Monday, February 2, 2015

And Now for Something Different

That something different has to do with electricity.

The electric bill for most people is simply. There is a charge for kilowatt hours. When it comes to electricity for those who have more than a 10KW demand, it becomes more complicated.

Their electric bill has two major components. First, like most other uses, there is the charge for KW hours used. The second major component is the demand charge.

How the demand charge is determined is probably most people have never thought about. This charge can have a large effect on the total of the electricity bill.

The first time I remember hearing about a demand charge was years ago when someone told me a story about radio transmitter up on mountain tops, miles away from anything. Many radio stations have two transmitters, one primary and a second for backup. The owner of the station told the people there that were repairing a transmitter not to power up the backup transmitted without first powering up the diesel generator. If they failed to do that, the owner would be paying a premium to the electric company for a year.

So how is the demand charge calculated? Here is the Dallas area Oncor takes the peak KWH used during a 15 minute time period and multiplies this by 4 to calculate the demand in KW. If you use 3 KWH of power during a 15 minute period, the KW demand would be 12KW (3KW*4=12KW). The demand charge in dollars would be about $120 for the month.

The demand charge can go up quickly, but it takes a year for it to goes down. So, if in January you have a high demand, the electric bill would include a demand charge that is at least 80% of the highest demand charge for the previous 12 months.

To give an example in 2012 our highest demand was in February for 22 KW and the lowest was in November for 7 KW. The demand charge in February was $195 (8.87/KW) and for the month with the lowest demand, November, the charge was $147.77 ($21.11/KW).

That one month where the demand was 22KW, for the rest of the year, increase cost by about $550.

Isolating why the demand was so high was fairly easy; our electric heater was the biggest draw on electricity. In the mornings when I would come in and turn on the heater it would run for about an hour or so before cycling off. A graph of power consumption show the first couple of hours in the morning had a high demand.

 In addition to the heater, other electrical loads would be the air conditioner condenser motor, the lights, air compressor and lift motors. The compressor is a 5 HP motor and draws about 3.7KW.

Here are estimated power draws:

A/C condenser motor – 4.4KW
Air Compressor – 3.7KW
Lights – 2.8KW

Electric Heater 14KW

Misc. – 1KW

In the winter months, the electrical demand could get up to about 22KW. The spring, fall and summer demand was much lower, about 8 KW. To get the electrical demand down what had to change was the high demand in the colder months.

The first step was to time the loads where two high demand items would not be powered up at the same time. My choice was to turn off the heater and A/C compressor motor any time the air compressor came on. The air compressor generally would run for about 5 minutes at a time. A transformer was wired in with the air compressor to control a relay near the thermostat which would disable either the heater or A/C whenever the air compressor came on.

The second step was to build and install a timer device based on a 555 timer. The 555 timer is control by a few resistors and capacitors and set to cycle a relay on and off in 7 minute 30 second intervals. So during a 15 minute period, it is on for 7.5 minutes and off for 7.5 minutes. By cycling on and off the heater every 7.5 minutes, the electric company sees the 14KW heater as a 7KW load. Overall, the same amount of energy is use. The modulating of the heater reduced the demand portion of the bill by 6KW. The savings per KW of demand is about $10.

The reason for cycling at 7.5 minute intervals was because of how Oncor determines how to charge for demand use, the highest KWH usage for any 15 minute period times 4.

More than a year has passed since making the changes and the results are in. The demand charges are down significantly and down by about $640 for the year. The trade-off? It takes longer to cool and heat.

Components to build the 555 timer circuit cost less than $10 and the relay cost about $10. While $640 is not a lot of money, it was worth spending the $20.

No comments:

Post a Comment