Energy consumption characteristics in residential houses in Danang, Vietnam

. Residential energy consumption has been increasing significantly in Danang, Vietnam, in recent years due to improvement in living standards. In order to plan housing energy conservation measures, it is necessary to clarify housing information, people's lifestyles, energy consumption style and usage of energy equipment. The purpose of this study is to clarify the energy consumption characteristics for further energy simulation analysis in the next step. For this purpose, a questionnaire survey of 300 households in Son Tra district, Danang was conducted in March 2022. The power consumption data during 2019 - 2021 were obtained through the local Department of Electricity (EVN). An analysis was performed on the information collected to provide insight into the current energy usage in Son Tra. The results showed that the main factors of household that effect the energy consumption were number of air conditionners, outdoor temperature, number of storeys and family size. Through this study, basic documents for energy saving measures and policy making have been obtained; Energy saving measures will be considered through simulation in the near future as the next step.


Introduction
Energy demand keep increasing in Vietnam in recent years, along with economic development. In the period 2011 -2021, the growth rate of energy demand was 7.2%, higher than the GDP growth rate (6.3%) of the same period. In 2021, energy demand was 4.32 Exajoules, increasing by 2.6% compared to 2020. It is expected that this trend will continue in the near future [1]. Since 2015, Vietnam has changed from an energy exporter to an energy importer. Although the domestic energy resources have been exploited to the maximum but still cannot meet the increasing demand of the economy and daily life. Fuel import rate will increase from 36% in 2020 to 60% in 2030 [2]. Due to population growth and development needs, increased demand for energy is inevitable, leading to depletion of resources, increasing in CO2 emissions and other serious consequences such as global warming related problems. Therefore, energy measures are needed in all fields, especially in the residential sector. Toward this goal, it is necessary to clarify the current state of energy use, and to find solutions to energy conservation in residential buildings as the next step.
The challenge here is that data on energy consumption characteristics for houses in Vietnam are * Corresponding author: le-dung-jv@ynu.jp limited. At macro level, there is only information on general status of energy: pathways for developing energy in Vietnam energy outlook report 2021 [2]; the owner ship of main electrical appliances in the house in National census 2019 [3]. The Cimigo team [4] conducted a detailed survey on ownership of home appliances and average energy consumption in 2013, but the study did not mention information on energy breakdowns. Murakoshi et al [5] conducted a survey across Southeast Asia countries, including Vietnam in 2017. In this research, details on energy consumption of Malaysia, Thailand, Vietnam, and Cambodia were clarified. Three Vietnamese cities in the research were Hanoi, Ho Chi Minh city, Hoa Binh. However, that is not sufficient to determine the residential energy consumption pattern in Vietnam, especially in cities like Danang with different climate types and urban sizes.
To understand the characteristics of energy consumption and contribute a piece to the overall picture of Vietnam's residential energy consumption, this study conducted a survey on the above topic in Danang, a city in the central of Vietnam. The study aims at: (1) Clarifying housing features, energy consumption, especially the state of electricity usage and devices in households. (2) Checking the factors affecting electricity consumption in houses. In this paper, the outline of the survey is described, and the main results are presented. Furthermore, factors influencing the energy consumption are analysed.

Outline of the survey 2.1 Location of survey
Danang is the major city in central Vietnam. This city is also facing the risk of energy shortage in the future due to the rapid urbanization process. Household is the second largest consumer of energy in Danang, after industry with 24% and 35% respectively [6]. Figure 2 depicts the steady growth of electricity consumption of Danang by sectors.  The survey was conducted in Son Tra, one of eights districts of Danang, Vietnam. Son Tra district has seven wards under, with a population of 157,415 in 2019. Son Tra District is located on the Son Tra Peninsula, with many newly built hotels for tourists, local residents' houses and old fishing villages. General information of Son Tra is shown in Figure 1. With two sides bordering the sea in the east and north, and on the west by the Han River, Son Tra is a vulnerable place to rising sea level caused by global warming. This area has a humid tropical monsoon climate with two main seasons: dry season during January -July, and rainy season in August -December. The average annual temperature in the recent 5 years is 26.5 0 C. The hot period of the year is from April to September, between the rainy season and the dry seasons. The highest average temperature is 29 -30 0 C in June-August [7]. Danang has a short "winter" from December to February, which is during the rainy season. The lowest average temperature in this time is 22 -23 0 C. The average humidity throughout the year is 80%, with the highest humidity of 84% in October -December, and the lowest of about 74% in June -July [7].

Methodology of the survey
Three hundred households participated in the survey. The questionnaires were distributed evenly among people in seven wards of this district. From the wards, the interviewers will contact the quarter leaders to introduce households for interviews. Buildings containing shops were excluded in our survey because the energy consumption for residential purposes is the main concern of this study. After contacting the households, the questionnaires were sent one day in advance and the interviewer will be conducted the next day to collect information. The interviewee can be a homeowner, or a person who understands the use of electrical appliances in the house.

Questionnaire items
As shown in Table 2, the questionnaire items include information on: Household 's characteristics, home 's appliances and energy consumption. The questions were multiple-choice style, except questions on the capacity, the name of the device's brands which were in the form of open-ended questions. Questionnaire filled out by interviewer through face-to-face communication.
Electricity consumption data of the above 300 households were provided by the local Department of Electricity (EVN), with the consent of the owners.  Figure 3, the average number of people in a household in Son Tra district is 4.9, higher than that of Danang (3.7 -2019), Hanoi (3.6 -2019), Kuala Lumpur, Bangkok (4.7 -2017) [3,4]. The most common family was a family of 4 (34.8%), of which 2 are employed (46%). Families with 5 and 6 members account for 18.7% and 15.4% respectively. Family size of 4 is also a popular model in Vietnam, however, the rates of a family of four in Son Tra was respectively 8.3% and 8.7% higher than the rates in Danang and Vietnam. (See in Table 3). This is an area that alternates between newcomers with medium-sized families, and long-term residents living with extended families for several generations in one house. Therefore, the rather large household size is not uncommon. Also as shown in Figure 3, monthly per capita income of the studied area is 3.2 million VND/person (~134.3 USD), lower than the average of Da Nang, which is 5.2 million VND (~ 212 USD, 2021) and Vietnam, which is 4.2 million VND (~ 170 USD). Since the purpose of the survey is to understand energy consumption characteristics for residential purposes, households with both residential and home business (presumed to have higher income) have been excluded from the survey, so it is possible that the real income of the area could be higher.
Tube houses are the dominant housing type in the area, with a rate of 95%. A so-called house is a house in contact with the walls of 2 neighboring houses on both sides. Detached house (with wall not in direct contact with the wall next door) accounts for 1.5%. This ratio of tube houses is 26% higher than the rate for hold country (69% in 2013) [4]. Two-story houses are twice as many as three-and one-story homes (53% versus 23 and 24%). A typical house in the survey area is a house with 2 floors, 5.2m wide, 16m long, with a total floor area of 145m 2 . The materials of the house was concrete frame, brick wall, corrugated iron roof or concrete roof. The two most popular types of roofs are corrugated iron (52%) and concrete roofs (46%). Tile roofs account for the lowest percentage (2%) in the study area.  Figure 5 shows the ownership of electrical appliances in houses. Seven popular devices with ownership rate over 80% at Son Tra are: fan, TV, refrigerator, electric rice cooker, mobile phone, washing machine, and air conditioner (AC). Excluding washing machine and air conditioner, the above 5 items are also present 80% in Vietnamese households [3,4]. Devices with large power consumption capacity such as iron, hair dryer, electric kettle, electric water heater had a presence rate in houses from 70 -50%. These are also items commonly found in households in Danang and Vietnam [3,4]. Less common appliances included: Dishwasher, DVD, TV game console, vacuum cleaner, ceiling fan with less than 10% ownership. Compared to Danang in 2019, the presence of air conditioner in Son Tra is 35% higher. This result may be due to the covid epidemic in 2020 -2021: the time spent at home in summer has impacted people to buy more air conditioners. Appliances equipped with more than 1 unit in each house include: fan (3.4), mobile phone (2.8), TV (1.4), air conditioner (1.3), refrigerator (1.1), electric rice cooker (1.05). This suggests that cooling, entertainment and cooking are the three biggest needs for the people in the study area. Table 3. Survey 's main results About the distribution of important appliances in Figure 6, 42 % households had an air conditioner, houses with 2 air conditioners and no air conditioner hold roughly the same ratio (23% and 20% respectively). As for light bulb, 61% of households using LED lights. The remaining 39% of households have the potential to save electricity if they are encouraged to switch to LED lights. Heating water is not as important as cooling air: 53% households had an electric water heater. Solar water heaters are not significant due to concerns that there is not enough solar energy in winter to heat water. For a tropical country, not prioritizing solar energy is a waste. People may need more support to be willing to use green energy.  Figure 7 presents the average daily use of electrical appliances per day. Except for refrigerators, which must be used 24/24, fans are the most used device during the day, with average daily use time almost twice as long as the second-ranked air conditioner † (12.3 hours and 6.8 hours). The remaining devices with a daily usage time of more than 1 hour include: ceiling fans, mobile phones, TVs, PC/ laptops, DVDs. On average, people in the surveyed area spend 4.7 hours on phones and TVs per day.

Appliances using time
Regarding schedule usage of cooling devices, fans are used all year round while air conditioners are seasonal used (Figure 8). Danang's hot period in recent years is from April to September, when the average † The daily air conditioning using time were calculated from April to September. outdoor temperature is above 27.5 0 C. This is also the period when air conditioners are heavily used. Figure 9 reveals the schedule for using ACs in a hot day. The time frames of using ACs the most are evening ( 21pm -3am) and noon (11am -1pm). In a typical hot season day, a family turns on the air conditioner for 5 hours: 2 hours for lunch (11am -1pm), and 3 hours for bedtime (11pm -2am). This schedule accounted for (18%) of households. There were 13% of households extending the time using the air conditioner at bedtime up to 3am, (6 hours a day). The same percentage of households (13%) turned on the air conditioner from 10pm -4am, (7 hours a day). 12% of households use air conditioners for 3 hours, mainly at bedtime (11-2am). Households using AC from 8 to 11 hours/day accounted for a total of 32%. There was no big difference in usage time between weekday and weekend. 4 Energy consumption Figure 10 shows that the main energy sources in household are electricity and liquefied petroleum gas (LPG). Electricity is used for most home appliances, and LPG is for cooking. The proportion of households using gas for cooking has decreased from 90% in 2014 [8] to 82% in 2022. Solar energy was limited used, only 6 in 300 families use solar energy for water heating purpose. One in 300 families use firewood for cooking. Coal and kerosene did not appear in the survey. Compared to 2014 [8], coal and firewood have almost disappeared, replaced by solar energy, although its percentage is negligible. Excluding gasoline for personal vehicles, annual energy consumption in Son Tra is 5298 KWh/ household, (~ 19.1 GJ), of which 82% was electricity and 18% is LPG. These results for total energy use and electricity sharing are similar to studies in Hanoi (5833 KWh, 80% electricity) and Ho Chi Minh city (5279 KWh, 85% electricity) [5] (Figure 11).  Table 4. Details on average consumption of electricity in a household by appliance.
As described in Table 4, cooling and cooking are the two areas which consume the most electricity (55% and 27%). Lighting comes down to the third position in power consumption with 6%. Other plug-in appliances count for 5% while laundry and water heating count for 4% and 3% of total electricity consumption, respectively. The total calculated electricity use of the equipment was 4404KWh, 54 KWh higher than the actual data taken from EVN (4350 KWh). This difference may be due to the fact that the calculation in the Table 4 is based on a full capacity of appliances, but it may not the case in reality.

Electricity consumption by devices
According to Figure 12, air conditioner (AC) is the largest power consumer in a year, nearly 2 times as large as the second-ranked and third-ranked appliances, the refrigerator and fans (1603KWh, 827KWh and 799KWh, respectively). These 3 appliances are the top consumers: they account for 73% the total electricity consumption in a year. Lighting ranks fourth, but with much less power consumption than fans, and ACs. The next groups of electricity consumption are kitchen appliances (excluding refrigerators), TVs, laundry.  Figure 13 shows that both the outdoor temperature and the electricity consumption increased during January and August, and decreased during September to December. In months with roughly the same temperature between years as March, the electricity consumption of those months is almost the same.It can be seen that power consumption varies with the outdoor temperature.

Electricity and outdoor temperature
In Figure 13, two peaks of energy consumption in August 2021 (543kWh) and September 2020 (480 kWh) correspond to the time of the Covid blockade in Da Nang. Electricity consumption in the months following the lockdowns was still higher than in 2019 may due to the trend of working from home.
During the hot period from April to September, the average monthly electricity consumption was 33% higher than the average of the remaining months (401 kwh compared to 301 kwh). This suggests that (1) the electricity consumption and outdoor temperature have a positive relationship, and (2) cooling demand in the hot season is higher than the heating demand in the cold season.

Electricity and air conditioner
In Figure 14, throughout the year, homes without AC consistently consume less electricity than homes with ACs. On average, a naturally ventilated house all year round consumed 44% less electricity than an airconditioned house (3108 kWh/year versus 5388 kwh/year). The yearly fluctuation of electricity used in houses without AC is less than other houses, only about 100 kWh from 200 to 300 kWh. Meanwhile, households with 3 air conditioners have a double change, ~ 200kWh between the hottest and coldest months. For households with 2 and 3 ACs, electricity consumption is almost the same, except for the 4 months with the highest temperature (June -September). Maybe for households with 3 ACs, when the temperature is not too high, they live like households with 2 ACs to save electricity and only use the 3rd air conditioner in the hottest months. The number of AC is correlated with the amount of electricity consumed.

Electricity and income
As seen in Figure 15, electricity expenditure increases with income level. The lowest income group pays 725,000 VND monthly (~ 31 USD), while the highest income group pays 1,443,000 VND (~ 60 USD). There is not much difference between groups with intermediate income, at about 850,000 VND. The last 2 groups have a clear increase in the amount of electricity to be paid: 1,225,000 VDN and 1,443,000 VND, respectively. The electricity consumption in households is directly proportional to income level.
In terms of the ratio of electricity expenditure to income, the lowest income group spends up to 18% of income on electricity electricity bills. The next two groups spend respectively 7% and 4% of their income on electricity. Meanwhile, the three highest income groups spend only 3% of there budget on electricity. Electricity expenditure accounts for a high proportion in the low-income groups and conversely, a low proportion in the high-income groups.  Figure 16 shows that electricity consumption is also proportional to household size and number of floors, but not to house orientation. The more floors the house had (that is, the larger floor area), the more electricity it consumed. The same is true for household size. As for orientation, the north and south facing houses consumed the most electricity (393, 385 kwh respectively) while the west and east facing houses consumed quite uniformly as other directions.

Conclusion
The article presented the results of a survey on energy consumption characteristics for 300 households in Sontra, Da Nang. Some main results are: -Main characteristics of a typical house: a tube house, 2 floors with 3 bedrooms. The width of the house is 5.2m, the length is 16m, the total floor area is 145m 2 . The house is concrete frame, brick wall, corrugated iron roof or concrete roof.
-Ownership of electrical appliances and time of use were calculated -Electricity and LPG are the main energy resources.
-Total average energy consumption of a household is 5298 kWh/ year, of which electricity is 4350 kWh/ year. Cooling and cooking account for 55% and 27% of total electricity use.
-Electricity consumption has the correlation with outdoor temperature, household's size, number of storeys and income level, while orientation of house has no clearly relation.
To improve energy efficiency, the followings should be noted: -High cooling demand, in which the fan is used all year round, suggests the need to improve indoor thermal conditions, with attention to ventilation.
-Kitchen appliances, which account for a large share of electricity consumption, will present more and consume more energy. Therefore, it is necessary to encourage energy efficient devices.
-Encourage solar enegy usage. This study gave an over view on enegy consumption of Danang. Based on this basic understanding, energy saving options will be carried out for the next step.