Resistive heating is more or less close to unity for electrical to thermal conversion efficiency. However the actual efficiency of transferring the heat into the pot or pan is another matter, as it can be as high as nearly 80% and as low as below 50% in the case of undersized pots on oversized burners and other factors. Natgas cooktops are somewhere around 30-40% and in my own personal testing with a heat exchanger kettle and a tight pot skirt at full power I am able to achieve 50% thermal efficiency assuming the burner BTU rating is accurate to the cooktop. By reducing the output by what feels like half from waving my hand high over the burner and "looks like" half from the output from the burner hob, I'm able to achieve boil for around 25% more time. Obviously thermal efficiency is less of a matter as for us as consumers we care more about what we're metered per unit of fuel or electricity but heating and cooking with combustion sources can be stretched out for efficiency if you're willing to wait a little longer for stuff like heating water or liquids. In regards to your friends and the Instant Pot or electric pressure cookers: I use one myself along with a normal pressure cooker. As power is close to 50 cents a kwh here, I use a natural gas burner in conjunction with a heat exchanger kettle and a tight skirt to bring the water to a boil before pouring the contents onto the liner pot. The lid goes onto it and then I bring that back to a near boil and drop the liner pot into an already pre-warmed cooker and quickly drop and rotate the lid before the pressure from the steam pops the safety up. What the cooker does is add the final bit of electrical heating to bring the contents to pressure I've attached an image of BTU/1 unit of currency from the spreadsheet along with a natgas one and regular unleaded gasoline for reference. Give me the numbers and I'll post back the results.
Those numbers should be pulled from this report, which in itself has a caveat about diameter mismatch to the coil on page 4. https://www.aceee.org/files/proceedings/2014/data/papers/9-702.pdf There are also further caveats for induction and mismatch of the cooking vessel: https://www.govinfo.gov/content/pkg/FR-2014-12-03/pdf/2014-28212.pdf#page=17
Sorry, I'm not great at the quote system. Thanks for that, so I *think* I was on the right lines, good enough to compare. Common sense regards pot/pan sizes always assumed, in reality quick boiling kettles always used. Brake cleaner can be had for £2.60 per litre here. The cheapest I have found meths at is 24 x 1L bulk packs of bio-ethanol working out at £2.67 per litre. I'm under the weather and struggling to get my head around the BTU business, so would go about it by trying to find the heat output and fuel consumption of an 8r and a Trangia. How clonky..!
Not sure what brake cleaner is energetically but if I assume it is naphtha/white gas like then it is around 11k BTU per £. Bio-ethanol would be 6.2k BTU per £ at £2.67/L. For reference, that'd be comparable to if electricity was about £0.55/kwh. 10ml of Ethanol: 230btu 10ml of Methanol: 180btu So to "start up" a stove you would be using 1/100th of the bottle assuming 10ml (overkill, but for the sake of calculations). At that level of usage, you're using about £0.02-0.03 as a fixed startup cost assuming 10ml. Realistically you'd use far less and if you intentionally let a little white gas out to mix into the alcohol during the warmup process the amount consumed could be further lowered. Realistically on some stoves I doubt you need more than 2-3ml to start up. There is also the option of using a butane or propane torch to gently massage the burner head for a little for rapid starts without the methanol.
Yes the brake cleaner mentioned is naptha/white gas. £0.78 per KWh 'in the pot' by my calculations. Still pricey compared to mains electricity. Rubbish, I was hoping to justify doing the first brew of the day on a 71 or 8r for convenience but not at that price! No suprise I suppose being twice the price of even premium paraffin, and less energy. Does that take into account the roughly 40% 'gas stove' efficiency? Sounds about right given the cost, plus lower energy content of the fuel. ...But of course you can't beat them for convenience or backup (we always have a meths stove wherever we go, just incase an odd leak appears in the soldered joints of a faithful pressure stove, or a NRV fails and nobody can be bothered to fix it when all that is wanted is a hot drink before bed.. On similar lines to a quick blast with a gas torch to preheat, if two paraffin stoves are needed for a meal we light the first with meths then preheat the second in the flame of the first to save on the expensive stuff. Small savings but they stack up with heavy use.
Use a heat exchanger pot/kettle and throttle down your stove as well as using a piece of aluminum flashing plus paper clips to form a pot skirt. I'm more than certain you can achieve 60-80% thermal efficiency (burner to contents of pot). I know that from a "non-optimal" setup using a gas burner with the horizontally dispersed flames was able to achieve nearly 50% thermal efficiency as I mentioned before by timing boil versus the reported BTU of the burner so to increase from that number would not be too hard under the right parameters. On increasing efficiency without any major expenditures (for heat exchanger pots) then I would try to "measure" the heat coming off the side of the pot with your hand. I've found that it is possible to dial back the stove as a lot of heat is just simply wasted over the side of the pot as it cannot absorb it fast enough as the hot gases go over the sides without contributing to much heating of the contents. By throttling the stove back to the point the gases are just a few degrees above the contents by the time they make it over the top of the kettle/pot's contents most of the heat has been absorbed and not wasted by being vented off into the environment. The lower setting also will produce less gas volume and thus less gas velocity if you're using a pot skirt. No, that number is BTU to currency only. Alcohol is generally not a cost efficient source of BTU/$. The highest number I've been able to get was 70% isopropyl back when it was $1.75 for 32oz, which gave me about 8.8kBTU/$. It was a significant reason as to why I drill a spare jet for most of my multifuel stoves so they can burn isopropyl alcohol. While not all of them can work off of 70% based on their generator design, a lot of them will readily burn 80-91% very well and they are just existing designs (The MSR Whisperlite Universal being one of those stoves) I will note that that electric stoves are realistically closer to 60-70% rather than the >80% from the above reporting. During the heating phases the measured efficiency isn't that high compared to when they are running at lower power settings and just holding temperature. Reading into another publication, it appears that to saute meat the efficiencies of induction and resistive cooktops can be a scatter of 38 to 54.8% (38, 47.8, 52.5, 54.8 respectively). The same publication has a gas stove at 22.8% in the same saute test but I can see the rings of the flame at the edge of the pot from the high angle shot so the pan is definitely undersized.
To think about efficiancy. A good camp stove will boil a litre of water in about 3 minutes. So will an electric kettle. Stove is about 3000 watts. Kettle is 1000 watts.
I think a search for some bashed up old copper pans in need of re-tinning is required, a set of different sizes, nice lightweight conductive copper, attach heat exchanger coils/skirt to the base with silver solder, re-tin.. maybe I'm taking all this a bit far. But then for a day's work, just think how much fuel might be saved in a lifetime of use? Even without such effort, your suggestions are all good sense for upping efficiency and would soon become second nature to incorporate into daily cooking. Thank you Reflector for your input on this subject, all food for thought, and the provision of better terminology than my past-my-bedtime attempts! Ahh, the advantage of immersing the heating element directly in the medium to be heated, and often surrounding the water with relatively insulating plastic. As you say, far more efficient than any camp stove, and more efficient than a kettle on an electric stovetop. But they never seem to last that long, and it is very difficult to find one without plastic parts contacting the water. Ever since material science studies at university I have been very wary of plastic in contact with food and drink and minimise it wherever possible. I do have some ancient copper electric kettles somewhere, but no means to properly test the insulation...
Ok, hope we've cleared this one up for you! Very roughly, at current prices, based on what has been discussed in this thread, the price per KWh delivered to the pan/kettle contents: Electricity £0.58 lowest efficiency, £0.44 highest efficiency Premium C1 paraffin (estimated current price of £1.70/l bulk) £0.42 lowest efficiency, £0.24 highest efficiency Heating oil C2 (Kerosene) £0.25 lowest efficiency, £0.14 highest efficiency Naptha (Holts brake cleaner at £2.60 per litre £0.78 lowest efficiency, 0.44 highest efficiency. Electricity efficiency based on 60-80% efficiency range. (Standalone electric kettle would surely be at least at the top end of this, but that's an aside) Liquid fuel efficiency based on 40-70% efficiency range. To achieve the higher end of this, you will need heat exchanger vessels, skirts, careful control of flame etc. Make a guess as where things are likely to fall in reality. If you can burn heating oil without clogging burners and deal safely with the fumes, there are big savings to be had. If anybody wants to get current quotes for Calor gas propane 13kg (portable-ish) and 47kg (def domestic!) I can work those out too, but I reckon paraffin will still win! Sorry, whiteturbo, to slightly hijack this thread...
A kerosene-naphtha mix would provide an intermediate option that is less likely to clog and easier to start up with. I've found a small amount (~5-10%) of alcohol also improves the burning characteristics of kerosene by reducing odor and making the flame bluer.
Kerosene-naptha in the context of multifuels, or paraffin pressure stoves? I've heard of folks adding a small percentage naptha to paraffin in classic pressure stoves and lamps, I'm sure it has been mentioned on here several times in the past, but never found the courage to try it myself. Might be a bit troublesome indoors with the need to keep the tank vented when not excuse, and a constant slight background of escaping naptha vapour?
The reason heat exchanger pots are not sold for domestic use is the potential for Carbon Monoxide production. CO is generated when combustion occurs at low temperatures. The heat exchanger fins can cool the flame and result in a massive spike in CO which is definitely no good indoors. If you are going to use them make absolutely sure that the flames do not reach the heat exchanger fins ie use a big pot on a small burner. I’m very jealous of the quoted prices for paraffin/ kerosene. That’s pretty much the price of petrol around here so it’s likely to be competitive against all comers, including electricity. By the way, electric kettles are efficient because they don’t take long to heat, which gives less time for the heat to be lost to the environment. In this context, they don’t really benefit from insulation, as the water isn’t losing heat for long enough. Even if you have a typical electric stove, it’s more efficient to boil the water in your electric kettle first for this reason. Not to mention a lot quicker. All combustion stoves are much less efficient because so much of the heat is lost in the combustion gases. If you use a smaller burner to reduce these losses then you take too long to heat the water, which causes heat losses by itself. It’s a no win situation unless the fuel is really cheap, as appears to be the case here…
@Blackdog Using 10% - 15% naphtha in kerosene pressure stoves and pressure lamps is a fairly common practice. I use it in my kero lamps and lanterns; but I mainly use straight kerosene in kerosene stoves. Tony
I have added 10% white gas to alcohol and used it in a standard alcohol burner with no negative as you would with straight white gas.
Sound advice, I was thinking along the lines of the coil based Simplex quick-boil kettles. I need to install decent extraction above our indoor cooking areas anyway for steam/fumes, but CO is to be avoided at all costs in the first place. The caveat is that those are bulk prices, pre-packed C1 paraffin was usually about £9 for 4 litres in the UK before the recent price rises, I hate to think what it is now! I will give it a try with the less refined C2 heating oil, and report back!
I use an Optimus Terra Express HE and a Bulin heat exchanger kettle on a natural gas stove, given the spacing of the flames to the heat exchanger fins and the range hood I'm confident that carbon monoxide buildup isn't a problem. However I would suggest that on an existing liquid fuel stove that you may want to create some kind of "standoff ring" to elevate the heat exchanger so the tips of the flame DO NOT touch the fins and/or have some space from it. This ensures combustion is complete and the flame is quenched. The downside is you lose a bit of efficiency versus having a "touching" flame to the bottom of the pot but you scavenge most of the heat using the fins which makes the pot effectively act like a large diameter pot. In regards to kerosene-alcohol and kerosene-white gas-alcohol mixes, I've done personal experimentation but I am using backpacking stoves. You may want to start out with those small amounts of naphtha and/or maybe add a tiny percentage of alcohol and look at the flame in a dark room. I've found that on some stoves that have occasional streaks of red or orange in the flames that it cleans the flame up to be a solid blue flame as if they were burning butane straight from the bottle. Start with small quantities. I did so using syringes and in my experimentation with "alternative fuels" like (American type, which is some kind of heavy naphtha and not kerosene) charcoal lighter fluid mixed with kerosene and alcohol that certain characteristics of a straight fuel can be pulled into a direction where they're a little more practical like being a little easier to heat up without it being a flammability hazard and being cleaner burning or otherwise eliminating the kerosene combustion smell in whole all while lowering cost for volume. https://classiccampstoves.com/threads/alcohol-in-white-gas-or-kerosene.45976/ There is also a home use and restaurant set of heat exchanger pots ("Turbo Pot") but they're nothing like the backpacking and camping type. I own two of the pans and a 3.5 quart pot. They're very heavy in construction with their massive thick milled fins but that heavy construction makes it slower to heat up due to the significant thermal mass. For the pans they're great for heat distribution but the pot is not so great if I want to heat up a smaller quantity ("about a liter") of fluid which means I tend to use the Optimus pot or the Bulin kettle much more. The other part of the pans that's helpful is it makes it so home cooktops feel less wimpy for frying since the combination of thermal mass and higher thermal absorption makes it act like there's more output.
