Patent Application
20160108763
This invention relates generally to power generation systems, and more particularly to a power generation system that uses supercritical carbon dioxide (SC-CO2) as the working fluid in a Rankine condensation power cycle with an integrated heat driven absorption refrigeration system (ARS) to condense the SC-CO2 without requiring an external cooling source.
During the last decade there has been a growing interest in supercritical carbon dioxide (SC-CO2) as a working fluid for the Brayton gas cycle. However, the use of SC-CO2 as a working fluid in a condensation power cycle (Rankine cycle) has been a challenge because of the low critical temperature (31° C.) of the SC-CO2, which makes it very difficult to be condensed in the absence of a source of cooling water or air with a temperature of about 10° C. Accordingly, this working fluid is rarely considered for a Rankine power cycle in spite of several advantages that CO2 may offer.
US patent application serial number 2012/0102996 attempts to solve this problem by using a Rankine cycle integrated at the desorber with an absorption chiller. In this system the desorber uses the heat available in the cycle working fluid after using an internal recuperator. All illustrations of the system disclosed in this application use a cooler and depend on an external cooling means such as water or atmospheric air to enhance the power cycle cooling.
US patent application serial number 2012/0125002 discloses a Rankine cycle integrated with an organic Rankine cycle and an absorption chiller cycle. In this application, a two Rankine or binary cycle power generator and a sort of cascaded heat utilization is proposed.
In a paper by H. Yamaguchi et. al., entitled “Solar Energy Powered Rankine Cycle Using Supercritical CO2”, published in 2006 in Applied Thermal Engineering 26 (2006) 2345-2354, the authors proposed using an ambient cooling system in two stages and direct heating through an evacuated solar collector.
Applicant is not aware of any prior system in which a Rankine power cycle with CO2 as the working fluid uses a heat driven absorption refrigeration system (ARS) to condense the CO2 at low temperatures, around −5° C., and that ensures continuous operation of the SC-CO2 Rankine power cycle independently of any external cooling water or other cooling media to condense the CO2.
It would be desirable to have a power generation system using a Rankine power cycle with SC-CO2 as the working fluid, in which an absorption refrigeration system (ARS) is integrated with the power cycle to condense the SC-CO2 without the need for an external low temperature cooling medium such as water or air.
For supercritical carbon dioxide (SC-CO2) to be used as a working fluid in a
Rankine cycle, a low temperature sink (around 15° C.) must be available. Satisfying this condition in many locations is almost impossible due to the variation in ambient temperature throughout the year. Applicant has developed an integrated cooling system derived from relatively low-grade thermal energy available in the system to continuously provide the cooling duties required by the power cycle, thus making the power cycle operation independent of environmental conditions and enabling the several benefits available through the use of SC-CO2 as the working fluid.
More specifically, the present invention is a power generation system comprising a Rankine power cycle with SC-CO2 as the working fluid, in which an absorption refrigeration system (ARS) condenses the SC-CO2 at a low temperature of around −5° C. without the need for an external low temperature cooling medium such as water or air.
The power generation system of the invention comprises two main subsystems: (1) a supercritical carbon dioxide (SC-CO2) Rankine power cycle; and (2) an integrated absorption refrigeration system (ARS). The SC-CO2 power cycle utilizes the thermal energy supplied by an external heat source to generate power, and the absorption refrigeration system cools the SC-CO2.
The foregoing, as well as other objects and advantages of the invention, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like reference characters designate like parts throughout the several views, and wherein:
As shown in
The supercritical carbon dioxide (SC-CO2) Rankine power cycle 41 has seven components: heater 43, reheater 44, a two stage turbine comprising high pressure turbine 45 and low pressure turbine 46, an internal heat exchanger 47, condenser 48, which is integrated with the cooling system 42, and a working fluid pump 49.
Heat transfer fluid enters the system at 3 and is split by a diverter valve 50 into two streams 5 and 7 for supply to the heater 43 and reheater 44, respectively. The two streams 6 and 8 leaving the heater and reheater, respectively, are combined at valve 51 into a single stream 9 that is fed through a desorber in the absorption refrigeration loop as described hereinafter.
The working fluid circulation loop of the Rankine condensation power cycle 41 comprises the heater 43 through which the heat transfer fluid and CO2 are circulated in heat exchange relationship to heat the CO2 to its supercritical temperature and pressure at first state 12. The high pressure turbine 45 is connected to receive the supercritical CO2 from the heater 43, and the CO2 expands in the high pressure turbine to a lower temperature and pressure at second state 13. The reheater 44 is connected to receive the second state CO2 from the high pressure turbine and heat it to a third state 14. The low pressure turbine 46 is connected to receive the CO2 from the reheater and expand the CO2 to a fourth state 15. The internal heat exchanger 47 is connected to receive the fourth state CO2 from the low pressure turbine and through which the fourth state CO2 passes and gives up some of its heat to leave the internal heat exchanger at a fifth state 16. Condenser 48 is connected to receive the CO2 from the internal heat exchanger and through which the fifth state CO2 passes and is condensed to a liquid sixth state 17. The working fluid pump 49 pumps the liquid CO2 back through the internal heat exchanger and to the heater 43 to repeat the cycle.
The absorption refrigeration system (ARS) 42, shown in
The ammonia/water absorption refrigeration circulation loop has ten components: the evaporator 48 (the condenser of the power cycle), an absorber 52, a condenser 53, a desorber 54, a rectifier 55, two heat exchangers 56 and 57, two expansion valves 58 and 59, and a solution circulation pump 60 (see
First, gaseous CO2 leaves the internal heat exchanger 47 at state point 19 and enters the heater 43, where it is heated to a high temperature (about 390° C.) by the heat transfer fluid (HTF) coming from the external heat source (the solar field 70 in the preferred example shown in
The CO2 expands in the low pressure turbine (LPT) 46 from state 14 to the pressure condenser level (about 3.77 MPa) at state point 15. Since the temperature of CO2 gas at state 15 is still high enough to be utilized, it is sent to the internal heat exchanger 47 to recycle its heat in a regenerative process before being sent to the condenser/evaporator 48. The CO2 is expected to leave the internal heat exchanger 47 at state 16 with a temperature of 22° C. and subsequently enter the condenser/evaporator 48 where the CO2 will undergo a refrigeration process and eventually is condensed to leave the condenser/evaporator as a liquid state 17 with a temperature of 3° C. At this stage, the liquid phase is easy to pump to the heater pressure level at state 18 with a reasonable power input to pump 49. The liquid CO2 is pumped to the internal heat exchanger 47 to be heated to state 19 by the hot stream coming from the low pressure turbine 46, and the cycle can then be repeated.
The operating principle of the power cycle 41 is based on an arrangement wherein heat is transferred to the system through a hot fluid (heat transfer fluid). In the heater 43 this heat is used to heat the CO2. The high temperature and pressure CO2 gas partially expands in the high pressure turbine 45 and is then sent to the reheater 44 to be heated and sent to the second stage turbine (low pressure turbine) 46 where CO2 gas expands to the condenser pressure. In order to increase the system efficiency, the available heat in the CO2 gas leaving the low pressure turbine is recovered through the internal heat exchanger 47 used to heat liquid CO2 coming from the feed pump 49. CO2 from the low pressure turbine leaves the internal heat exchanger at state 16 with a low temperature and is fed to the condenser 48 in which the CO2 is fully condensed and pumped back through the internal heat exchanger 47, increasing the pressure of the CO2 to the cycle's high pressure level. The pumped liquid goes through a transcritical phase change during the heating processes.
The operating principle of the absorption refrigeration system (ARS) 42 is as follows. The low grade heat available in the HTF streams 8 and 6 leaving the heater 43 and the reheater 44, respectively, is combined at valve 51 into a single stream 9 and further exploited by using it in the desorber 54 to condition the refrigerant to produce the required cooling effect in the power cycle condenser 48 to condense the CO2. Before returning to the external heat source (not shown in
In a preferred embodiment, the cycle is used as a power block in a concentrating solar power (CSP) plant and the overall plant is analyzed thermodynamically to assess its performance energetically and exegetically.
Use of a solar collector field 70 as the external heat source in the preferred embodiment is shown in
In operation, heat transfer fluid heated in the solar collectors is pumped through valves 74, 75 and 50 to the heater 43 and reheater 44 to heat the CO2 as discussed above. After passing through the desorber 53 the heat transfer fluid is returned to the solar collector field 70 to be reheated by solar energy. Heat transfer fluid with a reduced temperature as it leaves the power cycle circulation loop is returned to the solar collector field to be reheated. Some or all of the cooled heat transfer fluid can be diverted and pumped into the cold thermal energy storage tank 72 for eventual return to the solar collector field. Similarly, the heat transfer fluid heated by the solar collector field may be diverted by valve 74 into the hot thermal energy storage tank 73 to eventually be pumped through valves 75 and 50 into the power generation circulation loop.
The following discussion analyzes the performance of the integrated systems under different operating conditions. The mass, energy, and exergy balance equations are written for each component, and subsequently the energy losses, exergy destruction, and the energy and exergy efficiencies are evaluated.
The general forms of the mass, energy, and exergy balance equations over a control volume, enclosing involved components, are presented in the following under steady state conditions with neglected potential and kinetic energy changes.
Σ{dot over (m)}i=Σ{dot over (m)}e (1)
{dot over (Q)}−{dot over (W)}=Σ{dot over (m)}
e
h
e
−Σ{dot over (m)}
i
h
i (2)
Ėx
Q
−{dot over (W)}=Σ{dot over (m)}
e
ex
e
−Σ{dot over (m)}
i
ex
i
+Ėx
D (3)
where ĖxQ represents the net exergy transfer associated with the heat {dot over (Q)} transferred to/from the component at temperature T, which is calculated as
Ėx
Q=Σ(1−Ta/T){dot over (Q)} (4)
The specific exergy at point k is given by
ex
k
=h
k
−h
a
−T
a(sk−sa) (5)
and Ėk is the exergy rate at point k given by
Ėx
k
={dot over (m)} ex
k
={dot over (m)}[h
k
−h
a
−T
a(sk−sa)] (6)
The analysis of the system is conducted by solving the system's model under the assumption listed in Table 1. The software Engineering Equation Solver (EES), Klein, S. A., Engineering Equation Solver (EES) for Microsoft Windows Operating System; Academic Commercial Version, 2002, F-Chart Software: Madison, was used to model and obtain the properties of the different working fluids used in the system.
The heat transfer fluid used in the invention is Therminol-PV 1. Properties of this fluid can be found in Therminol®, Therminol VP-1 Vapor, Phase/Liquid Phase, Heat Transfer Fluid, [cited 2013; Available from http://www.therminol.com/pages/products/vp-1.asp. The mass flow rates, temperature and specific exergy of the HTF cycle are presented in Table 2 using the numbering system of
The state point data of the SC-CO2 cycle are listed in Table 3. The table presents mass flow rate, temperature, pressure, specific exergy, specific enthalpy, quality, and specific entropy for each point.
The main assumptions regarding the ARS that are made to facilitate the modeling listed as follows:
where {dot over (Q)}h, is the hot stream utility, and {dot over (Q)}c is the cold stream utility.
The properties of the state points of the absorption refrigeration cycle are listed in Table 4. The working fluid, mass flow rate, temperature, pressure, specific exergy, specific enthalpy, quality, specific entropy and concentration are identified according to each point.
Plots of some of the performance results for the system of the invention are shown in
The advantages of using CO2 as a working fluid according to the invention are apparent from the foregoing and include the following.
The advantages of using the absorption refrigeration system (ARS) in the invention are apparent from the foregoing description and include the following.
Assessment of the cycle performance of the invention was implemented in a solar system (CSP) and analyzed energetically and energetically. The performances of the cycle as well as the ARS were evaluated simultaneously under different operating conditions. The main conclusions from this study are summarized in the following points:
While particular embodiments of the invention have been illustrated and described in detail herein, it should be understood that various changes and modifications may be made in the invention without departing from the spirit and intent of the invention as defined by the appended claims.
