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.     

太阳能辅助地源热泵供热运行特性研究

介绍了太阳能辅助地源热泵,对其各装置性能进行了研究。分析了地下埋管换热器进出口水温及有、无蓄热水箱对太阳能辅助地源热泵性能的影响。太阳能辅助地源热泵制热性能系数随地下埋管换热器进口水温的升高呈下降趋势,随其出口水温的升高呈上升趋势。随地下埋管换热器出口水温升高,蒸发器传热量增大。当太阳能辅助地源热泵中无蓄热水箱时太阳能集热器的瞬时集热效率高于有蓄热水箱时的瞬时集热效率。就总体效果而言,有蓄热水箱要优于无蓄热水箱,这样可使地源热泵运行更加稳定。     

Enhancement of solar thermal energy storage performance using sodium thiosulfate pentahydrate of a conventional solar water-heating system

The time variations of the water temperatures at the midpoint of the heat storage tank and at the outlet of the collector in a conventional open-loop passive solar water-heating system combined with sodium thiosulfate pentahydrate-phase change material (PCM) were experimentally investigated during November and then enhancement of solar thermal energy storage performance of the system by comparing with those of conventional system including no PCM was observed. It was observed that the water temperature at the midpoint of the storage tank decreased regularly by day until the phase-change temperature of PCM after the intensity of solar radiation decreased and then it was a constant value of 45 °C in a time period of approximately 10 h during the night until the sun shines because no hot water is used. Heat storage performances of the same solar water-heating system combined with the other salt hydrates-PCMs such as zinc nitrate hexahydrate, disodium hydrogen phosphate dodecahydrate, calcium chloride hexahydrate and sodium sulfate decahydrate (Glauber's salt) were examined theoretically by using meteorological data and thermophysical properties of PCMs with some assumptions. It was obtained that the storage time of hot water, the produced hot water mass and total heat accumulated in the solar water-heating system having the heat storage tank combined with PCM were approximately 2.59–3.45 times of that in the conventional solar water-heating system. It was also found that the hydrated salts of the highest solar thermal energy storage performance in PCMs used in theoretical investigation were disodium hydrogen phosphate dodecahydrate and sodium sulfate decahydrate.     

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。     

Analysis on the optimum matching of collector and storage size of solar water heating systems in building space heating applications

The overall thermal performance of a solar water heating (SWH) system is significantly affected by the mismatch between the temporal distribution of solar radiation and the heating load. Therefore, a favorable correlation between the collector and storage size should be generated based on the dynamic characteristics of the system. This study focuses on the optimal matching of solar collector area with storage volume for an SWH system (with short-term heat storage capability) for a space heating application. A simplified model of an SWH system based on hourly energy flow is established. System control strategy is integrated into the model in a simple manner without sacrificing computing speed. Based on this model, the combined effect of collector area and storage volume on system thermal performance and economy is analyzed, and a simple procedure for determining the optimal system size is illustrated. A case study showed that for an SWH system utilized for space heating application, the optimized ratio between storage volume and collector area is dependent on the total collector area of the system, and is dominated by the requirement of overheating prevention. The minimum storage volume for a specific collector area that can prevent the storage tank from being overheated can be adopted as the optimum storage volume for that collector area. The optimum ratio between storage volume and collector area increases as the collector area increases. Therefore, a trade-off between heat collection and heat loss has to be made while attempting to increase solar fraction by improving collector area.     

小型别墅季节蓄热型太阳能供热技术研究

本文首先根据小型别墅的热水供应和采暖需要,建立了小型别墅季节性蓄热太阳能供热系统流程,进而根据某一特定城市的气象数据,进行全年逐时的计算机模拟。在此基础上,研究了全年热导平衡,各月太阳能供热量,不同蓄热容量对全年太阳能保证率、集热器集热效率、水箱温度的影响。并由此得出结论：蓄热容积是集热而积的3倍左右时,集热器年平均集热效率较高,且在进入冬季时可利用水温差较大;且采用大蓄热容积配合分隔水箱的方式,可以有效提高太阳能在冬季的利用。     

Calibrated simulation of a solar hot water system to match degraded performance over a 22-year period using two models

This paper reports on the measured performance of a residential solar water heating system over a period of 22 years and the modeling of the system to simulate its degradation over that period. The system consists of three fixed flat-plate collectors with a total of 5 m² of double-layer glass cover plates and black aluminum fin-tube absorber plates. The solar storage tank capacity is 303 L, which is used as a pre-heater to a 114-L conventional electric water heater. Measurements and simulations indicate that fogging of the glass cover plates has reduced the transmissivity by around 63% over the 22 years.     

太阳能供暖系统室内地下储热水箱的散热规律

针对太阳能地下储热水箱布置位置的不同,利用Fluent程序对太阳能储热水箱散热进行数值模拟和计算,得出在不同工况下,储热水箱周围土壤的温度场分布。同时,建立地下储热水箱的物理模型和数学模型,分析地下储热水箱的换热特性,并结合实际工程,验证其地下储水箱全年散热量和储能量,获得水箱顶板损失量与总散热量关系。计算结果表明,在相同工况下,冬季室外储水箱能量的散失量远高于室内,室内地下储水箱顶部散热量减少,因此该方式可以用来抵消这部分能量所需的集热器面积的减少,提高储热水箱的储热效率以及减少用户投资。     

Solar integrated energy system for a green building

Shanghai is characteristic of subtropical monsoonal climate with the mean annual temperature of 17.6 °C, and receives annual total radiation above 4470 MJ/m² with approximately 2000 h of sunshine. A solar energy system capable of heating, cooling, natural ventilation and hot water supply has been built in Shanghai Research Institute of Building Science. The system mainly contains 150 m² solar collector arrays, two adsorption chillers, floor radiation heating pipes, finned tube heat exchangers and a hot water storage tank of 2.5 m³ in volume. It is used for heating in winter, cooling in summer, natural ventilation in spring and autumn, hot water supply in all the year for 460 m² building area. The whole system is controlled by an industrial control computer and operates automatically. Under typical weather condition of Shanghai, it is found that the average heating capacity is up to 25.04 kW in winter, the average refrigerating output reaches 15.31 kW in summer and the solar-enhanced natural ventilation air flow rate doubles in transitional seasons. The experimental investigation validated the practical effective operation of the adsorption cooling-based air-conditioning system. After 1-year operation, it is confirmed that the solar system contributes 70% total energy of the involved space for the weather conditions of Shanghai.     

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.     

Exergetic modeling and assessment of solar assisted domestic hot water tank integrated ground-source heat pump systems for residences

The present study deals with the exergetic modeling and performance evaluation of solar assisted domestic hot water tank integrated ground-source heat pump (GSHP) systems for residences for the first time to the best of the author's knowledge. The model is applied to a system, which mainly consists of (i) a water-to-water heat pump unit (ii) a ground heat exchanger system having two U-boreholes with an individual depth of 90 m, (iii) a solar collector system composing of rooftop thermal solar collectors with a total surface area of 12 m², (iv) a domestic hot water tank with a electrical supplementary heater, and (v) a floor heating system with a surface of 154 m², and (vi) circulating pumps. Exergy relations for each component of the system and the whole system are derived for performance assessment purposes, while the experimental and assumed values are utilized in the analysis. Exergy efficiency values on a product/fuel basis are found to be 72.33% for the GSHP unit, 14.53% for the solar domestic hot water system and 44.06% for the whole system at dead (reference) state values for 19 °C and 101.325 kPa. Exergetic COP values are obtained to be 0.245 and 0.201 for the GSHP unit and the whole system, respectively. The greatest irreversibility (exergy destruction) on the GSHP unit basis occurs in the condenser, followed by the compressor, expansion valve and evaporator.     

直膨式太阳能热泵系统仿真

随着太阳能热利用和热泵技术的成熟及商品化,直膨式太阳能热泵技术将太阳能资源的清洁性、可再生性等特点和热泵系统的节能、高效的优点相结合,极具研究价值。但是目前直膨式太阳能热泵不能产品化推广的主要限制因素是系统设计不合理、运行不稳定、整体性能不佳等问题。现以直膨式太阳能热泵系统的优化和设计匹配为研究目标,同时,建立压缩机、集热器/蒸发器、热力膨胀阀、冷凝器及储热水箱的数学模型。从理论上分析集热器中集热面积、太阳能辐照度、环境温度、压缩机容积及冷凝温度等因素对直膨式太阳能热泵系统热工性能的影响,通过系统仿真及实验研究系统的整体热力性能,并在此基础上给出改善系统性能的建议。     

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.     

Design and performance of solar powered absorption cooling systems in office buildings

The paper contributes to the system design of solar thermal absorption chillers. A full simulation model was developed for absorption cooling systems, combined with a stratified storage tank, steady-state or dynamic collector model and hourly resolved building loads. The model was validated with experimental data from various solar cooling plants.As the absorption chillers can be operated at reduced generator temperatures under partial load conditions, the control strategy has a strong influence on the solar thermal system design and performance. It could be shown that buildings with the same maximum cooling load, but very different load time series, require collector areas varying by more than a factor 2 to achieve the same solar fraction. Depending on control strategy, recooling temperature levels, location and cooling load time series, between 1.7 and 3.6 m² vacuum tube collectors per kW cooling load are required to cover 80% of the cooling load.The cost analysis shows that Southern European locations with higher cooling energy demand lead to significantly lower costs. For long operation hours, cooling costs are around 200 € MWh⁻¹ and about 280 € MWh⁻¹ for buildings with lower internal gains and shorter cooling periods. For a Southern German climate, the costs are more than double.     

Computational model for a ground coupled space cooling system with an underground energy storage tank

A computational model for determining annual periodic performance of a cooling system utilizing a ground coupled chiller and a spherical underground thermal energy storage tank is developed. An analytical solution for the transient heat transfer problem outside the storage tank is obtained by the application of complex finite Fourier transform (CFFT) technique. Analytical expressions for heat gain to the space and energy consumption of the chiller are acquired, and these expressions are coupled with the transient temperature field problem to obtain computational model. Variation of water temperature in the storage tank is calculated using the transient solution of the problem over an entire year for different soil, chiller, and storage tank characteristics. Temperature profile of earth surrounding the storage tank and the COP of the cooling unit are also investigated under various assumptions and varying system design and operating conditions. The results show that water temperature in the storage tank remains under ambient air temperature during summer months, and thus the proposed ground coupled cooling system should yield higher COP values compared to a corresponding air source system.     

Energetic, economic and environmental performance of a solar-thermal-assisted HVAC system

A solar-assisted HVAC system was retrofitted in 2006-2009 onto an earlier (1980) energy-efficient building. A hybrid system of flat plate and vacuum tube solar collectors heats water in a large hot storage tank that is delivered to an absorption chiller in the cooling season or directly to heating coils in the heating season. Large chilled water storage tanks are charged off-peak and discharged during the day, cooling the building in parallel with the chiller. Measurements of the seasonal performance of the system are presented. Good overall agreement between actual measurements and earlier numerical modeling results is reported for our system, with one notable discrepancy attributable to the operation of the air terminal units, which requires tuning. In cold seasons, solar thermal energy can easily displace a large fraction of traditional heating sources. In the cooling season, the conversion of heat to cooling capacity incurs several parasitic losses, which if not accounted for properly in the design stage, have the capacity to completely offset any advantage gained from the solar system. The economics of building-scale solar thermal systems are strongly dependent on the cost of energy, and electricity in particular. The economics are favorable where electricity costs are high, and vice-versa.     

Heat loss characteristics for a typical solar domestic hot water storage

It is common practice to predict the performance of solar domestic hot water (SDHW) systems by computer simulation. This process relies on the accurate specification of the system's physical and thermal characteristics, and is often based on a number of simplifying assumptions. An important aspect of system performance is storage heat loss characteristics; however, these are often represented by an average heat loss coefficient or U-value that does not account for the complex geometry of the thermal storage or the interaction of the various inlet and outlet ports that may act as thermal conduits. In addition, most solar storage models assume that the tank temperature profile is one-dimensional and that conduction within the tank wall is negligible. To investigate these effects, tests were conducted on a typical thermal storage used in SDHW applications and included a cool-down test and a heat diffusion test sequence. The values derived from these test sequences were then compared to computer predictions based on estimated thermal properties. In addition, the basic assumptions typically used in the computer modelling of solar storage heat losses (e.g., one-dimensional temperature profiles, minimal tank wall conduction, uniform wall heat loss) were investigated, particularly in the context of a thermally stratified thermal storage.     

太阳能地板辐射供暖制冷系统模型仿真

近年来,我国北方地区新建住宅的采暖系统多数采用地板辐射供暖技术,而制冷系统仍采用传统的空调制冷技术,有必要对地板辐射制冷技术的应用进行研究,为此,针对一套实验用太阳房,搭建了太阳能地板辐射供暖制冷系统.通过计算确定了集热器面积、蓄热水箱体积、吸收式制冷机组的制冷量.利用TRNSYS仿真平台建立了系统仿真模型,并对控制策略进行了验证.仿真结果表明,该系统能有效地利用太阳能保持冬季室温18℃左右、夏季26℃左右的舒适温度.     

A solar heating system with annual storage

A heating system is described for a one-family house in Trento, Italy, using solar collectors with buried long-term storage water tanks, made of reinforced concrete and internally water-proofed, assisted by an electrical water—water type heat pump.

The following design parameters are described and evaluated:

a. the annual energy requirement for the house;

b. the solar energy available;

c. the average monthly efficiency of the collectors;

d. the storage tank—ground heat flows, taking storage insulation, water storage temperature and COP to be variable with time;

e. the overall energy balance of the system for the house in question.

The conclusion is that the proposed system can cover the annual energy required for this house with an electrical consumption equal to 20% of the total.     

Swedish solar heating with seasonal storage—design,performance and economy

At the present time two Swedish group solar heating plants with seasonal storage, connected to residential areas with about 50 single-family houses, have been in operation for 4 years. The Lambohov Plant has a total of 2700 m² of flat plate collectors and a 10 000 m³ rock pit store. The Ingelstad plant has a total of 1300 m² of concentrating collectors and a 5000 m³ free-standing tank. The emphasis of the assessment has been on system performance as a whole. Most of the emphasis has been placed on general conclusions concerning technology and economy for seasonal storage plants.