Patent Application
20100287978
Not Applicable
Not Applicable.
Not applicable.
1. Field
This invention pertains to the field of refrigeration, specifically to the utilization of low grade heat in the form of hot water by means of flat plate heat exchangers to power an absorption chiller that operates at low internal pressure, is cheap to build and scalable.
2. Prior Art
My invention improves on concepts of prior art that were first published in the USA by Einstein and Szilard in U.S. Pat. No. 1,781,541. U.S. Pat. No. 1,781,541 disclosed a refrigeration cycle and an arrangement of chambers and tube-in-shell heat exchangers wherein ammonia gas is bubbled through liquid butane to produce a refrigeration effect by evaporating butane. (
Surprisingly, there have been very few references to U.S. Pat. No. 1,781,541 in the patent literature in the 79 years since it was first published. A search of the patents referring to U.S. Pat. No. 1,781,541 returned 6 references, none of which pertain directly to advancing the mechanics which utilize the physical chemistry concepts introduced by U.S. Pat. No. 1,781,541.
Though the physical chemistry of U.S. Pat. No. 1,781,541 is wonderfully clever, the apparatus proposed for carrying out the chemistry is primitive. The prior art relies on a bubble pump to circulate the working fluids. The bubble pump is not scalable due to the well known square-cube relationship.
If the bubble bump in U.S. Pat. No. 1,781,541 is heated very slowly, the solution in the bubble tube will become depleted of ammonia and become locked. Subsequent application of high heat to the bubble tube will not un-lock the device. Slow heating is precisely the situation encountered with the rising sun each morning. Thus the device presented in U.S. Pat. No. 1,781,541 is probably not reliable for solar applications.
U.S. Pat. No. 1,781,541 does not give any hints for calculating the required capacity for any of the 5 heat exchangers in the original disclosure. The lack of full disclosure in U.S. Pat. No. 1,781,541 makes it impossible for one skilled in the art to construct a practical working machine. As will be demonstrated in this disclosure, the efficiency of the chilling is dependent on the correct sizing of the heat exchangers.
In U.S. Pat. No. 1,781,541, there is no provision for recovering heat from the ammonia gas leaving the generator before it enters the evaporator. Residual water vapor in the ammonia gas will condense in the evaporator reducing the overall efficiency of the system.
U.S. Pat. No. 1,781,541 specifies the use of butane as a refrigerant. Butane has the disadvantage of having a vapor pressure that is higher than atmospheric pressure at normal ambient temperatures. Butane's high vapor pressure requires an apparatus that can hold high pressure. If a leak were to develop in a device using butane, a noxious, flammable mixture of ammonia and butane could be aspirated into the surrounding environment with dangerous consequences.
The present invention addresses the deficiencies in U.S. Pat. No. 1,781,541 by applying modern advancements in flat plate heat exchanger design, most importantly to the advancement of brazed heat exchangers. Other areas of advancement include the development of seal-less magnetically driven pumps, and brazing metallurgy that did not exist in 1930 when U.S. Pat. No. 1,781,541 was granted.
Because of the deficiencies in the original patent disclosure, the Einstein Refrigeration Cycle has remained a dormant technology for 79 years. The present invention addresses the deficiencies to produce a chiller that can operate on low grade heat from solar collectors or waste heat from the cooling water of internal combustion engines.
Calculation methods disclosed herein have lead this inventor to the surprising conclusions that the refrigerant, specific ammonia concentrations, and heat exchanger capacities can be selected so that the system can operated at atmospheric internal pressure. This surprising conclusion was not reported by Einstein and Szilard and is essential for the Einstein Refrigeration Cycle to become practical, safe, working invention for the benefit of mankind.
a. The Linear Aspirator. The gas mixture (27) is mixed with the weak solution and distributed among the plates of the absorber (13) in the linear aspirator (32).
b. The Linear Bubbler. The ammonia gas (26) is introduced into the evaporator (11) below the level of the refrigerant (25) and distributed among the plates of the evaporator.
This invention employs an arrangement of flat plate heat exchangers and a pump that function as a thermal powered hydronic ammonia absorption chiller. Chilling is achieved by bubbling ammonia gas through a liquid refrigerant causing a reduction of the partial pressure of the refrigerant and evaporation with the absorption of heat. The refrigerant and working fluids can be selected to have a broad range of operating pressures. If the refrigerant and working fluid are selected so that the atmospheric boiling point is the about the same as the highest operating ambient temperature, the chiller can operate with low, or even no internal pressure. The low operating pressures allow the use of light weight materials, easy fabrication, low cost and safety. This chiller is especially suited for using solar heated water, cooling water from internal combustion engines or any source of hot water. The chiller is easily scalable to any size and will find wide application for comfort air conditioning and food preservation.
The arrangement of the heat exchangers and the fluid levels must follow a few rules for stable operation:
The general characteristics of flat plate heat exchangers that makes them useful in this invention are compact size, low internal volume, can be easily scaled to any size and are cheap to manufacture. Scalar adjustments covering orders of magnitude can be made by changing the size of the plates. Smaller adjustments can me made by changing the number of plates.
Many patents are on record that describe refinements to the development of flat plate heat exchangers. U.S. Pat. Nos. 4,872,578, 3,240,268, and 4,987,955 describe suitable designs, but the present invention is not limited to these designs.
The plates within the heat exchangers are corrugated. The corrugations are arranged so that when the plates are stacked, a series of wavy channels are created between the plates promoting turbulence. Turbulence within a heat exchanger is desirable as it disrupts the boundary layers between the solid plate and the flowing liquid promoting efficient heat exchange.
Flat plates heat exchangers can be brazed, gasketed or assembled using adhesives or sealants. Brazed heat exchangers have advantages in the present invention as they are resistant to the possibility of leakage of ammonia and refrigerant. Gaskets and sealants must be compatible with ammonia solution, and the refrigerant while operating at 80° C. and have no leakage. Copper containing heat exchangers are not suitable due to the aggressive action of ammonia toward copper.
Aluminums high thermal conductivity and low cost and passivity toward ammonia make it a desirable material for constructing heat exchangers for this invention. Aluminum brazing plate is especially a useful. Brazing plate consists of two alloys arranged in three layers. A high melting alloy core is clad on both sides with a low melting alloy skin. The brazing plate is formed into corrugated heat exchanger plates, stacked into a heat exchanger and fused in a furnace at temperature between the melting points of the two alloys.
Stainless steel brazed with nickel foil might be considered when the source of hot water or the cooling water is corrosive to aluminum. The heat exchangers might also be assembled out of a plastic material that is resistant to the refrigerant and ammonia such as PVC.
At the top of the absorber and the bottom of the evaporator, gas must be brought into intimate contact with liquid. Efficient operation of the chiller depends on mixing the gas and liquid in a way that evenly distributes the mixture across all of the plates of the heat exchanger.
a. shows the arrangement within the absorber (13) of a linear aspirator. The weak solution and the ammonia gas are rising vertically in separate tubes. The two separate tubes make a 90 degree bend and are combined into one tube which and then enters the absorber (13), extending horizontally across the breadth of the absorber. Along the length of the tube is a row of holes located along the side of the tube. As liquid and gas enter the tube, the liquid will fill the bottom half of the tube up to the row of holes and the gas will fill the top half of the tube. As the liquid and gas escape through the holes the velocity of the gas will be around 280 times the velocity of the liquid. As the gas and liquid escape the holes moving at different velocities, a shearing force will be applied to the liquid causing atomization and a dramatic increase in the surface area of the liquid promoting rapid absorption of the ammonia gas with the associated increase in temperature due to the heat of absorption. As the atomized liquid encounters the plates of the heat exchanger, the liquid will cascade downward under the force of gravity, transferring the heat to the cooling water and completing the absorption of ammonia to regenerate strong solution. At the same time liquid refrigerant is regenerated due to the insolubility of refrigerant in the strong solution.
The arrangement shown in
b shows how ammonia gas is introduced into evaporator (11) by means of a tube with a row of small holes along its length so that the bubbles are distributed equally among the plates of the evaporator (11). As the gas rises through the evaporator, it encounters the corrugations of the heat exchanger plates and is broken into small bubbles with high surface area which promotes rapid evaporation of pentane into the ammonia with the absorption of heat.
The pumps are very small in relation to the amount of low grade heat that they utilize. There are 4 pumps in the system.
The motive force to circulate the fluids within the chiller is provided by the strong solution pump (9). When the chiller is operating, ammonia weak solution (23) enters the top of the absorber (13) and cascades down the plates, and runs out under the force of gravity. To maintain the absorber in a state where the plates are continuously wet with fluid but not filled with fluid, a pump must overcome a fluid head that is the height of the absorber.
The pump must be absolutely leak proof, resistant to ammonia solution, resistant to refrigerant and able to withstand the maximum pressure that the system may encounter. The pump should also be very energy efficient. One suitable type of pump is the magnetically coupled centrifugal pump.
The pump can be surprisingly small. A centrifugal pump that delivers less than 5 l/min to a height of 300 mm and draws less than 20 watts of electricity can circulate fluid to utilize 5000 watts of hydronic thermal energy and produce 9,000 watts of cooling power. A pump so small can be economically powdered by solar voltaic cells.
The other pumps circulate fluids outside of the chiller and will likely be located remotely with respect to the chiller.
The chilled fluid pump (20) circulates cold fluid, to a fan coil or similar device to absorb heat and produce the desired cooling effect. The chilled fluid should have some additive, such as ethylene glycol, added to prevent freezing within the evaporator. the chilled fluid pump must be compatible chilled fluid and withstand constant exposure to cold.
The cooling fluid pump (22) circulates fluid between the cooling source and absorber.
The hot fluid pump (21) circulates fluid from the heat source to the chiller.
Description of Ammonia Solutions.
When ammonia gas dissolves in water the water gets hot. The dissolving releases 7.29 kJ/mol, the heat of solution. The reaction is reversible. If a saturated solution of ammonia is heated by an external source of thermal energy, it will release ammonia gas, consuming 7.29 kJ/mol of heat plus the heat required to raise the temperature of the gas and remaining solution.
The boiling point of aqueous ammonia is linear with respect to concentration over the range of 2% to 50%. The boiling point of a 23% solution is 42° C. and the boiling point of a 6% solution is 78° C.
If 100 g of a 23% solution of ammonia is heated to 78° C. at atmospheric pressure, it will release 17 grams of ammonia gas (1 mole) and will have a volume of 28.8 liters at 78° C., a 288 fold increase in volume.
The amount of heat required to release the 17 grams of ammonia (1 mol) from 100 ml of 23% solution will be the sum of three heats:
The actual work that we want done, the desorption of ammonia gas, requires 35% of the energy. The remaining 65% of the energy is used to heat the ammonia gas and the remaining ammonia solution.
The efficient recovery of thermal energy, through the use of flat plate heat exchangers, from the weak solution and ammonia gas is the central feature of this invention.
One advantage of this invention is the possibility to build a refrigeration device that that operates at low internal pressure. Low internal pressures allow the heat exchangers to be made of thin materials which improves the heat transfer, lowers the cost of materials. Lowering internal pressure will also increase the safety of the device. The internal pressure is set by the choice of refrigerant and the concentration of the ammonia solution.
N-pentane and cyclopentane are a good model compounds to demonstrate the properties of the refrigerant because the thermal properties are well understood, and widely published. N-pentane has a boiling point of 36° C. and cyclo pentane has a boiling point of 49° C. N-pentane can be blended with cyclo pentane to make a mixture that will not boil on a hot summer day thus insuring that the system will not become pressurized. The properties of a few refrigerants are listed in Table 2.
Higher molecular weight refrigerants with a higher boiling point and lower vapor pressure would be desirable if the chiller were installed at a high elevation. If the chiller were installed in an automobile that might move from low elevations to high elevations, the designer would have a choice as to whether to use higher molecular weight refrigerants or build the device to hold the higher internal pressure that would be encountered at higher elevations. This invention disclosure is not limited by the choice of refrigerant.
In practice, the refrigerant need not be a pure substance and can be a mixture of paraffinic petroleum distillates. White gas, such as the fuel that is commonly used in camping stoves and lanterns could be used as a refrigerant. One widely distributed brand of white gas, Colman® Camp Fuel has boiling point around 47° C. and a vapor pressure of 518 mm Hg @ 20° C. compared to n-pentane with a vapor pressure of 427 @20° C.
The principles of operation will be illustrated by calculating the required heat exchanger capacities and cooling power for 1 Watt of power input using n-pentane as a refrigerant. The calculations are shown in a in Table 1 in a spreadsheet format that is used by the widely available computer program Microsoft Excel.
Design calculations assume that it is possible to exchange 90% of the heat difference between two fluids and that fluids will be brought to within 2° C. of each other.
Table 1 is annotated with references to
Lines 1 through 7 list the design parameters for the specific application of this invention.
Lines 8 through 24 list the physical properties of the fluids.
Lines 24 through 40 shows the calculations for the fluid flows.
Lines 42 through 52 show the calculations for the required heat exchanger capacities.
The cooling process begins with a continuous stream of ammonia gas (27) entering the evaporator (11) through the linear bubbler (31) to bubble up through the n-pentane contained in the evaporator (11). Line 42 of Table 1 predicts that 1.83 W of cooling power will be produced from each watt of power input at the generator.
The cold gas mixture (27) exiting the evaporator is split between the cold economizer (12) and the gas chiller (17) where it cools the incoming refrigerant and gas. The cold economizer will require a heat transfer capacity of 0.318 W and the gas chiller will require a capacity of 0.150 W . Lines 43 and 45.
The gas mixture then moves to the linear aspirator (32) which is installed into the top of the absorber (13).
On line 29 we calculated that 1 W of power input would desorb 0.00198 g/s of ammonia gas would require 0.0138 g/s of strong solution heated from 42° C. to 78° C. to reduce the concentration of ammonia from 23% to 6%. Line 37. The strong solution pump (19) moves strong solution from the separator (18) to the gas cooler (16), where it absorb heat from the ammonia gas (26) at a rate of 0.138 W to increase it's temperature by 2.5° C. But more importantly, the process of removing water vapor from the ammonia gas is begun.
From the gas cooler (16), the strong solution moves to the hot economizer (14) where it absorbs recycled heat at a rate 1.73 W for every new watt of power input to the system. The remarkable size of the hot economizer compared to the generator is one of the surprising discoveries of this invention.
By the time the strong solution exits the hot economizer, it has released most of its ammonia gas using only heat that has been recycled in the hot economizer and gas cooler. Now the solution moves in to the generator to absorb 1 W of external power. Weak solution (24) and ammonia gas (26) exit the generator and are separated into different streams.
The ammonia gas (26) moves to the gas cooler (16) and then the gas chiller (17) to complete the gas cycle.
The weak solution (24) moves to the hot economizer (14) to preheat the strong solution (23) and then to the linear aspirator (32) to mix with the gas mixture (27) be injected into the absorber (13).
Overall, heat enters the chiller in two places: the generator (15) and the evaporator (11). The only place that heat can leave the chiller is through the absorber (13). The absorber is sized to transfer the combined the heat from the generator (15) and evaporator (11). The absorber (13) must transfer 2.83 W to the cooling water for every 1 W of heat input to the generator (15). Line 47 With 2.83 W of heat transferred by the absorber (13) to cooling water at 40° C., the strong solution (23) and refrigerant (25) will be returned to 42° C., the starting temperature. The refrigerant (25) and strong solution (23) drain out to the absorber into the separator (18) and the cycle repeats.
The spreadsheet used to calculate Table 1 was used to calculate the performance of other refrigerants based on their physical properties. Though cyclopentane and neohexane are slightly less efficient than n-butane or n-pentane, they have the advantage of having vapor pressures at 40° C. that are well below atmospheric pressure. A refrigeration system using cyclopentane or neohexane will not become pressurized at 40° C. and will be safer. Butane will develop considerable pressure under normal operating conditions and will require heavier materials to construct the heat exchangers. Off setting the added cost of materials and complexity of construction is the slightly more efficient operation. This invention is not limited by the choice of refrigerant. Any refrigerant can be used.
Accordingly, the reader will see that I have set forth an invention that can utilize low grade heat in the form of hot water to power a chiller that is made up of an arrangement of flat plate heat exchangers and a pump.
My invention provides a means to use water heated with solar energy to replace electricity for comfort air conditioning. Summer cooling demand is the highest peak demand on the national electricity grid. The widespread adoption of this invention will reduce the consumption of fossil fuels for electricity generation.
One obstacle to the widespread use of hydronic solar collectors is that if they are sized to produce enough heat in the winter, they produce too much heat in the summer. My invention provides a useful application for hydronic solar collectors in the summer time. Once solar collectors are installed and paid for by the savings in electricity from summer cooling, they are immediately available for collecting heat for winter heating. As a result, my invention will impact the burning of fossil fuel for heating and cooling year round.
Solar powered refrigeration using my invention provides a means to preserve food in remote areas that are not served by electricity. This will allow food that is produced by small farmers and hunters in remote areas to be accumulated until there is enough to transport to market economically.
My invention provides a means to co-generate cooling power together with electricity using an internal combustion engine. Electricity can be generated using the mechanical energy of an engine running on natural gas and cooling power can be generated using the thermal waste.
Automotive air conditioning using my invention will increase the gas mileage of cars by using waste thermal energy and eliminating the need for mechanical energy for air conditioning.
While the above description contains many specifics, these should not be construed as limitations on the scope, but rather as an exemplification of a few embodiments. Many other variations are possible.
