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The compression-assisted air source absorption heat pump (CASAHP) is a promising alternative heating system in severe operating conditions. In this research, parameter studies on the annual performance under various compression ratios (CRs) and source temperatures are performed to achieve the maximum energy saving rates (ESRs). Economic analyses of the CASAHP under different CRs and partial-design ratios are conducted to obtain an optimal design that considers both energy savings and economy improvements. The results show that the optimal CR becomes higher in colder regions and with lower heat source temperatures. For a source temperature of 130 °C, the optimal CR values in all of the cities are within 2.0. For source temperatures from 100 to 130 °C, the maximum ESR is in the range of 17.7–29.2% in the studied cities. The efficiency improvement rate (EIR) caused by compression in a severe source condition can reach 10.0–20.0%. From the viewpoint of economy, the relative investment of CASAHP is reduced to 30–60% with a CR of 2.0–3.0. With a 2–6% sacrifice in ESR, the payback period can be reduced from 12–32 to 5–9 years using compression. Partial-design of the CASAHP can further reduce the payback period to 3–6 years with a partial-design ratio of 50% and a CR of 2.8. Additionally, CRs and partial-design ratios are designed comprehensively by seeking the maximum ESR for a given acceptable payback period. [ABSTRACT FROM AUTHOR]

Highlights: [•] We propose a new concept, heat adaptor, to optimize thermal system performance. [•] It is a combination of heat engine, heat pump and conventional heat exchangers. [•] The most suitable device and process can easily be identified using the concept. [•] A typical problem is used to demonstrate the advantages of applying the concept. [Copyright &y& Elsevier]

Highlights: [•] A double-stage coupled air source absorption heat pump (ASAHP) is proposed. [•] The coupled ASAHP exhibits stable and high performance in very cold regions. [•] Energy-saving rate of the coupled ASAHP in all the typical cities is above 20%. [ABSTRACT FROM AUTHOR]

Heat supply systems based on absorption heat pump were assessed to have great potentials on energy savings and emissions reduction. A prototype of NH 3 -H 2 O water source absorption heat pump (WSAHP) designed for low temperature heating was experimentally investigated under different working conditions. The effects of driving source, hot water and source fluid temperature on the heating performance were studied. As driving source increases from 110 °C to 140 °C with 15 °C evaporator inlet and 45 °C hot water, COP increases from 1.429 to 1.552 and then decreases to 1.495, while the heating capacity increases from 32.23 kW to 88.35 kW. As hot water increases from 30 °C to 45 °C with 130 °C generator inlet and 15 °C evaporator inlet, COP decreases from 1.653 to 1.449, while the heating capacity drops from 94.55 kW to 60.37 kW. As the source fluid increases from −10 °C to 30 °C with 130 °C generator inlet and 45 °C hot water, COP increases from 1.203 to 1.609, while the heating capacity increases from 39.51 kW to 79.72 kW. Comparisons with former work indicate that the developed prototype can operate under evaporator inlet temperatures as low as −18 °C, which is significant to improve heating applicability in colder conditions. [ABSTRACT FROM AUTHOR]

Ground-coupled heat pumps are widely applied in cold regions, but soil thermal imbalances in some projects greatly deteriorate the systems’ practical performances and heavily inhibit the systems’ reasonable applications. This paper presents a comprehensive overview of the main problems caused by soil thermal imbalance and the existing solutions targeting on them. The caused problems mainly present in the following aspects: soil temperature decrease, heating performance deterioration, heating reliability decline, and even system failure. Three kinds of existing solutions can be classified. Ground heat exchanger-modified solutions including increasing borehole space, increasing borehole length, modifying borehole layout, and improving thermal properties, can retard the thermal imbalance to some extent, suitable to projects with slight imbalance. System-modified solutions integrating auxiliary energy sources like fossil fuel, solar energy, ambient air and waste heat with ground-coupled heat pumps or using ground-coupled absorption heat pumps, can eliminate serious thermal imbalance essentially by increasing soil thermal injection and decreasing the extraction. Thus, they are dominant in the applications and attract more attention along with the utilization of renewable energies. Apart from the modifications of system design, operation-modified solutions, like seasonal and intermittent operations, are also helpful to maintain better soil thermal balance. Based on the overview, the current application situations and economic performance of different solutions are discussed. Finally, some suggestions for the applications of ground source heat pumps in cold regions are proposed to make the overview helpful for the system research and applications. [ABSTRACT FROM AUTHOR]

• Efficient operating range of the original district heating substation is below 1℃. • Proposed system formed by original components operates efficiently in whole winter. • Control strategies are based on the lowest primary return water temperature. • The proposed system has advantages in energy and economic performance. With the rapid development of heating technology, heating terminals are developing toward low temperatures and diversification, resulting in the simultaneous presence of different temperature demands in district heating systems. To reduce the primary return water temperature and satisfy the temperatures of different terminals, a dual-temperature district heating substation supplying dual temperatures of hot water with two absorption heat pumps and two heat exchangers has been proposed based on the design conditions. However, whether the dual-temperature district heating substation can operate and how to control this system during the entire heating period have not been explored. In this paper, the models for the substation with two absorption heat pumps and two heat exchangers and its subsystems were established to analyze and compare the operational conditions for the substation and its subsystems at different outdoor temperatures. By comparing the primary return water temperatures of all subsystems, control strategies and a combined system for efficient operation during the heating season were determined. Then a case study and analysis of the combined system were conducted under the proposed control strategies and compared with two other systems – two heat exchangers and two absorption heat exchangers – used in dual-temperature heating stations. Results show that 1) The original system with two absorption heat pumps and two heat exchangers could only operate when the outdoor temperature was −8–1℃. 2) The subsystem with a high-temperature absorption heat pump and two heat exchangers and the subsystem with two heat exchangers which could achieve efficient operation under outdoor temperature ranges of 1–7 °C and 7–11 °C respectively were considered in the combined system to broaden the operating range of the original system. 3) The combined system had the lowest return water temperature (23.8℃) and the highest coefficient of performance (6.60) during the entire heating season. The payback period of the combined system was 1.54 years. This study provides a reference for the efficient operation of dual-temperature district heating substations at different ambient temperatures during the heating season. [ABSTRACT FROM AUTHOR]

The heat supply system combining a conventional boiler with an air source absorption heat pump (ASAHP) has been evaluated as having great potential in energy saving and emission reduction. However, an economic analysis needs to be performed before its wide applications. To improve the economy of ASAHP from a design perspective, a year-round simulation and techno-economic analysis are conducted. Results show that the energy saving rate (ESR) does not decrease much when the relative design capacity of ASAHP is within 50–100%. At a relative design capacity of 75% in Harbin, 35% in Shenyang, 50% in Beijing, and 50% in Zhengzhou, ESR can reach 21.6%, 24.3%, 26.2% and 26.3%. Given an ASAHP price of 1.5 CNY/W and a gas price of 4.0 CNY/Nm 3 , the payback period is 6.8, 6.6, 4.1 and 4.6 years in these four cities, at a 50% relative design capacity. There is a conflict between ESR and the payback period, but a small sacrifice (1.5–6.0%) in ESR can contribute a great reduction (38–48%) in the payback period. The heating period and energy price in the local area, as well as the expected ESR and acceptable payback period should be comprehensively considered for a better energy efficiency and economy. [ABSTRACT FROM AUTHOR]

The use of ammonia-based working fluids for absorption prevails in a wide range of applications due to the low freezing temperature of the refrigerant and the absence of crystallization as well as the lack of problems under vacuum conditions. This paper presents a comprehensive overview on the use of ammonia-based absorption chillers and heat pumps. The thermodynamic and physical properties of pure ammonia and binary and ternary ammonia mixtures are presented in correlation formulas. Developments and applications in subfreezing refrigeration, heating/domestic hot water, renewable energy utilization, waste heat recovery, thermal energy storage and miniaturization of absorption systems are presented and summarized. In subfreezing refrigeration, the evaporation temperatures for single-stage absorption lie mainly between −30°C and −5°C, and they can reach as low as −70°C in advanced absorption systems. Air-source and ground-source absorption heat pumps are suggested for heating/domestic hot water applications in cold regions. For renewable energy uses, ammonia-based solar absorption applications with various working fluids are quite popular, whereas geothermal and biomass energy systems are less studied. In thermal energy storage, ammonia-based working fluids are not advantageous for storage capacity or cycle efficiency, but they prevail for subfreezing energy storage. Additionally, ammonia-based fluids are also attractive options for the miniaturization of absorption systems due to the absence of crystallization. [ABSTRACT FROM AUTHOR]

As heating requirements continue to evolve and heating technology advances, radiators and radiant floor heating devices, which require different supply-water temperatures, have gradually become the two main heating terminals in district heating systems. However, the primary return-water temperature of current solutions based on heat exchangers and heat pumps is relatively high and fails to recover waste heat from the combined heat and power plants, owing to improper use of heat sources. To address this issue, a heat exchanger network optimisation method—the thermal energy discretisation and matching method—was used to design a dual-temperature water district heating system consisting of two absorption heat pumps and two heat exchangers. The primary supply water flows through the two generators, two heat exchangers, and two evaporators of the absorption heat pumps. The radiator and floor heating return water was divided into two parts and heated by the heat exchanger and the absorber and condenser of the absorption heat pump, respectively. The performance of the designed heating station was evaluated using the coefficient of performance and the primary return-water temperature and compared with that of other district heating systems. The changes in the district heating system's configuration when the boundary conditions changed were investigated. Results showed that (1) the designed system based on thermal energy discretisation and matching method -had the lowest primary return-water temperature of 18.8 °C and the highest coefficient of performance of 5.62. (2) When the low-temperature load ratio changed from 0% to 100% while maintaining a constant total load, the system's form changed regularly. (3) When the primary supply-water temperature increased, the heating station's form remained unchanged and the capacity of the heat exchange equipment changed regularly. (4) When the secondary supply and return-water temperatures increased, the evaporation temperature of the absorption heat pump increased, resulting in a change in the system's form. This study provides valuable insight into the design of district heating systems that supply dual-temperature water under various design conditions. • A dual-temperature DH system including two AHPs and HXs is designed by TEDM method. • Thermal energy of primary hot water can be used more reasonable in designed system. • Energy performance of designed system is better than that of other DH heating system. • The changes in DH system's form with different conditions are investigated. [ABSTRACT FROM AUTHOR]

To overcome the shortcomings of the conventional working fluids, ammonia/ionic liquids were analyzed for the compression-assisted absorption heat pump. The property models of different ammonia/ionic liquid mixtures and the thermodynamic model of the proposed cycle were established with verified accuracy. The primary energy efficiency and exergy coefficient of performance under various conditions were compared. N, N-dimethylethanolammonium acetate performed the best under favorable working conditions, while 1,3-dimethylimidazolium dimethylphosphate and 1-ethyl-3-methylimidazolium tetrafluoroborate performed better under other conditions. 1,3-dimethylimidazolium dimethylphosphate can operate under the lowest generation/evaporation temperatures. Compared to ammonia/water, ammonia/N, N-dimethylethanolammonium acetate showed quite similar or slightly higher efficiencies under favorable conditions, ranging in 1.026–1.491. The exergy efficiency generally increased first and decreased later with the increase of the generation/condensation/evaporation temperature and the compression ratio. With optimal compression ratios of 1.8–2.1, the maximum primary energy efficiencies were 1.339–1.382 for different ammonia/ionic liquids. The primary energy efficiencies of the compression-assisted absorption heating were improved by 36–69% compared to gas boilers, by 0.2–54% compared to the conventional absorption cycles. [ABSTRACT FROM AUTHOR]

The heat compensation unit with thermosyphon has been developed to eliminate the annual soil thermal imbalance of ground-coupled heat pump in heating-dominant buildings. But the issues on heating capacity deficiency at peak heating loads and on the high borehole investment are still unsolved in this non-coupled system. In this paper, a coupled operation of the heat compensation unit and ground-coupled heat pump system is proposed. That is, the heat compensation unit reheats the borehole outlet fluid and improves the temperature of the fluid entering the evaporator of heat pump during the heating season. By doing so, it can enhance the heating capacity at peak loads and reduce the number of boreholes required. The heat compensation required by the heat compensation unit in the non-heating season is also reduced, increasing the system efficiency. To demonstrate the effectiveness of the coupled system against the non-coupled one, the system models are built in TRNSYS to analyze the system reliability, efficiency and economy. Results show that, while the number of boreholes is reduced to 40%, the coupled system can maintain the soil thermal balance and meet the indoor heating demand. For the coupled system with different numbers of boreholes (40–100%), the seasonal average heating COPs of the ground-coupled heat pump and heat compensation unit are 4.39–4.70 and 3.67–3.80, respectively. The hourly heating capacity of the ground-coupled heat pump is also increased by 19–65%. The annual system COP of the coupled system is about 2.48–2.61, which is higher than that of 1.82–2.45 for non-coupled one. Compared to the conventional Boiler assisted ground-coupled heat pump system, the payback period of the coupled system is only 1 year. [ABSTRACT FROM AUTHOR]

• A method for constructing refrigerant cycle and heat exchange network is developed. • The method can obtain global optimal cycle structure and heat exchanger layout. • The method is verified by constructing reverse Carnot and two-stage cooling cycles. • Combined cooling and heating systems developed by the method improve COP by 38.6%. An integrated system of vapor compression cycle and heat exchanger network can comprehensively use heat transfer and improve the temperature grade of thermal energy to achieve efficient heating and cooling. Constructing an integrated system is necessary for saving energy and reducing CO 2 emissions. Existing construction methods of integrated systems cannot cover a global optimal cycle structure and are unsuitable for integrated optimization with external heat exchanger networks. This study proposes an innovative integrated construction method, named the GraPHsep method, to realize one-step optimization of integrated structures of vapor compression cycles and external heat exchanger networks. The proposed method is verified by constructing an ideal cycle and efficient two-stage compression cycle under two heat reservoirs. Furthermore, the proposed method is applied to construct a combined cooling and heating system with waste-heat recovery. The results show that the three novel systems constructed using the GraPHsep method exhibit 38.6%–40.3% higher coefficient of performance (COP) than the existing energy-efficient system. [ABSTRACT FROM AUTHOR]

Highlights: [•] A combined heating/cooling/DHW system based on GSAHP is proposed in cold regions. [•] The soil imbalance is effectively reduced and soil temperature can be kept stable. [•] 20% and 15% of condensation/absorption heat is recovered by GSAHP to produce DHW. [•] The combined system can improve the primary energy efficiency by 23.6% and 44.4%. [ABSTRACT FROM AUTHOR]

Highlights: [•] GSAHP with borehole free cooling is proposed to reduce the thermal imbalance. [•] Unguarded heating and guaranteed cooling hours are comparatively simulated. [•] Performance of GSAHP can be maintained at a high level by borehole free cooling. [Copyright &y& Elsevier]

Abstract: Ground Source Heat Pumps (GSHPs) are extensively used as renewable energy technology over the world. In heating-dominated buildings, the ground thermal imbalance leads to the decrease of the soil temperature and deteriorates the heating performance. Compared with Ground Source Electrical Heat Pump (GSEHP), Ground Source Absorption Heat Pump (GSAHP) extracts less heat from the soil and rejects more heat to it, which can reduce the ground thermal imbalance in cold regions. In this paper, applications of GSEHP and GSAHP in typical cities are comparatively analyzed based on thermal balance, soil temperature variation and energy efficiency. Results show that the thermal balance is well kept by GSAHP in severe cold cities, without obvious decrease of the soil temperature after 10 years of operation, and the Primary Energy Efficiency (PEE) of GSAHP is significantly higher than that of GSEHP. For buildings with heating load only, the average soil temperature of GSAHP can be 4–6°C higher than that of GSEHP, and the PEE can stay above 0.96 even in Harbin, the coldest city among analyzed typical cities. It can be concluded that GSAHP is advantageous over GSEHP on the point view of the average soil temperature and energy efficiency, which indicates that GSAHP may be a potential solution for ground thermal imbalance of GSHP in cold regions. [Copyright &y& Elsevier]