Solutions are presented in accordance with the stepped approach taken in the energy hierarchy. There are many solutions available within each step and their appropriateness and applicability will vary according the development type, nature and scale, likely building occupants, site constraints and the technical capability of the solutions themselves.
When designing for sustainable energy management it is important that issues and opportunities are identified at the outset as part of an on-going discussion between client, architect/designer and the planning authority.
Reducing demand and energy efficiency
There are many energy efficiency solutions that address the first steps on the energy hierarchy. The applicability of these solutions depends on the type of project, in particular, whether it is a new building or a refurbishment. Specific attention is drawn to considerations relevant to the residential, commercial, educational, health and industrial sectors.
In the case of new buildings, energy issues should be given consideration at the early stages of design (ideally at project inception) to enable the best technical and economic solutions to be achieved. These measures are influenced by decisions made throughout the design process.
Cost implications are variable. In many cases, these design choices can lower capital costs through reducing building service requirements. In addition, these passive energy saving measures typically require little or no maintenance and last throughout the lifespan of the building with no energy input.
At an early design stage there are many opportunities to develop an integrated approach to energy use and savings, including the determination of the physical form and characteristics of the building.
Consideration should be given to the following
- Orientation - Sun path around the site, prevailing wind direction and the need for clear views.
- Reducing the amount of exposed external area compared with enclosed internal volume.
- Incorporation of atria, courtyards or sunspaces can reduce energy consumption in deep plan buildings.
- Thermal mass should be fully considered and included or excluded as appropriate, to correspond with activity and occupancy patterns.
- Landscape design can influence the microclimate (and reduce energy demand) by providing shelter from driving rain and wind.
- The ratio of glazing to solid material in the external walls can be optimised to provide benefits from natural light and useful solar gains, whilst avoiding excessive heat losses or gains.
- Where appropriate and possible, these passive solutions should also be applied to refurbishment projects.
A - Warm air exists at high level due to natural 'stack' effect.
B - Well insulated and air tight building fabric.
C - Thermal mass in exposed concrete floors/ceiling.
D - Warm air rises naturally up the atrium. drawing cooler air in from outside.
E - Trees can provide shade in the summer and shelter form the wind and rain in exposed areas.
F - Overhanging eves to provide solar shade.
G - Integration of renewable energy, e.g. solar panels.
The majority of heat loss from a building historically occurred via conduction through the walls, roof, windows etc. However, Part L has improved the U-values of new buildings sufficiently such that the heat lost via air movement has become an important area to consider.
Warm and cold air moves around a building through both controlled means (ventilation design) and uncontrolled means (infiltration leakage). Infiltration is generally undesirable as it disturbs the ventilation strategy and results in excess heat loss in winter.
Careful detailing of the building fabric and junctions between building elements such as windows and external walls reduces heat losses from air leakage. Targets for air leakage rates should be specified at the design stage and verified during construction through pressure testing, which gives the opportunity to address any problem areas.
Part L 2010 includes a maximum permissible air-leakage rate (for both domestic and non-domestic) of 10 cubic metres of air every hour for every square metre of internal floor area (m3/m2/hr) when tested at an internal air pressure of 50 Pascals (which is achieved using fan blowers). Mandatory testing results evidence that many buildings perform significantly better than this figure, with leakage rates of 5m3/m2/hr@50Pa or less being commonplace.
Reducing air-leakage to 3m3/m2/hr@50Pa or less might introduce the need for mechanical ventilation, to ensure there is adequate ventilation at all times of year. Building Regulations Part F (ventilation) is therefore being updated in alignment with the air-leakage limits in Part L.
Mechanical ventilation is often regarded as an energy intensive solution; however, using mechanical ventilation with heat recovery (MVHR) in a very air-tight building with relatively low cooling loads can provide a very energy efficient solution. For instance, ‘PassivHaus’ buildings in Europe have completely avoided the need and cost of a conventional heating system by being extremely efficient and installing MVHR.
The increase in capital costs can be a few percentage points and it is claimed this can payback many times over within the lifespan of the building. The trade-off is the reliance on the mechanical ventilation system for heat and ventilation throughout the year.
Passive design and solutions
To achieve their energy efficiency benefits, passive solutions for ventilation, heating and cooling usually need to be applied in a controlled manner. In particular, the controlled use of daylighting, natural ventilation and passive heating and cooling can create a low energy building with reduced environmental impacts, whilst still achieving comfortable conditions for occupants. The following solutions can help achieve this:
Passive solar design
This is the capture of useful solar gains (heat) to offset heating energy requirements. Atria and sun spaces are typical examples of this approach.
Passive cooling techniques save energy, but also avoid the use of environmentally damaging refrigerants. The most simple and economic approach is the use of night purge ventilation in buildings with exposed thermal mass. However, this is not appropriate for every building and alternatives include the use of absorption cooling from waste heat, surface or ground water cooling, ground coupled air cooling, displacement ventilation, and evaporative cooling.
Night purge ventilation
During the day, heat gains from occupants, activities and solar energy are stored in the thermal mass of exposed concrete, stone or brick in the walls, floor and ceilings. This cools the space during the day without using energy.
At night, the heat that has built up during the day is released from the building fabric and leaves the space through an open window or vent. The space is then cool for the start of the next day. In winter, the vents can be closed and the heat retained.
Atria and courtyards
Enclosed glass volume, can be used to bring daylight and natural ventilation into the centre of deep plan buildings.
Void in the middle of a building, or group of buildings, which is open to the elements.
The use of natural light rather than artificial systems can offer significant energy savings. In addition to atria and courtyards, techniques to bring light further into deep plan buildings include sun pipes and solar reflectors. These can be either included during construction or can be retrofitted.
Natural ventilation can be used in situations where the external conditions are free from excessive noise or poor air quality and where the intended use of the space allows. In narrow plan buildings, a typical approach is to use openable windows. In larger spaces, additional measures would be required, for example the use of a ‘stack effect’ system (see earlier energy ef ficient design diagram).
Optimisation of glazed areas - balancing daylight against heat gains and losses
Uncontrolled solar gains can result in overheating and glare.
The use of solar shading or solar control glazing allows diffused light into the space and keeps excessive heat gains out.
Narrow floor plates (to facilitate natural ventilation)
Narrow floor plates: opening on single side
Maximum depth of 7m for effective natural ventilation via an opening on one side of the space.
Narrow floor plates: opening on opposite sides
Maximum depth of 15m for effective natural ventilation via openings on opposite sides of the space.
Materials such as concrete, brick and stone absorb heat, can help prevent a building from overheating.
Stored heat is released later when required.
Building services or active solutions
Building services equipment, such as boilers, air handling units and lighting systems use energy to provide comfortable internal conditions for building occupants. Where it has been determined that mechanical services are required to provide comfortable internal conditions, efficient plant should always be specified.
Minimum boiler efficiencies are recommended in the compliance guideline documents that accompany Part L: the ‘Domestic Building Services Compliance Guide’ and the ‘Non-Domestic Building Services Compliance Guide’.
The recommendations in these documents are for the minimum acceptable performance and should be bettered where possible. For instance, boilers with an operating efficiency of over 90% are readily available at no or minimal additional cost. The official boiler efficiency database is available from the Building Energy Performance Assessment.
For refurbishment projects, a building services survey will identify services that are underperforming or nearing the end of their life cycle. The opportunity to upgrade them with the most efficient replacements should always be assessed, particularly as making replacements during the refurbishment will almost always be more cost-effective than returning in future years.
‘Easy wins’ that pay back in a short timeframe, such as additional insulation to existing pipes and ducts or the upgrade of lighting ballasts, should not be overlooked in favour of ‘big ticket items’ such as new boilers or renewable energy.
If mechanical ventilation is being used then energy efficient fans should be specified. The building services compliance guides (referenced above) contain minimum performance values for specific fan power, which should be bettered where possible.
In air-tight buildings, the use of heat recovery should be investigated, in the form of plate heat exchangers, thermal wheels, or run-around coils, depending on the space availability and ventilation requirements. Note that using heat recovery in buildings that are not air-tight can lead to an increase in energy consumption. Ideally the building services designer would model the fan and seasonal energy consumption under several realistic scenarios.
Plate heat exchangers
Air (or water) moving in opposite directions is separated by thin sheets of metal, which allow heat transfer. In this way, heat can be recovered from the exhaust air and used to pre-heat incoming air.
The use of air conditioning and comfort cooling should be restricted to those areas where it is strictly necessary. Artificial lighting systems for use internally and externally should be as energy efficient as possible. Low energy light bulbs are widely available and use only 20% of the energy of standard bulbs, and can therefore offer significant energy and maintenance/replacement savings.
For refurbishment projects, services should be upgraded wherever possible. If costs are prohibitive, then more economic measures could be implemented, such as insulation of pipes and ductwork.
Building management systems/controls
The use of an appropriate control system can significantly reduce energy wastage. For example, sensors can be used to detect when there is sufficient daylight in a space and can dim or switch off the artificial lights in response.
Variable speed controls on pumps and fans can save a significant amount of energy. This allows building services to use only the energy that is required, rather than continuously operating at full capacity.
A BMS (Building Management System) is a sophisticated network of sensors and controls for the building services. A BMS can be programmed for optimum operation and minimal energy use. BMA are typically installed on larger projects.
Use of a BMS facilitates energy monitoring and can highlight areas of excessive use where savings could be made. In smaller buildings, a simpler approach could be to take regular readings of utility meters and any submeters. However, a benefit of having a BMS is that it frees up time that might otherwise be spent taking meter readings, so more time can allocated to the analysis of the data and to developing a plan for making improvements.
Many of these systems can be retrofitted, although this is not a reason to exclude them at the design and construction stage.
Consideration should be given to the maintenance implications of installing controls and sensors. In a few instances, the additional maintenance and replacement costs associated with e.g. additional sensors and motorised controls (such as daylight controlled blinds) can be greater than the cost savings arising from reduced energy consumption.
For refurbishment projects, a building survey will identify services that are underperforming or nearing the end of their life cycle. The opportunity to upgrade them with the most efficient replacements should always be assessed, particularly as making replacements during the refurbishment will almost always be more cost-effective than returning in future years.
‘Easy wins’ that pay back in a short timeframe should not be overlooked in favour of ‘big ticket items’ such as new boilers, windows or renewable energy.
- Draught proofing of windows, doors and service penetrations.
- Insulation to hot or chilled water pipework.
- Extra lagging to any hot water storage vessels.
- Sealing of ductwork leakage.
- Replacement of AC pumps and fans with variable speed DC/EC motors.
- Upgrade of burners in an existing boiler.
- Upgrade of light fittings (might also require new ballasts).
Renewable and low carbon energy solutions
The use of cost-effective renewable or low carbon (RLC) energy sources reduces the use of conventional energy and associated greenhouse gas emissions.
The suitability of renewable energy technologies varies from project to project and is dependent on site factors, location and funding availability. You will need to consider a number of questions when deciding which solution(s), if any, are most suitable:
- Is it right for this location and this type of building?
- Will it meet the energy needs of the occupier AND reduce CO2 emissions?
- Who will be responsible for maintenance and management?
- How difficult will it be to maintain?
- Will it have a detrimental impact on neighbours and the wider environment/landscape?
- Is it compatible with other design solutions being implemented?
- What other technologies and support will be required for occupiers to use it effectively?
- Is there enough space?
- Does it need separate planning permission, or other permissions?
Hertfordshire renewable and low carbon study
The study provided a high level analysis of the feasibility and appropriateness of a range of RLC technology solutions across Hertfordshire, including the solutions presented below.
Funding streams for RLC solutions
Increasing the use of renewable energy is important for Government to achieve its national and international targets. Various financial incentives have been recently introduced to enable renewable and low carbon solutions to compete on an economic basis with conventional fossil fuels. These include:
- Energy efficiency grants – discounts on wall and loft insulation and better boilers and controls. Developers and tax-paying building occupiers can gain cash flow discounts on energy efficiency products using Enhanced Capital Allowances (ECA). Existing building owners in Hertfordshire can benefit from the HEEP scheme.
- Feed In Tariffs (FIT) – incentives for low carbon electricity generation from micro-technologies such as photovoltaic panels and micro-CHP. The utility company pays the consumer for electricity generated.
- Renewable Heat Incentive (RHI) – to be introduced to incentivise low carbon heat generation from micro-technologies such as solar thermal, biomass boilers and ground source heat pumps.
- Renewable Obligation Certificates (ROCs) – tailored towards large energy installations such as wind farms and biomass power plants. The options above are more appropriate for building projects.
In addition to the consideration of technical and economic feasibility on projects, it is vital to address planning issues associated with the installation, including local environmental and visual impacts.
The following gives an overview of available RLC technologies:
Combined Heat and Power (CHP)
Both electricity and thermal energy (for space heating and/or hot water) are produced from a single energy source, which is typically natural gas. Although not a renewable energy technology, this can be a very efficient use of fuel for some buildings and reduces their overall carbon emissions.
This technology is most efficient when sized to operate at a thermal ‘base load’. Both CHP and Biomass boilers perform best when sized to meet the thermal base load. The thermal base load is the amount of heat continuously required by the building. This is illustrated in the following graph showing a generic demand profile:
The base load is that below the dotted line. Peak energy demands result from increased activity and heat requirement throughout the day. Peak demands are typically met by using conventional gas boilers.
Additional systems meet the peaks of demand. This technology is best suited to buildings where there is a heat demand that matches electrical demand, such as process industries, swimming pools, community heating systems and hospitals. Possible variations on the basic system include the addition of absorption chillers, or the use of biomass as a fuel source, although it should be noted that these options will impact on the economics of the system.
Solar collectors or solar thermal panels generate hot water using the thermal energy of sunlight which is used to offset conventional energy use for provision of hot water for showers and taps. This technology is well established, reliable and typically provides reasonable economic paybacks.
Panels are ideally applied to south facing roofs pitched at 30-45 degrees and can be freestanding or integrated into the roof, with pipework leading into the building and connected to a storage tank with a back-up heating supply.
Photovoltaic (PV) panels or tiles convert solar energy into electricity and are available in a variety of styles, colours and materials. Panels can be freestanding, or can be integrated into the south facing facades or roofs of buildings. Systems are best elevated at 30-45 degrees from horizontal. It is also possible to manufacture the PV cells into glass laminates providing the dual benefits renewable electricity and solar shading to internal spaces.
PV tiles are a relatively new form of the technology and have been designed to match the colour and appearance of conventional slate tiles. They therefore can be a good solution to use on buildings within conservation areas.
The installation of photovoltaic panels has been boosted by the Feed In Tariff for small scale electricity generation and the Green Deal in coming years. The FIT dramatically reduces the payback period of photovoltaic panels and has enabled very interesting options to emerge such as free PV using third-party finance. The applicability of these options depends on who actually owns the panels once installed.
Turbines generate electricity from wind and are available in a variety of sizes and scales. Suitable sites must be exposed and have an average wind speed of at least 6 metres per second for a large portion of the year.
There are various options for the configuration of turbines. Small scale models can be roof mounted. Typically they have a rated output of around 1.5kW. Alternatively, small scale freestanding models are available at rated outputs of 15-250kW. These range in size up to industrial scale models that can be seen on wind farms at rated outputs of 2-7 MW.
Turbines are categorised as either horizontal or vertical axis, which relates to the axis the blades rotate around. Horizontal axis turbines are more powerful for their size, although some people consider vertical axis turbines to be more aesthetically pleasing.
When considering wind turbines of any scale it is crucial to consider the difference between the rated power output and the annual average output. The rated output is the maximum possible output under perfect wind conditions, which rarely occur during the year.
The ratio between this maximum output and the average output over the year is called the ‘utilisation factor’, which typically ranges between 20-30% for a commercial wind farm. Small scale turbines in an inappropriate location without strong winds have been found to have utilisation factors of less than 10%.
Ground Source Heat Pumps (GSHP) and Air Source Heat Pumps (ASHP)
Heat pumps convert low grade thermal energy from a constant temperature source to higher grade energy that can be used for space heating or hot water. GSHP draw upon low grade thermal energy from the ground or an aquifer and ASHP draw low grade thermal energy from the air outside a building. Both GSHP and ASHP are available in different sizes, for both domestic heating and commercial premises, and the process can be reversed to provide cooling during summer months.
GHSPs are better suited to new build applications as their efficiency is highest when supplying low temperature distribution systems such as underfloor heating. The use of GSHP is restricted to sites with enough land to either lay pipework in long trenches or with access to a suitable body of water such as a lake or aquifer.
The high ‘coefficient of performance’ or COP of a heat pump means it is an energy efficient technology – for each unit of electricity used to operate the heat pump, around four units of heating energy is produced via a GSHP and 2-3 units of heating energy via an ASHP.
The average COP over the course of a year is known as the Seasonal Performance Factor. The CoP of GSHP over a year may gradually reduce if the thermal energy of the heat source (the ground or body of water) is not sufficiently 'recharged' naturally or managed.
The burning of energy crops in a biomass boiler to provide heating and hot water is considered to be a ‘carbon neutral’ process, as the amount of CO2 released during combustion is equivalent to that which is absorbed during the growing cycle of the crops.
Biomass fuel can be delivered in the form of either woodchips or pellets. Pellets contain much more energy within them, so a smaller volume needs to be stored between each delivery. Pellets can also be pumped like a fuel between the delivery lorry and the storage hopper, which makes deliveries quicker and more straight-forward.
Automated systems feed the fuel through to the combustion chamber from a hopper, which needs to be refilled on a regular basis. On domestic scale systems, this is typically once per week, but depends on the system size and energy demand. Ash also has to be removed approximately once every month.
Both wood pellets and wood chip require space for storage, with woodchips requiring a larger storage volume, and delivery. It is therefore important to consider the storage and delivery access arrangements, for both new build and retrofit projects, early in the design stage if a biomass boiler is to be used.
Although it is always desirable to grow the biomass crops locally, crops are almost always grown by a separate company who sell the crop via fuels agents on the open market. Installers of biomass boilers can advise on local options. The carbon and ecological footprint associated with the cultivation and transportation of biomass fuel is something that should therefore be considered before opting for a biomass solution.
If considering this technology on an urban site, care should also be taken to ensure that the specified equipment meets the requirements of any designated smoke control zone.
District heating networks
The circumstances of new development may make it difficult to utilise on-site RLC solutions to achieve significant carbon savings. Therefore, when seeking to achieve low or zero carbon developments, it may be necessary to consider off-site solutions or 'allowable' solutions. A recognised and widely used off-site solution in the UK and Europe is district heating networks.
District heating is an alternative method of supplying heat to buildings, using a network of super insulated pipes to deliver heat to multiple buildings from a central heat source. Heat is generated in an energy centre and then pumped through underground pipes to the building. Building systems are usually connected to the network via a heat exchanger (also known as a heat interface unit (HIU)), which replaces individual boilers for space heating and hot water.
Whilst there is some amount of thermal loss from the heat distribution infrastructure, the aggregation of small heat loads from individual buildings into a single large load allows the use of large scale heat technologies, including the capture of waste heat from industrial processes or power generation, or other large scale heat generation technologies which are not viable at a smaller scale. Of particular interest is combined heat and power (CHP) technologies.
Sector specific issues
Different types of buildings have individual requirements in terms of energy use.
Passive systems, such as natural ventilation and daylighting are traditional solutions for domestic buildings. Passive solar design can be particularly effective, with south facing sun spaces giving free energy and pleasant living conditions (note these are not the same as glazed conservatories). These must be design appropriately to avoid excessive solar gain and overheating.
An alternative low energy approach which is emerging from the influence of Scandinavian design is the construction of an extremely well insulated and air tight building with a mechanical ventilation system incorporating efficient heat recovery. This building design is sometime referred to as PassivHaus.
Small scale renewables can be appropriate. Technologies which lend themselves to this particular sector include solar thermal and photovoltaic panels, and biomass boilers. Micro combined heat and power (micro CHP) units for domestic use are gradually emerging onto the market.
Historic and listed buildings also present specific issues and circumstances that need to be taken into account when seeking to implement energy efficient and sustainable solutions.
Listed buildings and buildings within conservation areas are subject to tighter planning controls than the rest of the housing stock in regards to changes to facade and the fabric of the building. Specific guidance on how to achieve energy efficient refurbishment or renovation, and the use of sustainable solutions within historic and listed buildings can be found on Historic England.
Although natural ventilation is typically the preferred choice, it is not always appropriate in commercial developments. If the site is noisy, has poor air quality or high internal heat gains from intensive use of IT or other equipment, it may be more appropriate to seal the building and use a mechanical ventilation system. In this case, measures for energy ef fi ciency should be adopted, such as free cooling or heat recovery.
The use of daylight should be promoted and could be combined with control systems to switch off electric lighting when not required. This is typically a significant form of energy consumption in commercial buildings.
Atria can be particularly effective in this sector and can be used to bring natural light in and encourage air movement in deep plan spaces.
Renewable and low carbon energy technologies can be suitable, although biomass boilers for heat or combined heat and power do not tend to be suitable for most commercial developments due to their intermittent occupancy and relatively low heat demand.
As mentioned previously, it can be beneficial to reduce unregulated energy consumption in operation. IT and media equipment constitutes an increasing proportion of ‘unregulated’ energy loads, particularly in IT-rich environments such as offices and schools. IT equipment requires electricity to operate and releases waste heat, which can contribute to rooms overheating and becoming uncomfortable, or lead to the use of air-conditioning that requires even more energy to be consumed.
The procurement of efficient IT equipment reduces the energy consumed to run IT devices and helps to avoid the use of air-conditioning. Efficient IT features include:
- Low-energy flat screen displays (LED displays are emerging on the market).
- Thin client devices and virtual desktop management.
- Power saving measures and procedures, such as automatic hibernation mode or night-time power-down.
- Behaviour change campaigns, such as stickers/posters to encourage ‘switch off your PC for lunch’.
- Hosting IT server activities at an offsite ‘green data centre’, whose energy efficiency will be measured as a power usage effectiveness.
Passive measures are typically desirable in primary and secondary schools. The activities in further and higher education buildings can require a more intensive services strategy; efficient equipment should be specified. Controls systems that turn lights off in unoccupied classrooms, corridors and toilets should be incorporated.
The use of renewable and low carbon energy technologies can be particularly appropriate in education buildings, to raise awareness of energy issues. There are several examples of schools with small scale wind turbines in Hertfordshire and across the UK. It should however be remembered that even educational installations should only go ahead if they will generate a meaningful quantity of energy. The educational benefits can backfire if the energy meters show that little energy is being generated for the money spent on the installation.
Furthermore, the visible installation of renewable energy causes some people to consume more energy than they would do otherwise, on the premise that since it is ‘free’ green energy it can be wasted. This is one example of what is termed the ‘rebound effect’.
Renewable and low carbon energy technologies can be particularly appropriate for hospital buildings. A large roof space is typically available for installations of solar thermal or photovoltaic panels.
Combined heat and power systems lend themselves to this type of building, as there is a high and constant energy demand for both electricity and heat. Ground source heating/cooling can also be effective where site conditions permit.
The approach to energy efficiency on industrial buildings very much depends on the activities being carried out. For example, there may be processes on site which generate waste heat, which could be reclaimed and used to heat other areas of the building.
Large industrial buildings may benefit from using radiant heating rather than convective or warm air heating.