|Publication number||US20080264062 A1|
|Application number||US 12/104,797|
|Publication date||Oct 30, 2008|
|Filing date||Apr 17, 2008|
|Priority date||Apr 26, 2007|
|Also published as||US20090260361, WO2008134283A2, WO2008134283A3|
|Publication number||104797, 12104797, US 2008/0264062 A1, US 2008/264062 A1, US 20080264062 A1, US 20080264062A1, US 2008264062 A1, US 2008264062A1, US-A1-20080264062, US-A1-2008264062, US2008/0264062A1, US2008/264062A1, US20080264062 A1, US20080264062A1, US2008264062 A1, US2008264062A1|
|Inventors||Melvin L. Prueitt|
|Original Assignee||Prueitt Melvin L|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (11), Classifications (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This claims priority to and the benefit of Provisional U.S. Patent Application Ser. No. 60/914,036, filed Apr. 26, 2007, the entirety of which is hereby incorporated herein by reference.
Most of the heat engines in use today use adiabatic expansion of gases to produce power. The Rankine, Brayton, Otto cycles, and others use adiabatic expansions, and some of them also use adiabatic compressions. This disclosure shows that isothermal expansion and compression have some important efficiency advantages.
The isothermal cycle can use a working fluid such as water, ammonia, or propylene that are boiled and then expanded isothermally in an engine to produce power. The vapors are then condensed back to a liquid to repeat the cycle. It can also use a gas such as air or helium that can be compressed isothermally, heated, and then expanded isothermally in an engine that drives an electric generator or drives some other machine. Since the exhaust gas from the expander is still hot, its heat can be used to heat the compressed gas flowing from the compressor to the expander.
Some US patents that are somewhat related to embodiments of the present invention are U.S. Pat. Nos. 4,023,366, 4,207,027, 4,455,825, 4,676,067, 5,641,273, 6,186,755, 6,205,788, 6,225,706, 7,062,913, and 7,124,585.
In order to have isothermal compression and expansion of gases, heat must be removed from the gas while it is being compressed, and heat must be supplied to the gas during expansion. One purpose of this invention is to provide means of transferring heat to and from gases quickly.
By providing appropriate heat exchangers at appropriate points in the design, the disclosed isothermal systems provide a method that allows the exhaust from the expander engine to flow through the heat exchangers to supply all the superheat for the vapor flowing from the boiler or compressor to the expander engine. For a two-phase fluid, the gas from the expander that flows through the heat exchanger still has enough heat left over to partially preheat the feed liquid flowing to the boiler. This cannot be done with an adiabatic expander, because the vapor exhaust from the adiabatic expander is normally cooler than the superheat temperature.
Another important feature of the isothermal engine is that the same amount of gas can be expanded to much larger volumes than the adiabatic machine, and this provides more output energy.
We can consider an example to compare the isothermal steam engine with a Rankine cycle steam engine. Suppose we begin with liquid water and boil it at 400 K (127° C.). If one kilogram (0.731 m3) per second of steam flows from the boiler at a pressure of 2.455 bar and is superheated to 800 K (527° C.), the power produced by an isothermal expander is defined (for an ideal gas) by
W=P s V sln(V e /V s)
where Ps is the pressure of the superheated gas (same as the boiler pressure), Vs is the volume of the superheated gas, and Ve is the volume of the expanded gas flowing out of the expander. The “ln” is the natural log function. The volume Vs is 1.462 m3 (since the temperature is twice as high as the boiling temperature at constant pressure). If we let the gas expand in the isothermal expander to the condenser pressure, 0.03531 bar, the vapor volume will be 101.65 m3, and the power will be 1.522 MW. The efficiency will be 38.77%.
For an adiabatic expander, the power is defined by
W=(P s V s −P s V s)/(1−γ)
where γ is the ratio of specific heats of the gas. For this example, we let γ=1.32. If we let the superheated gas expand adiabatically to Pe=0.03531 bar (same as in the isothermal case), the volume will be only 36.35 m3. The power will be 0.72 MW. This is only 47% of the power that the isothermal system put out. The isothermal case requires 15% more heat input, but it uses that heat more efficiently. The efficiency of the adiabatic (Rankine) cycle is 21.16%.
For an ideal gas, the internal energy depends on only on the temperature. Thus, all the heat that is input to the isothermal engine is used to generate power as it maintains the gas temperature at a constant value (no change in internal energy of the gas). Then, when the gas is exhausted from the expander, it is still at the same temperature as the input temperature, and its energy can be used to superheat the boiler vapor in the counter-flow heat exchanger as its temperature theoretically drops down to the boiler temperature. In an actual machine, we would want the input heat to the isothermal engine to be at a little higher temperature than the superheat temperature in order to provide the temperature differential to cause adequate heat flow.
For the adiabatic case, after the gas expands to 36.35 m3, its temperature is only 286 K, which is too cold to supply any heat to superheat the gas flowing out of the boiler.
The following table gives some calculated values for the performance of the isothermal heat engine that uses steam as the working fluid. These calculations were made by a computer program called “Isoengine.exe.” For comparison, the next-to-the-last column gives the efficiency of an ordinary Rankine cycle steam engine. The last column gives the performance of a Rankine cycle steam engine with single reheat. The values in the table are theoretical values, but the comparison between the isothermal engine and the Rankine steam engines is valid. The power values are for a flow of one kilogram per second of steam. The values in the Rankine columns were calculated with a single gamma value and constant heat capacity of the steam, which provide slightly inconsistent values, since the gamma varies with temperature and pressure.
The first row in the table represents values that would be appropriate for a system that uses heat from a solar pond where the temperature of the pond might be 90° C.
The last five rows are for OTEC applications. No multi-staging is involved. Efficiencies could be higher if multi-stages were used. The Rankine cycle with reheat is not listed for these five rows, because the temperature differences are too small to use reheat.
Using a system that has isothermal compression and expansion of single-phase gases provides performances that are even better than isothermal system of the two-phase fluids, such as water-steam (Rankine cycle). Air can be used as the working fluid. Helium has high heat transfer properties and might be used. It is used in Stirling engines.
Table II gives some calculations of performance for this design that were made by a program called “Isotherm5.exe.” Again, these are theoretical calculations that do not include friction and thermal conduction losses. For each item in the table, the air pressure at the entrance to the isothermal compressor is 10 bar, the temperature is 27° C., and the volume is 1 cubic meter per second. The air is compressed in the compressor to 0.2 m3.
Note that at the same temperatures of superheat, the efficiencies for this device with isothermal compressor and isothermal expander are significantly higher than those for the device that has a boiler and an isothermal expander, as shown in Table I. In fact, when the single-phase simulations were run with the computer program, the efficiencies turned out to be Carnot efficiencies. When friction and heat conduction losses are included, the efficiencies will be less, but they should still be higher than the devices in Table I, when losses are included in those calculations.
It is therefore an object of the present invention to provide an economical means of producing power using isothermal compressors and isothermal expanders.
It is another object of the present invention to utilize solar energy or other heat sources to provide energy to generate electrical power using isothermal compression and expansion engines.
It is another object of the present invention to utilize solar energy or other heat sources to provide energy to generate electrical power using a boiler and an isothermal expansion engine.
It is another object of the present invention to utilize solar energy or other heat sources to provide the heat required to maintain near-isothermal conditions in isothermal expanders.
It is another object of the present invention to utilize the hot exhausts gas from an isothermal expander to superheat the compressed gas from an isothermal compressor in a counter-flow heat exchanger.
It is another object of the present invention to utilize the hot steam, or other two-phase working fluid, that exhausts from an isothermal expander to superheat the vapor from a boiler in a counter-flow heat exchanger.
It is another object of the present invention to provide means for reducing mechanical losses in the compression and expansion devices.
It is another object of the present invention to provide methods to effectively produce approximate isothermal compression and expansion of gases.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
For efficient heat transfer, the cooler can supply a liquid to channels beneath the surfaces in the compressor. The liquid evaporates as it removes heat from the surfaces. The vapor flows to the cooler, where it is condensed and returned to the compressor.
For adequate heat flow, the heater should supply heat at slightly higher temperature than the isothermal temperature of the expander, and the cooler should supply a fluid that is at slightly lower temperature than the compressor isothermal temperature.
For this design, the cooler 128, keeps the compressor cool. The cooler could use water-cooling, evaporative cooling, or even ambient air to provide a means to dispose of the heat. The isothermal expander is kept hot by heater 127. The heat could be supplied by solar energy or other heat source.
An isothermal expander can be used to produce power with steam (or other working fluid). In
The steam still has sufficient heat to preheat the boiler feed water in the counter-flow heat exchanger 143. The steam is then condensed in the condenser 144. The condensate is then pumped by pump 146 back into the boiler 140 through counter-flow heat exchanger 143.
Even though the engine is called “isothermal,” in order to have heat transfer into the expanding steam, the fluid flowing from the heater 147 is at a higher temperature than the “isothermal” temperature. The steam flowing out of the isothermal engine 142 is hotter than the steam coming from the counter-flow heat exchanger 141.
In my patent application Ser. No. 11/739,580 for the invention entitled “Water Extraction from Air and Desalination,” some designs for isothermal compressors and expanders are described. Some of the drawings and descriptions are repeated here in order to show the kinds of isothermal engines can be used to keep the gases close to isothermal.
In operation as a compressor, as the piston 21 in the isothermal engine 20 moves upward, it draws in air (or other gas) through check valve 26. When the piston reaches its maximum height, valve 26 closes, and the piston is forced downward, compressing the gas. As the gas is compressed, it tends to increase in temperature, but the tapered plates absorb the heat of the gas. Since the heat capacity of the metal plates is about 2000 times as great as the gas (per unit volume), the plates' temperature does not rise very much during one half cycle.
When the piston has traveled down far enough to provide the appropriate pressure the gas, check valve 36 opens to allow the gas to flow out. The piston continues to move downward to force the compressed gas out.
In operation as an expander, check valve 26 is replaced by a controlled valve. As the piston 21 in the isothermal engine 20 moves upward, it draws in air (or other gas) through controlled valve 26. When the piston reaches the designed height, valve 26 closes, and the piston continues to rise, expanding the gas and extracting energy from the gas. As the gas is expanded, it tends to decrease in temperature, but the tapered plates provide heat for the gas. When the piston 21 moves downward, it forces the gas out through check valve 36.
For the compressor, the tapered plates can have interior channels in which a cooling fluid can flow. The cooling fluid could be a liquid that evaporates to absorb the heat. For the tapered plates 22, the fluid would have to be delivered and retrieved through channels in the piston rod 27. The fluid can be delivered to tapered plates 23 through channels in the cylinder. Calculations show that the gas will remain near isothermal during both compression and expansion, since the gas has close proximity to the plates for heat transfer, and the motion of the plates create turbulence that further enhance heat transfer.
For the expander, heating fluids are required. The heating fluids could be gas or liquid, or they could be vapor that condenses to a liquid in the channels of the tapered plates to release the heat of condensation.
In this document, components that are used to increase the surface area in contact with the gas are mostly referred to as “tapered plates,” because it is easy to illustrate the tapered plates in the drawings. In many applications, it may be better to use tapered concentric circular forms that are approximately cylindrical.
One of the main sources of inefficiency for a compressor or expander that is needed for the isothermal engine is sliding friction of the piston. I have a U.S. Pat. No. 6,401,686 on a device that is often referred to as “MECH,” which stands for motor, expander, compressor, and hydraulics. Since it uses rolling friction between two rotating pistons rather than sliding friction of a standard piston engine, the friction losses are much less. The rotating pistons do not touch the cylinder walls. There is sliding friction on the ends of the pistons, but this can be relatively small by making the pistons long compared to the diameter.
It is well known that rolling friction is only about 1/100 as large as sliding friction. A MECH prototype demonstrated only 8% as much energy loss as a comparable size piston engine. It provides an engine with unprecedented economy for producing water from the air or for desalinating seawater.
The tapered plates provide large surface areas for the transfer of heat to and from the gas. The motion of the tapered plates relative to the stationary plates causes gas turbulence in the small gaps between them, and this enhances heat flow.
In order for the expander or compressor of
Instead of flat tapered plates, this embodiment of the invention could use tapered concentric circular forms as described above and illustrated in
In an isothermal turbine expander, the heating fluid can flow through the turbine walls, stator blades, and rotating blades.
From there, the vapor and liquid flow to the lower stator blades 165 and flow through channels in those blades. In the stator blade channels, more of the vapor condenses. The liquid and the remaining vapor flow into the liquid channel 166 on the lower half of the turbine. They return to the heating fluid boiler (not shown) through the condensed liquid outlet 167. After boiling, the vapor returns again to the isothermal expander turbine.
For an isothermal turbine compressor, the geometry looks much like the expander of
Finally, the vapor and any remaining liquid flow out the bottom and flow back to a cooler, which condenses the vapor to a liquid. The liquid is pumped back to the turbine compressor to repeat the cycle.
Alternatively, the liquid can be pumped into the bottom of the compressor. As it flows up through the lower stator blades, it boils. The vapor it creates blows liquid as a mist up through the rest of the system. The mist droplets strike surfaces and evaporate, removing heat and continuing the process of blowing liquid droplets up through the upper blades.
If the stator blades do not provide sufficient heat removal for the compressor, the rotating blades can also be designed to receive liquid that evaporates and removes heat. The liquid enters one end of the shaft that holds the turbine blades. The liquid then flows through a channel in the shaft until it reaches the channels in the blades. As it flows through the blade channels, it evaporates and removes heat. The vapor flows back to the shaft and flows through another channel in the shaft to the other end of the shaft and then flows back to the cooler.
It is not that simple for the isothermal expander turbine. If vapor is put into the rotating blades, it condenses to a liquid, which has high density. Centrifugal force causes the liquid to move to the tips of the blades. It would require high pressure to force the liquid back to the shaft against the centrifugal force. The density of the liquid can effectively be reduced by mixing it with the vapor. By supplying more vapor than is necessary to deliver the required heat, the extra vapor will flow back to the shaft through a small-diameter channel and will carry a mist of liquid with it.
A schematic drawing of a single rotating turbine blade 180 for an expanding turbine is shown in
Another alternative would be to use a liquid to transfer heat to the rotating blades. The liquid flowing from the shaft to the tip of the blade would provide the pressure to push the liquid back to the shaft. The liquid flowing back to the shaft would be slightly denser than the liquid flowing toward the tip, because it is cooler. Extra pressure in the input channel would be required.
In order to enhance the heat transfer between the turbine blades and the gas, the blades should probably be closer together and be wider than blades in standard turbines.
For an isothermal compressor, the tapered plates like those of
If we can increase the efficiency of standard power generating plants by replacing the adiabatic engines with isothermal engines, it would reduce the release of greenhouse gases into the atmosphere. For solar thermal power plants, the isothermal engines would produce more power for the same size solar collectors.
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|U.S. Classification||60/670, 60/671|
|International Classification||F01K27/00, F01K25/00|