Effect of buckling on the cooling performance of free-standing planar thermoelectric coolers

This article investigates the effect of buckling on the cooling performance of planar thermoelectric (TE) coolers (TECs). The TEC is made up of n-type and p-type TE elements with large length-to-thickness ratio. Each TE element is modeled as a fixed–fixed thin plate. Theoretical model for the solutions of temperature and electric potential fields of the TE element after buckling is established. The corresponding coefficient of performance (COP) that indicates the cooling performance of TEC is also given. Influence of Seebeck coefficient, thermal conductivity, temperature difference, and the ratio of length-to-thickness on the cooling performance are discussed. It is found that buckling of TEC will reduce its cooling performance. A bigger Seebeck coefficient and smaller thermal conductivity can both improve the value of COP. It is also found that there is no maximum COP when the temperature difference across the TEC is zero. However, the effect of buckling on the cooling performance of TEC can be ignored if the TEC achieves the maximum COP. The peak value of COP is independent of the ratio of length-to-thickness of the TEC. An optimized value of the electric current corresponding to the maximum COP of the TEC is obtained.     

Improved energy performance of air cooled centrifugal chillers with variable chilled water flow

This paper considers how to apply optimum condensing temperature control and variable chilled water flow to increase the coefficient of performance (COP) of air cooled centrifugal chillers. A thermodynamic model for the chillers was developed and validated using a wide range of operating data and specifications. The model considers real process phenomena, including capacity control by the inlet guide vanes of the compressor and an algorithm to determine the number and speed of condenser fans staged based on a set point of condensing temperature. Based on the validated model, it was found that optimizing the control of condensing temperature and varying the evaporator’s chilled water flow rate enable the COP to increase by 0.8–191.7%, depending on the load and ambient conditions. A cooling load profile of an office building in a subtropical climate was considered to assess the potential electricity savings resulting from the increased chiller COP and optimum staging of chillers and pumps. There is 16.3–21.0% reduction in the annual electricity consumption of the building’s chiller plant. The results of this paper provide useful information on how to implement a low energy chiller plant.     

Heat-exchanger design and switching-frequency effects on the performance of a continuous type solar adsorption chiller

A transient one-dimensional model, capable of describing the performance of a newly-introduced adsorption chiller with continuous operation, is developed. Since the cycle time and the switching frequency have a great influence on chiller performance, a non-dimensional switching frequency is introduced and a systematic parametric study is carried out in order to determine regions of optimal operation. An optimization based on the thermodynamic efficiency yields a lower switching frequency than an optimization based on the maximum cooling capacity. In addition, the effect of the heat-exchanger design parameters on system performance is explored. An increase of either the bed’s Fourier number or the thermofluid’s Nusselt number has a positive effect on both COP and cooling capacity. An improvement of system performance can also be achieved by decreasing either the thermofluid’s Fourier number or the bed’s Biot number. Finally, the effect of space velocity of the thermofluid exhibits the most interesting behavior; an increase of the space velocity has a positive effect on cooling capacity and a negative effect on COP.     

Cooling performance of a vertical ground-coupled heat pump system installed in a school building

This paper presents the cooling performance of a water-to-refrigerant type ground heat source heat pump system (GSHP) installed in a school building in Korea. The evaluation of the cooling performance has been conducted under the actual operation of GSHP system in the summer of year 2007. Ten heat pump units with the capacity of 10 HP each were installed in the building. Also, a closed vertical typed-ground heat exchanger with 24 boreholes of 175 m in depth was constructed for the GSHP system. To analyze the cooling performance of the GSHP system, we monitored various operating conditions, including the outdoor temperature, the ground temperature, and the water temperature of inlet and outlet of the ground heat exchanger. Simultaneously, the cooling capacity and the input power were evaluated to determine the cooling performance of the GSHP system. The average cooling coefficient of performance (COP) and overall COP of the GSHP system were found to be ～8.3 and ～5.9 at 65% partial load condition, respectively. While the air source heat pump (ASHP) system, which has the same capacity with the GSHP system, was found to have the average COP of ～3.9 and overall COP of ～3.4, implying that the GSHP system is more efficient than the ASHP system due to its lower temperature of condenser.     

Experimental investigation and numerical analysis for one-stage thermoelectric cooler considering Thomson effect

This study conducts experimental investigation and numerical analysis for one-stage thermoelectric cooler (TEC) considering Thomson effect. Three Seebeck coefficient models are applied to numerically and experimentally study the Thomson effect on TEC. Results show that higher current, higher hot side temperature, or lower heat load can increase the temperature difference between the cold and hot sides. Opposite trends are found for COP. Specific current should be chosen as the upper threshold in thermoelectric cooler design. The cooling performance can improve when the Thomson heat maintains positive.     

Change in energy performance as a result of a R422D retrofit: An experimental analysis for a vapor compression refrigeration plant for a walk-in cooler

In this paper, the energy performance of a walk-in cooler working with R22 and its substitute R422D are experimentally studied. The experimental investigation was carried out considering three different operating conditions; in particular, the AHRI standard has been used as reference for operating conditions. All tests were run at steady state conditions and keeping the external air temperature at 35 °C. The experimental analysis allowed the determination of cooling capacity, the electrical power absorbed, the COP and other variables characterizing the working of the plant. The results demonstrated that the cooling capacity for R422D was lower than for R22, while the electrical power absorbed with R422D was higher than that with R22. As consequence, the COP of R422D was lower than that of R22. Furthermore, technical proposals are introduced with the aim of improving the overall performances of those plants, which could be retrofitted with R422D.     

A solar ejector cooling system using refrigerant R134a in the Athens area

This paper describes the performance of an ejector cooling system driven by solar energy and R134a as working fluid. The system operating in conjunction with intermediate temperature solar collector in Athens, is predicted along the 5 months (May–September). The operation of the system and the related thermodynamics are simulated by suitable computer codes and the required local climatologically data are determined by statistical processing over a considerable number of years. It was fount that the COP of ejector cooling system varied from 0.035 to 0.199 when the operation conditions were: generator temperature (82–92 °C), condenser temperature (32–40 °C) and evaporator temperature (−10–0 °C). For solar cooling application the COP of overall system varied from 0.014 to 0.101 with the same operation conditions and total solar radiation (536–838 W/m²) in July.     

An investigation of thermoelectric cooling devices for small‐scale space conditioning applications in buildings

This paper presents the study of a thermoelectric cooler (TEC) designed for small‐scale space conditioning applications in buildings. A theoretical study was undertaken to find the optimum operating conditions, which were then applied in the laboratory testing work. A TEC unit was assembled and tested under laboratory conditions. Eight pieces of UltraTEC were shown to generate up to 220 W of cooling with a COP of 0.46 under the input current of 4.8 A for each module. Thermo‐economical analysis was carried out and results showed that a system with PV panel can compete with an equivalent system without a PV panel when PV costs fall down to or lower than £1.25 per Watt. For the cases without a PV panel, the system with a high level of TEC power input delivered a better performance in terms of the average cooling energy price than that system with a low level of TEC power input after critical interest rate (currently 4%). Copyright © 2009 John Wiley & Sons, Ltd.     

Feasibility of driving convective thermal wave chillers with low-grade heat

A theoretical investigation of a convective thermal wave adsorption chiller was completed. The working pair was activated carbon–methanol. The predicted axial profiles of loading and temperature exhibited the same features as those reported for ammonia-activated carbon beds. For practical purposes, the coefficient of performance (COP) and a dimensionless cooling power were insensitive to the heat capacity of the refrigerant vapour, or the effective thermal conductivity of the refrigerant. With regard to the bed, increasing either its effective heat capacity or its effective axial conductivity strongly impeded performance. The COP and the dimensionless cooling power were mapped against two composite dimensionless groups, formed from a Stanton number, a ratio of interphase heat transfer to axial conduction, and the group aL where a is surface area per unit volume and L is bed length. Realistic pumping power was possible only at the expense of relatively large machines and poor COP; the two attributes that the convective thermal wave machines are intended to enhance. The results discouraged the building of costly experiments.     

Performance characteristics of a magnetic Stirling cooler

The quantum mechanics model of a magnetic Stirling cooler is established. The optimal relation between the cooling load and the coefficient of performance (COP) is obtained. The maximum cooling load and corresponding COP as well as the maximum COP and the corresponding cooling load are derived. The optimization criteria for a magnetic Stirling cooler are determined. The results obtained here will be useful for the further understanding and the selection of the optimal operating conditions for a magnetic Stirling cooler. Published in 2002 by John Wiley & Sons, Ltd.     

Integrating high-temperature proton exchange membrane fuel cell with duplex thermoelectric cooler for electricity and cooling cogeneration

To harvest the waste heat from exothermic reaction processes, a novel hybrid system model mainly incorporating a high-temperature proton exchange membrane fuel cell (HT-PEMFC) and a duplex thermoelectric cooler is conceptualized to theoretically predict the potential performance limit. The duplex thermoelectric cooler is composed of a thermoelectric generator (TEG) and a thermoelectric cooler (TEC), where the TEG harvests the waste heat to generate electricity and the TEC utilizes the generated electricity for cooling production. A mathematical model is established to estimate the proposed system performance from both exergetic and energetic perspectives considering various irreversible effects, from which effectiveness and practicality of the proposed system can be examined. The hybrid system maximal output power density allows 14.1% greater than that of the basic HT-PEMFC, whereas the according destruction rate density of exergy is decreased by 7.7%. The feasibility and effectiveness of the proposed system configuration are verified. Moreover, substantial parametric analyses indicate that the proposed system performance can be improved by elevating the HT-PEMFC operating temperature, inlet relative humidity and doping level while worsened by enhancing the leak current density, electrolyte thickness and Thomson coefficient. The results acquired may be helpful in designing and optimizing such an actual hybrid system.     

The influence of heat resistance and heat leak on the performance of a four-heat-reservoir absorption refrigerator with heat transfer law of Q∝Δ(T)

On the basis of an endoreversible absorption refrigeration cycle model with Newton's heat transfer law, an irreversible four-heat-reservoir cycle model with another linear heat transfer law of Q∝Δ(T⁻¹) is built by taking account the heat leak and heat resistance losses. The fundamental optimal relation between the coefficient of performance (COP) and the cooling load, the maximum COP and the corresponding cooling load, as well as the maximum cooling load and the corresponding COP of the cycle with another linear heat transfer law coupled to constant-temperature heat reservoirs are derived by using finite-time thermodynamics. The optimal distribution relation of the heat-transfer surface areas is also obtained. Moreover, the effects of the cycle parameters on the COP and the cooling load of the cycle are studied by detailed numerical examples. The results obtained herein are of importance to the optimal design and performance improvement of a four-heat-reservoir absorption refrigeration cycle.     

Experimental study of horizontal ground source heat pump performance for mild climate in Turkey

The aim of this study is to evaluate the performance of horizontal GSHP by considering various system parameters for winter climatic condition of Bursa, Turkey. For this purpose, a previously used experimental facility on cooling cycle [Coskun S, Pulat E, Unlu K, Yamankaradeniz R. Experimental performance investigation of a horizontal ground source compression refrigeration machine. International Journal of Energy Research 2008; 32: 44–56] was modified for the heating cycle. Soil thermal conductivity estimation was expanded and discussed. Preliminary numerical temperature distribution around GHE pipes was obtained. Tests were performed under laboratory conditions for space heating from December 2004 to March 2005. Variations of entering and leaving antifreeze solution temperatures, extracted heat from ground and rejected heat to the test room, super heat rate in evaporator and subcooling rate in condenser, total power consumption and coefficient of performance (COP) values for both the entire system and only heat pump unit (HPU) were obtained. Effect of outdoor temperature on system capacities and COP values with respect to outdoor air and mean soil temperatures were also presented. The COP of the entire system and HPU lie between 2.46–2.58 and 4.03–4.18, respectively. GSHP system was compared to conventional heating methods in the economical analysis and it was shown that the GSHP system is more cost effective than the all other conventional heating systems.     

Solar-assisted absorption heat pumps feasibility

An absorption system can be used for space cooling as well as for space heating. This dual purpose may be achieved by using the system as heat pump in wintertime. Absorption heat pump heating may be an interesting alternative, particularly for countries where there is a shortage of electric power.When an absorption unit is used as heat pump, its mode of operation is not modified: the internal temperatures of the cycle are only raised. Commercially available LiBr units were tested as heat pumps. COP and heating capacity were considered as a function of cold source temperature for different temperatures of the useful heat. The COP arrived at 1.7, which must be considered a high value for a thermally driven heat pump.Simulations were carried out in order to compare the performance of “conventional” solar, solar assisted heat pump and the combined series system under two different climate conditions. The series system showed performance 25–75 per cent better than “conventional” solar alone.     

The effects of operation parameter on the performance of a solar-powered adsorption chiller

A solar-powered adsorption chiller with heat and mass recovery cycle was designed and constructed. It consists of a solar water heating unit, a silica gel-water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller includes two identical adsorption units and a second stage evaporator with methanol working fluid. The effects of operation parameter on system performance were tested successfully. Test results indicated that the COP (coefficient of performance) and cooling power of the solar-powered adsorption chiller could be improved greatly by optimizing the key operation parameters, such as solar hot water temperature, heating/cooling time, mass recovery time, and chilled water temperature. Under the climatic conditions of daily solar radiation being about 16–21 MJ/m², this solar-powered adsorption chiller can produce a cooling capacity about 66–90 W per m² collector area, its daily solar cooling COP is about 0.1–0.13.     

Performance comparison of natural refrigerants based cascade systems for ultra-low-temperature applications

Theoretical analyses and optimisation are carried out with ethane, ethylene and nitrous oxide as the low-temperature (LT) fluids in a cascade system for ultra-low-temperature refrigeration applications to examine the effects of design and operating parameters. Finally, performance improvement has been investigated employing an internal heat exchanger. Optimal intermediate temperature (IT) correlations have been developed. Ethane is superior in terms of coefficient of performance (COP), whereas nitrous oxide is superior in terms of volumetric cooling capacity as an LT fluid. With increase in compressor efficiency, COP increases; however, with little influence on the optimum IT. Using an internal heat exchanger in the LT circuit, the cooling COP can be increased for ethane and ethylene; however, there is marginal decrease in COP for N ₂O. Ammonia is not suitable as a HT fluid for some operating conditions where the optimum IT is lower than the normal boiling point and propylene may be a suitable substitute with a penalty on COP.     

Effects of operating parameters on the performance of a CO2 air conditioning system for vehicles

This paper deals with the effects of the operating parameters on the cooling performance that can be applied for a transcritical CO₂ automotive air conditioning system. The experimental conditions of the performance tests for a CO₂ system and components such as a gas cooler and an evaporator were suggested to compare with the performance of each at the standardized test conditions. This research presents experimental results for the performance characteristics of a CO₂ automotive air conditioning system with various operating conditions such as different gas cooler inlet pressures, compressor speeds and frontal air temperatures/flow rates passing through the evaporator and the gas cooler. Experimental results show that the cooling capacity was more than 4.9 kW and coefficient of performance (COP) was more than 2.4, at each optimum pressure of gas cooler inlet during idling condition. Also, the cooling capacity was about 7.5 kW and COP was about 1.7 at the optimum pressure of gas cooler inlet during driving condition when air inlet temperatures of gas cooler and evaporator were 45 °C and 35 °C, respectively. Therefore, we concluded that the automotive air conditioning system using CO₂ refrigerant has good performance. This paper also deals with the development of optimum high pressure control algorithm for the transcritical CO₂ cycle to achieve the maximum COP.     

An irreversible-thermodynamic model for solar-powered absorption cooling systems

The ideal three-heat-reservoir cycle with constant internal irreversibilities and external heat transfer irreversibilities is used to model the absorption refrigeration machine of a solar operated absorption cooling system. Analytical expressions are obtained for the variation of the entropy transfer with storage tank temperature and the variation of the coefficient of performance (COP) with the cooling capacity of the plant. These expressions give the operating points for the maximum cooling capacity and the maximum COP. The results for ideal irreversible cycles are compared with those obtained by detailed simulation of the absorption cooling system. The effect of internal and external irreversibilities on the second-law efficiency of the plant is examined. The ideal cycles that include internal and external irreversibilities are found to give realistic limits and trends for the cooling capacity and the COP of solar powered absorption cooling systems.     

The performance of a triple pressure level absorption cycle (TPLAC) with working fluids based on the absorbent DMEU and the refrigerants R22, R32, R124, R125, R134a and R152a

This paper compares the performance of a single-stage triple pressure level (TPL) absorption cycle with different refrigerant–absorbent pairs. Four HFC refrigerants namely: R32, R125, R134a and R152a which are alternative to HCFC, such as R22 and R124, in combination with the absorbent dimethylethylenurea (DMEU) were considered. The highest coefficient of performance (COP) and the lowest circulation ratio (f), were found as a function of the generator temperature for a given evaporating and cooling water temperatures. The sensitivity of the COP and f for evaporator and cooling water temperatures changes at the maximum COP for the best three working fluids were also examined. It was obtained that the preferable pair is R124–DMEU and among working fluids based on HFC the preferable pair is the R125–DMEU.     

Thermoeconomic analysis and optimization of residential split-type air conditioners

A simulation-based optimization methodology for designing unitary residential air conditioners with focus on both energy performance enhancement and cost savings is presented. A steady-state system simulation model was put forward for a 2.5-ton nominal cooling capacity split-type air conditioning unit operating with R-410A as the working fluid. The model predictions for cooling capacity, sensible heat ratio (SHR) and coefficient of performance (COP) were compared with experimental data, when it was found that the model is able to predict the experimental trends within a ±6% error band. The model was then used to find out the condenser and evaporator geometries (face area, heat transfer area) that enhance the system COP for a fixed cost. On one hand, it was observed that the COP can be increased by 7% if the cost is held fixed. On the other hand, cost savings of 33% were achieved when the system COP was held constant.