6.8 Design for Low Energy Use
6.8 Design for Low Energy Use
In domestic buildings total energy use can be broken down into consumption for space heating, water heating, cooking, lighting and small appliances. The largest element of this energy use is for space heating purposes. A typical breakdown is illustrated in table 6.7.1. The percentage figures for consumption and for CO2 differ because the majority of energy use for space-heating purposes in the domestic sector is for gas whilst the remaining consumption is by grid-produced electricity.
Table 6.8.1 – Domestic Energy Use
Use |
kWh/m2/pa |
% |
kgCO2/m2/pa |
% |
Space Heating |
120 |
61 |
23 |
41 |
Water Heating |
45 |
22 |
19 |
35 |
Cooking |
15 |
7 |
5 |
9 |
Appliances and small power |
20 |
10 |
8 |
15 |
Total |
200 |
100 |
55 |
100 |
Although relatively small in absolute terms the consumption of electricity for appliances has changed dramatically in recent years with the increased use of consumer goods in most households. Table 6.7.2 shows the breakdown of consumption in each of a number of categories.
Table 6.8.2 – Electrical Consumption of Household Appliances
Appliance |
% |
Consumer electronics – TVs, computers, phones |
14 |
Washing machines, dryers, dishwashers |
16 |
Cookers, kettles, microwaves |
16 |
Lights |
24 |
Fridges and freezers |
24 |
Total |
100 |
In order to provide low energy buildings the potential energy losses from a building need to be minimized and the potential gains to a building – such as that from solar radiation – need to be maximized. It is also essential that appliances and building services elements such as boilers and water heaters are as efficient as possible and controls are provided to support these aims and to facilitate and ensure user satisfaction.
Heat Losses
Heat can be lost from a building through the external fabric and these losses can be reduced by thermal insulation in any part of the external elements – the roof, the walls or the floor. Most common building materials have thermal resistance properties due to their porosity and the consequent presence of air in the pores. Most elements are multi-layered and therefore have thermal resistance due to air cavities between layers. Technically the thermal transmittance of any element is represented by its ‘U’ value, measured in Watts/m2ºC (a low U-value indicating better thermal resistance). A good low energy design necessitates a consideration of the relative areas and U-values of all the external building elements in a building in order to minimize fabric losses.
Pitched roofs can be simply insulated by means of glass fibre or mineral wool quilt laid between and over ceiling joists. An acceptable U-value for the roof of a property constructed in 2006 would be of the order of 0.25 W/m2ºC and that would require a depth of approximately 300mm of insulation quilt in the roof space.
Heat Gains
Adventitious or ‘free’ heating to a building is to be encouraged and utilized as long as it does not lead to overheating and the consequent need for cooling. Heat gains are obtained from a number of sources – occupants themselves, any electrical consumption such as for water heating, cooking, lighting and small appliances and from solar gain. It is the latter that can be the most beneficial in overall energy terms. A design that best utilizes solar radiation to supplement or replace space heating demand is termed ‘passive’ solar design – the term ‘active’ solar design is generally reserved for systems for water heating or photovoltaics.
Building Services
Modern heating systems comprise a primary heat producing unit – a boiler – and a distribution network, comprising pipework and heat emitters such as radiators. In domestic installations the system is designed and sized simply to provide sufficient heat even in the depths of winter. The system generally provides heating only; there is no requirement for cooling or for ventilation.
Renewable Energy
At the scale of an individual building it is possible to employ a number of technologies to generate energy locally, collectively known as ‘micro-generation’. Options for renewable connected to a domestic building are listed in table 6.8.1
Table 6.8.1 Renewable Energy Options for Buildings
Technology |
How it works |
Limitation |
Solar Photovoltaic |
PV cells secured to roof of property. Electricity produced by the PV cells contribute to buildings power requirements, any excess is fed into the grid via an inverter. |
Need a south facing roof without shading from trees. Can be prohibitively expensive for low income households. |
Small Wind Turbine |
Same as above, but power is produced by wind turbine mounted on a tower close to the property. |
Roof mounted and most urban environments unsuitable due to turbulence. Good wind resource required, usually found in urban areas. Planning difficulties if neighbours complain. Possible issues with noise |
Solar Thermal |
Evacuated tubes or flat plate collectors use energy from the sun to heat water which is fed into the buildings hot water system |
Difference in consumption and production time, need for effective storage of hot water. |
Ground Source Heat Pumps |
Air is pumped through an underground pipe, gains heat from ground temperature and is fed into the building. |
Only suitable for new builds, difficult to retrofit. Large civil works required. |
Combined Heat and Power (CHP) |
Gas powered sterling engine delivers both heat and electricity to the building |
Not renewable but efficient |
For a comprehensive overview of the technology as installed see the Energy Saving Trust report published in September 2010.
http://www.energysavingtrust.org.uk/Media/node_1422/Getting-warmer-a-field-trial-of-heat-pumps-PDF
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