Collected energy management tips, tricks and resources

Technical material:

Building fabric
Compressed air
Heat recovery
Metering, monitoring and targeting
Motors and drives
Tanks and vats
Travel and transport

Other information and resources:

Degree-day data (partly free)
Training courses from various sources
Energy quiz questions
cartoons and editorial snippets for newsletters

Economic thickness of pipe insulation

Thanks to Hannah Paddock for posing a question about how to calculate the economic thickness of pipe insulation.

My advice to her was to get suppliers to provide her with before-and-after calculations of heat loss. I advised checking their sums for reasonableness using common sense (for running hours, boiler efficiency, fuel prices, etc) and a reference book such as Handbook of Energy Data and Calculations (Peter D Osborn, pub. Butterworths 1985) for heat-loss coefficients. If you can't get hold of a textbook then (with caution) try websites such as and to corroborate their heat-loss assumptions. Hunting around, I did find an on-line pipe insulation calculator but rather annoyingly it does not allow you to specify zero insulation. If it had done, it would have been more useful. I have contacted the author to suggest it. I have since found what looks like a better article at V.V. 30 October 2006

Training aid for power factor

This is the design of a rig I use for teaching people about power factor. It consists of a long spring-loaded pointer, pivoted at the left-hand end. The trainee pulls a string to pull the pointer down to a horizontal position. A spring balance indicates tension in the string. Tension here represents the current, while deflection represents the useful power delivered. Trainees observe that the minimum tension is needed when the string is pulled vertically downwards, as in the left-hand diagram; this is analogous to the minimum current being required when alternating voltage and current are in phase. Pulling the string at an angle (centre diagram) simulates what happens when current and voltage get out of phase. The tension in the string needs to be higher to achieve the same deflection (higher current to deliver the same useful power).

The rig can even demonstrate the effect of fitting a capacitor. Running the string around a pulley, as shown in the right-hand diagram, reduces the tension (current) required to achieve the same deflection (useful power), regardless of the phase angle.

Myth-busting: why full freezers aren't more efficient

It's sometimes said that a full freezer uses less energy - not true. When the door is closed and the load is at the required temperature, all the refrigeration circuit is doing is removing heat that leaks into the compartment through the insulated walls. The freezer could be full or empty - the rate of heat transfer will be no different. It will only change if the room temperature changes. You can detect that it must be dubious advice because of what people suggest for filling the space: bread (high mass and volume); empty ice-cream cartons (low mass, high volume); and crumpled newspaper (low mass, low volume). The only thing they have in common is they impede air circulation (of which more later).

Some people say that by keeping the freezer full, you reduce the amount of chilled air that can spill out. Maybe, but door openings account for only 1 - 2% of annual running costs so the effect is small... In fact one kilowatt-hour of electricity would rechill all the air in a 100-litre freezer about a thousand times.

Much more important in causing high running costs are the following:

  • Having damaged door seals
  • Allowing the condenser coil on the back to get dirty
  • Obstructing air flow over the condenser coil
  • Not cooling produce before putting it in
  • Allowing ice to build up inside
  • Siting the freezer in a warm place
  • Freezing stuff that then gets out of date
  • Opening the door too frequently
  • Keeping the door open too long
This last one is the killer: that's exactly what happens if you keep the freezer full; you need to keep rummaging around to find what you are looking for. But there is an even more compelling technical reason why efficiency falls and running-costs rise if the freezer is chock-full. In a full freezer, air cannot circulate properly, which impedes the thermostat's operation and causes inefficient operation because the refigeration compressor runs on after it should have stopped cooling.

This myth is almost universal and is promulgated by organisations that should know better. So where did it come from? It probably started with the perfectly true observation that a larger freezer uses more power, so it is unwise to buy one that is bigger than you need. Related to this is the idea that the running costs per pound stored are lower for a freezer that is fuller; but this "operational" efficiency has then been confused with thermodynamic efficiency, which is affected by a lot of things, but not by the loading.

Conclusion: store as much or as little as you need; but leave room for air to circulate.

Voltage reduction

You may be able to save electricity by voltage reduction if your mains supply voltage is significantly above the statutory minimum. The method works particularly well if lighting is a significant part of your load; motors also benefit.

Compressed air systems

The US Department of Energy Industrial Technologies Program has a page containing a number of links to free 'tip sheets' on such things as: alternative strategies for low-pressure end uses; storage and control strategies; system maintenance; effect of intake air on compressor performance; minimizing leaks; and other relevant topics. (added 19 March 2006)

Electrical energy saving exercises

Sequencing fans with VSD

What's the best strategy for part-load operation of a bank of fans with variable-speed drives?
Try this simulation. (V.V. 7 Jan 06)

True or false?

If you are leaving the room for less than ten minutes, it's cheaper to leave the fluorescent lights on... find out, feeding in your own assumptions about the size and duration of the startup surge. (V.V. 6 Jan 06)

Why poor power factor is a problem

If an electrical circuit suffers from poor "power factor", a higher current than necessary will need to be drawn in order to deliver the required power. This can increase power charges, or stress the supply infrastructure: see a simulation of the effect. (V.V. 6 Jan 06)

Sankey diagram software

The product recommended by one of our readers in October has been upgraded: click here for feedback from the user and news from the supplier.
(V.V. 2 Dec 05)

Automatic controls

Controls can have a part to play saving energy for all users from the largest to the smallest. As well as the well-rehearsed topic of controlling heating lighting and other building services, there are often opportunities in industrial situations as well: see article. (V.V. 20/11/05)

Energy targeting for humidity-control systems

: if you manage energy consumption in a building with full air conditioning, you might need to take ambient humidity into account when monitoring for excess electricity consumption. This article describes a novel targeting method that has been successfully applied in two large sites of this kind. V.V. 10/11/05

Energy targeting tricks for industrial processes:

this article describes four scenarios in which it was not possible to use the simple straight-line activity-based targeting methods that would be used in some industries like papermaking and bread-baking. V.V. 31 Oct 05

Energy metering

I found this page on the Switch2 Energy Solutions web site rather useful: it summarises the attributes of various meters for electricity, gas, steam, compressed air, water, and oil. The Spirax Sarco Learning Centre has an extremely good module on flow metering. I've added both companies to my contacts page as well. As for electricity, Northern Design Electronics, a highly reputable supplier, have a downloadable PDF document on selecting and installing secondary kWh meters (V.V. 9/9/05)

Accounting for the weather

Energy management leaflet 7 (PDF format) for readers who would like basic information about degree-day data to print out and file for reference. V.V., 3rd July 2005