This invention relates to a device and a method for the operational and safety control of a heat engine. More particularly, it relates to a device for the operational and safety control of a heat engine, which has a working-fluid path including a high-pressure path and a low-pressure path, the heat engine using a condensable working fluid, which is in the liquid phase at least in part of the high-pressure path. The invention also relates to a method for the operational and safety control of a heat engine.

Below, a device for the operational and safety control of a heat engine is described. A method for the operational and safety control of a heat engine is described as well.

Heat engines exist in many different designs, and are based on different basic principles. More generally, heat engines are also referred to as motors. They share the characteristic of converting thermal energy into a form of higher-grade energy, for example mechanical or electric energy, which has a wider range of application. The majority of heat engines are based on internal combustion, and then at high temperatures (for example > 600 °C). Recently, it has become more and more relevant to use heat at low temperatures to drive heat engines.

There is a large amount of thermal energy available precisely at lower temperatures, and this energy often goes to waste or will have to be removed actively from different systems, for example from industrial processes or from the cooling systems of internal-combustion engines. Utilizing this energy to produce electricity, for example, may be very beneficial as it, as mentioned, often exists as a mere waste-heat product and therefore may be counted as free of cost. There are also several other examples of thermal-energy sources that may potentially be utilized in the same way, for example from gas, oil and biomass combustion, thermal solar collectors, geothermal sources and garbage incineration. Several of these heat sources may have relatively low temperatures, even under normal conditions. In this connection, several technologies based on, inter alia, Stirling and Rankine cycles have been developed, enabling their utilization to produce high-grade energy, generally in the form of electricity.

For particularly low temperatures (for example < 350 °C), motors based on so-called ORCs, the term ORC standing for "Organic Rankine Cycle", are generally used today. Rankine cycles are based on steam-engine processes, in which water is the working fluid, whereas ORCs are based on alternative working fluids, typically with lower boiling points than water, wherein the consequence is a more efficient utilization of the thermal energy. More often than not, these technologies are implemented in closed circuits, in which the working fluid remains in an internal and closed working-fluid circuit including, in the main, two or more heat exchangers, a fluid pump for working fluid and an expander, which may often be a turbine or a piston engine. Other expanders exist as well, such as various screw, vane, Wankel and spiral devices. In such closed motor systems, for the water flow, and thereby the energy flow, at least one heater section is required, typically in the form of a preheater, a boiler and a superheater, and a cooling section, generally consisting of a condenser, but further components may also be present. Exceptionally, a heater section may suffice, which is and was also the case in most steam locomotives, as the water (the working fluid) is then usually evacuated into and thereby indirectly cooled in the atmosphere (steam exhaust) after having carried out work by having expanded in the working cylinders.

In closed circuits for Rankine cycles, including ORCs as well, there is a working-fluid path in the form of a series of fluid passages and principal components in accordance with the working-fluid circuit described above. The fluid path consists, in the main, of the high-pressure path, which includes all the components from the fluid pump to an expander inclusive, and the low-pressure path, which includes all the components from the expander to the fluid pump inclusive, considering the normal direction of flow of the working fluid. This means that, in the main, the high-pressure path runs from the fluid pump via an outlet in the form of a pressure port, check valves, if any, at the outlet of the pump, connected pipes, further through the heater section which typically consists of the boiler and a superheater, and then to the expander through an inflow/injection valve. In the same way, the low-pressure path then typically runs from the expander, through an exhaust valve and exhaust passage(s), connected pipes and further through the cooler, which at least includes a condenser, a working-fluid reservoir, and then back to the pump through an inlet in the form of a suction port. The interfaces separating the high-pressure path from the low-pressure path will then be exactly the fluid pump and the expander. There may also be more components connected to each of the fluid paths, or fewer for that matter. Especially for Rankine motors, among them ORC motors as well, it may often involve an operational and safety risk if the energy transport through the motor should stop or encounter increased resistance in various ways. In systems based on Rankine motors, one will always find, directly or indirectly, a heat source and a heat sink, and a work receiver which may very likely be a shaft or a generator connected via a shaft. If the expander or the heat sink, for example, should be put out of action during operation, and then with the consequence that the mass and/or energy transport may stop as well, there will be a relatively immediate risk of the working fluid present in the heater section becoming superheated and/or an unacceptably high pressure building up in the engine.

This is a problem that concerns all heat-engine systems in which the heat-source temperature may be at, or may exceed, a level which in turn may lead to said fault condition in the motor system. By too high a temperature, some working fluids may easily degenerate into a condition in which they become unusable, or at worst hazardous to the safety of human beings or to the operation of the system, for example by toxic or corrosive degradation products developing. In the same way, an overpressure in the system could create dangerous situations, which could, at worst, lead to an explosion. A great number of serious explosions in steam boilers, for example, over the course of time are known. Corresponding elements of risk are also found in other heater and boiler types, such as in various ORC systems.

To increase the safety, it is standard design practice to place one or more safety valves in the system, wherein the safety valve(s) is (are) arranged to reduce the pressure and possibly the temperature of the working fluid in states of fault/emergency. The heated and evaporated working fluid can then be evacuated directly to the cooler, possibly to the working-fluid reservoir, without first having to flow through the expander, so that the temperature and pressure may be reduced as it is cooled by the colder surroundings here. If the cooler should be out of function, such a measure will not be sufficient in the long run. In that case, it must then be possible for the working fluid to be evacuated to an alternative destination, for example into the atmosphere or another open reservoir. With fluids other than water, this could not be a satisfactory solution, either, as several alternative fluids exhibit properties, which make them unfit for discharge into the local environment, either for the reason of human safety, environmental reasons or other reasons.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art. The object is achieved through features, which are specified in the description below and in the claims that follow.

An alternative to letting the heated and evaporated portion of the working fluid be returned to the cooler or an open reservoir is to ensure that, in terms of fluid flow, the working fluid can be drained and evacuated in front of the boiler section, so that the working fluid may then be evacuated from a point in the high-pressure path at which it has not yet undergone evaporation and therefore, in the main, is in the liquid phase.

This has the great advantage of enabling a removal of the working fluid at a point where it has not yet had very much energy added to it, which will be an effective method of preventing energy in the form of heat from accumulating in the working- fluid path. What will then be left in the high-pressure path is a small amount of superheated working fluid, a portion of which may also be evacuated, but this smaller amount constitutes only a small energy store, and the overpressure or elevated - temperature problem will then have been solved. In addition, in various systems, heating a small amount of working fluid to the maximum temperature achievable in the heater section could be allowable, as long as the amount is small enough. Superheated working fluid has a substantially lower density than the same fluid in liquid form and the residual amount could therefore constitute a minimal mass fraction in relation to the total amount of working fluid in the system.

In a normal Rankine process, the working fluid will be heated successively as it flows through the heater section. That is to say, the portion of working fluid that has flowed the farthest into a heater will normally have received the most heat and thereby reached the highest temperature to the point at which boiling starts, and then normally at a constant temperature. By placing the draining point in a portion early enough in the high-pressure path, for example just in front of the heater, or possibly in front of a recuperator, if such a one has been placed in the system, the flow of working fluid could be reversed in a possible need for evacuation. In addition to preventing further heat transfer to the working fluid in the boiler, it also means that the coldest portions of the working fluid present in the high-pressure path will be evacuated first. The working fluid that is evacuated will thereby have a minimum of energy, which gives a great advantage if it is going to be evacuated back into the working-fluid reservoir, possibly via the recuperator or cooler (condenser). This will help to limit the final pressure, and also the temperature, reached in the low-pressure path after the evacuation has been completed.

In a further connection, a drain loop like the one described above could be a very use- ful tool for stopping the operation of the motor in a quick and efficient way. In many Rankine systems, the high-pressure path must be drained of working fluid when the operation is to be stopped, and this requires in many cases that the evaporation of the fluid must be continued, while at the same time the working-fluid pump is stopped, in order then to evacuate the working fluid through the expander, possibly through a bypass, but wherein the working fluid will still be in the evaporated state as it is flowing out of the high-pressure path. By enabling drainage and evacuation of the working fluid at a point where it is still in liquid form, the advantages of being able to evacuate it substantially faster, as its density is higher, and of the energy level being low are achieved and the system may thereby be stopped relatively quickly.

According to a first aspect of the invention, a heat engine which has a working-fluid path including a high-pressure path and a low-pressure path is provided, the heat engine using a condensable working fluid which is in the liquid phase at least in part of the high-pressure path, and the heat engine being characterized by a fluid-drainage path, which is selectably open or closed, being connected to a portion of the high- pressure path in which the working fluid is mainly in the liquid phase.

By such a design of the heat engine, at least some of the unfavourable conditions described under the prior art are overcome. The design allows of further improvements as it will be described below.

The fluid-drainage path may be connected to the high-pressure path at a connection point located downstream of the fluid pump.

In its downstream portion, the fluid-drainage path may be connected to the low- pressure path.

By returning the working fluid from the high-pressure path to the low-pressure path, emissions of working fluid into the surroundings are prevented, which may be both environmentally and economically beneficial.

The fluid-drainage path may be provided with a drain valve, preferably in the form of a controllable valve. However, in some cases, the drain valve may be an overpressure valve, which is arranged to open at a predetermined working-fluid pressure.

In its activated state, when a signal is being supplied to it, the drain valve may be closed to fluid flow, and in its non-activated state, when it is not receiving any signal, it may be open to fluid flow. Such a "normally open" fluid valve contributes to increased safety in that, in the event of a signal drop-out, it will drain the high-pressure path so that the expander stops.

According to a second aspect of the invention, a method is provided for the operational and safety control of a heat engine which has a working-fluid path including a high- pressure path and a low-pressure path, the heat engine using a condensable working fluid which is in the liquid phase at least in part of the high-pressure path, and the method including the following steps:

providing the heat engine with a fluid-drainage path which is selectably open or closed, and which is connected to a portion of the high-pressure path in which the working fluid is mainly in the liquid phase;

detecting an operational condition in the heat engine that may cause the working fluid present in the high-pressure path of the heat engine to reach an undesirably high pressure and/or an undesirably high temperature, or, detecting that the working fluid present in the high-pressure path has reached an undesirably high pressure and/or an undesirably high temperature;

opening the fluid-drainage path; and

letting an amount of working fluid be drained and thereby evacuated from the high-pressure path via the fluid-drainage path.

The method may more specifically include providing the fluid-drainage path with a drain valve and driving the drain valve into the open position when there is a need to drain fluid from the high-pressure path.

The method may more specifically include:

letting an amount of working fluid be drained from the high-pressure path, wherein the flow direction of the portion of the working fluid which is being evacuated and which is still in the high-pressure path is principally reversed in relation to the flow direction during normal operation.

The method may further include connecting the fluid-drainage path to the low- pressure path and letting working fluid be drained from the high-pressure path into the low-pressure path.

The method may more specifically include:

letting working fluid be drained into the low-pressure path at a connection point which is placed in one of the following positions in the low-pressure path:

upstream of a recuperator;

upstream of a condenser; upstream of a working-fluid reservoir; or

on the working-fluid reservoir and with connection into it in terms of fluid .

It is emphasized that the recuperator does not constitute a necessary component, but is often used to increase the efficiency of a heat engine.

The method and the device according to the invention give markedly increased safety in a possible fault condition and are arranged to prevent unfortunate or dangerous situations in general. In addition, they are an effective means of stopping the heat engine in a quick, but controlled manner.

In what follows, an example of a preferred embodiment and method is described, which is visualized in the accompanying drawings, in which :

Figure 1 shows a block diagram of a heat-engine system including a heat engine, a heat source, a heat sink, an energy converter and an external control unit, in which interfaces between the components are shown;

Figure 2 shows a block diagram of a heat-engine system as shown in figure 1, in which the energy, electricity and signal flows are indicated;

Figure 3 shows schematically a heat engine in accordance with the invention with the associated main components; and

Figure 4 shows schematically the heat engine of figure 3, but the expander has been specified as being a piston engine.

In the drawings, the reference numeral 1 indicates a heat engine, which is connected via a heat-source interface 2 to a heat source 4, via a heat-sink interface 6 to a heat sink 8, via a power/electricity interface 10 to an electric-power converter 12 and via a signal interface 14 to an external control unit 16.

Some of the components in figures 3 and 4 are marked with the symbol "Z" . This ind icates that it is a heat exchanger of some form.

In figure 2, heat that is flowing from the heat source 4 to the heat engine 1 is indicated by Q _H . Residual heat that is being removed from the heat engine 1 and transferred to the heat sink 8 is indicated by Qc. Electric power that is being transferred from the heat engine 1 to the electric-power converter 12 is indicated by P _E L- Measurement and control signals that are being exchanged between the heat engine 1 and the external control unit 16 are indicated by S _c . The heat engine 1 preferably forms part of an ORC system and includes a fluid pump 20 with an inlet 22 and an outlet 24. From the outlet 24, a pressure-pump line 26 extends via a recuperator 28 and on to a heater 30. The recuperator 28 may consist of an, in the main, standard heat exchanger known per se, with two conventional opposite heat-exchanger sides, not shown, consisting of separate and heat-communicating internal fluid paths. The heater 30 typically includes an evaporator 32 and a superheater 34. The heater 30 is supplied with heat Q _H from the heat source 4 via the heat interface 2.

A steam line 36 is connected between the superheater 34 and the inlet 40 of an expander 38. The expander 38 may consist of, for example, a turbine, a piston engine or the like. An outlet 42 from the expander 38 constitutes an exhaust outlet. The components between the fluid pump 20 and the expander 38, including the pressure-pump line 26, the high-pressure side of the recuperator 28, the heater 30 and the steam line 36 constitute the high-pressure path 44 of the heat engine 1.

In this exemplary embodiment, the expander 38 drives a generator 48 via a shaft 46. Electric power P _EL is transferred via the power/electricity interface 10 to the electric- power converter 12. A motor-control unit 50 controls the expander 38 and the generator 48 among other things. Necessary transmitters and control lines, known per se, are not shown.

An outlet line 52 extends from the outlet 42 of the expander 38, via the recuperator 28, a condenser 54 to a working-fluid reservoir 56. The condenser 54 delivers residual heat Qc to the heat sink 8 via the heat-sink interface 6.

A suction line 58 connects the working-fluid reservoir 56 to the inlet 22 of the fluid pump 20. The components between the expander 38 and the fluid pump 20, including the outlet line 52, the low-pressure side of the recuperator 28, the condenser 54, the working-fluid reservoir 56 and the suction line 58 constitute the low-pressure path 60 of the heat engine 1.

A fluid-drainage path 62, which is here connected to the pressure-pump line 26 between the recuperator 28 and the heater 30, is connected via a drain valve 64 to the outlet line 52 between the expander 38 and the recuperator 28. The fluid-drainage path 62 is arranged to short-circuit the high-pressure path 44 with the low-pressure path 60 whenever necessary. The drain valve 64 is of an actively controllable kind, like an electromagnetically, mechanically, pneumatically or hydraulically activated on-off valve. Alternatively it may be a proportional valve or a servo-valve, for example. During normal operation, working fluid is sucked by means of the fluid pump 20 from the working-fluid reservoir 56 and is then pumped into the high-pressure path 44 under relatively high pressure.

The working fluid is first pumped through the recuperator 28, in which it is preheated by receiving residual heat from the exhaust which is flowing out of the outlet 42 of the expander 38 and which is directed into the low-pressure side of the recuperator 52 via the outlet line 52.

After first having passed through the recuperator 28, the working fluid flows into the heater 30 and, in the first step, into the evaporator 32 where it is heated up towards the boiling point and thereby evaporated . Further, the working fluid passes into the superheater 34 where the temperature is increased beyond the boiling point. After that, the working fluid is carried into the expander 38 where part of the added heat energy is converted into mechanical energy by the working fluid being expanded near- adiabatically, near-isothermally, near-isobarically or near-polytropically.

The mechanical energy is in turn converted into electric energy by means of the generator 48. The electric energy from the generator 48 is transferred as electric power PEL from the generator 48 via the power/electricity interface 10 to the electric-power converter 12.

Having completed the expansion in the expander 38, the expanded working fluid, which may now be defined as exhaust, is carried via the outlet line 52 to the low- pressure side of the recuperator 28, where part of the residual heat is returned to the working fluid in the high-pressure path 44 and recovered .

The working fluid is then directed into the condenser 54 in which the last portion of residual heat Qc that is to be removed flows via the heat-sink interface 6 to the heat sink 8. The working fluid is thereby condensed to the liquid phase before it is carried into the working-fluid reservoir 56.

When there is a risk of overpressure and/or superheating of the working fluid that might be present in the high-pressure path 44 during operation, or when there is a condition in which it may otherwise be desirable to stop the heat engine 1 in the quickest possible way, the motor-control unit 50 may drive the drain valve 64, by means of known control principles, into the open state by a control signal being communicated via a control-signal conductor 66 which is connected to a d rain-valve actuator 68, which in turn ensures that the drain valve 64 takes the open position . There is thereby a short-circuiting, in terms of fluid flow, between the high-pressure path 44 and the low-pressure path 60.

To be able to identify conditions in which it will be desirable to stop the heat engine 1 quickly, the heat engine 1 is provided with various known sensors, not shown, so that exactly these conditions can be registered and identified by the motor-control unit 50, which in turn may communicate the necessary control signals, and then in particular the control signal that ensures opening of the drain valve 64.

When short-circuiting then takes place, working fluid will be drained from the high- pressure path 44 in a position in which, normally, it is mainly in the liquid phase, up to the point when the entire liquid fraction has been evacuated almost completely. Thus, the greater portion of the mass of the working fluid will initially be drained in the liquid phase, and subsequent evacuation will then, in the main, consist of working fluid in gaseous form, either as saturated or superheated gas, representing only a minor mass fraction in relation to the total mass of the working fluid.

This will result in the working fluid being drained and thereby evacuated from the high-pressure path 44 in a state that means that a smallest possible amount of energy will have to be removed from the high-pressure path 44.

In figure 3, the fluid-drainage path 62 is shown connected to the high-pressure path 44 between the recuperator 28 and the heater 30. Depending on the conditions of operation, it may be advantageous to connect the fluid-drainage path 62 to the high- pressure path closer to the fluid pump 20, for example at a connection point 70 located downstream of the fluid pump 20. Likewise, it may desirable to connect the fluid- drainage path 62 to the low-pressure path in a position closer to the fluid pump 20, for example at one of the connection points 72 that are located upstream of the fluid pump 20.

As long as a condensable working fluid is used, it may be assumed that the fluid in the high-pressure path 44 is mainly in the liquid phase between the fluid pump 20 and the heater 30. This part of the high-pressure 44 thus constitutes a portion 74 in which the working fluid is, in the main, in the liquid phase.

In an alternative embodiment, see figure 4, the expander 38 consists of a piston engine. In this embodiment, the expander 38 is formed with at least one controlled inlet valve 76 and at least one controlled outlet valve 78 which together control the fluid flow through the expander 38, by the valves 76, 78 controlling the fluid flow through the at least one inlet 40 and the at least one outlet 42. In normal operation, the controlled valves 76, 78 ensure that said paths are never open simultaneously. Thereby there will not be a direct fluid short-circuiting across the expander 38 if the expander 38 should stop, whereby a direct short-circuiting between the high-pressure path 44 and the low-pressure path 60 is prevented from occurring through the expander 38. In many cases, the inlet valve 76 and the outlet valve 78 are controlled by respective valve actuators 80, and these will normally be synchronized in such a way that this form of short-circuiting is prevented.