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Solar Powered ARSs

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Solar energy is a renewable and ozone-friendly energy source. Solar cooling is the most attractive subject for many engineers and scientists who work on solar energy applications. Most of the research and development efforts have been carried out using an absorption cooling system. This system is usually a preferable alternative, since it uses thermal energy collected from the sun without the need to convert this energy into mechanical energy as required by the vapor-compression system. Besides, the absorption cooling system utilizes thermal energy at a lower temperature (i.e. in the range 80-110°C) than that used by the vapor-compression system.

Research and development studies on solar ARSs using different combinations of refrigerants and absorbents as working fluids have been done. These ARSs have good potential where solar energy is available as low-grade thermal energy at a temperature level of 100°C and above.

The principle of operation of a solar-powered absorption cooling system is the same as that of the absorption cooling system shown in Figure 3.57, except for the heat source to the generator. In Figure 3.57, we presented a solar absorption cooling system using an R-22 (refrigerant)-DMETEG (absorbent) combination as a working fluid (Dincer et al., 1996). Its operation can be briefly explained as follows. In the absorber, the DMETEG absorbs the R22 at the low pressure and absorber temperature supplied by circulating water, and hence a strong solution occurs (2). This strong solution from the absorber enters a solution pump, which raises its pressure and delivers the solution into the generator through the heat exchanger (3-6). The generator, which is heated by a solar hot water system, raises the temperature of the strong solution, causing the R-22 to separate from it. The remaining weak solution flows down to the expansion valve through the heat exchanger and is throttled into the absorber for further cooling as it picks up a new charge of the R22 vapor, becoming a strong solution (6-2) again. The hot R-22 vapor from the generator passes to the condenser and is released to the liquid phase (8-9). The liquid R-22 enters the second heat exchanger and loses some heat to the cool R-22 vapor. The pressure of the liquid R-22 drops significantly in the throttling valve before it enters the evaporator. The cycle is completed when the desired cooling load is achieved in the evaporator (10–12). Cool R-22 vapor obtained from the evaporator enters the absorber while the weak solution comes to the absorber continuously. The R-22 vapor is absorbed here (12–1). This absorption activity lowers the pressure in the absorber, causing the vapor to be taken off from the evaporator. When the vapor goes into liquid solution, it releases both its latent heat and a heat of dilution. This energy release has to be continuously dissipated by the cooling water.

Solar-operated ARSs have so far achieved limited commercial viability because of their high cost/benefit ratios. The main factor which is responsible for this drawback is the low COP associated with these systems, which generally operate on conventional thermodynamic cycles with common working fluids. It is essential to investigate the possibility of using alternative working fluids operating in new thermodynamic cycles. Also, development of more efficient, less expensive solar collectors will be a continuing need for solar energy to reach its full potential.

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November 27th, 2009 at 11:09 am

Solar Refrigeration Systems

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Many food products (e.g fruits, vegetables, meats, dairy products) are stored in cooling units for periods on the order of weeks at temperatures between 0 and 4°C in order to prevent spoilage and maintain freshness and quality. Food freezing systems are required for longer term storage at -18 to -35°C. Food storage and transport take place in chambers covering a wide range of sizes from cold stores to household refrigerators. Solar cooling is of great interest especially in developing countries, where food preservation is often as difficult a problem as food production.

From an energy saving view, a solar cooling system has the capability of saving electrical energy in the range of 25-40% when compared to an equivalent cooling capacity of a conventional water-cooled refrigeration system. Therefore, the use of solar cooling systems will save energy, especially during the summer season. The contribution of these systems to the food processing sector and consequently to the economy will be high.

Solar-powered mechanical cooling, of whatever type, is presently in the developmental phase. The technology is ready, but cost factors stand in the way of vigorous marketing programs. At present, active solar cooling is not in a reasonably competitive position with respect to conventional cooling systems (energized by electricity or fossil fuel). During the last decade the situation has changed quickly because of increasing interest in renewable energy sources, especially solar energy, for reducing the use of fossil fuels and electricity.

Solar energy can be used in different systems available for cooling applications. These systems are :

* Rankine cycle-vapor compression system,
* absorption cycle system,
* adsorption system,
* jet ejector system,
* Rankine cycle-inverse Brayton cycle system, and
* nocturnal radiation system.

Among these systems, the solar-powered absorption cooling cycle is the most popular system for solar cooling applications due to the following advantages:

* quiet operation,
* high reliability,
* long service life,
* effective and economic use of low-grade energy sources (e.g. solar energy, waste energy, geothermal energy, natural gas),
* easy implementation and capacity control,
* no cycling losses during on-off operations, and
* meeting the variable cooling load easily and efficiently.

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November 27th, 2009 at 11:08 am

Solar Refrigeration

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The developing worldwide shortage of petroleum emphasizes the need for alternative nergy sources which are both inexpensive and clean. There has been high interest in and high potential use of renewable energy sources since the energy crisis faced during the 1970s. During the last few decades, an increasing effort based on research and development has been concentrated on the utilization of renewable energy sources, e.g. solar energy, wind energy, tidal waves, biogas, geothermal energy, hydropower, and hydrogen energy. Among these sources, solar energy for refrigeration applications is very popular because it is direct and easy to use, renewable, and continuous, maintains the same quality, is safe and free, and is environmentally friendly.

The continuous supply of solar energy to the earth’s surface is equal to a power of about 100,000 TW. Approximately one-third of the radiation impinging on land area and accumulated over less than 2 hours should suffice to satisfy the entire primary energy demand by humans for the period of 1 year (Dincer, 1997). More than 25% of the total energy in the world is consumed for heating and cooling of buildings and providing hot water. Therefore, the diversion of this particular energy demand to an alternative source would result in a substantial reduction in the world’s dependence on fossil fuels. The annual incidence of solar energy on buildings in the United States is several times the amount required to heat these buildings; approximately 1015 kWh of solar energy is received on earth annually. It has been projected that by the year 2020 from 25 to 50% of the thermal energy for buildings could be provided from the sun. Consequently, solar energy is an available energy source for many applications ranging from electricity generation to food cooling.

Written by sam

November 27th, 2009 at 10:41 am

Thermoacoustic Refrigeration

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Garrett and Hofler (1992) pointed out that two recent events are responsible for the new em in refrigeration before the beginning of the twenty-first century. The most significant of these is the international agreement (signing of the Montreal Protocol) on the production and consumption of CFCs, which were found to be causing the depletion of the stratospheric ozone layer. The second event was the discovery of ‘high-temperature’ superconductors and the development of high-speed and high-density electronic circuits which require active cooling and hence a new approach to refrigeration, or thermoacoustic refrigeration, which was first discovered by Wheatley and others (1993) in August 1983. The simplicity of the hardware involved in thermoacoustic machines is best appreciated by examining a concrete example. In the mid-1990s, S.L. Garrett and his colleagues at the Naval Postgraduate School in Monterey, California developed two thermoacoustic refrigerators for the Space Shuttle. The first was designed to cool electronic components, and the second was intended to replace the refrigerator-freezer unit used to preserve blood and urine samples from astronauts engaged in biomedical experiments .

Thermoacoustic refrigeration is considered a new technology, attaining cooling without the need for refrigerants. The basic mechanism is very simple and efficient. A loudspeaker creates sound in a hollow tube which is filled with an ordinary gas. In fact, thermoacoustic refrigeration utilizes high-density sound waves to transfer heat due to the thermoacoustic effect (i.e. acoustic energy). Therefore, the working fluid in this system is acoustically driven gas. The process itself utilizes standing acoustic waves in an enclosed cavity to generate the mechanical compression and expansion of a working fluid (gas in this case) needed for the cooling cycle. The technique has the potential for high efficiency operation without the need for cooling liquids or mechanical moving parts. These factors make the concept amenable to miniaturization to chip-scale dimensions for thermal management of electronic components.

The interaction between acoustics and thermodynamics has been known ever since the dispute between Newton and Laplace over whether the speed of sound was determined by the adiabatic or isothermal compressibility of air. At the present time, the efficiency of thermoacoustic refrigerators is 20—30% lower than their vapor compression refrigerators. Part of that lower efficiency is due to the intrinsic irreversibilities of the thermoacoustic heat transport process. These intrinsic irreversibilities are also the favorable aspects of the cycle, since they make for mechanical simplicity, with few or no moving parts. A greater part of the inefficiency of current thermoacoustic refrigerators is simply due to technical immaturity. With time, improvements in heat exchangers and other sub-systems should narrow the gap. It is also likely that the efficiency in many applications will improve due only to the fact that thermoacoustic refrigerators are well suited to proportional control. One can easily and continuously control the cooling capacity of a thermoacoustic refrigerator so that its output can be adjusted accurately for varying load conditions. This could lead to higher efficiencies than for conventional vapor compression chillers which have constant displacement compressors and are therefore only capable of binary (on/off) control. Proportional control avoids losses due to the start-up surges in conventional compressors and reduces the inefficiencies in the heat exchangers, since such systems can operate over smaller temperature gaps between the coolant fluid and the heat load.

The research focus of the Thermoacoustics Laboratory in ARL at Pennsylvania State University in cooperation with Los Alamos Research Laboratory is the study of acoustically driven heat transport. Their goals include an improved understanding of fundamental thermoacoustic processes and the development of new thermoacoustic refrigerators and heat engines with increased power density, temperature span, and efficiency, and the commercialization of those devices. The Laboratory provides the infrastructure to support research on the basic processes required to understand this emerging, environmentally friendly refrigeration technology. This facility also supports the fabrication and testing required to produce complete, full-scale operational prototype refrigeration systems for military and commercial applications such as food refrigerators/freezers and air conditioners. Their prototypes have been flown on the Space Shuttle and have been used to cool radar electronics onboard a US Navy warship.  Thermoacoustic refrigerators with cooling powers ranging from a few watts to chillers with cooling capacities in excess of 10 kW are currently in operation or under construction. Figure 3.70a shows a thermoacoustic refrigerator developed by this Laboratory and it is operational for running a small fridge in Figure 3.70b.

Although thermoacoustic refrigerators have not been commercialized yet and are considered to be still at a developmental stage, it is known that they can be used for any kind of cooling. Conventional, single-stage, electrically operated thermoacoustic refrigerators can reach cold-side temperatures two-thirds to three-quarters of ambient, so they are not well suited to cryogenic applications below –40°. However, thermoacoustically driven pulse-tube style refrigerators can reach the cryogenic temperatures required to liquefy air or natural gas. In their early commercial stages, they will probably be limited to niche applications such as in military systems which are required to operate in closed environments and food merchandising where toxicity is an important issue. As global environmental mandates and legislations/amendments become essential, one can expect the scope of thermoacoustic applications to expand both domestically and in emerging markets.

thermoacoustic-refrigerator

Written by sam

November 27th, 2009 at 10:27 am

Thermoelectric Refrigeration

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This type of system is used to move heat from one area to another by use of electrical energy. The electrical energy, rather than the refrigerant, serves as a ‘carrier’. The essential use of thermoelectric systems has been in portable refrigerators, water coolers, cooling of scientific apparatus used in space exploration, and in aircraft. The main advantage of this system is that there are no moving parts. Therefore, the system is compact, quiet, and needs little service.

Thermoelectric coolers are solid state equipment used in applications where temperature stabilization, temperature cycling, or cooling below ambient are required. There are many products using thermoelectric coolers, including CCD (charge coupled device) cameras, laser diodes, microprocessors, blood analyzers and portable picnic coolers.

Thermoelectrics are based on the Peltier Effect, discovered in 1834, by which DC current applied across two dissimilar materials causes a temperature differential. The Peltier effect is one of the three thermoelectric effects, the other two being known as the Seebeck effect and Thomson effect. Whereas the last two effects act on a single conductor, the Peltier effect is a typical junction phenomenon. The three effects are connected to each other by a simple relationship (Godfrey, 1996).

The typical thermoelectric module is manufactured using two thin ceramic wafers with a series of P and N doped bismuth-telluride semiconductor materials sandwiched between them. The ceramic material on both sides of the thermoelectric adds rigidity and the necessary electrical insulation. The N type material has an excess of electrons, while the P type material has a deficit of electrons. One P and one N make up a couple, as shown in Figure 3.69. The thermoelectric couples are electrically in series and thermally in parallel. A thermoelectric module can contain one to several hundred couples. As the electrons move from the P type material to the N type material through an electrical connector, the electrons jump to a higher energy state absorbing thermal energy (cold side). Continuing through the lattice of material, the electrons flow from the N type material to the P type material through an electrical connector, dropping to a lower energy state and releasing energy as heat to the heat sink (hot side).

Thermoelectrics can be used to heat and to cool, depending on the direction of the current. In an application requiring both heating and cooling, the design should focus on the cooling mode. Using a thermoelectric in the heating mode is very efficient because all the internal heating (Joulian heat) and the load from the cold side is pumped to the hot side. This reduces the power needed to achieve the desired heating.

thermoelectric-coolers

Written by sam

November 27th, 2009 at 10:21 am

Steam Jet Refrigeration Systems

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In steam jet refrigeration systems, water can be used as the refrigerant. Like air, it is perfectly safe. These systems were applied successfully to refrigeration in the early years of this century. At low temperatures the saturation pressures are low (0.008129 bar at 4°C) and the specific volumes are high (157.3 m3/kg at 4°C). The temperatures that can be attained using water as a refrigerant are not low enough for most refrigeration applications but are in the range which may satisfy air conditioning, cooling, or chilling requirements. Also, these systems are used in some chemical industries for several processes, e.g. the removal of paraffin wax from lubricating oils. Note that steam jet refrigeration systems are not used when temperatures below 5°C are required. The main advantages of this system are the utilization of mostly low-grade energy and relatively small amounts of shaft work.

Steam jet refrigeration systems use steam ejectors to reduce the pressure in a tank containing the return water from a chilled water system. The steam jet ejector utilizes the energy of a fast-moving jet of steam to capture the flash tank vapor and compress it. Flashing a portion of the water in the tank reduces the liquid temperature. Figure 3.66 presents a schematic arrangement of a steam jet refrigeration system for water cooling. In the system shown, high-pressure steam expands while flowing through the nozzle 1. The expansion causes a drop in pressure and an enormous increase in velocity. Due to the high velocity, flash vapor from the tank 2 is drawn into the swiftly moving steam and the mixture enters the diffuser 3. The velocity is gradually reduced in the diffuser but the pressure of the steam at the condenser 4 is increased 5-10 times more than that at the entrance of the diffuser (e.g. from 0.01 bar to 0.07 bar).

This pressure value corresponds to the condensing temperature of 40°C. This means that the mixture of high-pressure steam and the flash vapor may be liquefied in the condenser. The latent heat of condensation is transferred to the condenser water, which may be at 25 °C. The condensate 5 is pumped back to the boiler, from which it may again be vaporized at a high pressure. The evaporation of a relatively small amount of water in the flash tank (or flash cooler) reduces the temperature of the main body of water. The cooled water is then pumped as the refrigeration carrier to the cooling-load heat exchanger.

steam-jet-refrigeration-system

An ejector was invented by Sir Charles Parsons around 1901 for removing air from steam engine condensers. In about 1910, the ejector was used by Maurice Leblanc in the steam ejector refrigeration system It experienced a wave of popularity during the early 1930s for air conditioning large buildings. Steam ejector refrigeration cycles were later supplanted by systems using mechanical compressors. Since that time, development and refinement of ejector refrigeration systems have been almost at a standstill as most efforts have been concentrated on improving vapor compression cycles (Aphornratana et al., 2001).

Furthermore, another typical gas-driven ejector is shown schematically in Figure 3.67a. High-pressure primary fluid (P) enters the primary nozzle, through which it expands to produce a low-pressure region at the exit plane (1). The high-velocity primary stream draws and entrains the secondary fluid (S) into the mixing chamber. The combined streams are assumed to be completely mixed at the end of the mixing chamber (2) and the flow speed is supersonic. A normal shock wave is then produced within the mixing chamber’s throat (3), creating a compression effect, and the flow speed is reduced to a subsonic value. Further compression of the fluid is achieved as the mixed stream flows through the subsonic diffuser section (b).

jet-ejector-refrigeration

Figure 3.67b shows a schematic diagram of an ejector refrigeration cycle. It can be seen that a boiler, an ejector and a pump are used to replace the mechanical compressor of a conventional system. High-pressure and high-temperature refrigerant vapor is evolved in a boiler to produce the primary fluid for the ejector. The ejector draws vapor refrigerant from the evaporator as its secondary. This causes the refrigerant to evaporate at low pressure and produce useful refrigeration. The ejector exhausts the refrigerant vapor to the condenser where it is liquefied. The liquid refrigerant accumulated in the condenser is returned to the boiler via a pump whilst the remainder is expanded through a throttling valve to the evaporator, thus completing the cycle. As the working input required to circulate the fluid is typically less than 1 % of the heat supplied to the boiler, the COP may be defined as the ratio of evaporator refrigeration load to heat input to the boiler as follows:

cal-1

Recently, Aphornratana et al. (2001) have developed a new jet ejector refrigeration system using R-ll as the refrigerant as shown in Figure 3.68. All vessels in the systems were constructed from galvanized steel. The boiler was designed to be electrically heated, two 4 kW electric heaters being located at the lower end. At its upper end, three baffle plates were welded to the vessel to prevent liquid droplets being carried over with the refrigerant vapor. The evaporator design was similar to that of the boiler. A single 3 kW electric heater was used to simulate a cooling load. A water-cooled plate type heat exchanger was used as a condenser. Cooling water was supplied at 32°C. The boiler was covered with a 40 mm thickness of glass wool with aluminum foil backing. The evaporator was covered with a 30 mm thickness of neoprene foam rubber. A diaphragm pump was used to circulate liquid refrigerant from the receiver tank to the boiler and the evaporator. The pump was driven by a variable speed 1/4 hp motor. One drawback of using the diaphragm pump is cavitation of the liquid refrigerant in the suction line due to pressure drop through an inlet check-valve. Therefore a small chiller was used to sub-cool the liquid R-11 before entering the pump. Figure 3.68c shows a detailed schematic diagram of the experimental ejector. The nozzle was mounted on a threaded shaft, which allowed the position of the nozzle to be adjusted. Two different mixing chambers with throat diameter of 8 mm were used: in mixing chamber no.l, the mixing section is constant area duct: in mixing chamber no.2, the mixing section is convergent duct.

Aphornratana et al.’s experiments showed that an ejector-refrigeration system using R-11 proved to be practical and could provide reasonably acceptable performance. It can provide a cooling temperature as low as -5°C. The cooling capacity ranged from 500 to1700 W with COP ranging from 0.1 and 0.25.

jet-ejector-refrigerator

Written by sam

November 27th, 2009 at 10:04 am

Air Standard Refrigeration Systems

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The air-standard refrigeration cycles are also known as the reverse Brayton cycles. In these systems, refrigeration is accomplished by means of a non-condensing gas (e.g. air) cycle rather than a refrigerant vapor cycle. While the refrigeration load per kilogram of refrigerant circulated in a vapor-compression cycle is equal to a large fraction of the enthalpy of vaporization, in an air cycle it is only the product of the temperature rise of the gas in the low-side heat exchanger and the specific heat of the gas. Therefore, a large refrigeration load requires a large mass rate of circulation. In order to keep the equipment size smaller, the complete unit may be under pressure, which requires a closed cycle. The throttling valve used for the expansion process in a vapor-compression refrigeration cycle is usually replaced by an expansion engine (e.g. expander) for an air cycle refrigeration system. The work required for the refrigeration effect is provided by the gas refrigerant. These systems are of great interest in applications where the weight of the refrigerating unit must be kept to a minimum, for example, in aircraft cabin cooling.

A schematic arrangement of a basic air-standard refrigeration cycle and its T-s diagram is shown in Figure 3.64. This system has four main elements:

• a compressor that raises the pressure of the refrigerant from its lowest to its highest value (e.g. isentropic compression: 1-2),
• an energy output heat exchanger where the high temperature of the refrigerant is lowered (e.g. isobaric heat rejection: 2-3),
• an expander where the pressure and temperature of the refrigerant are reduced (e.g. isentropic expansion: 3-4), and
• an energy input heat exchanger that raises the temperature of the refrigerant at a constant pressure (e.g. isobaric heat input: 4-1). This input is known as refrigeration load.

air-standard-refrigeration

The utilization of air as a refrigerant becomes more attractive when a double purpose is to be met. This is so in the case of air conditioning, when the air can be both the refrigerating and the air conditioning medium. Figure 3.65 shows an air standard refrigeration cycle using a heat exchanger and its T-s diagram. Furthermore, air-standard refrigeration cycle is commonly used in the liquefaction of air and other gases and also in certain cases where refrigeration is needed such as aircraft cooling systems.

air-standard-refrigeration-heat-exchanger

Written by sam

November 27th, 2009 at 10:00 am

Electrochemical Absorption Refrigeration Systems

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Recently, Newell (2000) has proposed a new electrochemical ARS as shown in Figure 3.52, which consists of four main components. An electrochemical cell is the heat absorber, equivalent to an evaporator in a conventional vapor compression refrigeration system. A fuel cell rejects heat in a manner similar to a condenser in a common vapor compression refrigeration cycle. The third component is a heat exchanger between gas streams and water flow stream. The fourth component is a current pump for elevating the fuel cell’s voltage output to a level sufficient for driving the electrochemical cell. The voltage required is sufficiently low such that the cycle may be one that is conveniently matched for solar photovoltaic cells or other direct current electric energy conversion systems. In fact, the system shown in Figure 3.52 can be used as a thermally driven power cycle by operating the fuel cell at a temperature lower than the electrochemical cell. The voltage supply becomes a load driven by the electric circuit. Lowering component irreversibilities is essential to reach a breakeven operating condition where the fuel cell is generating sufficient power for operation of the electrochemical cell. Newell’s system is based on a water/hydrogen/oxygen fuel cell and electrochemical cell combination. Other combinations are also considered. Each one has its own advantages or disadvantages. The configuration envisioned for the system operates near atmospheric pressure. The components could be operated at nearly uniform pressures with gravitation, surface tension, or low head pumping used for transporting the working fluids within and between components. Water may be moved from the electrochemical cell and fuel cell to external heat exchange surfaces, or the cells could be configured for direct heat exchange with their surroundings.

electrochemical-ars

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November 27th, 2009 at 7:12 am

Steam Ejector Recompression Absorption Refrigeration Systems

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The ejector recompression absorption cycle, which has recently been developed by Eames and Wu (2000), is similar to the conventional single-effect lithium bromide absorption cycle. The difference between them is that there is a steam ejector in this novel cycle for enhancing the concentration process. Because of the use of the steam ejector, the performance and the operating characteristics of the novel cycle are different from the conventional cycle.

The steam ejector recompression absorption cycle is shown schematically in Figure 3.51a. In this figure, the expansion of the high-pressure steam causes a low pressure at the exit of the primary nozzle of the steam ejector, therefore, the vapor at point 8 in the concentrator is entrained by the primary flow. The two streams are mixed in the steam ejector and condensed in the heat exchanger of the concentrator. The condensation heat is used to heat the solution in the concentrator. Obviously, the heat of the entrained vapor is recovered by the steam ejector in this process. Water at point 3 splits into two streams; one flows back to the steam generator and the other flows into the condenser. In stable operation, the mass flow rate of the first stream equals that of primary flow while the mass flow rate of the second stream equals that of the entrained vapor. The rest of the cycle is similar to that of the conventional single-effect lithium bromide absorption cycle. Figure 3.51b shows the novel cycle on a P-T-C diagram. As shown in Figure 3.51b, the cycle 6-7-9-10-6 takes up water at the absorber (10-6) and releases it as vapor at the concentrator (7-9). In the conventional absorption cycle, the vapor is condensed at 8′ and the condensation heat is rejected to the surroundings. In the novel cycle, this vapor undergoes a compression process through the ejector to point 2. Since the vapor temperature is greater than the solution temperature in the concentrator, this vapor is used to heat the solution by condensation to point 3. Therefore the heat otherwise wasted is recovered and the energy efficiency is improved.

Eames and Wu (2000) investigated the energy efficiency and the performance characteristics of the novel cycle and the theoretical results showed that the COP of the novel cycle is better than that of the conventional single-effect absorption cycle. The characteristics of the cycle performance show its promise in using high temperature heat source at low cost.

Recently, Kang et al. (2000) have undertaken a study to propose and evaluate advanced absorption cycles for the COP improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H2O-LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH3-H2O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH3-H2O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H2O-LiBr cycle is better from the high COP viewpoint for evaporation temperature over 0°C while the NH3-H2O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H2O-LiBr and NH3-H2O cycles.

steam-ejector-ars

Written by sam

November 27th, 2009 at 7:07 am

Crystallization Absorption Refrigeration Systems

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Some absorption chillers are notorious for ‘freezing up’ or crystallizing. The basic mechanism of failure is simple enough – the lithium bromide solution becomes so concentrated that crystals of lithium bromide form and plug the machine (usually the heat exchanger section). The most frequent causes are:

• air leakage into the machine,
• low temperature condenser water, and
• electric power failures.

The first two are actually very similar since they both drive the heat input up to the point that crystallization can occur. Whether air leaks into the machine or the condenser water temperature is too low, the water vapor pressure in the absorption chiller evaporator has to be lower than normal to produce the required cooling. This forces the heat input to the machine to be higher to increase the solution concentration. Air leakage into the machine can be controlled by designing the machine with hermetic integrity and routinely purging the unit using a vacuum pump.

Excessively cold condenser water (coupled with a high load condition) can also cause crystallization. While reducing condenser water temperature does improve performance, it could cause a low enough temperature in the heat exchanger to crystallize the concentrate. Sudden drops in condenser water temperature could cause crystallization. For this reason, some of the early absorption chillers were designed to produce a constant condenser water temperature. Modern absorption chillers have special controls that limit the heat input to the machine during these periods of lower condenser water temperatures.

Power failures can cause crystallization as well. A normal absorption chiller shutdown uses a dilution cycle that lowers the concentration throughout the machine. At this reduced concentration, the machine may cool to ambient temperature without crystallization. However, if power is lost when the machine is under full load and highly concentrated solution is passing through the heat exchanger, crystallization can occur. The longer the power is out, the greater the probability of crystallization.

Major absorption chiller manufacturers now incorporate devices that minimize the possibility of crystallization. These devices sense impending crystallization and shut the machine down after going through a dilution cycle. These devices also prevent crystallization in the event of power failure. A typical anti-crystallization device consists of two primary components: (i) a sensor in the concentrated solution line at a point between the concentrator and the heat exchanger, and (ii) a normally open, two-position valve located in a line connecting the concentrated solution line and the line supplying refrigerant to the evaporator sprays.

Written by sam

November 27th, 2009 at 7:02 am

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