Simulation and validation of a hybrid-power gas engine heat pump

Hybrid-power gas engine heat pump (HPGHP) combines hybrid power technology with gas engine heat pump, which can keep the gas engine working in the economical zone. In this paper, a steady-state model of the HPGHP in heating condition has been established, the optimal torque curve control strategy is proposed to distribute power between the gas engine and battery pack. The main operating parameters of the HPGHP system are simulated on Matlab/Simulink and validated by experimental data, such as operating temperature, coefficient of performance (COP), fuel-consumed rate, etc. Heating capacity and COP of the heating pump system are validated under different ambient temperatures and water flow rates. The simulation and experiment results shows acceptable agreement, the maximum difference is respectively 8.9%, 5.9%, 9.5% and 8.2% for engine torque, motor torque, reclaimed heat and fuel-consumed rate. Based on the simulation results, HPGHP has the lowest fuel-consumed rate of 283 g (kWh)⁻¹ at engine speed of 3000 rpm; the PER of HPGHP system is about 15.9% and 11.4% higher than the GHP under the same load in Mode C and D.     

Optimal operation of a complex thermal system: a case study

The paper focuses on the search for the optimal operation modes of a complex thermal plant. The system under analysis is basically made-up by four gas-fueled reciprocating engines with heat recovery. Each engine can drive simultaneously an electric generator as well as the compressor of a heat-pump/chiller. The plant is interconnected to the electric utility grid, both to receive additional power and to deliver power in excess. In addition, each heat-pump/chiller can be driven electrically, using the electric generator as a motor. For any given load condition, a large number of operation modes are possible. The problem of finding out the configuration that minimizes the economic cost of operating the system is dealt with. This cost is regarded as the objective function to be minimized in a typical constrained optimization problem. Statement and solution of this problem are discussed. Numerical examples are included and commented.     

Conception of combination of gas-engine-driven heat pump and water-loop heat pump system

Conception of combination of gas-engine-driven heat pump (GHP) and water-loop heat pump system (WLHPS) is firstly presented in the paper in order to reduce the energy consumption of air conditioning system further. Design of the new system is introduced through an actual project in China and compared with a conventional air-conditioning system (CACS) and conventional WLHPS (EHP-WLHPS) in terms of technical characteristics and payback period. It is found that the payback period of GHP-WLHPS is about 2 years when compared with CACS and 2.6 years with EHP-WLHPS on the average. So it is worth being more widely applied. And several barrier to application are also discussed.     

Thermal modeling of gas engine driven air to water heat pump systems in heating mode using genetic algorithm and Artificial Neural Network methods

The gas-engine driven air-to-water heat pump, type air conditioning system, is composed of two major thermodynamic cycles (including the vapor compression refrigeration cycle and the internal combustion gas engine cycle) as well as a refrigerant-water plate heat exchanger. The thermal modeling of gas engine driven air-to-water heat pump system with engine heat recovery heat exchangers was performed here for the heating mode of operation (in which it was required to model engine heat recovery heat exchanger). The modeling was performed using typical thermodynamic characteristics of system components, Artificial Neural Network and the multi-objective genetic algorithm optimization method. The comparison of modeling results with experimental ones showed average differences of 5.08%, 5.93%, 5.21%, 2.88% and 6.2% which shows acceptable agreement for operating pressure, gas engine fuel consumption, outlet water temperature, engine rotational speed, and system primary energy ratio.     

Heating performance enhancement of a CO2 heat pump system recovering stack exhaust thermal energy in fuel cell vehicles

A CO₂ heat pump system using recovered heat from the stack coolant was provided for use in fuel cell vehicles, where the high temperature heat source like in internal combustion engine vehicles is not available. The refrigerant loop consists of an electric drive compressor, a cabin heater, an outdoor evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the heat pump system were investigated and analyzed by experiments. The results of heating experiments were discussed for the purpose of the development and efficiency improvement of a CO₂ heat pump system, when recovering stack exhaust heat in fuel cell vehicles. A heater core using stack coolant was placed upstream of a cabin heater to preheat incoming air to the cabin heater. The performance of the heat pump system with heater core was compared with that of the conventional heating system with heater core and that of the heat pump system without heater core, and the heat pump system with heater core showed the best performance of the selected heating systems. Furthermore, the coolant to air heat pump system with heater core showed a significantly better performance than the air to air heat pump system with heater core.     

Modelling and simulation results for a domestic exhaust-air heat pump heating system

Energy consumption in residential buildings has gained an increasing interest the latest years due to the rising demand for efficient energy use and higher comfort standards. In tight building constructions with controlled ventilation, heat recovery with exhaust-air heat pump connected to floor heating is regarded as energy efficient heating system that optimises the energy use in buildings while maintaining an acceptable level of thermal comfort. In this study, we use the computational tools TRNSYS and EES to model and analyse the performance of a residential house, its ventilation system and its floor heating system based on an exhaust air heat pump. The system analysis focuses particularly on the influence of internal and solar gains on the operation of the heating system and the thermal comfort of the house. Furthermore, the way that gains influences the performance of the floor heating system is examined. Overall, the results bring to light the impact of factors that are not easy to predict on the indoor climate and the thermal comfort.     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.     

Thermal performance of cold storage in thermal battery for air conditioning

This article studies, experimentally and theoretically, the thermal performance of cold storage in thermal battery for air conditioning. Thermal battery utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. Experimental investigations are first conducted to study the cold storage thermal performance in two experimental systems: the ratio of distance between heat pipes to outer diameter of heat pipe W/D=6 and 2. Different heat transfer mechanisms including nucleate boiling, geyser boiling and natural convection are identified in different experimental systems with various liquid fills. A theoretical model to determine the thermal characteristics of the thermal battery has also been developed. Comparisons of this theory with experimental data show good agreements in the nucleate boiling stage of cold storage process.     

Compact heat exchangers, enhancement and heat pumps

In order to achieve widespread use of heat pumps across the full spectrum of potential applications, it is critical that the first cost of the units is acceptable. There are many factors influencing this cost, including the number of units manufactured, the ease of installation, the complexity of the control requirements, and the cost of the working fluid(s). A common feature of all heat pump cycles is the presence of at least one heat exchanger, indeed some heat-driven cycles are composed almost entirely of heat exchangers, each having a different but critical role to play. There are several important aspects of heat exchangers that can help to reduce first cost of these components and the system, (in addition to the possible positive impact on coefficient of performance). Two of these are discussed here — compact heat exchangers (CHEs) and heat transfer enhancement. The latter may be directly associated with CHEs but can be equally beneficial in reducing approach temperature differences in 'conventional' shell and tube heat exchangers. Both are essential features of many intensified processes, which the author argues need compatible heat pumps if the market for the latter is to flourish. In this paper, the most recent types of CHE are described, with emphasis on the benefits they can bring to heat pump first cost and performance. Heat transfer enhancement in heat pumps is also reviewed.     

Carnot-COP for sorption heat pumps working between four temperature levels

The theoretical efficiency limits of heat driven heat pumps operating between three and four temperatures are derived from the fundamental thermodynamical laws, i.e. the energy balance and the entropy balance. While in the three temperatures case the system is fully determined by specification of the three temperatures and the cooling capacity, a four temperature heat pump needs, in addition to the four temperatures and the cooling capacity, specification of an additional operating parameter. This can be, for example, the ratio of the two heat flows which are released at the two different intermediate temperatures. Various assumptions regarding this proportion are discussed with respect to their relevance for both the combination power cycle/vapor compression cycle as well as for single-effect sorption cycles. The present analysis shows that a single-effect sorption heat pump is principally not able to operate reversibly in an environment of four externally specified temperatures unless the four temperatures follow, incidentally, a correlation that is given by the equilibrium properties of the employed working fluids. Therefore, in endo-reversible models for four-temperature sorption cycles only three rather than four operating temperatures may be specified independently.     

“Heat pumps — status and trends” in Asia and the Pacific

In Asian and Pacific regions, economic growth in the last decade has propelled the use of air-conditioners for space cooling along with the use of reversible heat pumps for year round space conditioning. This has led to the rapid increase of electricity demand for air conditioning in summer. To cope with the increasing power demand and the requirement for efficient energy use for space conditioning, governments and energy supply utilities have encouraged effective use and leveling of power load using a heat pump with thermal storage systems and gas cooling systems, by enacting financial and promotional supports. Status and trends of heat pumps in Asian and Pacific regions, related to the use of heat pumps for space heating and cooling were surveyed from the view points of climate, energy consumption, technologies, markets and promotion measures.     

Propane heat pump with low refrigerant charge: design and laboratory tests

Independently of the choice of refrigerant, environmental and or safety issues can be minimised by reducing the amount of refrigerant charge per heat pump or refrigeration system. In the investigation reported here, a laboratory test rig was built, simulating a water-to-water heat pump with a heating capacity of 5 kW. The system was designed to minimize the charge of refrigerant mainly by use of mini-channel aluminium heat exchangers. It was shown that the system could be run with 200 g of propane at typical Swedish operating conditions without reduction of the COP compared to a traditional design. Additional charge reduction is possible by selecting proper compressor lubrication oils or by using a compressor with less lubrication oil.     

Frost retardation of an air-source heat pump by the hot gas bypass method

This study is concerned with a hot gas (refrigerant) bypass method to retard the formation and propagation of frost in an air-source heat pump. The feasibility of the hot gas bypass method was investigated experimentally and the method's performance is compared with that of a normal, 1.12 kW capacity air-source heat pump system with no defrost equipment such as an electric resistance heater. Results indicate that the hot gas bypass method is useful for retarding the formation and growth of frost at the outdoor coil. The best performance is shown under a bypass refrigerant flow rate of 0.2 kg/min (20% of the whole system refrigerant flow rate). During 210 min of heat pump operation, the hot gas bypass method improved COP and heating capacity at an average of 8.5% and 5.7%, respectively, relative to the normal system.     

Field test investigation of a double-stage coupled heat pumps heating system for cold regions

This paper reports a double-stage coupled heat pumps (DSCHP) heating system, which couples air source heat pump (ASHP) and water source heat pump (WSHP) together. The system is presented for the first time in open literature with the objective to improve the working condition and heating performance of the ASHP under cold environment. A practical project in Beijing firstly installed this system and field test has been performed for one month. The test results indicate that the DSCHP system can be smoothly and efficiently used for heating in cold regions. Compared with the traditional ASHP heating system, the operating characteristics of the DSCHP heating system are greatly improved, demonstrating that the system can offer considerable application potential in cold regions.     

A study on the performance of multi-stage condensation heat pumpsEtude sur la performance des pompes à chaleur à condensation en plusieurs étages

In this study, computer simulation programs were developed for multi-stage condensation heat pumps and their performance was examined for CFC11, HCFC123, HCFC141b under the same condition. The results showed that the coefficient of performance (COP) of an optimized ‘non-split type’ three-stage condensation heat pump was 25–42% higher than that of a conventional single-stage heat pump. The increase in COP differed among the fluids examined. The improvement in COP was due largely to the decrease in average temperature difference between the refrigerant and water in the condensers, which resulted in a decrease in thermodynamic irreversibility. For the three-stage heat pump, the highest COP was achieved when the total condenser area was evenly distributed to the three condensers. For the two-stage heat pump, however, the optimum distribution of total condenser area varied with working fluids. For the three-stage system, splitting the condenser cooling water for the use of intermediate and high pressure subcoolers helped increase the COP further. When the individual cooling water for the intermediate and high pressure subcoolers was roughly 10% of the total condenser cooling water, the optimum COP was achieved showing an additional 11% increase in COP as compared to that of the ‘non-split type’ for the three-stage heat pump system.     

Thermodynamic analysis of a liquid-flooded Ericsson cycle cooler

A novel approach to implementing a gas Ericsson cycle heat pump was developed. The concept, termed a liquid-flooded Ericsson cooler (LFEC), uses liquid flooding of the compressor and expander to approach isothermal compression and expansion processes. Analytical models of liquid-flooded compression and expansion processes were developed using ideal gas, constant specific heat, and incompressible liquid assumptions. Special considerations for use of positive displacement compressors with fixed volume ratios are detailed. The unique behavior of a liquid-flooded compressor was explored, including the discovery of an optimum liquid flooding rate that minimizes compression power. A computer model of the LFEC cycle was developed using ideal gas, incompressible liquid, and constant specific heat assumptions. The model was used for a thorough parametric study. The purpose of the study was to explore the feasibility of the concept, identify the optimum operating parameters, and to provide a basis for the design of an experimental system.     

Hybrid absorption heat pump with ammonia/water mixture – Some design guidelines and district heating application

An ammonia/water mixture can be used as an efficient working fluid in industrial-type heat recovery heat pumps and heat transformers. Several configurations of such systems are possible depending on the availability of the waste (thermal) and primary (thermal or electrical) energy sources. This article presents the configurations, the main thermodynamic and hydraulic parameters, and some design guidelines and operating experiences of a medium-temperature, ammonia/water-based compression/re-sorption heat recovery system for district domestic hot water production. In-field experiments have proven the advantages of the concept and its applicability limits in a particular economical environment, while hot water was produced at 55 °C with industrial cooling water at 36 °C as a waste heat source.     

Methodology for thermal design of novel combined refrigeration/power binary fluid systems

Refrigeration cogeneration systems which generate power alongside with cooling improve energy utilization significantly, because such systems offer a more reasonable arrangement of energy and exergy “flows” within the system, which results in lower fuel consumption as compared to the separate generation of power and cooling or heating. This paper proposes several novel systems of that type, based on ammonia–water working fluid. Importantly, general principles for integration of refrigeration and power systems to produce better energy and exergy efficiencies are summarized, based primarily on the reduction of exergy destruction. The proposed plants analyzed here operate in a fully-integrated combined cycle mode with ammonia–water Rankine cycle(s) and an ammonia refrigeration cycle, interconnected by absorption, separation and heat transfer processes. It was found that the cogeneration systems have good performance, with energy and exergy efficiencies of 28% and 55–60%, respectively, for the base-case studied (at maximum heat input temperature of 450 °C). That efficiency is, by itself, excellent for cogeneration cycles using heat sources at these temperatures, with the exergy efficiency comparable to that of nuclear power plants. When using exhaust heat from topping gas turbine power plants, the total plant energy efficiency can rise to the remarkable value of about 57%. The hardware proposed for use is conventional and commercially available; no hardware additional to that needed in conventional power and absorption cycles is needed.     

Capacity-controlled ground source heat pumps in hydronic heating systems

The objective of this study was to investigate the energy-saving potential of using variable-speed capacity control instead of the conventional intermittent operation mode for domestic ground source heat pumps. Variable-speed capacity control is commonly used in air-to-air heat pumps, but not in ground source heat pumps for hydronic heating systems, even though the energy-saving potential may be greater for this application. A theoretical analysis indicates how the energy efficiency is influenced by variable-speed capacity control of the compressor. The analysis shows that, to take full advantage of the capacity control, care should be taken to achieve the correct relationship between refrigerant flow and heat transfer media flows. Intermittent control and variable-speed capacity control were compared by laboratory tests on two capacity-controlled heat pumps and one standard heat pump with a single-speed compressor. Test data were then used for seasonal performance factor (SPF) calculations. The SPF calculations show that despite improved performance at part load the variable-speed controlled heat pump did not improve the annual efficiency compared to the intermittently operated heat pump. This is mainly due to inverter and compressor motor efficiencies and the need for improved efficiency and control of pumps used in the heating and ground collector systems.     

A theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.