Selection of Energy Improvement Factors and Economic Analysis of Standard MDU Complexes in Korean Metropolitan Regions

Abstract

In Korea, energy consumption within apartments in metropolitan areas accounted for more than 33% of the total energy consumption by buildings in 2020. In this study, in order to increase the energy efficiency of MDU (multi-dwelling unit) complexes in metropolitan areas, improvement factors and economic effects were analyzed using ECO2, a building energy efficiency evaluation program. Optimal improvement measures are proposed, to reduce the economic burden on users by applying energy saving technologies. This study was conducted in four stages; in the first stage, using ECO2 software, five types of apartments were selected as standards among 46 complexes. Standard MDUs were selected if more than two factors were satisfied from among the following: (1) household type, (2) average exterior wall insulation and window performance, (3) average energy consumption and demand per unit area per year, (4) average applied facility system, and (5) average monthly energy demand per unit area. In the second stage, improvement factors were derived by analyzing the 10 most recent energy efficient MDU complexes. The third stage involved analysis of the energy saving effect generated by the improvement of windows and total heat exchangers in five selected complexes. Primary energy consumption per unit area per year improved from 158.8 to 132. kWh/m2y in complex E, which had been upgraded from ‘floor heating system’ to ‘total heat exchanger’. Finally, in the fourth stage, optimal improvement factors were selected for economic analysis. By simultaneously applying the optimal improvement factors, such as windows and total heat exchanger, to the M complex, primary energy consumption per unit area per year was improved from 147.6 to 111.4 kWh/m2 y. When optimal improvement factors were applied to 59 m2, 74 m2, 84 m2 types in complex M, life cycle cost savings of energy consumption for 30 years became $1384~1970.

Full text

Citation: Lee, K.-W.; Kim, Y.I.
Selection of Energy Improvement
Factors and Economic Analysis of
Standard MDU Complexes in Korean
Metropolitan Regions. Energies 2022,
15, 4042. https://doi.org/10.3390/
en15114042
Academic Editors: Sławomir
Rabczak, Daniel Sły´s and Krzysztof
Nowak
Received: 25 April 2022
Accepted: 28 May 2022
Published: 31 May 2022
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Copyright: © 2022 by the authors.
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energies
Article
Selection of Energy Improvement Factors and Economic
Analysis of Standard MDU Complexes in Korean
Metropolitan Regions
Ki-Won Lee 1and Young Il Kim 2, *
1Department of Architectural Engineering, Graduate School, Seoul National University of Science and
Technology, Seoul 01811, Korea; giwonlee1104@naver.com
2School of Architecture, Seoul National University of Science and Technology, Seoul 01811, Korea
*Correspondence: yikim@seoultech.ac.kr
Abstract:
In Korea, energy consumption within apartments in metropolitan areas accounted for more
than 33% of the total energy consumption by buildings in 2020. In this study, in order to increase
the energy efficiency of MDU (multi-dwelling unit) complexes in metropolitan areas, improvement
factors and economic effects were analyzed using ECO2, a building energy efficiency evaluation
program. Optimal improvement measures are proposed, to reduce the economic burden on users
by applying energy saving technologies. This study was conducted in four stages; in the first stage,
using ECO2 software, five types of apartments were selected as standards among 46 complexes.
Standard MDUs were selected if more than two factors were satisfied from among the following:
(1) household type, (2) average exterior wall insulation and window performance, (3) average energy
consumption and demand per unit area per year, (4) average applied facility system, and (5) average
monthly energy demand per unit area. In the second stage, improvement factors were derived by
analyzing the 10 most recent energy efficient MDU complexes. The third stage involved analysis
of the energy saving effect generated by the improvement of windows and total heat exchangers
in five selected complexes. Primary energy consumption per unit area per year improved from
158.8 to 132. kWh/m
2
y in complex E, which had been upgraded from ‘floor heating system’ to
‘total heat exchanger’. Finally, in the fourth stage, optimal improvement factors were selected for
economic analysis. By simultaneously applying the optimal improvement factors, such as windows
and total heat exchanger, to the M complex, primary energy consumption per unit area per year was
improved from 147.6 to 111.4 kWh/m
2
y. When optimal improvement factors were applied to 59 m
2
,
74 m
2
, 84 m
2
types in complex M, life cycle cost savings of energy consumption for 30 years became
$1384~1970.
Keywords:
MDU (multi-dwelling unit; apartment); building energy assessment program ECO2;
energy demand; energy consumption; primary energy; energy efficiency; lifecycle cost (LCC)
1. Introduction
Given the global shortage of fossil fuel reserves and the emergence of climate change,
increased attention is being devoted to greenhouse gas reduction and energy conserva-
tion [
1
,
2
]. In October 2021, the Republic of Korea deliberated and decided on the ‘2050
Carbon Neutral Scenario Plan’ and the ‘2030 Nationally Determined Contribution (NDC)
Upgrade Plan’ [
3
,
4
]. As shown in Figure 1a, upgraded plans increased by 40% the total
greenhouse gas emission reduction goal, compared with the previously established 2018
target [
3
,
5
]. In particular, the reduction target in the building sector was strengthened by
32.8% compared with the 2018 reduction rate, with the goal of reducing 35.0 million tons of
CO2eq [3].
In order to achieve the CO
2
emission reduction target in the building sector, energy
and eco-friendly buildings are being evaluated worldwide [
6
,
7
]. Each country has de-
Energies 2022,15, 4042. https://doi.org/10.3390/en15114042 https://www.mdpi.com/journal/energies

Energies 2022,15, 4042 2 of 24
veloped and utilized standard indicators and systems for evaluation of building energy
efficiency [8,9].
To achieve the NDC upgrade plan within the building sector, various domestic incen-
tives have been launched, including strengthening the energy efficiency ratings of new
buildings, zero-energy building certification, energy diagnosis of existing buildings, and
green remodeling businesses, and their standards are being upgraded [5,10–12].
Figure 1.
(
a
) 2030 national greenhouse gas reduction goals (NDC) [
3
]; (
b
) Number of buildings
certified as preliminary zero-energy buildings (ZEBs) [13].
Considering zero-energy buildings certified in recent years, there have been sig-
nificantly fewer certification cases for residential buildings compared to nonresidential
buildings, as shown in Figure 1b [13].
Currently, zero-energy building certification is mandatory for new buildings, but it is
difficult to apply this rule to existing buildings [
13
,
14
]. In addition, based on an examination
of building status in the domestic ‘building life history management system’ (Figure 2a),
approximately 59% of buildings are older than 20 years [
5
,
15
,
16
]. With regard to the green
remodeling project, which is being carried out in Korea to improve the energy ratings
of existing buildings, support projects are actively being conducted, but it is difficult to
evoke positive perceptions owing to the economic burden [
17
–
20
]. In particular, in terms of
the energy consumption of buildings, it was observed that MDUs (multi-dwelling units)
accounted for the highest proportion [
20
,
21
]. As shown in Figure 2b, based on energy
consumption by building use in 2020 described in the ‘Green Together-Building Energy
Statistics’, the average building energy consumption for MDUs in metropolitan areas is 33%
or more [
22
]. It was also noted that MDUs accounted for 48% of the total energy consumed
in Gyeonggi-do province [23].
As mentioned previously, policies and R&D for energy saving in buildings are expand-
ing worldwide due to climate change and a decrease in fossil fuel reserves. In particular,
various domestic incentives are being actively carried out to improve the energy use of
buildings in Korea. However, the energy consumption of MDUs in the metropolitan area
accounted for more than 33% of the total energy consumption by buildings in 2020. There-
fore, research is needed to lower the energy consumption of MDUs in the metropolitan area.
As shown in Table 1, this study presents suggestions for the improvement factors for MDU
complexes with high energy consumption in metropolitan areas. To present reasonable
results, standard materials and equipment elements that are currently applied to MDUs
were studied. Futhermore, analysis of the improvement effects and proposals for optimal
improvement factors to reduce the economic burden on users were also studied. Reduction
of primary energy consumption per unit area per year, and life cycle cost savings, were
compared when optimal improvement factors were applied to selected complexes.

Energies 2022,15, 4042 3 of 24
Figure 2.
(
a
) Age status of domestic buildings as of March 2021 [
15
,
16
]; (
b
) Comparison of energy
consumption by region for MDUs and other uses [22].
Table 1. Comparison of literature review.
Reference Contents
[1,2] Shortage of fossil fuel reserves and emergence of climate changes.
[3] 2050 Carbon Neutral Scenario Plan.
[4] 2030 Nationally Determined Contribution (NDC) Upgrade Plan.
Major topic Energy technology to reduce CO2eq in buildings.
[5,12]Reinforcement of energy efficiency ratings focused on new buildings and zero-energy
building certification.
[15,16] Approximately 59% of buildings are older than 20 years.
Major topic Research into energy saving in existing buildings.
[20,21] The energy consumption of MDUs accounts for the highest proportion [20,21].
[22,23] High proportion of energy consumption in the metropolitan area [22,23].
Major topic Improvement of energy consumption in apartment complexes in the metropolitan area.
This work Selection of energy improvement factors and economic analysis of standard MDU
complexes in Korea metropolitan region.
2. Methods
This study was conducted on 46 MDUs that obtained building energy efficiency ratings
and received permits between 2014 and 2015. To suggest elements for improvement in
metropolitan areas, economic analysis was carried out for these MDUs [
23
]. In addition,
10 MDUs that obtained certifications and received building permits in 2021 were analyzed
to select improvement factors with regard to applied materials and facilities [
23
]. As shown
in the research methodology presented in Figure 3, the research process was divided into
four stages; standard MDU selection using ECO2, standard MDU improvement factor
selection, improvement factor application effect analysis and optimal improvement factor
selection, and economic analysis of the optimal plan.

Energies 2022,15, 4042 4 of 24
Figure 3.
Research methodology: selection of optimal improvement factors and economic analysis of
MDUs in metropolitan areas of Korea, using the building energy assessment program ECO2.
3. Selection of Standard MDU Using ECO2
3.1. Definition of ECO2
From 1 September 2013, domestic building energy efficiency rating certifications have
been issued to residential and nonresidential buildings that have been evaluated by the
building energy evaluation program ECO2. [
24
,
25
]. The ECO2 program utilizes weather
data of 13 regions in Korea in accordance with the ‘Rules for Building Energy Efficiency
Rating Certification and Zero Energy Building Certification’. Energy consumption sim-
ulations are carried out for heating, cooling, hot water supply, lighting, ventilation, and
new renewable systems. As a result, energy demand per unit area per year, energy con-
sumption, and primary energy consumption are calculated [
24
–
27
]. ECO2 is a building
energy consumption evaluation program based on the ISO 13790 and DIN 18599 [
25
–
28
].
The ECO2 program is mainly used for building energy evaluation, including zero energy
building evaluation [
29
,
30
]. It is a program similar to Energy Plus, a dynamic analysis
program that is widely used for building energy evaluation [
31
–
33
]. The ECO2 program
was applied to MDUs between 2014 and 2015 [24,25].
3.2. Standard MDU
For selection of standard MDUs, the floor area, exterior wall insulation, window
types, ventilation facilities, new and renewable energy facilities, energy demand, energy
consumption, and primary energy consumption were analyzed for 46 MDU complexes
that obtained building energy efficiency ratings in 2014 and 2015.
3.2.1. Floor Area
The floor areas of 46 complexes were analyzed, to study how area affects facility
capacity, wall insulation, and window size. A total of 73 area types were applied to
46 complexes, with the smallest type being 11 m
2
and largest type 273 m
2
. Figure 4
shows the results of the most frequently applied types across 46 complexes consisting
of 46,398 households in total. After analyzing 46 complexes, the most applied types
were 59 m
2
, 74 m
2
, and 84 m
2
. Among 46 complexes, 19 of them had 59 m
2
, 74 m
2
, and
84 m2types.

Energies 2022,15, 4042 5 of 24
Figure 4. Type distribution of 46,398 households in 46 complexes.
3.2.2. Insulation Performance and Facility Status
First, exterior wall insulation performance was studied to reveal how heating energy
demand, heating energy consumption and primary energy consumption were affected [
34
].
Insulation materials and thermal conductivity applied to the exterior walls were investi-
gated, as shown in Table 2. The most frequently applied exterior wall insulation in the
46 complexes was ‘bead method thermal insulation plate type 2 No. 2’.
Table 2. Results of investigation on external wall insulation in 46 MDU complexes.
Insulation Material Thermal Conductivity
(W/m2K)
Number of Applied
Complexes
Hard urethane type 1 No. 3 0.025 3
Hard urethane type 2. No. 2. 0.023 8
Glass wool thermal insulation plate 24K 0.037 2
Bead method thermal insulation plate type 1 No. 2 0.035 1
Bead method thermal insulation plate type 2 No. 1 0.031 2
Bead method thermal insulation plate type 2 No. 2 0.032 21
Bead method thermal insulation plate type 2 No. 3 0.033 5
Bead method thermal insulation plate type 2 No. 4 0.034 2
Extrusion method thermal insulation plate No. 1 0.024 1
Extrusion method thermal insulation plate special 0.027 1
Then, as a result of examining window insulation performance [
35
,
36
], which affects
insulation performance, heating energy demand, energy consumption and primary energy
consumption of buildings, it was found that ‘air-injected low-e double-glazed windows
(soft coating)’ were applied in 12 complexes. For balcony-extended windows, ‘argon-
injected low-e double glazed (soft coating) + air-injected low-e double glazed (soft coating)’
were applied in 18 complexes.
Third, when analyzing heating systems, cooling systems, ventilation, and new and re-
newable systems for the facility application status analysis, cooling systems were excluded
because these are the occupants’ responsibility. Among 46 MDU complexes surveyed,
no complex had adopted the new and renewable system. As a result of the analysis of
heating system status, it was found that 37 complexes used district heating. In ventilation
system status analysis, 27 complexes used total heat exchangers which have the feature of
ventilation energy recovery when exchanging indoor and outdoor air.

Energies 2022,15, 4042 6 of 24
3.2.3. Annual Energy Demand, Energy Consumption and Primary Energy Consumption
Following ECO2 program analysis, results were obtained for annual energy demand,
energy consumption, primary energy consumption, and CO
2
generation, per unit area per
year. Here, ‘energy demand’ means energy per unit area per year required for heating,
cooling, hot water supply and lighting of a building; ‘energy consumption’ means energy
per unit area per year consumed for heating, cooling, hot water supply, lighting, and venti-
lation [
24
,
37
]; ‘primary energy consumption’ is the standard for evaluating the building
energy efficiency rating certification, referring to energy per unit area per year including
energy conversion efficiency and losses in the fuel supply process [
24
,
38
,
39
]. Figure 5
shows the building energy efficiency rating certification grade, energy demand per unit
area per year, energy consumption per unit area per year, and primary energy consumption
per unit area per year for the 46 MDU complexes analyzed using the ECO2 program.
Figure 5.
Input and output of ECO2 program; (
a
) Analysis results: annual energy demand, energy
consumption, and primary energy consumption (
b
) Input of external wall insulation (
c
) primary
energy consumption per unit area per year by building energy efficiency grade.

Energies 2022,15, 4042 7 of 24
According to the analysis of 46 MDUs, the average energy demand per unit area per
year was 107.5 kWh/m
2
y, and the average energy consumption per unit area per year was
146.5 kWh/m
2
y. In addition, the average primary energy consumption per unit area per
year was 150.7 kWh/m2y.
This corresponded to Grade 2 of the building energy efficiency grading system.
Figure 5c
shows the primary energy consumption per unit area per year for each energy
efficiency grade.
3.2.4. Selection of Standard MDU Complex
Among the 46 MDU complexes, 19 complexes including 59 m
2
, 74 m
2
, and 84 m
2
types
were selected as standard groups due to their frequency of occurrence. Figure 6shows
five factors for selecting standard MDUs. Floor area, exterior wall and window insulation,
energy demand and energy consumption, facility system, and monthly energy demand
were analyzed. Among 19 MDUs, five complexes E, F, J, M, and P satisfied two or more of
the five factors, and were selected as standard MDU complexes for this study.
Figure 6.
The frequency analysis results showing occurrence of two or more of the five types of
1. TYPE, 2. exterior wall insulation, 3. windows, 4. ventilation, 5. energy demand and consumption,
out of 46 MDU complexes.
4. Selection of Improvement Factors
In order to improve the energy efficiency of E, F, J, M, and P complexes, selected as
standard MDUs, we analyzed the technology applied to the 10 latest MDUs that obtained
building energy efficiency Grade 1+ or higher in 2021. Through this analysis, improvement
factors were selected.
4.1. Energy-Saving Factors Applied to the Latest MDU Complexes
4.1.1. Insulation Performance and Facility System Status
Exterior wall insulation, window insulation, heating system, ventilation system, and
new and renewable system application status were analyzed for 10 MDU complexes that
obtained building energy efficiency grades of 1+ or higher. First, according to the analysis
of exterior wall insulation, ‘hard urethane foam type 1 No. 3
0
was applied to 9 complexes.
‘Hard urethane foam type 1 No. 3
0
has a thermal conductivity of 0.025 W/m
·
K. This
thermal conductivity is less by 0.007 W/m
·
K than the 0.032 W/m
·
K of the ‘bead method
warming plate type 2 No. 20, which was the most frequently applied insulation across the
46 complexes [
40
–
43
]. Energy saving effects can be expected after improvement of exterior

Energies 2022,15, 4042 8 of 24
wall insulation. Second, in the case of windows, ‘argon injection-(low-e double-glazed
glass + general double-glazed glass)’ was applied in five complexes. For balcony extension
windows, ‘argon injection-(low-e double glazing + double glazing (soft coating))’ was
applied in seven complexes. Third, heating systems, ventilation systems, and renewable
systems were analyzed. District heating systems were applied in eight complexes. Total
heat exchangers with energy recovery were applied in all 10 complexes. In terms of new
and renewable systems, PV(photo voltaic) was applied in nine complexes.
4.1.2. Energy Demand, Energy Consumption and Primary Energy Consumption Analysis
The 10 selected MDU complexes that acquired certification in 2021 were analyzed
using the ECO2 program. As a result, the average energy demand per unit area per year
was found to be 74.5 kWh/m
2
y. The average energy consumption per unit area per year
was 111.1 kWh/m
2
y. The average primary energy consumption per unit area per year was
102.9 kWh/m
2
y, which corresponded to building energy efficiency Grade 1+. Comparison
of the average monthly heating energy demand per unit area of 10 MDUs certified in 2021
and 46 MDUs certified in 2014–2015 was analyzed, as shown in Table 3. Monthly average
heating energy demand per unit area of MDU certified in 2021 was reduced by 1.7 kWh/m
2
when compared to complexes certified during 2014 and 2015.
Table 3. Comparison of monthly heating energy demand per unit area.
Heating Energy Demand per Unit Area (kWh/m2)
Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Avg.
2014~2015 17.5 13.0 7.9 1.9 0.0 0.0 0.0 0.0 0.0 0.6 7.2 14.0 5.2
2021 12.9 8.3 5.5 0.8 0.0 0.0 0.0 0.0 0.0 0.3 4.2 9.5 3.5
Savings 4.6 4.7 2.4 1.1 0.0 0.0 0.0 0.0 0.0 0.3 2.9 4.5 1.7
4.2. Analysis of Improvement Effect
Energy saving products including exterior wall insulation, windows, mechanical
facility systems, and renewable systems applied to 10 MDUs certified in 2021 as
Grade 1+ or
higher were analyzed. These products were applied to standard models E, F, J, M and P
to study their effects and also for selecting optimal improvement factors. In addition, to
verify the effect of the improvement factors, the results of ECO2 simulation were compared
with the actual measurements.
4.2.1. Effect of Exterior Wall Insulation Improvement
As a result of implementing exterior insulation material ‘hard urethane type 1 No. 3
0
in E, F, J, M, and P MDU complexes, the P complex saw the most significant effect in
reducing energy demand and energy consumption, as shown in Figure 7a. In the case of
the P complex, with ‘bead method warming plate type 2 No. 3
0
, insulation applied in the
past, the overall heat transfer coefficient of the exterior wall was 0.251 W/m
2
K, calculated
as shown in Figure 7b. The overall heat transfer coefficient became 0.194 W/m
2
K, an
improvement of 0.057 W/m
2
K. As a result of implementing higher exterior wall insulation
in the P complex, the annual primary energy consumption reduction per unit area became
3.2 kWh/m2y.
Table 4shows that the average annual primary energy consumption reduction per unit
area for E, F, M, and P was 1.9 kWh/m
2
y. J complex was excluded since in this complex,
‘hard urethane type 2 No. 2’ which has higher performance than ‘hard urethane type 1
No. 30was already applied and thus required no further improvement.

Energies 2022,15, 4042 9 of 24
Figure 7. ECO2 input for P complex. (a) Composition of exterior wall; (b) Exterior wall insulation.

Energies 2022,15, 4042 10 of 24
Table 4.
Changes in energy demand and energy consumption of E, F, J, M, and P MDU complexes
before and after exterior wall improvement.
Improvement
Status MDU
Energy Demand
per Unit Area per Year
(kWh/m2y)
Energy Consumption
per Unit Area per Year
(kWh/m2y)
Primary Energy
Consumption per Unit
Area per Year
(kWh/m2y)
Before
E 118.2 155.2 158.8
F 102.6 158.3 168.3
M 119.3 150.8 147.6
P 96.5 145.5 143.3
Average 109.15 152.45 154.5
After
E 116.5 153.2 157.3
F 100.5 155.4 166.2
M 118.4 149.7 146.8
P 93.2 141.1 140.1
Average 107.2 149.9 152.6
4.2.2. Effect of Window Improvement
For recently built energy-saving MDUs, ‘argon injection low-e double-glazed windows
(soft coating)’ with thermal conductance of 0.717 W/m
2
K were applied. To analyze the
window improvement effect, inputs for the ECO2 program were carried out by dividing
the window category into household windows and balcony extension windows. Figure 8a
shows the ECO2 input conditions for balcony extension windows in the M complex, which
showed the best result. In the case of the balcony extension type windows in the M complex,
‘argon injection low-e double-glazed (soft coating) + air-injected low-e double-glazed (soft
coating)’ of 1.3 W/m
2
K was applied. As shown in Figure 8b, by applying air to one side of
the filler, the thermal conductance of the improved window was reduced by 0.58 W/m2K
compared to the old type [
44
–
46
]. As a result of the ECO2 analysis of E, F, J, M, and P
complexes, the average annual primary energy consumption reduction per unit area per
year after improvement was found to be 8.5 kWh/m2y, as shown in Table 5.
Table 5.
Changes in energy demand and energy consumption of E, F, J, M, and P MDUs before and
after window improvement.
Improvement
Status MDU
Energy Demand
per Unit Area per Year
(kWh/m2y)
Energy Consumption
per Unit Area per Year
(kWh/m2y)
Primary Energy
Consumption per Unit
Area per Year
(kWh/m2y)
Before
E 118.2 155.2 158.8
F 102.6 158.3 168.3
J 92.7 121.9 137.5
M 119.3 150.8 147.6
P 96.5 145.5 143.3
Average 105.9 146.3 151.1
After
E 109.7 145.1 151.3
F 96.2 149.6 161.9
J 84.9 112.3 130.3
M 104.2 132.9 134.4
P 87.8 134.1 134.9
Average 96.6 134.8 142.6

Energies 2022,15, 4042 11 of 24
Figure 8.
(
a
) Window composition before and after improvement in M MDU; (
b
) Window input and
simulation calculation results before and after ECO2 simulation window improvement in M MDU.
4.2.3. Effect of Ventilation System
For the ventilation systems in new energy-saving MDUs, 100 cmh (cubic meter per
hour, m
3
/h) was applied to the 59 m
2
type, and 150 cmh was applied to the 74 m
2
and
84 m
2
types. Heat recovery efficiencies applied to the 59 m
2
type were heating 77% and

Energies 2022,15, 4042 12 of 24
cooling 60%, whereas those applied to the 74 m
2
and 84 m
2
types were heating 72% and
cooling 59%. Figure 9a lists input values for the ECO2 program for analyzing the ventilation
system improvement effect. Input factors were heat recovery rate (efficiency), air volume
flow rate, fan power, and fan pressure loss [
24
]. Figure 9b shows the input conditions of
the E complex which showed the best improvement result. In complex E, the ‘floor heat
supply system’ was applied before the improvement. This is a system that supplies outdoor
air to the inside of the house using floor heat [
47
]. If the floor heat supply system was
replaced by total heat exchanger (ERV) used in new energy-saving MDUs, the primary
energy consumption per unit area per year was reduced by 26.4 kWh/m2y.
Figure 9.
(
a
) Composition of ventilation systems before and after improvement of MDU E; (
b
) Ven-
tilation system input and simulation calculation results before and after improvement of ECO2
simulation MDU E.

Energies 2022,15, 4042 13 of 24
For ventilation systems, total heat exchangers were applied to E, F, and M complexes.
J and P complexes were excluded since they had already applied total heat exchangers
(ERV) [
48
] with higher heat recovery efficiency than the ventilation systems of new energy-
saving MDUs. Before improvement, the E complex applied an ‘underfloor air distribution
system’, the F complex used a total heat exchanger (ERV) with a heat recovery rate of
heating 72% and cooling 45%, and the M complex used a total heat exchanger (ERV) with
air flow rates of 200 cmh and 300 cmh, twice as high as those of ERVs in new energy saving
MDUs. By comparing before and after the improvements using the ECO2 program, the
average primary energy consumption reduction per unit area per year was found to be
18.7 kWh/m2y, as shown in Table 6.
Table 6.
Changes in energy demand and energy consumption of E, F, J, M, and P MDU complexes
before and after ventilation system improvement.
Improvement
Status MDU
Energy Demand
per Unit Area per Year
(kWh/m2y)
Energy Consumption
per Unit Area per Year
(kWh/m2y)
Primary Energy
Consumption per Unit
Area per Year
(kWh/m2y)
Before
E 118.2 155.2 158.8
F 102.6 158.3 168.3
M 119.3 150.8 147.6
Average 113.4 154.8 158.2
After
E 91.8 122.4 132.4
F 101.3 154.6 162.1
M 92.9 119.3 124.0
Average 95.3 132.1 139.5
4.2.4. Effect of New and Renewable System
Among 10 MDU complexes that obtained building energy efficiency Grade 1+ or
higher in 2021, 9 complexes installed solar PV modules on the roof. On the other hand,
new and renewable systems were not applied to all of the standard model E, F, J, M, and
P complexes before the improvement. For analysis of the application of the new and
renewable systems, it was assumed that solar PV modules were installed on the roof by
referring to the application status of 9 new energy-saving MDU complexes [47–49].
The average solar module capacity applied to the new energy saving MDU was a
horizontal type with a nominal power output of 455 W per module. Available area for PV
installation on the roof for each building of E, F, J, M, and P complexes for ECO2 simulation
was calculated under the assumption that no other facilities existed on the roof [47–49].
Among E, F, J, M, and P complexes, the F complex showed best energy performance
when solar PV system was installed, as shown in Figure 10, with primary energy con-
sumption reduction per unit area per year of 2.5 kWh/m
2
y according to ECO2 simulation.
In addition, by adoption of solar PV system the average primary energy consumption
reduction per unit area per year of E, F, J, M, and P complexes was 2.3 kWh/m
2
y, as shown
in Table 7.

Energies 2022,15, 4042 14 of 24
Figure 10.
Input and simulation results of ECO2 before and after application of solar PV renewable
system in MDU F.
Table 7.
Changes in energy demand and energy consumption of E, F, J, M, and P MDU complexes
before and after solar PV system application.
Improvement
Status MDU
Energy Demand
per Unit Area per Year
(kWh/m2y)
Energy Consumption
per Unit Area per Year
(kWh/m2y)
Primary Energy
Consumption per Unit
Area per Year
(kWh/m2y)
Before
E 118.2 155.2 158.8
F 102.6 158.3 168.3
J 92.7 121.9 137.5
M 119.3 150.8 147.6
P 96.5 145.5 143.3
Average 105.9 146.3 151.1
After
E 118.2 155.2 156.6
F 102.6 158.3 165.8
J 92.7 121.9 135.2
M 119.3 150.8 145.2
P 96.5 145.5 141.4
Average 105.9 146.3 148.8
The effect of improving energy demand, energy consumption, and primary energy
consumption by exterior wall-insulation material, window, ventilation system, and solar PV
renewable systems showed a different energy saving effect for each improvement element.

Energies 2022,15, 4042 15 of 24
The factor with the highest reduction in primary energy consumption per unit area per
year was the improvement of the ventilation system.
4.3. Selection and Verification of Optimal Improvement Factors
Figure 11 shows primary energy consumption per area per year after implementing
four factors (exterior wall insulation material, windows, ventilation system, and solar PV
renewable system) to E, F, J, M, and P complexes using the ECO2 program. Among the
four factors, window and ventilation system improvement showed the best energy saving
results. Thus, these two factors were selected as the optimal improvement factors. The
energy saving effect of applying these two improvement factors simultaneously was further
analyzed. In addition, for standard E, F, J, M, and P MDU complexes, it was determined
whether the improvement satisfied 20% improvement of energy consumption, which is
required to satisfy the criteria for Green Building Certification [50–52].
Figure 11.
Reduction in primary energy consumption per unit area per year by four improvement
factors (kWh/m2y).
4.3.1. Improvement Effect by Applying the Optimal Improvement Factor
Figure 12 shows the improvement rate of energy demand and primary energy con-
sumption per unit area per year by application of windows and ventilation facilities, which
were selected as the optimal improvement factors. By applying improved ventilation
systems, energy demand per unit area per year was reduced by 28.8% in the E complex. By
applying improved windows, energy demand per unit area per year was reduced by 14.5%
in the M complex.
When both improved window and ventilation system were simultaneously applied to
E, F, and M MDUs, average primary energy consumption reduction per unit area per year
became 26.6 kWh/m2y, as shown in Table 8.

Energies 2022,15, 4042 16 of 24
Figure 12.
Improvement rate (%) of primary energy consumption per area per year and energy de-
mand per unit area, after simultaneous application of window and ventilation system improvements.
Table 8.
Changes in energy demand, energy consumption and primary energy consumption of E, F,
and M MDU complexes before and after window and ventilation system improvement.
Improvement
Status MDU
Energy Demand
per Unit Area per Year
(kWh/m2y)
Energy Consumption
per Unit Area per Year
(kWh/m2y)
Primary Energy
Consumption per Unit
Area per Year
(kWh/m2y)
Before
E 118.2 155.2 158.8
F 102.6 158.3 168.3
M 119.3 150.8 147.6
Average 113.4 154.8 158.2
After
(Window and
ventilation system)
E 83.8 112.6 125.2
F 94.3 145.3 155.2
M 78.9 102.3 111.4
Average 85.7 120.1 131.6
The reduction rate of primary energy consumption and the change in building energy
efficiency rating after the simultaneous improvement of windows and ventilation systems
can be viewed in Figure 13. The building energy efficiency rating of the E and M complexes
was improved by one grade, and the primary energy consumption per unit area per year
reduction rates were 26.8% for the E complex and 32.5% for the M complex. Complexes M
and E met the condition of improving primary energy consumption by 20% per unit area
per year, which is the standard for green building conversion in the ‘Energy Performance
Improvement Standard for Existing Buildings’ [50].
Figure 13.
The reduction rate of primary energy consumption per unit area per year and changes in
building energy efficiency ratings, due to improvements in windows and ventilation systems.

Energies 2022,15, 4042 17 of 24
4.3.2. Examples of Calibration through Verification
Comparison with the actual measurement results for heating energy consumption
and ventilation energy consumption was carried out, to assess the reliability of the ECO2
simulation results for analyzing the improvement effect of windows and ventilation sys-
tems. Analysis showed these to be the most effective factors. Correction work was done to
improve the reliability of the simulation results [53–56].
First, heating energy consumption before and after improvement were corrected as
shown in Table 9, using ‘Comparison of heating energy consumption and actual usage in
MDU by the Building Energy Efficiency Rating Program’ [55].
Table 9.
Results of applying the heating energy correction factor suggested in ‘Comparison of heating
energy consumption and actual consumption of MDU by the Building Energy Efficiency Rating
Program’ (application of the correction factor for the central region).
Improvement
Status MDU Correction Factor
for Heating
Primary Energy Consumption per Unit Area per Year (kWh/m2y)
Heating Heating Lighting Ventilation Sum
Before
E
0.5
36.15 28.7 41.0 16.9 122.75
F 38.55 26.3 46.8 18.1 129.75
M 35.2 29.0 33.9 14.3 112.40
After
E
0.5
20.75 28.8 41.0 13.9 104.45
F 34.4 26.3 46.8 13.3 120.80
M 17.4 29.0 33.9 13.7 94.00
Second, to improve the reliability of the ECO2 analysis results for the application of
total heat exchangers, the energy consumption was derived by changing the total heat
exchangers applied in the study to natural ventilation. This was done by following the
methodology in ‘Comparative Analysis of Energy Performance by Ventilation Systems in
MDUs’ [
56
]. A correction factor of 4.22% was chosen to correct primary energy consumption
of ventilation [
56
]. Table 10 shows the results of the primary energy consumption per unit
area per year of E, F, M complexes, applying both the first heating energy correction factor
and the second ventilation system correction factor.
Table 10.
Results of the correction of the primary energy consumption of the final E, F, and M
complexes, applying the heating coefficient correction and the ventilation system correction factors.
Improvement
Status MDU
Primary Energy Consumption per
Unit Area per Year (kWh/m2y)
[Table 8]
After Correction
Correction
Factor
Primary Energy Consumption
per Unit Area per Year
(kWh/m2y)
Before
E 122.75
+4.22%
127.93
F 129.75 135.22
M 112.40 117.14
After
E 104.45
+4.22%
108.86
F 120.80 125.90
M 94.00 97.97
Average primary energy consumption per unit area per year of E, F, M complexes
before correction was 131.6 kWh/m
2
y. After applying corrections as suggested in [
55
,
56
],
the value became 110.9 kWh/m
2
y. As the next stage of the study process, economic analysis
was carried out.
5. Economic Analysis
To study the economic burden on users, the effects of windows and ventilation systems,
which were selected as the optimal improvement factors, on annual energy cost and life

Energies 2022,15, 4042 18 of 24
cycle cost were analyzed [
57
–
59
]. For economic analysis of heating and electricity energy
cost, 84 m
2
of complex M, which showed the highest cost reduction after improvement,
was analyzed. For life cycle cost analysis, annuity present value analysis was conducted
using only energy cost [57–59].
5.1. Annual Energy Usage Fee Analysis
The annual energy cost analysis used district heating fuel consumption and electricity
consumption derived from the ECO2 simulation. Primary energy consumption per unit
area per year using district heating can be calculated by Equation (1).
.
Epec,dh =0.728 ·Edh ·1
AMDU
(1)
The variables in Equation (1) are defined as follows:
.
Eped,dh: Annual primary energy consumption per unit area per year (kWh/m2y)
0.728: Primary energy conversion factor for district heating (-)
Edh: Annual district heating energy consumption (kWh/y)
AMDU : Total floor area of the MDU (m2)
Primary energy consumption per unit area per year using electricity can be calculated
by Equation (2).
.
Epec,p=2.75 ·Ep·1
AMDU
(2)
The variables in Equation (2) are defined as follows:
.
Eped,p: Annual primary energy consumption per unit area per year (kWh/m2y)
2.75: Primary energy conversion factor for electricity (-)
Ep: Annual power energy consumption (kWh/y)
AMDU : Total floor area of the MDU (m2)
Energy cost can be calculated with Equations (3) and (4). Cost of district heating
energy can be calculated with Equation (3).
Cdh =0.86 ·0.053 ·
.
Eped,dh Ah1.1 (3)
The variables in Equation (3) are defined as follows:
Cdh: District heating energy cost ($)
0.86: Unit conversion factor (Mcal/kWh)
0.053: Unit cost of district heating energy ($/Mcal)
.
Eped,dh: Annual primary energy consumption per unit area per year (kWh/m2y)
Ah: Floor area of house (m2)
1.1: Basic fee additional factor (-)
Cost of electricity can be calculated with Equation (4).
Cp=0.082 ·
.
Eped,p·Ah+0.758 (4)
The variables in Equation (4) are defined as follows:
Cp: Electricity cost ($)
0.082: Unit cost of electricity ($/kWh)
.
Eped,p: Annual primary energy consumption per unit area per year (kWh/m2y)
Ah: Floor area of house (m2)
0.758: Basic cost additional factor ($)
Energy cost by primary energy consumption per unit area per year before and after
improvement of the M complex was analyzed, as shown in Table 11 [60–62].

Energies 2022,15, 4042 19 of 24
Table 11.
Energy cost of district heating energy and electricity for the M complex before and after
improvement. (Currency: 1$
Energies 2022, 15, x FOR PEER REVIEW 20 of 25
0.053: Unit cost of district heating energy ($/Mcal)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
1.1: Basic fee additional factor (-)
Cost of electricity can be calculated with Equation (4).
𝐶= 0.082 ∙ 𝐸, ∙
𝐴
 + 0.758 (4)
The variables in Equation (4) are defined as follows:
𝐶: Electricity cost ($)
0.082: Unit cost of electricity ($/kWh)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
0.758: Basic cost additional factor ($)
Energy cost by primary energy consumption per unit area per year before and after
improvement of the M complex was analyzed, as shown in Table 11 [60–62].
Table 11. Energy cost of district heating energy and electricity for the M complex before and after
improvement. (Currency: 1$≒1200 KRW, as of 7 January 2022).
Complex Improvement
Status
Corrected Primary Energy Consumption per
Unit Area per Year (kWh/m2y) Type
(m2)
District Heating
Energy Cost
($/m2y)
Electricity
Cost
($/m2y)
Sum
($/m2y)
District Heating Energy Electricity Sum
M
Before 35.4 16.1 51.5
59 104 78.3 182.3
74 130.4 98.1 228.5
84 148.0 111.2 259.2
After 17.2 15.2 32.4
59 50.4 74.0 124.4
74 63.2 92.6 155.9
84 71.8 105.1 176.8
Cost analysis based on primary energy consumption of heating and ventilation was
carried out by referring to the heat cost table of the ‘Korea District Heating Corporation’
and electricity cost calculation standards of the ‘Korea Electric Power Corporation’ [60–
62]. As a result, the 84 m2 type was calculated to return an annual profit of $82.9 if im-
provements were implemented.
5.2. Life Cycle Cost Analysis
Life cycle cost analysis was performed for 30 years of operation with implementation
of energy saving windows and ventilation systems. Future costs were converted to pre-
sent costs using a ‘present value method’ that considers annual cost increase. Annual en-
ergy cost increase of gas and electricity was assumed to be 1.53%, which was derived con-
sidering consumer price increase during 2012–2021 [57–59,63].
Annuity present value coefficient can be derived with Equation (5).
𝐹=(1 + 0.0153)−1
0.0153(1 + 0.0153) (5)
Variables in Equation (5) are defined as follows:
𝐹: Electricity cost ($)
0.0153: Average consumer price inflation rate from 2012 to 2021 (-)
In order to include construction cost to life cycle cost analysis when applying window
and total heat exchanger improvements, product and construction costs of Companies A
and B were collected. Product and installation costs are shown in Table 12.
1200 KRW, as of 7 January 2022).
Complex Improvement
Status
Corrected Primary Energy Consumption
per Unit Area per Year (kWh/m2y) Type
(m2)
District Heating
Energy Cost
($/m2y)
Electricity
Cost
($/m2y)
Sum
($/m2y)
District Heating
Energy Electricity Sum
M
Before 35.4 16.1 51.5
59 104 78.3 182.3
74 130.4 98.1 228.5
84 148.0 111.2 259.2
After 17.2 15.2 32.4
59 50.4 74.0 124.4
74 63.2 92.6 155.9
84 71.8 105.1 176.8
Cost analysis based on primary energy consumption of heating and ventilation was
carried out by referring to the heat cost table of the ‘Korea District Heating Corporation’ and
electricity cost calculation standards of the ‘Korea Electric Power Corporation’ [
60
–
62
]. As
a result, the 84 m
2
type was calculated to return an annual profit of $82.9 if improvements
were implemented.
5.2. Life Cycle Cost Analysis
Life cycle cost analysis was performed for 30 years of operation with implementation
of energy saving windows and ventilation systems. Future costs were converted to present
costs using a ‘present value method’ that considers annual cost increase. Annual energy
cost increase of gas and electricity was assumed to be 1.53%, which was derived considering
consumer price increase during 2012–2021 [57–59,63].
Annuity present value coefficient can be derived with Equation (5).
FC=(1+0.0153)n−1
0.0153(1+0.0153)n(5)
Variables in Equation (5) are defined as follows:
FC: Electricity cost ($)
0.0153: Average consumer price inflation rate from 2012 to 2021 (-)
In order to include construction cost to life cycle cost analysis when applying window
and total heat exchanger improvements, product and construction costs of Companies A
and B were collected. Product and installation costs are shown in Table 12.
Table 12.
Window and total heat exchanger installation and product costs. (Currency: 1$
Energies 2022, 15, x FOR PEER REVIEW 20 of 25
0.053: Unit cost of district heating energy ($/Mcal)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
1.1: Basic fee additional factor (-)
Cost of electricity can be calculated with Equation (4).
𝐶= 0.082 ∙ 𝐸, ∙
𝐴
 + 0.758 (4)
The variables in Equation (4) are defined as follows:
𝐶: Electricity cost ($)
0.082: Unit cost of electricity ($/kWh)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
0.758: Basic cost additional factor ($)
Energy cost by primary energy consumption per unit area per year before and after
improvement of the M complex was analyzed, as shown in Table 11 [60–62].
Table 11. Energy cost of district heating energy and electricity for the M complex before and after
improvement. (Currency: 1$≒1200 KRW, as of 7 January 2022).
Complex Improvement
Status
Corrected Primary Energy Consumption per
Unit Area per Year (kWh/m2y) Type
(m2)
District Heating
Energy Cost
($/m2y)
Electricity
Cost
($/m2y)
Sum
($/m2y)
District Heating Energy Electricity Sum
M
Before 35.4 16.1 51.5
59 104 78.3 182.3
74 130.4 98.1 228.5
84 148.0 111.2 259.2
After 17.2 15.2 32.4
59 50.4 74.0 124.4
74 63.2 92.6 155.9
84 71.8 105.1 176.8
Cost analysis based on primary energy consumption of heating and ventilation was
carried out by referring to the heat cost table of the ‘Korea District Heating Corporation’
and electricity cost calculation standards of the ‘Korea Electric Power Corporation’ [60–
62]. As a result, the 84 m2 type was calculated to return an annual profit of $82.9 if im-
provements were implemented.
5.2. Life Cycle Cost Analysis
Life cycle cost analysis was performed for 30 years of operation with implementation
of energy saving windows and ventilation systems. Future costs were converted to pre-
sent costs using a ‘present value method’ that considers annual cost increase. Annual en-
ergy cost increase of gas and electricity was assumed to be 1.53%, which was derived con-
sidering consumer price increase during 2012–2021 [57–59,63].
Annuity present value coefficient can be derived with Equation (5).
𝐹=(1 + 0.0153)−1
0.0153(1 + 0.0153) (5)
Variables in Equation (5) are defined as follows:
𝐹: Electricity cost ($)
0.0153: Average consumer price inflation rate from 2012 to 2021 (-)
In order to include construction cost to life cycle cost analysis when applying window
and total heat exchanger improvements, product and construction costs of Companies A
and B were collected. Product and installation costs are shown in Table 12.
1200 KRW,
As of 7 January 2022).
Improvement Factors Type, Air Flow Rate Product Price ($) Installation Cost ($) Total Cost ($)
Ventilation
59 m2, 100 cmh 500 2608 3108
74 m2, 150 cmh 583 2781 3364
84 m2, 150 cmh 583 3090 3673
Window
59 m25898 500 6398
74 m27115 583 7698
84 m27068 667 7735
Based on M complex, profits for 30 years of use became $1385, $1736 and $1970
for types 59 m
2
, 74 m
2
, and 84 m
2
, respectively. As a result, it was confirmed that the
construction costs were greater than the profit from energy saving (Table 13) [57–59,64].

Energies 2022,15, 4042 20 of 24
Table 13.
Reflecting an average inflation rate of 1.53% for 2012–2021, life cycle cost analysis using
annuity present value factor for 30 years. (Currency: 1$
Energies 2022, 15, x FOR PEER REVIEW 20 of 25
0.053: Unit cost of district heating energy ($/Mcal)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
1.1: Basic fee additional factor (-)
Cost of electricity can be calculated with Equation (4).
𝐶= 0.082 ∙ 𝐸, ∙
𝐴
 + 0.758 (4)
The variables in Equation (4) are defined as follows:
𝐶: Electricity cost ($)
0.082: Unit cost of electricity ($/kWh)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
0.758: Basic cost additional factor ($)
Energy cost by primary energy consumption per unit area per year before and after
improvement of the M complex was analyzed, as shown in Table 11 [60–62].
Table 11. Energy cost of district heating energy and electricity for the M complex before and after
improvement. (Currency: 1$≒1200 KRW, as of 7 January 2022).
Complex Improvement
Status
Corrected Primary Energy Consumption per
Unit Area per Year (kWh/m2y) Type
(m2)
District Heating
Energy Cost
($/m2y)
Electricity
Cost
($/m2y)
Sum
($/m2y)
District Heating Energy Electricity Sum
M
Before 35.4 16.1 51.5
59 104 78.3 182.3
74 130.4 98.1 228.5
84 148.0 111.2 259.2
After 17.2 15.2 32.4
59 50.4 74.0 124.4
74 63.2 92.6 155.9
84 71.8 105.1 176.8
Cost analysis based on primary energy consumption of heating and ventilation was
carried out by referring to the heat cost table of the ‘Korea District Heating Corporation’
and electricity cost calculation standards of the ‘Korea Electric Power Corporation’ [60–
62]. As a result, the 84 m2 type was calculated to return an annual profit of $82.9 if im-
provements were implemented.
5.2. Life Cycle Cost Analysis
Life cycle cost analysis was performed for 30 years of operation with implementation
of energy saving windows and ventilation systems. Future costs were converted to pre-
sent costs using a ‘present value method’ that considers annual cost increase. Annual en-
ergy cost increase of gas and electricity was assumed to be 1.53%, which was derived con-
sidering consumer price increase during 2012–2021 [57–59,63].
Annuity present value coefficient can be derived with Equation (5).
𝐹=(1 + 0.0153)−1
0.0153(1 + 0.0153) (5)
Variables in Equation (5) are defined as follows:
𝐹: Electricity cost ($)
0.0153: Average consumer price inflation rate from 2012 to 2021 (-)
In order to include construction cost to life cycle cost analysis when applying window
and total heat exchanger improvements, product and construction costs of Companies A
and B were collected. Product and installation costs are shown in Table 12.
1200 KRW, as of 7 January 2022).
Type Annuity Present Value Factor Annual Profit with
Improvement ($)
Profit ($)
(30 Years of Use)
59 m2
23.914
57.9 1385
74 m272.6 1736
84 m282.4 1970
Table 14 shows the results of life cycle cost analysis reflecting the sharply increased
inflation rate of 4.8% as of April 2022 [
65
]. As shown in Table 14, the energy saving benefits
for 59 m2, 74 m2, and 84 m2were $911, $1142, and $1296, respectively.
Table 14.
Reflecting the inflation rate of 4.8% as of April 2022, life cycle cost analysis using annuity
present value factor for 30 years. (Currency: 1$
Energies 2022, 15, x FOR PEER REVIEW 20 of 25
0.053: Unit cost of district heating energy ($/Mcal)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
1.1: Basic fee additional factor (-)
Cost of electricity can be calculated with Equation (4).
𝐶= 0.082 ∙ 𝐸, ∙
𝐴
 + 0.758 (4)
The variables in Equation (4) are defined as follows:
𝐶: Electricity cost ($)
0.082: Unit cost of electricity ($/kWh)
𝐸,: Annual primary energy consumption per unit area per year (kWh/m2y)
𝐴: Floor area of house (m2)
0.758: Basic cost additional factor ($)
Energy cost by primary energy consumption per unit area per year before and after
improvement of the M complex was analyzed, as shown in Table 11 [60–62].
Table 11. Energy cost of district heating energy and electricity for the M complex before and after
improvement. (Currency: 1$≒1200 KRW, as of 7 January 2022).
Complex Improvement
Status
Corrected Primary Energy Consumption per
Unit Area per Year (kWh/m2y) Type
(m2)
District Heating
Energy Cost
($/m2y)
Electricity
Cost
($/m2y)
Sum
($/m2y)
District Heating Energy Electricity Sum
M
Before 35.4 16.1 51.5
59 104 78.3 182.3
74 130.4 98.1 228.5
84 148.0 111.2 259.2
After 17.2 15.2 32.4
59 50.4 74.0 124.4
74 63.2 92.6 155.9
84 71.8 105.1 176.8
Cost analysis based on primary energy consumption of heating and ventilation was
carried out by referring to the heat cost table of the ‘Korea District Heating Corporation’
and electricity cost calculation standards of the ‘Korea Electric Power Corporation’ [60–
62]. As a result, the 84 m2 type was calculated to return an annual profit of $82.9 if im-
provements were implemented.
5.2. Life Cycle Cost Analysis
Life cycle cost analysis was performed for 30 years of operation with implementation
of energy saving windows and ventilation systems. Future costs were converted to pre-
sent costs using a ‘present value method’ that considers annual cost increase. Annual en-
ergy cost increase of gas and electricity was assumed to be 1.53%, which was derived con-
sidering consumer price increase during 2012–2021 [57–59,63].
Annuity present value coefficient can be derived with Equation (5).
𝐹=(1 + 0.0153)−1
0.0153(1 + 0.0153) (5)
Variables in Equation (5) are defined as follows:
𝐹: Electricity cost ($)
0.0153: Average consumer price inflation rate from 2012 to 2021 (-)
In order to include construction cost to life cycle cost analysis when applying window
and total heat exchanger improvements, product and construction costs of Companies A
and B were collected. Product and installation costs are shown in Table 12.
1200 KRW, as of 7 January 2022).
Type Annuity Present Value
Factor
Annual Profit with
Improvement ($)
Profit ($)
(30 Years of Use)
59 m2
15.729
57.9 911
74 m272.6 1142
84 m282.4 1296
6. Discussion
In this study, energy improvement factors and economic feasibility were analyzed
for MDUs in the metropolitan areas of Korea, which account for 33% of the total energy
consumption by buildings in 2020. This study was conducted on 46 MDU complexes that
have obtained domestic building energy efficiency rating certification. Four factors were
used to derive a standard apartment complex: (1) applied household type, (2) average
exterior wall insulation and window performance, (3) average energy consumption and
demand per unit area per year, (4) average applied facility system, and (5) average monthly
energy demand per unit area. In the case of deriving improvement factors, 10 MDU
complexes that recently acquired a building energy efficiency rating above Grade 1+ were
targeted. Primary energy consumption comparison analysis and economic feasibility
analysis were conducted by applying the improvement factors to the standard MDU
complexes using the ECO2 program.
This study is a case study applied to domestic MDU complexes, and is limited in the
extent to which it can reflect various renewable energy systems [
66
–
68
]. Using present
value method economic analysis, this study has found a limit to the profit obtained only
from energy saving when compared to construction costs [
67
,
68
]. Therefore, future studies
are necessary to analyse diverse renewable energy systems, and it is important that their
calculations include change in asset values after improvements are made.
7. Conclusions
In this study, analysis was conducted by using ECO2, a building energy efficiency
rating program, to present improvement factors for MDU complexes with high energy
consumption in the metropolitan areas of Korea, to reduce the economic burden on users.
Factor effects and energy costs were analyzed; in particular, standard complexes were
identified, by analyzing buildings with building energy efficiency ratings from 2014 to
2015, and energy improvement factors were derived by analyzing buildings that obtained
building energy efficiency ratings of Grade 1+ or higher in 2021 after the strengthening of
domestic building energy standards. By applying the improvement factors to the standard
complex, the energy cost was derived from the primary energy consumption before and

Energies 2022,15, 4042 21 of 24
after the improvement of the ECO2 program results. Life cycle cost analysis was also
carried out, using the present value method.
(1)
As a result of analyzing 46 MDU complexes that obtained building energy efficiency
rating certification from 2014 to 2015, the average energy demand per unit area per
year was found to be 107.5 kWh/m
2
, the average energy consumption per unit area
per year was 146.5 kWh/m
2
, the average primary energy consumption per unit area
per year was 150.7 kWh/m
2
. The building energy efficiency rating was analyzed to
correspond to an average of Grade 2.
(2)
The monthly heating energy requirement per unit area was analyzed, revealing that
the monthly average heating energy demand of new energy saving MDUs in 2021
was reduced by 1.7 kWh/m2y compared to MDUs certified in 2014–2015.
(3)
As a result of analyzing the primary energy consumption reduction rate due to the
simultaneous improvement of windows and ventilation systems, the building energy
efficiency ratings of E and M complexes were improved by one grade. The reduction
rate of primary energy consumption was analyzed as 26.8% for E complex and 32.5%
for M complex.
(4)
As a result of life cycle cost analysis using the present value method for 30 years, it
was confirmed that a profit of $1384.6~$1970.5 was acquired for M complex, which
had undergone simultaneous improvement of the windows and ventilation system.
In this study, life cycle cost analysis for energy consumption was presented; for future
research, we plan to include demolition costs as well as product and construction costs.
Furthermore, to improve life cycle analysis, we plan to continue research on increases in
asset costs if improvements are implemented.
Author Contributions:
Conceptualization, K.-W.L.; methodology, K.-W.L.; experiment, K.-W.L.;
software, K.-W.L.; validation, Y.I.K.; formal analysis, K.-W.L.; investigation, K.-W.L.; resources,
K.-W.L.
; data curation, K.-W.L.; writing—original draft preparation, K.-W.L.; writing—review and
editing, Y.I.K.; visualization, K.-W.L.; supervision, Y.I.K.; project administration, Y.I.K.; funding
acquisition, K.-W.L. All authors have read and agreed to the published version of the manuscript.
Funding:
This study was supported by the Research Program funded by the SeoulTech (Seoul
National University of Science and Technology).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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