High Efficiency HVAC for Marine Vessels
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Description of technology
Since the energy for the entire ship’s electrical load is produced from diesel generator sets, each electrical load has a direct impact on overall fuel economy and emissions. Key areas for fuel efficiency improvement for 100 m class passenger vessel were identified through completion of a study to quantify the annual fuel consumption associated with each of the major electrical loads. Second to the propulsion load is HVAC. The HVAC load can account for 15% of annual fuel consumption.
HVAC marine architecture is typically very conventional and is usually put together based on designs for similar, previous ship builds. However, there are significant opportunities for efficiency upgrades using approaches that are a major departure from conventional practice. This report summarizes a number of commercially available HVAC technologies for efficiency enhancements and are summarized in the following figure:
High Efficiency HVAC Features
- • Dedicated intermediate temperature cooling of electronic and mechanical equipment
- • Demand-controlled ventilation of cabins and compartments, perhaps based on CO2 sensing
- • Smaller central chillers and air handling equipment with increased efficiency of HVAC
- • Comfort, efficiency, and volume saving advantages of radiant heating and cooling
- • No recirculation of return air; energy and humidity exchange with compartment air
- • Additional energy recovery opportunities not shown in the figure:
- • Waste heat from gensets for production of chilled and fresh water
- • Waste heat from chiller condensers for domestic hot water heating
Four key elements to reduce HVAC energy consumption while maintaining a comparable level of flexibility and function are discussed below. The proposed technology is used on land-based systems, but much less on ships. It appears that the technology, however, is quite appropriate for ship applications.
1. Intermediate temperature cooling loop for electronics and equipment – a significant portion of the HVAC chilled water load will be dedicated for space conditioning to accomplish electronic and mechanical equipment cooling. This cooling is typically accomplished by drawing compartment air at 70-80° F through hot equipment and exhausting the heated air back into the conditioned space for subsequent recirculation through the air handling ductwork. Cooling of equipment using the primary HVAC system in this way is clearly inefficient. A portion of the added burden is caused by the additional fan power required to move significantly more hot air through compartments in which these equipment loads are located.
Fortunately, there is an alternative thermal management strategy that is more energy efficient. A dedicated intermediate temperature cooling loop could be installed for direct cooling of equipment loads. The intermediate sink fluid would be circulated through installed equipment using either a conventional cold plate design or a cooling coil and closed system recirculation fan within the equipment cabinet. By de-coupling equipment cooling from the comfort conditioning load, the following benefits are possible:
- • Reduced energy consumption by optimizing the intermediate cooling loop specifically for installed equipment
- • Increased efficiency of equipment cooling through point-source conduction and convection waste heat removal
- • Lowered power consumption in the primary cooling loop which is unburdened from cooling electronics and installed mechanical equipment
- • Reduced recirculation-fan power requirements in the primary loop
- • Elimination of some of the “hotel load” recirculation ductwork
- • Enhanced efficiency for cooling equipment, thus extending its service life
An initial assessment of the benefits of an intermediate temperature cooling water loop aboard a 100 m class vessel was completed. The relative energy savings from chiller operation are about 13%.
2. Demand controlled ventilation – The conventional HVAC systems are designed to supply ventilation (outside) air based on assumed, rather than actual, occupancy. This often results in over-ventilation, thus wasting both energy and placing an additional burden on the primary HVAC system. A demand controlled ventilation (DCV) system is specifically tailored to monitor conditions in every conditioned zone and then deliver required ventilation when and where it is needed. Using DCV, excessive over-ventilation is avoided while providing the required volume (cfm/person) of outside air specified by applicable codes and standards.
In direct digital control systems, the required controls are easily implemented because all of the necessary real-time information can be already available. CO2 occupancy sensing is particularly attractive and is increasingly being used to modulate outside air ventilation based on real-time occupancy.
Energy savings in a given application depend on actual occupancy level patterns, as well as type of conditioned space and the ambient climate. A well-designed DCV ventilation system capable of meeting exact occupancy needs could be expected to save on-the-order-of 25% of HVAC operating costs.