Energy and exergy analyses of space heating in buildings

In the present study, energy and exergy analyses are presented for the whole process of space heating in buildings. This study is based on a pre-design analysis tool, which has been produced during ongoing work for the International Energy Agency (IEA) formed within the Energy Conservation in Buildings and Community Systems Programme (ECBCSP) Annex 37. Throughout this paper, in all of the calculations such as heat losses and gains were taken according to Turkish Standards Institution TSE, which is in accordance with the European Standard TS EN ISO 13789. In the analysis, heating load is taken account but cooling load is neglected and the calculations presented here are done using steady state conditions. The analysis is applied to an office in Izmir with a volume of 720 m³ and a net floor area of 240 m² as an example of application. Indoor and exterior air temperatures are 20 °C and 0 °C, respectively. It is assumed that the office is heated by a liquid natural gas (LNG) fired conventional boiler, an LNG condensing boiler and an external air–air heat pump. With this study, energy and exergy flows are investigated. Energy and exergy losses in the whole system are quantified and illustrated. The highest efficiency values in terms of energy and exergy were found to be 80.9% for external air–air heat pump and 8.69% for LNG condensing boiler, respectively.     

Low exergy (LowEx) heating and cooling systems for sustainable buildings and societies

Heating, cooling and lighting appliances in buildings account for more than one third of the world's primary energy demand and there are great potentials, which can be obtained through better applications of the energy use in buildings. In this regard, the building sector has a high potential for improving the quality match between energy supply and demand because high temperature sources are used to meet low-temperature heating needs. Low exergy (or LowEx) systems are defined as heating or cooling systems that allow the use of low valued energy, which is delivered by sustainable energy sources (i.e., through heat pumps, solar collectors, either separate or linked to waste heat, energy storage) as the energy source. These systems practically provide heating and cooling energy at a temperature close to room temperature while the so-called LowEx approach, which has been and still being successfully used in sustainable buildings design.The present study comprehensively reviews the studies conducted on LowEx heating and cooling systems for establishing the sustainable buildings. In this context, an introductory information is given first. Next, energy utilization and demand in buildings are summarized while various exergy definitions and sustainability aspects along with dead (reference) state are described. LowEx heating and cooling systems are then introduced. After that, LowEx relations used to estimate energy and exergy demand in buildings and key parameters for performance assessment and comparison purposes are presented. Finally, LowEx studies and applications conducted are reviewed while the last section concludes. The exergy efficiency values of the LowEx heating and cooling systems for buildings are obtained to range from 0.40% to 25.3% while those for greenhouses vary between 0.11% and 11.5%. The majority of analyses and assessments of LowEx systems are based on heating of buildings.     

Geothermal space heating

The use of geothermal resources for space heating dominates the direct use industry, with approximately 37% of all direct use development. Of this, 75% is provided by district heating systems. In fact, the earliest known commercial use of geothermal energy was in Chaudes-Aigues Cantal, France, where a district heating system was built in the 14th century. Today, geothermal district space heating projects can be found in 12 countries and provide some 44,772 TJ of energy yearly. Although temperatures in excess of 50 °C are generally required, resources as low as 40 °C can be used in certain circumstances, and, if geothermal heat pumps are included, space heating can be a viable alternative to other forms of heating at temperatures well below 10 °C.     

Analysis of cumulative exergy consumption

An analysis of exergy losses appearing in all the links of the chain of production processes, from the winning of natural resources to the fabrication of the product under consideration, has been proposed. These exergy losses inform about the influence of the imperfection of particular intermediate processes on the thermodynamic imperfection of the considered chain of processes. A graphical scheme and band diagram for the presentation of the results of analysis have been proposed. A numerical example has been included.     

Comparative effects of building envelope improvements and occupant behavioural changes on the exergy consumption for heating and cooling

Much focus is put on measures to improve the building envelope system performance to reduce the impact of the building sector on the global environmental degradation. This paper compares the potential of building envelope improvements to those of a change in the occupant's behavioural pattern. Three cases of improvements together with a base case were analysed using exergy analysis, because the exergy concept is useful to understand the underlying processes and the necessary adjustments to the calculation of the heat-pump system. The assumptions for the occupant behaviour were set up based on our field measurements conducted in a dormitory building and the calculation was for steady-state conditions. It was found that the potential of occupant behavioural changes for the reduction in exergy consumption is more affected by the outdoor temperature compared to building envelope improvements. The influence of occupant behaviour was highly significant (more than 90% decrease of exergy consumption) when the temperature difference between indoors and outdoors is small, which is the case for long periods in regions with moderate temperatures during summer and/or winter. Nevertheless, both measures combined lead to a reduction from 76% up to 95% depending on the outside conditions and should be the final goal.     

Energy and exergy performance of residential heating systems with separate mechanical ventilation

The paper brings new evidence on the impact of separate mechanical ventilation system on the annual energy and exergy performance of several design alternatives of residential heating systems, when they are designed for a house in Montreal. Mathematical models of residential heating, ventilation and domestic hot water (HVAC–DHW) systems, which are needed for this purpose, are developed and furthermore implemented in the Engineering Equation Solver (EES) environment. The Coefficient of Performance and the exergy efficiency are estimated as well as the entropy generation and exergy destruction of the overall system. The equivalent greenhouse gas emissions due to the on-site and off-site use of primary energy sources are also estimated. The addition of a mechanical ventilation system with heat recovery to any HVAC–DHW system discussed in the paper increases the energy efficiency; however, it decreases the exergy efficiency, which indicates a potential long-term damaging impact on the natural environment. Therefore, the use of a separate mechanical ventilation system in a house should be considered with caution, and recommended only when other means for controlling the indoor air quality cannot be applied.     

Optimum glazing combination for solar space heating

A study is made for comparing the maximum seasonal energy yield obtainable by solar collectors for space heating application. Different glazing combinations with glass and plastic as glazing materials are considered. The study is made for four different locations. The performance of eight glazing combinations with covers ranging in number from one to three is compared to obtain the optimum combination for each location. The results show that selecting the optimum glazing combination improves the performance significantly. In general, plastic covers give higher yield. The study confirmed that the use of two covers is justified in cold, cloudy climates while a single cover is suitable for temperate climates. In most cases three covers lead to a significant reduction in the yield. Replacing plastic by glass as a top cover for longer life results in a small yield reduction. Some of the other conclusions are that the ratio of average to normal transmittance-absorptance product changes significantly with location and month of the year. However, the seasonal average value of this ratio is almost constant for any number of covers but changes with location.     

Residential space heating: An analysis of energy conservation

The energy signature technique is used to analyse the marginal benefits of various types of energy conservation in residential space heating and certain systematic errors frequently committed by utility companies in estimating these benefits. The results indicate that the marginal price of energy and the change in the balance point temperature of a dwelling are the most significant factors in determining the value of energy saved each year. Utilities in the 27 cities studied do not consider the effects of changes in the balance point temperature of a structure when estimating energy savings. Regional differences are also examined.     

集中供暖热计量模式多样化的必要性分析

通过工程实例对我国节能建筑集中供暖热计量系统设计所存在的问题进行讨论。目前我国居住建筑集中供暖系统热计量设计模式单一,没有充分考虑我国国情和不同类型居住建筑的特点,加之我国的供热体制改革滞后,使得新建居住建筑的节能效果无法保证,还增加了工程建设费用。相关的技术规程予以补充和完善。     

Latent heat thermal energy storage tanks for space heating of buildings: Comparison between calculations and experiments

Latent heat thermal energy storage tanks, where carbon fiber brushes are inserted to improve the heat transfer rates in the phase change materials, are installed in an air conditioning system of a building as a space heating resource. The measured outlet fluid temperatures are compared with the numerical ones predicted by a previously developed three dimensional heat transfer model. The preliminary numerical results had unallowable prediction errors, which probably resulted from poor contact between the brushes and the heat transfer tubes due to an installation problem of the brushes. However, the numerical results predicted by a corrected model agree well with the experimental ones under various operating conditions. The effect of the brushes on the thermal outputs of the tanks is then investigated using the corrected model. The result shows that the brushes contribute to saving space and reducing the cost of the tanks.     

Comprehensive exergy analysis of a ground-source heat pump system for both building heating and cooling modes

This paper presents a comprehensive exergy analysis of three circuits and whole system of a ground-source heat pump (GSHP) for both building heating and cooling modes. The purpose is to search out the key potential energy saving components. The analytical formulae of exergy loss, exergy efficiency, exergy loss ratio, exergy loss coefficient and thermodynamic perfect degree are derived, respectively. The results show that these exergy indexes should be used integratively, and in the whole system the location of maximum exergy loss ratio is the compressor, while the location of minimum exergy efficiency and thermodynamic perfect degree is the ground heat exchanger, so that the compressor and the ground heat exchanger should be primarily improved. The results also indicate that the exergy loss of a GSHP system for building heating mode is bigger than that of cooling mode, and the exergy efficiency of a whole GSHP system is obviously lower than those of its components for both building heating and cooling modes. Therefore, a comprehensive exergy analysis of a GSHP should be paid more attention to. The results may provide guidelines for the design and optimization of GSHP systems.     

地板辐射式供暖的能耗分析

依据辐射供热室内辐射与对流综合作用的传热规律,计算和分析了地板辐射式采暖房间能耗的构成及影响因素,给出在相同舒适度条件下,几种常见的围护结构地板辐射式供暖房间的能耗随地表面加热温度的变化关系,并与相应的对流式采暧进行了对比。     

Simulation models for the analysis of space heat consumption of buildings

This study develops and analyzes an original methodology for the simulation and prediction of space heating energy consumption in buildings connected to a district heating system, characterized by lack of individual control systems for end-users. The identification of the input parameters is based on both classical engineering equations and statistical analysis of collected data. Two main factors play important roles in the model: (i) climate and (ii) human behavior. Model validation was undertaken through the analysis of field data collected during the winter, via a monitoring system working in a partially-controlled district heating system. The comparison between the results obtained with the proposed model versus classical methods points out the possibility to implement, using the proposed methodology, management policies for a district that offer significant cost-effective energy savings opportunities.     

Application of a ground source heat pump system with PCM‐embedded radiant wall heating for buildings

This paper deals with the utilization of a renewable energy‐based integrated system with the latent heat storage option for building thermal management systems. Both energy and exergy‐based assessments of the current combined system are conducted. For this purpose, phase change material (PCM)‐embedded radiant wall heating system using solar heating and ground source heat pump (GSHP) is studied thermodynamically. Heat is essentially stored within the PCMs as used in the panels to increase the effectiveness. The stored heat is released when the solar energy is not available. In the thermal energy storage analyses, four different PCMs are considered. The present results show that the overall first ‐ law (energy) and second ‐ law (exergy) efficiencies of the PCM‐free radiant heating system are much lower than the case with the PCM‐embedded radiant heating system. Therefore, it is confirmed that the energy efficiency increases from 62% to 87% while the exergy efficiency rises from 14% to 56% with the option where SP26E PCM is employed accordingly.     

Optimisation of a Swedish district heating system with reduced heat demand due to energy efficiency measures in residential buildings

The development towards more energy efficient buildings, as well as the expansion of district heating (DH) networks, is generally considered to reduce environmental impact. But the combined effect of these two progressions is more controversial. A reduced heat demand (HD) due to higher energy efficiency in buildings might hamper co-production of electricity and DH. In Sweden, co-produced electricity is normally considered to displace electricity from less efficient European condensing power plants. In this study, a potential HD reduction due to energy efficiency measures in the existing building stock in the Swedish city Linköping is calculated. The impact of HD reduction on heat and electricity production in the Linköping DH system is investigated by using the energy system optimisation model MODEST. Energy efficiency measures in buildings reduce seasonal HD variations. Model results show that HD reductions primarily decrease heat-only production. The electricity-to-heat output ratio for the system is increased for HD reductions up to 30%. Local and global CO₂ emissions are reduced. If co-produced electricity replaces electricity from coal-fired condensing power plants, a 20% HD reduction is optimal for decreasing global CO₂ emissions in the analysed DH system.     

Energy and exergy analysis of a micro-compressed air energy storage and air cycle heating and cooling system

Energy storage systems are becoming more important for load leveling, especially for widespread use of intermittent renewable energy. Compressed air energy storage (CAES) is a promising method for energy storage, but large scale CAES is dependent on suitable underground geology. Micro-CAES with man-made air vessels is a more adaptable solution for distributed future power networks. In this paper, energy and exergy analyses of a micro-CAES system are performed, and, to improve the efficiency of the system, some innovative ideas are introduced. The results show that a micro-CAES system could be a very effective system for distributed power networks as a combination that provides energy storage, generation with various heat sources, and an air-cycle heating and cooling system, with a energy density feasible for distributed energy storage and a good efficiency due to the multipurpose system. Especially, quasi-isothermal compression and expansion concepts result in the best exergy efficiencies.     

欧洲采暖热计量述评

本文从供暖热计量的供热收费法规,热费收缴机构和服务体系,热费分摊的方法,热费收缴信息传输管理系统四个方面介绍了欧洲的供暖热计量情况,这些成功经验对我国供暖收费制度的改革有很大的参考价值。     

Improved underground heat exchanger by using no-dig method for space heating and cooling

This paper describes experiments and analyses on an improved underground heat exchanger by using a no-dig method for the purpose of the cost reduction of a space heating and cooling system using underground thermal energy. First, the improved underground heat exchanger was installed on the campus of Hokkaido University, and it was shown that a ground source heat pump system utilizing the heat exchanger was sufficient for space heating and cooling. Second, evaluation program of the heat exchanger was developed, and the program was verified to give good predictions by comparing with experimental results. As a result of system simulations, an energy reduction for a system installation relative to a conventional vertical earth heat exchanger reached 78%. The primary energy reduction rate including the system installation and operation relative to a typical air source heat pump was 29%.     

An estimation of the energy and exergy efficiencies for the energy resources consumption in the transportation sector in Malaysia

The purpose of this work is to apply the useful energy and exergy analysis models for different modes of transport in Malaysia and to compare the result with a few countries. In this paper, energy and exergy efficiencies of the various sub-sectors are presented by considering the energy and exergy flows from 1995 to 2003. Respective flow diagrams to find the overall energy and exergy efficiencies of Malaysian transportation sector are also presented. The estimated overall energy efficiency ranges from 22.74% (1999) to 22.98% (1998) with a mean of 22.82±0.06%$22.82\pm 0.06\%$ and that of overall exergy efficiency ranges from 22.44% (2000) to 22.82% (1998) with a mean of 22.55±0.12%$22.55\pm 0.12\%$. The results are compared with respect to present energy and exergy efficiencies in each sub-sector. The transportation sector used about 40% of the total energy consumed in 2002. Therefore, it is important to identify the energy and exergy flows and the pertinent losses. The road sub-sector has appeared to be the most efficient one compared to the air and marine sub-sectors. Also found that the energy and exergy efficiencies of Malaysian transportation sector are lower than that of Turkey but higher than Norway.     

分时供暖在高校类建筑中的节能率分析

针对高校类建筑的使用特点及其供暖所存在的问题,提出了分时供暖节能控制方案。以天津地区某大学办公楼为例,结合天津地区气候特点对采用分时供暖方案后节能率随室外温度的变化规律进行分析。通过分析表明,在室内控制温度与间歇周期不变的条件下,节能率随室外温度的升高不断增加。