Slashdot Mirror
Converting More Heat To Useful Energy
An anonymous reader writes "New Scientist has an article
about a technology proposed by Wow Energies which can nearly double the efficiency of power stations, utilise waste heat from many industrial processes, and reduce toxic pollution. The secret is to use propane vapour, which boils at much lower temperatures than steam, and so can convert more heat to useful energy. Even better, it uses existing pump and turbine technology. Could this be a big weapon in the fight against global warming?"
*puts on skeptic's hat and steps up on soapbox*
The design here seems to scavange heat off of the flue gas. The problem with this is that you can only remove so much energy from the waste gasses before you create problems.
Combustion of either coal or oil procudes carbon dioxide, water, soot (unburnt carbon) and nitrogen and sulphur compounds (From impurities in the fuel).
The boilers of the power plant are typically designed very well to remove as much heat as possible from the fuel and resulting gaseous waste. Easly over 80% efficient. (80% is par for most large commercial boilers as used in schools and office buildings. Some can get up to 90%). So while the temperature may be 450F, remember this is a gas. Temperature is only half the story when you talk about energy.
Attempting to extract this energy causes two big problems: draft and condensation. The whole point of a chimney is to create a draft from the hot waste gas rising up (which uses energy in the process... so the gas is cooling as it rises). This draft helps stoke the fire and prevents the fumes from accumulating inside the plant. Removing too much energy from the stream will DESTROY the draft, which means you will need a fan to make up for it (which will use more energy than you extract!) Remember that the waste is mostly water vapor? Removing too much heat will cause this to condense. Also remember that the gas cools as it rises up the chimney, so there is a minimum temperature at the chimney base that is required to maintain your draft and prevent condensation.
Condensation is a real issue, too. The now liquid water starts to absorb those sulphur and nitrogen compounds to create some very strong acids. If you get this, it won't be long before that chimney rots out. You can line the chimney with stainless steel to help prevent this, but it needs to be a specific alloy (oil and gas burners require different materials because of the different products they create). But you still have to deal with the acid itself and even a stainless steel lining won't last without regular maintenance. They estimate the flue gas will be "a relatively cool 55C", and they correctly state that nearly ALL the water will condense out. At least they plan to treat the waste properly...
So how much energy can you really extract from the flue gasses? Certaintly not 20% of the plant's total output!
The biggest problem I have is the second stage turbine they propose. Supposedly they plan to use the leftover heat from the first propane stage to power a second stage to "capture almost all the remaining energy." Clearly this second stage must operate at a MUCH lower pressure than the first, because if there was enough energy in the propare at outlet of the first turbine to boil more propare, it would still be a gas and not a saturated mix like it should be. Pressure drops, operating temperature drops, efficiency drops, output drops.
I would be extremely impressed if they managed to increase a plant's efficiency by 5%, let alone the 20-35% they are claiming.
*takes off hat, steps down*
=Smidge=
Thus in the long run a logarithmic decrease cannot overcome an exponential increase.
This is true, which means the only long-term solutions are either solar or fusion. Any fossil fuel-based plan is doomed from the start. Solar can be anything from using ocean thermals to tapping the Sun's core itself, but it has to use the Sun's output in real-time.
Vote in November. You won't regret it.
It's not a scary as you might think.
I'm a bladesmith, and I work with a propane fired forge on a regular basis. Even my properly aspirated burners don't burn well without containment and back pressure. If you pull a burner out of the forge, it sputters real bad and doesn't produce nearly as much heat.
Most of the time, a straight propane leak (without
proper aspiration) will blow it self out, even with an open flame nearby. Sure, it's stinky, and if you get the mix with atmostphere wrong it could blow up if contained properly, but generally, LPG is pretty safe stuff.
I think you mean a flammable gas rather than a volatile one. Volatile means that something easily turns into gas or vapor at relatively low temperatures, which is exactly what you want for heat exchange. It doesn't imply anything at all the flammability of the substance.
To be fair, most commonly encountered volatile chemicals are also flammable, and once something is in a gaseous state (and mixed with air) it gets a lot easier to ignite, but the volatility and flammability aren't the same.
There are several problems with the technology proposed in the article:
1) The efficiency limit of *ANY* thermal cycle is determined by the source temperature and the sink temperature, and are independent of the working fluid used in the cycle (water or propane). The source temperature is limited by the highest temperature in the gas stream and by the sink temperature (the condenser cooling water).
2) Working fluids other than water have been discussed in engineering textbooks for many years. The use of Propane as proposed in the article is NOT a new idea. I remember working problems using propane, mercury, water, freon(s) and others back in the mid 1970s. The Mercury cycle was actually built as a topping cycle for a (very) few power plants in the early 1950s or so. Thankfully these are now retired! Ammonia was used as the primary working fluid for refrigeration in the early days and abandoned for safety reasons. I expect these same safety reasons would work against propane or other flamable hydrocarbon as the working fluid in any industrial or larger scale plant.
3) Current (from the 1950s to present) technology for steam based power plants is able to reduce flue gas temperatures below the acid and water dew points. We often had a stack temperature of 180F, and had to keep it UP to prevent water condensation from turning the flyash to mud and bringing everying to a halt.
4) The sink temperature has a reasonable limit of about 100F based on cooling towers and the wet bulb temperature of the air in summertime. Anything below about 100F is *VERY* expensive in extra hardware.
With the water/steam cycle able to exploit the environmental limits of sink temperature and extract heat from any source up to the thermal limits of alloy steel piping (1000F steam temperature), there are few reasons to invest in a working fluid that is flamable.
A combined cycle gas turbine uses the waste heat from a Brayton gas-turbine cycle as the heat source for a Rankine steam cycle. In the "cascading closed-loop cycle" described in the article, a similar idea is used except that two Rankine cycles are involved -- they just use different working fluids. This should work, both in theory and in the real world, but I wonder about the cost and the additional complexity.
Another alternative that is proven, and makes good use of waste heat, is the combined heat and power cycle... for example, the waste heat can be used for district heating. Still another alternative that extracts more usable heat in the first place is the Kalina cycle, which uses a variable mixture working fluid.
Here's some basic info on heat engine cycles that may be useful for comparison purposes:
Getting tired of Slashdot... moving to Usenet comp.misc for a while.
Actually, logarithmic functions do not have a limit or an asymtote. They keep increasing to infinity, but do so much more slowly than exponential functions.
Both you and Malthus made two mistakes:
Both your models are overly simplistic and overly pessimistic.
They are simplistic in thinking that a complex system with feedback loops (population, food supply, energy demands, energy supply) can be modeled with simple math functions.
They are overly pessimistic in thinking that just because there will be eventual difficulties humanity is doomed and there is nothing we can do, so we should stop trying.
Malthus believed that people were sex machines and as long as you fed people they would reproduce without limit until a population crash came about.
The fact is that a rich gluttonous society like those in the West creates low birth rates. Most of Europe, Japan and a lot of the US have negative birth rates. The richer a society is the fewer kids they have.
Your tone, even if you did not intend it, gave the impression that the people who try to squeeze efficiencies out of current systems are doomed to failure and shouldn't bother trying. Yes, fossil fuels cannot last indefinitely and we should stop using them ASAP. But this same system can be used with any steam turbine system including Solar tower systems, biogas systems, and even fusion if it was ever made practical.
Maybe this system will lower the cost of energy produced by the method of focusing sunlight via mirrors on solar towers to produce steam. If this propane method could make the other system more efficient it will lower the price point where solar systems will be cheaper than current oil/natural gas plants (which are rising daily). Once it is cheaper to produce a watt by solar than by oil, the free market will start chipping away at fossil fuel methods.
You and Malthus sound defeatist and pessimistic. Innovation in any form should be applauded and encouraged.What I hear from some factions of the greens and environmentalists (not necessarily from you, but others that share your views on power plants) is a general attitude along the lines of, modern society/technology is evil, mankind is evil,the world is going to hell in a handbasket so there is no use doing anything about it. The only active thing they say is stop "sinning" by using technology and go back to eating roots and berries.
This sort of attitude was what led to the Dark Ages. St. Augustine wrote his "City of God" after Rome was sacked. He said the cities of men were corrupt and worthless and we should turn our back on them and think about the "City of God". If he instead wrote a book examining the social structure and economics of the Roman Empire and proposed innovative new ways to produce goods, generate tax revenue and supply an effective army, Europe would have avoided 1000 years of poverty and death.
If you were alive then you would have been standing right next to Augustus pointing out that the Gross Domestic Product of the Empire had been declining for many generations while the borders were expanding thus the Empire was doomed so it is right that people give up.