Search

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]

The low-temperature district heating system based on the air source absorption heat pump (ASAHP) was assessed to have great energy saving potential. However, this system may require smaller temperature drop leading to higher pump consumption for long-distance distribution. Therefore, the applicability of ASAHP-based district heating system is analyzed for different primary return temperatures, pipeline distances, pipeline resistances, supplied water temperatures, application regions, and working fluids. The energy saving rate (ESR) under different conditions are calculated, considering both the ASAHP efficiency and the distribution consumption. Results show that ASAHP system is more suitable for short-distance district heating, while for longer-distance heating, lower supplied hot water temperature is preferred. In addition, the advantages of NH 3 /H 2 O are inferior to those of NH 3 /LiNO 3 , and the advantages for warmer regions and lower pipeline resistance are more obvious. The primary return temperatures are optimized to obtain maximum ESRs, after which the suitable distances under different acceptable ESRs are summarized. To improve the applicability of ASAHP, the integration of cascaded heat exchanger (CHX) and compression-assisted ASAHP (CASAHP) are proposed, which can reduce the primary return temperature. The integration of CHX can effectively improve the applicability of ASAHP under higher supplied water temperatures. As for the utilization of CASAHP, higher compression ratio (CR) is better in longer distance, while lower CR is more advantageous in shorter distance. For a distance of 50 km, the maximum ESR is improved from 13.6% to 20.4–25.6% under different CRs. [ABSTRACT FROM AUTHOR]

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]

This paper presents a new scheme of an inverter air cooling heat pump system with domestic hot water. A water reheater is placed between the compressor outlet and the four way valve inlet to utilize the sensible heat of the superheated gas exhausted from the compressor, and a water preheater is placed between the condenser and the throttling device to use the sensible heat of the subcooled liquid flowing out of the condenser. With these two parts of heat, the domestic hot water can be heated to a temperature high enough for domestic use. In order to maintain the system efficiency in the period of part load, an inverter compressor is adopted as the substitute for the constant speed one used in the conventional heat pump system. A hot water storage tank with a circulation pump is placed in the system to reduce the peak load of the system. Compared with the traditional system, this new design is able to reduce energy consumption by 31.1% and decrease thermal pollution to the environment. [Copyright &y& Elsevier]

• 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]

Because of high thermal storage density and little heat loss, absorption thermal energy storage (ATES) is known as a potential thermal energy storage (TES) technology. To investigate the performance of the ATES system with LiBr-H2O, a prototype with 10kWh cooling storage capacity was designed and built. The experiments demonstrated that charging and discharging processes are successful in producing 7°C chilled water, 65°C domestic hot water, or 43°C heating water to meet the user's requirements. Characteristics such as temperature, concentration and power variation of the ATES system during charging and discharging processes were investigated. The performance of the ATES system for supplying cooling, heating or domestic hot water was analyzed and compared. The results indicate that the energy storage efficiencies (ESE) for cooling, domestic hot water and heating are 0.51, 0.97, 1.03, respectively, and the energy storage densities (ESD) for cooling, domestic hot water and heating reach 42, 88, 110kWh/m3, respectively. The performance is better than those of previous TES systems, which proves that the ATES system using LiBr-H2O may be a good option for thermal energy storage. [ABSTRACT FROM AUTHOR]

Highlights • A novel heat recovery solution regeneration system is proposed for heating tower. • Regeneration performance of internally heated regenerator is experimentally studied. • Correlations of heat and mass transfer coefficients are developed based on results. • A low-grade heat source can be used for heating tower solution regeneration. Abstract Owing to their high cooling efficiency in summer and frost-free characteristics in winter, heating tower heat pumps are becoming increasingly popular. However, the solution will be diluted during heating due to absorbing moisture from the air, and should be properly regenerated for continuous operation. In this study, a novel heat recovery solution regeneration system using an internally heated regenerator is proposed for antifreeze solution regeneration. A test bench is fabricated to investigate the regeneration performance of the internally heated regenerator. Correlations of the heat and mass transfer coefficients are proposed, and agree well with the experimental data. The influences of inlet parameters on the moisture removal rate, latent heat ratio, and humidity effectiveness are evaluated and compared to those for internally heated liquid desiccant regeneration. The results indicate that: the regeneration performance of the internally heated regenerator is affected mainly by the air flow rate, heating water flow rate, and inlet temperature of heating water. The moisture removal rate increases remarkably with increasing the heating water flow rate and inlet temperature of heating water. The latent heat ratio decreases significantly with increasing the inlet temperature of heating water. The humidity effectiveness decreases remarkably with increasing the air flow rate. The influences of the above inlet parameters on performance indices in the present study exhibit the same trends as those reported for internally heated liquid desiccant regeneration. However, compared with internally heated liquid desiccant regeneration, the regeneration temperature for internally heated antifreeze solution regeneration is much lower. This suggests that a low-grade heat source can be used to meet the antifreeze solution regeneration demands. [ABSTRACT FROM AUTHOR]

Heat pump technologies play significant roles in building energy saving and emission reduction. Combing the opposite characteristics of the electrical and absorption heat pumps, a novel internally hybrid absorption-compression heat pump was proposed for performance improvement. The thermodynamic models for the hybrid heat pump using two cycles (single-effect and generator-absorber-heat-exchange) were built for component design and performance simulation with various refrigerant distribution ratios. Results show that a higher absorption-side refrigerant ratio generally makes the primary energy efficiency less sensitive and the capacity more sensitive to working condition. In the heating mode, a higher absorption-side refrigerant ratio brings about a higher efficiency when the evaporator inlet temperature is below 16 °C and 20 °C for the two cycles. In the cooling mode, a lower absorption-side refrigerant ratio contributes to a higher efficiency. The hybrid absorption-compression heat pumps can realize flexible heating/cooling capacity ratios to accommodate the design heating/cooling loads as necessary. Generally, a higher absorption-side refrigerant ratio is preferred in colder conditions owing to the higher heating efficiency and the higher heating/cooling capacity ratio, well matching the heating-dominant building loads. This study focuses on the principle and performance of the novel hybrid heat pump, the applications in actual buildings will be conducted through year-round simulations in future studies. [ABSTRACT FROM AUTHOR]

The heat and mass transfer simulation model of a ground heat exchanger (GHE) directly affects the design and operation performance of a ground-coupled heat pump system. The GHE models based on the response function (like the Green function and g-function) can achieve a fast calculation speed. However, the heat sources in these models are limited to points or whole boreholes, leading to low calculation accuracy in heat transfer during a short time period and limitation to a certain GHE. A general distributed parameter model for a ground heat exchanger (RF model) is proposed based on the principle of response factors in this paper. A sandbox experimental platform is then built to test the temperatures of typical points in the double-layered soil and to validate the RF model. After that, the calculation of the RF model is simplified by determining suitable positions for the soil boundaries and the numbers of sub pipes and sub soil boundaries. Finally, the RF model is applied in different scenarios to demonstrate its characteristics. The results show that: (1) the RF model is suitable for different kinds of GHEs with arbitrary shape and type of heat sources releasing heat in arbitrary time steps; (2) the RF model has only 0.01 °C and 0.23 °C temperature response errors compared to those from numerical solutions and experiments, respectively; (3) the general RF model has similar accuracy to the numerical solution in calculating the distributed temperatures of the borehole and pipes, heat transfer in the short term, and heat transfer of borehole groups and the energy pile. [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]

Underground thermal imbalance is a significant problem for ground-coupled heat pump (GCHP) systems that serve predominately heated buildings in cold regions, which extract more heat from the ground and inject less heat, especially in buildings requiring domestic hot water (DHW). To solve this problem, a previously developed heat compensation unit with thermosyphon (HCUT) is integrated with a GCHP unit to build a hybrid GCHP system. To improve the energy savings of this hybrid GCHP system, the HCUT unit is set to have multiple functions (heat compensation, direct DHW and direct space heating) in this paper. To analyze the improved system performance, a hotel requiring air-conditioning and DHW is selected and simulated in three typical cold cities using the dynamic software DeST and TRNSYS. The results indicate that the hybrid GCHP system can maintain the underground thermal balance while keeping the indoor air temperature within the design range. Furthermore, the HCUT unit efficiently reduces the energy consumption via its multi-functional operations. Compared to the previous system that only used HCUT for heat compensation, adding the direct DHW function further saves 7.5–11.0% energy in heat compensation (HC) and DHW (i.e., 3.6–4.8% of the whole system). Simultaneously adding the direct DHW and space heating functions to the HCUT can save 9.8–12.9% energy in HC and DHW (i.e., 5.1–6.0% of the whole system). The hybrid GCHP system with a multi-functional HCUT provides more energy savings while maintaining the underground thermal balance in cold regions that demand both air-conditioning and DHW. [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]