Membrane desalination of ballast water using thermoelectric energy from waste heat

Figures & data

Figure 1. (A) Principle of thermoelectric generation from waste heat source; (B) the effects of ZT values and temperature differentials on the electrical conversion efficiency of a TEG; (C) Typical energy efficiency profile and losses in ship operations; and (D) Energy efficiency profile with waste heat recovery through thermoelectric generation.

Table 1. Waste heat sources on the M/V Rosita.

Heat Source	Medium	Temp. (°C)	Quantity	Estimated TEG Power (kW)
Electrical generating unit	Flue gas	340	0.69 Nm^3/s	10.0
Incinerator	Flue gas	340	0.68 Nm^3/s	9.7
Main engine exhaust after boiler	Flue gas	210	14.18 Nm^3/s	42.4
Main engine scavenge air cooling	Air	162	13.92 Nm^3/s	46.4
Excess steam from boiler	Sat. steam	159	0.087 kg/s	5.9
Main engine cooling water	Fresh water	83	18 kg/s	11.8
FW generator unit, boiling water	Fresh water	61	8.3 kg/s	4.3
Lubrication oil cooler	Lubrication oil	49	46 kg/s	2.1
Fresh water generator unit, condenser	Salt water	37	25 kg/s	0.4
Total energy recovered				133 kW

Figure 2. Process schematic of a reverse osmosis (RO) desalination unit with or without energy recovery device: (a) without energy recovery device; with energy recovery devices (b) Pelton turbine; and (c) pressure or work exchanger units (refer to Table 2 for process locations and flow/pressure/energy values).

Table 2 Specific energy consumption calculations without an energy recovery device.

				Efficiency (%)
Process point	Description	Flow rate (m^3/h)	Pressure (bar)	Pump	Motor	power required (kWh)	Time (h/d)	Energy consumed (kWh/d)	% of Total energy
1	Seawater intake	2.22	4	80	92	0.34	24	8.04	5.35
2	Pre-treatment supply	2.22	2	80	95	0.16	24	3.89	2.59
3	Pre-treatment chemicals dosing	–	–	–	–	0.10	24	2.40	1.60
4	Microfiltration + HP pump supply	2.22	4	80	96	0.32	24	7.71	5.13
5	High pressure pumping	2.22	64	87	–	4.64	24	111.44	74.16
6	Post-treatment chemicals dosing	–	–	–	–	0.10	24	2.40	1.60
7	Treated water pumping	1	10	85	96	0.34	24	8.17	5.44
8	Concentrate discharge	1.22	2	80	92	0.09	24	2.21	1.47
9	Filters backwashing pump			80	96	2.00	2	4.00	2.66
							Total = 150.26 kWh/d
							SEC = 6.26 kWh/m^3

Table 3. Specific energy consumption calculations with a Pelton turbine energy recovery device.

				Efficiency (%)
Process point	Description	Flow rate (m^3/h)	Pressure (bar)	Pump	Motor	power required (kWh)	Time (h/d)	Energy consumed (kWh/d)	% of Total energy
1	Seawater intake	2.22	4	80	92	0.34	24	8.04	5.35
2	Pre-treatment supply	2.22	2	80	95	0.16	24	3.89	2.59
3	Pre-treatment chemicals dosing	–	–	–	–	0.10	24	2.40	1.60
4	Microfiltration + HP pump supply	2.22	4	80	96	0.32	24	7.71	5.13
5	High pressure pumping	2.22	64	–	–	4.64	24	111.44	74.16
	Energy recovered by Pelton Group (η= 89%); shaft (η= 85%)	1.22	63			1.60	24	−38.43	−25.58
6	Post-treatment chemicals dosing	–	–	–	–	0.10	24	2.40	1.60
7	Treated water pumping	1	10	85	96	0.34	24	8.17	5.44
8	Concentrate discharge	1.22	2	80	92	0.09	24	2.21	1.47
9	Filters backwashing pump			80	96	2.00	2	4.00	2.66
							Total = 111.83 kWh/d
							SEC = 4.66 kWh/m^3

Table 4. Specific energy consumption calculations with a pressure or work exchanger unit.

				Efficiency (%)
Process point	Description	Flow rate (m^3/h)	Pressure (bar)	Pump	Motor	Power required (kWh)	Time (h/d)	Energy consumed (kWh/d)	% of Total energy
1	Seawater intake	2.22	4	80	92	0.34	24	8.04	5.35
2	Pre-treatment supply	2.22	2	80	95	0.16	24	3.89	2.59
3	Pre-treatment chemicals dosing	-	-	-	-	0.10	24	2.40	1.60
4	Microfiltration + HP pump supply	2.22	4	80	96	0.32	24	7.71	5.13
5	High pressure pumping;	1	64	-	-	1.93	24	46.38	30.86
	Circulation pump (η= 95%) Pressure exchanger recovered energy from 55% brine	1.22	4			0.16	24	3.83	2.55
6	Post-treatment chemicals dosing	-	-	-	-	0.10	24	2.40	1.60
7	Treated water pumping	1	10	85	96	0.34	24	8.17	5.44
8	Concentrate discharge	1.22	2	80	92	0.09	24	2.21	1.47
9	Filters backwashing pump			80	96	2.00	2	4.00	2.66
							Total = 89.03 kWh/d
							SEC = 3.71 kWh/m^3

Figure 3. A generic relationship between the feed water temperature and the permeate flow (A); the effect of feed water temperature on permeate flow rate (B); the effect of feed water temperature on specific energy consumption (C); and the specific energy consumption including thermal energy requirements.