ENERGY REQUIREMENTS COMPARISON

Since most salts have an affinity for water, it takes energy to remove these salts from water. (The predominant salt involved is NaCl.)

It is theoretically possible to calculate the minimum work or energy needed for separation of pure water from salty water. This minimum energy is equal to the difference in free energy between the incoming feed (i.e. seawater) and outgoing streams (i.e. product water and discharge brine). For the normal seawater (3.45 per cent salt) at a temperature of 25_C, for usual recoveries the minimum work has been calculated as equal to about 0.66 KCal per liter. (Click here for a detailed calculation of this idealized condition.) Note that this idealized energy level is related to the salt content, since the free energy (and related osmotic pressure) varies with salt concentration.

However, for real processes this factor is mostly academic since the actual energy required is likely to be many times the theoretically possible minimum. In the distillation processes there is always a substantial heat loss as well as a pumping requirement to remove soluble and in-leakage gasses in order to lower the boiling point and avoid corrosion on the large heat exchange surfaces. Therefor there is both a heat and an electricity energy requirement. In the reverse osmosis process additional pressure (beyond the osmotic pressure) is requred to maintain an adequate flow rate through the many very tiny pores in the semi-permeable membranes. This energy will also be several times the osmotic pressure - which will be greater when the feed water salinity is greater due to the lack of fresh water feed into the upper end of the Bay during dry seasons. There is also a more substantial pre-cleaning operation in order to remove particles that might plug the small pores.

The following table summarizes the energy requirements for the various desalination processes. (Click here to see the detailed reference article.)

PROCESS TYPE	ENERGY TYPE	ENERGY		PLANT SCALE (MGD)
		per Cubic Meter	per 100 Cubic Feet
Multi-Stage Flash (MSF)	Electric	3.5-5.0 KWHr	9.9 - 14.1 KWHr	7.1
	Thermal	282 MJ = 2.67 MMBTU	7.56 MMBTU
Multi-Effect Distillation (MED)	Electric	1.8 KWHr	5.1 KWHr	2.64
	Thermal	263 MJ = 2.56 MMBTU	7.25 MMBTU
Vapor Compression Distillation	Electric	11 KWHr	31.1 KWhr	0.8
	Thermal	0	0
Reverse Osmosis	Electric	6.6 KWHr	18.7 KWHr	1.6
	Thermal	0	0

Note: The source document provides the energy requirements in metric units. For convenience, the above table also shows the energy values in terms of MMBTU (million BTU) and per 100 Cubic Feet of product water. The choice od 100 Cubic Feet was based on the fact that the MMWD water rates are also based on $ per 100 Cubic Feet.

At $0.12 per KWHr for electricity, the energy costs for the RO process is $2.24.

While the electric energy consumption of an MED plant is 13.6 KWHr per 100 cubic feet less than the Revrse Osmosis plant, causing a savings of $1.62 per 100 cubic feet in electricity consumption, the additional cost of the thermal energy if natural gas were used (at $3.50 per MMBTU) would be $25.38. Accordingly, large scale distillation plants are not used unless thermal energy is available at essentially no cost. This is true when the thermal energy can be obtained from the exhaust steam from a steam-electric generation plant (the exhaust steam has considerable energy and must be condensed for recycle to form more steam for the electrical plant).

Alternatively, since only a relatively low temperature is required of the thermal energy needed by a distillation plant, the use of thermal solar panels could be considered. Further, it is recognized that the cost of thermal solar panels is substantially less than the cost of solar photo voltaic panels that produce electricity directly. Further, their efficiency in converting solar energy into heat is approximately 80 per cent versus the 20 per cent efficency of solar photo voltaic panels.

As a result, small scale thermal solar desalination plants are commercially available (click here to view one such system).

While no large scale thermal desalination plants exist, it is possible to make a rough comparison of the cost of energy for a Reverse Osmosis plant powered by photo voltaic solar panels versus a distillation plant powered by thermal solar panels.

Assuming a solar insolation average value of 1200 BTU per sauare foot per hour for 12 hours per day, the energy production from a 4 foot by 8 foot panel would be as follows:

Total energy input to panel = 38,400 BTU per hour

REVERSE OSMOSIS

Photo voltaic - 38,400 BTU per hour X 20% = (7,680 BTU/hour) / (3413 BTU/KWHr) = 2.25 KWHr/hour = 2.25 KW

Panels required = 18.7 KWHr per 100 cubic feet / 2.25 KW = 8.3 panels to generate 100 cubic feet per hour.

DISTILLATION

Electricity requirement:

Panels required = 5.1 KWHr per 100 cubic feet / 2.25 = 2.27 photo voltaic panels

Thermal energy = 38,400 BTU per hour X 80 % = 30,720 BTU/hour

Panels required = 7.25 MMBTU per 100 cubic feet / 30,720 BTU/hour = 236 panels.

DIFFERENCE

Since the distillation process requires 6.03 less photo voltaic panels, to be competitive (assuming the capital costs of the RO and distillation plants are comparable) the cost 6.03 panels saved must be equal to or less than the cost of the 236 thermal panels or a themal panel must cost only 6.03/236 = 0.025 (2.5%) of the cost of a photo voltaic panel.

Obviously this does not work - and solar desalination will likely be used only for small applications that have site specific problems.

CAVEATS:

The design of most distillation systems assumes nearly free thermal energy, thereby allowing the use of a minimum of heat transfer surface and heat utilization efficiency. If thermal energy were not free the systems would undoubtedly be differently designed to minimize energy costs. This would involve a trade off between lowering energy costs but increasing capital costs by use of increased heat transfer surface area. No cost estimates relative to such an approach are available.

Thermal solar panels are made of copper and other good heat conductors - but which would be badly corroded by sea water. Changing to less corrodable stainless steel would be possible but extremely expensive. The more logical approach if solar panels were used would be to use an intermediate heat exchange fluid, much as is used in most automobiles, which would in turn transfer the heat corrosion resistant heat transfer surfaces in the desalination unit. In addition, it would be possible to store the heat over-night when the sun is not shining by storage of hot heat transfer fluid in a large well insulated tank.