The invention relates to a method for improving the efficiency of a small-size power plant preferably based on a closed, i.e. hermetic Organic Rankine Cycle (ORC) process, whereby the ORC medium, such as freon, toluene or the like, is vaporized in a vaporizer, condensed in a cooler and returned by a feeding device back to the vaporizer, whereby the small-size power plant, i.e. an energy converter unit or several of the same, comprises a high-speed machine which is formed of at least a turbine and a generator changing the form of energy mounted on a joint rotor.

The small-size power plant based on the ORC process was developed particularly for recuperation of heat lost from different heat-producing processes or machines, whereby the lost heat cannot be used as such by heat transfer means or the like, due to the temperature of the lost heat in question or the conditions of the environment. In a small-size power plant, waste energy is usually converted by means of a turbine and a generator to electricity which can be easily utilized for different purposes. If high efficiency of the small-size power plant is achieved, the plant can also be used for small-scale energy production of fuel burned for the purpose, e.g. of wood chips.

It can shown thermodynamically that converting such energy is best performed by a Rankine or ORC process based on circulation of an organic medium. The organic medium has a relatively small vaporization heat as compared with e.g. water, and the drop of its specific enthalpy in the turbine is small and the mass flow rate in relation to the output is high, whereby it is possible to achieve a high turbine efficiency even at small output rates.

A hermetic or fully closed circuit process has the advantage that there are no leaks and the process is thus reliable and durable in operation. The utilization of high-speed technology, whereby the turbine is directly coupled with a generator rotating at the same speed and thus producing high-frequency current, has made it possible to further simplify the process in a way that e.g. a separate reduction gear required by conventional processes as well as shaft inlets are not needed.

A hermetic energy converter unit of this kind, operating on high-speed technology and based on the ORC process, is known from the publication FI-66234, according to which the bearing of the rotor of the high-speed machine is carried out by an organic circulating medium, wherein the circulating medium is in a gaseous state. A previous patent application by the Applicant, FI-904720, discloses a method for securing the lubrication of the bearings in a hermetic high-speed machine.

The output of a single energy converter unit being used for applications in this connection is below 500 kW mainly because of constructional reasons. Naturally the total output of a small-size power plant may be significantly bigger by combining several energy converter units. Respectively the speed of rotation may vary considerably, in customary applications being generally over 8000 rpm, in power range from 200 kW to 400 kW most suitably between 18000-12000 rpm.

The process efficiency rates of small-size power plants are typically within the range of 10-21% depending on the size of the power plant, the circulating medium, the temperature of the incoming waste heat, and other similar factors, whereby the maximum efficiency that can normally be attained by an ORC process is 20-24%.

It is generally known that the efficiency of an aqueous steam process can be raised by reheating, because the average temperature of incoming heat is raised as explained for example in the Finnish publication Tekniikan Käsikirja II, p. 630. However, reheating is commonly used in relatively large power plants only, because e.g. two turbines operating at a different pressure level are needed. Similarly, a method is known from the source mentioned above for reducing the superheating of the superheated aqueous steam by spraying water in it. Also this arrangement is utilized in relatively large power plants only.

Finally, a power plant operating in accordance with the preamble of claim 1 is disclosed in EP-A-0 003 264.

The present invention makes it possible to obtain a significant improvement in efficiency compared to small-size power plants of classical designs based on the ORC process. For achieving this aim, the invention proposes a method of operating a small-size power plant based on a closed hermetic organic rankine cycle (ORC) process with improved efficiency, said power plant comprising at least one energy converter unit comprised of first and second turbines together with a generator mounted on a common rotor, and at least one burner for the combustion of fuel for producing energy for said at least one energy converter unit; said method comprising the steps of: A) supplying an ORC medium to a vaporizer and vaporizing said ORC medium by utilizing the energy derived from the combustion of fuel in the burner of the vaporizer; B) expanding the vaporized ORC medium in the first turbine of the energy converter; C) reheating the ORC medium leaving the first turbine, using a reheater located in said vaporizer and thereby utilizing the energy derived from the combustion of the fuel in the burner of the vaporizer; D) expanding the reheated ORC medium in the second turbine of the energy converter to produce electric energy; E) leading the ORC medium from the second turbine to a cooler for condensing said ORC medium; and F) preheating the ORC medium to be returned from said cooler to said vaporizer, characterized in that the ORC medium is preheated in a recuperator provided in a fluid line between the second turbine and said cooler.

The invention also covers a small-size power plant based on the aforementioned method.

The most important advantages of the method of the invention are its simplicity and reliability of operation, whereby the method enables the application of a conventional technique, known as such, in connection with the ORC process for improving the efficiency of a small-size power plant operating on high-speed technology.

The other dependent claims disclose advantageous embodiments of the method according to the invention.

The invention relates also to an apparatus for applying the method. The apparatus is defined more closely in the introductory part of the independent claim related to the apparatus. The apparatus is mainly characterized by the features shown in the characterizing part of the corresponding claim.

Using the apparatus according to the invention, it is possible to utilize the ORC process in a simple and efficient manner in a small-size power plant giving a significantly better output than the present solutions. The efficiency is raised by means of the apparatus of the invention, whereby the net output of the small-size power plant is increased. Consequently, despite the capital investment in the additional arrangements required by the method, the total operating costs of the apparatus are significantly lower than with present solutions.

In the following description, the invention is illustrated in detail with reference to the appended drawings, in which

Fig. 1

shows an operating chart of the apparatus applying the method in principle,

Figs. 2a and 2b

show advantageous alternative operating charts of apparatuses applying the method of the invention, and

Fig. 3

shows a partial cross-section of an advantageous high-speed machine for use in the apparatus in longitudinal direction.

The invention relates to a method for improving the efficiency of a small-size power plant based on an Organic Rankine Cycle (ORC) process. In a preferably closed, i.e. hermetic ORC process, the ORC medium, such as freon, toluene or the like, is vaporized in a vaporizer 1, expanded in a turbine 2, condensed in a cooler 3 and returned by a feeding device 4 back to the vaporizer 1. The small-size power plant, i.e. an energy converter unit, comprises a high-speed machine 7 which is formed of at least a turbine 2 and a generator 9-changing the form of energy-mounted on a joint rotor 8. According to the invention, ORC medium is intercooled by an intercooler 6b, 6c substantially in connection with turbine 2 and/or reheated by a superheater (or reheater) 5 in the vaporizer 1, whereby the first and second phase of the two expansion phases in the turbine 2 are carried out by the first 2a and second 2b turbine wheels of turbine 2 mounted on the rotor 8 of the high-speed machine 7.

The operating chart shown in Fig. 1 illustrates an advantageous embodiment of the apparatus applying the method, wherein the ORC process is utilized in a small-size power plant supplied with fuel F, such as wood chips. The first expansion phase in turbine 2 is carried out by the first turbine wheel 2a and the second expansion phase by the second turbine wheel 2b mounted on rotor 8 of the high-speed machine 7. The reheater is formed of a superheater 5 comprising a heat exchanger in the vaporizer 1.

Figure 3 shows, in a side view, a partial cross-section of an advantageous high-speed machine 7 of a small-size power plant, wherein the first turbine wheel 2a of the turbine 2, mounted on rotor 8 on the first side of generator 9, operates on the principle of axial flow, and the second turbine wheel 2b mounted on the second side of generator 9 is radially operated. The solution of this kind is very advantageous in practice, whereby in both expansion phases, advantageous turbine wheel constructions are optimally utilized with respect to both manufacturing and operation.

As an advantageous embodiment shown in Fig. 1, the fluid medium to be returned from cooler 3 to vaporizer 1 is arranged to be preheated by a recuperator 6a placed in the cycle between turbine 2 and cooler 3.

As a consequence of reheating, the efficiency of the recuperator 6a is increased, and the ORC medium is hot upon entering vaporizer 1. Consequently, it is advantageous to arrange the combustion air P to be fed to the burner of vaporizer 1 to be preheated by means of a preheater 10 (Luftvorwärmer). The preheater 10 is formed by a heat exchanger in the vaporizer 1.

In the present embodiment, there is provided a feeding device 4 formed of a separate, preferably hermetic feeding pump 4a and a pre-feeding pump 4b, such as an ejector. The pre-feeding pump 4b can also be used for developing pressure for the lubrication of bearings. The feeding pump 4a can naturally be mounted also on the joint rotor 8 of the high-speed machine 7, in addition to the turbine wheels 2a, 2b.

Applying conventional calculation techniques, an efficiency rate higher than 30% can be achieved by the apparatus of the operating chart shown in Fig. 1. The efficiency rate has been calculated with the following values:

• preheating the combustion air P in the preheater (Luftvorwärmer) 10 from about 20°C to about 290°C,
• flue gas S exiting from vaporizer 1: about 100°C,
• vaporized ORC medium to the first turbine wheel 2a: about 382°C/50 bar,
• ORC medium after the first turbine wheel 2a: about 289°C/2 bar,
• ORC medium after superheater 5 (reheating), to the second turbine wheel 2b: about 382°C/ 2 bar,
• ORC medium after the second turbine wheel 2b: about 310°C,
• ORC medium after the recuperator 6a: about 68°C, and
• ORC medium returned to vaporizer 1 after the recuperator 6a: about 226°C.

The electric power supply of the generator 9 being 100 kW, the net efficiency rate of the apparatus thus obtained is about 32.3%.

As the maximum efficiency rate obtained by conventional small-size power plants is about 20-24% as described above, it is surprising that a significantly better efficiency rate exceeding 30% can be achieved by a small-size power plant utilizing an ORC process with reheating. In spite of reheating, the maximum temperature required of the steam is about 380°C, which is still reasonable. This is because organic cycle media do not sustain very high temperatures; in addition, the valves, pipework etc. needed for high steam temperatures (400...500°C) will be too expensive in relation to the small size of a power plant.

Consequently, a small-size power plant which operates on the ORC process and is equipped with reheating is well adapted for combustion of a variety of fuels, such as wood chips, gas, oil or the like. The small-size power plant of the invention can be used as a compact and reliable power source supplied by solid fuel, e.g. in heavy vehicles. Thus e.g. wood chips can be used as fuel and fed by an automatic burner. In addition, the invention can be applied e.g. in deconcentrated energy management in developing countries using local solid fuel.

Further on the basis of continuous product development, it has proved advantageous to reduce the superheating of the ORC medium by spraying fluid ORC medium to the superheated ORC medium by the intercooler 6b, 6c. Consequently, as shown in the alternative operating chart shown in Fig. 2a, the intercooler 6b is used to reduce the superheating of the ORC medium by spraying fluid ORC medium returned from cooler 3 to the vaporizer 1 by the feeding device 4 to the at least partially superheated ORC medium passing from the turbine 2 to the cooler 3.

The said arrangement is suitable for use in apparatuses with no recuperator or with a low rate of recuperation. Thus the reducing of the superheating of the ORC medium used, such as toluene vapour, increases the efficiency of the heat transmission surface of the cooler, because the heat transfer coefficient is at least five times smaller with removal of the superheating than with cooling. By spray-cooling toluene vapour into a saturated state, only cooling takes place in the cooler, not removal of the superheating any longer. Due to the high value alpha, a smaller heat transmission surface is sufficient, although the mass flow rate is higher. The lower temperature is naturally advantageous in view of material technology.

Similarly, Fig. 2b shows also an advantageous alternative arrangement, whereby intercooler 6c is used for reducing the superheating of ORC medium by spraying fluid ORC medium returned from cooler 3 to vaporizer 1 by the feeding device 4 to the superheated ORC medium passing from the first turbine wheel 2a to the second turbine wheel 2b.

This embodiment is advantageous in that the mass flow rate and thus also the efficiency of the turbine is increased by the spraying. Although a fall in the temperature decreases the drop in enthalpy on one hand, it can be shown by calculations that the power output of the turbine may increase as much as 10%. In addition, the degree of superheating of the vapour passing from the turbine 2b to the cooler 3 is thus very small, which decreases the heat transmission surface of cooler 3 as described above.

It is obvious that the invention is not restricted to the embodiments presented above but it can be modified within the basic idea to a great extent, due to the large extent of the method and the apparatus applying the method. Naturally, for applying the method, it is possible only to intercool the superheated ORC medium without reheating as described above. The superheating apparatus can also contain several phases, in which case a cooling device with one or several phases can be arranged between the said phases. By means of a cooling device as described above or an intercooler placed after the superheater as shown in Fig. 2b, and by an oversized heat transmission surface of the superheater, it is possible to maintain the temperature of the vapour constant in a large range of loading and simultaneously to prevent overheating of certain parts of the superheater. The cooling device may be either of the spraying or surface type in a manner known as such. Similarly, the apparatus presented above can be supplemented by conventional e.g. automatically-operated equipment, such as back-pressure valves, deaerators, etc.