Energy saving
There are many ways to reduce a ship’s energy consumption, some of them which uses today for waters systems are waste heat utilization from diesel engine high temperature jacket cooling water, uses of variable frequency drives, high-pressure boosters and pressure exchangers, etc. Below described their principals and application
Waste heat utilization
This way of energy saving well-known
and widely uses on merchant fleet. Conventional FWG plants as with plate type
of heat exchangers as with shall-and-tube type of
heat exchangers by themself are energy saving plants on ship due to
opportunity to utilize waste heat from diesel engine high
temperature jacket cooling water to produce
distil water.
Variable frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to provide effective control AC motor speed and torque by manipulating voltage and frequency.
Controlling the speed of a motor ensure improved process control, reduced wear on machinery, increased power factor and energy savings.
There are many ways to save energy by utilizing variable frequency drives on shipboard, depending on the type of application. For instance, variable torque application, like fans and pumps, simply reducing the speed will lead to a significant reduction of energy consumption. The cube law relationship between speed and power means that reducing a fan’s/pump’s speed in a variable torque load application by 20 % can achieve energy savings of 50 %.
As for applying VFD for water systems this type of motor drive well recommended itself for energy saving of RO FWG plants, the flow through the membranes can be controlled and optimized in a better manner as well as improved the efficiency of the motors.
The HPB (hydraulic turbocharger)
allow to reduce the energy consumed in a seawater RO system by recover the
wasted high pressure brine energy due to reduce the size of the high-pressure
feed pump and lowering the motor electrical consumption. In the hydraulic
turbocharger, the high-pressure reject brine from the RO membranes enters the
turbine side of the unit (see figure below). This high-pressure flow rotates the
turbine impeller of the rotor. The rotor converts the hydraulic energy into
mechanical energy used by the pump side impeller. This mechanical energy
provides a pressure boost to the feed raw water flow. This boost reduces the
pressure requirement of the RO system high-pressure feed pump.
High-pressure booster (Hydraulic turbocharger)
1 – pump side, 2 – hydraulic turbine side
In figure below is shown process diagram of RO FWG plant with energy recovery by HPB.
Source: https://www.watman.fi/pages/en/sea-water-desalination
WatMan RO 45000 SW 2-pass reverse osmosis FWG plant with energy recovery by HBP
The pressure exchanger transfers hydraulic energy from a high-pressure fluid flow to a low-pressure fluid flow. Pressure exchanger unit can be different type. One of them is rotary type and depicted in figure below.
Rotary type pressure exchanger schematic diagram
a – high-pressure side, b – low pressure side, c – rotor rotation, d – sealed area, 1 – low-pressure raw water inlet, 2 – pressurized raw water outlet, 3 – high-pressure reject brine inlet, 4 – low-pressure reject brain outlet
Rotary type pressure exchanger generally consists of cylindrical rotor with longitudinal channels parallel to its rotational axis (see figure above). The rotor rotates inside a sleeve between two end covers. Pressure energy is transferred directly from the high-pressure flow to the low-pressure flow in the channels of the rotor. Some fluid that remains in the channels serves as a barrier that inhibits mixing between the feeding raw water and rejected brain flows. The channels of the rotor charge and discharge as the pressure transfer process repeats itself.
One
application in which pressure exchangers are widely used is RO FWG plant. In a
RO FWG, pressure exchangers are used as energy recovery unit (figure below).
Source: https://www.watman.fi/pages/en/sea-water-desalination
WatMan RO 45000 SW 2-pass reverse osmosis FWG plant process diagram