We can plan, design, and build for low carbon living
As the drivers towards zero carbon development increase, we need to understand which planning, design and construction options can help to achieve it. By Dr Paul Rowley
Although the principles of designing buildings with good thermal efficiency are well known (if not well applied), the role of renewable energy technologies are less well understood – and nor are the masterplanning and spatial design principles that enable renewables to be used effectively. Zero carbon means there must be zero net emissions of CO2 resulting from energy consumption through the year. For example, the UK Code for Sustainable Homes (CSH) gauges the sustainability of a dwelling across a number of categories. Points awarded in each category are added together, and this is translated into a rating for the building.
Not all CSH categories relate to building mechanics. Many address low carbon lifestyle issues such as provision for home working and cycle storage. There is also guidance on developing low carbon communities, for example the creation of local amenities within walking distance, providing space and facilities that encourage the growing and consumption of local produce, and planning for cycleways and shared energy schemes.
Crucially, planning categories in CSH level 6, which will be mandatory for all new residential buildings by 2016, include the requirement for the building to be ‘zero carbon’. Any imported electrical power must be balanced by exported power generated from domestic renewable energy technologies. Likewise, for thermal demand, 100 per cent of heating and hot water requirements must by covered by renewables on site. These criteria have major implications for masterplanning and spatial design principles.
It is important to realise that definitions of ‘zero carbon’ differ. In addition to CO2 emitted during use, all buildings contain ‘embodied carbon’ as a result of energy consumed in extracting, processing, transporting and constructing the constituent parts. Typically, 10 per cent of a modern building’s lifetime carbon emissions are embodied in the structure. But, as buildings become more energy efficient in use, a much higher proportion of lifetime carbon emissions will be embodied. This means that products certified as coming from low-carbon sources will become the market norm.
On site renewables
Currently, where legislation designed to reduce carbon emissions has included a renewable energy requirement, this is specified as on-site generation of heat or electrical power. There are several on-site options in terms of heat and electrical energy options. _
Biomass: some pointers
Biomass is increasingly specified in low carbon developments due to its relatively low cost and high energy yield – up 100 per cent of space heating and hot water requirements. However, building designers and planners must be aware of some key issues for biomass:
The Arup-designed Vauxhall Cross transport interchange features as solar PV energy roof
Wind PV biomass
As buildings become more energy effecient, their 'embodied-energy' will represent an increasingly significant proportion of the building's through-life carbon 'liability'
- Consider fuel quality, delivery and storage. Biomass fuel consumption can be considerable. For pellets or wood chip, around 3-5 tonnes per year per thermally efficient dwelling. Enough covered storage space needs to be designed in to prevent the need for frequent deliveries and the resulting local traffic movements.
- Factor in fuel costs and availability. Fuel is generally cheaper if bought in bulk. Make sure you can access a local wood fuel supply chain able to supply your needs at a reasonable price. Otherwise, payback may not be what you hoped for.
- Make sure that fuel quality (including moisture content and source) meets recommended benchmarks. High moisture content means a lower net energy density, whilst imported biomass has high embodied carbon. Waste derived fuel should come from clean sources, including forestry residue and clean timber industry by-products.
Solar thermal In the UK, a well-sited, efficient solar thermal hot water system can deliver up to 60 per cent of annual hot water requirements. One drawback is that the energy shortfall during months when solar radiation is low must be filled. For zero carbon developments, in many cases this shortfall will be supplied by a biomass installation. As with all renewables, it’s best to consider the solar thermal option at the concept design stage – a suitable mounting area with good solar access is essential, along with space for a suitable hot water energy store.
Heat pumps Some issues emerging from the application of CSH level six are interesting, if not problematic. Under current requirements, the potential for heat pumps is limited. This is because heat pumps require electrical power to move thermal energy from outside to inside, typically at a ratio of 1:3 or 1:4. So, even a very thermally efficient dwelling will consume up to 3,000kWh of ‘enabling’ electrical energy each year for heating and hot water purposes. To generate this via renewables on site will, in most cases be very difficult, either from a siting or an economic perspective. For low, rather than zero, carbon development, however, both ground source and air source heat pump technologies will still find a role. Both technologies may be appropriate for the retrofitting of existing homes.
Renewables for some electricity
Wind power The bad news is that small-scale wind turbines can perform very poorly in urban locations and other complex terrains due to low windspeeds and turbulence. However, for larger, more open sites, a larger wind turbine, tall enough to harvest faster, less turbulent winds, may be a good option. At this scale (from 50kW upwards), on-site wind speed assessment to assess potential energy yields is recommended.
Photovoltaics (solar electricity) Solar PV is often the only feasible option for on-site renewable electricity generation. This is not cheap. Costs can reach £30,000 per dwelling to install a PV system sized to meet 100 per cent of electrical demand, even if energy efficiency measures are optimised and sufficient roof area or other mounting space exists for the area of PV required – up to 40m2 per dwelling. Bear in mind that where high cost facing materials are specified, this can be replaced by PV, thus reducing PV net cost significantly.
Biomass CHP for communities
For larger community-scale developments, biomass CHP is gaining increased interest. However, this technology is still relatively untested and capital costs are high. The number of sub-1MW installations (relatively small scale) is low, and the operation of biomass CHP has had its technical hitches.
For community-scale systems, the economics of biomass CHP look marginal. This is where creative spatial planning can make a difference. For developments which include a facility with a high summer heat demand (such as leisure centres, catering or hospitality), the CHP system can be run at a much higher capacity factor, thereby maximising revenues from electricity and Renewable Obligation Certificate (ROC) sales.
Our understanding of all the issues important in achieving zero carbon performance is continually evolving. In time, the requirement for 100 per cent on-site renewable energy generation will probably be modified, to be replaced with at least a partial off-site option. If through-life verification of the energy source can be implemented, this will be a sensible move, as gains in net unit carbon emission reductions become obvious. Whatever the future holds, it is becoming more clear by the day that a transition towards a low if not zero carbon society is no longer an option, but is essential.
Dr Paul Rowley is knowledge transfer coordinator at CREST
The Centre for Renewable Energy Systems Technology (CREST) was established in 1993 at Loughborough University. Its primary activity is to undertake research and education in renewable energy. Research activities at CREST cover a range of technical applications, including wind power, solar PV, energy in buildings, grid connection & integration and energy storage (including hydrogen).
http://www.lboro.ac.uk/departments/el/research/crest/index.html
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