Cold Water Source Cooling
Cold water source cooling is not an energy source, but it can enable significant energy savings by providing a cooling source for regions with high air conditioning loads. It is considered a potentially viable option if cold water from the ocean exists in close proximity to large load centers. Unlike OTEC, cold water source cooling only requires a cold water source, it does not need a thermal gradient. Its economics are largely determined by the infrastructure cost needed for implementation, which are difficult to obtain for a generic application. Because of the high thermal capacity of water, cold ocean water has the potential to reduce energy demands of cooling processes in commercial and industrial applications, especially in populated regions of the GoM where air conditioning is a primary load.
Cold water source cooling uses cold water in the chiller of an air conditioning (A/C) system instead of an electricity-driven compressor as shown in the following two images. Using cold water reduces the electricity use of A/C systems by up to 85% to 90% (Makai 2017). This provides a significant benefit because electricity used in A/C is often from periods of peak generation and is the highest cost.
The most effective form of cold water source cooling is district cooling. District cooling is much like district heating, but rather than having a central source of heat, a central source of cold is used to chill a cooling fluid. This cooling fluid is then circulated between buildings and is used for air conditioning (War 2011; Makai 2017). For this to work, cold-water is pumped from ocean sources that are usually deeper than 300-m and brought to shore. Seawater sources can be as warm as 8 ˚C (Ascari et al. 2012; Makai 2017), but colder water sources yield higher efficiency and require less water.
It is also worth noting that the chiller efficiency for an air conditioning system is related to the temperature of the ambient source for which the heat from the hot refrigerant is dumped. This is typically air or an evaporative cooling tower. Cooler water near shore can thus be used to increase the efficiency of conventional air conditioning systems with gains between 10 and 25% (personal correspondence with Makai Ocean Engineering).Cold water source cooling systems are not common, but they have been implemented in several commercial applications, including:
- City of Stockholm (>100,000 tons )
- City of Toronto (75,000 tons)
- Cornell University (20,000 tons)
- Purdy’s Warf – Nova Scotia (1,000 tons)
- Intercontinental Hotel – Bora Bora (450 tons)
- Natural Energy Laboratory of Hawaii Authority (50 tons)
While there are no large technical risks in cold water source cooling, the primary hurdle within the GoM is developing a low cost offshore pipe that can transport water to shore with minimal heat transfer to the cold water and low power pumping.
In the states that border the GoM, a large proportion of the electricity use goes to air conditioning. For example, the percent of electricity used for air conditioning, based on an annual average in the residential sector up is to 27% (EIA 2009). On a seasonal basis, this number is much higher in the summer when air conditioning use is the greatest. Also, it is often the most expensive electricity (peak generation) that supplies air conditioning. Replacing or increasing the efficiency of conventional electricity-based air conditioning systems has the potential to significantly reduce energy use along the GoM; however, the primary costs of a cold water source cooling system are in the pipe used to transport the water to shore, pumping system and the cold water distribution system.
Favorable economics for cold water source cooling depend on the following factors:
- A source of cold water close to shore, ideally within 10 km
- A concentrated air conditioning load (e.g., dense city core) near the source, and
- High electricity costs.
When the cold water source is close to shore (within 10km), the heat exchanger and pumping stations can be located on shore where they are easier to install and service, with minimal heating of the cold water as it is brought to shore. For further offshore installations, costs will increase due to the need for larger diameter pipes and intermediate underwater pump stations – both major cost drivers. Additionally, heat gain through long runs of pipe through warm water can be significant. For these reasons, cold water source cooling systems have not been considered when an intake pipe exceeds 10 km (personal correspondence with Makai Ocean Engineering). For new installations, the trenching and tunneling work to run pipes can be very expensive if infrastructure does not already exist.