KIEAE Journal

World Watch Institute reported buildings took up 40% global consumption of raw minerals and rocks as well as 55% of virgin wood each year. (Roodman and Lenssen, 1995). In the United States, the building sector accounts for the largest consumption of material and annual energy use (U.S. EIA, 2012). Accordingly, no longer should the notion of sustainability be simply a question of some of architects as built environment significantly changes our globe.

Recent growing attention on sustainability has been led to development of green building rating systems (e.g. LEED, Energy Star, CIBSE, etc.), and among many environmental indices, the dominant player is the operational energy performance that is estimated by building simulation or prescribed methods. In this case, adaptation of brand-new technology or enhancement of performing efficiency are the crucial matters to guarantee a system’s lower environmental impact.

However, as Willis (Braham, 2013) points out, characterization of building sustainability is not just limited to energy efficiency of a building, rather it should be understood as a macro phenomenon beyond the physical building boundary, since environmental impact is likely to be shifted from a building site to a neighborhood or even global societies (Zhang et al., 2010). Particularly for architectural practices, decisions in a building project, made by energy-driven procedures therefore occasionally result in higher construction costor being heedless of nonrenewable inputs (Malin, 2010).

In recognition of such problems, many studies to assess environmental loads in terms of both direct and indirect have already been attempted with various metrics. For example, Thormak (2002) analyzed a building's lifetime energy demand within the framework of life cycle analysis(LCA),and Pajchrowski et al. (2014) also characterized building energy use in aggregation with embodied energy of material flows. In addition, hybrid methods of LCA and embodied energy or exergy have recently been drawn upon to account for dynamic potential of sustainability in building design disciplines (Sakulpipatsin et al. 2010, Ramesh et al. 2010, Hernandez et al. 2010, Baek et al. 2013).

Nevertheless, as Srinivasan and Braham (2012) have pointed out, in the purview of right measurement on limited capacity of biosphere resources, above aggregation indicators leave much to be desired. It is because, even if those metrics were developed to pursue sustainability assessment in a larger scale, they fail to grasp the extensive boundary. To be specific, embodied energy does not consider quality of energy, and LCA begins with material extraction, not including initial formation process by natural work.

In this perspective, emergy (spelled with "m") pioneered by H.T. Odum, is a much more useful and can be a universal metric that offers a unique aggregate indicator scaling the most extensive impacts of building construction.

To explore emergy-driven assessment of building sustainability as well as clarification of competing sustainable design goals, it is obvious that designers have to keep track of the whole building retrofit process, and feeding it back for sufficient quantification.

However, as for building emergy analysis, unfortunately, fairly little research has been conducted. Instead, some case studies have attempted emergy evaluation on building manufacturing (Li et al. 2011, Meillaud et al. 2005, Pulselli et al. 2007), material recycling (Buranakarn 1998), or building envelopes (Pulselli et al. 2009, Price et al. 2010, Srinivasan et al. 2012). Meillaud et al. (2005) and Pulselli et al. (2007) carried out emergy analysis in accordance with building construction and maintenance. Srinivasan and Braham detailed long-term expectation of a building emergy with renewable inputs. Li et al. also utilized emergy indices to appraise ecological balance with respect to building manufacturing, but they only considered six or less materials which appear amiss to describe complex behavior of building construction. Schramski et al.(2008) and Pulselli et al. (2009) analyzed specific building envelopes (roof and wall) to gauge trade-off effects between energy improvement and compensating emergy variation. However, it reveals clear limit in that other major components acting towards the whole building analysis were not compiled into the study.

Remainder of this research is to answer the following questions:

● Is it still valid to rate a building with operational energy performance estimation?

● What is ranked priority of building components among multiple design factors in light of the most extensive environmental impacts?

● Given a baseline model, how do we develop retrofitting scenarios with emergy indices?

Conception of emergy was first coined by H.T.Odum in early 1980s. As initiated from the study of cost benefits of energy and ecological environment, it has been developed to intend for modeling and evaluation of the work of systems of various scales in common metric. The idea of emergy centers on the assumption that systems of different types interact to work as being gained energy from one source-solar energy.

Analogy of all systems to biospherical networking allows to illustrate energy and materials, processed in a environmental system, through the holistic way of evaluation. Therefore, the use of emergy as an indicator of resource intensity has significant benefits since it reduces the various inputs and environmental services required for material cycles to a common denominator (Buranakarn, 1998).

Introduction of emergy analysis method is quite new to built environment as well as architecture. As an environmental accounting method, building emergy synthesis (BES) considers quantities of resources and quality of energy, which is termed as unit emergy value (UEV) or transformity. Calculating emergy indices of a building provides a new scale for evaluating environmental impacts by integrating energy use, material consumption, and other environmental services as represented by solar-emjoule (sej).

Solar-emjoule is a metric of emergy based on the principle that all embodied energy in both natural(e.g. water, rocks, etc.) and man-made products on the earth source from the solar inputs. 9.26×10²⁴sej/yr is the baseline for all emergy accounting (Odum, 1996), and the measure of emergy is in units of the solar-emjoule.

H.T. Odum clearly illustrated dissimilarity in the behavior of energy and emergy with an example of typical basic configuration of a steady-state environmental system (Fig. 1.). In a typical ecosystem network, energy is always conserved from a source to a sink (Fig. 1(a)). However, emergy flows constantly through the chain in the same amount and does not be disrupted or disappear at the sink (Fig. 1(b)). This difference makes introduction of the notion of transformity useful to assess the quality of energy. In Fig. 1(c), numbers on the paths represent unit value of emergy. As the energy of feedback flow becomes the smallest (0.1J), transformity indicates the largest value (10,000 sej/J).

Fig. 1 
Concept of Emergy flow: This diagram comprises of four components; a circle stands for a source, a half-circular box is a producer of services, a hexagon is a consumer, and the rest is a sink (adapted from H.T. Odum, 1998).

For BES, emergy value(Em) of a building can be obtained as follows,

Emergy of materials is estimated by multiplication of its quantities and transformity. Accordingly, emergy demanded for manufacturing can be formulated by,

where ρ is unit weight of a material, V is volume, Tr is UEV of a material, and T is life time expectancy of a material. n is number of building components, and m is number of building materials.

To analyze retrofit decisions, a single-story building with common characteristics was attempted. The building under study is a real project pursuing green home certification but not yet built (Project named BK house, 386-8, YoungIn-city, South Korea). In order to provide generalized results, the sample was targeted to be a typical case of single-family house in South Korea. The site area is 474.0 m², and building is oriented to the south. Floor area is 101.9 m² which is approximately the average (102.3 m²) occupied by a four-strong Korean family. The building space is comprised of three bed rooms (9.1~13.2 m²) and a living room (19.3 m²) with a partitioned dining kitchen (19.8 m²). The structure is typical 2×6 light-weight wood frame with batt insulation infill (R-19, R-30). The split gable was adopted for roof (Fig. 2).

Based on the data sampled in the project, the baseline energy and emergy analysis were conducted respectively. For the whole building energy simulation, e-Quest 3.64 was utilized as recommended in ASHRAE Standard 90.1-2007. The space plan and design elements such as window size and location are client-dominated systems, but material properties affecting insulation should be guided.

Therefore, local building code of Energy-saving Design Guideline (Korea National Ministry of Land, 2012) was employed to set the base thermal conductivities such that, grad wall (150 mm) - 0.270W/m²K, roof (250mm) - 0.180W/m2K, ground floor - 0.230W/m²K, and glazing - 2.10W/m2K. Meanwhile, no specific regulation for operation schedule was found for the detached house. Accordingly, typical office daytime use planning was inputted: occupancy-08~19h, cooling- 13~17hour, and heating-09~19h for weekdays. For cooling, Split system single zone DX was implemented. Cooling coil make condensed air expand, and the electric motor supplies the cooled air into each room by a supply fan. On the other hand, thermal zones are heated by radiation produced by hot water coil in a boiler. Korean houses mostly adopt the radiant floor heating system, but US system-based eQuest does not provide the same equipment. Selection of an alternative system functioning similar is controversial thus far. Some argues that it can be modeled with low-temperature ventilator with zonal baseboard modules (Brahme et al., 2009), yet others find a solution in mimicking the system structure¹⁾. However, there is no such a rule set for temperature and specification of baseboard. Therefore, this study found the forced-air hot water heater with no ventilation approximates closest to this case, since radiation boards that hot water circulates work as terminal space heaters in the system, and no ventilation as well adds close analogy to the Korean heating method²⁾.

Each demand of heating and cooling was calculated separately, and combined into total energy loads.

3D modeling in ECOTECT was used for material take-off, and having extracted quantities tabulated. For convenience of accounting, building emergy synthesis can be categorized into five different domains: structure, external skin, interior/ finishes, HVAC/electrical/ plumb system, and fixture/furniture /equipments. Each category is assumed to undergo the similar depreciation process and run out in exact life expectancy. External skin and structure, for instance, were assumed to last 50years, and 25years for interior /finishes. Input quantities multiplied by unit emergy values are summed within each category, and divided by presumed life cycle periods.

Fig. 2 
Building model for the case study (101.9 m², YongIn, South Korea)

Fig. 3 
Plan of the test building

To estimate annual utility use, emergy synthesis is coupled with energy simulation results. The case study house is so simple and following local tradition that the experiment did not take airflow ducts or auxiliary ventilating machinery into account.

For comparative analysis, 11 passive-design items of interest are presented independently as variants, and each has three sub options according to presumed level of energy performance such as Good, Fair, and Baseline. Collection of 22 items is identified in the end for comparative evaluation as indicated in Table 3. As a decision-making level moves from Baseline to “Good”, it ensures the use of high-efficient building assemblies and lower operational energy consumption.

Emergy calculation results obtained from the baseline is tabulated in Table 2. Table-form evaluations are added together to obtain total amount of emergy.

Table 4 summarizes the results of emergy evaluation of building manufacturing on each design item. According to the category to which it belongs, the amounts are scaled by their life time as a denominator. It varies from 5.9 × 10⁴ sej/yr to 1.64 × 10¹⁴ sej/yr.

Combinations of each design item are incorporated into eight assemblages to come up with eight design scenarios as shown in Table 5. Metrics of natural gas use for heating (BTU) and electricity for cooling (kWh) were integrated into kWh for normalization. To convert energy consumption to emergy, UEVs of 4.00×10⁴ sej/J and 2.69×10⁵ sej/J are used for electricity and natural gas respectively. Note that transformity of electricity is higher than natural gas due to secondary processing to generate electricity from non-renewable and renewable resources. That is, using electricity for space conditioning deplete capacity of natural resources more. Considering that electricity is used for space cooling in residential buildings, how to reduce cooling demands can be more critical than heating. Total emergy of baseline is calculated as 9.85 × 10¹⁵ sej/yr. Design scenario 1 reinforced wall insulation, thereby saving operational energy to 9.6%. This, after all, led to the whole emergy improvement of 9.81 × 10¹⁵ sej/yr. Tightening up air leakages, in scenario 2, total emergy was reduced more without having any environmental loads from building manufacturing. However, 16% increment of energy efficiency in scenario 3 though, total emergy increased again to similar state as the baseline. It is due to the fact that energy saving did not compensate emergy rise from new material attachment involving various embodied energy consuming processes. Likewise, even though scenario 8 would be the best option in energy retrofitting of the building, it must not be the best in holistic perspective, since the total emergy value does not show the lowest. Such unbalance between energy driven retrofits and ones associated with BES is illustrated in Fig. 4. As though energy seems constantly decreased all over the scenarios, the emergy value of each alternative goes ups and downs without regard to energy efficiency.

To understand this results, investigation on each design component is inevitable. Table 4 shows total emergy value of components. (1) and (2) refers to level of energy performance. Plus sign means increment of total emergy. The most influential component is glazing type(G) regardless of energy performance level. For level (1), glazing increased emergy to 1.64 × 10¹⁴ sej/yr, and to 1.04 × 10¹⁴ sej/yr for level (2) in comparison with the baseline. Wall insulation(Iw) showed the largest difference between (1) and (2) (1.51 × 10¹⁴ and 0.60 × 10¹⁴ sej/yr). This result clearly demonstrates that energy reduction can benefit from high level of insulation, but over a certain point, the advantage may be directed to degradation of overall environmental soundness due to large increment of indirect energy required to produce high-performance insulation materials (Fig. 5).

Fig. 4 
Comparison of energy/emergy values according to results of design scenario (assemblies)

Fig. 5 
Contribution of each component to total emergy value

It should be noted that change in ceiling height (Ch) is good for both operational energy and emergy. Reducing space volume benefits material flow reduction as well as increasing low-energy potential.

Fig. 5 describes contribution level of each design component in conjunction with total emergy values. The highest contribution level to emergy reducement is Ch for level (1), and the second is roof insulation(Ir) in level (2). Except for Ch, all others have reverse relations or even are irrelevant between energy and emergy. While blind installation (B) has little effect on construction emergy, it significantly affects operational energy use. Comparing wall insulation (Iw) and roof insulation (Ir), material energy inflow from high level of roof insulation is counterbalanced by low operational energy. Accordingly, one may notice that investment of high-performing roof takes advantage in terms of low environmental impact for the residential building.