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Diesel or electric?

21 July 2011

Diesel or electric?

 

 

July – as farms across the UK start harvest, diesel tanks will be emptied into combines and grain driers. But is the fuel a cheaper source of power than electric for irrigation pumps? FEC Services’ Andrew Kneeshaw urges growers to get the calculator out.

It’s a commonly held belief that even at its present price, diesel oil is a cheaper fuel than electricity, unit for unit.  And to some extent that’s correct.  The maths is simple.  There are roughly 11kWh of energy in a litre of diesel oil, so at a pump price of 67p per litre that equates to about 6.2p per kWh.  Therefore, with electricity now pushing 10p per kWh, on the face of it diesel looks the cheapest.

However that’s not the end of the story.  You also need to take into account the efficiency of conversion of the fuel to what you require it to do to get the full picture.  And if that’s to drive a pump for instance, things look a little different.

Diesel engines are not the most efficient things at producing ‘shaft power’.  In fact they could be described as heaters which produce mechanical power as a by-product.  Most of the energy either comes out of the radiator or the exhaust pipe leaving no more than 40% as useful power for pumping.

In contrast an electric motor is the model of high efficiency, converting 90% or more of its energy input to output power.

Time to get your calculator out…

The calculation goes:

  Input fuel price per kWh / efficiency  = output power cost
For diesel this is  6.2p / 40% = 15.5p/kWh
For electricity it’s  10p / 90% = 11.1p/kWh

So electricity wins hands down.

To add insult to injury, electric pumps are cheaper, need less maintenance and are more reliable.  So why would you use anything else you might ask?

Well, there’s always the sticky point of getting somewhere to plug them in of course.  And that can be a real problem.  Electricity supply connection costs have reached astronomic levels, with the average supply for a modest three-phase pump, reasonably near to a high voltage line, costing at least £15,000.  Go further from the supply, and for every span of electricity line you can add another £3,000.  The consequence is that, despite the cheaper cost of buying the pump total capital cost can go through the roof.

So the message is simple.  Do your sums before going either way.  Weigh up the running costs, maintenance requirements and set them against capital outlay and then and only then, armed with the figures, make your decision.

Andrew Kneeshaw (FEC Services Ltd)
July 2008

What’s your experience?  Do your figures agree with Andrew’s?  What’s your annual spend on pumping irrigation water?  Do you have a question for Andrew or one of the Sutton Bridge team?

E-mail the AHDB Potatoes Energy Hub now at sbeu@potato.org.uk – we’ll respond within one working day.