The present invention concerns a method for operating a power plant which is designed to generate energy with the features of the preamble of claim 1 and a power plant with the features of the preamble of claim 11.

Power plants known in the state of the art comprise at least one stationary internal combustion engine, which is configured to deliver a mechanical power by burning a fuel. Typically types of fuels for operating a stationary internal combustion engine are burnable gases (like natural gas), diesel, gasoline or heavy fuels.

To increase the overall efficiency of at least one stationary internal combustion engine it is known to connect or integrate the internal combustion engine to a district heating system. This allows to exchange heat between the district heating system and the at least one internal combustion engine, wherein on the one hand the at least one internal combustion engine is cooled and, on the other hand, heat is supplied to the district heating system.

To reach a better overall efficiency of the power plant it is common to operate the internal combustion engine with an increased engine temperature to provide the district heating system with as much thermal energy as possible. This leads automatically to a decreased efficiency of the internal combustion engine in isolation. This not optimal operation of the internal combustion engine is tolerated because the overall efficiency of the power plant increases.

In most cases, the district heating system comprises a plurality of sources of heat (beside the at least one internal combustion engine). Further, district heating systems comprise a very high thermal inertia because of the large amount of thermally affected masses and their thermal storage properties.

Therefore, if the internal combustion engine is inactive or operates in idle, the internal combustion engine may maintain a temperature given by the district heating system. In other words: the internal combustion engine does not cool down in an inactive operation state or during idle. This leads to the advantage that the internal combustion engine does not have to be pre-heated before starting.

A disadvantage of such a configuration is that the whole internal combustion engine is heated by the district heating systems and therefore also the intake manifold of the internal combustion engine and thus the charge air or air-fuel mixture have an undesired higher temperature level right at the start of the internal combustion engine. Higher temperature of the charge air or air-fuel mixture leads to a lower density of the same, which affects the power output of the internal combustion engine. This, however, leads to an increase of starting time and also to an increase of transient time at change of load of the internal combustion engine. Additionally, the generated exhaust pollution - like the NO _x -level - increases. To ensure that the NO _x limits (and other pollutants limits), which are given by the national legislature, are not exceeded a very slow start has to be chosen to keep the NOx emissions low.

The present invention's purpose is to provide a method for operating a power plant and a power plant, wherein a faster reaction corresponding to a transient performance requirement is possible with reduced or constant emitted emissions.

This is being achieved by a method for operating a power plant with the features of claim 1 and a power plant with the features of claim 11. Some advantageous embodiments of the invention are defined in the dependent claims.

According to the invention it is provided that cooling of the at least one internal combustion engine is effected - at least partially - using the at least one additional cooling device when a transient performance requirement for the at least one internal combustion engine occurs.

For example, to ensure a reliable operation of the internal combustion engine in most cases additional cooling devices (next to the district heating system and possible other regular cooling systems of the at least one internal combustion engine) are provided. In case that the cooling of the internal combustion engine no longer can be done by the district heating system (for whatever reason), the additional cooling devices can take over the cooling of the internal combustion engine to prevent an overheating of the internal combustion engine ensuring reliable operation. Additional cooling devices -in principle known in the prior art - therefore could be viewed as cooling emergency units for the internal combustion engine.

These already present, but usually inactive additional cooling devices can for example be used by the invention to increase the power output of the internal combustion engine (and/or to decrease emissions) for a short term, essentially during the occurrence of a transient performance requirement.

Alternatively, separate additional cooling devices can be provided, e.g. as thermal reservoir, preferably as at least one roof top cooler and/or thermal storage mass.

The term “district heating system” does not necessarily mean that this heating system has to be used for heating a district of a city or a town. A district heating system can be used to provide heating for any number of structures (e.g. residences, industrial or agricultural buildings).

The district heating system usually uses a tempering medium as heat carrier to facilitate the transfer/transport of heat. This tempering medium could for example be oil, air, water with or without additives and so on. Tempering mediums can be used for heating and/or cooling systems.

It can be provided that the transient performance requirement relates to a start of the at least one internal combustion engine. The start of at least one internal combustion engine means transferring the internal combustion engine from an inactive state until a predeterm inable engine speed or a predeterm inable power output is reached.

It can also be provided that the transient performance requirement relates to the performance increase of the at least one internal combustion engine e.g. for creating additional electrical power. Also, if an increased performance requirement for the internal combustion engine occurs the emissions generated by the increase can be reduced using the additional cooling devices according to the invention. It is also possible to achieve the performance increase faster with unchanged emitted emissions. It can be provided that a connection between the at least one additional cooling device and a cooling system for the combustion air and/or the fuel - preferably at least one intercooler and or a thermal reservoir - of the at least one internal combustion engine is established when a transient performance requirement occurs. The cooling of the fuel, the fuel mixture or the combustion air has a direct effect on the combustion process and the combustion temperature. In this way the emissions emitted by the combustion can be controlled.

It can be provided that a connection between the at least one additional cooling device and an engine cooling system is established when a transient performance requirement occurs.

It can be provided that during operation of the at least one internal combustion engine the at least one internal combustion engine is cooled by the district cooling system and the at least one internal combustion engine is separated from the district cooling system when a transient performance requirement occurs. Therefore it can be provided that the whole internal combustion engine is cooled by the at least one additional cooling device during a transient performance requirement.

Preferably, at least one intake manifold of at least one internal combustion engine is cooled by an additional cooling device during a transient performance requirement.

Cooling at least one intake manifold leads to a temperature reduction of the charge air or the charge fuel-air mixture, which leads to an increase of mass of air or fuel-air mixture into at least one cylinder of the internal combustion engine and therefore results in an increase of the power output of the internal combustion engine. According to the invention the intake manifold is cooled by the at least one additional cooling device, preferably in addition to a common cooling device, which cools the intake air or fuel-air mixture.

It can be provided, that the at least one additional cooling device comprises a reservoir, with which heat is exchanged. Such a reservoir can be a fluid reservoir with fluid, e.g. water, being preferably colder than the engine part (e.g. the intake manifold) which has to be cooled during a transient performance requirement. It can be provided that the at least one additional cooling device is provided for a plurality of internal combustion engines. It is quite conceivable that also other energy systems which have to be cooled and deliver thermal energy can be coupled to the at least one additional cooling device, e.g. combustion turbines.

It can be provided that an electric generator is driven by the at least one internal combustion engine, which generator preferably delivers electrical energy to an energy supply network (grid).

It can be provided that the - at least partial - cooling of the at least one internal combustion engine using the additional cooling device is stopped after a transient performance requirement when a quasi-stationary performance requirement occurs.

It can be provided that that the additional cooling device is provided for a safe operation of the at least one internal combustion engine.

A quasi-stationary performance requirement means a nearly constant or slowly changing performance requirement, wherein performance requirement changes slow enough, so that the operation of the internal combustion engine can be viewed as stationary at each point in time of a given time period.

A power plant according to the invention comprises:

- at least one stationary internal combustion engine, wherein the at least one internal combustion engine is configured to deliver a mechanical power by burning a fuel,

- a district heating system connected to the at least one internal combustion engine, wherein through a heat exchange between the district heating system and the at least one internal combustion engine on the one hand the at least one internal combustion engine is cooled and on the other hand heat is supplied to the district heating system,

- at least one additional cooling device and

- at least one open or closed loop control unit, characterized in, that the at least one open or closed loop control unit is configured to open or closed loop control a cooling of the at least one internal combustion engine by activating and/or deactivating of the additional cooling device when a transient performance requirement the at least one internal combustion engine occurs.

Further details and advantages of the invention are apparent from the accompanying figures and the following description of the drawings. The figures show:

Fig. 1 a stationary internal combustion engine,

Fig. 2a an internal combustion engine in conjunction with district heating system and

Fig. 2b an internal combustion engine in conjunction with district heating system when a transient performance requirement occurs.

Fig. 1 shows a stationary international combustion engine 1, wherein the internal combustion engine 1 comprises a turbo charger 4. By means of the turbo charger 4 air or an air-fuel mixture can be charged for the combustion in the internal combustion engine 1. This air or air-fuel mixture is charged by the compressor 7 of the turbo charger 4. The turbo charger 4 further comprises an exhaust gas turbine 26, which is connected to the compressor 7 by a shaft 23. The exhaust gas turbine 26 is driven by exhaust gas coming from the internal combustion engine 1, where the exhaust gas is produced by combustion of the air fuel mixture.

This combustion normally takes place in the combustion chambers 2 of the internal combustion engine 1. For combustion in mixed charged internal combustion engines a charged air-fuel mixture is fed to the combustion chamber 2 of the internal combustion engine 1. For combustion in supercharged internal combustion engines having a fuel port injection a charged air is fed to the internal combustion engine 1, a fuel is separately fed to the internal combustion engine 1 by means of port injection nozzles. When charging air or an air-fuel mixture the air or the air-fuel mixture also is heated by the compression operation. For reducing the temperature of the air or the air-fuel mixture an intercooler 5 is provided. After passing the intercooler 5 the air or the air-fuel is guided to the combustion chambers 2 via the intake manifold 3.

The exhaust gas turbine 26 can be bypassed by means of a bypass conduct and a bypass valve 6. This bypass valve 4 can be connected with a control unit of the internal combustion engine 1, which is configured to open or closed loop control the bypass valve 6. The control unit of the internal combustion engine 1 can be configured to determine a pressure difference, from pressure measurements downstream and upstream of the exhaust gas turbine 26 and control the charge pressure by control an opening degree of the bypass valve 6.

Furthermore, the internal combustion engine 1 comprises an engine cooling system 8 for cooling the internal combustion engine 1 during operation. In this embodiment of an internal combustion engine 1 the cooling system 8 comprises a supply line 9 (coming from the district heating system 20 - explained in the following figures in more detail) entering the internal combustion engine 1 and passing through the intercooler 5 cooling the air or the air-fuel. Here the intercooler 5 acts as a heat exchanger. After passing the intercooler the tempering medium of the cooling system 8 is guided to the engine block 27, for cooling the engine block 27. Before entering the return line 10 - in the shown embodiment of Fig. 1 - a heat exchange takes place between the medium of the cooling system 8 and the exhaust gas in the exhaust heat exchanger 19 for heating the tempering medium as much as possible in order to increase the efficiency of the district heating system 20.

Fig. 2a shows the internal combustion engine 1 (embodiment for example as in Fig. 1) in conjunction with a district heating system 20 during normal operation. A tempering medium is pumped by the circulation pump 14 coming from a supply line 21 of the district heating system 20 to the internal combustion engine 1, wherein through a heat exchange between the district heating system 20 and the at least one internal combustion engine 1 on the one hand the at least one internal combustion engine 1 is cooled and on the other hand heat is supplied to the district heating system 20. After passing through the internal combustion engine 1 the tempering medium is led through the return line 22 of the district heating system 20 to a field of application (not shown by the figures), where the thermal energy from the district heating system 20 is used for heating or is used for another thermal process.

An emergency cooling device (used as additional cooling device 12 in this embodiment) is provided for cooling the internal combustion engine 1 if the cooling of the internal combustion engine 1 can no longer be performed by the district heating system 20 (e.g. because of a failure of the district heating system). This emergency cooling device ensures a safe operation of the internal combustion engine 1.

The additional cooling circuit comprises a plurality of cooling devices (in this embodiment designed as roof top coolers 11). Using another circulation pump 13 the tempering medium in the additional cooling circuit is circulated. The circulation pump 13 can be controlled by an open or closed loop control unit 24. The open or closed loop control unit 24 is connected to the circulation pump 13 via signal lines 25 (shown in the figure as dashed lines).

The open or closed loop control unit 24 is also connected to several valves 16, 17, 18 to control the medium flow of the district heating system 20.

During normal operation - quasi-stationary performance requirement - (as shown by Fig. 2a) the medium flow is controlled by the open or closed loop control unit 24 by closed valves 16, 18 and open valve 17. This allows the tempering medium to flow directly from the supply line 21 of the district heating system 20 to the internal combustions engine 1 and after a heat exchange with the internal combustion engine 1 from the internal combustions engine 1 to the return line 22 of the district heating system 20.

The open or closed loop control unit 24 is configured to close valve 17 and open valve 18 when a transient performance requirement occurs for the at least one internal combustion engine 1 (as shown in Fig. 2b), wherein the district heating system 20 is separated from the cooling system 8 of the internal combustion engine 1. At the same time the open or closed loop control unit 24 is configured to open valve 16 and activate the circulation pump 13 (if this pump does not already work). By opening the valve 16 the engine cooling system 8 is connected to the additional cooling device 12, wherein the internal combustion engine 1 is cooled by the additional cooling device 12 via the heat exchanger 15.

It can be provided that after the occurrence of a transient performance requirement to the internal combustion engine 1 the open or closed loop control unit 24 closes the valves 16, 18 and opens the valve 17 to continue with normal operation (as explained in connection with Fig. 2a).

It can be provided that the at least one additional cooling circuit 12 comprises at least one (thermal) reservoir 11 ^' (also referred to roof top coolers), wherein an exchange of heat between the cooling medium coming from the internal combustion engine, preferably coming from intake manifold of the internal combustion engine, and said reservoir takes place during a transient performance requirement. The (thermal) reservoir can be any fluid like air or liquid.

Reference signs:

1 internal combustion engine

2 combustion chamber

3 intake manifold

4 turbo charger

5 intercooler

6 bypass valve

7 compressor

8 engine cooling system

9 supply line of the engine cooling circuit

10 return line of the engine cooling circuit

11 roof top cooler

11 ^' (thermal) reservoir

12 additional cooling device

13 circulation pump

14 circulation pump

15 heat exchanger 15

16 valve

17 valve

18 valve

19 exhaust heat exchanger

20 district heating system

21 supply line of the district heating system

22 return line of the district heating system

23 shaft

24 open or closed loop control unit

25 signal lines

26 exhaust gas turbine

27 engine block