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Thermodynamic Geoengineereing: The Fourth Way

Geoengineering is the deliberate and large-scale intervention in the Earth’s climatic system with the aim of reducing global warming. It is most often thought of in terms of carbon dioxide removal or solar radiation management but a third approach using the cooling of ocean surface waters and the surrounding atmosphere with cold water brought up from the oceans depths has also been considered.

Thermodynamic geoengineering is a fourth way, recently submitted to the MIT climatecolab competition.

Thermal stratification of the oceans induced by global warming presents the opportunity to convert warming heat to productive energy.

The oceans are storing about 93 percent of the energy of climate change.  When water is heated it becomes less dense and rises thus the oceans are increasingly, thermally stratified.  This stratification presents the opportunity to create work in accordance with the 2nd law of thermodynamics.

The 2nd law dictates that heat flows from warm to cold.  The majority of warming heat is accumulating in tropical waters. The ocean depths are a vast cold sink but the density issue makes it difficult for tropical heat to flow into the abyss. The avenue of least resistance is for tropical heat to move towards the poles where it is melting icecaps and permafrost that is locking in methane; a potent greenhouse gas the release of which would create a dangerous warming feedback.   

Damaging tropical storms are one of the main mechanisms of heat transport from tropics to the poles.

Although CO2 emissions have continued to rise, it is thought that increased ocean heat uptake has caused an atmospheric warming slowdown this century.  The forces believed to have brought about the mixing of surface heat into deep waters are wind and density issues associated with the melting of polar ice. Both of these are believed to be temporary phenomena that when reversed will see much of the so called missing heat returned to the atmosphere.

Heat pipes are sometimes described as thermal superconductors because they rapidly move heat through phase changes of a working fluid. They are a highly efficient way of conducting heat away from a region where it can do harm, as with the ocean’s surface.  The heat pipe’s efficiency stems from the fact they have no moving parts yet they can transport heat at speeds approaching that of sound. They can move heat counter to the forces of gravity and when a turbine is situated in the vapor stream heat can be converted to work.

With enough of these devices the hiatus can be perpetuated while generating as much energy as is currently derived from fossil fuels.

The efficiency of a heat engine is equal to 1 – the absolute temperature of the cold reservoir divided by the absolute temperature of the hot reservoir.

Ocean thermal energy conversion (OTEC) requires a delta T of at least 20 degrees.

Universally the oceans at a depth of 1000 meters are about 4oC or 277K so the theoretical Carnot efficiency of a minimally operative OTEC system would be about 6.75%.  Realistically 5% is about the best that can be achieved, which means 20 times as much heat has to be moved away from the surface of the ocean as energy produced.

It has been estimated the oceans are capable of an output of about 14 terawatts (TW) of primary energy without creating environmental damage. This is about what is currently derived from fossil fuels. To convert ocean heat to that much work would require the transmission of an additional 280 TWth into the deep through heat pipes and since NOAA estimated in 2010 that the oceans are accumulating about 330 TWth due to climate change, virtually all of this excess energy can be converted or relocated to the safety of the abyss with heat pipe OTEC.

This would short circuit the movement of heat towards the poles by moving it to an ocean depth where the coefficient of thermal expansion of sea water is half that of the tropical surface. By sapping as well the energy of tropical storms these systems would ameliorate the two greatest risks of climate change; sea level rise and storm surge.

It is estimated that at depths from 500 to 2000 meters, the oceans are warming by about .002 degrees Celsius every year, and in the top 500 meters, they’re gaining .005 degrees C.  In contrast the atmosphere has been warming about 3 times faster than the deep ocean and the poles 3 times faster than that. It is apparent therefore that the deep oceans have the greatest capacity to accept the heat of global warming while producing the least temperature increase because of their huge heat capacity.

To get ocean derived energy to shore requires the conversion of electricity to an energy carrier. Electrolysis of sea water can be done in such a way that not only is hydrogen produced, carbon dioxide is sequestered with the formation of carbonates and bicarbonates and these in turn neutralize the increasing acidity of the oceans brought on by increasing CO2 uptake.

Heat pipe OTEC addresses both the cause and effect of climate change.

 

(100 MW Plant)

Parties to the Copenhagen Accord agreed to raise $100 billion a year by 2020, to help developing countries cut carbon emissions.  Thomas Peterson of the National Oceanic and Atmospheric Administration recently pointed out however, “There are factors other than CO2 governing surface temperature and therefore global warming. . . These include cloud cover, the amount of heat absorbed by the ocean, El Niño events and more.”

Unless climate change is addressed in accordance with sound scientific principles the money spent nominally to address the problem will be wasted.

Energy is one of the largest sectors of the global economy. The Carbon Brief has suggested that to meet the two degree planetary warming limit agreed to by the parties to the Copenhagen accord, 35% of global petroleum reserves, 52% of the world’s natural gas reserves and 88% of its coal must remain in the ground. These cutbacks will create an energy void that will have to be filled with emissions free and renewable energy sources that are a huge opportunity for business.

Heat pipe OTEC is the only way to produce revenue generating energy that in turn offers a 2000% climate dividend.

As cost is a constraint in a declining carbon market, so too will it be in an increasing renewable energy sector. Heat pipe OTEC potentially has the lowest levelized cost of the renewables.

It is geoengineering that pays for itself with the energy produced.

There is a roll for academia in modeling the impact of massive heat transfers in the ocean and the mechanics of the process as well as in pressing policy makers to spend scarce dollars on efforts that comport with sound scientific principles.

The roll of the proponent of this solution is to leave no stone unturned in the effort to advance the concept.  

The largest economies are the largest contributors to climate change and therefore are rightfully expected to be the greatest contributors to the solution.

It has been suggested every mine and every shipyard on the planet needs to be conscripted into this effort.

Tropical waters are where the action will take place and while they are remote from most markets they are also in no one’s backyard.

Heat uptake in the deep oceans is believed to be the reason for the atmospheric warming slowdown experienced this century.  This natural phenomenon is replicated with heat pipes that are part of a system that produces energy in a heat engine. Although the natural phenomena are expected to reverse within a few decades, heat moved to an ocean depth of 1000 meters would take about 250 years to return given that upwelling in the Pacific is estimated at about 1cm/day.

As this is an emissions free approach to producing energy, atmospheric concentrations of CO2 would be reduced by the time the heat reemerged, at which point it could be returned to the deep with the same process.

Heat moved into the deep is no longer available to drive tropical storms or to migrate towards the poles where it would melt icecaps and permafrost.

The thermal coefficient of expansion of sea water is half at an ocean depth of 1000 meters that it is at the tropical surface thus sea level rise would be reduced.

The MIT masters thesis of Shylesh Muralidharan illustrates the high capacity factor of OTEC as well as its competitive levelized capital cost with respect to other technologies. (Although not shown in the following table from the thesis, the paper points to a study that shows that the deep water condenser architecture – as in the heat pipe design – can bring down the installed capital cost of a 100 MW plant ship from 4000 $/kw to 2650 $/kw.)

 

The thesis also shows that a doubling of plant size leads to a cost/kW reduction of OTEC plants by approximately 22%.

Using CO2 as a working fluid allows for plants of gigawatt capacity.

Extrapolating from the thesis a 1 GW plant of heat pipe design would cost $86*2650/4000*78/100*(1-(.22*(200/800))) or 42 $/MWh for the lowest levelized capital cost of all energy sources but for combined cycle natural gas and by a considerable margin it would be the cheapest renewable.

Headlines surrounding a recent Carnegie Institution study suggest that funneling massive amounts of cold water to the surface, as is the case with conventional OTEC, would initially cool the atmosphere but after about 50 years global warning would be exacerbated due to changes in cloud cover.

That study however uses the extreme example of vertical diffusivity of 60 cm2  s-1, which is about 5 million times anything possible.  

We would be lucky if we could convert and move the 330TWth the oceans are absorbing with the result sea surface temperatures would remain about what they are today so there would be little impact on cloud formations.

The only real cost of this approach would be incurred in the initial prototyping and R&D. Full scale production plants would be self financing and supporting from revenues.

Damage to marine life and the out gassing of dissolved CO2 incurred in the upwelling of large volumes of deep cold water inherit in both the approach considered by the Carnegie Institution and conventional OTEC are not incurred with the heat pipe design.

The first time consideration of this proposal is every watt of energy produced is a thermal watt of global warming heat converted to productive use and at least 20 more thermal watts moved to the safety of the abyss.

Gerard Nihous of the University of Hawaii estimates the oceans are capable of supporting about 250,000 100MW plants.

During the Second World War the allies built 637,248 planes and 54,932 ships. As only about 8,000 of these ships were considered large, for arguments sake, it is assumed about 16,000 equivalent to OTEC plants worth of ships were built and about half that in plane equivalents. 

It would take therefore a full war time effort the rest of this century to reach OTEC’s full potential.

It also has to be noted that at the end of the war the vast majority of the ships and planes either had been destroyed or were obsolete and written off, whereas OTEC plants will be revenue generators from day one.  

Initially it will be necessary to prototype, at lab scale, the system and then produce a small ocean plant for testing, which can be accomplished within the first 5 years of taking action.

Content Discussion

Alistair Newbould's picture
Alistair Newbould on April 20, 2015

Jim, I wonder if a major shipping company would be interested in backing a prototype. They may be interested in a source of fuel (Hydrogen or ammonia) over which they have full control. They may also have the odd container ship suitable for the job. I could see htem establishing a network of “gas stations” around their shipping routes

Bruce McFarling's picture
Bruce McFarling on April 21, 2015

This get back to the comment above, but how is the energy harvested and delivered for use?

It seems that an electrofuel target, either ammonia or methane, would fit a system of regular delivery by ship, where direct electrical transmission requires underground cables would have a substantial additional fixed cost, as well as more constraint on location.

A methane electrofuel would also be able to use LNG shipping and import terminals and methane distribution infrastructure and consumption equipment that will otherwise be stranded assets when we have to stop using fossil methane.

Bruce McFarling's picture
Bruce McFarling on April 22, 2015

The carbon that is emitted from methane that was produced from hydrogen produced by electrolysis from a renewable source of electricity is carbon that was taken from the atmosphere when the methane was produced. The problem with natural gas is that the carbon that was emitted came out of the atmosphere millions of years ago and is being dumped back in at a much faster rate than it was originally sequestered.

The argument for methane as an electrofuel is that we have the infrastructure in place. While it requires refrigeration for LNG, it does not required it for CNG or methane delivered by pipeline, and has much better handling characteristics than hydrogen.

One argument against might be that fugitive emissions of methane will be a greater GHG gas than the atmospheric CO2 that is used to produce in the methane, so it might still be a net GHG emitter. One would have to have an good estimate of fugitive emissions of methane as an electrofuel to know how much of an issue that is.

There is ongoing research and development of ammonia, whether for ICE or fuel cell applications. Independent of using ammonia as an electrofuel, it would still be useful to replace natural gas inputs into ammonia production for fertilizers, but methane for electricity generation has a lot of installed capacity already in place and a long research and development head start. If you are converting hydrogen to ammonia as a more convenient carrier and then cracking some or all of the ammonia back to hydrogen, as in one possible ammonia fuel system that got some notice last summer, it would seem burning methane directly would have an advantage on the power production side.

Bruce McFarling's picture
Bruce McFarling on April 23, 2015

Bruce I am happy to leave to others the best way to market the energy you get from moving the heat away from the ocean’s surface to the deep. I am content with the benefits derived from reducing atmospheric warming, sapping the strength of tropical storms and reducing sea level rise that you get from this process.”

Fair enough. The closer it is to break-even on a commercial basis on current costs/benefits, the less government support or less dramatic institutional reforms are required to make it a self-propagating system.

I am intrigued though with thesupergreen hydrogen technique developed by a Lawrence Livermore team, which captures carbon dioxide from the atmospheric and produces an alkaline stream that reduces the acidity of the oceans. The synergy of this process with heat pipe OTEC is pretty compelling.”

Yes … any electrofuel production with this would be salt-water electrolysis, and that approach seems to take a challenge of salt-water electrolysis and turn it into a benefit.

The reason that electrofuels tend to focus on hydrogen rich carrier gases that can be produced from a flow of hydrogen, like methane or ammonia, is that H2 gas has such bad handling characteristics as a fuel (very high pressure and low temperature for liquid hydrogen, flammable in transport and burns with an invisible flame, a very light molecule so very prone to leaking, etc.) … ammonia can be stored liquid at 20 atmospheres, like LPG, and CNG at 20-25 atmospheres (although CNG is not liquid, CNG and liquid ammonia have similar energy densities, as both the hydrogen and carbon component of CNG are combustible). 

The economic challenge for electrofuels proposed to use “surplus” variable renewable energy and put it into a storeable form is that in that proposed role, the capital equipment is idle a substantial part of the time. So the economics are better when taking a slice of some fixed output supply and making it dispatchable … and a plant in the middle of the ocean that has a transport ship periodically show up and haul the produced fuel away sounds like as good a competitive target as an electrofuel is going to find. 

If the product can be safely used to moderate acidification of the oceans, so much the better … if that is simply a side-product that can be sold as a chemical feedstock, that still might be turning a challenge of sea-water electrolysis and turning it into a net benefit.

Bruce McFarling's picture
Bruce McFarling on April 23, 2015

Yes, there is an efficiency loss with each step, but there are also substantial economic costs in transport and storage at higher pressures and efficiency losses in additional pressurizing, additional refrigeration, and in leakages.

As far as whether the problems are insurmountable, that has been part of the R&D in ammonia, since one way to use H2 is to transport it in a more convenient carrier fuel and crack it at the place of use. There are certain types of hydrogen fuel cells that are poisoned by trace amounts of ammonia left over in an H2 stream that was carried by ammonia and cracked at a previous stage, others that do not seem to be, and some that are able to use ammonia directly, as they operate at a high enough temperature that the ammonia stream is effectively cracked for the hydrogen by the fuel cell itself.

Bruce McFarling's picture
Bruce McFarling on April 25, 2015

One thing that occured to me was that if the hydrogen to carrier fuel process takes place at the pressurization for transport of ammonia / CNG / LNG of 20-25 bars, then if the hydrogen comes from depth at 100 bars, instead of having to be further pressurized for transport as GH2, it would be depressurized.

If the electrofuel of choice was methane, a heat exchanger between the depressurized GH2 at the input to the process and generated methane at end of the process might then reduce the amount of additional energy required to cool the CNG to LNG for transport. Depending on the details of the process used, the heat exchange might be useful on both sides, if warmer H2 gas benefits the conversion process.

There was a small flurry of interest in electrofuels when DOE’s ARPA-E was funding research, but with the low price of natural gas, that’s taken an unfortunate swerve into generation of liquid fuels from natural gas.