24 August 2006

Pluto a Dwarf Planet


21 August 2006

Water and Plug-ins Aren't Miscible

One issue that hides under the table of the plug-in concept is that your hydrocarbon fuel may end up sitting in the tank for months if you don't extend the electric range of your vehicle very often. This portends badly for gasoline which can gum up as solids settle out. Summer and winter formulations of gasoline are different (with winter gas containing hydrocarbons that can evaporate in summer temperatures).

Water accumulation can cause greater problems. Gasoline can only contain about 0.02 % water by volume before the water will separate. This memo from the Environmental Protection Agency details some of the issues (thanks to Robert Rapier who tracked this down for me). If a car is left outside where it can go through many hot/cool cycles condensation can increase the amount of water in the tank. When it gets cold in the winter time the water can freeze or simply stall the engine as it drops to the bottom of the tank. Water expands when it freezes so you don't want freezing to occur in your fuel lines or pump.
Ethanol mixed with gasoline complicates the situation. Ethanol likes to absorb water. Anyone who knows anything about liquor probably is aware that ethanol and water are miscible, i.e. they will mix in any proportion. For a vehicle running on pure ethanol this wouldn't be a serious problem. The presence of water in the fuel reduces fuel economy because it is dead weight that needs to be vapourized and brought up to high temperature in the cylinder. However, that's all it does.
When you mix gasoline, ethanol, and water you get a ternary solution as long as the water content is low enough. As the proportion of water increases it will reach a critical point at which time the ethanol and water will separate from the gasoline and form a new solution. A water/ethanol solution is heavier than gasoline or water, so it will drop to the bottom of the tank.

As you can see from the graph the tolerance drops with lower temperatures. If water manages build up in the tank over the summer and fall then in the winter as the temperature drops so does the tolerance. Where previously you were just sipping gasohol with 0.5 % water content you might end up with half a litre of ethanol and water in the bottom of your tank after an overnight cold snap. Then when the driver stomps on the pedal to get the power of the engine along with the electric drive the engine will turn over but then splutter and not fire.

It's generally quite difficult to separate water and ethanol. Ethanol has a higher vapour pressure than water (which means it likes to evaporate faster) and it's more polar. It's not practical to use any sort of desiccant to remove water from gasohol. The best solution is generally to keep the tank full and use a better seal for the gas cap. Stations will also have to take steps to insure that their storage facilities don't allow water to infiltrate before they sell it to their customer. Overall it's a fairly minor problem that can be overcome with good design.

20 August 2006

Baby Steps Along the Plug-in Path

As I mentioned in my last post on the drawbacks of hydrogen, it has problems with production, storage, and distribution. It is well known that electric vehicles thump hydrogen on production efficiency. With regards to storage, the problems for batteries are greater than hydrogen but are still technical rather than theoretical in origin. Distribution is not a problem at all for electrics and plug-ins.

Not only does the electrical grid already exist, but also progress towards a full-fledged plug-in hybrid can be made in small baby steps that do not require any massive investments of capital.
  1. The battery capacity can be tailored to the user. Ideally a plug-in hybrid should have enough electric range for the daily commute. Since battery storage is expensive no one will want to buy more then is economical for them
  2. As transportation energy consumption is shifted to electricity the proportion of hydrocarbon fuel that is derived from biomass or synthetic sources can be increased at the expense of fossil fuels. This does not benefit climate change greatly but does have a major mitigating impact on the problems of peak oil.
  3. Improvements in hybrid technology that boost the tank-to-wheel efficiency of cars can be gradually introduced. This includes adding ultracapacitors to increase the power that can be recouped by braking, lightening of the body, etc.
It is easy to imagine the battery capacity of the plug-in advancing in small steps. Right now the biggest markets for high power batteries is power tools and laptops. Widespread introduction of hybrids with their large battery packs should drive the price down (as long as it doesn't cause any material shortages). The Toyota Prius currently carries a 1500 Wh battery pack. Assuming it can discharge 50 % of its capacity and it consumes 160 Wh/km then the electric range is already almost 6 km. The main reason the Prius doesn't have an all-electric switch (in North America) is that the battery and transmission can't provide enough power to achieve high speed. That requires a series hybrid setup, where there is no transmission: the propulsion is provided solely by electric motors.

Switching from a parallel to a series hybrid is a big step. It transforms the engine into a generator, which implies that it runs at a constant speed. This will benefit diesel engines significantly on the side of pollution emissions. It also means that the size of the internal combustion generator component of the car can shrink substantially. The engine only needs to be able to provide enough power to keep the vehicle at highway cruising speed. For a car such as the Prius traveling at 110 km/h that's only about 15 kW (or 20 hp) to overcome the drag and rolling resistance. Note that is power needed at the tires, not what's developed on the crankshaft. All of the surge acceleration can be handled by the stored electrical energy. The torque of electrical motors is extremely high.

I exaggerate a touch but the hydrogen path benefits little from hybrid technology. One can't take all the benefits of the hybrid path and then at the end of the road decide to switch to the less efficient hydrogen vehicle. Does not compute, QED.

One reaction to the general malaise around hydrogen hype lately comes from Hydrogenics, which has joined the Plug-in Development Consortium (hat tip to Green Car Congress). This is a fairly major step down for a major Proton Exchange Membrane fuel cell equipment manufacturer. The plug-in concept marginalizes the on-board generator by attempting to use battery capacity to its maximum.

At some time in the far future we might find ourselves in the situation where electricity can be produced extremely cheaply in prodigious quantities. The most likely candidate would be solar power that uses thin-film technology that can be produced without high vacuum manufacturing. In this case the efficiency of energy storage might become irrelevant. One might assume that a hydrogen vehicle could be produced for significantly less, even if it consumed 2-3 times more energy. Currently the Honda H2 fuel cell vehicle costs about $2000000 so they have a long way to go. In fact the concept is so far into the future, what possible business does a commercial company have doing research on the subject? It's futurist stuff, which is why we see hydrogen often talked about in the same breath as carbon nanotubes and superconductors. The path to improving the status quo clearly lies along the path Toyoto chose of the hybrid vehicle. We have to keep kicking car builders in the pants so they continue to move forward.