Water Desalination Energy Requirements 20,000
18,000
16,000
Sea Water contains about 35 grams of salt per Kg of Water.
Energy per Kg of Water Produced (Joules)
14,000
12,000
10,000
8,000
6,000
4,000
About 6,000 Joules per Kg of Water produced is required for desalination - at 40C, with input being concentrated to 70 grams of salt per Kg of Water remaining.
2,000
0 0
25
50
75
100
125
150
175
200
225
250
Concentration of Salt (grams of Salt per Kg of Water) At Temp = 100C
(C) Colin Dunstan July 5, 2004
At Temp = 40C
At Temp = 25C
35 grams per Kg of Water
70 grams per Kg of Water
THE CABINET OFFICE NEW SOUTH WALES
AA04/14636 - CABSEC
19/07/04
Mr Colin Dunstan
Dear Mr Dunstan The Premier has received your recent letter concerning water desalination. As the matter you have raised primarily concerns the administration of the Minister for Infrastructure and Planning, and Minister for Natural Resources, the Hon CJ Knowles, MP, the Premier has arranged to bring your approach to the Minister’s attention. You may be sure that your comments will receive close consideration. Yours sincerely
Roger B Wilkins Director-General
LEVEL 39, GOVERNOR MACQUARIE TOWER. 1 FARRER PLACE, SYDNEY 2000, AUSTRALIA. TEL: (02) 9228 5300 FAX: (02) 9228 3062 G.P.O.BOX 5341, SYDNEY 2001
Colin Dunstan
5th July 2004 Mr Bob Carr, Premier of New South Wales NSW Parliament Macquarie Street SYDNEY NSW 2000 Dear Mr Carr, Subject: Desalination In a television news report shown on Thursday, 1st July 2004, you said that the NSW Government had ruled out desalination for increasing the water supply because too much energy was needed. From one perspective, this is true. I have enclosed a chart showing the energy required for desalination. Briefly, the chart shows that energy needed for desalination increases with: 1. The temperature at which the process is carried out. 2. The proportion of the input seawater that is extracted as fresh water. (One of the enclosed spreadsheets may be used to verify the correct use of the physical models from which the chart is prepared. The other spreadsheet provides the performance targets for a simple, reliable desalination process.) However, there is another perspective for looking at this problem. With changes in the efficiency and economics of energy production, an energy requirement that is “too high” at present can become quite affordable. NSW has a substantial capital investment in coal-fired power generating capacity. This capital investment is more than machinery, steel and concrete. The skill and experience of the workforce that manages, operates and maintains these power stations is also very valuable. There is a relatively simple adaptation that may be made to this existing investment with which to increase the power generating capacity. An increase of 10-20 times the current output with the same input of coal should be considered a reasonable target. Sceptics will dismiss this idea for 2 reasons: 1. It is theoretically impossible to increase energy output from thermal power stations by more than a few percent above what is already achieved by the States’ most efficient coal-fired power stations. 2. Even if it were theoretically possible to convert 100% of the heat energy in the coal to mechanical/electrical energy, this would only allow output to be increased by 2-3 times above what is currently achieved.
These following examples are more complex than what I’d suggest, and their economic viability is doubtful. They are only to illustrate some strategies that sceptics may have overlooked: 1. Different types of fuel cells have been tested that operate at elevated temperatures. Say, 200°C, just for an example. A source of heat energy is needed to maintain these fuel cells at their most efficient operating temperature. The electrical energy produced by such fuel cells is many times the heat energy needed for their operation. The theoretical limits for thermal efficiency of heat engines do not apply to non-thermal energy conversion processes. “But where does the additional energy come from?” the sceptics may ask: 2. Low-grade energy may be accumulated in one process, and then delivered as high-grade energy in another process. For example, a sugar cane plant growing in the middle of a paddock can accumulate solar energy over a number of months. An ethanol factory may, from the sugar extracted from the sugar cane plant, create a bottle of ethanol that your car engine can burn in a matter of minutes. Note that there are far less costly methods of accumulating low-grade energy — Methods that do not involve taking agricultural land away from food production. The adaptation of existing power stations that I have in mind is not very complex. As a guide to the engineering task, the additional module would be on about the same level as a “catalytic cracker”, examples of which can be found in every oil refinery throughout the world. The science and engineering was developed in the 19th century, and is, sadly, simple and old-fashioned. (No chance for a Nobel Prize nomination.) Even though the science and engineering of the solution is unimpressive, the social and economic benefits are staggering. For one thing, the amount of energy needed for water desalination should no longer be a concern.
Yours sincerely, Colin Dunstan
2
Attachment — Considerations in Efficient Operation of Desalination Plants A couple of additional points to keep in mind: 1. Calcium carbonate (that is, limestone) can be deposited on heat transfer surfaces inside desalination plants. This slows heat transfer, and reduces efficiency. The depositing of limestone occurs quickly if desalination is carried out above 50°C, or if the concentration of calcium becomes too high in the water being distilled. For seawater, this occurs when about 50% of the input seawater has been extracted as fresh water. 2. The enclosed chart has a marker showing the energy required for desalination of seawater at 40°C, with 50% of the input being extracted as fresh water (leaving 70 grams of salt per kilogram of seawater to be rejected.) This should be considered an advisable operating limit. Otherwise, “de-scaling” measures become necessary. For example, use of chemical additives in the input flow. Cost of desalination would then be greater than is necessary. 3. The “energy” on the enclosed chart is mechanical energy, not heat (thermal) energy. Mechanical energy has traditionally been more expensive than thermal energy. This is because heat engines (eg. Steam turbines) produce most mechanical energy, and even very efficient versions, such as large-scale electric power stations, must consume 2-3 times more heat energy than the mechanical energy they produce.
3
Water Desalination Energy Requirements 20,000
18,000
16,000
Sea Water contains about 35 grams of salt per Kg of Water.
Energy per Kg of Water Produced (Joules)
14,000
12,000
10,000
8,000
6,000
4,000
About 6,000 Joules per Kg of Water produced is required for desalination - at 40C, with input being concentrated to 70 grams of salt per Kg of Water remaining.
2,000
0 0
25
50
75
100
125
150
175
200
225
250
Concentration of Salt (grams of Salt per Kg of Water) At Temp = 100C
At Temp = 40C
At Temp = 25C
35 grams per Kg of Water
70 grams per Kg of Water
At Temp = 100C Relative NaCl (Wt Humidity % %)
NaCl
NaCl
RT * ln(Po/P) (T = 373.15K, =100C) (R=8.314472)
(grams / Kg (Moles / Kg (Joules / Mole) H2O) H2O) 100.0000 98.3816 96.6842 93.1447 89.4737 85.3947 81.1842 76.7763
0.00 2.84 5.52 10.46 14.91 18.94 22.60 25.95
At Temp = 40C Relative NaCl (Wt Humidity % %)
0.00 29.20 58.40 116.80 175.20 233.60 292.00 350.40
NaCl
0.0 0.5 1.0 2.0 3.0 4.0 5.0 6.0
NaCl
0.0000 50.6230 104.6181 220.3291 345.0826 489.8476 646.7237 819.9220
RT * ln(Po/P) (T = 313.15K, =40C) (R=8.314472)
(grams / Kg (Moles / Kg (Joules / Mole) H2O) H2O) 100.0000 98.3816 96.6842 93.1447 89.4737 85.3947 81.1842 76.7763
0.00 2.84 5.52 10.46 14.91 18.94 22.60 25.95
At Temp = 25C Relative NaCl (Wt Humidity % %)
0.00 29.20 58.40 116.80 175.20 233.60 292.00 350.40
NaCl
0.0 0.5 1.0 2.0 3.0 4.0 5.0 6.0
NaCl
0.0000 42.4832 67.7962 184.9017 289.5956 411.0834 542.7349 688.0841
RT * ln(Po/P) (T = 298.15K, =25C) (R=8.314472)
(grams / Kg (Moles / Kg (Joules / Mole) H2O) H2O) 100.0000 98.3816 96.6842 93.1447 89.4737 85.3947 81.1842 76.7763
0.00 2.84 5.52 10.46 14.91 18.94 22.60 25.95
0.00 29.20 58.40 116.80 175.20 233.60 292.00 350.40
0.0 0.5 1.0 2.0 3.0 4.0 5.0 6.0
0.0000 40.4482 83.5907 176.0448 275.7239 391.3923 516.7377 655.1246
Energy for Vapour Compression
Osmotic Pressure
(Joules / Kg H2O)
(kPa)
0.00 2,812.39 5,812.12 12,240.50 19,171.25 27,213.75 35,929.10 45,551.22
0.00 2,812.39 5,812.12 12,240.50 19,171.25 27,213.75 35,929.10 45,551.22
Energy for Vapour Compression
Osmotic Pressure
(Joules / Kg H2O)
(kPa)
0.00 2,360.18 4,877.57 10,272.31 16,088.65 22,837.97 30,151.94 38,226.89
0.00 2,360.18 4,877.57 10,272.31 16,088.65 22,837.97 30,151.94 38,226.89
Energy for Vapour Compression
Osmotic Pressure
(Joules / Kg H2O)
(kPa)
0.00 2,247.12 4,643.93 9,780.27 15,317.99 21,744.02 28,707.65 36,395.81
0.00 2,247.12 4,643.93 9,780.27 15,317.99 21,744.02 28,707.65 36,395.81
Carnot Cycle - Theoretical optimum performance of heat pumps 1
For Cooling Temperature Kelvin Centigrade T(hot)
313.15
40
T(cold)
312.15
39
1
1
Difference Coefficient of Performance [T (cold) / Difference]: 2
For Heating
312.15 Temperature Kelvin Centigrade
T(hot)
313.15
40
T(cold)
312.15
39
1
1
Difference
Coefficient of Performance [T (hot) / Difference]: 313.15