs. The set of recreational needs discussed by this article, within the context of solar refrigerators, consists of spending time within a cabin as well as marine recreation. It discusses the fact that solar refrigerators come within a variety of sizes. The largest size which this particular article mentions is 19 ft.³ which is approximately equal to the volume of what the article refers to as a "conventional 120 volt AC refrigerator/freezer you may find in most homes". In contrast, this article goes on to say that a 12 ft.³ solar refrigerator corresponds to the size of a refrigerator found within an apartment. Finally, this article makes suggestion concerning the price range individuals may wish to pay depending on whether or not they will have access to a generator during their recreation. The fact that such an article can be found on the Internet demonstrates the popularity of the use of solar refrigerators for recreational purposes.
One of these applications concerns the storage of blood and vaccines. This may be particularly important in rural or less developed regions. The first step in discussing this important application is to document the need and demand for storage of blood and vaccines to rural areas. The next paragraph will discuss the need to preserve vaccines, using refrigeration, within less developed areas. The paragraph following the next will discuss the importance of preserving blood, using refrigeration, within less developed regions. Following that paragraph, this chapter will describe some of the current technologies used to refrigerate blood and vaccines as well as to briefly discuss some of the relevant issues associated with this general application of refrigeration.
The importance of vaccines can be seen through report issued by UNICEF (the United Nations children's fund) in 2004. The use of vaccines may have saved as many as 20 million lives during the last two decades of the 20th century. Furthermore, this same report estimated that 2.5 million lives were saved in the year 2003 by itself. Fewer than 10% of all children within less developed countries were vaccinated in the year 1974 before WHA (World Health Assembly) when this organization began its immunization program. Toward the end of the 20th century, and continuing into the beginning of the 21st century, this percentage dramatically increased to 75%. This translates into approximately 100 million infants being vaccinated every year. Nonetheless, UNICEF estimates that almost 33 million children still require vaccination each year as the result of conditions occurring within less developed countries. Many of these diseases can be prevented through the use of vaccines. Examples of such preventable diseases include measles, pertussis, tetanus, diphtheria, tuberculosis, and polio. Finally, it needs to be mentioned that providing children with vaccines may help lower the incidence of infectious diseases among the general population of a region. As will be elaborated upon within a following paragraph the primary technologies which are presently used to refrigerate (and therefore preserve) vaccines are refrigerators which make use of kerosene, refrigerators which operate on bottled gas, and solar refrigerators.
Similarly, there is an important need to preserve and transport blood. This can be demonstrated by the statistics provided by WHO (World Health Organization, which is a division of the United Nations). This organization estimates that roughly 1% of a region's population needs to donate blood if that same region is able to provide a sufficient amount of blood for that region's needs. In actuality this same organization found that 65% of all blood donations were made within developed nations which only account for 25% of the global population. This information is for the year 2007. This information implies that blood may need to be transported over a considerable distance. In turn, this means that blood may need to be transported across long distances of rural areas.
It is imperative that blood be kept at a temperature of between 2° Celsius and 6° Celsius while being transported. There are two reasons for this. First, it helps maintain the ability of blood to keep its ability to transport oxygen via the circulatory system. Second, it helps to minimize the presence of bacteria within the blood being transported. Of course, this requires the use of 1) refrigeration and 2) the use of some form of thermostat to ensure that the blood is kept within the temperature range listed within the first sentence of this paragraph. Therefore, from the standpoint of refrigeration, it is paramount that the blood being transported is maintained within the aforementioned range of temperatures. One final comment concerning the safe transport of blood is that if a compressor is used by the refrigerator being used to transport the blood, then it is important to ensure that any refrigerant gas being used should not contain CFCs (i.e., chlorofluorocarbons). Whatever the refrigeration mechanism, one important concept relevant to the transportation of blood (as well as vaccines) is the hold time of the refrigerator. The hold time is equal to, by definition, the length of time that biological products (e.g., blood and vaccines) can be stored safely by the refrigerating unit being used to transport this biological product.
As was mentioned above, there exist three classes of refrigerators which are often used to transport biological products such as blood and vaccines. One of these three classes is that of solar refrigerators that is the topic of this dissertation. The second two classes are kerosene refrigerators and bottled gas refrigerators. It will be important to briefly discuss kerosene refrigerators and bottled gas refrigerators at this point within this chapter of this dissertation. This will allow us to determine, within a later chapter, if any of the types of solar refrigerators (and, if so, which particular type) offer any advantages over use of kerosene refrigerators and/or bottled gas refrigerators within the context of transporting blood, vaccines, and/or other biological products.
Kerosene refrigerators make use of the absorption mechanism which was discussed in relation to solar absorption refrigerators within the chapter titled Types of Solar Refrigerators. That is, kerosene refrigerators make use of a thermal mechanism to create refrigeration. However, kerosene refrigerators make use of burning kerosene to produce the thermal mechanism, which creates the refrigeration, instead of using the radiation produced by the Sun.
As a result, there are two problems associated with the use of kerosene refrigerators to create the required cooling effect. First, they can pose a hazard because of the required flammable nature of kerosene which is obviously required for the use of this material to produce the thermal mechanism required by absorption refrigerators. Second, kerosene refrigerators are not reliable [Hill and Hill]. Their cost may be less than, conquerable to, or greater than solar refrigerators. For example, at the time of the writing of the text by Hill and Hill, had an initial price of between $300 and $800. However, this initial may have been as much as $1500 when installation was included. This should be compared with the cost of a solar PV type refrigerator at the time of the writing of Hill and Hill which was approximately equal to $5,000. Furthermore, the maintenance expense of a kerosene type refrigerator made it much more expensive than a solar PV type refrigerator at that time. One more drawback of kerosene refrigerators is that it is difficult to maintain the range of their temperatures. As was mentioned within an earlier paragraph, of this chapter, this is an extremely important consideration when dealing with the transport of blood.
Next, this portion of this chapter will examine the class of refrigerators which make use of bottled gas. As was mentioned earlier, this is important because it will allow the later determination of the merits of the different types of solar refrigerators within the context of the transportation of blood, vaccines, and/or other biological products. The term bottled gas refrigerator refers to the use of bottled gases to run an absorption refrigerator. Therefore, instead of making use of kerosene to fuel the thermal mechanism, associated with an absorption refrigerator, bottled gas refrigerators employ substances such as propane or butane. That is, this type of refrigerator producers a flame using propane or butane, to create the cooling effect [Bottled Gas].
Just as within the case of kerosene refrigerators, there are several drawbacks to the use of bottled gas refrigerators to transporting blood, vaccines, and/or other biological products within developing countries. For one thing, there may not be an available source of either propane or butane to refuel the tanks of the bottled gas refrigerator. Second, there may be a fire hazard as the result of the combustible nature of the fuel which is being burned to produce the refrigeration effect (of course, one of the requirements of the gas being used is that it is combustible which is needed to produce the flame which, in turn, will produce the refrigeration effect). Neither of these two drawbacks, associated with the use of bottled gas refrigerators, is present within the use of solar radiation to produce the refrigeration effect of absorption refrigerators. That is, solar radiation is readily available at any time during which the sun is out. Also, sunlight does not pose any combustible hazards as do propane and butane. Finally, the use of bottled gas refrigerators is connected with the extra mass required to produce the flames of these absorption type refrigerators. This extra mass can be substantial especially if a sufficient amount of gas is required for lengthy trips as may be needed to transport blood and vaccines across less developed countries. PV panels are likely to be much less massive.
Economics of Solar Refrigerators
This dissertation research has already discussed some of the costs associated with solar refrigerators. However, these discussions of costs have been sprinkled throughout this dissertation. This chapter will 1) allow the collection of this information into a single location within this dissertation and 2) allow the presentation of new information which will better equip this examination to make recommendations concerning solar refrigerators.
In general, some experts feel that the use of solar energy is high in general. This is illustrated by the book authored by Ramsey and Hughes. These two authors claim that non-solar energy efficient devices can be only between 20% and 33% of the cost of the equivalent solar devices. This past statement regards all solar devices including solar refrigerators. However, it is interesting to note that the third type of solar device which Ramsey and Hughes recommends on purchasing, from the perspective of efficiency and economics, are solar refrigerators. These two authors provide the example of a 19 ft.3 traditional (i.e., non-solar) refrigerator in comparison with a 19 ft.³ solar refrigerator. These authors suggest that a typical 19 ft.3 traditional many use 2.5 kWh every day. These same authors claim that the equipment necessary to transform this 19 ft.3 refrigerator into one which will allow this refrigerator to be powered using sunlight will cost approximately $5,000. Finally, these authors state that this cost, to run a 19 ft.³ refrigerator, can be reduced by placing 4 inches of insulation around the refrigerator. What is interesting here is that Ramsey and Hughes does not state what type of solar refrigerator is used to compare these two refrigerators. It only makes sense if the two refrigerators make use of the same physical cooling mechanism. This is because differences within the costs, of these two refrigerators, may result simply from the fact that they make use of distinct principles to produce their refrigeration.
Of course, it is important to remember that even if a more traditional type of refrigerator is compared with the corresponding solar type of refrigerators, then there may still be a significant amount of cost difference between them. Such is the case with absorption type refrigerators. Absorption type refrigerators can be fueled by sunlight, kerosene, or some form of bottled gas (e.g., propane or butane). Solar absorption refrigerators do not require a source of fuel to drive their cooling mechanisms which produce their refrigeration. In contrast, many other categories of absorption refrigerators do make use of some fuel (e.g., kerosene or some sort of bottled gas such as propane or butane). This can add to the maintenance cost of an absorption refrigerator. For example, even though propane may be inexpensive it can still add on to the cost of an absorption refrigerator as the result of the inefficiency of this gas. This cost, of such a propane driven absorption refrigerator may add up. For example, an 8 ft.³ tank may go through between 1 and 2 gallons of propane every. An even larger tank having a volume of 18 ft.³ may require between 1 and 2 gallons of propane every week. Finally, it should be noted that all of the cost considerations, discussed within this paragraph, are relative to the year 2007.
However, while the aforementioned (bottled gas and kerosene) absorption type refrigerators require some form of chemical fuel, all types of solar refrigerators require the use of PV panels to capture sunlight. Therefore, the cost to operate a solar refrigerator should include the cost required to produce the necessary solar panels. It is estimated and predicted that the cost of these solar panels may lower as much as by 50% by the year 2020, relative to the year 2007.
Recommendations and Conclusion
The body of this dissertation has been leading up to the present chapter. It is in this chapter which this dissertation will make a sequence of recommendations concerning solar refrigerators. Of course, along with these recommendations, this chapter will also provide the reasons for these recommendations.
The first recommendation concerns the use of solar energy technology to create the phenomenon of refrigeration by driving a compressor which makes use of a Rankine cycle. This type of solar refrigerator drives this Rankine cycle by heating a fluid through the use of solar collectors. An earlier chapter, within this dissertation, referred to this type of solar refrigerator as a solar mechanical refrigerator. Recall that this type of solar refrigerator is termed a solar mechanical refrigerator. More details concerning the physical principles upon which solar mechanical refrigerators operate can be found within the chapter titled Types of Solar Refrigerators.
The primary issue with regards to solar mechanical refrigerators is simply an issue of efficiency. The efficiency of this type of solar refrigerator was discussed elsewhere within this dissertation (please see the chapter titled Types of Solar Refrigerators as well as the chapter titled Solar Refrigerator Efficiencies). As was mentioned much earlier within this dissertation, the efficiency of the mechanical component (of this type of refrigerator) monotonically increases with temperature (this is to be expected as the result of the thermodynamic nature of solar mechanical refrigerators). On the other hand, as it was also mentioned much earlier, the efficiency of the solar collectors (used to heat the fluid used to drive the Rankine cycle) decreases with temperature. Therefore, it may be expected that the increasing nature of the mechanical portion of the efficiency with respect to temperature competes with the decreasing portion of the decreasing nature of the efficiency of the solar collectors which heat the aforementioned fluid for the Rankine cycle. Indeed, it does and maximum for the efficiency results from this mathematical competition. This maximum of efficiency is roughly equal to 4.5% for an ambient temperature of approximately 30° Celsius. Therefore, it may be concluded that solar mechanical refrigerators should not be used if a solar type of refrigerator is required for some particular application. Furthermore, it has been pointed out that such solar mechanical refrigerators require larger temperatures to be truly efficient. These same two authors have stated that this last statement implies that solar mechanical refrigerators would need to be very large in size for this technology to be feasible. In fact, they suggest the fact that such a solar mechanical refrigerator would either need to weigh 1000 tons. This last statement makes solar mechanical refrigerators impractical for even commercial applications let alone for those applications which this dissertation discussed within earlier chapters. Because of this, not only are solar mechanical refrigerators much less preferable to other types of solar mechanical refrigerators (for all applications), solar mechanical refrigerators should only be chosen if there is not some other more traditional alternative power source (such as chemicals or the AC electrical power which can be obtained from the utility company to drive the refrigeration).
The next type of solar refrigerator which will be discussed by this chapter concerns PV solar refrigerators. Right from the start, it needs to be noted that PV solar refrigerators are preferable to solar mechanical refrigerators as the result of the comments appearing within the previous paragraph. As was mentioned within an earlier chapter, there are actually two subtypes of PV solar refrigerators which needs to be considered within this final chapter of the dissertation. The first type of PV solar refrigerators store energy (for night and/or cloudy weather) using a battery. The second type of PV solar refrigerator stores its energy thermally.
The first subtype of PV solar refrigerator makes use of batteries to store its energy. One advantage in making use of batteries is that this form of energy storage technology has been around for quite some time and it is well understood. Indeed, this advantage has been mentioned by other authors such as Ghassemi. Nonetheless, as was pointed out earlier within this dissertation, the use of batteries can have its own issues. For example, the batteries will need to be periodically charged whereas the thermal storage version, of PV direct solar refrigerators, does not.
The second subtype of PV solar refrigerators makes use of a thermal storage system. This type of PV solar refrigerator is called a PV direct solar refrigerator and was discussed (as well as all of the other major classes solar refrigerators) within the chapter titled Types of Solar Refrigerators. PV direct solar refrigerators relieve the need for a battery, but then needs to be a sufficient amount of sunlight during some fraction of some set of days. This is not only to produce refrigeration. The energy within the thermal storage device will also become depleted if it isn't replaced. Therefore, a temperature variation within this thermal storage device will need to be reestablished periodically to maintain the refrigeration effect. This will need to be accomplished by ensuring that the photovoltaic panels, of the solar refrigerators, receive a sufficient amount of sunlight over the course multiple days. These statements remain true even though the thermal storage device may possess a sufficient amount of energy to maintain the internal temperature of the solar refrigerator for several days (please see the earlier portions of this dissertation for a discussion of the characteristics of the thermal storage device).
Of course, a comparison between those PV solar refrigerators which make use of batteries to store their energy and those PV solar refrigerators which use a thermal device to store energy needs to be made. Both PV solar refrigerators which make use of batteries to store energy as well as those PV solar refrigerators which make use of a thermal device to store energy are available. An example of a company which provides both types of solar refrigerators is SunDazer. In an event, one of the considerations within a comparison of these two subtypes of solar refrigerator concerns the length of time which the thermal storage device is able to maintain the internal temperature of the solar refrigerator versus the corresponding length of time which the battery is able to maintain this same temperature. This may not only be an important consideration within the preservation of food and beverages, but also within the context of preserving food, vaccines, and/or other biological products.
Another consideration between these two types of PV refrigerators (i.e., between those which make use of batteries and those which make use of a thermal storage device) concerns the mass of the refrigerator. This may be an important consideration in applications such as the recreational use of solar refrigerators as well as the transport of blood, vaccines, or other biological products. To be specific, there are times in which it may be important to compare the weight of batteries to thermal storage devices. There is no detailed information within the literature which would allow us to make general statements concerning this comparison. It depends upon several different factors including the material used within the construction of the battery versus the material used within the construction of the thermal storage device and the size of the battery versus the size of the thermal storage device. As a result of these factors, the consideration of the mass can only be performed on a case-by-case basis.
Therefore, the comparison of those PV solar refrigerators which make use of batteries to store electrical energy and those PV solar refrigerators yields mixed results. On one hand, those PV solar refrigerators which do make use of a thermal energy storage device provide certain benefits. The most important of these benefits is simply the fact that the solar refrigerator does not require any electrical source of power to maintain the colder temperatures which reside within it. One implication of this is that, assuming that there is a sufficient amount of sunlight reaching the panels of a PV solar refrigerator, the contents of this type of refrigerator can remain well preserved indefinitely. On the other hand it was mentioned earlier, within this chapter, that battery technology is well established. This implies that the maintenance of this type of storage device (i.e., batteries) are less likely to experience any issues and/or difficulties.
There is one important recommendation that this dissertation will make concerning one of the applications of those PV solar refrigerators which make use of a thermal storage device. In particular, this type of solar refrigerator may be important for the transport of blood, vaccines, and/or other biological materials within countries which are less developed. It is likely that such transport will take place over lengthy distances. There is one important benefit within the context of having to transport such items over long distances. To be more specific, these specific subtypes of solar refrigerators do not require any fuel nor do they require any form of external power supply such as batteries. This was discussed at greater length within the chapter titled Applications of Solar Refrigerators. This implies that solar refrigerators, which make use of a thermal device to store energy, can in principle run indefinitely.
This is an important advantage over kerosene refrigerators and bottled gas refrigerators which need to be refilled with the appropriate combustible substance which will produce the flame which, in turn, will derive the physical process of the absorption refrigerators which they fuel. Similarly, those PV solar refrigerators which contain batteries will need to have their batteries replaced occasionally. However, as was just mentioned, those subtypes of PV solar refrigerators have no need to replenish any substance or process which drives the mechanism producing the cooling effect. Therefore, the operators of those PV solar refrigerators which make use of thermal energy storage devices do not need to concern themselves with needing to find a location which can be used to purchase fuel or batteries to run the corresponding absorption refrigerator. Instead, such operators only concern themselves with the requirement that the panels of the solar refrigerator receive sufficient sunlight to "recharge" the thermal storage device of the solar refrigerator. This ability to not be concerned about the need to find locations by which fuel or batteries may be purchased can be an important consideration when transforming blood, vaccines, and/or other biological products across rural areas.
Of course, there are other smaller advantages in using those PV solar refrigerators which store energy through the use of a thermal storage device. One of these smaller advantages consists of the fact that kerosene refrigerators and bottled gas refrigerators (which make use of propane or butane) may present a fire hazard as the result of the combustible nature of their fuel. Of course, their fuel needs to be combustible in order to drive the absorption mechanism associated with this form of refrigerator. However, PV solar refrigerators can create the required cooling effect without the necessity of making use of a combustible substance. Actually, none of the three major types of solar refrigerators, discussed within this dissertation, require the use of combustible substances. Nonetheless, when combined with the comments presented within the previous paragraph, those PV solar refrigerators which make use of thermal storage devices deserve special mention because the advantage discussed up until this point of this paragraph adds to its benefits.
The next portion of this chapter will make recommendations concerning the use of solar absorption refrigerators. This chapter has already mentioned the comparison between those absorption refrigerators which do not make use of solar energy and those solar refrigerators which do not make use of absorption to create the effect of refrigeration. Thus, the last objective of this chapter is to examine the benefits solar absorption refrigerators and to make recommendations concerning these same type of appliances.
Solar absorption refrigerators produce the necessary cooling effect through the use of sunlight as opposed to the fuel which is used by other traditional solar refrigerators. As has been discussed elsewhere within this dissertation, these other traditional absorption refrigerators typically make use of some sort of chemical agent to produce the flame to produce the required cooling effect within the interior of the refrigerator. As a result of this last fact, one of the issues (at least in principle) concerning solar absorption refrigerators concerns the temperature at which the absorbent material changes phases (either from liquid to solid or from gas to liquid). This temperature implicitly means that there exists some limit concerning the available operation of the solar absorption refrigerator. Of course, this statement will also be true of other forms of absorption refrigerators (e.g., those absorption refrigerators which make use of kerosene, propane, or butane). However, what is important within the context of solar absorption refrigerators concerns the ability of the PV panels to raise temperature within the generator sufficiently to allow the efficient separation of the refrigerant from the absorbent. For example, an absorption refrigerator which makes use of ammonia and water as the absorbent and refrigerant desorbs within a temperature range of between 120° Celsius and 130° Celsius. As was discussed much earlier in this dissertation, temperatures within this range can be readily obtained by making use of inexpensive solar tracking devices.
Therefore, solar absorption refrigerators may also be an alternative for those applications for which no external source of a chemical fuel is desired for any reason. This same conclusion was arrived at for the case of a PV direct solar refrigerator which made use of a thermal storage device. However, as was mentioned earlier, PV direct solar refrigerators are readily available through companies such as SunDazer. That is, an important conclusion of this dissertation is that if it is desired that some (likely portable) refrigerator not possess some form of fuel such as kerosene, propane, or butane, then either a solar absorption refrigerator or a solar PV direct refrigerator should be considered.
Finally, this last portion of this chapter will provide a summary of those recommendations which were discussed earlier within this chapter. In particular, this chapter has made the following recommendations:
- Solar mechanical refrigerators should not be used for any application: the primary reason for this is that such solar refrigerators are inefficient unless they are so large as to be impractical.
- Either PV solar refrigerators or solar absorption refrigerators should be used for those applications for which solar refrigerators are to be used: of course, the question then becomes which of these two types of solar refrigerators should be used? To answer this question, the users of such technology should consider the following items (it should be stressed that the following items are only two be considered when addressing the aforementioned no definitive method of determining whether a solar absorption refrigerator, a PV solar refrigerator, or a PV solar refrigerator which makes use of a thermal storage device is best suited for a particular application):
- If a solar absorption refrigerator is to be used for a given application, then the characteristics of both the refrigerant and the absorbent should be considered. To be specific, their characteristics with respect to temperature need to be considered. In particular, will the energy supplied by the solar collection be sufficient to maintain its phase properties.
- If a PV solar refrigerator is to be used, then what will be the availability of batteries? If there is no available source of replacement batteries, then it may be advantageous to make use of a PV solar refrigerator which contains a thermal storage device. This is because such solar refrigerators do not require the use of batteries to store energy for the nighttime or for those days on which the weather is sufficiently cloudy to prevent the use of sunlight to power the refrigeration effect of the appliance.
- If the availability of batteries is not an issue, then PV solar refrigerators which make use of batteries should be used for the application. This is because batteries are a well-developed technology whereas thermal storage technology is less developed. As a result, there is a greater risk that there will be problems with a given PV solar refrigerator which contains a thermal energy storage device than there will be with a PV solar refrigerator which contains batteries.
This dissertation has provided a thorough literary review of the present state of the technology concerning solar refrigerators. It has examined solar refrigerators from the perspectives of the different types of these appliances, their efficiencies, their economics, and their applications. From the conflation of these perspectives, a set of recommendations were developed within the present chapter.
It is inevitable that the future will continue to see further developments that will be relevant to solar refrigerators. Therefore, the author of this dissertation suggests that such a literary review should be periodically performed in the future. Therefore, solar absorption refrigerators may also be an alternative for those applications for which no external source of a chemical fuel is desired for any reason. This same conclusion was arrived at for the case of a PV direct solar refrigerator which made use of a thermal storage device. However, as was mentioned earlier, PV direct solar refrigerators are readily available through companies such as SunDazer. That is, an important conclusion of this dissertation is that if it is desired that some (likely portable) refrigerator not possess some form of fuel such as kerosene, propane, or butane, then either a solar absorption refrigerator or a solar PV direct refrigerator should be considered.
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Isobaric: refers to any process which occurs at constant pressure [Tuckerman].
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