Cutting desalination’s energy habit

Droughts cost the United States billions of dollars each year. Desalination could bring relief - but energy-guzzling technologies like reverse osmosis are plagued by financial and environmental drawbacks. Here Joseph Ellsworth, CTO of, proposes an alternative.

Tapping the vast amount of salty water available in the oceans is becoming increasingly popular as a solution that can supply adequate amounts of water in areas affected by drought and increasing populations. Unfortunately, this can actually contribute to global warming and cause more drought.

Existing approaches to desalination consume large amounts of electricity which contributes billions of pounds of greenhouse gases to global warming , blamed for droughts which made the desalination project necessary in the first place. This creates a vicious circle that could end up destroying cultures and economies world wide. New reverse osmosis membranes have dramatically reduced the energy required for desalination but energy use remains high.

The immense amount of power consumed by major water projects makes it necessary to deploy new approaches capable of meeting water needs without contributing to global warming. Large scale water projects should seek ways to deliver safe water while emitting zero carbon either directly or indirectly.

A possible solution

A group of emerging technologies is becoming available that uses a variety of renewable energy approaches to deliver water while requiring zero electricity and emitting no greenhouse gases. Some of these are actually capable of going beyond zero carbon emissions by reducing greenhouse emission while delivering water cleaner that what is delivered by large scale reverse osmosis systems.

The new technologies tend to be more capital and land intensive than traditional electricity intensive approaches but with energy prices increasing world wide the higher capital costs are easily repaid within 20 year life cycles.

Industrial scale desalination plants start at about 10 million gallons per day with some scaling to 10 times that amount. This requires so much power that traditional renewable solutions such as photovoltaic solar panels are not economical and that demands at that scale would increase demand on high purity silicon and rapidly drive prices up. Wind energy could be viable provided reliable wind generation is located close enough maintain low transmission costs.

The new renewable strategies mentioned here use solar heat which has already been absorbed by water and air for a majority of their energy which allows them to be economically installed in sufficiently large scale and in some instances produce a surplus of electricity.

Climate change intensifying drought

In the last couple of years England and Australia have experienced record droughts and are facing water restrictions or severe shortages while Europe has experienced record heat waves. Droughts and heat waves are aggravated by human-induced climate change, while at the same time 2005 greenhouse emissions reached record levels.

In the USA the states such as New Mexico, California and Texas and even normally moist north east have published research papers showing global warming will cause a larger number of droughts that will be more severe and last longer. These droughts will drain already stressed water supplies. India is providing a perfect example of how overtaxed ground water contributes to large scale contamination of the ground water placing people across entire regions at risk.

The USA has already experienced droughts that cost over 5 billion USD so it is easy to see costs over 100 billion per year if more droughts materialize in multiple regions simultaneously. If the economic impact of these droughts is factored in deploying technologies which radically reduce greenhouse gas emissions suddenly become cost effective.

Major incentives are needed to encourage the replacement of traditional approaches with new technologies that can dramatically reduce greenhouse emissions. This is especially true for the water industry where rapid growth is poised to increase power demands and related emissions by billions of pounds.

Desalination is one of the most popular techniques to replace water lost due to global warming induced drought. It is also used to meet ever increasing demands from growing populations. Unfortunately one of the realities of desalination is that it consumes large amounts of power which is largely generated by burning fossil fuels.

A report published by the state of Texas indicates desalination uses 10.6KWh per thousand gallons produced. A 10 million gallon per day facility will consume 106,000 KWh per day and will produce 72,822 pounds of carbon dioxide per day or 26.6 million pounds of carbon dioxide per year. Hundreds of desalination plants these sizes are needed all around the world which represents truly staggering environmental costs.

When compared to automobiles a single 10 million gallon per day desalination facility produces as much greenhouse gas as 2,299 average cars or close to 10,000 energy efficient cars. The 2,299 grows to 4,500 when EPA estimates for greenhouse emissions per KWh are used.

It is relatively expensive to replace the fleet of cars, but eliminating the emissions from desalination is feasible and economically attractive when using the XDOBS solar thermal technology.

The major question is how to utilise the immense body volume of salty water to produce safe fresh water without investing huge amounts of energy and indirectly causing the emission of large amounts of greenhouse gas.

XDOBS has one answer which is a patent pending distillation process which uses solar energy that has previously been absorbed by the air and water to aid in the evaporation process. This heat energy is recaptured by Stirling (heat differential) motors which are subsequently used to drive the process. Excess energy is manifest as surplus mechanical energy. Thermo-electric generators can also be used to directly convert the heat differential to electricity which drives electric pumps. These generators are rapidly increasing in efficiency to where they will soon be competitive for use in lieu of the Stirling engines.

Distillation systems have historically produced the cleanest water and in most instances produce water that has lower TDS (salt) levels than membrane systems. Some of the modern membrane systems are producing water that contains over ½ the allowed salt level. In fact if membrane systems are enhanced to produce water that has TDS levels as low as that produced by the better distillation systems their energy costs increase substantially due to higher pressures and lower filtration rates.

With a minimal amount of solar or geothermal heat added to the process it can actually produce a net surplus of electricity in which is the opposite of what occurs during other desalination approaches which must invest additional energy.

The XDOBS process does not use any expensive membranes and as such avoids a major cost involved with Reverse Osmosis. It is immune to scaling and clogging that cause problems and degraded efficiency with traditional heat distillation systems. It is also immune to calcium based clogging that has plagued the membrane based systems. The system also has a high resistance to most chemical spills which would completely disable membrane system or even worse ruin the entire set of membranes.

How it works

Any distillation process depends on the ability to evaporate water and then recapture the humidity in the form of condensate. Considerable amounts of energy in the form of heat are required to evaporate water and this insensible heat is recovered when the humidity condenses back to water. The XDOBS process utilizes this factor which allows the heat recovered during the condensation process to be used as a heat source which powers our heat differential motors.

Evaporation can be increased in a number of ways including increasing heat, increasing the effective surface area of the water in contact with air and decreasing pressure in the form of a partial vacuum. In fact a popular physics experiment is to freeze a body of water simply by subjecting it to vacuum which increases evaporation which effectively sucks the heat out of the water until it freezes.

The XDOBS process uses billions of tiny air bubbles to increase surface area. These bubbles are pulled through the system by a partial vacuum which increases evaporation even further. The use of vacuum would normally cause the water to freeze but heat from ambient air is continually injected into the water by the water to air contact of the bubbles. Ultimately this combination produces a humidity stream that exceeds 100% RH even at temperatures way above ambient and it accomplishes this without an intensive energy investment. The bubbles provide the entire heat exchange which eliminates heating elements and the associated scaling problems.

Most of the evaporating energy is coming from the surrounding air which has previously absorbed solar heat. A unique aspect of this process is the billions of tiny bubbles when measured and added together provide a very large surface area almost like spreading a gallon of water across the surface of an entire soccer field where it would obviously evaporate rapidly.

The rich humidity stream can now be condensed at temperatures from 10F to 50F above ambient because the condensation chambers are under a pressure higher than ambient which works in the reverse of the vacuum mentioned above. The latent heat recovered from the condensation chamber is used in a multi stage process where the difference between the heat in the condenser and the cold ocean water can be used to drive heat differential motors which supply the pressure and partial vacuum.

This process will work in many conditions with very little additional energy input and will produce a net surplus of electricity in ideal conditions where the ambient air is very warm 90+F and dry.

The electricity surplus can be increased by pre-heating the input air which heats the water in the evaporation chamber and increases both the evaporation rate. The moisture stream still exits the evaporation chamber at 100+% humidity but with a higher temperature. 100% humidity at the higher temperature represents more water per cubic foot of air and a higher condensation temperature.

This higher dew point and positive pressure allows the condenser to operate at temperatures over 100F above ambient. The greater thermal differential between the condenser and the cold water which increases the efficiency of the thermal differential motors while increasing their total energy output which creates higher levels of vacuum and pressure which further increase efficiency.

The easiest way to heat the input air is with simple flat plate collectors which are inexpensive to install in large volumes and would average less than 9% the cost of photovoltaic systems. In warmer climates these simple collectors receive 1,000 watts per square meter or 4,046,000 watts per acre. At 30% efficiency this will allow them to gather 1,213,800 watts per acre which when multiplied by 8 hours of sun per day provides 9,710,400 watt hours per acre in the form of air heated to 180F or 100F above ambient. 200 watts per of heat energy per gallon is sufficient to guarantee a significant energy surplus which means the solar energy from 1 acre will drive production of 48,552 gallons per acre per day. Thermal collectors with higher efficiencies are available but there is a trade off between costs and increased efficiency and small increases in efficiency can cause substantial increases in costs. In areas with limited or expensive land the cost of the higher efficiency collectors may be worth the extra cost.

To produce 10 million gallons per day would require 206 acres worth of inexpensive thermal collectors. In addition to the 10 million gallons per day the system would produce in approximately 40 megawatts of electricity per day based on a surplus efficiency of 2%. The land under the collectors can be used for other purposes such as homes and offices and their shaded air space will help cool the buildings. Depending on operating conditions and the efficiency of the thermal differential motors electricity efficiency in excess of 10% could be possible which would increase the 40 megawatts to over 200 megawatts per day.

During the 20 year life the system will produce 292,000 MWh of electricity at 2% efficiency and billions of gallons of water.

Costs compared

XDOBS estimates a single 10 million gallon per day system can be installed for 1.5 million per acre which includes collectors, evaporators and condensers. This could drop substantially with larger volumes.

At 206 acres this would approximately $309 million or roughly twice the cost of the grid powered membrane system if electricity prices remain constant for the next 20 years. Since electricity prices are likely to continue increasing this cost may actually be lower than the RO system once the power bill and membrane replacements are factored in.

The 292,000 MWh worth of excess power valued at $.08 per KWh would have a market value of $23.6 million which when deducted from the $309 million brings the cost down to $285 million.

When installed in a 100 million gallon systems XDOBS believes the costs can drop into the range of $0.9 million per acre which would bring the cost per 10 million gallons per day to 185.4 million and with the 23.6 million value of the produced electricity drops to 161.8 million. At 161.8 million the cost is lower than the cost of membrane desalination plus grid power over a 20 year period event without greenhouse gas taxes and constant electricity prices. If power costs continue to go up and responsible governments start imposing a greenhouse gas taxes the XDOBS system could out perform the RO + grid system by 300%.

In summary the XDOBS process has the ability to produce large amounts of fresh water using a low energy distillation process which can be used to reverse the age old problem of energy costs. Even though the capital costs are higher the savings in energy costs make it extremely competitive economically. With the reduced greenhouse emissions this process can be an essential component in reducing global warming and represents a responsible action for those in power.

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