Seawater cooled chillers are becoming more and more common for big district cooling plants close to the seashore. The high efficiency of these chillers together with a zero fresh water consumption make this technology as extremely interesting from the OPEX and environmental points of view. For sea water cooled district cooling plants, seawater intake systems are crucial in the mentioned areas. A typical seawater intake system is made up of piping, pumps, intakes, screens and channels among others. At present, two main intake systems exist: Subsurface Intakes and Open Intakes. This post presents these two alternatives, discussing the challenges associated with subsurface intakes compared to the suitability of open intakes for district cooling.
Open Seawater Intakes
An open seawater intake collects the water directly from the ocean through an offshore or onshore inlet structure. A pipeline interconnects the desalination plant to the structure. Typically, this system comprises of the following components:
- Fore bay (inlet structure)
- Coarse bar screen
- Source water conveyance channel
- Concrete screen chamber
- Mechanical fine screens
The inlet structure is located at an adequate sea depth and convenient position. After the water has entered the underwater pipelines, it is transported to land, whereby it is pumped to the desalination plant. The sea-located structure is made from plastic or concrete, but not metal because of corrosion. Open intake seems to be the more popular alternative because it guarantees quality water and has unlimited hydraulic capacity.
For district cooling applications, open surface intake is preferable because of not only their flexibility and cost effectiveness, but also minimal environmental impact as compared to subsurface sea intake. Open surface intakes will even suit densely populated areas because they do not need to fulfill a long list of suitable hydrogeological conditions before installation. Furthermore, installation of the intake requires minimal land, thus making this alternative ideal for coastal communities where land availability is a big issue.
A subsurface water intake consists of pipes and wells set beneath the ocean floor or shoreline. The intake pipe draws seawater through the subsurface. It can be either infiltration galleries or wells. This system is different in that as the seawater slowly filters through the ocean floor, it undergoes a natural pretreatment. As a result, the water usually contains lower levels of aquatic organisms, organic contaminants, solids, oil & grease and slit.
The advantages of subsurface water intake include:
- Regulatory agency support
- Low marine life growth inside the intake pipeline
- Natural filtration pre-treatment possibility
- Entrainment and impingement issues absent
However, this system is not without challenges. For instance, if the source is an alluvial aquifer, chances are that the water will contain not only extremely low oxygen concentration, but also abnormally high level of contaminants such as hydrogen sulfide, iron and manganese. There is also this assumption that subsurface intake offers better seawater quality. This holds true only in specific cases, for example, when the location is a well-flushed ocean bottom. In summary, the following issues have to be considered before opting for subsurface water intake:
- Water quality
- Site feasibility
- Possible beach erosion
- Maintenance and operation
- Impact of infiltration galleries on environment
- Discharge problems
- Aesthetic and visual impact
And so on.
Choosing Between the Alternatives
While both alternatives offer a number of advantages and disadvantages, open sea intakes are more common globally because they have low environmental impact, are available in virtually any size and can be built anywhere. Usually low profile in nature, the open surface system does very little harm to the surrounding coastal environment. If the chiller plant is already in the vicinity, there might be no need to build a new onshore facility or structure. Generally, the environmental impact of the system is usually minimal. Important to remember is that you need to consider the following factors to take full advantage of these cost-effective benefits of this system:
- Plant size
- Geology and depth of sea floor
- Water quality impact
- Ease of installation
Designing a seawater intake system for district cooling purposes is an intricate process that has massive repercussions. You need to perform a thorough study of the surrounding accompanied by an equally good dimensioning. Design the structure incorrectly and you put yourself at risk of losing more than even the construction costs. It is important to have collaboration with the installation company. To know how you can generate district cooling water using seawater intakes, contact the company today.