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

https://doi.org/10.1016/j.rser.2011.07.138Get rights and content

Abstract

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.

Introduction

In many countries, global warming considerations have led to efforts to reduce fossil energy use and to promote renewable energies in the building sector. Energy use reductions can be achieved by minimizing the energy demand, by rational energy use, by recovering heat and cold and by using energy from the ambient air and from the ground. To keep the environmental impact of a building at sustainable levels (e.g., by greenhouse gas neutral emissions), the residual energy demand must be covered with renewable energy. In this theme integral concepts for buildings with both excellent indoor environment control and sustainable environmental impact are presented [1].

Buildings play an important role in consumption of energy all over the world. Building sector has a significant influence over the total natural resource consumption and on the emissions released. Building energy consumption keeps rising in recent years due to growth in population, increasing demand for healthy, comfort and productive indoor environment, global climate changing, etc. Nowadays, the contribution from buildings towards global energy consumption is approximately 40%. Most of energy use in buildings is for the provision of heating, ventilation and air conditioning (HVAC). High-level performance of HVAC systems in building lifecycle is critical to building sustainability. A building uses energy throughout its life, i.e., from its construction to its demolition. The demand for energy in buildings in their life cycle is both direct and indirect. Direct energy is used for construction, operation, rehabilitation and demolition in a building; whereas indirect energy is consumed by a building for the production of material used in its construction and technical installations [2], [3].

Various policies have been formulated in many countries around the world to aim at decreasing carbon dioxide emissions, while many countries have also established policies towards increasing the share in renewable energy utilization. Both are parts of a global response to the climate change [4]. Especially in analyzing 100% renewable energy systems, which will be technically possible in the future, and may even be economically beneficial compared to the business-as-usual energy system, energy savings, efficient conversion technologies and the replacement of fossil fuels with renewable energy are essential elements to consider [5].

As a consequence of the latest reports on climate change and the need for a reduction in CO2 emissions, huge efforts must be made in the future to conserve high quality, or primary energy, resources [6], [7]. A new dimension will be added to this problem if countries with fast growing economies continue to increase their consumption of fossil energy sources in the same manner as they do now. Even though there is still considerable energy saving potential in building stock, the results of the finished IEA ECBCS Annex 37, Low Exergy Systems for Heating and Cooling of Buildings, show that there is an equal or greater potential in exergy management [8].

With the urgent need to reduce the economic and environmental cost of energy consumption, investigations covering many aspects related to thermal comfort in indoor environments have attracted many investigators for decades [9]. In this regard, exergy analysis was also applied to human heat and mass exchange with the indoor environment [10], while various exergetic indexes have been recently developed to assess the performance of sustainable buildings [7], [11].

A high-performing sustainable building needs to maximize its energy, exergy, and comfort performances with little or no compromise among them, while the environmental footprint is minimized. Until now these factors were treated separately at best, if the concept of exergy was not ignored. In fact, exergy is a long forgotten concept in building and HVAC technology so much so that energy balances are made purely by the first law of thermodynamics. Exergy, which is the useful work potential of a given amount or stream of a given energy resource, is very important in metrication of the building carbon footprint. For example, the rational exergy management efficiency, which is a measure of how much the resource exergy, is balanced with the demand like space heating is only in the range of 6%. This shows that without factoring in the exergy concept, major environmental problems and solutions remain hidden in the building sector [11].

Buildings consume energy throughout their whole lifecycles, and many aspects and stages of building development and utilization impact their energy and environmental performance, from planning, design, construction and installation to test, commissioning, operation and maintenance [12].

The environment-oriented design of buildings is a complex task. Energy and environmental performances of buildings strictly depend on many factors related to the choice of construction materials, HVAC plants and equipment, design, installation and use. By definition, an eco-building closely interacts with its environment. In such a building natural phenomena, such as natural ventilation, day lighting, passive cooling and heating, and renewable energy sources, are integrated in a thermal insulated envelope framework with energy efficient systems. Then interactions between building and climate, plants, and users have to be taken into account. This aspect is evident in new buildings design process, but it is even more important in the design phase of an existing building renovation, during which energy saving actions are developed. Several studies on the design phase of buildings have been carried out, but few analyses have developed the environmental implications of retrofit and refurbishment actions [13].

In addition, in recent years exergy efficient design concept has been studied and developed in an increasing manner. In this regard, there has been pioneering work done by Shukuya (1994, 1996), an architectural engineer by background, who has studied various aspects including fenestration, building services and more recently the human body [14], [15], while since then, different studies have been undertaken [16], [17], [18].

A guidebook on low exergy heating and cooling systems was issued in 2003 [16]. This summarizes the work of the LowEx cooperation. An other result of the LowEx cooperation was the funding of the International Society for Low Exergy Systems in Buildings (LowExNet). LowEx, the international research programme for Low Exergy Systems for Heating and Cooling of Buildings, is part of the International Energy Agency's (IEA) Implementing Agreement Energy Conservation in Buildings and Community Systems (ECBCS). The aim of the programme was to promote rational use of energy by encouraging the use of low temperature heating systems and high temperature cooling systems of buildings. These systems that are suitable for office buildings, service buildings and residential buildings, can use a variety of fuels and renewable energy sources. These systems use energy efficiently while providing a comfortable indoor climate. They should be widely implemented now.

In this regard, a new methodology for prediction models of the thermal behavior of thermally activated building components was derived. The exergy concept was applied to a whole building analysis, while a mathematical model to estimate air flows under natural cross ventilation conditions was derived [17]. In the scope of Schmidt's Ph.D. thesis, various lox exergy concepts were studied in terms of design, optimization and performance assessment aspects [19], [20], [21], [22], [23], [24].

The exergy concept was also applied to building and building services design. The applicability of existing exergy-related definitions was systematically investigated in built-environment conditions (e.g., smaller temperature differences between a system and environment) and incorporated to existing exergy calculation models [18], as also reported in other associated studies [25], [26], [27].

As part of the measures taken for reducing the emissions from energy utilization processes, efforts have also been made to reduce energy consumption in buildings because buildings account for a major fraction of the world's annual energy demand. This has been achieved by constructing heavily thermally insulated buildings, improving the quality of window glazing, and using the thermal storage of the construction itself. To find and further quantify potentials in energy conservation, the thermodynamic concept of exergy can be beneficial. Energy, which is entirely convertible into other types of energy, is called exergy (high valued energy such as electricity and mechanical work load). Energy, which has a very limited convertibility potential; for example, heat close to room air temperature, is a low valued energy. Low exergy heating and cooling systems use low valued energy, which could also easily be delivered by sustainable energy sources (e.g., by using heat pumps, solar collectors or other means). Common energy carriers like fossil fuels deliver high valued energy [17].

The main objective of this study is to comprehensively review low exergy heating and cooling systems and applications for sustainable buildings and societies. In this regard, the structure of the paper consisting of eight sections is organized as follows: the first section gives some introductory information; Section 2 summarizes energy utilization and demand in buildings; various exergy definitions used and sustainability are presented in Section 3; the definition of dead (reference) state, with respect to which exergy is always evaluated, is described in Section 4; Section 5 includes LowEx heating and cooling systems; LowEx relations used to estimate energy and exergy demand in buildings and key parameters for performance assessment and comparison are given in Section 6; the LowEx studies and applications conducted are reviewed in Section 7 while the last section concludes.

Section snippets

Energy utilization and demand in buildings

The world's primary energy demand has increased rapidly due to the increase of the industrialization and population. More than one third of the world's primary energy demand use in residential sector. Space heating, cooling and lightening in the residential sector are considered one of the main parts of the energy consumption in buildings. Worldwide energy consumption by HVAC equipment in buildings ranges 16–50% of total energy consumption [28], [29], depending on the countries and their

Various exergy definitions and sustainability

Exergy analysis is relevant in identifying and quantifying both the consumption of useful energy (exergy) used to drive a process as well as the irreversibilities (exergy destructions) and the losses of exergy. The latter are the true inefficiencies and, therefore, an exergy analysis can highlight the areas of improvement of a system. Exergy measures the material's true potential to cause a change. Throughout the years such analysis has been extensively discussed and applied to a wide variety

Dead (reference) state

Exergy is a thermodynamic concept that has been widely promoted for assessing and improving sustainability, notably in the characterization of resources and wastes [68]. In the analysis, a parametric study is undertaken to investigate the effect of varying dead-state properties on energy and exergy efficiencies (i.e., [76], [77], [78], [79]). In this context, it should be noticed that exergy is always evaluated with respect to a reference environment (i.e., dead state). When a system is in

Low exergy (LowEx) heating and cooling systems

Over the last two decades various so-called “energy saving” measures have been conceived, developed, and implemented in building envelope systems and also their associated environmental control systems such as lighting, heating, and cooling systems. Those measures can be categorized into two groups: those for “passive” systems and those for “active” systems. “Passive” systems are defined as building envelope systems to make use of various potentials to be found in the immediate environment such

Estimation of energy and exergy demand in buildings

As stated above, an important step in the entire analysis is the estimation of the energy demand of the actual building. The calculation of the heating (or cooling) energy demand of the building itself is included, that is without any energy demand from the building services systems. The heat demand is a key figure in the analysis, as it corresponds to the building's exergy load. A low exergy load means a thermally well constructed building envelope. The energy requirement for the service

Reviewing the LowEx studies and applications conducted

The first LowEx-related studies conducted date back to almost 17 years ago. In this regard, there have been pioneering works done by Shukuya [14], [15], an architectural engineer by background, who has been studying different aspects including fenestration, building services and more recently the human body [18], and Shukuya and Komuro [97]. In this regard, Shukuya [14], [98] discussed the terms of energy, entropy and exergy, which were defined in Table 1 and the working system consisting of

Conclusions

Buildings play an important role in consumption of energy all over the world. LowEx systems allow utilization 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. In this study, low exergy heating and cooling systems were comprehensively reviewed in terms of the previously performed studies and applications while their analysis and assessment relations

Acknowledgements

The author would like to thank both Dr. Dietrich Schmidt and Prof. Dr. Masanori Shukuya due to their kind invitations to being a member in LowEx.Net: Network of International Society for Low Exergy Systems in Buildings (http://www.lowex.net/). He also would like to express his appreciation to his wife Mrs. Fevziye Hepbasli and his daughter Ms. Nesrin Hepbasli for their continued patience, understanding and full support throughout the preparation of this paper as well as all the other ones while

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