A heat-exchanging device for a primary heat source is described, which is arranged to deliver heat energy at a first temperature via at least one primary heat exchanger to a heat distribution network. A method of operating a combined heat and power plant is described as well .

Modern district heating stations use, to a great extent, biomass as their energy source, the biomass being burnt and the energy released being utilized to heat water to a suitable temperatu re. To avoid coming under regulatory requirements for steam boilers or the like, for example, such district heating plants are, as a rule, operated at a temperature in the range of 80-120 °C and a working pressure of up to 2 bars. If the district heating station is provided with an energy converter, for example a thermodynamic engine driving an electric generator, or a thermoelectric generator converting heat energy into electrical energy, the energy converter utilizing the hot water from the district heating station, the plant may be operated as an independent unit without electrical energy from an external distribution network for the operation of pumps et cetera. The heat energy available may also be converted into other forms of energy where appropriate.

The drawback of such systems is that the energy converter has a relatively low efficiency when the heating-fluid temperature lies within the range of 80-120 °C.

It may be an advantage to use water as the heating fluid, as water can withstand high local temperatures. By comparison, thermo-oils often cannot withstand contact with elements having a surface temperature above 350 °C as the thermo-oils are inflammable and, in additio^, require extra safety measures.

When water is used as the heating fluid, regard must be had to official regulations concerning steam boilers and the like. It will therefore be beneficial to operate with hot-water circuits with limited volumes and pressures, in which, typically, the product p*V<200 (p = pressure [bars], V = heating-fluid volume [litres]).

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.

The invention relates to a primary heat source in which the combustion of fuel, typically by burning biomass, in a combustion chamber may provide for a primary heat distribution fluid to achieve a prescribed first temperature, typically 80-120 °C, via a primary heat exchanger. The primary heat distribution fluid may be used for heating buildings et cetera via heat transfer means known per se, for example radiators. In the combustion chamber, a secondary heat exchanger is arranged, in which a secondary heat distribution fluid circulates, achieving a second, higher temperature, typically in the range of 150-250 °C. The secondary heat exchanger is in fluid communication with a thermodynamic engine which is arranged to convert a portion of the heat energy of the secondary heat distribution fluid into a different energy, typically mechanical work, electrical energy et cetera. By this arrangement, an ordinary primary heat source, typically a biomass burner, adapted for distributing a heat distribution fluid at a normal temperature (80-120 °C) and pressure (up to approximately 2 bars), could be used to provide more high-grade heat energy for the efficient operation of a thermodynamic engine, without extensive and expensive adaptations of the primary heat source being required.

In an alternative embodiment, the invention includes several secondary heat exchangers connected in parallel arranged in the combustion chamber. The size of each individual secondary heat exchanger may thereby be limited, so that the safety requirements for the system will be reduced. This means, among other things, that water may be used as the heat distribution fluid with the high temperature achieved and with a higher pressure than what the primary heat source in itself has been built for.

In a first aspect, the invention relates more specifically to a heat-exchanging device for a primary heat source which is arranged to deliver heat energy at a first temperature via at least one primary heat exchanger to a heat distribution network, characterized by at least one secondary heat exchanger being thermally connected to the at least one primary heat source and being arranged to deliver heat energy at a second temperature to an energy converter, the second temperature being higher than the first temperature. The second temperature may advantageously be 20-220 °C higher than the first temperature, more advantageously 30-180 °C higher than the first temperature, and most advantageously 30-160 °C higher than the first temperature.

The energy converter may be arranged in a combined heat and power plant which is arranged to generate electrical energy.

At least two secondary heat exchangers may be thermally connected to the primary heat source, and the secondary heat exchangers may each, via separate heat- exchanger fluid circuits, be connected to a respective second secondary heat exchanger.

An internal electricity distribution network and an external electricity distribution network may be electrically interconnected via an electrical interfacing device arranged for transferring at least parts of the amount of electrical energy generated, from said energy converter to the external electricity distribution network.

The electrical interfacing device may be arranged for transferring an amount of electrical energy at least corresponding to the amount of electrical energy generatable in said energy converter, from the external electricity distribution network to the internal electricity distribution network.

The heat distribution network may include at least one tertiary heat exchanger which is thermally connected to said energy converter and may be arranged to transfer an amount of residual heat energy from said energy converter.

Said tertiary heat exchanger may be arranged upstream of said primary heat exchanger.

An air preheater may be thermally connected to said energy converter and may be arranged to receive a portion of an amount of residual heat energy from said energy converter.

The ratio between the nominal thermal power capacity of the primary heat source and the nominal thermal power capacity of the secondary heat exchanger may be in the range of 2: 1-20: 1.

In a second aspect, the invention relates more specifically to a method of operating a combined heat and power plant, characterized by the method including the following steps:

a) making an amount of heat energy available to one or more heat-energy con- sumers connected to a heat distribution network,

al) in order to, by thermal contact between a primary heat source and the heat distribution network and via one or more primary heat exchangers, transfer heat energy from said primary heat source to a heat distribution fluid in the distribution network at a first temperature;

b) by means of one or more energy converters, converting into electrical energy an amount of heat energy which is supplied to said energy converter at a second temperature from at least one secondary heat exchanger arranged in the primary heat source, the second temperature being higher than the first temperature; and c) transferring the electrical energy from said energy converter to an electricity distribution network.

The method may include the further step of:

a2) by thermal contact between the at least one secondary heat exchanger and the heat distribution network, supplying an amount of heat energy via at least one tertiary heat exchanger in the form of residual heat energy from the conversion, by said energy converter, into electrical energy of the amount of heat energy supplied from said secondary heat exchanger.

The method may include the further step of:

providing a second temperature which is advantageously 20-220 °C higher than the first temperature, more advantageously 30-180 °C higher than the first temperature, and most advantageously 30-160 °C than the first temperature.

The method may include the further step of:

d) supplying the amount of heat energy via said tertiary heat exchanger upstream of said primary heat source.

The method may include the further step of:

e) supplying heat energy to an air supply for said primary heat source by means of an air preheater, the heat energy being, at least in part, residual heat energy from said energy converter.

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

Figure 1 shows a principle drawing of a prior-art district heating plant; Figure 2 shows a principle drawing of a first embodiment of a district heating plant provided with secondary heat exchangers according to the invention connected to a thermodynamic engine;

Figure 3 shows a principle drawing of a second embodiment of a district heating plant provided with one secondary heat exchanger according to the invention connected to a thermodynamic engine;

Figure 4 shows a principle drawing of a third embodiment of a district heating plant provided with one secondary heat exchanger according to the invention connected to a thermodynamic engine, and an air preheater; and

Figure 5 shows a principle drawing of a fourth embodiment of a district heating plant provided with a secondary heat exchanger according to the invention connected to a thermodynamic engine, and a separate distribution network.

Reference is made in particular to the figures 2, 3, 4 and 5 as regards the description of an exemplary embodiment of the present invention. The prior art, as it appears from figure 1, exhibits some principal features in common with the invention, and equal elements are indicated by the same reference numerals.

The reference numeral 1 indicates a district heating plant according to the invention . A primary heat source 11 is connected to a heat distribution network 15 arranged to deliver heat energy Q to a heat consumer 16. The primary heat source 11 may be a boiler normally dimensioned per se, with a nominal output of 0.1-2 MW. The primary heat source 11 may be heated through the burning of a supplied fuel 18 suitable therefor, for example biomass, under a supply of air 181, an amount of heat energy QLI being made available to a primary heat exchanger 111 which constitutes part of the circulation circuit formed by the heat distribution network 15. Via the primary heat exchanger 111, the primary heat source 11 heats a first heat distribution fluid 152 suitable therefor, for example water or a thermo-oil, which circulates in the heat distribution network 15 with an output temperature limited to approximately 120 °C at a pressure not exceeding approximately 2 bars. The district heating plant 1 is connected to an external electricity distribution network 17 delivering electrical energy EL for the operation of pumps et cetera.

A first secondary heat exchanger 121 is arranged in the primary heat source 11, typically in the combustion chamber of the primary heat source 11. The secondary heat exchanger 121 heats a second heat distribution fluid suitable therefor, typically water under overpressure, circulating in a heat-exchanger fluid circuit 122, to an output temperature considerably higher than 120 °C, typically 150-300 °C. To a person skilled in the art, extracting heat from the combustion chamber of a primary heat source 11 in which the biomass is being burnt at a temperature of approximately 1000 °C is common general knowledge and will not be referred to in further detail. An amount of heat energy Q _H is transferred to a second secondary heat exchanger 131 in an energy converter 13, typically formed as a thermodynamic engine, which generates electrical energy P _EL , or mechanical energy (not shown), by means of the heat energy Q _H supplied. The electrical energy P _EL delivered to an internal electricity distribution network 19 is used for the operation of electric components (not shown) connected to the primary heat source 11 and any other electrical consumers in the district heating plant 1 or other connected consumers. The mechanical energy alternatively generated may be used to operate a machine (not shown).

In figure 2, the use of two first secondary heat exchangers 121 and 121', respectively, connected in parallel is shown, each connected to a respective second secondary heat exchanger 131 and 131', respectively, arranged in the energy converter 13. Said secondary heat exchangers 121 and 121', respectively, transfer an amount of heat energy Q _H and Q _H ', respectively, via separate heat-exchanger fluid circuits 122 and 122', respectively, to the respective second secondary heat exchanger 131 and 131', respectively. Even if one energy converter 13 provided with two secondary heat exchangers 131, 131' is shown here, it is obvious and lies within the scope of the invention for the secondary heat exchangers 131, 131' to be arranged in separate energy converters 13.

An electrical interfacing device 14, for example an inverter, is connected to the internal electricity distribution network 19 of the district heating plant 1 and the external electricity distribution network 17 in such a way that a surplus of electrical energy P _EL from the energy converter 13 may be supplied to the electricity distribution network 17, and a deficiency in energy P _EL from the energy converter 13 may be covered by supply from the electricity distribution network 17, for example in a situation in which a shutdown of the first secondary heat exchanger 121 or the energy converter 13 requires supply from external electrical-energy sources.

The heat distribution network 15 forms a closed circuit for the circulation of the first heat distribution fluid and the transfer of an amount of heat energy Q to one or more heat-energy consumers 16, shown schematically here as one heat-energy consumer 16. In the embodiments shown in the figures 3 and 4, the heat distribution network 15 is additionally connected to a tertiary heat exchanger 151 arranged in the energy converter 13 and arranged to transfer residual heat energy Q _L from the energy converter 13 and thereby preheat the return flow of the cooled, first heat distribution fluid circulating in the heat distribution network 15. The residual heat energy Q _L is advantageously supplied upstream of the primary heat exchanger 111 in the primary heat source 11 to achieve a lowest possible heat-sink temperature for the energy converter 13 and thereby high efficiency.

It may be an advantage to preheat the air supply 181, in particular to increase the efficiency of the primary heat source 11. For this purpose, an air preheater 182 may be used, which, in the embodiment shown according to figures 4 and 5, is supplied with heat from the heat distribution network 15 which, downstream of the tertiary heat exchanger 151, is laid in a loop passing through the air preheater 182. In an embodiment not shown, the air preheater 182 may be connected to a separate heat distribution circuit (not shown) which is in thermal contact with the energy converter 13, for example via the tertiary heat exchanger 151 or a further heat exchanger (not shown) arranged in association with the energy converter 13 for the transfer of a portion of the residual heat energy Q _L .

In the exemplary embodiment shown in figure 5, the residual heat energy Q _L from the energy converter 13 is supplied to a heat consumer 16' in a separate heat distribution network 15'. In this exemplary embodiment, the heat distribution network 15 is directly connected to the air preheater 182 and is not connected to the energy converter 13.