WO2004020793A1 - Energy conversion method and corresponding device - Google Patents

Abstract

The invention relates to an energy conversion method in accordance with a closed thermodynamic cyclic process, in particular in accordance with the Carnot cycle, said method using a working gas, to which heat is supplied in order to perform the work. The temperature of the working gas is controlled by a mechanical counter-pressure acting on the working gas, said pressure being proportional and working in opposition to the pressure of the working gas.

Description

 Energy conversion method and apparatus therefor

description

The invention relates to a method for energy conversion according to a closed thermodynamic cycle, in particular according to the Carnot process, using a working gas, to which heat is supplied to perform work, and a device therefor.

The Carnot cycle has a relatively high thermal efficiency, which is only dependent on the upper and the lower limit temperature. However, the technical implementation of this cycle is difficult because an isothermal change of state is practically impossible and because of the pressures that occur, compression is only possible within small temperature ranges.

In practice, it is necessary for power plant operators or energy supply companies to keep the voltage in the electrical network constant to ensure the supply to their customers. In order to compensate for fluctuations in consumption, it is necessary to constantly provide an energy reserve in addition to the energy required to secure a base load is at least partially maintained in pumped storage power plants. In times of relatively low energy consumption, mostly at night, electrical energy is used to pump water in the pumped storage power plant to a relatively high level in order to store potential energy for driving water turbines to generate electricity in times of relatively high energy requirements. The use of pumped storage power plants with a relatively poor efficiency reduces the maintenance costs for the thermal power plants, the energy of which is used to cover peak consumption. The use of alternative energy, i.e. wind power or solar radiation, also proves to be problematic in the case of constant power supply, since if there is no wind or insufficient solar radiation, the capacity of a wind or solar power plant must be provided by a conventional power plant, which is sufficient if wind power or solar radiation is sufficient to generate electricity is only in a standby state. This means that energy supply companies and power plant operators incur high costs for providing energy to cover peak loads in electricity consumption, which represent a significant part of the purchase costs of electricity for consumers.

It is an object of the invention to provide a method and a device of the type mentioned at the outset with which an optimized process control is ensured.

According to the invention the object is solved by the features of claims 1 and 3.

The subclaims represent advantageous embodiments of the Invention.

With the type of temperature control according to the invention in the cyclic process, efficient process control is achieved, which results in a relatively high energy yield with a high degree of efficiency. This process control results from considerations regarding the validity of Newton's third axiom in thermodynamics.

It goes without saying that the features mentioned above and still to be explained below can be used not only in the respectively specified combination but also in other combinations. The scope of the present invention is defined only by the claims.

The invention is explained in more detail below on the basis of several exemplary embodiments with reference to the associated drawings. Show it:

Fig. 1 representations of a cycle according to the inventive method, Fig. 6th

Fig. 7 is a schematic representation of an inventive device for performing the process and

8 shows a schematic illustration of the device according to FIG. 7 in an alternative embodiment.

Fig. 1 shows a Carnot process that starts at point 1. In this idealized process flow, a quantity of heat supplied is partially converted into technical work W, with a working gas being expansive from point 1 to point 2. diert. With increasing temperature i, the area and thus also the heat conversion below the Tj isotherm increases. For a technical application, a process must be repeated as often as required. A one-time operation provides only limited work. Starting point 1 should therefore be controlled periodically. Carnot chooses the lowest possible temperature T ₂ in the vicinity of the ambient temperature. The working gas is compressed isothermally from point 3 to point 4. The amount of heat expelled Q ₂ corresponds to the work done on the gas. From point 4 to point 1 there is an adiabatic compression of the working gas, the temperature of which increases from T ₂ to Ti.

To illustrate the compression and expansion of the working gas, a pressure cylinder 10 with a piston is shown below the cycle. On the way from point 3 to point 4, heat is expelled from the working gas. The internal energy of the working gas is smaller at point 1 than at point 3. Without the supply of heat, the imagined, friction-free piston in the pressure cylinder 10 cannot reach the starting point 3 of the compression. The possible return path that the piston will cover due to the internal energy of the working gas is shown by the dashed line. At temperature i the isentropic expansion begins in the direction of point 4. In contrast to the compression, the piston of the pressure cylinder 10 does not change the downward direction at point 4 but does isentropic work up to point 5. The work performed corresponds to the area under the isentropes 1-4 -5. At point 5 there is a change of direction. The pressure force of the working gas acts on the piston from one side and the air pressure p _Lc ι of the environment from the other side. Both pressures and the corresponding forces are in amount and Direction the same size but directed in opposite directions. The working gas temperature T ₃ at point 5 is lower than the working gas temperature T ₂ and has reached the minimum.

Assuming that the walls of the pressure cylinder 10 are heat-permeable, the working gas uses the temperature difference to the environment and absorbs heat. The temperature of the working gas rises and its volume increases under the action of the piston isobarically. The piston is moved to the right in the direction of point 3. The process ends when the temperature of the working gas has reached the ambient temperature T2. The left-handed cycle ends at point 3. The expansion work done by the working gas is less than the compression work done.

The process flow for reproductive work efficiency between points 5 and 3 is significant for the technical use of ambient heat. The working gas temperature is T ₃ for the piston position in point 5 and T ₂ for the piston position in point 3. The working gas expands from T ₃ to T ₂ , where T ₃ <T ₂

In Section 5-3, the working gas absorbs the heat quantity Q _at. Heat is not dissipated with this isobaric process control. It can therefore be assumed that the heat quantity Q _2U supplied is completely converted into mechanical work (W = Q _zu = ΔV * p). The mechanical work is done to the environment and cannot be used for energy. Nevertheless, the crucial starting points for the use of ambient heat can be found here. The piston of the pressure cylinder 10 shown in FIG. 2 moves from point 5 to point 3 during the isobaric change of state. The volume increases by the amount ΔV at constant pressure p and does work (W = p * ΔV). The pressure acting on the piston is calculated from p = F / A, where A describes the area of the piston. The change in volume is defined by ΔV = A * s, where A describes the area of the piston and s the distance traveled by the piston due to the amount of heat Q _2U supplied.

The direct relationship between the amount of heat Q _zu and the isobaric work (W _t h) of the working gas is described as follows:

Wth = _mec h = Qzu = F * S.

The piston does mechanical work on the environment. This work cannot be used technically. Nevertheless, there are other starting points here that show how a thermal process can be controlled in order to use the ambient heat or generally use thermal energy more efficiently.

If a body exerts a force on another, it experiences an opposite, opposite force from it. Forces always occur in pairs and in opposite directions (F '= -F). This principle of reaction or interaction is of a fundamental nature. If the points of attack of F and -F 'lie in two different bodies, this state corresponds to Newton's principle of reaction or interaction.

If two forces act on one body (piston) (equal or unequal in amount, opposite), then it is not about force and reaction force in the sense of Newton's third axiom. In the case of the same amount, these are referred to as compensation forces. Newton's basic law must also be demonstrated in thermodynamics.

2 shows the isobaric change in state between points 5 and 3 that Newton's axiom is valid. According to the equation _th = W _meC h = F * s, a force F acts over the path s.

If a thermal force F _t h (gas pressure) _acts on the piston, this force must be counteracted by a mechanical force F _meCh .

According to the assumptions relating to FIG. 2, the atmospheric air pressure acts on the piston at point 5 or at piston position 5 as an equally large, oppositely directed force. The process flow direction changes. Along the isentrope 1-4-5, the piston does mechanical work due to the internal energy of the working gas. The working gas temperature drops from Ti to T ₃ . From point 5 or the piston position 5, the working gas absorbs ambient heat and performs mechanical work due to the amount of heat supplied. The working gas temperature rises and the gas volume increases. Due to the infinitesimally higher gas working pressure, the piston moves in the direction of point 3 and does mechanical work. At point 3 or in the piston position 3, the working gas reaches the temperature T ₂ . Further heat absorption is no longer possible. Reproductive energy generation ends at the expense of ambient heat. According to Carnot, a working gas along the isotherms i takes heat from a heat container, the temperature of which does not change during the process, and does the corresponding amount of mechanical work. The work generated corresponds to the area under the isotherms Ti according to FIGS. 2 and 3 and is limited by the curve points 1-2-bal.

An enormous problem arises when considering force theory. The working gas expands - left to itself - isentropically from point 1 towards point 5. The reaction force is generated by gas molecules. The random movement of the gas molecules changes on average to a targeted movement. The inertia of the gas molecules must be overcome for this change in direction.

According to FIG. 3, an external force F _{mech x} acts on the piston at point x, which force can be generated, for example, by a hydrostatic water column, the height of which does not change during the process.

At point x, the internal working gas force F _t hx _r acts on the piston and is calculated from the working gas pressure at x multiplied by the piston area A. The working temperature is T _x <Ti. The internal force F _{th x} acting on the piston forms an equilibrium of forces with the external force F _meC h _x - the isentropic expansion of the working gas _stops . The isentropic mechanical work corresponds to the area 1-xfal. Thermal energy generation has not yet ended.

Although there is a momentary balance of forces at point x the system is in a thermal imbalance state. According to Carnot, the process should run at a working gas temperature Ti. This working gas temperature cannot be assumed. Instead, a working gas temperature T _x (T _x <Ti) is to be used as the temperature in the pressure cylinder. Heat is supplied to the working gas via the heat-permeable walls of the pressure cylinder and the working gas temperature rises.

The supply of heat to the working gas creates a change in state that is isobaric. The piston then moves to the right when the condition force Fth> F _mΘ ch is fulfilled. The thermal action force F _t h will only differ infinitesimally from the attacking reaction force F _meCh . The piston strives for the equilibrium of forces and covers the distance Δs. The amount of heat supplied is converted into mechanical work. In Fig. 3, this conversion rate corresponds to the area under the isobar xy.

A further statement can be derived from FIG. 3. At the beginning of the isobaric change of state at point x, the effective temperature gradient is ΔTi> ΔT ₂ . This allows direct statements to be made about the performance of the process. The heat transfer and thus the transmittable quantity of heat Q _to, by a warmedurchlassige wall on the following factors dependent:

Q = Qzu = k A t ΔT = W _mech with:

Q the amount of heat transferred through the flat wall k heat transfer coefficient A passage area t time or duration of heat transmission

ΔT temperature difference in front of and behind the wall.

3 shows that the temperature difference ΔT between the Ti isotherms and the working gas temperature is variable. From the maximum at process point x, the temperature difference ΔT decreases continuously to zero up to the end point y. The thermal imbalance state, the prerequisite for the heat flow, no longer exists. The system has reached equilibrium. Heat can no longer be converted into mechanical work.

As the equation above shows, the theoretical possibility of converting heat into mechanical work ends at point y. Technical problems arise at an earlier point in time when the performance of the process is assessed. Performance is the ratio of the work performed to the time required.

P = W / t = Q _to / t Thus the thermal conversion rate from heat to mechanical work decreases when ΔT becomes very small. Although the process is still energetically fruitful, the time t in which a quantity of heat Q _is converted becomes very long. A large amount in the denominator means that the amount of heat Q is converted _to very poor performance and ultimately becomes technically unusable.

Thermodynamics takes this fact into account. After thermodynamic considers a process is supplied to the quantity of heat Q _to as thermal energy. This thermal see energy (Q _zu ) cannot be completely put into work after the assumption according to Newton's third axiom. Thermodynamics describes the deficit that arises from the energy conservation law as the entropy quantity.

As can be shown below, the amount of heat Q _is more of a type of thermal field property in which the heat conversion process takes place. Such an assumption makes perfect sense. The amount of heat Q _zu cannot be completely transferred into work based on force theoretical assumptions. However, as far as conversion is theoretically possible, the energy conservation rate applies without exception.

The process of converting heat into mechanical work is clearly not solely dependent on thermal efficiency. Heat is then converted into mechanical work as long as the pair of forces F _th = -F _{mech is} in an unbalanced state. The pair of forces strive for equilibrium and either produce mechanical work or work is done on the system.

4 shows the isobaric change in state of a gas between the Ti and T ₂ isotherms. An imaginary frictionless, mass-free piston moves within the pressure cylinder 10 on the path s. The piston in the pressure cylinder 10 is in position X. The pressure cylinder 10 is in a heat bath with the temperature Ti. The temperature of the working gas is T ₂ <i. The external force F _meCh (hydrostatic column, the height of which does not change during the process) _acts on the piston, _creating a balance of forces Working gas pressure forms (F _t h) - The walls of the pressure cylinder 10 are heat-permeable.

Under these assumptions, heat is added to the working gas via the walls and the piston moves isobarically in the direction of position Y. The mechanical work F _meC * Δs is carried out. The temperature gradient ΔTi decreases during the process and finally reaches zero at point Y. The working gas temperature in the pressure cylinder 10 has increased from T ₂ to Ti, the thermal equilibrium has been reached. Without a change in force or temperature, the piston remains immovably in position Y. All imbalance states acting on the system have reached the desired equilibrium state. If all forces acting on a body are in balance, the system has reached the static state. Static systems are energetically sterile and cannot do any work.

In the second case, the pressure cylinder 10 is accommodated in a heat bath with the temperature T ₂ . The working gas has the temperature Ti. Under these circumstances, the applied force F _mech shifts (hydrostatic column) the piston from Y to X. The mechanical force F _mech performs work on Ar ^¬ beitsgas adäguate and a quantity of heat is dissipated via the temperature gradient .DELTA.T ₂ to the environment. When the piston reaches point X, a state of equilibrium occurs again. The working gas temperature is identical to the ambient temperature T. The system becomes static again and the movement of the piston ends.

6, the working gas is initially at the process starting point 1. The working gas pressure _± with the working gas temperature Ti forms a balance of forces with the external mechanical force -F _mech . According to Carnot, the working gas expands isothermally from point 1 to point 2. Such a process is not technically possible because an entropy gap must be kept open.

Thermal charging of the working gas from point 1 to point 2a is more energy-efficient. Heat is added to the working gas and the gas temperature rises from Ti to T ₄ . The attacking external mechanical force -F _meC h is shifted _to isobaric to point 2a due to the heat quantity Q supplied and performs mechanical work. At point 2a, the heat supply to the working gas is interrupted and the external force -F _meC h is reduced in accordance with the course of the isentropic pressure. The working gas is relaxed isentropically and does work due to its internal energy, whereby the working gas temperature drops from T ₄ to T ₂ = T _u . The supplied amount of heat Q _zu is converted into mechanical work if all heat conduction and radiation losses are neglected.

Such a process is possible with a thermal-hydromechanical or torque-controlled system.

7, a refrigerant is mechanically liquefied in a heat pump 11. This refrigerant has to evaporate again in a cycle. This process is also used in the cold steam power plant for energy reproduction. A hydrostatic pressure regulates the vapor pressure of the refrigerant. The necessary for the evaporation process of the refrigerant almost unlimited heat is available in the form of ambient heat.

A pressure vessel 12 firmly anchored in a lake, river course or body of water, including associated risers 13, is filled with water. In order to control the process in a targeted manner, the system has valves 14 to 18, which are initially closed. The pressure vessel 12 is connected to the heat pump 11 via two transport lines 19 for supply and return. In order to keep height losses as low as possible, high tanks 20, 21 coupled to the risers 13 and arranged at different heights and the pressure tank 12 are built relatively flat. The vapor pressure of the refrigerant is above 5 bar at ambient temperature (approx. 280 K). The refrigerant is compressed and liquefied by the heat pump 11. The density of the liquefied refrigerant is lower than the density of water and the refrigerant is chemically neutral to water.

In the system, the valves 16, 18 are opened so that the refrigerant passes from the heat pump 11 via the one feed line 19 to the pressure vessel 12, which is now connected to the upper elevated tank 20 via the one riser line 13. If the refrigerant filled with water is liquefied with a vapor pressure of> 5 bar at ambient temperature, the feed line 19 is fed in and the valve 18 inserted in the feed line 19 is closed, the refrigerant begins to evaporate and displaces a corresponding volume of water to the elevated tank 20 Evaporation process ends when the gas pressure of the refrigerant and the hydrostatic pressure of the water column has reached a stable equilibrium. A stable equilibrium state is established when the working gas can no longer absorb heat at the external pressure and the state ΔT = 0 is reached. This state is reached according to the assumption conditions if the working gas has expanded to 1/5 of the normal volume.

The potential energy of the water obtained in the elevated tank 20 corresponds to the amount of heat absorbed and is higher than the mechanical work involved. The only source of energy is environmental heat. If no heat is supplied from the outside, the water temperature of the displaced water quantity drops. Deviating from classic processes, the heat of vaporization of the refrigerant is already being worked on. A real expansion process takes place at a temperature gradient ΔT = 10-20 ° C. The second law of thermodynamics becomes apparent, according to which heat is only transferred into work as long as there is a thermal imbalance condition. With a temperature gradient of ΔT = 0, the working gas, the refrigerant, reaches the equilibrium state. A dynamic, energy-generating process is transformed into a static system. Entropy must increase to keep the process dynamic.

The energy yield at the expense of environmental heat has not yet ended. If the valve 16 to the upper elevated tank 20 is closed and the valve 15 to the lower elevated tank 21 is opened, a new state of imbalance arises. The working gas again strives for equilibrium. According to the assumptions made, the external pressure that the water Sersaule, reduced and the working gas volume will increase due to the lower back pressure and water displaced to the lower elevated tank 21. Further potential work is gained at the expense of environmental warmth.

The system presented is mainly used for simple reasoning. An absolute energy yield is not considered here. The process control for using the environmental heat ends prematurely here. After temperature and pressure equalization in the second work cycle, the valve 15 to the lower elevated tank 21 is closed and the valve 17 inserted into the feed line 19 designed as a return is opened. The working gas, ie the refrigerant, is supplied to the heat pump 11 for compression due to the prevailing excess pressure via the feed line 19. As soon as the ambient water pressure and the gas pressure in the pressure vessel 12 are in equilibrium, valve 1 opens. The denser water pushes the remaining refrigerant via the feed line 19 to the heat pump 11 and at the same time fills the pressure vessel 12. After the filling process has ended, the valves 14, 17. The refrigerant circuit is closed. The potential energy of the water obtained from the process is converted into electrical energy by means of a water turbine 22 if required.

According to FIG. 8, a power plant 23 is provided for generating electrical energy, the power plant 23 being able to be designed, for example, as a thermal, wind or photovoltaic power plant, which is connected via electric lines 24 to compressors 25 for compressing gas (air) as well as a water turbine 22 is connected to generate electricity. In times of relatively low power consumption, the power plant 23 Electricity for driving the compressors 25 is supplied. By compressing the gas by means of the compressors 25, a corresponding amount of heat is obtained for the compression work performed on the gas (air), this statement always being valid if the compressed gas can emit heat via non-adiabatic walls. The compression work is converted into heat and given off for heating purposes.

Since the gas has been driven out of heat, the internal energy of the gas has decreased. In order to use the expansion work of the gas in a reversible process, the expelled heat must be added to the gas again. With a lower heat input, the yield of the expansion work is reduced. Warm is available as ambient heat for technical purposes in practically unlimited quantities, but only in a temperature range between -10 ° C and + 30 ° C. Assuming that the heat expelled from the gas occurs at a compression temperature of about 70 ° C (343 K), about 80% of the heat required for the expansion work of the gas can be supplied from the natural energy source “ambient heat”, which is done in mechanical work is to be converted.

In order to take advantage of the expansion work of the gas, it is passed through a compressed air line 26 from the compressors 25 into a compressed air store 27, in which the gas is stored at ambient temperature as a medium for storing potential energy. When there is an energy requirement, the compressed air reservoir is tapped, the gas expands and at the same time absorbs ambient heat, which increases the work of expansion and provides mechanical work. To the mechanical To use the work of the expanding gas to generate electricity, the compressed air reservoir 27 is connected via a control valve 28 to the pressure tank 12 filled with water. The pressure vessel 12 is coupled to a water reservoir 31 via a feed line 29 and a return line 30. There is a height difference Δh between the water surface in the water reservoir 31 and the lower edge of the pressure vessel 12. At the beginning of the process of power generation, shut-off valves 32, 33 inserted into the feed line 29 and the return line 30 are closed. By opening the control valve 28, the gas acts on the water inside the pressure vessel 12 due to its expansion, so that the hydrostatic pressure (Δh) inside the pressure vessel 12 rises to the gas pressure p. When the shut-off valve 32 inserted into the feed line 29 opens, the water flows from the pressure vessel 12 with the pressure into the water turbine 22, which converts the mechanical energy into electrical energy. The control valve 28 is closed when a pressure of Δp = p - Δh is reached at a temperature T _o . The gas expands from Δp to Δh, absorbing ambient heat. The heat exchange between the water and the gas is influenced by appropriately dimensioning the flow line 29 or adjusting the flow rate by appropriately adjusting the control valve 28 or shut-off valve 32 or by inserting a control valve into the flow line. The water turbine 22 supplies the electrical energy obtained from the mechanical work via the electrical lines 24 to the power plant 23 or end user.

As soon as the pressure vessel 12 is empty of water, this is in the The supply line 29 shut-off valve 32 is closed and the water present in the system is located in the water reservoir 31. A valve 34 inserted in the return line 30 is then opened and the water flows freely in the pressure vessel 12. With sufficient water pressure, the return line 30 can an additional water turbine will be provided.

Claims

claims 1. A method for energy conversion according to a closed thermodynamic cycle, in particular according to the Carnot process, using a working gas to which heat is added to perform work, characterized in that the temperature of the working gas is controlled by a mechanical back pressure acting on the working gas which is proportional to and opposed to the pressure of the working gas. 2. The method according to claim 1, characterized in that the back pressure is generated using the ambient heat. 3. Device for performing the method according to claim 1 with a heat pump (11) or at least one compressor (25) for liquefying a refrigerant or for compressing a gas, characterized in that - The heat pump (11) is coupled to a pressure vessel (12) in which the refrigerant evaporates under the action of a counter pressure or - The compressor (25) is coupled to a pressure vessel (12) in which the gas expands under the action of a counter pressure. 4. The device according to claim 3, characterized in that the pressure vessel (12) is connected to a device generating the back pressure for storing potential energy. 5. The device according to claim 3 or 4, characterized in that the pressure vessel (12) for controlling work cycles via valves (14-18) with the heat pump (11) and the device is connected. 6. Device according to one of claims 3 to 5, characterized in that the pressure vessel (12) is exposed to the ambient temperature in an open reservoir. 7. The device according to claim 4 or 5, characterized in that the device at least one elevated tank (20, 21) for receiving water, which is coupled to the pressure vessel (12) via a riser (13). 8. The device according to claim 3 or 4, characterized in that the pressure vessel (12) via a control valve (28) is coupled to the compressor (25). 9. The device according to claim 4, characterized in that the device comprises at least one water reservoir (31) which is coupled to the pressure vessel (12) in a valve-controlled manner via a flow line (29) and a return line (30). 10. Device according to one of claims 3 to 9, characterized in that the device is assigned at least one water turbine (22) connected to the elevated tank (20, 21) or the water reservoir (31).