Category Archives: Energy technologies

Power factor

In electrical power systems using alternating current, the voltage and current can get out of step. This has the effect of reducing the real power in watts; to make up the shortfall the magnitude of the current must increase.

In this six-minute ‘kitchen tabletop talk‘, I try to explain the phenomenon and illustrate it with apparatus made from wire, a spring, string and a chopstick.

Poor power factor wastes energy mainly through excessive losses in transformers and distribution cables. So if you pay for electricity at high voltage and own your own transformers, it definitely pays to do something about poor power factor. But even if you don’t, your electricity supplier will claw back extra money from you through any invoice charges denominated in kVA (kilovolt amperes) or kVAh rather than kWh or kW.

Voltage reduction and cyclical heating

Although it is perfectly true that reducing the supply voltage to an electric heater will reduce its power consumption, this is not the same as saying its energy consumption will decrease. In fact, if it is thermostatically controlled, it will merely run for longer to maintain the energy input needed to balance the thermal energy requirement of the process or space being heated. If the heat output is regulated, energy consumption will not reduce with lower voltage.

But for some heating processes the effect of reduced voltage is actually perverse. For example, consider a laser printer. One of the components in the printer, the fuser unit, contains a heated roller maintained at around 200°C whose job is to melt the toner particles on the paper surface. The heater in the roller is likely to be rated at 500 watts or more, and to minimise energy consumption it is turned off when the printer goes into standby. That is why it can take 20 seconds or so to wake the printer up: the heater element has to boost the roller back up to temperature.

Suppose the element has a resistance of 100 Ω at its normal operating temperature. At 240 V, the current flowing will be 2.4 A (by Ohm’s law) so its power will be 240 x 2.4 = 576 W. Reduce the voltage to 230 V and the current drops to 2.3 A, yielding 529 W, a power reduction of just over 8%.

However, reduced power output means it takes a little longer to reach operating temperature. In the simple simulation illustrated below, the warm-up time increases from 19 to 22 seconds.

laser_printer_warmup

Unfortunately, 22 seconds at 529 watts is 11.6 kJ, whereas 19 seconds at 576 watts (the higher-voltage scenario) is only 10.9 kJ. So the warm-up cycle uses more electricity at lower voltage. Sadly this additional startup energy consumption  is not counteracted by savings elsewhere, since all the other electronic components of the printer will be fed from stabilised power supplies and therefore do not respond to fluctuations in mains voltage.

OK, the energy waste penalty of operating at reduced voltage is absolutely tiny – I quite accept that – but the point is that it is not a saving, let alone a saving of 8%. Furthermore there are plenty of cyclical heating processes from kettles to school pottery kilns where minimising warm-up time will save more than downrating the heater output.

 

Voltage reduction and IT equipment

The vast majority of computing and communications equipment consists of circuitry that requires very stable supply voltages, usually at 3V, 5V and 12V DC. Because the electronics is supplied at fixed voltage, its power consumption depends entirely on what it is doing and is completely insensitive to mains supply voltage. Fluctuations in mains voltage are dealt with by having stabilised power supply units (PSUs), which these days are commonly engineered to accept AC inputs anything between 100V and 240V while giving the rock-steady DC output that the equipment needs.

psu_efficiencyPSUs are not loss-free. At 87% efficiency, 100W of useful DC output incurs 14.9W of heat loss (100/114.9=0.87). There is much competition between manufacturers to reduce these losses and the sales specification sheets now usually sport a chart like the one above, showing how PSU efficiency changes with loading. Peak efficiency (minimum loss) typically occurs at about half load. But what these charts also disclose is that losses are higher at lower voltage. The difference in this case (230V versus 115V) is extreme, with losses increasing from roughly 15% to 18% as a percentage of output power.

Admittedly a reduction of a few volts would only add a fraction of a percentage point to the PSU losses, but neveretheless although conventional wisdom is that voltage reduction has no effect on energy consumption in computer equipment, the truth is that it should actually incur a slight penalty.

If the IT equipment is in an air-conditioned environment, then what effect will voltage reduction have on chiller power? It depends how it is controlled but either the chillers’ outputs will fall, causing them to run longer to make up the deficit, or their control systems will maintain the requisite output, causing them to demand the same electrical input power. Either way, no saving on input power; you cannot get out more than you put in, and in fact the reduction of condenser cooling fan flow might well compromise the coefficient of performance, causing the chiller installation to use rather more input energy than at the higher voltage.

All in all, not a good prognosis for voltage reduction with IT equipment; and that is without mentioning the fact, often glossed over, that the voltage-reduction gear itself incurs energy losses.

Estimating savings from building-fabric improvements

If you improve a building’s insulation, or reduce its ventilation rate, the resulting energy saving can be estimated using simple formulae in combination with relevant weather-data tables. In the case of an improvement to insulation of an individual element of the building envelope, the approximate formula for annual fuel savings is

0.024 x (UOLD – UNEW) x A x DDA / EFF                         (kWh)

where  UOLD and UNEW are the original and improved U-values (W/m2K), and A is the area of building element being improved (m2).  EFF is heating-system efficiency, for which it would be reasonable to assume a value in the range of 0.8 to 0.9, reflecting the fact that 10-20% of the fuel used is accounted for by combustion losses.

DDA meanwhile is the annual heating degree-day figure, which is a measure of how cold the weather was in aggregate. Degree-day totals tend to be higher in the north and lower in the south; and they also depend on the outside temperature below which a given building’s heating needs to be turned on (the ‘base’ temperature). Selected totals are given in Table 1 for various regions and base temperatures. Buildings with high space temperatures and low casual heat gains have higher base temperatures, implying higher annual degree-day totals and thus bigger expected savings for a given improvement to their insulation.

Turning to the effect of reducing the building’s ventilation rate, we need to know the reduction in air throughput, RDV. If we express RDV in m3/day, the annual energy savings are given by this approximate formula:

(0.008 x RDV x DDA) / EFF                   (kWh)

DDA and EFF have the same meanings as before.

Use for air conditioning

The same techniques can be used to gauge the effect of reduced cooling load. In this case we use cooling degree days (examples in Table 2) and EFF is likely to be in the range 2 to 4, representing the chiller coefficient of performance. Saving one kWh of cooling effect saves much less than a kWh of electricity.

Base temperatures

The base temperature for heating depends on the temperature set-point, the construction of the building, how it is used, how densely it is populated and how much casual heat gain it experiences from lighting and equipment. It is invariably below the internal set-point temperature. How far below can be determined in various ways but there would typically be about 4°C difference.

Similar considerations apply to cooling: the cooling base temperature is the temperature above which it becomes necessary to run air conditioning. If you know air-conditioning is used throughout the year, a very low base (say 5°C) is appropriate. Otherwise something of the order of 15°C could be a reasonable assumption.

Table 1: Annual heating degree days1

Base temperature: 20°C 15°C 10°C
South West   3,189   1,576      503
Midland   3,632   2,033      860
N E Scotland   4,075   2,355   1,003

Table 2: Annual cooling degree days1

Base temperature: 25°C 15°C 5°C
South West          2      233   2,386
Midland          6      274   2,111
N E Scotland    0      111   1,649

1 The full tables can be downloaded from www.vesma.com. Click on ‘D’ in the A-Z index and look for ‘degree days’.

Voltage reduction: the short answer

I am somewhat conscious of taking my life in my hands in this issue, but as so many readers have asked me what I think about voltage “optimisation” (or reduction, to use a more accurate term), let me answer the question with the following three guidelines, which 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.

Link: Energy management training

How to waste energy No. 4: compressed air

1. Use compressed air for dusting off overalls, sweeping the yard and other cleaning duties. This not only wastes energy but blows debris into people’s eyes.

2. If you have individual applications that require a higher pressure, run the entire system to satisfy them rather than fitting local boosters.

3. Set overall system pressure as high as you can (check that the safety valves are lifting frequently). As a rule of thumb, every 2 psi increase in operating pressure requires an additional 1% energy.

4. For low-grade duties such as tank agitation, use clean dried compressed air at high pressure rather than fitting local blowers.

5. Locate air inlets in the hottest place possible – remember every 6C increase in temperature adds 1% to the electricity consumption.

6. Never clean your air filters and avoid fitting low-loss types.

7. Make sure you do not dry the air.

8. Allow all your compressors to run in parallel, sharing the load however small.

9. Do not shut the system down if the premises are closed at night; but if you do, empty the air receiver at the end of the day so that it needs to be repressurised in the morning.

10. Leave air-receiver drain cocks cracked open.

11. Bypass the air receiver so that the compressors have difficulty matching the load and need to start and stop frequently. This is a marvellously inefficient mode of operation, and abrupt swings in pressure will also help to maximise the number of leaking joints and fittings.

12. Maximise pressure drops in the distribution system by undersizing all pipework.

13. Ignore leaks: fixing one probably causes another to appear somewhere else. If you have a routine for tagging and repairing leaks, do not repair any that people find. As well as wasting energy this will discourage people from reporting air loss.

14. When specifying new equipment, give preference to models that continuously vent air. Use air tools if electric equivalents are just as good.

15. Look for opportunities to use compressed air inappropriately. Dusting off overalls may not waste enough; try using it for cooling motor bearings that are running hot, or to cool people working in hot locations.

16. Do not recover free heat from compressor exhausts if it is possible to use heat from a boiler system (or better still, electric heaters) instead.

Link: Energy management training

How to waste energy No. 3: 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.

Link: Energy management training

How to waste energy No. 2: automatic control of 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 setpoint. 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.

Link: Energy management training