Central solar heating plants with seasonal duct storage and short-term water storage: design guidelines obtained by dynamic system simulations

A central solar heating plant with seasonal ground storage is analysed by dynamic system simulations. A reference system, involving a collector area, water buffer storage and ground duct storage, is defined for typical Swiss conditions and simulated for several types of heat load. A methodology is established for the optimisation of the main system parameters. The thermal behaviour of such a system is highlighted. The short-term heat requirements are covered by the buffer unit, whereas the seasonal heat requirements are covered by the ground duct storage. As a consequence, a system such as this is intended to supply a large solar fraction (>50%). Optimal ratios between the main system parameters are sought for an annual solar fraction of 70%. An optimal buffer volume of 110 to 130 l per m² of collector area is obtained. The optimal duct storage volume and collector area vary respectively from 4 to 13 m³ per m² of collector area and from 2 to 4 m² per MWh (3.6 GJ) of annual heat demand. They depend mainly on the specific heat losses from the duct storage unit. A large annual heat demand (>3600 GJ or 1000 MWh) and/or low temperatures in the heat distribution are essential for satisfactory system thermal performance. The spacing of the boreholes which form the ground heat exchanger of the duct store is fairly constant and is found to be about 2.5 m for a ground thermal conductivity of 2.5 Wm⁻¹ K⁻¹. Some improvements of the system control are also investigated to assess the influence on the overall thermal performances of the system. They indicate that the system thermal performances are only slightly improved in contrast to the improvement brought by a simple but optimised system control.     

Design of a seasonal thermal energy storage in the ground

Longterm storage of high quantities of thermal energy is one of the key problems for a widespread and successful implementation of solar district heating and for more efficient use of conventional energy sources. Seasonal storage in the ground in the temperature range of up to 90°C seems to be favourable from a technical and economical point of view. Preferably duct systems with vertical heat exchangers can be built in areas without ground water or low flow velocity compared with the geometry of the store and the storage period.

The thermal performance of such systems is influenced by the heat and moisture movement in the area surrounding the heat exchangers. Thermal conductivity and heat capacity are strongly dependent on the water content. This combined heat and moisture transport was simulated on the computer for temperatures up to 90°C. This model calculates the effective heat transfer coefficient and the heat capacity of the soil depending on water content, mineral composition, dry bulk density and shape of soil components. The computer simulation was validated by a number of laboratory and field experiments.

Based on this theoretical work a pilot plant was designed for seasonal storage of industrial waste heat. A heat and power cogeneration unit (174 kW_th) delivers waste heat during summer to the ground storage of about 15 000 m³ with 140 vertical heat exchangers of 30 m depth. About 418 MWh/a will be charged into the ground at a temperature level of 80°C, about 266 MWh/a should be extracted at temperatures between 40°C and 70°C and delivered directly to the space heating system. With this design an economic calculation gave energy prices of 39 US$/MWh which is of the same order as conventional energy prices.     

基于TRNSYS的太阳能谷电蓄能供热采暖系统性能研究

为了研究太阳能谷电蓄能供热采暖系统运行特性,采用TRNSYS软件建立系统各部件模型,分析了太阳能辐照强度、集热面积和空气流量对系统太阳能保证率的影响,对系统进行优化研究。结果表明：太阳能辐射强度对系统太阳能保证率的影响较大,拉萨全年太阳能保证率波动比上海和北京小;太阳能保证率与集热面积呈正相关;空气流量对太阳能保证率影响较小,当空气流量为40 m³/(h∙m²) 时太阳能保证率最大,相比36 m³/(h∙m²)工况提高了0.26%;选择集热面积为650 m²、最佳空气流量为40 m³/(h∙m²) 的优化系统,相比集热面积为716 m²、空气流量为36 m³/(h∙m²) 工况下的年均太阳能保证率降低了1.22%。本研究可为太阳能谷电蓄能系统的后续研究提供参考。     

Thermal performance evaluation of an energy and environmental research center building during winter

The building is constructed on a ground area of 3300 m² and consists of five floors of total area of 6361 m² with an air conditioned area of 3351 m². The heating system comprises 1577 evacuated-tube collectors, two accumulation tanks of 15 m³ each, two thermal storage tanks of 150 m³ each, two auxiliary boilers of 443,000 kcal/h each, pumps, heat exchangers and control equipment. The thermal performance evaluation shows that the inside temperature was kept at the desired value (22°C) which demonstrates the system's ability to meet the building load. The performance also shows that the collection efficiency for the whole season was 50% and the collected and stored energy was higher than the required load which indicates that the solar fraction to the load was 100%.     

跨季蓄热多方式可调节供暖系统的模拟分析及可行性研究

针对太阳能跨季蓄热量利用不充分的问题,提出了一种多方式可调节供暖系统模型,利用温差控制原理实现系统供暖方式的自动转换.以乌鲁木齐地区某公共建筑为例,采用TRNSYS动态模拟软件对多方式可调节供暖系统进行了全年运行分析.结果表明:15 100 m3的蓄热基坑体积与6 040 m2的集热器面积恰好匹配;在非供暖期蓄热基坑内...     

Operation of a commercial solar gel pond

The solar gel pond is an innovative concept which overcomes many of the shortcomings of the conventional salt-gradient solar pond. In this paper, the design, construction and start-up details of a commercial scale pond (400 m²), built for a publishing company in Albuquerque, New Mexico will be presented. A pond with trapezoidal cross section was designed so that shadowing could be minimized and also the ratio of surface area to the volume of the storage zone could be maximized. The publishing company required a minimum of 1 GJ/day (1MBTU/day). Generally it has been noted that in ponds with large volume a lesser percentage of retained energy is lost as edge losses. Based on the above consideration a pond size of 400 m² and 5 m deep with a gel thickness of 60 cm and a mass flow rate (for the heat extraction loop) of 1 × 10⁻⁴ to 1 × 10⁻³ kg/m²·sec was determined to be the optimum size for the publishing company's needs. Two to seven percent salt water was used mainly to keep the gel bags floating on the surface. Tedlar bags were used to contain the gel. During the first year of operation, while the pond was still heating up, the pond obtained a temperature of 60°C and the gel showed no signs of degradation.     

A REVIEW OF LARGE-SCALE SOLAR HEATING SYSTEMS IN EUROPE

Large-scale solar applications benefit from the effect of scale. Compared to small solar domestic hot water (DHW) systems for single-family houses, the solar heat cost can be cut at least in third. The most interesting projects for replacing fossil fuels and the reduction of CO₂-emissions are solar systems with seasonal storage in combination with gas or biomass boilers. In the framework of the EU–APAS project “Large-scale Solar Heating Systems”, thirteen existing plants in six European countries have been evaluated. The yearly solar gains of the systems are between 300 and 550 kWh per m² collector area. The investment cost of solar plants with short-term storage varies from 300 up to 600 ECU per m². Systems with seasonal storage show investment costs twice as high. Results of studies concerning the market potential for solar heating plants, taking new collector concepts and industrial production into account, are presented. Site specific studies and predesign of large-scale solar heating plants in six European countries for housing developments show a 50% cost reduction compared to existing projects. The cost–benefit-ratio for the planned systems with long-term storage is between 0.7 and 1.5 ECU per kWh per year.     

The solar heating system with seasonal storage at the Solar-Campus Jülich

The Solar-Campus Jülich is an area of 14 ha in the north of the existing University of Applied Sciences. Three independent partners are constructing low energy buildings on this site (heating demand 144 MJ m⁻² a⁻¹). To date (June 2000) an auditorium with a library has been completed as well as an additional laboratory building. The Students’ Association Aachen has erected 23 houses with accommodation for 136 students. The houses are arranged in five rows, each of them demonstrating different kinds of modern energy-saving technologies for heating and ventilation. The energy utility in Jülich intends to build industrial buildings and move their complete organisation from southern Jülich to the Solar-Campus. A solar district heating concept, with seasonal storage, is planned to cover about 50–60% of the heating demand of all the buildings. The paper gives details of the design of the pyramidal seasonal storage tank including cost analysis. The top cover of the 2500 m³ tank will be constructed of several insulated floating pontoons, which are connected to each other, so that it is possible to walk on it. The 1200 m² of collectors are distributed over the different buildings and coupled to the underground storage tank by either a 2-, 3- or 4-pipe distribution network. Due to the low energy demand, the overall energy consumption will be low (2124 GJ a⁻¹). The heating cost consequently will be high: 0.17 DM/kWh with a conventional gas-based system, and 0.54 DM/kWh with the solar system including seasonal storage.     

Fertilizer solar ponds as a clean source of energy: Some observations from small scale experiments

Commercially available NH₂CONH₂ is used to establish a salinity gradient solar pond in a small 1 m² outdoor tank. With a salinity difference of 35% between the upper and lower zone, a temperature difference of 23°C was obtained without any instabilities in the gradient zone. The difference in concentration of solution required to sustain a temperature difference of 40°C across the gradient zone is 520 kg/m³. By economically using runoff into the fertilizer cycle of an agricultural system the estimated cost of fertilizer solar pond generated heat is Rs. 1.10/kWh.     

光伏/光热-地源热泵联合供热系统运行性能研究

为解决太阳电池的发电效率随温度升高而下降以及地源热泵系统供热引起的土壤热失衡问题,以典型居住建筑的光伏/光热-地源热泵（PV/T-GSHP）联合供热系统为研究对象,基于TRNSYS软件,采用土壤温度、地源热泵机组季节能效比、光伏发电效率和太阳能保证率为评价指标,对该联合供热系统进行运行性能分析。研究结果表明：夏热冬冷地区（以长沙为例）太阳能保证率相对较高,PV/T组件面积为满屋顶最大化安装（900 m²）时,第20年末土壤温度相比初始地温仅升高0.8 ℃,热泵机组季节能效比约为5.1,太阳能保证率为97.0%~98.7%;不同气候地区的太阳能保证率与PV/T组件面积和建筑全年累计供热量有关,通过定义单位建筑全年累计供热量PV/T组件面积指标,得到中国不同气候地区的太阳能保证率与该指标的耦合关系,回归方程的决定系数R²为0.983,得出在已知建筑全年累计供热量和太阳保证率设计目标值的条件下所需PV/T组件面积的计算方法。PV/T-GSHP联合供热系统的全年运行能耗显著小于平板太阳能集热器-地源热泵联合系统（最小降幅为沈阳,49.7%）,远小于空气源热泵（最小降幅为石家庄,79.8%）和燃气壁挂炉（最小降幅为沈阳,65.1%）。     

Sunstations

A method of obtaining insolation data is described which provides information directly useable in the sizing of solar equipment. The method is particularly valuable for the collection of data at a large number of sites where equipment cost and availability of technically trained personnel are critical. The device consists of a calibrated solar cell, and electrochemical accumulator and associated electronic equipment having the following characteristics: capacity 13 GJ/m² (3600 kW hr/m²); temperature range −55 to +71°C; low cost; no field maintenance; reproducibility ±5 per cent.     

High performance–low cost seasonal gravel/water storage pit

Seasonal storage of solar energy is necessary to achieve high solar fractions of 50 and more per cent of the total heat demand in housing developments. One main requirement for heat-storage units is a low specific cost so that the solar heat price can be reduced and becomes more competitive in the near future with the heat prices of conventional fossil fuels. Another important target is the integration of large seasonal storage units in housing areas to save ground area. The gravel/water storage pit is one of the most promising long-term heat-storage techniques with respect to the mentioned topics.

A gravel/water storage pit of the third generation, which is currently being built in Steinfurt, near Münster/Germany, is presented in this paper. The technical innovations of the new generation type are described compared to the state of technology. The ecological compatibility of the used materials is proved. The high cost-reduction potential of the introduced storage technique is shown by a detailed calculation of the cost for the storage pit in Steinfurt and an upscaled storage unit with 16,000 m³ volume of water-equivalent.

The main innovations of the new technique are the thermal insulation, which consists of granulated recycling glass and a new lining material that is installed in double layers. By using new materials, the operating temperatures of the storage can be risen up to 90°C.     

直接空冷型槽式太阳能发热电系统技术经济分析

以50 MW槽式太阳能直接空冷发电系统为研究对象,基于光电效率、发电量、度电成本评价指标,对空冷光热机组和水冷机组的年热力性能与经济性进行比较分析研究。结果表明:在储热时长9 h下,当以年光电效率最高为优化目标时,空冷机组和水冷机组聚光器采光面积分别为425100 m²(集热场回路数130)和398940 m²(集热场回路数122),对应的年光电效率分别为13.19%和13.84%;当以发电成本最低为目标时空冷机组和水冷机组聚光器采光面积分别为647460 m²(集热场回路数198)和623100 m²(集热场回路数190),对应的发电成本分别为1.118元/kWh和1.069元/kWh。     

跨季节蓄冷特性与系统设计优化

考虑现有采暖供冷技术能耗高或适用性低等特点,提出以冰源热泵为核心的跨季节蓄冷技术。以5种系统方案为研究对象,建立系统模型定量评价分析不同方案在不同区域的能耗特性以及经济效益。结果表明：1）方案3（跨季节蓄冷）、方案4（跨季节蓄冷与夜间蓄冷）、方案5（土壤源热泵）的一次能源消耗量和污染物排放量比方案1（空气源热泵）和方案2（集中供热与冷水机组）少50%以上;2）跨季节蓄能技术可有效提高系统的全年COP值,其存储效率和循环次数可达到96%和2.1次;3）上海适用空气源热泵方案,北京适用跨季节蓄冷方案,沈阳适用集中供暖方案;4）在北京,对于3万m²以上供暖供冷面积的大型建筑,采用方案4的增量投资回收期只有约3 a。     

Central solar heating plants with seasonal storage in Germany

In the house building sector, central solar heating plants presently offer the most cost-favourable application of all possibilities of solar-thermal systems. By the integration of seasonal heat storage, more than 50% of the annual heating demand for space heating and domestic hot water can be supplied by solar energy. Since 1995, eight central solar heating plants with seasonal heat storage have been built in Germany within the governmental R&D-programme ‘Solarthermie-2000’. This report describes the technology of central solar heating plants and gives advice about planning and costs. The pilot and demonstration plants for seasonal heat storage already built in Germany are described in detail to give an idea about possible system design and applications of central solar heating plants.     

Evaluation of CPC-collector designs for stand-alone, roof- or wall installation

An asymmetrically truncated non-tracking compound parabolic concentrator type collector design concept has been developed. The collector type has a bi-facial absorber and is optimised for northern latitudes. The concept is based on a general reflector form that is truncated to fit different installation conditions. In this paper collectors for stand-alone, roof and wall mounting are studied. Prototypes of six different collectors have been built and outdoor tested. The evaluation gave high annual energy outputs for a roof mounted collector, 925 MJ/m², and a stand-alone collector with Teflon, 781 MJ/m², at an operating temperature of T_op = 75 °C. A special design for roofs facing east or west was also investigated and gave an annual energy output of 349 (east) and 436 (west) MJ/m² at T_op = 75 °C. If a high solar fraction over the year is the objective, a load adapted collector with a high output during spring/fall and a low output during summer can be used. Such a collector had an output of 490 MJ/m² at T_op = 75 °C. Finally a concentrating collector for wall mounting was evaluated with an estimated annual output of 194 MJ/m² at T_op = 75 °C. The concentrator design concept can also be used for concentrators for PV-modules.     

A highly-advanced solar house with solar thermal and sky radiation cooling

A unique energy-independent house (‘HARBEMAN house’; HARmony BEtween Man And Nature) incorporating solar thermal, underground coolers, sky radiation cooling, photovoltaic electricity generation and rain-water collection was built in Sendai (latitude; 38° 17′00″ north and longitude; 140° 50′14″ east), Japan during July, 1996. The average solar energy received on a horizontal surface there in January is 7900 kJ/m²/day. This paper reports the experimental results since September 1996 to date. The annual variations of water temperature in the underground main tank, heating /cooling/domestic hot water demands, collected and emitted heats by solar collector and sky radiator, were measured. The paper also clarifies the method of computer simulation results for the HARBEMAN house and its results compared with the annual experimental data. The proposed HARBEMAN house, which meets almost all its energy demands, including space heating and cooling, domestic hot water, electricity generated by photovoltaic cell and rainwater for standard Japanese homes. The proposed system has two operational modes: (i) a long-term thermal energy storage mode extending from September to March and (ii) a long-term cool storage mode extending from April to August. The system is intended to utilize as little energy as possible to collect and emit the heat. This paper also clarifies the primary energy consumption, the external costs (externalities) and the means for the reduction of carbon-dioxide (CO₂) emissions. The primary-energy consumption and carbon-dioxide emissions of the proposed house are only one-tenth of those of the conventional standard house. Moreover, the thermal performance of this house will be compared with the results of the IEA solar low-energy house TASK 13. Finally, this paper validates the external costs of this house, which have been intensively discussed in recent years in European countries. The present energy-sufficient house will be attempting in 21st century to reduce carbon dioxide emissions, which will be one of the key factors for mitigating global warming.     

Biomass equations for determining fractions of European aspen growing on abandoned farmland and some practical implications

A study was conducted in order to construct functions for aboveground biomass of fractions of young European aspens (Populus tremula L.). The constructed functions were designed to be used for predicting the amount of biofuel produced from small areas. Biomass production was estimated in 11 stands of European aspen growing on abandoned farmland. The stands were located in Sweden at latitudes ranging from 65° to 60° N, and their total age varied from 5 to 24 years. A modified “mean tree technique” was used to estimate biomass production; i.e. depending on the plot area size, the number of sampled trees was chosen. The mean total dry weight above stump level for aspen stands was 78 tonnes ha⁻¹ with a range of 14–162 d.w. ha⁻¹. Mean annual increment for the stands was 5.56 tonnes ha⁻¹ year⁻¹ (2.86–9.15). Aspens growing on silt soils produced more than on fine sand soils. In addition to estimating conventional dry weights of trees and tree components, specific leaf area, total surface area and LAI, among other measures, were estimated. The mean LAI was 2.58 and the specific leaf area was 9.4 m² kg⁻¹. Some practical implications are given.     

城市大气污染对到达地表的太阳辐射的影响研究

在晴空（无云）的条件下,大气污染是影响到达地球表面太阳辐射的重要因素之一。选择中国6个典型城市（北京、沈阳、上海、武汉、广州和成都）,利用2014年1月—2020年12月的空气质量日监测数据以及地面太阳辐射、日照时数等逐日观测数据,定量分析晴空条件下大气污染指数（AQI）与地表太阳总辐射、散射辐射的关系。结果表明：1）大气污染会降低清晰度指数,增加散射系数,对于地表太阳总辐射有衰减作用,对于散射辐射有增强作用。2）2014—2020年,大气污染（AQI>100）使得晴天地表太阳总辐射的年衰减总量和相对衰减量（共7 a）较大的是北京（212.40 MJ/m²,4.01%）、沈阳（184.16 MJ/m²,3.00%）、上海（123.80 MJ/m²,4.37%）和武汉（106.36 MJ/m²,3.04%）,而成都（58.03 MJ/m²,3.82%）和广州（18.76MJ/m²,0.96%）的衰减总量较小。3）大气污染（AQI>100）使得晴天散射辐射的年增加总量和相对衰减量分别是北京256.64 MJ/m²（12.96%）、沈阳134.45 MJ/m²（7.10%）、武汉22.62 MJ/m²（1.36%）、成都43.40 MJ/m²（9.71%）、上海94.74 MJ/m²（8.25%）和广州37.79 MJ/m²（5.90%）。     

Thermal design of a roof as an inexpensive solar collector/storage system

This paper presents the thermal performance of a roof as a solar collector/storage system which is important for the thermal design of buildings. The system consists of a mass of concrete or concrete insulation, one face of which is blackened/glazed and exposed to solar radiation and ambient air, while the other is in contact with room air at constant temperature. The heat can be extracted by the passage of water through the network of tubes in this block. It is seen that, by increasing the depth of the tubes, the rise in water temperature decreases but the time difference between the maxima of the solair temperature and that of the outlet water temperature increases. At a tube depth of 0·10 m, the maximum temperature rise of the water is 33·5°C. The corresponding efficiency of the system is 28·0% while the flow rate of water is 5·0 litre/h m²; the heat flux entering the room is also reduced considerably.