Sustainable Energy in Building Design

Introduction

Statistics have it that up to 50 % of the total energy used in domestic buildings is for heating and cooling purposes, so as to maintain the thermal comfort of the occupants. Such energy expenditure is not the best for the environment. The recommendable solution is design for climate. This involves architects and engineers designing buildings with careful consideration of the climate of the area, so that the heating and cooling mechanisms for the building do not consume more than enough energy. If well designed, energy use in the building for thermal comfort could be cut up to almost zero. For proper design of the office building in Bournemouth on the South Coast of England, it is important to first of all analyse the climate of the area to understand its characteristics and how it affects the thermal comfort of the buildings in the area. The climatic data for the year 1970 was obtained from the weather station at Portsmouth, which is the nearest to the office block. Below is a close analysis of the climatic data.

1.1 Air Temperature

This is also referred to as the dry bulb temperature. It is the ambient air temperature measured by using a normal thermometer freely exposed to the atmosphere but protected from radiation and moisture. Dry bulb temperatures were recorded every hour for the entire year. Below is a summary of the minimum, maximum, and mean dry bulb temperatures for the different months of the year 1970.

The above data for monthly minimum, maximum, and mean air temperature can be graphically illustrated as below:

From the graph, it is clear that the winter months run from December to February. The minimum temperature recorded during the winter months is -8 0C, whereas the maximum temperature during the same period is about 18 0C. After that, Spring sets in and runs from March to May. The approximate temperature range during this period is -1.2 0C to 22.3 0C. The summer months are evidently from June to August. The maximum summer temperature recorded is 29.6 0C. The minimum summer temperature recorded is 6.7 0C. After Summer follows the Autumn between the months of September and November. The temperature range during this period is -1.2 0C to 23.8 0C.

A further analysis of the temperature data yields the following:

Below is also a histogram for the annual dry bulb temperature.

The climatic data for Portsmout is near symmetric. The extreme temperatures - those below 0 0C and those above 26 0C - are minimal. Temperature in Portsmouth mostly varies between 8 - 18 0C. The climatic data exhibits normal distribution. A plot of the cumulative frequency curve for the dry bulb temperature is shown below:

This confirms that the very few days in Portsmouth experience the extreme temperatures - i.e. below 0 0C and above 26 0C.

From the above temperature analysis, passive ventilation can comfortably be adopted for the office building for a better part of the year. Passive ventilation involves the natural flow of air into and out of the building through openings in the building. This will greatly reduce energy costs for the office block.

2.2 U value for external wall

NB:

Thermal resistance = thickness of material / thermal conductivity of material

Overall U value = 1/R = 1/6.11 = 0.16 Wm-2K-1

The lower the U value, the better.

2.3 Calculating heat loss

Indoor design temperature T1 = 21 0C = 69.8 F

Outdoor Design temperature T2 = -2 0C = 28.4 F

Design temperature = T1 - T2 = 21 - -2 = 23 0C = 73.4 F

Assumption - adjacent spaces are at the same temperature - so we won’t consider heat loss through floor and one wall

Surface area of wall = Room surface area - Window Area

= [(3 x 6) + 2(3 x 10)] - (2 x 5 x 0.5 x 1.2) = 72 m2 = 775 ft2

U value of wall = 0.16 Wm-2K-1

Wall surface heat loss = U-value x Wall area x Design Temperature

= 0.16 x 775 x 73.4 = 9,101.6 BTUH

Window heat loss = 0.65 x (2 x 5 x 0.5 x 1.2 x 10.7639) x 73.4 = 3,081.3 BTUH

(Double glazed window, U value = 0.65)

Ceiling heat Loss = 0.15 x (6 x 10 x 10.7639) x 73.4 = 7,110.6 BTUH

(Timber roof, U value = 0.15)

Total Wall Heat Loss = 9101.6 + 3081.3 + 7110.6 = 19,293.5 BTUH

Air Infiltration heat loss

= Room volume x design temperature x Air changes per hour x 0.018

= (10 x 6 x 3 x 35.3147) x 73.4 x 0.75 x 0.018 = 6,298.8 BTUH

2.4 Heat Loss Coefficient for the room

The total fabric contribution to the overall heat loss coefficient is given by:

Qf/ΔT = ΣUxAx W K−1

Effect of solar radiation on fabric heat loss

The sol-air temperature can be defined as the temperature at which, if there is no direct solar radiation and no moving air, will result in the same heat transfer into the room as that caused by the combination of the prevailing atmospheric conditions. The value of sol air depends on the solar radiation falling on the outside surface of the building. On a sunny winter day, the sol air temperature at 0900h will be almost equal to that at 1600h because the sun hits the building at the same angle, only in different directions.

3.1 Finding sun position

Time - 1430h

Month - October

Day - 15

Latitude = 50.818

Longitude = -1.113

GMT offset = 0

3.2 Sun position details

Day number = 288

EoT = 14.86 min

Time Corr = 10.86 min

Declination = - 9.6 deg

Hour angle = 40.21 deg

Azimuth = 22.69 deg

Sunrise = 0637h

Sunset = 1701h

Incidence angle of sun on south west facade

The angle of incidence of the sun on SW facade is given by:

Where 𝛃 is angle of tilt of surface from horizontal, Azs is Azimuth angle, δ is the declination angle, and Φ is the observer’s latitude.

Substituting the respective values gives the angle of incidence on the SW facade as 59 deg and on the roof as 149 deg.

3.3 Heat flows in and out of the building

On 15th October, at 1430h, the following are the main ways in which heat flows in and out of the building:

Conduction - Heat flows out through the walls and roof due to the temperature difference between the inside and outside of the building.

Convection - Heated air within the building rises and escapes through the ventilation openings, as cooler air from outside gets into the building through openings on the windows and doors.

Radiation - Heat from the sun directly heats up the building.

3.4 Effect of colour of wall on heat flow

The shade of the wall on both the exterior and interior can determine the amount of heat that flows through the wall. Darker colours can absorb up to 90 % of the solar radiation and transfer the heat to the interior of the building. Lighter colours lead to absorption of only 35% of the solar radiation and thus the building gains very little heat.

Conclusion and Recommendations

From the detailed analysis of the climate and the thermal properties of the building, it is now possible to come up with the most energy efficient design that can save energy. With a U-value of 0.16 0.16 Wm-2K-1 , the heat loss of the building is very little. This will ensure that the building remains thermally comfortable even during the winter months of low intensity solar radiation. Given the total heat loss of about 20,000 BTUH, the HVAC systems can now be rightly sized. It is recommended to choose HVAC systems with sensors that can detect the temperature and humidity of the room and have the ability to automatically turn on or off, so that they get turned on only when it is necessary.