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 Post subject: Re: LFTR and Water?
PostPosted: Oct 02, 2014 2:42 am 
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KitemanSA wrote:
bensoon wrote:
Yes, the wiki pages do state that the energy requirements of MED/MSF are between 50-110 kWh/m^3. The 23-25 figures come from some other assertions, but in anycase, taking a best case and worst case scenario would allow us to understand the economics in either case.

Please remember that factors like temperature differences have as much effect of the efficiency of desalination as they do with electricity generation. Saying something uses 50-110 kWh/m^3 of thermal energy tells you nothing if you don't know the quality of the heat. 5.5kWh of electricity made with 100C steam will consume similar large quantities of thermal energy.


Yes I do understand that, we will definitely need a more precise set of parameters to determine desalination efficiency. Perhaps you could help define the discussion in this area?

Say for example you can get 300-400MW or 500-600MW usable waste thermal energy at 60 or 120 degrees C?


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 02, 2014 5:59 am 
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KitemanSA wrote:
bensoon wrote:
Yes, the wiki pages do state that the energy requirements of MED/MSF are between 50-110 kWh/m^3. The 23-25 figures come from some other assertions, but in anycase, taking a best case and worst case scenario would allow us to understand the economics in either case.

Please remember that factors like temperature differences have as much effect of the efficiency of desalination as they do with electricity generation. Saying something uses 50-110 kWh/m^3 of thermal energy tells you nothing if you don't know the quality of the heat. 5.5kWh of electricity made with 100C steam will consume similar large quantities of thermal energy.


I think this is the most important (and underate...) point, most of MED and MSF systems need an input of heat, whatever that amount is, at a temp of 100 °C or even less (about 70 °C in the case of MED, if I remember correctly) that implies a tiny loss in power production; hence, a single MSR plant can produce electricity AND - at the same time - fresh water, unlike RO processes where all the electricity used to produce clean water is eventually lost


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 04, 2014 5:43 am 
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Alex P wrote:
KitemanSA wrote:
bensoon wrote:
Yes, the wiki pages do state that the energy requirements of MED/MSF are between 50-110 kWh/m^3. The 23-25 figures come from some other assertions, but in anycase, taking a best case and worst case scenario would allow us to understand the economics in either case.

Please remember that factors like temperature differences have as much effect of the efficiency of desalination as they do with electricity generation. Saying something uses 50-110 kWh/m^3 of thermal energy tells you nothing if you don't know the quality of the heat. 5.5kWh of electricity made with 100C steam will consume similar large quantities of thermal energy.


I think this is the most important (and underate...) point, most of MED and MSF systems need an input of heat, whatever that amount is, at a temp of 100 °C or even less (about 70 °C in the case of MED, if I remember correctly) that implies a tiny loss in power production; hence, a single MSR plant can produce electricity AND - at the same time - fresh water, unlike RO processes where all the electricity used to produce clean water is eventually lost


If I understand you correctly, I think your point sums up exactly what makes LFTRs so compelling for water production, the fact that you (potentially) could use the waste thermal energy that would have otherwise been dissipated to the environment without needing to incur an opportunity cost in electrical production to desalinate water (co-generation), and whether it would be better than existing co-generation examples.

The question now is roughly how much water can be produced in such a manner and whether it is actually better to incur some losses in electrical generation to increase that production under certain circumstances. This also allows us to debate what would be the preferred methodology, all this should be based off (reasonable) assumed 'brackets' of temperature/total usable energy from the system.


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 06, 2014 2:53 pm 
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If RO requires 5.5 kWh/m3 then a 500 MWe plant dedicated 100% to RO will produce 2.18 million m3/day.

500 x 24 x 1000 = 12,000,000 kWh/d 12,000,000/5.5 = 2.182 million m3/day. Which is a lot of fresh water.

For steam turbine cycles there isn't any waste heat that's usable aside from a small warming of seawater prior to a RO plant or some other desalination process. There is lots of low pressure steam available, but every kg/s of steam extracted above condenser pressure reduces the STG output. So that's not waste heat as I would define it, and when you get to 70C there is a lot of energy in that low pressure steam which can be extracted in a LP turbine, more than you might think.

From what I've read the most economic combination is to combine your RO and power plants on one site, share common raw water infrastructure and use the prewarmed water leaving the condenser as as feedstock for the RO plant. That may also provide a great opportunity to do some limited load shifting to rebalance variations in generation or demand.


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 06, 2014 2:56 pm 
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bensoon wrote:
The question now is roughly how much water can be produced in such a manner and whether it is actually better to incur some losses in electrical generation to increase that production under certain circumstances.


I can't say about the economics, that I guess depend on the specific site needs (for example, middle east and maybe south east Asia, too), but it remains a smart idea for sure, particurally considering that there are very tiny loses in a high temp system, like the MSR ones, i.e. something in the range of no more than about 0.10-0.15 MW electric per MW thermal used. Obviously in colder climates, district heating is an other interesting and useful way to use low temp heat, needing hot wat or steam at a temp not higher than 90-120 °C in the worst case


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 06, 2014 10:04 pm 
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One big missing component here is the capital cost of an RO system if it is only going to run ~1/3 of the time to load balance.

This could substantially affect the economics of the water produced even if the input electricity is cheap.

I simply do not have a good feel for the relative fixed vs variable costs in an RO system to know if this could be a show stopper or not...

Also, would a RO system entail extra wear/costs from cycling up/down or on/off to load balance?


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 07, 2014 8:51 am 
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Is it cheaper to produce potable water via RO than it is through the use of process heat to flash distillers? Does anyone know the economics. i still maintain that load balancing is more effective using a leak off of process heat from the first loop where all the heat is transferred to massive flash distillers than it is using the mechanical method of RO.

David

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 Post subject: Re: LFTR and Water?
PostPosted: Oct 07, 2014 1:55 pm 
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dwalters wrote:
Is it cheaper to produce potable water via RO than it is through the use of process heat to flash distillers? Does anyone know the economics.

David

UC Berkeley did some work on this some time ago IIRC, I will try to find the paper and post it.


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 07, 2014 2:06 pm 
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thx.

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 Post subject: Re: LFTR and Water?
PostPosted: Oct 07, 2014 2:09 pm 
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BTW...the great advantage to nuclear water cracking is that there are almost no moving parts. This is true for front end process heat or back end for the low side of the turbine's waste heat. Essentially it's just a series of metal plates of different sizes and a low pressure pump to pump water around (potable water out, cooling water in and brine out). You could build one the size of 10 football fields for a few million bucks. RO requires high pressure pumps and lots of solenoid valves (at the least the ones I operated for PG&E).

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 Post subject: Re: LFTR and Water?
PostPosted: Oct 07, 2014 9:28 pm 
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dwalters wrote:
BTW...the great advantage to nuclear water cracking is that there are almost no moving parts. This is true for front end process heat or back end for the low side of the turbine's waste heat. Essentially it's just a series of metal plates of different sizes and a low pressure pump to pump water around (potable water out, cooling water in and brine out). You could build one the size of 10 football fields for a few million bucks. RO requires high pressure pumps and lots of solenoid valves (at the least the ones I operated for PG&E).


That was my thoughts about MSF/MED vs RO exactly. Of course I would love to see the paper that lindsay was talking about, that's pretty much what I was wanting to figure out.

Which methodology would present the best economics, and how much water would it produce?


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 08, 2014 9:10 am 
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Something to think about. RO requires electricity (or possible steam pumps) which means an automatic 50% thermal hit on energy produced by the reactor. A flash distiller taking direct process heat from the reactor is, in theory, 100% efficient (minus line lost or conversion loss from the first loop). Still, very efficient.

David

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 Post subject: Re: LFTR and Water?
PostPosted: Oct 08, 2014 3:01 pm 
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dwalters wrote:
Something to think about. RO requires electricity (or possible steam pumps) which means an automatic 50% thermal hit on energy produced by the reactor. A flash distiller taking direct process heat from the reactor is, in theory, 100% efficient (minus line lost or conversion loss from the first loop). Still, very efficient.

David


As we already said, it' s even better than this because with MED/MSF we can use the low temp "waste" heat from the reactor (eventually, with tiny loss of nominal electric power), particurally a high temp nuclear reactor like MSR ones - we can't say the same for a LWR/HWR, being a pretty low temp system


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 11, 2014 12:05 am 
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Attachment:
This is not from UCB, but it seems to indicate that RO is the most efficient method. 5.5 kWh/m3 vs 9.7 kWh/m3 for Muli-Effect Distillation (MED)

If you have some kind of closed Brayton cycle, the heat rejection temperature is ideal for desalination, and that energy is in effect free.


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 Post subject: Re: LFTR and Water?
PostPosted: Oct 11, 2014 10:31 am 
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Lindsay wrote:
Attachment:
Energy Consumption and Performance for Various Desalination Processes.pdf
This is not from UCB, but it seems to indicate that RO is the most efficient method. 5.5 kWh/m3 vs 9.7 kWh/m3 for Muli-Effect Distillation (MED)

If you have some kind of closed Brayton cycle, the heat rejection temperature is ideal for desalination, and that energy is in effect free.


True enough. but that's the working energy comparison at the point of the RO and flash distiller. Does the electrical side include the 50%+ hit you get on thermal inefficiency? That is, from the heat energy generated by the reactor, how does that now compare with the figures you site above? I think you'll find the numbers different when this is now calculated into that comparison.

David

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