What does it cost to run a green wall?

I have been asked what it costs to run a green wall; often, people imagine extreme costs and infra structure, so think a green wall can’t be justified on those grounds alone.  The fact is, early systems were highly consumptive of water and electricity (ie felt based systems) but modern systems are not.  I base the following on the Biowall system, because that is what I know; this was a case study done on the wall I designed at Edgware Road for TfL:

Green Wall at Edgware Road

Green Wall at Edgware Road

Water & Power Consumption

To realise the on going benefits of the wall there are a number of necessary inputs.  The two inputs that are often raised as a concern are water and power.  The following calculations confirm the minimal amounts required by an efficient living wall system:

Water

The unique design of the hydroponic Biowall system provides fully comprehensive planting at an average of 1 litre per m2 per day.  This is by far and away the least water use of any of the living wall systems available today.  Note: traditional summer ground planting in beds requires 3 to 4 litres per m2

200m2 x 1 litre per day x 365 days = 73m3 of water in a year.
At an average supply rate of £1.50 per m3 this gives an annual water bill of £110.00.  If this is harvested rainwater then there is not supply rate but there are extra installation costs.

Drainage

Remote sensing and remote control irrigation system allow fine tuning of the system.  To keep the wall flushed through, 10% of what is put onto the wall comes off the bottom and goes to drain.
7.3m3 of water going to drain at an average charge rate of £1.00 per m3 gives a drainage bill of £7.30

Power

The irrigation system is driven by a 4bar pressure pump rated at 0.8kW.

There are three irrigation zones on the wall at Edgware Road and during the summer months each zone may operate for up to 9 minutes per day.  During the winter months this will reduce to approximately 1 minute per day.  An annual average of 4 minutes per day per zone is expected.

4 minutes x 3 zones x 365 days = 4,380 minutes of pump use per year

This equates to 73 pump hours per year.  73 x 0.8 = approximately 60 kWh annually.  At an average supply rate of £0.10 per kWh this equate to electricity usage of £6 per year.

Exclusive of standing charges, the overall annual costs for water, drainage and electricity for this 200m2 wall is therefore expected to be £123.00

Therefore guideline running costs for a living wall (excluding irrigation and horticultural maintenance) can be assumed to be less than £1 per m2 per year.

Update:

I now run a new company Vertology Living Walls and have an advanced, patent-pending green wall system, Viridiwall™ which improves further on the Biowall system.

April 14th, 2013 by