Hydronic heating systems: transient modelling,validation and load matching control

A dynamic model of a hydronic heating system is developed. The system consists of a boiler, baseboard terminal units, domestic hot water (DHW) heat exchanger coil and an environmental zone. The model is described by a set of time varying nonlinear coupled differential equations. Predicted responses from the model are compared with the measured data gathered on an on–off controlled hydronic heating system installed in an apartment building. Results show that the model predictions compare well with the field data. Using this validated model, feedback controllers are designed to achieve better regulation of zone air temperature, boiler water temperature and DHW temperature. A load tracking setpoint control strategy is proposed to regulate boiler temperature as a function of outdoor air temperature. Results showing the simulated responses of the system with the designed controllers subject to step changes in space heating and DHW loads are given. © 1998 John Wiley & Sons, Ltd.     

Simulation analysis for the active solar heating system of a passive house

The active solar heating system consists of the following sub-systems: (1) a solar thermal collector area, (2) a water storage tank, (3) a secondary water circuit, (4) a domestic hot water (DHW) preparation system and (5) an air ventilation/heating system. An improved model for the secondary water circuit is proposed and two interconnection schemes for sub-systems (4) and (5) are analyzed. The integrated model was implemented to Pirmasens passive house (Rhineland Palatinate, Germany). Both interconnection schemes show that (almost all) the solar energy collected is not used for space heating but for domestic hot water preparation. The classical water heater operates all over the year and the classical air heater operates mainly during the nights from November to April. The yearly amount of heat required by the DHW preparation system is about 77% of the yearly total heat demand of the passive house and the classical water heater provides about 20% of the yearly heat required by the DHW preparation system. The solar fraction lies between 0.247 in January and 0.930 in August, with a yearly average of 0.597.     

Design and transient-based analysis of a power to hydrogen (P2H2) system for an off-grid zero energy building with hydrogen energy storage

In this study, zero energy building (ZEB) with four occupants in the capital and most populated city of Iran as one of the biggest greenhouse gas producers is simulated and designed to reduce Iran's greenhouse emissions. Due to the benefits of hydrogen energy and its usages, it is used as the primary energy storage of this building. Also, the thermal comfort of occupants is evaluated using the Fanger model, and domestic hot water consumption is supplied. Using hydrogen energy as energy storage of an off-grid zero energy building in Iran by considering occupant thermal comfort using the fanger model has been presented for the first time in this study. The contribution of electrolyzer and fuel cell in supplying domestic hot water is shown. For this simulation, Trnsys software is used. Using Trnsys software, the transient performance of mentioned ZEB is evaluated in a year. PV panels are used for supplying electricity consumption of the building. Excess produced electricity is converted to hydrogen and stored in the hydrogen tank when a lack of sunrays exists and electricity is required. An evacuated tube solar collector is used to produce hot water. The produced hot water will be stored in the hot water tank. For supplying the cooling load, hot water fired water-cooled absorption chiller is used. Also, a fan coil with hot water circulation and humidifier are used for heating and humidifying the building. Domestic hot water consumption of the occupants is supplied using stored hot water and rejected heat of fuel cell and the electrolyzer. The thermal comfort of occupants is evaluated using the Fanger model with MATLAB software. Results show that using 64 m² PV panel power consumption of the building is supplied without a power outage, and final hydrogen pressure tank will be higher than its initial and building will be zero energy. Required hot water of the building is provided with 75 m² evacuated tube solar collector. The HVAC system of the building provided thermal comfort during a year. The monthly average of occupant predicted mean vote (PMV) is between ?0.4 and 0.4. Their predicted percentage of dissatisfaction (PPD) is lower than 13%. Also, supplied domestic hot water (DHW) always has a temperature of 50 °C, which is a setpoint temperature of DHW. Finally, it can be concluded that using the building's rooftop area can be transformed to ZEB and reduce a significant amount of greenhouse emissions of Iran. Also, it can be concluded that fuel cell rejected heat, unlike electrolyzer, can significantly contribute to supplying domestic hot water requirements. Rejected heat of electrolyzer for heating domestic water can be ignored.     

太阳能地面采暖系统蓄热水箱容积分析

通过分析太阳能采暖系统所需蓄热鼍与建筑热负荷、太阳能集热量日变化规律之间的关系,得出太阳能采暖系统所需蓄热水箱容积的理论算式.根据拉萨、银川、西宁、西安等地的太阳辐射强度及建筑热负荷的日变化规律,模拟得出系统所需蓄热量变化规律;并对各种蓄热温差下对应的蓄热水箱容积进行了模拟分析,结果表明:太阳能采暖系统所需蓄热量随太阳集热器的集热量与建筑热负荷之间的差值增大而增加;蓄热水箱容积随蓄热温差增大而减小,当蓄热水温达到80℃时,在各种地面采暖系统取水温度下,单位集热器面积所需蓄热水箱容积趋于相等.     

炼油装置余热回收集中热水供应技术研究

为回收炼油装置丰富的低温余热用于生活区的集中供热,对余热资源和生活热水供应热负荷分别进行了综合标定和统计计算,在分析比较的基础上确定采用热管换热器回收余热和双管制生活热水集中供应系统方案,并对余热流股热回收的热管换热器进行了设计计算。集中热水供应系统的技术经济分析表明,该方案在技术上是可行的,节能效果和经济效益十分显著。     

Online Fouling Detection of Domestic Hot Water Heat Exchangers

Due to hardness of cold water supply in many countries, there is a risk of fouling in domestic hot water (DHW) counterflow plate heat exchangers. The scaling will result in increased resistance to heat transfer, which has negative effects on the economics of the district heating network. A common approach is to clean or change the heat exchanger periodically, which can be expensive if only limited fouling has occurred (unnecessary) or if a higher than expected scaling layer has formed (inefficiency). A better approach is to monitor the state of the heat exchangers and clean them when actually required. This would result in more energy-efficient operation and provide an optimum schedule for heat exchanger cleaning. This can be simple if the heat exchangers are operating under steady-state conditions; however, if large variations in the inlets are experienced, as is the case with the mass flows in DHW heat exchangers, it quickly becomes impossible with standard methods. In this paper it is proposed to monitor the state of the heat exchanger online by using measurements that are easily obtainable under normal operation and applying fast mathematical models to estimate the overall heat transfer coefficient of the heat exchanger. The results show that the methods proposed can be used to detect fouling in DHW heat exchangers.     

分层-掺混切换式蓄热水箱热特性实验及评价研究

稳定分层、充分掺混是蓄热水箱实现高效供暖和恒温出水2种功能的重要手段。该研究设计一种分层-掺混一体式蓄热水箱,可实现2种功能的有效切换,满足分层高效供暖和恒温生活热水在不同时段、不同季节的灵活需求。搭建一套蓄热水箱热力学特性测试实验系统,利用分层效率、效率等蓄热水箱热性能评价指标,研究不同尺寸、流量、温度下分层-掺混式蓄热水箱的热力学性能及动态响应特征。以125 L的实验蓄热水箱为例,结果表明：在分层模式下,热分层速率、稳定性显著优于常规水箱,效率和分层效率明显提高,效率可达90%以上;在掺混模式下,掺混速度明显提高,分层效率迅速降低到0.10,实现了蓄热水箱的完全混合,结果对分层-掺混双效水箱的开发与应用具有一定指导。     

熔盐单罐双盘管释热换热器布置方式优选

为了获得单罐内盘管换热器最佳的布置方式,利用数值计算的方法研究相同换热器面积不同盘管换热器布置方式对单罐蓄热系统释热性能的影响规律。结果表明,叉排双螺旋盘管换热器的释热性能优于单排盘管换热器,且内盘管高外盘管低（hi=90 mm,ho=174 mm）的布置方式换热器其释热功率、出口温度以及累计释热量最大。     

空气源热泵热水器储水箱动态性能试验研究

以热泵热水器储水箱的动态性能为对象进行了试验研究。试验结果表明:在加热过程中,循环水与储水箱中的水混合较好,储水箱内水温度均匀升高,系统供热系数随循环水温度的升高而降低。放水过程中,储水箱内冷、热水分层较好,当放水流量小于800L/h时,放水效率在80%以上。放水加热过程中,放水流量在400~600L/h范围内变化时,对有效水量影响较小,循环水量和温控点位置对有效水量的影响较大,当循环水量由550L/h减小至300L/h以及温控点位置由高位降至低位时有效水量会有所增大。     

An energy-independent house combining solar thermal and sky radiation energies

A unique energy-independent house incorporating solar thermal, underground coolness, and sky radiation cooling energies is presented. An experiment on the long-term thermal energy storage (TES) performance was carried out for a model solar house at Tohoku University, and its characteristics, such as heat loss from the reservoir and long-term variation of the water temperature are reported. An experiment on the long-term sky radiation cooling was also performed and the radiative cooling performance of the sky radiator is reported.

It was shown that the proposed system provides almost all the heating, cooling, and domestic hot water (DHW) needs for a standard Japanese home.     

PCM storage for solar DHW: From an unfulfilled promise to a real benefit

The present numerical study is concerned with the use of phase change materials (PCMs) in solar-based domestic hot water (DHW) systems. During the last decade, the majority of the studies related to that issue concluded that the recourse to PCMs-based storage units was quite promising in order to enhance the overall performances of solar-based DHW systems. One recently interesting published numerical study (Talmatsky and Kribus, 2008), suggested though that this beneficial impact is not guaranteed since the gains observed over the day period brought by the presence of PCMs to store the solar energy were compensated by the losses undergone by the storage tank during the night. The origin of this absence of any beneficial impact of the use of PCMs in a DHW system has to be clearly understood in order to reconcile studies which indicated apparently contradictory findings. In that framework, the goal of the present contribution is to analyze the conditions under which such an absence of advantage of the use of PCMs in a DHW system were obtained in order to propose some possibilities of improvement for demonstrating the interest in using PCMs in solar-based DHW systems. Thus, the mathematical model based on the one reported in Talmatsky and Kribus (2008) is considered. This model describes the heat storage tank with PCM, collector, pump, controller and auxiliary heater. Realistic environmental conditions and typical end-user requirements are imposed.     

Influence of supply and return water temperatures on the energy consumption of a district cooling system

In a district heating and cooling system, for example, the Beijing combined heating, cooling and power (CHCP) system studied here, high temperature water generated by cogeneration plants circulates through a network between the plants and the heating substations. In heating substations, high temperature supply water from the network drives absorption chillers for air-conditioning in the summer, satisfies space heating demands in the winter and provides domestic hot water using heat exchangers throughout the year. This paper studies the significant effect of the parameters, i.e. the supply and return water temperatures in the network, on the CHCP system energy consumption for cooling and for domestic hot water.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

强制循环式太阳热水系统动态特性分析

基于集热器，水箱及换热器等部件热,地晴天无负荷条件下运行的强制循环式太阳能热水系统进行数值模拟，分析了贮热水箱内温度分层，水量，高径比和水流率等对瞬时集热效率和系统日效率的影响。特别探讨了带热交换器的复合回路系统在两种介质热容流率比值改变时，系统热性能变化规律。对设计和控制运行强制循环式太阳热水系统提出了一些建议。     

Parametric Analysis of Solar Heating and Cooling Systems for Residential Applications

Abstract

In this paper, a parametric analysis of two solar heating and cooling systems, one using an absorption heat pump and the other one using an adsorption heat pump, was performed. The systems under investigation were designed to satisfy the energy requirements of a residential building for space heating/cooling purposes and domestic hot water production. The system with the absorption heat pump was analyzed upon varying (i) the solar collectors’ area, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The system with the adsorption heat pump was evaluated upon varying (i) the inlet temperature of hot water supplied to the adsorption heat pump, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The analyses were performed using the dynamic simulation software TRNSYS in terms of primary energy consumption, global carbon dioxide equivalent emissions, and operating costs. The performance of the solar heating and cooling systems was compared with those associated with a conventional system from energy, environmental and economic points of views in order to evaluate the potential benefits.     

太阳能吸收式空调及供热系统的设计和性能

一套太阳能吸收式空调及供热综合系统已在山东省乳山市建成。该系统由热管式真空管集热器、溴化锂吸收式制冷机、储水箱、循环、冷却塔、空调箱、辅助燃油锅炉和自动控制系统等内部分组成,具有夏季制冷、冬季供热和全年提供生活用热水等功能。太阳能集热器总采光面积540m∧2,制冷、供热功率100kW,空调、采暖建筑面积1000m∧2供生活用热水量32m∧3／d。文中着重介绍了系统的设计特点和测试性能。     

An experimental study on a modified air conditioner with a domestic hot water supply (ACDHWS)

The recovery of condenser heat in air conditioners is attractive because of its great economy and environmental value. This work experimentally studies a modified air conditioner with a domestic hot water supply (ACDHWS) that operates in the space-cooling and water-heating mode. The working principles and the basic features of the ACDHWS are introduced in this paper. This is followed by an experimental study on dynamic operation characteristics, hot water supply performance, energy efficiency and the temperature distribution of hot water in the storage tank of the unit. The results show that the ACDHWS can reliably be used to heat domestic hot water without losing its cooling capacity when it is controlled well in different operation conditions. Comparatively, the coefficient of comprehensive energy performance (COP₂) of the ACDHWS is about 38.6% higher than that of the original unit. Furthermore, it is proved that the ACDHWS can continuously supply hot water for householders if a suitable hot water storage tank is installed. All these may be much helpful to develop a perfect ACDHWS product.     

A new version of a solar water heating system coupled with a solar water pump

This research target was to improve the thermal efficiency of a solar water heating system (SWHS) coupled with a built-in solar water pump. The designed system consists of 1.58-m² flat plate solar collectors, an overhead tank placed at the top level, the larger water storage tank without a heat exchanger at the lower level, and a one-way valve for water circulation control. The discharge heads of 1 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 10–18 kPa. It was found that water circulation within the SWHS ranged between 15 and 65 l/d depending upon solar intensity and discharge head. Moreover, the max water temperature in the storage tank is around 59 °C. The max daily pump efficiency is about 0.0017%. The SWHS could have max daily thermal efficiency of about 21%. It is concluded that the thermal efficiency was successfully improved, except for the pump one. The new SWHS with 1 m discharge head or lower is suitable for residential use. It adds less weight to a building roof and saves electrical energy for a circulation pump. It has lower cost compared to a domestic SWHS.     

带饮水机的多功能电冰箱设计

在传统电冰箱的基础上,另独立设置2个保温箱贮存冷热饮用水。冷水制取是在冷水箱中设置蒸发器,将冷水箱中的水制冷。热水制取是在热水箱中设置逆流套管式冷凝器,利用制冷系统的排热量将饮水机中的水加热到一定温度,如温度不够则再通过电加热器,可将饮用水加热至沸腾,冷凝管通热水箱后可采用风冷式和水冷却两种方式相结合。根据设计和理论计算,多功能电冰箱的冷凝器可将饮用水加热到65℃左右,再用电加热器,可将饮用水加热至沸腾,比市场上纯粹用电加热的饮水机可节电22.31W;利用制冷系统将冷水箱中的水制冷,每产生4℃冷饮用水2L比市场上一般的压缩式制冷饮水机可节约大约2.3×105J的能量。     

Identifying key design parameters of the integrated energy system for a residential Zero Emission Building in Norway

This study examined an integrated solution of the building energy supply system consisting of flat plate solar thermal collectors in combination with a ground-source heat pump and an exhaust air heat pump for the heating and cooling, and production of domestic hot water. The supply energy system was proposed to a 202 m² single-family demo dwelling (SFD), which is defined by the Norwegian Zero Emission Building standard. The main design parameters were analyzed in order to find the most essential parameters, which could significantly influenced the total energy use. This study found that 85% of the total heating demand of the SFD was covered by renewable energy. The results showed that the solar energy generated by the system could cover 85–92% and 12–70% of the domestic hot water demand in summer and winter respectively. In addition, the solar energy may cover 2.5–100% of the space heating demand. The results showed that the supply air volume, supply air and zone set point temperatures, auxiliary electrical volume, volume of the DHW tank, orientation and tilt angle and the collector area could influenced mostly the total energy use.