Exergia: tool to promote energy efficiency in buildings

2021/03/01 Picallo Perez, Ana - ENEDI ikerketa-taldea Energia Ingeniaritza saila (EHU) | Sala Lizarraga, José M. - ENEDI ikerketa-taldea Energia Ingeniaritza saila (EHU) | Iribar Solaberrieta, Eider - Eraikuntza Kalitatearen Kontrolerako Laborategia Eusko Jaurlaritza Iturria: Elhuyar aldizkaria

• Arkitektura
• Energia
• Ingeniaritza
• Ekonomia

A flow of energy, besides having a quantity of energy, has a quality, that is, a useful working capacity, for example: lighting a light, air conditioning a stay or moving a car. Thus, the chemical energy capacity of 1 kWh stored in a natural gas container is greater than the thermal energy of 1 kWh in an air mass of 20ºC; in short, the chemical energy of natural gas works with the use of the chemical energy of natural gas (air conditioning, mechanical work, lighting of a bulb, etc. ), but with the thermal energy of the air can only be heated, although both in quantity can have an energy of 1 kWh (see).

The energy demand of buildings has high levels of quality. On the one hand electricity is consumed, such as lighting, appliances, elevators, etc. On the other hand, the supply of domestic hot water and the satisfaction of heating and cooling demands are generally carried out with high quality energies such as natural gas (chemical energy). However, as the demand for heating or cooling is satisfied with low quality energies (thermal energy), the final energy quality does not conform to the quality of demand (see figure 2).

Until now, although most energy systems have been designed quantitatively, they should be designed taking into account quality (i.e. exertion), with the benefits that this entails. In addition, in addition to promoting energy efficiency, it is necessary to incorporate the environmental aspect to achieve energy systems with less environmental impact, that is, to promote sustainability.

The aim of this work is to reflect on the benefits of applying the concept of exergia in the area of buildings. Promoting energy efficiency and building sustainability is critical considering the high energy consumption of the sector and its potential for improvement.

What is exergia?

Exertion represents the ability of an energy to transform into work. Some forms of energy can be completely transformed, for example, into electric energy where all energy is exerted. However, there are other forms of energy such as heat, in which only a part can become work, so that the flow of exergia is only a part of a heat flow [1].

Moreover, the capacity to transform energy into work corresponds to the degree of energy imbalance with the environment. For example, while the internal energy of a lake's water is enormous, its potential for generating work is zero. The further away from the environment, the greater the capacity to transform into work. Therefore, a mass of hot water at 60ºC presents a greater exergia than if it is at 40ºC, since it is farther from the ambient temperature.

Figure . Comparison of quantity and quality between different types of energy. Image courtesy of: Ana Picallo.

These ideas of quality are collected in the second principle of thermodynamics: although energy is not produced or destroyed (first principle of thermodynamics), the quality of this energy is increasingly less and unrecoverable. This loss of quality is related to the imperfections of equipment and processes, calling irreversibility or destruction of exertions.

How are the concepts exergia and economics related?

Our society needs more and more energy, but natural resources are limited. Therefore, it is essential for systems improvement to understand the mechanisms of degradation of energy and resources and to develop systematic procedures to reduce their environmental impact. If exergoeconomics is combined with economics, a powerful system analysis and optimization tool called exergoeconomics is obtained [2].

Because exergia is a useful energy for society, it has economic value and we must take care of it. When we pay for energy consumption, we are actually paying for its availability, that is, its exaggeration. Therefore, exertion is a rational basis for evaluating system resources, processes, equipment and efficiencies and evaluating system product costs.

How are the concepts exergia and environment related?

Almost all energy (and therefore exertion) reaches the earth's surface from the Sun. The exertion absorbed by the Earth is gradually destroyed, but in this destruction the cycles of water, wind and soil life are managed. Plants absorb exertion from the sun and make it a chemical exertion through photosynthesis. Through the food chain, this chemical exertion passes through organisms in ecosystems.

On the other hand, energy and environmental problems are currently notable, such as global warming, air pollution, surface and groundwater pollution, solid waste, soil degradation, etc. However, instead of associating this degradation with energy, it must be associated with exertion.

Exergian analysis is a powerful tool for making processes and facilities more efficient and, at the same time, reducing the consumption of resources (exergia) and therefore reducing the waste generated. Therefore, the use of exergative methods implies an adequate assessment of air pollution, liquid or solid waste, etc., a discipline known as exergoenvironmics [3].

Figure . Inadequate relationship between energy use and sources. Image courtesy of: Ana Picallo.

How do the concepts of exergy and sustainability relate?

The sustainable development of society and, in particular, of the construction sector involves the sustainable provision of natural resources. The limitation of most natural goods obliges its efficient use to keep them in a longer term. In addition, the mineral can be considered a vector of exergia: a deposit of minerals contrasts with the environment, and the higher the concentration of the mineral, the greater the contrast and the greater the potential of work (exergia).

Taking these ideas into account, the exergative analysis allows us to analyze the degree of efficiency of a society and the balance in the consumption of its physical resources. In this way, it is possible to compare global societies and analyze the international system if we want to distribute resources in a more just way in the world.

The application of energy in buildings promotes energy efficiency

The construction sector is closely related to natural resource consumption and emissions into the atmosphere. If we analyze buildings throughout their life cycle (construction, use and demolition), they reduce the ozone layer due to the use of different chemicals. They also contribute to climate change due to high CO2 emissions both during the construction phase and during service life.

The exérgic analysis is very useful both for the study and for the design of the systems of the buildings and the whole of the building. As already mentioned, when considering the qualitative aspect of energy (energy quality) we obtain information about the adequacy of the energy used and the energy demand. Thus, low quality energy sources, such as waste heat, can be used to meet low quality demands such as air conditioning. Therefore, the minimum exertion consumption required to meet demand is quantified (see Figure 3. Therefore, exergia enhances energy efficiency and renewable sources.

In addition, exertion losses are quantified along the energy chain and the potential for energy improvement is highlighted, which is not possible with energy analyses. On the other hand, it provides a common basis for comparing the energy efficiency of building systems and installations. In this way it can be compared with the exergia the heat of the combustion fuel of a boiler and the solar gain of a window.

Exertion losses and destructions identify the places and causes of system ineffectiveness. This facilitates decision-making for the implementation of improvement measures.

Figure . Proper relationship between energy use and sources, taking into account its quality. Image courtesy of: Ana Picallo.

Although exérgic analysis is interesting, a few years ago began to study its application in buildings. Some professionals may find it complex and, unfortunately, calculations of exergative methods seem annoying, and their results can sometimes be difficult to interpret or understand.

In addition, the exergian analysis highlights the low exergic yields of conventional systems. For example, if a conventional gas boiler has an energy yield of 85-90%, its energy yield is 18-20%. In fact, the exergian analysis highlights that some of the processes and systems commonly used are basically wrong, which can go against the interests of some.

Therefore, exergia is a variable of great meaning

This paper presents the key ideas for the application of exergological analysis in buildings, both from an energy and economic and environmental point of view.

Dermal analysis is used to analyze the behavior of buildings, in order to accurately quantify losses. This information, essential for the promotion of energy efficiency and the best use of resources, is only possible through the application of the second law of thermodynamics. All these characteristics make exergia a good tool to promote sustainability in the field of engineering and architecture.

Therefore, it is necessary to have detailed models of exergonomic analysis and calculation methodologies specifically designed in buildings, in order to update the concept and use it by professionals in the sector, objective of the recently published book “Exergic analysis and thermoeconomics in buildings: analysis and design of sustainable energy systems” [4].

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