Transient Conduction in a Fin-Wall Assembly with Harmonic Excitation--Network Thermal Admittance

The classical performance indicators for extended surfaces, efficiency and effectiveness, cannot be used for the proper design of finned systems subject to time-dependent processes, such as heat exchangers or electric devices. Based on the network simulation method, a network model of a fin-wall assembly, whose admittance is identical to the thermal admittance of the system, has been designed for the whole system. A new fin performance indicator, output admittance, is proposed, and frequency analysis of the system is carried out. The simulated numerical response is rapidly obtained by running the network in the appropriate circuit resolution software. This method is especially useful for studying complex thermal transmission functions such as admittance, evaluating modulus, phase, and real and imaginary components of the thermal signal.     

Time-dependent heat transfer in a fin-wall assembly. New performance coefficient: thermal reverse admittance

Transient thermal fields and heat fluxes due to step-harmonic temperature excitation and their dependence on frequency are studied in a fin-wall assembly. Application of the currently used efficiency coefficient to transient-harmonic processes is discussed. A new coefficient, thermal reverse transfer admittance, and others, including the augmentation factor, have been used to characterise the behaviour of the system. In a thermal frequency response analysis, module, phase, real and imaginary components have been obtained. For the calculation a network model (whose admittance is identical to the thermal admittance of the system) has been designed for the whole system. The network simulation method provides the numerical response of the system by running the network in circuit resolution software.     

Optimization of hybrid – ground coupled and air source – heat pump systems in combination with thermal storage

Ground coupled heat pumps are attractive solutions for cooling and heating commercial buildings due to their high efficiency and their reduced environmental impact. Two possible ideas to improve the efficiency of these systems are decoupling energy generation from energy distribution and combining different HVAC systems. Based on these two ideas, we present several HVAC configurations which combine the following equipments: a ground coupled heat pump, an air to water heat pump and a thermal storage device. These HVAC configurations are linked to an office building in a cooling dominated area in order to evaluate in these conditions the total electrical consumption of each configuration to obtain which one satisfy the thermal demand more efficiently. The results of our simulations show that the electrical energy consumption obtained when the system employs a suitable configuration is of around the 60% compared with an HVAC system driven by an air to water heat pump and around the 82% compared with an HVAC system driven by a ground coupled heat pump.     

A cost‐effective operating strategy to reduce energy consumption in a HVAC system

The operation of the building heating, ventilating, and air conditioning (HVAC) system is a critical activity in terms of optimizing the building's energy consumption, ensuring the occupants' comfort, and preserving air quality. The performance of HVAC systems can be improved through optimized supervisory control strategies. Set points can be adjusted by the optimized supervisor to improve the operating efficiency. This paper presents a cost‐effective building operating strategy to reduce energy costs associated with the operation of the HVAC system. The strategy determines the set points of local‐loop controllers used in a multi‐zone HVAC system. The controller set points include the supply air temperature, the supply duct static pressure, and the chilled water supply temperature. The variation of zone air temperatures around the set point is also considered. The strategy provides proper set points to controllers for minimum energy use while maintaining the required thermal comfort. The proposed technology is computationally simple and suitable for online implementation; it requires access to some data that are already measured and therefore available in most existing building energy management and control systems. The strategy is evaluated for a case study in an existing variable air volume system. The results show that the proposed strategy may be an excellent means of reducing utility costs associated with maintaining or improving indoor environmental conditions. It may reduce energy consumption by about 11% when compared with the actual strategy applied on the investigated existing system. Copyright © 2007 John Wiley & Sons, Ltd.     

Modeling of a hydronic ceiling system and its environment as energetic auditing tool

As a part of a commissioning study, the chilled ceiling system of a large commercial building located in Belgium is evaluated. A representative office has been instrumented and data on the chilled ceiling system operating in real conditions have been collected. The simulation of the whole system is performed by means of a transient thermal model of the building and its HVAC system. The model considers the hydronic panels as a transient-state finned heat exchanger connected to a simplified lumped transient model of the building. The behavior of the hydronic ceiling system and the interactions with its environment (walls, ventilated facade, internal loads and ventilation system) has been experimentally and numerically evaluated. Commissioning test results show that the influence of surfaces temperatures inside the room, especially the facade, is considerable. Then, it is clear that the hydronic ceiling system must be evaluated together with its designed environment and not as a separate HVAC equipment.     

Thermoelectricity analogy method for computing the periodic heat transfer in external building envelopes

Simple and effective computation methods are needed to calculate energy efficiency in buildings for building thermal comfort and HVAC system simulations. This paper, which is based upon the theory of thermoelectricity analogy, develops a new harmonic method, the thermoelectricity analogy method (TEAM), to compute the periodic heat transfer in external building envelopes (EBE). It presents, in detail, the principles and specific techniques of TEAM to calculate both the decay rates and time lags of EBE. First, a set of linear equations is established using the theory of thermoelectricity analogy. Second, the temperature of each node is calculated by solving the linear equations set. Finally, decay rates and time lags are found by solving simple mathematical expressions. Comparisons show that this method is highly accurate and efficient. Moreover, relative to the existing harmonic methods, which are based on the classical control theory and the method of separation of variables, TEAM does not require complicated derivation and is amenable to hand computation and programming.     

Energy saving by integrated control of natural ventilation and HVAC systems using model guide for comparison

Integrated control by controlling both natural ventilation and HVAC systems based on human thermal comfort requirement can result in significant energy savings. The concept of this paper differs from conventional methods of energy saving in HVAC systems by integrating the control of both these HVAC systems and the available natural ventilation that is based on the temperature difference between the indoor and the outdoor air. This difference affects the rate of change of indoor air enthalpy or indoor air potential energy storage. However, this is not efficient enough as there are other factors affecting the rate of change of indoor air enthalpy that should be considered to achieve maximum energy saving. One way of improvement can be through the use of model guide for comparison (MGFC) that uses physical-empirical hybrid modelling to predict the rate of change of indoor air potential energy storage considering building fabric and its fixture. Three methods (normal, conventional and proposed) are tested on an identical residential building model using predicted mean vote (PMV) sensor as a criterion test for thermal comfort standard. The results indicate that the proposed method achieved significant energy savings compared with the other methods while still achieving thermal comfort.     

The map of energy flow in HVAC systems

Heating, ventilation and air conditioning (HVAC) systems are the most energy consuming building services representing approximately half of the final energy use in the building sector and between one tenth and one fifth of the energy consumption in developed countries. Despite their significant energy use, there is a lack of a consistent and homogeneous framework to efficiently guide research and energy policies, mainly due to the complexity and variety of HVAC systems but also to insufficient rigour in their energy analysis. This paper reviews energy related aspects of HVAC systems with the aim of establishing a common ground for the analysis of their energy efficiency. The paper focuses on the map of energy flow to deliver thermal comfort: the HVAC energy chain. Our approach deals first with thermal comfort as the final service delivered to building occupants. Secondly, conditioned spaces are examined as the systems where useful heat (or coolth) is degraded to provide comfort. This is followed by the analysis of HVAC systems as complex energy conversion devices where energy carriers are transformed into useful heat and coolth, and finally, the impact of HVAC energy consumption on energy resources is discussed.     

Simple tool to evaluate energy demand and indoor environment in the early stages of building design

A simplified building simulation tool to evaluate energy demand and thermal indoor environment in the early stages of building design is presented. Simulation is performed based on few input data describing the building design, HVAC systems and control strategies. Hourly values for energy demand and indoor temperature are calculated based on hourly weather data. Calculation of the solar energy transmitted through windows takes into account the dependency of the total solar energy transmittances on the incidence angle, shades from far objects and shades from the window recess and overhangs. Several systems including heating, cooling, solar shading, venting, ventilation with heat recovery and variable insulation can be activated to control the indoor temperature and energy demand. Predicted percentages of dissatisfied occupants are calculated for a given time period to support decisions concerning the thermal indoor environment. The simplified building simulation tool gives reliable results compared to detailed tools and needs only few input data to perform a simulation. The tool is therefore useful for preliminary design tasks in the early design stages where rough estimates of the building design are given and rough estimates of energy use and thermal indoor environment are needed for decision support.     

Practical support for evaluating efficiency factors of a space heating system in cold climates

Plenty of technical norms, included in the EPBD umbrella, assesses the performance of buildings or its sub-systems in terms of efficiency. In particular, EN 15316 and its sub-sections determine the efficiency factors of a space heating system. This paper focuses on the estimation of efficiency factors for hydronic panel radiators. The assessment of efficiency factors occurs by evaluating the amount of heat emitted from the heat emitter and the thermal losses towards building envelope. A factor that influences the heat emitted is the location of radiator connection pipes. Connection pipes can be located on opposite side or at the same side of the radiator. To better estimate the heat emitted from the radiator with different location of connection pipes, a transient model with multiple storage elements is implemented in a commercial building simulation software and validated versus available experimental measurements. Sensitivity analysis encompasses the variations of heat losses due to the building location in different climates, the changing of the active thermal mass and the type of radiator local control. The final outcome of this paper is a practical support where designers and researchers can easily assess the efficiency factors for space heating system equipped with hydronic panel radiators of buildings located in Sweden. As main results, (i) the efficiency factor for control is higher in Northern climates (Luleå) than in Southern climates (Gothenburg), (ii) heavy-weight active thermal masses allow higher efficiency factors than light active thermal masses, and (iii) connection pipes located on the same side of the hydronic panel radiator enable higher efficiency factors than pipes located on opposite side.     

建筑室内人体太阳辐射得热特性及数值模拟

太阳辐射对建筑室内人体热舒适和建筑能耗有着显著影响.通过实测验证了daylight coefficient(DC)算法模拟太阳辐射强度的准确性.随后基于假人仿真模型采用DC算法计算室内人体平均辐射温度增量(ΔMRT),与SolarCal(SC)算法结果作对比,并对SC算法进行改进.相比原SC算法,改进SC算法与DC算法...     

Combination of ground source heat pumps with chemical dehumidification of air

In the present paper the possible synergies provided by the combination of an underground thermal energy storage (UTES) system with a desiccant based air handling unit (AHU) are analysed. Differently from the conventional solutions, the summer humidity control is obtained here by chemical dehumidification of the ventilation airstream performed by liquid desiccants in a packed column. Being the water temperature of the boreholes heat exchangers generally suitable to meet the sensible load without any integration with the chillers, the plant can operate in a complete free-cooling mode. In winter, the main benefits are due to the higher temperature level at which the UTES works and to the AHU configuration allowing sensible and latent heat recovery. For the same reasons, the required UTES size is sensibly smaller, reducing in this way not only the operation but above all the investment costs. The UTES system competitiveness is then increased. The described solution is investigated by a computer simulation referring to a modern office building in the climate of northern Italy and its performance has been compared to a traditional HVAC plant and to a traditional ground source heat pump (GSHP) system. Finally, some economic evaluations are reported, showing the competitiveness of the proposed configuration.     

Comparison of energy and exergy analysis of fossil plant,ground and air source heat pump building heating system

The energy and exergy flow for a space heating systems of a typical residential building of natural ventilation system with different heat generation plants have been modeled and compared. The aim of this comparison is to demonstrate which system leads to an efficient conversion and supply of energy/exergy within a building system.The analysis of a fossil plant heating system has been done with a typical building simulation software IDA–ICE. A zone model of a building with natural ventilation is considered and heat is being supplied by condensing boiler. The same zone model is applied for other cases of building heating systems where power generation plants are considered as ground and air source heat pumps at different operating conditions. Since there is no inbuilt simulation model for heat pumps in IDA–ICE, different COP curves of the earlier studies of heat pumps are taken into account for the evaluation of the heat pump input and output energy.The outcome of the energy and exergy flow analysis revealed that the ground source heat pump heating system is better than air source heat pump or conventional heating system. The realistic and efficient system in this study “ground source heat pump with condenser inlet temperature 30 °C and varying evaporator inlet temperature” has roughly 25% less demand of absolute primary energy and exergy whereas about 50% high overall primary coefficient of performance and overall primary exergy efficiency than base case (conventional system). The consequence of low absolute energy and exergy demands and high efficiencies lead to a sustainable building heating system.     

Optimization of an HVAC system with a strength multi-objective particle-swarm algorithm

A data-driven approach for the optimization of a heating, ventilation, and air conditioning (HVAC) system in an office building is presented. A neural network (NN) algorithm is used to build a predictive model since it outperformed five other algorithms investigated in this paper. The NN-derived predictive model is then optimized with a strength multi-objective particle-swarm optimization (S-MOPSO) algorithm. The relationship between energy consumption and thermal comfort measured with temperature and humidity is discussed. The control settings derived from optimization of the model minimize energy consumption while maintaining thermal comfort at an acceptable level. The solutions derived by the S-MOPSO algorithm point to a large number of control alternatives for an HVAC system, representing a range of trade-offs between thermal comfort and energy consumption.     

Performance of PV-Trombe wall in winter correlated with south façade design

PV-Trombe wall (PVTW) is a novel version of Trombe-wall. Photovoltaic cells on the cover glazing of the PVTW can convert solar radiation into electricity and heat simultaneously. A window on the south façade can also introduce solar heat into the room in the winter season. Experiment has been conducted to study the temperature field of a building with both southern facing window and the PVTW. A dynamic numerical model is developed for the simulation of the whole building system. The temperature of the indoor air is found to be vertically stratified from the measurement. The nodal model is adopted to calculate the temperature profile in the room. The simulation results are in good agreement with the experimental data. The different south façade designs affect the thermal efficiency of the PVTW significantly from the numerical simulation. With a southern facing window, the thermal efficiency of the PVTW is reduced by 27% relatively. The increase of PV coverage on the glazing can reduce the thermal efficiency of the TW by up to 17%. By taking account of electric conversion, the total efficiency of solar utilization is reduced by 5% at most while the glazing is fully covered with PV cells. The electric conversion efficiency of the PVTW achieves 11.6%, and is slightly affected by south façade designs.     

Solid oxide fuel cell/gas turbine trigeneration system for marine applications

Shipping contributes 4.5% to global CO₂ emissions and is not covered by the Kyoto Agreement. One method of reducing CO₂ emissions on land is combined cooling heating and power (CCHP) or trigeneration, with typical combined thermal efficiencies of over 80%. Large luxury yachts are seen as an ideal entry point to the off-shore market for this developing technology considering its current high cost.This paper investigates the feasibility of combining a SOFC-GT system and an absorption heat pump (AHP) in a trigeneration system to drive the heating ventilation and air conditioning (HVAC) and electrical base-load systems. A thermodynamic model is used to simulate the system, with various configurations and cooling loads. Measurement of actual yacht performance data forms the basis of this system simulation.It is found that for the optimum configuration using a double effect absorption chiller in Ship 1, the net electric power increases by 47% relative to the electrical power available for a conventional SOFC-GT-HVAC system. This is due to more air cooled to a lower temperature by absorption cooling; hence less electrical cooling by the conventional HVAC unit is required. The overall efficiency is 12.1% for the conventional system, 34.9% for the system with BROAD single effect absorption chiller, 43.2% for the system with double effect absorption chiller. This shows that the overall efficiency of a trigeneration system is far higher when waste heat recovery happens.The desiccant wheel hardly reduces moisture from the outdoor air due to a relative low mass flow rate of fuel cell exhaust available to dehumidify a very large mass flow rate of HVAC air, Hence, desiccant wheel is not recommended for this application.     

Neural computing thermal comfort index for HVAC systems

The primary purpose of a heating, ventilating and air conditioning (HVAC) system within a building is to make occupants comfortable. Without real time determination of human thermal comfort, it is not feasible for the HVAC system to yield controlled conditions of the air for human comfort all the time. This paper presents a practical approach to determine human thermal comfort quantitatively via neural computing. The neural network model allows real time determination of the thermal comfort index, where it is not practical to compute the conventional predicted mean vote (PMV) index itself in real time. The feed forward neural network model is proposed as an explicit function of the relation of the PMV index to accessible variables, i.e. the air temperature, wet bulb temperature, globe temperature, air velocity, clothing insulation and human activity. An experiment in an air conditioned office room was done to demonstrate the effectiveness of the proposed methodology. The results show good agreement between the thermal comfort index calculated from the neural network model in real time and those calculated from the conventional PMV model.     

Control of thermally-activated building systems (TABS)

Integrating the building structure to act as an energy-storage, thermally-activated building system (TABS) has proved to be energy efficient and economically viable for cooling and heating of buildings. However control has remained an issue to be improved. In this paper, a method is outlined allowing both for dimensioning and for automated control of TABS, with automatic switching between cooling/heating modes for variable comfort criteria. The method integrally considers both HVAC and building automation design aspects, as well as the fact that during design and operation heat-gains are unknown, but that bounds of them normally can be specified. This integral method is termed the Unknown-But-Bounded or UBB method. Applying the method guarantees that comfort can be maintained, as long as the actual heat-gains stay within the predefined range between the lower and upper bounds. The UBB method can also handle non-predictable day-to-day variations as well as room-to-room variations of the heat gains. The paper outlines the underlying thermal models and assumptions, and gives the procedure and an example for the application of the method.     

On the ground temperature below buildings

A transient, numerical model for the prediction of the ground temperature at various depths below buildings is presented in this paper. The proposed model was developed by calculating the heat flow to the ground from a building, which depends on the complicated three-dimensional thermal process in the ground. The main difficulties in obtaining manageable solutions of the heat flow problem were: The three-dimensionality of the thermal process, the strong temporal variability of the outdoor temperature as well as the large number of parameters involved in describing the building foundation geometry as well as the thermal insulation. The techniques of superposition and numerical analysis were used to cope with these difficulties. The model was validated against experimental data and it was found that it could accurately predict the ground temperature under a building.