At last week's party, one of the discussions lead to the power demand of your typical petrol station1. What would the power requirement of a typical station be, if the energy from the fuel were supplied by electricity?
This is assuming you have some form of local storage, such as batteries under the ground where the tanks currently are, that would average out the daily peaks and troughs in demand.
Typical UK Petrol Station Throughput
To find out the typical demand I asked a colleague, who works in the retail business, what the typical petrol station usage was and got the following response:
the average fuel volume per site is 6 million litres but this hides the difference between smaller independents at 2.4 million litres per annum and supermarket sites averaging 10 mlpa. The fuel split is about [55% Diesel, 45% Petrol] although the filling of trucks at filling stations clouds the split between petrol and diesel cars as there are no statistics that split these two vehicle types.
So rather than just calculate the average figure (6 million liters per annum) I will do the calculation twice to show the range. Once using the 2.4 mlpa (independents) and once using 10 mlpa (supermarkets).
I am happy to take the 55%/45% split between Diesel and Petrol (so including the truck figures) as these and are included in the total volume2 and would also need to recharge as well.
Enthalpy of combustion
While I initially struggled to find heat of combustion values from what I considered a reliable source, I eventually found a reference to a document from Oak Ridge National Laboratory. Unfortunately when I followed the link I got a 404 not found error. Luckily the wayback machine had saved it so I could extract the values I needed:
|Fuel||LHV (MJ/kg)||HHV (MJ/kg)||Density (g/gal)|
|Reformulated or low-sulfur gasoline||42.358||45.433||2,830|
I have used the low-sulfur3 versions as I think these are the closest to what we actually sell, although the properties don't actually vary that much anyway.
So do we use the Lower Heating Value (LHV) or the Higher Heating Value (HHV) for the heat of combustion?
- The lower value is the amount of energy released when burning something starting at 25°C and cooling it back to 150°C.
- The higher value is the amount of energy released when burning something starting at 25°C and cooling it back to 25°C.
Essentially the higher value includes the energy from returning any water vapour back to liquid. I will use the Higher Heating Value and calculate the total energy delivered by the station.
Luckily this source also provides the density of the different fuels although it is given in grams per US gallon. Therefore we need to divide by 3.78 to convert it to grams per liter.
|Fuel||HHV (MJ/kg)||Density (g/l)||Ratio|
|Throughput (total vol)||2.4||10||mlpa|
|Throughput (petrol vol)||1.08||4.5||mlpa|
|Throughput (diesel vol)||1.32||5.5||mlpa|
|Throughput (petrol mass)||25.6||106.6||g/s|
|Throughput (diesel mass)||35.4||147.6||g/s|
|Energy (petrol)||1160||4835||KJ/s (or KW)|
|Energy (diesel)||1614||6727||KJ/s (or KW)|
So independents use on average 2.8 MW and supermarkets use 11.6 MW.
How big is this?
Newark sugar factory consumes around 9 MW of electricity when operating at full capacity. Its link to the national grid is only rated for around 1 MW4. So even the independent retailers are using a lot of electricity and this is assuming that local battery storage would perfectly level out the usage.
Does this mean that if everyone switched over to electric cars we would have to install many miles of extra power lines to all the 'petrol' stations?
- Using the term 'petrol station' because that is the normal name for it here even though they can sell more diesel. ↩
- although the split would be different for independents and supermarkets ↩
- or low-sulpher ↩
- The link is mostly used to import when the factory is offline in maintenance mode. When the factory is operating, electricity is generated on site using a steam turbine and a small amount is exported to the grid. ↩
- and I think it is pretty obvious why. ↩