700 miles on LPG without needing to refuel

[LPGC]

200 psi, don't cut the mustard, cng requires 200barg.

Its a folly anyway and we ALL know that

Yes I meant 200 bar not 200psi, sorry for that mistake.

[Gilbertd]

We / Gilbert has been saying that a tank has to be chemically suitable for whatever we put in it, plus it also has to be capable of withstanding the pressure of whatever we put in it.

You have got hung up on the whether it is suitable for CNG at the pressures you use, rather than if a cylinder is suitable for a gas, any gas, under pressure. I have in front of me at the moment, a coffee cup but just because it is called a coffee cup doesn't mean I can only put coffee in it. If I choose to I can fill it with water, tea, orange juice, beer or any other non-corrosive substance that is liquid at room temperature without any problems. In the same way, we could fill an LPG tank with any gas, or any liquid for that matter, as long as we don't pressurise it beyond the 10bar pressure it is designed for. OK, it would be pretty useless if filled with Methane as a CNG tank as the quantity it can hold is negligible but it would hold it, hence the comment of the tank doesn't know or care. It doesn't, even though it may be totally unsuitable for the purpose.

You keep on saying that the tanks are heavy and need a crane or a truck, but how heavy are we talking?

You also haven't said how much mains gas you use when you fill your tank. From what I have been able to find online, 1 cubic metre of Methane weighs 0.7kg, but when compressed to 200 bar, 1 cubic metre weighs 180 kg, so you are using 257 cubic metres of gas. Obviously your tank isn't that large but as a ratio it will be the same, so you are paying for that quantity via your domestic gas bill.

[Pinger]

, so you are paying for that quantity via your domestic gas bill.

And, the electric meter will be whirling at a fair old rate when the compressor is running.
For hydrogen - as an example - something like one third of its energy value is required to compress it to storable pressure. Not insignificant.

[LPGC]

I just Googled 'average electricity and gas prices' and got the following Google AI generated results...
As of October 2024, the average prices for gas and electricity in the UK are:
Gas: 6.24 pence per kilowatt hour (p/kWh)
Electricity: 24.50 p/kWh
Gas standing charge: 31.66 pence per day
Electricity standing charge: 60.99 pence per day

If an engine is 30% efficient while an electric motor is near 100% efficient, at the above prices it would still be cheaper to power a compressor from an engine than from an electric motor, perhaps even if the engine powered an electricity generator to turn an electric motor turning a compressor. Could have a set up that doesn't use any electricity to compress the cng, effectively the cng compresses itself but at a cost (smaller than electric cost) reflected as a fraction increase in price per mile of cng vehicle use.

Mindful not to step on Gilbert's toes with his heat pump business and wouldn't say this on a heat pump forum but I've said it on LPG forum before - The engine is 30% efficient because 70% is wasted heat energy (cooling system and exhaust) but that heat could be captured and used to do something useful such as heat a house. In fact at the above prices why not use an engine to turn a heat pump (compressor) to heat a house and use the 70% 'wasted' heat to heat the house...

1 litre of LPG is just over 7kwh, so at the above prices natural gas would be as cost effective as getting LPG at around 45p per litre (before having to pay any additional cost to compress it and if the intention is to run a vehicle on it assuming the engine makes the same input kw/h to road wheels kw/h as when running on LPG). Electric at the above prices is like paying £1.74 per litre of LPG.

[Gilbertd]

I don't do heat pumps and never will. Some time ago there was a lot of publicity about the lack of qualified engineers meaning it wouldn't be possible to meet the Government targets. As I already have F-Gas qualifications, it would only require an add on but having looked into heat pumps I could not honestly sell them. The AC units I supply and install will give 3kW of heat for 550W of electrical input but yes, it could be run from an engine running on gas driving a generator and the engine would supply some additional heat. Quite how you would arrange it when it is cooling though as the systems are reversible. Heat in winter, cooling in summer.

[LPGC]

I don't do heat pumps and never will. Some time ago there was a lot of publicity about the lack of qualified engineers meaning it wouldn't be possible to meet the Government targets. As I already have F-Gas qualifications, it would only require an add on but having looked into heat pumps I could not honestly sell them. The AC units I supply and install will give 3kW of heat for 550W of electrical input but yes, it could be run from an engine running on gas driving a generator and the engine would supply some additional heat. Quite how you would arrange it when it is cooling though as the systems are reversible. Heat in winter, cooling in summer.

Yes, my setup idea wasn't for cooling. But as you said an engine powered generator could provide the 550W needed to run your AC setup while simultaneously heating hot water and we could count one of those functions (electricity generation to run your AC setup or hot water heating) as a byproduct of the other so effectively free?

[Gilbertd]

Although a much simpler way of heating for free is, as a number of my customers have done, power the unit from solar panels.....

[LPGC]

Although a much simpler way of heating for free is, as a number of my customers have done, power the unit from solar panels.....

Thinking of winter short daylight hours but can see lots of solar panels and battery storage could help with that.

3kw equivalent output from 550W seems very impressive. I know you explained it to me before, I don't remember all the details, when I get a bit of time I'll try to find our earlier conversation.

[Gilbertd]

The theory is actually very simple, but the designs these days are pretty complex to get the best efficiency as possible. You have 3 main components, an evaporator, a compressor and a condenser. When you draw lots of Propane vapour from your red bottle, ice forms on the outside due to the cooling effect of the liquid vaporising (although in thermodynamics there is no such thing as cooling, it is a lack of heat, so it draws heat). The refrigerant circulates around the system, passes through an orifice at the inlet to the evaporator that causes it to vaporise so it gets cold. The evaporator has a fan behind it to distribute that cold and to help raise the temperature of the evaporator. The refrigerant is then sucked out by the compressor, compressed in the compressor and fed into the condenser where it condenses back into a liquid and generates heat which is dissipated by airflow through the condenser, so it starts to cool until it reaches the evaporator again. So although you are generating heat (and drawing heat so the evaporator gets cold), the only power you are consuming is to drive the compressor. As the systems are reversible, when cooling the evaporator is in the unit inside the house with the condenser outside but when heating the flow direction is reversed so the indoor unit becomes the condenser and the outdoor unit becomes the evaporator.

This always amuses me when you see systems in apartments in Spain, and similar countries with a hotter climate than ours, where the outdoor unit is located on a balcony. Outside temperature may be up in the mid 30's C and the AC is on inside to keep it down to a nice cool 22 degrees C. But if you go out on the balcony you've got a big box with a fan in it, blasting air out at something in the region of 40 degrees so far too hot to sit out on the balcony.....

Solar panels and storage batteries are the way to go definitely. Last December I was asked to install a system in a bungalow out in the sticks where there was no mains gas so they had solar panels and two storage batteries. It was one of these 1930's bungalows with 4 almost equal sized rooms so it was divided into quarters. They also had a extension on one side for a living room. Living room wasn't a problem with a single 3.5kW wall unit but doing the same with the 4 original rooms would have been overkill and also expensive. I put in a ducted system with a single 5kW unit in the loft with ducts feeding vents in the ceiling of each of the 4 rooms. I was in the area so called in there one day earlier this year to see how it was going to be told it was the best thing they had ever done. Prior to it being put in, they had electric heaters in each room which were all set to come on for an hour five times a day. Since I had put their system in, they hadn't switched it off, it had been on 24 hours a day, they had been able to walk around in tee shirts all winter and their electricity consumption had halved. To the point where they were generating enough all year round to keep the batteries fully charged and still have some left over to sell back to the grid.

But, getting back on topic (ish), when Methane is compressed to put it into a tank then it is going to heat up surely? So that is something else, on top of the electricity needed to drive the compressor, that is going to reduce the efficiency as some of the energy being used will be wasted as heat.

[LPGC]

I know the basic theory of heat pumps that you described above, I think of / bracket all the usual type AC systems, beer coolers, plus all the big industrial freezer warehouses etc as heat pumps, home heating setups with a evaporator outside pretty much the same only run in reverse. I'd even bracket cheap portable coolers that use the thermo-electric principal or 3way 240v / 12v / gas absorption fridge/freezers as heat pumps except the 'pumping mechanism' is a different type. I understand the equipment you fit differs from the usual type domestic heat pumps (and yours are reversible so can be run in AC or heating mode) but they still use the same principal using a compressor? I also understand that in principal a heat pump doesn't make any heat, it simultaneously heats the condenser while cooling the evaporator so the total heat in the system remains the same (in reality a bit more heat than cooling effect partly due to the compressor never being 100% efficient), so it can be a lot more energy efficient than hot wire electric heating because you're only using energy to run the compressor... But I didn't know they could be 3000/550=5.45 times more efficient than a hotwire, I thought it was more like 3 times in ideal conditions where ideal conditions are about the temperatures involved (probably dependent on how close to the phase change temperature and pressure of the refrigerant gas we use in the system the external and internal temperatures are)? I thought some (maybe older) heat pumps get close to 3 times hot wire efficiency if for example external temp is (say) 15C and you want to heat a room to (say) 21C but the efficiency falls close to 1 times hot wire if the external temp is below 0C and you want to heat the room to 21C, then if your only source of heat is the heat pump it won't be able to heat a decent sized room from 0C external to 21C internal temp because the compressor is only rated at say 1kw which isn't enough at 1 times efficiency to heat the room sufficiently? Do your pumps remain at 5.45 times efficiency regardless of external/internal temps when in heating mode and have the same efficiency regardless of internal/external temps when in AC mode?

Of course yes when @CNG is compressing his ng into cng the compression is going to create heat, wonder if he could put some of that heat to good use in winter, maybe it heats the garage a bit. We wouldn't expect this compressor to make more than 1 x hot wire heat though, a lot of the work it does is to cram a lot of stuff into a small space and the heat is only a by-product of doing that, it isn't a closed system with an evaporator and even if it were there is no phase change. There is some cooling effect when the CNG pressure is decreased at the pressure reducer in the engine bay before it goes to the injectors / mixer, which is why the CNG reducer needs heating and is plumbed to the engine's heater water circuit, but a CNG reducer doesn't need as much hot water flow as an LPG reducer because there is no phase change... Still every bit of heating the reducer needs due to the cooling effect of lowering the pressure of the CNG will have had similar/opposite heating effect during compression.

@CNG how much electricity does your CNG compressor use (by how much does that increase cost of running your vehicles on CNG)?

[Pinger]

The equation for temp post compression (T2) is:

T2 = T1 x (PR^(Gamma - 1)) where T1 is original temp, ie, ambient in this case in degrees Kelvin (288K), Gamma is the 'ratio of specific heats' ; 1.31 for methane .

So, final temp of pressurising to 200bar from ambient is : T2 = 288 x (200^0.31) = 288 x 5.17 = 1488 degK. Subtract 273 to get degrees C and T2 = 1215 degC. That though would be in the very last stages of compression as earlier in the process the compressor would be compressing to a much lower (but increasing as the tank filled) pressure.
Huddling round the neck of the receiving tank looks like the way to utilise that significant (circa double that of diesel air compression at CR = 18:1) temp rise.

[LPGC]

Interesting, though we're talking the peak temperature inside the compressor and obviously not the average temp of the receiving tank etc. E.g. if it's a piston type compressor that'll be the peak temp at the top of the stroke but the piston bore/stroke will be small so it's only taking a little ng at a time and compressing it, won't the cylinder and piston be at closer to average temp of peak and environment temps and the receiving tank much cooler as it radiates / convects heat around it?

Interested to know how much electricity the compressor uses though if CNG can tell us or if we can calculate it?

[hitman2]

I ran a twin train compression set up, 1st and 2nd stages took 9.5 bar up to 28b then up to 80b.

3rd stage 80 up to final discharge of 220b

4 Siemens 1mw 11kv motors driving the compressors.

Discharge temperature was quite toasty in between stages with each stage cooled immediately after compression.

Final stage about 180c before being cooled to about 50c through a cooler not much bigger than a wheelie bin (pche)

Final stage gas went onto a twin train reciprocating compressor station that took the gas up to 340b

That was the fuel gas for 8 v32 diesel engines, no gas and we used about 120m3 a day diesel. Engines had a small slipstream of diesel at all times to provide ignition. Engines provided the electricity, can't remember how many mw it came to but could have provided a large town I guess

[Pinger]

Interesting, though we're talking the peak temperature inside the compressor and obviously not the average temp of the receiving tank etc. E.g. if it's a piston type compressor that'll be the peak temp at the top of the stroke but the piston bore/stroke will be small so it's only taking a little ng at a time and compressing it, won't the cylinder and piston be at closer to average temp of peak and environment temps and the receiving tank much cooler as it radiates / convects heat around it?

Yes, pretty much. I'd only expect to see the high temp in the delivery pipe. I forgot that 200 bar would require intercooled multi-stage compression. Hitman's post shines more light on this.

Interested to know how much electricity the compressor uses though if CNG can tell us or if we can calculate it?

In my 2T endeavour I (roughly) calculated the power absorbed compressing air at a compression ratio of 8;1 with a swept volume of 644cc at 3000 rpm. I used Gamma = 1.4 which gives a lower absorbed power than methane's 1.31, and an initial temp of 85 degC. It worked out at 30 kW. At a mere pressure ratio of 8.

[CNG]

It still works for me because of the 'sunk' cost. As covered earlier, the benefit-argument used to be insurmountable. Today my personal circumstances have changed.Since Mr Putin's endeavour, gas cost has near tripled, as has costs to compress it. LPG and petrol/ diesel has not tripled. This means, the Putin factor, alongside the reasons already discussed, and the same today is nothing like as attractive. Today I'd probably get a bulk tank and one of Simon's pumps.

[Gilbertd]

But you still haven't said how much gas you actually use to fill your car or how much power your pump uses. All along you haven't actually said it but you've implied that the costs are considerably cheaper than other fuels but never come up with any actual figures for it. I know that the pump price of LPG is around triple what I was paying 17 years ago when I first started running a car on it. Petrol prices have gone up over the years too but it is only in the last year or so that it is no longer roughly half the price of petrol but we can thank Mr Putin and MFG for that. When petrol prices shot up in 2022, LPG prices did the same, the difference is that when petrol came back down again, LPG didn't.

But, onto Simon's queries. Over the last 4 years since I have been installing AC, the efficiency, (listed as the COP) has gone up with each new version (I started installing C variant, the current ones are G variant). I've installed a system today in a nightclub chill out room. A large space so needed a larger 24,000 Btu unit that is nominally rated at 8kW but will provide between 0.9 -10.1 kW depending on demand. That is one of the ways the power consumption is kept down. You set what temperature you want the room to be and, if the existing temperature and desired temperature are wildly different, it will initially run flat out and then slow down as it approaches the desired temperature, not only the fan but also the compressor speed so the power consumption is only at max when flat out. At the maximum 10.1 kW this particular unit draws 1.91 kW giving a COP of 5.29. When doing the commissioning test it is first run flat out cooling and then switched to flat out heating. Although the air temperature outside was 4 C the outdoor unit was in sunshine so was at 8.6C. On flat out cooling the evaporator in the indoor unit was blowing air out at -0.9C. When run for flat out heating, it was at 70.6C, easily enough to raise the room temperature to the desired 20C in a matter of minutes by which time the fan (and compressor) had slowed down to what can best be described as tickover. So while it may draw 1.91 kW initially, once the room reaches the desired temperature it is down to around 250 W when running at minimum. Manufacturers data says it will cool at an ambient of anything between -10 and 46C and can heat between -15 and 24C. Unlike your fridge which is only designed to work properly over a much more limited range so uses something far cheaper like R600a which is isobutane or R290 which is Propane (but without the stenching agent).

[hitman2]

Gilbert

One of my old colleagues started up an ashp company and was using that many Panasonic units they gave him the main distribution contract up in Scotland, he's making an absolute fortune in the Islands as no mains gas and virtually no competition plus everyone jumping on the green bandwagon. Definitely a market for them up in Scotland where mains gas isn't available...

We have solar pv, it runs the pool pump and a 20kw ashp during the summer for free. Seriously thinking about having split ac units for heating the house in the winter, we have a log burner and biomass (pellets) but I think ac will be easier and quicker for getting the house warmed up plus can be for cooling during the odd very hot nights in the summer (I'm on Portugal silver coast) Pellets doubled a couple years ago but have since returned to a more reasonable price, still costing too much to heat the whole house though.

Q for you if I may, what size units for, main room 10x8m, how many outlets required?

4 double bedrooms, about 6x5m

Sorry for the thread swerve

[LPGC]

But you still haven't said how much gas you actually use to fill your car or how much power your pump uses. All along you haven't actually said it but you've implied that the costs are considerably cheaper than other fuels but never come up with any actual figures for it. I know that the pump price of LPG is around triple what I was paying 17 years ago when I first started running a car on it. Petrol prices have gone up over the years too but it is only in the last year or so that it is no longer roughly half the price of petrol but we can thank Mr Putin and MFG for that. When petrol prices shot up in 2022, LPG prices did the same, the difference is that when petrol came back down again, LPG didn't.

But, onto Simon's queries. Over the last 4 years since I have been installing AC, the efficiency, (listed as the COP) has gone up with each new version (I started installing C variant, the current ones are G variant). I've installed a system today in a nightclub chill out room. A large space so needed a larger 24,000 Btu unit that is nominally rated at 8kW but will provide between 0.9 -10.1 kW depending on demand. That is one of the ways the power consumption is kept down. You set what temperature you want the room to be and, if the existing temperature and desired temperature are wildly different, it will initially run flat out and then slow down as it approaches the desired temperature, not only the fan but also the compressor speed so the power consumption is only at max when flat out. At the maximum 10.1 kW this particular unit draws 1.91 kW giving a COP of 5.29. When doing the commissioning test it is first run flat out cooling and then switched to flat out heating. Although the air temperature outside was 4 C the outdoor unit was in sunshine so was at 8.6C. On flat out cooling the evaporator in the indoor unit was blowing air out at -0.9C. When run for flat out heating, it was at 70.6C, easily enough to raise the room temperature to the desired 20C in a matter of minutes by which time the fan (and compressor) had slowed down to what can best be described as tickover. So while it may draw 1.91 kW initially, once the room reaches the desired temperature it is down to around 250 W when running at minimum. Manufacturers data says it will cool at an ambient of anything between -10 and 46C and can heat between -15 and 24C. Unlike your fridge which is only designed to work properly over a much more limited range so uses something far cheaper like R600a which is isobutane or R290 which is Propane (but without the stenching agent).

Thanks for the info Gilbert, I see what you mean.

I know LPGShop have listed CNG compressors on their site for a long time but never seemed to have any in stock (so expect would be a special order). Only basic info on their website not including electricity use, so I looked up one of their compressors, the Coltri CNG5, which LPG shop's site says can do up to 250bar at 85L per minute (5 cubic meters per hour) and Coltri's site says uses 4kw of electricity, seems this is the most powerful single phase one this firm (Coltri) do. Not entirely sure how to work out how long it would need to run to charge a tank but expect the maths would be (250 bar / flow rate) x tank capacity, so for an 85L tank would be 250 / 85 x 85 so 250 minutes? That's if the flow rate is consistent at 85L / minute and doesn't slow as tank pressure increases. So 4 hours and 10 minutes at 4kw = 16.17kwh to charge an 85L tank to 250bar? I haven't checked this any other way (e.g. work involved to compress 85L to 250 bar) to see if the maths seem feasible but intuitively it seems about right. Actually the electrical spec says between 3kw and 4kw, so maybe it starts at 3kw when pressure is low and increases to 4kw under load when pressure is high, so maybe a bit less than the 16.17kwh to charge an 85L tank.

If the tank is 85L, and if it takes 16kwh to charge it to 250bar, and if electricity is 24.5p/kwh, it adds £3.92 to the price of the 85L tank of CNG at 250bar. There are 11.1868kwh per cubic meter of ng, = 0.011868kw per litre, we'd be cramming 250x85=21250 litres into the 85litre tank so the tank would contain 21250x0.011868 = 252.195kwh of CNG costing 252.195 x 0.0624 = £15.74 and the 16kwh work to compress it using 16kwh of electricity at 24.5p per kwh would cost £3.92. £15.74+£3.92 = £19.66 which is the total price for the 85L tank fill to 250bar with the electricity cost being 23.5% of the total. Including the electricity cost (and all other things being the same such as the engine making as much use of CNG kwh as LPG kwh) this makes the true cost of running on CNG the equivalent in cost per mile as running on LPG at 54.49p per litre (45p per litre for the fuel plus 23.5% for electricity to compress it).

[Gilbertd]

Hitman has a PM rather than swerve the thread even further.

So although CNG hasn't admitted to it, that's nearly 4 quid on electricity to fill his tank and the cost of the gas. At 5 cubic metres per hour, on a 250 minute fill that would be just over 20 cubic metres. 20 cubic metres appears to be 211 kWh so will cost around £11. So £15 a tankful which will go how far? Without going back through the thread(s), I have a feeling CNG has mentioned less than 100 miles to the tank, so yes, it is cheaper than LPG but it isn't the running on free fuel he has implied.

[LPGC]

Hitman has a PM rather than swerve the thread even further.

So although CNG hasn't admitted to it, that's nearly 4 quid on electricity to fill his tank and the cost of the gas. At 5 cubic metres per hour, on a 250 minute fill that would be just over 20 cubic metres. 20 cubic metres appears to be 211 kWh so will cost around £11. So £15 a tankful which will go how far? Without going back through the thread(s), I have a feeling CNG has mentioned less than 100 miles to the tank, so yes, it is cheaper than LPG but it isn't the running on free fuel he has implied.

Your post and my edit crossed, I included the pricings in my edit but they closely agree with your figures, my maths including the electricity comes to 54.49p per litre in equivalent LPG terms with the electricity to compress it being 23.5% of that cost.

Another aspect is we've just worked out the 85L CNG tank at 250 bar contains around 250kwh of fuel... An LPG tank containing 85L of liquid LPG would have around 601kwh. Even if the 85L tank only fills to 80% it's still 480kwh so still around double range from same size tank as CNG.

Kind of doing fag packet maths and might not have had the best info to base the maths on... CNG might tell us if we're correct with our figures?

Just dropped Miles' wife off at the station, she dropped their P38 off for me to fix a switching back to petrol under load problem, hence the delayed edit.