Understanding energy use: A ‘green’ perspective

04 House_Icon_by_Yuninaa
04 House_Icon_by_Yuninaa

Each type of building will have a different pattern of energy consumption throughout each day and throughout the year. Predicting the energy used by a building is complicated because of the wide range of factors that affect its energy consumption. These include: Building type: Buildings enclose a wide range of human activity. For example, they can be places of education, work, entertainment or living. Within each building type there are variations, for example, dwellings may be flats, terraced houses or detached homes. The prime difference here is the physical size of the building, specifically areas and volumes.

04 wall detail
04 wall detail

Building fabric: One role of the building fabric is to protect the occupants from the discomfort of the external environment whilst letting in those elements that are beneficial, such as fresh air and daylight. Thermal insula­tion, thermal mass, air tightness, and quality of workmanship all have an effect on energy consumption.

04 radiator
04 radiator

Building services: It is the primary building services, space and water heating systems, cooling systems and lighting systems that use energy to create comfort in buildings. It follows that the type, capacity, efficiency and method of control of these services will all have a significant effect on the amount of energy used by a building.

04 House-in-the-snow_web
04 House-in-the-snow_web

Load on the services: The principle factor, making the operation of energy consuming services necessary, is the variation in external climate. This could be too hot, requiring cooling, or too cold, requiring heating. The amount of sunlight may fluctuate, varying how much solar energy is deposited in the building. It may also be too dark requiring the use of artificial lighting.

Woman running on treadmill in gym
Woman running on treadmill in gym

Human behaviour: When humans interact with the building services they can modify the amount of energy used by them. The extent of the interaction varies between building types. For example in the home there is every opportunity to turn heating and lighting on or off as required. No two sets of occupants are alike. One person may be energy aware and use energy frugally, another may be profligate with the energy and use it wastefully. In other buildings there is little opportunity for modify­ing energy using systems for example when working in an office the services are often controlled centrally, with no accessible manual controls.

The effect of all the above is to create a continuous variation in the amount of energy used by buildings. It is necessary, therefore, to consider statistical indicators of energy consumption to get an idea of the amounts of energy being consumed. The coarsest method of considering energy use in buildings is to consider two broad categories of building: domestic buildings (dwellings) and non­domestic buildings. This is a broad grouping of many different types of building within each category but is often used when describing UK energy use. For example, government statistics reveal that of the 761.7 TWh of energy used each year in buildings, roughly 70% (530.3 TWh) is used in housing and 30% (231.4 TWh) is used in the services' sector (non-domestic buildings - note the services sector also includes a small amount for agriculture).

The pie charts below show that space heating energy consumption is by far the largest contribution to the total energy consumed in both the domestic building and air conditioned office, comprising 78% and 48% of the total respectively. It can be said that the proportion of the total energy used for space heating diminishes as the building complexity increases from domestic to non-domestic. This change can be attributed largely to the working environment of the non­domestic building, which requires increased electricity consumption to provide higher standards of lighting and thermal comfort. In particular there is additional electricity consumption by the air conditioning chillers and fans.

04 pie charts
04 pie charts

The pie charts have been drawn the same size to illustrate where the energy goes and to allow a comparison between the different building types. The area does not represent the amount of energy used. If the area did represent quantities of energy used (per m2), the mechanically ventilated and air conditioned office chart would have a much bigger area than that for the domestic building. This is because the total energy consumption in any building depends on a range of factors including:

• The localised climate

• Installed services

• The insulative quality of the building fabric

• The behaviour of the occupants

• The overall size of the buildings

To consider energy use in buildings in more detail it is necessary to look at average values for a number of similar buildings over the course of a year. To be able to make a comparison between two types of building it is necessary to normalise the energy consump­tion. It is usual to divide the total amount of energy used over the year by the building's treated floor area. This gives energy consump­tion in kilowatt hours per square metre per year (kWhm2/y). For our two building types, typical energy consumptions are:

Type                                                       kWh/m2/y

Domestic                                               150

Air conditioned office                         460

The aim of low energy design is to reduce these values as much as possible. Interim target consumption can be set based on good practice values that have been found by looking at energy efficient buildings. The target good practice energy consumptions on this basis are:

Type                                                       kWh/m2/y

Domestic                                               88

Air conditioned office                         260

It is clear then that space heating uses the lion's share of our energy and, as building complexity increases, then so it is likely that overall energy consumption will increase too. We have also shown that a number of measurable factors influence total energy consumption and that these are often not appliances but behaviour. The figures above show that a low energy building design can reduce energy costs by around 50% without any real behaviour changes needed. If these measures were coupled with behaviour changes (the way we use energy) then further savings would be found. Concern over climate change has begun to motivate people to think more carefully about how they use and manage buildings. Aside from the need to reduce CO2 emissions and stave off global warming, there are other reasons why it is important to reduce the amount of energy consumed by our buildings but the reason that engages most people's attention is cutting costs, especially as fuel costs are rising so fast at the moment.