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التبريد الامتصاصي بالطاقة الشمسية

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    عبدالله الفلاح
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    CHAPTER 13
    ABSORPTION
    REFRIGERATION
    Kevin D. Rafferty, P.E.
    Geo-Heat Center
    Klamath Falls, OR 97601
    13.1 INTRODUCTION
    The absorption cycle is a process by which refrigeration
    effect is produced through the use of two fluids and
    some quantity of heat input, rather than electrical input as in
    the more familiar vapor compression cycle. Both vapor
    compression and absorption refrigeration cycles accomplish
    the removal of heat through the evaporation of a refrigerant
    at a low pressure and the rejection of heat through the
    condensation of the refrigerant at a higher pressure. The
    method of creating the pressure difference and circulating
    the refrigerant is the primary difference between the two
    cycles. The vapor compression cycle employs a mechanical
    compressor to create the pressure differences necessary to
    circulate the refrigerant. In the absorption system, a
    secondary fluid or absorbent is used to circulate the
    refrigerant. Because the temperature requirements for the
    cycle fall into the low-to-moderate temperature range, and
    there is significant potential for electrical energy savings,
    absorption would seem to be a good prospect for geothermal
    application.
    Absorption machines are commercially available today
    in two basic configurations. For applications above 32oF
    (primarily air conditioning), the cycle uses lithium bromide
    as the absorbent and water as the refrigerant. For applications
    below 32oF, an ammonia/water cycle is employed with
    ammonia as the refrigerant and water as the absorbent.
    13.2 LITHIUM BROMIDE/WATER CYCLE
    MACHINES
    Figure 13.1 shows a diagram of a typical lithium
    bromide/water machine (Li Br/H2O). The process occurs in
    two vessels or shells. The upper shell contains the
    generator and condenser; the lower shell, the absorber and
    evaporator.
    Heat supplied in the generator section is added to a
    solution of Li Br/H2O. This heat causes the refrigerant, in
    this case water, to be boiled out of the solution in a
    distillation process. The water vapor that results passes into
    the condenser section where a cooling medium is used to
    condense the vapor back to a liquid state. The water then
    flows down to the evaporator section where it passes over
    tubes containing the fluid to be cooled. By maintaining a
    Figure 13.1 Diagram of two-shell lithium bromide
    cycle water chiller (ASHRAE, 1983).
    very low pressure in the absorber-evaporator shell, the water
    boils at a very low temperature. This boiling causes the
    water to absorb heat from the medium to be cooled, thus,
    lowering its temperature. Evaporated water then passes into
    the absorber section where it is mixed with a Li Br/H2O
    solution that is very low in water *******. This strong
    solution (strong in Li Br) tends to absorb the vapor from the
    evaporator section to form a weaker solution. This is the
    absorption process that gives the cycle its name. The weak
    solution is then pumped to the generator section to repeat the
    cycle.
    As shown in Figure 13.1, there are three fluid circuits
    that have external connections: a) generator heat input, b)
    cooling water, and c) chilled water. Associated with each of
    these circuits is a specific temperature at which the
    machines are rated. For single-stage units, these temperatures
    are : 12 psi steam (or equivalent hot water) entering
    the generator, 85oF cooling water, and 44oF leaving chilled
    water (ASHRAE, 1983). Under these conditions, a coefficient
    of performance (COP) of approximately 0.65 to 0.70
    could be expected (ASHRAE, 1983). The COP can be
    thought of as a sort of index of the efficiency of the machine.
    It is calculated by dividing the cooling output by the
    299
    required heat input. For example, a 500-ton absorption
    chiller operating at a COP of 0.70 would require: (500 x
    12,000 Btu/h) divided by 0.70 = 8,571,429 Btu/h heat
    input. This heat input suggests a flow of 9,022 lbs/h of 12
    psi steam, or 1,008 gpm of 240oF water with a 17oF D T.
    Two-stage machines with significantly higher COPs
    are available (ASHRAE, 1983). However, temperature
    requirements for these are well into the power generation
    temperature range (350oF). As a result, two-stage machines
    would probably not be applied to geothermal applications.
    13.3 PERFORMANCE
    Based on equations that have been developed
    (Christen, 1977) to describe the performance of a singlestage
    absorption machine, Figure 13.2 shows the effect on
    COP and capacity (cooling output) versus input hot-water
    temperature. Entering hot water temperatures of less than
    220oF result in substantial reduction in equipment capacity.
    The reason for the steep drop off in capacity with
    temperature is related to the nature of the heat input to the
    absorption cycle. In the generator, heat input causes boiling
    to occur in the absorbent/refrigerant mixture. Because the
    pressure is fairly constant in the generator, this fixes the
    boiling temperature. As a result, a reduction in the entering
    hot water temperature causes a reduction in the
    temperature difference between the hot fluid and the boiling
    mixture. Because heat transfer varies directly with temperature
    difference, there is a nearly linear drop off in absorption
    refrigeration capacity with entering hot water temperature.
    In the past few years, one manufacturer (Yazaki,
    undated) has modified small capacity units (2 to 10 ton) for
    increased performance at lower inlet temperature. However,
    low-temperature modified machines are not yet available
    in large outputs, which would be applicable to
    institutional- and industrial-type projects. Although COP
    and capacity are also affected by other variables such as
    condenser and chilled water temperatures and flow rates,
    generator heat input conditions have the largest impact on
    performance. This is a particularly important consideration
    with regard to geothermal applications.
    Because many geothermal resources in the 240oF and
    above temperature range are being investigated for power
    generation using organic Rankine cycle (ORC) schemes, it
    is likely that space conditioning applications would see
    temperatures below this value. As a result, chillers
    operating in the 180 to 230oF range would (according to
    Figure 13.2) have to be (depending on resource temperature)
    between 400 and 20% oversized respectively for a
    particular application. This would tend to increase capital
    cost and decrease payback when compared to a conventional
    system.
    An additional increase in capital cost would arise from
    the larger cooling tower costs that result from the low COP
    of absorption equipment. The COP of singe effect equipment
    is approximately 0.7. The COP of a vapor compression
    machine under the same conditions may be 3.0 or
    higher. As a result, for each unit of refrigeration, a vapor
    compression system would have to reject 1.33 units of heat
    at the cooling tower. For an absorption system, at a COP of
    0.7, 2.43 units of heat must be rejected at the cooling tower.
    This results in a significant cost penalty for the absorption
    system with regard to the cooling tower and accessories.
    Figure 13.2 Capacity of a lithium bromide absorption chiller (Christen, 1977).
    300
    0
    100
    200
    300
    400
    500
    600
    Installed Cost in $ *1000
    0 200 400 600 800 1000
    Capacity in Tons
    Abs chlr
    Elec chlr
    Abs twr
    Elec twr
    In order to maintain good heat transfer in the generator
    section, only small D Ts can be tolerated in the hot water
    flow stream. This is a result of the fact that the machines
    were originally designed for steam input to the generator.
    Heat transfer from the condensing steam is a constant
    temperature process. As a result, in order to have equal
    performance, the entering hot water temperature would have
    to be above the saturated temperature corresponding to the
    inlet steam pressure at rated conditions. This is to allow for
    some D T in the hot water flow circuit. In boiler coupled
    operation, this is of little consequence to operating cost.
    However, because D T directly affects flow rate, and thus
    pumping energy, this is a major consideration in geothermal
    applications.
    For example, assuming a COP of 0.54 and 15oF D T on
    the geothermal fluid, 250 ft pump head and 65% wire-towater
    efficiency at the well pump, approximately 0.20 kW/t
    pumping power would be required. This compares to
    approximately 0.50 - 0.60 kW/t for a large centrifugal
    machine (compressor consumption only).
    The small D T and high flow rates also point out
    another consideration with regard to absorption chiller use
    in space conditioning applications. Assume a geothermal
    system is to be designed for heating and cooling a new
    building. Because the heating system can be designed for
    rather large DTs in comparison to the chiller, the
    incremental cost of the absorption approach would have to
    include the higher well and/or pump costs to accommodate
    its requirements. A second approach would be to design the
    well for space heating requirements and use a smaller
    absorption machine for base load duty. In this approach, a
    second electric chiller would be used for peaking. In either
    case, capital cost would be increased.
    13.4 LARGE TONNAGE EQUIPMENT COSTS
    Figure 13.3 presents some more general cost
    information on large tonnage (>100 tons) cooling equipment
    for space conditioning applications. The plot shows the
    installed costs for both absorption chillers (Abs. chlr.),
    centrifugal chillers (Elec. chlr.), and auxilliary condenser
    equipment (cooling tower, cooling water pumps and cooling
    water piping) for both absorption chillers (Abs. twr.) And
    centrifugal chillers (Elec. twr.). As shown, both the chiller
    itself and its auxilliary condenser equipment costs are much
    higher for the absorption design than for electric-driven
    chillers. These are the primary capital cost differences that
    a geothermal operation would have to compensate for in
    savings.
    13.5 SMALL TONNAGE EQUIPMENT
    To our knowledge, there is only one company (Yazaki,
    undated) currently manufacturing small tonnage (<20
    tons) lithium bromide refrigeration equipment. This firm,
    located in Japan, produces equipment primarily for solar
    applications. Currently, units are available in 1.3, 2, 3, 5,
    7.5, and 10 ton capacities. These units can be manifolded
    together to provide capacities of up to 50 tons.
    Because the units are water cooled chillers, they
    require considerably more mechanical equipment for a
    given capacity than the conventional electric vapor
    compression equipment usually applied in this size range.
    In addition to the absorption chiller itself, a cooling tower is
    required. The cooling tower, which is installed outside,
    requires interconnecting piping and a circulation pump.
    Because the absorption machine produces chilled water, a
    cooling coil and fan are required to deliver the cooling
    Figure 13.3 Chiller and auxiliary equipment costs - electric and absorption (Means, 1996).
    301
    capacity to the space. Insulated piping is required to connect
    the machine to the cooling coil. Another circulating
    pump is required for the chilled water circuit. Finally, hot
    water must be supplied to the absorption machine. This
    requires a third piping loop.
    In order to evaluate the economic merit of small
    absorption equipment compared to conventional electric
    cooling, Figure 13.4 was developed. This plot compares
    the savings achieved through the use of the absorption
    equipment to its incremental capital costs over a
    conventional cooling system. Specifically, the figure plots
    cost of electricity against simple payback in years for the
    five different size units. In each case, the annual electric
    cost savings of the absorption system (at 2,000 full load
    hours per year) is compared to the incremental capital cost
    of the system to arrive at a simple payback value. The
    conventional system to which absorption is compared in this
    case is a rooftop package unit. This is the least expensive
    conventional system available. A comparison of the
    absorption approach to more sophisticated cooling systems
    (VAV, 4-pipe chilled water, etc.) would yield much more
    attractive payback periods.
    The plot is based on the availability of geothermal fluid
    of sufficient temperature to allow operation at rated capacity
    (190oF or above). In addition, other than piping, no costs
    for geothermal well or pumping are incorporated. Only
    cooling equipment related costs are considered. As a result,
    the payback values in Figure 13.4 are valid only for a
    situation in which a geothermal resource has already been
    developed for some other purpose (space heating and
    aquaculture), and the only decision at hand is that of
    choosing between electric and absorption cooling options.
    Figure 13.4 also shows that the economics of small
    tonnage absorption cooling are attractive only in cases of 5
    to 10 ton capacity requirements and more than $0.10 kW/h
    electrical costs. Figure 13.4 is based on an annual cooling
    requirement of 2,000 full load hours per year. This is on the
    upper end of requirements for most geographical areas. To
    adjust for other annual cooling requirements, simply
    multiply the simple payback from Figure 13.4 by actual full
    load hours and divide by 2,000.
    The performance of the absorption cooling machine
    was based on nominal conditions in order to develop Figure
    13.4. It should be noted that, as with the larger machines,
    performance is heavily dependent upon entering hot water
    temperature and entering cooling water temperature.
    Ratings are based on 190oF entering hot water, 85 oF entering
    cooling water and 48oF leaving chilled water. Flow
    rates for all three loops are based upon a 9oF D T.
    Figure 13.4 Simple payback on small absorption equipment compared to conventional rooftop equipment.
    302
    Figure 13.5 Small tonnage absorption equipment performance.
    Figure 13.5 illustrates the effect of entering hot water
    temperature and entering cooling water temperature on
    small machine performance. At entering hot water
    temperatures of less than 180oF, substantial derating is
    necessary. For preliminary evaluation, the 85oF cooling
    water curve should be employed.
    13.6 COMMERCIAL REFRIGERATION
    Most commercial and industrial refrigeration applications
    involve process temperatures of less than 32oF and
    many are 0oF. As a result, the lithium bromide/water cycle
    is no longer able to meet the requirements, because water
    is used for the refrigerant. As a result, a fluid which is not
    subject to freezing at these temperatures is required. The
    most common type of absorption cycle employed for these
    applications is the water/ammonia cycle. In this case, water
    is the absorbent and ammonia is the refrigerant.
    Use of water/ammonia equipment in conjunction with
    geothermal resources for commercial refrigeration applications
    is influenced by some of the same considerations as
    space cooling applications. Figure 13.5 illustrates the most
    important of these. As refrigeration temperature is reduced,
    the required hot water input temperature is increased.
    Because most commercial and industrial refrigeration
    applications occur at temperatures below 32oF,
    required heat input temperatures must be at least 230oF. It
    should also be remembered that the required evaporation
    temperature is 10 to 15oF below the process temperature.
    For example, for a +20oF cold storage application, a 5oF
    evaporation temperature would be required.
    Figure 13.6 suggests a minimum hot water temperature
    of 275oF would be required. There is not a large number of
    geothermal resources in this temperature range. For geothermal
    resources that produce temperatures in this range,
    it is likely that small scale power generation would be competing
    consideration unless cascaded uses are employed.
    Figure 13.7 indicates another consideration for refrigeration
    applications. The COP for most applications is
    likely to be less than 0.55. As a result, hot water flow
    requirements are substantial. In addition, the cooling tower
    requirements, as discussed above, are much larger than for
    equivalently sized vapor compression equipment.
    13.7 CURRENT ABSORPTION RESEARCH
    Recent work at the Lawrence Berkeley Laboratory
    (LBL)(Wahlig, 1984) has resulted in significantly improved
    absorption cycle performance. Researchers at LBL,
    working to improve absorption cycle performance for solar
    application, have developed two advanced versions of the
    ammonia/water machine. Ammonia/water was chosen as
    the working fluid pair in order to allow the use of an aircooled
    condenser for potential heat pump operation.
    303
    Figure 13.6 Required resource temperatures for ammonia/water absorption equipment (Hirai, 1982).
    Figure 13.7 The COP for ammonia/water absorption equipment in refrigeration applications (Hirai, 1982).
    304
    Figure 13.8 Ammonia/water single and double effect regenerative cycle performance (Wahlig, 1984).
    The two cycles that have been developed are designed
    1R and 2R for single effect regenerative cycle and double
    effect regenerative cycle, respectively. As shown in Figure
    13.8, these two cycles show substantially higher COP, over
    a much broader range of generator input temperatures than
    the conventional lithium/bromide cycles. The superior
    performance is achieved by operating the chiller input stage
    at constant temperature, rather than constant pressure as in
    conventional systems. This has the effect of reducing the
    thermodynamic irreversibilities in the absorption cycle
    (Wahlig, 1984).
    It is not known to what extent this technology has been
    incorporated by the major manufacturers of indirect fired
    absorption equipment.
    13.8 MATERIALS
    The generator section is the only portion of the
    absorption machine that is likely to be exposed to the
    geothermal fluid. In this section, the heating medium is
    passed through a tube bundle to provide heat to the
    refrigerant/absorbent mixture located in the shell.
    The generator tube bundle is generally constructed of
    copper or a copper alloy (90/10 cupro nickel). These alloys,
    as discussed in Chapter 8, are not compatible with
    most geothermal resources, particularly if hydrogen
    sulphide (H2S), ammonia (NH3) or oxygen are present. Because
    most resources contain some or all of these
    dissolved gases, exposure of standard construction chillers
    to these fluids is not recommended. Two available options
    are:
    1. Special order chiller with corrosion resistant tubes.
    2. An isolation heat exchanger and clean water loop.
    Conversations with at least one major large tonnage
    absorption machine manufacturer indicate that the first
    option may be the most cost effective (Todd, 1987).
    Although a 316 stainless steel tube would appear to be the
    most cost effective, the manufacturer suggest the use of
    titanium. Because titanium tubes are more generally
    available in the enhanced surface configurations necessary
    for this application, their cost is very competitive with the
    stainless steel tubes. In addition, the use of unenhanced
    stainless steel tubes would, according to the manufacturer,
    result in a large de-rating of the chiller because of less
    effective heat transfer.
    The incremental capital cost for this type of
    construction (titanium generator tubes) would amount to
    approximately 10 to 15% of the basic machine cost. In most
    cases, this would be far less than the cost associated with
    the heat exchanger, circulating pump, piping, and controls
    necessary for an isolation loop. An additional advantage is
    that the alternate generator construction avoids the losses
    associated with the heat exchanger.
    305
    13.9 CONCLUSION
    In conclusion, it is necessary to evaluate the following
    factors when considering a geothermal/absorption cooling
    application for space conditioning.
    Resource temperature
    Substantial derating factors must be applied to
    equipment at temperatures less than 220oF. Very high
    resource temperatures or two-stage are required for
    low-temperature refrigeration.
    Absorption machine hot water requirements
    compared to space heating flow requirements
    Incremental well and pumping costs should be applied
    to the absorption machine.
    Refrigeration capacity required
    Larger machines have lower incremental capital costs
    on a $/ton basis. Coupled with the larger displaced
    energy, this result in a more positive economic picture.
    Annual cooling load for space conditioning, in full
    load hours or for process cooling, in terms of load
    factor
    Obviously higher utilization of the equipment results in
    more rapid payout.
    Pumping power for resources with unusually low
    static water levels or drawdowns
    Pumping power may approach 50% of high efficiency
    electric chiller consumption.
    306
    Utility rates
    As with any conservation project, high utility rates for
    both consumption and demand result in better system
    economics.
    REFERENCES
    American Society of Heating, Refrigeration and Air
    Conditioning Engineers, 1983. "1983 Handbook of
    Fundamentals," ASHRAE, Atlanta, GA, pp. 14.1-14.8.
    Christen, J. E., 1977. "Central Cooling - Absorptive Chillers,"
    Oak Ridge National Laboratory, Oak Ridge TN.
    Hirai, W. A., 1982. "Feasibility Study of an Ice Making
    and Cold Storage Facility Using Geothermal Waste
    Heat," Geo-Heat Center, Klamath Falls, OR.
    Means, R. S., 1985. "1985 Means Mechanical Cost Data,"
    R. S. Means, Inc., Kingston, MA.
    Todd, M., Sales Engineer, 1987. Personal communication.
    Airefco Inc., Portland, OR.
    Wahlig, M., 1984. Lawrence Berkeley Laboratory. Personal
    communication with Gene Culver, Geo-Heat
    Center.
    Yazaki Corporation, undated. Yazaki Gas & Solar Air
    Conditioning Equipment - Cat. No. 15.3 AME, Yazaki
    Corporation, Tokyo, Japan.

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    ebalahmr
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    اخي العزيز هناك موقع اسمه (ملتقى التدريب العربي ) فيه مواضيع عن الطاقه الجديده والمتجدده حيث فيه عن الطاقه الشمسيه

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    Adsorptive Refrigeration cycle

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    ارجو ان يكون مفيدا.

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