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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=
Nuclear reactors are currently even less efficient than coal-burning powerplants, and produce even more medium grade waste heat. This technology should be especially useful there. It should help fusion power even more, since the cost of a fusion core (per thermal MW) is projected to be many times that of a fission core, so getting the most power out of it will be very important.
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.
Anyone who has seeded a sterile medium with bacteria realizes that growth is only quasi-exponential. After a while the population hits carrying capacity, stalls out, then crashes.
"I'm so moist I'm sticking to the leather." -Kermit the Frog on The Late Late Show
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:
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