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Low Energy Buildings An Overview. Dr Nick Kelly Energy Systems Research Unit (ESRU) Mechanical and Aerospace Engineering University of Strathclyde. Content. Buildings and Energy Consumption Legislating for Better Performing Buildings Focus on Housing PassivHaus

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low energy buildings an overview

Low Energy BuildingsAn Overview

Dr Nick Kelly

Energy Systems Research Unit (ESRU)

Mechanical and Aerospace Engineering

University of Strathclyde

  • Buildings and Energy Consumption
  • Legislating for Better Performing Buildings
  • Focus on Housing
    • PassivHaus
    • What is a zero carbon building?
  • Zero Energy/Carbon Housing
    • Case Studies
    • Simulations
  • Implications of Zero Carbon Housing
    • LV Network
    • Heat Networks
buildings and energy
Buildings and Energy

The built environment accounts for over 50% of delivered energy consumption

The majority of energy consumed is for space and water heating, not electricity in most buildings

The domestic sector in the UK accounts for almost 37% of final energy consumption

The energy efficiency of the bulk of the housing stock is poor with an average SAP rating of 52 [scale of 1-100]

Generally the older the building the poorer the energy performance: ~70% of the UK housing stock is pre-1970

There is significant scope for energy efficiency improvements with savings in space heating of up to 90% achievable

improving energy efficiency
Improving Energy Efficiency

There have been various pieces of EU-wide legislation to improve the energy performance of housing (and buildings in general) in an attempt to mitigate their GHG emissions

Energy Performance in Building Directive (2002)

Energy Efficiency Action Plan (2006)

End-use Efficiency and Energy Services Directive (2006)

These have been implemented at a sub-national level through mechanisms such as the building regulations (England, Scotland, Wales and Northern Ireland)

The UK has been active in developing its own targets (Energy White Papers 2003, 2007), Climate Change Act (2008); all made clear that radical improvements in domestic energy efficiency was key to achieving deep carbon reductions (80% by 2050)

improving energy efficiency1
Improving Energy Efficiency

The EPBD was a milestone in energy conservation legislation and set out minimum energy performance targets for buildings

Many EU member states have gone beyond the EPBD and set out their own criteria for low-energy buildings

  • The PassivHaus standard is gaining popularity around Europe as a benchmark for energy efficient buildings
  • The requirements for a Northern European PassivHaus are:
    • very high levels of insulation (wall U-values less than 0.15 W/m2K);
    • high-quality building construction (thermal bridge free, air-tightness of construction better than 0.6 air changes per hour [ACH] measured at 50 Pa with ventilation openings closed, equivalent to 0.03 ACH under normal conditions);
  • The requirements for a Northern European PassivHaus are (contd.) :
    • high-performance glazing (U-value less than 0.85 W/m2K including installation, frame and glazing edge losses, solar transmittance greater than 50%);
    • a high efficiency ventilation system with heat recovery (MVHR);
    • construction mass, ventilation openings, thermal capacity and shading designed for comfortable summer temperatures; high efficiency appliances and lighting.
zero carbon housing
Zero Carbon Housing
  • The PassivHaus standard focuses on reducing energy demand
  • Whilst microgeneration can be integrated with a PassivHaus, their contribution is not counted
  • A step beyond PassivHaus is to offset the energy demand of the building using local zero-carbon generation
zero energy carbon housing
Zero Energy/Carbon Housing
  • What does zero energy or zero carbon mean? There are multiple definitions:
    • Autonomous Zero Energy Buildings – all demand are met by on-site generation, no external network connections
    • Net-zero site energy – local generation completely offsets on-site demand, demand and supply are not temporally matched but balance over a year
    • Net-zero source energy - local generation completely offsets primary energy demands, demand and supply are not temporally matched but balance over a year
zero energy carbon buildings
Zero Energy/Carbon Buildings
  • What does zero energy or zero carbon mean? There are multiple definitions:
    • Lifecycle net-zero energy buildings - local generation completely offsets primary energy demands AND embodied energy, demand and supply are not temporally matched but balance over the lifetime of the building
    • NB For a building to be zero carbon (as opposed to just zero energy) then the local generation needs to be carbon free: e.g. PV, solar thermal, biomass, SWEC
zero energy carbon housing1
Zero Energy/Carbon Housing

Primary Energy

Site Energy

zero carbon housing standards
Zero Carbon Housing Standards
  • England and Wales have been one of the first areas in Europe to develop specific zero carbon standards for new homes
  • Code for Sustainable Homes (CSH) (DCLG, 2008a) defined voluntary levels of performance (Codes 1 to 6)
  • Two of these levels are described as zero-carbon buildings:
    • Code 5 equates to a 100% reduction in regulated energy use, with these being offset by renewable heat and power generation. Energy use from appliances is not offset (so-called unregulated energy use).
zero carbon housing standards1
Zero Carbon Housing Standards
  • Code 6 proposed zero-carbon for all energy use including appliances by 2016.
  • This equates to a 140% reduction in calculated regulated energy uses such as heating and cooling (i.e. 100% reduction in regulated energy use plus 40% from local electrical generation technologies such as PV to offset the unregulated energy use by appliances)
zero carbon housing standards2
Zero Carbon Housing Standards
  • To encourage the development of a zero-carbon housing market, the UK Government has announced a tax incentive for new zero-carbon buildings to become effective in 2016
  • There is also a feed-in tariff (FIT) for small-scale renewable electricity generation and a proposed renewable heat incentive (RHI) to replace the defunct low-carbon building programme
  • Technologies covered under the FIT include PV, CHP and SWECs
  • Note there is growing doubt as to whether the RHI will actually appear in the proposed form
supply technologies
Supply Technologies





Biomass boiler


Heat Pump

storage +

storage +

storage +

Non despatchable


Solar thermal


class exercise
Class Exercise
  • Get into a group of 4
  • Consider a zero-carbon house for one of the following areas
  • Rural Scotland
  • Urban Scotland
  • Urban Greece
  • Rural Australia
  • Bullet the 4 main features you think should be present in the house design
zch examples
ZCH: Examples
  • BedZED, UK (2002)
  • The Beddington Zero Energy Development (BedZED) project in Hackbridge, London was an attempt to create a zero-energy community
  • Consisted of 99 super-insulated dwellings of various sizes, workspaces and community facilities.
  • on-site zero-carbon generation provided by a prototype 120kWe wood-waste fuelled combined heat and power (CHP) system with 777m2 of photovoltaic panels
  • designed to meet all of the energy demands of the residents along with providing the potential to power up to 40 electric vehicles at some future date.
zch examples1
ZCH: Examples
  • Monitoring indicated:
    • that space heating demand was 90% lower than the UK average
    • electrical power consumption for appliances and lighting was 33% lower than average, however this was still higher than anticipated due to residents using back-up electrical water heating
  • The development did not achieve zero-carbon operation, primarily due to the fact that the prototype biomass CHP system was unreliable and never operated effectively (it was shut down in 2005)
  • The remaining PV system only offset around 20% of the total energy demands of the community with the remainder being drawn from external supplies
zch examples2
ZCH: Examples
  • Portland House, Australia (2009)
  • Demonstration family dwelling in Portland Victoria with a super-insulated and tightly sealed fabric, thermal mass and a cooling tower (promoting stack ventilation) for summer cooling
  • All appliances in the dwelling were low-energy
  • Energy was provided by a 1.4kW PV installation and 3 solar thermal panels augmented with an electric back-up heater were used to meet the dwellings’ hot water demands
  • A reversible heat pump met the dwelling’s small space heating and cooling requirements.
zch examples3
ZCH: Examples
  • Monitoring over the period September 09 to June 10 indicated that the PV installation provided 93% of the dwelling’s electrical energy consumption
  • Note that this period did not cover all of the winter months and it is likely that the quantity of imported electricity will be slightly greater over the course of a full year.
  • The developers have since indicated that the PV installation is due to be doubled in size to a capacity 2.8kW. Given the current performance, this is likely to turn the building into a net electricity exporter.
zch examples4
ZCH: Examples
  • Wheat Ridge, Colorado (2005)
  • The Habitat for Humanity home in Wheat Ridge, Colorado is a 119m2 super insulated dwelling e.g. the building features low-e solar glass with argon fill and a U-value of 1.14W/m2K
  • Renewable heat provided by a 9m2 solar collector with a 760 l thermal store, backed up by a gas-fired water heater
  • Renewable electricity provided by a 4 kWp roof-top PV system
  • The building also features a mechanical ventilation system with heat recovery.
zch examples5
ZCH: Examples
  • Monitoring indicated that during the period April 2005 to the end of March 2007, the PV system produced an excess of 1542 kWh
  • Gas consumption for the water heater amounted to some 1670 kWh.
  • The net site energy requirement for all fuels of the home was approximately 0.6 kWh/m2/year.
zch lessons
  • From the evidence of the case studies achieving zero-carbon operation is not straightforward!
  • A building designed to be zero-carbon does not necessarily perform as zero carbon
  • Most fail to achieve this due to 1) under-prediction of demands at the design stage 2) over-prediction of energy yields from renewables or 3) poorly performing equipment in-situ
  • PV seems to be the ubiquitous option for electricity generation
  • Most achieve significant reductions in thermal demand … less success with curbing electrical and demands
zch simulating performance
ZCH: Simulating Performance
  • In addition to data emerging from the limited number of demonstration “zero-carbon” buildings simulation can provide useful performance information
  • This study investigated a typical detached dwelling with a floor area of 136m2. The building was assumed to be occupied by a family of 4 in which the parents work;
  • Occupancy was therefore intermittent;
  • The building performance was assessed for a Scottish West-coast climate.
  • NB The mechanics of building simulation will be covered in more detail in the next presentation
zch simulating performance1
ZCH: Simulating Performance
  • modelling indicated that for the detached house to achieve the 140% reduction target as set out in the Code for Sustainable Homes required the following measures:
    • 46m2 PV electrical generation (approx 6 kWp)
    • passive house building envelope
    • high efficiency mechanical ventilation heat recovery (MVHR)
    • 4m2 Solar thermal panels
    • 2kW Biomass heating
    • high A+ rating efficiency appliances
    • high efficiency lighting
zch simulating performance2
ZCH: Simulating Performance
  • To illustrate the change in energy performance of housing, a comparative simulation was undertaken with the proposed zero-carbon house and a more conventional dwelling (built in UK post 1997)
  • Dynamic simulation of the building energy flows over a 1-year period @ 10 minute time intervals, west coast climate (solar radiation, temperature, wind speed, wind direction), pre-defined occupant and equipment heat gains, control settings (on/off times, set point temperatures), etc.
  • Produces large volumes of time series data: temperatures; heat fluxes, electrical production (PV), etc.
zch simulation results
ZCH: Simulation Results
  • Aggregating the data from the simulation gives the following annual performance characteristics
zch analysis
ZCH: Analysis
  • Dramatic reduction in demand for space heating, dropping from 3083 to 372 kWh
  • The heat-to-power ratio of the simulated dwelling shifts from 1:1 in the base case dwelling to 0.7:1 in the zero carbon dwelling
  • The calculated electrical demand reduces from 5776 to 3240 kWh (due to the adoption of high efficiency appliances)
  • The modelled electrical demand for the zero carbon dwelling also indicated a drop in peak demand of around 10%
zch analysis1
ZCH: Analysis
  • The addition of 6kWp of PV generation to the simulated dwelling gives rise to striking seasonal variations in electrical energy flows: in winter 768 kWh of electrical energy is imported, whilst in summer a total of 1,756 kWh if exported to the network
  • There are also significant daily variations. For example, on an average summer day, a peak electrical demand of 4 kW occurs in the morning. By mid-day this has changed to a peak electrical output of 5 kW
  • The variability in solar hot water production is as pronounced with 40kWh produced by the solar collectors in January, offsetting around 18% of the total thermal demand compared to 219kWh in June, which exceeds the total thermal demand in the zero carbon dwelling.
  • The dramatic reduction in demand for space heating raises questions as to how space heating could be provided in future dwellings.
    • Often in current UK dwellings, low space heating demands in premises such as flats have tempted developers to install low-cost technologies such as electric resistance heating or storage heating.
    • Any move to electrically-based heating technologies will further change the energy demand patterns of zero carbon dwellings as heating energy loads are displaced from the gas grid to the electrical grid
  • Thermal energy demands for the zero carbon dwelling shift from space heating to hot water
    • This offers an opportunity for load shifting with regards to when heating is provided – a particularly useful feature if those thermal loads were met by cogeneration, heat pumps or direct electric heaters - enables a high degree of control over heating-related electrical loads and/or generation.
  • The low overall heating demand seen in the zero-carbon house also raises some serious questions regarding the development of heat networks (e.g. zero-carbon district heating schemes).
  • The very low heat requirements make the prospect of heat distribution unlikely in all but the highest density housing developments such as flats: elsewhere the small revenue from heat sales per dwelling would be unlikely to offset the capital costs associated with the installation of metering, piping, heat sources, thermal storage and other balance of plant.
  • The heat-to-power ratio of the simulated dwelling shift from 1:1 in the base case dwelling to 0.7:1 in the zero carbon dwelling.
  • This seems to favour lower heat-to-power ratio equipment such as bio-fuelled internal combustion engines or (in future) fuel cells
  • Electrical energy becomes the predominant demand in the dwelling - currently over 80% of demand is thermal energy
  • The reduction in electrical demand in zero-carbon houses is less dramatic than is seen for thermal demands, with in the case examined here.
  • The modelled electrical demand for the zero carbon dwelling also indicated a drop in peak demand of around 10%
  • Using a large areas of roof-mounted PV could lead to power management problems in climates with the highly variable daily and seasonal solar insolation levels such as those seen in the UK.
  • To curb carbon emissions from dwellings building standards are being tightened leading to the development of a raft of low-energy building definitions
  • In Europe, the Passive House standard is seen as a reference for low-energy buildings and has been demonstrated to reduce space heating demands by up to 90% (compared to conventional dwellings)
  • Zero-carbon and zero-energy building standards are beginning to emerge such as those defined in the UK’s Code for Sustainable Homes
  • There are different definitions as to what constitutes a zero-carbon or zero-energy building; the most common definition appears to be net-zero-source-energy where on-site renewable sources are used to offset a building’s primary energy demands.
  • There are numerous examples of zero-carbon demonstrations throughout Europe and North America.
  • Monitoring of actual performance indicates that most do not actually achieve zero-carbon operation.
  • Zero-carbon buildings will have radically different energy demand characteristics compared to existing housing.
    • space heating demand is minimal
    • electricity for appliances and lighting becomes the major energy demand
  • In Northern latitudes there are very significant seasonal variations in PV electrical production the high levels of export to the network in summer could result in significant power management problems in areas with high levels of PV penetration.
  • Significant swings from export to import were also evident throughout the course of a day.
  • The change in the primary thermal demand from space heating to hot water provision affords opportunities for significant demand/supply control opportunities if adequate hot water storage is provided and if some of the heat demand is met by heat pumps or micro-cogeneneration devices.