Investigating control strategies for a domestic low-temperature heat pump heating system

Despite energy conservation regulations and efforts for improving HVAC operations, numerous domestic buildings do not perform energy efficiently and many times the indoor environment is far away from specified comfort levels. Especially in houses served from low-temperature heating systems the low ability of the heating system to respond to fast changing thermal loads is common. In such cases, the implementation of new, sophisticated controls is an important issue. In this study, we use a reference model of a domestic low temperature heat pump heating system developed in TRNSYS–EES and analyse its operation. Several methods of control strategies have been applied for specified time periods in order to keep the comfort within reasonable ranges. Prognostic climatic control and increased ventilation rates when required are some of these methods. The results depict the influence of the control method on the indoor temperature and the comfort indexes of PMV and PPD. The highest indoor temperature difference for a chosen day reaches 4 °C when there is no shading and when there is internal shading with the option of applying prognostic climatic control. Generally, the findings highlight the importance of dynamics in controlling functions and the difficulty of incorporating in models unpredictable factors as the solar radiation.     

A novel parallel-type hybrid-power gas engine-driven heat pump system

This paper presents a novel concept to integrate a heat pump system and a power system which form a hybrid-power gas engine-driven heat pump (HPGHP) system. The power system of the HPGHP system includes an engine, a motor, a set of battery packs, a continuous variable transmission device and a power-control module. The engine in the power system is capable of operating constantly with high thermal efficiency and low emissions during the four different operating modes: for operating mode A, the ICE powers directly to match the compressor's demand load by throttling the natural-gas flow or adjusting the speed of the ICE, correspondingly the battery packs are disengaged and the ICE operates alone; for operating mode B, the ICE operates in the unique condition with the lowest fuel consumption ratio, meanwhile, the battery packs discharge to provide the supplementary power by the power-control module; for operating mode C, the ICE operates in the unique condition with the lowest fuel consumption ratio, and the redundant power provided by the ICE is converted by the motor to charge the battery packs, here, the motor is used as a generator; for operating mode D, the ICE is disengaged and the battery pack is used alone. Simulation results of the power system showed that for a conventional gas engine-driven heat pump (GHP) system the maximum and minimum thermal efficiencies of the power system are 33% and 22%, respectively; compared with the conventional GHP system, the power system in the novel HPGHP system has superior performance with the maximum and minimum thermal efficiencies of 37% and 27%, respectively.     

Modelling a ground-coupled heat pump system using adaptive neuro-fuzzy inference systems

The aim of this study is to demonstrate the usefulness of an adaptive neuro-fuzzy inference system (ANFIS) for the modelling of ground-coupled heat pump (GCHP) system. The GCHP system connected to a test room with 16.24 m² floor area in Fırat University, Elazığ (38.41°N, 39.14°E), Turkey, was designed and constructed. The heating and cooling loads of the test room were 2.5 and 3.1 kW at design conditions, respectively. The system was commissioned in November 2002 and the performance tests have been carried out since then. The average performance coefficients of the system (COPS) for horizontal ground heat exchanger (GHE) in the different trenches, at 1 and 2 m depths, were obtained to be 2.92 and 3.2, respectively. Experimental performances were performed to verify the results from the ANFIS approach. In order to achieve the optimal result, several computer simulations have been carried out with different membership functions and various number of membership functions. The most suitable membership function and number of membership functions are found as Gauss and 2, respectively. For this number level, after the training, it is found that root-mean squared (RMS) value is 0.0047, and absolute fraction of variance (R²) value is 0.9999 and coefficient of variation in percent (cov) value is 0.1363. This paper shows that the values predicted with the ANFIS, especially with the hybrid learning algorithm, can be used to predict the performance of the GCHP system quite accurately.     

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.     

Detailed simulation of fluted tube water heating condensers

Fluted tube-in-tube condensers are key components in advanced energy efficient water heating heat pumps. Therefore, there exists a need for a computer design tool that incorporates all the essential features of these heat exchangers. This paper describes the development of a detailed model to simulate fluted tube refrigerant-to-water condensers. The model allows the surface area to be divided into any number of sections for which all the refrigerant and water properties can be evaluated. This allows for the extension of the model to simulate heat exchangers for cycles employing zeotropic refrigerant mixtures. For the waterside existing empirical equations are used for both friction and heat transfer. However, on the refrigerant side no correlations are available in the literature to calculate friction and heat transfer coefficients. The approach followed in this paper is therefore to use existing smooth tube correlations combined with enhancement ratios based on correlations available for helical coils as well as enhancement factors based on empirical data for fluted tube condensers. The model is validated with the aid of results from independent tests on two commercial fluted tube heat exchangers. The average difference between the simulated and measured pressure drops is 7.27% and the average difference for the log mean temperature difference (LMTD) is 4.41%.     

Prediction of the heat transfer rate of a single layer wire-on-tube type heat exchanger using ANFIS

In this paper, we applied an Adaptive Neuro-Fuzzy Inference System (ANFIS) model for prediction of the heat transfer rate of the wire-on-tube type heat exchanger. Limited experimental data was used for training and testing ANFIS configuration with the help of hybrid learning algorithm consisting of backpropagation and least-squares estimation. The predicted values are found to be in good agreement with the actual values from the experiments with mean relative error less than 2.55%. Also, we compared the proposed ANFIS model to an ANN approach. Results show that the ANFIS model has more accuracy in comparison to ANN approach. Therefore, we can use ANFIS model to predict the performances of thermal systems in engineering applications, such as modeling heat exchangers for heat transfer analysis.     

Modelling the performance of a transcritical CO2 heat pump for high temperature heating

A prototype transcritical CO₂ heat pump was constructed for heating water to temperatures greater than 65°C while providing refrigeration at less than 2°C. The heating capacity was 115 kW at an evaporation temperature of +0.3°C and a hot water temperature of 77.5°C, with a heating coefficient of performance (COP) of 3.4. Performance data is presented for each of the compressor, the gas cooler, and the recuperator as well as for the overall heat pump system. Equipment performance data was incorporated into a computer model to enable parametric investigations of heat pump performance. Model predictions showed that the hot water temperature could be increased from 65 to 120°C with a relatively small reduction in heating capacity and heating COP of 33 and 21%, respectively. Model predictions also highlight the potential for significant capacity improvements by eliminating the recuperator in favour of a larger gas cooler.     

Modelling heat and mass transfer in frozen foods: a review

This paper reviews mathematical methods for modelling heat and mass transfer during the freezing, thawing and frozen storage of foods. It starts by considering the problems in modelling heat transfer controlled freezing (the Stefan problem): release of latent heat, sudden changes in thermal conductivity. The author gives a unified overview of the common numerical methods: finite difference, finite element and finite volume. Mass transfer is then considered, involving different phenomena and approaches for dense and porous foods. Supercooling, nucleation and trans-membrane diffusion effects during freezing, and recrystallization during frozen storage are considered next. High pressure thawing and thawing are considered in view of their recent popularity. Finally, the paper offers a brief look at mechanical stresses during freezing, a much neglected area. It is concluded that while modelling heat transferred controlled freezing is a settled problem, much work remains to be done in modelling associated phenomena in order to gain the ability to predict changes in food quality at the micro-level.     

Charge optimisation study of a reversible water-to-water propane heat pump

This paper presents the results and conclusions regarding the optimisation of the charge obtained throughout a thorough development study performed with a reversible water-to-water heat pump using propane as refrigerant. The paper first describes the influence of the charge on the capacity and COP of the unit at given operating conditions and then analyses the causes of such influence. Then, the theoretical study carried out with a detailed mathematical model to estimate the amount of refrigerant in each part of the unit is discussed. The second part of the paper is devoted to analysing the influence of the design of the evaporator (BPHE with and without integrated orifice distributor) and the operating conditions on the optimal charge. Finally, the effect of employing a POE or a mineral oil for lubrication on both performance and optimal charge is discussed.     

Numerical study of electroosmotic (EO) flow in microfabricated EO pump with overlapped electrical double layer (EDL)

Micropump is an important component for the development of micro absorption heat pump systems. To study the performance of microfabricated EO pump with the dimension comparable to the electrical double layer (EDL), a model avoiding the use of the Boltzmann equation was developed to investigate the EO flow instead of using the Poisson–Boltzmann (PB) equation. Generally used Debye–Huckle approximation and symmetric condition are also not involved so this model can be applied in the more complicated EO micropumps. Numerical simulation was carried out to study the distributions of potential, ions and net charge density in the EO micropump. Comparison between the present model and the PB equation was conducted. Using the present model, the EO flow in the EO micropump with overlapped EDL was investigated. It shows that the flow is quite different from the channel with dimension much larger than the EDL, which exhibits plug-like flow characteristics. The errors induced by the PB equation for the pumps with different depths were evaluated. It shows that the accuracy of the PB equation is not good when the EO pump is very thin.     

Internal energy flow analysis within a single effect sorption heat pump

The knowledge of the characteristics of unused, excess and untapped exergy allows a thorough analysis of internal energy flows distribution within a sorption heat pump. It can be applied to any system based on gas–liquid absorption, adsorption or solid–gas reaction as well as to any process based on the internal recycling of the energy flux. It can also be applied for the case of a simple effect ideal machine, in particular in the definition of processes where the COP is larger than 2: the levels at which the initial exergy is downgraded on the one hand, as well as, the upgraded excess exergy produced on the other allows the designer to make a judicious choice of a system.     

Exhaust air heat recovery in buildings

The technique of heat recovery from ventilation air in dwellings started in Sweden in late 1979. This was due to an energy crisis and new building codes. The competing heat recovery system, air to air heat exchangers, had a firm grip on the market. Today the situation is on the contrary. Almost all new single family houses are equipped with exhaust air heat pumps. This paper describes the development of the market in Sweden and Germany and also the different techniques of supplementary heating due to national differences in electricity prices. Germany has a situation very similar to Sweden concerning new building codes concerning the allowable energy use for space heating. Starting in 1976 and continued from 1982 to 1995, the building code has prescribed tighter and more insulated houses. The new building code for the year 2000 contains requirements for well insulated and tight buildings so the energy demand for heating from ventilation air tends to reach about 60% of the total annual energy demand for the building. Under these circumstances new buildings must have ventilation systems with heat recovery. Different means of heat recovery from the ventilation system, and the benefit for the environment, by using heat pumps are described. The German market for heat recovery systems is approx. 5–10.000 units/year. Most important for the efficiency of a ventilation system is to maintain the quality criterias concerning:equipmentplanning, installation, taking into operationoperation.VEW ENERGIE AG has accomplished a field survey of 60 units from 1994 to 1996. As the result was not statistically sufficient, the field survey is followed by an investigation into air quality and reliability.     

Experimental study on the performance of a simultaneous heating and cooling multi-heat pump with the variation of operation mode

The cooling load in the winter season becomes significant in commercial buildings and hotels because of the wide usage of office equipment and improved wall insulation. In this study, a simultaneous heating and cooling multi-heat pump having four indoor units and an outdoor unit was designed and tested in five operation modes: cooling-only, heating-only, cooling-main, heating-main, and entire heat recovery. The performance of the system with R410a was optimized by adjusting the system's control parameters. In the cooling-main mode, the rate of the bypass flow to the heating-operated indoor unit was optimized by controlling the EEV opening of the outdoor unit. In the heating-main mode, the mass flow rate to the cooling-operated indoor unit was optimized by adjusting the EEV opening in the outdoor unit. In the entire heat recovery mode, the compressor speed was controlled to improve the system COP with appropriate heating and cooling capacities.     

A novel special distributed method for dynamic refrigeration system simulation

A novel dynamic mathematical model based on spatially distributed approach has been developed and validated in this paper. This model gives good agreement in predicting the system COP and other parameters. The validated model has been used to enhance the prediction of the micro variations of superheat and sub-cooling. The novel spatial distributed model for the condenser and evaporator in refrigeration system, calculates the two-phase region in gas and liquid field separately since the gas and liquid in the two-phase region have different velocities. Previous researchers have used a pre-defined function of the void fraction in their spatially distributed model, based on experimental results. This approach results in the separate solution of the mass and energy equations, and less calculation is required. However, it is recognized that the mass and energy equations should be coupled during solving for more accurate solution. Based on the energy and mass balance, the spatial distribution model constructed here solves the velocity, pressure, refrigerant temperature, and wall temperature functions in heat exchangers simultaneously. A novel iteration method is developed and reduces the intensive calculations required. Furthermore, the condenser and evaporator models have shown a parametric distribution along the heat exchanger surface, therefore, the spatial distribution parameters in the two heat exchangers can be visualised numerically with a two-phase moving interface clearly shown.     

Simulation of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications

A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.     

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.     

A numerical simulation model for plate-type, roll-bond evaporators

This study presents a first-principles mathematical model developed to investigate the thermal behavior of a plate-type, roll-bond evaporator. The refrigerated cabinet was also taken into account in order to supply the proper boundary conditions to the evaporator model. The mathematical model was based on the mass, momentum and energy conservation principles applied to each of the following domains: (i) refrigerant flow through the evaporator channels; (ii) heat diffusion in the evaporator plate; and (iii) heat transmission to the refrigerated cabinet. Empirical correlations were also required to estimate the shear stresses, and the internal and external heat transfer rates. The governing partial differential equations were discretized through the finite-volume approach and the resulting set of algebraic equations was solved by successive iterations. Validation of the model against experimental steady-state data showed a reasonable level of agreement: the cabinet air temperature and the evaporator cooling capacity were predicted within error bands of ±1.5 °C and ±6%, respectively.     

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.     

Optimal shape of the multi-passage branching system in a single-phase parallel-flow heat exchanger

A numerical simulation is performed to examine the heat and fluid flow characteristics of the branching system in a single-phase parallel-flow heat exchanger (PFHE) and to obtain its optimal shape. The relative importance of the design parameters [injection angle of the working fluid (Θ), inlet shape and location (Y_c), and height of the protruding flat tube (Y_b)] is determined to decide the optimization sequence. The optimal geometric parameters are obtained as follows: Θ=−21°, Type A, Y_c=0 and Y_b=0. The heat transfer rate of the optimum model compared to that of the reference model is increased by about 55%. The optimal values of the parameters can be applicable to the Reynolds number ranging from 5000 to 20,000.     

Development of a design tool for display case evaporators

A model for simulating a fin-and-tube display case evaporator has been developed to serve as a design tool for improving performance in frosting conditions. It is capable of simulating cross-counterflow evaporators with multiple modules having completely different geometries. Quasi-steady heat and mass transfer calculations provide local values of all relevant variables, including heat and mass transfer coefficients, air and refrigerant-side pressure drops and fin and tube frost thicknesses. A multi-lump method enables the model to simulate both tube and fin frost thicknesses and surface temperatures. The results presented in this paper reflect interactions with the display case and its air curtains and predict local and overall effects of frost accumulation, to facilitate optimization of the larger display case system.