Patent Grant
6854279
The commercial building business continues to place a strong focus on Active Humidity Control (AHC) equipment for commercial air conditioning (AC) applications. This has grown out of an understanding that for building applications, the independent control of humidity and temperature afforded by AHC offers marked advantages over the traditional “cool first” approach. AHC conserves energy while affording building occupants immediate and lasting improvements in comfort, health and indoor-air quality. However, such AHC is not presently used in ships, though ships could also benefit from such systems.
Current AC systems aboard ship use the traditional “cool first” approach, in which the air in the ship's compartments is simultaneously dehumidified and cooled to prescribed environmental conditions. The demand on the cooling system is especially acute in a hot and humid marine environment where moisture levels in the compartment replenishment air delivered to the AC system are higher than those encountered on land. AC systems, therefore, are designed/rated for the abnormally high heat loads needed to accommodate these environmental conditions. Because the resulting systems are very large and severely taxed in producing enough chilled water to lower both the absolute humidity (moisture content) and temperature of the compartment air to the prescribed conditions, they consume a tremendous amount of the generated electrical power on the ship.
Traditional vapor-compression AC systems are designed to remove both sensible heat and latent heat by cooling the outside air below the dew point to condense out water vapor. A large amount of electricity is required to provide the additional chilled water required for this large latent heat load. Dynamic-desiccant-based AC systems, on the other hand, use a desiccant to remove moisture from the outside air prior to cooling the air with traditional chilled water. Some type of heat source then regenerates the desiccant.
Desiccants are a class of materials that have a great affinity for capturing and retaining water and are used in many applications where the presence of water or water vapor would be detrimental. Desiccants fall into two broad categories: solids and liquids. Liquids are usually absorbents, which means they undergo a physical or chemical change when they collect moisture. Sodium chloride, commercial table salt, absorbs moisture from humid air and eventually becomes crystallized a chemical and physical change. In contrast, solid desiccants are usually adsorbents, which means they collect water vapor on their surface but do not undergo a chemical or physical change. The low-cost granules used in pet litter boxes are alumina silicate clay, which is also a dry desiccant material. Silica gel is a typical dry adsorbent desiccant in which the crystals appear to have a smooth sealed surface, yet a microscope reveals a massive internal network of passages and crevices. Desiccant materials such as these that adsorb moisture from humid air can collect between 20 and 40% of their dry weight in water vapor.
There are two processes in which desiccants are used: dynamic desiccation and static desiccation. Dynamic desiccation is a continuous and cyclic process in which a desiccant material that has adsorbed moisture from a supply-air stream is subjected to a hot-air stream which dries out, and thereby regenerates, the desiccant for reuse in supply-air desiccation. Static desiccation also removes moisture from air, but there is no regeneration process. The desiccants are simply removed and discarded when saturated with moisture.
Static desiccants are primarily used in packaging, storage, and preservation of medicines, electronic and mechanical equipment, and other materials that have adverse or undesirable reactions to the presence of moisture. Dynamic desiccants are used in various building dehumidification applications, refrigeration systems, air-handling equipment, compressed-gas generation (air, nitrogen), vessel lay-up, reduction gears, etc. The primary desiccants used for these applications are molecular sieve, activated alumina, and anhydrous calcium or hygroscopic salts; these materials can be formed into granular beds or rotary wheels.
The wheel form of the dynamic desiccant is commonly known as the desiccant wheel and is used extensively in the design of desiccant-based, air-conditioning systems. The desiccant wheel is typically a circular device that is composed of a plurality of thin sheets of plastic or metal that are coated with a desiccant. The wheel, which is situated in the ducts of the air-conditioning system, is perforated to allow for the passage of air. The duct system is split to provide two countercurrent flow passages, one of which furnishes the supply air being dehumidified and the other which furnishes the air needed to regenerate the desiccant. The wheel slowly rotates to facilitate the transfer of moisture from the saturated desiccant to the regeneration air.
While supply air treated by dynamic desiccation has a moisture content much lower than that achieved by conventional chiller coils, commercial applications of the desiccation process generally do not exhibit high energy savings since a heat source, such as fossil-fuel-fired water or air heater or an electric heater, is required for indirect or direct heating of the regeneration air.
The present invention describes a method and apparatus for controlling the temperature and absolute humidity of air supplied to compartment cooling coils on a gas-turbine-powered ship through a dynamic-desiccation system (DDS). The system passes supply air through a desiccant wheel, which dries and concomitantly heats the supply air. This supply air stream is then passed through a rotatable thermal wheel whereby heat is transferred from the dry supply air to an exhaust-air-stream to finally condition the supply air for delivery and circulation to a plurality of cooling coil units in a plurality of compartments. The exhaust air from the compartments, which is mixed with some of the conditioned supply air to achieve the absolute humidity needed for effective regeneration of the desiccant wheel, is passed through an indirect evaporative cooler to meet the predetermined heat load on the rotatable thermal wheel. After this exhaust-air-stream mixture has been preheated by the thermal wheel, it is then heated to the desiccant regeneration temperature by passing it through a second heat exchanger, wherein engine-exhaust waste heat is transferred to the exhaust air stream mixture or regeneration air. After this heated regeneration air regenerates the desiccant wheel by fully drying out the desiccant, it is then expelled from the fan room.
Optionally, the method could be implemented in a land based installation where a sufficient waste-heat source is collocated with an HVAC system for a building. AHC based on dynamic-desiccant technologies in accordance with the present invention could realize significant benefits in energy reduction and health and comfort for personnel in conditioned spaces.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.
Referring to
As shown in
The exothermic reaction between the desiccant and the moist supply air in desiccant wheel 20 effects significant heating of the supply air. This reaction necessitates the introduction of a means for cooling the supply air to a predetermined dry-bulb temperature. This cooling process consists of a revolving wheel or thermal rotor 30 that transfers heat from the supply-air stream to the regeneration-air stream, thus preheating the regeneration air before delivering it to heat exchanger 60. The thermal rotor 30, divided by a split-duct system similar to that in desiccant wheel 20, is preferably a metal wheel which has a plurality of perforations and spins slowly about a central axis to convey heat from the supply air to the exhaust air. After cooling by the thermal rotor 30, the supply-air stream is delivered to the compartment cooling coils 50.
Since the absolute humidity of the compartment exhaust air returned to the fan room is higher than that needed for desiccant regeneration, a portion of the dry supply air exiting thermal rotor 30 is mixed, via valves and control circuits (not shown), with the exhaust air at a point upstream of evaporative cooler 40. The process of mixing a large portion of treated supply air with the exhaust air reduces the absolute humidity of the regeneration-air mixture to the predetermined value needed to fully dehydrate desiccant wheel 20. However, since this regeneration air is too hot to cool the supply air leaving thermal rotor 30 to its predetermined temperature, it is first passed through evaporative cooler 40. By an indirect process, evaporative cooler 40 cools the regeneration air to its wet-bulb temperature without altering its absolute humidity. However, since the regeneration air leaving thermal rotor 30 is below the predetermined temperature needed for fully dehydrating the desiccant, it must be heated in heat exchanger 60. The heat transferred in exchanger 60 is supplied by heat exchanger 64, which transfers to a liquid stream, such as water, some of the waste heat in the exhaust gas of the engines 70 normally employed for ship-service (SS) power. Preferably, heat exchanger 64 would have a finned-tube design, wherein the liquid flows inside tubes equipped with external transverse fins across which the gas flows. The waste heat conveyed by this liquid stream is used to preheat the regeneration air in heat exchanger 60, at a point immediately upstream of desiccant wheel 20. This regeneration-air stream moves parallel to, but in a direction opposite, that of the supply-air stream. The heating process in heat exchanger 60 is essential in raising the temperature of the regeneration air to a predetermined value needed for complete dehydration of desiccant material in the regeneration side of desiccant wheel 20. As in a conventional fan room, the regeneration air is ejected overboard after leaving the regeneration side of desiccant wheel 20.
EXAMPLE: In applying the above DDS to a shipboard system, the potential for energy savings is based on the utilization of waste heat for heating the regeneration air to the predetermined temperature needed for desiccant dry out. Therefore, gas turbine powered ships, which have an abundance of waste heat in the engine exhaust, are among the best candidates for utilizing this energy-saving invention. To characterize savings, the present invention has been applied to a DDG-51 class ship that contains five fan rooms that furnish replenishment air to the ship's compartment recirculation coils 50.
The example characterizes a minimum-energy AC system that utilizes a DDS in accordance with the present invention to deliver replenishment air at the optimum dry-bulb and wet-bulb temperatures needed by the compartment coils 50. A psychometric analysis was conducted to determine the new AC system heat loads due to both the fan-room coils (due to the new airflow arrangements) and the compartment coils that will operate under the above inlet conditions. The analysis assumes that all airflow rates remain constant and that the required compartment air temperatures, relative humidity, and sensible-heat factors remain unchanged. Replication of any energy pickup processes associated with a particular compartment was also essential.
In view of the effort involved in analyzing all 73 compartment recirculation coils in the AC system, a method was devised whereby the system cooling load could be predicted by extrapolation, through a suitable scale factor, from the cooling load prediction for a judiciously selected subsystem of coils. It was concluded that the appropriate coil subsystem should be one served by a single fan room and should exhibit an average product of coil load and replenishment fraction close to that for the entire system. It was found that this simulation parameter could be satisfied by a 16-coil subsystem that included those coils having the largest values of the parameter.
The predetermined value for the moisture content of the replenishment air delivered to the compartment coils was found by examining the baseline coil exit conditions given in the subsystem coil performance data. These data showed that the optimum moisture content for minimizing the latent heat load on all subsystem coils was 50 gr/lb. To estimate the dimensions of the required DDS machinery, the fan room delivering the largest replenishment airflow was selected. On the basis of this airflow and a predicted DDS replenishment-air, dry-bulb temperature of 90° F., preliminary machinery sizes and airflow conditions were determined. These data were then utilized to assess overall AC system performance.
In order to meet fan-room dimensional constraints and provide redundancy, two parallel desiccant circuits were necessary, as shown in FIG. 2. The exothermic nature of the desiccant moisture adsorption process causes marked heating of the supply air. Since chiller coils are not used in these circuits, the supply (replenishment) air is cooled to the desired temperature by a rotary heat exchanger 30, whose cold side receives regeneration air that has been cooled by an indirect evaporative process. A rotary exchanger 30 employed in this manner also serves as a preheater for reducing the load on the regeneration-air heater 60. However, to enhance the desiccant-regeneration process, the absolute humidity of the regeneration air stream is reduced by combining a portion of the conditioned supply air with the air exhausted from the compartments.
Table 1 presents an energy capitulation based on the above analysis and assumptions. It is seen that the total energy consumed by both the fan-room coils and the compartment recirculation (replenishment air) coils has dropped from 507 tons to 372 tons, the latter value obtained by applying the above scale factor (defined as the baseline total-system-to-subsystem energy ratio) to the load found for the 16-coil DDS subsystem.
#Re-circulation systems are those systems being replenished by fan room air
As shown in Table 1, the scale factor is closely approximated by the ratio of the total coil throughput for the two systems. However, it must be noted that since the nearly 135-ton energy savings is based on a projection of a small number of cooling coils, it must be viewed as an approximation subject to verification by rigorous psychometric analysis of the entire 73-coil recirculation system. The approximate nature of the results is underlined by the observed differences in the baseline values of overall replenishment fraction and sensible-heat factor found for the two systems.
From a review of available data on chiller-plant coefficient of performance (CoP), it was determined that the values given for a seawater temperature of 75° F. were most appropriate for a ship operating in a 90° F. environment. The data also show that the CoP is a function of plant load; however, it was found that, over the range of 100-200 tons, the CoP varies by only 11%. Thus, little error is incurred in selecting a CoP of 0.75 at a load of 150 tons. By applying this value, the above DDS heat-load savings, Q, can be converted to savings in ship-service (SS) power, P, by the formula
P=Q×CoP=101 kW
The above power saving was expressed in terms of an annual savings in fuel by applying average-ship data that cites the portion of the annual operating time each SS engine spends at discrete values of specific fuel consumption (SFC) and load. From these data, the engine profile could be characterized by a mean SFC (denoted by SFC), weighted with respect to both load and operating time, equal to 0.787 lb/hp-hr. On the basis of a total annual operating time (T) of 3000 hr, the annual fuel savings, E, is given by
E=SFC×P×T×1/0.746=319,653 lb
To compute the total annual fuel consumed by the SS engines, the average-ship data were again applied in summing the products of the discrete values given for SFC, load Li, and time-interval Δti, where i represents the ith time interval. Since U.S. Navy destroyers routinely operate with two SS engines on line, the annual fuel consumption (W) of the SS engines can be mathematically expressed as
WSS=2×Σ(SFCi×Li×Δti)=8,604,000 lb.
By a procedure analogous to that for the SS engines, the annual fuel consumption of the propulsion engines can be similarly expressed as
Wprop=4×Σ(SFCi×Li×Δti)=9,849,000 lb.
From the above results, the fraction (f) of fuel saved annually (E) by the average ship is given by
f=E/(WSS+Wprop)=319653/(8,604,000÷9,849,000)=0.0173 or 1.73%.
The accuracy of the above result is somewhat dependent on how closely the available engine data matches that for a ship whose entire annual deployment was in warm climates. Moreover, it also assumes that the annual operating times for a chiller plant and an SS engine are the same. Consequently, the above result should only be construed as an upper limit on the DDS payoff.
This energy study shows that a DDS has the potential to markedly improve the energy efficiency of the AC system aboard gas-turbine-powered ships. Such a system requires only a very small amount of energy for removing a substantial amount of water vapor from the outside “makeup” air entering the system. While the existing AC system would require a great deal of electrical energy to remove this water from the air, the application of a DDS avoids this energy penalty by greatly enhancing the performance and energy efficiency of the AC system. Also, due to the fact that the DDS in accordance with the present invention utilizes a regeneration process that does not require an additional primary heat source, energy cost savings are magnified. The DDS in accordance with the present invention may also greatly mitigate a current problem with U.S. Navy vessels operating in extremely warm waters. AC systems aboard Navy ships utilize seawater coolant in the refrigerant condenser. When the incoming seawater temperature is over 95° F., the cooling capacity of the AC system is markedly reduced, as shown in FIG. 6. The resulting elevation of the temperature and humidity of the processed air causes significant problems with electronic instrumentation and personnel comfort. With this DDS, makeup air supplied to the AC system is substantially lower in humidity, therefore allowing the system to provide cooler and dryer air through increased performance.
While there have been described what are believed to be the preferred embodiments of the present invention, those skilled in the art will recognize that other and further changes and modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications that fall within the true scope of the invention.
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefore.
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