Permeable paving is already one of the most accepted forms of sustainable urban drainage systems, but can it crack the renewable energy market. B&E reports on what could be a great leap forward.

THE UK has suffered drought and the worst flooding for over a half a century in the last five years.

Several EU and UK commitments are in place to encourage more sustainable construction to mitigate these effects and to control our water more effectively.

The Planning Policy Statement 25 Development and Flood Risk and the Code for Sustainable Homes (CSH) have put the onus on developers and specifiers to incorporate Sustainable Urban Drainage Systems (Suds) into new developments.

As ever, the barriers seem to be cost. The drive to make CSH Level 6 of the code by 2016 is placing an enormous burden on developers and contractors to meet the requirements, to incorporate renewable energy technology and control water management.

In order to reach any level within the code a Suds system needs to be implemented.

Research project
Hanson Formpave exhibited its combined Suds and Ground Source Heat Pump system (GSHP) at the Building Research Establishment’s (BRE) Offsite 2007 exhibition.

The system recycles and reuses rainwater, using permeable paving as storage, which reduces the domestic reliance on traditional sources of energy and water.

Exhibited as part of the Hanson Ecohouse, it is hoped the system will provide over 50% of the mains water for non-portable uses, and when full could provide a family of four with 30 days water without any extra rainfall.

Hanson Formpave is collaborating with Coventry University to monitor the BRE site for the next three years. Funding has been found for a PhD studentship to determine the effectiveness of the combined Suds/GSHP system.

Part-time lecturer and research manager for Hanson Formpave, Dr Stephen Coupe is notably excited about the system, although keen to establish its validity with hard data.

“In some ways we are looking at this as a test case,” he says. “Obviously permeable paving is one of the best-accepted Suds techniques, but we’ve added significant other benefits. It should massively reduce the need for mains power.”

The anticipated efficiency of the schemes justifies Dr Coupe’s excitement.

Up to 80% of the costs incurred for annual heating and cooling can be removed by GSHP technology. These benefits are in addition to the Suds good practice inherent in permeablepaving, including flow attenuation and water quality improvements.

Dr Coupe says the multi-purpose system justifies the real or perceived high up-front costs of installing sometimes unfamiliar technology such as Suds. “Someone’s always got to take a chance on something at the start,” he says. “GSHP technology is quite well established. What is not established and has
not been done before is to combine that with Suds and a system that offers multiple benefits.”

Paving efficiency
Dr Coupe said at last year’s Sudsnet conference the operational efficiency of GSHP technology buried within a permeable pavement is expected to be more efficient than when buried in soil for two reasons.

Firstly, the large water holding capacity of the pavement system, when full and subject to the installation of a properly sealed tank system, ensures that the heat extraction pipes will constantly be buried in water, unlike in soil water where fluctuations of the water table or soil drying could dramatically reduce heat transfer efficiency.

Secondly, the termination of down pipes in the paving system is likely to lead to a replenishment of local heat in the stored water by mixing and the introduction of kinetic energy from the flow.

In a domestic setting at certain times of the year, Dr Coupe says that more than 50% of water could be provided by the use of rainwater. And during times of heavy rainfall has the added advantage of being a floodwater control method. “Any Suds technology you put in is going to mitigate flooding. Obviously the intensity of rain we had over the summer, you’d struggle to contain that with any one design. In general Suds are right for combining in many different ways and I think this is no exception, as the system is reducing energy costs and recycling your water.”

Multiple potential
The potential combination of Suds with other renewable energy technologies is something Dr Coupe feels has huge potential. “There is no reason why you couldn’t combine our system with photovoltaics in order to heat some of the water or combine it with a wind turbine to produce extra electricity, but even on its own, it’ll do the job anyway.”

Ciria associate Paul Shaffer admits the technology is relatively new to him, but if robust enough makes sense. “If the technology is robust these kinds of things are welcomed,” he says. “The Government’s Planning Policy Statement 25 on development and flood risk talks about delivering multiple objectives. So in the whole approach to climate change management, linking pervious paving and heat source pumps makes sense.

“Hanson Formpave is promoting it very much on its sustainability principles and I haven’t seen anyone else take that forward.

“The policy statement talks about the opportunities for getting these multiple objectives, which at the time was talking about linking sustainable drainage with improvements of water quality, flood risk management and, also, better places for people to live. If you throw into there resource and energy efficiency, it obviously makes sense to combine ground source heat pumps and pervious paving.”

Dr Coupe says the system has benefits over any extra time and cost during installation. “The market has dictated that permeable paving is a viable option and it often gets chosen over other Suds techniques because people can understand it. Once you start grafting on other geothermal or water recycling benefits too, they’ll be willing to wait that bit longer for it to be finished off.”

Risk Assessment
Although he recognises the system is logical and works in principle, Phil Chatfield, soil policy manager at the Environment Agency raises concerns about GSHP systems and the potential for groundwater contamination.

“It depends on what sort of system and fluid you’re using,” he says. “Clearly there is a benefit from a climate change point of view. Are there any risks to groundwater? Well, we need to assess that.”Chatfield’s concern is with the heat exchange medium. “What you’d typically put into a GSHP system would be a glycol fluid, which is collecting heat from the ground. That goes into a sealed coil that is layed into the paving. Now, if for any reason that coil was to leak and the material in it was to leak out, would it have a polluted potential?

That’s the question we’re asking.

“We want to make sure that there isn’t a potential for water pollution from these systems and that’s something that’s being looked at.”

Dr Coupe hopes the analysis, which begins at the end of January, will provide clear guidance on whether it is feasible to combine Suds elements and renewables energy systems. Some of the variables to be analysed include temperatures inside and outside the eco-house and the water quality discharged, from both halves of the site, with particular reference to the EU Water Framework Directive. “It will look at volumes of water being reused and recycled compared with if you didn’t have a technology in place. We think it is a good bet for the future, but of course the evidence and actual proof of that is something that we are looking to investigate right now. We can see going forward it is a good technology that will come more and more accepted as time goes by.”

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