THE WATER ENERGY NEXUS
by Herbot Mesnard
ARTICLE III
Water & Electricity Synergies
The water and energy sectors are closely linked: it’s what is commonly called the water-energy nexus. In this series of articles, we will study the interactions between water and electricity and how changes in one sector are affecting the other. The development of renewable energies (RE) and lower power consumption water treatment technologies are not only deeply transforming the water sector, they are also increasing the possible synergies between the water and energy sectors.
“The water sector is defined as the sector that provides drinking water and wastewater treatment services to different end users (communities, businesses, industries). This sector can be divided into 4 sub-sectors following the water cycle: water collection, treatment, distribution, and overall management.”
The technologies used in water projects are very dependent on the scale, the raw water quality, the location and the end-use. In this series of articles, we will only consider small to medium scale, decentralised water treatment plants in developing countries as this is our core business at Impact Water Solutions (IWS).
The two previous articles of the “water-energy nexus” series have illustrated how the reduced energy consumptions of water treatment plants as well as the development and better integration of renewable energies (RE) have led to new decentralised water projects. Such projects used to be unaffordable, unsustainable or with poor performances, which is not the case anymore. New synergies have now emerged between RE generation and water treatment making decentralised water projects increasingly relevant and affordable. To study these synergies, we will mostly focus on solar desalination projects here as it is our expertise as IWS.
FLEXIBLE LOAD
Whilst historically, water desalination plants required constant power supplies, innovative technologies such as the OSMOSUN® desalination unit enable variable power supplies and therefore 100% RE supplied plants (see previous article). This technology therefore makes fully off-grid water treatments projects possible, but it also benefits on-grid projects, especially for electrical grids with high RE penetration. Indeed, the OSMOSUN® can cope with important power drops in just a few seconds: it can therefore act as a flexible load and offer partial or complete load shedding.
Consequently, for grids with important RE penetration, when the power supply suddenly drops or the consumption abruptly peaks, then the OSMOSUN® can disconnect and therefore regulate the grid (“load shedding”).
IWS plant installed in 2018 in Witsand (South Africa)
These flexible water treatment technologies also enable small scale networks to increase their share of RE without having to invest in battery storage or diesel generators (all this while producing clean water!). Whilst electrical grids tend to be highly unstable with more than 30% RE production, it is possible to go above this threshold for small scale grids integrating the OSMOSUN® desalination technology.
WATER/ENERGY STORAGE
Grids with an important share of renewables often require either battery storage or diesel generators to handle periods of low renewable energy production. However, storing energy in a battery or using a fuel generator as a backup are both costly options. On the contrary, water is extremely cheap to store. Consequently, for such grids, using a flexible desalination system like the OSMOSUN® is economically relevant as it can produce water during RE production periods and store this water for periods when the power supply is interrupted (“load shifting”). Therefore, instead of storing energy with a battery to provide a constant power supply to the desalination plant, water is produced whenever power is available and it is then stored for later consumption. Water storage tanks can act as buffers when there is no power supply and enable water supply to always match demand. However, to match water consumption at all times without using batteries or backup power, the desalination plant and the storage tanks need to be sized correctly (using RE production predictions).
For example, in Djibouti, IWS is currently developing an off-grid desalination plant and distribution system that will be capable of matching the water needs of the local community thanks to solar power only. Indeed, the water desalination plant is slightly oversized so that enough excess water is produced during sunlight hours. To avoid water shortages, the excess water produced is then stored (under safe conditions) for periods when there is no solar power available. Therefore, these communities will have a reliable water supply whether the sun shines or not, and they are using environment friendly RE generation to produce the water.
WATER & ELECTRICITY MINI-GRID
Scheme of a typical off-grid Water & Electricity mini-grid (IWS)
Water and Electricity are very much inter-linked and inter-dependant. As we understand these inter-dependencies, we have found that looking at the integration of both utility supplies allow for further optimisation. In the specific case of new developments (e.g. property, farming, hospitality), when water and electricity supplies are designed jointly, it is then possible to increase the efficiency of the system, decreasing overall cost and reaching an optimum whilst keeping flexibility.
Optimising water and electricity makes obvious sense in the case of off-grid systems (such as islands) with no external supply of water and electricity. Yet it’s also relevant for systems connected to the grid as it decreases the need for (and cost of) external supply and accelerates the transition to complete autonomy. The first step is to use this external grid supply only as backup, and then to completely disconnect it from the client’s local utility grid.
At IWS we design customised models integrating the water and electricity load profiles that we estimate with our client. This joint design leads to an optimised sizing of the power supply, water treatment plant and water or energy storage; and therefore an overall cost decrease of up to 20%.
The following methodology is used by IWS to match its clients’ water and electricity needs:
There are therefore many synergies between the water and electricity sectors as water treatment technologies can act as flexible loads and a trade-off can be made between energy and water storage.
The next step to optimise the water and electricity supply is to do “demand management” by influencing end-users’ consumption. This helps maximise these synergies and achieve the most optimal system, delivering the even more affordable, sustainable and performant water and electricity supply.
At IWS we believe our client’s water and electricity needs can now be fully met using only RE generation, innovative treatment technologies and water storage.
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