Patent Application
20130160449
The present disclosure relates generally to Organic Rankine Cycle (ORC) systems and, more particularly, to a cascaded organic Rankine cycle.
The Organic Rankine Cycle (ORC) is a vapor power cycle with an organic fluid refrigerant instead of water/steam as the working fluid. The working fluid is heated in an “evaporator/boiler” by a source of waste or low quality heat. The fluid starts as a liquid and ends up as a vapor. The high-pressure refrigerant vapor expands in the turbine to produce power. The low-pressure vapor exhausted from the turbine is condensed then sent back to the pump to restart the cycle.
The simple rankine cycle used for power generation follows the process order: 1) Adiabatic pressure rise through a pump; 2) Isobaric heat addition in a preheater, evaporator and superheater; 3) Adiabatic expansion in a turbine; and 4) Isobaric heat rejection in a condenser, although other cycle modifications are possible such as the addition of a vapor-to-liquid recuperator.
A main thermodynamic irreversibility in organic Rankine cycles is caused by the large temperature difference in the evaporator between the temperature of the waste heat stream and the boiling refrigerant. The higher the waste heat stream temperature the greater this irreversibility becomes. One way to reduce this loss is to cascade two thermodynamic cycles together where a cycle operating at higher temperatures rejects heat to a cycle operating at lower temperatures.
A cascaded Organic Rankine Cycle (ORC) system according to an exemplary aspect of the present disclosure includes a bottoming cycle in thermal communication with a topping cycle through a condenser/evaporator in which a bottoming cycle working fluid is first evaporated and then superheated and a topping cycle working fluid is first desuperheated and then condensed such that a percentage of total heat transfer from the topping cycle fluid that occurs during a saturated condensation is equal to or less than a percentage of total heat transfer to the bottoming cycle fluid that occurs during a saturated evaporation.
A method of operating a cascaded Organic Rankine Cycle (ORC) system in which a bottoming cycle is in thermal communication with a topping cycle according to an exemplary aspect of the present disclosure which includes maintaining a percent saturation for a fluid in the topping cycle at less than a 40 percent saturation for a fluid in the bottoming cycle.
Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows:
The topping cycle 22 generally includes a power producing turbine 26 which is driven by the working fluid to drive a generator 28 that produces power. A refrigerant pump 30 increases the pressure of the working fluid from a condenser/evaporator 32. The heat exchanger group that transfers heat from the topping cycle 22 to the bottoming cycle 24 is referred to herein as the “condenser/evaporator” 32, although it should be understood that it may also include desuperheating and subcooling of the working fluid in the topping cycle 22, and preheating and superheating of the working fluid in the bottoming cycle 24.
An evaporator 34 such as a boiler receives a significant heat input from, for example, an oil circuit 36 to vaporize the Siloxane MM working fluid with the vapor thereof passed through to the turbine 26 to provide motive power. Upon leaving the turbine 26, the relatively lower pressure working fluid vapor passes to the condenser/evaporator 32 and is condensed by way of a heat exchange relationship with the bottoming cycle 24 such that the condenser/evaporator 32 operates as a condenser in the topping cycle 22 as well as an evaporator in the bottoming cycle 24.
In the disclosed non-limiting embodiment, the turbine 26 is a radial inflow turbine that expands the topping cycle working fluid vapor down to a lower pressure and generates power by the extraction of work from this expansion process. The vapor is still superheated so that its heat potential is utilized in the condenser/evaporator 32. The condenser/evaporator 32 actually de-superheats the working fluid and ultimately condenses the working fluid back to liquid for communication through the pump 30. The condensed working fluid is then circulated to the evaporator 34 by the pump 30 to complete the topping cycle 22.
The bottoming cycle 24 generally includes a power producing turbine 36 which is driven by the working fluid in the bottoming cycle and in turn drives a generator 38 that produces power. A refrigerant pump 40 increases the pressure of the working fluid from a recuperator 40. The bottom cycle working fluid is in thermal communication with a cooling system such as a water circuit 42 through a water cooled condenser 44.
By the nature of the proposed cycle, the vapor entering and leaving turbine 36 is highly superheated. The energy potential of the superheated vapor at the turbine exit is not wasted, but is fed into a recuperator 46. The recuperator 46 transfers heat from the low-pressure hot vapor from the turbine exit to the high pressure liquid at the pump exit.
The recuperator 46 uses this superheat to preheat the liquid working fluid downstream of the pump 40. That is, if a cycle is driven to high turbine inlet superheat, then turbine outlet superheat will be high. The availability of this heat is thereby captured to maintain cycle efficiency as the recuperator 46 is an internal heat exchanger. When the low pressure side of the topping cycle 22 is de-superheated, it is essentially recuperated into the bottoming cycle 24 which is where high superheat is achieved. Matching of the working fluids and the pressures thereof facilitates this interaction.
The recuperator 46 is only in the bottoming cycle 24. As the topping cycle 22 is not recuperated, its waste heat is captured by the condenser/evaporator 32. Both cycles are highly superheated yet avoid heat-exchanger pinches to minimize the heat-transfer temperature difference and minimize process irreversibility
{dot over (m)}
A(hA in−hA out)={dot over (m)}B(hB out−hB in)
Where the subscripts A and B refer to streams A and B respectively, m is the mass flow rate, and h is the enthalpy of the fluid.
In
The point where the temperature profile transitions from flat (saturated) to increasing (vapor) will be identified herein as the “knee.” For the above goals to be achieved, the “knee” of fluid A must lie equal to or slightly to the left of the “knee” of fluid B in the normalized enthalpy plot. If the “knee” lies far to the left then the saturated section may have a good heat transfer difference (typically 5 to 15 F; 3 to 8 C), but the heat transfer difference of the vapor section will be too large. If the “knee” lies too far to the right then a “pinch” condition will be created between the two fluids. Practically the temperature difference will increase and the saturated temperature difference will be too high.
The effect of the recuperator 46 on the condenser/evaporator 32 in the proposed cycle is to change the inlet enthalpy, and therefore temperature, of the colder fluid, B. By increasing the size of the recuperator 46, the enthalpy of the inlet of B increases by recovering heat from the turbine exit. This results in a smaller percentage of the total heat transfer for Fluid B occurring to the left of the knee, shifts the knee of B to the left and results in a pinch condition. Conversely, if the recuperator heat exchange is reduced or eliminated, this shifts knee of B to the right and therefore increases the temperature difference in the vapor section. That is, a percentage of total heat transfer from the working fluid in the topping cycle 22 that occurs during a saturated condensation is equal to or slightly less (within 10%) than a percentage of total heat transfer to the working fluid in the bottoming cycle 24 that occurs during a saturated evaporation.
The selection of a high superheat cascaded cycle with a condenser/evaporator heat exchanger transferring heat from the topping cycle to the bottoming cycle, and the selection of refrigerants for the topping and bottoming cycles and recuperator in the bottoming cycle allows for optimized heat exchanger temperature profiles.
It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting.
It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom.
Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present disclosure.
The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.
