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 6.8.1Domestic 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 6.8.2Electrical Consumption of Household Appliances

Appliance

%

Consumer electronicsTVs, 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 buildingsuch as that from solar radiationneed 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 elementsthe 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 itsUvalue, 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.

External walls are typically multi-layered and can be insulated in a number of ways, between the layers or by the substituting of relatively poor insulators with materials of higher thermal resistance. In traditional masonry construction the internal blockwork may have to be lightweight and the cavity may have to be fully filled in order to achieve a reasonable U-value. Perhaps the most straightforward method is to change the method of construction and to use a timber frame solution with a heavily insulated internal timber frame and a brickwork external leaf. The U-value for a wall should be a maximum of 0.35 W/m2ºC.

Floors may take one of two formssolid or suspended timber. Solid concrete floors can be insulated below the slab, using a non-compressible insulant, and depending on the construction detail, additionally beneath the floor finish. Suspended floors can be insulated between the timber joists with the insulation, such as glass fibre or mineral wool, supported by netting draped over the joists. A target U-value for a floor would be a maximum of 0.25 W/m2ºC.

Windows are traditionally the largest thermal bridge in the external envelope of a building with older properties having single glazed windows with timber frames and poor draught-proofing. To reduce heat losses through doors and windows requires double or triple glazed elements, as is common in Scandinavian countries, as well as seals for opening lights and thermal breaks in the surrounding frames. Traditional windows have a U-value of approximately 5.7 W/m2ºC and this can be improved to less than 2.0 by careful design and specification.

Heat is also lost by means of ventilation and infiltration. Ventilationis the controlled removal of pollutant-laden air and its replacement by fresh air. In a domestic situation ventilation can be natural, through windows, chimneys, flues and the like or mechanical, by means of extract fans. Infiltrationis the uncontrolled movement of air in a building. It can arise though gaps in the building fabric, which may seem insignificant but may account for up to 50% of the heat loss from a house.

 Heat Gains

Adventitious orfreeheating 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 sourcesoccupants 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 termedpassivesolar designthe termactivesolar design is generally reserved for systems for water heating or photovoltaics.

Passive solar design takes into account site factors such as orientation and layout and the relationship of one house to another adjoining house in order to avoid overshadowing. It also requires a particular consideration of glazed areas and the correct sizing and balance of glazing according to orientation and room use. Selection of glazing types and shading and the use of thermal mass in a building are also necessary in order to reduce the likelihood of overheating. Glazed building elementswindows in domestic properties and curtain walling in commercial propertiesprovide the best opportunities for maximum solar gains but conversely are the worst thermal insulators. It is necessary to consider the net balance of energy through such elements on a month-by-month basis, particularly to determine likely heating loads across the season and to avoid excessive summer temperatures.

 Building Services

Modern heating systems comprise a primary heat producing unita boilerand 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.

Boilers may use a variety of fuel types, including mains gas, oil (kerosene or gas oil) or bulk LPG (butane or propane). Independent boilers can also be fueled by coal, anthracite or wood. The key consideration is the design of the heating system in order to operate at optimum load conditions and, linked to this, the selection of a boiler with high seasonal efficiency. Information on boiler efficiencies can be obtained from manufacturers or, in the UK, from the SEDBUK (Seasonal Efficiency of Domestic Boilers, UK) website.

Condensing boilers are generally more efficient boilers, designed to utilise the latent heat released by the condensation of water vapour in the combustion flue products.

Water heating may be provided by the main heating system or it may be supplied using an independent water heating systemsuch as an immersion heater in a hot water storage cylinder. Some boilers in domestic use in the UK are combination orcombiboilers, which have the capacity to provide domestic hot water directly, in some cases containing an internal hot water store.

Heating systems may be controlled in a number of ways. In the simplest systems a room thermostat is used. This is a sensing device that measures the air temperature within the building or in different rooms and switches the space heating system on or off. Systems may also incorporate a time clock enabling a user to choose one or moreonperiods in a daily or weekly cycle. The most sophisticated systems will have a variety of controls, including for example, thermostatic radiator valves (TRVs) for each radiator, an external thermostat for weather compensation, a boiler thermostat and a programmer. These may all be brought together in a boiler energy manager system.


Renewable Energy

At the scale of an individual building it is possible to employ a number of technologies to generate energy locally, collectively known asmicro-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


Section 6.8 is sourced from ORBEE under a Creative Commons Licences Attribution-Non Commecial-Share Alike License
http://www.orbee.org/teaching-learning-resources.html?view=oerareas&expand=3%3A25