Conventional Heat Pump Research Articles

Experimental performance evaluation of a novel anti-fouling wastewater source heat pump system with a wastewater tower

Using wastewater source heat pumps to recover heat from the sewage discharged from buildings could help improve building energy utilization efficiency remarkably, which is a promising technology in near-zero energy buildings and should be widely used. However, fouling deposit would occur on a heat exchanger surface due to the poor water quality of wastewater, which would affect the heat transfer performance and may even block the heat exchanger, reducing the reliability of a wastewater source heat pump unit. Fouling poses a challenge for the wider application of wastewater source heat pumps. A novel anti-fouling wastewater source heat pump system with a wastewater tower was proposed in this work and its operational performances were experimentally investigated based on a long-term test of 41 days (492 h). Experimental results showed that the heat transfer rate in the evaporator of the novel wastewater source heat pump was stable during the 41-day test at an average of 3.80 kW. Unlike in the evaporator of a conventional wastewater source heat pump unit, the total thermal resistance in the evaporator of the novel wastewater source heat pump unit was not increased but only fluctuated around an average of 0.0267 K m2/W throughout the entire 41 days test. The evaporating temperature of the refrigerant stayed stable around an average of 2.74 °C. Furthermore, the coefficient of performance of the unit was also stable at an average of 3.32 and the coefficient of performance of the system maintained an average of 2.13. The condensate drained away from the bottom of the evaporator did not contain any foulant. In addition, the photos for the evaporator showed that its surface was still clean after the 41-day test. Therefore, the experimental results suggested that the novel wastewater source heat pump system not only had superior performance of heat recovery but also achieved effective anti-fouling on its evaporator surfaces.

A combined moving boundary and discretized approach for dynamic modeling and simulation of geothermal heat pump systems

The major goal of this study is to demonstrate the feasibility of the dynamic modeling and simulation of both conventional and direct exchange geothermal heat pump applications particularly with regard to performance evaluations. A direct exchange geothermal heat pump system is a geothermal heat pump system in which the refrigerant circulates through tubing placed in the ground. In this research, dynamic models for the simulation of geothermal heat pump systems with the working fluid propane for the application in building-scale energy systems have been developed based on the Moving Boundary approach and the challenges of dynamically evaluating these kind of energy systems have been hereby met. Additionally, the performance of both horizontal (slinky-coil) and vertical (borehole) geothermal heat exchangers, which have been modeled based on the Conventional Discretized approach, for such systems has been evaluated. Since these two modeling approaches are not compatible with one another, the coupling of the moving boundary model with the discretized model is therefore another challenge and of great importance. On the basis of a case study, the complete conventional and direct exchange geothermal heat pump systems with vertical and horizontal heat exchangers have been simulated and energetically and exergetically analyzed.

Prospective environmental and economic assessment of solar-assisted thermal energy recovery from wastewater through a sequencing batch biofilter granular reactor

The integration of an off-grid solar-assisted heat pump (SHP) and a sequencing batch biofilter granular reactor (SBBGR) for thermal energy recovery from wastewater was assessed by means of a prospective life cycle assessment (LCA) and life cycle costing (LCC), by theoretically scaling up a pilot installation in Bari, Italy, to a full-scale unit designed for 5000 person-equivalents. The LCA and LCC included all activities in the life cycle of the SHP and wastewater treatment plant (WWTP), namely construction, operation and end-of-life. The thermal energy produced by the SHP was assessed as supplying heating and cooling for an air-conditioning system, displacing a conventional air-source heat pump powered by electricity from the grid. This integrated system was compared to a reference situation where wastewater is treated in a conventional WWTP applying activated sludge with no thermal energy recovery system, showing clear environmental benefits in all impact indicators, such as a 42% reduction in greenhouse-gas emissions and a cost reduction of 53%. Several sensitivity analyses confirmed these findings, with the exception of the price rebound effect, which showed that the lower cost of the integrated system could lead to overturning the environmental benefits. As a limitation of the study, the distribution of the supplied air-conditioning to meet a demand off-site the WWTP premises, such as in residential buildings or hotels, was not included. Therefore, our results constitute only a preliminary positive outcome that should be validated in a real-life application.

Hardware in the Loop Real-Time Simulation for Heating Systems: Model Validation and Dynamics Analysis

Heating systems such as heat pumps and combined heat and power cycle systems (CHP) represent a key component in the future smart grid. Their capability to couple the electricity and heat sector promises a massive contribution to the energy transition. Hence, these systems are continuously studied numerically and experimentally to quantify their potential and develop optimal control methods. Although numerical simulations provide time and cost-effective solutions for system development and optimization, they are exposed to several uncertainties. Hardware in the loop (HiL) approaches enable system validation and evaluation under different real-life dynamic constraints and boundary conditions. In this paper, a HiL system of a heat pump testbed is presented. It is used to present two case studies. In the first case, the conventional heat pump testbed operation method is compared to the HiL operation method. Energetic and dynamic analyses are performed to quantify the added value of the HiL and its necessity for dynamics analysis. In the second case, the HiL testbed is used to validate a model of a single family house with a heat pump participating in a local energy market. The energetic analysis indicates a deviation of 2% and 5% for heat generation and electricity consumption of the heat pump model, respectively. The model dynamics emphasized its capability to present the dynamics of a real system with a temporal distortion of 3%.

Experimental study on the performance of a novel in–house heat pump water heater with freezing latent heat evaporator and assisted by domestic drain water

The temperature of domestic drain water is relatively high, but its flux is sometimes insufficient as a heating source. Thus, the conventional wastewater source heat pump water heater is incapable of meeting user heating demand given the limited sensible heat temperature difference in many situations. To address this problem, a novel in–house heat pump water heater with a freezing latent heat evaporator and assisted by domestic drain water was proposed. A prototype of the system was first constructed to determine the performance of the system. An experiment was then conducted, in which a parallel helical coil tube evaporator was designed and developed to extract freezing latent heat from domestic drain water. Experimental data was obtained in different experimental conditions. Results of the investigation show that the heat pump water heater with a freezing latent evaporator recovers 3.4 times more energy than the conventional wastewater source heat pump water heater, and saves 60% energy compared with the traditional electric water heater. Furthermore, an interesting result is that the heat transfer coefficient increased significantly at the moment the wastewater was just frozen. This paper provides a reference for the wastewater source heat pump water heater to extract freezing latent heat from domestic drain water.

Waste heat recovery of power plant with large scale serial absorption heat pumps

Large amount of waste heat is dissipated in industries, resulting in energy waste and environment pollution. Waste heat recovery with absorption heat pump is one of the attractive solutions. In this paper, we present the theoretical study and test of a power plant waste heat recovery system with large scale LiBr-water absorption heat pumps for district heating. Waste heat from steam condenser with temperature of ∼35 °C is recovered by the absorption heat pumps driven by steam with pressure of ∼0.25 MPa from the turbine. Heat output from the absorption heat pumps is used to preheat the return water of district heating network from ∼45 °C to ∼80 °C. The return water is further heated by the steam to ∼105 °C for heating supply. Large temperature lift of the return water is achieved by the serial absorption heat pumps. Theoretical analysis of the system is presented, showing advantages compared to the conventional absorption heat pump and original heating supply system. On-site test showed that COP of 1.77 and heating capacity of 63.57 MW were achieved by one serial absorption heat pump, with waste heat inlet/outlet temperature of 34.63/28.33 °C and return water inlet/outlet temperature of 45.94/81.34 °C.

Proposal and thermodynamic analysis of a combined open-cycle absorption heat pump and thermal desalination system driven by high-humidity exhaust gas

A combined system based on an open-cycle absorption heat pump (OAHP) and a low-temperature multi-effect evaporation (LT-MEE) water desalination process is configured, modeled, and analyzed. The system is applicable to be run by high-humidity gases, and the flue gas from the combustion of natural gas is used here. Compared with conventional absorption heat pumps, the OAHP is characterized by a gas-solution direct-contact absorber, where not only the sensible heat but also the latent heat and water vapor in flue gas are partially recovered. The OAHP-MEE system has good internal synergy, as demonstrated by a 36%–74% water production gain over the simple system when driven by flue gas with temperature of 120 °C–250 °C. With the increase of the feed-gas temperature, the combined system becomes increasingly favorable over the simple system in energy utilization and water production. For a specified feed gas condition, an optimal water recovery rate of OAHP and a maximum number of effects of MEE exist, mutually leading to a maximum water output. Using a cooler to cool the strong solution entering the absorber not only has a favorable effect on water production, but also widens the parameters' ranges, thus offering good flexibility for design and operation.

High efficiency sub-critical carbon dioxide supplementary heat pump for low temperature climates (energy and exergy analysis)

Abstract Nowadays heat pumps are crucial elements of air conditioning systems and still gaining popularity, but they suffer from limitations in very cold climates such as low coefficient of performance (COP) and air leaking. Recently-developed trans-critical CO2 heat pumps are proper replacements for their (H)CFC counterparts, especially in environmental concerns, but they have not answered the temperature limitations yet. The temporal solution is to bridge the temperature gap between low temperature ambient air (lower than −15 °C) and normal operating temperature of heat pump (about 0 °C) by intermediate systems such as supplementary heaters or geothermal sources. The main idea of this study is to define a highly-efficient heat pump instead of intermediate systems. Consequently, instead of a heat pump with auxiliary heater, there is a cascade of heat pumps which operates properly in very cold climates. In this regard, two sub-critical CO2 heat pumps were studied. One uses expansion valves and another utilizes expanders. COPs more than 3.7 were obtained in −40 °C. Using the first and second laws of thermodynamics, analyses on the performance characteristics of the cycles were carried out. Also, a parametric study was conducted to optimize the performance of each cycle under various operating conditions. The results show that these cycles perform properly in both energy and exergy points of view and expander increases COP and reduces lost exergy. Inter-stage pressure and performance of internal heat exchanger were studied and optimal values were extracted. Lastly, a thermo-economic comparison between the purposed cycles and conventional fossil fuel heating systems were done. The findings show that this sub-critical CO2 heat pump can make a conventional heat pump to remain functional at low temperatures without any significant fall in COP.

Economic optimization and comparative study of solar heat pumps

In this paper, an economic study of solar heat pumps and an investigation of differences between solar heat pumps and conventional heat pumps—based on their performances and energy consumption—are conducted for a residential apartment located in Austin, Texas, USA. Heating in the apartment is provided via a solar heat pump during the cold months of a year. Solar collectors are used to meet the domestic hot water requirement of the apartment during the other months. In order to carry out a rational comparison, the base system comes with an air-to-air domestic heat pump and a boiler to provide domestic hot water. The comparison includes both energy consumption (fuel and power) and economic aspects. The simulation is performed through the commercial software TRNSYS, in which optimization offers a collectors’ surface of 28 square meters. The proposed configuration requires almost half of the electrical energy that the conventional system consumes. Its fuel consumption is about a quarter of the non-solar system, which offers a double value of COP in comparison with the traditional system. The economic calculations reveal that the payback period is about two years.

Modeling and theoretical analysis of a CO2-propane autocascade heat pump for electrical vehicle heating

Heat pump and its efficiency are of great significance to the energy saving of electric vehicle in cold climate. This paper proposes a modified autocascade heat pump (ACHP) system using natural CO2-propane mixture. The main purpose is to enhance the heating performance compared with that of conventional heat pump. Using a thermodynamic model of the proposed ACHP cycle, the transcritical and subcritical cycle performance under various conditions are carried out from the perspective of first and second law of thermodynamics, and the transcritical operating characteristics of ACHP system is introduced for the first time. Additionally, an exergy performance comparison has been carried out. Results indicate that the ACHP system can produce more heating capacity, lower compression pressure ratio, lower heat rejection temperature for low ambient temperature conditions and it consumes less compressor work for all the temperature range researched. For transcritical ACHP cycle, the best COP is 2.7 obtained when CO2 mass fraction is 0.8 at −20 °C, which is 12.3% higher than that of CO2 single stage heat pump. For subcritical ACHP cycle, the best COP is 3.97 while heating capacity is much lower than that of transcritical cycle in the same condition. Finally, the exergy analysis confirms the feasibility of the ACHP system and points out exactly the further optimization locations in the ACHP system.

Development of a novel unbalanced ammonia-water absorption-resorption heat pump cycle for space heating

Ammonia-water absorption-resorption heat pump (ARHP) is a promising technology for promoting efficient utilization of low-temperature thermal for space heating. This paper proposes a novel unbalanced ARHP cycle by replacing the condenser and evaporator in the conventional absorption heat pump (AHP) cycle with a high-pressure absorber and a low-pressure generator, respectively. The proposed cycle can work just depending on the concentration difference of ammonia-water solution under different pressure levels. A numerical model has been developed to investigate the feasible high-pressure/low-pressure (PH/PL) values to effect the thermodynamic cycle. The cycle's coefficient of performance (COP) under different PH/PL pair values and other given working conditions are studied. The maximum COP is 1.550 and the corresponding optimum PH/PL pair to effect the cycle is 1.40 MPa/0.38 MPa at a heat source temperature of 95 °C. When PH/PL pair value is 1.40/0.38 MPa, heat supply temperature of 44.5 °C and COP value of 1.331 can be obtained, which can basically meet the temperature demand of building floor heating. In addition, effects of different PH/PL pair values on solution circulation ratios and vapor discharge scopes of generators are discussed. It is concluded that the ideal PH/PL candidates yield large concentration difference between the inlet and outlet of generators and absorbers. Meanwhile, the solution circulation ratios of the two generators at ideal PH/PL pairs are acceptable values between 2.0 and 12.0. The cycle can work when the ambient temperature is above −7.5 °C and the driving heat source temperature is larger than 85 °C, which is potential in efficient utilization of commonly used stationary solar collectors for winter heating.

Dynamic modelling and energy performance analysis of an innovative dual-source heat pump system

In this paper the energy performance of a Dual-Source Heat Pump (DSHP) system able to use both air and ground as external heat source is analysed by using TRNSYS and the experimental data obtained by testing a DSHP prototype. The DSHP seasonal and annual performance factors are compared with those offered by the same DSHP in which only ground (ground-source mode) or only external air (air-source mode) is used as external heat source in order to evaluate the benefits achievable with the exploitation of a double external heat source (ground and air) with the same unit. Yearly dynamic simulations have been carried out by coupling the DSHP to a detached residential building located in Bologna, characterized by unbalanced heating and cooling loads, and coupled to a geothermal loop based on borehole heat exchangers (BHEs). With the help of the dynamic simulations it has been demonstrated that DSHPs can be very useful in order to solve the problems linked to the ground temperature drift which can be originated by the presence of an undersized borehole heat exchanger field and/or by unbalanced heating and cooling loads. In fact, the use of external air as auxiliary heat source with respect to ground during the most severe season enables to obtain more stable energy performance even in presence of undersized BHEs. In this paper it is shown how an optimal trade-off in terms of annual energy performance and investment costs can be obtained by reducing the size of the DSHP borehole field of 15–55% with respect to the borehole field needed by a conventional ground-coupled heat pump having the same size. In this way the DSHP can be used during the retrofitting of thermal plants based on ground-coupled heat pumps in which an undersized BHEs field is present.

A comprehensive investigation of using mutual air and water heating in multi-functional DX-SAMHP for moderate cold climate

Solar energy assisted heat pump systems (SAHP) have been used in this application. SAHP system with solar collectors and the heat pump are combined into one unit so as to convey the solar energy to the refrigerant. The solar collector is used as the evaporator, where the refrigerant is directly vaporized by solar energy input. Due to the complicated technical issues associated with a combined system that provides air for space heating and domestic hot water, most of the previous studies have concentrated on water heater heat pump mechanism. The current work is aimed at examining the use of a new multi-functional heat pump (DX-SAMHP), air for space heating mutually with solar for domestic hot water without employing an auxiliary heater. Comprehensive experimental and analytical studies in the first of its kind have been performed on the new system. The novel system with ternary panels and the thermal performance of the collector has been examined in this study. Results indicate that the DX-SAMHP using solar inner and outer panels for space and water heating is a promising substitute for the existing DX-SAHP water heater. Compared to the conventional solar-assisted SAHP heat pump systems, the coefficient performance of the new design doubles that of the conventional DX-SAHP systems.

Organic Rankine Cycle-assisted ground source heat pump combisystem for space heating in cold regions

This paper presents an Organic Rankine Cycle (ORC)-assisted ground source heat pump combisystem for the cascaded utilization of low-grade energy and shallow geothermal energy in cold regions. The ORC unit was added to conventional ground source heat pumps to solve the cold accumulation problem, which can lead to performance degradation of heat pumps. The existing types and novel types of the combisystem were then modeled with the Transient System Simulation Program (TRNSYS). During the heating season, the ORC unit and heat pump unit were combined for space heating, and during the non-heating season, the ORC unit was connected to ground heat exchangers for seasonal storage. System performance was simulated over periods of one year and twenty years. The twenty-year simulation results showed that the proposed combisystem could maintain a higher annual average coefficient of performance (COP) of approximately 3.8 because of the steady soil temperature, whereas the annual average COP of the conventional ground source heat pump system decreased from 3.7 to 3.2. Additionally, the total power consumption per unit heating area of the heating system decreased from 2.2 × 103 kW·h/m2 to 3.3 × 102 kW·h/m2 when the ORC unit was added to the conventional ground source heat pump. This translates to electricity cost savings of ¥ 908/m2 over twenty years. Moreover, in the combisystem, the ORC unit provides 55.6% of the total heating capacity; compensates for 78.5% of the heat pump unit’s power consumption. And 94.1% of the thermal energy storage from the non-heating season can be used during the heating season by the heat pump unit.

120℃ 스팀 생성을 위한 100 kW급 히트펌프의 실험적 연구

Recently, the development of a heat pump technology to recover process waste heat and to generate steam of 120℃ or higher required for industrial processes, has attracted attention. The research of conventional heat pump utilizing the available energy is used primarily for air conditioning, and the production temperature is about 60℃, so it is difficult to utilize it for industrial use. Therefore, in this study, we developed a steam heat pump (SGHP) which recovers the waste heat of process and generates steam at 120℃. The low-pressure refrigerant R245fa, considered to be an eco-friendly refrigerant, has been selected as the refrigerant for SGHP in this study since its Ozone Depletion Potential (ODP) is zero and the Global Warming Potential (GWP) is relatively low. A flash tank functioning as a phase separator was installed in the rear stage of the condenser, and the saturated water of high temperature was decompressed to generate steam. It was started at the initial temperature of 70℃, and it was confirmed that 120℃ steam was produced after the system stabilized. We have conducted experiments by modifying the system, and ultimately achieved a heating capacity of 101.4 kW and a COP of 3.05.

Dynamic modelling and simulation of a slinky-coil horizontal ground heat exchanger using Modelica

Ground source heat pumps are attractive alternatives for both conventional heating and cooling systems because of their higher energy efficiencies. A ground source heat pump system consists of a conventional heat pump coupled with a ground heat exchanger where water or a water-antifreeze mixture exchanges heat with the ground. Several major design options are available for ground heat exchangers, which are classified by layout to open and closed loops. Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells. Slinky-coil horizontal ground heat exchangers are a compromise between vertical ground heat exchangers and conventional horizontal ground heat exchangers. They combine the relatively small installation cost of conventional horizontal ground heat exchangers and the high heat exchange capacity of vertical ground heat exchangers. These traits make a slinky-coil horizontal ground heat exchanger a suitable alternative for space heating in areas with limitations on land space usage. However, caused by a lack of appropriate dimensioning tools, these systems are often undersized or overdesigned. In this research, the feasibility of modelling and simulation of slinky-coil geothermal heat exchangers in the modelling language Modelica has been demonstrated. A model of a slinky-coil horizontal ground heat exchanger has been developed to simulate the performance of such systems. The model includes a ground heat exchanger and a soil cell. The ground heat exchanger has been modelled in two levels of detail, one with coil-like pipes and one with thin plates instead of pipes. In order to be able to assess the validity of the model, it has been developed based on a field test in Japan, and simulated under the same conditions. Reasonably good agreement has been obtained between the numerical predictions and published experimental measurements, which demonstrates the reliability and accuracy of the model.

Theoretical investigation of a novel thermoelectric laundry dryer concept

The environmental impact of conventional heat pump dryers lies, apart from its energy consumption, in the use of a noxious refrigerant. By utilizing a thermoelectric heat pump laundry dryer, a refrigerant-free heat pump process is realized making especially the disposal of a laundry dryer more convenient. Moreover, the drying process is more efficient compared to a conventional condensation dryer as both the hot and cold side of the thermoelectric heat exchanger are used in the process.A novel cycle is introduced that combines thermoelectric heat pumping and air cooling by means of ambient air. This cycle is compared theoretically to two conventional cycles of a heat pump dryer and a condensation dryer. Moreover, a comprehensive system model is developed to evaluate the performance of such a dryer. Within the fully transient model temperature dependent material properties and contact resistances are considered. The performance of the thermoelectric heat exchanger is analyzed and significant variables are identified. Simulation results are compared with validated simulation results of a condensation and heat pump dryer. It is shown that the energy consumption of a thermoelectric heat-pump based tumble dryer lies between the two conventional dryers and thus can be seen as a promising technology.

Systematic optimization for the utilization of low-temperature industrial excess heat for district heating

Because of extensive heat shortage and serious air pollution, industrial excess heat (IEH) has attracted much attention in the district heating (DH) markets of Northern China, as it has great potential for recycling and satisfying the large demand. Most of the available IEH is released at low temperature by steel plants, chemical plants, etc., which are located far away from the heat users. The energy consumption and pipe investment for heat transportation should be optimized to improve the comprehensive efficiency. This study will use systematic models to make optimization to primary-network temperatures and corresponding connecting forms. Several suggestions will be proposed to help improving the energy efficiency. Furthermore, based on a real case study, two new schemes are proposed and compared with the existing scheme. The connecting form of the optimized scheme is different to conventional heat pump systems using sources such as sewage and ambient water. The rated COP is estimated to be 6.16, and the annual electricity consumption is 40.78 kWh/GJ. The system has significant advantages in terms of energy savings and reductions in pollution emissions in comparison to conventional source systems, implying that it can be worth exploiting IEH even at low temperatures and long distances.

A simulation study for evaluating the performances of different types of house-hold radiant air conditioning systems

As one potential solution of independent temperature and humidity control in small-to-medium scaled residential buildings, systems of outdoor air dehumidifier with radiant terminals attracted lots of researchers’ interests. In this paper, three types of house-hold radiant air conditioning (A/C) systems, a conventional air source heat pump (ASHP) based, a high-temperature ASHP based and a novel variable refrigerant flow rate (VRF) based radiant A/C system, were proposed. Simulation models for three systems were established using Energy Plus. A prototype of the VRF based radiant A/C system was built and tested in an existed psychometric room. Testing results were compared with the simulated results showing that the average relative errors between them were less than 8%. Therefore, simulation studies were carried out to evaluate the energy performances of the three types of house-hold radiant A/C systems. Compared to the conventional ASHP based system, the high-temperature ASHP based system achieved 14.99% annual energy saving and the VRF based system 19.34%. Therefore, the VRF based system achieved the best energy efficiency, providing a promising solution for the house-hold radiant A/C system to realize the independent control of temperature and humidity in small-to-medium scaled residential buildings.

Operational Energy Saving Potential of Thermal Effluent Source Heat Pump System for Greenhouse Heating in Jeju

Massive thermal effluent energy from power plant is mostly released to the sea, and only a little is used for fishing culture and agriculture in South Korea. The thermal effluent from the power plant can be an efficient heat source of the heat pump system to provide heating energy for the greenhouse, but energy loss and pump power by long distance pipeline installation from a power plant to the greenhouse should be considered. In this paper, an operational energy saving potential of a thermal effluent source heat pump system for the greenhouse heating was investigated. For the estimation of thermal load, three cases of greenhouse were categorized, and the thermal performance and operating energy consumption during the heating season were compared with those of a conventional ground source heat pump (GSHP) system. The model for heat pump system was newly derived to estimate the energy performance of the proposed system, and then detailed simulations for each system under three cases of greenhouse were conducted. The results showed that the operational energy of the proposed system can be saved by 17–20% than that of the conventional GSHP system.