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, the United States, the United Kingdom, 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.

Legislation

In the United Kingdom by 2016 all new homes will have to be zero emission on heating and cooling. The United Kingdom Government’s ‘Code for Sustainable Homes’ legislates binding regulations for energy reduction with impressive targets: 25 per cent more efficient by 2010, 44 per cent by 2013, and 100 per cent, or zero emissions by 2016 (Matt Wright, 2008).

The California Energy Commission (CEC) is charged with implementing new building standards under its ‘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 United States 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 United States as a whole has grown relentlessly at 2 per cent per year to more than 12,000 kWh today. Consequently, in California, gains from improved energy efficiency have cancelled out the growth that would have otherwise occurred, while as a whole, the United States has a poor record.

In Figure 1 (Rosenfeld 2007) the green wedge represents energy savings to date as a result of mandatory, improved building standards. Meanwhile, Australia’s 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 per cent a year.

As Professor Ross Garnaut has warned, 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.

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:

1. Minimising on-site energy consumption as far as possible by means of energy conservation and energy efficiency 2. 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 per cent renewable.

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 second 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 conducted by the CSIRO indicate that high-performance windows lift an otherwise reasonable design (5 or 6 stars) to very good, or around 8 stars. This is consistent with findings in a study performed by Peter Lyons & Associates for the Australian Glass and Glazing Association 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 (more than 0.55 and 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 solar heat gain coefficient is required (less than 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 2-2.5 stars over that achieved using the predominant residential window sold in Australia today, which is single-glazed clear in a thermally unbroken aluminium frame.

Conclusions

Case studies from a wide range of climates share windows whose U-value is typically around 2 watts/square metre kelvin 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 1. Wright, Matthew. 2008. www.beyondzeroemissions.org 2. California Energy Commission. 2007 Integrated Energy Policy Report. www.energy.ca.gov/2007_energypolicy/index.html 3. Fraunhofer Institute for Solar Energy Systems. Freiburg, Germany. www.ise.fhg.de/publications. 4. Rosenfeld, Arthur, 2008. Energy End-Use Efficiency. www.energy.ca.gov/2008publications/CEC-999-2008-005/CEC-999-2008-005.ppt 5. Foliente, Greg, 2008. CSIRO Sustainable Ecosystems Zero-Emission House. www.csiro.au/news/ps2i8.html