KIEAE Journal

According to a report on UNFCCC, the CO2 level in the atmosphere exceeds 400ppm (as of 2013) for the first time since the observation began. Also, the average global temperature has increased by 0.85, the sea level 19cm (as of 2010) for the past 1333 years. These are the warning signs that catastrophic results might be caused if there is no measure against global warming. In this context, Korea, the 9th largest carbon emitter in the world, is under pressure to take concrete measures to reduce carbon emissions. Therefore, the Korean government has adopted low-carbon, green growth as a key policy direction, and pushed forward the development of renewable energy and a sustainable growth engine in cooperation with enterprises. Therefore, it is required for the building energy sector, which consumes about 1/3 of the total energy, to develop practical energy reduction measures.

In the past, school buildings were designed and constructed to provide relatively low-level indoor environment compared to other types of buildings. However, the number of school buildings with heating and cooling systems has rapidly increased in a bid to improve the education environment, resulting in energy issues.

The problem of energy consumption is more serious in university buildings than primary, middle and high school buildings. Because university buildings have generally heating and cooling systems, and various incidental facilities.¹⁾ In particular, university libraries spent more energy than any other university buildings, but there are only a few studies that identify whether energy is efficiently spent and suggest improvements, calling for more research on this area.

This research aims to assist an architect to make an optimal decision on designs in terms of energy saving when he/she tries to provide a guideline for a low-energy library design. Sejong city was chosen as a target location, and a fundamental case study was conducted in the previous research.²⁾ This time, space composition, use schedule, performance of the structure, etc. of the library in the same city were identified, and an energy simulation was performed by applying several measures that affect the improvement of energy performance. The results of the each case simulation on heating and cooling load and energy consumption were compared. The K library uses EHP system for heating and cooling, but the energy analysis by the characteristics of the system was not included in this research. It will be covered in the next research after a careful inspection.

This program calculates the solar radiation that reaches to the entire surface of a building at every step of every hour. The solar radiation is divided into two – direct solar radiation and diffuse solar radiation – and then calculated as shown below.

Direct solar radiation I_dir Idir is calculated as follows:

I_beam represents the vertical incidence component, θthe incidence angle.

Diffuse solar radiation is divided into two – sky radiation (I_sdiff) and terrestrial radiation (I_gdiff) and then calculated as below.

Ihdiff represents diffuse sky radiation on the horizon, β the surface tilt, and ρg albedo. The total solar radiation on the horizon can be described as below.

But, here, α is the solar altitude.

The indoor conduction, convection, radiation, and infiltration were calculated with the following formula.

The spatial temperature distribution of a solid without internal heat source (Q_cdQcd)(W/m²) is calculated as below.

Here, λ represents thermal conductivity (W/m2K), and T temperature (℃).

As for the convective heat transfer (Q_cv)(W/m2),

Here, hc is the convective heat transfer rate, Ta the air temperature (℃), and Ts the average surface temperature (℃).

A radiant heat emitted from the surface (Q_r) is

Here, ε represents surface heat dissipation, A the surface area,σ the Stefan–Boltzmann constant, and θ a surface absolute temperature (°K).

The heat transfer rate related to an air current that moving around spaces (Q_vent) is

Here, m is air volume (kg/s), c_p the specific heat under constant pressure of air (J/kg/K), and Ti the air temperature which is supplied (℃).

After consideration on physical figures, solar gain, internal heating value and others, the heating and cooling load is calculated.

After calculating the heating and cooling load, the heating and cooling systems are set. The efficiency of the systems, auxiliary power use, and power use of an outdoor unit and fan were considered in the calcaulation of the consumption of fuel and/or electricity.

The subject of the simulation is the 6-story (including a basement floor) library of K university in Sejong city. The basement floor consists of research rooms and seminar rooms. Reading rooms, stack rooms, reference. etc. are located in ground floors. Fig. 1 shows the campus map and Fig. 2 the library’s 1st floor plan.

Fig. 1.  
K university campus

Fig. 2.  
library 1 floor plan

Excluding the basement floor, the simulation modeled the ground floors from 1^st to 5^th floors, and zoning by use was conducted – the library and service zones. The library zone consisted of the reading room, stack room, and office, reference, etc. while the service zone includes the elevator core, warehouse, corridors, stairs, etc. The building contents are shown in Table 1, and the model zoning by each zone and their area is described in Table 2.

The subject building changed its heating and cooling system to EHP in response to the government’s energy saving policies. Before the change, the building used the central system installed in 1989 together with individual air conditioners that were added to the building when necessary. However, there have been no architectural improvement for energy saving, and control measures such as outdoor air cooling have not been implemented.

The property of heating and cooling system is shown in Table 3. By analyzing the floor plan and conducting an actual enumeration, the detailed information on the building was identified. Then, the building was modeled using the Model IT of the VE (Virtual Environment). The image is shown in Fig. 3.

Fig. 3.  
Simulation model image

The material property of the building is shown in Table 4, and windows used low-e glass with 24mm thick, as shown in Table 5. During the simulation, the standard climate date provided by the Korean Solar Energy Society was used, and heating and cooling temperatures were set 18℃ and 26℃ respectively.

Related research³⁾ was used in identifying the amount of internal heat. Based on interviews, week day, weekend, and monthly schedules were surveyed to find out the overall operation schedule. More specifically, the stack room controls temperature for 24 hours while the reading room has different schedules during week days and vacation. During week days, it provides air conditioning from 8:30 to 24:00 while during vacation, from 8:30 to 24:00 for only some reading rooms in 1st and 2nd floors. (see Table 6)

This research reviewed the standard Case (Case 1) by applying the operation result based on the result of the on-site inspection under the condition that the material properties of the existing walls and windows were applied when windows were closed. To check whether the energy performance of the building was improved, Case 2 with a strengthened wall insulation and Case 3 with roof afforestation were reviewed. Also, to identify changes caused by the change of glass, Case 4 with clear glass instead of low-e insulating glass was examined. The effectiveness of outdoor air conditioning was examined by opening the windows in May when the outside temperature is at 18℃to 26℃ (Case 5).

Table 8 shows materials used in Case 2 (wall) and Case 3 (roof material for roof afforestation). Also, Table 9 shows the property of glass used in Case 4 where clear glass was used instead of insulating glass.