Coefficient Of Performance Research Articles

ENERGY AND EXERGY ANALYSIS OF AN AIR SOURCE HEAT PUMP UNDER VARIABLE AMBIENT CONDITIONS

Air source heat pumps (ASHPs) are becoming an increasingly popular heating source for buildings. The paper presents an evaluation of the experimental data from ASHP operation during the heating season in Lithuania when the problem of the evaporator’s surface freezing is visible. The performance of the air-to-water heat pump is examined using energy and exergy analyses performed by a coefficient of performance (COP), COPCarnot, exergy efficiency, and primary energy ratio. Analysis results show that the existing difference between the ideal and actual operation of ASHP represents the demand to improve the performance of ASHP evaporator. The actual COP was from 3.5 to 4.7 times lower than the Carnot COP. At 0 °C and 95% humidity, the ASHP’s performance was least favourable, with an average exergy efficiency of 0.21 and a COP of 1.49.

Dynamic simulation of a novel liquid desiccant air-conditioning system for greenhouse cooling and water recovery

We present dynamic, hour-by-hour modelling of a multi-stage nanofiltration regeneration system for liquid desiccant air-conditioning (NF-LDAC) of horticultural greenhouses in 17 locations covering four climate types. Four technologies are compared: fan ventilation, evaporative cooling (EvapC), conventional air-conditioning (AC), and NF-LDAC. The comparison is based on acceptable conditions for cultivation and coefficient of performance (COP). On average, fan ventilation and EvapC achieve acceptable conditions for 2 months per year, compared to 5.4 and 10.5 months for AC and NF-LDAC respectively. The highest COP value of 7.6 for NF-LDAC is reached at Colombo (Sri Lanka), followed by 5.3 at Mecca (Saudi Arabia). The permeate of the multi-stage regenerator can be recycled for irrigation, providing water savings. The highest water saving of 63% is at Mecca. These results are inferior but more realistic than those from an earlier idealised and steady-state model, which predicted a COP of 12.4 and water savings of 100% at Mecca. Nevertheless, in hot desert climates, NF-LDAC maintains acceptable conditions for year-round cultivation and saves water that is scarce in such climates. Future advances in nanofiltration membrane fabrication could increase the COP of NF-LDAC to 12.1 at Colombo and 7.4 at Mecca.

Analysis and optimization of a medium-depth ground source heat pump heating systems with heat storage and borehole heat exchangers

The medium-depth ground source heat pump (MD-GSHP) heating technology is considered a clean energy solution with the potential to mitigate air pollution issues attributed to winter heating in northern China. However, it currently suffers from problems such as a mismatch between heat supply and demand and a lack of optimal operation. To overcome these limitations, this study focuses on a 3000 m2 residential building in Xi’an and implements two configurations: coupling MD-GSHP heating systems with heat storage tank and shallow borehole heat exchanger (BHE). Additionally, an optimization strategy based on the improved particle swarm algorithm is proposed, integrating chaos optimization and dynamic parameters. Finally, noteworthy outcomes were revealed by optimizing the MD-GSHP heating systems under different objective functions. In system coupled with shallow BHE, the variation trend of flow in the medium-depth BHE was similar to that of heating load when the heating cost was taken as the objective function. After the optimization, heating cost reduced by 10.02%, while system coefficient of performance (COP) and medium-depth geothermal utilization coefficient increased by 10.43% and 2.00%, respectively. For system coupled with heat storage tank, the optimized flow exhibited a generally decreasing-increasing-decreasing trend, with the peak preceding the heating load curve. This resulted in a 12.82% reduction in heating cost, along with increases of 10.35% and 1.96% in system COP and the medium-depth geothermal utilization coefficient, respectively. Comprehensive consideration of heating cost and system COP suggested a favorable weight of 2.8 for continuous system operation. In conclusion, when prioritizing heating cost, the system coupled with heat storage tank yields greater economic benefits compared to the system coupled with shallow BHE. The findings of this study can provide a guidance for the applications of the MD-GSHP heating technology.

Modelling and Experimental Characterisation of a Water-to-Air Thermoelectric Heat Pump with Thermal Energy Storage

Nowadays, increasing the penetration of renewable heat technologies is an important approach to minimise global primary energy use and reduce CO2 emissions for a sustainable future. Thermoelectric heat pumps, which have some unique characteristics in comparison with conventional vapour compression heat pumps, can be integrated with solar thermal energy storage to form a promising renewable heat technology. However, currently, a reliable numerical model for TeHPs suitable for building energy simulation is lacking and the benefits achievable for a TeHP thanks to the integration with heat storage are unclear. To solve these issues, in this work, an experimental apparatus consisting of a water-to-air TeHP unit with a heat storage tank is modelled and tested for the first time, under the scenarios with thermal energy storage and without thermal energy storage, respectively. The results found that the developed numerical model could well predict the output performance of the TeHP unit, with deviations within 12%. Additionally, the output performance of the TeHP unit when combined with a heat storage tank is better than that of the TeHP unit without heat storage, in terms of the maximum temperature achieved in the testing box, the temperature response speed of the testing box, and the coefficient of performance (COP) of the TeHP unit. This work not only paves the way for the following building-integrated simulations of TeHP units, but also provides guidance for the design of the integrated systems that include TeHPs and thermal energy storage.

Numerical Study of a Three-Bed Adsorption Chiller Employing an Advanced Mass Recovery Process with Different Cycles

In this paper, the performance comparison between two different cycles has been numerically studied. In cycle1, the configuration of beds in the three bed chiller with mass recovery were taken as uniform in size but in cycle2 the configuration of Hex3 is taken as half of Hex1 or Hex2 (where Hex1 and Hex2 are identical). In the present numerical solution, the heat source temperature variation is taken from 70°C to 90°C (for both cycle) and along with coolant inlet temperature at 30°C and the chilled water inlet temperature at 14°C. Silica gel-water is chosen as adsorbent-refrigerant pair. In the new strategy, if any one bed (3rd bed) is connect with the evaporator during pre-heating or pre-cooling time then it will give better performance than that of existing system. In this strategy, mass recovery process also occurs in all bed. Results show that the cooling capacity (CC) and coefficient of performance (COP) of the cycle1 is much better than that of the cycle2 in the range of heat source temperature from 700C to 900C.

Tunable Low-Pressure Water Adsorption in Stable Multivariate Metal-Organic Frameworks for Boosting Water-Based Ultralow-Temperature-Driven Refrigeration.

The green water-based adsorption refrigeration is considered as a promising strategy to realize near-zero-carbon cooling applications. Although many metal-organic frameworks (MOFs) have been developed as water adsorbents, their cooling performance are commonly limited by the insufficient water uptakes below P/P0 = 0.2. Herein, the development of multivariate MOFs (MTV-MOFs) is reported to highly modulate and boost the low-pressure water uptake for improving coefficient of performance (COP) for refrigeration. Through ligand exchange in the pristine MIL-125-NH2 , a series of MTV-MOFs with bare nitrogen sites are designed and synthesized. The resulting MIL-125-NH2 /MD-5% exhibits the significantly improved water uptake of 0.39g g-1 at 298 K and P/P0 = 0.2, which is three times higher than MIL-125-NH2 (0.12g g-1 ) and comparable to some benchmark materials including KMF-1 (0.4g g-1 ) and MIP-200 (0.36g g-1 ). Combined with its low-temperature regeneration, fast sorption kinetics and high stability, MIL-125-NH2 /MD-5% achieves one of the highest COP values (0.8) and working capacities (0.24g g-1 ) for refrig-2 under an ultralow-driven temperature of 65°C, which are higher than some best-performing MOFs such as MIP-200 (0.74 and 0.11g g-1 ) and KMF-2 (0.62 and 0.16g g-1 ), making it among the best adsorbents for efficient ultralow-temperature-driven refrigeration.

The study of dynamic characteristics on a novel air source heat pump coupled with liquid-storage gas-liquid separator under non frosting and refrigeration conditions

Air source heat pump is widely used for heating as it has high efficiency and environmentally friendly advantages, but there is frosting issue in low temperature and high humidity conditions. In order to improve the performance of air source heat pump (ASHP) when its evaporator is subject to frosting, a novel ASHP system coupled with liquid-storage gas-liquid separator was proposed in previous work, which was conducted at a constant temperature. However, the environment temperature is variable during the actual operation, the influence mechanism of the temperature change on the dynamic characteristics of the system and operation characteristics is still unclear. Therefore, experimental method is employed in this work, the aim is to investigate the dynamic performance of the novel ASHP in heating and refrigeration modes in actual environment, including suction/exhaust pressure, suction/exhaust temperature, inlet/outlet water temperature, the coefficient of performance (COP) etc. to improve the adaptability of the heat pump unit. The results indicate that the COP is 2.78–3.49 when the ambient temperature is 6.5–12.5 °C under the heating condition. Also, in refrigeration mode, the COP of the system is 3.28–3.04 under the ambient temperature of 22∼38 °C. This study shows that the novel ASHP has a good dynamic performance during heating/cooling operation in the actual environment. Also, some suggestions for improvement are put forward to solve the existing problems in this work.

Impact of condensate water recycling on the performance of open refrigerated display cabinet

The escalating global energy concerns have underscored the urgent need for energy conservation, particularly within the refrigerated industry. The imperative of optimizing the performance of open refrigerated display cabinets (ORDC) has become increasingly pronounced. This study sought to investigate the impact of condensate water recycling on ORDC operations through experiment, focusing on the influence of condensate water flowrate, temperature, and heat exchanger length on key performance indicators such as thermal entrainment factor (TEF), coefficient of performance (COP), and heat transfer efficiency enhancement (η). The findings revealed that the flowrate of condensate water wielded the most substantial impact on system performance, leading to noteworthy increases of 10.42% in COP and 7.05% in η, while TEF experienced a reduction of 4.33% within a flow rate range from 0 ml/min to 54 ml/min. Notably, an optimal flowrate of 54 ml/min was identified. Conversely, the temperature of condensate water exhibited minimal influence on system performance. Increasing the length of the heat exchanger demonstrated a linear correlation with improved system performance. In practical applications, the enhancements in COP, η, and TEF, compared to scenarios without a condensate water recycling system, ranged from 2.23% to 5.80%, 1.21% to 3.68%, and 0.37% to 2.09%, respectively.

Improving hydrogen liquefaction efficiency based on the temperature-distributed refrigeration method in regenerative refrigerators

Liquid hydrogen has more advantages than gaseous hydrogen in terms of economy and convenience of storage and transportation owing to its higher energy density. Therefore, the hydrogen liquefaction technology is conducive to promotion and application for hydrogen energy in many sectors. The small-scale regenerative refrigerators usually use helium as the working fluid, which is of the high-low temperature cycle, and bring about a significant entropy generation during the cooling process. In this work, the study on the liquefaction rate and FOM (figure of merit) is implemented with a working fluid of hydrogen. It shows that the entropy generation much decreases when applying the distributed refrigeration method comparing to the high-low temperature cycle, and the COP (coefficient of performance) reaches 80 % when cooling the sensible heat load and conversion heat. The calculation results indicate that the liquefaction rate of the temperature-distributed cycle reaches 34.32 L/d at a refrigeration power of 15 W (a representative case). While the maximum of FOM is 0.483, which is improved by 21.7 % compared with the high-low temperature cycle.

Enhanced boiling heat transfer via microporous copper surface integration in a manifold microgap

Heat flux dissipation from electronic devices has increased with their miniaturization owing to the increasing performance demands and development of microfabrication technologies. Improving the heat transfer performance is crucial for enhancing heat dissipation. However, this often leads to an increase in pressure drop, which reduces energy efficiency. Microgap heat sink is a promising approach in this regard owing to its geometrical simplicity and facile fabrication while providing heat transfer performance comparable to that of microchannel heat sinks with substantially lower pressure drops. In this study, we develop a manifold microgap heat sink integrated with a porous copper surface that effectively enhances heat transfer while using significantly low pumping power. A three-dimensional liquid routing manifold is used to achieve better flow distribution and alleviate temperature non-uniformity while providing improved heat transfer by enabling jet impingement and mixing of the thermal boundary layer on the microgap surface with minimal additional pressure drop. Moreover, a microscale inverse opal structure is used to facilitate nucleate boiling on the microgap surface, which improves heat transfer while maintaining a relatively low flow resistance. A maximum heat flux of 322.8 W/cm2 was achieved at the mass flux of 472 kg/m2 s, and the corresponding pressure drop and maximum heater temperature were 0.6 kPa and 140 ℃, respectively. The coefficient of performance (COP) achieved herein was significantly higher than those reported in previous relevant studies, indicating that the proposed cooling technique can potentially be used for energy-efficient thermal management in electronics devices.

Potential assessment of biomass-based single-stage adsorption systems for refrigeration, cooling, and heat-pumping applications

ABSTRACT This article presents the performance estimation of a single-stage vapor adsorption system employing various carbon-based adsorbents with ethanol refrigerant, coupled with a small-scale biomass-based heating unit for space cooling, refrigeration, and heat-pumping applications. A detailed parametric investigation highlights the effects of operating temperatures, and heat exchanger to adsorbent mass ratio on specific cooling energy (SCE), coefficient of performance (COP), uptake efficiency (ηu), and mass of adsorbent to combustion fuel (mad/mcf) to identify the suitable adsorbent and sorption bed designs for each application. The numerical findings reveal that the flat-finned tube (FFT) adsorber outperforms traditional-finned tube (TFT) and tube & shell-type (T&S) adsorbers, achieving 19% and 30% higher COP, respectively, with biomass carbon-ethanol pair. The H2-treated Maxsorb-III shows the greatest optimal cooling COP (0.73), followed by the biomass-carbon (0.72) for refrigeration and space cooling. For heat pumping applications, biomass carbon-ethanol achieves the highest heat pumping COP (1.84) at a desorption temperature of 90°C, surpassing H2-treated Maxsorb-III. Further, the biomass-derived activated carbon requires the lowest amount of adsorbent per unit mass of combustion fuel in the biomass heating unit. The present thermodynamic analysis is a precursor for optimization of the potential heat-exchanger geometries and operating parameters, with the high-performance carbon adsorbents.

A Look into the Boil‐Off Gas Reliquefaction in Propane Storage Plants

AbstractThe reliquefaction process of boil‐off gas (BOG) in a propane plant using the reverse Brayton cooling cycle was examined by simulating the operational parameters and system performance of a propane reliquefaction plant in Aspen HYSYS software. The simulation considered various process and design parameters and their effects on the BOG production rate, energy consumption demand, and coefficient of performance (COP) of the cycle. The results demonstrate that the inclusion of methane in the propane product has a considerable impact on the generation of BOG, but this comes at a cost because power consumption increases significantly and the cycle's COP decreases. Furthermore, heat transmission from the surrounding air through the tank insulation and the temperature at which propane is returned from the jetty influence the BOG generation.

The optimization and energy efficiency analysis of a multi-tank solar-assisted air source heat pump water heating system

This article undertakes an optimization study of a solar-assisted air source heat pump water heating system by harnessing the advantages inherent in a multiple water tank arrangement. The Hooke-Jeeves algorithm was adopted to optimize the system parameters. The optimization variables encompass pivotal parameters such as water tank volume, collector area, dip angle, azimuth angle and heat pump power, while the objective function is the annual value of life cycle cost. By research and analysis of the monthly power consumption, electricity cost, solar fraction, average coefficient of performance (COP) and annual value of carbon emissions of the single-tank, dual-tank, and triple-tank system, it is discovered that the triple-tank system exhibits superior performance in overall aspects. Compared to the pre-optimized version, it achieves an annual energy consumption reduction of approximately 3.9%, leading to a 22.5% decrease in electricity expenses. Furthermore, it brings a 5.2% reduction in the annual lifecycle cost and a 6.3% decrease in carbon emissions, while simultaneously improving exergy efficiency by 4.8%.

An efficient and eco-friendly heat-driven thermoacoustic refrigerator with bypass configuration

Developing high-efficiency and environmentally friendly cooling technology is crucial for sustainable life. Heat-driven thermoacoustic refrigerator (HDTR) is an emerging cooling technology characterized by no moving components, eco-friendly working substances, and high reliability. This study reports an innovative efficient HDTR with bypass configuration, which employs eco-friendly and economical working substances, the nitrogen and water. The bypass configuration enables good matching of acoustic power between the engine and cooler units at high heating temperatures, thus enhancing the efficiency. In addition, the employment of liquid resonator significantly reduces the working frequency and reduces the loss. An experimental investigation is performed on the onset characteristics and operating performance of the proposed system. The results show that a highest coefficient of performance (COP) of 0.49 and a cooling power of 726 W are achieved at the heating, ambient, and cooling temperatures of 500, 35, and 7 °C, respectively, exhibiting a high level for the HDTRs employing nitrogen. This work supplies an effective way to the efficiency enhancement of the HDTRs, indicating substantial potential of the proposed HDTR in eco-friendly air-conditioning applications.

A validated model for variable-speed multi-chillers to assess different operation modes

Environmental protection drives the replacement of environmentally harmful refrigerants with potentially flammable alternatives, necessitating efficient cooling strategies for varying loads. Legislation limits flammable refrigerant charge, leading to the adoption of multi-chiller systems with smaller charges and therefore also smaller loads per chiller. However, operating such systems requires complex strategies, addressable through fast and reliable models. This study presents a semi-empirical model predicting the behaviour of variable-speed multi-chiller systems. The model focuses on predicting cooling capacity and coefficient of performance (COP) for cooling, with input parameters of evaporation and condensation temperatures, and compressor speed. Experimental validation of the parameterized model shows COP predictions within 3 % for a two-module system using R-290 (propane) under different conditions and compressor speeds. The model assesses various operation strategies, including the conventional mode with fewer chiller modules running at high speed, compared to modes where all chillers run but at reduced speeds. Comparing COP values in these modes reveals energy-saving potentials of up to 14.57 % for constant condensation temperatures. Moreover, strategies involving condensation temperature reduction, or a combination with compressor speed reduction are analysed for energy-saving potentials. Reducing the condensation temperature by 3 K results in savings of up to 13.25 % and combined with the compressor speed reduction savings of up to 23.54 % are feasible. These findings emphasize the importance of considering compressor speed when operating multi-chiller systems to maximize energy savings. The presented model effectively evaluates various strategies and implements variable-load control in multi-chiller systems, serving as a valuable tool for optimising energy efficiency and promoting environmentally friendly cooling practices.

An experimental investigation of vapor compression refrigeration cooling and energy performance for CPU thermal management

This case study experimentally investigates a R134a charged Vapor Compression Refrigeration (VCR) assisted liquid cooling of high-powered CPUs. In this hybrid system, water-based liquid cooling of the CPU is achieved in a pumped loop with a commercial cold plate affixed to the simulated CPU. The heat is then transferred to the evaporator heat exchanger of a miniature VCR system that rejects the energy to ambient air. Tests are performed at room temperature over a range of operating conditions, including CPU heat loads (12 W/cm2 - 75 W/cm2), liquid volumetric flow rates (0.5 Litre/min - 8 Litre/min), fan air speeds (2.1 m/s – 4.2 m/s), and compressor speeds (2540 rpm–6500 rpm). Both cooling performance and additional energy consumption implications are considered. Overall, it is shown that the miniature hybrid VCR system can cool the primary liquid to below ambient temperature, even at the highest tested heat load. As a result, cooling levels that far exceed both a conventional Fan-Fin Air Cooled heat exchanger and a Hybrid Liquid-Air Cooled system are achieved. Finally, optimal combinations of liquid flow rate and compressor speed are identified for minimum energy consumption of the cooling system to maintain a setpoint CPU die temperature. It is shown that management and control of pump, compressor and fan speeds can result in significant improvement in the overall system Coefficient of Performance (COP).

Seasonal performance of an energy pile heat pump system and prediction of building thermal load

Ground source heat pumps (GSHPs) are widely used for cooling and heating buildings, which can reduce the burning of fossil fuels and contribute to the reduction of carbon emissions. Energy piles, as underground heat exchangers, have advantages over conventional buried borehole pipes, saving drilling costs and accelerating the construction period. However, few studies have focused on the actual operation of an energy pile heat pump system, and the prediction of the building thermal load can reduce the instrument arrangement and test recording time. Hence, a heat pump system was built in this study to serve a single lab using energy piles with a cap structure as the energy supply, and summer cooling and winter heating tests were conducted. Under the test conditions in this study, the GSHP had a faster start-up speed and less power consumption than the air source heat pump (ASHP), and the average coefficient of performance (COP) of the energy pile heat pump system was 3.78, which is 40.0 ∼ 51.2% higher than that of the corresponding ASHP (the normative reference COP is 2.50 ∼ 2.70). The methods of higher-order surface fitting and an artificial neural network model were both suitable for predicting building thermal load.

Maximizing cooling/heating performance of thermoelectric modules across variable thermal loads via optimal control based on COP curves

The thermoelectric module has high potential as a compact electrothermal actuator in the generation of cooling/heating effects without any moving part. However, one difficulty of the design is that the fundamental principles of optimal cooling/heating performance of thermoelectric modules are not yet fully understood and implemented. The purpose of this paper is to propose a mathematical optimization to gain insight and implement the most effective usages. In the proposed analysis, it is found that the coefficients of performance (COP) are dependent on the driving voltage and the ceramic-substrate temperatures of the thermoelectric module at both the cold and hot sides. The thermoelectric properties of a 91.2W thermoelectric module are used to simulate the proposed performance analysis to find the optimal driving voltage at desired operating temperatures at both sides on COP curves. The coefficient of determination R2 of 0.9832 indicates strong agreement on the coefficients of performance between the analysis and the experiments under one hundred thirty-seven operating conditions. With the proposed methodology, the 91.2W thermoelectric module can be operated at the maximum coefficient of performance across variable thermal loads of airflow where the optimal driving voltage is determined from desired operating temperatures at both sides.

Investigation of Solar Powered Single Effect Absorption System

Cooling processes like refrigeration and air conditioning are known for their high energy consumption. Since most Indian states experience abundant sunshine year-round, solar refrigeration is a suitable technology for the country. Solar-powered absorption refrigeration systems offer a viable alternative to traditional refrigeration, providing not only cooling and ice-making capabilities but also energy savings and environmental benefits. However, further research is necessary to enable widespread industrial adoption and the replacement of conventional refrigeration systems with solar powered absorption system. Using solar energy for an air-conditioning system is typically more cost-effective, when it can fulfil both heating and cooling needs. This research focuses on the thermodynamic modelling of a system, specifically the effect of various temperature on COP. Thermodynamic modelling was done with the help of first law of thermodynamics. Using engineering equation solver software, study simulated the cycle and equations were developed for energy and mass flow for each component. Further, before proceeding to analysis some assumptions were also made. We then analysed how the Coefficient of Performance (COP) changed when varying the temperatures across each component. Our findings show that COP increases with increase in condenser, evaporator, and absorber temperatures, but it is declined with decrease in generator temperature.

Tribological and residential air conditioning performance using SiO2-TiO2/PVE nanolubricant

Applying nanoparticles as an additive may increase the lubrication performance of the oil. The present paper evaluates the tribological performance of SiO2-TiO2 nanoparticles in PVE lubricant and its effect on the system's performance in residential air conditioning (RAC). The hybrid nanolubricants with a concentration from 0.003 to 0.01% at a 50:50 ratio were tested using the four-ball tribological method and then applied in a customised RAC test rig. The friction coefficient and wear scar diameter were evaluated to synthesise nanolubricant anti-wear effect on a paired surface. The coefficient of performance (COP), cooling capacity, power consumption, and energy efficiency ratio (EER) were analysed to understand the influence of SiO2-TiO2/PVE nanolubricant in the RAC system. About 25% reduction in frictional coefficient was discovered when the nanoparticles were added into lubricant with a concentration of 0.005%. Applying nanolubricant in the RAC system can reduce compressor work and power consumption, increase the refrigerant flow rate and cooling capacity, and increase the COP and EER. Maximum enhancement of 39.2% and 52.7% for COP and EER were found for nanolubricant at concentrations of 0.005%. The 0.005% concentration is the nanoparticle's concentration threshold for SiO2-TiO2/PVE, as the higher concentration may lead to the performance of a pure lubricant.