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Raindrops keep falling...and should be harvested By: Dr Susanne Charlesworth, Low Impact Buildings Grand Challenge Initiative, Coventry University

Dr Susanne Charlesworth

Rainwater, as a resource, is mistrusted. It is something to get rid of as quickly as possible, hurried from roof and pavement into concrete pipes.Yet research has shown that some rainwater, and even rainwater which has been stored in a tank, is as safe to drink as treated water provided through taps. More importantly, when rainwater is "harvested" it has the potential to significantly reduce the mains water needs of commercial properties, lowering bills and putting less strain on drainage. It can also be used as a potential source of heating.

Our research (myself alongside Stephen Coupe and Ernest Nnadi at Coventry University) has studied water quantity, quality and resource use considerations provided by a permeable pavement system (PPS).

A PPS provides a porous surface for rainwater, however it can also be constructed with a reservoir structure underneath, which traps the water for rainwater harvesting. As a general rule, a total pavement depth of 500 mm can provide 1000 litres of water within 10 m2 of paving.

Ground Source Heat Pumps (GSHP) are an established technology to provide low CO2 source heating for buildings. A set of double tube ground loops filled with ethylene glycol and water are placed in the soil and the liquids in the pipe are pumped around the loop extracting heat from water. The only energy required to power the GSHP is electricity for the heat pump. The system extracts heat energy from the wet ground and moves the heat via a pump to the building where, following contact with a compressor (which increases the temperature of the water) is moved around the building, typically via underfloor heating. The energy available in the sub base of the PPS has been calculated to provide 1 kW of energy per 12-15 m2 of paving.

In 2008, the GSHP PPS system was specified for a new three floor office development with around 7000m2 of total area in Bedford. The proposed design featured 6500 m2 of car parking to go with the office and the GSHP paving was analysed for its suitability for the heating system. The drainage and heating system design were combined and the suppliers of GSHP equipment calculated that the PPS would provide sufficient heat to completely heat and cool the building. The design included:

  • Infiltration of rainwater at the surface by the use of permeable block paving
  • Storage and flow control of the infiltrated rainfall in the lined pavement sub base to a depth of 300 mm
  • Recycling of roof water for non-drinking uses within the office development. On this site the
  • storage element was not the permeable pavement but roof based tanks.

Together, these criteria prevented rapid runoff from the site and reduced both the rate and the volume of water discharging into the stormwater network. The plant room was established to hold the five heat pumps, which were 130 kW in capacity. The systems led to:

  • A typical 70% reduction on overall building carbon emissions
  • A reduction in utility bills of 50 %
  • No local emissions or pollution
  • Lower risk of fires and explosions than for gas boilers
  • No external equipment & noise units exceed NR35
  • Significant planning approval advantages

Permeable Pavement Systems have enormous potential to provide added benefits to their role as a source control device without compromising efficiency in the long run, including provision of void spaces for storage, recycling of stormwater, serving as a source of irrigation water and a supply of renewable energy.

Features May 2012

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