I was asked recently by a friend of a friend to comment on the practicality of building a carbon neutral office tower.

Without knowing any better, I bravely suggested that a carbon neutral office tower was not yet possible. The root problem I surmised is that we can’t control the source of energy delivered by the grid. Thus any would-be carbon neutral building needs to generate all of its power on site using renewables to be certain that its energy use is not causing carbon emissions.

Assuming that power generation of any kind is likely to take up a lot of space, it seems highly unlikely that a tall energy intensive building on a tight urban would be able to generate all of its power on site. The only viable way to be carbon neutral is therefore to generate as much as possible on site using renewables and use carbon offsetting or carbon capture to neutralise the remaining carbon emissions. Obviously having an energy efficient building will make emissions targets easier to achieve.

Having made these comments I thought it prudent to do a little post-verification just in case I had got it wrong. Luckily my old colleague and friend Mark Weintraub was able to confirm most of what I had surmised. Being something of a high rise guru Mark was able to point me in the direction of current best practice in sustainable design of tall buildings.

Apparently the best we can do at the moment is achieving 20% of a building’s ongoing energy requirement generated on-site using renewables. This figure is included in a BREEAM ‘excellent’ rating or LEED ‘platinum’ rating. BREEAM (Building Research Establishment Environmental Assessment Method) and LEED (Leadership in Energy and Environmentally Design) are the sustainable design standards for the UK and USA respectively.

Mark suggested several approaches to on site generation. The favourite seems to be space efficient hydrogen fuel cells which can provide a large proportion of electrical energy and also cooling through absorption chillers. From what I can gather, absorption chillers use the heat output from the fuel cells’ electricity generation to dehumidify air prior to cooling by evaporation, but I’m not very clear on this at the moment.

Another option is a combined heat and power (CHP) plant run on biofuels but this takes up much more space than fuel cells.

In considering these options it is obvious to me that neither fuel cells nor CHP are automatically carbon neutral. We still need to get the fuel from somewhere. If you have read other pages of my blog you will see that I am very sceptical about carbon neutral biofuel. Similarly hydrogen does not simply occur; it must be produced by separating water molecules, a process which uses energy. Only if this energy comes from a truly carbon neutral source (such as geothermal, hydro or nuclear) can the resulting hydrogen claim to also be carbon neutral. These fuels also need to be transported to the building, a process which presumably will use a diesel engine truck. Everything is interconnected which makes it all very complicated.

Purists might argue that we should therefore rely on our building sheathed in photo-voltaics and bristling with wind turbines. This might avoid the pitfalls of certified carbon neutral fuel, but the fact that we haven’t done this yet suggests that either the technology is inadequate or that there is just not enough solar or wind energy acting on a single high density building to provide all of it’s energy requirements.

One advantage that is clear to me is that efforts to be carbon neutral are pushing for better designed and more energy efficient buildings. In rural and lower density suburban sites, carbon neutral and/or off grid buildings are already a reality, but in high density urban centres we are still waiting for either sustainable design technology to improve or our grid electricity to be carbon neutral.

Related Posts