Posts Tagged 'solar energy'

Purifying water with solar energy (pasteurisation and solar disinfection)

Back at University [in 2004] we did a project investigating water purification systems for developing countries. I have summarised some of the interesting sections that I worked on here…

It is estimated that over a quarter of the world’s population do not have access to clean drinking water(1). Even in water-rich countries contamination of drinking water supplies promotes the often easily preventable spread of waterborne diseases. Additionally, the communities affected are frequently unaware of the link between water quality, health and sanitation. The need is simple, but the solution is not necessarily as straight forward, with hundreds of researchers around the world working towards technically, culturally and socially appropriate methods of water purification.

Solar pasteurisation

Pasteurisation involves the purification of water by heating it to a certain temperature for a specific length of time. Contrary to what many people believe, it is not necessary to boil water to effectively purify it. Extensive research has found that heating water to 65ºC for 6 minutes, or to a higher temperature for a shorter time, will kill all germs, viruses, and parasites(2).

Even more surprising is the effectiveness of lower temperatures at making water safe to drink. Some research suggests that samples with a maximum temperature of only 55ºC became completely disinfected within 7 hours(3).

Solar disinfection (also called ‘SODIS’ or ‘Solar UV’)

Ultraviolet (UV) filters have been successfully used for many years in the developed world, however, their relative costly nature and need for a reliable electricity source has prevented a transition to use in developing countries. The doses of UV created in these filters are much higher than those produced by the sun, but the time spent in the filter is quite short. This fact led researchers to experiment with the prolonged exposure of water to the sun’s UV light as an effective method of purification.

The advantage of the technology is its simplicity as it only requires relatively inexpensive 1-2 litre clear plastic (PET) bottles, or something similar, to hold the water. Field testing of the technology has indicated its appropriateness to climates with average solar intensity of above 500W/m2 for 3-5 hours of the day. Testing in Haiti showed a 52% effectiveness for one day exposure and a 100% effectiveness for a two day exposure(4).

Combined solar thermal/solar disinfection

Interestingly, the treatment of water using solar disinfection is enhanced as water temperature increases. A group of researchers from the Nestle Product Technology Centre experimented with this idea using relatively cheap food packaging material to make two types of containers(5). The ‘pouches’ had a UV transmitting upper layer, with the back layers made from either metallised plastic to reflect light or from black plastic to increase temperature. The pouches were protected from the wind with small three-sided reflective boxes.

Although the experiments were conducted in October (going into winter) in Connecticut, Ohio, the samples showed positive results after six hours of exposure to sunlight. Their results indicated that light reflection (in the reflective packages) was more important than a modest temperature increase (in the packages with a black backing).

Experiments

To experiment with some of the above ideas I constructed several ‘low tech’ solar water purifiers. Pasteurisation temperatures were reached relatively easily with simple flat plate and parabolic collectors (below left)… in fact I got up to 80 degrees fairly quickly! Other techniques using the combined method can be constructed using insulated reflective boxes with water containers inside (below right).

Parabolic trough solar water pasteuriser purifier insulated box drinking water purification UV plus pasteurisation

(1) Murcott, S, Clean water for 1.7 billion people?, Submission for ‘Development by Design’ Workshop, July 22, 2001.

(2) Ciochetti, D. A., Metcalf, R. H., Pasteurization of Naturally Contaminated Water with Solar Energy, Applied and Environmental Microbiology, 47:223-228, 1984

(3) Joyce, T.M., McGuigan, M., Elmore-Megan, M., Conroy, R.M., Inactivation of fecal bacteria in drinking water by solar heating, Applied and Environmental Microbiology, 62:399-402, 1996

(4) Oates, P.M., Shanahan, P., Polz, M.F., Solar disinfection (SODIS): simulation of solar radiation for global assessment and application for point-of-use water treatment in Haiti, Water Research, 37:42-54, 2003

(5) Walker, D.C., Len, S., Sheehan, B., Development and evaluation of a reflective solar disinfection pouch for treatment of drinking water, Applied and Environmental Microbiology, 70:2545-2550, 2004

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A brief introduction to photovoltaics

Photovoltaic cells generate electricity from sunlight, typically by means of a p-n junction semiconductor material. Their ability to generate energy from a free fuel, using no moving parts, creating no noise or on-site emissions, with minimal maintenance in a highly predictable and reliable fashion, is unparalleled. Application of the technology is modularised and deployment can take place where energy is most valuable: at the point of use. In a world of widespread energy inequality, dominated by unbalanced and typically centralised energy systems, photovoltaics’ time has come.

Knowledge of photovoltaic technology has progressed rapidly over the last half-century. Although the photovoltaic effect was discovered in the 1800’s, the silicon solar cell was first developed in a useful form by the Bell Telephone Laboratories in 1954. This wafer-based silicon cell has become the mainstay of the solar industry since that time and has certainly led the charge to production levels of a giga-watt per annum scale (reached in 2004). The global bottleneck in supply of high purity solar grade silicon experienced in recent years highlighted the point that this technology is not likely to lead the charge to a world with a terra-watt of installed photovoltaic capacity.

Photovoltaics is, however, not without its limits. Just as the amount of oil left in the earth’s crust is limited, there is only a finite amount of energy available from sunlight. Thankfully, unlike oil and other fossil fuels, this amount of energy is highly predictable, enormous on a global scale and quite obviously, constantly replenished.

This is an edited excerpt from Opportunities for Vehicle Integrated Photovoltaics.

If you are interested in buying an in-depth book on the topic I suggest Applied Photovoltaics from Amazon.


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