Process and apparatus for generating work

The invention relates to a process and an apparatus for generating work.

In current power plants, work is typically generated by means of a Carnot cycle, also referred to as "steam cycle", employing a high temperature source and a low temperature source. In practice, a high temperature medium, typically superheated steam, is fed to a turbine, which generates work, and is subsequently condensed, (super)heated and once more fed to the turbine. I.e., the difference between the amount of heat contained in the high temperature medium and the amount of heat sunk to the low temperature source is converted into work, in accordance with the first law of thermodynamics.

At higher temperature differences between the high and low temperature sources, more heat can be converted into work and the efficiency of the process improves. Typically, the environment (earth) serves as the low temperature source (heat sink) and the high temperature medium is generated by burning fossil fuels or by nuclear fission.

It is an object of the present invention to provide a process for generating work at a relatively high efficiency.

To this end, the process according to the present invention comprises the steps of expanding a gas, preferably substantially adiabatically and/or at the critical pressure and temperature of the gas, causing part of the gas to condensate and form a liquid phase, and separating, during or after expansion, at least part of the liquid phase from the gas phase.

Thus, a temperature difference and a substantial difference in heat content between the liquid and the gas phase can be generated with little work, preferably by subsequently compressing the gas phase, causing the temperature of the gas phase to increase to a temperature higher, preferably at least 20°C higher, than that of the liquid phase.

The said differences in temperature and heat content can be employed to generate substantial amounts of work, as will be explained in more detail below.

A preferred process further comprises the step of raising the pressure of the separated liquid.

It is further preferred that the process comprises the steps of heating a further medium by means of the gas phase and generating work by expanding the further medium, preferably in a Carnot or steam cycle.

The invention further pertains to an apparatus for producing work comprising at least one cylinder of turbine for expanding a gas, preferably substantially adiabatically and/or at the critical pressure and temperature of the gas, thus causing part of the gas to condensate and form a liquid phase, and means for separating, during or after expansion, at least part of the liquid phase from the gas phase.

It is preferred that the apparatus further comprises a compressor for compressing the gas phase, preferably substantially adiabatically or cooled, causing the temperature of the gas phase to increase to a temperature higher, preferably at least 20°C higher, than that of the liquid phase. Consequently, a substantial difference in heat content between the liquid and the gas phase can be generated with little work.

It is further preferred that the said components form a main cycle for generating a relatively low temperature liquid and a relatively high temperature gas and that the cycle is coupled to a further cycle for generating work.

The invention will now be explained in more detail with reference to the drawings, which schematically show a presently preferred embodiment.

Figure 1 is a schematic layout of a power plant in accordance with the present invention.

Figures 2A to 2C show cross-sections of a preferred cylinder and piston assembly suitable for use in the power plant of Figure 1.

Figure 3 shows a side view and a cross-section of a preferred centrifuge for separating liquid and gas at the outlet of the assembly of Figures 2A to 2C.

Identical parts and parts performing the same or substantially the same function will be denoted by same numeral.

Figure 1 shows a layout of a power plant 1 including two systems, a first system operated in accordance with the present invention and referred to as "heat swing cycle" and a second system operated in accordance with a Carnot or steam cycle.

In this example, the first system comprises twelve pairs of cylinders 2¹ - 2²⁴, each cylinder 2 containing a piston 3 connected, via a rod 4, to a common crankshaft 5. This particular crankshaft 5 is modular, i.e. it comprises a crank section for each pair of cylinders 2 interconnected by means of e.g. pre-tensioned splines (not shown). Thus, the total power of the plant 1 can be de- or increased by respectively removing or adding (pairs of) cylinders 2.

Each cylinder 2 comprises a non-return valve 6, connected to a gas pressure line 7 and allowing gas to be expelled from the cylinder 2, and a control valve 8, connected to a suction line 9 and allowing fluid to be drawn into the cylinder 2. A further control valve 10 is provided to remove liquid from the cylinder 2. The further control valves 10 of each pair of cylinders 2 are connected to a pump 11.

The gas pressure lines 7 are mutually interconnected and in communication with compensators 12 to reduce pulsations in the lines 7. The same is true for the suction lines 9 and the liquid pressure lines 13 of the pumps 11, i.e. these lines 9, 13, are interconnected and connected to respective compensators 12. The compensators 12 in turn are connected to a common pressure vessel 14 and serve to equalise pulsations generated by the cylinders 2. Further, the gas pressure lines 7 and liquid pressure lines 13 are connected to respectively first and second reverse current heat exchangers 15, 16, and, downstream from the heat exchangers 15, 16, connected to a central (collecting) duct 17, which in turn is connected to the suction lines 9.

The second system, referred to as "steam cycle", comprises an evaporator, i.e. the first heat exchanger 15, a condenser, i.e. the second heat exchanger 16, an additional heater 18, and a heat engine, known in itself and comprising, in this example, a plurality of piston generators 19, and a pump 20, which serves as a boiler feed pump. The system further comprises, downstream from both the heat engine 19 and the pump 20, compensators 21, which are connected to common pressure vessels 22.

During start-up, the heat swing system is filled, for example, with Argon, whereas the Carnot cycle is filled, for example, with CF₄. Both systems are cooled to a temperature preferably at least 50 °C lower than ambient temperature, e.g. to 150 K, and the pressure and temperature in the heat swing system are controlled to be substantially equal to the critical pressure (P_c) and temperature (T_c) of the medium, in this example, Argon. At or close to the critical pressure and temperature the heat swing cycle will be at its most efficient.

During operation, each of the cylinders goes through a cycle comprising the following steps:

• Expanding the gas substantially adiabatically (shown in uneven cylinder 2¹; and even cylinder 2¹²), generating work and causing the greater part of the gas to condensate and form a liquid phase (cylinders 2³, 2⁵). As mentioned, it is preferred that the gas is in its critical state (P_c, T_c) at the start of the expansion cycle. Typically, over 50% of the gas is converted to liquid.
• Removing, at the end of the expansion stroke of the piston, the liquid phase from the cylinder by means of the corresponding liquid pump (cylinder 2⁷). Alternatively, the liquid is removed during the expansion stroke.
• Compressing the remaining gas to a pressure slightly higher than the system pressure (preferably P_c), thus expelling substantially all of the gas from the cylinder, and to a temperature significantly higher than the starting temperature (uneven cylinders 2⁹ - 2¹⁹).
• Charging gas into the cylinder, again preferably at the critical pressure and temperature (cylinders 2²¹, 2²³), thus generating work. The volume charged into the cylinder is substantially equal to the initial volume (cylinder 2¹).

The gas expelled from the cylinders is fed to the first reverse current heat exchanger, i.e. heat of the gas is used to heat condensate in the steam cycle. Further, the temperature of the gas is decreased to T_c or near T_c.

Upon removal from the cylinders after expansion, the liquid pressure is increased to P_c by use of the pumps. As the compressibility of the liquid is very low, an increase in pressure will substantially not result in an increase of the liquid temperature. Subsequently, the liquid is fed to the second heat exchanger, i.e. the heat content of the (relatively cold) liquid is used to convert gas in the condensers of the steam cycle to liquid. Further, the temperature of the liquid in the heat swing cycle is increased to T_c or near T_c.

The heated liquid and cooled gas in the heat swing cycle are subsequently mixed to obtain a mixture having a pressure and temperature substantially equal to the pressure and temperature of the mixture that was present in the cylinder at the start of the expansion stroke (shown in uneven cylinder 2¹; and even cylinder 2¹²).

The steam cycle is operated thus: The medium, e.g. CF₄, in the steam cycle is pre-heated in the evaporator 15 and, since the amount of heat is not sufficient to completely evaporate the medium, subsequently heated in the additional heater 18, which preferably comprises a heat exchanger that absorbs heat from the environment, such as a river or the atmosphere. The heated medium is fed to the heat engine, where it generates work, and discharged to the condenser 16, where the medium is cooled and condensed by means of the liquid in the heat swing cycle. Finally, the medium is pumped to the evaporator 15 and the cycle is complete.

As thermal losses result in a relatively low efficiency, all equipment described above and indeed any auxiliary equipment is thermally isolated in a manner known in itself.

If the same type of fluid, e.g. Argon, is used in both the heat swing- and steam cycles, it is preferred that the gas in the heat swing cycle is cooled during compression. Such cooling can be achieved e.g. with the cold condensate after it has been used for gas to liquid conversion in the condenser of the steam cycle.

Figures 2A to 3 show a preferred cylinder and piston assembly 25 suitable for use in the power plant 1 of Figure 1 and a preferred centrifuge 26 for separating liquid and gas at the outlet of this assembly 25.

The assembly 25 comprises two pistons 3, rigidly interconnected by means of a rod 4 and received inside corresponding cylinders 2. At its centre, the rod 27 is provided with a slot 28 extending in a direction substantially perpendicular to the direction of reciprocating movement of the pistons 3 and allowing a crank of a crankshaft 5 to pass. The pistons 3 are connected to the crankshaft 5 by means of respective pairs of rods 29A, 29B. The cylinders 2 each comprise a spiral cylinder head 30 having a tangential outlet 31 and an axial inlet duct 32. The inlet duct 32 is fixed with respect to the cylinder head 30 and slidingly received in a central bore 33 in the piston 3. Furthermore, the piston 3 itself comprises a plurality of return ducts 34 that extend substantially parallel to the central bore 33 and open, via vanes 35, into the spiral cylinder head 30.

Figure 3 shows an active cyclone 26 for separating the gas and the liquid and comprising an inlet 40, connected to the tangential outlet 31 of the cylinder 2, a central rotor 41 having a plurality of U-shaped ducts 42, an annular filter 43 surrounding the outlets of the ducts 42 and providing an annular outlet duct 44 for the liquid, and, beneath the filter 43, an annular outlet duct 45 for collecting the gas.

During operation, a gas, preferably at its critical pressure and temperature, is continuously fed, via the inlet duct 32 to one of the cylinders 2 and allowed to expand, causing the piston to move towards the crankshaft and causing part of the medium to condensate, forming a liquid phase. Furthermore, the piston will cause the piston in the other (opposite) cylinder, which is in a different stage of the cycle, to compress the medium contained in that cylinder. The medium containing the condensate subsequently flows through the return ducts 34 and passed the vanes 35, which impart a centrifugal force on the condensate and entrain droplets. The flow is then fed to the inlet 41 of the cyclone 26 and separated into a liquid (outlet 44) and a gas (outlet 45).

Compared to the cylinders discussed with respect to Figure 1, the assembly according to Figures 2A to 3 more readily allows continuously separating the liquid, during expansion, from the gas.

The invention will now be illustrated by way of a numerical example. The calculations below are based on the following assumptions and conditions: the pistons move without friction, the process is considered to be adiabatic and reversible (isentropic), heat can be transferred at negligible differential temperatures, and the flow of the media is constant (no pulsations). The starting temperature (T_h) of the medium in the heat swing cycle, Argon, is 150 K (-123°C) and the temperature after expansion (T_l) is 110 K (-163°C). Work inputted (W_inp) in the heat swing cycle to compress the medium amounts to 1000 kJ.

During expansion, part of the medium condensates thus forming a liquid. The heat withdrawn from the liquid (the low temperature source) is calculated by: $$\left(\frac{1}{1-\frac{T_l}{T_h}}\right)\cdot W_{\mathrm{inp}}=\left(\frac{T_h}{T_h-T_l}\right)\cdot W_{\mathrm{inp}}=3.75\cdot 1000\hspace{0.17em}\mathit{kJ}=3750\hspace{0.17em}\mathit{kJ}$$

The heat added to the gas (the high temperature source) is calculated by $$\left(\frac{T_h}{T_h-T_l}\right)\cdot W_{\mathrm{inp}}=3750\hspace{0.17em}\mathit{kJ}$$

Adding the work inputted in the system yields a total amount of heat of 3750 + 1000 = 4750 kJ.

The exergy of the system is 4750 - 3750 = 1000 kJ, which is the work inputted to complete the cycle. However, the energy of the total system will be much higher if environmental heat is used as a high temperature source in the steam cycle.

The efficiency factor of heat to work conversion in such a cycle with an environmental temperature of 275 K (2°C) is defined as: $$1-\frac{T_l}{T_h}=\frac{T_h-T_l}{T_h}=\frac{275-110}{275}=0.6$$

The amount of heat required from the high temperature source (e.g. a river) is: $$\frac{1}{1-0.6}\ast 3750\hspace{0.17em}\mathit{kJ}=9375\hspace{0.17em}\mathit{kJ}$$

The amount of work that can be derived from the steam cycle is: $$Q_h-Q_c=9375-3750=5625\hspace{0.17em}\mathit{kJ}$$

The amount of heat available in the high temperature source is 4750 kJ. The amount of heat withdrawn from the environment is 9375 - 4750 = 4625 kJ. The amount of work generated in the steam cycle is calculated at 5625 kJ. However, it took 1000 kJ to sustain the heat swing cycle. So the total result is 5625 - 1000 = 4625 kJ, not surprisingly the same amount, as the amount withdrawn from the environment.

The heat swing efficiency factor amounts to $$\frac{W_{\mathrm{out}}}{W_{\mathrm{in}}}=\frac{4625}{1000}=4.625$$

It is noted that this is a calculated factor, which will be lower in an actual power plant.

The invention is not restricted to the above-described embodiments, which can be varied in a number of ways within the scope of the claims. For instance, instead of cylinder and piston assemblies, the heat swing cycle can be operated using rotary converters. Also, the gas and the liquid can be separated by means of a filter or electrostatically.