Voltage Optimisation

VOLTAGE REDUCTION PART 1: THE SHORT ANSWER

The following three guidelines apply to everything from heating and lighting to motive power:

1. If the equipment is regulated in any manner, don’t expect voltage reduction to save energy.
2. If it is unregulated and you don’t mind reduced output, voltage reduction will save energy.
3. If it is a thermal application used on an intermittent cycle, voltage reduction will have a perverse effect, increasing energy consumption.

VOLTAGE REDUCTION PART 2: A MORE DETAILED DISCUSSION

I will concede at the outset that if you have an electrical appliance with fixed resistance, then the power that it draws will vary with the square of the applied voltage, so that for example reducing the voltage by 5% will drop the power by nearly 10% (95% x 95% = 90.25%). But power is not the same as energy, and not every load is simply resistive.

Let’s take the distinction between power and energy first, and consider the case of an electric heater (a classic purely resistive device). If the heater is unregulated—running all the time regardless—then yes, reduced power consumption equates to reduced energy demand (fewer kilowatts multiplied by the same number of hours). But if the heater is thermostatically controlled, it will run for longer at lower power in order to deliver the heat output required to balance the heat demand. Result: no energy saving. More generally, whenever the output of the equipment in question is regulated, no savings should be expected because electrical energy input in such cases is dictated by the required energy output, be it heat from heaters or mechanical energy from motors. Conversely, if you do see a reduction in electrical energy input, it has to be because you have accepted reduced output.

You will sometimes see claims that voltage reduction reduces the energy consumed in motors. Actually it works only with some motors, specifically, those whose output is unregulated. Toilet extract fans would be a good example: at lower voltage they slow down and draw less air through. The same, however, is not true of a motor-driven system where there is a variable-speed drive (VSD) regulating mechanical power output. The VSD will increase the motor’s supply frequency to keep its rotor turning at whatever speed is demanded by the driven equipment, delivering the required mechanical power. And to be fair, most voltage-reduction vendors recognise this. But VSDs are not the only way that motors can be regulated. Suppose you had a booster pump delivering water to the top of a tall building. The load on its motor will be dictated by water demand, and as the motor does nothing more than convert electrical energy into mechanical energy (and a bit of heat), its electrical energy input must balance its mechanical energy output plus thermal losses. If it does draw less power, it will deliver water at a lower rate and have to run longer to make up the shortfall. Refrigeration compressor? Air compressor? Same logic. Lots of motor-driven systems have regulated mechanical outputs (and hence defined electricity inputs), not just those with variable-speed drives. So no saving in any of these cases, unless the losses in motors are reduced by running at lower voltage. But firstly those losses themselves are very small (limiting the impact of reducing them) and in any case they could well increase rather than decrease at lower voltage. This is because to deliver the same power at reduced voltage a motor must draw more current. This will increase what are known as the ‘copper’ losses (resistive losses in the windings) and although counteracted by reduced ‘iron’ losses (eddy currents in the magnetic circuits) the truth is that the sum of all the losses is a minimum at the motor’s rated voltage and operating away from its nominal voltage—higher or lower—will marginally increase the electrical energy required to deliver the required mechanical energy output.

The other type of ‘regulated’ load is one which will tolerate a wide range of supply voltages. The notebook computer I am using works equally well at 100 or 240 volts because its power supply delivers the same output voltage regardless. It is therefore going to be relatively insensitive to variation in mains voltage, and the same is true of other kit like fluorescent lights with electronic control gear.

Which brings me to my final category: intermittent thermal loads, of which the domestic kettle is a fine (if trivial) example. When you put electrical energy into a kettle, part of it goes into raising the temperature of the water, and part is lost as heat from the kettle’s surface. I’ll ignore what happens after you reach boiling point because I am going to focus on the heating-up phase during which the surface temperature of the kettle rises from say 10 to 100 degrees. Simplifying a lot, let’s say the average temperature of the surface will be 55 degrees regardless how long the process takes. That implies the same average heat flux from the surface, which in turn means the longer the heat-up cycle, the greater the aggregate heat loss and hence—oh dear—the greater the energy input. OK, kettles may not be a significant energy use, but if you have electric catering equipment, or a pottery kiln, or heat-treatment furnaces, or anything else whose warm-up time you would like to minimise, raising the voltage will be more energy-efficient than lowering it (hence my objection to the term “optimisation”).

For all these reasons it is essential to have a thorough load survey to determine what mix of responses you are likely to get (reduction, neutral or increase), always bearing in mind your future plans. Savings from yesterday’s voltage reduction could be slashed by tomorrow’s lighting project.

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How to waste energy – Frost Protection

Heating controls shows that frost protection running continuously is been a major cause of energy waste!. As an extreme example, some years ago a friend took over as energy manager of a substantial office building and discovered that its air-handling units had electric frost-protection preheaters which were accounting for 40% of the building’s electricity consumption. At another major building, one with BREAM excellent rating, the electricity consumption was 3 times higher than it should be because heating the air for ventilation to 18C all year around! In a third example, an engineering services manager came to work one rainy morning and noticed all the front steps were dry (except one; work it out…). So unnecessary running of frost-protection is something to watch out for all year.

HOW TO WASTE ENERGY: LIGHTING

  1. If your light fittings are the type with translucent diffusers, fill them with dead flies.
  2. Avoid replacing tungsten-filament light bulbs with compact fluorescent equivalents. Although it is now illegal to sell most general lighting service (GLS) filament lamps, one can still buy “rough service” equivalents which have the great advantage of being even less energy-efficient.
  3. Keep your external lighting on 24 hours a day. This encourages a culture of not caring about leaving things running when idle, and will help waste many times more energy than is used in the lights alone.
  4. Also keep your internal lights on continuously, not least because doing so will increase the demand for air conditioning.
  5. Provide excessive light levels in working areas and try to ensure that corridors and stairwells are even brighter (this removes one of the vital cues that prompt people to turn lights off when they leave empty rooms).
  6. Be careless when specifying automatic lighting controls. Choose the wrong sensor technology, so as to maximise nuisance switching. This has a dual benefit – it encourages people to override the control, and it also antagonises them so they won’t cooperate with other energy-saving initiatives.
  7. In shared workplaces, paint over any labels identifying which switch controls which zone.
  8. Choose automatic lighting controls with remote control handsets that cannot be understood without training. Then lose the instructions and the remotes.

HOW TO WASTE ENERGY : CONTROL SYSTEMS IN BUILDINGS

  1. Set your frost-protection thermostat at too high a temperature.
  2. Override your time control to run the plant continuously.
  3. Set heating controls for maximum air temperature.  The aim should be to make it so hot that occupants are forced to keep the doors and windows open, increasing the heat loss.
  4. Alternatively, place a baked-potato oven under the space temperature sensor. This will hold the heating off and encourage people to bring in electric heaters.
  5. If you have adaptive optimum-start control, set  the timings as if it were a conventional time-switch (i.e. with start of occupancy at the same time you would previously have asked the plant to start up).
  6. Also if you have adaptive optimum-start control, set a target temperature above the daytime control set point. The control will add more and more preheat every day because it never achieves the target temperature.
  7. If you have air conditioning, set it to cool to a lower temperature than your heating, so that the two systems run simultaneously providing perfect comfort at infinite cost.
  8. If you have humidity control, set it for the narrowest range conceivable. This will ensure you are nearly always either humidifying or dehumidifying.
  9. Remove or jam the linkages on valve and damper actuators.
  10. Do not commission your building energy management system; do not document the control philosophy or agreed settings; and as a backstop, lose the operating manuals.

HOW TO WASTE ENERGY No. 1: MOTOR-DRIVEN EQUIPMENT

  1. When a motor fails, have it rewound, as this will reduce its efficiency.
  2. If you need to replace a motor, use the cheapest and least efficient unit available (preferably oversized). Efficiency standards of new motors are being continuously improved, so you may need to shop on eBay.
  3. Shift motors slightly on their mounting plates so that any drives and couplings are misaligned.
  4. Ensure that drive-belts are slack. On multi-belt drives it can help to remove some belts. If possible, use the wrong kind of belt for the pulleys fitted.
  5. Change pulley ratios to drive fans and pumps at higher speed: on centrifugal fans and pumps, a 20% speed increase adds over 70% to the load.
  6. Neglect lubrication of bearings and gearboxes.
  7. Allow equipment to run continuously, whether it is needed or not. This has the added advantage of accelerating wear and tear, and reducing your power factor.
  8. When the driven equipment is decommissioned, at least leave its motor behind, energised and running.
  9. In dirty environments, do not clean any debris off motor cooling air inlets. The extra resistance to air flow will increase mechanical losses in the motor and, as a bonus, accelerate its failure by causing it to overheat.
  10. In situations where the mechanical output of a fixed-speed motor is controlled and regulated, run the motor below its rated voltage in order to increase the motor current and associated copper losses.

Reduce costs through LED lighting

LED lighting can greatly reduce energy consumption.  Here’s how:

  1. Survey your lighting and calculate the annual cost of running your lighting for each area. This will identify priority areas for changing as well as those areas where no change is required.
  2. Work out how to measure the energy consumption before and after the solution is implemented. Take weekly meter readings or use a sub meter on the lighting circuits if possible.
  3. Try behavioural change 1st and modify the cost calculation according to the new usage pattern.  Its best to try behaviour change 1st since it will impact on the ROI for new lighting.
  4. Select reputable suppliers for a shortlist.  This will reduce the issues with the new lighting if it doesn’t last as long as promised
  5. Allow the shortlisted suppliers to carry out their own surveys and propose a new solution.  A 1 for 1 replacement might not be required and lighting controls correctly applied will further reduce energy usage.
  6. Choose an apporpriate lighting colour.  Your current lighting will be yellow and you can choose a new colour more appopriate to your working environment.  Using daylight coloured lighting will provide a more attractive working environment in offices.
  7. Select a new supplier and implement the solution.  Measure the change in energy consumption and confirm the savings have been realised.

Top 4 Energy Efficiency Tips

These are my top 4 tips for reducing energy consumption:

  1. Insulation for cavities and roof voids can reduce heat loss by up to 35%
  2. Heating
    1. Check heating controls are set appropriately
      1. 16C for corridors or sports halls
      2. 20C for offices
      3. Fit Thermostic Radiator Valves to enable fine tuning of radiators
      4. Use zoning to turn heating off in areas to close them down when people leave rather than heat all of the building.  Try to keep one area operational only if some people need to stay late or for lettings.
      5. Check boiler controls are working properly so that boilers don’t operate when there is no demand
  3. Lighting
    1. Turn lights off when not required
  4. Baseload
    1. Check that all equipment is switched off at the end of the working day and don’t turn them back on until they are needed