US20100307169A1

US20100307169A1 - Trigeneration system and method

Info

Publication number: US20100307169A1
Application number: US12/611,888
Authority: US
Inventors: Gerardo C. Diaz
Original assignee: University of California
Current assignee: University of California
Priority date: 2007-11-15
Filing date: 2009-11-03
Publication date: 2010-12-09
Also published as: US20090126381A1

Abstract

A trigeneration system comprising a cooling loop and a heat/power loop connected by a heat exchanger. Energy available at the high side of the cooling loop is transferred from cooling loop to the heat/power loop by the heat exchanger. This energy is put to use by the heat/power loop to efficient produce heat and power. The system can be run transcritical and use environmentally friendly working fluid, such as carbon dioxide.

Description

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 11/941,037, filed Nov. 15, 2007, incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to heating, cooling, and/or power generation systems. More particularly, embodiments of the present invent relate to combined heating, cooling and power generation systems with a transcritical carbon dioxide cycle.

BACKGROUND INFORMATION

This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Ozone layer and/or global warming problems have focused considerable attention on the nature of refrigerants employed in refrigeration systems of various sorts. Some such systems, particularly those that do not have sealed compressor units are prone to refrigerant leakage. Older refrigerants, HFC 12, for example, are thought to cause depletion of the ozone layer while many of the replacements, HCFC 134a, for example, are believed to contribute to the so-called “greenhouse effect” and thus global warming.
As a consequence, a considerable effort is underway to develop refrigeration systems employing “natural” refrigerants such as carbon dioxide and/or propane. Carbon dioxide is plentiful in the atmosphere and may be obtained therefrom by conventional techniques and employed as a refrigerant in such systems. Should the systems leak the CO₂refrigerant, because it was originally obtained from the atmosphere, there is no net increase of the refrigerant in the atmosphere, and thus no increase in environmental damage as a result of the leak.
As carbon dioxide has a low critical point, systems utilizing carbon dioxide as a refrigerant usually require the refrigeration system to run partially above the critical point, or to run transcritical. Transcritical refrigeration systems operate at relatively high pressures and require, in lieu of a condenser in a conventional vapor compression refrigeration system, a gas cooler for the refrigerant. One of the drawbacks of this cycle is that it has a lower coefficient of performance (COP) than comparable refrigeration cycles operating at high ambient temperatures.
Cogeneration systems are systems which combine cycles to generate electricity together with refrigeration and/or heat. These systems offer the opportunity to reduce the cost of electrical energy for a building complex, factory, hospital, or local group, and to ensure the continuous availability of electrical energy during blackouts or “brownouts,” while simultaneously providing cooling and/or heating.
Today, many standard industrial processes use energy cogeneration to increase the efficiency of their heating and electrical generation processes. One application of a cogeneration systems is to use “wasted” energy, in the form of heat or exhaust, from a conventional power generation system, to heat or cool a liquid which, in turn, is used to heat or cool a building. While cogeneration is useful, much of the heat produced by the electrical and/or heat generation techniques goes unused. Moreover, many industrial applications also desire the production of a cooled fluid to utilize in cooling applications, such as air-conditioning of buildings. Therefore, there has been effort to utilize cogeneration waste heat to also generate a cooled fluid. This energy concept is called trigeneration. While attractive, there are many hurdles to producing an efficient trigeneration cooling system.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to an apparatus for providing cooling, heat and power, comprising a cooling loop for providing cooling, a heat/power loop for providing heat and power, and a heat exchanger connecting the cooling loop to the heat/power loop such that available energy from the cooling loop is transferred to the heat/power loop by the heat exchanger. The cooling loop can be a closed loop cooling system comprising cooling loop components, conduit connecting the cooling loop components, and cooling working fluid flowing through the conduit and cooling loop components. The cooling loop components can include a compressor for compressing the cooling working fluid, an expansion valve for expanding the cooling working fluid, and an evaporator for absorbing heat from ambient air into the cooling working fluid. The cooling loop components can also include a suction line heat exchanger and/or expander for increasing the coefficient of performance of the cooling loop. The cooling loop can be configured to run transcritical and, in one embodiment, the cooling working fluid can comprise carbon dioxide.
In one embodiment, the heat/power loop can be a closed loop system comprising heat/power loop components, conduit connecting the heat/power loop components, and heat/power working fluid flowing through the conduit and heat/power loop components. The heat/power loop components can include a pump for increasing the enthalpy of the heat/power working fluid, a turbine for producing power from the heat/power working fluid, and a condenser and hot water heat exchanger for heating water from energy extracted from the heat/power working fluid. The heat/power loop components can also include an auxiliary energy heat exchanger for adding auxiliary heat from an external system, for example a solar collector, to the heat/power working fluid. The heat/power loop can be configured to run transcritical and, in one embodiment, the heat/power working fluid can be carbon dioxide. In systems in which both the cooling and heat/power working fluids comprise carbon dioxide, the heat exchanger can comprise a carbon dioxide to carbon dioxide heat exchanger.
Another embodiment of the invention comprises a method for producing cooling, heat, and power. Cooling can be produced by compressing the cooling working fluid to produce high pressure/high enthalpy cooling working fluid, rejecting heat from the high pressure/high enthalpy cooling working fluid to produce high pressure/low enthalpy cooling working fluid, expanding the high pressure/low enthalpy cooling working fluid to produce low pressure/low enthalpy cooling working fluid, and absorbing heat from ambient air into the low pressure/low enthalpy cooling working fluid to cool the ambient air. Heat and power can be produced by increasing pressure of a heat/power working fluid to produce a high pressure/low enthalpy heat/power working fluid, adding the heat rejected from the high pressure/high enthalpy cooling working fluid to the heat/power working fluid to produce a high pressure/high enthalpy heat/power working fluid, using energy from the high pressure/high enthalpy heat/power working fluid to produce power, and using remaining energy from the heat/power working fluid to produce hot water. Additional energy can also be added to the heat/power working fluid from an external system. The external system can include solar collectors and/or the additional energy can comprise waste heat generated by the external system.
These and other advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a trigeneration system according to the present invention.

FIG. 2 is a graph illustrating pressure-enthalpy diagrams for one embodiment of a trigeneration system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described as being useful in the environment of a trigeneration system employing a trancritical cycle using a refrigerant such as CO₂. However, it is to be understood that the principals disclosed and claimed herein may find use in refrigeration systems using nontranscritical and/or conventional cycles. In addition, refrigerants other than CO₂can be employed. Accordingly, no limitation to transcritical refrigeration systems or CO₂refrigerants are intended except insofar as expressly stated in the appended claims.
Referring to FIG. 1, one embodiment of a trigeneration system according to the present invention is generally designated with reference numeral 100. The system 100 includes a refrigeration loop 102 and a heating/power loop 104, connected by a heat exchanger 106. The refrigeration loop 102 includes heat exchanger 106, as well as a compressor 108, an expansion valve 110, and an evaporator 112. The heat/power loop 104 includes a pump 114, a heat exchanger 116, a turbine 118, a second heat exchanger 120 and a condenser 122.
In the refrigeration loop 102, refrigerant is circulated through the closed loop system. In one embodiment, the refrigerant is CO₂. Because CO₂has a low critical point, systems utilizing CO₂as a refrigerant usually require the refrigeration loop to run transcritical. In this embodiment, refrigerant at point 1 is compressed above its critical point by a compressor 108. As a result, high pressure and high temperature refrigerant exits the compressor 108 at point 2. Heat is rejected from the high-pressure side of the refrigeration loop 102 to the heating/power loop 104 by the heat exchanger 106 which produces a low enthalpy-high pressure working refrigerant fluid at point 3. The working refrigerant fluid is then expanded in an expansion valve 110 to produce an expanded low pressure refrigerant at point 4. The refrigerant enters the expansion valve 110 through an expansion valve inlet 109 and exits through an expansion valve outlet 111. The expansion inlet 109 can be controlled to regulate the high side pressure to achieve the optimal coefficient of performance.
After expansion, the refrigerant enters an evaporator 112 through an evaporator inlet. In the evaporator 112, the refrigerant receives heat from ambient air. The ambient air flows through the evaporator in a direction opposite to, or perpendicular to, the flow of the refrigerant. A fan can be used to move the ambient air across the evaporator 112. After exchanging heat with the refrigerant, the cooled ambient air exits the evaporator. The refrigerant exits the evaporator 112 at high enthalpy and low pressure and temperature. The superheated refrigerant then re-enters the compressor 108, completing the refrigeration cycle. The environmental working conditions of the refrigeration loop 102 are defined, at least in part, by the ambient air temperature at the evaporator inlet.
In the heat/power loop 104, CO₂can also be used as the working fluid. A pump 114, can be used to increase the pressure of the working fluid between points a and b. Heat from the refrigeration loop 102 can be added at a constant pressure between points b and z by the heat exchanger 106. In one embodiment, where CO₂is used as the working fluid in both the refrigeration loop 102 and the heat/power loop 104, the heat exchanger 106 can be a CO₂-to-CO₂heat exchanger. If solar collectors and/or waste heat are available, then additional energy can be added by another heat exchanger 116 to increase the available potential to generate power at the turbine 118. Power is generated at the turbine 118 between points c and d. For example, the turbine 118 can include an output drive shaft which drives an electric generator for producing electricity.
In one embodiment, the temperature at point d remains high enough to allow the system 100 to produce hot water. A heat exchanger 120 and condenser 122 can be used to extract energy from the working fluid to heat up water flowing through the heat exchanger 120 and condenser 122. Water flows from outside the system 100 into the condenser 122 where it is heated from temperature T_w ⁱⁿto T_w ^out. It then flows through the heat exchanger where it is heated up from temperature T_w ^outto T_w ^HW. The heat exchanger 120 can be used to extract energy from the working fluid between points d and e. At point e, the working fluid is generally still in vapor state so the condenser 122 can be used to reject latent heat and complete the heat/power cycle.
The embodiment described above effectively utilizes the energy available at the high-side of the refrigeration loop 102 to produce power and hot water. This energy is usually wasted as heat rejected to the ambient in a standard air conditioning system. Instead, using heat exchanger 106, this energy is transferred to and utilized in the heat/power loop 104. By putting this available energy to use, the power obtained at the turbine 118 in the heat/power cycle is more than the power used by the pump 114 so the net COP of the refrigeration cycle can be increased. The system 100 can also be operated in winter as a heat pump. Since in this case the priority is to obtain very hot water for heating purposes, the location of the turbine 118 can be modified to be placed downstream of the heat exchanger 120. The cycle can operate with an environmentally friendly refrigerant, such as CO₂. In addition, the system 100 can also take advantage of waste heat or heat generated from the utilization of optional solar collectors. The system's COP can be further improved by adding a suction line heat exchanger and/or an expander in the refrigeration loop. The cycles can also be optimized in terms of operating pressures and amount of charge in the system 100 to suit particular applications.
FIG. 2 shows a pressure-enthalpy diagram for the combined refrigeration-heat/power cycles described above. For the illustrated set of operating conditions, an improvement in COP of 15% can be realized and the heat/power cycle can generate hot water at 40° C. Of course, performance numbers can vary depending on the operating conditions. As shown in FIG. 2, a vapor refrigerant exits the compressor 108 at high pressure and enthalpy, shown by point 2 in FIG. 2. As the refrigerant flows through the heat exchanger 106 at high pressure, it loses heat to the heat/power loop working fluid, exiting the heat exchanger 106 with low enthalpy and high pressure, indicated as point 3 in FIG. 2. As the refrigerant passes through the expansion valve 110, the pressure drops to point 4 in FIG. 2. The refrigerant passes through the evaporator 112 and exchanges heat with the outdoor air, exiting at a high enthalpy and low pressure, represented by point 1 in FIG. 2. The refrigerant is then compressed in the compressor 108 to high pressure and high enthalpy, completing the refrigeration cycle.
On the heat/power loop 104 side, low pressure-low enthalpy working fluid, at point a is pumped through pump 114 to produce high pressure-low enthalpy working fluid at point b. Excess heat from the cooling loop is passed through the heat exchanger 106 and is absorbed into the working fluid in the heat/power loop 104 side raising the temperature of the working fluid at point z. Additional energy can be added to the working fluid by heat exchanger 116. This additional energy can be waste heat (if available) or it can be supplied by solar collectors or other additional heat sources. This additional energy increases the enthalpy of the working fluid at point c as shown in FIG. 2.
The high pressure-high enthalpy working fluid available at point c can be fed into the turbine 118 to produce power. The working fluid exiting the turbine 118 at point d is low-pressure, but high enthalpy fluid which can be used to produce hot water. Heat exchanger 120 and condenser 122 can be used to extract energy from the working fluid to produce hot water. Partially heated water flows into the heat exchanger 120 and exits as hot water. As energy is extracted from the working fluid by the heat exchanger 120, the working fluid temperature drops from point d to point e on FIG. 2. The condenser 122 partially heats the water being fed into the heat exchanger 120. Latent heat rejected by the condenser 122 is absorbed by the water further reducing the enthalpy of the working fluid from point e to point a thus completing the heat/power loop.
The foregoing description of embodiments has been presented for purposes of illustration and description. The foregoing description is not intended to be exhaustive or to limit embodiments of the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various embodiments. The embodiments discussed herein were chosen and described in order to explain the principles and the nature of various embodiments and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products.

Claims

1. A method for producing cooling, heat, and power, comprising:

producing cooling by:

compressing cooling working fluid to produce high pressure/high enthalpy cooling working fluid;

rejecting heat from the high pressure/high enthalpy cooling working fluid to produce high pressure/low enthalpy cooling working fluid;

expanding the high pressure/low enthalpy cooling working fluid to produce low pressure/low enthalpy cooling working fluid; and

absorbing heat from ambient air into the low pressure/low enthalpy cooling working fluid to cool the ambient air;

and

producing heat and power by:

increasing pressure of a heat/power working fluid to produce a high pressure/low enthalpy heat/power working fluid;

adding the heat rejected from the high pressure/high enthalpy cooling working fluid to the heat/power working fluid to produce a high pressure/high enthalpy heat/power working fluid;

using energy from the high pressure/high enthalpy heat/power working fluid to produce power;

using additional energy from the heat/power working fluid to heat water to produce heat.

2. The method of claim 1 further comprising adding additional energy to the heat/power working fluid from an external system.

3. The method of claim 2 wherein the external system comprises at least one solar collector.

4. The method of claim 2 wherein the additional energy comprises waste heat generated by the external system.