Patent Application
20120324885
1. Field of the Invention
The present invention generally relates to an apparatus and method for producing power using geothermal fluid in an Organic Rankin Cycle (“ORC”) system, and more particularly, to an apparatus and method to optimize heat release from brine extracted from a geothermal reservoir and minimize the ORC system equipment.
2. Description of the Related Art
In general, there is a constant drive to increase the operating efficiency of geothermal power plants. Geothermal fluid recovered from a geothermal reservoir typically contains a mixture of steam and hot liquid. The liquid is often in the form of brine. In many high temperature brine geothermal fields, the prior art practice has been to flash the steam at a lower pressure and separate the geothermal steam from the geothermal brine and utilize the steam in a steam turbine power system. Specifically, power was produced by using the steam in a traditional steam turbine while the hot brine along with the condensed steam condensate was generally returned back to the ground to replenish the geothermal field. For lower temperature fields, the use of a binary Organic Rankin Cycle (“ORC”) power system often utilize heat extracted from the hot brine to vaporize an organic working fluid, which in-turn drives a turboexpander/generator. More recently there has been limited use of a combined cycle approach, whereby the separated incoming hot brine is used to provide heat to a superheater of an ORC system before being returned back to the ground, while the steam is used in a conventional steam turbine. In these prior art systems, the exhaust steam from the steam turbine is first used to vaporize a preheated organic working fluid and then to preheat the working fluid. One drawback to the systems of the prior art is that they are inefficient in some applications in that they do not fully utilize the brine heat, particularly in geothermal fields where a significant amount of the extracted fluid's heat is contained in the brine. In other words, the prior art systems are more useful for “steam dominant” resources but are less effective in geothermal fields where a greater portion of the heat may be carried by the brine. For “brine dominant” resources, the prior art utilized ORC systems without a steam turbine topping cycle or the prior art utilizes more complex ORC designs such as multiple ORC systems or additional heat exchangers, thereby requiring more equipment and expense. Therefore, there is a need for a more efficient geothermal ORC power plant that maximizes use of heat from the steam as well as the extracted brine while minimizing the need for multiple ORC systems, reduces the number of turboexpanders and reduces the number of heat exchangers required for efficient operation of the power plant while also reducing the heat exchange path required for the steam condensate flow.
The present invention improves upon the prior art by allowing a more efficient means of using heat release curves from the separated steam and hot brine streams, thereby minimizing the need for multiple ORC systems and the number of heat exchangers through which the steam and hot brine must flow. More specifically, the present invention is generally directed to an improved method for recovering heat from a combined steam/hot brine resource by separating the steam from the hot brine and then utilizing the steam first to drive a steam turbine and then, utilizing the steam turbine exhaust, to provide heat to the working fluid vaporizer heat exchanger, which vaporizer is preferably sized to absorb enough heat from the steam turbine exhaust that the cooled steam condensate leaving the vaporizer may be returned to the ground without having to pass through any additional heat exchangers. The separated hot brine is utilized first to provide heat to the superheater through which vaporized working fluid passes and then to provide additional heat to the working fluid preheater, both of which are preferably sized so as to maximize heat extraction from the brine before the subcooled brine from the preheater is returned to the ground. Because the preheater extracts such a large amount of the remaining usable heat, the subcooled brine can be directly injected into the ground without the need to pass the brine through additional heat exchangers. Preferably, the vaporizer is sized to utilize most of the remaining heat in the steam condensate, such that there is no need to pass this steam condensate leaving the vaporizer though any additional heat exchangers. The condensed steam from the vaporizer may be mixed with the subcooled brine leaving the preheater and this mixed fluid stream may then be reinjected back into the reservoir to maintain hydraulic pressure. An important aspect of the invention is that the sizing of the heat exchangers, as well as the arrangement of the sequence of flow of all three fluids (separated brine, separated steam, and working fluid) must be accomplished in such a way that the heat from the geothermal liquid, as well as the heat contained in the separated steam, are utilized to closely match the heat release curve of the working fluid as discussed below. An additional advantage of using all of the steam condensate heat in a single heat exchanger, i.e., the vaporizer, rather than combining this condensate with the separated brine (to then flow as a combined stream through other heat exchangers) is that combining the streams would require equivalent pressure for both the steam condensate and brine, which requirement would restrict the operating conditions available to those two streams and result in a less optimized system.
The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:
According to Carnot Cycle theory, whenever there is a heat Q transferred from the hot temperature Th reservoir to cold temperature Tc reservoir, the Exergy E, the available energy that can be converted into power rather than heat, is defined by the following formula:
If the cold reservoir is at atmosphere temperature T0, the hot reservoir temperature is T1, and Exergy is E1, then the formula becomes:
Similarly, when heat Q is transferred from T2 (lower than T1) to T0 then,
Then during the heat transfer process from T1 to T2, an Exergy loss occurs.
Based on the foregoing, it is clear that exergy loss is proportional to the temperature difference between the two heat transfer fluids. This suggests it is preferable to design a process where the heat exchanger system employs a tight approach throughout the temperature range to minimize the exergy loss.
With reference to
With the process scheme described above, the objective of the invention is to arrange the preheater, vaporizer and superheater in such a way, and to design the heat exchanger surface area of all three of these exchangers, so as to optimize the temperature match between the heat source fluid and the working fluid throughout the temperature gradient so as to transfer as much heat as possible to the working fluid system for a given amount of heat exchanger surface area since this relates to cost. In other words, the preheater, vaporizer and superheater are designed and arranged so curve A and curve B match one another as much as possible to minimize the area between the two curves (5) while maintaining an approximately equal temperature difference between curve A and curve B throughout the entire heat exchange process. This will reduce the lost heat energy for a given amount of heat exchanger area thus maximizing utilization of heat from the available heat source and resulting in improved system performance. In one embodiment, the temperature difference between the two curves is no more than 20%, and in another preferred embodiment, the temperature difference is no more than 10%.
Referring to
The steam stream (20) leaves separator (16) and is first expanded in a steam turbine (22) to produce power. The steam turbine exhaust (26) is used to provide heat to vaporizer (28), which will heat and vaporize preheated working fluid (30) to produce vaporized working fluid (32). The steam turbine exhaust (26) upon cooling in vaporizer (28) is condensed back into steam condensate (34). The vaporizer heat exchanger (28) is preferably sized to extract a majority of the useful heat contained in steam turbine exhaust (26) such that the steam condensate (34) leaving the vaporizer (28) may be reinjected back into the ground without the need for further cooling, i.e., stream (34) need not be passed through additional heat exchangers before reinjection into the ground. The working fluid, which is preferably a fluid other than water, such as an organic fluid, is thus heated in three separate, sequential stages, which stages are arranged to maximize the heat release curve in all three heat exchangers.
More specifically, with continued reference to
In one embodiment, the expanded working fluid (44) is then directed to a condenser (46), where the working fluid will be condensed. Although not intended as a limitation of the invention, condenser (46) may utilize heat transfer with ambient air or from water or similar device to transfer heat from the expanded working fluid (44) to the ambient atmosphere. Alternatively, this heat may be utilized in a separate process which utilizes the rejected heat. In any event, the condensed working fluid (48) is circulated by a pump (49) back to the preheater (50) to repeat the process.
The subcooled brine (52) leaving the preheater (50) may be reinjected back into the reservoir along with the steam condensate (34) to help maintain the reservoir pressure. This reinjection may occur in two separate reinjection wells, or preferably, the subcooled brine (52) and the steam condensate (34) will be mixed together at 54 so they may be reinjected as a single stream into one or multiple reinjection wells (56). Those skilled in the art will appreciate that the arrangement of the three heat exchangers as described above maximizes heat transfer from geothermal fields where geothermal liquid is the dominant heat storage fluid of the field through the circuiting of the steam turbine exhaust and the hot brine so as to best match the heat release curve. Although not intended as a limitation of the invention, those skilled in the art will further appreciate that all three of the heat exchangers of the system, namely the preheater, the vaporizer, and the superheater, are preferably sized such that the differences in the heat release curves can be minimized and such that the most effective heat transfer can occur with the available exchanger surface area, thereby minimizing the need for additional heat exchangers.
In one preferred embodiment, the working fluid is contained in a closed loop system, such as for example, an ORC system. In one preferred embodiment, the working fluid is a refrigerant other than water. In another preferred embodiment, the working fluid is R-134a or R-245fa or ammonia. In another preferred embodiment, the working fluid is an organic fluid.
Referring now to
The system of the invention is particularly useful for geothermal fields where a significant portion of the field's heat is contained in hot liquid extracted, such as brine, from the field (as opposed to steam). In this regard, in certain embodiments, the system of the invention provides a “hot water dominant” system so as to maximize heat recovery from these types of geothermal fields.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
