The present invention relates to a method for optimising the efficiency of a transcritical cooling installation, and the installation itself.
Because of the adverse effects on the environment of refrigerants consisting of halogenated hydrocarbons or NH3, recent years saw a revival of the “old-fashioned’ refrigerant CO2. Under certain circumstances this has certain disadvantages. These can however be overcome by allowing the cooling cycle to be run transcritically, i.e. above as well as below the critical temperature. An example is the process described in U.S. Pat. No. 4,205,532. In a lot of literature attention is paid to the efficiency of the cooling process (COP, coefficient of performance) at full load. However often the COP is not only important at full load but also at partial load. This applies in particular to the cooling installations in the air conditioning industry and in particular for air treatment units.
A simple cooling cycle with. CO2 as a refrigerant is indicated in
In
The compressor sucks the CO2 gas from the CO2 evaporator at suction pressure Po(1) and increases the pressure to the discharge pressure Pd (2). In the CO2 cooler the CO2 gas is cooled from the discharge gas temperature (2) to temperature (3). Temperature (3) is a number of degrees (e.g. 5 K) above the entrance temperature of the medium with which the CO2 is cooled. After cooling the CO2 passes the high-pressure buffer vessel and the pressure of the CO2 is lowered from the discharge pressure to the suction pressure (4) by means of an expansion device. In the CO2 evaporator the liquid CO2 is evaporated, whereby the expansion devices assures that the CO2 gas leaves the evaporator superheated (1) (a couple of degrees e.g. 7 K above the corresponding evaporation pressure P0). Points (1), (2), (3) and (4) are also indicated in the mollier diagram.
Because of the particular course of the isotherms above the critical point different laws apply to the COP of transcritical CO2 installations compared to a subcritical installation. This will be clarified by means of
For transcritical CO2 installations the following aspects are important in order to achieve a maximum COP under partial load conditions:
In order to increase the thermodynamic efficiency (COP) of the system it is important to control the pressure in the high pressure part of the cooling cycle. The prior art supplies a number of methods for this. E.g. WO-A-97/27437 and WO-A-94/14016 propose to do this by varying the refrigerant charge of the system. However this does not achieve the desired improvement in the efficiency of the installation but only serves to avoid pressure problems during inactivity of the installation at high ambient temperatures.
Because of the fact that in CO2 installations the evaporation, like with halogenated hydrocarbons and NH3 takes place in the co-existence area, the same rules apply regarding the variation of the evaporation temperature.
In order to improve the COP at partial load the following operational conditions should be aimed for:
Ad 1. the increase of the evaporation temperature at partial load is countered by a reduction of the mass flow density, as a result of which the internal heat transfer coefficient (α) decreases. As a result the evaporation temperature increases less strongly than would be expected on the basis of the logarithmical temperature difference.
Ad. 2 at partial load the discharge pressure will decrease for two reasons:
The increase in the amounts of refrigerant in the evaporator is extracted from the high pressure side of the installation as a result of which the discharge pressure at partial load decreases.
In various patents and other scientific literature systems are described that superficially are comparable with the system according to the invention:
EP-A-1207361. In this system the pressure of the system is controlled but this is done by means of a valve at the discharge end of one or more cooler circuits. This will not lead to a higher COP because the disconnected circuit fills with the relatively cold CO2 with a high density. As a result the pressure will actually decrease in the cooler because less CO2 is available in the other circuits. According to the ideal gasses law the pressure will decrease in such cases.
Hafner et al, IIR-Gustav Lorentzen Conference on natural working fluids. Proceedings, XX, XX Jun. 2,1998, pp 335-345
The system described here consists of two separate evaporators. These are not controlled via superheating but directly control the pressure on the high-pressure side. The purpose of this system is not to optimise the COP but to easily and quickly vary the cooling/heating capacity.
US-A-2004123624. This is a system with two evaporators that work at different pressures. It is described that the pressure at the high-pressure side can be optimised via valves to achieve an optimum COP. However the two evaporators serve different spaces or parts of spaces.
U.S. Pat. No. 6,095,379. This system also has two evaporators and here too the system is controlled by directly opening or closing a valve.
US-A-2001037653 In one system two evaporators are used, each with its own evaporation pressure. One evaporator has an adjustable valve, the other evaporator uses a turbine. Both systems have their own compressor. Also a system with a turbine and an adjustable expansion valve is described.
All solutions mentioned above do not aim—nor are capable of—achieving and maintaining an optimum COP. However for reasons of energy economy this is desirable.
The purpose of the present invention is to optimise the COP of a transcritical installation at partial load. In order to achieve this the invention proposes an intelligent control of the installation, characterised in that the intelligent control system optimises
Another aspect of the present invention is that the COP can be further improved by optimising the difference between discharge and suction pressure by including an expansion vessel with an adjustable pressure in the system, connected to a superfeed in case of a screw compressor, and in case of multi-stage compression, set at one of the intermediate pressures.
Furthermore the invention offers a transcritically working cooling installation, comprising a compressor, cooler, one or more temperature transmitters, one or more pressure transmitters, one or more valves, a capacity control of the compressor (frequency control, cylinder or control valve) characterised in that it further comprises
An essential difference with the systems of the state of the art is that there separate evaporators are used for different spaces or parts of spaces (in order to achieve different temperatures in each), where as the invention uses one evaporator with several circuits for one space.
This fact alone makes it impossible to optimise the COP with the systems of the state of the art, because the different processes are going on in different evaporators under different circumstances.
Another aspect is that in the systems of the state of the art the superheating is a function of the pressure, and not the other way round as in the invention. By taking the superheating as the variable to be controlled it is possible to set the other variables in the system (pressure, compressor performance) in such a way that an optimum condition in terms of energy consumption and efficiency is always maintained.
The invention will be further explained by means of the following figures in which
a shows a circuit including a turbine
b shows a circuit with a high-pressure buffer vessel with adjustable intermediate pressure (simplified representation of
a-f steps in the control cycle
a full load
b increase of superheating
c increasing the suction pressure
d lowering of the discharge pressure
e disconnecting a circuit
f lower temperature
In these FIGS. the accents (e.g. 3″) have the following meaning
′≡ increased superheating
″≡ desired standard superheating with high suction pressure and lower discharge gas temperature
″′≡ lower discharge pressure
″″≡ higher discharge pressure
″″′≡ reduced load, further cooling
The starring point is a full load situation as represented in
The result is that a smaller cooling capacity is generated with a higher COP than at full load because
The above is illustrated by means of a example on the basis of
Evaporator
Spiralised copper pipe
Pipe pattern: Ø ⅜″* 1 mm (Cu alloy) 40 rows high, 8 rows deep
Fins: 0.3 mm Al
The pipes has been divided over 4 independently controllable circuits
CO2—gas cooler
The design is similar to that of the evaporator. The circuits are connected in a different way.
Compressor
From the above h will be apparent to a person skilled in the art that in this way it is possible to maintain an optimum COP under all circumstances, i.e. also at partial load. In other words the installation is working as efficiently as possible under all circumstances. By selecting the superheating as a control value instead of the pressure a situation is achieved in which the pressure of the system is at all times adapted to the targeted condition (temperature) to achieve maximum efficiency. Thus, it will also be appreciated that this cannot be achieved by the systems of the state of the art, where the pressure is adjusted only to achieve a certain ambient temperature under full load, not to run the installation efficiently under other circumstances.
In the above CO2 has been mentioned as a refrigerant but it will be obvious to the person skilled in the art that the invention can also be used on installations with other refrigerant with a low critical temperature. Also it will be apparent that variants and modifications are possible within the scope of the invention.
Number | Date | Country | Kind |
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1026728 | Jul 2004 | NL | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/NL05/00542 | 7/25/2005 | WO | 00 | 7/28/2008 |