大型制药厂热电冷三联供工程设计(英文文献翻译+开题报告+CAD图纸) 第2页
designed for heating and cooling a new building. Because the heating system can be designed for rather large delta Ts 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.
LARGE TONNAGE EQUIPMENT COSTS
Figure 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.
Figure 3. Chiller and auxiliary equipment costs - electric and absorption (Means, 1996).
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 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 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.
Figure 4. Simple payback on small absorption equipment compared to conventional rooftop equipment.
The plot is based on the availability of geothermal fluid of sufficient temperature to allow operation at rated capacity (190°F 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 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 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 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 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 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 190°F entering hot water, 85°F entering cooling water and 48°F leaving chilled water. Flow rates for all three loops are based upon a 9°F delta T.
Figure 4 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 180°F, substantial derating is necessary. For preliminary evaluation, the 85°F cooling water curve should be employed.
COMMERCIAL REFRIGERATION
Most commercial and industrial refrigeration appli-cations involve process temperatures of less than 32°F and many are 0°F. 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 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 32°F, required heat input temperatures must be at least 230°F. It should also be remembered that the required evaporation temperature is 10 to 15°F below the process temperature. For example, for a +20°F cold storage application, a 5°F evaporation temperature would be required.
Figure 5. Small tonnage absorption equipment performance.
Figure 5 suggests a minimum hot water temperature of 275°F 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 5 indicates another consideration for refrigeration applications. That is 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.
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 220°F. 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.
Utility rates
As with any conservation project, high utility rates for both consumption and demand result in better system economics.
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