ENERGY REQUIREMENTS

REVERSE OSMOSIS

Chapter 5, Energy Analysis, of the MMWD Environmental Impact Report reviews the energy requirements for their proposed reverse osmosis desalination plant. Click here to see that Chapter.

Those requirements, and their relative magnitude compared to the total Marin County electrical energy requirements are summarized here from that Chapter for the readers convenience.

The greatest energy requirement occurs in draught years when the salinity of the feed water is the greatest and when the plant operates year around. The total power consumption for this case is 76.650 million kilowatt hours per year.

By comparison, the total annual electric energy consumption by Marin County users is 1,421 million kilowatt hours.

Thus, the proposed desalination energy requirement is about 5 per cent of the total county use.

While this number is likely to be insignificant to the total usage, the specific power distribution system to provide it to the plant from its source outside of Marin has yet to be developed. This may or may not be significant depending on the source and existing transmission systems. Thus, the subject is similar to the related question of how externally generated solar and wind power would be transmitted and distributed to Marin customers.

It is also noted that their are two quite different proposed modes of operation. During draught years the desalination plant would be operated at full capacity for 365 days per year - less reduced production for maintenance operations. During wet years the plant would be operated at full capacity for only a few months with the plant being shut down or being kept at standby (low power consumption) conditions the remaining months.

If operated at full capacity wholesalers of electric power are likely to offer pricing based on "base load" usage - meaning that they can recover their capital costs throughout the full year.

However, this is not likely since there would be strong pressure to shut down or reduce the power consumption during late summer afternoons when the total county consumption is peaking. While this would ease the peaking power requirement for the overall system, it would result in some penalty for not being a totally base loaded system.

Further, when the plant is only operated for a few months during wet years, wholesalers of power would recognize that the plant is not base-loaded - meaning that they would not be able to recover continuing capital and standby operation costs during periods of no or lower power consumption.

See the section on economics for a further discussion of base load versus peak load power pricing.

ALTERNATIVE MEHODS and COMPARISONS

The electrical energy requirement for MMWD proposed reverse osmosis plant is about 10 KHh per 1000 gallons of fresh water product.

MSF

MSF plants use about 5 KWh/cubic meter, or about 1.89 KWh/1000 gal. This is about 1/5 of that used by Reverse Osmosis plants

Of course, MSF requires a large amount of lower grade thermal energy. This is normally provided from otherwise waste heat from an electrical generation plant.

However, the temperature required is that which is required to evaporate water. MSF plants typically operate with a top temperature of a little less than 100 deg C, a limit set to avoid scaling on the large heat exchange surfaces. In turn, this requires that they operate at a slightly reduced pressure - requiring exhaust systems to remove dissolved air and air inleakage. (Part of the electrical energy requirement.)

Click here to view Solar Incidence, Conversion Factors and MSF Thermal calculations.