Thermodynamic Geoengineereing: The Fourth Way
- Apr 17, 2015 4:00 am GMT
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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.