Sunday, 16 August 2009

GM Chevy Volt Spurious Claim Number 5: Low CO2 Emissions

The USA Environmental Agency

So, how much CO2 will the Chevy Volt actually produce (or save)? According to GM, to travel 40 miles the Chevy Volt uses 8kWhrs. Note that this is energy from the battery pack. The amount of energy needed to charge it will be more than this depending upon the efficiency pf the charger circuit/battery combination.

However, according to the US Environmental Agency, in America 1kWh of electricity typically generates 7.18 x 10-4 metric tonnes of CO2, so 8kWh obviously generates 5.75 x 10-3 metric tonnes of CO2. A gallon of petrol produces 8.81*10-3 metric tons. Given that the Chevy Volt travels 50mpg during its petrol phase, this means it would generate 0.8x8.81*10-3 metric tons or 7*10-3 metric tons travelling 40 miles in the petrol phase. So, although the Chevy Volt is less polluting during its electrical phase, this is not zero emission and it is not as significant a saving as it might first appear.

In terms of petrol equivalence for CO2 emissions, in reality during the electrical phase, the vehicle is doing the petrol equivalent of 60.9mpg. Effectively, what we are seeing here is the cleaner generation of energy from electricity in terms of CO2 than from directly burning petrol in a vehicle. This is well known and as more electricity is generated by renewables or nuclear, this benefit increases. However, once again, GM are being slightly economical with the truth in implying that the Chevy Volt is a zero-pollution solution to commuter travel below its electric 40 mile range (assuming even this is, in real driving conditions, actually achievable).

But what if however the range isn't 40 miles in real world conditions? What is the pivot point for range on electric where petrol actually takes the lead in terms of CO2? The answer is easy to arrive at. The Chevy Volt does 50mpg on petrol @ 8.81*10-3 metric tons, the electric phase generates 5.75 x 10-3 metric tonnes of CO2. So, the pivot point is 50*5.75 x 10-3 /8.81 x 10-3 or 36.5 miles. I find this is both surprisingly close to the 40 mile range and that it causes immediate alarm bells given the real world performance of this car in real driving with real loads and real drivers, not admen engineers. In reality, this vehicle, if it achieves less than 36.5 miles in the real world, could actually generate MORE co2 in the electric cycle section, not less. And these figures have not included the inefficiencies in the charging process.

Even people travelling only in the electric cycle totally may contribute to the overall CO2 increase if they continuously recharge the battery pack without fully discharging it to make sure it is topped up. I would suspect the energy losses become disproportionate the less you top it up, effectively the recharge cycle being better optimised for full charges although I have no figures for this and it may prove to be insignificant or not of any great issue.

Saturday, 15 August 2009

GM Chevy Volt Spurious Claim Number 4: The petrol saving equivalence

As referenced in GM Claim 3, the Volts battery pack is rated at 16kWhrs but the Chevy Volt only uses 8kWhrs to protect the battery pack. Although it will obviously vary slightly, petrol has an energy content of 36.6 kWh/US gal. Thus, the battery pack is equivalent to 8/36.6=0.22 US gallons of petrol. Given that the Chevy Volt (from GM data) has a real mpg figure of 50mpg during the petrol part of the cycle, this is equivalent to saving 11 miles worth of petrol per journey.

None of the GM figures at this stage seem to be making sense or adding up, but significant differences between the claims and real world driving will soon show up when customers actually have one in the real world. This will do more harm than good and help to damage the public image of electric vehicles in general and the GM Chevy Volt in particular.

Energy content of petrol/gasoline:

GM Chevy Volt Spurious Claim Number 3: The 40 mile battery pack

The Volts battery pack is rated at 16kWhrs. However, to protect the battery pack it isn't deep discharged and is only used to the 8kWhr. limit.

Let assume though that the battery is fully used to get the maximum range the Volt can achieve. The reference states that GM have quoted the petrol engine/generator combo as "The electricity, at around 50 kw, serves primarily to propel the car.". So, they are using 50kW to propel the car under the petrol cycle. As 1hp=746W, 50kW is equivalent to 67hp (a reasonable ball park figure for bhp rating for an equivalent standard internal combustion car engine would obviously need to be higher to allow for losses to through the drive train). Given that the car will take just as much energy to propel it normally, we can assume it needs 50kW during the electric only part of the cycle. So, 8kWhrs of battery would last 8/50 hrs or approximately 9.6 minutes. Assuming the car is averaging 40mph in commuter traffic, this gives an electric only range of 6.4 miles. Alternatively, for the Chevy Volt to return a range of 40 miles during this 9.6 minutes, it would need to be travelling at a fairly brisk 250mph.

This also means at a 6.4 miles electric range from a 50mpg petrol cycle car, the Volt is saving 0.12 gallons of fuel per journey in real world situations using the figures that GM are providing or approximately 1 litre per journey. This is not really planet saving stuff and nowhere near GM's claims or even worthy of all the hype that this vehicle is currently being given or generating, most of it through Viral Advertising I suspect.

Are GM making up the figures as they go along?

Ref. for power used in propelling Chevy Volt:

GM Chevy Volt Spurious Claim Number 2: Takes Less Electricity To Run Than A Fridge/Freezer

Mark DuVall of the Electric Power Research Institute (EPRI)
"If was pointed out that daily charging of the Chevy Volt will increase the typical single family home electric usage by about 25%. The electric draw is less than that of a refrigerator/freezer or central air conditioning system."

The Volt will take less electricity than the fridge and freezer? A typical modern fridge in the UK takes 300kWhrs per annum, a freezer 200kWhrs per annum. This give a total of 500kWhrs per annum, or approximately 2kW per day. We can double this to allow for the USA propensity to have everything bigger but not necessarily better. So, at 1000kWhrs per annum, this works out at approximately 3kWhrs per day.

If we assume this 3kWhrs is used with 100% efficiency to charge the Volt and is delivered to the Volt propulsion system with 100% efficiency, this means it will travel for 40 miles at 40mph using 3kWhrs. As 1hp is 746W (roughly, depending on other issues: we can approximate this to a 4hp engine.

So, GM apparently have found a way to power a family sized car 40miles on the equivalent to a 4hp engine. Why did they bother with the electricity cycle part of the vehicle? They could have simply put in a moped engine, a motor mower engine or even a modified chainsaw engine and probably got even more mpg overall.

Once more it seems that GM are making spurious claims. However, it IS totally possible to charge the Volt for less than the energy used by a fridge and freezer. Just don't expect to get more than the end of your street and back, or as GM would put it "the end of your block".

Friday, 14 August 2009

GM Chevy Volt Spurious Claim Number 1: achieving 230mpg

How can a car that has a 12 gallon tank and a range of 640 miles get any number of miles per gallon up to infinity? Easy, just get a GM Marketing mathematician to do the maths. Because the Chevy Volt does the first 40 mile on battery and doesn't use any petrol, if you only drive 40 miles a day your tank will never need refilling and it will get infinite mpg. How good is that? However, the marketing gurus of GM are making some assumptions here. Firstly, that the electricity doesn't have a petrol equivalent, in effect it doesn't count. If this method of calculating mpg was viable we'd all be driving milk floats and getting infinite mpg from those too. Secondly, that the general public can't do simple maths.

Unfortunately, all they mean is that the petrol is only hit after 40miles. This is a bit like having a current account and a savings account and saying that so long as you only take money from the current account, you are not spending anything. In reality, the pure petrol consumption (after 40 miles) is easily calculated at 50mpg. The bit before that is slightly more complicated, but it certainly doesn't save fuel or energy, not until the whole electricity generating system is renewable that is. Fuel is still used to generate the electrical energy needed to drive the first 40 miles. It's just that the fuel use is hidden in the generation of the electricity. How far will it really travel on a gallon of petrol? Easy, put in a gallon and set off. If it's fully charged, you'll actually get to 40 miles and not use any. After that, the petrol engine will kick in and you'll get another 50 miles. Total, 90miles, only 140 miles short of the 230 miles range. To get the next 140 miles you'd need nearly 3 more gallons. So, to actually drive 230 miles would take 4 gallons of petrol in total or about 60mpg. Maybe that small efficient diesel engine isn't looking so bad after all - and you can drive it without having to plug it in every night.

Why are GM being allowed to make these spurious statements and are the American public really so dumbed down and innumerate that they will believe them?

Spurious claim number 2 follows!