The integration of power produced by solar means in combined-cycle power stations with a gas turbine and a water/steam circuit to form an integrated solar combined-cycle (ISCC) power station makes it possible to reduce CO2 emissions from such power-station facilities. Solar-thermal solutions are in this case—in contrast to photovoltaic technologies—particularly highly suitable for such integration. Solutions such as these are distinguished in that the solar energy is used in order to generate steam (so-called solar steam) directly or indirectly, and this can be used in various ways in a combined-cycle circuit with a gas turbine. One preferred form is for the solar steam to be added to the water/steam circuit (WSC), instead of using it in the gas turbine itself. Furthermore, the solar steam is preferably generated in solar arrays which are fitted with parabolic groove collectors.
FIG. 1 shows a highly simplified example of a power station layout of such an integrated solar combined-cycle power station. The integrated solar combined-cycle power station 10 shown in FIG. 1 comprises a gas turbine 11 with sequential combustion, a water/steam circuit 20 with a steam turbine 21, and a solar circuit 30 with a solar array 33. In the present example, the gas turbine 11 consists of two compressors 13a, 13b which are connected one behind the other, that compress the combustion air sucked in via an air inlet 12, and pass it to a first combustion chamber 14, for burning a fuel. The hot gas that is produced is expanded in a first turbine 15, producing useful power, is heated once again in a second combustion chamber 16, and is passed through a second turbine 17. The compressors 13a, 13b and the turbines 15, 17 are connected to a generator 19 via a shaft 18.
The exhaust gas which emerges from the second turbine 17 and is still hot is passed through a heat recovery steam generator (HRSG) 26, where it generates steam for the water/steam circuit 20 which includes the heat recovery steam generator 26. After flowing through the heat recovery steam generator 26, the exhaust gas flows to the outside via an exhaust gas line through an exhaust gas chimney 27. Fundamentally, the water/steam circuit 20 is formed from the steam turbine 21 which is connected to a generator 25, a condenser 22, a feedwater boiler 24, a feedwater pump 23 and the heat recovery steam generator 26.
Thermal energy is additionally supplied to the water/steam circuit 20 from the solar circuit 30, which is formed from the solar array 33 with the parabolic groove collectors 37, a pump 31 and a heat exchanger 32. Storage means for storage of solar heat for operation during periods when there is little or no sun can, of course, additionally be associated with the solar circuit 30. It is likewise feasible to use collectors equipped with Fresnel mirrors or heliostat installations instead of the parabolic groove collectors 37. The solar heat can be introduced into the water/steam circuit 20 at various points; therefore, as the representative of various solutions, FIG. 1 shows only one connection 28 as a double-headed arrow between the heat exchanger 32 and the heat recovery steam generator 26.
Previous investigations and studies have confirmed that integrated solid combined-cycle power stations such as these are both technically and economically feasible and worthwhile, and are suitable for the use of solar energy, not least because use can be made of proven technologies. In fact, integrated solar combined-cycle power stations have a number of advantages, which are listed below.
For a combined-cycle KA26 type power station from the Assignee of the present application, which is based on the use of GT26-type gas turbines, the overall efficiency can be increased from approximately 57% to approximately 65%, with the contribution of the solar energy being only approximately 15%. The CO2 emissions are thus drastically reduced.
Components which already exist in conventional combined-cycle power stations (CCPSs) can largely be used for utilization with solar steam, thus considerably reducing the cost of electricity (CoE) in an integrated solar combined-cycle (ISCC) power station in comparison to pure solar power stations (from, for example, C=300/MWh to C=180/MWh in the case of an ISCC).
Large amounts of power from a reliable supply can be generated 24 hours a day and 7 days a week, independently of the climatic conditions.
The power station starts to operate as soon as the solar array emits heat, as a result of which maximum use can be made of the solar energy.
A multiplicity of investigations and proposals have already been made in the prior art as to how the solar steam generated in a solar array can be integrated in a combined-cycle power station:
U.S. Patent Application Publication Nos. 2006/0260314(A1) and 2006/0174622(A1) propose intermediate circuits in order to generate solar steam using the heat from a solar array.
Others investigate the economic and power aspects of the integration of parabolic groove solar arrays in a combined-cycle power station (Dersch et al., “Trough Integration into Power Plants”, Energy, Vol. 29, pages 947-959, 2004).
It is also proposed that steam be produced from a solar array via an intermediate circuit and that supplementary firing (SF) be used, in order to regulate out the load changes (Hosseini et al., “Technical & economic assessment of the ISCC power plants in Iran”, Renewable Energy, vol. 30, pages 1541-1555, 2005).
Publication No. WO95/11371(A1) also proposes the use of supplementary firing for adaption to load changes.
U.S. Patent Application Publication No. 2008/0127647(A1), in particular, describes numerous options, referring back to previous proposals (see above), for the combination of solar combined-cycle power stations and combined-cycle power stations equipped with gas turbines. The aim is to maximize the solar component from the combined-cycle power station and to maximize the total output power by operating existing or retrofitted power stations, which are equipped with overdesigned heat recovery steam generators and steam turbines, with a high level of supplementary firing.
While the inclusion of solar steam in a combined-cycle power station undoubtedly represents a positive step in the direction of increasing the power output while at the same time reducing the CO2 emissions per power unit, internal investigations have shown that the known solutions are not optimized in terms of effective fuel utilization (and therefore CO2 avoidance) and economic parameters (electricity costs). However, these aspects are particularly important for acceptance and implementation of new technologies such as these, for example in the case of an integrated solar combined-cycle power station.
As already mentioned, the prior art is based on supplementary firing, in order to match the load on the power station and/or to increase the output power. However, supplementary firing involves combustion of additional fuel (for example by means of channel burners) in the heat recovery steam generator (supplementary firing 34 in FIG. 1), in order to produce additional steam for electricity generation by the steam turbine. While this undoubtedly increases both the output power and the flexibility of operation, thermodynamic considerations indicate that this reduces the overall efficiency of the power station (because the additional heat is produced at comparatively low temperatures). This means that neither the specific CO2 emissions nor the fuel costs are minimized. The supplementary firing can admittedly increase the electricity generation by about 10%; however, at the same time, the specific output of CO2 from a typical integrated solar combined-cycle power station is also increased by about 3% (350 kgCO2/MW without supplementary firing, 360 kgCO2/MW with supplementary firing).
The prior art postulates overdesigning of both the heat recovery steam generator (HRSG) and the steam turbine (by up to 50%), in order to make it possible to process the additional steam from the solar array and the supplementary firing. This results in higher investment costs. Furthermore, the overall efficiency decreases, when the power station is not being operated at full power (that is to say when the supplementary firing and/or the solar heat do(es) not reach the full 100%), because operation does not take place at the nominal operating point.
The aforementioned U.S. Patent Application Publication No. 2008/0127647(A1) in fact proposes the conversion of existing power stations, which already have a large extent of supplementary firing (15-50%) and use an overdesigned heat recovery steam generator and an overdesigned steam turbine (that is to say the heat recovery steam generator and the steam turbine are designed for operation with 100% exhaust gas heat from the gas turbine and, in addition, the solar heat and the supplementary firing).
FIG. 2 shows a diagram for this situation of the total electrical output power (total gross output in MWel) plotted against the relative load on the gas turbine (GT Relative Load in %). A dashed line bounds the intended design space DS1. Although the power station must be designed for consumption of the peak power from the solar array (+110 MW) and supplementary firing (+110 MW)(operating point A′ in FIG. 2), the power station (because of the change between day and night and changes in the atmospheric and weather conditions) will only rarely receive 100% solar heat. This design means that, whenever less than 100% solar heat is offered, the power station is overdesigned and is being operated away from the intended (optimum) operating point (that is to say between A′and C′).
Although, in its own right, solar energy costs nothing, the equipment, the infrastructure, the land and the other requirements (for example the water for cleaning the mirrors in the solar array) for the use of solar energy are very expensive. Therefore, simply maximizing the solar system of an integrated solar combined-cycle power station does not necessarily represent an optimum solution in terms of the balance between environmental protection, performance and economic aspects. In the corresponding manner, the previously described power stations do not maximize efficiency while at the same time minimizing the economic and environmental costs, and therefore also do not exploit the full potential of solar energy. In fact, the previous solutions attempt only to maximize the solar component.