WO1999008055A1

WO1999008055A1 - Method of producing cooling power

Info

Publication number: WO1999008055A1
Application number: PCT/FI1998/000609
Authority: WO
Inventors: Seppo Leskinen
Original assignee: Abb Power Oy
Priority date: 1997-08-12
Filing date: 1998-08-03
Publication date: 1999-02-18
Also published as: CZ2000499A3; FI102565B; AU8544098A; PL338609A1; FI102565B1; EP1005622A1; FI973310A0

Abstract

The invention relates to a method of producing cooling power for one or more buildings by means of hot district heating water and an absorption aggregate (5, 8, 9, 10) obtaining energy from the district heating network. To provide a compact solution, the absorption aggregate (5, 8, 9, 10) is designed in view of a power that is lower than the peak consumption. When the consumption exceeds the design power, the temperature of the water discharged to the boiler part (5) of the absorption aggregate is raised by means of an external source or sources of heat (26).

Description

METHOD OF PRODUCING COOLING POWER

The invention relates to a method of producing cooling power for one or more buildings by means of hot district heating water and an absorption aggregate obtaining energy from the district heating network. Today buildings are generally cooled by cooling aggregates based on a compressor aggregate, dispersed to the places of use. Cooling power is produced in them by electricity. The proportion of the cooling of buildings in the consumption of electricity is now fairly important; in the Southern European countries, for example, the electricity consumption peaks in the summer. With regard to production, the consumption also occurs at an unfavourable time. The heat inevitably generated in connection with production of electricity cannot be used for much else than production of hot tap water, and so it has to be condensed and supplied to waterways by brine condensers, for example, or to air by cooling towers. Cooling power could also be produced by waste heat produced in the production of electricity in absorption aggregates, the best known of which are lithiumbromide/water and ammonia/water aggregates. The consumption of electricity and thus, for example, emissions of CO₂ could be reduced by these aggregates, and the waste heat, which is now completely wasted, could be utilized.

The preferred way of generating chill would be a so-called district cooling system, in which cooling power would be generated concentrated ly in power plants and supplied to users via a pipe system in the same way as district heat. This kind of system may be worth building in new communities where all buildings that require cooling will be connected to the system. The proportion of such building, however, is small in industrialized countries, and its proportion in relation to all building is growing smaller. Most building activity today comprises extension or renovation of existing communities. It is therefore not possible to connect a significant number of buildings simultaneously to a district cooling system if such a system is built in the area. A small number of buildings connected is not sufficient to cover the investment costs of the district cooling system and of the generation of district chill, which hampers the building of district cooling systems in existing communities.

A similar problem has been encountered when district heating networks have been built. The problem has been solved by movable heating stations in which heat is produced only for a limited area, whereby the costs of a distribution system remain small and can be covered immediately. When a sufficient number of areas has been connected, a main network is built, and the areas are connected to a power plant via the system. The movable heating stations are shifted to new areas or maintained in the area as heating stations that are used during maximum heat demand. The same idea cannot be readily applied to the building of a district cooling system. It is true that the costs of building a main network are eliminated, but the use of return water as condensation water is here not possible. Because of this, cooling towers, ground water, etc. would have to be used. For example, it is often impossible to place cooling towers in urban areas for architectural reasons, lack of space, etc.

Some plants of the above type based on absorption aggregates have been built, and technically such systems work well, but their competitiveness with compressor cooling is questionable, and the smaller the number of hours of use (i.e. in mild and cold climatic zones, where district heating systems are common), the less competitive they are. The reason for this is that the investment costs of an absorption heat pump, a cooling tower and a distribution system are much higher than the costs of a corresponding compressor aggregate. Even when the energy, i.e. heat, is almost free and the electricity used by the compressor aggregate is expensive, a reduction in the costs of use is not sufficient to cover the difference in the investment costs if the number of hours of use is not sufficiently great. The situation is made worse by a high short-term load peak in the cooling demand, the peak being more than double the average load during a cooling period. This is due to the fact that in the mild and cold zones, the design outdoor temperature prevails only in the afternoon on a few days a year. The average cooling load is also short-term. Cooling, unlike heating, is not needed round-the-clock but only at midday and in the afternoon. Since the consumption of electricity in the countries situated in the cold and mild zones peaks in the winter, high investment costs cannot be substantiated by a reduction in the investments on electricity production machinery, like in the countries situated in the hot zone. Only a few such plants have thus been built in Middle and Northern Europe, for example, for test and research purposes, although they are common in the hot zone.

Further, the investment costs of the production and distribution of heat are dependent on the peak consumption, which is primarily dependent on the outdoor temperature. The design outdoor temperature, however, is measured rather seldom. For example, the design temperature of Helsinki is -26°C. On the average, however, the temperature prevails for the duration of less than 18 hours a year. Further, a temperature of -20°C or below prevails for about 88 hours on the average, whereas the total length of the heating period is from 5000 to 6500 hours, depending on the building. The situation is thus very similar to summer. The temperature duration curve exhibits a high short-term peak value.

With regard to the production and distribution of heat, the situation is made worse by the diurnal variation of the consumption. About half of the buildings are used only in the working hours. The ventilation systems of these buildings are usually switched off or adjusted to the minimum for the nights and weekends. When the proportion of ventilation in relation to the heat consumption in the buildings is about half, the diurnal heat consumption in these buildings varies constantly between 50% and 100%. This further increases the difference between the average and the peak consumption of heat. Further, the indoor temperature of such a building is often dropped when the building is not used, which makes the situation even worse.

Only in recent years has it been noted that the energy saving operations carried out in the buildings have worsened the situation. The yearly consumption of heat has dropped drastically in the last 20 years. The peak consumption, however, has not dropped nearly as much for various reasons. Perhaps the primary reason is that heat cannot be recovered from the exhaust air at maximum efficiency during the peak load due to the risk of freezing. Another major reason is that the indoor temperature is dropped when the buildings are not used. With regard to the production and distribution of heat, the situation is difficult. The heating plant and the distribution system should be designed in view of peak consumption, but their average degree of use would not be higher than about 25 to 35%. The situation is growing even worse.

In practice, the expensive plant and the distribution system are not designed in view of peak load but of much lower power. The power for the peak consumption of heat is generated in heating stations used during maximum heat demand, built in different parts of the distributing network; these heating stations may generate a notable portion of the overall heating power. In Helsinki, for example, the degree of use of the heating stations used during maximum heat demand is low: at worst, they are only used for a few dozen hours a year. The unit price of the heat produced in them is very high due to the high investment costs.

Finnish Patent Application 954,949 discloses an arrangement by which the investment costs of cooling can be substantially reduced and the reliability of the plant simultaneously improved as compared with the applications used earlier. These advantages are achieved by cutting the peak load by using evaporative cooling provided in the air-conditioning units of a building and by levelling the diurnal variation of consumption by providing the system with a tank, from which power stored in the night or some other time when there is little or no consumption can be drawn during the day-time peak consumption.

The evaporative cooling and the tank in particular cause extra costs, but these costs amount to less than what is saved by reducing the size of the absorption heat pump, cooling tower, pipe system, etc. However, the extra costs impair the system's competitiveness with compressor cooling.

Finnish Patent Application 954950 discloses an arrangement by which the diurnal variation of heat consumption can be levelled so that the buildings connected to the system do not use heat from the district heating network at all, or in some cases may even be able to supply power to the district heating network when there is peak consumption in the other buildings. Correspondingly, they take power from the network when the consumption in the other buildings is low. The system is based on the use of a cooling power tank also for storing heating power at a temperature that is higher than the temperature of the heat-consuming units of the buildings. The load peaks caused by the other buildings can be levelled by the system, and the uneconomic heating stations used during maximum heat demand can be made smaller or even disposed of.

Finnish Patent Application 954951 discloses a method in which at least some of the return water coming from the air-conditioning unit or other apparatus consuming cooling energy is conducted to an absorption aggregate or some other aggregate that generates heat to be condensed, and there the return water absorbs the condensation heat generated in the aggregate. The primary advantage is that either no condensers are needed at all in the system, or the size and/or number of condensers can be decisively reduced as compared with the known solutions. The costs of an absorption aggregate are reduced considerably, which improves the competitiveness of the cooling energy generated by waste heat as compared with compressor cooling.

The above measures render the cooling power produced by district heat in an absorption aggregate competitive with compressor cooling. In a system that is being built, the maximum power of the cooling can be dropped from 2 MW to 1 MW, and the maximum power of the heating from 4 MW to 3 MW, and the power peak can also be slid so that it occurs in the night, and the condensation power can be dropped from 4.8 MW to 2 MW. The preliminary comparison of costs seems very promising. The tanks, however, pose a problem. Even though it is somewhat easier to arrange tanks than condensers in urban areas, there is not always space for them. About half of the above 1 MW power cut is effected by tanks whose net volume is 300 m³. The gross volume is slightly higher because of mixing; and since the pipe systems, etc. need space, the volume needed is about 600 m³. Often it proves impossible to find, or unduly expensive to obtain such space in a densely built urban area.

If the tanks cannot be placed anywhere, the advantages presented in Finnish Patent Applications 954,950 and 954,951 will not be achieved, and the advantages presented in Finnish Patent Application 954,949 will be achieved only in part. In the above example, the maximum power of the cooling would therefore drop from 2 MW to 1.5 MW and the condensation power from 4.8 MW to 3.6 MW. The design power of the heating would not drop at all.

Another problem is the low temperature in the district heating network in the summer. The temperature is usually not higher than 80°C, which is also the lowest temperature at which the absorption process can be made to work by the known absorbents and absorption aggregates so that cooling water of about 10°C can be produced. A particular problem is posed by condensation water, the temperature of which is difficult to maintain at a sufficiently low level particularly when there is peak load. The condensers are thus rendered big and expensive, and so it is even more difficult to find space for them than before.

The simplest solution to the above problems would be to raise the temperature of the district heating network, whereby the condensation temperature could also be higher, the power of the absorption aggregate would rise, etc. Indeed, this has been done in Gothenburg, where waste heat is obtained from industry. This, however, does not work in common backpressure power plants, where the electricity supply/fuel unit grows smaller as the temperature of the district heating water rises. Consequently, the electricity saved by replacing compressor cooling with absorption cooling would therefore be lost many times.

The object of the invention is to provide a method by which most of the drawbacks of the prior art can be eliminated, and by which the power peaks can be cut both in the heating and in the cooling when there is no space for the tanks. The object is achieved by the method of the invention, which is characterized in that the absorption aggregate is designed in view of power that is lower than the peak consumption, and that when the consumption exceeds the design power, the temperature of the water discharged to the boiler part of the absorption aggregate is raised by an external source or sources of heat. The simple basic idea of the invention is that the temperature of the district heating water is raised locally, for example, by a gas-heated boiler or some other heating apparatus used as a heating station during maximum heat demand in the winter. The temperature of the whole district heating network need then not be raised, and yet all the advantages are achieved that would be achieved by raising the temperature in the network, i.e. the power of the absorption aggregate is raised, the temperature of the condensation water may slide slightly, etc. In addition, the power peak can be cut even in the winter. A small boiler can usually be fitted into the space that has become available as the size of the absorption aggregate has been reduced, particularly when it is realized that the efficiency of the boiler is not relevant because the number of hours of use is so small. The whole water flow discharged to the absorption aggregate need not be heated; only a small partial flow is heated to a temperature that is much higher than the desired discharge temperature to the absorption aggregate. The desired discharge water temperature is obtained by mixing hot water coming from the boiler with cooler district heating water.

Because of the small number of hours of use, it is usually not worthwhile to cool the exit gases to a low temperature. In order to make the boiler small and thereby less expensive, the boiler can naturally also be provided with a convection part, which is then connected in series with a heat exchanger of a furnace in a manner known per se. The temperature of the exit gas is then low, which may be a significant advantage when the boiler has a much higher number of hours of use for some other reason than what would be necessary for cutting the peak load of the cooling.

The invention will now be described in greater detail by means of the embodiments illustrated in the attached drawing, in which Fig. 1 is a schematic view of a prior art solution, Fig. 2 is a schematic view of a first embodiment of the invention, Fig. 3 is a schematic view of a second embodiment of the invention, Fig. 4 is a schematic view of a third embodiment of the invention, Fig. 5 is a schematic view of a fourth embodiment of the invention,

Fig. 6 is a schematic view of a fifth embodiment of the invention, and

Fig. 7 is a schematic view of a sixth embodiment of the invention. Fig. 1 shows an example of prior art solutions. The solution of Fig. 1 operates, in principle, as follows. Hot water is drawn from a supply pipe 1 of the district heating system through a pipe 2 to a boiler part 5 of an absorption aggregate and returned through a pipe 4 to a return pipe 3 of the district heating system. For the sake of controllability, the boiler 5 is usually provided with a circulation pump 6 and a control valve 7. A refrigerant is evaporated from the absorbent in the boiler 5 with hot district heating water. The refrigerant is supplied to a condenser 8, where it is cooled so that it liquefies. From the condenser 8 the refrigerant is supplied to an evaporator 9, and the pressure is thereby reduced such that the refrigerant evaporates, whereby the temperature drops and cools the circulation water in the cooling system of the building. From the evaporator 9, the refrigerant is conducted to an absorber 10, to which is also conducted the absorbent from the boiler 5 through a heat exchanger 11. The refrigerant is allowed to absorb to the absorbent in the absorber 10, whereby reaction heat is released. A solution of the absorbent and the refrigerant is pre-heated in the heat exchanger 11 and pumped to the boiler 5 with a pump 12 at an elevated pressure.

Heat is conducted from the outside to the boiler 5 and the evaporator 9 of the absorption aggregate, and it has to be conducted away in order that the aggregate would operate continuously. Cooling is usually implemented using water, which - heated - is conducted from the absorber 10 through a pipe 13 to a cooling tower 14, in which it is cooled evaporatively. Naturally, however, for example a brine condenser or some other apparatus known per se can be used. From the cooling tower 14, the water is pumped through a pipe 15 to the condenser 8 of the absorption aggregate and from there - somewhat heated - through a pipe 16 to the absorber 10, and from there back to the cooling tower 14. In the evaporator 9, the chilled cooling water of the building is conducted through a pipe 33 to the cooling water network of the building, from which it returns -heated - through a pipe 32 to the evaporator 9.

The building usually comprises many apparatuses that use cooling water, but for the sake of clarity, only one air-conditioning unit is shown in Fig. 1. The cooling water flows through a control valve 18 to a heat exchanger 23, by which the cooling power is transferred to a heat transfer circuit of the air- conditioning unit, from which it is returned by a pump 20 either through control valve 18 to heat exchanger 23 or to condenser 9. Heat exchanger 23 is not necessarily needed: the cooling water can also be supplied directly to a circulation water pipe 21 or even directly to a cooling radiator 19, if the air- conditioning unit does not comprise a recovery radiator 22 and thus a circulation water circuit.

The above solution has the drawbacks presented in the general part describing the prior art.

Fig. 2 shows a first preferred embodiment of the invention. Like reference numbers indicate similarly as in Fig. 1.

The absorption aggregate is here designed so that it operates in the above manner up to a certain load. For example, in the above example the aggregate would operate conventionally up to a power of 1.0 MW, i.e. 90 to 95% of the total number of hours of use. The typical temperatures could then be

District heat discharge water in pipe 2 80°C return water in pipe 4 70°C

Cooling discharge water in 33 10°C return water in pipe 32 20°C Condensation discharge water in pipe 15 22°C return water in pipe 13 35°C.

As the load continues to increase, the power of the absorption aggregate will not suffice. Because of this, the burner of a boiler 26 and pump

6 are started and valve 7 allows district heating water to pass to boiler 26, where the water is heated and subsequently returned to the supply pipe 2 of the district heating. The temperature of boiler 5 rises, whereby the power of the absorption aggregate rises and the temperature of condenser 8 can also slide upward. As the load increases further, valve 7 opens wider, until all water required by boiler 5 passes through boiler 26 in the case of peak load. The typical temperatures could then be, for example:

District heat discharge water in pipe 2 90°C return water in pipe 4 80°C

Cooling discharge water in 33 10°C return water in pipe 32 20°C Condensation discharge water in pipe 15 26°C return water in pipe 13 38°C.

The valve 7 can also be made to supply only part of the district heating water needed by boiler 5 to boiler 26, in which the water is then heated to an elevated temperature. If the water in boiler 26 is heated up to 170°C, for example, only 12.5% of the amount of water to be supplied to boiler 5 needs to be supplied to boiler 26.

In the example of Fig. 2 the district heating water circulates through boiler 26. Another alternative is illustrated in Fig. 3: the temperature of the district heating water is raised by a heat exchanger 27, whereby a control valve 28 and a pump 29 can be arranged in the circulation water circuit of the boiler as shown in Fig. 3. This kind of solution is particularly useful when the source of heat is other than a boiler, for example a solar cell, a heat pump or the like, whereby the heat-carrying liquid is seldom water.

Also, the heat exchanger can be designed in view of a greater temperature difference and a lower flow rate and connected as shown in Fig. 2, in which boiler 26 is replaced with heat exchanger 27, to which for example control valve 28 and pump 29 are naturally connected as shown in Fig. 3.

Fig. 4 shows an embodiment in which the temperature of boiler 4 can be raised more in the case of peak load than in the examples illustrated in Figs. 2 and 3, without increasing the energy consumption. The apparatus operates such that when the load is heavy, valve 7 closes the intake of district heating water from the supply pipe 2, whereby the boiler is driven only by heating energy supplied by heat exchanger 27. The temperature then depends only on the temperature of the heat carrying liquid circulated by pump 29. For example, the following temperatures can be easily provided:: Discharge water after heat exchanger 27 105°C

Return water before pump 6 95°C.

The temperature of the return water can thus be higher than the 80°C supply temperature of the district heating water in pipe 2. The temperature difference of 10°C need not necessarily grow, if, for example, the temperature of condenser 8 is allowed to rise considerably. Usually, however, the temperature of boiler 5 is raised to increase the power of the absorption aggregate, so either the temperature difference between the supply water and the return water of boiler 5 or the water flow through pump 6 increases. Fig. 5 shows an embodiment in which only part of the water flow of pump 6 is conducted through heat exchanger 27 by valve 30. Particularly when the source of heat is a boiler, the overall costs can thus usually be minimized.

The embodiments of Figs. 2 to 5 show only examples relating to a cooling situation. In all these embodiments, a by-pass can be arranged for winter use in accordance with the principle illustrated in Fig. 6. In the figure, pump 6 draws water from the return pipe 3 of the district heating network through pipe 4 at a temperature of 40 to 55°C. The water is conducted by valves 7 and 30 to heat exchanger 27, where it is heated, for example, up to 100°C, after which it is conducted by valve 31 through pipe 2 to the supply pipe 1 of the district heating network.

In the embodiments of Figs. 2 and 3, the cooling and heating power can be cut in a desired manner, but condensation power can be cut only within certain limits. Instead, in the embodiments of Figs. 4 to 6, the boiling temperature in the boiler part 5 is not restricted. The temperature of the condensation part 8 can thus also rise more, which as such increases the power of the cooling tower 14. Sometimes it may be worthwhile to increase the water flow in the condensation part 8 and the absorption part 10 to lower the costs of piping and to arrange a by-pass valve 34 as shown in Fig. 7 and to conduct part of the water flow past the cooling tower 14.

The invention thus makes it possible to achieve equal cuts in the cooling, heating and condensation power as have been achieved by tanks in the example described earlier. For this, a 2.1 MW source of heat is needed, so the maximum power of the heating drops from 4 MW to 1.9 MW. The heat usually has to be generated using fuel, but since the maximum power both in the cooling and in the heating is used only for a short period of time, definitely for less than 100 hours a year in all, the costs of use are not unduly high. The size of a 2 MW boiler is typically 4 x 2.2 x 2.5 m (length x breadth x height), i.e. the boiler fits into the space that becomes available as the size of the absorption aggregate can be reduced because of the boiler. In any case, the space demand is less than one tenth of the space demand of the tanks.

The performance characteristics achieved by the solutions described in Finnish Patent Applications No. 954,949, 954,950 and 954,951 can thus also be achieved by the method of the present invention with the additional advantage that the extra space required by the tanks in the above applications is here not needed. The fuel consumption and the servicing of the boiler or another source of heat cause extra costs. However, since the number of hours of use is small, the costs are usually at least not higher than what is saved by saving space. The investment costs of the boiler seem to be somewhat lower than the investment costs of the tanks. The overall costs are thus at most of the same order as in the solutions described in the above applications, and the advantages achieved in them are also achieved when the space required by the tanks is not available. The primary advantages are that the investment costs of absorption cooling are substantially reduced by cutting the peak load of the cooling, as a result of which the absorption heat pump, condensers, piping, etc. can be made substantially smaller than in the previously known solutions, and that the load peak of the heating is cut by about half, which means that substantially smaller investments are needed in the heating stations used during the maximum heat demand and in the district heating network.

Further, when the nominal power of the absorption aggregate is reduced, the lower limit of the range of adjustment is notably lower than in the conventional systems.

The invention is described above by means of a few examples. It should be noted that the above examples are not to be understood as restricting the invention in any way, but that the invention can be varied quite freely within the scope of the claims. For example, in all the figures there is one absorption aggregate and one boiler. Naturally, there may be more than one of each to improve reliability, facilitate positioning, or for other reasons. Further, the by-pass connection of the cooling tower 14 in Fig. 7 is implemented by arranging valve 34 in pipe 13 following the absorption part 10, the valve conducting part of the water flow past the cooling tower 14 to pipe 15. In some cases valve 34 can also be placed in pipe 16 to conduct part of the water flow past both the absorption part 10 and the cooling tower 14 to pipe 15. Likewise, in the summer/winter use illustrated in Fig. 6, valve 31 , for example, can be replaced with stop valves placed in the discharge and return pipes of the boiler part 5, valves 7 and 30 can be passed in the winter use, etc. Generally, the 3-way valves can be replaced with 2-way valves in all the examples, shunting can be arranged between all discharge and return pipes, etc. Naturally, a combination in which a power peak is cut both by evaporative cooling, which is inherent in the air-conditioning units of the building, and by raising the temperature of the boiler part of the absorption aggregate falls within the scope of the invention, and so does the use of tanks to cut the power peak somewhat as the temperature of the boiler part is raised. Likewise, protecting of the district heat return pipe 3 when the return temperature has the allowed maximum value due to heat expansion or some other reason also falls within the scope of the invention. This can be effected in many ways, for example by conducting the water returning from the boiler part 5 of the absorption aggregate to the district heat supply pipe or by stopping the intake of district heating water altogether as described in connection with Fig. 4 when the temperature of the return water exceeds the allowed maximum value. The return water can also be cooled, for example, by a storing tap water heat exchanger placed in return pipe 4, or in another suitable manner. All connections and arrangements of this kind thus fall within the scope of the invention.

Claims

1. A method of generating cooling power for one or more buildings by means of hot district heating water and an absorption aggregate (5, 8, 9, 10) obtaining energy from the district heating network, characterized in that the absorption aggregate (5, 8, 9, 10) is designed in view of power that is lower than the peak consumption, and that when the consumption exceeds the design power, the temperature of the water discharged to the boiler part (5) of the absorption aggregate is raised by an external source or sources of heat (26, 27).

2. A method according to claim ^characterized in that only part of the water discharged to the boiler part (5) of the absorption aggregate is conducted to the external source of heat (26, 27), where it is heated to a temperature that is higher than the desired discharge temperature to the boiler part (5) and is then conducted back and allowed to mix with the unheated part of the water.

3. A method according to claim 1, characterized in that as the consumption approaches the peak value, the district heating water intake is closed and the water returning from the boiler part (5) of the absorption aggregate is heated using the external source of heat (26, 27), and that after the heating the water is conducted back to the boiler part (5).

4. A method according to any one of claims 1 to 3, characterized in that when the design value of the absorption aggregate (5, 8, 9, 10) has been exceeded, the temperature of the condensation water supplied to the condensation part (8) is also raised.

5. A method according to any one of claims 1 to 4, characterized in that part of the peak consumption is covered by evaporative cooling inherent in the air-conditioning units of the building or buildings.

6. A method according to any one of claims 1 to 5, characterized in that part of the peak consumption is covered by a tank arranged in conjunction with the absorption aggregate.

7. A method according to any one of claims 1 to 6, characterized in that part of the water needed for cooling the condensation and absorption parts (8, 10) is conducted after the condensation part (8) or the absorption part (10) to a supply pipe (15) following a cooling tower (14) or the like.

8. A method according to any one of claims 1 to 7, characterized in that an external source of heat (26, 27) is used to heat the building or buildings and/or the ventilation air needed by them.

9. A method according to any one of the preceding claims, characterized in that when the water discharged to the boiler part (5) of the absorption aggregate is heated with an external source of heat (26, 27), the water returning from the boiler part (5) is conducted to the supply side (1) of the district heating network.