Exergetic modeling and performance evaluation of solar water heating systems for building applications

Solar water heating (SWH) is a well-proven renewable energy technology and has been used in many countries of the world. The basic technology is straightforward, although there are a variety of various types of SWH systems. In the performance assessment of SWH systems, energy analysis (first law) method has been widely used, while the number of the studies on exergetic assessment is relatively low. The SWH system investigated consists of mainly three parts, namely a flat plate solar collector, a heat exchanger (storage tank) and a circulating pump. The main objectives of the present study are as follows, differing from the previously conducted ones: (i) to model and assess SWH systems using exergy analysis (second law) method as a whole, (ii) to investigate the effect of varying water inlet temperature to the collector on the exergy efficiencies of the SWH system components, (iii) to study some thermodynamic parameters (fuel depletion ratio, relative irreversibility, productivity lack and exergetic factor) and exergetic improvement potential, and (iv) to propose and present an exergy efficiency curve similar to the thermal efficiency curve for solar collectors. The system performance is evaluated based on the experimental data of the Izmir province, Turkey, which is given as an illustrative example. Exergy destructions (or irreversibilities) as well as exergy efficiency relations are determined for each of the SWH system components and the whole system. Exergy efficiency values on a product/fuel basis are found to range between from 2.02 to 3.37%, and 3.27 to 4.39% at a dead (reference) state temperature of 32.77 °C, which is an average of ambient temperatures at eight test runs from 1.10 to 3.35 p.m., for the solar collector and entire SWH system, respectively. An exergy efficiency correlation for the solar collector studied was also presented to determine its exergetic performance. It is expected that the model presented here would be beneficial to the researchers, government administration, and engineers working in the area of SWH systems for residential applications.     

A computer study of solar water heating

A computer model of a domestic solar water heating system was used to investigate the effect of changes in design parameters on the performance of the system. The theor y underlying each component of the model is described and the approximations made are discussed. The model showed that annual heat output was highly dependent on the total collector area of the system but that other changes, in collector slope, orientation, storage tank volume etc, had relatively small effect. The daily pattern of demand for hot water was also shown to influence performance.     

较大规模集中式住区级跨季节蓄热型太阳能供热系统参数研究

建筑供暖面积、集热器面积、蓄热水箱容积等系统参数的确定是跨季节蓄热太阳能供热系统的关键。通过对虚拟案例的计算分析,初步得出了这些参数的取值范围,为较大规模跨季节蓄热型太阳能供热系统的设计和建设提供基本选型条件。     

哨楼太阳能供暖设备设计与计算

对西北边疆某哨楼进行了建筑热工分析和计算,计算出围护结构的得热量和耗热量,对平板型空气集热器、卵石床蓄热器及其它系统设备进行了设计与计算,简述了太阳能供暖系统集热、蓄热、供暖的运行控制方法。     

基于典型用水模式下太阳能热水系统性能分析与优化

对典型的太阳能热水系统建立了数学模型.结合工程实例和长沙地区太阳能辐射特征,运用该模型分析了在给定单个家庭典型用水模式下集热器面积和贮热水箱体积对太阳能热水系统热利用性能的影响.优化集热嚣面积和贮热水箱体积,在减少投资的同时,提高太阳能热水系统的综合性能.集热器面积和水箱体积的优化匹配设计有利于提高系统的能量转换效率.     

Renewable energy for passive house heating: Model of the active solar heating system

The large windows on the south-oriented façade of a passive house strongly contribute to building space heating. These windows constitute the passive solar heating system. This paper studies the active heating system of a passive house, which includes the following sub-systems: (1) solar thermal collectors, (2) a water storage tank, (3) a secondary water circuit, (4) a domestic hot water preparation system and (5) an air ventilation and heating system. Models for all sub-systems are presented. The integrated model was implemented to Pirmasens Passive House (Rhineland Palatinate, Germany). The active solar heating system provides a smaller amount of heat than the heat provided by the passive solar heating system. Almost all the solar energy collected is not used for space heating but to domestic hot water (DHW) preparation. However, there is still a need for the classical water heater to operate all over the year. Almost all space heating thermal load is covered by using the classical air heater that operates mainly during the nights from November to April. The solar fraction lies between 0.180 in February and 0.679 in October, with a yearly average of 0.446. The study reveals that on a yearly basis it is more advantageous to use vertical south-oriented solar collectors instead of roof placed collectors.     

Study on the performance of a solar assisted air source heat pump system for building heating

This paper proposed a new solar assisted air source heat pump system with flexible operational modes to improve the performance of the heating system. A mathematical model was established on the solar assisted air source heat pump system for building heating with a heating capacity of 10 kW, and an air source heat pump unit was developed to validate the model. The effect of the solar collector area on the performance of the system running in Nanjing was studied. The results showed that the COP of the heat pump unit was enhanced with the increase of the solar radiation density during the typical sunny day in the heating season. In addition, the COP also increased in proportion to the solar collector area. Compared with the case when the solar collector area was 0 m², the COP increase of the heat pump and the energy-saving rate were 11.22% and 24% respectively when the solar collector area was 40 m². Meanwhile, the solar equivalent generation power efficiency could reach 11.8%.     

Analysis of feasibility on heating single family houses in rural areas by using sun and wind energy

Households in Lithuania consume about 1/3 of total final consumption of fuel energy. In order to reduce imports of fossil fuel and emissions of dangerous pollutants, solar energy could be used for the above-mentioned needs. That would require large collector areas and volumes for seasonal heat storage. In wintertime the wind speed velocity is much higher than in summertime in Lithuania. Therefore, it is advisable to study meeting the thermal needs of single family houses by combining use of wind and solar energy. To this end analytical research has been made by using deterministic method. The analysis has been carried out for the case when 1 m² of heated room area requires 0.25 m² of solar collector area and 0.5 m² working area of wind turbine rotor. Heat storage is planned for 24 h. By using such a hybrid system during the heating season 42.6–56.2% of heating needs for space and domestic hot water are satisfied. However, for individual days (especially from May to October) a surplus of generated heat is formed and it reaches about 53.6% of space heating needs per year. This relative surplus of energy could be used for transmitting wind power-plant energy to the electric network or in a household and thermal energy can be used for drying agricultural produce, heating greenhouses, open swimming pools and satisfying other needs.     

Thermal and economical analysis of an underground seasonal storage heating system in Thrace

Economical analysis of the solar heating system with seasonal storage, which was established in Edirne (41°39′54″N) in order to provide the heat requirement of buildings, has been fulfilled. Optimum collector area for the heating system has been determined. Total heat requirement of 69% has been met by means of heating system concerning the space heating and domestic water heating. In the accordance with the results of the economical analysis, the payback time of the heating system has been determined as 19-20 years.     

西藏地区太阳能直接供暖系统的设计

结合拉萨市西藏科技厅太阳能供暖示范工程,对太阳能直接供暖系统中太阳能集热器、集热器侧传热介质、蓄热方式、蓄热水箱、散热设备、控制系统的设计进行了研究。     

加拿大卡尔加里市Okotoks小镇太阳能小区建设

简要介绍了北美目前最大的跨季太阳能储存项目——加拿大0kotoks小镇的太阳能小区建设，对其太阳能供热系统的工作原理及利用土壤床作为储能体进行大规模跨季节太阳能储存的方法进行了分析。该项目根据不同季节可利用太阳能数量的不同，分别设置了短期（临时）太阳能储箱（STTS）及跨季节太阳能储存箱（BTES），以提高太阳能的利用率。其中，BTES的效率可达50％以上。小区太阳能家用热水系统（DWH）可满足住户60％的热水需求，而太阳能采暖系统则可满足90％采暖要求；建成后每幢住宅每年可减排5t温室气体，整个小区可减排260t／年。     

Dynamic simulation and parametric optimisation of a solar-assisted heating and cooling system

In this paper, a complete transient simulation model of a solar heating and cooling plant is presented. The system under analysis is based on the coupling of evacuated solar collectors with a single-stage LiBr-H₂O absorption chiller. An auxiliary heater, circulation pumps, storage tanks, feedback controller, mixers, diverters, ON/OFF hysteresis controller, single lumped capacitance building and controllers are also included.

The simulation was performed using the TRNSYS environment. This software also includes a detailed database with weather parameters for several cities all over the world. The system was simulated using specially designed control strategies and varying the main design variables. In particular, a variable speed pump on the solar collector was implemented, in order to maximise the tank temperature and minimise heat losses. A cost model was also developed in order to calculate operating and capital costs. A case study is presented and discussed, aiming at determining the performance of the system, from both energetic and economic viewpoints, in a specific application. A thermoeconomic objective function was also introduced, and finally a sensitivity analysis was performed, in order to calculate the set of synthesis/design parameters that maximise the global efficiency of the system or the above-mentioned objective function, for the case under analysis. The results of the case study showed that a good selection of the solar collector (SC) area and of the volume of the storage tank (TK1) are mandatory. The Primary Energy Saving (PES) is positive in the case of high solar field area, while the optimal thermo-economic volume of the storage tank was found to be 75 l/m². The parametric optimisation also showed that it is important to lower the SC and auxiliary heater (AH) set-point temperatures, as much as possible.     

Model of a solar-assisted heat-pump system for space heating integrating a thermal energy storage unit

Details about a model developed to analyze a complex space heating system are given. Solar air heaters provide thermal energy for driving a vapor compression heat-pump and for other purposes. A (sensible heat) thermal energy storage (TES) unit ensures a more efficient usage of the collected solar energy. The TES operation is described by two non-linear coupled partial differential equations for the temperature of storage medium and heat transfer fluid, respectively. The two main operating modes of the space heating system are presented. The electric power required by the motor of heat-pump’s compressor is lower in case of the space heating system that includes a TES unit as compared to the simple solar-assisted heat-pump system. The saving in electric energy varies between 20–35% (during the months when the TES unit is uncharged) and more than 50% (for the months when the TES unit is charged).     

建筑消防水池蓄热的主动式太阳能采暖系统

结合西宁地区的气象资料,计算太阳能系统集热量与建筑热负荷之间的相对关系;并分析太阳能热泵为热源的地板辐射供暖系统在实际应用中的可行性.提出利用建筑消防水池作为太阳能热泵为热源的地板辐射供暖系统蓄热水池的一些现点.     

太阳能热泵蓄热系统的实验研究

文章介绍了带有相变蓄热水箱的太阳能热泵系统的运行实验。该系统是在原有的太阳能一土壤源热泵的基础上通过增加一个蓄热装置建立起来,包含太阳能集热器、相变蓄热水箱、双热源（太阳能和土壤）热泵以及末端装置（风机盘管）四个主要部分实验在供暖期末期进行,在整个实验阶段,系统供暖满足率为0．6,平均供热COP达到6.5。     

Long term operation of a solar assisted ground coupled heat pump system for space heating and domestic hot water

This paper introduces a solar-assisted ground-coupled heat pump (SAGCHP) system with heat storage for space heating and domestic hot water (DHW) supply. The simulation results of the system's detailed operating performance are presented. The optimization of the system design is carried out by the TRNSYS and a numerical simulation is performed for continuous operation of 20 years under the meteorological conditions of Beijing. Different control strategies are considered and the operational characteristics of each working mode are studied. The simulating results show that the long term yearly average space heating efficiency is improved by 26.3% compared to a traditional ground coupled heat pump (GCHP) system because the solar thermal collecting system is used to elevate the thermal energy in the soil and to provide direct space heating with heat storage. At the same time, the underground heat load imbalance problem for a heating load dominated GCHP is solved by soil recharging during non-heating periods, while extra solar energy is utilized to supply DHW. The flexibility and high efficiency of the SAGCHP system could offer an alternative for space heating and DHW supply by heat pump technology and solar energy in cold winters of northern China.     

Solar powered absorption air conditioning

The design of buildings to provide a suitable thermal environment is discussed and the reasons for artificial heating or cooling introduced. The problem of sizing a solar-powered cooling plant is investigated. An iterative method of estimating heat flow and resultant temperatures in buildings subject to fluctuating heat loads is described. A model is developed to allow investigation of the performance of a solar collector and thermal storage system and some of the basic relationships between performance and physical parameters are determined.

An iterative method of predicting the cooling output from a lithium bromide-water absorption refrigeration plant having variable heat input is described.

The design of a solar collector/thermal storage) absorption cooler system, its performance on a particular building and its fine tuning are examined.     

Experimental investigation and theoretical analysis of solar heating and humidification system with desiccant rotor

In this paper, a solar heating system, which combines the technologies of evacuated tube solar air collector and rotary desiccant humidification together, has been configured, tested and modeled. The system mainly includes 15 m² solar air collectors and a desiccant air-conditioning unit. Two operation modes are designed, namely, direct solar heating mode and solar heating with desiccant humidification mode. Performance model of the system has been created in TRNSYS. The objective of this paper is to check the applicability of solar heating and evaluate the feasibility and potential of desiccant humidification for improving indoor thermal comfort. Experimental results show that the solar heating system can convert about 50% of the received solar radiation for space heating on a sunny day in winter and increases indoor temperature by about 10 °C. Compared with direct solar heating mode, solar heating with desiccant humidification can increase the fraction of the time within comfort region from about 10% to 20% for standalone solar heating and from about 30% to 60% for solar heating with auxiliary heater according to seasonal analysis. It is confirmed that solar heating with desiccant humidification is promising and worthwhile being applied to improving indoor thermal comfort in heating season.     

蓄热型空气式太阳能集热-空气源热泵复合供暖系统在寒冷地区的应用研究

设计了一种蓄热型空气式太阳能集热-空气源热泵复合供暖系统.该系统具有太阳能供热、太阳能辅助热泵供热和热泵供热3种运行模式,可根据环境工况及供暖负荷的变化自动切换运行模式,保证室内供暖的稳定性.在通辽市的实验研究结果表明:在整个供暖季内,该系统可持续提供42.6 ℃的热水,维持室内温度在21.3?24.1 ℃之间;平均C...     

双水箱太阳能热水系统贮热水箱容积的比较研究

针对设置双蓄热水箱的集中式太阳能热水系统,以辅助热源能耗为目标,分析了不同贮热水箱容积的设置对系统辅助热源能耗的影响。并通过运用TRNSYS软件,建立动态分析模型,比较分析了不同太阳能保证率条件下,单位面积集热器对应的贮热水箱容积对系统性能的影响。分析结果表明:单位面积集热器对应的蓄热水箱容积最优值分布在50~70 L/m~2范围内;贮热水箱的容积对系统能耗的影响较小,在单位面积集热器对应的蓄热水箱容积为40~100 L的推荐值范围内,其辅助热源的最大值与最小值差异在1%以内。