Energy Audit of Two Multifamily Buildings and Economic Evaluation of Possible Improvements

Abstract

The energy use of buildings is gradually increasing, which is due to economic growth and an increase in population. Several studies have indicated that the implementation of energy-saving measures (ESMs) such as thermal insulation results in more energy saving; however, most ESMs are not economically viable. This chapter outlines ESMs using the IDA ICE computer software. The evaluation of the energy performance of two multifamily buildings is conducted, and possible ESMs are suggested such as thermal insulation, changing windows, installing a new air handling unit, installing a heat exchanger in showers, improving thermal bridges, replacing lighting bulbs, increasing external insulation plus temperature reduction, and changing schedules for air discharge control. The economic feasibility of these suggestions is assessed using the life cycle cost analysis to determine their economic viability. This involves the determination of the life cycle cost and life cycle cost saving to decide the best option. The most important factor in determining life cycle cost saving is the modified uniform present value. The addition of the attic insulation, installing a heat exchanger in showers, replacing lighting bulbs, and changing schedules meet the economic requirement within a feasible time frame.

1. Introduction

Energy plays an important role in economic growth and other daily humans’ activities. Global energy use is mainly supplied from fossil fuels such as oil, gas, and coal. This accounts for 80% of the total energy use worldwide. The share of fossil fuels includes 33% of crude oil, 27% of coal, 22% of natural gas, and 18% of other sources [1]. Fossil fuels are not useful energy resources owing to their limited availability and impact on climate. In recent years, energy prices are increasing. This is due to the gradual increase in economic and population growth. To reduce this impact, the United Nations has considered essential measures through Paris Agreement to combat climate change. The central aim of the agreement is to strengthen the global response to climate change by keeping the temperature rise below 2°C [2].

Global energy use is mainly divided into three sectors which include industry, transportation, and residential and service buildings. Both the industry and building sectors show a gradual increase when compared with transport [3]. The increased environmental awareness and energy analysis of buildings are the tools that would drive the design of buildings with low environmental impact and energy use.

Buildings are made of enclosure that separates the internal environment from the external. Energy is used for lighting, cooking, running appliances, thermal comfort, and many other applications in buildings. Energy use in the building sector is rapidly growing. This may cause a serious environmental problem [4] in Sweden and a challenge for the European Union’s (EU) directions. The energy used by buildings is approximately 40% of the total energy in the EU [5]. To reduce this, the EU proposed a directive on the energy performance of buildings, and this was implemented in 2006. The main purpose of the directive is to improve the total energy efficiency of buildings. This includes new and existing buildings.

In the EU, energy use of buildings is becoming the fastest-growing sector. Energy is needed for various purposes which include thermal comfort, lighting, cooking, etc. The need for energy saving is of great significance especially considering the fluctuation in energy prices, and the population and economic growth [6]. In this study, Doukas et al. evaluated the decision-making process for selecting energy-saving measures (ESMs). The systematic approach was integrated based on key areas of energy management systems of buildings such as load, demand, and user requirements.

An energy investigation was carried out on a multifamily building in Sweden [7]. There a simulation of the building was conducted by using IDA ICE. The results included various ESMs and analysis of the individual measure elaborated that the building had the potential to reduce the energy by 50%. This would further reduce CO₂ emissions by more than 43.3%.

Air leakages through building elements can result in a change in temperature [8]. The research performed in [8] presented a critical review of the use of the infrared thermography (IRT) survey in the building energy audit. IRT identifies leakages and thermal bridges. It was indicated that after identifying the leakages when used together with the blower door method in a building and then applying retrofitting measures, they would result in substantial energy reduction for the building.

An energy audit conducted in [9] using IDA ICE demonstrated that lowering indoor temperature could provide advantages when reducing energy use. However, lowering the indoor temperature should be combined with the insulation of the external wall. This retrofitting as a package could achieve a 53.3% reduction in the total energy delivered.

Studies [10, 11] reported that one of the biggest wastes of energy was caused by inefficient lighting. Moreover, lighting accounts for a great part of the total energy use in buildings. Using energy-efficient lights with demand and proper daylighting controls could help reduce electrical demand. This would contribute to visual comfort and green building development. Furthermore, it was illustrated that LED lighting systems reduced total power use by up to 21.9% [10]. However, in most apartments, human behavior and switching on/off depending on the need played an important role in selecting a more efficient light.

The energy audit carried out on a Swedish multifamily building using IDA ICE presented a change in the overall heat energy demand for ventilation when demand control was used [9]. This resulted in an approximately 50% reduction in the annual heating demand. Several studies [12, 13] suggested that when a heat recovery system was utilized in buildings, it was done by the air handling unit (AHU) which recovered heat from the exhaust air.

The selection of ESMs depends on the capital investment and benefit achieved after the implementation of ESMs. Various economic analysis methods can be utilized to evaluate the economic viability of each ESM [8]. However, there are various ways to analyze the economic feasibility of ESMs generated from an energy audit. Several studies [7, 8] employed life cycle cost to assess the profitability of ESMs. Life cycle cost consists of investment, energy, and maintenance costs. According to [7], when assessing which ESM is the most profitable, the outcomes are achieved when the life cycle cost is lower than the life cycle saving.

It is also important to link ESMs to environmental urban transition and urban building energy conservation [14] since reducing energy usage in buildings in urban area contributes greatly to a sustainable and environmental urban transition.

This chapter focuses on the energy audit of two multifamily buildings owned by the company Älvkarlebyhus AB located in Skutskär, Sweden. Also, some ESMs are proposed for these buildings such as thermal insulation, changing windows, installing a new AHU, installing a heat exchanger in showers, improving thermal bridges, replacing lighting bulbs, increasing external insulation plus temperature reduction, and changing schedules for air discharge control. These ESMs have also been promoted in research [15, 16, 17]. The life cycle costs of these proposed ESMs are analyzed and discussed.

2. Materials and methods

The following approach was adopted in this research. Firstly, the research involved ventilation measurements and data collection of the actual building. This includes the design plans of the buildings, site, materials, ventilation systems, energy use for hot water, and energy bills. This was further used as the input in IDA ICE for the simulation and validation of the base models. Lastly, an IR-thermal camera was utilized to detect leakages.

2.1 Field study objects

The study was conducted on two buildings owned by the housing company named Älvkarlebyhus AB located in Skutskär, Sweden. The company is publicly owned. The company is taking ESMs to decrease the energy use in these two multifamily buildings. The buildings are Centralgatan 14 and Tebogatan 5 which were constructed in 1968. Centralgatan 14 is both a residential and service building. It has five business shops on the first floor and sixteen apartments on the others. While Tebogatan 5 is a residential building with eighteen apartments. Heat is supplied by the district heating (DH) company Bionär AB, while electricity is supplied by Vattenfall AB. Centralgatan 14, Tebogatan 5, and Tebogatan 6 share one central heating system. The total heating energy demand for all the buildings is 710.082 MWh/year obtained from the invoices. According to the financial accountant of the company, the energy demand for each building is obtained from the ratio 2:2:1, respectively, as recommended by the DH company. The buildings of this study are displayed in Figures 1 and 2.

Figure 1.

Studied building, Tebogatan 5.

Figure 2.

Studied building, Centralgatan 14.

2.2 Field measurements

The measurements were done in Centralgatan 14 and Tebogatan 5. Ventilation flow rates were measured using electronic instruments. Two sets of instruments were utilized which were the hot wire anemometer (VelociCalc Plus 8386, TSI incorporated Ltd.) and hood (Testo 420, Swena flow air hood). The airflow hood is an electronic air instrument that was used for air volume measurement passing through the mechanically ventilated duct (uncertainty ±6% l/s) [7]. The hot wire anemometer measured the speed per second (uncertainty ±0.1 m/s). Figure 3 illustrates the measuring instruments employed for the field measurements.

Figure 3.

Ventilation measuring devices; thermo-anemometer (TSI) on the left side and airflow hood on the right side.

2.3 Inspection of thermal bridges and leakages

To identify the thermal bridges and leakages in the buildings, an IR camera (Thermal CAM S60, FLIR Systems) was used. This is because the air infiltration in buildings cannot be seen by visual inspection in most cases. Therefore, special equipment is needed to detect them. The IR-thermal camera is a fast and reliable tool to identify leakages in buildings [15]. The inspection of thermal bridges and leakages was conducted which demonstrated that the leakages were distributed on the buildings’ envelopes exposed to the external environment. Figures 4 and 5 indicate examples of IR thermography pictures collected during the inspection.

Figure 4.

IR-thermography picture showing thermal bridges around the window in Centralgatan 14.

Figure 5.

IR-thermography picture showing thermal bridges around the entrance of Centralgatan 14.

2.4 IDA ICE simulation models

The models for Centralgatan 14 and Tebogatan 5 were built in the IDA ICE simulation software. IDA ICE is an important computer tool for the simulation and optimization of buildings’ energy use [9]. The process of modeling involved importing the architectural drawings of each level of the buildings. Then, the zones representing each apartment within the buildings were created. The required information about the buildings was taken from the drawings. This included the dimension of the buildings, the size of the windows, the height of the buildings, and other required inputs. Söderhamn’s (Sweden) weather file was utilized in the software. Then, the boundary conditions which were needed to be adopted in the software were building materials, lights and equipment, air leakage areas, ventilations, occupants of the buildings, weather data, indoor and outdoor temperatures, and the ventilation system and the temperature in corridors. Figures 6 and 7 display the base models created in IDA ICE for Tebogatan 5 and Centralgatan 14, respectively.

Figure 6.

IDA ICE model created for Tebogatan 5.

Figure 7.

IDA ICE model created for Centralgatan 14.

2.5 Parameters of buildings materials

Table 1 lists the materials that were used as the input to the models of Tebogatan 5 and Centralgatan 14. Apart from the wall surface, windows consisted of wood, covering the areas between windows and the surrounding surface.

Structural elements	Material	Thickness (mm)	U-value (W/m^2·K)
External walls	Gypsum	9.5	0.33
	Concrete	200
	Mineral wool	150
Roof	Lightweight concrete	150	0.18
	Attic insulation	180
	Air gap	200
	Wood	20
Floor	Concrete	150	3.17
	Floor coating	5
Internal wall	Lightweight concrete	200	0.85
External wall for basement	Concrete	200	0.33
	Insulation	100

Table 1.

Materials of buildings.

2.6 Other input parameters

Other input parameters considered for the models are explained here.

2.6.1 Ventilation system

The base model was developed using a mechanical exhaust ventilation system for Tebogatan 5. Centralgatan 14 has both a supply and exhaust unit with a heat exchanger for the shops and a mechanical exhaust ventilation unit for the apartments. In the exhaust type of ventilation system, no supply is required. In this system, the air was entering through adjustable slots located on top of the windows.

2.6.2 Temperature and air infiltration of buildings

The room temperature of the models was set at 20°C. The air infiltration rate was taken as 0.36 l/(s.m²) at 50 Pa.

2.6.3 Ground properties

The ground specification was modeled using ISO standard 13,370. The basement floor was taken as 200 mm concrete, and the ground layer outside the basement was assumed by default values having a ground layer of 0.1 m.

2.6.4 Internal gains and masses

The occupancy, lighting system, and equipment are among the sources of internal gain in a building. The activity levels in the created zones were assumed as 1MET, and the number of people in each apartment was determined using SVEBY standard: 1rk = 1.42, 2rk = 1.63, 3rk = 2.18, 4rk = 2.79, and 5rk = 3.51. Therefore, it involved counting the number of bedrooms creating a zone, and depicting the number from the standard to set it as the input to the model. The occupancy was assumed to use the apartments 14 hours/day. The lighting and equipment used electricity, and 70% of that energy was converted to heat. In addition, occupants contributed to the internal heat gain.

The portion of stairs and the internal mass was assumed as an area of 2.72 m² per stair height with the heat transfer coefficient of 1.7 W/m·K of concrete.

2.6.5 Heating and cooling systems

Each apartment was modeled using the existing ideal heaters in the modeling software, and no cooling system was present. The heating system was provided by DH and supplied to the buildings.

2.6.6 Assumptions

• The hot water consumption was assumed as 25 kWh/m² floor area [18].

• The electricity used in the apartment was also taken as 30 kWh/m² floor area [18].

• 4 kWh/m² year was added as an extra heat loss due to the opening of windows [18].

• From the produced heat, 25% was from lighting and 70% was from the equipment. These were derived from the Swedish household electricity use [3] assuming that the operating loads related to the periods were neglected when occupants were not in the buildings.

• The reduction of solar radiation due to the internal shading was modeled by reducing the g-value of the windows by 70% [18].

• The thermal bridges and the DH losses were considered typical.

• The U-value for the windows was assumed as 1.9 W/m²·K with triple pane glazing, solar heat gain value of 0.68, and solar transmittance value of 0.6.

2.7 Economic analysis

After proposing ESMs, an economic evaluation was carried out on the measures taken. This was performed to ensure that savings were at least greater than the investments. Life cycle costs were determined using the equation of the net present value (NPV), Eq. (1). NPV is generally used to calculate the cost when it is evenly distributed during n years. It takes into account the parameter r as the interest rate and p as the estimated rate of the increase in energy prices. Therefore, the net interest rate between the real interest rate and real energy price, f, is expressed as in Eq. (2) which is utilized in NPV, Eq. (1) [7]:

$$\mathit{NPV}=\frac{{\left(1+f\right)}^n-1}{f{\left(1+f\right)}^n}$$E1

$$f=\frac{r-p}{1+p}$$E2

3. Results and discussion

The results obtained from the analysis are represented and discussed in this section.

3.1 Validity of base models

Based on the simulation results, the total heating loads for Tebogatan 5 and Centralgatan 14 were 263.2 MWh/year and 301.2 MWh/year, respectively. The results from the models are summarized in Table 2. They were compared with the real value of the total consumption of 284 MWh/year for heating and domestic hot water of both buildings. The reference value for the heating demand was obtained from the calculations (Table 2). This consisted of domestic hot water and the energy used for zone heating. The table also gives the total electricity demand in a year for both buildings. As can be seen from the table, the percentages of the errors between the reference values of the real buildings and the simulation results were acceptable which validated the modeling results with good accuracy.

Building	Energy source	Reference value (MWh/year)	Simulation result (MWh/year)	Error (%)
Tebogatan 5	DH	284	263.2	−7.9
	Electricity	22.9	23.5	+2.6
Centralgatan 14	DH	284	301.2	+5.7
	Electricity	32.9	31	−5.8

Table 2.

Comparison of base models with reference values.

3.2 Energy balance

Energy balance is one of the most important parts of the verified base models. It allows identifying the influence of each part of the buildings contributing to high losses to the buildings. It is also the key factor in implementing retrofitting measures. Figures 8 and 9 display the energy balances of the base models. They allow for the identification of the specific areas required to reduce heat losses. They illustrate that higher losses were caused by the buildings’ envelopes. In addition, the heat was lost through the air created by the ventilation system. It was needed to improve the insulation of the buildings’ envelopes to reduce the high energy losses.

Figure 8.

Energy balance for Tebogatan 5.

Figure 9.

Energy balance for Centralgatan 14.

3.3 Transmission loses

Heat losses are transferred from buildings through different parts. Figures 10 and 11 illustrate huge transmission losses through the buildings’ envelopes, especially through the walls, windows, and roof. Thermal bridges such as balconies slabs also contributed to the additional heat losses. From the energy balance of both buildings, it was observed that the largest share of heat losses was due to the buildings’ envelopes and thermal bridges. Apart from the transmission losses taking up the largest heat losses, some additional heat losses were contributed by the air infiltration of the buildings. Identifying the air infiltration of the buildings required the combination of the IR-thermal camera and blower door method. In the study, as observed on the IR-thermal camera, the result demonstrated that thermal bridges were concentrated on the joints formed by the external walls and doors. Implementing retrofits is an effective solution that can promote energy saving. Therefore, various ESMs were needed to reduce heat transmission losses through the buildings and to optimize energy use.

Figure 10.

Transmission losses for Tebogatan 5.

Figure 11.

Transmission losses for Centralgatan 14.

3.4 Energy-saving measures (ESMs)

The energy performance was evaluated to improve the overall energy efficiency of the buildings. Several ESMs were proposed to optimize the energy use of both buildings. They included thermal insulation (increasing both external wall and attic insulations), changing windows, installing a new AHU, installing a heat exchanger in showers, improving thermal bridges, replacing lighting bulbs, increasing external insulation plus temperature reduction, and changing schedules for air discharge control.

3.4.1 Scenarios A₁ and A₂: thermal insulation

The addition of external insulation to buildings results in high energy savings. It reduces the flow of heat out of buildings and promotes energy saving of buildings [6]. Thermal insulation was added to the external walls and roof to provide an effective ESM that reduced the overall heat loss of the buildings [19]. On both buildings’ models, 200 mm mineral wool insulation (0.036 W/m·K) was added to the external surface. The results indicated a decrease in the energy use for heating in both buildings. The net saving of energy by adding 200 mm of external insulation was 10% for both Centralgatan 14 and Tebogatan 5. To reduce the heat loss through the roof, attic insulation was utilized with a thermal conductivity of 0.036 W/m.K and a thickness of 200 mm. The net energy saving for the heating energy demand was achieved at 6% for Centralgatan 14 and 7% for Tebogatan 5. These energy savings are listed in Table 3.

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
External insulation	10	10
Attic insulation	6	7

Table 3.

Energy saving when thermal insulation was implemented.

3.4.2 Scenario B: changing windows

The windows were changed by replacing them with a lower U-value (U = 0.85 W/m²·K). The energy saving for the heating energy demand after replacing the windows was 10% for Centralgatan 14 and 7% for Tebogatan 5, as presented in Table 4. The net losses through the windows were reduced.

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
Replacing windows with U = 0.85 W/m^2·K	10	7

Table 4.

Energy saving when windows with U = 0.85 W/m²·K were erected.

3.4.3 Scenario C: installing a new air handling unit

Various control systems are employed in the AHU. The base model utilized constant air volume (CAV) on the mechanical exhaust system. This system requires high flow rates and higher energy for heating. This system was replaced with a standard AHU having variable air volume (VAV) with temperature control and a heat recovery exchanger with an efficiency of 85%. This is more appropriate for achieving good thermal comfort as well as reducing energy usage in buildings. The net energy savings of heating energy demand was 30% for Centralgatan 14 and 34% for Tebogatan 5 when compared with the base models. Table 5 summarizes the percentages of energy saving by installing a new AHU.

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
Installing a new AHU	30	34

Table 5.

Energy saving by installing a new AHU.

3.4.4 Scenario D: installing a heat exchanger in showers

The installation of a heat exchanger in the showers resulted in reduced hot water use by 20%. When this was implemented, the heating energy demand was reduced and led to a net saving of 5% for Centralgatan 14 and 6% for Tebogatan 5 (Table 6).

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
Installing a heat exchanger in showers	5	6

Table 6.

Energy saving by installing a heat exchanger in showers.

3.4.5 Scenario E: improving thermal bridges

The presence of thermal bridges in buildings’ envelopes affects the energy use and thermal comfort of occupants. To reduce the typical leakages displayed in Figures 4 and 5, it was required to change external windows and doors to the ones with low thermal bridges. Thermal bridges are parts of the buildings’ envelopes that have a major effect on thermal performance [20]. When the thermal bridges were improved, the energy demand for the zone heating was reduced with a net saving of 5% for Centralgatan 14 and 3% for Tebogatan 5 compared with the base models, as indicated in Table 7. The comfort of the buildings remained the same as the base models.

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
Improving thermal bridges	5	3

Table 7.

Energy saving by improving thermal bridges.

3.4.6 Scenario F: replacing lighting bulbs

The lighting bulbs utilized in the buildings were fluorescent and candescent based on the site visits. The average power rating for the lamps is 60 W in the apartments. To improve the efficiency of the buildings’ lighting systems, the lighting bulbs should be upgraded to energy-saving bulbs. When this was replaced by 20 W LED lighting with the same luminous flux, it would lead to an electrical energy saving of 21.3% for Tebogatan 5 and 24.2% for Centralgatan 14 (Table 8). Using LED bulbs resulted in a small increase in the heat demand for both buildings. However, the amount of energy savings obtained from implementing this ESM was higher when compared with the heat demand.

ESMs	Electricity energy saving (%)
	Centralgatan 14	Tebogatan 5
Replacing lighting bulbs (LED)	24.2	21.3

Table 8.

Energy saving by replacing lighting bulbs.

3.4.7 Scenario G: increasing external insulation plus temperature reduction

The indoor temperature was lowered by 2–18°C. Maintaining the energy balance for the buildings needed increasing the external insulation to 200 mm. The total energy saving was 20% for Centralgatan 14 and 21% for Tebogatan 5 compared with the base models. This energy saving is reported in Table 9. The change was the result of reduced heating value for the zones, while the domestic hot water remained the same. Though, this ESM had a high impact on thermal comfort.

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
Increasing external insulation plus temperature reduction	20	21

Table 9.

Net energy saving by external insulation plus temperature reduction.

3.4.8 Scenario H: changing schedules for air discharge control

The schedule for the air discharge control is different from VAV because it works on the demand control principle. Air is extracted depending on the demand inside the buildings. The schedule was changed during the period when the demand was low. The supply of heating energy demand was reduced because of the reduced heat generated. Therefore, running the ventilation system when it is not required, results in high energy use. When the schedules were integrated into the air discharge control, it led to a heat energy saving of 14% for Centralgatan 14 and 12% for Tebogatan 5. These percentages of energy saving are provided in Table 10.

ESMs	DH energy saving (%)
	Centralgatan 14	Tebogatan 5
Changing schedules for air discharge control	14	12

Table 10.

Energy saving by changing schedules for air discharge control.

3.5 Summary of ESMs and possible outcomes

Table 11 summarizes the energy savings when the mentioned ESMs were implemented.

Analysis scenarios	ESMs	Energy savings (MWh/year)
		Centralgatan 14	Tebogatan 5
A1	Adding 200 mm external insulation	28.3	21.4
A2	Adding 200 mm attic insulation	16.19	14.5
B	Replacing windows with less U value (U = 0.85 W/m^2·K)	26.8	16.42
C	Installing a new AHU (with VAV plus temperature control)	82.7	74.6
D	Installing a heat exchanger in showers (reduced hot water)	15.2	13.06
E	Improving thermal bridges	12.99	6.77
F	Replacing lighting bulbs	7.51	5
G	Increasing external insulation plus temperature reduction	55.84	46.3
H	Changing schedules for AHU control	40.0	26.1

Table 11.

ESMs and possible energy savings.

3.6 Thermal comfort for base models

IDA ICE integrates many standards which include ISO 7730 for computing the thermal comfort of buildings. From the analysis of the base models, the predicted percentage of dissatisfied (PPD) index was 9% for the base model of Centralgatan 14 and 14% for Tebogatan 5. This is within the acceptable standard of EN 15251. When the operative temperature was above 27°C, the percentage of hours was 3% for Centralgatan 14 and 1% for Tebogatan 5. Table 12 illustrates PPDs for the base models.

Buildings	PPD (%)
Centralgatan 14	9
Tebogatan 5	14

Table 12.

PPD for base models.

3.6.1 Effect of ESMs on thermal comfort

Using the proposed ESMs in the base models to improve the energy efficiency of the buildings led to changes in thermal comfort. The PPD of the base models was increased depending on the implemented ESMs. When it was compared with the EN ISO 7730 which states that the acceptable thermal dissatisfaction in buildings should be smaller than 15%, various measures were therefore within the limit.

ESMs introduced in the models resulted in substantial energy savings for the buildings. The use of these measures had less impact on thermal comfort, apart from the combined effect of adding the external insulation plus lowering the indoor temperature. The combined effect of adding the external insulation plus lowering indoor temperature affected the thermal comfort considerably. The percentage of the total occupant hours with thermal dissatisfaction increased from 8% to 15% in Centralgatan 14, and from 8% to 28% in Tebogatan 5. This occurred when the external insulation was increased by 200 mm plus lowering the temperature by 2°C.

The percentage of occupants with thermal dissatisfaction was reduced to 13% as compared with 14% of the base model when the external insulation was added for Tebogatan 5, while it remained the same (9%) for Centralgatan 14. However, when the operative temperature was above 27°C, the percentage of hours was increased from 3% to 8% for Centralgatan 14, whilst it remained the same (1%) for Tebogatan 5.

Reducing the U-value to 0.85 W/m²·K for the windows led to PPD remaining the same as the base models, 14% for Tebogatan 5, but it was increased from 9% to 10% for Centralgatan 14. When the operative temperature was above 27°C, the percentage of hours was increased from 3% to 5% for Centralgatan 14.

When AHU with VAV plus temperature control was used for both buildings, the percentage of the total occupant hours with thermal dissatisfaction was increased by 1% for both buildings. However, the percentage of hours, when the operative temperature was above 27°C in the worst zone, was increased from 1% to 2% for Tebogatan 5. Figure 12 depicts the summary of the thermal results after implementing each ESM in the base models.

Figure 12.

Thermal comforts after implementation of each ESM and their comparison with the standard requirement.

3.7 Economic feasibility of ESMs

The implementation of ESMs largely depends on capital, and it should be analyzed in such a way that the investment could be viable. Therefore, it is important to optimize energy use by improving areas with huge heat loss. In the base models, the heat was mainly lost through walls, windows, and thermal bridges of roofs and floors. After implementing ESMs, it was important to carry out the economic feasibility of ESMs. Life cycle cost determines the most cost-effective approach from a series of alternative ESMs.

The life cycle analysis was done based on Eq. (3). This equation is the modified uniform present value (NPV. Factor) which is obtained by modifying NPV [21]:

$$\mathit{\text{Modified uniform present value}}\left(\mathit{NPV}.\mathit{\text{Factor}}\right)=\frac{{\left(1+r-p\right)}^n-1}{\left(r-p\right){\left(1+r-p\right)}^n}$$E3

where p is the estimated rate of increase in either electricity or DH, and r is the interest rate.

Tables 13 and 14 provide life cycle costs achieved for both buildings and indicate which ESMs are economically viable. Despite high energy savings by implementing certain ESMs, the results demonstrated that some ESMs were not economically viable. This was because of higher life cycle costs compared with their life cycle savings. ESMs that were not viable for both buildings include adding 200 mm external insulation, replacing windows with less U-value, installing a new AHU, improving thermal bridges, and increasing external insulation plus temperature reduction. However, according to the tables, adding 200 mm attic insulation, installing a new heat exchanger in showers, replacing lighting bulbs, and changing schedules for both buildings represented economical ESMs. This was thanks to lower life cycle costs than life cycle savings.

ESMs	Energy saving (MWh/year)	Life cycle cost (SEK)	Life cycle saving (SEK)	Working life (years)	Is this a good investment?
Scenario A1	28.3	3.68× 10^6	5.76 × 10^5	40	No
Scenario A2	16.19	1.2 × 10^5	3.31 × 10^5	40	Yes
Scenario B	26.8	1.545 × 10^6	4.28 × 10^5	30	No
Scenario C	82.7	1.15 × 10^6	9.25 × 10^5	20	No
Scenario D	15.2	1.15 × 10^5	1.31 × 10^5	15	Yes
Scenario E	12.99	4.4 × 10^5	7.8 × 10^4	20	No
Scenario F	7.51	2.13 × 10^4	2.52 × 10^4	4	Yes
Scenario G	55.84	3.68× 10^6	1.137 × 10^6	40	No
Scenario H	40.0	1.08 × 10^5	4.477 × 10^5	20	Yes

Table 13.

Results of life cycle cost analysis for Centralgatan 14.

ESMs	Energy saving (MWh/year)	Life cycle cost (SEK)	Life cycle saving (SEK)	Working life (years)	Is this a good investment?
Scenario A1	21.4	2.822 × 10^6	4.36 × 10^5	40	No
Scenario A2	14.5	6.092 × 10^4	2.95 × 10^5	40	Yes
Scenario B	16.41	1.035 × 10^6	2.63 × 10^5	30	No
Scenario C	74.6	9.5 × 10^5	8.34 × 10^5	20	No
Scenario D	13.06	9 × 10^4	1.12 × 10^5	10	Yes
Scenario E	6.77	5.056 × 10^5	7.57 × 10^5	15	No
Scenario F	5	1.6 × 10^4	1.67 × 10^4	15	Yes
Scenario G	46.3	2.822 × 10^6	1 × 10^6	40	No
Scenario H	26.1	5.4 × 10^4	2.92 × 10^5	20	Yes

Table 14.

Results of life cycle cost analysis for Tebogatan 5.

4. Conclusions

The buildings’ envelopes and thermal bridges greatly affected the energy performance of the buildings. Most of the heat losses in the buildings occurred through them. To analyze the buildings’ performance, various ESMs were studied to demonstrate how the energy efficiency of the buildings would be improved. The addition of thermal insulation, changing windows, installing an AHU, installing a heat exchanger in the showers, improving thermal bridges, and changing air discharge schedules could improve the energy performance of the buildings. The suggested ESMs affected the buildings’ energy performance positively. All the implemented ESMs contributed to energy saving for the buildings, but their economic feasibility depended on the economic aspect of their life cycle. This was based on the investment cost and life cycle savings of ESMs. The amount of energy reduction from ESMs varied; the combination of external insulation plus temperature reduction had the highest impact on the energy reduction of the buildings. However, from the individual ESM, the highest energy reduction was recorded from the analysis of installing a new AHU, and this reflected the importance of ventilation with a heat recovery system in the buildings. The economically viable ESMs included the addition of attic insulation, installing a heat exchanger in the showers, replacing lighting bulbs, and changing the schedules for AHU control.

This study links ESMs in existing buildings to urban sustainable development and energy efficiency in buildings. Since many buildings are old, they do not meet the requirements of being energy efficient according to the latest building codes or contributing to urban sustainability, however, this can be bridged with the implementation of ESMs.