The invention is encompassed within the technical sector of thermosolar technology, more specifically, within the cylindrical collectors sector, as well as within the plants where said collectors are installed, whether they are direct steam generation plants or plants which use a heat carrying fluid to produce steam in a subsequent exchanger.
The general principle of thermosolar technology is based around the concept of concentrating solar radiation in order to heat a heat carrying lid and generate electricity. The collection and concentration of solar energy is one of the biggest challenges in the development of thermosolar plants. There are two primary types of concentrator technology: point concentration and linear concentration. Linear concentration technology is easier to install since it has fewer degrees of freedom but it has a lower concentration factor and may therefore reach lower temperatures than point concentration technology.
Within the point concentrator category, parabolic dish concentrators and central towers can be distinguished. Within the linear technology category, the Parabolic Trough Concentrator (PTC) is the most mature concentration system but some new Fresnel-type Linear Collectors (FLC) are now emerging.
Fresnel collectors are composed of a primary and a secondary system. The primary system is formed by a series of parallel lines of reflecting mirrors, flat or slightly curved, with mobile structures responsible for emitting and directing the solar radiation towards the secondary system. The radiation reaches the opening of the secondary system and is redirected by some mirrors to an imaginary focal “tube”, which is where the absorber tube is placed.
This secondary system is elevated several meters above the field of mirrors and is responsible for concentrating the solar radiation emitted by the primary system and directing it towards an absorber tube.
The absorber tube remains inert in the space independently of the collector which serves to track the sun. The reconcentrator has a double positive effect: it enables the collecting surface to be increased, preventing rays from escaping and facilitates concentration increase in the primary field. In addition, this type of collector facilitates the more compact use of land, since the lines are very dose together.
Eventually, the land under the fields of mirrors may be used for purposes other than energy production. The lines are small in comparison with a parametric cylindrical collector, thereby wind loads are lower and the structures are lighter.
Parabolic trough technology is a more mature technology than Fresnel collector technology and has an extensive history serving to prove that it is ready to be installed on a large scale. This technology has been installed commercially since the 1980s and has exceptional performance. Since then, it has improved significantly in terms of both cost and output. Currently, there are 300 MWs in operation, 400 in construction and about 6 GWs being promoted on a global scale.
Parabolic trough technology operates based on solar tracking and by concentrating solar rays in high thermal efficiency receiver tubes located in the focal line of the parabolic trough collectors.
The main components of the solar field of the parabolic trough technology are as follows:
Traditionally, a heat carrying fluid or heat transmitting fluid, generally synthetic oil, circulates through the absorber tube, which is heated to approximately 400° C. by the concentrated solar rays. This oil is pumped through a series of heat exchangers at high temperatures in order to produce superheated steam.
However, in the last few years, the direct generation of steam with parabolic trough collectors has been in development. This technology (known as DSG), eliminates the need for an intermediary heat transmitting fluid in the solar field. The water is introduced directly through the interior of the receiver tubes and absorbs the energy reflected by the collectors, changing from its liquid phase into saturated steam and subsequently into superheated steam.
In both cases, the heat present in the steam is converted into electric energy by means of a conventional steam turbine and an alternator.
In terms of synthetic oils, the DSG has the following characteristics:
Whereby DGS plants are concerned, the steam produced in the solar field, in pressure conditions ranging from 60 to 100 bar and temperature conditions ranging from 400° C. to 525° C., supplies the turbine directly without the need for an exchanger. In this type of plant, the solar field is divided into two parts. One part (around 80% of the field) is used to generate saturated steam at temperatures of approximately 300° C. and the rest of the field (approximately the remaining 20%) is used for overheating. In addition, the temperature of the water upon entering the solar field is reduced to around 240° C.
The average working temperature of DSG plants is therefore lower than in the case of synthetic oil. This limits radiation losses and opens the door for collectors with receivers, without a vacuum or other types of receivers allowing convection losses to be minimized, to be used, at least in areas of the solar field working at lower temperatures.
From an optical standpoint, whereby current parabolic trough collectors are concerned, all rays reaching the trough within the angle of incidence of the design are reflected towards the absorber tube. The collection efficiency of this concentrator, defined as the fraction of incident power within the angle of acceptance of the primary, reaching the absorber tube, is 100%. It is possible to check that the concentration of current parabolic trough collectors is about 26 suns, whilst the second principle of thermodynamics makes it possible to affirm that, by keeping current acceptance angles, it is possible to reach 83 suns. In order words, the concentration of current collectors may theoretically increase 3.2 times.
Through recent history there have been various attempts to increase the concentration of parabolic trough collectors by means of secondary reconcentrators:
There is another solution in the state of the art which does not have the aforementioned disadvantages and increases concentration in the collectors. This solution consists of using concentrators designed with the optical design method known as “The Simultaneous Multiple Surface (SMS)” by Miñano, Benitez, et al. The method is well described in document U.S. Pat. No. 6,639,733 B2 and in other publications and it enables the design of a surface for the primary and secondary reflectors which is no longer parabolic and maintains 100% collection efficiency for rays falling on the collector, within the acceptance of the design, whilst facilitating the optimization of the concentration ratio C/Cmax. In this case, the number of solar ray reflections is two.
Therefore, the present invention focuses on the development of a new collector, based on the SMS optical design, which considerably improves the efficiency thereof but in this case applied to multi-tube receivers without vacuum and introducing improvements in its geometry, thus overcoming the difficulties encountered in the state of the art by controlling convective losses in such a way that these collectors may be used at low temperatures, whereby convection losses in relation to radiation are controlled.
In addition, the invention develops a concentration thermosolar plant containing the claimed collectors, combined with collectors having tubular receivers, which are either parabolic trough or parametric cylindrical collectors. This plant, having a combination of both types of collectors, increases efficiency in relation to existing plants because, owing to the combination of both technologies, the operation of each type of collector is optimized and convective losses are minimized.
The present invention relates to the design of a collector having a multi-tube receiver without vacuum and to the thermosolar plant combining said collectors with collectors having tubular receivers, with or without vacuum, for the direct or indirect generation of steam.
The parametric cylindrical collector having a multi-tube receiver without vacuum, object of the invention, is a symmetrical collector, formed by a primary reflector having an optically optimized geometry to maximize total concentration, an optimized secondary reconcentrator, and a receiver consisting of several tubes with a circular cross-section, each characteristic as described below:
In order to design the primary and secondary reflectors of the collector, the design method known as “The Simultaneous Multiple Surface (SMS)” by Miñano, Benitez, et al., is applied. The method is well described in document U.S. Pat. No. 6,639,733 B2 and in other publications and it enables the design of a surface for the primary and secondary reflectors which is no longer parabolic and maintains the collection efficiency at 100% for rays falling on the collector, within the acceptance of the design, whilst allowing the concentration ratio C/Cmax to be optimized.
In any of the previous configurations, the tubes can be joined to one another by means of welding with or without additional flaps or by using clamps and in all cases, the inclusion of mechanisms allowing the longitudinal dilation of the tubes will be necessary.
In short, the geometry of the multi-tube collector being proposed optimizes the concentration ratio C/Cmax of the parametric cylindrical tube to up to values of 0.63, has a collection efficiency of 100% and a number of reflections of solar rays prior to reaching the receiver less than or equal to two. In addition, it controls convection losses in such a way that this collector may be used at low temperatures.
The collector being proposed differs from current conventional parabolic geometry but keeps the structural requirements and has a similar or lower cost.
Regarding the plant having said multi-tube collectors, two options are considered: a plant with changes of state for direct steam generation and a plant without changes of state.
The direct steam generation plant uses a fluid that changes state during its course along the plant until it arrives at the turbine in the steam form. The most common fluid for this type of plants is water.
The claimed plant is designed by dividing the solar field into two parts: one area of the solar field is used for the generation of saturated steam at temperatures of approximately 300° C. and the other area of the solar field is used for the generation of superheated steam at temperatures of approximately 500° C.
The collectors operating in the saturated field at low temperatures are of the tubular parabolic trough type, with or without vacuum or of the parametric type having a multi-tube receiver without vacuum and are supplied by the supply water of the circuit at low temperatures. This steam is heated and passes from one collector to the next until it reaches the saturation temperature of about 300° C.
In order to improve the efficiency of the cycle, working at higher temperatures is convenient. For this reason, the collectors in the overheating field are of the parabolic trough or parametric cylindrical type having a tubular receiver with vacuum. Overheated steam at an elevated temperature (above 500° C.) is obtained at the outlet of these collectors. The fluid at this temperature is sent directly to the turbine in order to produce electricity.
The second type of concentration plant, the plant whose fluid and working temperature range do not produce changes of state, is designed by following a circuit with several collectors connected in series.
The first collectors, that is to say, those working at low temperatures, are parabolic trough collectors with or without vacuum or parametric collectors having a multi-tube receiver. The first of all of the collectors in the series is supplied with the supply fluid of the circuit at low temperatures. This fluid is heated and passes from one collector to the next until it reaches a temperature above 300° C.
Parabolic trough or parametric collectors with vacuum tubular receivers are connected in series with these collectors in order to work at higher temperatures. At the outlet of these collectors, the fluid has a temperature of above 500° C., with pressures reaching up to 200 bars whereby the heat carrying fluid is CO2. The fluid at this temperature is made to circulate through a heat exchanger to generate steam at temperature of approximately 500° C. or higher. This steam is sent to the turbine to produce electricity.
In order to complete the description being made and with the purpose of aiding better understanding of the invention, a set of drawings representing the following in an illustrative rather than limitative manner has been attached:
The references being:
In order to reach a better understanding of the invention, the thermosolar plant, object of the invention, as well as the multi-tube collector, will be described below according to a preferred embodiment.
Firstly and as shown in
The first collectors, that is to say, those working at low temperatures, are of the parabolic trough concentrator-type having a tubular receiver, with or without vacuum or parametric concentrator-type having a multi-tube receiver (1), the first of them being supplied by the supply fluid of the circuit (3) at a low temperature. This fluid is heated and passes from one collector (1) to the next, until it reaches a temperature above of 300° C.
A series of parabolic trough or parametric collectors with tubular receiver and with vacuum (2) is connected in series to the multi-tube collectors (1) to work at higher temperatures. At the outlet of these collectors, the fluid is found at an elevated temperature (4), above 500° C. The fluid at this temperature is used, thanks to a heat exchanger, to generate steam at temperatures of approximately 500° C. or above.
The fluid without change of state circulating through this type of plant may be CO2.
Secondly, for a thermosolar plant with fluid with change of state, such as water, for direct steam generation and as shown in the configuration in
Working at higher temperatures is convenient in terms of increasing the efficiency of the cycle. Therefore, the collectors of the overheating field (19) are of the parabolic trough or parametric type, with tubular receiver, with vacuum (2). At the outlet of these collectors, overheated steam at a high temperature (4), above 500° C., is obtained in form of steam. The fluid at this temperature is sent directly to the turbine for the production of electricity. As already indicated, the tubular collectors, whether parabolic trough or parametric and with or without vacuum, are known in the state of the art and therefore no figure representing them is included.
In terms of the new parametric cylindrical collector for multi-tube receiver, without vacuum or Multitube (1), it is shown in
As shown in
The structure is optimized to bear the different loads to which the collector is subjected and in addition, the collector's center of gravity is very close to the axis of rotation of the collector itself.
The geometry of the secondary reconcentrator (6) is optimized and the collection efficiency of the collector is 100%. Depending on the selected solution, all or the majority of the rays (8) are reflected on the secondary reflector (6) before reaching the receiver (1).
The multi-tube receiver (1) is formed by multiple tubes with a circular cross-section, without a vacuum (7), several design options being considered:
In any of the previous configurations, the tubes (7) are joined to each other by welding. In some cases one tube is welded (13) directly to another (see
Therefore, the claimed solar plant posits the combination of two different types of collectors, one new one and one known one, which assumes significant progress in the solar energy production sector and increases the efficiency thereof considerably by making each collector work within the most appropriate temperature range.
Number | Date | Country | Kind |
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P201001406 | Nov 2010 | ES | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/ES2011/000321 | 11/2/2011 | WO | 00 | 7/8/2013 |