Pneumatic and thermal design procedure and analysis of earth-to-air heat exchangers of registry type

An analytical pneumatic and thermal design procedure is proposed for earth-to-air heat exchangers (EAHEs) of registry type. The procedure allows to choosing between different EAHE geometrical configurations and between the two usual air circulation paths inside the EAHE (i.e. the Z- and Π-paths, respectively). The implementation of the design procedure is made for the EAHE of a large passive house (PH) built near Bucharest, Romania (AMVIC PH). A time-dependent simulation of EAHE’s operation is performed. It allows to computing the soil temperature profile at the surface and at various depths and the air temperature distribution inside the EAHE. This simulation is validated by comparison with experimental results. The EAHE heating and cooling potential during the year is investigated. The energy delivered by the EAHE depends significantly on its geometrical configuration. A computer fluid dynamics (CFD) analysis is also performed. This analysis is validated by comparison with experimental results. There is good agreement between the results predicted by the design procedure and the CFD analysis concerning the air pressure drops in the EAHE. From a thermal point of view the Z-path should be preferred to the Π-path. The CFD analysis results confirm the conclusions stressed out from the simple analytic design procedure and the time-dependent simulation.     

Simple and accurate model for the ground heat exchanger of a passive house

This paper develops previous research on passive house (PH) space heating. A simple and accurate ground heat exchanger model is developed. It is based on a numerical transient bi-dimensional approach that allows to computing the ground temperature at the surface and at various depths. The new model was integrated into the existing theoretical approach and implemented within the computer code used to simulate the heating system operation in Pirmasens PH (Rhineland Palatinate, Germany). The heating and cooling potential of the system under real climatic conditions was investigated. The energy delivered by the ground heat exchanger depends significantly on different design parameters like pipe's depth, diameter and material.     

Optimal design of plate-and-frame heat exchangers for efficient heat recovery in process industries

The developments in design theory of plate heat exchangers, as a tool to increase heat recovery and efficiency of energy usage, are discussed. The optimal design of a multi-pass plate-and-frame heat exchanger with mixed grouping of plates is considered. The optimizing variables include the number of passes for both streams, the numbers of plates with different corrugation geometries in each pass, and the plate type and size. To estimate the value of the objective function in a space of optimizing variables the mathematical model of a plate heat exchanger is developed. To account for the multi-pass arrangement, the heat exchanger is presented as a number of plate packs with co- and counter-current directions of streams, for which the system of algebraic equations in matrix form is readily obtainable. To account for the thermal and hydraulic performance of channels between plates with different geometrical forms of corrugations, the exponents and coefficients in formulas to calculate the heat transfer coefficients and friction factors are used as model parameters. These parameters are reported for a number of industrially manufactured plates. The described approach is implemented in software for plate heat exchangers calculation.     

Earth-to-air heat exchanger for NZEB in Mediterranean climate

Nowadays the study of Net Zero Energy Buildings (NZEBs) is fundamental, because they are the main strategy to reduce the building energy demand and CO_2-equivalent emissions.This paper analyses a case study concerning a multipurpose building located in Palermo (Southern Italy), and evaluates the benefits related to the use of an earth-to-air heat exchanger in a NZEB, in terms of energy saving and reduction of CO_2-equivalent emissions, for a Mediterranean climate.The chosen building envelope is thermally performing. The HVAC system consists in fan-coil units connected to an air-to-water heat pump, and mechanical ventilation. The energy demand is reduced by an earth-to-air heat exchanger that pre-heats the ventilation outside air in winter and cools it in summer. The optimization of the heat exchange with the ground is a key-element to reduce the primary energy requirements and CO_2-equivalent emissions, especially during summer. Moreover, this paper assesses the possibility to obtain a NZEB using only on-site renewable energy (on the roof of the building), with and without the earth-to-air heat exchanger. The energy analysis is carried out by means of a dynamic building simulation engine, namely EnergyPlus. In addition, a thermal performance and an environmental analysis are performed.     

地源热泵在室内游泳池供暖空调中的应用研究

在分析了地源热泵系统的特点和概算了室内游泳池的冷热负荷的基础上，提出采用地源热泵系统可同时满足室内游泳池的供暖、空调及池水加热3项需求；分析了不同季节地源热泵在游泳池的运行工况。通过分析比较表明，地源热泵系统的运行费用比传统的冷水机组加燃油锅炉系统的运行费用节省约50％，比冷水机组加燃气锅炉系统的运行费用节省21％。     

Numerical analysis of earth air heat exchangers at operating conditions in arid climates

This paper presents the modeling and simulation of an earth air heat exchanger (EAHE), employed as an air-conditioning device for buildings in the climate conditions of the south of Algeria. The earth tubes buried in the ground can offer considerable advantages in terms of energy savings. The appropriate depth of the buried tubes was calculated taking into account the physical properties of the soil in the region under study and using a specific program developed by the authors. A parametric analysis was carried out taking into account the length and the radius of the pipe and the velocity of the air in the pipe. The results of performance and overall energy savings are presented. The maximum daily cooling capacity of the EAHE studied was 1.755 kWh. Results showed that a simple EAHE system can provide 246.815 kWh in a period of one year.     

Seasonal cooling performance of a ground-coupled heat pump system in a hot and arid climate

The goal of the present study is to validate the cooling performance of a ground-coupled heat pump system established in Fırat University, Elazığ (38.41°N, 39.14°E), Turkey. The cooling load of the test room was 3.1 kW at design conditions. The experimental results were obtained from June to September in cooling season of 2003. The ground heat exchanger was used, and it was buried with in 2 m depth trench. The average cooling performance coefficient of the system (COP_overall) was obtained to be 2.01. The results obtained from experimental measurement showed that these systems could be used safely, reliably and efficiently at the lowest possible cost for Elazığ, Turkey climatic conditions. Especially, the seasonal energy efficiency ratio (SEER) of this system is moderate at longer-term testing.     

基于预测的复合地源热泵系统控制方法实现

在冬冷夏热且夏季冷负荷远大于冬季热负荷的地区常采用带有冷却塔的复合式地源热泵系统,其控制策略存在极大的优化空间。文章提出了直接比较冷却塔和与土壤换热器相连的板式换热器的出口温度的控制方法,并通过人工神经网络预测板式换热器机组侧的出口水温来实现此控制方法。通过FLUENT软件建立复合式地源热泵系统动态数值模型,获取建立神经网络的数据,采用3层BP网络,建立了多个预测板式换热器机组侧出口温度的模型。研究结果表明,采用神经网络可以准确实现此预测,绝对误差不超过0.4℃。     

集中供热(冷)的冷热源选择

在以天然气为燃料的前提条件下,提出了热冷联供的3种冷热源方案,从技术经济方面论述了各方案的特点,提出了选择建议。     

Optimal configuration design for plate heat exchangers

A screening method is presented for selecting optimal configurations for plate heat exchangers. The optimization problem is formulated as the minimization of the heat transfer area, subject to constraints on the number of channels, pressure drops, flow velocities and thermal effectiveness, as well as the exchanger thermal and hydraulic models. The configuration is defined by six parameters, which are as follows: number of channels, numbers of passes on each side, fluid locations, feed relative location and type of channel flow. The proposed method relies on a structured search procedure where the constraints are successively applied to eliminate infeasible and sub-optimal solutions. The method can be also used for enumerating the feasible region of the problem; thus any objective function can be used. Examples show that the screening method is able to successfully determine the set of optimal configurations with a very reduced number of exchanger evaluations. Approximately 5% of the pressure drop and velocity calculations and 1% of the thermal simulations are required when compared to an exhaustive enumeration procedure. An optimization example is presented with a detailed sensitivity analysis that illustrates the application and performance of the screening method.     

Numerical simulation and performance assessment of a low capacity solar assisted absorption heat pump coupled with a sub-floor system

A prototype low capacity (10 kW) single stage Li–Br absorption heat pump (AHP), suitable for residential and small building applications has been developed as a collaborative result between various European research institutes and industries. The primary heat source for the AHP is supplied from flat plate solar collectors and the hot/chilled water from the unit is delivered to a floor heating/cooling system. In this paper we present the simulation results and an overview of the performance assessment of the complete system. The calculations were performed for two building types (high and low thermal mass), three climatic conditions, with different types of solar collectors and hot water storage tank sizes and different control systems for the operation of the installation. The simulations were performed using the thermal simulation code TRNSYS. The estimated energy savings against a conventional cooling system using a compression type heat pump was found to be in the range of 20–27%.     

Effectiveness of the earth tube heat exchanger system coupled to a space model in achieving thermal comfort in rural areas

The aim of this work is to investigate by modelling the possibility of reducing the operational energy of a typical house without negatively affecting its embodied energy. This is done through consideration of different building materials coupled with the use of an earth to air heat exchanger (EAHE) for fresh air supply and cooling. For known indoor and outdoor conditions and for given building materials (thermal capacity and conductance), a ventilation controller determines the amount of flow rate needed to temperate the indoor air temperature to achieve thermal comfort. Different wall configurations are assumed for each of the living zone and the bedroom zone of the apartment. It is found that the use of an optimal wall configuration in each zone coupled with the EAHE results in 76.7% energy savings compared with the reference case with conventional cooling.     

Experimental study on the heat transfer enhancement of MWNT-water nanofluid in a shell and tube heat exchanger

Heat transfer enhancement of multi-walled carbon natube(MWNT)/water nanofluid in a horizontal shell and tube heat exchanger has been studied experimentally. Carbon nanotubes were synthesized by the use of catalytic chemical vapor deposition (CCVD) method over Co–Mo/MgO nanocatalyst. Obtained MWNTs were purified using a three stage method. COOH functional groups were inserted for making the nanotubes hydrophilic and increasing the stability of the nanofluid. The results indicate that heat transfer enhances in the presence of multi-walled nanotubes in comparison with the base fluid.     

Modelling earth-to-air heat exchanger behaviour with the convolutive response factors method

This paper shows a new numerical model of earth-to-air heat exchanger. The system is discretized into “n” sections perpendicular to the exchanger pipe. In each section, the problem of conduction is solved using response factors method in order to reduce computational time. Each response factor is calculated using a finite elements program that solves 2D conduction problems. The particularity of this problem is that the time-constants are very high, making it impossible to use conventional properties of response factors to reduce the number of calculations. We will set out a new approach to solve this particular problem. Heat flux entering the pipe is then expressed as a function of the temperature of the air crossing the pipe and the external solicitations. A heat balance is then applied for each layer to find the resulting outlet air temperature. The model is then compared to an analytical model and a 3D model based on the dynamic finite volume approach. Finally an example of coupling between an earth-to-air heat exchanger and a low-consumption building is presented.     

Transient 3D analysis of borehole heat exchanger modeling

This paper presents the development and application of a three-dimensional (3D) numerical simulation model for U-tube borehole heat exchangers (BHEs). The proposed model includes the thermal capacities of the borehole components, viz., the fluid inside the tubes, as well as the grouting material, making it possible to consider the transient effects of heat and mass transports inside the borehole. In this approach, the use of simplified thermal resistance and capacity models (TRCMs) provides accurate results while substantially reducing the number of nodes and the computation time compared with fully discretized computations such as finite element (FE) models. The model is compared with a fully discretized FE model which serves as a reference. Furthermore, the model is used to evaluate thermal response test (TRT) data by the parameter estimation technique. Comparison of the model results with the results of an analytical model based on the line-source theory further establishes the advantage of the developed 3D transient model, as the test duration can be shortened and results are more accurate.     

燃气轮机内环水余热回收研究

通过对燃机内环水所携带热量的研究,设计换热器,论证了用内环水加热燃机天然气进气的可行性,从而达到回收内环水余热,节约能源的目的。     

A novel method for the design of CHCP (combined heat,cooling and power) systems for buildings

The design of capacity and operation of CHCP (combined heat, cooling and power) plants applied to HVAC (heating, ventilation and air conditioning) in buildings entails a considerable difficulty, because efficiency and economic aspects frequently interact in a complex way. Due to the strong fluctuations in thermal demands, the evaluation of a given design usually requires detailed simulations and a significant amount of input data. This paper proposes simplified approaches to estimate the main parameters characterising the thermal performance of the plant (ATD_e method) as well as to identify optimal designs for a given application under certain encouragement policies (annual PES (primary energy savings) strategy). In the ATD_e method, the duration curve of ATD (aggregated thermal demand) is used to estimate, among others, the amount of heat and cooling effectively supplied to the final user for a given design of the plant. This procedure serves to achieve a quick, global evaluation of the thermal performance of CHP (combined heat and power) or CHCP plants with little computational effort. The annual PES strategy searches the optimal values for the engine capacity, the OP (operation period) or both for CHP and CHCP plants in a particular application, defined by its energy demands. Both methods have demonstrated a notably good performance in several test cases with different patterns of the thermal demands.     

Comprehensive exergy analysis of a ground-source heat pump system for both building heating and cooling modes

This paper presents a comprehensive exergy analysis of three circuits and whole system of a ground-source heat pump (GSHP) for both building heating and cooling modes. The purpose is to search out the key potential energy saving components. The analytical formulae of exergy loss, exergy efficiency, exergy loss ratio, exergy loss coefficient and thermodynamic perfect degree are derived, respectively. The results show that these exergy indexes should be used integratively, and in the whole system the location of maximum exergy loss ratio is the compressor, while the location of minimum exergy efficiency and thermodynamic perfect degree is the ground heat exchanger, so that the compressor and the ground heat exchanger should be primarily improved. The results also indicate that the exergy loss of a GSHP system for building heating mode is bigger than that of cooling mode, and the exergy efficiency of a whole GSHP system is obviously lower than those of its components for both building heating and cooling modes. Therefore, a comprehensive exergy analysis of a GSHP should be paid more attention to. The results may provide guidelines for the design and optimization of GSHP systems.     

Thermodynamic optimization of a coiled tube heat exchanger under constant wall heat flux condition

In this paper the second law analysis of thermodynamic irreversibilities in a coiled tube heat exchanger has been carried out for both laminar and turbulent flow conditions. The expression for the scaled non-dimensional entropy generation rate for such a system is derived in terms of four dimensionless parameters: Prandtl number, heat exchanger duty parameter, Dean number and coil to tube diameter ratio. It has been observed that for a particular value of Prandtl number, Dean number and duty parameter, there exists an optimum diameter ratio where the entropy generation rate is minimum. It is also found that with increase in Dean number or Reynolds number, the optimum value of the diameter ratio decreases for a particular value of Prandtl number and heat exchanger duty parameter.     

Experiments on solid state hydrogen storage device with a finned tube heat exchanger

The absorption and desorption performances of a solid state (metal hydride) hydrogen storage device with a finned tube heat exchanger are experimentally investigated. The heat exchanger design consists of two “U” shaped cooling tubes and perforated annular copper fins. Copper flakes are also inserted in between the fins to increase the overall effective thermal conductivity of the metal hydride bed. Experiments are performed on the storage device containing 1 kg of hydriding alloy LaNi₅, at various hydrogen supply pressures. Water is used as the heat transfer fluid. The performance of the storage device is investigated for different operating parameters such as hydrogen supply pressure, cooling fluid temperature and heating fluid temperature. The shortest charging time found is 490 s for the absorption capacity of 1.2 wt% at a supply pressure of 15 bar and cooling fluid temperature and velocity of 288 K and 1 m/s respectively. The effect of copper flakes on absorption performance is also investigated and compared with a similar storage device without copper flakes.