Low-temperature Heat Pump Research Articles

A Review on Sludge Drying after High Pressure Filter Pressing

Sludge drying is a key step in municipal sludge treatment. After mechanical dewatering, the water content of sludge is usually between 50% and 70%, which needs further drying. The main drying technologies include thermal drying, solar drying, ultrasonic drying and microwave drying, among which thermal drying technology is the most mature. Research shows that low-temperature drying technology has obvious advantages in reducing the release of harmful substances, energy consumption and cost. In terms of technology application, domestic sludge drying process is mainly divided into direct drying and indirect drying. Direct drying directly evaporates water through high-temperature gas, while indirect drying makes use of heat conduction effect to contact with high-temperature media. At present, indirect drying technology is mainly used in China, such as paddle type, disc type and thin layer drying. In particular, the low-temperature heat pump drying technology excels in terms of safety, footprint and energy consumption, with its ability to operate at lower temperatures, reduce the release of hazardous gases, and achieve environmental and economic benefits in a closed-cycle mode. This technology is suitable for small and medium-sized wastewater treatment plants and shows clear advantages in cases of budgetary constraints.

Numerical analysis of scroll compressor performance with pre-discharge valve for low-temperature heat pumps

Scroll compressors are a crucial component of automotive air conditioning systems. When operating at off-designed working conditions, over-compression can occur in the scroll compressor. A pre-discharge valve (PDV) is proposed to enhance the performance of the scroll compressor under off-design working conditions. The performance of the scroll compressor with a PDV for a low-temperature heat pump system is simulated. The results show that the scroll compressor with the PDV structure has lower pressure in the compression chambers and improved over-compression compared to conventional scroll compressors. The use of the PDV also leads to a reduction in the temperature of the refrigerant gas in the compression chambers and a decrease in the leakage between the compression chambers. As the evaporating temperature increases from −25 °C to −5 °C, the mass flow rate and volumetric efficiency increase, and the discharge temperature decreases. At an evaporating temperature of −5 °C, the discharge temperature at the center discharge port of the scroll compressor with the PDV is reduced by 2.4 °C compared to the conventional scroll compressor. The input power of the scroll compressor is reduced by 165.4 W–897.7 W. The volumetric efficiency and the isentropic efficiency are improved by 0.13 % and 6.63 % respectively.

Study on drying characteristics of sludge under different conditions based on the low-temperature sludge heat pump drying model

The study established a model for a closed low-temperature heat pump drying (HPD) system and structured a sludge drying experimental platform to validate it. The effects of incoming air temperatures of the drying cabinet, circulating air volume, and ambient temperature on both drying rate and system performance were investigated. The findings revealed that the drying rate and specific moisture extraction rate (SMER) rise with higher incoming air temperatures of the drying cabinet and circulating air volume, while the coefficient of performance (COP) exhibited a contrasting trend. The sludge dried the fastest with a drying rate of 6.01 kg/h when the incoming air temperatures of the drying cabinet and circulating air volume were set to 65 °C and 900m³/h, respectively. At the incoming air temperature of the drying cabinet was 65 °C and the circulating air volume was set to 850 m3/h, the SMER reached the maximum value, 2.782kg/(kW·h). In addition, the COP increased as the ambient temperature rose, while the drying rate and SMER both decreased gradually. The drying rate demonstrated an average decline of 0.29 kg/h for every 2 °C rise in ambient temperature, while the SMER exhibited an average decrease of 0.148kg/(kW·h). These works offer guidance for the practical implementation of the sludge HPD system.

Research on Energy Consumption Model of Heat Pump Air Conditioning System for New Energy Vehicles Based on Digital Technology

Energy shortages and environmental degradation are issues that are getting more and more significant globally. New energy vehicles are being promoted by the state due to the advantages of low pollution and low fuel consumption. However, due to battery technology, the range of new energy vehicles cannot meet the needs of users. As the most energy-efficient auxiliary device, the energy consumption of air conditioning will significantly reduce the range of new energy vehicles. In low temperature environments, heating energy consumption will reduce the range of vehicles by more than 50%. Therefore, the research aims to reduce the energy consumption of air conditioning systems in new energy vehicles by reducing load demand and improving operating efficiency. The study designs a low-temperature heat pump air conditioning system based on digital technology and then uses a computational algorithm to construct an energy consumption model for the heat pump air conditioning system of a new energy vehicle. According to the test results, the system’s average increase in heat production after activating the enthalpy charge is 35% and its average COP is 0.14% lower than when switched off. At -5oC, the air outlet temperature of the system reaches up to 50.0oC. Summer cooling energy consumption increases exponentially with temperature, while winter heating energy consumption decreases linearly with temperature. In addition, the range decreases significantly as the ambient temperature deviates from the human comfort zone. The decline in the winter range is more severe than that in summer, moreover, the range of modern energy vehicles is reduced by 30% at an average winter temperature of -10oC. In summary, the low-temperature heat pump system offers greater performance. It is more useful in real-world applications and can offer a rational alternative to air conditioning’s energy-saving tactics.

Multi-source district heating system full decarbonization strategies: Technical, economic, and environmental assessment

Implementing innovation and sustainable development of district heating requires sound energy planning at the national and local levels. However, a research gap still exists on technically and economically viable solutions for developing fully decarbonized heat supply systems and assessing their environmental impact to provide a holistic analysis. Therefore, the case study-based research includes in-depth analyses of the current situation and existing operating conditions of the solar field, biomass and natural gas boilers, and thermal storage tank. The future fossil-free scenarios analyze the additional thermal storage capacity, and integration of heat pumps and local waste heat sources to reduce fossil fuel consumption, increase energy efficiency and minimize heat production costs.The results show that the waste heat utilization and heat pump integration allow to phase-out of natural gas for the peak load coverage. However, increasing thermal storage capacity by more than 50 % is crucial to store the thermal energy for peak demand-supply. The increased thermal storage tank volume allows higher biomass production rates and aligns the heat production with the available waste heat sources. The total heat production costs in a low-temperature heat pump scenario decrease by 5 %. The environmental impact assessment shows that the low-temperature heat pump scenario decreases the resource category by 61 % and the climate change category by 17 % in comparison with the Reference scenario due to higher solar yield and higher overall efficiency.

Study on the Performance of Photovoltaic/Thermal Collector–Heat Pump–Absorption Chiller Tri-Generation Supply System

The solar energy supply system has played an increasingly substantial role in realizing nearly zero-carbon buildings. In order to overcome the impact of solar randomness on the energy supply of a distributed solar system, this paper proposes a solar tri-generation supply system which integrates a photovoltaic/thermal collector (PV/T), a heat pump (HP), and an absorption chiller (AC). The PV/T-HP integration system is adopted to provide stable heating for a building and AC. The system model is established in TRNSYS software, and its performance is evaluated based on energy, exergy, and economic aspects. The results demonstrate that the system effectively meets the load demand, with an energy efficiency of 32.98% and an exergy efficiency of 17.62%. The payback period (PP) is 7.77 years. Compared with the systems proposed in the other literature, the performance of the proposed system has a certain extent of advantage. Furthermore, the equipment and system exergy performance decline with an increase in the intensity of solar radiation. Increasing the PV/T area effectively improves the system’s profitability within the actual roof area limitation of the building. Moreover, increasing the capacity of the low-temperature heat pump after 68 kW improves the system efficiency and reduces the payback period. In summary, this paper proposes an efficient distributed solar energy system that is suitable for urban building energy supply.

Classification, potential role, and modeling of power-to-heat and thermal energy storage in energy systems: A review

Most of the power-to-heat and thermal energy storage technologies are mature and impact the European energy transition. However, detailed models of these technologies are usually very complex, making it challenging to implement them in large-scale energy models, where simplicity, e.g., linearity and appropriate accuracy, are desirable due to computational limitations. In the literature, the main power-to-heat and thermal energy storage technologies across all sectors have not been clearly identified and characterized. Their potential roles have not been fully discussed from the European perspective, and their mathematical modeling equations have not been presented in a compiled form. This paper contributes to the research gap in three main parts. First, it identifies and classifies the major power-to-heat and thermal energy storage technologies that are climate-neutral, efficient, and technologically matured to supplement or substitute the current fossil fuel-based heating. The second part presents the technology readiness levels of the identified technologies and discusses their potential role in a sustainable European energy system. The third part presents the mathematical modeling equations for the technologies in large-scale optimization energy models. We identified electric heat pumps, electric boilers, electric resistance heaters, and hybrid heating systems as the most promising power-to-heat options. We grouped the most promising thermal energy storage technologies under four major categories. Low-temperature electric heat pumps, electric boilers, electric resistance heaters, and sensible and latent heat storage show high technology readiness levels to facilitate a large share of the heat demand. Finally, the mathematical formulations capture the main effects of the identified technologies.

Lubrication of R290 Rotary Compressor Used in Low-Temperature Air-Water Heat Pumps

Owing to the environmental friendly and low discharge temperature of propane (R290), R290 rotary compressor was proposed to use in low-temperature heat pump (ambient temperature of 7 ∼ -30°C). At present, research on lubricants of R290 rotary compressors used in low-temperature heat pumps is still lacking. Therefore, in this study, the oil sump thermophysical properties (temperature, solubility, and viscosity) of R290 rotary compressors used in low-temperature air-water heat pumps (LT-AWHP) and their influence on the oil film thickness of the bearings were studied experimentally and numerically. The study was conducted under different ambient temperatures (7, -12, -20, and -30°C) with different lubricants (PAG40 and POE68). The results show that under any condition, the oil sump solubility is uniform, and is close to the oil sump inlet saturation solubility (Soup-sat), while the viscosity in the oil sump outlet is higher than that in the oil sump inlet. Additionally, the oil film thickness and performance comparison show that POE68 is more suitable for low-temperature heat pumps than PAG40. We suggest that measures should be taken to decrease the oil sump viscosity under lower ambient temperatures and increase it under higher outlet water temperatures when using POE68. These results would provide guidelines for the choice of oil in the R290 rotary compressor for low-temperature heat pump and thus improving its performance and reliability.

Approximate Thermal Calculation of the Superheated Steam Condenser of Heat Pumps

A comparative analysis of the methods of approximate thermal calculation of the superheated steam condenser of the steam compression heat pump of the heat supply system is presented. The working substance of the heat pump and the condensing steam is the refrigerant R410a. When the single-zone method is applied, the condenser is calculated by one area with the inclusion of the heat of overheating in the heat of condensation and the use of the overheating coefficient. The two-zone method assumes the calculation of the condenser in two separate areas, viz. the cooling of superheated steam and its actual condensation. The approbation was carried out during a numerical study of a condenser of a low-temperature heat pump system for heating and hot water supply, with a heat exchange surface in the form of a spiral coil pipe in a pipe immersed in a heated liquid. In the first approach, the flow rate and temperature of the heated water are limited by the saturation temperature of the condensing refrigerant, regardless of the flow pattern of the working media. The method of two-zone calculation of the superheated steam condenser with a counter-current or cross-counter-current flow scheme of working media makes it possible to obtain real results of the temperature of the heated water that exceeds the temperature of the saturated refrigerant vapor, taking into account the flow rate of the heated water. In this case, the wall temperature in the cooling area is higher than the saturation temperature, and during condensation it is lower, which further confirms the adequacy of the presented technique. The use of a two-zone technique with a separate averaging of the physical properties of the working media in the areas of superheated steam and condensation cooling, as well as temperature pressures, provides a more accurate value of the heat exchange surface, which in the case under consideration is reduced to 20%. Based on the conducted studies, it is recommended to use a two-zone technique that makes it possible to obtain reliable data on the parameters of the superheated steam condenser.

Comprehensive assessment of operational performance of coupled natural ventilation and thermally active building system via an extensive sensor network

The thermally active building system (TABS) has emerged as an energy-efficient measure for cooling and heating buildings since it facilitates the use of low-energy technologies, such as free cooling with geothermal sources and heating with low-temperature heat pumps. However, the control of TABS is challenging due to its high thermal inertia. Though a considerable amount of research has been done using simulations or small-scale experiments, few studies have published detailed long-term measured data of TABS operation. In addition, there has been little discussion on the control integration of slow-acting TABS with other fast-acting systems, such as natural ventilation. To fill these gaps, this paper presents a comprehensive assessment of the operational performance of the coupled NV and TABS using one-year high-fidelity measured data of an experimental small office building with an extensive sensor network located in Cambridge, Massachusetts. First, the coupled system and control design are described. Then, the thermal, energy and detailed control performance are assessed by comparing with design intents and simulated predictions. It is found that the coupled system demonstrates its effectiveness to control indoor thermal environment while achieving high energy efficiency. The annual energy consumptions are 11.5 kWh/m2 for heating and 1.3 kWh/m2 for cooling. The heating COP, including heat pump and two water pumps, is 3.5, and the cooling COP is 18.3 by utilizing free cooling from geothermal resources. The issues in operation are identified, including system/equipment faults, implementation of controls, and settings by building operators. Finally, lessons learned from this project are discussed and the strategies for improvement are summarized.

Evaluation of The Use R450A as an Alternative to R134a in Low and Medium Temperature Heat Pump Systems: 4-E (Energy, Exergy, Environmental and Enviro-Economic) Analysis

This investigation presents a theoretical evaluation of the use R450A replace to R134a in the low and medium temperature heat pump systems. The energy, exergy, environmental and enviro-economic analyzes of heat pumps have been performed for two refrigerants (R134a and its alternative R450A). The thermodynamic and environmental analyses have been made for two heat source temperatures (10 oC and 20 oC) and six heat sink temperatures (from 30 oC to 55 oC). It has been observed that R450A's mass flow rate is lower than R134a. Because the density of R450A in the heat pump suction line is lower than R134a. Although the heating capacity of R134a is more than R450A, the reason why R134a has a similar COP value with R450A is that R450A's compressor energy consumption is lower than R134a. Exergy destruction occurred mostly in the compressor under all operating conditions for both refrigerants. At low heat sink temperatures, minimum exergy destruction occurs in the expansion valve, as the minimum exergy destruction occurs in the evaporator at high heat sink temperatures. The exergy results show that R450A behaves similarly to R134a. In conclusion, as alternative refrigerant R450A can be used in heat pumps replace to R134a according to energy, exergy, environmental and environmental-economic analysis results.

Comprehensive selection and assessment methodology of compression heat pump system

In the process of “Carbon peaking” and “Carbon neutrality”, compression heat pump is an effective technology to supply heat while improving energy utilization efficiency and shifting CO2 emission. In order to promote the application of compression heat pump and provide the most suitable heat pump system, the favorable design logic and selection strategy is important for researchers and designers. Therefore, a comprehensive selection and assessment system (S&A) of compression heat pump is proposed in this article. Taking waste heat temperature, required supply temperature, heating capacity, energy efficiency as input parameters, the system first classifies heat pump according supply temperature to select refrigerants, followed by choosing compressor and heat exchanger types based on heating capacity, and last selects configuration by efficiency requirement. All the satisfied combinations of refrigerant, compressor, heat exchanger, system configuration can be output, also the optimal combination based on the highest coefficient of performance (COP). Further, the selected heat pump systems will be assessed from economic and environmental aspects. Two case studies are carried on to show the selecting and simulating process for low-temperature and high-temperature heat pump, and to prove the reliability of compression heat pump S&A system.

Dynamic characteristics improvement of the single screw compressor with odd-grooves screw rotor

Recently, the single screw compressor (SSC) has been applied in the low-temperature heat pump system with the purpose of achieving higher system efficiency. However, the traditional SSC with even-grooves screw rotor behaves high torque fluctuation to the motor, thanks to its work synchronously and doubled compressed gas-induced torque on the screw rotor axis. It is more serious in the low-temperature heat pump system because of the higher pressure ratio. In this paper, an odd-grooves screw rotor design is proposed to smooth the compressed gas-induced torque on the screw rotor axis, by arranging the two compression chambers work alternatively. A dynamic model for the screw rotor is established to calculate the gas force and the gas-induced torque on screw rotors under different working conditions. The results show that the odd-grooves screw rotor design can remarkably reduce the gas-induced torque fluctuation of the SSC. Meanwhile, the maximal load on the bearing caused by the odd grooves design could be controlled below the maximum allowable load of the bearing.

Performance and Optimization Study of R290 as Alternative Refrigerant for R22 in Low Temperature Heat Pump System

This paper mainly studies the replacement performance of R290 in R22 low temperature heat pump system from the experimental point of view. By comparing the performance differences under different working conditions, it is found that when R22 is directly extracted from the original system and filled with R290, the heat capacity and COP of the system are attenuated, and the compressor discharge temperature and pressure of the R290 system are higher than those of the original R22 system in low temperature environment. Through the analysis of the system components, it can be considered that the main reason for the above phenomenon is that the compressor displacement of the R22 system is too large and does not match the R290 system. Therefore, in order to meet the safety requirements of the system and improve the overall performance of R290 in the low temperature heat pump system at the same time, it is considered to replace the compressor with a smaller displacement which is more matched with R290 in the system. The experimental results show that the compressor displacement optimization of the R290 low temperature heat pump system can effectively reduce the exhaust temperature and pressure of the system and improve the overall performance of the system. The COP of the optimized R290 low temperature heat pump system is 6.5% higher than that of the original R22 system, and the exhaust temperature in the low temperature environment is reduced by 36% to below 80 C.

Development of single screw compressor technologies and their tendency

Single screw compressor (SSC), a strong competitor of the twin screw compressor, is regarded as an excellent gas compressor because of symmetrical structure and balanced gas forces on the screw rotor. However, the market for the SSC is tiny compared with that for the twin screw compressor because of the serious abrasion of star-wheel teeth. In recent years, this problem has been solved due to the development of the new profiles of meshing pairs, and the SSC has attracted more attention due to its high application potential for low-temperature heat pump, mechanical vapor recompression, and two-phase expansion. Nevertheless, there are still many technical problems remained to be solved for applying SSC to these fields urgently. In this paper, a comprehensive review on the progress and development of SSC technologies is presented. The review first combs the current situation of SSC products. Besides, the design and processing of meshing pairs are clarified and analyzed. Moreover, the working process of SSC and its performance in new applications are discussed in detail. Finally, the further research direction of the SSC is also discussed, which will be beneficial for designing high-efficiency SSC.

Air Source Low Temperature Heat Pump Hot Water Technology

Air Source Low Temperature Heat Pump Hot Water Technology

Numerical investigation of thermal deformation of meshing pairs in single screw compressor

Single screw compressor serves as a core component in air compression system, mechanical vapor recompression system, low-temperature heat pump system and so on. However, under high temperature conditions, thermal deformation will lead to misaligned meshing between the vertical meshing pairs of the screw rotor and the star wheels. It would also lead to the serious abrasion of the components. Thus, it is necessary to investigate the thermal deformation mechanism of the single screw compressor to provide a guiding for clearance setting and strategy of assembling. In this paper, the thermal deformation mechanism of the meshing pairs in SSC is analyzed by using finite-element methods under the actual thermal boundary conditions tested by the infrared temperature camera. The results point out the deformation laws of the meshing pairs and torsional deformation of the screw rotor which cause the misaligned engagement. Influence of the thermal deformation on most of the clearances could be eliminated or controlled by setting reserved clearances. Such as the misaligned meshing could be controlled by reserving an axial assembling clearance and adjusting the bearing position. However, the torsional deformation of the screw rotor could only be modified by compensation machining. The method and results in this paper can be adopted for designing the meshing pairs and setting the design clearance of the compressor.

Investigating the kinetics at low temperature heat pump drying and conventional drying of osmotic dehydrated blueberries

The possibilities for low-temperature heat pump and conventional drying of blueberries during preliminary osmotic dehydration was studied. A full factor experiment of type 22+3 was performed. The kinetics of the process during variation of the main mode parameters: inlet temperature and thickness of the drying layer (load on the scaffolding) was studied. The curves of the drying process U = f (τ) and the curves of the drying speed dU / dτ = f (U) were constructed. The influence of the regime parameters on the drying time was established. Regression models of the drying coefficient during the second period of drying were presented.

Design and Transient Analysis of a Natural Gas-Assisted Solar LCPV/T Trigeneration System

This paper proposes a natural gas assisted solar low-concentrating photovoltaic/thermal trigeneration (NG-LCPV/T-TG) system. This novel system simultaneously provides electrical, thermal and cooling energy to the user. The design and dynamic simulation performance of the NG-LCPV/T-TG system is completed using Transient System Simulation (TRNSYS) software. The results show that the system can satisfy the requirements of the cooling and heating load. The proposed system maintains the experimental room temperature at about 25 °C under the cooling mode, at about 20 °C under the heating mode. The electrical and thermal energy produced by the low-concentrating photovoltaic/thermal (LCPV/T) system are 3819 kWh and 18,374 kWh. Meanwhile, the maximum coefficient of performance (COP) of the low temperature heat pump (LHP), high temperature heat pump (HHP) and chiller are 5, 2.2 and 0.6, respectively. This proposed system realizes the coupling of natural gas and solar energy in a building. In summary, this trigeneration system is feasible and it promotes the implementation of building integrated high-efficiency energy supply system.

Performance evaluation and multi-objective optimization of a low-temperature CO2 heat pump water heater based on artificial neural network and new economic analysis

For the advantages of high efficiency and low impact to the environment, CO2 air source heat pump water heater (ASHPWH) is applied to produce domestic water, which also reveals good potential in cold regions. In order to boost the system performance and practicability under low ambient temperature, optimization for CO2 ASHPWH is conducted using non-dominated sorting genetic algorithm (NSGA-II). A validated artificial neural network (ANN) predicts energy parameters for the optimization. And an economic model provides economic and environmental parameters, which considers the influence of housing price, tank volume, and on/off-peak electricity price, rarely taken into account in published studies. Then the optimizing progress is conducted under −20 °C ambient temperature and 9–65 °C water temperature, in which four optimized variables are selected: gas cooler outlet temperature (Tgc), heat rejection pressure (Pgc), compressor displacement (qvh) and water tank volume (Vwt). The final solution of Tgc = 15 °C, Pgc = 8294.1 kPa, Vwt = 0.3647 m3, qvh = 401.33 mL/s results in two objectives (CO2 emission and total annual cost) of 8599.4 kg and 1626.9 $/year, revealing advantages both in energy and economy. It is noteworthy that the cost of the space occupied by system is the fourth important factor in capital cost. These results lay solid foundation for further studies and system application.