Monday, August 23, 2010

Nuclear Co-generation: Good Idea Coming Back in the Czech Republic.

Maya Wind

Tiffany Chantel, Contemporary American

From the gallery of the artist.

By NNadir.

Unfortunately the vast majority of the world's nuclear reactors have a thermodynamic efficiency well less than 40%. Thermodynamic, or Carnot, efficiency, of course, is a measure of work divided by thermal energy input in a heat engine, where work is typically thought of as the mechanical energy recovered via the expansion of a heated fluid.

The work - expressed as mechanical energy or electrical energy - can never even equal] the heat energy input: This is the crux of the second law of thermodynamics.

Although 100% efficiency in a heat engine would violate the 2nd law of thermodynamics - Einstein said that the laws of thermodynamics were the only physical laws he never expected to be overturned - one can raise the efficiency to values much higher than the traditional 30-40% found in most industrial and small heat engines around the world.

Although I favor the immediate phase out of all dangerous fossil fuels, many fossil fuel plants have achieved fairly high efficiency making their horrible properties slightly less worse, although they can never be as safe as nuclear energy. The most successful approach has been the combined cycle gas engine, which uses two different types of heat engine cycles, the Brayton cycle and a Rankine cycle. Brayton cycles involve a heated gas driving a turbine - a jet engine is an example - and a Rankine cycle is a familiar steam engine.

In the Brayton cycle a gas is compressed, then heated to a high temperature and allowed to expand against a turbine. Since the gas is at a high temperature - although expansion causes it to cool slightly - it can still remain hot enough to boil water. If the boiling water is connected to a steam engine, further energy can be collected by turning a turbine with the steam so generated. The limitation of these types of machines historically has involved materials science, but materials science advanced enough in the twentieth century that by the 1980's, many combined cycle dangerous fossil fuel plants had actually been built, many in Great Britain.

In theory, this type of option is available to nuclear energy, particularly with a type of reactor that is not particularly popular - although many actual examples are were known - the high temperature gas cooled reactor, which can be operated on Brayton cycles. (Another option is what I believe is Charles's favorite reactor - and in fact mine - the molten salt reactor which can also run Brayton cycles.)

My friend Rod Adams designed a type of reactor - the Adams Atomic Engine - that is designed as a Brayton nuclear driven engine, but these reactors have not been built.

However the laws of thermodynamics while placing limits on the amount of work that can be obtained from a heat engine, place far less restrictions on how much energy can be useful, particularly if one needs heat for some purpose. Of course, we do need heat to heat spaces, living and office spaces.

The original Calder Hall reactor, which connected to the grid in 1956 and ran until 2003, with an overall capacity utilization of 83.10%. The reactor generated 270 MWs of thermal power and only 50 MWs of electricity - it was a very small reactor in modern terms - and thus had a thermal efficiency of 50/270 = 18.5%. Most modern PWR type reactors have thermal efficiencies of around 33%; the EPR is said - if my memory serves me well - at close to 40%. So it would seem that the small Calder Hall reactor wasted better than 80% of the energy that nuclear energy provided.

Well, yes and no, quite and not quite. The Calder Hall reactor, the first truly commercial reactor in the world, and regrettably of a type that was dual use, weapons and commercial, used its waste heat to heat buildings.

This idea - which is not all that new - New York City has been using waste heat as district heat for over a century, is called cogeneration.

Even though nuclear reactors produce a lot of waste heat - as do coal and gas plants - very little of the heat has been used for district heating. It was so used in the former Soviet Union - including at Pripyat, where Chernobyl was located - and is still used today and the wonderful 3 operating heavy water reactors (CANDUS) operating at Cernavoda in Romania. (Talks are underway to complete two more CANDUS in Romania.)

This is an idea that should have been more widely exploited and, happily, there is a proposal to do just that in the Czech Republic.

To wit:

Plans are under consideration for a district heating network for the city of Brno, 40 kilometres from the Dukovany nuclear power plant.

Dukovany features four VVER reactors with a total thermal power of 5500 MW. Plant systems convert 1760 MW of this into electricity for transmission over the grid, but some of the leftover heat could in future be piped to homes and businesses.

An environmental impact assessment for plans by plant owner CEZ was put to regional officials at the end of July, which is expected to take up to two years to evaluate. Should it get the go-ahead CEZ would need another two years or more to install the feeder pipeline, which would be more than 40 kilometres long.

Benefits for the residents of Brno would come in the shape of reduced emissions and stabilized heating prices. The supply should also be very reliable: There have been no unplanned shutdowns at Dukovany's four reactors in the last ten years.

The Dukovany reactors all have capacity utilization factors in the mid 80% range, meaning that they are very reliable machines.

Czech Reactors Could Supply Heat.

Note that there are other ways to increase the efficiency of nuclear reactors by using them to produce process heat to drive chemical reactions, including those that produce fluid fuels. This is the real strength of fluid phase reactors, including LFTR's which are frequently discussed here and elsewhere on the internet.

It is very possible for these type of reactors to produce thermal efficiencies nearing - or even exceeding 60% - and if the heat losses are used - possibly in connection with heat pumps - one conceivably could utilize a very large fraction of the heat energy generated.


donb said...

The biggest limit to cogeneration is the large loss of heat in pipes -- it is not possible to transport the heat very far. By comparison, electricity can be transported over large distances with much smaller losses.

At the end of the posting NNadir makes reference to heat pumps. This is the enabling technology. At some distance, not too far from the heat source, it is more econonmical and efficient to transport electricity rather than heat, and then use heat pumps. Especially good are geo-sourced heat pumps. Their efficiency is so high that they can more than overcome the thermodynamic losses of the generating plant, yielding effective heating efficiency over 100%.

This is the beauty of the conversion of lower grade energy (heat) to higher grade energy (electricity and mechanical work) by means of the laws of thermodynamics. The high grade energy can do more things, including pumping heat to provide space heating and water heating at effective efficiencies over 100%.

The reject heat from an electrical power plant is a free waste product, but making use of it is not free. The key then is to use it where it make economic sense. Those uses must compete with things like heat pumps, and economics will dictate the point where the crossover is made.

The bigger point is the siting of nuclear power plants. If society can overcome it irrational fear of nuclear power, especially for something as inherently safe as a LFTR, nuclear power plants could be sited underground in the middle of cities, providing close-by demand for the reject heat from the electrical generation, and providing electricity with minimal losses.

Yordan Georgiev said...

The biggest obstacle is not technology, but public acceptance. More education and efficient PR are needed ...

Jim Baerg said...

If the reactor is sited a to far from a city for district heating the waste heat could still be used for warming greenhouses.

Alex P. said...

Just curious, any figure about the loss of electric power in a such project, considering LWRs are quite low temp engines (are the long distance from the nuclear site to the end city users) ? I guess something in the 0,10-0,15 MWe per MW thermal produced, at least.

However, agree that molten salt reactors - being higher temp reactors (than LWR) - can do the same work much better (including desalination in costal sites)


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