Economic and environmental impacts of insulation in district heating pipelines

The determination of optimum thickness of insulation is often applied to energy technologies and building projects. In this study, the energy, economic and environmental evaluations of thermal insulation in district heating pipeline are discussed. The optimum insulation thickness, energy saving over a lifetime of 10 years, payback period and emissions of CO₂, CO and SO₂ are calculated for nominal pipe sizes and fuel types based on heating loads in Afyonkarahisar/Turkey. The life cycle cost analysis is used to determine the optimum thickness of the pipeline material in order to take into account the change in inflation that directly affect both the cost of pipeline material and fuels depending on fuel type. The results show that the highest value of optimum insulation thickness, energy savings, emissions and the lowest payback period are reached for a nominal pipe size of 200 mm. About three times more energy saving results by making 200 mm nominal pipe instead of 50 mm. Considering the economical and environmental advantages, the geothermal energy is a better choice and then fuel-oil. When thermal insulation is done in a district heating pipeline, there will be a significant reduction of 21% in the amount of CO₂ emitted to the atmosphere.     

Pre-insulated district-heating pipelines: Design and operational advice

Steady-state temperature distributions have been measured within a cylindrical duct enclosing two different diameter pipes—one for district heating and the other for the supply of domestic hot-water; the gaps between the pipes being filled with well-packed fibrous thermal insulant. The procedure was repeated successively with each of three sets of different-diameter pipes. The recommended configuration was with the DH pipe coaxial with the centre of the pipeline and the DHW one located vertically below it: this corresponded to the minimum rate of heat loss from the district-heating pipe under the considered conditions. An overall improvement of 8% in the thermal insulation of the DH pipe has been achieved compared with the current conventional practice of installing the DHW pipe above the DH pipe.     

Determining the optimum economic insulation thickness of double pipes buried in the soil for district heating systems

The insulation thickness (IT) of double pipes buried in the soil (DPBIS) for district heating (DH) systems was optimized to minimize the annual total cost of DPBIS for DH systems. An optimization model to obtain the optimum insulation thickness (OIT) and minimum annual total cost (MATC) of DPBIS for DH systems was established. The zero point theorem and fsolve function were used to solve the optimization model. Three types of heat sources, four operating strategies, three kinds of insulation materials, seven nominal pipe size (NPS) values, and three buried depth (BD) values were considered in the calculation of the OIT and MATC of DPBIS for DH systems, respectively. The optimization results for the above factors were compared. The results show that the OIT and MATC of DPBIS for DH systems can be obtained by using the optimization model. Sensitivity analysis was conducted to investigate the impact of some economic parameters, i.e., unit heating cost, insulation material price, interest rate, and insulation material lifetime, on optimization results. It is found out that the impact of sensitivity factors on the OIT and MATC of DPBIS for DH systems is different.     

Single,twin and triple buried heating pipes: on potential savings in heat losses and costs

In order to make district heating systems competitive in areas with single family houses or in other areas with low heat demands it is necessary to reduce the heat losses from the pipes. In recent years the twin pipe has become popular in the Nordic countries. In the article we describe how the heat loss and the heat loss coefficients can be calculated. We introduce the triple pipe with three media pipes (two supply pipes and one return pipe). The temperature dependency and the ageing of polyurethane insulation are briefly discussed. A comparison is made for different 80 mm distribution pipes and for different service pipes with respect to heat losses and to resources, i.e. materials needed for the casing and polyurethane insulation and the gravel in the excavations. For the distribution pipe we found that an egg‐shaped twin pipe can reduce the heat loss by 37% and the investments by 12% compared with a pair of single pipes. For the service pipes we found that the triple pipe reduces the heat loss by 45% compared with a common pair of single pipes and by 24% compared with circular twin pipes. The reduction in investment index is 21%. The article also addresses the question of the heat exchange between the two media pipes in a twin pipe. Copyright © 2005 John Wiley & Sons, Ltd.     

有空气保温层的蒸汽直埋管道保温厚度计算与分析

对不同介质温度条件下有空气保温层的"钢套钢"蒸汽直埋管保温结构厚度进行了数值计算,并将计算结果与无空气保温层的保温结构厚度进行了比较.计算结果若用于实际工程,可在确保保温效果的前提下,减小保温层厚度和外套钢管的使用量,以达到减少材料的使用、节约投资的目的.     

Finite time thermal analysis of ground integrated‐collector‐storage solar water heater with transparent insulation cover

A transparent honeycomb insulated ground integrated‐collector‐storage system has been investigated for the engineering design and solar thermal performance. The system consists of a network of pipes embedded in a concrete slab whose surface is blackened and covered with transparent insulation materials (TIM) and the bottom is insulated by the ground. Heat may be retrieved by the flow of fluid through the pipe. A simulation model has been developed; it involves the solution of the two‐dimensional transient heat conduction equation using an explicit finite‐difference scheme. Computational results have been used to determine the effect of such governing parameters as depth as well as pitch of the pipe network and collector material on the thermal performance of the system. The pipe network depth of 10 cm and the TIM cover made of 5 cm compounded honeycomb seem suitable for the proposed system. Solar gain (solar collection efficiency of 30–50% corresponding to collection temperature of 40–60°C) and the diurnal heat storage characteristics of the system are found to be of the right order of magnitude for solar water heating applications. Copyright © 1999 John Wiley & Sons, Ltd.     

热电联产机组工程中厂区供热蒸汽管道的设计新探索

本文介绍了热电联产机组工程中厂区供热蒸汽管道在设计上的两项新探索,厂区蒸汽管道通过采用综合管架加无推力旋转补偿器的敷设方案,并通过隔热支吊架的设置可以满足电厂设计可靠、美观、经济、节能的要求,对于类似厂区热网设计项目具有一定的参考价值。     

真空隔热油管传热性能研究

真空隔热油管是稠油注蒸汽开采的主要设施之一,其隔热性能直接影响热采效果,因此,分析隔热油管隔热层内部的传热过程,研究隔热层各种结构参数对隔热油管隔热性能的影响,对改善隔热油管隔热性能有重要的指导作用。本文在测试隔热油管视导热系数的实验模型基础上,建立了隔热层传热的物理及数学模型,计算得到不同结构参数下隔热油管的视导热系数;研究了玻璃丝布孔隙比、隔热层层数、支撑材料导热系数以及铝箔发射率对隔热油管隔热性能的影响,研究表明:隔热层层数宜选为4~6层之间;选用的铝箔发射率应在0.01~0.05之间;在缠绕玻璃丝布工艺中,尽量不要让玻璃丝布对角线方向受力,以确保较大的孔隙比;在隔热油管加工过程中,应尽量使玻璃丝布和铝箔保持干燥。     

Experimental and numerical investigation of the thermal performance of a protected vacuum-insulation system applied to a concrete wall

A concrete wall externally insulated with six expanded polystyrene boards, each containing three vacuum insulation panels, was investigated both experimentally and numerically. The main goal of this study was to determine the thermal performance of vacuum-insulation panels applied to walls in building constructions. Comparisons were made with conventional insulation and also with systems including damaged, i.e., vented vacuum panels. Since the vacuum insulation panels are encased in a metallized laminates as barriers against permeation of moisture and gas, special attention was given to the edge effects. Stepwise adjustment of the measured and calculated results reported here provide a general assessment of the efficacy of this insulation system applied on different wall materials. A functional representation of the measured data, for steady-state conditions, is introduced. Moreover, infrared thermography was used to confirm the three dimensionally calculated temperature distributions on the surface. The present investigation was part of the research programme “High Performance Thermal Insulation in Buildings and Building Systems” of the international energy agency (IEA).     

EVALUATION OF THE DYNAMIC PERFORMANCE OF A HOT WATER TANK WITH BUILT-IN HEATING COIL

Hot water tanks with a built-in water-heating coil are commonly used in district heating house stations in Denmark for domestic hot water (DHW) production and storage. In this study, an evaluation of the dynamic performance of a hot water tank with built-in heating coil is carried out by applying a dynamic simulation programme which has been made previously, based on a simple dynamic model developed by the authors. System evaluation of the way in which system parameters, such as control valve size, heat loss coefficient of the DHW circulation pipe, position of the temperature sensor (for DHW temperature control) and fouling of the heating coil, affect the domestic hot water capacity and the average district heating water cooling for a given hot water tank is presented and discussed in this paper. The evaluation results show the importance of the correct design of the control valve size, the reduction of heat loss from DHW circulation pipes, the careful adjustment of temperature sensor position and temperature sensor set-point, and the reduction of the heat coil fouling growth rate in order to operate the hot water tank in an efficient way and to achieve significant cooling of the district heating water. © 1997 by John Wiley & Sons, Ltd.     

The influence of different ground covers on the heating potential of earth-to-air heat exchangers

Ten years' hourly measurements of air and ground temperature values at various depths below bare and short grass soil at Dublin Airport have been used in order to investigate the impact of different ground surface boundary conditions on the efficiency of a single and a multiple parallel earth-to-air heat exchanger system. The heating potential of both these systems buried under bare soil has been assessed and compared with the heating potential of the same systems buried under short-grass-covered soil. The results of this comparison revealed that soil surface cover might be a significant controllable factor for the improvement of the performance of earth-to-air heat exchangers. The heating system consists of a single pipe or multiple parallel pipes laid horizontally, through which ambient or indoor air is propelled and heated by the bulk temperature of the natural ground. The dynamic thermal performance of these systems during the winter period and their operational limits have been calculated using an accurate numerical model. Finally, a sensitivity analysis was performed in order to investigate the effect of the main design parameters, such as pipe length, pipe radius, air velocity inside the tube and the depth of the buried pipe below the earth's surface, on the system heating capacity. Cumulative frequency distributions of the air temperature at the pipe's exit have been developed as a function of the main input parameters.     

基于航空散热板的内嵌热管传热模型分析

对一种非规则截面的航空散热板轴向槽道热管,分别建立了基于SINDA软件HEATPIPE算法的一维模型和基于QFLOW算法的二维模型,比较了两种传热模型和算法下热管的暂态和稳态传热特性。结果表明,两种模型对热管平均工作温度的计算结果吻合较好,由于一维模型没有考虑热管壳体的周向传热以及热管的结构对边界条件施加的影响,最大传热能力的计算结果小于二维模型。一维模型适于对航空热管进行系统级的初步热设计,二维模型适于对非规则截面热管的定量传热分析。     

集中供热热管太阳能集热系统

在原有平板集热器的基础上研制了一种与建筑结合的太阳能集中供热热管系统，并对策略热管的传热机理进行了分析，提出正常工况下该热管的集总参数模型，给出了热管的倾角范围。文中给出了该集热系统的理论预测和实验结果对比。热管太阳能集热器的理论模型以Duffie 和Beckman理论（1980）为基础，修改后用于能量传输。该文还给出了该集热器和传统的太阳能集热器的对比试验结果。热管太阳能集热器的瞬时效率在早上低于传统的太阳能集热器的，而当热管达到工作温度时，前者高于后者。     

供热管道保温材料的选择及经济保温层厚度计算

合理选择供热管道的保温材料及经济保温层厚度,对减少管道在输送热媒过程中的散热损失,降低工程造价尤为重要。本文根据国家现行标准和有关规定,对热网保温材料的选择、经济保温层厚度的简便计算方法进行了阐述。为工程实践中在管道保温方面,达到节能降耗,节约投资的目的提供参考。     

Piping network design of geothermal district heating systems: Case study for a university campus

Geothermal district heating system design consists of two parts: heating system and piping network design. District heating system design and a case study for a university campus is given in Yildirim et al. [1] in detail. In this study, piping network design optimisation is evaluated based on heat centre location depending upon the cost and common design parameters of piping networks which are pipe materials, target pressure loss (TPL) per unit length of pipes and installation type. Then a case study for the same campus is presented.     

Optimal flow control of a forced circulation solar water heating system with energy storage units and connecting pipes

This paper focuses on pump flow rate optimization for forced circulation solar water heating systems with pipes. The system consists of: an array of flat plate solar collectors, two storage tanks for the circulation fluid and water, a heat exchanger, two pumps, and connecting pipes. The storage tanks operate in the fully mixed regime to avoid thermal stratification. The pipes are considered as separated components in the system so as to account for their thermal effects. The objective is to determine optimal flow rates in the primary and secondary loops in order to maximize energy transfer to the circulation fluid storage tank, while reaching user defined temperatures in the water storage tank to increase thermal comfort. A model is developed using mainly the first and second laws of thermodynamics. The model is used to maximize the difference between the energy extracted from the solar collector and the combined sum of the energy extracted by the heat exchanger and corresponding energies used by the pumps in the primary and secondary loops. The objective function maximizes the overall system energy gain whilst minimizing the sum of the energy extracted by the heat exchanger and corresponding pump energy in the secondary loop to conserve stored energy and meet the user requirement of water tank temperatures. A case study is shown to illustrate the effects of the model. When compared to other flow control techniques, in particular the most suitable energy efficient control strategy, the results of this study show a 7.82% increase in the amount of energy extracted. The results also show system thermal losses ranging between 5.54% and 7.34% for the different control strategies due to connecting pipe losses.     

On transient heat losses from buried district heating pipes

The theory of steady‐state heat loss determination from buried heating pipes has been reviewed as well as previous approaches to treat transient heat losses in the case of constant water temperatures. A new method has been developed to find an undisturbed ground temperature by which the transient heat loss can be calculated by the use of the steady‐state heat loss equations. By numerical simulations, as well as by experiment, the position of this undisturbed ground temperature has been found. The position of this ground temperature is closer to the ground surface in the case of uninsulated pipes—or pipes with the insulation in poor condition—than in the case of insulated pipes. For pre‐insulated pipes the position corresponds approximately to the top of the pipe casing. Copyright © 2000 John Wiley & Sons, Ltd.     

Development of tree-shaped flows by adding new users to existing networks of hot water pipes

This paper describes the optimization of a tree-shaped system of insulated pipes for the distribution of a stream of hot water over an area. The area is covered uniformly by users who must receive the same flow rate of hot water. The network of pipes is developed in steps. Each step consists of attaching to an existing network an extension (one new user) that is placed in the position that maximizes the temperature of the water received by the new user. The network grows `one-by-one', i.e., by one new user at a time. Networks with up to 16 users are optimized in this manner, and their geometric features and thermo-fluid performance are documented. These one-by-one trees of hot water flows are compared with corresponding `constructal' trees that are obtained in steps of pairing (doubling), i.e., connecting together two identical area constructs of the same size. It is shown that although the constructal trees perform the best (uniform water delivery at the highest temperature), the one-by-one trees approach the same level of performance as they become more complex. It is also shown that the geometry of the insulated tree structure is relatively insensitive to how the insulation is distributed over all the pipes. The thermal performance of the structure is relatively insensitive to how finely the distribution of pipe sizes and insulation radii is optimized.     

Temperature control using variable conductance closed two-phase heat pipe

The concept of using variable conductance heat pipes (VCHP) for controlling the temperature of solar collectors is introduced. This closed system does not need any external force, is self-controllable and therefore ensures high reliability in thermal control. A copper/water heat pipe equipped with a cold reservoir and buffered with air as non-condensable gas (NCG) has been tested for temperatures from 25°C up to 85°C. Using thermodynamic water properties, the operation of the VCHP is analysed. Temperature distributions along the VCHP are presented and a simplified model for calculation of the required mass of NCG is included. The model predictions agree well with experimental data. It was found that there is no hysteresis associated with heating and cooling of VCHP. The effect of mass diffusion at the vapour and non-condensable gas interface has been analysed. Test results show that the starting point of operation for VCHP is mainly dependent on the amount of NCG present in the pipe and on the superheat required for boiling.     

Simulated and experimental performance of a heat pipe assisted solar wall

Performance was evaluated for a passive solar space heating system utilizing heat pipes to transfer heat through an insulated wall from an absorber outside the building to a storage tank inside the building. The one-directional, thermal diode heat transfer effect of heat pipes make them ideal for passive solar applications. Gains by the heat pipe are not lost during cloud cover or periods of low irradiation. Simplified thermal resistance-based computer models were constructed to simulate the performance of direct gain, indirect gain, and integrated heat pipe passive solar systems in four different climates. The heat pipe system provided significantly higher solar fractions than the other passive options in all climates, but was particularly advantageous in cold and cloudy climates. Parametric sensitivity was evaluated for material and design features related to the collector cover, absorber plate, heat pipe, and water storage tank to determine a combination providing good thermal performance with diminishing returns for incremental parametric improvements. Important parameters included a high transmittance glazing, a high performance absorber surface and large thermal storage capacity.An experimental model of the heat pipe passive solar wall was also tested in a laboratory setting. Experimental variations included fluid fill levels, addition of insulation on the adiabatic section of the heat pipe, and fins on the outside of the condenser section. Filling the heat pipe to 120% of the volume of the evaporator section and insulating the adiabatic section achieved a system efficiency of 85%. Addition of fins on the condenser of the heat pipe did not significantly enhance overall performance.The computer model was validated by simulating the laboratory experiments and comparing experimental and simulated data. Temperatures across the system were matched by adjusting the model conductances, which resulted in good agreement with the experiment.