Water and Environment - Water, water everywhere...01 August 2005

With rapidly rising populations, coupled with the effects of global warming, water is becoming an increasingly sought-after commodity in nations both developed and developing.

It thus becomes increasingly important to make the best possible use of this precious liquid, and also of its heat (or cold) carrying capacity.

Since even summaries of all relevant recent technical developments would fill a thick textbook, we propose to concentrate on three that happen to encompass lessons to which all plant engineers should pay attention. These are: using waste heat from cooling to assist heating; simple self cleaning filters which require absolutely no additional water; and solar powered systems for greenhouses in hot climates, which use sea water to both cool and irrigate them.

The simplest, and with hindsight the most obvious, is an idea brought to our attention by Londoner Gurpreet Singh, who has just graduated in Industrial Design Engineering from Brunel University. His 'EcoFridge' project involves attaching water-cooling pipes to a standard refrigerator condensation coil. His basic idea is to take the mains water, which in the UK normally comes in at 4ºC, use this to lower the temperature of the coil, enhancing refrigerator efficiency, and then supply the warmed water as feed to the hot water system. He told us that he has conducted many tests over six-hour periods, measuring the total time the compressor was running for, and thus the total energy consumed by the refrigerator, in three different scenarios.

In conventional use, the energy consumption was 0.185kWh. With stagnant water in the heat exchanger, it was 0.15kWh, but if the water was flowing into the domestic hot water system, the energy consumption was only 0.09kWh. He says that, in an average UK household, the saving would translate into a reduction of 179kg of CO2 emissions over the course of a year.

Filtering with a rotary flush

A new self-cleaning filter, initially developed for pumps for use on farms, wastes not a drop of water, has no mechanical scrapers or removable elements, and has just made it into submersible pumps suitable for use in waste water treatment.

Jim Hosford says that he developed his first self-cleaning filter when he was a dairy farmer in 1994. The filter on the pump that pumped water contaminated with cow manure, straw and silage from his dairy unit kept blocking, sometimes as often as once every half hour.

He therefore came up with the idea of making the pump self cleaning by installing a rotor on the inside of the drum, with two jets that blasted water back through the screen. The jets were angled to make it spin at 60rpm, cleaning the whole filter screen every half second. The output of the pump was divided, with 25% of the flow being sent back to the cleaning water and the remaining 75% forming the usable flow.
The initial development took first prize in the Farmers Weekly invention competition and, encouraged by feedback from other farmers resulting from the publicity, Hosford went into production.

This led to a smaller version that could be fitted to garden pond pumps, 25,000 of which were delivered in the first six months, and to Hosford licensing out his patents for this market.

Subsequently, however, he was approached by a company selling waste water treatment equipment. They wanted a filter that could be sited in the intake channel in a waste water treatment plant to provide a filtered supply of wash water for cleaning their intake screens. While the eventual solution works well in many situations, the pumps sometimes lose prime in dry periods when flows are low. The overall system installation costs are high, involving a surface-mounted pump, with frost protection and plumbing work to supply a flow to the cleaning rotor, which adds cost and causes significant friction losses.

This has led to the latest development, a submersible pump driven by an electric motor, with a shaft extended to drive an integral back-washing rotor within a circular intake screen - which works in the same way as in the earlier standalone filter products.
As there is no pipework, only about 10% of the pump power is used to drive the self cleaning filter, as opposed to 25% in the separate filter design. Furthermore, this 10% is recouped, because the pump impellers can be made with smaller clearances, increasing pump efficiency, due to the fact that it is possible to work with a finer filter screen: in some cases, down to 50 microns.

The present design comes into two versions, a three-phase motor-driven pump, capable of delivering 120 litres/minute at up to 7 bar, and a single-phase version capable of delivering 120 litres/minute at 6.5bar. So far, about 50 have been sold, 35 for use in waste water treatment works, plus a number supplied to dairy farmers for pumping contaminated water for land irrigation. One is on trial in Milan, providing a supply of filtered water to an online analyser. Another has been supplied to a steeplejack company that uses water-cooled chain-saws to fell chimneys. A new version under development will have a capacity of 30cm3/hour (500 litres/minute).

Using the sun to produce crops from sea water

The ideas we describe above are intrinsically simple, but that is in no way true of the technologies being developed to use the sun to power water-based air conditioning systems, or to turn sea water into both a source of irrigation water and a means of cooling greenhouses in very hot climates.

The basic idea of the 'Seawater Greenhouse' is simple enough. However, as in so many alternative energy-powered systems, it requires advanced control and a high degree of technological innovation in order to get it to work efficiently. The idea is the brainchild of Charlie Paton, and his company Light Works, spinning into Seawater Greenhouse, both headquartered in East London.

In the basic scheme, sea water is trickled down via an evaporation screen, through which is drawn ambient air. The screen also traps dust, salt spray, pollen and insects. In the greenhouse, plants grow in a clean, humidified air environment at a lower temperature than they would experience outside. The air from the main greenhouse is then drawn through a second evaporation screen, where it is humidified to near saturation, before it passes to a condenser which recovers distilled water with which to water the plants.
In the latest version, there are two separate sea water circuits. The first takes the water from the first evaporator and passes it to the condenser, after which it re-circulates to the first evaporator. The second passes sea water through tubes in the greenhouse roof, where it is heated by the sun before being passed to the second evaporator. Here, it evaporates more readily, because it has been solar heated, prior to being condensed to form the irrigation water.

After initial use of expensive cupro-nickel tube and fin condensers, the team gas found that better results can be achieved by using all-plastic 'Watermaker' condensers, specially developed for the project, which cost less and yield more water per kJ of heat transferred. The evaporators are made of cardboard and crystallise calcium carbonate from the sea water and harden like sea shells. The greenhouse glazing is not glass, but polyethylene, specially treated to incorporate ultraviolet-reflecting and infrared-absorbing properties, and able to be 100% recycled at the end of its useful life. Plumbing is ABS and MDPE (medium density polyethylene).

Computer modelling has been an essential part of the development process. At the outset, the first prototype greenhouse built in Tenerife was modelled using MATLAB and Simulink from The MathWorks in Cambridge. This modelled and simulated all the processes, including the operation of pumps and fans, evaporation of water and cooling of air, transpiration from crops, solar heating, heat transfer through the roof, and condensation of fresh water.

This work was based on formulae supplied by what is now the Silsoe Research Institute at Wrest Park and information supplied by the Met Office. Much of the data was supplied in Microsoft Excel spreadsheets and transferred directly into MATLAB. Each of the processes was modelled within a Simulink block, each block representing a physical object, such as an evaporator or condenser. The completed blocks were connected as in the real greenhouse, the links between them corresponding to mass and energy fluxes.
"Because there was an obvious visual correspondence to the real system, it was difficult to make mistakes in the connections", says Charlie Paton.
The Tenerife prototype was then built and equipped with 30 sensors to validate the model. These measured temperature, humidity, wind speed, solar radiation and flow rate, logged at 10-minute intervals over several months, and demonstrated a good correlation between actual and modelled performance.

One the things to come out of working with the Tenerife prototype was that sufficient ventilation and cooling could be achieved by wind alone, without having to use fans at all, at least at some sites. This led on to the need to undertake further modelling studies, using CFD (Computational Fluid Dynamics) analysis with 'Flovent' from Flomerics, based in Hampton Court.

The Seawater Greenhouse project has received many accolades. London's Design Museum presented its first £40,000 Design Sense Award for Best Practice in Sustainable Design in 1999. As a result, the team was asked to build another seawater greenhouse in Abu Dhabi. Development continued.

"The heat exchanger we were using to condense the water was not as suited to the purpose as we would have liked," concedes Paton, "yet it was accounting for half of the cost of the greenhouse. We started to think about our own Watermaker, which would be cheaper, more efficient and could be flat-packed for assembly on site."

However, he soon realised he needed support and a friend recommended NESTA, The National Endowment for Science, Technology and the Arts. They gave the project a £75,000 Invention and Innovation award in 2002. Following the Abu Dhabi project, another was funded by His Majesty's Research fund at the College of Agricultural and Marine Sciences, Sultan Qaboos University, in Oman in 2003. In the same year, Dr Philip Davies, principal engineer at Seawater Greenhouse, was awarded a two-year Industrial Fellowship by the Royal Society, in order to continue research work at the University of Warwick.

Paton reports that this latest greenhouse is currently producing a bumper crop of cucumbers.

"This is excellent news, as May is one of the hottest months and long past the traditional growing season for Oman. I found a number of things that needed repair, cleaning or replacement - the sun in Oman has a voracious appetite! I also found various techniques for improving both the climate cooling and water production through minor modifications to the design, which I plan to implement fully in the autumn."

Research by Dr Davies is also continuing into a possible improved cycle involving the use of regenerable desiccants. Investigations elsewhere include a scheme that would allow the growing of cucumbers within the Arctic Circle.

The world's growing population, and its growing hunger and thirst against a background of global warming, can only mean that developments such as these now need to move quite quickly from the interesting research phase to worldwide deployment. There is much to do.

Gurpreet Singh
gurps7@yahoo.com
Rotorflush
www.rotorflush.com
Seawater Greenhouse
www.seawatergreenhouse.com
The MathWorks
www.mathworks.co.uk
Silsoe Research Institute
www.sri.bbsrc.ac.uk
Met Office
www.met-office.gov.uk
Flovent
www.flovent.com
NESTA
www.nesta.org

SOE

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