Eating lunch and catching up on email, I came across a question on the significance of the recent Tesla fires. It isn't something I'm comfortable commenting on as anything I might say would be speculation as I don't know the specifics. That said a fair amount of energy is stored in a large battery - up to 80 kilowatt-hours in the largest model - and one should treat stored energy with respect.
It got me thinking about the energy in my sandwich and comparing it to the battery. As it happens my sandwich, two slices of homemade rugbrød with my own cashew butter and some store-bought jam - has about twice the energy density of dynamite - nearly 11 vs about 5 megajoules per kilogram.1 My sandwich packs nearly twice the energy of a stick of dynamite and 131 would hold the same amount of energy as the Tesla.
We tend to be careful around explosives like dynamite but don't worry about shrapnel from accidential sandwich detonations. The difference is dynamite can convert its stored energy to another form quickly enough to impart damaging forces to nearby objects. Power is the amount of energy released per unit time. Exploding dynamite produces a lot of power, a digesting and metabolizing sandwich doesn't.
Lithium ion automotive batteries have an energy density of about 0.7 MJ/kg.2 Much worse than dynamite, a sandwich and especially gasoline. In the case of dynamite and sandwiches energy is stored in molecular bonds and is released when a chemical reaction takes place. As it happens hydrocarbons are a dandy material for storing chemical energy. Their energy density is fairly high, they are relatively easy to handle and natural has built up an enormous supply of them in the form of fossil fuels.3 An interesting feature of hydrocarbons is their creation and disassembly are a form of rechargeable chemical battery. A process light photosynthesis captures energy and rearranges water and carbon dioxide to form a hydrocarbon and oxygen. This is stored until a bit of energy is added to cause the hydrocarbon and oxygen to reform giving off water, carbon dioxide and the energy used to initially run photosynthesis. Terribly nifty. Sadly we have yet to make our own "good enough" version of photosynthesis although some progress is being made.
There are other ways to store energy - if you've ever dropped anything on your toe, gravitaty storage is painfully familiar. Water allowed to follow downhill releases potential energy converting it to kinetic energy in the process. We spin turbines and create electricity with this. Normally the storage part of the cycle is run by the Sun. Water evaporates, rain falls at high elevations and we don't have to work to move the water uphill. But sometimes we do. You can pump water up to a reservoir and release it at a more opportune time to smooth out energy production and demand cycles. The fact that electricity costs vary throughout the day can make this an attractive financial arrangement - assuming you have the right geography to build artificial lakes in the hills. The energy density is very low - a 100 meter head of water packs about 0.001 MJ/kg - but it is cheap as utility scale storage goes.
IHere is some grounding if power and energy aren't things you think about every day. I don't have the time to make a proper table, but for rough comparisons a few common energy densities are (all in megajoules per kilogram and a few common hydrocarbons highlighted4
53.6 natural gas
43 E10 gasohol
42.8 turbine A
33 E85 gasohol
32 coal - anthracite
24 coal -bituminous
17 carbohydrates and proteins
16 dry wood
14 coal - lignite
3 gun powder
0.7 Li-Ion battery (rechargeable)
0.5 compressed air at 300 atmospheres
0.335 latent heat of fusion of water
0.3 NiMH battery
0.17 lead acid battery
0.02 super capacitor
0.001 water at 100 meters
Natural gas and hydrogen may look attractive, but they are normally in gaseous form. To make them practical for applications like transportation you need to compress or liquify them and that often renders their application impractical. Also energy densities have nothing to do with the conversion efficiency. That is a function of how the energy is extracted and must answer to physics. The gasoline engine in your car running in its sweet spot manages to move about 20% of the energy from the fuel to the wheels while an electric car can get over 75%.
1 The sandwich weighed about 200 grams and had about 525 nutritional calories of stored energy in it. A good stick of dynamite has an energy density of about 5 megajoules per kilogram. Converting calories to joules (calories and joules are both units of energy - we could have used BTUs, electron volts, ergs or more obscure forms, but joules are convenient), the sandwich's energy density is about 11 MJ/kg.
A single stick of dynamite stores about 1 MJ .. my sandwich is about 2.2... One reason why dynamite is so low is that it packs its own oxidizer for it can burn - er explode - so quickly.
2 There is a range depending on battery chemistry, operating conditions and so on. Battery energy densities are usually expressed in terms of kilowatt-hours/kilogram. 190 kWh/kg is close to 0.7 MJ/kg and sort of the high end for current electric cars.
3 A quick primer. In hydrocarbons energy is released by combining them with oxygen and somehow igniting the mixture. Chemical bonds rearrange and carbon dioxide and water are formed with energy being given off. A nice way to think of it is to consider a very simple hydrocarbon - methane - with its single carbon atom bonded to four hydrogen atoms. Combine it with two oxygen molecules and you get
CH4 + 2O2 -> CO2 + 2H2O + energy
It takes energy to break each of the four C-H bonds - you need to start the reaction. Breaking the two O=O bonds takes additional energy. A fair amount so a hydrocarbon like methane is reasonably stable even in an oxygen rich atmosphere. Now you have some components wandering around and a couple of C=O bonds are formed liberating a lot of energy. Also four O-H bonds form giving off even more energy.
The energy released by the reaction is greater than what was required to get it going. There is enough extra energy to keep the reaction going splitting up other methane and oxygen molecules.
4 These are mostly average values. For some (batteries, supercaps etc) there are a better values, but these are industrial average numbers .. just use for rough comparisons
If you are cooking for Thanksgiving you probably already have a good idea of what you're making, so rather than the recipe corner I'll offer a link to an old post that speaks about the rate of energy transfer - power. Our foods are hydrocarbon based and we're impressive compared to our household electrical needs when it comes to power. Consider that it is no big thing to eat my 525 calorie sandwich in five minutes (we can often consume food at rates two or three times higher) - that's over 7,300 watts! To put things in perspective a 15 amp circuit breaker blows at 1800 watts and the self cleaning mode on my over is 5,300 watts. Check out the post for the power involved in fueling your car.
To impress and possibly appall your visitors estimate how many calories someone eats in some amount of time. Now multiply by a conversion factor (4187 watts/calorie) and divide by time:
calories * 4,187/time in seconds == watts.
example: 2,500 calories in 2 hours is (2500*4187)/(2*3600) or about 1,454 watts. enough to light 14 one hundred watt lightbulbs over the period... I've convinced some people do well over 2,500 calories.