Electrical Coefficient Of Performance Research Articles

Comparative study on two low-grade heat driven ejection-compression refrigeration cycles with evaporator-condenser and evaporator-subcooler

The ejection-compression cooling cycle has gained attention due to its superior performance. However, there is limited research on combining the ejector refrigeration cycle with the vapor compression subcooling cycle (ES) driven by a low-grade heat source. Therefore, this study compares the proposed ES system with traditional vapor compression refrigeration and ejection-compression refrigeration cycles with an evaporator-condenser (EC). The aim of this comparative analysis is to evaluate the combined cycles using various performance metrics such as global coefficient of performance (COP), electrical COP, low-grade heat conversion ratio, and discount payback period. The evaluation is conducted under different operating conditions to provide recommendations for specific applications. The results show that ES exhibits better economic performance when solar heat is utilized, while EC focuses more on electricity saving when waste heat is used. In the base case, ES achieves a 2.24 times higher electrical COP than EC while consuming only 20% of the low-grade heat. The global COP of EC is 0.74, whereas ES achieves 2.08. The total exergy destruction in EC is 18.32 kW, which indicates a 165.51% increase compared to ES (6.90 kW), highlighting better exergy performance for ES. The discount payback periods for EC range from 25.33 to 9.24 years, while for ES, they range from 12.06 to 2.85 years. Based on these findings, ES shows more promise as a technology for investment and further development. This comparative analysis provides valuable insights into the strengths and weaknesses of the two combined cycles, aiding decision-making processes for implementing these systems in different applications.

Performance analysis of no-insulation long distance thermal transportation system based on single-stage absorption-resorption cycle

High source temperature demand (>100 °C) and inflexible working pressure restrict the widespread application of solution transportation systems. Absorption-resorption heat pump cycles stand out in significantly lowering the demand of heat source temperature (to 80 °C), as well as multiplying working pressure choices (lower to 500 kPa) attributed by solution concentration adjustment and simplifying system structure (no rectifier). In this paper, a novel solution transportation resorption system is proposed based on the single-stage absorption-resorption heat pump cycle by assigning high-pressure generator and high-pressure absorber on the source site while placing low-pressure generator and low-pressure absorber on the user site. Feasible working pressure combinations and outlet temperature of high pressure-absorber to effect the system are investigated, illustrating the pressure range difference between resorption cycle and cycle -based heat transportation system. Thermodynamic parameters such as thermal coefficient of performance, electrical coefficient of performance, heat supply temperature range of the system are revealed in this paper. Under the condition that transporting distance is 20 km, the maximum COP value of the proposed system could reach 0.53 when high/low pressure pair is 700 kPa/405 kPa. The highest supply water temperature could reach 53.84 °C with COP of 0.477, which is suitable for floor heating. Besides, hydrodynamic and economic analysis of the proposed system are conducted, indicating that when the transportation distance exceeds 4 km, the proposed system is more economical than conventional sensible heat transportation systems. According to restriction to pump power, the longest distance is supposed to be no more than 113 km when loading power is 20 MW.

Termoelektrik Soğutucunun Modelleme Çalışması ve Performans İncelemesi

Thermoelectric refrigerators are widely used in electronics, medical, and food industry application areas. A refrigeration effect can also be achieved without using any moving parts by merely passing a small current through a closed circuit made up of two dissimilar materials. This effect is called the Peltier effect, and a refrigerator that works on this principle is called a thermoelectric refrigerator. They consist of several thermoelectric legs sandwiched between two thermally conductive plates, one cold and one hot. Thermoelectric refrigerators presently cannot compete with the vapor-compression refrigeration system because of their low- coefficient of performance (COP). However, some applications have been preferred because of their small size, simplicity, quietness, and reliability. In this study, a thermoelectric cooler having a maximum cooling power of 50 W, having a dimension of 40mmx40mmx3.6 mm, is modeled in multi-physics software. Also, the performance of a thermoelectric refrigerator is investigated. It is computed the temperature difference between ceramics plates versus electric current and COP for a temperature difference between ceramics plates. The simulation results are compared with experimental data. The data obtained from the analyses have been compared with the experimental results and found to agree with each other. For the surface temperatures of 25 oC and 50 oC, the maximum coefficients of performance have been computed to be 1.091 and 1.445, respectively. In general, as the temperature of hot surfaces has increased for the same temperature differences, the COP of the thermoelectric cooler has increased.

Efficient absorption cooling for low district heating return temperatures

In the past years scientists of Technische Universität Berlin, developed a new generation of single stage H2O/LiBr absorption chillers in the range of 50 to 500 kW nominal cooling capacity. In 16 installations 25 absorption chillers have been monitored within a broad field test (FeldtestAbsorptionskälte für Kraft-Wärme-Kälte-Kopplung-Systeme, FAkS, funded by BMWi as project FKZ 03ET1171a-d) in the years 2013 to 2018. Annual performance figures of thermal coefficient of performance (COPth) in the range of 0.7 kWh 0/kWh 2 and electrical coefficient of performance (COPel) superior to 20 kWh 0/kWh el have been proven, serving district heat (DH) return line temperatures in the range of 62 °C to 68 °C.While the FAkS project focused on installation and operation of absorption chillers themselves, the follow-up project called ReKs (Regelungenergie-aufwandsoptimierterKälteerzeugungssysteme, FKZ 03ET1583), concentrates on overall system control and efficiency. Controlling absorption chiller cooling supply at low district heat return line temperatures is extended to an energy efficient control of all participating components including hot water, chilled water, heat sink pump and cooling tower fans. Models and strategies have been developed on thermal science theory and implemented in industrial control software. Proof of concept has been evaluated by monitoring field test installations while in parallel particular field test related parameters have been varied. Without diminishing quality of chilled water temperature and/or cooling capacity, electrical energy efficiency can at least be doubled, compared to the figures of FAkS installations. Moreover, new strategies enable efficient absorption cooling with DH return line temperatures in the range of 50 °C during moderate weather conditions periods. Field test results from 2020 and 2021 are presented here.

Seasonal energy performance evaluation with new perspective on partial-coupling ejection-compression solar cooling system for modern city buildings

Space cooling accounts for high percentage of energy consumption in modern city building. To improve energy utilizing efficient of space cooling, a new system based on solar partial-coupling ejection-compression refrigeration cycle is studied. Builds are assumed to be located at modern city of Shanghai, with area of 100 m2 for each floor. With a model combining cooling system, building, and weather information, comparative analysis is carried out hourly, using new perspective of total primary energy, during the whole cooling season. For the building with a common height of six-storey in modern city, partial-coupling ejection-compression refrigeration cycle reveals obvious superiority in electric coefficient of performance, solar factor, thermal coefficient of performance, total primary energy, with best value of 6.2, 1, 3.6, and 14 kW, respectively, which helps to reduce carbon dioxide emission. The remarkable difference in auxiliary heat between partial-coupling ejection-compression refrigeration cycle and traditional ejection-compression refrigeration cycle results in the great primary energy saving of partial-coupling cycle, which reveals significant superiority and good suitability of partial-coupling cycle used in multi-stroey buildings in modern city.

Energy Analysis of a Novel Ejector-Compressor Cooling Cycle Driven by Electricity and Heat (Waste Heat or Solar Energy)

Low-grade heat is abundantly available as solar thermal energy and as industrial waste heat. Non concentrating solar collectors can provide heat with temperatures 75–100 °C. In this paper, a new system is proposed and analyzed which enhances the electrical coefficient of performance (COP) of vapour compression cycle (VCC) by incorporating low-temperature heat-driven ejectors. This novel system, ejector enhanced vapour compression refrigeration cycle (EEVCRC), significantly increases the electrical COP of the system while utilizing abundantly available low-temperature solar or waste heat (below 100 °C). This system uses two ejectors in an innovative way such that the higher-pressure ejector is used at the downstream of the electrically driven compressor to help reduce the delivery pressure for the electrical compressor. The lower pressure ejector is used to reduce the quality of wet vapour at the entrance of the evaporator. This system has been modelled in Engineering Equation Solver (EES) and its performance is theoretically compared with conventional VCC, enhanced ejector refrigeration system (EERS), and ejection-compression system (ECS). The proposed EEVCRC gives better electrical COP as compared to all the three systems. The parametric study has been conducted and it is found that the COP of the proposed system increases exponentially at lower condensation temperature and higher evaporator temperature. At 50 °C condenser temperature, the electrical COP of EEVCRC is 50% higher than conventional VCC while at 35 °C, the electrical COP of EEVCRC is 90% higher than conventional VCC. For the higher temperature heat source, and hence the higher generator temperatures, the electrical COP of EEVCRC increases linearly while there is no increase in the electrical COP for ECS. The better global COP indicates that a small solar collector will be needed if this system is driven by solar thermal energy. It is found that by using the second ejector at the upstream of the electrical compressor, the electrical COP is increased by 49.2% as compared to a single ejector system.

Dynamic modelling and analysis of an adsorption-based power and cooling cogeneration system

In this study, an electricity and cooling power cogeneration system is investigated to harvest low-grade heat below 80 ℃, which consists of a two-bed adsorption-based desalination (AD) system for thermally separating the salt solution into diluted and concentrated solutions while offering cooling power and a reverse electrodialysis (RED) system for converting the Gibbs free energy of mixing of the generated solutions into electricity. The dynamic response of the cogeneration system is presented first, and the asymmetric operation period is analyzed. Then the effects of adsorption/desorptiontime, switching time, working concentration, working fluid mass, and adsorbents on the electric efficiency, coefficient of performance (COP) and exergy efficiency of the cogeneration system are systematically evaluated. Results reveal that longer adsorption/desorption time leads to degraded electrical efficiency and exergy efficiency, and upgraded COP. Extended switching time contributes to COP and exergy efficiency, however, decreases the electric efficiency. Larger salt concentration improves the electric efficiency, however degrades the exergy efficiency and COP. Increasing working solution mass can augment the electrical efficiency, exergy efficiency and COP. Furthermore, refrigeration performance conflicts with power generation performance for different adsorbents. With CAU-10 as the adsorbent, an exergy efficiency of 30.04% is achieved, meanwhile the electric efficiency and COP is 0.39% and 0.84, respectively at adsorption/desorption time of 900 s, switching time of 10 s and working concentration of 8 mol/kg.

Performance evaluations of an adsorption-based power and cooling cogeneration system under different operative conditions and working fluids

In order to harvest low-grade waste heat below 80 °C, we present an alternative adsorption-based power and cooling cogeneration system, which consists of an adsorption-based desalination system for generating concentrated and diluted salt solutions as well as providing cooling power, and a pressure retarded osmosis system for converting the produced salinity gradient energy into electricity. Effects of operation conditions, adsorbents, salt types and solvents are systematically investigated to evaluate the coefficient of performance (COP), electric efficiency, and exergy efficiency of the hybrid system. Results reveal that there exists an optimal desorption temperature leading to the maximum COP and electric efficiency. Larger salt concentration results in upgraded electrical efficiency and exergy efficiency, and degraded COP. Adsorbents with larger relative pressure where the adsorbent achieves half of the maximum absorption uptake and moderate adsorption enthalpy are more appealing. Solvents with high vaporization latent heat and specific heat capacity contribute to COP, however, decrease electrical efficiency and exergy efficiency. Salts rendering a larger osmotic coefficient improve electric and exergy efficiencies, however degrade the COP. When operating at a desorption temperature of 50 °C, the maximum exergy efficiency can reach 33.9%, meanwhile the electric efficiency and COP are 1.63% and 0.87, respectively.

Energy and Exergy Analysis of LiBr-aq and LiCl-aq Liquid Desiccant Dehumidification System

In this study, energy and exergy analysis of experimental results obtained from a dehumidification system using LiBr-aq (lithium bromide-water) and LiCl-aq (lithium chloridewater) as desiccant was made. In dehumidifier and regenerator columns polycarbonate sheets, which have not been used before, were used as packing material to increase contact area in purposed liquid desiccant dehumidification system. In the analysis, variation of electrical coefficient of performance and exergy efficiency with airflow rate for different solution mass flow rates were investigated. Because of investigation, maximum values of electrical coefficient of performance and exergy efficiency were calculated approximately as 2.8 and 18%, respectively.

Integrative modelling and optimisation of a desiccant cooling system coupled with a photovoltaic thermal-solar air heater

An integrated model of a desiccant cooling system with a hybrid photovoltaic thermal collector-solar air heater (PVT-SAH) was developed and coupled with a building model to enable a performance evaluation and optimisation of the system. A double pass PVT-SAH system that incorporates heat pipes was used to increase the air temperature of the PVT-SAH outlet in order to regenerate the desiccant in a desiccant wheel. The integrated model was then used to quantify the performance of the hybrid desiccant cooling system in terms of Solar Fraction (SF) and the Coefficient of Performance (COP) and its response to the various sizes of PVT-SAH systems. The model outputs showed that the selection of the PVT-SAH design parameters can be critical in improving the system’s utilisation in a desiccant cooling process. For a commercial building case study in a hot and humid climate, the annual Solar Fraction and the electrical COP of the cooling system reached up to 96.6% and up to 19.8 respectively, when the design of the PVT-SAH was optimised. The optimal electrical COP was higher than those for non-optimal designs that ranged from 0.6 to 15.1. The minimum size of the PVT-SAH that is required for the hybrid desiccant cooling system to exceed a typical COP for commercial buildings (2.6–3.0) was found to be 0.35 m2 per m2 of the conditioned floor area.

The effects of regeneration temperature of the desiccant wheel on the performance of desiccant cooling cycles for greenhouse thermally insulated

The use of solar energy for cooling greenhouses in the hot period in Mediterranean climate is an important issue. Desiccant evaporative cooling (DEC) system is advantageous because it uses a low grade thermal energy and preserves the merits to be friendly environmentally technology. In this paper, a numerical investigation was carried out on a desiccant cooling system powered by air solar collectors coupled to an insulated greenhouse. The influence of the regeneration temperature on the air stream properties at every system component state point was studied. The performance of the desiccant cooling system was evaluated in terms of thermal and electric coefficient of performance. Results show that the best performance of the system (COPel = 14 and COPth = 0.94) was obtained for a 60 °C regeneration temperature and a supply flow rate ratio of 0.2. An economic analysis shows that the use of the DEC system for greenhouse cooling is attractive and profitable since the payback period is 1 years. The use of the proposed system allows saving 9396 kWh/year of electric energy compared to conventional system.

Experimental investigation on binary ammonia–water and ternary ammonia–water–lithium bromide mixture-based absorption refrigeration systems for fishing ships

Heat recovery of marine engine exhaust gas is an effective way of improving the onboard fuel economy and environmental compliance of fishing ships. Among such heat recovery techniques, the absorption refrigeration cycle shows potential as it can convert the exhaust thermal energy into refrigeration output and meet the onboard refrigeration requirement. However, the severe operating conditions on the shipboard poses a great challenge for its application. This paper presents an experimental investigation of an absorption refrigeration system for the heat recovery of marine engine exhaust gas. To overcome the adverse effect of the severe onboard condition on the rectification process of the absorption refrigeration system, a ternary ammonia–water–lithium bromide mixture is selected as the working fluid. A prototype of the absorption system is designed and an experimental investigation is conducted. Then, the performances of both the ternary ammonia–water–lithium bromide-based system and binary ammonia–water-based system are compared. The results show that the rectifier heat exchange area can be reduced by approximately 16% under the experimental working condition. Furthermore, the ternary system operates at a relatively lower pressure, with a refrigeration temperature of less than −15.0 °C, which is higher compared to the temperature of less than −23.6 °C associated with the binary system. Nevertheless, the ternary system achieves a remarkably higher cooling capacity. Moreover, by using the ternary ammonia–water–lithium bromide mixture, the heat loss of the prototype is reduced while the coefficient of performance and electric coefficient of performance are increased, indicating that the ternary system has a higher energy conversion efficiency.

Liquid desiccant air conditioning system with natural convection

After introducing the concept of natural convection air dehumidifier system and establishing the feasibility of eliminating desiccant pump, this study aims to approve the application of natural liquid desiccant as an air conditioning system in real working conditions. The system consists of two heat/mass transfer modules: the absorber and the regenerator which are connected to constitute a closed loop. In each module hollow fiber membranes separate the air and desiccant streams. The liquid desiccant is aqueous solution of lithium chloride which is on the shell over the fiber surfaces and the air flows inside fiber lumens. The cold/hot water streams flow in the outer jackets of absorber/regenerator modules. The loop is redesigned to resolve the shortcomings of the previous design. The loop performance is studied experimentally under two inlet air conditions which resemble two climate conditions with relative humidity of 78% and 37% and temperature of 31.5°C. The evaluation indexes of system such as moisture absorption rate, cooling capacity and coefficient of performance (COP) are presented. The results show that the loop can act as a natural convection liquid desiccant air conditioning system with electrical COP values of 3.95 and 4.56 for higher and lower humidity ratio air inlets respectively. Also, it is resulted that the system has better performance in higher humidity air inlet. It is observed that main electric power consumption of the system is required for supplying cold water stream and the performance of system could enhances remarkably if a free cold water supply is available.

Multi-stage liquid-desiccant air-conditioner: Experimental performance and model development

An experimental study evaluated the performance of a novel multi-stage liquid-desiccant air-conditioner featuring parallel inter-stage flows of cooling and heating water, and semi-series flows of desiccant. The system was tested at different inlet cooling water temperatures, ranging from 8 °C to 20 °C, using a solution of LiBr and water as the desiccant. Average total air cooling rates between 8.4 kW and 19.7 kW were observed, with higher cooling rates achieved with lower cooling water temperatures. The system was able to provide high sensible cooling rates of up to 8.8 kW. The cooling water temperature impacted the sensible cooling, but did not have a discernible impact on the latent cooling rate. The inlet air humidity was shown to have an impact on the latent cooling rate, with higher humidity leading to higher latent cooling rates. The energy consumption of the system was low, with an average thermal coefficient of performance (COP) of 0.58 and electrical COP of 4.7. A variable effectiveness model was developed in TRNSYS and proven to be accurate at predicting the air cooling, but further experimental work is required to improve the thermal energy consumption model. The modular design philosophy of the model allows for easy changes to the design (e.g., number of stages, fluid flow paths, operating conditions, etc.) enabling future work to improve the regenerator modeling once further experimentation is completed.

Investigation of a solar energy driven and hollow fiber membrane-based humidification–dehumidification desalination system

A solar energy driven and membrane-based air humidification–dehumidification desalination (MHDD) system is proposed. A test rig is designed and constructed to investigate the performance of the system. The test rig consists of a U-tube evacuated solar collector, a heat storage water tank, a membrane-based humidifier (hollow fiber membrane module) and a dehumidifier (a fin-and-tube heat exchanger). A theoretical model for the whole system simulation is developed and validated. Performance indices of the system, such as the specific water production rate on the basis of unit area of membrane (SWR), specific electric energy consumption on unit volume of water production (SEC), coefficient of performance (COP) and electric coefficient of performance (COPE) are investigated. The effects of various parameters including the saline water flow rate, the air flow rate and the packing fraction of the membrane module, etc., on system performance are examined. It indicates that solar energy accounts for 92.0% of the energy consumption by the whole system. Sensible heat losses account for most of the energy losses from the system. High-purity water is produced by this system at a SWR of 25.88kgm−2d−1, a SEC of 19.23kWh/m3, a COP of 0.75 and a COPE of 36.13. The feasible operating parameters investigated are: hot saline water flow rate, 236L/h for per unit area of membrane; air flow rate, 25m3/h for per unit area of membrane; module packing faction, 30%.

A high performance desiccant dehumidification unit using solid desiccant coated heat exchanger with heat recovery

A new type of solid desiccant dehumidification cycle with internal heat recovery based sorptive material coated heat exchanger (SCHE) is proposed. This system can realize waste heat recovery from exhausted air in regeneration process, in which a heat recovery device (HRD) is adopted. The performance evaluation of the novel cycle is compared with that of the conventional cycle without HRD in terms of moisture and energy transfer. Based on the experimental results, the thermal coefficient of performance of the system has been improved to 1.2, almost twice the thermal coefficient of performance of the conventional cycle without HRD. The electrical coefficient of performance can reach about 13.83. When HRD is involved in the system, the supply air temperature is close to the inlet temperature of process air (ambient air), dehumidification capability meets the design requirements (lower than 8gkg−1) and the regeneration temperature can satisfy the regeneration requirement of low grade heat source. The thermal efficiency of the HRD is calculated as 0.88 on average. As a conclusion, SCHE with HRD system could be a potential alternative with effective operation which can be promoted in higher humidity and temperature areas.

Integration of an adsorption chiller in an open-cycle desiccant-assisted air-conditioning system

In a pilot installation at Hamburg University of Technology, the coupled operation of an open-cycle desiccant-assisted air-conditioning system with an adsorption chiller in combination with cooling ceilings to remove sensible loads is investigated. This article presents experimental results of the year 2013 with a special focus on the performance of the adsorption chiller. Furthermore, the system is compared to two reference systems relying on vapor compression chillers. The average thermal coefficient of performance of the adsorption chiller during the investigated period is satisfying (COPth = 0.54), while the average electrical coefficient of performance does not meet expectations (COPel = 6.2). The fan control of the dry cooler can be identified as a main cause for the non-satisfying electrical performance. However, depending on the heat supply, considerable primary energy savings are possible through the investigated system, and the advantages of combined heat and power (CHP) engines can be extended to the summer period.

Experimental results of a solar absorption cooling plant in Greece

Over the last years, the thermal comfort level during the summer period has been significantly increased due to the use of conventional cooling and air-conditioning systems, leading to higher electricity consumption. Solar cooling systems may provide the solution and become the leading technology in the future. The aim of this paper is to present a small-scale solar thermal system for cooling an office building in Athens, Greece. The study documents the system design, the monitoring procedure and equipment, and presents the experimental results from the first complete summer period. The daily electrical coefficient of performance (COP) of the absorption chiller of 48.6 and the electrical COP of the solar system 10.9 indicate the potential of solar cooling in small-scale systems.

Performance investigation of a solar heating system with underground seasonal energy storage for greenhouse application

This study reports the performance of a demonstrated 2304 m2 solar-heated greenhouse equipped with a seasonal thermal energy storage system in Shanghai, east China. This energy storage system utilises 4970 m3 of underground soil to store the heat captured by a 500 m2 solar collector in non-heating seasons through U-tube heat exchangers. During heating seasons, thermal energy is delivered by the heat exchange tubes placed on the plants shelves and the bare soil. The system can operate without a heat pump, which can save electricity consumption and further enhance the solar fraction. It was found that in the first operation year, 331.9 GJ was charged, and 208.9 GJ was later extracted for greenhouse space heating. No auxiliary heating equipment was installed so that solar energy covered all the heating loads directly or indirectly. It was demonstrated that this system was capable of maintaining an interior air temperature that was 13 °C higher than the ambient value when the latter temperature was −2 °C at night. The ECOP (electrical coefficient of performance) of the first operation year was approximately 8.7, indicating a better performance than the common heat pump heating system.

The performance of a large-scale rotary magnetic refrigerator

Astronautics has constructed a large-scale rotary magnetic refrigerator which was designed to provide 2 kW of cooling power over a temperature span of 12 K with Electrical Coefficient Of Performance (COPe) > 2. The system uses a NdFeB magnet assembly with peak field of 1.44 T which rotates over twelve beds arranged circumferentially. Each bed was packed with six layers of LaFeSiH of different Curie temperatures, chosen to optimize system performance over the desired span. We report here on the performance of this system at flow rates ranging from 12.5 to 21.2 L min−1. At the largest flow rate, the system produced 3042 W of cooling power at zero span and peak performance of 2502 W over a span of 11 K. To our knowledge, this represents the largest cooling power yet observed for a magnetic refrigeration system. We show that the measured performance is in good agreement with theoretical prediction.