Carbon dioxide—new uses for an old refrigerant

Carbon dioxide has been used as a refrigerant in vapour compression systems of many types for over 130 years, but it is only in the last decade that inventive minds and modern techniques have found new ways to exploit the uniquely beneficial properties of this remarkable substance. This paper traces the development of the old carbon dioxide systems, considers the technical, commercial and social reasons for their slow development and subsequent decline and examines the recent renaissance across a surprisingly broad range of applications, from trans-critical car air conditioners to low temperature industrial freezer plants. The paper then concentrates on industrial refrigeration systems, which were the basis of early developments in the period 1865–1885, but which have been somewhat overlooked in the current renaissance. The paper concludes with a review of possible future developments, indicating the areas of research and product development required to maximise the potential of the only non-toxic, non-flammable, non-ozone-depleting, non-global-warming refrigerant available for Rankine cycle vapour compression systems in the 21st century.     

Residential CO2 heat pump system for combined space heating and hot water heating

A theoretical and experimental study has been carried out for a residential brine-to-water CO₂ heat pump system for combined space heating and hot water heating. A 6.5 kW prototype heat pump unit was constructed and extensively tested in order to document the performance and to study component and system behaviour over a wide range of operating conditions. The CO₂ heat pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW.

The CO₂ heat pump was tested in three different modes: space heating only, DHW heating only and simultaneous space heating and DHW heating. The heat pump unit gave off heat to a floor heating system at supply/return temperatures of 33/28, 35/30 or 40/35 °C, and the set-point temperature for the DHW was 60, 70 or 80 °C. Most tests were carried out at an evaporation temperature of −5 °C, and the average city water temperature was 6.5 °C. The experimental results proved that a brine-to-water CO₂ heat pump system may achieve the same or higher seasonal performance factor (SPF) than the most energy efficient state-of-the-art brine-to-water heat pump systems as long as: (1) the heating demand for hot water production constitutes at least 25% of the total annual heating demand of the residence, (2) the return temperature in the space heating system is about 30 °C or lower, (3) the city water temperature is about 10 °C or lower and (4) the exergy losses in the DHW tank are small.     

Instability of automotive air conditioning system with a variable displacement compressor. Part 2. Numerical simulation

A test system is built first in order to investigate the instability of the automotive air conditioning (AAC) system with a variable displacement compressor (VDC), and hunting phenomena caused by the large external disturbance in the AAC system with a VDC and a thermal expansion valve, and in the AAC system with a VDC and a fixed-area throttling device are investigated experimentally in part 1 of this paper. The experimental results indicate that there also exist the hunting phenomena in the AAC system with a fixed-area throttling device. The system stability is found to be dependent on the direction of the external disturbance, and the system is apt to cause hunting when the condensing pressure decreases excessively since it may cause two-phase state at the throttling device inlet and make a large disturbance to the system. The piston stroke length will oscillate only when the oscillation amplitudes of forces acting on the wobble plate are great enough, otherwise the piston stroke length will be kept invariable, and then the system instability rule is also suitable for the AAC system with a fixed displacement compressor. From the experimental results, it is concluded that the two-phase flow at the throttling device inlet or at the evaporator outlet is the necessary condition but not sufficient condition for system hunting. Finally, a new concept, conservative stable region, is proposed based on the experimental results and theoretical analysis.     

A generalized correlation for refrigerant mass flow rate through adiabatic capillary tubes

A capillary tube is a common expansion device used in small sized refrigeration and air-conditioning systems. A generalized correlation for refrigerant flow rate in adiabatic capillary tubes is developed by implementing dimensionless parameters based on extensive experimental data for R-22, R-290, and R-407C measured in this study. Dimensionless parameters are derived from the Buckingham Pi theorem, considering the effects of tube inlet conditions, capillary tube geometry, and refrigerant properties on mass flow rate. The generalized correlation yields good agreement with the present data for R-22, R-290, and R-407C with average and standard deviations of 0.9 and 5.0%, respectively. Approximately 97% of the present data are correlated within a relative deviation of ±10%. Further assessments of the correlation are made by comparing the predictions with measured data for R-12, R-134a, R-152a, R-410A, and R-600a in the open literature. The correlation predicts the data for those five refrigerants with average and standard deviations of −0.73 and 6.16%, respectively.     

A DDC-based capacity controller of a direct expansion (DX) air conditioning (A/C) unit for simultaneous indoor air temperature and humidity control – Part II: Further development of the controller to improve control sensitivity

The development of the novel direct digital control (DDC)-based capacity controller for a direct expansion (DX) air conditioning (A/C) unit having variable-speed compressor and supply fan to simultaneously control indoor air temperature and relative humidity (RH) in a conditioned space served by the DX A/C unit has been reported in Part I of the two-part series. The results of preliminary controllability tests for the novel capacity controller presented in Part I, however, suggested that the controller developed was operational, with acceptable control accuracy but rooms for improvement with respect to control sensitivity. This paper, the second part of the two-part series, reports on the further development of the controller to improve its control sensitivity and the associated controllability test results. Both control accuracy and reasonable control sensitivity were achieved by incorporating a traditional Proportional–integral (PI) controller into the DDC-based capacity controller.     

Choice of sink for air conditioning systems. Part 2

The technical superiority of water-cooled air conditioning systems for different applications in terms of overall power rating have been established by the authors in an earlier publication. This paper deals with the generalized approach for evaluating the techno-economic choice of sink for an air conditioning system. A single parameter of unit cooling cost in fils/kWh_c (100 fils = 1 Kuwaiti dinar = US$3.25), combining the system's technical performance and economic aspects, is an effective cost-benefit criterion for making the choice for the type of sink. The results of an analysis, applied to example case studies in Kuwait, favour the water-cooled system for both the direct expansion (mostly used in the residential sector) and the chilled water (commonly used for non-residential applications) air conditioning systems.     

Seasonal performance of a central air conditioner in Dhahran, Saudi Arabia. A computer simulation study

Seasonal performance of four standard, residential-sized, central air conditioners in a typical hot and humid climate of Gulf environment was measured. A computer model was developed to stimulate the residential building cooling loads and the resulting seasonal energy consumption. For an average 100 m² residence located in Dhahran, equipped with a 17.6 kW air conditioner, simulations show a consumption of ≈22200 kWh of electricity for space cooling over 2600 operation hours, this being within 13% of the measured value. The seasonal performance of three lower capacity (10.5, 12.3 and 14.0 kW) air conditioners for the same duty was also predicted. With the 14.0 kW capacity unit the electricity consumption is reduced by ≈ kWh and can satisfy the load for 96.6% of the time. The smallest (10.5kW) of the four systems can supply the house cooling load for only ≈78% of the time with a seasonal energy consumption of 17 350 kWh.     

Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner

As per the Montreal Protocol, CFCs and HCFCs are being phased out. HCFC-22 is used in window air conditioners. This paper presents the experimental performance study of a window air conditioner with propane (HC-290), a natural refrigerant, as a drop-in substitute to HCFC-22. Experimental results showed that HC-290 had 6.6% lower cooling capacity for the lower operating conditions and 9.7% lower for the higher operating conditions with respect to HCFC-22. The coefficient of performance for HC-290 was 7.9% higher for the lower operating conditions and 2.8% higher for the higher operating conditions. The energy consumption of the unit with HC-290 was lower in the range 12.4–13.5% than HCFC-22. The discharge pressures for HC-290 were lower in the range 13.7–18.2% than HCFC-22. For HC-290, the pressure drop was lower than HCFC-22 in both heat exchangers.This paper also presents simulation results for the heat exchangers of an HCFC-22 window air conditioner with HC-290 as a drop-in substitute. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by NIST [National Institute of Standards and Technology. EVAP-COND: simulation models for finned-tube heat exchangers, Maryland, USA (2003). http://www2.bfrl.nist.gov/software/evap-cond/ [18]]. The simulated evaporator capacities are within ±4% of the experimentally measured cooling capacities for both refrigerants. Simulation results for HC-290 and HCFC-22 are compared. The exit temperatures of HC-290 are lower by 0.3–1.2 °C in the condenser and are higher by 2.1–2.4 °C in the evaporator than HCFC-22. Evaporating pressures of HC-290 are lower by 2.1–3.3% as compared to HCFC-22. The pressure drops of HC-290 are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and HC-290 in both heat exchangers are nearly the same.     

The use of CFD to improve the performance of a chilled multi-deck retail display cabinet

Maintaining food temperatures below critical values is the key to maximising the high quality display life of chilled foods. Studies were carried out to see if computational fluid dynamic (CFD) modelling could be used to rapidly identify the changes that would be required to an existing multi-deck display cabinet so that it would meet a higher test specification. Implementing the changes on a Pastorfrigor MV 200TP display cabinet reduced the average power consumption from 1.37 to 1.29 kW as well as significantly reducing the number of test packs which spent any time above 4 °C, from 12 to 1.     

A novel experimental investigation of a solar cooling system in Madrid

This paper reports novel experimental results derived through field testing of a part load solar energized cooling system for typical Spanish houses in Madrid during the summer period of 2003. Solar hot water was delivered by means of a 49.9 m² array of flat-plate collectors to drive a single-effect (LiBr/H₂O) absorption chiller of 35 kW nominal cooling capacity. Thermal energy was stored in a 2 m³ stratified hot water storage tank during hours of bright sunshine. Chilled water produced at the evaporator was supplied to a row of fan coil units and the heat of condensation and absorption was rejected by means of a forced draft cooling tower. Instantaneous, daily and period energy flows and energy balance in the installation is presented. System and absorption machine temperature profiles are given for a clear, hot and dry day's operation. Daily and period system efficiencies are given. Peak insolation of 969 W m⁻² (at 12:30 solar time on 08/08/03) produced 5.13 kW of cooling at a solar to cooling conversion efficiency of 11%. Maximum cooling capacity was 7.5 kW. Cooling was provided for 8.67 h and the chiller required a threshold insolation of 711 W m⁻² for start-up and 373 W m⁻² for shut-down. A minimum hot water inlet temperature to the generator of 65 °C was required to commence cold generation, whereas at 81 °C, 6.4 kW of cooling (18.3% of nominal capacity) was produced. The absorption refrigeration machine operated within the generation and absorption temperature ranges of 57–67 and 32–36 °C, respectively. The measured maximum instantaneous, daily average and period average COP were 0.60 (at maximum capacity), 0.42 and 0.34, respectively. Energy flows in the system are represented on a novel area diagram. The results clearly demonstrate that the technology works best in dry and hot climatic conditions where large daily variations in relative humidity and dry bulb temperature prevail. This case study provides benchmark data for the assessment of other similar prototypes and for the validation of mathematical models.