2.2.2 All-CO 2 systems
2.2.2.2 Integrated CO 2 system solutions
In the integrated system solutions, both MT and LT circuits are connected to the same heat rejection circuit. The arrangement of the three circuits depends on the system solutions chosen. These are described in the following sections.
Transcritical CO2 booster system
A simplified diagram of a transcritical CO2 booster system is shown in Figure 2.10. The system is divided into 4 pressure levels. The high pressure is in the range between 60 and 90 bara. It incorporates the discharge of HT compressor, gas cooler and high pressure expansion device (ICMT). The intermediate pressure is from 30 to 40 bara. This section begins from downstream the ICMT valve incorporates the liquid receiver and the bypass valve (ETS) and ends at the expansion valves of the MT and LT circuits.
The medium pressure level ranges from 26 to 29 bara and includes the discharge of the LT compressor, the MT evaporator and suction of the HT compressor. The low pressure section in the pressure range from 12 to 14 bara incorporates the LT evaporator and suction line to the LT compressor.
The main function of the bypass valve (ETS) is to keep the pressure in the intermediate pressure section relatively constant by bypassing saturated gas from the receiver to the medium pressure section. The bypass valve also ensures a differential pressure exists
LT display cabinet Liquid
receiver
IHX Gas cooler
(b) LT system Inter
cooler ICMT
EXV MT display
cabinet Gas cooler
Liquid receiver
(a) MT system ICMT EXV
IHX
between the liquid line after the liquid receiver and the MT evaporator coils for the proper operation of the MT expansion valve.
Figure 2.10 All-CO2 booster system with gas bypass (Adapted from: Danfoss, 2008a)
In a CO2 booster system, the lower the intermediate pressure the better it is for system efficiency which can be explained as follows: The intermediate pressure does not influence the pressure ratio of the LT and HT compressors which means the power consumption of the compressors remains constant. The refrigeration effect of the MT and LT evaporators, however, will increase when the intermediate pressure reduces.
This will improve the refrigeration capacity of the systems which leads to a higher system efficiency.
Figure 2.11 All-CO2 booster system installed in the Refrigeration Laboratory, Brunel University MT cabinet
LT cabinet
ETS
ICMT IHX
Liquid receiver
compressorLT
compressors HT Gas cooler
EXV EXV
High pressure
Medium pressure Intermediate
pressure
Low pressure
LT compressor HT compressors
ICMT
Gas bypass valve
For the operation of the MT expansion valve, at least a 4 bar pressure difference is maintained over the MT evaporator pressure (Danfoss, 2008a). Other factors that influence the operation of the bypass valve and control of the intermediate pressure include the gas cooler exit pressure and temperature and the requirement to have superheated CO2 vapour at the entry to the HT compressor.
Figure 2.11 shows a small size plant with integrated all-CO2 booster system. The CO2
system employs an air cooled gas cooler, two parallel compressors for the high pressure system and a single compressor for the low pressure cycle. ICMT and ICM valves are utilised for the high pressure expansion device and the gas bypass valve respectively.
The CO2 plant incorporates a Danfoss controller for the system control and monitoring.
Cascade CO2 system with flash gas bypass
A cascade CO2 system with flash gas bypass is shown in Figure 2.12. The high stage or heat rejection section is similar to the booster system. The difference is that the LT system is cascaded as an individual circuit with the HT system for heat rejection. The MT evaporators are divided into two groups: one provides refrigeration to the MT display cabinets and the other condenses the CO2 gas for the LT systems.
Figure 2.12 Integrated cascade all-CO2 system with flash gas bypass
This solution has been implemented in a number of UK supermarkets (Campbell, 2009).
Performance investigations have shown its seasonal energy performance to be about the
MT display cabinets
LT pack LT display
cabinets
HT pack LT
condenser Gas cooler
HT liquid receiver
LT liquid receiver
Two LT condensers are cascaded to
other HT packs
IHX EXV
EXV EXV ICMT
ICM
same as that of conventional R-404A systems (Suamir and Tassou, 2010). In order to gain full advantage of subcritical operation when the ambient temperature is low a bypass valve is required in parallel with the ICMT valve (the bypass valve is not shown in the diagram). The bypass valve closes when the ambient temperature is above a certain value (the switching point between subcritical and transcritical operation) to allow the ICMT valve to regulate the flow of CO2 gas out from the gas cooler and opens below the switching point to allow the liquid CO2 to freely flow from the condenser (gas cooler) to the liquid receiver.
Cascade CO2 system with a suction receiver
The cascade CO2 system with low pressure receiver is an improvement on the cascade system with flash gas bypass. As can be seen in Figure 2.13, a suction receiver is added on the high temperature side of the system. A back pressure valve is used instead of bypass valve to minimise the fluctuation in the intermediate pressure due to pressure and temperature variations at the outlet of the gas cooler.
Figure 2.13 Integrated cascade all-CO2 system with suction receiver
With this arrangement the MT evaporators can be set at zero superheat which can increase the refrigeration capacity and the COP of the system. To ensure no liquid enters the compressors, the system is equipped with oil cooler HX at the suction line of
ICMT
HT Pack HT
Receiver
Gas cooler
Suction receiver
MT display cabinets
LT pack LT display
cabinets
condensersLT
LT liquid receiver
Two LT condensers are cascaded to other
HT packs
IHX EXV
EXV Back EXV
pressure valve
Oil cooler HX
each HT compressor. According to Campbell (2009) this system is adaptable to variable load; is less sensitive to charge and is easy to service.
Figure 2.14 presents an integrated cascade all-CO2 system with suction receiver implemented in a UK supermarket. The store has sales area of 2,300 m2. The CO2
refrigeration plant of this store constitutes 4 HT packs and 2 LT packs. Every single HT pack comprises 3 compressors, an air cooled gas cooler and a suction receiver (0.26 m3 volume). The LT pack incorporates 3 LT compressors, a liquid receiver (0.13 m3 volume) and 3 condensers. For reliability of the LT refrigeration system, each LT pack is cascaded to three different HT packs as shown in Figure 2.13.
Figure 2.14 Integrated cascade all-CO2 plant in Tesco Ramsey, UK
2.2.33 SSuummmmaarryy
This chapter outlines advantages and disadvantages of the CO2 as a natural refrigerant.
The CO2 refrigerant, with ODP of zero and GWP of one, has lower impact to the environment compared to HCFC and HFC refrigerants. Having attractive thermo-physical properties, the CO2 refrigerant can provide good heat transfer in heat exchangers of a refrigeration system which allows selection of smaller equipment than HCFC and HFC refrigerants. The CO2 refrigerant is also non-toxic and non-flammable which make it more advantageous than other natural refrigerants such as ammonia and hydrocarbons. Moreover, the CO2 refrigerant is relatively cheap.
LT-pack of 20 kW refrigeration duty HT pack of 40 kW
refrigeration duty
The main downside of the CO2 refrigerant is its high working pressures. The problem of the higher working pressure, however, can be overcome by using smaller and stronger components. Some practical techniques to protect a CO2 refrigeration system against high pressure are also explained in this chapter.
The chapter also describes different solutions and arrangements of the CO2 refrigeration systems for supermarket applications which fall into two major categories: subcritical cascade systems and transcritical systems. Subcritical cascade systems operate at moderate pressures and employ two refrigerants one for refrigeration and another for heat rejection whereas transcritical systems operate at high pressures at high ambient temperatures but employ only CO2 as refrigerant. Recent developments of the CO2
refrigeration systems and their applications in supermarkets are also presented.
The following chapter will explain the design and the construction of the test facility and will include the integration arrangement, system design and modelling, component calculations, the components used, refrigeration load system and the test chamber.
ChChaapptteerr 33
D D ES E SI I G G N N A A ND N D C CO ON NS S TR T RU UC CT TI IO ON N O OF F T TH HE E T TE ES ST T F F A A C C I I L L I I T T Y Y
A small size CO2 refrigeration system for low and medium temperature applications was designed and constructed to enable it to be integrated with the existing trigeneration system to form an overall test facility. Instrumentation and monitoring systems were also fitted to comprehensively monitor the performance of the system for evaluation purposes.
For design purposes, mathematical models were established in the Engineering Equation Solver (EES) platform which include pipe sizing, liquid receiver, evaporator-coil models and system integration. The EES models were used to determine the dimensions and capacity of the components. The models were also used to estimate the performance of the integrated system at different operating conditions. Based on the dimensions of the components a 3D drawing of the system was drawn using AutoCAD.
The CO2 refrigeration plant and the integration circuit were constructed in the Refrigeration Laboratory of Brunel University. Technical drawings were produced which include piping, instrumentation diagrams, liquid receiver, evaporator coils and electrical control systems.
This chapter presents the mathematical models produced for the design of the CO2
refrigeration system. The evaporator coil models will be presented separately in Chapter 4. This chapter also details the construction of the test facility which
incorporates mechanical, electrical, control and monitoring systems. The loading systems and the environmental test chamber are also briefly described.
3.3.11 IInntteeggrraattiioonn aarrrraannggeemmeenntt
The test facility consists of three main modules; CHP module, absorption refrigeration system module, and a refrigeration load module as described in Section 1.4. The main construction is very similar to the previous trigeneration test facility (see Figure A-1 in the Appendix A). The difference is only on the refrigeration load module. A subcritical CO2 refrigeration system with chilled and frozen food display cabinets was used to replace the water based secondary loop medium temperature display cabinet.
Figure 3.1 Integration arrangement of CO2 refrigeration and trigeneration systems
Figure 3.1 shows a schematic diagram of the integration arrangement of the CO2
refrigeration and trigeneration systems. The arrangement employs a water based secondary loop which bridges the CO2 refrigeration system to the absorption chiller of the trigeneration facility. Main components of the secondary loop include cascade condenser, brine pump and evaporator of the absorption chiller. The CO2 refrigeration system rejects heat to the secondary loop in the cascade condenser which is a brine/CO2
heat exchanger. The rejected heat is then released to the atmosphere from the condenser of the absorption chiller which is driven by recovered heat from the CHP system.
CHP Plant LT load MT load
Absorption chiller
Exhaust gas
Air Fuel Electricity
Boiler HX
Generator Set HTF
pump
CO2 Refrigeration section Cascade condenser Water-based
secondary loop
brine pump
3.3.22 MMaatthheemmaattiiccaall mmooddeellss
3.2.1 Integration model
The integration model uses the ambient conditions and required product temperature as boundary conditions. The temperature for the products was assumed to be -1 oC to 5 oC for medium temperature (MT) system and -15 oC to -18 oC for the low temperature (LT) system. Evaporating temperatures for the medium and low temperature systems were assumed to be -8 oC and -32 oC respectively. London weather data was assumed for the purposes of heat rejection.
The integration model involved all components which influenced the performance of the system and each component was treated as a single control volume. Mass and energy balance principles were applied to the control volumes which can be expressed as follows: