Cascade Heat Pump Research Articles

Optimizing working fluids for advancing industrial heating performance of compression-absorption cascade heat pump

Independent cascade hybrid heat pump (ICHHP) can bridge the large temperature gap between low-grade air sources and high-temperature industrial demands. However, the working fluids used in previous studies are just considered sufficient as long as they perform their functional role. They may not always be the most suitable choices in different situations, and the maximum efficiency of ICHHP has not been achieved. Ionic liquids (ILs) as alternative absorbents have shown the potential to enhance ICHHP performance with their unique ability to tailor thermal properties by freely combining anions and cations. However, the scarcity of IL thermodynamic properties data, underscored by limited vapor-liquid equilibrium experiments, has impeded the full exploitation of this inherent advantage. Obviously, the lack of dedicated research on screening optimal working fluids limits ICHHP performance. To address the identified limitations, the optimal fluid is recommended in this paper by comprehensively evaluating the performance among 100 fluid combinations under different working conditions. The results indicate that R161 is the best choice for the compression subloop. For the absorption subloop, the performance improvement is more sensitive to the anionic species, with the order of influence generally being [OAC]− > [Br]− > [OMS]− > [TFA]−. Specifically, H2O/[EMIM][OAC]—R161 stands out, with maximum improvements in coefficient of performance (COP) and exergy coefficient of performance of 9.4% and 5.6%, respectively, compared to other candidates. Furthermore, it doubles the COP relative to the reference fluid H2O/LiBr—R134a. Consequently, H2O/[EMIM][OAC]—R161 is the superior working fluid, significantly advancing industrial heating efficiency for ICHHP in large temperature lift conditions.

Optimal intermediate temperature of two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling

This paper presents a model for finding the optimal intermediate temperature to achieve the highest coefficient of performance of the standard two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling. The optimal intermediate temperature remains unaffected by the pinch point temperature at the cascade heat exchanger, though this temperature does impact heat pump performance. The optimal intermediate temperature rises with increasing high-side condensing and low-side evaporating temperatures and the value is deviating from the average temperature between the condensing and evaporating temperatures of the high and low-temperature sides of the cascade heat pump, respectively by a correction factor. The factor is found to be dependent on the critical temperature ratio of the working fluids in the low-temperature side and high-temperature side including the condenser temperature of the high-temperature side of the cascade heat pump cycle. A developed model consisting of the average temperature with the correction factor for estimating the optimal intermediate temperatures were examined with a set of non-azeotropic refrigerants with different compositions at different high-side condensing (65–95 °C) and low-side evaporating (5–15 °C) temperatures and pinch point (2–11 °C) temperatures. The results on the optimal intermediate temperatures and the maximum COPs agreed very well with those calculated from the enthalpy method within ±7 % and ±3 % deviations, respectively.

HAPPENING: an efficient cascade heat pump system for multifamily buildings

An innovative hybrid heat pump system is presented as a novel solution for multifamily building heating system retrofitting. This system is very efficient on the one hand thanks to its innovative configuration in cascade, minimum thermal losses and on the other hand, the maximization of local solar energy self-consumption due to decoupling generation and consumption and applying smart control strategies. It is aimed at facilitating retrofitting of existing heating systems in multifamily buildings, enabling an efficient decarbonization. Three variations of the system have been designed, installed, and tested in real buildings across Europe, within the H2020 project “HAPPENING”. The project is now under monitoring phase and the preliminary results are positive, demonstrating the high efficiency of the concept

Vapor absorption, compression and cascade heat pumps for carbon capture plants: Multi-criteria analysis and techno-economic assessment with different working fluids

Post-combustion CO2 capture plants include high costs that are associated with intense energy consumption. Heat pumps can recover heat from the flue gas and other sources in the CO2 emitting plant and upgrade it to reduce the use of fossil-based steam. The few studies regarding heat pumps in CO2 capture systems include arbitrary selection among limited working fluids, without optimizing the underlying cycle. This work implements a multi-criteria investigation of the performance of 20 working fluids for vapor compression heat pumps (VCHP) and 21 refrigerant/absorber pairs for absorption heat transformers (AHT), considering the operating optimization of each cycle for each fluid. A cascade cycle combining the two configurations (AHT-VCHP) is proposed, using the optimum fluids cyclopentane and water/potassium nitrate identified for the VCHP and the AHT. The annual operating costs are higher than the annualized capital expenditures. The proposed fluids outperform the conventional options. The VCHP with cyclopentane covers 58 % (2.68 GJ/t CO2) of the reboiler duty with recovered heat and exhibits the lowest cost of $33.8/tCO2. This is $18.7/tCO2 lower than the cost of the conventional direct contact cooling unit that is used in capture plants. The VCHP remains competitive for up to 57 % increase in the electricity price.

Synthesis of heat-integrated water networks: Integration of heat pumps

A typical heat-integrated water network (HIWN) synthesis problem aims to minimise freshwater and utility consumption while establishing trade-offs between operating and investment costs by exploring different water and heat integration opportunities. Heat integration between hot and cold water streams is performed to save external heating and cooling utilities required to achieve the target temperatures of these streams. This paper proposes a mathematical programming approach to integrate water-to-water heat pumps into the HIWNs. A simple cascade heat pump system with isobutane as a natural working fluid enables a larger temperature lift between the heat pump source and the sink. The objective function of the proposed mixed-integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater, heating/cooling utilities, electricity, and investment in heat exchangers and heat pumps. A sensitivity analysis is performed to evaluate the economic feasibility of the system by varying the heating/electricity cost ratio and depreciation period for the capital investment. The results show that additional utility savings can be obtained by integrating heat pumps into the HIWNs. For threshold problems, a heat pump is feasible for heating/electricity price ratios from 1.175, and heating utility is replaced by electricity for a 1:1 ratio. However, for pinched problems, the feasibility of the heat pump is shifted to lower heating/electricity price ratios, from 0.3 to 0.5, depending on the annualisation factor for the investment.

Development and performance evaluation of a high solar contribution resorption-compression cascade heat pump for cold climates

Absorption-resorption heat pumps (ARHPs) have great potential in utilizing low-temperature solar heat for efficient winter heating, but are limited by their weak adaptability to cold climates relative to absorption heat pumps (AHPs) and vapor compression heat pumps (VCHPs). In this work, a resorption-compression cascade system consisting of ARHP and VCHP subsystems is proposed and investigated, with the goal of achieving efficient winter heating with improved solar contribution in cold climates. Thermodynamic models of the two subsystems and of the whole cascade system are developed for system feasibility verification and performance evaluations. Feasible operation temperature and pressure conditions are identified in terms of internal subsystem matching. Based on these operation conditions, the primary energy ratio (PER) – which is a key performance evaluation index – is investigated over a range of solar fractions (f). The results show that for cases without solar contribution, the PER can be generally >0.95 under feasible operating conditions. In addition, the primary energy saving ratio (PESR) and heating capacity lift ratio (εlift), as well as the heat source temperature (Th) and ambient temperature (Tamb) restriction, are all investigated and compared to those of other, competing heating systems. The results show that the proposed system can reduce the Th demand to 69 °C (with a minimum Tamb of −21 °C) and extend the operational Tamb to −31 °C (with a minimum Th of 90 °C), while importantly achieving PESR > 0.20 and εlift > 20 % under the above extreme conditions. Moreover, when integrating the system with solar heat with f > 43 %, the proposed solar-assisted system has a clear PER advantage over an equivalent VCHP system in the Tamb range from −31 °C to 10 °C, highlighting the possibility of achieving higher solar contribution in cold climates.

Performance analysis of a novel air source ammonia/water cascade heat pump for heating buildings in subarctic climate

Performance of available air-source heat pumps (ASHP) for buildings is extremely influenced by the outdoor air temperature (OAT). When OAT falls below –20 °C, especially in subarctic climate, most commercially available ASHPs lose their defined functionality. To address this issue, a novel cascade heat pump (CHP) is proposed that utilizes two natural (with zero global warming index) refrigerants: ammonia, aka R717, as the low stage (LS) refrigerant, and water, aka R718, as the high stage (HS) working fluid. The proposed system is equipped with two rotary vane compressors for LS and HS. To desuperheat the compressor discharge temperatures and increase the HS heating capacity, liquid refrigerant is injected into the chamber of both LS and HS compressors. The wet compression is thermodynamically modeled and validated against experimental data available in the literature. Optimum design parameters such as the maximum allowable temperatures during compression and the minimum vapor quality after injection are obtained where the COP is maximized. Total exergy destruction, number/location of injectors and the injected amounts are also investigated. The proposed CHP can deliver 6.35 kW heating load at 50 °C with a COP of 1.985 when OAT is –40 °C. From a broader perspective, the proposed CHP not only proves its efficacy in extremely cold subarctic climates but also emerges as a pivotal player in addressing the global warming issue. This is accomplished through a dual approach to decarbonization—firstly, by electrifying heating processes and secondly, by substituting phased-out refrigerants with environmentally friendly natural alternatives.

Applicability performance evaluation of cascade heat pump for building electrification in winter

This study aimed to evaluate the performance of indoor air heating and domestic hot water supply using a cascade heat pump system. The focus was on using existing heat pump equipment to satisfy system requirements in the context of building retrofitting. The experimental setup was located in nearly net-zero experimental building of the South Korea. The system was configured to heat air in the low-cycle heat pump and supply hot domestic water in the high-cycle heat pump. The study compared three operational modes: a low-cycle-only mode; a hot water-only mode; and a combined mode. While low-cycle-only mode and hot-water-only mode fulfilled the required condition with 3.94–5.38 COP and 60 °C hot water temperature with a flow rate of 15.8 LPM separately, the combined mode failed to reach the target air temperature fulfilling only hot water temperature. In conclusion, the system demonstrated satisfactory performance when heating indoor air and providing hot water separately. However, the indoor heating performance declined when both functions were operated simultaneously. To address this issue, possible solutions could include revising the system operation logic to better align with heating and domestic hot water usage schedules or altering the refrigerant flow path.

Thermodynamic analysis of high temperature cascade heat pump with R718 (high stage) and six different low-GWP refrigerants (low stage)

Decarbonizing industrial heating globally is a significant challenge, driving the integration of high-temperature heat pumps for heat recovery and electrification in the industrial sector. However, current high-temperature heat pumps with low global warming potential refrigerants face limitations in delivering high-temperature heat due to their restricted compressor discharge temperature or require several compressors/intercoolers. To address these issues, a novel high-temperature cascade heat pump system is proposed, utilizing water (R718) in the high stage, while selecting one among six low GWP refrigerants (R1234ze(Z), R1233zd(E), R1336mzz(Z), R600, R600a, R601) in the low stage, with single compressor at each stage. To manage high discharge temperatures of high stage, liquid water injection is applied during high stage compression, modeled for both wet and dry steam. The model is validated against available experimental data with maximum error less than 5 %. Further, the effectiveness of the internal heat exchanger in low stage cycle is assessed for all investigated low stage refrigerants. Results reveal that R718 + R600 offers the optimal refrigerant working pair, achieving a maximum COP × VHC of 2895.1 kJ m−3 for delivering high temperature heat at water condensation temperature of 190 °C and recovering heat from low temperature waste energy of 50 °C.

Energy performance analysis of a cascade heat pump system for heating in a semiconductor fabrication plant

To reduce carbon emissions, active research is being conducted on replacing the heating equipment using fossil fuels with heat pumps. While heat pumps still utilize electricity and have associated carbon emissions, they possess the potential to consistently decrease carbon emissions owing to the growing availability of renewable energy sources.In this study, the applicability of a heat pump system capable of supplying 86 °C hot water for heating a semiconductor fabrication plant was verified. In addition, there are a few results considering the energy saving effect that can be obtained from the heat source. Therefore, reflecting the characteristics of a semiconductor fabrication plant, which requires a huge cooling load to remove heat from equipment even in winters, a cascade heat pump system using a chiller as a low-cycle heat pump was proposed. Compared to a boiler, the heat pump system has the advantage of reducing the load on the typical chiller system as it can supply cold and hot water.On comparison, the cascade heat pump system, which incorporates the energy-saving effect derived from the heat source, demonstrated 66 % of primary energy and 56 % carbon emissions than the boiler. Consequently, the cascade heat pump outperformed the boiler system.

Comparative study of the cascade absorption heat pump running optimal ternary ionic liquid working pair

Novel working pairs are one of the significant measures to enhance the performance of the absorption heat pump. Ternary ionic liquid working pairs (Refrigerant-Salt-Ionic liquid) can alleviate the economic constraint of ionic liquids, but the influence of additional salt-absorbent on the performance of binary ionic liquid working pairs (Refrigerant-Ionic liquid) has not been evaluated. Furthermore, the number of candidates for ternary ionic liquid working fluids is still limited. To fill the above research gaps and advanced absorption technology, a novel ternary ionic liquid working pair is proposed in this work first, and its thermal properties are measured experimentally. Then taking the compression-absorption cascade heat pump as the subject, the differences in cooling performance between various ternary ionic liquid working pairs and binary reference fluids are simulated. The results show that the addition of salt-absorbent does not affect the applicable source temperature range of original ionic liquid working fluids, but it helps to improve the system efficiency and cooling capacity. In particular, the ternary ionic liquid working pair proposed in this work deserves priority recommendation, as it can achieve up to 12.59% higher maximum second law efficiency and 1.95% ∼ 87.87% higher cooling output than other candidates, as well as make cascade heat pump earlier freed from the assistance of compression sub-loop.

Influence of composition of binary mixed working fluids on the performance of cascade heat pump

The optimized design of the cascade heat pump facilitates the recovery and reuse of waste heat in printing and dyeing wastewater. This paper constructs a cascade heat pump system with two-stage cycles sampling the same binary mixed working fluids. The effects of three kinds of combinations of working fluids, including R134a/R152a, R134a/R22, and R152a/R22, on the COP and heating power, are discussed through the thermodynamic cycle process simulation when the component ratio varies. It is found that, for the R134a/R152a mixed working fluids, the system performance can be adjusted approximately linearly by changing the component ratio. For the two mixed working fluids of R134a/R22 and R152a/R22, the performance of the cascade heat pump system is closer to the case of using pure R134a and R152a as corresponding working fluids. The system’s heating capacity is significantly reduced, compared with the case of pure R22 employed. Besides, with the component ratio of working fluid increasing, the COP and heating power change with opposite trends.

A semi-cascade heat pump system for different temperature lifts

Conventional cascade heat pumps are widely applied to large temperature lifts between the heat sink and the heat source (ΔTsource-sink). However, this system configuration struggles to maintain energy efficiency over a wide range of operating conditions, especially for small ΔTsource-sink or large temperature lifts on the heat sink side (ΔTsink). To address this challenge, a novel semi-cascade heat pump system is proposed for flexible operation across varying temperature lifts. The system can operate in three modes, including single-stage mode, conventional cascade mode, and semi-cascade mode with stepped heating. To verify the advantages of multi-mode operation, various conditions are considered for ΔTsource-sink of 20–110 °C and ΔTsink of 10–100 °C. A steady-state thermodynamic system model using GREATLAB solver is developed and validated. Based on the parameter optimization, the energy performance of each mode is analyzed and mode switching correlations are developed. The results show that the single-stage mode performs best for both small ΔTsource-sink and ΔTsink, while the conventional cascade mode is suitable for relatively large ΔTsource-sink and small ΔTsink. The semi-cascade mode maintains high efficiency over a wider range of ΔTsource-sink and ΔTsink. The novel system achieves up to 22.3 % efficiency improvement versus the conventional cascade system, demonstrating promise for applications with a wide range of temperature lifts.

Thermodynamic analyses and performance improvement on a novel cascade-coupling-heating heat pump system for high efficiency hot water production

In the background of global warming and reducing carbon emission, air-source heat pump water heater is used as an energy efficient technology to improve the efficiency of building hot water production, which may account for 21–41% of building energy cost. However, there are still space for the energy efficiency improvement of heat pump water heater, as there is large temperature difference between water and refrigerant, which may limit the energy performance (especially at low ambient temperature) but receives little attention. Therefore, a novel cascade-coupling-heating heat pump system with less water-refrigerant temperature different is proposed in this paper, where part of the heat is first provided at the low-temperature stage, reducing the heat load at the high-temperature stage and the high-temperature stage compressor power, and increasing system energy efficiency. To study the system, a theoretical model was established and verified through experiments. Experimental and theoretical results show that: in the ambient temperature range of 243.15 K ∼ 323.15 K, the average and maximum improvement in the coefficient of performance of the new system reach 34.8% and 107.7%, respectively, comparing with cascade heat pump; and in the ambient temperature range of 248.15 K ∼ 323.15 K, when comparing with single-stage heat pump, the new system reveals average and maximum improvement in the coefficient of performance as high as 25.8% and 55.6%, respectively. This study provide a promising potential technology for efficiency water heating, and lay the basis for further study and application of the new system.

Technical-economic-environmental analysis of high temperature cascade heat pump with R718 (high stage) and six different low global warming potential refrigerants (low stage)

High temperature cascade heat pump with two separate refrigerants is a crucial system for decarbonizing industrial heating through electrification. The selection of optimal working refrigerants and temperature lifts relies on not only the thermodynamic analysis outputs such as coefficient of performance but also economic and environmental analyses. In this study, a technical–economic-environmental investigation was conducted for the proposed high-temperature cascade heat pump employing water as the high-stage refrigerant and six low global warming potential refrigerants in the low stage. To facilitate a detailed economic-environmental analysis, precise equipment sizing was undertaken. A comprehensive heat transfer model was developed to size the falling film shell-and-tube cascade heat exchanger, and empirical correlations were employed to size the internal heat exchanger at various cascade temperature lifts. The cascade heat exchanger model is validated against experimental data with less than 5 % error. The distribution of refrigerant charged mass was illustrated using instantaneous density and temperature data. To consolidate all thermal, economic, and environmental sub-indicators into a single parameter, the coefficient of performance, volumetric heating capacity, total annual cost, and total equivalent warming impact were combined as (COP × VHC)/(TAC × TEWI). This proposed comprehensive indicator can be applied to high-temperature cascade heat pumps to identify the optimum working pair and conditions. The results highlighted that R718 + R1234ze(Z) and R718 + R600 were the best working refrigerant pairs, considering all energy, economic, and environmental criteria.

Waste heat recovery of the hydrogen–water mixture from high‐temperature water electrolysis by cascade heat pump for steam generation

AbstractWaste heat recovery is common in high‐temperature water electrolysis production, but the latent heat from the hydrogen–water mixture is not fully utilized, and the electric heater to generate steam consumes much energy. In this research, a cascade heat pump is proposed to recover the latent heat from electrolysis products and generate steam. The heat pump can save as much as 65% electric energy compared with a single electric heater. Although in some cases, the latent heat is not enough to generate sufficient steam, it can still save 46.1% of energy. Also, this research emphasizes the significant influence of hydrogen and water proportion in electrolysis products. Compared with 80% water proportion, 50% water proportion can save 1.67 MW energy just in the water vaporization process for a 10 MW electrolyzer. The payback period is 3.43 years, which makes it worth investing.

Thermodynamic analysis of cascade high-temperature heat pump using new natural zeotropic refrigerant mixtures: R744/R600 and R744/R601

Many Industrial applications demand clean, high-temperature heating to meet their requirements. High coefficient of performance (COP) and low-pressure ratios are the keys to the development of cascade heat pump systems. The present work investigates the high-temperature heat pump (HTHP) using a two-stage cascade refrigeration system to produce hot water of more than 100 °C and up to 118 °C. MATLAB was used to design and simulate the HTHP model. The results of the simulation were analyzed to explore the production of high-temperature water. The composition of mixed natural zeotropic refrigerants, such as CO2+butane and CO2+pentane mixtures were introduced in the Low-stage (LS) and High-stage (HS) cycle, respectively. Water was used as the secondary fluid and as a heat source with a temperature range of 10–50 °C. The temperature lift, heating capacity, heat sink, hot water delivery temperatures, HS, LS, and total COP, were investigated to identify the performance of the system and expounded in detail. The maximum heating capacity of the system can reach 205 kW. The system's total COP was 4.5, one of the top-notch results in this HTHP research. Overall, the system's use of natural refrigerants, highly efficient performance, and high-temperature water heating abilities make the investigation of high importance. A comparison of the present and published results indicates a substantial improvement of 36% of the total COP. The pressure levels in both LS and HS indicate that the system pressure requirements are in good agreement to operate the system effectively with excellent performance.

사무실 건물의 다기능 히트펌프 적용에 대한 성능 및 에너지 소비량 분석

This study aimed to evaluate the energy consumption performance of a multi-functional air-source heat pump for the indoor air-conditioning, heating, and hot water supply of an office building. Based on the investigation, four heat pump system configurations are proposed for the comparative evaluation of each system. Two reference systems are used for comparisons in the energy performance evaluation. The detailed thermal load for a 100 ㎡ office building was conducted using commercial energy simulation tools. Thermodynamic theoretical equations were used for heat pump simulation. The results show that case 2, the cascade heat pump system, can reduce primary energy consumption by 41.7 % and 54.0 % when compared with two reference systems and showed the highest energy-saving performance in the proposed cases. This study validated the use of the multi-functional cascade heat pump system for use in an office building.

Energy, exergy, exergoeconomic and environmental (4E) analysis of cascade heat pump, recuperative heat pump and carbon dioxide heat pump with different temperature lifts

Air source heat pump is recognized as an effective environmental protection and energy-saving technology. The heating temperature lift is one of the key factors affecting the performance of heat pump systems. In this paper, an in-depth quantitative thermodynamic comparison of cascade heat pump, recuperative heat pump and CO2 heat pump for direct-heating and space heating was conducted. Aspen HYSYS was used to invoke advanced genetic algorithm in MATLAB to optimize the system parameters. The temperature matching in heat exchangers was employed to reveal the essence of the performances of different heat pump systems. The recuperative heat pump is more suitable for direct-heating with the temperature lift 60 °C, and its COP is up to 4.81 at the ambient temperature of 20 °C. The cascade heat pump performs best in space heating with the maximum COP of 2.47. The exergoeconomic factor of the compressor is less than 7%, which illustrated that the investment in compressor should be increased to further improve the system efficiency. The annual CO2 and SO2 lowest emissions is 4431 kg and 130 kg of recuperative heat pump for direct-heating, and the cascade heat pump has the lowest annual 4234 kg CO2 and 127 kg SO2 emissions for space heating.

Comparative analysis of sizing procedure for cooling and water heating cascade heat pump applied to a residential building

An appropriate design strategy suitable for the purpose of a building should be considered to apply a cascade heat pump system for multiple functions to buildings. The purpose of this study is to propose a sizing process and evaluate a capacity calculation simulation of air-to-air/water cascade heat pump systems applied in residential buildings that supply indoor cooling and hot water. A simulation study was performed by dividing the capacity calculation method suitable for the building conditions into two cases when this multi-functional cascade heat pump system was applied to a residential building. Case 1 was a sizing method that considered a low-cycle heat pump for indoor cooling. The existing heat pump sizing method for the hot-water supply used in the previous study was set to Case 2. The heat pump simulation and heat calculations were conducted based on thermodynamic theoretical equations. A simulation tool capable of dynamic thermal load analysis was used for model building. Detailed building information was entered in accordance with design standards. The results showed that Case 1 completely satisfies both the cooling and hot-water supply performance of the heat pump compared to Case 2. Based on Case 1, when the temperature of the intermediate heat exchanger was 10 °C higher than the outdoor temperature, the air volume of the indoor unit and the outdoor unit was found to be 941 m3/h at an appropriate air volume rate. In addition, the energy consumption was also the lowest at 2.29 kW h, and the maximum system COP of 6.0 was achieved.