Bigger is better?

The adage is that Bigger is always Better. Well, it isn't true. Some examples of where the right size in't the biggest and being too big can be a wasteful exercise.

Bigger is better?

Those of you who know me will be very aware that I'm a rather large individual (and I've been told "larger than life"). So I'm naturally inclined to the view that larger items are better than smaller items. "A good Big 'Un will always beat a good little 'Un", "there's no cure for cubes" and a lot more adages promote the concept of "economy of scale" and that a larger package is better value than a smaller one.

Trouble is, I work in two fields where that isn't always true; in Sustainability it is much more likely that a proportion of a large consignment will be wasted because it goes off before it can be consumed. Similarly, contamination will spoil more in total if it affects part of a large package rather than one of a number of smaller packages (One bad apple spoils the whole barrel, not what the Jackson 5 sang!).

I'll add that when we're talking about renewable generation (solar PV, wind turbine) the best return on investment (ignoring FiT questions) comes when you can use all the generated power on-site and gain the value of the electricity not bought rather than the much lower tariff for supplying to the grid (basically you get the best return per kW that way). So if you exceed your demand, your rate of return (though not the quantity) goes down!

And in energy efficiency, "right-sizing" isn't just a euphemism for "down-sizing" (aka reducing numbers), but is a valid concept.

When I first started designing heating systems, I was told the engineer/consultant's principle: "no one ever complained that the plant was too big, always opt for one size larger than your calculations say it should be" and I like most engineers followed the rule of over engineering everything I did to "add redundancy" and make sure nothing was "unduly stressed". A general rule is that for plants where you need “n” you always install “n+1” (at least). Unfortunately, I've realised over time that sometimes those simple rules don't apply.

I became Catterick Garrison's ENMAN and became aware that we had nearly 30MW of transformer capacity with 7MW or so of maximum input to the system. "What's wrong with that?" you ask. Well, transformers have fixed "copper losses" which only depend on the size of the transformer not the amount of energy moving through it, so an oversized transformer can easily lose more energy that way than it delivers.

One extreme example (and one of my predecessor ENMEN was most vociferous about the consultant involved) was a 1MW transformer with a MAXIMUM of 100kW load on it (the fact that there was another transformer with twice that amount available less than 50m away just added to his vexation) - the transformer never got its efficiency up to acceptable levels.

Another case related to a design for CHP which originally involved three small engines (small enough that their capital cost took a premium) which were "value engineered" in favour of a larger unit where the cost per kW was much smaller. "What's wrong with that? You're getting more bang for your buck, aren't you?" Well, no. Most CHP sets can deliver (either to the site or exported to the grid) all the electricity they can generate, so the electrical size is rarely a limiting factor. What is crucial is the heat demand and three small units can operate much more flexibly (and therefore longer) than one big unit which has limited capability to turn down and will quickly satisfy the heat load and therefore turn off and not generate electricity.

So the one large unit will be much less economically viable than the three small units despite its lower capital cost and higher output because it will run for less hours and generate less valuable electricity as a result. CHP system designers know this and generally size their units to maximise the electricity generated by taking as much of the heat load as possible – but not always!

The next issue relates to biomass boilers. Despite wood burning having been Man’s first source of heat and the technologies having been around even longer than me, it seems that the understanding of non-spcialists of how to install and operate biomass boilers  can be lacking in comparison with their gas and oil counterparts.

Biomass boilers often do not have the ability to turn off and relight automatically (or reliably) so unlike gas systems they do not generally turn off but go into a slumbering/smouldering mode where no fuel is added and air is limited to control combustion. The trouble is in that state a large boiler can still produce more heat than the average domestic gas boiler. Getting rid of that heat from the boiler into an already satisfied system is inherently not energy efficient.

That predicates to making the biomass smaller than one would for a gas or oil boiler where they can smoothly modulate down or turn off and equally smoothly turn back up again. So designers need to understand that for biomass (like CHP) “rightsizing” is important if the system is to operate efficiently and effectively. The old rule of “next size up” will actually result in energy wastage, unreliable operation and potentially a much shorter life for the plant.

So please: next time, get the size RIGHT, not just “big enough”.

Andy Clarke

Topics: Energy efficiency & low-carbon
Tags: apple | biomass | Energy Efficiency | gas | solar
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