Makemo Atoll, French Polynesia
Project Makemo is a system for irrigating corals with cooled, alkalized water. Right now this is only lightly sketched out. Over time this page will be filled in to estimate how practical and efficient this design is.
The water which will be delivered to the corals is produced by first enclosing surface seawater in a reservoir. This seawater is alkalized by growing plants in it: photosynthesis removes carbon dioxide from the water and raises its pH as a result. Once the seawater reaches the desired pH, it is pumped through one side of a heat exchanger. In parallel, a pipeline is used to pump seawater out of the deep ocean, whose water is much colder than at the surface. This cold water is sent through the other side of the heat exchanger. Within the heat exchanger, the cold water acts to cool the alkalized water to the desired temperature without intermixing. On the outlet of the heat exchanger, the cold water is sent back to the ocean, and the cooled, alkalized water is sent to the corals themselves with a network of irrigation pipes.
This system is best suited for reefs that are nearby both deep water for pumping and protected waters that can store a reservoir of alkalized water. Atolls and islands with fringing reefs encircling a lagoon both meet these criteria.
Biological processes have a large effect on the amount of dissolved in seawater, and on the resulting pH. In closed/small bodies of water, pH can vary quite a lot diurnally, swinging between 8.2 and 8.9 . This is due to differing levels of photosynthesis and respiration: photosynthesis consumes and alkalizes water, whereas respiration produces and acidifies water. During the day, more photosynthesis is happening and pH increases, while at night more respiration is happening and pH decreases.
By creating a reservoir, an area of the lagoon that has minimal water exchange with its surroundings, and then managing the biota within the reservoir to maximize photosynthesis and minimize respiration (essentially an aquaculture operation), it seems like it should be possible to increase the alkalinity of the reservoir significantly above that of surrounding areas. How exactly this will work is yet to be determined.
Using seawater pumped from the deep ocean for cooling is an established technique. The main existing use is for air conditioning systems: cold water is pumped out from the deep ocean or a deep lake, and run through a heat exchanger with a closed loop of cooling system water on the other side . There are several large systems that have been built (e.g. one for all of Cornell University ), and several that have been installed for resorts near corals in French Polynesia. These systems are very efficient, with reported energy reductions of around 90% compared with conventional air conditioning  .
Because this is is an established technique, the main task here is to figure out the appropriate engineering for a given cooling system size. Deeper water will be colder but require more piping, and larger cooling systems will be more cost effective than smaller ones. The piping will probably be the most expensive part of the whole system. A system on Tetiaroa in French Polynesia pumps water from 930m deep which arrives at 5° C .
The heat exchanger is also expensive and it would be good to figure out how necessary it ultimately is. The main reason for using a heat exchanger is that deeper ocean water is more acidic than surface water. It's hard to say how much more acidic, as the effect could vary from a drop of .2 - .6 at 1000m depth , depending on one's location. Mixing the cold acidic ocean water with the alkalized water would require a lot of care to make sure the result has the right temperature and pH and may require a lot more alkalized water than would otherwise be needed. Instead, the existing design uses a heat exchanger to make the deep water pH irrelevant.
Once seawater has been alkalized and cooled, it can be delivered to the corals. It is probably most efficient, albeit pretty ugly and invasive, to do this in the same manner as irrigation pipes, where the treated seawater is released directly next to the corals themselves. It's unknown how effective this will actually be at adjusting the temperature and pH of the water the coral is exposed to, and there are several factors working against this approach. The treated and untreated seawater will soon reach equilibrium even if they don't mix, and if they do mix they will reach equilibrium much faster. If there is much current or surge the treated water will be quickly swept away from the corals, and baffles will almost certainly be needed to reduce these effects.
The efficiency of this project directly corresponds with how efficient the irrigation system is: the less water that is required to treat an area of corals, the more corals that can be treated by a system of a given size. The next step here is to model the chemical and physical processes in this irrigation and gain more understanding of it.
- F. Millero, Chemical Oceanography, (CRC Press, 2013), 286
- F. Millero, Chemical Oceanography, (CRC Press, 2013), 289