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High Perf Windows: Help Realise The Zero Emmission Home
Published: 16/08/2010 by Dr Peter Lyons
GA article Final…Peter Lyons 21 April 2008
High-Performance Windows Help Realise the Zero-Emission Home
Towards Zero-Emission and Zero-Energy Homes
Energy efficiency is one of the first ways to cut greenhouse gas emissions, to reduce heating, cooling and lighting bills and to improve occupant comfort inside buildings. Understanding this, governments in many countries are developing new building standards design to slash and eventually eliminate emissions from buildings altogether, within 15 years or less. These countries include Australia, USA, UK, Germany, Japan and many others. High-performance windows are an essential part of the strategy in all climates. Such is the urgency with which we must address global warming.
This is the first of two articles in Glass Australia exploring opportunities and performance benefits from using high-performance windows in Zero-Energy Housing around the world.
Legislation
In the United Kingdom by 2016 all new homes will have to be zero emission on heating and cooling. The UK Government's 'Code for Sustainable Homes' legislates binding regulations for energy reduction with impressive targets: 25% more efficient by 2010, 44% by 2013, and 100%, or zero emissions by 2016 (Matt Wright, 2008)
In California the California Energy Commission (CEC) is charged with implementing new building standards under their ‘Title 24’ code which will require all new homes in that state to be ‘net zero energy’ by 2020 and all new commercial buildings to be zero energy by 2030 (CEC, 2007). The California designs will draw on knowledge gained under the Building America program, funded by the US Department of Energy.
In Germany the Fraunhofer Institute for Solar Energy Systems has been developing self-sufficient, near-zero-energy homes for over 15 years.
Do tougher energy-efficiency standards really work?
There are few long-term studies of the impact of higher efficiency standards for appliances and buildings. However, we can look to California for hard data and evidence of what has been achieved. California has a range of climates similar to Australia’s, apart from humid tropical. Remarkably, since the 1970s, California has held per-capita electricity consumption constant at 7,000 kilowatt hours (kWh) a year, while consumption for the US as a whole has grown relentlessly at 2% per year to more than 12,000 kWh today. This means that in California, gains from improved energy efficiency have cancelled out the growth that would have otherwise occurred, unlike the US as a whole which has a poor record.
Figure 1. California vs. whole of USA – electricity sales 1960 – 2008 (Rosenfeld 2007).
In Figure 1 (Rosenfeld 2007) the green wedge represents energy savings to date as a result of mandatory, improved building standards. Meanwhile back here in Australia our per capita electricity consumption is also more than 12,000 kWh per annum – similar to that of the United States. It is also growing at 2 – 3% a year.
As Professor Ross Garnaut has warned us, the challenge we face is not just to hold consumption and carbon emissions steady but to put our carbon emissions into reverse. Energy efficiency is one of the first ways to do this.
Energy-efficient windows have contributed to California’s energy savings, and California was one of the first US states to require NFRC (National Fenestration Rating Council) ratings for windows, skylights and doors. Australia has the AFRC which uses the NFRC certification system under the Building Code of Australia (BCA). Overall, efficiency gains in California have resulted from a raft of stringent energy-efficiency standards for buildings and appliances. In addition, their utilities are now provided with incentives to sell less energy, not more. Energy-efficient windows also help to reduce peak loads - especially peak cooling loads - which have resulted from the rapid growth of residential air conditioning in Australia. Peak-load stress on our generators is becoming a regular occurrence in many Australian states.
Before getting into the detail, it is worth considering what is meant by “zero-energy house” and “zero-emission house”.
One of the most stringent targets is the concept of the Zero-Emission Building / Zero-Emission House / Zero-Emission Home (ZEB, ZEH). Simply put, this is a building which emits no greenhouse gases as a result of activities on the site. This is achieved by
- minimising on-site energy consumption as far as possible, by means of energy conservation and energy efficiency;
- supplying any residual energy requirements from local renewable sources. Since such technologies are still relatively expensive, it is vital for the financial viability of the project that residual energy needs be minimised by having the most efficient design in the first place.
The term ‘Zero-Energy House’ denotes a slightly less stringent criterion, where energy may be purchased from off-site sources provided they are 100% renewable.
ZEB sounds simple enough, but what is the best way to get there? This article reviews the state of the art in several countries and suggests what sort of glazing and windows are needed to achieve ZEB. There are many other ZEH and ZEB examples not covered by this article. It is worth noting that if there are any remaining electricity needs, beyond those supplied by efficient design and on-site generation, they can be sourced from purchased Green Power, paid for by the savings realised through efficient design. This leverages growth in renewable energy generation and helps to displace fossil-fuel power stations.
Aus-ZEH: CSIRO Australian Zero-Emission House Project
Led by Dr Greg Foliente of CSIRO Sustainable Ecosystems, the Aus-ZEH project aims to deliver a blueprint for Zero-Emission Houses that will work anywhere in the country and can be built by existing builders. To satisfy the energy efficiency objective described above, Aus-ZEH designs need to score very highly – around 8 stars – using the AccuRate simulation software. Using AccuRate under Australia’s 2nd Generation NatHERS, a ‘perfect’ home that scores 10 stars requires no energy for heating or cooling. The Aus-ZEH home’s 2-star deficit will be more than made up using on-site photovoltaic (PV) panels, micro wind turbines, or other renewable energy sources. Enough surplus energy will be generated on-site to power all lights and appliances so that each Aus-ZEH home draws no electricity from the grid. A special software design tool called Homer, from National Renewable Energy Laboratory in Colorado, is being used to explore renewable energy mixes and trade-offs for the Aus-ZEH project.
Extensive AccuRate simulations done by CSIRO indicate that high-performance windows lift an otherwise reasonable design (5 or 6 stars) to very good: around 8 stars. This is consistent with findings in a study performed by Peter Lyons & Associates for AGGA last year. In this context, “high performance windows” means low-e, argon-filled double glazing in an insulating frame (wood, uPVC or thermally broken aluminium). The corresponding U-value will be 2.5 or less, measured for the whole window (NFRC rating).
In cool and cold climates, a high-performance window needs to have a high solar heat gain coefficient (> 0.55; higher if possible) achieved with a low-e coating on the third glass surface having a high solar transmittance. In warm and hot climates, or for large unprotected east- or west-facing orientations in general, a low SHGC is required (< 0.35). This is achieved by means of an insulating glass unit with toned (body tinted) outer glass followed by low-e inner glass, or alternatively spectrally selective low-e outer glass. Both these configurations cut solar gain dramatically while maximising light transmission. In temperate and cool-temperate climates (thus, most of south-eastern Australia) the use of such windows lifts a home’s energy rating by 2.0 – 2.5 stars over that achieved using the predominant residential window sold in Australia today, which is single-glazed clear in a thermally unbroken aluminium frame.
Lakeland ZEH Project, Florida (hot, humid climate)
The Florida Solar Energy Center and a local builder in Lakeland, Florida built two near-zero-energy homes side-by-side in 1998 in a residential subdivision (Figure 2). One home was the builder's standard model (the ‘Control’) and the other house was a super-energy-efficient photovoltaic residence with high-performance windows (‘PVRES’). The two homes were monitored simultaneously for more than a year. The goal “was to determine the extent to which contemporary energy efficiency technologies could reduce the demand for electricity in Florida homes.”
Figure 2. Bird’s-eye view of both Lakeland ZEH homes: completed control and near-zero-energy homes in the Windwood Hills development of Lakeland, Florida (www.fsec.ucf.edu/en/research/buildings/zero_energy/lakeland).
The windows in the Control house were standard, 3mm clear single glazing in thermally unbroken aluminium frames, overall U = 7.1 and SHGC = 0.75, while the super-efficient PVRES house had low-SHGC, spectrally selective windows with U = 2.0 and SHGC < 0.35.
The results were inspiring, with the PVRES home using 92% less electricity over a year than the Control home. If the houses were rated under AccuRate, this would translate to performance in excess of 8 stars. Furthermore the air-conditioner in the PVRES home was half the size of that in the Control house (thus saving on capital as well as running costs) and needed to be run only sparingly.
Habitat for Humanity Homes, Lenoir City, Tennessee (humid, mixed, temperate climate)
In a collaborative project between Oak Ridge National Laboratory, Tennessee, the Florida Solar Energy Center and the local utility (TVA), a Habitat for Humanity team constructed four low-cost, high-efficiency, three-bedroom homes over 2000 – 2004 (see Figure 3). A major goal was to see whether energy savings could pay for efficiency improvements in the homes and result in affordable homes for low-income families. The four high-efficiency houses have windows with U = 1.9 and SHGC = 0.33. These specifications are very similar to those in other projects. A base-case, control house, built for comparison, was (as it turns out) built to slightly above minimum code performance for the area with windows of U = 3.1 (SHGC not stated but probably around 0.65, assuming clear double glazing). Similar to the emerging Aus-ZEH homes and the Lakeland home in Florida, the Habitat homes have rooftop PV panels to offset residual electricity needs.
Figure 3. Habitat for Humanity low-cost ZEH homes in Lenoir City, Tennessee (http://160.91.4.30/info/ornlreview/v40_2_07/article02.shtml).
As with Lakeland, the monitored results from the Habitat project are very encouraging. On an annual basis the four low-energy homes use 40 – 60% less energy than the control house down the street. However as noted above, the control house has significantly better windows than the single-glazed norm in Australia. When the adjustment is made for this, effective annual savings are equivalent to as much as 75%. This translates to 7 stars or more if the homes were to be rated under AccuRate.
BedZED Project, United Kingdom (cool temperate, heating-dominated climate)
The Beddington Zero Energy Development (BedZED) is a very successful and well documented case study in the UK. Located in south London and completed in 2002, BedZED consists of 82 super-insulated row houses with supplementary PV on-site power generation (see Figure 4). Annual heating energy needs are 88% less than the UK norm. BedZED houses have low-e, argon-filled, wood-framed, high-solar-gain windows with U = 1.8. SHGC is not stated but given the heating-dominated climate and the need for maximum passive solar gain in winter, the BedZED windows would have an SHGC in excess of 0.60. The BedZED homes would easily rate 8 stars under AccuRate.
Figure 4. Aerial view of BedZED development, south London (www.inhabitat.com/2008/01/17/bedzed-beddington-zero-energy-development-london)
Conclusions
These case studies, from a wide range of climates, share windows whose U-value is typically around 2.0 W/m2.K or less In addition to the low U-value, the required solar heat gain coefficient varies according to climatic needs: high for heating climates and low for cooling climates or for orientations with severe solar exposure. With highly optimised, site-specific passive solar design it is sometimes possible to relax the window specifications, but such design and construction is quite rare in the mass housing market for new construction. It is even rarer in the vast existing stock of residential buildings which so badly need retrofitting. High-performance windows will play a big role in transforming Australia’s housing stock from amongst the least energy-efficient in the world towards true, exemplary carbon-neutral performance in heating and cooling.
References
- Wright, Matthew. 2008. www.beyondzeroemissions.org.
- California Energy Commission. 2007 Integrated Energy Policy Report. www.energy.ca.gov/2007_energypolicy/index.html
- Fraunhofer Institute for Solar Energy Systems. Freiburg, Germany. www.ise.fhg.de/publications.
- Rosenfeld, Arthur, 2008. Energy End-Use Efficiency. www.energy.ca.gov/2008publications/CEC-999-2008-005/CEC-999-2008-005.ppt.
- National Fenestration Rating Council, 2008. www.nfrc.org.
- Australian Fenestration Rating Council, 2008. www.afrc.org.au.
- Foliente, Greg, 2008. CSIRO Sustainable Ecosystems Zero-Emission House. www.csiro.au/news/ps2i8.html
- ZEH: Lakeland, Florida. www.fsec.ucf.edu/en/research/buildings/zero_energy/lakeland.
- Oak Ridge National Laboratory - Christian, Jeff, 2005. Pushing The Envelope. www.ornl.gov/info/ornlreview/v38_1_05/article08.shtml.
- Oak Ridge National Laboratory, 2005. Energy Savings from Small Zero-Energy Houses. www.ornl.gov/btc/pdfs/ZEB_TIR_27Sept05.pdf.
- BedZED Factsheet. www.peabody.org.uk/pages/GetPage.aspx?id=179
