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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentTimeApr 14th 2017
    Thanks - edited Vitesse to read GT6 mk3
    • CommentTimeApr 14th 2017
    Posted By: skyewright
    Dragging things vaguely back to 'green' if not exactly back On Topic my experience of Herald, Spitfire & GT6 was that they were very good on mpg for their day (if driven sensibly).

    They were certainly very good for maintenance. The offside front wheel was an excellent place to sit whilst balancing the carbs on my Spit, for example. I can't speak for the fuel consumption whilst driving sensibly, I'm afraid. :shocked:
    • CommentTimeApr 14th 2017 edited
    Um, SU carburettors, we don't have to balance fuel injection systems these days, they do it themselves.

    I started my academic engineering career by studying automotive engineering. Cars have moved on so much in 35 years that they are not comparable.
    I followed a Rover P6 V8 the other day, not a very large car is it. And the boot on it was tiny, so small that the spare wheel was bolted to the boot lid.
    They had DeDion Tube rear suspension, what a nonsense.

    Here is a bit about 30 years old cars:
    • CommentTimeApr 14th 2017 edited
    Cut off the boot and bonnet and you have a Citroen DS - almost exact. Front suspension very strange too - massive bell-cranks. But it all worked very well. 'The best sedan in the world' - Car&Driver.
    You will probably never own a H2 fuel cell Toyota Mirai.. But I saw one at the traffic lights today which apparently the co-wheels car club is hiring out for £4.50/ hr. Bit cheaper than the Riva?

    • CommentTimeApr 21st 2017 edited
    Posted By: WillInAberdeenBit cheaper than the Riva?
      2006-02-13 032.jpg
    • CommentTimeApr 21st 2017
    Posted By: WillInAberdeenYou will probably never own a H2 fuel cell Toyota Mirai
    why? same deal?
    :-) oops Rasa/Riva

    Posted By: fostertomwhy? same deal?


    The ones driving round Aberdeen look pretty neat but I haven't driven one.
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2017
    Had a read through various regurgitated press releases posing as journalism but didn't find anything saying what the Aberdeen hydrogen stations where producing their hydrogen from. Any idea if it's methane or water?
    The first refueling station at the council's depot is an electrolysis plant, it was apparently put in for the hydrogen buses by this company: it's visible from the railway and from Great Northern Road if you are ever travelling down this way.


    The second refueling station in the south of the city is by the same company but I couldn't see how it works.

    Apparently they have a third contract for an electrolyser station west of town.

    Just what I read on the web, I have no connection with any of this.
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2017
    Thanks Will, interesting site.

    They claim 65 kWh/kg (for 350 bar storage/supply, 68 kWh/kg for 700 bar) [¹]. That's 234 MJ/kg.

    According to Wikipedia [²] the enthalpy of combustion of molecular hydrogen gas is 286 kJ/mol. The mass of 1 mol of molecular hydrogen is 2 g (give or take the odd isotope) so the enthalpy of combustion would be 143 MJ/kg.

    So that's 143/234 ~= 61% efficient. Not wonderful but not as bad as I feared. Particularly if you could make use of much of the remaining 39% (which is waste heat, I'd guess).

    It's also interesting to compare that enthalpy with those of petrol or diesel fuel which are 43.2 and 43.1 MJ/kg respectively so the hydrogen, mass-for-mass, stores about three times as much energy. By the time you've got the mass of the tank to store it at pressure I don't suppose it makes so much difference to the vehicle but it does put the quantities of hydrogen their stations can produce and store into context. I thought they were silly small until I worked out how much more energy was involved.

    [¹] http://www.hydrogenics.com/wp-content/uploads/renewable-projects-references---fueling-stations.pdf

    [²] https://en.wikipedia.org/wiki/Hydrogen#Combustion
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2017
    OTOH the tank-to-wheel efficiency of fuel-cell cars is also pretty low. Maybe 36%. That's a bit depressing. I'm pretty sure a battery-electric car would go quite a bit further per input kWh.

    [¹] https://en.wikipedia.org/wiki/Fuel_cell#In_practice
    Interesting numbers! Be careful to compare the Higher or Lower heating values (whether the end product is steam, or can usefully be condensed). You want the the LHV of hydrogen 242 kJ/mol or 120 MJ/kg to compare with hydrocarbon fuels LHV 43 MJ/kg.

    The Autocar link I posted found the Mirai does 300 miles on a 5kg tank of hydrogen. At 68kWhe/kg thats 1.1kWh electricity per mile travelled.

    At grid intensity around 300gCO2/kWh, that comes to 200gCO2 per km travel which is comparable with average petrol car same size, but 2 or 3 times a petrol-electric hybrid.

    The same car mag reviews the TeslaX doing 180miles on a 90kWhe battery so 0.5kWhe/mile.
    Doubtless those numbers will improve as the tech matures and grid intensity reduces.

    Interesting that the H2 cars do have a battery for regenerative braking, acceleration etc, so are more comparable with petrol hybrids - long range and quick refuelling. Maybe there will be plugin versions later?
    • CommentAuthorEd Davies
    • CommentTimeApr 23rd 2017
    Yes, higher or lower heating values do matter. Obviously want the lower for H₂ fuelled internal combustion engines but I'm not sure which, if either, is appropriate to a fuel cell.

    Another consideration in favour of hydrogen in the frozen north is that, presumably, waste heat from the fuel cell can be used for cabin heating whereas with a battery-electric vehicle that has to come straight out of the battery (other than pre-heat while parked on charge).

    A plug-in hybrid H₂ fuel-cell car with a plug-in range of 30 km or so would be quite interesting to those who mostly do short journeys but sometimes need much longer range with quick refuels. My guess at the moment would be that it'll be mostly battery EVs but there are important niches for fuel cells. The Aberdeen car club having both will make for interesting comparison.
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