Comparison of analytical and numerical modeling approaches for sizing of seasonal storage solar heating systems

Analytical and numerical approaches for the predesign of central solar heating plants with seasonal storage (CSHPSS) systems are compared. The results indicate that the analytical approach employed in SOLCHIPS predesign tool is significantly faster and more powerful than the traditional numerical methods for predesign of high solar fraction seasonal storage solar heating systems.     

German central solar heating plants with seasonal heat storage

Central solar heating plants contribute to the reduction of CO₂-emissions and global warming. The combination of central solar heating plants with seasonal heat storage enables high solar fractions of 50% and more. Several pilot central solar heating plants with seasonal heat storage (CSHPSS) built in Germany since 1996 have proven the appropriate operation of these systems and confirmed the high solar fractions.Four different types of seasonal thermal energy stores have been developed, tested and monitored under realistic operation conditions: Hot-water thermal energy store (e.g. in Friedrichshafen), gravel-water thermal energy store (e.g. in Steinfurt-Borghorst), borehole thermal energy store (in Neckarsulm) and aquifer thermal energy store (in Rostock). In this paper, measured heat balances of several German CSHPSS are presented. The different types of thermal energy stores and the affiliated central solar heating plants and district heating systems are described. Their operational characteristics are compared using measured data gained from an extensive monitoring program. Thus long-term operational experiences such as the influence of net return temperatures are shown.     

Effect of climate on major CSHPSS design parameters

The effects of site location on the sizing of the main components of a central solar heating system with seasonal storage (CSHPSS) are discussed. Results for optimum storage volume, collector area, and cost of produced energy versus latitude are shown for a generic CSHPSS type. The study indicates a decrease in the storage capacity requirement per unit collector area (V/A_c-ratio) when moving towards north though showing simultaneously a declining system cost-effectiveness.     

Verification of a CSHPSS simulation program with emphasis on system control

A comparison between one-year measured and simulated thermal performance of a full-scale seasonal storage solar heating system (CSHPSS) is presented: To minimize simulation inaccuracies, special attention has been paid to the system control and operational strategies. Then the discrepancies on monthly basis are less than 15% and the predicted yearly energy flows differ only by a few percent.     

Performance analysis & energy benefits of a desiccant based solar assisted trigeneration system in a building

In this paper, performance details and operational benefits of a large scale solar trigeneration system that provides for solar assisted desiccant cooling, heating and hot water generation installed in a teaching institute building have been reported. A two-rotor desiccant system designed for handling 12 000 m3/hr of air was integrated into existing plant to provide a net reduction in energy consumption over the pre-existing heating ventilation and air-conditioning and domestic hot water systems. The system is controlled and monitored by a building management system which has been used to investigate and analyse the typical system behaviour. Heat from solar energy contributed consistently to reduce gas usage for water heating and on an annual basis showed a reduction of 21% of consumed energy. The solar energy contribution for space heating varied over winter months and during some months it was observed to contribute more than 50% of the total energy requirements for space heating. Under suitable ambient conditions, approximately 35% of total building cooling load was met by the solar driven desiccant cooling system. Continuous monitoring has also helped understand some of the operational issues of the system.     

An Application of Solar Energy Storage in the Gas: Solar Heated Biogas Plants

Abstract

Temperature is an important factor that may affect the performance of anaerobic digestion. Therefore, biogas plants without heating system work only in warmer regions for the whole year. In regions with extreme temperature variations, for instance in Turkey, the biogas plant should be built with heating system. One of the methods is to use solar energy to increase the reactor temperature. In this study, solar heated biogas plants were reviewed. Furthermore, the optimization of insulation thicknesses and solar energy systems for 5 m³ biogas reactor were carried out for two different cities for three different climatic zones in Turkey. Based on the obtained results, the ratio of annually produced biogas used for reactor heating was calculated for each city, with and without solar heating system. Obtained results indicate that the biogas consumption for reactor heating is decreased by approximately 19% for average of six cities when solar heating system is used. This means that available biogas potential would be increased.     

High temperature water pit storage projects for the seasonal storage of solar energy

Central solar heating plants with seasonal storage (CSHPSS) are capable of covering more than 75% of the annual heat demand of housing areas if appropriate storage technologies are available. The maximum design temperature should be 90–95° and the long term cost goal is 100 DM m⁻³ for a storage volume larger than 10 000 m³ water equivalent. Three pilot projects are presently under construction and planning in Germany with 600, 4500 and 12 000 m³ volume. The storage medium in all three cases is water.A first pilot heat storage with about 600 m³ volume is being built in Rottweil. This small scale project will be applied as short term storage in connection with a combined heat and power (CHP) plant. The storage container is made of concrete, water tightness is achieved by a stainless steel linear and mineral wool is used as insulation. The aim of this project is to demonstrate the feasibility of the technology and to gain practical experience for the construction of larger stores.During 1995/96 two full scale central solar heating plants with seasonal storage (CSHPSS) of this type will be built in Hamburg, North Germany and Friedrichshafen, South Germany, with 4500 and 12 000 m³ storage volume, respectively.     

Review of long-term fault detection approaches in solar thermal systems

This paper presents an overview, assessment and comparison of automated fault detection methods that check if solar heating systems are functioning correctly. Fault detection in solar thermal systems is important to minimize the time when the system is not functioning well, thereby ensuring an optimal energy (and economic) yield.During the past decades many systems have been monitored, mainly for scientific or demonstration projects by logging measurement data which was subsequently analysed by an expert. Automation of fault detection is necessary to reduce costs and minimize experts’ time needed for analysis of a system. An overview of existing fault detection approaches is given; these are evaluated and compared with a multi-criteria analysis.The only commercially available automated method, the Input-Output Controller, detects faults causing more than 20% energy loss in the solar loop. The function control approach is cheap without a heat meter, and only relies on few sensors to check how several components in the solar loop are functioning with algorithms. The approach developed at Kassel University checks how well a solar plant is functioning both with plausibility checks and with energy balances based on simulations. This method includes a larger part of the solar heating system and therefore requires more measurement equipment.Further research and application of several fault detection methods should improve the effectiveness and costs of these methods.     

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.     

Fossil fuels substitution by the solar energy utilization for the hot water production in the heating plant “Cerak” in Belgrade

The main objective of this paper is to evaluate energy and environmental benefits of the large-scale solar heating system connection with district heating system. The assessment of fossil fuels substitution by the solar energy for the hot water production for domestic use, during the summer period, is done. Hot water for district heating and domestic use is produced in heating plant “Cerak” placed in the suburb of Belgrade. The existing production and distribution system are based on fossil fuel energy, mainly on the natural gas. In the first phase of the project plan was to install about 10,000 m² of solar collectors to substitute nearly 25% of natural gas consumption. During the summer period, the saving of natural gas calculated for presented system is approximately 430,000 m³ and in this way 900 t of the CO₂ emissions would be reduced.     

A novel prediction algorithm for solar angles using solar radiation and Differential Evolution for dual-axis sun tracking purposes

This paper deals with a dual-axis sun tracking system for a photovoltaic system. Its trajectories are determined by an optimization procedure. The optimization goal is the maximization of the electrical energy production within a photovoltaic system, by considering the tracking system consumption. The procedure used for determining the tilt angle and azimuth angle trajectories is described as a nonlinear and bounded optimization problem. Since an explicit form of the objective function is unavailable, a stochastic search algorithm called Differential Evolution is applied as the optimization tool. In order to evaluate the objective function, models for calculating the available solar radiation and tracking system consumption are applied together with the efficiencies of solar cells, a DC/DC converter and inverter. A new algorithm is introduced for the time dependent prediction of available solar radiation. It is based on the length of a sunbeam’s path through the atmosphere and the statistical data of a pyranometer measured total and diffuse solar radiation at a given location on the Earth. The optimization bounds are given in the form of angular speed, lower and upper bounds for both angles and angle quantization. The results presented in this paper show, that the optimal trajectories can help to increase the electrical energy production within photovoltaic systems by sun tracking.     

太阳能相变蓄热应用于吸收式制冷的分析

介绍了一种将太阳能相变蓄热技术应用于两级吸收式制冷的新型空调系统,简要分析了该系统的装置结构、工作原理和使用优点。对相变蓄热装置放热过程中放热盘管出水温度随放热时间的变化关系进行了实验测量,并对两级吸收式制冷系统效率进行了分析。通过研究可知,该太阳能空调系统有效解决了以往系统不稳定性和间断性问题;太阳能相变蓄热装置具有体积小、蓄热量大、放热速率大、连续放热温度均匀、便于控制热源加热温度等特点,适合储存太阳能并为吸收式制冷系统提供加热热源。综合考虑系统设备简单,加工要求低的制造特点,所以吸收式制冷以太阳能等低品位热源驱动有着良好的发展前景。     

Load optimization in solar space heating systems

When load variables, such as window and insulation types, are included in the economic optimization of a solar space heating system, the over-all cost is lower than that resulting from optimization of collection area for a fixed load (as by FCHART [1] and SOLCOST [2]). In this paper an algorithm is derived for choosing insulation levels, as well as solar collection area, so as to minimize the over-all cost of constructing and heating a building. The general algorithm is applicable with any solar performance prediction method, and with any economic criterion where the “cost” is a linear function of collection area and of auxiliary energy consumption. A specific algorithm is also derived for active solar systems using the Relative Areas method of performance prediction [3] and a conventional present worth life cycle cost analysis. The degree-day model is used for the load calculations.     

Energy and economic analysis of an integrated solar absorption cooling and heating system in different building types and climates

The use of solar energy in buildings is an important contribution for the reduction of fossil fuel consumption and harmful emissions to the environment. Solar thermal cooling systems are still in their infancy regarding practical applications, although the technology is sufficiently developed for a number of years. In many cases, their application has been conditioned by the lack of integration between cooling and heating systems. This study aims to evaluate the potential of integrated solar absorption cooling and heating systems for building applications. The TRNSYS software tool was used as a basis for assessment. Different building types were considered: residential, office and hotel. The TRNSYS models are able to run for a whole year (365 days), according to control rules (self-deciding whether to operate in heating or cooling modes), and with the possibility of combining cooling, heating and DHW applications. Three different locations and climates were considered: Berlin (Germany), Lisbon (Portugal), and Rome (Italy). Both energy and economic results are presented for all cases. The different local costs for energy (gas, electricity and water) were taken into account. Savings in CO₂ emissions were also assessed. An optimization of solar collector size and other system parameters was also analysed.     

Techno-economic analysis of a local district heating plant under fuel flexibility and performance

Brovst is a small district in Denmark. This paper analyses the use of local renewable resources in the district heating systems of Brovst. The present use of fossil fuels in the Brovst district heating plant (DHP) represents an increasing environmental and climate-related load. Therefore, an investigation has been made to reduce the use of fossil fuels for district heating system and make use of the local renewable resources (biogas, solar, and heat pump) for district heating purposes. In this article, the techno-economic assessment is achieved through the development of a suite of models that are combined to give cost and performance data for this district heating system. Local fuels have been analyzed for different perspectives to find the way to optimize the whole integrated system in accordance with fuel availability and cost. This paper represents the energy system analysis mode, energyPRO, which has been used to analyze the integration of a large-scale energy system into the domestic district heating system. A model of the current work on the basis of information from the Brovst plant (using fossil fuel) is established and named as a reference option. Then, four other options are calculated using the same procedure according to the use of various local renewable fuels known as “biogas option,” “solar option,” “heat pump option,” and “imported heat option.” A comparison has been made between the reference option and other options. The greatest reduction in heat cost is obtained from the biogas option by replacing a new engine, where 66 % of the current fuel is substituted with biogas.     

Thermo-economic analysis and sizing of a PV plant equipped with a compressed air energy storage system

Photovoltaic (PV) farms are widely used around the world to provide required electricity. Compressed air energy storage (CAES) system has been already proposed for energy storage applications in large scales. In this work, employing a CAES unit equipped with an ancillary solar heating system for a large scale PV farm in Brazil is proposed. A PV farm with 100 MWp (megawatt peak) capacity is proposed to be built in the most suitable point within Brazil. The sizing of the CAES unit and the solar heating system, which has not been investigated, along with selecting the best power sales strategy for the power plant, which has been always a challenge for renewable energy source power plants, are carried out emphasizing energy-economic considerations. In order to prove the proficiency of the proposal, the performance of the power plant and energy storage unit is assessed over a sample year. In order to have a comprehensive economic analysis, Net Present Value (NPV) method is employed and all the possible uncertainties in the system have been taken into account.     

Optimization study of integration strategies in solar aided coal-fired power generation system

Parabolic trough solar-aided coal-fired power generation system has been developed to achieve efficient use of solar energy resources by coupling conventional coal-fired power plant with solar energy. However, there are no appropriate evaluation criteria about the thermal and economic performance of the system presently. This paper proposes the evaluation standard of the solar aided coal-fired power plant, and then based on this standard an optimization of this system is applied by the genetic algorithm model. According to the optimization research of this system, a series of parameters such as the solar collector area; the way of the coupling the power plant; whether storage is needed and the relevant storage capacity are obtained.     

Economic evaluation and optimization of solar heating systems

A procedure is developed for assessing the economic viability of a solar heating system in terms of the life cycle savings of a solar heating system over a conventional heating system. The life cycle savings is expressed in a generalized formby introducing two economic parameters, P₁ and P₂, which relate all life cycle cost considerations to the first year fuel cost or the initial solar system investment cost. Using the generalized life cycle savings equation, a method is developed for calculating the solar heating system design which maximizes the life cycle savings. A similar method is developed for determining the set of economic conditions at which the optimal solar heating system design is just competitive with the conventional heating system. The results of these optimization methods can be presented in tabular or graphical form. The sensitivity of the economic evaluation and optimization calculations to uncertainties in constituent thermal and economic variables is also investigated.     

Solar multi-mode heating system based on latent heat thermal energy storage and its application

为克服太阳能间断性和不稳定性的缺点进而实现太阳能集热与采暖的能量供需调节和全天候连续供热,提出了基于相变储热的太阳能多模式采暖方法（太阳能集热直接采暖、太阳能集热采暖+相变储热、太阳能相变储热采暖）,并在西藏林芝市某建筑搭建了太阳能与相变储热相结合的采暖系统,该系统可根据太阳能集热温度和外界供热需求实现太阳能多模式采暖的自动控制和自动运行。实验研究表明：在西藏地区采用真空管太阳能集热器可以和中低温相变储热器很好地结合,白天储热器在储热过程中平均储热功率为10.63 kW,储热量达到92.67 kW·h,相变平台明显;晚上储热器在放热过程中供热量达85.23 kW·h,放热功率和放热温度平稳,储放热效率达92%,其储热密度是传统水箱的3.6倍,可连续供热时间长达10 h,从而实现了基于相变储热的太阳能全天候连续供热,相关研究结果对我国西藏地区实施太阳能采暖具有一定的指导作用。