WO2012007640A1

WO2012007640A1 - Chp plant which uses wood chips and functions as an island

Info

Publication number: WO2012007640A1
Application number: PCT/FI2011/050634
Authority: WO
Inventors: Juha Sipilä; Jarno Haapakoski
Original assignee: Fortel Components Oy
Priority date: 2010-07-12
Filing date: 2011-07-05
Publication date: 2012-01-19
Also published as: FI20105791A0; FI20105791A

Abstract

The CHP plant (100) according to the invention utilises accumulators (34) of electric cars as electric energy storage. Inside the CHP plant all the electric energy generation means (11, 12, 13) and the accumulators (34) of the electric cars are connected together in a direct voltage point (30) of the CHP plant. Depending on the electric energy balance of the CHP plant, electricity is either transferred from the direct voltage point (30) to the accumulators (34) of the electric cars or from the accumulators (34) of the electric cars to the direct voltage point (30). The direct voltage in the direct voltage point (30) is converted with an inverter (20) into alternating current, which is supplied to the electric network (1) of the consumers.

Description

CHP plant which uses wood chips and functions as an island

The invention relates to a combined heat and power plant (CHP), which functions as an independent island for producing and distributing energy. Prior art

Energy conservation measures relating to habitation affect both solutions realized in energy applications and solutions related to energy production. Traditionally the energy needed for habitation is divided into two parts. There are energy production and distribution solutions related to habitation heat and warm service water. The other part consists of the production and distribution of electric energy into apartments. In order to reduce the heat energy needed in the apartments, the building norm has been changed so that the aim is a low energy apartment, where the need for external heat energy is smaller in comparison to standard building. At the same time also the factors in the apartment which use electric energy can be intervened with and thus the need for electric energy in a specific apartment can be reduced. The need for electric energy can be optimised for example by using energy-efficient electric devices, by controlling the electricity consumption of several apartments so that all the apartments do not simultaneously use a device requiring a lot of energy, or so that certain devices, which consume a lot of energy, cannot be installed in the apartments.

In large energy-consuming units, such as cities, heat and electricity are generally produced in power plants, in which heat energy and electric energy are produced at the same time. The energy source is some material, which is suitable for burning. Possible surplus heat energy can temporarily be stored in heat accumulators for example in summertime, when the need for heat energy is at its lowest. Comparable storage of electric energy is not common, but, depending on the local energy balance, produced electric energy is instead either sold to the open markets or more electric energy is bought from the open markets, if the local energy production is not enough to satisfy the consumption. On a national level care is taken that there is always a suitable amount of electric energy available in the electric trunk network. Hydropower plants, which can quickly be connected to the electric network or detached from the electric network, are often used as sources for regulation energy. In the future one rising application for electricity is the electric car. Charging points will be created for charging the accumulators of the electric car, from which charging points energy from the electric network is transferred into the accumulator of the electric car. The solution requires a rectifier, with which the alternating voltage from the electric network is converted into a direct voltage suitable for the accumulators.

An idea has also been presented that the transfer of electric energy from the accumulators of an electric car back into the electric network should be possible. Such a solution requires that an inverter is also needed at the charging point, with which inverter the direct voltage in the accumulators of the electric car is transformed into alternating voltage used in the electric network. Power losses of a magnitude of 3-5% are generated in the inverter. One such solution is presented in application publication US 2010/0076615. The publication describes an electricity supply system, where a consumer has a direct voltage network (47) available in addition to the alternating current network (43). The presence of a parallel direct voltage network (43) is in the publication justified by the fact that several electric devices in practice utilize only direct voltage. A parallel 12-24 volt "intelligent" direct voltage network (43) removes the need for separate rectifiers or power-supply apparatuses, which would be needed, if only a common alternating current network was utilised.

In Figure 2 in the application publication is shown how the alternating voltage supply (43) required by the described direct voltage network (47) is connected to the common alternating voltage network in a fuse box (3). The alternating voltage network is through the fuse box (3) also supplied by a boiler unit (40), solar panels (25) and a standby accumulator (42), which are connected to the alternating voltage of the fuse box (3) through a separate control unit (50), where a DC/AC conversion is performed if necessary.

Also an accumulator (36) of an electric car (28) has been suggested to be connected to its own separate charging unit (27), which performs an AC/DC or DC/AC conversion between the alternating current network (43) from the fuse box (3) and the direct voltage of the accumulator or the electric car (28). It is possible to transfer energy through the charging unit (27) in any direction. The direct voltage point of the accumulator (36) of the electric car and the described intelligent DC network (43) are not depicted to have a common DC current connection point. The connection from the direct voltage point of the accumulator (36) of the electric car to the direct voltage point of the "intelligent" direct voltage network (47) always passes through the alternating voltage network (43). In the described DC/AC or AC/DC conversions a loss of power of the magnitude described above is however always generated in the charging unit (27) and the AC/DC converter of the central unit ( ) of the direct voltage network.

As an alternative energy production system to large energy production units have been suggested local small energy production units, the main purpose of which is to produce the heat and electric energy needed by some particular area. Thus the local energy production unit and its distribution system functions as an independent island, where the used energy is also produced locally. Several energy production alternatives are needed, so that the energy supply can be ensured. Energy can be produced with different turbine plants, where for example wood gas can be used as energy source. These plants provide both heat and electric energy.

Electric energy for the requirements of the island can also be produced by utilising wind and solar energy. A reserve power machine functioning with liquid fuel can be used as a reserve power source.

Surplus heat energy generated on the island can be stored for example in a boiler, or it can also be used to pre-dry bio fuel to be used in the power plant. Surplus electric energy generated on the island can be stored in separate accumulators, from which the stored electric energy can be discharged into the electric network of the island in a situation, where the capacity of the proper production means of electric energy is not sufficient to satisfy the need for electric energy in the island.

One such combined system for producing and storing heat and electricity is presented in publication WO 155577.

Functioning as an energy-independent island raises investment costs, because a reserve power system and an accumulator must be built in connection with the power plant. A cost-effective operation requires that all the produced electric energy is used as precisely as possible in the island, if the supply tariff of the surplus energy to be fed into the national network is low. Different possibilities of utilising surplus energy during different times of the day are in a key position in securing the overall economy of the plant. Such a situation sets strict requirements on the management of the energy balance of the CHP plant. Uninterrupted heat energy must be ensured for the users. Additionally disturbances occurring in the supply of electric energy must be kept at a minimum, because every electricity supply disturbance can immediately be seen by the consumers.

Summary of the invention

It is an object of the invention to provide a new CHP plant functioning as an island, where special attention has been paid to the uninterrupted and sure availability of electric energy. The island can be completely detached from the national network.

The objects of the invention are attained with a CHP plant, wherein in addition to the accumulator of the CHP plant the accumulators of the electric cars used in the island are utilized as a storage place for electric energy. In the island according to the invention the accumulators of the electric cars are charged during the times, when other consumption of electricity in the island is low. When the electricity consumption in the island grows to be larger than the electric energy obtained from the proper energy source, the missing electric energy is taken from the accumulators of the electric cars. In one case the entire supply of electric energy of the island can at times be based only on the electric power obtainable from the accumulators.

In the energy production and supply system according to the invention the different energy sources are joined together in one direct voltage point, whereby the joining of several different electric energy sources is easy. Thus the transformation into alternating current for the user is performed in the CHP plant according to the invention only in one point in the electricity network. By proceeding thus the transfer of the energy production from the generator to the accumulators or vice versa or their functioning simultaneously as electric energy sources thus always occurs uninterruptedly for the user.

It is an advantage of the CHP plant according to the invention that the number of accumulators functioning as a fixed reserve power source can be reduced in the CHP plant. This reduces the investment and maintenance costs of the CHP plant. It is further an advantage of the invention that in a situation, where the production of electric energy exceeds the need for electric energy in the electric network, the surplus electric energy can be stored in the accumulators of the electric cars. By proceeding thus, the device which produces electric energy in the CHP plant can function all the time at an optimal efficiency, and its electric power production does not need to be controlled all the time based on changes in the load of the electric network.

It is further an advantage of the invention that the conversion from direct voltage to alternating voltage used in the electric network is done only once, whereby the conversion losses in the CHP plant can be minimised.

It is further an advantage of the invention that the electric network can be supplied from the accumulators of the electric cars for several hours without having the end users experience disturbances.

The CHP plant functioning as an island according to the invention, where surplus electric energy produced by the electric energy production means is arranged to be charged into the accumulators of an electric car, is characterised in that the joining together of the direct voltage outlets of the electric energy production means and the accumulators of the electric car is arranged to be done in one direct voltage point, which is also the electricity network supply point of the CHP plant and the voltage level of which direct voltage point is arranged to indicate the state of the electric energy balance of the CHP plant.

The operating method of the CHP plant according to the invention, where the accumulators of electric cars are charged with the surplus electric energy, is characterised in that the electric energy balance of the CHP plant is indicated to the electric energy production means and the accumulators of the electric cars from the voltage of a common direct voltage point.

Some advantageous embodiments of the invention are presented in the dependent claims. The basic idea of the invention is the following: The CHP plant according to the invention utilizes the accumulators of electric cars connected to the island as storage means for electric energy. When electric energy exceeding the momentary consumption is produced in the CHP plant, the electric energy is advantageously steered to the accumulators of the electric cars connected to the energy production system of the CHP plant. On the other hand, when the momentary need for electric energy exceeds the electric energy production, the missing electric energy is primarily taken from the accumulators of the electric cars. In the CHP plant according to the invention it is also possible that the entire supply of electric energy of the island can at times be based only on the electric power obtainable from the accumulators.

In order for the transfer from one electric energy source to the other to occur as undisturbed as possible for the consumer, all the energy sources are connected within the CHP island to one direct voltage point. The electric energy departing to the electric network from this direct voltage point is in one point transformed into a three-phase alternating current, which is supplied to the consumers belonging to the island.

In the following, the invention will be described in detail. In the description, reference is made to the appended drawings, in which

Figure 1a shows as an example a CHP plant according to a first embodiment of the invention,

Figure 1 b shows as an example a CHP plant according to a second embodiment of the invention and Figure 2 shows as an example an operating method of a CHP plant according to the invention.

The embodiments of the invention shown in the following description are given as examples only and someone skilled in the art may carry out the basic idea of the invention also in some other way than what is described in the description. Though the description may refer to a certain embodiment or embodiments in several places, this does not mean that the reference would be directed towards only one described embodiment or that the described characteristic would be usable only in one described embodiment. The individual characteristics of two or more embodiments may be combined and new embodiments of the invention may thus be provided. Figure 1a shows an example of the means 100 related to the production and distribution of electric energy in a plant producing electric and heat energy, a CHP plant, according to the first embodiment of the invention. Figure 1a does not show devices and functions of the CHP plant, which mainly relate only to the production and distribution of heat. The operation and energy balance of the CHP plant are advantageously controlled with a measuring and control unit 10. In the following description the island made up by the exemplary CHP plant comprises ten exemplary apartments, the entire energy service of which is based on the energy obtainable from the CHP plant.

The CHP plant according to the invention can advantageously function as an entirely self-sufficient energy production and distribution island. The invention is however not limited to an energy self-sufficient island, but the CHP plant according to the invention may also be connected to the national electric trunk network. In this embodiment it is also possible to supply the surplus electric energy generated in the island to the national electric network.

In Figure 1a the end user of the electricity is supplied with three-phase current, the main voltage of which is 400 volts, via the electric network 1 comprised in the CHP plant. Thus the end users can utilise conventional electrical devices with 230 volt phase voltage available on the market. The energy efficiency of the used electrical devices is however advantageously such that they consume as little energy as possible. Thus the daily need for electric energy in the above-mentioned CHP plant servicing ten households is about 350 kWh.

In the CHP plant according to the invention energy can be generated in several alternative ways. Three energy production means 11 , 12 and 13 are shown in the example of Figure 1a. It is however clear to someone skilled in the art that the invention is not restricted to the three energy production alternatives shown in Figure 1a.

Heat and electric energy can be generated simultaneously with a wood gas generator 11 or alternatively with a gas turbine. In a wood gas generator 11 it is advantageous to use wood chips or the like as an energy source, which wood chips are gasified, and the generated gas is used as fuel in the wood gas motor. The wood gas motor rotates the actual electricity-producing generator, the electric energy generated by which is supplied into the electric network 1 of the CHP plant. Heat energy is also generated in the wood gas generator 11 in connection with the process, which heat energy can be utilised for warming up apartments.

Additionally the heat energy generated in the combustion process can advantageously be used to warm up the apartments and the service water of the island. Possible unused heat energy generated in the combustion process can be stored for example in a boiler proportioned to the size of the island.

In the example in Figure 1a (which may for example be a CHP plant servicing ten apartments) the wood gas generator 11 has a power of about 25 kW. It generated three-phase alternating current, the main voltage of which is advantageously 400 V. The power and production ratio of electricity and heat of the wood gas generator 11 is controlled by a measuring and control unit 10 via a two-way connection 3. The alternating current produced by the wood gas generator 11 is advantageously rectified with a rectifier 21. The output of the rectifier 21 is connected to a shared direct voltage point 30 of the CHP plant.

The voltage of the direct voltage point 30 of the CHP plant is advantageously 400- 800 volts. In the example of Figure 1 the direct voltage point 30 has a voltage of 730 volts.

In Figure 1a a wind generator 12 is shown as a parallel or alternative means for producing electric energy. In the exemplary CHP plant servicing ten apartments the power of the wind generator may advantageously be about 26 kW. Also the operation of the wind generator 12 is controlled by a measuring and control unit 10 via a connection 3. Because the operation of the wind generator 12 is dependent on the force of the wind, it is advantageous to use some other energy production means alongside it, for example a wood gas generator 11. The wind generator 12 also generates three-phase alternating current, the main voltage of which is advantageously 400 V. The alternating current generated by the wind generator is rectified with a rectifier 22. The outlet of the rectifier 22 is also connected to a shared direct voltage point 30 of the CHP plant.

A third means for generating electric energy shown in Figure 1a, an aggregate 13, functions as a reserve power machine of the CHP plant. Rapeseed oil or diesel is advantageously utilised as fuel for the aggregate 13. In the above-described CHP plant with ten apartments the capacity of the aggregate may advantageously be about 100 kW. Thus it can in all situations alone produce all the electric energy used in the CHP plant. The operation of the aggregate 13 is controlled by a measuring and control unit 10 via a connection 3.

The main voltage of the three-phase current obtained from the aggregate 13 is in the example in Figure 1 also 400 V AC. Also the alternating current generated by the aggregate 13 is rectified with a rectifier 23. The direct voltage output of the rectifier 23 is connected to a shared direct voltage point 30 of the CHP plant. The aggregate 13 is arranged to be connected as a producer of electricity only in a situation, where no electric energy is available from any other source of electric energy.

Surplus electric energy generated in the CHP plant is advantageously charged into accumulators 34 and 31. In one advantageous embodiment of the invention the utilised accumulators 34 are all accumulators of electric cars. In the example in Figure 1a the terminal voltage of the accumulators 34 of the electric cars is 400 V direct voltage.

In a second advantageous embodiment of the invention the CHP plant comprises, in addition to the accumulators 34 of the electric cars, also a fixed accumulator 31. The terminal voltage of the fixed accumulator 31 may also advantageously be 400 V direct voltage.

In both the above-described embodiments the electric energy can be transferred in two directions; either from the direct voltage point 30 to the accumulators 31 and 34 or from the accumulators to the direct voltage point 30. The transfer direction of the electricity is controlled by the measuring and control unit 10 by using direction switches 33 and 36 via a connection 5.

The charging level of the accumulators 31 and 34 is monitored by the measuring and control unit 10 via a connection 4.

Between the shared direct voltage point 30 of the CHP plant and the accumulators there are direct voltage converters 32 and 35, which adapt the terminal voltages of the accumulators 31 and 34 and the direct voltage point 30 of the CHP plant to each other.

If the accumulators 31 and 34 are charged or discharged depends on the charging level of the accumulators and on whether all the electric energy generated in the CHP plant is consumed in the electric network 1 or not. If more electricity is generated in the CHP plant than what is consumed, then the surplus electricity can be used to charge the accumulators 34 and/or 31. Such a situation is likely at night. The accumulators 34 and 31 are discharged when less electric energy is generated than what is consumed in the electric network 1 of the CHP plant. This is likely in the morning and afternoon, when people perform household tasks at the same time.

The measuring and control unit 10 monitors the voltage of the direct voltage point 30 via a connection 6. The measuring and control unit 10 compares the voltage of the direct voltage point 30 to the voltage which prevails in a situation, where the electricity generation and the electricity consumption are in balance. When more electric energy is produced in the CHP plant than what is used, the voltage in the direct voltage point 30 increases. On the other hand if more electricity is consumed than what is generated, then the voltage in the direct voltage point 30 decreases. Based on the change in the direct voltage point 30 the measuring and control unit 10 can control the direction switches 33 and 36 so that in a case of overproduction of electricity the accumulators 31 and 34 are charged and in a case of underproduction of electricity the accumulators 31 and 34 are discharged into the electric network 1.

The state of the electric energy balance is indicated by the magnitude of the voltage of the direct voltage point 30, when it is compared to the direct voltage prevailing when the electric energy is in balance.

If a situation arises, where the charging level of the accumulators 34 and/or 31 does not allow for any further discharging from them into the electric network 1 or where for some other reason the power of the main producer of electric energy, the wood gas generator 11 , is not sufficient, the measurement and control unit 10 advantageously turns on electric energy restriction measures in the electric network 1. Alternatively the users are either informed about a restriction need or secondary electricity applications are automatically disconnected from the electric network 1. In one advantageous embodiment of the invention the direction switches 36 and 33 are advantageously a part of the rectifiers 35 and 32. This embodiment is shown with the rectangles according to references 35a and 32a in Figure 1a. In this embodiment an operational voltage has been set in the rectifier 35 and 32, the crossover of which voltage automatically causes a turn in the direction of the current toward the lower potential. For example, if the voltage in the direct voltage point 30 decreases, the voltage in the accumulators/rectifiers 34 and 31 is higher, and thus the direction of the electric current turns from the accumulators 31 and 34 to the direct voltage point 30. In this situation the accumulators 34 and 31 are discharged. If, on the other hand, the voltage on the direct voltage point 30 is higher than the voltage of the accumulators 31 and 34, then the direction of the electric current turns from the direct voltage point 30 toward the accumulators 31 and 34. In this advantageous embodiment of the invention the measuring and control unit 10 can advantageously monitor the direction of the electric current between the accumulators 31 and 34 and the direct voltage point 30, but it does not actually control the transfer direction of the current.

Alternating current for the customers of the CHP plant is generated with a three- phase inverter 20, with which the 730 V direct voltage in the direct voltage point 30 in the CHP plant is converted into three-phase alternating voltage suitable for the users, the main voltage of which is 400 V and which is supplied to the electric network 1 utilised by the users. It is an advantageous technical feature of the invention that the accumulators 34 and 31 can continuously be used in the direct voltage system. Nowhere in the accumulator utilising stage is it necessary to convert the direct current supplied by the accumulators into alternating current or vice versa. Such an energy production and distribution system of the CHP plant is structurally simple. Additionally, because the direct current does not need to be converted into alternating current or vice versa during charging and discharging of the accumulators, the conversion losses can also be minimised.

Figure 1 b shows an example of the means 100 according to a second embodiment of the invention related to the production and distribution of electric energy in a plant producing electric and heat energy, a CHP plant. The example in Figure 1 b differs from the embodiment on Figure 1a in that there are no direct voltage converters or direction switches between the accumulators 31 and 34 and the direct voltage point 30. Thus in this embodiment of the invention the direct voltage point 30 of the CHP plant "floats" at a direct voltage optimized for the accumulator 34 of the electric car. The accumulators 34 and 31 are charged when the voltage in the direct voltage point 30 rises, and the accumulators 31 and 34 are discharged when the voltage in the direct voltage point 30 decreases. In this embodiment of the invention the' charging profile of the accumulators 31 and 34 is only adjusted by the difference between the electricity generation and the electricity consumption. Such an embodiment has the technical advantage that there are fewer components causing electric losses.

Figure 2 shows as an exemplary flow chart the main stages of a method utilised in the management of the electric energy balance in a CHP plant according to the invention. The CHP plant is started up in stage 41. At least the wood gas generator 11 is thus advantageously started, which generator can be used to produce both electric and heat energy.

In stage 42 the on- and off-switching of the energy generation means can be performed. The power of the energy production means producing electric energy can also be adjusted there.

In stage 43 the voltage of the direct voltage point 30 of the CHP plant is measured, which voltage indicates the ratio between production and consumption of electricity.

In stage 44 a comparison is made on the state of the electric energy balance of the CHP plant; is more electricity generated than what is consumed or vice versa. The ratio between the electricity generation and consumption is advantageously measured by comparing the voltage of the direct voltage point 30 to a preset comparison voltage level. The comparison voltage level prevails in the direct voltage point 30 of the CHP plant when the electricity generation is as large as the electricity use. If the voltage of the direct voltage point 30 is higher than the set comparison voltage level, then more electricity is generated than what is consumed. If, on the other hand, the voltage of the direct voltage point 30 is lower than the set comparison voltage level, then more electricity is consumed than what is generated.

If more electricity is generated than what is consumed, the process moves to stage 46, where surplus electric energy is used to charge the accumulators 34 of electric cars connected to the CHP plant. It is additionally also possible to charge fixed accumulators 31 , if such are a part of the CHP plant. Stage 47 consists of measuring if the accumulators are already fully charged or not. If the charging of the accumulators can be started or it can be continued still, the process returns to stage 43. In this situation the measuring and control unit 10 switches the transfer direction of electricity from the shared direct voltage point 30 of the CHP plant to the accumulators 34 and 31.

If it is in stage 47 found that the accumulators are already fully charged, the process returns to stage 41 (reference 47a). In this situation the measuring and control unit 10 opens up the direction switch(es) 36 and/or 33 between the shared direct voltage point 30 of the CHP plant and the accumulators 34 and 31. In such a situation the power of the energy production device producing electricity can advantageously be decreased.

In one advantageous embodiment of the invention the electric network comprised in the CHP plant is in this situation supplied only from the accumulators 31 and/or 34 as long as their charging levels allow it.

Alternatively in this situation the produced surplus electric energy can be used to either warm up water or pre-dry fuel for the wood gas generator 11.

If it is in stage 44 found that less electric energy is generated than what is consumed in the network 1 , the process moves to stage 45. Stage 45 consists of checking if the missing electric energy can be taken from the accumulators 34 and/or 31 or not. If the charging level of the accumulators enables the supply of electric energy to the consumer electric network 1 , then the discharge of the accumulators to the electric network 1 is begun (reference 45a). The measuring and control unit 10 thus switches the transfer direction of electricity from the accumulators 34 and 31 to the shared direct voltage point 30 of the CHP plant by using the direction switches 36 and/or 33.

If it is in stage 45 found that the charging level of the accumulators 34 and/or 31 does not allow for their discharge, then the process returns to stage 42 (reference 45b), where the measuring and control unit 10 cuts off the transfer of electricity from the accumulators 34 and/or 31 to the shared direct voltage point 30 of the CHP plant by opening the direction switch(es) 36 and/or 33.

In this situation, where the charging level of the accumulators 34 and/or 31 no longer allows for discharge of electric energy from them to the electric network to replace a shortage of electric energy, the measuring and control unit 10 advantageously turns on electric energy restriction measures in the electric network 1. The users are either informed about a restriction need or secondary electricity applications are automatically disconnected from the electric network 1.

In one advantageous embodiment of the invention the CHP plant in this situation takes an amount of energy from the national electric network, which is the size of the shortage in the own production, until the energy production and consumption of the CHP plant are again balanced.

Some advantageous embodiments of the CHP plant according to the invention and devices utilised therein have been described above. The invention is not limited to the embodiments described above, but the inventive idea can be applied in numerous ways within the scope of the claims.

Claims

1. An energy plant producing heat and electricity, a CHP plant, functioning as an island, wherein electric energy is configured to be generated with at least one electric energy generation means (11 , 12, 13) and wherein surplus electric energy generated by the electric energy generation means is configured to be charged into accumulators (34) of electric cars, from which electric energy charged into them is configured to be discharged into an electric network (1 ) of the CHP plant when the load on the electric network exceeds the output of the electric energy generation means (11 , 12, 13), which generates electric energy, characterised in that a connection between direct voltage outputs of the electric energy generation means (11 , 12, 13) and a direct voltage point in the accumulators (34) of the electric cars is configured to be done in one direct voltage point (30), which is also the feed point of the electric network (1 ) of the CHP plant and the voltage level of which direct voltage point (30) is configures to indicate the state of the electric energy balance of the CHP plant.

2. The CHP plant according to claim 1 , characterised in that there is a direct voltage converter (35) and a direction switch (36) between the accumulators (34) of the electric car and the direct voltage connection point (30) for connecting the accumulators (34) of the electric car with a two-way connection to the direct voltage point (30) of the CHP plant.

3. The CHP plant according to claim 1 , characterised in that there is a rectifier (21 , 22, 23) between the voltage output of the energy generation means (11 , 12, 13) and the direct voltage connection point (30) for connecting the voltage output of the energy generation means (11 , 12, 13) to the direct voltage connection point (30) of the CHP plant.

4. The CHP plant according to claim 1 , 2 or 3, characterised in that the voltage of the direct voltage connection point (30) is about 400-800 V.

5. The CHP plant according to claim 4, characterised in that there is an inverter (20) between the electric network (1 ) of the CHP plant and the direct voltage connection point (30), with which inverter voltage of the direct voltage connection point (30) is converted into three-phase alternating voltage, which can be used in the electric network (1).

6. The CHP plant according to any of the claims 1-5, characterised in that it comprises a measuring and control unit (10), which is configured to control the energy generation means (11 , 12, 13) and the charging level of the electric car accumulators based on the direct voltage measured from the direct voltage connection point (30) comprised in the CHP plant.

7. The CHP plant according to claim 2, characterised in that it also comprises a fixed accumulator (31 ), which is connected to the direct voltage connection point (30) of the CHP plant with a direct voltage converter (32) and a direction switch (33).

8. The CHP plant according to claim 1 , characterised in that the energy generation means comprise at least one of the following: a wood gas generator (11 ), a wind generator (12) or an aggregate (13).

9. A method for maintaining electric energy balance in an energy plant generating heat and electricity, a CHP plant, which functions as an island, which method comprises

- starting up (42) at least one energy generation means (11 , 12, 13)

- measuring (43) the electric energy balance of the CHP plant (1)

- charging (46) accumulators (34) of electric cars with surplus electric energy, if the energy generation means (11 , 12, 13) produce more electric energy than what is consumed in the electric network (1 ) of the CHP plant, and

- discharging (45, 42) the accumulators (34) of the electric car, if the energy generation means generate less electric energy than what is consumed in the electric network of the CHP plant,

characterised in that in the method the electric energy balance of the CHP plant is indicated (44) by the voltage of the direct voltage connection point (30), which is shared between the electric energy generation means (11 , 12, 13) and the accumulators (34) of the electric car.

10. The method according to claim 9, characterised in that the state of the electric energy balance is indicated by the magnitude of the voltage of the direct voltage connection point (30), when it is compared to a direct voltage prevailing when the electric energy is in balance.

11. The method according to claim 9, characterised in that the energy generation means (11 , 12, 3) for electric energy is used at its optimal efficiency.

12. The method according to claim 9, characterised in that if the charging level (45, 45b) of the accumulators (34) of the electric car does not make possible discharging thereof, then the electricity generation capacity of the used energy generation means is either raised or at least one other energy generation means (11 , 12, 13) is started (42) in order to produce electric energy.

13. The method according to claim 9, characterised in that if the charging level (45, 45b) of the accumulators (34) of the electric car does not make possible discharging thereof, then the consumption of electric energy in the electric network (1 ) is restricted (42).

14. The method according to claim 9, 11 , 12 or 13, characterised in that the electric network (1 ) of the CHP plant is supplied with three-phase alternating voltage, which is obtained with an inverter (20), which is fed from the direct voltage connection point (30) of the CHP plant, to which point the energy generation means (11 , 12, 13) or the accumulators (34) of the electric car supply electric energy.