Offices

In order to account the energy spent in the office a Simulink model was built to keep the office running in comfort condition (temperature and lighting), the simulation was divided by month (due to limitations in the software), as results four graphs were plotted for each month with: total energy saved by the heating system; total energy spent of aircon, lights and computers; costs related to this monthly energy spent; inside temperature of the office.

The Figure 1 shows the overall model:

Figure 1- Office Simulink model

(1)- Heat gains from people of 100W per person [1] that varies with the number of people inside the office, the occupancy rate can be seen in the lighting section.

(2)- Heat gains from lights of 31.5W per LED that is 70% of total LED power [2], the total heat from light varies with the natural light control, that shutoff the lights during the day, the function can be seen in the lighting section.

(3)- Heat gains from computers, in this case an average of 155W of heat per computer [3] the number of computers vary with the number of people inside the office.

(4)- Is the outside temperature, in this simulation were used temperatures with one hour of resolution on the entire year, the simulation was divided in months due to software limitations.

(5)- Is the solar gains from the south part, the south facade is composed half of wall and half of windows, the solar radiation gained in a vertical surface is a function of: direct horizontal radiation, latitude, date, solar azimuth and wall azimuth (in this case 0 for a vertical wall), the solar radiation is separated in tree separately parts: reflected radiation, diffuse radiation and direct radiation, and the calculation was based in [4] and [5]. In Figure 2 can be seen a result sample of the vertical global radiation for tree different days 21 of June, 21 of March and 21 of December:

Figure 2 - solar radiation, results sample

(6)- Thermal mass of the office, in which the heat capacity is an average of the heat capacity of people, computer and air inside the office based on [6], [7], [8] and [9]

(7)- Is the heat losses based air tightness (measure in m^3 of air/m^2 of floor area) of passivehaus standard [10], knowing this value it is possible to measure the air flux that enters the building at outside temperature, in order to simulate this effect, an air flow at the outside temperature (updated hourly) enters the building each second exchanging heat with the air inside the office, the air flow can be seen in table Table 2.

(8)- Simulates the fabric losses, that is the heat exchanged between the outside environment and inside air, the wall, windows and roof are simulated with a thermal resistance that can be seen in Figure 3.

The setup for the thermal resistances of wall, roof and windows are as follows:

Figure 3- wall thermal resistance

Table 2 - air tightness

Table 1 - Office thermal mass

(9)- Is the aircon unit that pumps outside air inside the data centre, outside air can be used to cool the office thanks to a psychometric analysis done previously, where outside air is usually at lower temperatures than the set temperature for room comfort (22°C), to became more realistic the aircon unit turn on if temperature passes above 22°C and shuts down if the temperature reaches 22°C, this is controlled by a thermostat. The air mass flow and Simulink model for the aircon unit can be seen below:

(10)- Is the heating system pumps air coming from the servers room at 34°C to heat the offices without requiring an electrical or gas heater, saving energy. The Simulink model is very similar to the aircon unit, the difference is that the heating system is programed to turn on when the inside temperature stays below 22°C and shuts down when reaches 22°C, the air flow can be seen in Table 5 and is much lower than the air flux in the server room, meaning that it has enough air to keep a comfort temperature without requiring any other type of heating.

Figure 4 - aircon unit

Table 4 - air flow for the aircon

Table 5 - air flux for the heating system

Table 3 - building physical properties

In Figure 5 to Figure 8 are a results sample from December, after that each month was calculated separately the results were coupled using excel that can be seen in the results section of this website, it can be seen there the total energy savings from the heating system, total energy spent in one year to operate the office and total cost relate to this energy spent.

Figure 5 - inside temperature and heat gains

Figure 7 - costs of energy for office

Figure 8 - energy savings from the heating system

Figure 6 - monthly energy for the office

References

[1] engineering toolbox, "People and Heat Gain," engineering toolbox, [Online]. Available: http://www.engineeringtoolbox.com/persons-heat-gain-d_242.html. [Accessed 05 2015].

[2] "LED heat sink calculation simulation thermal design," [Online]. Available: http://www.mechatronix-asia.com/LED_heat_sink_calculation_simulation_thermal_design.html. [Accessed 05 2015].

[3] ASHRAE, 2015.

[4] engineeringtoolbox, "emissivity-coefficients," [Online]. Available: http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html. [Accessed 05 2015].

[5] tradeframe, "U value," [Online]. Available: http://tradeframe.com/upvc-triple-glazing. [Accessed 05 2015].

[6] engineeringtoolbox, "human-body-specific-heat," engineeringtoolbox, [Online]. Available: http://www.engineeringtoolbox.com/human-body-specific-heat-d_393.html. [Accessed 05 2015].

[7] wolframalpha, "volume of a person," [Online]. Available: http://www.wolframalpha.com/input/?i=volume+human+body. [Accessed 05 2015].

[8] syska, "Thermal mass," [Online]. Available: http://www.syska.com/cms/docs/articles/KKhankari.ASHRAETransPaper._01.28.11.pdf. [Accessed 05 2015].

[9] supermicro, "rackcabinet 42u spec," [Online]. Available: http://www.supermicro.co.uk/products/rack/rackcabinet_42u_spec.cfm. [Accessed 05 2015].

[10] passivhaustagung, "Air Tightness to Avoid," [Online]. Available: http://www.passivhaustagung.de/Passive_House_E/airtightness_06.html. [Accessed 05 2015].

[11] D. Clark, "Ventilation rates in offices - mechanical and natural," Cundal Jhonston , 2013.